investigation of acoustical and mechanical properties of ......investigation on chemical treatment...
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Investigation of Acoustical and Mechanical
Properties of Epoxy Based Natural Fibre Composites
Muhammad Khusairy Bin Bakri
(4222644)
A thesis submitted in fulfilment of the requirement for the degree of
Master of Engineering (by Research) (MEng)
Faculty of Engineering, Computing & Science
Swinburne University of Technology, Sarawak Campus
August/2015
i
ABSTRACT
Noise is unwanted sound, which is harmful to our auditory system. Sound absorbing
materials are used in reducing, eliminating or insulating the noise. Existing sound
absorbing materials are mostly made from asbestos and synthetic materials, which
are known to be harmful for the environment and health. Thus, this has motivated the
initiative to develop alternative materials. In this research, natural waste product such
as banana, luffa and oil palm fibre were blended with epoxy to form natural fibre
composites. Mechel, and Delany and Bazley empirical models were used as a
preliminary theoretical analysis in order to gain an understanding of the effect of
thickness, fibre size, flow resistivity and types of fibre on sound absorption. The
modelling showed that banana, luffa, and oil palm have almost similar sound
absorption behaviour. Based on the empirical models, it showed that with increasing
fibre diameter, the flow resistivity decreased, but the sound absorption still increased
as long as the porosity is high. Furthermore, with increasing material thickness, an
increase in the sound absorption is expected. Lastly, the modelling results showed
that the sound absorption, increased with increasing frequency. In the case of
acoustical and mechanical characterisation, it was revealed that the fibre loading and
alkaline treatment have a significant effect. It was found that the sound absorption
coefficient for the composites tested increased with increasing fibre loading. The
result also showed that the treated fibre gives better sound absorption compared to
the untreated counterpart. The treatment caused removal of certain wax, oil, and
certain structure (i.e. such as hemicellulose, cellulose, and lignin), which improved
the tensile and yield strength of the natural fibre composite materials. Chemical
treatment caused break, stretch or bend in the H, OH and CH bond structure of the
fibres which were validated by using Fourier transform infrared spectral analysis.
Furthermore, the changes on the surface roughness and the structure of the fibre can
be seen using scanning electron microscope and light/optical microscope. It showed
that the treated fibre have a rougher surface and larger lumen structure than the
untreated fibre. Based on the result of acoustical and mechanical test, it was
concluded that the novel composites developed would find new applications for
agricultural waste and reduced oil-derived polymers.
ii
ACKNOWLEDGEMENT
First of all, I am grateful to The Almighty God for establishing me to complete this
research and thesis. I wish to express my sincere thanks to Swinburne University of
Technology, Sarawak Campus and Universiti Malaysia Sarawak for providing
me with all the necessary facilities.
I would like to express my gratitude to my supervisors and collaborators, Dr.
Elammaran Jayamani, Dr. Soon Kok Heng, Dr Basil Wong, Professor Dr. Sinin
Hamdan and Dr. Md Rezaur Rahman for their support and supervision in pursuing
this research and provided me with an extended vision of the research. Furthermore,
their valuable comments, helpful advice, technical guidance and valuable feedback
throughout the development of the work especially related to the research.
I would like to express my gratitude to Swinburne University of
Technology, Sarawak Campus for providing the financial support for my
international conferences. Furthermore, I would like to express my gratitude to
Universiti Malaysia Sarawak for the collaboration in the international journal
publications and providing me with their lab facilities. I would also like to thank the
lab technician Mr. Fadillah Johari, Mr. Thomas Manggie and other Staff
Member at Swinburne University of Technology, Sarawak Campus and
Universiti Malaysia Sarawak for their provision of the experimental facilities and
technical assistance.
Finally, I wish to express my special thanks to My Family for their great
encouragement throughout my research, thesis and supporting funds, as well as my
friends and counterparts, Mr. Toh Ming Hui, Mr. Kong Wai Loon, Mr. Tan Yi
Zhou, Mr. Kelvin Yong, Mr. Mark Tee Kit Tsun, Mr. Aaron Isaac Kuek, Mr
Jackson Wong, Mr. Akshay Kakar, Mr. Win Ko Min, Dr. Lee Ker Chia, Mr.
Mohd Elfy Mersal, Mr. Izzul Islam Sazali, Mr. Abang Adib Izzaniq Abang
Iskandar, and Mr. Mohamad Azhar Zulkifli Handeri for their continual supports
and encouragement.
iii
AUTHOR DECLARATION
I hereby declare that this thesis entitled “Investigation of Acoustical and
Mechanical Properties of Epoxy Based Natural Fibre Composites” is the result of
my own research work except for quotations and citations which have been duly
acknowledged. I also declare that is, it has not been previously or concurrently
submitted for any other master students at Swinburne University of Technology
(Sarawak Campus).
Name: Muhammad Khusairy Bin Bakri
ID: 4222644
Date: 30th
August 2015
iv
LISTS OF JOURNAL PUBLICATIONS AND CONFERENCES
Journal Papers:
Bakri, MKB, Jayamani, E, Kok Heng, S, & Hamdan, S, 2015, ‘Reinforced
Oil Palm Fiber Epoxy Composites: An Investigation on Chemical Treatment of
Fibers on Acoustical, Morphological, Mechanical and Spectral Properties’, Materials
Today: Proceedings, vol. 2, no. 4-5, pp. 2747-2756.
Bakri, MKB, Jayamani, E, Kok Heng, S, Hamdan, S, & Kakar, A, 2015, ‘An
Experimental and Simulation Studies on Sound Absorption Coefficient of Banana
Fibers and Their Reinforced Composites’, Journal of Biomimetics, Biomaterials and
Biomedical Engineering, vol. 0, no. 0, pp. 0. (Accepted and Reviewed – Waiting for
Publication)
Bakri, MKB, Jayamani, E, Hamdan, S, Rahman MR, & Kakar, A, 2015,
‘Fundamental Study on the Effect of Alkaline Treatment on Natural Fibers Structures
and Behaviours’, ARPN Journal of Engineering and Applied Sciences, vol. 0, no. 0,
pp. 0 (Accepted and Reviewed – Waiting for Publication)
Jayamani, E, Hamdan, S, Rahman, MR, Kok Heng, S, & Bakri, MKB, 2014,
‘Processing and Characterization of Epoxy/Luffa Composites: Investigation on
Chemical Treatment of Fibers on Mechanical and Acoustical Properties’,
BioResources, vol. 9, no.3, pp. 5542-5556.
Jayamani, E, Hamdan, S, Rahman, MR, & Bakri, MKB, 2014, ‘Comparative
Study of Dielectric Properties of Hybrid Natural Fiber Composites’, Procedia
Engineering, vol. 97, no. 1, pp. 536-544.
Jayamani, E, Hamdan, S, Rahman, MR, & Bakri, MKB, 2014, ‘Investigation
of Fiber Surface Treatment on Mechanical, Acoustical and Thermal Properties of
Betelnut Fiber Polyester Composites’, Procedia Engineering, vol. 97, no. 1, pp. 545-
554.
Jayamani, E, Hamdan, S, Rahman, MR, & Bakri, MKB, 2015, ‘Study of
Sound Absorption Coefficient and Characterization of Rice Straw Stem Fibers
Reinforced Polypropylene Composites’, BioResources, vol. 10, no. 2, pp. 3378-3392.
v
Jayamani, E., Hamdan, S, Rahman, MR, Bakri, MKB, & Kakar, A, 2015,
‘An Investigation of Sound Absorption Coefficient on Sisal Fiber Poly Lactic Acid
Bio-Composites’, Journal of Applied Polymer Science, vol. 132, no. 34, pp. 1-8.
Jayamani, E, Hamdan, S, Rahman, MR, & Bakri, MKB, 2015, ‘Dielectric
Properties of Lignocellulosic Fibers reinforced Polymer Composites: Effect of Fiber
Loading and Alkaline Treatment’, Materials Today: Proceedings, vol. 2, no. 4-5,
pp.2757-2766.
Jayamani, E, Hamdan, S, Ezhumalai, P, & Bakri, MKB, 2015, ‘Investigation
on Dielectric and Sound Absorption Properties of Banana Fibers Reinforced Epoxy
Composites’, Jurnal Teknologi, vol. 0, no. 0, pp. 0 (Accepted and Reviewed –
Waiting for Publication)
Jayamani, E, Hamdan, S, Rahman MR, Bakri, MKB, Kok Heng, S, & Kakar,
A, 2015, ‘Analysis of Natural Fiber Polymer Composites: Effects of Alkaline
Treatment on Sound Absorption’, Journal of Reinforced Plastics and Composites vol.
0, no. 0, pp. 1-9.
Jayamani, E., Bakri, MKB, Kok Heng, S, and Kakar, A, 2015, “Acoustical,
Thermal and Morphological Properties of Zein Reinforced Oil Palm Empty Fruit
Bunch Fiber Bio-Composites,” Journal of Applied Polymer Science, vol. 0, no. 0, pp.
0-0. (Under Review)
Book Chapter:
Jayamani, E, Hamdan, S, & Bakri, MKB, 2015, ‘Sustainable Noise Control
Materials Based on Bamboo/Unsaturated Polyester Composites: Analysis of Factors
Affecting the Sound Absorption Coefficients’ Apple Academic Press, New Jersey.
(Accepted and Reviewed – Waiting for Publication)
Conferences Attended:
Bakri, MKB, Jayamani, E, Kok Heng, S, & Hamdan, S, (2015), ‘Reinforced
Oil Palm Fiber Epoxy Composites: An Investigation on Chemical Treatment of
Fibers on Acoustical, Morphological, Mechanical and Spectral Properties’, (4th
International Conference on Materials Processing and Characterization, ICMPC 2015,
14th – 15th March 2015, Organized by Department of Mechanical Engineering,
vi
Gongkaraju Rangaraju Institiute of Engineering & Technology, GRIET, Hyderabad,
India). – Paper Presentation for Reinforced Oil Palm Fiber Epoxy Composites: An
Investigation on Chemical Treatment of Fibers on Acoustical, Morphological,
Mechanical and Spectral Properties
Bakri, MKB, Jayamani, E, Kok Heng, S, & Hamdan, S, (2015), ‘An
Experimental and Simulation Studies on Sound Absorption Coefficients of Banana
Fibers and Their Reinforced Composites’, (Recent Advances in Nano Science and
Technology, 2015, RAINSAT 2015, 8th – 10th July 2015, Organized by Sathyabama
University in association with Central Leather Research Institiute , Chennai, India). –
Certificate of Presentation for An Experimental and Simulation Studies on Sound
Absorption Coefficient of Banana Fibers and Their Reinforced Composites.
Paper Award:
Jayamani, E, Hamdan, S, Kok Heng, S, Rahman, MR, Bakri, MKB, & Kakar,
A, 2015, ‘The Effect of Natural Fibres Mercerization on Natural
Fibres/Polypropylene Composites: A Study of Thermal Stability, Morphology and
Infrared Spectrum’, (International Conference on Material, Mechatronics,
Manufacturing, and Mechanical Engineering 2014, ICMMMM 2014, 24th – 25th
October 2015, Organized by International Postgraduate Network (IPN.org), Kuching,
Sarawak, India). – Best Paper Award for The Effect of Natural Fibres Mercerization
on Natural Fibres/Polypropylene Composites: A Study of Thermal Stability,
Morphology and Infrared Spectrum
vii
TABLE OF CONTENTS
ABSTRACT i
ACKNOWLEDGEMENT ii
AUTHOR DECLARATION iii
LISTS OF JOURNAL PUBLICATIONS AND CONFERENCES iv
TABLE OF CONTENTS vii
LIST OF FIGURES xii
LIST OF TABLE xv
CHAPTER 1 1
1.0 INTRODUCTION 1
1.1 SOUND 1
1.2 NOISE 3
1.3 SOUND ABSORBING MATERIAL 4
1.4 TYPE OF SOUND ABSORBER 7
1.5 SOUND ABSORPTION MECHANISM 8
1.6 MAIN AIM OF RESEARCH 9
1.7 OBJECTIVES OF RESEARCH 9
1.8 RESEARCH FLOWCHART 10
CHAPTER 2 11
2.0 LITERATURE REVIEW 11
2.1 OVERVIEW 11
2.2 FACTORS AFFECTING SOUND ABSORPTION OF ABSORBING
MATERIALS 11
2.2.1 FIBRE SIZE 12
2.2.2 POROSITY 13
2.2.3 FLOW RESISTIVITY 14
2.2.4 DENSITY 15
2.2.5 THICKNESS 15
2.2.6 TORTUOSITY 16
2.2.7 COMPRESSION 17
2.2.8 SURFACE IMPEDANCE 18
2.2.9 PLACEMENT/DESIGN 18
2.2.10 TEMPERATURE 19
viii
2.3 EMPIRICAL MODEL FOR SOUND ABSORBING MATERIALS 19
2.3.1 EMPIRICAL MODEL FOR THE FLOW RESISTIVITY 20
2.3.1.1 MECHEL MODEL 20
2.3.1.2 BIES & HANSEN MODEL 21
2.3.1.3 GARAI & POMPOLI MODEL 22
2.3.2 EMPIRICAL MODEL FOR THE SOUND ABSORPTION
COEFFICIENT 22
2.3.2.1 DELANY& BAZLEY MODEL 22
2.3.2.2 GARAI & POMPOLI 23
2.4 METHODS TO MEASURE SOUND ABSORPTION COEFFICIENT 24
2.4.1 REVERBERATION METHOD 24
2.4.2 STANDING WAVE METHOD 25
2.4.3 TWO-MICROPHONE TRANSFER FUNCTIONS METHOD 26
2.5 COMPOSITE CHARACTERISATIONS 28
2.5.1 ACOUSTICAL PROPERTIES 28
2.5.2 MECHANICAL PROPERTIES 31
2.5.3 CHEMICAL MAPPING ANALYSIS BY FOURIER TRANSFORM
INFRARED SPECTROSCOPY (FTIR) 34
2.5.4 MICROSTRUCTURE ANALYSIS 36
2.6 FIBRE TREATMENT 36
2.7 SUMMARY AND RESEARCH GAP 37
CHAPTER 3 39
3.0 METHODOLOGY 39
3.1 OVERVIEW 39
3.2 SOUND ABSORPTION MATERIALS 40
3.3 EMPIRICAL MODELS USED 41
3.4 CALCULATIONS 42
3.5 MATLAB EMPIRCAL MODELLING 44
3.6 MATERIAL PREPARATION 45
3.7 FIBRE PREPARATION 45
3.8 FABRICATION OF NATURAL FIBRE SOUND ABSORPTION TEST
SPECIMENS 48
3.9 FABRICATION OF NATURAL FIBRE EPOXY COMPOSITE
MATERIALS 48
ix
3.10 DESIGN CRITERIA FOR TWO-MICROPHONE TRANSFER
FUNCTION IMPEDANCE TUBE METHOD 49
3.10.1 THE SOUND SOURCE 50
3.10.2 THE TEST SPECIMEN 50
3.10.3 THE CONSTRUCTION OF TUBE 50
3.10.4 THE MICROPHONES 51
3.11 FABRICATION OF THE TWO-MICROPHONE TRANSFER
FUNCTION IMPEDANCE TUBE METHOD 52
3.12 CALIBRATION OF THE TWO-MICROPHONE TRANSFER
FUNCTION IMPEDANCE TUBE METHOD AND SOUND ABSORPTION
CALCULATION 55
3.13 SOUND ABSORPTION TEST 58
3.14 TENSILE TEST 59
3.15 LIGHT/OPTICAL MICROSCOPE 59
3.16 SCANNING ELECTRON MICROSCOPE 60
3.17 FOURIER TRANSFORM INFRARED SPECTROSCOPY 61
CHAPTER 4 62
4.0 RESULTS AND DISCUSSION 62
4.1 EMPIRICAL MODELLING 62
4.1.1 BANANA FIBRE 62
4.1.1.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF
BANANA FIBRE ON SOUND ABSORPTION 62
4.1.1.2 EFFECT OF THICKNESS OF BANANA FIBRE ON SOUND
ABSORPTION 65
4.1.2 LUFFA FIBRE 67
4.1.2.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF
LUFFA FIBRE ON SOUND ABSORPTION 67
4.1.2.2 EFFECT OF THICKNESS OF LUFFA FIBRE ON SOUND
ABSORPTION 70
4.1.3 OIL PALM FIBRE 72
4.1.3.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF OIL
PALM FIBRE ON SOUND ABSORPTION 72
4.1.3.2 EFFECT OF THICKNESS OF OIL PALM FIBRE ON SOUND
ABSORPTION 74
4.2 EXPERIMENTATION 76
x
4.2.1 ACOUSTICAL PROPERTIES OF BANANA, LUFFA, OIL PALM
FIBRE (SOUND ABSORPTION ANALYSIS) 76
4.2.2 BANANA FIBRE EPOXY COMPOSITES 80
4.2.2.1 MORPHOLOGICAL OF BANANA FIBRE EPOXY
COMPOSITES (SCANNING ELECTRON MICROSCOPE AND
LIGHT/OPTICAL MICROSCOPIC ANALYSIS) 80
4.2.2.2 ACOUSTICAL PROPERTIES OF BANANA FIBRE EPOXY
COMPOSITES (SOUND ABSORPTION ANALYSIS) 82
4.2.2.3 MECHANICAL PROPERTIES OF BANANA FIBRE EPOXY
COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS) 86
4.2.2.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS
OF BANANA FIBRE EPOXY COMPOSITE 88
4.2.3 LUFFA FIBRE EPOXY COMPOSITES 92
4.2.3.1 MORPHOLOGY OF LUFFA FIBRE EPOXY COMPOSITES
(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL
MICROSCOPIC ANALYSIS) 92
4.2.3.2 ACOUSTICAL PROPERTIES OF LUFFA FIBRE EPOXY
COMPOSITES (SOUND ABSORPTION ANALYSIS) 94
4.2.3.3 MECHANICAL PROPERTIES OF LUFFA FIBRE EPOXY
COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS) 96
4.2.3.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS
OF LUFFA FIBRE EPOXY COMPOSITES 98
4.2.4 OIL PALM FIBRE EPOXY COMPOSITES 101
4.2.4.1 MORPHOLOGY OF OIL PALM FIBRE EPOXY COMPOSITES
(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL
MICROSCOPIC ANALYSIS) 101
4.2.4.2 ACOUSTICAL PROPERTIES OF OIL PALM FIBRE EPOXY
COMPOSITES (SOUND ABSORPTION ANALYSIS) 104
4.2.4.3 MECHANICAL PROPERTIES OF OIL PALM FIBRE EPOXY
COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS) 107
4.2.4.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS
OF OIL PALM FIBRE EPOXY COMPOSITES 108
4.3 COMPARISONS BETWEEN BANANA, LUFFA AND OIL PALM
FIBRE AND ITS COMPOSITES 111
4.3.1 COMPARISONS BASED ON EMPIRICAL MODEL 111
4.3.2 COMPARISONS BASED ON EXPERIMENTATION 112
4.3.3 COMPARISONS BASED ON EMPRICAL MODEL AND
EXPERIMENTATION 113
xi
CHAPTER 5 115
5.0 CONCLUSIONS AND FUTURE WORKS 115
5.1 CONCLUSIONS 115
5.2 APPLICATIONS 116
5.2 FUTURE WORKS 117
REFERENCES 118
APPENDICES 134
GANTT CHART FOR RESEARCH 134
SODIUM HYDROXIDE DATASHEET 135
EPOXY RESIN PART A DATASHEET 145
EPOXY RESIN PART B DATASHEET 148
xii
LIST OF FIGURES
Figure 1: Typical structure of human ear (Cheremisinoff, 1996) 2
Figure 2: The typical behaviour of absorption of three different type of sound
absorber (Jacobsen et al., 2011) 7
Figure 3: The flowchart of the overview research 10
Figure 4: The flowchart showing the overview of literature review 11
Figure 5: Standing wave method 25
Figure 6: The two-microphone transfer function impedance tube method 27
Figure 7: Composites schematic views (Mohanta and Acharya, 2013) 33
Figure 8: Flowchart of methodology 40
Figure 9: Creating a new M-file on MATLAB 44
Figure 10: Banana fibre: (a) untreated, and (b) treated 47
Figure 11: Luffa fibre: (a) untreated, and (b) treated 47
Figure 12: Oil palm fibre: (a) untreated, and (b) treated 47
Figure 13: Hydraulic press machine 49
Figure 14: Mould for: (a) tensile test, and (b) sound absorption test (circular
cavities) 49
Figure 15: Schematic diagram of two-microphone method 52
Figure 16: The two-microphone transfer function impedance tube test rig 53
Figure 17: Sinusoidal sound wave for microphone A and B 54
Figure 18: Fast Fourier transform graph of sound wave: (a) real part (the
incidence peak), and (b) imaginary part (the reflective peak) 54
Figure 19: Tensile test machine 59
Figure 20: Light/optical microscope 60
Figure 21: Equipment for morphological test (a) scanning electron microscope,
and (b) sputter coating machine 61
Figure 22: Infrared spectrometer 61
Figure 23: The effect of fibre diameter of banana fibre on the sound absorption
(5 mm thickness) 63
Figure 24: The effect of material thickness of banana fibre on the sound
absorption (32 µm fibre diameter) 66
Figure 25: The effect of fibre diameter of luffa fibre on the sound absorption (5
mm thickness) 68
xiii
Figure 26: The effect of material thickness of luffa fibre on the sound
absorption (19 µm fibre diameter) 71
Figure 27: The effect of fibre diameter of oil palm fibre on the sound absorption
(5 mm thickness) 73
Figure 28: The effect of material thickness of oil palm fibre on the sound
absorption (22 µm fibre diameter) 75
Figure 29: The sound absorption coefficient: (a) untreated banana, luffa, and oil
palm fibre without epoxy (5 mm thickness), and (b) treated banana, luffa, and oil
palm fibre without epoxy (5 mm thickness). 78
Figure 30: Comparison between untreated and treated sound absorption
coefficient: (a) banana fibre (5 mm thickness) (b) luffa fibre (5 mm thickness),
and (c) oil palm fibre (5 mm thickness). 79
Figure 31: SEM micrographs of banana fibres: (a) tangled banana fibres, (b)
hollow lumen structure, (c) untreated banana fibre, and (d) treated banana fibre 81
Figure 32: Light/optical micrographs of banana fibre: (a) untreated banana fibre
(b) treated banana fibre 82
Figure 33: The effects of fibre loading on sound absorption for banana epoxy
composites: (a) composites of untreated fibre (5 mm thickness), and (b)
composites of treated fibre (5 mm thickness) 83
Figure 34: Illustration of tortuosity in fibre 84
Figure 35: Illustration of type of pores in hollow fibre 85
Figure 36: The effects of chemical treatment on banana fibre for banana fibre
epoxy composites at 15 wt% (5 mm thickness) 86
Figure 37: The effects of banana fibre loading on: (a) tensile strength, and (b)
yield strength 88
Figure 38: Fourier transforms infrared spectroscopy analysis of: (a) untreated
banana fibre; and (b) treated banana fibre 89
Figure 39: SEM micrographs of luffa fibres: (a) randomized tangled luffa fibres,
(b) hollow lumen structure, (c) untreated luffa fibre, and (d) treated luffa fibre 93
Figure 40: Light/optical micrographs of luffa fibre: (a) untreated luffa fibre (b)
treated luffa fibre 94
Figure 41: The effects of fibre loading on sound absorption for luffa epoxy
composites: (a) composites of untreated fibre (5 mm thickness), and (b)
composite of treated fibre (5 mm thickness) 95
xiv
Figure 42: The effects of chemical treatment on luffa fibre for the luffa fibre
epoxy composites at 20 wt% (5 mm thickness) 96
Figure 43: The effects of fibre loading on: (a) tensile strength, and (b) yield
strength 97
Figure 44: Failure of the luffa fibre epoxy composite. Failure on the epoxy has
initiated whereas a significant portion of the fibre is still intact 98
Figure 45: Fourier transforms infrared spectroscopy analysis of: (a) untreated
luffa fibre; and (b) treated luffa fibre 99
Figure 46: SEM micrographs of oil palm fibres: (a) untreated oil palm fibres, (b)
treated oil palm fibres, (c) hollow lumens structure of oil palm fibre, and (d)
agglomeration of oil palm fibre 102
Figure 47: Light/optical micrographs of oil palm fibre: (a) untreated oil palm
fibre (b) treated oil palm fibre, and (c) fracture part of oil palm fibre epoxy 103
Figure 48: The effects of fibre loading on sound absorption for oil palm epoxy
composites: (a) composites with untreated fibre (5 mm thickness), and (b)
composites with treated fibre (5 mm thickness) 105
Figure 49: The effects of chemical treatment on oil palm fibre for oil palm fibre
epoxy composites at 15 wt% (5 mm thickness) 106
Figure 50: The effects of oil palm fibre loading: (a) tensile strength, and (b) yield
strength 107
Figure 51: Fourier transforms infrared spectroscopy analysis of: (a) untreated oil
palm fibre, and (b) treated oil palm fibre 109
xv
LIST OF TABLE
Table 1: Speed of sound in different mediums (Salvato et al., 2003) 2
Table 2: Typical noise levels in different environments 4
Table 3: Value of the eight coefficient equation from new impendence model
(NMI) compare with Delany & Bazley model value (Garai and Pompoli, 2005) 23
Table 4: Summary of acoustical properties for natural fibre 30
Table 5: Summary of acoustical properties of natural fibre composites 31
Table 6: Luffa fibre epoxy composites mechanical properties (Mohanta and
Acharya, 2013) 32
Table 7: Summary of mechanical properties of natural fibre composites 34
Table 8: The properties of natural fibre materials 41
Table 9: Maximum recommended frequency based on microphone size diameter
(ASTM E1050-12, 2012) 51
Table 10: Properties of the banana fibre and the respective flow resistivity at
different fibre diameters 63
Table 11: Properties of the banana fibre and flow resistivity value used in
empirical calculations 65
Table 12: Properties of the luffa fibre and the respective flow resistivity at
different fibre diameters 68
Table 13: Properties of the luffa fibre and flow resistivity value used in empirical
calculations 70
Table 14: Properties of the oil palm fibre and the respective flow resistivity at
different fibre diameters 73
Table 15: Properties of the oil palm fibre and flow resistivity value used in
empirical calculations 75
Table 16: The characterisation of banana fibre using infrared spectral analysis 91
Table 17: The characterisation of luffa fibre using infrared spectral analysis 100
Table 18: The characterisation of oil palm fibre using infrared spectral analysis 110
1
CHAPTER 1
1.0 INTRODUCTION
1.1 SOUND
A simple common definition of sound is something that can be heard by the animal
and human auditory system (Talty, 1988) which includes the auditory parts of
sensory system and sensory organs (i.e. the ears). In scientific terms, sound was
created because of the variation of oscillation wave due to the changes in pressure in
a medium that composed of different sets of frequency range, that can be diagnosed
both by animal or human auditory system that later be transmitted to brain to be
analysed. This was described by Cory (2005), Crocker and Arenas (2007) and Talty
(1988) in their publication of fundamental of sound. Sound travels, transfers or
transmits in different types of medium such as air, water and solid from one to
another except in vacuum medium. The sound wave is a transverse wave as in a solid
medium. If the sound wave is in other mediums apart from solid, it is then
transmitted in the form of longitudinal waves and with some mixture of transverse
waves.
One of the most common sources of sound is a speaker. When a speaker is on,
it creates shock wave and vibration motion on the surface area of the solid diaphragm
of the speaker. The motion on the surface area of the solid diaphragm forces air
molecules to vibrate and move. Thus, these create a fluctuating up and down (or back
and forth) pressure in the surrounding air (Talty, 1998). Similar fluctuating wave can
also be created in pulsating gases in-car exhaust system or moving fluids in flowing
water in a river (Wakefield Acoustics Ltd., 2004).
It is known that the structural mechanisms of human hearing in the inner and
middle parts of the ears are very sensitive to the rapid change of fluctuating air
pressure. A rapid change of fluctuating air pressure caused the vibration in the inner
and middle parts of the ears. This vibration is also called as hearing sensation. This
sensation produces a signal that later being transcended and transmitted into the brain
to be analysed and recognized as sound. Figure 1 shows the typical structure of the
human ear. A sound can travel in any directions radiated from its source. According
to Wakefield Acoustics Ltd. (2004), the stronger the source of the sound created, for
2
example through the motions; the bigger the pressure waves, and hence the louder
the sound, thus faster vibration of the source and the higher sound frequency.
Ear Drum
Hammer
Anvil
Eustachian Tube
Ossicles
Cochlea
Fluid
Nerve
MIDDLE
EAR
INNER
EAR
OUTER
EAR
Organ of Corti
Figure 1: Typical structure of human ear (Cheremisinoff, 1996)
A normal young person normally can receive approximately 15 Hz to 16 kHz
range of audible frequency. However, the sensitivity of normal hearing can detect
sound frequency approximately 3 kHz to 6 kHz and at a pressure approximately 20
µPa. When comparing this to an atmospheric pressure (101.3 kPa), it may vary in
fractional variation approximately 2x10-10
(Vér and Beranek, 2006). Based on
Crocker and Arenas (2007), the sound level is normally expressed in term of Decibel
(dB) and sound frequency is measured in terms of cycles per second or Hertz (Hz).
As the sound wave enters different mediums, the speed will vary accordingly. The
sound that move, transfer or transmitted either fast or slow is dependent on the
density of the medium it enters or produces. Table 1 shows the speed of sound in
different mediums.
Table 1: Speed of sound in different mediums (Salvato et al., 2003)
Medium Speed
ms-1
fps
Air (at 21°C) 344 1129
Wood (soft) 3350 10991
Concrete 3400 111155
3
1.2 NOISE
In terms of unsophisticated understanding, noise was considered as unwanted sound
that can be heard by human or animal. According to Cheremisinoff (1996), noise is a
result of discordant sound because of the change and non-periodic vibration in a
medium, in term of physical and mechanical views. Unfortunately, the definition of
noise is completely dependent on the hearer level of understanding. A noise can
create an emotional feeling as responses, depending on the level of consciousness
and subconsciousness of the feeler (Cheremisinoff, 1996). A certain variation and
level of sound causes the hearer to feel anger, frustrated, distracted and stressed and
this effect cause changes in physiological and psychological emotion. Thus, the noise
effect can affect our behaviour, health, environment, communication and etc.
(Cheremisinoff, 1996).
Hearing damage is normally results from the long exposure to noise in a
certain duration and intensity. Hearing loss can either be permanent or temporary,
depending on the situation and sensitivity of hearing. In general, it is believed that
the exposure to loud noise can cause significant temporary hearing loss or threshold
shift that may lead to permanent hearing loss for prolonged or recurring exposure.
Unfortunately, the exact relationship between permanent and temporary hearing loss
could not be clarified. Hearing loss in the high frequency ranges might seriously
affect the way and understanding of speech. Thus, distinguish the consonants of
speech that carried information on the higher frequencies of hearing is very
important (Cheremisinoff, 1996).
Although evidence to support non-auditory on health effects cases may not be
as complete as the hearing loss cases, there should be a concern regarding on this
matter. This is because noise can alter the normal functioning of the endocrine,
neurological and cardiovascular systems. It may cause a rise in blood pressure and
affect the in-equilibrium disorder in body system, constriction of blood vessels, and a
change in heart rhythm. Noise may produce similar effects, or compound effects of,
other more common stresses; as studies performed on animals to prove such
phenomena. However, it is not clear enough to support, whether a change in
physiology and psychology are permanent due to recurring noise (Cheremisinoff,
1996). The effects on behavioural may range from the dulling response to certain
4
specify auditory signals in noisy environments that might produce a frustration,
falling to a sensitizing of annoyances that normally would be ignored.
Noise can magnify the minor exasperation especially in a work environment.
Furthermore, it is known that human performance is affected by the presence of
noise, especially those who involved in information gathering, processing and
analysing. It is acknowledged that prolonged exposure to noise could affect the
health. The issue of exposure to noise and its undesirable effect on health is only
limited to the workplace, such as industrial and office work. Technological advances
have provided a lot of new design and engineering system such as engines, tires and
subways that contributed a lot of noise to the environment. Sometimes, household
noise sources can be as severe as industrial sources. For example, household
machinery such as chain saw, lawn mowers, hand tools, televisions, stereos, fans and
air conditioners are considered as noise sources. Table 2 shows the typical noise
levels in different environments. These noise levels are sometimes left unmonitored
and uncontrolled. It may subsequently affect the unregulated and unprotected
occupants. Thus, these noise levels may be significant in causing hearing impairment.
Table 2: Typical noise levels in different environments
Source of Noise Sound Level (dB) References
Iron factory 100.4 Ali (2011)
Power plant 91.5 Ali (2011)
Residential areas (in city) 55 Ali (2011)
School yard 55 Sarantopoulos et al. (2014)
Roadside 70 Sarantopoulos et al. (2014)
1.3 SOUND ABSORBING MATERIAL
The different structure of sound absorbing materials, including porous structure,
hollow structure, multi-dimensional size and length structure or solid composite
materials have their unique sound absorbing characteristics upon interacting with a
variety of sound frequency and intensity. Thus, in order to understand the
development of sound absorbing material, a brief review on the asbestos based sound
absorbing materials and the reason it was replaced with more advanced alternatives
are given below.
5
Asbestos was the first materials that being used as sound insulating or sound
proofing materials. Asbestos is also considered as fibrous mineral materials which is
not easily burn and corrode. It is easily deformed with other materials and it is
generally considered as a good electrical insulator. According to National
Toxicology Program (2011), the term asbestos is the common name for a specific
generic mineral that occurs in the mineral that consists of natural occurring mineral
fibre such as actinolite, chrysotile, crocidolite, tremolite, amosite and anthophyllite.
According to National Toxicology Program (2011) and National Park Service (1999),
the first application of asbestos is reported in the early 1800’s in the United States in
line with the booming of industrial revolution in the country. Although at the first
stage introduction of asbestos was very challenging and rare, it managed to reach its
highest peak demand in the 1940-1975 eras. Asbestos was widely used in building as
fire insulator and sound proof product for roof and ceiling in school, houses, office
and plant.
Unfortunately, during the introduction stage of asbestos to industry, there is a
lack of understanding on the harmfulness of asbestos toward human, animal and
environmental health. After a decade, because of the advancement in the sound
proofing technologies and research, the threat of asbestos started to reveal. The
asbestos materials when reacts with air or water in a long period of time may release
small particle poisonous fibrous molecules from the fibre materials which volatile
and yet insoluble. In other words, it means that the small particle poisonous fibrous
molecules released are unreactive and has a slow degradation rate in the environment
(Clement International Corporation, United States Public Health Service, and United
States Agency for Toxic Substance and Disease Registry, 2002).
The small particle released from the asbestos material caused health problem
when it is inhaled for a long period of time. This will cause the deformation of small
growth, which is called a cancer. According to Churg et al. (1984), McDonald and
McDonald (1997) and Bernstein et al. (2005), the extreme bio-persistence,
inflammatory and strong scouring action when inhaled, asbestos (especially tremolite)
is considered to be co-responsible for lung cancer and mesothelioma even when it is
present as a contaminant phase embedded within other asbestos fibres such as
chrysotile and also within non-asbestos materials, as for example talc and vermiculite.
6
With the increasing awareness on the effect on health posed by asbestos
material, most usage and applications of asbestos material have been banned in most
countries. According to Kogel et al. (2006), countries within the European Union
have banned the use of asbestos. Tight restriction in importing and exporting causes
some factories to stop asbestos production and eventually has to terminate their
operation due to the low demand on asbestos. However, some company in the
industry manages to pursue other alternative material such as synthetic fibre
materials made from artificial materials. As a result, the related industry of synthetic
fibres or artificial fibres has expanded during their introduction as the main substitute
for asbestos fibres. It is known that there are more than hundred types of synthetic
fibre in the world today.
According to Rouette (2001), synthetic fibres are often manufactured with
cellulose as starting material that commonly made from natural polymers. Due to the
large demand and the urgent need to replace asbestos material, the worst effect
possessed by synthetic fibre was taken slightly due to the lack of on this area. After a
research conducted in lab, it was found that the synthetic fibre was having similar
behaviour, characteristic and hazardous properties as asbestos material, specifically
the inhalation of synthetic fibre may cause injury in the lung and formation of growth
called cancer, according to Su and Cheng (2009). It is also known that the usage of
synthetic fibre is bad for the environment because it has slow degradation and
deterioration rate. After many health and environmental cases about synthetic fibre
and asbestos reported, the research and development of alternative sound absorbing
materials based on natural fibre are emerging. The replacement of synthetic fibre
with natural fibre materials can help to reduce the health and environmental problem
cause by synthetic fibre and asbestos fibre materials. It is known that natural fibre is
affordable, renewable, process able, recyclable, sustainable and biodegradable
(Fragomeni and Venkatesan, 2010).
A lot of researchers reported for natural fibre composite materials, however,
most of them only reported the acoustic behaviour of the material without taking
consideration of other properties (i.e. mechanical, chemical, morphological and etc.).
These properties, especially the findings of the mechanical properties, the availability
of certain chemical functional group in the fibre-polymer interface, and the
morphological information enable further interpretation of the structure-properties
7
relationship. Thus, research on using natural fibre composite material as alternatives
in the application, such as sound proof insulation materials with constructive
structure is very promising.
1.4 TYPE OF SOUND ABSORBER
Sound absorber can be referred to the model or method of the sound being absorbed
in the material. It can be categorized into three types of sound absorber which are
porous absorber, panel absorber (membrane) and resonators (Helmholtz). Figure 2
shows the typical behaviour of absorption of three different types of sound absorber.
There are a lot of common porous absorber materials, such as carpet, fibrous mineral
wool, open-cell form and etc. Generally, this type of material allows air or sound
wave to flow inside of the material structure which comprises of many cavities,
interstices and channel. It also can be categorized as cellular, granular and fibrous
according to Rey et al. (2012). Throughout this report, it studies focuses on fibrous
porous composite material.
Figure 2: The typical behaviour of absorption of three different type of sound
absorber (Jacobsen et al., 2011)
Panel absorber is characteristically a non-rigid or non-porous type of sound
absorber which modelled over an airspace that vibrated in a flexure mode in response
to sound pressure exerted by adjacent of air molecules. This panel normally takes on
honeycomb structure and has a smooth membrane cover that help to shield it from
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 500 1000 1500 2000
Sou
nd
Ab
sorp
tio
n C
oe
ffic
ien
ts (α
)
Frequency (Hz)
Porous Absorber Helmholtz Resonator Panel Absorber
8
humidity. Because of the characteristic that it has, according to Kang & Fuchs (1999),
this panel absorber type is also being used as silencers or as surface treatment in
architectural applicants. Panel absorber is also called as membrane absorber.
Resonators used to absorb sound in a low and narrow-type frequency band
range. Resonators type of sound absorber normally has the characteristic like
perforated material and opening end (holes and slots) materials. An example of
resonators is Helmholtz resonators. Helmholtz resonators have a connecting cavity in
the main duct that connected in series that produce two resonances at low-frequency.
Due to some limitations in Helmholtz resonator, it is not efficient in absorbing and
controlling noise that does not have tonal characteristics (Tang et al., 2012).
1.5 SOUND ABSORPTION MECHANISM
When sound is produced, it generates a vibration in the molecule of air. The
vibration in the air molecule causes the energy be transferred from one molecule to
another molecule in the air. When it reaches to the absorber material, it will transfer
the energy into the sound absorbing material from air medium to solid medium
through the collision particle of air and sound absorbing material. The energy
transfers from air to the sound absorbing material causes the sound absorbing
material to vibrate as well. The vibration and collision between the sound absorbing
material and air cause friction which generates heat.
Therefore, almost all the heat generated is absorbed by the sound absorption
material and some of them are reflected back in the air. According to Cox and
D’Antonio (2009), the flow of sound wave expands and contracts when flowing
through irregular pores. Furthermore, there is a change in the direction of flow and
this change causes the sound wave to lose momentum. The perfect sound absorber
will absorb almost all the noise created and reflects a minimal quantity of sound.
According to Vér and Beranek (2006), the sound energy degenerated after being
absorbed by the sound absorbing material before it reaches a surface due to reflection
from the rigid backed of the material. This also implies that the porous materials used
should be sufficiently thick in order to reduce the noise reflected.
9
1.6 MAIN AIM OF RESEARCH
The aim of this project is to study the novel sound absorbing materials, specifically
those based on agricultural waste/product natural fibre and polymer matrix
composite material, and understand their absorbing mechanism by using empirical
modelling and experimentation.
1.7 OBJECTIVES OF RESEARCH
In order to achieve the main aim, the following objectives have to be fulfilled:
Empirical modelling for predicting sound absorption coefficient of fibres.
Design and fabrication of the test rig and subsequently characterize the sound
absorption coefficient of novel sound absorbing composites.
Fabrication of agricultural waste natural fibres reinforced polymer matrix
composites.
Investigation on the cause or factor (i.e. fibre loading, chemical treatment and
etc.) that affecting the sound absorption of natural fibres and composite
material.
Evaluation on the effects of fibre-polymer interface adhesion on acoustical,
mechanical, spectral, and morphological properties of composites.
10
1.8 RESEARCH FLOWCHART
Investigation of Acoustical and
Mechanical Properties of Epoxy Based
Natural Fibre Composites
Literature Review
Research Proposal
Higher Research Degree Committee
Approval
No
Yes
For Empirical Modelling For Experimentations
Aim and Objectives
Identifying Empirical Model For Sound
Absorbing Materials: Delany and Bazley Garai and Pompoli Mechel Bies and Hansen
Modelling Sound Absorbing Materials Using
MATLAB
Fabrication of The Two-Microphone Transfer
Function Impedance Tube (ASTM E1050-12)
Fabrication of The Natural Fibre Composites
Calibration and Improvement of The Two-
Microphone Transfer Function Impedance Tube
(ASTM E1050-12)
Sound Absorption Test (ASTM E1050-12)
Light/Optical Microscopic Test (ASTM E2015-04)
Tensile and Yield Test (ASTM D638-14)
Scanning Electron Microscopic Test (ASTM E2015-04)
Fourier Transform Infrared Spectroscopy Test (ASTM, E168-06 and ASTM E1252-98)
Results and Discussions
Conclusions
Future Works
Thesis PreparationJournal Publications and
Conferences
Figure 3: The flowchart of the overview research
11
CHAPTER 2
2.0 LITERATURE REVIEW
2.1 OVERVIEW
In this literature review, the understanding of the current knowledge, including the
substantive findings, either in term of theoretically or methodologically in the aspect
of sound absorbing materials and related influence on the sound absorption will be
reported. This literature review covers the factors that influence the sound absorbing
material needs to be identified and the underlying fundamental needs to be
comprehended. Apart from that, the uses of empirical models to predict the sound
absorption coefficient were also covered. Figure 4 shows the flowchart of literature
review covered.
Literature Review
Empirical Models to Predict Sound Absorption
for Materials
Experiment Validation and
Characterisation
Empirical Models to Predict Sound Absorption Coefficient: Delany & Bazley Garai & Pompoli
Fibre Size Porosity Flow Resistivity Density Thickness Tortuosity Compression Surface
Impedance Placement/
Design Temperature
Reverberation Method
Two-Microphone Transfer Function Impedance Tube Method
Standing Wave Method
Factors Affecting Sound Absorbing
Materials
Empirical Models to Predict Flow Resistivity: Mechel Bies & Hansen Garai & Pompoli
Acoustical Mechanical Spectral Morphological
Type of Sound Absorption Testing
Treatment
Figure 4: The flowchart showing the overview of literature review
2.2 FACTORS AFFECTING SOUND ABSORPTION OF ABSORBING
MATERIALS
Sound absorption occurs when the sound wave passes through a material and a
reduction in the acoustic energy is resulted. The materials capable of reducing
acoustic energy significantly are also called as sound absorptive materials. The
change in the parameters (i.e. porosity, density and etc.) is an important thing that
12
needs to be understood deeply because it can influence the properties of the materials
to decide the outcome of sound absorption properties of material such as the
absorption coefficient. Sound absorbing materials were normally used to control an
acoustic environment into a certain level of an enclosed volume by reducing the
amplitude of the reflected waves. Common factors, including fibre size, porosity,
flow resistivity, density, thickness and etc. that affect sound absorption of the
materials will be investigated and discussed.
2.2.1 FIBRE SIZE
Fibre sizes may vary depending with materials that were usually used. The changes
in the fibre size can either be the changes in the size of the fibre diameter or the size
of the fibre length itself. It is also one of the main factors that influenced the sound
absorption properties of porous materials. The effect due to changes in fibre size can
be observed through related studies reported in the literature. Lee and Joo (2003)
managed to conduct an experiment based on the usage of three different types of
non-woven fabric materials that were made of recycled polyester fibres that having
the same fibre length but different in denier. The fibre deniers used were 1.25 den, 2
den and 7 den in diameter with a 38 mm in length. For binding purposed, low
melting point polyester staple fibre was used, that having a 6 den in diameter with 42
mm in length. Denier (den) is the unit measurement of the fibre linear mass density
of fibre size. Lee and Joo (2003) concluded that the fine fibre content increases the
sound absorption coefficient values of the materials due to an increase in the airflow
resistance by mean of friction of viscosity through the vibration of the air.
Koizumi et al. (2002) also reported that an increase in the sound absorption
coefficient coherent with a decrease in fibre diameter size. The reason behind this
was due to that sound waves can easily move within the thin fibre compared to the
thick fibres. Meanwhile, according to Sun et al. (1993), by using fine fibre denier,
more fibres are needed to fulfil or reach equal of its original fibre content or same
volume density. This resulted in higher airflow resistance and more tortuous path. A
study by Koizumi et al. (2002) also showed that the fine fibre denier that range from
1.5 to 6 dpf (denier per filament) performed better acoustically compared to the
coarse denier fibres. Furthermore, it is also reported that the micro denier fibres (less
13
than 1 dpf) can provide a dramatic increase in the acoustical performance of sound
absorption in the materials (Koizumi et al., 2002).
2.2.2 POROSITY
The simplest definition of porosity is an empty void or space in materials. According
to Allard et al. (1989), the porosity of materials is defined as the ratio of the volume
of the voids in the porous materials to its total volume. In terms of mechanical and
mathematical perspective, porosity is the fraction of the volume of medium voids (i.e.
air). Porosity normally traps gas (i.e. air), liquid (i.e. water) or solid (i.e. starch) form
of materials. The porosity in materials is higher, especially for materials which are
having a foam type structure or open pores such as porous silicon and porous rubber.
Furthermore, according to Shoshani and Yakubov (2001), for designing an absorber
of non-woven webs which has a high sound absorption coefficient, the structure of
the absorber should have increasing porosity along the direction of propagation of
the sound wave. Based on Canham (1997), porosity can be identified in many ways
and it depends on the shape and size packed inside a material. The formula for
porosity can be identified as below:
Porosity, 𝜙 =𝑉v
𝑉t (1)
Whereas, 𝑉v is the volume of medium voids (i.e. air) in the acoustical materials (m3),
and 𝑉t is the total volume of the acoustic materials (m3).
According to Sagartzazu et al. (2008), the values of porosity are limited in the
range of 0 to 1. In other word, it is measured from 0 % to 100 %. It is known that
some porous absorber like polymer foams and fibrous polymer, the porosity can lie
between the ranges of 0.95 to 0.99. The effect of porosity can be supported through
research conducted by Knapen et al. (2003). In their research, the porosity was
manipulated and the effect on the sound absorption coefficient against frequency was
investigated. The high porosity in a material caused the material surface contact with
sound wave to increase and causes more acoustical energy being dissipated. Other
than that, the number, size and types of pores are important factors that should be
considered while studying sound absorption mechanism in porous materials. Thus, to
allowed sound dissipation by friction, the sound waves have to enter the porous
14
materials. In other words, there should be enough pores within the material and on
the material surface for the sound to pass through and get dampened.
2.2.3 FLOW RESISTIVITY
A determination on how much the air can enter a porous absorber and the resistance
to the air flow from the entering the structure is called flow resistivity. Thus, flow
resistivity is one of the most important aspects that influence the characteristics of
sound absorption in materials. It is also known that the specific flow resistance per
unit thickness of the materials influences the sound absorbing properties of fibrous
materials. Interlocking within the fibres in non-woven materials provides the friction
that resists acoustic wave motion. Generally, when a sound wave enters these
materials, the amplitude will decrease as friction happens when the waves travels
through the tortuous and rough passages. The acoustical or sound wave energy was
converted into heat energy as sound waves energy pass through the frictional rough
and tortuous passage of the materials (Conrad, 1983). Friction quantity can be
expressed by the resistance of the materials to airflow that is called airflow resistance
and it is defined as the following:
Flow Resistivity, 𝑅f =∆𝑃
∆𝑇𝑢 (2)
Where, ∆𝑃 is the differential sound pressure across the thickness of the sample (Pa),
∆𝑇 is the incremental thickness (m), and 𝑢 is the particle velocity through the sample
(ms-1
).
Based on Crocker and Arenas (2007), the high flow resistivity materials had a
high resistance of air flow through the materials. This showed that materials with
high flow resistivity sometimes tend to reflect more sound than absorbed it.
Furthermore, according to Ren and Jacobsen (1993), because of the impedance and
propagation constant characteristic, which normally describes as the acoustical
properties of porous materials, the flow resistance of the materials were governed to
a great extent. For a porous material, the airflow resistivity per unit thickness is
proportional to the shear viscosity coefficient of the fluid (i.e. air) involved and
inversely proportional to the square of pore size characteristics of materials. Thus,
for fibrous materials with given porosity, this showed that the flow resistivity per
unit thickness is inversely proportional to the square of fibre diameters (Uno Ingard,
15
1994). Meanwhile, according to Cox and D’Antonio (2009), flow resistivity may
vary between common porous absorber, thus this can imply that it is one of the most
important factors in the research.
2.2.4 DENSITY
Density is considered to be the important parameter that influences the behaviour of
sound absorption coefficient in the materials. According to the studies done by
Koizumi et al. (2002), there was an increase of sound absorption coefficient as the
material density increased, especially in the middle and higher frequency. Similar
results were shown by Ballagh (1996) research on wool materials with varying bulk
density of the materials. Based on both results, it showed that the low density type of
material absorbs more sound of low frequency compare to the higher density
materials that normally absorb middle and high frequency. Furthermore, the material
starts to perform well with a big margin or gap range in higher density starting from
500 Hz and above. This is because the energy loss will increase as the surface
friction increases in a denser material which allows an increase in the sound
absorption coefficient. Moreover, when the apparent density is high, the quantity of
fibres per unit area increases. The sound absorption coefficient increases as the
energy loss increases when the surface friction increases. The density of a material
composite can be determined according to the formula:
Density, 𝜌 =𝑊a
𝑊a+𝑊w−𝑊b(𝜌water) (3)
Where, 𝑊ais the specimen weight in air (kgm-3
), 𝑊w is the partly immersed wire
specimen holder weight (kgm-3
), 𝑊b is the specimen weight fully immersed in
distilled water along with the partly immersed wire specimen holder (kgm-3
) and
𝜌water is the density of distilled water at testing temperature (kgm-3
).
2.2.5 THICKNESS
There is an enormous amount of study done for porous materials, especially on the
sound absorption behaviour and the effect of its thickness. According to Coates and
Kierzkowsld (2002), an effective sound absorption of a porous absorber was
achieved when the thickness of the materials is around one tenth of its incident sound
wavelength by following the rule of thumb. Meanwhile, according to Hirabayashi et
16
al. (1995), peak absorption might occur at one quarter incident sound wavelength at
the resonance frequency. Thus, these showed that there is a relationship between
thickness either at low, medium or high frequency sound absorption. According to
Ibrahim and Melik (1978), as the material increases in the thickness, at the low
frequency, the sound absorption will increase indirectly. However, at certain high
frequency, the sound absorption showed an inconsistent effect.
The effects of thickness against sound absorption were also included in
Hakamada et al. (2006) report. According to Hakamada et al. (2006), based on their
experiment on the three samples of material with different thickness at 1 mm, 2 mm
and 20 mm, the result clearly showed the effect of thickness difference on the sound
absorption coefficient. It showed that the thicker the materials, the more sound would
be absorbed. Material thickness can enhance the acoustic impedance for porous
materials (Huang et al., 2008). It can be seen clearly in Fatima and Mohanty (2011)
results, where the natural rubber latex jute composites showed higher sound
absorption coefficients in thick composites as compared with thin composites.
Meanwhile, Jiang et al. (2012) also reported that the chlorinated polyethylene (CPE)
and seven-hole hollow polyester fibres (SHPF) composites show improved sound
absorption when the thickness increased. Ersoy and Küçük (2009) also reported that
a linear increase in sound absorption coefficient can be observed in the sample
results, as the thickness increased.
2.2.6 TORTUOSITY
In general, the measurement of the elongated passage way through the pores in
certain thickness of the sample was called tortuosity. However, there is a lot of
definition describing tortuosity. According to Yamashita et al. (2009), tortuosity was
defined as a parameter in the internal structure of porous materials that represent in
the quantity of sound waves that enter the materials air space. Meanwhile, according
to Knapen et al. (2003), tortuosity is the internal structures that influence the
acoustical properties of the material. According to Wassilieff (1996), tortuosity is the
measurement of how far the pores deviate from the normal, or twist bout the material
axis. Tortuosity equation can be defined as below:
Tortuosity, 𝜏 = 𝜙𝑅s
𝑅f (4)
17
Where, 𝜙 is the porosity, 𝑅s electrical resistivity of the saturated sample (N.s.m-4
)
and 𝑅f is the resistivity of the fluid used (N.s.m-4
).
Based on Sakagami et al. (1996) report, it appears that the optimum sound
absorption is achieved by the less tortuous materials rather than the more tortuous
materials. Knapen et al. (2003) stated that the more tortuous the materials, the lower
absorption coefficient it had. Higher tortuosity causes larger fluctuations in the sound
absorption coefficient. The fluctuations will start to manifest from the lower
frequencies as the tortuosity increased. However, Sakagami et al. (1996) stated that
tortuosity only had a small effect on the noise transmission coefficient. Horoshenkov
and Swift (2001) reported that the tortuosity only affects the location about a quarter
wavelengths of its peaks, whereas the flow resistivity and porosity may involve in
affecting the height and width of the peaks. Furthermore, the value of tortuosity
might determine the high frequency response of sound absorbing porous materials.
2.2.7 COMPRESSION
The effect of compression can play a vital role in the field of sound absorption or
acoustical absorption. The results of manipulating porous materials such as fibrous
materials or cellular materials can create different variants of physical parameters
that can affect sound absorption. The compression of materials caused the sound
absorption may vary in different porous materials. According to Castagnede et al.
(2000) and Wang et al. (2008), the sound absorption properties decreased as the
fibrous mats or porous materials being compressed. Under compression condition,
the various fibres in the mat were brought nearer to each other without deformation
(without any change in fibre size) (Castagnede et al., 2000). The compression also
resulted in a decrease in thickness and indirectly cooperates with other physical
variation (Castagnede et al., 2000).
Based on the study by Castagnede et al. (2000), compression caused an
increase in flow resistivity and tortuosity, and indirectly decreased the shape factor
(include characteristic length) and porosity. Thus, this sometimes caused
inconsistency in sound absorption. According to Wang et al. (2008), the large
compression rate caused the effect on the sound absorption at the lower frequency, as
the final thickness of porous materials is invariable. Unfortunately, these physical
parameter variations in compressed materials do not caused the reason for the drop in
18
sound absorption directly because it normally related to the decrease in sample
thickness based on the experiment Castagnede et al. (2000) conducted. According to
Keshavarz and Ohadi (2013), compression may either give negative or positive
effects on sound absorption coefficient depending on the compression method.
2.2.8 SURFACE IMPEDANCE
Jayaraman (2005), has investigated the influenced of surface impedance on sound
absorption by using air laid samples with different configurations. It can be seen that
there is an increase in the sound absorption coefficients when the surface impedance
of the materials were altered. Significantly, the surface impedance can influence the
sound absorption by altering the tortuous path and porosity of the material. Higher
sound resistivity was expected when the material with a given layer of thickness
produced higher dissipation. The reason behind this was due to the surface
impedance that increased at the same time with resistivity that caused a greater
amount of reflection, especially on the surface layer that indirectly produces a lower
absorptive capability. Moreover, according to Simón and Pfretzschner (2004), the
whole process must be frequency dependent, as the lower frequency bands on the
necessary layer thickness increases, the resistivity will decrease.
2.2.9 PLACEMENT/DESIGN
According to Everest (2001), the different types of sound absorbers used in different
places such as sides, ends and ceilings at different axial modes (can either in
transverse, vertical or longitudinal axis) may influence the sound absorption of
materials. Typically, absorbing materials placed near corners or along edges of the
room surfaces demonstrated effective sound absorption, especially in a rectangular
room. Taking example of speech studios, the absorbing materials should be placed at
head height on the walls, so that it will be more effective at higher audio frequencies.
At a certain point, the sound absorption can be twice more effective as the same
material placed elsewhere if it were applied at lower portions of high walls (Everest,
2001). Moreover, the material surfaces should never face each other.
If there is an air cavity behind the materials, the materials will act as a
membrane of a certain mass. This design is frequency dependent. The air inside the
materials cavity is analogous to mechanical spring. Significantly, the sound
19
absorption of materials is enhanced by embedding an air gap between the rigid back
surface and the sample (Hakamada et al., 2006). In the research work by Hakamada
et al. (2006), it was observed that the air gap increase when the thickness decrease.
Aso and Kinoshita (1965) concluded that the greatest absorption coefficient in a thin
sample of large porosity slightly decreased as in the depth of the air space increased.
Besides, their study revealed that the absorption coefficient in a thick sample is
almost constant, independent with the profundity of its air space.
2.2.10 TEMPERATURE
Srivastava et al. (2006) and Harris (1966) examined the effect of temperature on the
mineral sound absorption and the outcomes showed that sound absorption was
affected when there was a change in temperature. Furthermore, Srivastava et al.
(2006) had established a theoretical connection between the thermal conductivity
using a least-square regression method and the noise reduction coefficient. These
relations were done under the distinctive temperature condition.
2.3 EMPIRICAL MODEL FOR SOUND ABSORBING MATERIALS
It is known that the sound absorbing material properties can be anticipated by
microstructural and phenomenological models. However, the validity limited to a
certain condition. In the empirical models formulation, the theoretical considerations
were not generally considered, nor are the inside structure of the material taken into
account (Ballagh, 1996).
In the past few years, there were several models developed (Qunli, 1988;
Dunn and Davern, 1986) based on the model and principles laid down by Delany and
Bazley (1970). These models were subsequently extended by Mechel (Mechel and
Ver, 1992). Impedance and propagation coefficient characteristic of the material can
be predicted by taking the flow resistivity of the materials and normalized it by the
frequency of desired. From the complex quantities, another prediction can be made
for acoustic properties. Although, this method was used extensively, however, some
materials like natural wool are outside the limits of the available empirical models. It
is best to use microstructure models in some of the cases (Ballagh, 1996). It is
because that it is more detailed in knowledge as the consideration of physical
properties and internal structure of the material were taken in consideration to predict
20
the acoustic performance. The models used normally correlated with such parameters
like tortuosity, porosity, shape factors, and flow resistivity. There is a lot of
development on different models over the past few years by Attenborough (1982),
Rayleigh (1896), Biot (1956), and Zwikker and Kosten (1949). Other complex model
structure used shape factors also have been modelled by Allard and Champoux
(1992), and Johnson et al. (1987).
2.3.1 EMPIRICAL MODEL FOR THE FLOW RESISTIVITY
The influenced of flow resistivity in a material for sound absorption is significant.
The increased and decreased of sound absorption coefficient are normally related
with the change of flow resistivity that profoundly in the materials. Thus, flow
resistivity plays a role in modelling sound absorption. It is known that the flow
resistivity can be manipulated by either make it as constant or variable in the formula
in modelling of sound absorption coefficient. The model to determine the flow
resistivity can be taken from Mechel (2008), Garai and Pompoli (2005), and Bies and
Hansen (1980). The flow resistivity is normally derived from related calculation and
measurement based on the fibre radius, porosity and the bulk density of the materials
relates.
2.3.1.1 MECHEL MODEL
Based on Mechel (2008) models, the derivation of the relationship between the fibre
and the sound incidence that is perpendicular to it is by having the same radii for
flow resistivity. Normally, the prediction equations were based upon the fibre radius
and bulk density or material porosity of the materials. As stated below, the equations
based on a model created by Mechel (2008):
Flow Resistivity, 𝜎 = 10.56 (𝜂
𝑟2) (
𝜇1.531
𝑟2(1−𝜇)3) (5)
and
Flow Resistivity, 𝜎 = 6.8 (𝜂
𝑟2) (𝜇1.296
𝑟2(1−𝜇)3) (6)
Where, 𝜂 is the viscosity of the medium (i.e. air viscosity for 1.84 × 10−5𝑃𝑎. 𝑠), 𝑟 is
the radius of the fibre (m) and 𝜇 is the massitivity (𝜇 = 1 − 𝜀, where 𝜀 is the porosity
of the material). Equation 5 and 6 can be simplified as below:
21
Flow Resistivity, 𝜎 =10.56𝜂(1−𝜀)1.531
𝑟2𝜀3 (7)
and
Flow Resistivity, 𝜎 =6.8𝜂(1−𝜀)1.296
𝑟2𝜀3 (8)
Where, 𝜀 is the porosity of the material and 𝑟 is the radius of fibre (m).
As for fibre diameter range from 6 µm to 10 µm, Equation 5 was used and for
a fibre diameter range from 20 µm to 30 µm, Equation 6 was used. According to
Oldham et al. (2011), it stated that for material having a small amount of binder or no
closed cell, the porosity, 𝜀 is given by the equation below:
Porosity, 𝜀 ≈ 1 −𝜌b
𝜌m (9)
Where, 𝜌b is material bulk density (kgm-3
) and 𝜌m, is the material matrix density
(kgm-3
).
2.3.1.2 BIES & HANSEN MODEL
Bies & Hansen (1980) model is based on the materials which have the same matrix
materials with the bulk density as the main functions of porosity. According to Bies
and Hansen (1980), the flow resistivity can be expressed as below:
Flow Resitivity, 𝜎 =𝐾
𝑑2𝜌m−1.53 (10)
Where, 𝐾 is the constant value (3.18 × 10−9), 𝑑 is the fibre diameter (m), and 𝜌𝑚 is
the bulk density of the material (kgm-3
). The model was revised by Oldham et al.
(2011) and is given as below:
Flow Resistivity, 𝜎 =3.18×10−9
4𝑟2𝜌−1.53 (11)
Where, 𝑟 is the radius of fibre (m). Based on Oldham et al. (2011), the expression
above was measured and obtained by using fibreglass and fibre mineral with a matrix
material density approximately 2600 kgm-3
.
22
2.3.1.3 GARAI & POMPOLI MODEL
According to Oldham et al. (2011), the improved Garai & Pompoli model is based on
Bies & Hansen model that is for fibre with a variable diameter (i.e. between 18µm to
48µm), a mean value (i.e. around 33µm) with a matrix density material
(approximated 1350 kgm-3
). Below is the equation based on Garai and Pompoli
(2005):
Flow Resistivity, 𝜎 =28.3×10−9
4𝑟2𝜌−1.404 (12)
Where, 𝑟 is the radius of fibre (m). The equation created by Garai and Pompoli (2005)
before the update is based on Bies & Hansen model formula as below:
Flow Resistivity, 𝜎 = 𝐴𝜌m𝐵 (13)
Where, 𝜌mis the bulk density of the material (kgm-3
), and both A and B are the free
parameters of the materials. It is best to decide an ideal value for A and B. Garai and
Pompoli (2005) managed to obtain A and B values as 25.989 and 1.404 by using
least-square best fitting method.
2.3.2 EMPIRICAL MODEL FOR THE SOUND ABSORPTION
COEFFICIENT
There are two models which predict sound absorption coefficients were presented by
Garai and Pompoli (2005) and Delany and Bazley (1970).
2.3.2.1 DELANY& BAZLEY MODEL
According to Oldham et al. (2011), Delany & Bazley model were obtained by a
simple power-law relations by best-fitting a big amount of experimental data for a
range of fibrous porous absorbers. The data obtained were normalized based on
dimensional variable (which is the frequency and the flow resistance) and
represented in power-law relations. By that, they manage to create the empirical
formula below.
Characteristic Impedance, 𝑍c = 𝜌0𝑐 (1 + 0.0571 (𝜌0𝑓
𝜎)
−0.754
− 𝑗0.087 (𝜌0𝑓
𝜎)
−0.732
)
(14)
23
Complex Wave Number, 𝑘c =𝜔
𝐶(1 + 0.0978 (
𝜌0𝑓
𝜎)
−0.754
− 𝑗0.187 (𝜌0𝑓
𝜎)
−0.732
)
(15)
Where, 𝜌0 is the air density (kgm-3
), 𝜎 is the flow resistivity (Nsm-4
), 𝑓 is the
frequency (Hz), and 𝜔 = 2𝜋𝑓 is the angular frequency (rads-1
). The equation, is
limited in the range of 0.01 < 𝜌0𝑓
𝜎 < 1. Based on Delany and Bazley (1970), it was
shown that the propagation of sound was influenced by two complex quantities
which are characteristic impedance, 𝑍 = 𝑅 + 𝑗𝑋 and coefficient propagation 𝛾 =
𝛼 + 𝑗𝛽.
2.3.2.2 GARAI & POMPOLI
Garai & Pompoli model is based on Delany & Bazley improved formula. Garai and
Pompoli (2005) claimed that the empirical formula made by Delany and Bazley
(1970) is not suitable for other material like polyester fibre material. The equations
are derived from the best-fit model’s value from the new impedance model (NMI)
for polyester fibre materials. As shown in Table 3, the comparison between Delany
& Bazley result and the eight coefficient value of polyester material are given.
Table 3: Value of the eight coefficient equation from new impendence model
(NMI) compare with Delany & Bazley model value (Garai and Pompoli, 2005)
Model 𝑪𝟏 𝑪𝟐 𝑪𝟑 𝑪𝟒 𝑪𝟓 𝑪𝟔 𝑪𝟕 𝑪𝟖
Delany
and
Bazley
0.057 0.754 0.087 0.732 0.189 0.595 0.098 0.700
NMI 0.078 0.623 0.074 0.660 0.159 0.571 0.121 0.530
The new equation was then rewritten so it can be related in the range of 0.05<𝜌0𝑓
𝜎
<8.4 based on the power-law relationship from data obtained by Garai and Pompoli
(2005) according to Oldham et al. (2011). Below is the simplified equation:
Real Part of Characteristic Impedance, 𝑍R = 𝜌0𝐶0 [1 + 𝐶1 (𝜌0𝑓
𝑟)
−𝐶2
] (16)
Imaginary Part of Characteristic Impedance, 𝑍I = −𝜌0𝐶0 [𝐶3 (𝜌0𝑓
𝑟)
−𝐶4
] (17)
Real Part of Propagation Constant, 𝛾, 𝛼 =2𝜋𝑓
𝐶0[𝐶5 (
𝜌0𝑓
𝑟)
−𝐶6
] (18)
24
Imaginary Part of Propagation Constant, 𝛾, 𝛽 =2𝜋𝑓
𝐶0[1 + 𝐶7 (
𝜌0𝑓
𝑟)
−𝐶8
] (19)
Where, 𝜌0 is the air density (kgm-3
) and 𝑓 is the frequency (Hz).
From the equation, at normal incidence, the sound absorption coefficient for a
rigid backed fibrous layer can be obtained by using the equation below:
Surface Impedance, 𝑍l = (𝑍R + 𝑖𝑍I)[coth(𝛼 + 𝑖𝛽) 𝑙] = 𝑍𝑙R + 𝑖𝑍𝑙I (20)
Sound Absorption at Normal Incidence, 𝛼𝑛 =4𝑍𝑙𝑅𝜌0𝐶0
|𝑍𝑙|2+2𝜌0𝐶0𝑍𝑙R+(𝜌0𝐶0)2 (21)
Where, 𝑙 is the layer thickness (m) and 𝜌0 is the density (kgm-3
). From the equation
above, it showed that the simplified procedure can be used in most applications,
where usually the best fit is based on the values of both propagation constant and
characteristic impedance that works usefully in noise control purposes (Delany and
Bazley, 1970; Dunn and Davern, 1986). Furthermore, according to Gardner et al.
(2003), in analogous ways, the neural network of the measured values of the surface
impedance and absorption coefficient was obtained and resulting in optimized on the
sound absorption coefficient.
2.4 METHODS TO MEASURE SOUND ABSORPTION COEFFICIENT
Reverberation and impedance tube are two methods used to measure sound
absorption coefficient. Meanwhile, the impedance tube method is divided into two
sub-methods, which are the standing wave and two-microphone transfer function
method. The method mentioned generally use concentrate ratio of absorbed sound
energy to incident energy. Thus, one method focussed on this research that is the
two-microphone method.
2.4.1 REVERBERATION METHOD
Reverberation is a method that uses the concept of echoes in receiving and hearing
the sound in certain locations from a sound source as the outcome of reflections
occurs in a compartment or room. The method like reverberation in a room is being
considered in terms of reverberation time. The factor that affected reverberation time
is the sound absorption coefficient of the room interior surface and the room size.
According to Chen (2011), the reverberation time was concluded at the time requisite
25
for reflections of the direct sound to attenuate by 60 dB below the level of the direct
sound. In general and based on Jambrosic et al. (2008), the testing procedure of
reverberation is to measure the reverberation time in an empty room. The specimen
sample is placed in an empty room and the reverberation time is taken again
repeatedly. Then the average of the reverberation time obtained is used to calculate
the absorption performance of the test sample.
2.4.2 STANDING WAVE METHOD
The standing wave method is created earlier compared to the other known methods.
Based on this method, sound absorption is measured though measuring the sound
speed using a tube, (Oldfield, 2006). One of the method standing waves is standing
wave ratio (SWR). It is when the traversing microphone used to determine the
magnitude and the location of sequential minimum and maximum of the standing
wave pattern in the tube. This will deduce the normal acoustic impedance and
reflection coefficient. In other words, the main fundamental concept of the standing
wave method is to create a standing wave in the tube where the source of sound is
being transmitted in the tube before sound wave is reflected at the end side of the
tube where the specimen is mounted at. The microphone is being moved to utilize the
measurement of the amplitude to get the minimum and maximum of the standing
wave which helps in the calculation of sound absorption coefficient as in relation to
the change in frequency. Figure 5 shows the impedance standing wave diagram.
Standardize standing wave method normally standardized in accordance with ISO
10534-1 (1996) and ASTM C384-04 (2011).
Sound Source
Specimen
Moveable Microphone
Figure 5: Standing wave method
26
According to the concept of standing wave method, when the source of sound
is being switched on, it creates a standing wave. The pressure point for the minimum
and maximum can be found by moving back and forth (up and down) along the tube
by using a microphone. The microphone is then connected to a voltmeter to get a
reading of measurement. The computation of the sound coefficient of the specimen
requires data based on the phase and the magnitude of standing wave relative to the
specimen location. Due to the varieties of position and value needed to be noted
separately for each related frequency, the standing wave method will consume a lot
of time. However, the result of the standing wave method is still reliable and easy to
be interpreted.
2.4.3 TWO-MICROPHONE TRANSFER FUNCTIONS METHOD
According to Huiping et al. (2006), the sound absorption can be calculated
experimentally. The two-microphone method is also known as the two-microphone
transfer function impedance tube method. According to ISO 10534-2 (1998), the
two-microphone transfer function method has replaced the standing wave ratio
method that is popular during the introduction of it. This is because the two- transfer
function impedance tube method has several advantages against standing wave ratio
method. This method uses an impedance tube that acts as a waveguide cylinder for
plane wave which is generated by the source of sound. The sound pressure is
measured at two locations where the two microphones are located near the specimen
sample as shown in the Figure 6. According to Suhanek et al. (2008), the frequency
of this method is depending on the size of the diameter of the tube and the distance
between the microphone positions. According to Lefebvre et al. (2007), the
impedance of an object was evaluated when the measurement was taken at two
different locations along a waveguide connected to the object in the two-microphone
transfer function impedance tube.
According to Putra et al. (2013), in their reported work, two microphones
method was employed to test the coefficient of sound absorption for bio-degradable
sample based on sugar cane fibres. The sugar cane is dried and cut off before being
heated at 80 °C for five minutes to remove the remaining water. Binders like
polyester and polyurethane are used during fabrication of test specimens with
different composition of sugar cane fibre. After that, a hot-pressed mould is used
27
with different pressure to create different levels of porosity. By using the two-
microphone method, the result showed that the binder with up to 40 % of the total
composition displayed no effect on the performance of the absorption regardless of
different type of the binder in the sugar cane. At a frequency of 3500 Hz, the fibres
started to show a good result in coefficient of absorption (α > 0.5). The absorption
kept increasing until it reaches 0.78 at 4500 Hz. From the research made by Putra et
al. (2013), it managed to provide a lot of information on the fabrication of bio-
composites material. Furthermore, the result obtained showed that the two-
microphone method is reliable to characterise the sound absorption properties of
composite material.
Two-Microphone
Sound Source
Specimen
Figure 6: The two-microphone transfer function impedance tube method
The impedance tube method is based on the mathematical theory of one
dimensional planar pressure wave propagation. According to Seybert (1988), this
technique is driven by random sound in the tube. Thus, this method gives more
advantage in creating fast measurement of the normal incidence wave as a change in
parameters either by using small samples that are easy to disassemble or assemble.
Furthermore, the results demonstrated by Horoshenkov et al. (2007) shows that the
result obtained varied at different laboratory conditions. The advantage of the
transfer function impedance tube is that the apparatus is smaller and therefore it can
become more practical for small test specimens. On the other hand, it allows the
surface impedance to be determined in addition to the sound absorption (Oldham et
al., 2011). However, the disadvantages are that the sound absorption can only be
measured at normal incidence wave and uncertainties are introduced when measuring
28
the sound absorption of materials having porous structure. Thus, other related
characterisation needed to be done to reduce the uncertainties.
2.5 COMPOSITE CHARACTERISATIONS
Several characterisation needs to be done in order to understand the overall
performance of composite materials. It is best worthwhile to gain a thorough
understanding on the development of these materials and their characterisation
techniques.
2.5.1 ACOUSTICAL PROPERTIES
Khedari et al. (2003), Khedari et al. (2004), and Zulkifli et al. (2008) have reported
successful fabrication of a composite particle board using agricultural wastes.
Furthermore, according to Koizumi et al. (2002), specimens of bamboo fibre
demonstrated almost similar sound absorption properties as glass wool fibres. The
bamboo fibreboard materials with enclosed surface yielded higher sound absorption
properties than the plywood materials, although it had a similar density. Yang et al.
(2003) also obtained a similar result in randomly cut rice straws and wood particles
composite boards. In the frequency range of 500 Hz to 8000 Hz, it tended to exhibit
higher sound absorption properties as compared with fibreboard, plywood and
particleboard. Thus, these studies implied that there is a great attention in using
natural fibres to create composite materials. The reason behind this is due to the
superior performance that reinforced natural fibre composites compared to the
reinforced synthetic fibre composites, especially in the consideration of lightweight,
combustible, recyclable and biodegradable. According to Avella et al. (2000),
because of the good physical properties that it have, natural fibre composites have
been ranked among high performance composites that have an advantages in the
environment and economy.
Sound absorbing materials are chosen based on the frequency range it can
control, materials, types and dimensions (Simón and Pfretzschner, 2004). Taking the
example of poly (l-lactic) acid reinforced ramie fibre, at the frequency of 250 Hz to
1600 Hz, the sound absorption coefficient obtained were from 0.089 to 0.353 (Chen
et al., 2010). Furthermore, Zou et al. (2010) also showed a higher sound absorption
coefficient from 0.03 to 0.2 within the range of 300 Hz to 1800 Hz in the
29
polypropylene reinforced with wheat straw as compared with polypropylene
reinforced with jute fibre. Reddy and Yang (2011), showed a higher sound
absorption coefficient form 0.06 to 0.8 for the range of 1000 Hz to 5000 Hz for zein-
jute composites as compared with polypropylene-jute composites within the
frequency range from 1000 Hz to 5000 Hz. Composite boards made from rice straw
and wood particle reinforced commercial urea formaldehyde showed better sound
absorption coefficient in the frequency range of 500 Hz to 8000 Hz as compared to
plywood, particleboard and fibreboard (Yang et al., 2003). A higher sound
absorption coefficient at the frequency in the range of 2000 Hz to 8000 Hz were also
seen in commercial polyurethane reinforced rice straw and waste tire particle
composite that also show superior performance than fibreboard, particleboard and
rice straw-wood particle composite board (Yang et al., 2004).
Nowadays, raw materials from agricultural waste are widely being used to
replace synthetic fibres and wood-based materials for acoustic absorption purposes.
In Malaysia for example, plenty of agricultural waste such as coconut, rice husk, and
oil palm frond fibre are expected to be useful in various material applications that
can be used in the near future (Mohd Nor et al., 2004). As similar to wood particle or
fibre, agricultural lignocellulose fibres such as wheat straw, oil palm frond and rice
straw can be easily crushed into particles or chips. Thus, this show it is a good
substitute for wood-based raw materials. The advantage of these fibres is that they
are cheaper, abundant, less toxic, nonabrasive and renewable. Thus, through research,
acoustical materials based on this renewable resource can lead to viable alternatives
to conventional materials for future and current applications (Nick et al., 2002;
Asdrubali, 2006). Table 4 and Table 5 summarized the acoustical properties of raw
natural fibre and composites available in the research literature.
30
Table 4: Summary of acoustical properties for natural fibre
Materials Fabrication
Methods
Sound
Absorption
Coefficient
Specimen
Parameters References
Bamboo Compression 0.02 - 0.4 (at
3000 Hz)
Thickness of 50
mm
Koizumi et al.
(2002)
Wood Compression 0.1 (at 4000
Hz)
Thickness of
25mm
Wassilieff
(2003)
Cellulose
(isofloc)
-
1.10 (at 3000
Hz) Thickness of 60
mm Asdrubali
(2006) Sheep wool
0.80 (at 3000
Hz)
Mineral wool 0.90 (at 3000
Hz)
Thickness of 40
mm
Arenga
pinnata -
0.40 - 0.80 (at
5000 Hz)
Thickness of 10 -
40 mm and
diameter of 28
mm
Ismail et al.
(2010)
Cotton Compression 0.50 (at 500
Hz)
Thickness of 50
mm
Oldham et al.
(2011)
Kapok - 0.67 - 0.99 (at
4000 Hz)
Thickness of 20
mm - 60 mm,
density of 8.3 -
41.7 kg/m3, and
combined
diameter of 30
mm and 60 mm
Xiang et al.
(2013)
Coir
Compression
0.78 (at 4000
Hz) Thickness of 20
mm and combined
diameter of 100
mm and 28 mm
Fouladi et al.
(2013)
Sugarcane
Grass 0.70 (at 4000
Hz)
Corn 0.90 (at 4000
Hz)
31
Table 5: Summary of acoustical properties of natural fibre composites
Materials Fabrication
Methods
Sound
Absorption
Coefficient
Specimen
Parameters References
Rice straw/
wood urea
formaldehyde
composites
Hot
compression
(500 psi 140
°C)
0.05-0.5 (at
8000 Hz)
Thickness of 10 mm,
diameter of 25 mm,
and fibre loading of 0
– 30 wt%,
Yang et al.
(2003)
Hemp brick
(66%)
-
0.50 (at
2000 Hz) Thickness of 300 mm
Asdrubali
(2006)
Polystyrene
(PST)
0.20 (at
3000 Hz) Thickness of 40 mm
Recycled latex +
coco
0.50 (at
3000 Hz)
Rami poly (L-
lactic acid)
composites
Hot
compression
0.01-0.02
(at 1800
Hz)
Thickness of 3mm,
diameter of 110 mm
and various grade of
PLLA.
Chen et al.
(2010)
Wheat straw/
polypropylene
composites
Hot
compression
0.03-0.23
(at 3000
Hz)
Thickness of 3.2 mm,
fibre loading of 40 -
80 wt%, and
diameter of 63 mm
Zou et al.
(2010)
Polyester fibres
polyethylene
composites
Hot
compression
0.03-0.35
(at 10000
Hz)
1st condition:
Thickness of 1 - 3
mm and fibre loading
of 0 - 20 wt% ,
2nd
condition:
Thickness of 20mm
and diameter of 60
mm
Jiang et al.
(2012)
Flax/epoxy Compression
(laminated)
0.11 (at
2000 Hz) Thickness of 3 mm
and diameter of 100
mm
Prabhakaran
et al. (2014) Glass/flax/epoxy
Compression
(laminated)
0.15 (at
2000 Hz)
2.5.2 MECHANICAL PROPERTIES
The reason that bio-composite was preferred is due to their environmental
friendliness and carbon dioxide free (Saw et al., 2013). Besides that, natural fibres
were also known for their low cost, renewability, biodegradability and etc. that
would benefit to the society. Specimen like luffa reinforced epoxy composite had
been tested for mechanical tensile testing as part of research conducted by Mohanta
and Acharya, (2013). The composite was fabricated with different layer from single,
double and triple layer for testing under mechanical testing. The weight composition
32
was varied from 8 wt%, 12 wt% and 18 wt% with a ratio of 1:10 hardener to epoxy.
The result showed that there was an increase in tensile test for double and single
layer composite and decrease in triple layer as shown in Table 6.
Table 6: Luffa fibre epoxy composites mechanical properties (Mohanta and
Acharya, 2013)
Luffa Fibre Layer Fibres Composition (wt%) Tensile Strength (MPa)
0 Neat epoxy 13.50
SL 7 16.29
DL 13 16.76
It is understandable that the fibre strength and fibre content are the main
influences that affect and enhance the strength of composite material itself as the
different load applied to it. However, there was an argument on the result, whereas
the triple layer composite did not meet the requirement performance on the strength
where the single and the double layer composite outperformed triple layer
counterpart. Nevertheless, the result obtained by Mohanta & Acharya (2013) is
useful as a guideline for our research. Figure 7 shows the typical schematic diagram
for the composites reported in their research
Natural fibre reinforced composites properties depend on certain parameters
such as a fibre aspect ratio, stress transfer at interface, volume fraction of the fibres,
fibre-matrix adhesion, orientation, structure, fibre chemical composition, defects,
micro fibril angle, cell dimensions, and mechanical properties (Faruk et al., 2012).
The properties of natural fibre can differ due to the method of fabrication, testing,
fibre, and moisture conditions. Thus, it is important to know that certain mechanical
properties of natural fibre composite materials. It is known that the tensile strength is
more sensitive to the matrix properties. This is because that the modulus is dependent
on the fibre properties. Thus, to improve the tensile strength, low stress concentration,
good fibre concentration and orientation, high fibre aspect ratio and fibre wetting in
the matrix phase can help to determine the tensile modulus. According to Van Der
Jagt and Beukers (1999), the deformation characteristics of composites in tension are
normally governed by the reinforced fibre strength and the behaviour between the
fibre and the matrix interface bonding.
33
Epoxy Resin + Hardener
Luffa Fibre
Epoxy Resin + Hardener
Epoxy Resin + Hardener
Luffa Fibre
Epoxy Resin + Hardener
Luffa Fibre
Epoxy Resin + Hardener Epoxy Resin + Hardener
Luffa Fibre
Epoxy Resin + Hardener
Luffa Fibre
Epoxy Resin + Hardener
Luffa Fibre
Epoxy Resin + Hardener
Single Layer Composite Double Layer Composite Triple Layer Composite
Figure 7: Composites schematic views (Mohanta and Acharya, 2013)
The use of chemical treatments of the fibres can help to improve the
interfacial bonding and surface roughness that promote strong bonding between
fibres and matrix. Sathishkumar et al. (2012) examined the mechanical properties of
reinforced snake grass fibre polyester composites by using a simple lay-up technique
with various fibre length and volume fraction. It was noted that the optimum tensile
and flexural properties was obtained at the fibre length of 30 mm and 150 mm.
Venkateshwaran et al. (2011) also investigated the polyester reinforced with
randomly oriented banana fibres fabricated using a simple lay-up method with
different fibre length of 5 mm, 10 mm, 15 mm and 20 mm with various weight
fractions of 8 %, 12 %, 16 % and 20 %. It showed that the 10 mm and 15 mm fibre
length at the weight fraction of 12 % composites have exhibited a maximum tensile
strength. Table 7 summarized the mechanical properties for natural fibre composites
available in the research literature.
34
Table 7: Summary of mechanical properties of natural fibre composites
Materials Fabrication
Methods
Maximum
Tensile
Strength
Specimen
Parameters References
HDPE/
henequen
Hot
compression
21 MPa – 27
MPa
Various types of
fibre surface
treatment
Herrera-
Franco and
Valadez-
González
(2005)
Rice straw/
PHBV
copolymer with
hydroxyvalerate
Melt mixing 5 MPa – 6
MPa
Fibre loading of
20 - 30%, and
thickness of 3.5
mm
Buzarovska
et al. (2008)
Kenaf/
Polyurethane
Composites
Hot
compression
85 MPa – 20
MPa
Various
treatment
concentration
and additive (i.e.
alkaline
treatment,
pMDI)
El-Shekeil et
al. (2011)
Luffa/Epoxy
Composites Hand-lay up -
Various layer
(i.e. single,
double, triple),
and thickness of
5 mm
Mohanta and
Acharya
(2013)
Asian palymyra/
epoxy
composites
Hot
compression
14 MPa – 27
MPa
Volume
percentage of 20
- 80 %
Balakrishna
et al. (2013)
Jute/ PLA
composites
Injection
moulding
28 MPa –
90.7 MPa
Various fibre
lengths (i.e.
short and long).
Thickness of 8
mm
Arao et al.
(2015)
2.5.3 CHEMICAL MAPPING ANALYSIS BY FOURIER TRANSFORM
INFRARED SPECTROSCOPY (FTIR)
An effective analytical tool used to identify the functional groups that present in the
fibre structure is the Fourier transforms infrared (FTIR) spectroscopy. In FTIR, the
infrared radiation is directed pass through the sample. Thus, this cause the bond in
the functional groups to vibrate vigorously depending on the reactivity of the bond to
infrared radiation and the absorbance of the radiant energy in a specific frequency
range (from strong to weak frequency and vice versa). In relation to the vibration
frequencies, the energy absorption produced peaks and the samples for molecular
35
fingerprints can be generated. The sample fingerprint can be presented in either
transmission or absorbance versus frequency curve, where the peaks explain the
molecular structure of the fibres (Kabir et al., 2012). Generally, for composites, the
FTIR spectra were presented in the mid infrared region of 4000 cm-1
to 400 cm-1
.
Furthermore, it is also known that the mid infrared spectrum can be divided into four
regions and the nature of the functional group’s presence is determined based on the
region where it is located. According to Stuart (2005), the four regions are the
fingerprint region (1500 cm-1
to 600 cm-1
), the double bond region (2000 cm-1
to
1500 cm-1
), the triple-bond region (2500 cm-1
to 2000 cm-1
) and X-H stretching
region (4000 cm-1
to 2500 cm-1
). In the vibration due to infrared radiation, the X-H
stretching region (4000 cm-1
to 2000 cm-1
) may involve O-H, C-H and N-H
stretching.
To determine the presence of methylene or methyl groups in a sample, the
exhibition of the C-H stretching band due to vibration is used as a diagnostic tool to
show the location of the groups. Meanwhile, because of the high force constants on
the bonds, the region of 2500 cm-1
to 2000 cm-1
falls for triple bond stretching
absorption. Where, C≡ C bond peak absorptions are seen in the region of 2300 cm-1
and 2050 cm-1
and C≡ N bond occurs in between the region of 2300 cm-1
and 2200
cm-1
. The double bond region is also called as a principle band region in the 2000
cm-1
to 1500 cm-1
is due to C=O and C=C stretching. It is known that the C=C double
bond groups are useful to identify the group for alkenes whereas the general range
for this vibration located in the region of 1680 cm-1
to 1630 cm-1
. Almost all the
natural fibre composites FTIR spectrum shows various bands at aliphatic
hydrocarbon. Aliphatic hydrocarbon does not contain any aromatic ring. Three
common groups of hydrocarbons are alkynes, alkenes and alkanes (Smith, 1998).
Among the three of these groups, alkanes are the simplest type of hydrocarbon.
Alkanes contained carbon-hydroxide (C-H) and carbon-carbon (C-C) single bond
molecular groups. Meanwhile, alkenes contained a carbon=carbon (C=C) double
bond molecular groups. It also called as olefins groups. While, alkynes contained
carbon≡ carbon (C≡ C) a triple bond molecular group. Thus, this contributes the
factor that affected the property enhancement of natural fibre composites (in term of
interaction between the fibre and polymer). In this context, FTIR analysis gives the
insight about the interactions between the fibre and polymer.
36
2.5.4 MICROSTRUCTURE ANALYSIS
The best way to examine the surface morphology either for the fibre or the fracture
structure of the natural fibre composites are by using scanning electron microscope
(SEM) and light/optical microscope. The SEM images showed two different opposite
morphological situations. It either shows (Fornasieri et al., 2011):
(a) The domain structure of dispersed fibre in the polymer matrix and the
interfacial interaction between the fibre and the polymer that prevented the
fibre released from the matrix, and/or
(b) The poor interfacial interaction between polymer and fibre that caused the
fibre released from the polymer matrix.
Meanwhile, for light/optical microscope, it showed the absorption of polymer inside
the fibre and the physical structure of fibre.
According to Reddy et al. (2010), by using SEM, it revealed that the addition
of coupling agent increased the surface adhesion between the fibres and polymer
matrix interface that further showed an increased in the mechanical properties of the
composites. Thus SEM showed that the surface modifications improved the adhesion
between fibre and polymer matrix. Verma et al. (2013) also observed the increased in
adhesion between fibre and polymer matrix by using SEM. SEM also showed the
weak interaction between fibre and polymer matrix as can be seen in SEM analysis
of Perinovic et al. (2010). SEM and light/optical microscope also help in showing the
lignocellulose fibre hollow structure, the cell walls of fibres, spirally oriented
cellulose in the hemicellulose and lignin matrix. The constituents at the outer surface
of the cell wall are a layer of lignocellulose material and waxy substances that bond
the cell to its adjacent neighbours (Jacob et al., 2005).
2.6 FIBRE TREATMENT
Treatment is used to remove impurity in the materials especially fibres. Normally it
involves using chemicals as reactant. One of the common treatments was called
mercerization or alkaline treatment. The alkaline treatment helps to create better
adhesion between the fibre and the binder and thus improves better bonding and
interlocking reactions. Through research conducted by Demir et al. (2006), they have
used pre-treated luffa fibre with 0.1M sodium hydroxide (NAOH) solution for 20
37
minutes at boiling temperature before re-treating the fibre with silane-coupling agent.
Then, it was mixed with polypropylene (PP) composite which contained 2 wt% to 15
wt% of treated and pre-treated luffa fibre. Then it was hot pressed at 190 °C at 100
bars for 10 minutes before cooling down to 40 °C in 6 minutes under the same
pressure.
Through scanning electron microscope (SEM), it showed that 15 wt%
untreated luffa fibre showed the presence of the void between the matrix and fibre
which indicated a poor adhesion. It also showed that fracture happened due to matrix
failure and not from fibre breakage. However, the 15 wt% treated luffa fibre showed
better fibre-matrix adhesion although void presence still remained. From the result
published by Demir et al. (2006), it can be concluded that alkalisation treatment of
natural fibre improved the adhesion and created a better mechanical properties of
reinforced composite material. However, our main focus is on the acoustical
properties of luffa fibre reinforced composite. Hence, the effect of treatment on luffa
fibre is still unknown as none literature was found.
2.7 SUMMARY AND RESEARCH GAP
A considerable amount of literature regarding the acoustic property of fibrous
materials has been published. However, very little literature reports on the modelling
of acoustic property using empirical model. Furthermore, although there was a lot of
treatment used to treat the fibre but only a few of the researches used the alkaline
treatment (i.e. sodium hydroxide) as treatment agent. To date, there is very few
systematic studies conducted to investigate the sound absorption behaviour for
natural fibre composites. On the other hand, the effect of fibre chemical treatment on
the acoustic property is seldom reported in the literature. The important parameters
affecting the acoustic property include the fibre types, porosity, fibre loading,
chemical treatment, and etc. Thus, more tests are needed to fully explore the
properties of natural fibre composites. The current study is done in order to
understand the behaviour and performance of banana, luffa and oil palm and its
composites. In addition, SEM and chemical mapping analysis via FTIR were
performed in order to obtain microstructural and fibre-polymer matrix interface
information. It is believed that these findings could give an insight on how the
microstructure and interface adhesion affect the mechanical and acoustic properties.
38
The efforts devoted to the understanding of these materials may enhance
understanding of sustainable materials as noise reduction elements in future. Natural
waste fibres, such as banana, luffa, and oil palm have considerable potential to be
used as structural material to replace wood in construction industry. Thus, this led to
the present work to investigate the acoustic and mechanical properties of natural
fibre composites.
39
CHAPTER 3
3.0 METHODOLOGY
3.1 OVERVIEW
The main aim and objectives of this research is to understand the behaviours and
characteristics of sound absorption properties in sound absorbing materials such as
natural fibre composites. Both experimentation and modelling are required to achieve
the aim. MATLAB was used to run an empirical model for sound absorption
coefficient of fibre. The model created was based on combinations of two empirical
models, (i.e. the Delany and Bazley, and Mechel models). The reason behind the
combination of these two models is to create more input on the variable and constant.
By using this empirical model, the flow resistivity (in which is a function of fibre
size) and sound absorption can be predicted. For the experimentation, the two-
microphone transfer function impedance tube method was used to validate the
modelled sound absorption coefficient incorporating the effect of fibre loading,
treatment, and fibre type (this includes fibre orientation and structure). Thus, the
fabrication of the test rig needs to be done accordingly to an established standard.
In this research, a thermoset polymer resin, epoxy was used as the matrix
materials. Meanwhile, lignocellulose type of fibres including the banana, oil palm
and luffa fibre were used as reinforcing materials. The lignocellulose fibres used
were chemically treated while untreated fibres were used as an experimental control
in order to observe the effect of treatment. The untreated and treated fibres were then
used to prepare natural fibre composite materials. These natural fibre composite
materials were then used to examine the potential of using lignocellulose fibre as
reinforcement and as sound absorption materials. Several tests were used to
determine the effect of certain factors that affects the sound absorption. Sound
absorption coefficients were evaluated using the two-microphone transfer function
impedance tube fabricated in house. The mechanical property was characterized
using tensile test. It is crucial to check if the mechanical property is not deteriorated
in order to realize any practical application. The scanning electron microscope was
used to analyse the morphological properties that influenced the sound absorption.
Fourier transform infrared spectroscopy (FTIR) helped to evaluate the compatibility
at the molecular level. Figure 8 shows the methodology workflows.
40
For Empirical Modelling For Experimentations
Empirical Model for Sound Absorbing Materials:
Mechel Delany and Bazley
Modelling of Sound Absorbing Materials Using
MATLAB
Fabrication of The Two-Microphone Transfer
Function Impedance Tube (ASTM E1050-12)
Fabrication of The Natural Fibre Composites
Calibration and Improvement of The Two-
Microphone Transfer Function Impedance Tube
(ASTM E1050-12)
Sound Absorption Test (ASTM E1050-12)
Light/Optical Microscopic Test (ASTM E2015-04)
Tensile and Yield Test (ASTM D638-14)
Scanning Electron Microscopic Test (ASTM E2015-04)
Fourier Transform Infrared Spectroscopy Test (ASTM E168-06 and ASTM E1252-98)
Results and Discussions
Methodology
Figure 8: Flowchart of methodology
3.2 SOUND ABSORPTION MATERIALS
There are three types of natural fibre materials that were used in this research. They
are the banana, luffa and oil palm fibre. Sound absorption modelling was conducted
on all three of these natural fibre materials. It is to predict these natural fibre
materials having the highest sound absorption coefficient. The reason behind this is
to understand the independent behaviour of standalone natural fibre materials before
the fabrications of natural fibre composite materials through modelling. Therefore,
before conducting any experimental test, it is best to know the basic properties of
each natural fibre material. The natural fibre materials that were selected and used in
this research are expected to help to bring down the level of pollution in addition to
exhibiting good, economical, sustainable, and degradable quality. To create sound
absorption materials based on natural fibre materials, the main properties such as the
41
fibre diameter, bulk density, matrix density and porosity need to be identified. This
data are available in the literature. Table 8 shows the data tabulated for natural fibre
materials.
Table 8: The properties of natural fibre materials
Material
Range of
Fibre
Diameter
(µm)
Mean of
Fibre
Diameter
(µm)
Bulk
Density
(kg/m3)
Matrix
Density
(kg/m3)
Porosity References
Banana 14-50 32 154 1325 0.884
Sfiligoj
Smole et al.
(2013),
Tholkappiyan
et al. (2014),
and Kulkarni
et al. (1983)
Luffa 8-30 19 - - 0.930
Laidani et al.
(2012), and
Saeed and
Iqbal (2013)
Oil Palm 8-36 22 - - 0.636
Abdul Khalil
et al. (2012),
and
Evbuomwan
et al. (2013)
3.3 EMPIRICAL MODELS USED
Based on the literature reviews, there are a few methods to calculate the flow
resistivity and sound absorption coefficient of the natural fibre materials. In this
research, in order to calculate the flow resistivity of the natural fibre materials,
Mechel empirical model was used. The reason is because the Mechel model is
derived from the diameter range applicable to the natural fibre involved in this
research. According to Oldham et al. (2011), the diameter range for Mechel model
covered from 6 µm to 30 µm. This can be confirmed and shown in the obtained
tabulated data in Table 8. From Table 8, it shows that the luffa, banana and oil palm
fibre diameter are relatively closer to the lower limit of Mechel empirical models.
According to Oldham et al. (2011), there is not much different between the two
predicted values based on Garai and Pompoli, and Mechel empirical model. However,
there are slightly different values expected when the fibre diameter reached the
42
middle range value or above. Unfortunately, this slight difference does not affect the
sound absorption coefficient significantly.
Meanwhile, Delany and Bazley empirical model were used to calculate the
sound absorption coefficient of the natural fibre materials. Although Garai and
Pompoli empirical models were also used to calculate the sound absorption, the
limits between the two empirical models were different. Delany and Bazley
empirical model have a limit range in between 0.01 <𝜌0𝑓
𝜎< 1 and for Garai and
Pompoli empirical model have a limit range in between 0.05 <𝜌0𝑓
𝜎< 8.4. Thus, all
the computed results based on the Delany and Bazley model were validated using the
results obtained from the Garai and Pompoli model. According to Oldham et al.
(2011), the accuracy of Delany and Bazley empirical model is more accurate if the
value of the fibre diameter is acceptable in its 𝜌0𝑓
𝜎 range. Whereas, the Garai and
Pompoli model is more suitable for bigger range results, based on the Mechel
empirical model for the flow resistivity.
3.4 CALCULATIONS
Before applying any data into the MATLAB, it is best to simplify and identified all
the parameter that related to the sound absorption empirical model calculation. In
both modelling, the data obtained in Table 8 were inputted into the empirical models
to compute the flow resistivity and sound absorption. For modelling the effect of
flow resistivity toward material sound absorption, all variables were held constant
except for the fibre diameter, since this will create a variable change in the flow.
Meanwhile, for modelling the effect of material thickness, the mean diameter and the
flow resistivity are held constant while the thickness is taken as variable.
The flow resistivity, 𝜎 of natural fibre materials is related to the material
porosity, bulk density and fibre radius. The major factor that affected the sound
absorption for a wide range of porous or fibrous materials is the flow resistivity. This
relationship can also be seen in the empirical model presented by Mechel (Mechel
and Ver, 1992). For sound incidence perpendicular to the direction of the fibre
materials, the flow resistivity is given by Equation 8. The fibre size indirectly
influences the flow resistivity value obtained. The radius of the fibre, 𝑟 is given by:
43
𝐹𝑖𝑏𝑒𝑟 𝑅𝑎𝑑𝑖𝑢𝑠, 𝑟 =𝑑
2 (22)
Whereas, 𝑑 is the diameter of the fibre (m).
Assuming there is no closed cells in the natural fibre material and having a
small amount of binder, the porosity is given by Equation 9. It is known that
common sound absorbing materials especially porous materials have many varieties
and forms. It is known that the material porosity depends upon their pore structure.
Commonly, sound absorbers are highly homogenous and porous with the structure
that consists of fibrous and open cell foam. Thus, many empirical models were
created based upon the work of Delany and Bazley empirical model. The
characteristic impedance can be identified using Equation 14 by taking the air
density at 1.20 kgm-3
and speed of sound at 340.29 ms-1
. Meanwhile, to determine
the complex wave number, Equation 15 was used.
Based on the Delany and Bazley empirical model that is Equation 14 and
Equation 15, the characteristic impedance, 𝑍c and complex wave number, 𝑘c needs
to be solved since it involved an imaginary number. To remove this imaginary
number, it is best to apply a constant and variable value in it that indirectly simplifies
the empirical model by eliminating the imaginary number. The constants applied in
the equation are the speed of sound value, 𝑐, frequency, 𝑓, density of air, 𝜌0 and the
angular frequency, 𝜔 . Meanwhile, the variable that can be added in is the flow
resistivity value. After the simplified equation of 𝑍c and 𝑘c been obtained, the
simplified equation will be applied in the surface impedance equation below:
Surface Impedance, 𝑍s = −𝑗𝑍c cot(𝑘c𝑡) (23)
Where, 𝑡 is the thickness (m) of natural fibre materials or the thickness for single
layer backed rigid porous absorber.
The set of thickness in the equation is depending on the factor it was going to
be tested. Thus, the thickness of the samples, the complex wave number and the
characteristic impedance may indirectly influence its absorption characteristics. The
surface impedance will be substituted in the pressure reflection coefficient equation
below:
44
Pressure Reflection Coefficient, 𝑅 =𝑧s−𝜌0𝑐
𝑧s+𝜌0𝑐 (24)
It is known that the intensity of the plane wave reflected from the surface of
absorbent materials tend to be much smaller than the incident wave by a factor of
|𝑅2|. The sound absorption coefficient, α is thus given by:
Sound Absorption Coefficient, 𝛼 = 1 − |𝑅2| (25)
3.5 MATLAB EMPIRCAL MODELLING
MATLAB software was used to generate and run the empirical model to obtained
predicted sound absorption coefficient. MATLAB is the high level mathematical
languages and interactive software that can help shorten the calculation time for
empirical modelling. The MATLAB modelling was carried out based on the
simplified equation in Section 3.3 and Section 3.4 that is a combination of Mechel
and Delany and Bazley empirical models, and natural fibre materials data in Table 8.
The modelling will help to investigate the influence of change in flow resistivity as a
change in fibre diameter and change in thickness against the sound absorption
coefficient of the material to be tested. Furthermore, the reason MATLAB being
used to calculate the flow resistivity and the sound absorption coefficient of the
material is because it can handle imaginary numbers. Figure 9 shows the new file on
MATLAB.
Figure 9: Creating a new M-file on MATLAB
45
When MATLAB empirical modelling software started, a new file needs to be
created. After new file was created, all the empirical models and data were added to
the new file. All variables and constant value are added in the file whether it is the
change in flow resistivity due to a change to the diameter size or a change in the
thickness. For modelling the effect of thickness, the flow resistivity is set to a
constant value, whereas the thickness of fibre materials is taken as a variable.
Meanwhile, for modelling the effect of flow resistivity, the thickness of the materials
is set to a constant and the flow resistivity is set as a variable. Finally, the file being
saved and run to get the result of sound absorption coefficient against the frequency
for various thickness and flow resistivity. All the data obtained from the modelling
will be plotted using a spreadsheet (i.e. Microsoft Excel).
3.6 MATERIAL PREPARATION
Pre-mixed types of epoxy resin with product code ‘BBT 7892 A’ and epoxy hardener
with product code ‘BBT 7892 B’ were supplied by Borneo Indah (Malaysia) Sdn.
Bhd, one of the branch suppliers for epoxy (the main supplier was Berjaya Bintang
Timur (Malaysia) Sdn. Bhd). This product contained bisphenol-A for part ‘A’ resin
and polyoxypropylene diamine and diethylenetriamine for part ‘B’ hardener. These
types of epoxy resin had a low reactivity, slow curing and yellowish in colour. The
oil palm fibres were directly obtained from local agricultural and industrial waste
from Federal Land Consolidation and Rehabilitation Authority (FELCRA) (Malaysia)
Bhd. Oil Palm Mill Centre at Kota Samarahan, Sarawak, Malaysia. Luffa fibre and
banana fibre were obtained from local farms in Kuching, Sarawak, Malaysia.
Malaysia. Caustic soda types of sodium hydroxide (NaOH) with product code
‘S/4920/AP1’ were supplied by Fisher Scientific, UK. The pellet forms caustic soda
of sodium hydroxide completely soluble in water, high alkalinity (before soluble in
water) and odourless.
3.7 FIBRE PREPARATION
Musa paradisiaca or musa balbisiana is the scientific name of bananas that were
commonly found in Malaysia, especially in Borneo. The banana fibres were
extracted from the tree trunks. Banana fibres were also called as pseudo stem fibres.
The tree trunk of banana was cut down and sliced into small long rectangles sizes up
to 10 to 15 cm long. Banana tree trunks were then cleaned with water to remove dirt
46
and sap colour. Later it was dried and shredded using shredding machine before
being chopped into small sizes varied from 1 to 10 mm long. Elaeis guineensis is a
scientific name of oil palms that were used. The oil palm fibres were usually
extracted from the oil palm empty fruit bunches wastes in agriculture and industries.
The oil palms empty fruit bunches wastes were then pressed before sent into
decorticator for shredding the fibres. A decorticator was also used to remove the skin
and bulk of the empty fruit bunches. Later, the shredding oil palm fibres transferred
into the drum dryer to remove the moisture by 15 %. Lastly, the oil palm fibres being
sieved to remove the dust and the fibres were sorted into different sizes. The fibre
sizes obtained and used were normally varied from 1 to 10 mm long. Luffa
cylindrical is the scientific name for luffa. The luffa fibres were usually extracted
either two ways, by either naturally drying on the plant itself or by cutting it when it
had matured and then allowing it to dry under the sun. Once it dried, the hardtop was
removed by cutting off the hard top shell with scissors. The seed inside the luffa was
removed by striking the luffa against hard objects. It also indirectly removed the skin.
Later, the luffa was soaked and sprayed with clean water to remove the sap colour.
The luffa fibres were then dried before chopped into smaller sizes varied from 1 to
10 mm long.
Two sets of composite, with either untreated or treated fibres, for each sample
were fabricated. For untreated samples, the luffa, oil palm and banana fibres were
rinsed with distilled water and dried in an oven at 60 °C for 48 hours. For the treated
samples, the luffa, oil palm and banana fibres immersed in a 5 wt% sodium
hydroxide solution at room temperature for 24 hours. The sodium hydroxide
solutions were obtained by dissolving the pellet forms of caustic soda with distilled
water. Immersing the fibre with sodium hydroxide solution enabled the removal of
impurities and increased the surface roughness of the fibre. The immersed luffa, oil
palm and banana fibres in the sodium hydroxide solution were later cleaned with
distilled water and dried in an oven at 60 °C for 48 hours. The drying oven with
model name ‘ECOCELL EC55’ and brand ‘MMM Group’ equipped by Fisher
Scientific, UK was used in the process. Figure 10, Figure 11, and Figure 12 shows
the untreated and treated banana, luffa and oil palm fibre.
47
Figure 10: Banana fibre: (a) untreated, and (b) treated
Figure 11: Luffa fibre: (a) untreated, and (b) treated
Figure 12: Oil palm fibre: (a) untreated, and (b) treated
(a) (b)
(a) (b)
(a) (b)
48
3.8 FABRICATION OF NATURAL FIBRE SOUND ABSORPTION TEST
SPECIMENS
Both untreated and treated raw fibres were filled in the mould. The moulds used for
sound absorption test has circular cavities of 25±0.5 mm in diameter and 5±0.5 mm
depth that complies with ASTM E1050-12, (2012) standard. The raw fibres were
compressed using hydraulic hot/cold press machine with model code ‘LS-22071’
equipped by Lotus Scientific (Malaysia) Sdn. Bhd. under a pressure of 100 psi (0.69
MPa) (i.e. same as normal human finger/palm press pressure). The moulds were then
left in the press machine for 30 minutes under standard room temperature 24±1 °C to
let the fibre maintained in circular form. Figure 13 shows the hydraulic hot/cold
press machine and Figure 14 (a) and (b) show the moulds used.
3.9 FABRICATION OF NATURAL FIBRE EPOXY COMPOSITE
MATERIALS
Epoxy was then mixed with the hardener at a ratio of 4:1 resin to hardener (4 amount
of part ‘A’ and 1 amount of part ‘B’). Both untreated and treated fibres, acts as fillers
being mixed together with epoxy achieved 5 wt%, 10 wt%, 15 wt% and 20 wt% fibre
loading. The moulds used for sound absorption test has circular cavities of 25±0.5
mm in diameter and 5±0.5 mm depth that complied with ASTM E1050-12, (2012)
standards. Meanwhile, tensile mould with a width of 13.0±0.5 mm and thickness of
3.2±0.4 mm (<7 mm) were also fabricated according to ASTM D638-14, (2014)
standards. The moulds were waxed before filled with the mixture of composite
ingredients. The releasing wax contained carnauba that used to prevent the sample
stick and stuck in the mould when removing the samples out from the mould.
Mixture of epoxy and fibres were then poured into the mould and compressed by
using hydraulic hot/cold press machine with model code ‘LS-22071’ equipped by
Lotus Scientific (Malaysia) Sdn. Bhd. under a pressure of 1000 psi (6.89 MPa). The
moulds were then left in the press machine for 24 hours under standard room
temperature 24±1 °C for curing purposes. Figure 13 shows the hydraulic hot/cold
press machine and Figure 14 (a) and (b) show the moulds used.
49
Figure 13: Hydraulic press machine
Figure 14: Mould for: (a) tensile test, and (b) sound absorption test (circular
cavities)
3.10 DESIGN CRITERIA FOR TWO-MICROPHONE TRANSFER
FUNCTION IMPEDANCE TUBE METHOD
Fabrication of the two-microphone transfer function impedance tube is required for
characterizing sound absorption materials. Thus, the sound source, tube, test
specimen and microphone need to be constructed, fabricated and designed according
to certain criteria as required by the ASTM E1050-12 (2012).
(a) (b)
50
3.10.1 THE SOUND SOURCE
According to ASTM E1050-12 (2012), the sound source must be supplied with
constant power over the range of frequency desired. It can be connected to the
impedance tube either by using a straight tampered, co-axially, or exponential
transition in between the tube and sound source. To reduce the effect of sound
leaking, it is best to isolate and seal the outer tube, the sound source cover and
transition path.
3.10.2 THE TEST SPECIMEN
The specimen must be in shape with the tube cross section area, based on ASTM
E1050-12 (2012). The specimen must be fitted ideally to the tube (i.e. not too loose
that there is a gap between specimen; and not too tight that there is a bulge at the
centre). It is recommended to seal any gap between specimen and the tube wall with
modelling clay or petroleum jelly. The test specimen is fitted normal to the tube axis.
At least two specimens need to be made and tested in the same mounting conditions.
The average results of tests were taken to improve the accuracy of the result in order
to minimize the potential variability in the surface quality of the specimen which can
influence the acoustic response. The thickness is allowed to be varied depending on
the test scope (ASTM E1050, 2012). The specimen specification used in this study is
reported in Section 3.9 and Section 3.13.
3.10.3 THE CONSTRUCTION OF TUBE
By referring to ASTM E1050-12 (2012), the tube must be constantly straight end-to-
end with the negligence of the type of sound transmitted either it is in rectangular or
circular shape interior. The tube material can be in a form of plastic, metal, wood or
cement as long as the inside surface is free of dust, non-porous, solid and smooth.
Sealing the tube with smooth coating will sustain the low sound attenuation. The
source of sound should be capable of generating both plane wave and non-plane
wave sound (i.e. desired and undesired wave) at the same time. Thus, the tube must
be long (i.e. the length of the distance from the sound source to the receiver which is
the microphone should be at least three times the tube diameter) to make sure that the
plane wave is developed when it reached the receiver microphone and the test
specimen. This setup will reduce the non-planar wave intensity to a minimum. The
51
tube diameter can be calculated by taking consideration of upper working frequency
by referring to the working frequency range as below:
𝑓l < Operating Frequency, 𝑓 < 𝑓u (26)
𝑑 <𝐾𝑐
𝑓u (27)
Where, 𝑓u is the upper working frequency of the tube (Hz), 𝑓l is the lower working
frequency of the tube (Hz), while 𝐾 has the value of 0.586 and 𝑐 is the speed of
sound (m/s) (ASTM E1050-12, 2012). The details of the impedance tube used in this
study are reported later in Section 3.11.
3.10.4 THE MICROPHONES
As stated in ASTM E1050-12 (2012), the microphone diameter should be smaller
than the spacing of microphone ports so that the spatial average is at a minimum at
high frequency across the face diaphragm. Furthermore, it must be lesser by twenty
percent of the highest wavelength working frequency. For high sensitivity
microphone, larger diameter of microphone can be used especially for large tube at
low frequency. The maximum frequency limits recommended are tabulated in Table
9 at room temperature based on ASTM E1050-12 (2012). Although the large spacing
between microphones can improve the accuracy of the measurement, it is still limited
by the equation below for spacing between the two-microphones:
Microphone Spacing, 𝑠 ≪𝑐
2𝑓u (28)
Where, 𝑐 is the speed of sound (m/s) and 𝑓u is the highest working frequency (Hz). It
is advised that the maximum microphone spacing, 𝑠 to be 80 % of 𝑐
2𝑓u.
Table 9: Maximum recommended frequency based on microphone size
diameter (ASTM E1050-12, 2012)
Nominal Diameter
(inch.)
Diaphragm Diameter
(mm)
Maximum Frequency
(Hz)
1 22.70 3000
0.5 12.2 5600
0.25 5.95 11500
To calculate the small distance between the specimen and the nearest
microphone, two identical microphones must be mounted depending on the surface
52
characteristics of the specimen (either it is asymmetrical or flat surface). For
specimen with a flat surface, the nearest distance the microphone can be placed in
front of the specimen is within one-half of the tube diameter in order to maintain the
best signal-to-noise ratio. However, for asymmetrical surface, the minimum distance
between the microphone and the specimen must be at least two times the tube
diameter (ASTM E1050-12, 2012). The specification of the microphone used in this
study is reported later in Section 3.11.
3.11 FABRICATION OF THE TWO-MICROPHONE TRANSFER
FUNCTION IMPEDANCE TUBE METHOD
The best method used to calculate the sound absorption coefficient of materials is by
using the two microphone transfer function impedance tube method. In order to
fabricate the test rig for the two-microphone transfer function impedance tube
method, all of the criteria mention in ASTM E1050-12 (2012) was used as a standard
reference. Figure 15 shows the schematic diagram of the two-microphone transfer
function impedance tube method.
Microphone BLoudspeaker
Microphone A
Sample
Frequency Generator
Sound Analyzer (PicoScope)
PC Oscilloscope Software Computer
Amplifier B Amplifer APower Supply
Switchable
Cylindrical Tube
Sound Wave Generated
Figure 15: Schematic diagram of two-microphone method
53
SPECIMEN HOLDER
AMPLIFIER A AND B
FREQUENCY GENERATOR
POWER GENERATORPICOSCOPE
INSULATED TUBE PIPE
SPEAKER
COMPUTER
MICROPHONE A & B
Figure 16: The two-microphone transfer function impedance tube test rig
Figure 16 shows the actual two-microphone transfer function impedance tube
test rig. The frequency generator (GW Instek SFG-2110 Synthesized Function
Generator) is connected to the speaker (8 Ω, 50 W) to generate a sinusoidal wave at
different frequency in acoustic range. Then, the end of the tube is connected with two
microphones, A and B. The function of the two microphones is used to detect the
sound transmitted in the tube. Each microphone is connected to an amplifier which is
also connected to a stable power supply of 9 V ~ 12 V direct current (GW GPC-3030
D.C Power Supply). Lastly, both amplifiers is connected to a PicoScope (PC
Oscilloscope ADC-216) that obtained the reading and converted it to digital form
wave which can be seen in the PicoScope software on the computer. From sinusoidal
sound wave, fast Fourier transforms (FFT) graph were obtained. It is plotted in
decibel (dB) versus frequency (Hz) graph. The two highest peaks generated, which is
the real part (the incidence peak) and imaginary part (the reflection peak) was
obtained. Figure 17 shows the sinusoidal wave graph and Figure 18 shows the real
part (the incidence peak) and imaginary part (the reflective peak).
54
Figure 17: Sinusoidal sound wave for microphone A and B
Figure 18: Fast Fourier transform graph of sound wave: (a) real part (the
incidence peak), and (b) imaginary part (the reflective peak)
The tube for the test rig was made from bronze and it is cylindrical, straight,
non-porous, dust free, has smooth inner and outer surface, and thick. This helps to
maintain low attenuation. Attenuation is the gradual loss in intensity throughout the
medium that disturb the propagation wave and the reading obtained on the
microphone. The tube was insulated using synthetic sound proof layer that prevents
outer noise or sound entering the tube. A sound source was placed at one end and the
sample holder at the other end. The two sets of microphone were mounted at two
(a)
(b)
55
different locations along the wall of the tube. For data processing and acquisition, a
two-channel, digital frequency analysis device was used. The working frequency
range is given by:
𝑓lower < Working Frequency, 𝑓working < 𝑓upper (29)
Where, flower is the lower working frequency of the tube (Hz) and fupper is the upper
working frequency of the tube (Hz). To maintain the plane wave generated in the
tube, the upper limit is given by:
Upper limit, 𝑓upper <𝐾𝑐
𝑑 (30)
Where, K is constant (0.586), c is the speed of sound (m/s) and d is the diameter of
the tube (m).
For accuracy of the measurements, a large spacing between the microphones
is desired. Where, the microphone spacing must be less than the shortest wavelength
of desired frequency. The microphone spacing, s is given by:
Microphone Spacing, 𝑠 ≪𝑐
2fupper (31)
According to ASTM E1050-12 (2012), the maximum spacing of the two
microphones must be 80 % of the equation. The frequency range for the two-
microphone transfer function impedance tube test rig has the working frequency
range of 500 Hz to 6000 Hz. Although the apparatus can support from 200 Hz to
6000 Hz, from the range from 500 Hz to 6000 Hz is selected in order to reduce
inconsistent data from lower frequency because of the lower sound level created by
the sound source.
3.12 CALIBRATION OF THE TWO-MICROPHONE TRANSFER
FUNCTION IMPEDANCE TUBE METHOD AND SOUND ABSORPTION
CALCULATION
Calibration needs to be done in order to get better enhancement on the results in the
sound absorption. It may be done in both physical and theoretical ways. The
calibration must follow the criteria stated in ASTM 1050-12 (2012). One of the best
ways to get an accurate result in obtaining good sound absorption coefficient is by
56
using a sound insulator. Sound insulator was used to insulate the impedance tube and
the outer cover speaker. It also helps to provide enhancement for the test rig. The
insulation of the tube is done to restrict the sound generated by the speaker to be
leaking away from the impedance tube and to prevent outside, external sound or
noise from disrupting the signal detected by the two microphones. Furthermore, it
also used to avoid airborne flanking transmission to the microphone. Insulation for
the speaker reduces the resonances of the air column in the impedance tube that were
arises when the mechanical impedance of the speaker membrane or diaphragm is at
high frequency. As referred to the ASTM E1050-12 (2012) criteria, the sound
produce from the speaker must be in respective position that can be detected by both
microphones which is at least 10 dB greater than the noise from the background
which is the environment area. Apart from that, the usage of sound insulation can
also reduce distortion produce by the speaker.
Another method of calibration that was done on the two microphones method
is by interchanging the location of the two microphones. By interchanging the
location of the two microphones, it rectifies the phase mismatch and internal
amplitude between both two microphones according (Wolkesson, 2013). This
technique is also called switching technique. Nevertheless, the switching of the two
microphones needs to be handled with care to get the exact location of normal
configuration as mentioned in ASTM E1050-12 (2012). Furthermore, the reading
from the interchange and original location of the two microphones were also
corrected by using a calculation as stated in ASTM E1050-12 (2012) indirectly for
small mismatch.
The frequency range is taken from 500 Hz to 6000 Hz. It is known that the
speed of air changes depending on the temperature. The speed of sound is given by:
Speed of Sound, 𝑐 = 20.047√273.15 + 𝑇 (32)
Where, T is the temperature (°C) such as room temperature. In this case, calibrations
need to be done to reduce error and link two different measurements. For calibration,
to find the complex amplitude mismatch is given by:
Complex Amplitude Mismatch, |𝐻C−| = (|𝐻−I|/|𝐻−II|)
1
2 (33)
57
Complex Amplitude Mismatch, |𝐻C−| = [(𝐻r
−I)2 + (𝐻i−I)2] × [(𝐻r
−II)2 + (𝐻i−II)2]
1
4
(34)
Where, Hr-I is the complex real part (incidence peak data) from the microphones
reading before the switch, Hi-I is the complex imaginary part (reflective peak data)
from the microphones reading before the switch, Hr-I is the complex real part
(incidence peak data after the switch) from the microphones reading after the switch
and Hi-II
is the complex imaginary part (reflective peak data after the switch) from
the microphones reading after the switch. Meanwhile, calibration to find the complex
phase mismatch is given by:
Complex Phase Mismatch, 𝛷c− =
1
2(𝛷−I − 𝛷−II) =
1
2tan−1
[(𝐻i−I𝐻r
−II)+(𝐻r−I𝐻i
−II)]
[(𝐻r−I𝐻r
−II)−(𝐻i−I𝐻i
−II)]
(35)
After calibration, the values for the real and imaginary part of the microphone
is corrected by finding the complex real part and complex imaginary parts that are
given by:
Complex Real Part, 𝐻r =1
𝐻c− [(𝐻r
− cos(𝛷c−)) + (𝐻i
− sin(𝛷c−))] (36)
Complex Imaginary Part, 𝐻r =1
𝐻c− [(𝐻r
− cos(𝛷c−)) + (𝐻i
− sin(𝛷c−))] (37)
In the meantime, to complex reflection coefficient part is given by:
Complex Reflection Coefficient, 𝐷 = 1 + 𝐻r2 + 𝐻i
2 − 2[𝐻r cos(𝑘𝑠) + 𝐻i sin(𝑘𝑠)]
(38)
Where, s is the microphone spacing and, k is the attenuation constant that is given by:
Attenuation Constant, 𝑘 =2𝜋𝑓
𝑐 (39)
The complex imaginary reflection is given by:
Complex Imaginary Reflection, 𝑅i =2𝐻r sin[𝑘(2𝑙+𝑠)]−sin(2𝑘𝑙)−(𝐻r
2+𝐻i2) sin[2𝑘(𝑙+𝑠)]
𝐷
(40)
Meanwhile, the complex real reflection part is given by:
58
Complex Real Reflection, 𝑅r =2𝐻r cos[𝑘(2𝑙+𝑠)]−cos(2𝑘𝑙)−(𝐻r
2+𝐻i2) cos[2𝑘(𝑙+𝑠)]
𝐷
(41)
Lastly, the sound absorption coefficient is given by:
Sound Absorption Coefficient, 𝛼 = 1 − |𝑅|2 = 1 − 𝑅r2 − 𝑅i
2 (42)
MATLAB was used to generate and run the calculation to obtain the sound
absorption coefficient of banana epoxy composites. MATLAB can help to complete
with less time.
3.13 SOUND ABSORPTION TEST
The sound absorption coefficients of the natural fibre composites were tested in the
acoustic frequency range of 500 Hz to 6000 Hz for acoustical testing. The sound
absorption coefficients were obtained from two-microphone transfer function
impedance tube test rig fabricated in house, according to ASTM E1050-12 (2012)
standards. The inner diameter of the impedance tube is 25 mm and therefore the
circular specimens of the same size were fitted into the tube. The thickness of the
specimen is 5 mm. It should be noted that specimen thickness has a significant effect
to the measured sound absorption coefficient even though the material formulation
has not been changed. This was reported by Ismail et al. (2010). Therefore, the
measured sound absorption coefficient based on this thickness are expected to be
lower than those reported in the literature which have specimens thickness in the
range of 20 to 300 mm. Charts of sound absorption coefficients of different
composition in terms of fibre content and chemical treatment were plotted and
comparisons were made. A sample was tested for each fibre loading. The standard
deviation for repeated tests (i.e. 5 run for each sample) is around ± 2 %. It is known
that the transfer function estimates are made from sample records of finite duration
and frequency resolution, and susceptible to random and bias errors. The random
error (i.e. distortion and etc.) generally kept low by creating frequency smoothing,
that is by averaging together the results of the test repeatedly (ASTM E1050-12,
2012). Meanwhile, for bias error, this may arise from uncorrected phase and
amplitude mismatch. Thus, these errors were corrected by using the calibration
calculations mentioned in Section 3.12 and as stated in ASTM E1050-12 (2012).
59
3.14 TENSILE TEST
The tensile tests were conducted using a universal testing machine with model code
‘UTM107’ according to ASTM D638-14 (2014) standards. The tensile test machine
was originally built and equipped by T-Machine Technology (Taiwan) Co. Ltd. Five
good samples were tested for each weight loading. The results of the five tests were
then averaged. Figure 19 shows the tensile test machine.
Figure 19: Tensile test machine
3.15 LIGHT/OPTICAL MICROSCOPE
Samples were taken from each of the untreated luffa, epoxy, oil palm epoxy and
banana epoxy composites and the treated counterparts. Images for each of the
samples were taken at the breaking point and the middle section of the natural fibre
composites. Samples were typically tested using industrial microscope with model
code ‘Eclipse 3x2 LV140’ equipped by Nikon (Japan) Corporation. Test preparations
were typically done according to the ASTM E2015-04 (2014) standards. Figure 20
shows the light/optical microscope.
60
Figure 20: Light/optical microscope
3.16 SCANNING ELECTRON MICROSCOPE
For the scanning electron microscope, the samples were sliced and mounted on
aluminium stubs. Then, they were fine coated by sputtering the samples with metal
particle coating (i.e. gold) for a one to two minutes to a thickness about 10 nm under
0.1 torr and 18 mA by using auto fine coaters with the model code of ‘JFC-1600’
equipped by JEOL (Japan) Ltd. The morphological studies of the untreated and
treated natural fibre composites were clearly observed using a Hitachi Analytical
Table Top SEM (bench top) with model code ‘TM-3030’ equipped by Hitachi High-
Technologies Europe (Germany) GmbH. It used a field emission gun and with
accelerating voltage of 5 kV and 15 kV to collect images of the surface of
composites. The test preparations were coherence with ASTM E2015-04 (2014)
standards. Figure 21 (a) shows the scanning electron microscope and Figure 21 (b)
shows the spurred coated machine.
61
Figure 21: Equipment for morphological test (a) scanning electron microscope,
and (b) sputter coating machine
3.17 FOURIER TRANSFORM INFRARED SPECTROSCOPY
The Fourier transform infrared spectroscopy (FTIR) was conducted using a
spectroscopy with the model name of ‘IRAffinity-1’ equipped by Shimadzu (Japan)
Corporation. The scanning was conducted in the wavenumber range of 400 to 4000
cm-1
for both untreated and treated fibre samples. Test prepared according to ASTM
E168-06 (2006) and ASTM E1252-98 (2013) standards. An infrared spectrum of the
wavenumber range was collected and the functional groups of fibre were identified.
Figure 22 shows the devices used to obtain the spectral data.
Figure 22: Infrared spectrometer
(a) (b)
62
CHAPTER 4
4.0 RESULTS AND DISCUSSION
4.1 EMPIRICAL MODELLING
Several tests were carried out by manipulating the fibre characteristics in order to
understand the basic relationship between the sound absorption coefficient and those
parameters under investigation. These tests included the change in flow resistivity,
the change in fibre size, and the change in thickness. Three types of fibres were used
and analysed using the Mechel, and Delany and Bazley empirical models (Section
3.2 to Section 3.5). The fibres considered were banana, luffa and oil palm fibre.
Other factors which might influence the sound absorption coefficient were also
discussed and analysed.
4.1.1 BANANA FIBRE
4.1.1.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF BANANA
FIBRE ON SOUND ABSORPTION
Figure 23 shows the effects of the fibre diameter of banana fibre on sound absorption.
The results were obtained by using a combination of Mechel, and Delany and Bazley
empirical models. As shown in Figure 23, the sound absorption, increased with
increasing flow resistivity which is the results of decreasing fibre diameter. It is
observed that at the lower frequency range (i.e. from 500 Hz to 2000 Hz), with
increasing fibre diameter, the sound absorption exhibited an increasing trend.
Meanwhile, at medium to higher frequency range (i.e. from 2000 Hz to 6000 Hz),
with decreasing fibre diameter, the sound absorption is increased. It was also
observed that the gap between the lower to medium frequency is nearer and the gap
between the medium to higher frequency is greater as the sound absorption increases
due to increase in frequency. A crossover point can be observed at 2000 Hz. At the
frequency of 6000 Hz, the fibre size of 20 µm has the highest sound absorption
coefficient (i.e. 0.64) whereas the fibre size of 50 µm has the lowest sound
absorption coefficient (i.e. 0.32). Beyond 2000 Hz, with increasing frequency, the
sound absorption coefficient for all fibre sizes exhibits increasing trends. Table 10
shows the properties of banana fibre and the flow resistivity values used in modelling
the sound absorption. The porosity and the fibre material thickness were taken as a
63
constant, whereas the fibre diameter and the flow resistivity were manipulated, when
predicting the sound absorption coefficients using the empirical models.
Table 10: Properties of the banana fibre and the respective flow resistivity at
different fibre diameters
Type of
Fibre Thickness Porosity Fibre Diameter Flow Resistivity
Banana 0.005 m 0.884
20 µm 111410 Pa.s.m-2
30 µm 49517 Pa.s.m-2
40 µm 27853 Pa.s.m-2
50 µm 17826 Pa.s.m-2
Figure 23: The effect of fibre diameter of banana fibre on the sound absorption
(5 mm thickness)
At the lower frequency range from 500 Hz to 2000 Hz, a higher sound
absorption coefficient is observed for the larger fibre diameter. This is because a
larger fibre diameter, the pore within the fibre lumen structure is expected to be
larger if compared with fibre of a smaller diameter, even though the flow resistivity
can be lower. Meanwhile, at the frequency range of 2000 Hz to 6000 Hz, it showed
the higher sound absorption coefficient is expected for larger diameter fibre. This is
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1000 2000 3000 4000 5000 6000
Sou
nd
Asb
orp
tio
n C
oe
ffic
ien
t (α
)
Frequency (Hz)
20µm 30µm 40µm 50µm
64
because the higher flow resistivity values at larger fibre diameters have a more
significant effect compared to the case at the lower frequency range. According to
Ballagh (1996), the fibre diameter size is inversely proportional to the flow resistivity.
Thus, this support the behaviour of the high flow resistivity happened at a lower fibre
diameter or vice versa. In other words, the increase in the fibre diameter size will
eventually reduce the flow resistivity and vice versa.
By comparing the sound absorption coefficient with other parameters used in
the empirical model calculations, with increasing complex wave number, 𝑘c and
characteristic impedance, 𝑍c , a minor change in the flow resistivity value of the
Mechel empirical model was expected. A low pressure reflection coefficient, 𝑅 was
produced because of the higher surface impedance, 𝑍s (Oldham et al., 2011). Thus,
with a lower reflection coefficient exhibited by a material, a higher sound absorption
coefficient is expected. However, at medium and higher frequencies, the exhibition
of higher sound absorption coefficient is ascribed to the small fibre diameter that
produced a comparatively larger impact on the flow resistivity value of the Mechel
empirical model which subsequently caused a higher sound absorption. According to
Koizumi et al. (2002), the reason behind the high sound absorption coefficient is that
the decrease in the fibre diameter indirectly increases the number of fibre per unit
area inside the samples. Moreover, the energy loss also increases with increasing
surface as a result of higher fibre per unit area.
The relationship between the fibre diameter and the flow resistivity may be
used to indicate the permeability of a medium through the sound absorbing materials.
A material with high flow resistivity will be less permeable to sound. The reason
behind this is that the sound wave that projected onto the fibrous materials was
highly resisted to enter, move or pass throughout the materials. This characteristic of
the fibrous materials tends to reflect a certain amount of sound wave or air flow.
Usually, the fibre agglomeration forms a network or web that acts as a natural fibre
filter. This network or web had an interlocking characteristic between the fibres that
not only provides strength, but also help to become a natural resistance to the
acoustic wave motion (Seddeq, 2009). In general, as sound entered the materials, the
amplitude started to decrease due to the friction, as the result of waves tried to enter,
move or pass through the tortuous path (Conrad, 1983). Tortuosity is a measurement
of elongation path through the pores that created by the fibres, especially on the fibre
65
lumen structure, the fibre hollow structure or the structure created by fibre
agglomeration forms. As the matter of facts, Knapen et al. (2003) described that the
internal structure of the materials may influence the sound absorption coefficient of
the materials.
4.1.1.2 EFFECT OF THICKNESS OF BANANA FIBRE ON SOUND
ABSORPTION
Figure 24 shows the effects of material thickness of banana fibre on the sound
absorption coefficient. The results were obtained by using a combination of Mechel,
and Delany and Bazley empirical models. Based on Figure 24, it showed that the
increases in sound absorption were due to the increasing in the material thickness. It
was observed that at the lower to medium frequency range (i.e. from 500 Hz to 4000
Hz), with increasing material thickness, the sound absorption followed the same
trend. Whereas, at a higher frequency range (i.e. from 4000 Hz to 6000 Hz), there
was a drop in sound absorption for thickness of 0.020 m. This behaviour does not
correlate with the basic theoretical concept and understanding. This is because, as the
materials become thicker, it should absorb more acoustic energy as compared to the
thinner materials. Beyond 4000 Hz, for banana fibre material of thickness more than
0.015 m, the fibre sound absorption coefficient stabilized in the range of 0.90 to 0.99.
In overall, the banana fibre material thickness of 0.015 m has the highest sound
absorption coefficient (i.e. 0.97) whereas the banana fibre material thickness of 0.005
m has the lowest sound absorption coefficient (i.e. 0.46). With increasing frequency,
the sound absorption coefficient followed the same trend. Table 11 shows the
properties of banana fibre and the thickness values used in these empirical
calculations. In this case, the porosity, the mean fibre diameter and the flow
resistivity were taken as constants whereas the fibre material thickness was varied.
Table 11: Properties of the banana fibre and flow resistivity value used in
empirical calculations
Type of
Fibre Thickness Porosity Fibre Diameter Flow Resistivity
Banana
0.005 m
0.884 32 µm 43521 Pa.s.m-2 0.010 m
0.015 m
0.020 m
66
Figure 24: The effect of material thickness of banana fibre on the sound
absorption (32 µm fibre diameter)
It is known that a thicker material tends to produce a higher sound absorption
coefficient. Based on Zent and Long (2007), thicker samples caused a higher sound
absorptions or vice versa (as thinner samples caused lower sound absorptions). This
may be true for other type of fibre materials based on the empirical model observed.
However, based on Figure 24, it shows the opposite way of understanding. It is
observed that the sound absorption coefficient of banana fibre with a lower thickness
of 0.015 m can achieve a higher sound absorption coefficient at higher frequency
ranges. The reason behind this was due to the porosity of banana fibre has a value
close to the matrix density or the bulk density. This explanation can be supported
based on Ren et al. (2011), it stated that material with low porosity or a very close
porosity with the matrix density and its bulk density will tend to have a higher sound
absorption coefficient ranging from medium to high frequency at low thickness with
the same material used. Apart from that, when the thickness of the material sample
increased, the improvement in the sound absorption performance at the lower to
medium frequency range is more significant compared to the result in the higher
frequency range. A similar result is reported in another study. According to Ren et al.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1000 2000 3000 4000 5000 6000
Sou
nd
Asb
orp
tio
n C
oe
ffic
ien
t (α)
Frequency (Hz)
0.005m 0.010m 0.015m 0.020m
67
(2011), there is little influence of thickness to the sound absorption performance at
high frequency range, especially for fibre thickness in the range of 0.010 m to 0.020
m. The results for banana fibre as in Figure 24, supports the conclusion drawn by
Ren et al. (2011). In conclusion, the basic understanding that a thicker material has a
higher sound absorption coefficient only applied at lower frequency. This theory is
further supported by the work conducted by Ibrahim and Melik (1978). They showed
that the increased sound absorption with increasing thickness happened at low
frequency. However, at higher frequency, the thickness of the materials had no
significant effect on the sound absorption.
4.1.2 LUFFA FIBRE
4.1.2.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF LUFFA
FIBRE ON SOUND ABSORPTION
Figure 25 shows the effect of the fibre diameter of luffa fibre on sound absorption.
The results were obtained by using a combination of Mechel, and Delany and Bazley
empirical models. Based on Figure 25, it can be observed that the fibre diameter has
a significant effect on the sound absorption. It is also observed that at the lower
frequency range (i.e. from 500 Hz to 1500 Hz), with increasing the fibre diameter,
the sound absorption coefficients increased slightly. Meanwhile, at medium to higher
frequency (i.e. from 1500 Hz to 6000 Hz), with decreasing as the fibre diameter, the
sound absorption followed an increasing trend, which is a result of increased flow
resistivity. The results gap of each sound absorption coefficient between the lower to
medium frequency is nearer as the frequency increased until at a certain point it will
start to overlap with each other. Once it overlaps, the results gap of each sound
absorption coefficient is greater, especially at medium to higher frequency. The fibre
size of 10 µm has the highest sound absorption coefficient (i.e. of 0.74), whereas the
fibre size of 25 µm has the lowest sound absorption coefficient (i.e. 0.40). Table 12
shows the properties of luffa fibre and the flow resistivity values used in the
empirical calculations. In this case, the porosity and the fibre material thickness were
taken as constant when comparing the sound absorption coefficient of luffa fibre
material of various fibre diameters.
68
Table 12: Properties of the luffa fibre and the respective flow resistivity at
different fibre diameters
Type of
Fibre Thickness Porosity Fibre Diameter Flow Resistivity
Luffa 0.005 m 0.930
10 µm 198240 Pa.s.m-2
15 µm 88105 Pa.s.m-2
20 µm 49559 Pa.s.m-2
25 µm 31718 Pa.s.m-2
Figure 25: The effect of fibre diameter of luffa fibre on the sound absorption (5
mm thickness)
It can be seen that at the lower frequency, ranging from 500 Hz to 1500 Hz,
the sound absorption coefficient increased with increasing fibre diameter. Although
the value of the flow resistivity is lower for larger fibre diameter, it helps to maintain
the porous lumen structure in the fibre as compared to the smaller diameter fibre. If
compared with banana fibre, luffa fibre had higher porosity which caused the shifted
of the crossover point that is 2000 to 1500 Hz for luffa fibre. Furthermore, the
frequency range of 1500 Hz to 6000 Hz, it showed a higher sound absorption
coefficient because of the higher flow resistivity for luffa fibres. With a decreased
fibre diameter, the more porous structure of the sound absorbent is expected. Thus, it
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indirectly increased the sound absorption coefficient. Furthermore, according to
Ballagh (1996) and Seddeq (2009), the fibre diameter is inversely proportional to the
flow resistivity. Thus, this support the behaviour of high flow resistivity is due to the
decreased in fibre diameter.
Furthermore, compared with banana fibre, it is observed that the fibre
diameter of luffa fibre is smaller and hence more porous. This caused luffa fibre to
have a higher sound absorption. The relationship between the fibre diameter and flow
resistivity may be used to indicate the permeability of a medium through the sound
absorbing materials. Materials with high flow resistivity will be less permeable to
sound. The reason behind this is that the sound wave that projected onto the fibrous
materials was highly resisted to enter, move or pass throughout the materials. This
characteristic of the fibrous materials tends to reflect a certain amount of sound or air
flow. Usually, the fibre agglomeration forms a network or web that acts as a natural
fibre filter. This network or web had an interlocking characteristic between the fibres
that not only provides strength, but also help to become a natural resistance to the
acoustic wave motion (Seddeq, 2009).
In general, as sound entered the materials, the amplitude decreased due to the
friction, as the result of waves tried to enter, move or pass through the tortuous path
(Conrad, 1983). As already known, tortuosity is a measurement of elongated path
through the pores that created by the fibres, especially in the fibre lumen structure,
fibre hollow structure and/or structure created by fibre agglomeration forms. As a
matter of fact, Knapen et al. (2003) described that the internal structure of the
materials may influence the sound absorption coefficient of the materials. The luffa
fibre has a larger pore compared to the banana fibre. In fact, the banana fibre is
flatter than the luffa fibre if the dimensions of both types of fibres are compared.
Thus, it is expected that the sound absorption for the luffa fibre is better than banana
fibre. Furthermore, the density of luffa fibre is different to banana fibre. The density
caused a change in porosity of the fibre itself. A lower ratio of density caused
increases in the porosity whereas a higher ratio of density caused decreases in the
porosity.
70
4.1.2.2 EFFECT OF THICKNESS OF LUFFA FIBRE ON SOUND
ABSORPTION
Figure 26 shows the effects of the material thickness of the luffa fibre on the sound
absorption coefficient. The results were obtained by using a combination of Mechel,
and Delany and Bazley empirical models. Based on Figure 26, it showed that the
increases in the sound absorption were due to the increasing material thickness. It
was observed that at the lower to medium frequency range (i.e. from 500 Hz to 3500
Hz), with increasing material thickness, the sound absorption followed the same
trend. Whereas, at the higher frequency range (i.e. from 3500 Hz to 6000 Hz), there
was a drop in the sound absorption for the thickness of 0.020 m. However, this
behaviour does not correlate with the basic theoretical concept and understanding.
This is because, as the materials become thicker, it should absorb more sound energy
if compared to the thinner materials. At high frequency more than 3500 Hz, for fibre
material having thickness more than 0.015 m, the sound absorption coefficient
stabilized in the range from 0.90 to 0.99. Overall, the fibre material thickness of
0.015 m has the highest sound absorption coefficient (i.e. 0.99), whereas the fibre
material thickness of 0.005 m has the lowest sound absorption coefficient (i.e. 0.50).
With increasing frequency, the sound absorption coefficient followed the same trend.
Table 13 shows the properties of luffa fibre and the thickness values used in these
empirical calculations. In this case, the porosity, the mean fibre diameter size and the
flow resistivity were taken as constants whereas the fibre material thickness was
varied.
Table 13: Properties of the luffa fibre and flow resistivity value used in
empirical calculations
Type of
Fibre Thickness Porosity Fibre Diameter Flow Resistivity
Luffa
0.005 m
0.930 19 µm 54913 Pa.s.m-2 0.010 m
0.015 m
0.020 m
71
Figure 26: The effect of material thickness of luffa fibre on the sound
absorption (19 µm fibre diameter)
It is known that a thicker material tends to produce a higher sound absorption
coefficient. Based on Zent and Long (2007), thicker material samples created caused
higher sound absorptions or vice versa (as thinner samples caused lower sound
absorptions). This may be true for other type of fibre materials based on the
empirical calculations observed. However, based on Figure 26, it shows the opposite
way of understanding. It is observed that the sound absorption coefficient of luffa
fibre with a lower thickness of 0.015 m can achieve a higher sound absorption
coefficient at higher frequency ranges. The reason behind this was due to the
porosity of luffa has a value closed with to the density. This explanation can be
supported based on Ren et al. (2011), it stated that material with low porosity or a
very close porosity with the density will tend to have a higher sound absorption
coefficient ranging from medium to high frequency at low thickness with the same
material used.
Apart from that, when the thickness of the sample material increased, the
improvement in the sound absorption performance at lower to medium frequency
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range is more significant compared to the result in the higher frequency range. A
similar result is reported in another study. According to Ren et al. (2011), there is
little influence in the change of thickness to the sound absorption coefficient
performance at high frequency range, especially for fibre thickness in the range of
0.010 m to 0.020 m. The results obtained in Figure 26, supports the conclusion
drawn by Ren et al. (2011). In conclusion, the basic understanding a thicker material
has a higher sound absorption coefficient only applied at low frequency. This theory
is further supported by the work conducted by Ibrahim and Melik (1978). They
showed that the increased sound absorption with increasing thickness happened at
low frequencies. However, at higher frequency, the thickness had no significant
effect on the sound absorption.
4.1.3 OIL PALM FIBRE
4.1.3.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF OIL PALM
FIBRE ON SOUND ABSORPTION
Figure 27 shows the effects of the flow resistivity and the fibre diameter of the oil
palm fibre on the sound absorption. The results were obtained by using a
combination of Mechel, and Delany and Bazley empirical models. As shown in
Figure 27, the sound absorption increased, with decreasing flow resistivity. The
behaviour is different than the banana fibre and the luffa fibre due to its solid
structure and also because of the low porosity value it has. It is also observed that at
the lower frequency range (i.e. from 500 Hz to 2000 Hz), there is instability of sound
absorption. It showed that the fibre of 20 µm had a higher sound absorption in the
range of 500 Hz to 1250 Hz and followed by 25 µm diameter in the frequency range
of 1250 Hz to 2000 Hz. Meanwhile, at medium to higher frequency range (i.e. from
2000 Hz to 6000 Hz), as the fibre diameter increased, the sound absorption increased.
It was also observed that the gap between the lower to medium frequency is nearer
and the gap between the medium to higher frequency is greater as the sound
absorption increases due to increase in frequency. The fibre size of 30 µm had the
highest sound absorption coefficient (i.e. 0.65), whereas the fibre size of 15 µm had
the lowest sound absorption coefficient, (i.e. 0.28). Generally, as the frequency
increased, the sound absorption coefficient also increased. Table 14 shows the
properties of the oil palm fibre and the flow resistivity values used in the empirical
73
calculations. The porosity and the fibre material thickness were taken as constants
whereas the fibre diameter and flow resistivity were manipulated, when predicting
the sound absorption coefficients using the empirical models.
Table 14: Properties of the oil palm fibre and the respective flow resistivity at
different fibre diameters
Type of
Fibre Thickness Porosity Fibre Diameter Flow Resistivity
Oil Palm 0.005 m 0.636
15 µm 2333600 Pa.s.m-2
20 µm 1312600 Pa.s.m-2
25 µm 840080 Pa.s.m-2
30 µm 583390 Pa.s.m-2
Figure 27: The effect of fibre diameter of oil palm fibre on the sound
absorption (5 mm thickness)
At the lower frequency range from 500 Hz to 2000 Hz, it can be observed
that there is instability in sound absorption caused by the tortuosity in the fibre
materials. Random tortuosity caused the sound absorption for 15 µm, 20 µm, 25 µm
and 30 µm to be overlapped with each other and caused the fluctuation in sound
absorption. Furthermore, it is known that the structure of oil palm is denser and
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contains more oil compared to other fibres. These physical and chemical differences
affect the sound absorption behaviour. The almost-solid structure caused the fibre to
act efficiently in increasing the apparent density and hence the sound absorption
behaviour. Knapen et al. (2003) described that the internal structure of the materials
may influence the sound absorption coefficient. Meanwhile, at the frequency range
of 2000 Hz to 6000 Hz, it showed that the higher sound absorption coefficient is
because of the lower flow resistivity values at bigger diameter size. At an increasing
fibre diameter, the structure of the fibre became denser and hence the highest sound
absorption was obtained.
Furthermore, according to Ballagh (1996), the fibre diameter size is inversely
proportional to the flow resistivity. Thus, this support the behaviour of the higher
flow resistivity happened at lower fibre diameter and vice versa. In other words, as
the increase in the fibre diameter size will eventually reduce the flow resistivity and
vice versa. According to the studies done by Koizumi et al. (2002), there was an
increase of sound absorption coefficient as the material density increased, especially
in the middle and higher frequency. Similar results were shown by Ballagh (1995)
research on wool materials at various densities of the materials. Based on both results,
it showed that the less density type of material absorbs more sound at low frequency
compare to the higher density materials that normally absorbed at middle and high
frequency.
4.1.3.2 EFFECT OF THICKNESS OF OIL PALM FIBRE ON SOUND
ABSORPTION
Figure 28 shows the effects of the material thickness of the banana fibre on the sound
absorption coefficient. The results were obtained by using a combination of Mechel,
and Delany and Bazley empirical models. Generally, the increases in the sound
absorption were due to increases in the material thickness. However, based on Figure
28, it showed the material thickness does not affect the sound, especially at medium
to higher frequency. It is observed that at the lower frequency range (i.e. from 500
Hz to 1500 Hz), as the material thickness increased, the sound absorption increased.
Meanwhile, at medium to higher frequency range (i.e. from 1500 Hz to 6000 Hz), the
results of 0.010 m, 0.015 m and 0.020 m fibre overlapped. This behaviour does not
correlate with the basic theoretical concept and understanding. This is because, as the
75
materials becoming thicker, it is expected to absorb more sound as compared to
thinner materials. The fibre material thickness of 0.005 m has the highest sound
absorption coefficient (i.e. 0.45), whereas the fibre material thickness of 0.010 m,
0.015 m and 0.020 m have the lowest sound absorption coefficient (i.e. 0.43).
Generally, as the frequency increased, the sound absorption coefficient also
increased. Table 15 shows the properties of oil palm fibre and the thickness values
used in these empirical calculations. In this case, the porosity, the mean fibre
diameter size and the flow resistivity were taken as constants whereas the fibre
material thickness was varied.
Table 15: Properties of the oil palm fibre and flow resistivity value used in
empirical calculations
Type of
Fibre Thickness Porosity Fibre Diameter Flow Resistivity
Oil Palm
0.005 m
0.636 22 µm 1084800 Pa.s.m-2 0.010 m
0.015 m
0.020 m
Figure 28: The effect of material thickness of oil palm fibre on the sound
absorption (22 µm fibre diameter)
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76
It is known that the thicker material tends to produce higher sound absorption
coefficients. But for oil palm fibre, the sample showed an opposite. It is observed
that the sound absorption coefficient of oil palm fibre with a lower thickness (i.e.
0.005 m) can achieve a higher sound absorption coefficient at medium to high
frequency range. The reason behind this may be due to the fibre agglomeration. The
fibre agglomeration is not expected for the luffa fibre and the banana fibre. The
degree to which the material is consolidated or compacted also becomes the main
factor that may affect the results due to further agglomeration. According to Oldham
et al. (2011), the relatively poor sound absorption may also because of the uneven
nature of the fibres and the difficulty to compact the sample that caused or resulting
in high porosity that indirectly give an opposite result than expected. This is probably
due to the diameters of these fibres differing considerably from those one that the
predictive models have been developed and the possibility that the surfaces of the
very uneven fibres might also contain micro-pores.
4.2 EXPERIMENTATION
Several tests were done to investigate the relationship and the effect of chemical
treatment on sound absorption, mechanical, spectral and morphological properties.
Furthermore, the interrelationship between properties was also investigated.
Composites were fabricated using moulds, with epoxy as matrix and three types of
natural fibre were used as fillers. The fibres are banana fibre, luffa fibre and oil palm
fibre.
4.2.1 ACOUSTICAL PROPERTIES OF BANANA, LUFFA, OIL PALM
FIBRE (SOUND ABSORPTION ANALYSIS)
Figure 29 shows the sound absorption of raw untreated and treated banana, luffa and
oil palm fibre. The fibres were compressed until it reached 5 mm thickness without
adding any adhesive. The compression was done carefully in order to maintain the
original structure of the raw natural fibre as mentioned in Section 3.8. According to
Piggott (2013), the structure of the fibre will be maintained if the pressure is less than
1MPa (less than 150 psi). Based on Figure 29, the sound absorption coefficients
increased with increasing frequency. It is observed that raw luffa fibre had better
sound absorption frequency range of 500 Hz to 2500 Hz, and 3200 Hz to 5000 Hz.
Meanwhile, raw banana fibre showed the highest sound absorption coefficient at the
77
frequency of 5000 Hz to 6000 Hz. The increase in the sound absorption is believed to
be related to the structure of the raw fibre. Natural fibre had a hollow lumen structure
that tends to absorb entities (i.e. nutrition, water, and etc.). Based on Pickering et al.
(2015), most of hydrophilic hollow structure tends to absorbed more entities than the
hydrophilic solid structure due to greater number of open and through pores in the
fibre as illustrated later in Figure 34 in Section 4.2.2.2. A larger hollow lumen
structure on the fibre is believed to absorb sound better. This speculation is
consistent with the result shown in Figure 29. The raw luffa fibre has larger hollow
lumen structure compared to the raw oil palm and banana fibre. The fluctuation of
sound absorption curves in Figure 29 is believed to be the results of the raw fibres
having a wide range of diameters in contrast to an uniform diameter as reported by
Fouladi et al. (2011) in his study of coir fibre sound absorption. According to Fouladi
et al. (2011), the coir fibre has a varied diameter from 0.1 mm to 0.4 mm. Although
different types of fibre were used in this study, it is likely the fibres have a wide
diameter range. The wide distribution of fibre diameter caused the flow resistivity to
be varied significantly especially the samples were exposed to sound emission at
varying frequency.
Similar fluctuations behaviour of sound absorption is reported by Ismail et al.
(2010). Furthermore, based on their research, the sample with 10 mm thickness can
only achieved up to 0.3 - 0.4 sound absorption coefficient. Thus, based on their
sample, it is expected that by using 5 mm thickness, the sample should show the
same sound absorption properties as the raw banana, luffa and oil palm fibre given in
Figure 29 (i.e. fall in the range of sound absorption coefficient of 0.1 - 0.2). Other
than that, among the three types of raw fibre used, raw luffa fibre has a 3D-
interconnected structure which will be shown in Figure 40 (a) in Section 4.2.3.1. The
raw luffa fibre has the main stems interconnected by side branches that work as
natural occurring network. Another reason for higher sound absorption of raw luffa
fibre was due to the tortuosity of the fibre. Tortuosity will increase the sound
absorption path that will lead to high sound absorption. The raw fibre surface
structure also played an important role. For example, the raw oil palm fibre had a lot
of impurities (i.e. such as wax, dry oil and etc.), which caused the surface of the raw
fibre to be smooth. Compared to raw luffa fibre, the fibre has a bit rougher than the
raw banana and oil palm fibres. Furthermore, increased in sound absorption may be
78
due to high density of raw luffa fibre. According to Zulkifli et al. (2008), it showed
that raw oil palm had a higher sound absorption coefficient than raw coir fibre due to
higher density.
Figure 29: The sound absorption coefficient: (a) untreated banana, luffa, and
oil palm fibre without epoxy (5 mm thickness), and (b) treated banana, luffa,
and oil palm fibre without epoxy (5 mm thickness).
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(a)
(b)
79
Figure 30: Comparison between untreated and treated sound absorption
coefficient: (a) banana fibre (5 mm thickness) (b) luffa fibre (5 mm thickness),
and (c) oil palm fibre (5 mm thickness).
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Based on Figure 30, it is observed that there are no significant changes at
lower frequency of 500 Hz to 2000 Hz for all fibres. The only significant changes
happen in the middle and upper frequency of 2000 Hz to 6000 Hz, It is also observed
that the treated luffa and oil palm fibre has the sound absorption coefficients
increased by 10 - 20 %, while the treated banana fibre has the sound absorption
coefficient decreased by 10 - 20 %. The reason behind the low sound absorption
coefficient may be due to shrinkage of banana fibre, which resulted decreased in
fibre size. The fibre is taken from pseudo stem part, which is known to be sensitive
with the chemical reactions. Shrinkage of fibre causes reduction in size of pores.
Meanwhile, for the treated luffa and oil palm fibre, the alkaline treatment cause
increased in size of pores and fibre size, which resulted in increased sound
absorption coefficients. The comparison between empirical model and experiment
were discussed in Section 4.3.
4.2.2 BANANA FIBRE EPOXY COMPOSITES
Composites of epoxy and banana fibre were fabricated at 5 wt%, 10 wt%, 15 wt%
and 20 wt% fibre loading. The properties such as acoustical, mechanical and the
infrared spectral of absorption as well as the morphological were investigated and
analysed. Besides that, the comparisons between the untreated and the treated banana
fibre were conducted.
4.2.2.1 MORPHOLOGICAL OF BANANA FIBRE EPOXY COMPOSITES
(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL
MICROSCOPIC ANALYSIS)
Figure 31 (a), (b), (c) and (d) shows the scanning electron microscope (SEM)
micrographs of the composites. It is found that the banana fibres are made up of a
bundle of hollow sub-fibres which have a lumen structure. Banana fibre contains
numerous connected air pores or air cavities. These air cavities are believed to
contribute to the sound absorption of the composites. From Figure 31 (a), it is
obvious that the fibre were entangled in the composite. Thus, this concludes that
there were fibres network structures in the composites (Jiang et al., 2012). However,
raw fibres have dry and liquid substance (i.e. oil, wax and etc.) and impurities that
may fill the structure and covering the fibre as interpreted from Figure 31 (c). The
alkaline treatment caused the re-construction of a hollow lumen structure by
81
removing the dry and liquid (i.e. oil, wax and etc.) substance impurities inner and
outer surface of the fibre. Thus, this will indirectly create rough surface on the fibre
as interpreted from Figure 31 (d).
Figure 31: SEM micrographs of banana fibres: (a) tangled banana fibres, (b)
hollow lumen structure, (c) untreated banana fibre, and (d) treated banana
fibre
When the sound wave energy generated projected onto the surface of the
composite structure, the air motion, vibration and compression caused friction within
the microspores walls and limited the movement of air molecules. According to Chen
et al. (2010), because of the viscous and frictional forces, some parts of the energy
created were converted to heat energy and cause reduction in the sound energy
projected on the composites. As can be seen in Figure 31 (b), the alkaline treatment
allowed absorption of epoxy into the hollow lumen structure due to removal of a
certain impurities on the surface and inner structure of fibre. This indirectly
(a) (b)
(c) (d)
Smooth
surface
Rough
surface
Epoxy was absorbed
inside the lumen
structure
Twisted
fibre
82
improved the mechanical properties of banana fibre indirectly. Figure 32 (a) and (b)
shows the light/optical micrograph of both the untreated and treated banana fibres. It
can be seen that the untreated banana fibre structure is more intact compared to the
structure of treated fibre. The alkaline treatment caused the banana fibre to have an
uneven structure. Thus, this caused a certain reduction in the cross sectional area of
the fibre. The untreated fibres have a rough surface which easily that traps air and
this feature tends to improve sound absorption. However, it will not absorb too much
sound as expected if the presence of long path tortuosity within the raw fibre itself
was being eliminated through treatment.
Figure 32: Light/optical micrographs of banana fibre: (a) untreated banana
fibre (b) treated banana fibre
4.2.2.2 ACOUSTICAL PROPERTIES OF BANANA FIBRE EPOXY
COMPOSITES (SOUND ABSORPTION ANALYSIS)
Figure 33 (a) and (b) shows the effect banana fibre loading of composites, on the
sound absorption coefficient. The results were obtained using the two-microphone
transfer function impedance tube method according to ASTM E1050-12 (2012)
standards. By comparing the acoustics behaviour of the raw fibre and the composites,
blending the raw fibre with epoxy resin has reduced the sound absorption coefficient
to a lower level (i.e. from 0.18 to 0.12 at frequency of 5000 Hz to 6000 Hz). The
reason that caused the low sound absorption coefficient may be due to the reduction
in the quantity of fibre used. Lower sound absorption coefficient were obtained
because the fibre involved are less than 20 wt% than the raw fibre is which is 100
wt%. The other reason that caused the low sound absorption coefficient may be due
to process/method used to fabricate the samples. The compression caused the epoxy
(a) (b)
20 µm 20 µm
83
being forced into the hollow lumen of the banana fibre as shown earlier in Figure 31
(b) in Section 4.2.2.1.
Figure 33: The effects of fibre loading on sound absorption for banana epoxy
composites: (a) composites of untreated fibre (5 mm thickness), and (b)
composites of treated fibre (5 mm thickness)
It can be observed form Figure 33 (a) and (b) that the sound absorption
coefficient increases with increasing fibre loading. According to Markiewicz et al.
(2011), the sound absorption is defined as the ratio of the acoustical wave energy that
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84
is absorbed by the composites to the total energy incident to the sample. The
reduction in the sound pressure is due to the absorption of sound wave within the
material. It is believed that the sound energy is converted to heat energy. The
fluctuating behaviour at 2000 Hz, 3000 Hz, and 4000 Hz was due to the specific
characteristic (i.e. tortuosity path) of banana fibre epoxy that tends to reflect and
absorbing sound at that frequency (Yang et al. 2003) as illustration in Figure 34.
Another factor that might influence the fluctuation was due to different fibre
diameter size, uneven structure and crumpled structure of the banana fibre. Overall, it
was observed that banana epoxy composites with 15 wt% tend to absorb more sound
energy than other composites.
Twisted elongation path
Twisted fibre on it axis
Figure 34: Illustration of tortuosity in fibre
At high fibre loading, the banana fibres were more compact within the epoxy
matrix. It is speculated that the banana fibre were packed closer to each other within
the matrix. This compact structure caused a reduction in the size and the volume of
the air void fractions within the composites. This indirectly produced narrower
passages for sound wave and increase in the tortuosity of sound propagation. Thus, it
will cause the sound to travel in a longer distance than the usual distance. Jiang et al.
(2012) showed similar results in their study of the acoustic properties of seven-hole
hollow polyester fibres (SHPF). At increasing fibre loading, the sound absorption
coefficient of the SHPF composites shows an increasing trend. Furthermore,
Markiewicz et al. (2011) also reported that the sound absorption coefficients of PP-
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long hemp fibre composites, PP-crumble hemp plant composites, PP-rapeseed straw
kaszub composites, and PP-long flax fibre composites increased significantly with
the change in the fibre loading, especially in the medium and high frequencies of
sound. Moreover, porosity is another factor that may influence the sound absorption.
Beside the size of the pores, the number and types of pores also influenced the sound
absorption of the materials (Seddeq, 2009). Figure 35 shows types of pores for
hollow lumen structure. Meanwhile, some pores tend to propagate the sound and
dampened it. At fibre loading of 20 wt%, it is found that the composites have the
sound absorption coefficient lowered than the 15 wt%. The reason behind this may
be due to the difference of the microstructures between the two composites. For the
composite at 20 wt% fibre loading, the hollow lumen structure is collapsed, thus this
shorted the sound to travel inside the natural fibre epoxy composites. This
characteristic is not observed for the 15 wt% fibre loaded composite.
Closed pores
Through pores
Inner hollow
structure
Open pores
Blind pores
Rough surface
Outside of the fibre
Figure 35: Illustration of type of pores in hollow fibre
Figure 36 shows the effects of the chemical treatment on sound absorption of
banana fibre epoxy composite. Based on Figure 36, it shows that the alkaline
treatment of the banana fibre decreased the sound absorption coefficient of banana
fibre epoxy composites. It was observed that composite at 15 wt% untreated banana
fibre loading tends to absorb more sound compared to the counterpart with treated
86
banana fibre. Significant difference in sound absorption can be seen in the frequency
ranges from 2000 Hz to 3000 Hz; and from 5000 Hz to 6000 Hz. This is because the
chemical treatment using sodium hydroxide and heat treatment cause the fibre to
shrink, and hence affects the structure of banana fibre. Changes in the structure of
banana fibre caused the elimination of liquid-structure (i.e. moisture) inside the
banana fibre. Thus, this caused the epoxy to absorb into the fibre easily. The
absorption of epoxy caused the closing in the porous structure of the fibre and hence
reduces the porosity. Furthermore, according to Fouladi et al. (2011), the mix
between the matrix and the fibre during fabrication causes the matrix and the fibre
act as one part of the materials itself. This indirectly will create a less porous
structure with lower tortuosity for sound propagation.
Figure 36: The effects of chemical treatment on banana fibre for banana fibre
epoxy composites at 15 wt% (5 mm thickness)
4.2.2.3 MECHANICAL PROPERTIES OF BANANA FIBRE EPOXY
COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS)
Figure 37 (a) and (b) shows the effect of fibre loading of the composite on the tensile
strength and yield strength. Figure 37 (a) and (b) also compares the tensile and yield
strength for the untreated and the treated banana epoxy composites. The results were
obtained by using the tensile testing machine according to ASTM D638-14 (2014)
standards. Based on Figure 37 (a) and (b), it shows that increasing in the fibre
loading has a decreased tensile and yield strength of banana epoxy composites. The
0.00
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0.14
0 1000 2000 3000 4000 5000 6000
Sou
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oe
ffci
ein
t (α
)
Frequency (Hz)
Untreated 15 wt% Banana Epoxy Treated 15 wt% Banana Epoxy
87
optimum fibre loading is at 5 wt%. At 5 wt% fibre loading, the composite has the
highest tensile and yield strength followed by 20 wt%, 10 wt% and lastly 15 wt%.
The increased in the tensile and yield strength is due to the inclusion of fibres (Liu et
al., 2009). However, because the banana fibre has a flatter structure unlike other
natural fibres which are rounded, the fibre caused a different effect on the composite
tensile and yield strength. The randomized distribution of the banana fibre inside the
epoxy matrix during the moulding may cripple of fibre structure, crumples the fibre.
It is speculated that this randomization caused the tensile and yield strength to
behave in the opposite way as expected for other types natural fibre composite. The
mechanical properties of composites normally depend on the fibre matrix interaction.
It is known that when the fibre loading is higher, the resin will be insufficient to
completely wet the fibre. However, in this case, the overlapping between the flat
surfaces of the fibres may cause incomplete wetting and caused the reduction in the
tensile and yield strength test results.
Another factor that caused the decrease in the tensile and yield strength was
due to the improper fracture of the composite. This happened as the flat banana fibres
started to break first before the epoxy started to break. Improper fracture was also
caused due to the small holes, void or internal crack caused by poor dispersion of
fibre inside the composites. However, according to Boynard & D’Almeida (2000),
this may be considered as controlled fracture which does not break suddenly like
normal fracture. According to Prasad et al. (2014), it may be also because of the poor
dispersion of the fibres. Other than that, the weak banana fibre itself caused the
decreasing in the composites strength. The composites based on the treated banana
fibre have a higher tensile and yield strength compared to the counterpart with
untreated banana fibre. The alkaline treatment modified the structure of banana fibre.
It caused the surface to be rough. It also removes moisture and change in the
molecular structure of fibre. The two effects later indirectly increased the fibre
strength (Balakrishna et al., 2013).
88
Figure 37: The effects of banana fibre loading on: (a) tensile strength, and (b)
yield strength
4.2.2.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS OF
BANANA FIBRE EPOXY COMPOSITE
Figure 38 shows the FTIR spectral of the untreated banana fibres and the treated
banana fibres. In Figure 38 the region of the broad absorption band at 3500 cm-1
to
3000 cm-1
for the both treated and untreated banana fibres are characterized with O-H
stretching and H- bonding structure that contains phenols, alcohols and water
functional group. The O-H stretching, and H- bonding broad absorption band in the
regions is decreased after alkaline treatment. The decreased of the broad absorption
0
5
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15
20
25
5wt % 10wt % 15wt % 20wt %
Ave
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Te
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ngt
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a)
Weight Composition (wt%)
Untreated Banana Epoxy Treated Banana Epoxy
0
5
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15
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25
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Ave
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a)
Weight Composition (wt%)
Untreated Banana Epoxy Treated Banana Epoxy
(b)
(a)
89
band promote addition of two small peaks for the treated banana fibres at 3431.36
cm-1
and 3300.20 cm-1
. The decrease of functional group phenolic or aliphatic
hydroxyl in the fibre was due to the reaction with sodium hydroxide which promotes
almost hydroxyl free bond structure. A small peak at 2924.09 cm-1
and 2870.08 cm-1
for the untreated banana fibre was attributed to the C-H stretching and O-H
stretching of alkanes (cellulose and lignin) and carboxylic acids. According to Saw et
al. (2013) small peak in the region of the C-H stretching includes the functional
group of methyl and methylene (cellulose and lignin). These peaks were reduced due
to alkaline treatment.
Figure 38: Fourier transforms infrared spectroscopy analysis of: (a) untreated
banana fibre; and (b) treated banana fibre
(a)
(b)
90
Table 16 shows the characterisation of the untreated banana fibres and the
treated banana fibres as extracted from Figure 38. For the untreated banana fibre, it
shows that the small peak band at 2403.30 cm-1
is characterized as the C≡ C
stretching and C≡ N stretching of the functional group of nitriles and alkynes. The
peaks at 1720.50 cm-1
and 1635.64 cm-1
are characterized as the C=C stretching of
the functional group of alkenes (lignin). The small peaks at 1323.17 cm-1
and
1247.94 cm-1
are characterized as the C-H bending of the functional group of alkanes
(cellulose, hemicellulose and lignin). The peak at 1018.41 cm-1
is characterized as
the C-O stretching of the functional group of alcohols (cellulose, hemi-cellulose and
lignin) and the small peak at 883.40 cm-1
is characterized as C-H bending, C-H rock,
N-H wags of the functional group of aromatic rings (out-of-plane), alkanes, 1°, 2°
amines (lignin). Compared with the treated fibre in Figure 38 (b), there were a
reduction in the intensity of the absorption bands at 1720.50 cm-1
and 1635.64 cm-1
and shifting of the absorption peak to 1662.64 cm-1
that are characterized as C=C
stretching from the functional group, alkenes, and another shifting from 1323.17 cm-1
and 1247.94 cm-1
to 1419.61 cm-1
which are characterized as C-H bending from the
functional group, alkanes due to the reaction of sodium hydroxide. The alkaline
treatment caused removal of lignin, hemicellulose and cellulose structure. Thus, this
created hollow lumen structure and rough surface of banana fibre that tends to give
better absorption of epoxy resin into the pore and surface. Better absorption of epoxy
caused greater reduction in sound absorption coefficient. Thus, this caused reduction
in the sound absorption for the treated composites as compared with raw banana fibre
and untreated banana fibre epoxy composites. The process also caused a swelled
structure due to the changes in the structure, morphology, mechanical and dimension
properties; whereas some part of the fibre tend to absorbed more entities (i.e. epoxy)
than the other part of it due to change in diameter (Bledzki and Gassan. 1999).
Swelling increased the banana fibre diameter size and porosity that increased the
sound absorption coefficient.
Due to alkaline treatment, the alkaline sensitive hydroxyl groups (O-H bond
structure) present in the natural fibre molecules was broken. It then reacted with
water, phenols or alcohols molecule groups (H-O-H bond structure) and moved in or
out from the fibre structure depending on the characteristic of fibre toward the
reaction of alkaline. Thus, the remaining of the reactive molecules indirectly formed
91
the fibre cell of -O-Na bond structure between the cellulose molecular chains (John
and Anandjiwala, 2008). Thus, some hydrophilic hydroxyl groups were reduced and
the fibre moisture resistance properties were increased. Some of the impurities,
pectin, hemicelluloses, hemi-cellulose, lignin, wax and oil that covered the fibre
surface were removed as due to alkaline treatment (Li et al., 2007). Alkaline
treatment also caused fibrillation that broke the composites fibre bundle into smaller
fibres and reduced the fibre diameter and length. An increased in the aspect ratio of
the fibre led to enhanced interface adhesion between fibre and matrix due to higher
surface roughness (Maiti et al., 2014). The sound absorption, mechanical, water
absorption and thermal behaviours of the composites are expected to improve
significantly by this treatment. According to Kabir et al. (2012), the treated fibres are
known to have lower lignin content than the untreated fibres. Furthermore, the
chemical treatment also partially removes the oil and wax that cover fibres and
distension of crystalline cellulose order (Kabir et al., 2012).
Table 16: The characterisation of banana fibre using infrared spectral analysis
Bond - Functional Group
Untreated banana
fibre (Wavenumber,
cm-1
)
Treated banana
fibre
(Wavenumber, cm-1
)
O-H stretching , Free hydroxyl -
Alcohol, Water, Phenols 3797.34, 3684.04 3855.70, 3728.40
O-H stretching , H- bonded, -
Alcohol, Water, Phenols 3284.77 3431.36, 3300.20
C-H stretching, O-H stretching -
Alkanes (CH; CH2; CH3),
carboxylic acids
2920.23, 2870.08 -
C≡ C stretching, C≡ N stretching –
Nitriles, Alkynes 2403.30 2418.74, 2287.58
C=C stretching - Alkenes (lignin) 1720.50, 1635.64 1662.64
C-H bending - Alkanes (cellulose;
hemi-cellulose; lignin) 1323.17, 1247.94 1419.61
C-O stretching - Alcohol
(cellulose; hemi-cellulose; lignin),
Carboxylic acids, Esters, Ethers.
1018.41 1029.99
C-H bending, C-H rock, N-H wag -
Aromatic rings (out-of-plane ),
Alkanes, 1°, 2° Amines (lignin)
883.40 844.82
92
4.2.3 LUFFA FIBRE EPOXY COMPOSITES
Composites of epoxy and luffa fibre were fabricated at 5 wt%, 10 wt%, 15 wt% and
20 wt% fibre loading. The properties such as acoustical, mechanical and the infrared
spectra of absorption as well as the morphology were investigated and analysed.
Besides, the comparisons between the untreated and the treated luffa fibre were
conducted.
4.2.3.1 MORPHOLOGY OF LUFFA FIBRE EPOXY COMPOSITES
(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL
MICROSCOPIC ANALYSIS)
Figure 39 (a), (b), (c) and (d) shows the scanning electron microscope (SEM)
micrographs of the composites. A single luffa fibre is made up of a bundle hollow
sub-fibre as can be seen in Figure 39 (b). Meanwhile, according to Yang et al. (2012),
the sub-fibre cell wall is made up of millions nano-fibrils luffa fibre. It also contains
numerous connected air pores or air cavities. The nano-sized fibril would lead to
extra vibration that caused higher energy dissipation. The distribution of fibres in the
composites also produced a porous structure that promoted sound absorption. It is
speculated that the porous structure is the major contribution for sound absorption.
From Figure 39 (a), it is obvious that the fibre were randomly entangled in the
composite. Thus, this concludes that there were fibres network structures in the
composites (Jiang et al., 2012). However, raw fibres have oil, wax or other impurities
that may fill in the hollow lumen structure and covering the fibre as can be
interpreted from Figure 39 (c). The alkaline treatment caused the re-construction of a
hollow lumen structure by removing the oil, wax or other impurities inner and outer
surface of the fibre. Thus, this will indirectly create rough surface on the fibre as
interpreted from Figure 39 (d). Figure 40 (a) and (b) shows the light/optical
micrographs of luffa for untreated and treated.
93
Figure 39: SEM micrographs of luffa fibres: (a) randomized tangled luffa fibres,
(b) hollow lumen structure, (c) untreated luffa fibre, and (d) treated luffa fibre
As the sound wave projected incident on the surface of the porous luffa epoxy
composite structure, the air compression and motion tend to cause vibration and
friction between microspores walls and restrict the movement of the air. Because of
the friction and viscous forces, some parts of the sound energy are converted to heat,
which could subsequently cause sound energy attenuation. Likewise, the heat loss
caused by the heat exchange between the air, microspores and the microspore wall
will also cause sound energy attenuation. Due to the bonding between luffa and
matrix, the sound tends to reflect as it hit the composite wall because of its heavy
bonding and tighter tangle arranged structure. The higher reflection and lower sound
absorption is due to the dense layer created by the pectin, hemicellulose, and other
low-molecular substance on the surface and internal structure of the fibre in the
untreated version of the composites. According to Yilmaz et al. (2012), the treatment
may also cause the fibre flow resistivity to be reduced (as this created a better bond
between matrix and fibre), which makes it one of the primary factors influencing the
reduction in the sound absorption coefficients.
Tangled
fibre
Hollow lumen
structure of fibre
Smooth
surface
Rough
surface
(a) (b)
(c) (d)
94
Figure 40: Light/optical micrographs of luffa fibre: (a) untreated luffa fibre (b)
treated luffa fibre
4.2.3.2 ACOUSTICAL PROPERTIES OF LUFFA FIBRE EPOXY
COMPOSITES (SOUND ABSORPTION ANALYSIS)
Figure 41 (a) and (b) shows the sound absorption coefficient of untreated and treated
luffa epoxy composites. Comparing the acoustics property of the raw luffa fibre and
the composites, it showed that blending epoxy resin with the raw luffa fibre reduced
the sound absorption coefficient (i.e. from 0.16 to 0.08 at the frequency of 4000 Hz
to 6000 Hz). The reason that caused the low sound absorption coefficient may be due
to the reduction in the quantity of fibre used. Lower sound absorption coefficient
were obtained because the fibre involved are less than 20 wt% than the raw fibre is
which is 100 wt%. Other than that, the reason for the low sound absorption
coefficient can be related to the process/method used to fabricate the samples. For
the range of fibre loadings concerned, it can be observed that increasing fibre content
resulted in higher sound absorption coefficients at increasing frequency. Figure 41 (a)
and (b) also show that the sound absorption of 20 wt% untreated and treated luffa
epoxy composite have a higher sound absorption coefficient compared to other fibre
loading. At high fibre loading, it is expected that the composite is compacted with
fibres, and this structure should have reduced air voids and makes the air passages
much narrower and more tortuous. Thus, this caused the sound wave to travel at a
longer distance. This resulted in the reduction of sound energy (Huang et al., 2013).
This was also supported by Jiang et al. (2012), whereas there is an increase in the
sound absorption coefficient as the seven-hole hollow polyester fibre content
increased.
(a) (b)
20 µm 20 µm
95
Figure 41: The effects of fibre loading on sound absorption for luffa epoxy
composites: (a) composites of untreated fibre (5 mm thickness), and (b)
composite of treated fibre (5 mm thickness)
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96
Figure 42 shows the comparison between untreated and treated luffa epoxy
composites. Chemical treatment was used to remove the moisture and impurity of the
fibre, and promotes better adhesion of the epoxy. This alkalisation or chemical
treatment indirectly gave a big impact on the sound absorption properties of the
material. Specifically, the treated luffa epoxy composite an exhibits higher sound
absorption if compared to the untreated luffa fibre reinforced epoxy composite. The
soaking of luffa fibre in alkaline during the chemical treatment and the drying
process using an oven, caused swelling of the fibre and hence increase sound
absorption property.
Figure 42: The effects of chemical treatment on luffa fibre for the luffa fibre
epoxy composites at 20 wt% (5 mm thickness)
4.2.3.3 MECHANICAL PROPERTIES OF LUFFA FIBRE EPOXY
COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS)
From Figure 43 (a) and (b), it is clear that there was an increase in the tensile and
yield strength of both the untreated and treated luffa epoxy composites. The
maximum tensile and yield strength were achieved at the fibre loading of 15 wt% for
the untreated luffa epoxy composites. For the treated luffa epoxy composite, the
optimum fibre content occurred at 10 wt% fibre. The increase in tensile and yield
strength of the composites was due to the increase in the fibre loading (for the both
types of fibre) in the composites. The increase in the fibre content caused the load to
0
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Untreated 20wt % Luffa Epoxy Treated 20wt % Luffa Epoxy
97
be distributed uniformly within the composites. After reaching the optimum fibre
loading, the tensile and yield strength decreased at increasing fibre loading. This
reduction after the optimum point is because of the decrease in the binder content.
The decrease in the epoxy content was due to excessive luffa fibre in the composites
at high fibre loading. According to Liu et al. (2009), the increase in the tensile
strength was due to the presence of fibres dispersed within the matrix that allowed
uniform distribution of stress and load on the materials. Also, a decrease in the
tensile and yield strength after the optimum level was due to the high excessive of
fibre in the materials, which contributes to the initiation of cracks. Cracks have been
reported to cause non-uniform stress transfer due to the fibre agglomeration within
the fibre matrix (Liu et al. 2009).
Figure 43: The effects of fibre loading on: (a) tensile strength, and (b) yield
strength
0
5
10
15
20
25
5wt % 10wt % 15wt % 20wt %
Ave
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Te
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a)
Weight Percentage (wt%)
Untreated Luffa Epoxy Treated Luffa Epoxy
(a)
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5wt % 10wt % 15wt % 20wt %
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a)
Weight Percentage (wt%)
Untreated Luffa Epoxy Treated Luffa Epoxy
(b)
98
On the other hand, the highest strength is obtained at the treated fibre loading
of 10 wt%. The lignocellulosic characteristic (lignin, hemicellulose, and cellulose
removal) of was affected by alkaline treatment. This is supported by the other work
from Balakrishna et al. (2013), specifically that the chemical treatment removed the
moisture and impurity substance in the fibre and hence increased the strength.
Besides, the decrease in the tensile and yield strength is more drastic for composites
with treated luffa fibre. The reason for the decrease in the tensile and yield strength is
the low matrix-filler interface. This happens as the matrix binder started to break
before the fibre failed, as can be seen in Figure 44. According to Boynard and
D’Almeida (2000), this type of failure or fracture is considered as controlled fracture.
Specifically, the failure will not occur suddenly.
Figure 44: Failure of the luffa fibre epoxy composite. Failure on the epoxy has
initiated whereas a significant portion of the fibre is still intact
4.2.3.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS OF
LUFFA FIBRE EPOXY COMPOSITES
Figure 45 shows the FTIR spectral of the untreated luffa fibres and the treated luffa
fibres. In Figure 45 the region of the broad absorption band at 3500 cm-1
to 3000 cm-1
for the both treated and untreated luffa fibres are characterized with O-H stretching
and H- bonding that contains phenols, alcohols and waters functional group (Gañán
et al. 2008). Based on Ramadevi et al. (2012) and Saw et al. (2013), the O-H
stretching and H- bonding was ascribed to the presence of carbohydrates (cellulose
99
and hemicellulose). The O-H stretching, and H- bonding broad absorption band in
the regions is decreased after alkaline treatment. The decreased of the broad
absorption band promote addition of two small peaks for the treated luffa fibres at
3861.49 cm-1
and 3597.24 cm-1
. The decrease in functional group, phenolic or
aliphatic hydroxyl in the fibre was due to the reaction with sodium hydroxide which
promotes almost hydroxyl free bond structure. A small peak at 2918.30 cm-1
for the
untreated luffa fibre was attributed to the C-H stretching and O-H stretching of
alkanes (cellulose and lignin) and carboxylic acids. According to Saw et al. (2013)
small peak in the region of the C-H stretching includes the functional group of
methyl and methylene (cellulose and lignin). These peaks were reduced due to
alkaline treatment.
Figure 45: Fourier transforms infrared spectroscopy analysis of: (a) untreated
luffa fibre; and (b) treated luffa fibre
(a)
(b)
100
Table 17: The characterisation of luffa fibre using infrared spectral analysis
Bond - Functional Group Untreated luffa fibre
(Wavenumber, cm-1
)
Treated luffa fibre
(Wavenumber, cm-1
)
O-H stretching , Free hydroxyl -
Alcohol, Water, Phenols 3782.41
3861.49, 3732.26,
3597.24
O-H stretching , H- bonded, -
Alcohol, Water, Phenols 3305.99 3321.42
C-H stretching, O-H stretching -
Alkanes (CH; CH2; CH3),
carboxylic acids
2918.30 2906.73
C≡ C stretching, C≡ N stretching –
Nitriles, Alkynes 2146.77 2276.00
C=C stretching - Alkenes (lignin) 1734.01, 1612.49 1658.78
C-H bending - Alkanes (cellulose;
hemi-cellulose; lignin)
1423.47, 1323.17,
1244.09 1506.41, 1431.18
C-O stretching - Alcohol
(cellulose; hemi-cellulose; lignin),
Carboxylic acids, Esters, Ethers.
1029.99 1024.20
C-H bending, C-H rock, N-H wag -
Aromatic rings (out-of-plane ),
Alkanes, 1°, 2° Amines (lignin)
898.83 -
Table 17 shows the characterisation of the untreated luffa fibres and the
treated luffa fibres as extracted from Figure 45. For the untreated luffa fibre, it shows
that the small peak band at 2146.77 cm-1
is characterized as the C≡ C stretching and
C≡ N stretching of the functional group of nitriles and alkynes. The peaks at 1734.01
cm-1
and 1612.49 cm-1
are characterized as the C=C stretching of the functional
group of alkenes (lignin). The small peaks at 1423.47 cm-1
, 1323.17 cm-1
and
1244.09 cm-1
are characterized as the C-H bending of the functional group of alkanes
(cellulose, hemicellulose and lignin). The peak at 1029.99 cm-1
is characterized as
the C-O stretching of the functional group of alcohols (cellulose, hemi-cellulose and
lignin) and the small peak at 898.83 cm-1
is characterized as C-H bending, C-H rock,
N-H wags of the functional group of aromatic rings (out-of-plane), alkanes, 1°, 2°
amines (lignin). Compared with the treated fibres in Figure 45 (b), there were
reduction in the intensity of the absorption bands at 1734.01 cm-1
and 1612.49 cm-1
and shifted of the peak 1658.78 cm-1
that are characterized as C=C stretching from
functional group alkenes and another reduction in the intensity of the absorption
101
bands and shifting from 1423.47 cm-1
, 1323.17 cm-1
, and 1244.09cm-1
to 1506.41 cm-
1 and 1431.18cm
-1 are characterized as C-H bending from the functional group,
alkanes, due to the reaction of sodium hydroxide. Furthermore, there was a
reduction in the intensity of the bands at 898.83 cm-1
characterized as C-H bending,
C-H rock, N-H wag of the functional groups of aromatic rings (out-of-plane), alkanes,
1°, 2° amines (lignin). This caused disappearing of smell from the luffa fibre.
The alkaline reaction caused the removal of lignin, hemicellulose and
cellulose structure. Thus, this hollow lumen structure and rough surface of luffa fibre
are believed to give better absorption of epoxy resin (i.e. on the surface and through
the pores). Although, better absorption of epoxy caused reduction in the sound
absorption coefficient, the multi-layered cells of the luffa fibre prevented the epoxy
to be absorbed due to its high resistivity and high tendency toward alkaline treatment
(i.e. whereas luffa fibre contained sub-layered structure within the fibre structure).
Thus, this caused increased in the sound absorption inside the treated luffa fibre
epoxy composites as compared with untreated luffa fibre epoxy composites. The
process also caused a swelled structure due to the changes in the structure,
morphology, mechanical and dimension properties; whereas some part of the fibre
tend to absorbed more epoxy than the other part of it (Bledzki and Gassan. 1999).
Swelled increased the banana fibre diameter size and porosity that increased the
sound absorption coefficient.
4.2.4 OIL PALM FIBRE EPOXY COMPOSITES
Composites of epoxy and oil palm fibre were fabricated at 5 wt%, 10 wt%, 15 wt%,
and 20 wt% fibre loading. The properties such as acoustical, mechanical, and the
infrared spectra absorption as well as the morphology were investigated and analysed.
Besides, the comparisons between the untreated and the treated oil palm fibre were
conducted.
4.2.4.1 MORPHOLOGY OF OIL PALM FIBRE EPOXY COMPOSITES
(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL
MICROSCOPIC ANALYSIS)
Figure 46 (a), (b), (c) and (d) shows the scanning electron microscope (SEM)
micrographs of the composites. As shown in Figure 46 (a), the fibre has a smooth
102
outer surface layer. From the perspective of the interface strength, this may cause
poor adhesion between the epoxy matrix and the fibres. Meanwhile, Figure 46 (c)
shows the hollow lumen structure of oil palm fibre after treatment. Figure 46 (b)
shows the treated oil palm fibre epoxy composites with the interface of fibre and
matrix.
Figure 46: SEM micrographs of oil palm fibres: (a) untreated oil palm fibres, (b)
treated oil palm fibres, (c) hollow lumens structure of oil palm fibre, and (d)
agglomeration of oil palm fibre
The alkaline treatment caused the smooth outer surface layer to be removed
together along with the oil and impurities. In addition, this treatment caused the fibre
surface to be rough, cripple and tear out. The structure on the outer surfaces
promotes better adhesion of fibre and epoxy. The cripple and tear out fibre caused
interlocking within the fibre and epoxy. Meanwhile, the rough surface created
friction that decreases the movements of fibre agglomeration. Thus, this resulted in
the high tensile and yield strength properties of the composites. Also, there are
potholes and small bubbles observed. These defects are also reported in the published
Smooth
surface
Better adhesion
between fibre and
matrix
Rough
surface
Void/bubbles
Hollow
lumen
structure
Fibre
(a) (b)
(c) (d)
103
works of Matta et al. (2014) and Nayak (2014). Nayak et al. (2014) stated that the
traces of the potholes or the small bubbles were resulted from to the hand lay-up
fabrication technique. The mixing technique of the pre-mixed epoxy with fibre
contributed in likely to produce the potholes or small bubbles. This happened when
the vigorously stirred epoxy caused the air being trapped. This problem can be
alleviated by letting the mixture settle for a couple of minutes before being
compression moulded with pressure. As seen in the SEM images in Figure 46 (d), the
alkaline treatment which caused the fibre cripple and tear out also contributed to
potholes or small bubbles. The cripple and tear out fibre may trap the air indirectly.
Figure 47: Light/optical micrographs of oil palm fibre: (a) untreated oil palm
fibre (b) treated oil palm fibre, and (c) fracture part of oil palm fibre epoxy
Figure 47 (a), (b) and (c) show light/optical microscope images of oil palm
epoxy composites. In Figure 47 (a), it shows that the untreated oil palm fibre had less
adhesion to the epoxy matrix if compared to the treated fibre in Figure 47 (b). The
treated oil palm epoxy composite showed better adhesion between the epoxy matrix
and the fibres because of alkaline treatment. The removal of impurities and oil on the
fibres caused the fibres to become highly adhesive to the matrix because of the
hollow structures of the fibre. Furthermore, the drying process of the fibres caused
the moisture into the fibres to be removed and increased the span of absorption of the
fibres. Although the untreated fibres were dried too, the impurities and the oil
(a) (b)
(c)
20 µm 20 µm
20 µm
104
prevented and reduced the adhesion. Figure 47 (c) shows the fracture surface of oil
palm fibre epoxy. These types of fractures showed that the epoxy was semi-
amorphous (Odegard and Bandyopadhyay, 2011). The traces of epoxy at the fibre
showed that fibre absorbed a certain amount of epoxy and resulted in a more brittle
behaviour.
4.2.4.2 ACOUSTICAL PROPERTIES OF OIL PALM FIBRE EPOXY
COMPOSITES (SOUND ABSORPTION ANALYSIS)
Based on the acoustics behaviour of the raw oil palm and the composites in Figure 48
(a) and (b), it showed that blending epoxy resin with the raw fibre caused a reduction
to the sound absorption coefficient from 0.0.12 to 0.09 at frequency range of 4000
Hz to 6000 Hz. The reason that caused the low sound absorption coefficient may be
due to the reduction in the quantity of fibre used. Lower sound absorption
coefficients were obtained because the fibre quantity involved is less than 20 wt%.
These fibre loadings were low if compared to the sample fabricated at 100 wt% raw
fibre. The reason for this reduction is the same to the case of banana fibre and luffa
fibre reported in Section 4.2.2.2 and Section 4.2.3.2. The compression moulding
process is believed to force the epoxy resin into the hollow lumen structure of the oil
palm fibre.
Considering the effect of fibre it can be observed that higher fibre content
exhibits a higher sound absorption coefficient at increasing frequency. It is also
shown that the sound absorption of the composite at 15 wt% fibre loading has a
higher sound absorption coefficient than the others. The increase in the sound
absorption coefficients is believed to relate to the microstructure as shown earlier in
Figure 46 (b) in Section 4.2.4.1. At increasing fibre loading, the structure of the
composites becomes more compact. In addition, the amount of fibres that brought
nearer to each other caused a decreased in the size and volume of air voids. Thus,
these indirectly created narrower air passages and increased the tortuosity. However,
at 20 wt% of fibre loadings for both untreated and treated composites, the sound
absorption coefficients were found to decrease. This is because of the shrinkage of
fibres as a consequent of being compressed nearer to each other.
105
Figure 48: The effects of fibre loading on sound absorption for oil palm epoxy
composites: (a) composites with untreated fibre (5 mm thickness), and (b)
composites with treated fibre (5 mm thickness)
Besides that, the shrinkage caused the hollow fibre structure to collapse. The
hollow structure of the fibre is expected to trap and absorb sound that entered the
composites. Thus, in other words, the sound waves created tends to bounce back if
the hollow lumen structure is collapsing. Apart from that, the increasing sound
absorption with increasing fibre loading can be supported by the research done by
Jiang et al. (2012) on seven-hole hollow polyester fibre (SHPF) composites. The
0
0.02
0.04
0.06
0.08
0.1
0 1000 2000 3000 4000 5000 6000
Sou
nd
Ab
sop
rtio
n C
oe
ffic
ien
t (α
)
Frequency (Hz)
5wt% 10wt% 15wt% 20wt%
0
0.02
0.04
0.06
0.08
0.1
0 1000 2000 3000 4000 5000 6000
Sou
nd
Ab
sorp
tio
n C
oe
ffic
ien
t (α
)
Frequency (Hz)
5wt% 10wt% 15wt% 20wt%
(a)
(b)
106
SHPF composites showed an increased in sound absorption coefficients as the SPHF
content increased. Another factor that contributed in the sound absorption
coefficients was the random orientation distribution of oil palm fibres within the
composites. Randomized in size and length was also another reason that contributes
in the sound absorption coefficients. Indirectly, this created a different angle of fibre
orientation in the composites. Based on Chen et al. (2010) research, the sound
absorption of ksFIB/PLLA was comparably higher to PLLA and ksFAB/PLLA
because of no specific orientation and uniform distributed of ksFIB in the composites.
When the sound waves hit the surface of the composites, it dispersed in the material
at a random direction and thus being absorbed after being reflected at different angles
by the fibres in the composites.
Figure 49: The effects of chemical treatment on oil palm fibre for oil palm fibre
epoxy composites at 15 wt% (5 mm thickness)
Based on Figure 49, in terms of the effects of fibre treatment, it is observed
that treated oil palm epoxy composites absorbed more sound compared to untreated
oil palm epoxy composites. Sodium hydroxide causes the oil, impurities and fatty
acids being removed completely. According to Li et al. (2007), the hydrogen bonds
in the network structure of the fibre were disrupted when reacted with sodium
hydroxide. The alkaline treatment increased the fibre surface roughness. Furthermore,
according to Prasad et al. (2014), sodium hydroxide exaggerated the de-
0
0.02
0.04
0.06
0.08
0.1
0 1000 2000 3000 4000 5000 6000
Sou
nd
Ab
sorp
tio
n C
oe
ffic
ien
t (α
)
Frequency (Hz)
15wt% untreated oil palm-epoxy 15wt% treated oil palm-epoxy
107
polymerization of cellulose. The rough surfaces caused friction which promotes
sound absorption. The treatment also initiated better adhesion of oil palm fibre and
epoxy. Besides that, the rough surface of the oil palm fibres indirectly allowed the air
to be trapped and created small bubbles that cause disruption of sound propagation in
the composites by creating a medium that reflected sound within the composites
itself.
4.2.4.3 MECHANICAL PROPERTIES OF OIL PALM FIBRE EPOXY
COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS)
Figure 50: The effects of oil palm fibre loading: (a) tensile strength, and (b) yield
strength
0
2
4
6
8
10
12
14
16
18
20
5wt% 10wt% 15wt% 20wt%
Ave
rage
Te
nsi
le S
tre
ngt
h (
MP
a)
Weight Composition (wt%)
Untreated Oil Palm Epoxy Treated Oil Palm Epoxy
(a)
0
2
4
6
8
10
12
14
16
18
20
5wt% 10wt% 15wt% 20wt%
Ave
rage
Yie
ld S
tre
ngt
h (
MP
a)
Weight Composition (wt%)
Untreated Oil Palm Epoxy Treated Oil Palm Epoxy
(b)
108
Figure 50 (a) and (b) shows the results of average tensile and yield strength for
untreated and treated oil palm epoxy composites. It can be observed that the tensile
and yield strength increased with increasing fibre loading. The highest strength is
observed at fibre loading 10 wt%. The increased in the tensile and yield strength was
because of the reinforcing effects of fibres (Liu et al. 2009).
According to Liu et al. (2009), the dispersed fibre allowed uniform
distribution of load and stress within the materials. However, the tensile and yield
strength started to show a decrease trend at fibre loading beyond 10 wt% because of
the excessive amounts of fibre contents. According to Prasad et al. (2014), excessive
fibre loading could reduce the uniformity in fibre dispersion. Furthermore, the
excessive fibre loading also promoted initiation the cracks. The cracks caused non-
uniform stress transfer. The similar explanation is also proposed by the published
work by Liu et al. (2009). Another reason for the decreased in the tensile and yield
strength was due to premature fracture of epoxy matrix components of the
composites. Specifically, the epoxy started to fracture while the fibre is still intact.
According to Boynard and D’Almeida (2000), this fracture mode was known as
controlled fracture. As a contrary, a sudden and complete fracture will pose a safety
issue. Besides that, the treated oil palm epoxy showed a higher tensile and yield
strength than the untreated counterpart. This was because of the removal of moisture
and impurity of fibre that increased the fibre strength (Balakrishna et al. 2013).
4.2.4.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS OF OIL
PALM FIBRE EPOXY COMPOSITES
Figure 51 (a) and (b) shows the FTIR spectral of the untreated oil palm fibres and the
treated oil palm fibres. According to Khan et al. (2005), the peak locations shown in
the absorption spectra of the oil palm fibre are characteristics of its constituents. In
Figure 51 (a) and (b), the region of the broad absorption band from 3500 cm-1
to
3000 cm-1
for the untreated oil palm fibres and the treated oil palm fibres are
characterized with O-H stretching and H- bonding that contains functional groups of
phenols, alcohols and waters (Gañán et al. 2008). Based on the published works of
Ramadevi et al. (2012) and Saw et al. (2013), the O-H stretching and H- bonding was
due to the presence of carbohydrates (cellulose and hemicellulose). The intensity of
the O-H stretching, and H- bonding broad absorption band is decreased after alkaline
109
treatment. Two smaller peaks at 2916.37 cm-1
and 2848.86 cm-1
for the untreated
luffa fibre were attributed to the C-H stretching and O-H stretching of the functional
group of alkanes (cellulose and lignin) and carboxylic acids. According to Saw et al.
(2013), the smaller peak in the region of the C-H stretching represents the functional
group of methyl and methylene (cellulose and lignin). This peak is reduced after
alkaline treatment.
Figure 51: Fourier transforms infrared spectroscopy analysis of: (a) untreated
oil palm fibre, and (b) treated oil palm fibre
Table 18 shows the characterisation of the both untreated and treated oil palm
fibres and the treated oil palm fibres as extracted from Figure 51. For the untreated
oil palm fibre, it shows that the peak at 1708.93 cm-1
is characterized as the C=C
stretching of the functional group of alkenes (lignin). The small peaks at 1369.46 cm-
1, 1313.52 cm
-1, and 1240.23 cm
-1 are characterized as the C-H bending of the
functional group of alkanes (cellulose, hemicellulose and lignin). The peaks at
(a)
(b)
110
1159.22 cm-1
and 1031.92 cm-1
are characterized as the C-O stretching of the
functional group of alcohols (cellulose, hemi-cellulose and lignin) and the small peak
at 896.90 cm-1
is characterized as C-H bending, C-H rock, N-H wags bond structure
of the functional group of aromatic rings (out-of-plane), alkanes, 1°, 2° amines
(lignin). Compared with the treated fibre in Figure 51 (b), there were reduction in the
intensity of the absorption bands at 1159.22 cm-1
and 1031.92 cm-1
and shifting of
the absorption peak to 1033.85 cm-1
that are characterized as C-O stretching of the
functional group alcohols (cellulose; hemi-cellulose; lignin), carboxylic acids, and
esters, as a result of reaction with sodium hydroxide. Furthermore, there was a
reduction in the intensity of the bands at 896.90 cm-1
characterized as C-H bending,
C-H rock, N-H wag of the functional groups of aromatic rings (out-of-plane), alkanes,
1°, 2° amines (lignin) which caused disappearing of smell from the oil palm fibre.
Table 18: The characterisation of oil palm fibre using infrared spectral analysis
Bond - Functional Group
Untreated Oil Palm
Fibre (Wavenumber,
cm-1
)
Treated Oil Palm
Fibre
(Wavenumber, cm-1
)
O-H stretching , H- bonded, -
Alcohol, Water, Phenols 3340.71 3344.57
C-H stretching, O-H stretching -
Alkanes (CH; CH2; CH3),
carboxylic acids
2916.37, 2848.86 2922.16
C=C stretching - Alkenes (lignin) 1708.93 1639.49, 1598.99,
1512.19
C-H bending - Alkanes (cellulose;
hemi-cellulose; lignin)
1369.46, 1313.52,
1240.23
1460.11, 1419.61,
1369.46, 1267.23
C-O stretching - Alcohol
(cellulose; hemi-cellulose; lignin),
Carboxylic acids, Esters, Ethers.
1159.22, 1031.92 1033.85
C-H bending, C-H rock, N-H wag -
Aromatic rings (out-of-plane ),
Alkanes, 1°, 2° Amines (lignin)
883.40 -
The alkaline reaction caused the removal of lignin, hemicellulose and
cellulose from the surface and some in the internal structure. Thus, this created
hollow lumen structure and rough surface of oil palm fibre that tends to give better
absorption of epoxy resin in the fibre and on the surface of the fibre. Although, better
absorption of epoxy caused reduction in the sound absorption coefficient, the multi-
111
layered cells of the oil palm fibre prevented the epoxy to be absorbed due to its high
resistivity and high tendency toward alkaline treatment. Thus, this caused reduction
in the sound absorption for the treated composites as compared to the raw oil palm
fibre and untreated oil palm fibre epoxy composites. The process also caused a
swelled structure due to the changes in the structure, morphology, mechanical and
dimension properties; whereas some part of the fibre tend to absorbed more entities
(i.e. epoxy) than the other part of it (Bledzki and Gassan. 1999). Swelled increased
the banana fibre diameter size and porosity that increased the sound absorption
coefficient. These factors are believed to influence the sound absorption of the
composites.
4.3 COMPARISONS BETWEEN BANANA, LUFFA AND OIL PALM FIBRE
AND ITS COMPOSITES
The purpose of the empirical modelling is to identify the potential effect of the
various types of raw natural fibre on sound absorption. As there is no suitable
empirical model for natural fibre composites, parametric study was done on those
empirical models for raw fibre in order to identify the significant parameters
affecting sound absorption. It is acknowledged that the raw fibre and natural fibre
composites exhibit different structure and formulation. Thus, the sound absorption
behaviour of the materials is expected to be different. However, it would be
fascinating if the composites showed consistent trend to the case of raw fibre. In this
section, the findings from empirical modelling will be compared and discussed with
experimental results.
4.3.1 COMPARISONS BASED ON EMPIRICAL MODEL
For effect of fibre size, it is found that luffa fibre has a higher sound absorption
coefficient (i.e. 0.74 at 6000 Hz for fibre size of 10 µm) compared to the banana
fibre (i.e. 0.64 at 6000 Hz for fibre size of 30 µm) and oil palm fibre (i.e. 0.64 at
6000 Hz for fibre size of 20 µm). Luffa fibre has a smaller diameter compared to
banana and oil palm fibre. The smaller diameter caused higher flow resistivity. It also
allowed more fibre to be filled in specific specimen mould. The porosity is
influenced by the number and size of pores inside the fibre. Larger pores in the fibre
will resulted in a more porous fibre. By comparing banana fibre and oil palm fibre,
both of them were having the similar result of sound absorption coefficient (i.e. 0.64
112
at 6000 Hz). The oil palm fibre has a smaller diameter than the banana fibre. Smaller
diameter caused more fibre can be filled in the specimen even though the oil palm
fibre is less porous than banana fibre. Thus, it is speculated the abovementioned
reasons caused the increased in the sound absorption of oil palm fibre to be similar to
the banana fibre.
For effect of thickness, it is found that banana and luffa had a higher sound
absorption coefficient (i.e. 0.99 at 6000 Hz for thickness of 0.015m) compared with
oil palm fibre (i.e. 0.44 at 6000 Hz for thickness of 0.015m). The reason behind this
low sound absorption for oil palm fibre was due to the lower porosity value of oil
palm fibre, which causes the fibre to be less sound absorbing. Comparing the sound
absorption trend for banana and luffa fibre, luffa fibre tends to absorb more sound
than the banana fibre. It can be seen that luffa fibre achieved higher sound absorption
at the frequency range from 500 Hz to 3000 Hz if compared to the banana fibre. The
reason behind this was due to the small fibre diameter of luffa fibre (i.e.19 µm)
which is smaller than banana fibre (i.e. 32 µm). Smaller fibre diameter tends to have
higher flow resistivity. Even though the luffa fibres have a lower porosity, it is
believed that the smaller fibre diameter of luffa is the main reason for its better sound
absorption compared to banana fibre and oil palm fibre.
4.3.2 COMPARISONS BASED ON EXPERIMENTATION
For acoustical properties (i.e. sound absorption analysis), it is found that the raw
luffa fibre has a higher sound absorption coefficient than raw banana and oil palm
fibre at the frequency range from 500 Hz to 5000 Hz. As it reached higher frequency,
banana is observed to have a higher sound absorption coefficient. The reason behind
this may be due to natural swelling on the fibre. However, as it mixed with epoxy
resin and fabricated into composites, all of the raw fibres sound absorption
coefficients were reduced by 20-40%. It is found that the highest reduction happened
to the raw luffa fibre. The reason behind this may be due to the nature of luffa fibre
that tends to absorb entities (i.e. water, nutrients and etc.). The nature of luffa fibre is
known to be highly porous which resulted in increase of absorption properties, which
allowed it to become denser after absorption. Furthermore, the fibre has a 3-D
interconnected structure which is believed to be related to the increase in sound
113
absorption. This allowed the sound to be redirected in any direction inside the
composites.
As for natural fibre composites, it is found that banana fibre epoxy
composites has a higher sound absorption coefficient (i.e. 0.12 at 5500 Hz for 15 wt%
untreated banana fibre loading) than luffa fibre epoxy composites (i.e. 0.08 at 6000
Hz for 15 wt% untreated luffa fibre loading) and oil palm fibre epoxy composites (i.e.
0.08 for 15 wt% untreated oil palm fibre loading). The reason behind the higher
sound absorption observed in Section 4.2 may be due to the banana fibre used in the
fabrication of composites has higher density than luffa and oil palm fibre. For the
treated natural fibre epoxy composites, the sound absorption was found to increase
for luffa and oil palm fibre. However, for banana fibre, the alkaline treatment caused
the sound absorption to decrease. The chemical treatment caused structural change
that increased the size and number of pores in the fibre especially to luffa and oil
palm fibre. Furthermore, the surfaces of the fibre were found to be rough as can be
seen in the micrographs in Section 4.2 after the removal of oil, wax and impurities as
verified by FTIR. Surface roughness caused better adhesion that indirectly increased
the tensile and yield strength of the natural fibre composites. It is also known that
chemical treatment caused changes in the H-, C-H, and O-H bonding and related
functional groups at the fibre surface. Alkaline treatment caused stretch, break and
re-bond to the molecular structures. Taking luffa fibre as an example, the reduction
of intensity of O-H bond at 3500 cm-1
to 3000 cm-1
cause breaking of O-H bond
which cause an addition of an extra peak for treated fibre was due to the removal of
waterish (hydroxide) structure. This caused the fibre to be more absorbent which
may cause better absorption. Thus, this shows that the chemical treatment improved
the strength and sound absorption of the composites.
4.3.3 COMPARISONS BASED ON EMPRICAL MODEL AND
EXPERIMENTATION
Based on the empirical and experimental results, it showed that the sound absorption
coefficient of banana, luffa and oil palm fibre for were varied due to the change in
porosity and etc. It is found that the sound absorption predicted for empirical model
is higher than the experimentation by 20 % – 30 %. Meanwhile by comparing the
raw fibre with natural fibre composites, the results were dropped by 20-30%. There
114
are a few reasons behind this difference. It is well known that the natural fibre used
in the experiment or in real-world applications does not have a wide distribution of
fibre diameter. It is expected that the fibre diameter will vary depends on the part of
the crop it is derived. The natural fibre size depends on the nutrient the plant gained
from the soil. On the other hand, the empirical model considers the fibre to have a
uniform diameter which may be obtained from literature or measurement, which is
based on fibres from different sources (Oldham et al. 2011). Moreover, the variation
of fibre diameter will create random flow resistivity due to wide and narrower path
for sound to travel which resulted in the fluctuating pattern in sound absorption as
shown in Section 4.2.2, Section 4.2.3 and Section 4.2.4. Another reason for the
difference is that the porosity of an individual fibre is different to another fibre in the
same batch based on results obtained in literature. Furthermore, some empirical
model used different ways to predict and obtained the porosity values, whereas the
empirical model porosity valued was measured by the total change in volume
(Oldham et al. 2011, Mechel 2008). Although there are a few differences between
the results of the empirical modelling and experimentation, it still identifies the
significant parameters influencing the sound absorption, including fibre size, fibre
types and etc. Other than that, it can be used as early prediction and be used as basic
concept of understanding of relationship of sound absorption between theoretical and
experimental result. It is able to predict the sound absorption of banana, luffa and oil
palm fibre. Since the model does not assume a materials structure and formulation
other than fibrous materials, it cannot be used in prediction of natural fibre composite
sound absorption behaviour.
115
CHAPTER 5
5.0 CONCLUSIONS AND FUTURE WORKS
5.1 CONCLUSIONS
Based on the empirical calculations conducted, it can be concluded that the higher
the flow resistivity, the higher the sound absorption coefficient. It showed that luffa
fibre had a higher sound absorption (i.e. 0.74 at 6000 Hz for fibre size of 10µm) than
banana fibre (i.e. 0.64 at 6000 Hz for fibre size of 30 µm) and oil palm fibre (i.e. 0.64
at 6000 Hz for fibre size of 20 µm). This is due to the higher flow resistivity as fibre
diameter is smaller. The material thickness has a direct effect on the sound
absorption. An increase in the thickness of the materials will result in higher sound
absorption coefficient. It showed that luffa fibre and banana fibre have the higher
sound absorption (both fibre having 0.99 at 6000 Hz for thickness of 0.015m) than
oil palm fibre (i.e. 0.44 at 6000 Hz for thickness of 0.015) at increasing thickness.
Although the modelling gave different values to the experiment results, it showed a
comparable trend sound absorption. Specifically, for the fibre studied, the sound
absorption coefficients increased with increasing frequency.
Meanwhile, for the natural fibre polymer composites, the samples exhibited
almost similar trend at all frequencies, specifically the sound absorption coefficient
for natural fibre polymer composites increased with increasing frequency. Compared
with the results obtained for raw natural fibre, the sound absorption coefficients were
20 - 30 % lower. For the case of the composites, the sound absorption coefficients
increased with increasing fibre loading from 5 wt% to 15 wt%. The fibre loading has
an effect on the structure of the composites. Specifically, at excessive fibre loading,
the lumen structure of the fibre is found to be collapsed. Thus, this structure resulted
in lower sound absorption. It is speculated that, the collapsed of hollow lumen
structure was due to over-packed fibre in the specimens due to compression.
Further experimentation on the composites revealed that the treated fibre with
alkalisation altered the characteristics of the natural fibre. Alkalisation caused the
fibre surface to be rough due to the removal of impurities and wax from the fibre and
subsequently improved the interfacial adhesion between the fibre and matrix.
Alkalisation treatment separated the micro-fibril structure and reduced the fibre
116
diameter because of the removal of cement materials (i.e. cellulose, hemicellulose,
lignin and etc.). Reduction in the diameter increased the flow resistivity, which led to
higher sound absorption. It also increased the effective surface area available to
adhere with the matrix and hence improved the interfacial adhesion. Furthermore,
chemical treatment caused increased porosity within the fibre. It is observed that the
treated natural fibre composites have increased in sound absorption, tensile and yield
strength (depending on the behaviour of the fibre towards alkalisation).
Through the chemical mapping analysis via FTIR, it was observed that
alkaline treatment caused re-construction of fibre structure and removal of certain
impurities that prevented or decreased the tensile and yield strength and sound
absorption. The mapping confirmed that the alkalisation resulted in removal of
lignin, cellulose and hemicellulose. The SEM revealed the micro-phase separation in
the composites, the porous structure of the natural fibre bundle and the distribution of
natural fibres in the polymer matrix composites. These special structures and the
fibre distribution are believed to be the main reasons for better sound absorption.
5.2 APPLICATIONS
The applications of sound absorbing materials are not only limited in the studio or
lab applications. It can also be applied in housing and automotive industries. In
industrial environments, the sound created from noise sources (i.e. engine, water and
drain pipes, generator, etc.) that can cause a noise problem resulting in sound
disruption, or even worse, hearing damage. An effective solution to this problem is to
enclose the noise source in order to reduce noise pollution without hindering its
operations. With proper design, it can meet a variety of processes and applications
from small to large item if structural design. For condominium/apartment and home
theatre design, natural composites can be used to replace asbestos or fibreglass
materials applications (i.e. roof deck, ceiling, woofer box, etc.).
It can also be used as undercoat layer. For marine and petroleum applications,
the uses of natural composites can help reduce the noise created from the engine
room (i.e. exhaust, HVAC system, etc.). Furthermore, it has and advantages of
restraining from corrosion due to salt water (i.e. non-corroding and lower electrical
conductivity). In automotive, road noise and engine noise can be reduced greatly
with the application natural composites as sound absorbing material. This application
117
has the potential to reduce the noise level in the vehicle. It also provides better
performance for car audio system. The applications are covered from the doors,
interior walls, panels and etc. Other than that, it can be used in interior design of
aircraft. The new materials are safer compared with the traditional absorbing
materials produced in the market. Furthermore, it is lighter and more technologically
optimized. In addition, the concept of environmental friendly, recyclability, and
sustainable materials will be an important part of the marketing the sound absorbing
natural fibre polymer composite materials.
5.2 FUTURE WORKS
In this research, a compression moulding technique was used to fabricate the
composites. However, there are other existing manufacturing processes for polymer
composites (i.e. extrusion, reaction injection moulding and etc.). Thus, final
conclusion can be drawn from there after analysed. The results provided in this thesis
can act as a base for the optimization of the composites. From this work, it was found
out that chemical treatment such as alkaline modified the fibre. It improves the
performance of the composites. However, there are other chemical treatments that
can be used such as silane, acetylation, acylation, isocyanates, permanganate and etc.
that can be used. In the current tests for morphology, the current measurement is
limited in micrometre (µm). This work can be further extended to the other smaller
measurement (i.e. nanometre and etc.). Thus, the extended study thereafter will
clarify in extending the behaviour of the fibre polymer composites. Current research
only focused on the materials itself without consideration of design work and
placement of the materials. Thus, in future, by using the existing materials, a
prototype can be fabricated and its design can be custom made so that it may replace
existing materials in the markets.
118
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APPENDICES
GANTT CHART FOR RESEARCH
ID Task Name2014 2015
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Simulations MATLAB
Fabrication of Luffa Epoxy Composites
Sound Absorption Testing for Luffa Epoxy Composites
Tensile and Yield Strength Testing for Luffa Epoxy CompositesScanning Electron Microscope and Light/Optical Microscope Test for Luffa Epoxy Composites
Fourier Transform Infrared Spectroscopy Test for Luffa Epoxy Composites
Fabrication of Banana Epoxy Composites
Sound Absorption Testing for Banana Epoxy Composites
Tensile and Yield Strength Testing for Banana Epoxy Composites
Scanning Electron Microscope and Light/Optical Microscope Test for Banana Epoxy CompositesFourier Transform Infrared Spectroscopy Test for Banana Epoxy Composites
Fabrication of Oil Palm Epoxy Composites
Sound Absorption Testing for Oil Palm Epoxy Composites
Tensile and Yield Strength Testing for Oil Palm Epoxy CompositesScanning Electron Microscope and Light/Optical Microscope Test for Oil Palm Epoxy CompositesFourier Transform Infrared Spectroscopy Test for Oil Palm Epoxy Composites
Journal Publication
Conference ICMMMM2014
Conference ICMPC2015
Conference RAINSAT2015
Thesis Preparation
135
SODIUM HYDROXIDE DATASHEET
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137
138
139
140
141
142
143
144
145
EPOXY RESIN PART A DATASHEET
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147
148
EPOXY RESIN PART B DATASHEET
149
150
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152
153
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