acoustical characteristics of oil palm mesocarp...
TRANSCRIPT
ACOUSTICAL CHARACTERISTICS
OF OIL PALM MESOCARP FIBRES
HANIF BIN ABDUL LATIF
A thesis submitted in
fulfillment of the requirement for the award of the
Degree of Master of Mechanical Engineering
Faculty of Mechanical and Manufacturing Engineering
Universiti Tun Hussein Onn Malaysia
OCTOBER 2016
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“Special dedicated to my beloved parent and my sibling who always gave me support
and encouragement”
iv
ACKNOWLEDGEMENT
Alhamdulillah, all praise and thanks goes to Allah S.W.T who was gave the strength
and blessings for me to make it possible to complete the thesis title Investigation on
Acoustical Performance of Oil Palm Mesocarp fibres.
First, I am heartily thank to my supervisor, Dr. Musli Nizam Bin Yahya, who
gives encouragement, guidance and support from the initial to the final stage of the
research. Without his leading and knowledge contributed, especially in the field of
noise and vibration, the research will not be finished as fast as two years. A special
thank to him for looking the funds to support this research.
In addition, I would like to thank to Mr. Md Zainorin Bin Kasron who helps
me to set-up the impedance tube and acoustic instrument. Thank to Mr. Fazlannuddin
Hanur Bin Harith and Mr. Shahrul Mahadi Bin Shamsudin who help me during the
sample fabrication stage and also during physical testing.
The most important, thank to my beloved parent and sibling for their concern
and support. A special thank to Nurul Afiqah Bin Ahmad Puad for his
encouragement, understanding, supports and helps when I faced difficult in the
research.
Last but not least, I would like express my regard and blessing to all of those
who support me in any aspect during the completion of the research.
THANK YOU
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ABSTRACT
Natural fibres are availability biomass, plentiful in agricultural waste and eco-
friendly materials. Currently, it is become the major reasons for emerging renewable
materials in sustainable technology. Therefore, this study discusses the acoustical
characteristics of agro-waste biomass fibres from oil palm Mesocarp as fibrous
acoustic material. Incorporated polyurethane (PU) had improved the sound
absorption coefficient (SAC), especially at low frequency range. The oil palm
Mesocarp fibres were mixed with four different percentages of PU, namely 10%,
20%, 30% and 40%. The measurement of SAC was done using an impedance tube
method (ITM). Moreover, there were two analytical models which are Delany-
Bazley model and Johnson-Champoux-Allard model to validate the experimental
outcomes. There were several internal characteristics of the prepared samples
investigated namely flow resistivity, porosity, tortuosity, characteristic lengths, fibre
diameter, air gaps and thickness of sample. The flow resistivity, tortuosity, fibre
diameter, density of samples and characteristic lengths tend to increase with
percentage of PU binder. But, porosity was slightly decrease by adding more binder
into the sample. This study confirms that by increasing the thickness of samples, the
SAC were improved. Air gaps had great influence in adjusting amount of low
frequency range and moved the peaks toward lower frequency. The internal
characteristics have a positive and significant effect on Noise Reduction Coefficient
(NRC). The sample with 10% PU binder showed the greatest sound absorption
performance in most of low to mid frequency range and demonstrated highest value
of NRC of 0.66. Besides, sample with 20% PU binder demonstrated optimum SAC
which is very close to 1 at 1000 Hz. There is still space to improve the performance
of the Delany-Bazley Model, especially for natural fibre. Besides, the Johnson-
Champoux-Allard model gives similar pattern of graph to ITM results and the model
predicts very well with better fit to acoustic behaviour.
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ABSTRAK
Serat semula jadi adalah biomass ketersediaan, banyak di sisa pertanian, dan bahan
mesra alam. Pada masa kini, ia telah menjadi sebab utama penghasilkan bahan-bahan
yang boleh diperbaharui dalam teknologi mampan. Lantaran itu, kajian ini
membincangkan ciri-ciri akustik serat biomass agro-buangan dari Mesokap kelapa
sawit sebagai bahan akustik berserat. Hasil dari gabungan pengikat Polyurethane
(PU) telah meningkatkan pekali penyerapan bunyi (SAC), terumatanya pada julat
frekuensi rendah. Serat Mesokap kelapa sawit diadun dengan empat peratusan
pengikat PU yang beza-beza, iaitu 10%, 20%, 30% dan 40%. Pengiraan SAC telah
dilakukan dengan menggunakan tiub galangan (ITM). Tambahan pula, terdapat dua
model analisis seperti model Delany-Bazley dan Johnson-Champoux-Allard
dijalankan untuk mengesahkan keputusan yang diperolehi dari ITM. Terdapat
beberapa ciri dalaman sampel yang telah dianalisis antaranya kerintangan aliran,
keliangan, kedalaman keliangan, bentuk keliangan, diameter serat, ruang udara dan
ketebalan sampel. Kerintangan aliran, kedalaman keliangan, diameter serat,
ketumpatan dan bentuk keliangan cenderung untuk meningkat dengan peratus
pengikat. Tetapi, keliangan sedikit berkurang dengan penambahan lebih banyak
pengikat ke dalam sampel. Kajian ini mengesahkan bahawa penambahan ketebalan
sampel dapat memperbaiki SAC. Ruang udara dibelakang sampel mempengaruhi
sejumlah julat difrekuensi rendah dan mengalih puncak ke arah frequency lebih
rendah. Sampel dengan 10% pengikat menunjukkan prestasi penyerapan bunyi yang
baik dalam lingkungan frequensi rendah dan memiliki nilai pekali pengurangan
bunyi (NRC) iaitu 0.66. Selain itu, sampel dengan 20% pengikat menunjukkan SAC
optimum yang mana amat hampir dengan 1 di 1000 Hz. Masih terdapat ruang untuk
meningkatkan prestasi model Delany-Bazley terutamanya untuk serat asli. Selain itu,
model Johnson-Champoux-Allard menunjukkan bentuk seakan dengan graf ITM dan
model ini boleh meramal dengan padanan yang lebih sesuai kepada ciri-ciri akustik.
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CONTENTS
TITLE i
DECLARATION ii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF SYMBOLS xv
LIST OF ABBREVIATIONS xvii
LIST OF APPENDICES xviii
CHAPTER 1 INTRODUCTION 1
1.1 Research Background 1
1.2 Problem Statement 3
1.3 Research Questions 4
1.4 Research Objectives 4
1.5 Scope of Research 5
1.6 Organization of the Thesis 5
CHAPTER 2 LITERATURE REVIEW 6
2.1 Introduction to Acoustics 6
2.2 Sound Absorption 8
2.3 Porous Absorbent 10
2.4 Natural Fibres 12
2.4.1 Previous Research on Natural Porous Absorbent 15
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2.4.2 Oil Palm Mesocarp Fibre 18
2.5 Binder or Adhesive 21
2.5.1 Polyurethane 22
2.6 Factor Influencing of Sound Absorption 25
2.6.1 Flow Resistivity 25
2.6.2 Porosity 29
2.6.3 Tortuosity 32
2.6.4 Characteristic Lengths 33
2.6.5 Density 35
2.6.6 Fibre Diameter 36
2.6.7 Air Gap 36
2.6.8 Thickness 38
2.7 Acoustical Characteristics of Porous Absorbents 39
2.7.1 Sound Absorption Coefficient (SAC) 39
2.7.2 Noise Reduction Coefficient (NRC) 40
2.8 Impedance Tube Method (ITM) 42
2.9 Prediction Models of Porous Absorbent 43
2.9.1 Delany-Bazley Model 45
2.9.2 Johnson-Champoux-Allard Model 47
CHAPTER 3 METHODOLOGY 49
3.1 Introduction 49
3.2 Materials Preparation 52
3.2.1 Oil Palm Mesocarp Fibre Preparation 52
3.2.2 Polyurethane (PU) Preparation 52
3.3 Sample Preparation 54
3.3.1 Sample with Binder 56
3.3.2 Sample without Binder 58
3.4 Parameters for the Study 59
3.4.1 Flow Resistivity Measurement 59
3.4.2 Porosity Measurement 59
3.4.3 Tortuosity Measurement 60
3.4.4 Characteristic Lengths Measurement 61
3.4.5 Density Measurement 62
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3.4.6 Fibre Diameter Measurement 63
3.4.7 Air Gap and Without Air Gap 63
3.4.8 Thickness of Sample 63
3.5 Acoustical Characteristics Measurement 64
3.5.1 Sound Absorption Coefficient Measurement 64
3.5.2 Noise Reduction Coefficient Measurement 66
3.6 Prediction Models of Porous Absorbent Measurement 66
CHAPTER 4 RESULTS AND DISCUSSION 69
4.1 Introduction 69
4.2 Samples 69
4.3 Physical Characteristic of Oil Palm Mesocarp Panel 71
4.3.1 Flow Resistivity of Samples 71
4.3.2 Porosity of Samples 72
4.3.3 Tortuosity of Samples 74
4.3.4 Characteristic Lengths of Samples 75
4.3.5 Density of Samples 75
4.3.6 Fibre Diameter of Samples 76
4.3.7 Summary of Physical Characteristic of Samples 77
4.4 Acoustical Characteristic 78
4.4.1 Influence of Percentage Binder 78
4.4.2 Influence of Thickness 81
4.4.3 Influence of Air Gap 82
4.5 Results of Noise Reduction Coefficient 84
4.6 Influence of Physical Properties on NRC 85
4.6.1 Influence of Flow Resistivity on NRC 85
4.6.2 Influence of Porosity on NRC 86
4.6.3 Influence of Tortuosity on NRC 87
4.6.4 Influence of Characteristic Lengths on NRC 88
4.6.5 Influence of Density on NRC 90
4.6.6 Influence of Fibre Diameter on NRC 91
4.7 Acoustical Characteristics Using Analytical Models 92
4.7.1 Results of Delany-Bazley Model 92
4.7.2 Results of Johnson-Champoux-Allard Model 96
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CHAPTER 5 CONCLUSION AND RECOMMENDATIONS 100
5.1 Conclusion 100
5.2 Future Recommendation 102
REFERENCES 103
APPENDICES 113
LIST OF PUBLICATIONS 141
VITA
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LIST OF TABLES
2.1 World availability on non-wood fibres 14
2.2 Mechanical properties of natural fibre 15
2.3 The disadvantages of using synthetic fibre 16
2.4 Total planted area (hectare) of oil palm, 1974 – 2011, Malaysia 20
2.5 Oil palm Mesocarp chemical composition analysis 20
2.6 Various empirical equations to predict flow resistivity 28
2.7 Variation of porosity values for some materials 31
2.8 Various tortuosity values 33
2.9 General rating of NRC 41
2.10 SAC and NRC for common material 41
2.11 Common prediction model for large number of models 44
3.1 Specification of polyol 53
3.2 Specification of isocyanate 54
3.3 Mixing formulations for the 10mm thickness 57
3.4 Mixing formulations for the 20mm thickness 57
3.5 Mixing formulations for the 30mm thickness 57
3.6 Mixing formulations for the 40mm thickness 58
3.7 Mixing formulations for the 50mm thickness 58
4.1 Summary of physical characteristic of samples 77
4.2 Influence of percentage of binder on NRC 84
4.3 Comparison between all samples and from other researchers 85
4.4 Physical characteristics of samples 96
xii
LIST OF FIGURES
2.1 The illustration of wavelength and peak amplitude 8
2.2 Incident sound, reflection, absorption and transmission of sound 8
2.3 Sound hitting a surface 10
2.4 Schematic cross-section of a porous solid material 11
2.5 Three main types of porous absorbing materials 12
2.6 Classification of natural fibres 13
2.7 A close up of oil palm fruit 19
2.8 Set-up direct airflow method for measuring flow resistivity 26
2.9 The apparatus for measuring acoustical porosity 29
2.10 The absorption coefficient for with and without air gap 37
2.11 Effect of varying absorbent thickness on the sound absorption coefficient 38
2.12 Absorption coefficient 40
2.13 Schematic sketch Impedance Tube Method 43
2.14 Common prediction model with different material morphology and
number of parameters 45
3.1 Theoretical framework 50
3.2 Methodology chart of the research 51
3.3 Oil palm Mesocarp fibre received from the Oil Palm mill 52
3.4 Two components used to prepare PU foams; (a) polyol, (b) isocyanate 53
3.5 Mould with two different of diameter 54
3.6 Weighted machine 55
3.7 Mixer machine 55
3.8 Polyurethane; (a) Polyol, (b) Isocyanate 55
3.9 Cleaned oil palm Mesocarp fibre 56
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3.10 Density kit machine (Mettler Toledo) 62
3.11 The separation between the back plate and test sample that creates a
cavity which represents the air gap thickness 63
3.12 Impedance Tube 65
3.13 The algorithm chart of Delany-Bazley model 67
3.14 The algorithm chart of Johnson-Champoux-Allard model 68
4.1 Sample without binder for low frequency 70
4.2 Sample without binder for high frequency 70
4.3 Sample with binder for low and high frequency 71
4.4 Flow resistivity of samples with 50mm thickness samples 72
4.5 Porosity of samples with 50mm thickness samples 73
4.6 Tortuosity of samples with 50mm thickness samples 74
4.7 Characteristic lengths of samples with 50mm thickness samples 75
4.8 Density of samples with 50mm thickness samples 76
4.9 Fibre diameter of the samples 77
4.10 Influence of percentage of PU binder with 50 mm thickness 79
4.11 Comparison between two optimum results of samples and glass fibrous
wool 81
4.12 Influence of thickness; sample without binder 81
4.13 The sound absorption coefficient for three different air gap thickness;
percentage of binder = 20% binder, thickness = 50 mm 82
4.14 Schematic illustration of the quarter-wavelength effect (thickness of
material and air gap) 83
4.15 Influence of flow resistivity on NRC of samples with 50mm thickness
samples 86
4.16 Influence of porosity on NRC of samples with 50mm thickness samples 87
4.17 Influence of tortuosity on NRC with 50mm thickness samples 88
4.18 Influence of viscous characteristic length on NRC with 50mm thickness
samples 89
4.19 Influence of thermal characteristic length on NRC with 50mm thickness
samples 90
4.20 Influence of density on NRC with 50mm thickness samples 91
4.21 Influence of fibre diameter on NRC with 50mm thickness samples 92
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4.22 SAC for sample without binder 93
4.23 SAC for sample with 10 % binder 94
4.24 SAC for sample with 20 % binder 94
4.25 SAC for sample with 30 % binder 95
4.26 SAC for sample with 40 % binder 95
4.27 SAC for sample without binder 97
4.28 SAC for sample with 10 % binder 97
4.29 SAC for sample with 20 % binder 98
4.30 SAC for sample with 30 % binder 98
4.31 SAC for sample with 40 % binder 99
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LIST OF SYMBOLS
HZ - Hertz
dB - Decibels
- Wavelength
- Velocity of sound, air velocity
- Frequency
- Specific flow resistance
- Particle velocity through sample
- Sound pressure differential across the thickness of the sample
measured in direction of particle velocity
- Incremental thickness
- Static pressure differential between both faces of the sample, power
incident, power transmitted
- Thickness of sample, total length of fibre
- Porosity
- Volume of the air in the voids
- Total volume of the sample of the acoustical material being tested
- Tortuosity
- Effective channel length or effective sample thickness
- Thickness of the sample
- Sound absorption coefficient
- Incident sound energy
- Sound absorbed
- Sound transmission coefficient
- Reverberation time
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- Volume of the room
- Total absorption, total area of the object
- Average area density
- Total mass of the object
- Weight wet
- Weight dry
- Weight float
- Weight of sample (saturated)
- Weight of the sample (dry)
- Weight of sample immersed (saturated)
- Characteristic of impedance
- Wave-number, complex wave-number
- Density of air
- Speed in air
- Angular frequency
- Flow resistivity
- Frequency dependent sound speed
- Density of a highly porous material
- Effective density
- Density of saturating liquid
- Effective bulk modulus of the air
- Air as the ratio of specific heat
- Atmosphere pressure
- Viscosity of air
- Prandtl number
- Thickness size of viscous
- Thermal boundary layer
- Specific heat capacity of air at constant pressure
- Pore volume
- Bulk volume
- Viscous characteristic length
- Thermal characteristic length
xvii
LIST OF ABBREVIATIONS
NRC - Noise reduction coefficient
STL - Sound transmission loss
SAC - Sound absorption coefficient
NRI - Noise Reduction Index
ITM - Impedance Tube Method
PU - Polyurethane
ISO - International Organization for Standardization
ASTM - American Society for Testing and Materials
UTHM - Universiti Tun Hussein Onn Malaysia
MPOB - Malaysian Palm Oil Board
FRIM - Forest Research Institute Malaysia
Pa - Pascal
M1 - Microphone 1
M2 - Microphone 2
MATLAB - Matrix Laboratory
Sdn - Sendirian
Bhd - Berhad
xviii
LIST OF APPENDICES
A Flow resistivity 113
B Porosity 121
C Tortuosity 124
D Characteristic lengths 125
E Density of samples 127
F Fibre diameter 128
G Influence percentage binder 133
H Influence of thickness 136
I Influence of air gaps 138
J MATLAB code for Delany-Bazley 139
K MATLAB code for Johnson-Champoux-Allard 140
1
CHAPTER 1
INTRODUCTION
1.1 Research Background
Over the past centuries, great efforts have been made to investigate sound absorption
panel. Over this time, the focus of attention is on the use of material in making sound
absorption panel. Most absorption panels used for sound absorption that available in
the market at present are a variety of traditional synthetic fibres such as glass wool,
rock wool, and asbestos, because of their high performance at mid-high frequency
range. Moreover, these materials offer outstanding thermal and absorption properties,
unique flexibility in design capabilities, and ease of fabrication. Despite its efficacy,
the synthetic fibres produced several major drawbacks. Since 40 years ago, the
European Council Directive has stated that the synthetic fibre has several
disadvantages where the fibre can lay down in the lung alveoli and cause skin itching
(Council Directive, 1967). According to Rapisarda et al., (2015), they found that the
synthetic fibres exposure significantly to the human related to increased oxidative.
Due to the environmental concerns, there are a lot of needs and demands to
find replacement or renewable materials in the making of acoustical panel. There was
an increasing interest in green technology, which is to replace the synthetic fibre with
agrowaste and agroforest materials. Moreover, natural fibres are being used in many
products such as automotive industry, interior lining for apartments, aircrafts, ducts
and biocomposite products (Chandramohan & Marimuthu, 2011).
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The utilization of natural fibres has drawn attentions many researchers due to
the environmental pollution, reduction of waste disposal problems (Nishino,
Matsuda, & Hirao, 2004) and their low density, biodegradable, and low cost (Zhu et
al., 2013).
Normally, porous materials are often used in absorbing sound. A porous
material means a material that contains many voids and not be interconnected. The
material contains a solid matrix that is connected, so that it can resist shear stresses,
and within which are small pores filled with a compressible fluid (Rossing &
Beranek, 2009). The porous absorber will reduce the speed of sound and
consequently a given thickness of absorber will improve the lower frequency. In
order to understand the theoretical model of sound propagation through a porous
material, it is necessary to set down the measurement characterizing which
determining the acoustic behaviour of sound within porous absorbents. There are two
basic material characteristics within porous absorbents, namely flow resistivity and
porosity. The flow resistivity and porosity are usually the most important parameters
for determining the material characteristic (Cox & D’Antonio, 2010). These
parameters can change the absorption behaviour (Seddeq, 2009).
In recent decades, new ways to predict and measure absorptive materials have
been developed. A commonly used model in the prediction of waves in porous media
was developed by Biot (1956a). This model was based on the Hamilton’s principle.
A widely and simple model which involves a single parameters (flow resistivity) for
the acoustical properties was developed by Delany & Bazley (1970). It is also
possible to predict the characteristic impedance by considering the microscopic
propagation within the pores when given the material properties; flow resistivity,
porosity, tortuosity and characteristic lengths. This simple phenomenological model
has been developed by many of people in the development of the models (Allard &
Atalla, 2009; Attenborough, 1971; Champoux & Allard, 1991; Johnson, Koplik, &
Dashen, 1987). In order to gain an accurate prediction of the sound characteristic is
problematic, and consequently it is necessary to have the comparison between
theoretical and experimental results.
3
1.2 Problem Statement
Regarding to environmental and sustainability concerns, a great effort has been made
to produce a product-based on the green materials, especially in the acoustical panel.
This is because, normally a commercial products are made from man-made vitreous
fibres refers to a group of synthetic fibre including glass wool, rock wool, slag wool
and asbestos. Unfortunately, these materials known to be hazardous to our health and
also contribute higher to Global Warming Potential (GWp) kg CO2 (Asdrubali,
2006). Approximately, 27% of mineral wool (glass), 30% of mineral wool (stone)
and 40% of foam plastic has been estimated in European building insulation
(Schmidt et al., 2004). A number of epidemiological studies have found an increased
risk of human respiratory system cancer when man-made fibre exposed to workers
(Berrigan, 2002; Marsh et al., 2001; Stone et al., 2004).
Since in 1987, the definition of sustainability was developed by the World
Commission on Environmental and Development which became widely known as
the Brundtland Report. “Sustainable development meets the needs of the present
without compromising the ability of future generations to meet their own needs”, was
stated in that report (Brundtland, 1987). However, sustainability is still a relatively
new concept in many developing countries that are heading toward industrialization,
including Malaysia. The implementation of the sustainability concept in Malaysia is
still in early stage (Abidin, 2010). Implementing sustainable development becomes
problematic due to lack of sustainable materials, method and technologies (Shafii et
al., 2006). Thus, in order to implement sustainability, a strong financial supports and
aids are needed due to the cost is often higher than anticipated.
Natural fibres can be considered as a good potential to replace the synthetic
fibres and to implement the sustainability materials. However, most of the natural
fibres in the country are left abandoned. Approximately 22% of fibres produced from
palm oil industry have been wasted (Badri et al., 2005). Over the last decade, the
increasing demands on palm oil have made Malaysia as a major player in the world’s
oil palm industry. As of 2011, there are about 57% of total production occurs in West
Malaysia and 99% in Sabah and about 5 million hectares of area palm oil has been
planted (Malaysia Palm Oil Board, 2011). Thus, this has largely contributed to
abundant source of biomass materials. Forest Research Institute of Malaysia (FRIM)
and Malaysia Palm Oil Board (MPOB) have conducted fundamental research for
4
many years to find the potential use of palm oil residues as raw materials for various
products. Raw materials obtained from palm oil trees have been used widely in
biomass media, mattress, fibre board, cushion, rugs, carpets and rope manufacturing.
Unfortunately, there is a lack of scientific information on acoustical characteristic of
oil palm Mesocarp fibres. Hence, the purpose of this research is to investigate the
potential of oil palm Mesocarp fibres on acoustical performance.
1.3 Research Questions
There are some important questions based on the discussion above. These questions
are:
1. Are oil palm Mesocarp panels feasible to be applied for acoustical panels?
2. If so, what are the acoustical characteristics of the oil palm Mesocarp panels?
3. What are the physical characteristics and how it is influencing on acoustical
characteristics?
4. How can analytical models support the results of acoustical properties of oil
palm Mesocarp panels?
1.4 Research Objectives
The objectives of the study are:
1. To determine the acoustical characteristics of oil palm Mesocarp panels.
2. To determine the physical properties of oil palm Mesocarp panels.
3. To investigate the effects of physical characteristics, air gap and thickness on
acoustical characteristics of oil palm Mesocarp panels.
4. To determine the acoustical characteristics of oil palm Mesocarp panels using
analytical models.
5
1.5 Scope of Research
The scope of the study is limited to:
1. The acoustical panels are made from oil palm Mesocarp natural fibre and
mixed with Polyurethane (PU) as a binder.
2. Four percentages of binder which are 10%, 20%, 30% and 40%.
3. The parameters implemented in this study such as flow resistivity, porosity,
tortuosity, characteristic lengths, density, fibre diameter, air gap and
thickness.
4. The acoustical characteristics are sound absorption coefficient (SAC) and
noise reduction coefficient (NRC) which is carried out experimentally.
5. Two analytical models involved which are Delany-Bazley Model and
Johnson-Champoux-Allard Model which is carried out analytically.
1.6 Organization of the Thesis
This section provided a brief summary of the thesis organization. There are five
chapters were organized in this thesis. This thesis first gives an overview of the
background, problem statement, research questions, objectives and scope of research.
Chapter 2 begins by laying out the comprehensive literature review about the
acoustical characteristic, natural fibres and related parameters outcomes. The
theoretical models about an equivalent fluid model of porous absorbent are also
explicated.
Chapter 3 describes the research methodology, including material
preparation, sample preparation, all parameters and impedance tube method setup. A
depth explanation about the equivalent fluid model of porous absorbents
measurement is also presented in this chapter.
Chapter 4 discusses the experimental results and the analytical predictions of
equivalent fluid model of porous absorbent. The influences of physical characteristic
are presented in this chapter.
Finally, Chapter 5 highlights and summarizes the results of the research and
provides some further works.
6
CHAPTER 2
LITERATURE REVIEW
2.1 Introduction to Acoustics
The word “acoustic” is comes from the Greek word akouein which means to hear
(Lindsay & Shankland, 1973). Rossing & Beranek (2009) defined acoustic as the
science of sound that deals with the development of sound, the propagation of sound
and the detection and perception of sound. They also described acoustic as the
interdisciplinary science, involving the study of physics, engineering, physiology,
speech, audiology, music, architecture, physiology, and neuroscience. Bies & Hansen
(2009) stated that acoustic field is the study of small pressure fluctuations in the air.
Acoustic or sound can be defined as a movement of wave in air or other medium
such as the excitation of the hearing mechanism that results in the perception of
sound (Everest & Pohlmann, 2001). The acoustics is the field that deals, in the
broadest sense, with the interaction of sound fields or mechanical vibratory
phenomena with organisms (Rosenblith & Stevens, 1953). In the acoustic field,
various definitions of acoustic are found. Although, there are several definitions
defined by acousticians, these definitions actually describes the same meaning but
with different view and concept.
The acoustic also overlaps with law of physics. There are several
fundamentals and studies that included in the physics of acoustic and produced a
change of acoustical properties which is sound wave phenomena, transmission,
reflection, absorption, refraction, dispersion, interference, diffraction and scattering.
7
The term “sound” is used to describe auditory sensation in the ear and
disturbance in a medium that can cause this sensation. Vibrating bodies, changing
airflow, time-dependent heat sources and supersonic flow are the processes that can
produce sound (Rossing & Beranek, 2009). Sound waves propagate and travel
through the surrounding medium; gases, liquids and solids. Thus, without medium,
sound cannot propagate. Sound also can be categorized into different types which
include, the sound that can be detected by ear, ultrasound (sound waves with
frequency above the range of human hearing; >4000Hz) and infrasound (sound
waves below the frequency of human hearing; <4000Hz), environmentally (likes
bird, dog and waterfall) sound, and aesthetic sound (music). The minimum sound
pressure audible that can be detected by a young human is approximately
Pa or atmospheres (1 atmosphere equals . For a normal
human ear, it can be considered painful when the sound pressure is
atmospheres (Bies & Hansen, 2009).
Generally, there are three characteristics used to visualize a sound wave;
amplitude, frequency and wavelength. Frequency is defined as properties of periodic
waves and it is measured in . Amplitude is a measurement of the amount of
energy travelled in the wave. Wavelength is the distance of sound propagation in the
one cycle with certain time. A wavelength can be determined from a point of phase
to another point on the next phase (Bies & Hansen, 2009). Frequency and
wavelength can be expressed as illustrated in equation 2.1 and 2.2.
(2.1)
or can be written as:
(2.2)
Normally, equation 2.1 and 2.2 is used frequently in the field of acoustic.
Figure 2.1 shows the illustration of wavelength and peak amplitude.
8
Figure 2.1: The illustration of wavelength and peak amplitude
(Everest & Pohlmann, 2001)
2.2 Sound Absorption
For over a decade, there has been great effort to studying surface absorption. Over
this time, a lot of innovative absorbers have been designed. Sound absorption is
defined as a sound incident on a material and it can be reflected, absorbed and
transmitted, as indicated in Figure 2.2. The sound direction refracted downward
because the material is denser than air. The amount of energy is that going to be
reflected, absorbed and transmitted depends on the surface of sound absorption
properties. Everest & Pohlmann (2001) stated that based on the rule of the
conservation of energy, the energy can neither be created nor destroyed, but can be
changed from one form to another.
Figure 2.2: Incident sound, reflection, absorption and transmission of sound
(Everest & Pohlmann, 2001)
9
There are four interrelated acoustic quantities that can be characterized
which are impedance, pressure reflection coefficient, admittance and absorption
coefficient. The information about the magnitude and phase that change on reflection
can be obtained from impedance, admittance and pressure reflection coefficient.
While, the information about the energy change on reflection can be obtained from
the absorption coefficient (Bies & Hansen, 2009). These four fundamentals
contribute to understanding of sound absorption. In order to consider a wave
propagation within two media; layer 0 and layer 1, as illustrated in Figure 2.3, these
fundamentals can be defined mathematically (Cox & D’Antonio, 2010). The
incident , reflected and transmitted pressures are defined in equation 2.3 –
2.5.
(2.3)
(2.4)
(2.5)
where , and are the magnitude of the plane waves incident, reflected and
transmitted, respectively. Consider a plan wave incident at an angle to boundary
between two acoustic media at as illustrated in the Figure 2.3. A simple model
for a porous absorber assumes that it behaves as an acoustic medium like air, only
with a different speed of sound . This must be true for all times and for all values
of as this was a plane wave. The relationship relating to the angles of propagation
is obtained as illustrated in equation 2.6 and 2.7, commonly known as Snell’s law:
(2.6)
(2.7)
where is angle of incidence and reflection, is reflection and is transmitted. The
complex wavenumber is defined as and . The characteristic of the sound wave
therefore depends on the relative values of the speeds of sound in two media. The
speed of the sound is much less than that in air. Thus, the angle of propagation in
medium is smaller than in the air. In equation 2.8, the pressure reflection
coefficient is defined as the ratio of the reflected and incident pressure:
(2.8)
10
Figure 2.3: Sound hitting a surface (Cox & D’Antonio, 2010)
Therefore, the magnitude and phase obtained from the pressure reflection
coefficient can give information about the reflection of sound.
2.3 Porous Absorbent
In general, there are three common types of sound absorbing material used in noise
control which are membrane resonator, Helmholtz resonator and porous absorber.
The membrane resonator and Helmholtz resonator are not used in this study. Porous
absorbers are often used for sound absorbing material. It is used to avoid a resonance
of the air cavity. If the cavities of the resonances are not eliminated by damping,
sound will pass easier through the partitions, thus the sound absorption will be poor
(Cox & D’Antonio, 2010). Porous absorber is any material where sound propagation
occurs in a network of interconnected pores in such a way that viscous effects cause
acoustic energy to be dissipated as heat. Porous absorbers are also used as part of
silencers and mufflers used to attenuate sound within pipe work.
The transmission loss for porous materials is directly proportional to the
thickness traversed by the sound. This loss is about 1 dB (100Hz) to 4 dB (3000Hz)
per inch of thickness for a dense, porous material like rockwool and less for lighter
material. This direct dependence of transmission loss on thickness for porous
materials is in contact to the transmission loss for solid, rigid walls, which is approx
5 dB for each doubling of the thickness (Everest & Pohlmann, 2001).
11
Those pores that are totally isolated from their neighbours are called “closed
pores”. They have an effect on some macroscopic properties of the material whereas
bulk density, mechanical strength and thermal. However, closed pores are
substantially less efficient than open pores in absorbing sound energy. This is
because closed pores have less of cavities, channels or interstices so that sound
waves are unable to enter through them. Besides that, the open pores have great
influence on the absorption of sound and they have a continuous channel of
communication with the external surface of the body. A schematic cross-section of a
porous absorber is shown in Figure 2.4. The figure can classify the pores according
to their availability to an external fluid. There is a simple convention to be
considered that a rough surface is not porous unless it has irregularities that deeper
pores than their wide (Arenas & Crocker, 2010; Rouquerolt et al., 1994).
Figure 2.4: Schematic cross-section of a porous solid material
(Rouquerolt et al., 1994)
Based on microscopic configuration, the porous absorber can be classified as
cellular, fibrous or granular. Cellular materials include those made from open-celled
polyurethane and foams. The common porous absorber panels are made from
synthetic fibres such as glass wool, rock wool, asbestos and mineral wool (Bies &
Hansen, 2009; Cox & D’Antonio, 2010; Everest & Pohlmann, 2001; Kazragis et al.,
2002). These fibres are excellent sound absorbers and good heat insulators;
unfortunately they are of limited value in insulating against sound. Granular
materials are used in controlling outdoor sound propagation (Attenborough, 1982).
12
Materials that some kinds of porous concrete, sands, soils, asphalt, granular clays and
gravel are examples of granular materials (Asdrubali & Horoshenkov, 2002; Magrini
& Ricciardi, 2000). These three main types of porous absorbing material are shown
in Figure 2.5. To describe the absorbing mechanisms, it is necessary to know their
typical microscopic arrangements and some physical models.
Configuration Types of Porous Absorbing Material
Cellular Fibrous Granular
Surface
Microscopic
Arrangement
Physical Models
Cubic cells with
connecting pores
Parallel fibre bundles Stacked identical
sperhes
Figure 2.5: Three main types of porous absorbing materials
(Arenas & Crocker, 2010)
2.4 Natural Fibres
Natural fibres include those made or obtained directly from plant, animal or mineral
sources. According to their origin, natural fibres can be classified into three types, as
illustrated in Figure 2.6. The three types of classifications are: (i) vegetable fibres
which are mainly of cellulose and are found in the manufacture of paper and cloth,
(ii) animal fibres which generally consist of protein including animal hair, silk fibre
and avian fibre, and, (iii) mineral fibres which are naturally obtained fibre produced
from minerals and are categorized into three types which are asbestos, ceramic fibres
and metal fibres.
13
Figure 2.6: Classification of natural fibres (Chandramohan & Marimuthu, 2011)
In general, natural fibres are lignocelluloses in nature consisting of helically
wound cellulose micro fibrils in a matrix of lignin and hemicelluloses (Dastoorian et
al., 2008). Cellulose-based natural fibres have high specific properties such as
stiffness, impact resistance, flexibility and modulus (Ca et al ., 2001; Nair et al.,
1996; Sydenstricker et al., 2003). Moreover, they are abundant, renewable,
biodegradable, low cost, low density, less equipment abrasion, less skin and
respiratory irritation, vibration damping and enhanced energy recovery (Karnani et
al., 1997; Maldas et al., 1988; Mohanty et al., 2000; Toriz et al., 2002).
The great amount of natural fibres is available and produced in many
developing countries. For example, approximately 28.8 million hectares of land are
occupied by banana plantations and about 14.2 million tons of banana fibre are
available in India (Database of National Horticulture Board, 2002). Another
example, in China, about 15.4 million hectares of land are occupied by bamboo
plantations, which is the biggest grove coverage in the world (Büyükakinci et al.,
2011). Thus, it is important to take the opportunity of the factor in the availability of
any plant for the fibre is its collectable yield per unit land area. Table 2.1 shows such
estimated yield for a number of non-wood plants. However, the species of these plant
fibres are difficult to estimate as accurate data are not available.
Plant/Lignocellulosic Fibres
Wood Stem/Bast Leaf Seed/Fruit Grass
Softwood
Hardwood
Flax Jute
Hemp
Kenaf
Ramie
Sisal Abaca
Pineapple
Banana Fique
Henequen
Palm
Cotton
Coconut
Bamboo
Rice
Animal Fibres
(Protein)
Silk
Wool
Mineral Fibre
Asbestos Ceramic
Metal
14
Table 2.1: World availability of non-wood fibres (Yang et al., 2004)
Nonwood Worldwide production (million, ton/yr)
Wheat straw 600
Rice straw 360
Barley straw 195
Oats straw 55
Rye straw 40
Grass seed straw 3
Seed flax straw 2
Sub-total straw 1255
Sugar cane bagasse 102.20
Reed 30
Bamboo 30
Cotton staple fibre 18.30
Cotton linters (1st and 2nd cut) 2.70
Cotton stalks 68
Other stem fibre (e.g hemp, jute, kenaf) 13.70
Leaf fibre 0.50
Abaca 0.08
Hemp 0.20
Com stalks 750
Grain and sweet sorghum stalks 252
Papyrus 5
Sabai grass 0.20
Grand Total 2527.88
Several investigations have been done to investigate the potential of natural
fibres (Bachtiar et al., 2010; Bledzki et al., 2001; Büyükakinci et al., 2011; Valadez-
Gonzalez et al., 1999). Table 2.2 shows the physical and mechanical properties of
some of natural fibres.
15
Table 2.2: Mechanical properties of natural fibre
(Chandramohan & Marimuthu, 2011)
Fibre Types Density (kg/m3) Water
absorption (%)
Modulus of
elasticity,
E(Gpa)
Tensile
strength (Mpa)
Sisal 800-700 56 15 268
Roselle 800-750 40-50 17 170-350
Banana 950-750 60 23 180-430
Date Palm 463 60-65 70 125-200
Coconut 145-380 130-180 19-26 125-200
Reed 490 100 37 70-140
Hemp 1480 80 70 590-900
Jute 1460 74 10-30 400-800
Ramie 1500 63 44 500
Flax 1400 72 60-80 800-1500
Cotton 1510 61 12 400
2.4.1 Previous Research on Natural Porous Absorbent
For the past centuries, there are many research have been made to study about sound
absorption panel. Moreover, in recent decades, many innovative absorber designs to
measure absorptive material have been achieved. Over this time, the focus of
attention has been done on the uses of material for sound absorption. Recently,
developments in sound absorber have heightened the need for replacement or
renewable materials in the making of sound absorber. The common acoustical panels
are made from synthetic fibres such as glass wool, rock wool, asbestos and mineral
wool. Although the synthetic fibres had excellent performance on the physical,
mechanical and absorption characteristic, but it is difficult to devise suitable disposal
methods for them.
Due to the environmental problems, there are increasing concerns that some
synthetic fibres have major drawbacks. Man-made vitreous fibres which refer to a
group of synthetic, such as rock wool, slag wool, glass fibres are widely used as
thermal and acoustic insulation material in building and industrial application.
16
Approximately, 27% of mineral wool (glass), 30% of mineral wool (stone) and 40%
of foam plastic have been estimated in European building insulation (Schmidt et al.,
2004). Table 2.3 lists some of the disadvantages of using man-made vitreous fibre.
Table 2.3: The disadvantages of using synthetic fibre
Disadvantages of synthetic fibre References
Fibres are difficult to recycle and expansive
synthetic fibres.
(Zhu et al., 2013)
The fibre can lay down in the lung alveoli
and cause skin irritation.
(Council Directive, 1967)
Exposure significantly to the human related
to increased oxidative.
(Rapisarda et al., 2015)
Fibres contribute greatly to global warming
potential.
(Asdrubali, 2006)
There are increased risks of human
respiratory system cancer when man-made
fibres are exposed to workers.
(Berrigan, 2002; Marsh et al., 2001; Stone at
el., 2004)
Fibres diffuse pleural mesothelioma and
asbestos exposure
(Wagner et al., 1960)
Hence, researchers have now drawn their attention to finding natural or
renewable materials instead of producing and using synthetic fibres. In recent years,
sustainability has become an important issue. This issue has also led to porous
absorbers being constructed from natural fibres. Numerous studies have been done to
investigate natural fibres for sound absorbing material.
Alrahman et al., (2014) conducted a research to compare acoustic
characteristics of date palm fibre and oil palm fibre. Both these fibre sheets were
treated with latex. There were two different thickness involved in their study namely;
50 mm and 30 mm thickness. The density for the date palm fibre sheet was 150
kg/m3 and 130 kg/m
3 for the 50 mm and 30 mm thickness, respectively. In contrast,
the density of oil palm fibre was 75 kg/m3 and 65 kg/m
3 for the 50 mm and 30 mm,
respectively. They found that the date palm fibre exhibits high sound absorption
coefficient (SAC) which was 0.93 at 1356 Hz and 0.99 at 4200 - 4353 Hz for the
17
50mm thickness. They concluded that the date palm fibre had a better performance
of SAC for high and low frequencies than oil palm fibre.
Veerakumar & Selvakumar (2012) studied the sound absorption properties of
Kapok fibre with polypropylene fibre for development of sound absorptive
nonwoven materials. The Kapok fibres were obtained from southern part of India.
Kapok fibres were blended with the polypropylene in three different blend
proportions which are 30%, 40% and 50% of Kapok fibres. There were two different
types of composite samples, uncompressed from and compressed form. The physical
characteristics result showed that the bulk density increased as the content of Kapok
fibre in the composite increased from 30% to 50%. Moreover, compressed
composites showed higher bulk density than that of uncompressed composites. The
result of SAC showed that the maximum SAC was between 250 Hz and 2000 Hz and
it was obtained from uncompressed composites of 30% of Kapok with high bulk
density and low porosity.
Abdullah et al., (2011), conducted a study on the potential of using dried
paddy straw fibre as raw material for sound absorbing material. They constructed
two stages to form the absorber sample, namely the pre-treatment stage and
preparation stage. During the pre-treatment stage, the selected paddy straws were cut
into 1 to 3 mm length. Then the paddy straws were sun dried for 1 week and heated
again in the oven at 115 degrees for 10 minutes. In the preparation stage, the paddy
straws were mixed with different composition of binders and blowing agents,
gypsum, aluminium oxide, methycellulose and water. The result indicated that a
larger amount of methycellulose improved the SAC, especially at low frequency
region, but reduced it at high frequency region. They also compared their results with
commercial glass wool and found that the samples with paddy absorber had
comparable performance.
Büyükakinci et al., (2011) investigated the thermal conductivity and acoustic
properties of three different natural fibres which are cotton, bamboo and wool mixed
with polyurethane (PU). The natural fibres were used as reinforcement agents in a
polyurethane matrix to improve the SAC and thermal conductivity. They found that
by increasing the fibres content SAC decreased, especially at high frequencies. The
thermal conductivity of the samples show does not significantly changing by adding
more fibres. The optimum thermal conductivity value was obtained from the sample
with 4% cotton fibre.
18
Ismail et al., (2010) investigated the potential of using Arenga Pinnata fibre
as raw material for sound absorbing material. There were four different thicknesses,
10 mm, 20 mm, 30 mm and 40 mm. The panel with 40 mm thickness showed the
optimum SAC from 2000 Hz to 5000 Hz. Comparisons were also made between coir
fibre and palm oil fibre. The result demonstrated that Arenga Pinnata had good
properties at above 2000 Hz, better than that of coir fibre, but slightly lower than that
of palm oil fibre. These results indicated that Arenga Pinnata fibre is capable to be
used as a sound absorption panel at a low cost, with less weight and its
biodegradable.
Mahzan et al., (2009) investigated on sound absorption of Rice-Husk
reinforced composite. The Rice-Husks went through several treatment processes of
cleaning and drying up at room temperature. The dried rice-husks were mixed
together with Polyurethane (PU) as a binder. There were six different percentages of
binder. The result indicated that the sample with 20% of Rice-Husk showed the best
performance at a low frequency region. A comparison was made with other materials
such as rubber particles and woods shaved. The results demonstrated that the Rice-
Husk, blended with PU, had better SAC.
Ersoy & Küçük, (2009) examined the characteristics of tea leaf waste on
sound absorption and developed it into three different layers with or without a single
backing layer of woven textile cloth. The result showed that the tea leaf fibre had a
better sound absorption when compared to polyester and polypropylene fibre at 500-
3200 Hz.
2.4.2 Oil Palm Mesocarp Fibre
Presently, Malaysia is famous in the production of palm oil in the international
market. The oil palm trees do not only provide oil, but also raw material such as oil
palm frond (OPF), oil palm trunk (OPT), empty fruit bunch (EFB), palm kernel shell
(PKS), palm kernel cake (PKC), palm kernel expeller (PKE), palm oil mill effluent
(POME), dry decanter cake (DDC), ash and Mesocarp fibre (Cuah, 2009). Among
the alternative material resources, agricultural waste like oil palm Mesocarp fibre is
selected and is known as lignocellulosic materials. Oil palm Mesocarp fibre, also
known as press fibre. The fibre is obtained after the pressing and squeezing process,
to obtain the crude oil using Cyclone Separator Machine. In this process, the oil palm
19
fruit is heated using live steam and it is continuously stirred to loosen the oil-bearing
Mesocarp from the nuts. Figure 2.7 shows a close up of oil palm fruit that contain
endocarp, kernel and Mesocarp.
Figure 2.7: A close up of oil palm fruit (Hai, 2002)
Among the 10 major oilseeds, oil palm plantation is found about 5.5% of
global land use for cultivation. However, the palm oil produced 32% of global oils
and fats output in 2012 (Sime Darby, 2014). Oil palm trees had achieved aggressive
growth in production and they had doubled from 1990 to 2001. Approximately 85%
of the world palm oil was produced by Malaysia and Indonesia, followed by other
countries including Thailand, Columbia, Nigeria, Papua New Guinea and Ecuador.
According to Malaysia Palm Oil Board (2011), Malaysia is the largest
producer of palm oil and the world’s largest exporter of palm oil, approximately
11.80 million tons or 50.9% of total production and accounting for about 61.1% or
10.62 million tons in 2001. Table 2.4 shows the total planted area hectare of oil palm
from 1974 to 2011 in Malaysia. Furthermore, Malaysian government had projected
to double the total production from 2000 to 2020 with exceeding 40 million tonnes.
Thus, Malaysia will continue to keep its status as the largest production in the world.
20
Table 2.4: Total planted area (hectare) of oil palm, 1974 – 2011, Malaysia
(Malaysia Palm Oil Board, 2011)
Year P.Malaysia (Hectare) Sabah (Hectare) Sarawak (Hectare) Total (Hectare)
1975 568561 59139 14091 641791
1976 629558 69708 15334 714600
1977 691706 73303 16805 781814 1978 755525 78212 19242 852979
1979 830536 86683 21644 938863
1980 906590 93967 22749 1023306 1981 983148 100611 24104 1107863
1982 1048015 110717 24065 1182797
1983 1099694 128248 25098 1253040 1984 1143522 160507 26237 1330266
1985 1292399 161500 28500 1482399
1986 1410923 162645 25743 1599311 1987 1460502 182612 29761 1672875
1988 1556540 213124 36259 1805923
1989 1644309 252954 49296 1946559 1990 1698498 276171 54795 2029464
1991 1744615 289054 60389 2094028
1992 1775633 344885 77142 2197660 1993 1831776 387122 87027 2305925
1994 1857626 452485 101888 2411999 1995 1903171 518133 118783 2540087
1996 1926378 626008 139900 2692286
1997 1959377 758587 175125 2893089 1998 1987190 842496 248430 3078116
1999 2051595 941322 320476 3313393
2000 2045500 1000777 330387 3376664 2001 2096856 1027328 374828 3499012
2002 2187010 1068973 414260 3670243
2003 2202166 1135100 464774 3802040 2004 2201606 1165412 508309 3875327
2005 2298608 1209368 543398 4051374
2006 2334247 1239497 591471 4165215 2007 2362057 1278244 664612 4304913
2008 2410019 1333566 744372 4487957
2009 2489814 1361598 839748 4691160 2010 2524672 1409676 919418 4853766
2011 2546760 1431762 1021587 5000109
Unfortunately, by increasing of cultivation and palm oil production, in the
same time the fibres that obtained from oil palm trees also increased. According to
Badri et al., (2005), 22% fibre produced from oil palm industry were left abundant in
Malaysia. In order to use the oil palm Mesocarp fibre, it is necessary to know about
their properties. According to Sabil et al., (2013), the oil palm Mesocarp fibre has 5
components as summarized in Table 2.5.
Table 2.5: Oil palm Mesocarp chemical composition analysis (Sabil et al., 2013)
Components Oil Palm Mesocarp
Holocellulose 0.3180
Cellulose 0.3440
Lignin 0.2527
Ash 0.0350
Moisture 0.3700
21
However, there has been a lack of discussion about the properties of oil palm
Mesocarp fibres. Most studies have only been carried out in pre-treatment and the
reaction of the fibre after treated with certain chemical. In addition, no research has
been found that studied about the potential of oil palm Mesocarp on sound absorbing
material. So far this area of investigation is rarely studied.
2.5 Binder or Adhesive
Over the past 38 years, the use of adhesives based on polymer materials has greatly
increased as a result of the availability of improved adhesives (Tout, 2000; Wake,
1978). Adhesives or binders are not replacing the nails, screws, rivets, but it is an
agent to becoming a preferred alternative to soldering, spot welding and brazing. The
use of adhesive allows the assembly of components without the need to drill or
perforate them and produce joints of higher strength.
In 1600s, the first commercial basis adhesive was producing animal glue. The
glues were used for joint production as well as for decorative purpose. Until 1700s
the crude forms of animal or fish glues were used by furniture makers of the era. Up
to that time, adhesives started to have a major impact on the furniture production and
design. In the 20th century, there was still no alternative for producing animal glues.
However, in 1930s, a revolution on the adhesive has been made which produced
synthetic adhesive over the second half of the century. During this period, the
development and introduction of urea formaldehyde (UF) adhesives became the most
important development, as far as the furniture industry is concerned (Tout, 2000).
Over this time, a considerable library of adhesives were tabulated based on the
particular operation, be it jointing, veneering or edge-banding and improving bond
quality and durability properties (Tout, 2000; Wake, 1978). In the recent decade, the
development continues with varying types of adhesives which have been developed
and applied in several of the application. There are several of adhesive types for
various applications and it is may be classified into a variety of ways depending on
their chemistries, form, type and their unique characteristics. It is difficult to think of
the use of adhesive without polymeric material.
In order to create a solid structure (sample), it is necessary to mix them with
an adequate adhesive. There are a lot of factors, for examples, types or properties of
adhesive and their percentage ratio, which affect and govern the acoustical
22
properties. Hence, it is important to have some basic knowledge on binder used in
this research, so that the way they affect acoustical performance can be understood
clearly in the later chapter.
2.5.1 Polyurethane
Polyurethane (PU) is known as polymeric that consist of urethane in their backbones.
The difference of properties and production of PU is distinguished by the type and
degree of poly-addition polymerization. Basically, PU is made from a formula that
contains a host of ingredients selected to aim in achieving the desired grade of foam.
These components included water, surfactants, cross-linkage agent, blowing agent,
polyol and isocyanate. Among all, polyol and isocyanate are the main components
used to forms PU linkage. Generally, the isocyanate is a basic chemical component
of PU reaction and the unique and basic monomers in segmented PU elastomers can
be either aromatic or aliphatic. Besides, a major part of the PU is composed of polyol
which determine the properties of the final PU polymer. Polyol is generally an
alcohol function which low molecular weight polymer. The end products of PU can
be applied to many categories which are PU elastomers, PU foams, PU adhesive,
binders, thermoplastics, coating, castables, millables and so on (Klempner & Frisch,
1991).
Polyurethane (PU) is used in a wide range of applications due to their
superior versatility. PU has been used in various of products, for example,
automotive industry (truck beds, seating, dashboards, door panels, mirror surrounds
and steering wheel), thermal insulation (rigid PU foam), furniture (flexible PU
foam), construction (rigid PU foam and advanced wood composites), footwear (PU
adhesives); seals, synthetic leather, shoe soles and as elastomers (Randall & Lee,
2002; Szycher, 1999). During World War II, PU foam was widely used in jet. Foams
turned out to be popular because of their inexpensive polyester polyols.
In this research, the PU is used as an adhesive or a binder agent to consolidate
the compound in order to create a solid sample. Among the adhesives, material
resources, PU is selected as an adhesive in this research. It is due to the high
performance properties and it is essential for a multitude of end-use applications.
Moreover, PU does not aggravate allergies during handling, it is light, quiet, and it
resists mildew (Randall & Lee, 2002). PU also has some defects when used in
23
applications which is unable to withstand in-use temperature and humidity condition
(Klempner & Frisch, 1991). However, by introducing polyether-based polyol, these
defects can be improved. Polyester-based polyol can be improved on tensile strength,
tear strength, oil resistant, more durable and heat aging properties (Randall & Lee,
2002)
The use of adhesive or binder to produce porous absorber is not a new
method. Many studies have shown the incorporation of binder such as polyurethane,
polyester, latex, epoxy, urea formaldehyde, phenol formaldehyde, isocyanate with
varying natural fibres likes pine sawdust, sugarcane, date palm fibre, kenaf, durian
peel, coconut coir, rubber crumbs for acoustical performance (Alessandro & Pispola,
2005; Alrahman et al., 2014; Asdrubali et al., 2008; Borlea et al., 2011; Khedari et
al., 2003; Putra et al., 2013). In order to create a better sound absorber, it is
important to know the mechanical strength and the environmental condition that the
absorber will be applied to. The absorber must not only be good in absorbing sound,
but also sufficiently robust over time.
Maderuelo et al., (2013) determined the acoustical performance of recycled
rubber and polyurethane resin. The purpose of their study was to analyse and
evaluate the physical characteristics, the influence of sample thickness and the
different percentage of PU binder. Acoustical properties were obtained using
experimentally and prediction model which is ITM and Champoux-Stinson model,
respectively. The model was consists of flow resistivity, porosity, tortuosity and two
shape factor. Based on their study, the result indicated that the porosity decreased
when the percentage of binder increased. Sample with 5% of PU binder presented
higher values of porosity, which was the lowest composition of the composite. The
researchers mentioned that it was due to the voids between individual grains that
were filled by the binder. The result also represented that by reducing the porosity
values was increased the flow resistivity values. From the acoustical characteristics
with different thickness sample, the results revealed that the thickness of the samples
increased and improved the absorption at low frequency range. They also pointed out
that the predicted results followed closely the experimental result, practically through
the considered frequency range. The researchers claimed that the model was a good
tool in order to design a new porous absorber. The optimum absorption coefficient
obtained for the sample with 5% and 12% of PU binder was 1.00 and 0.96,
respectively, achieved at the same frequency, 2560 Hz. Therefore, it can be
24
concluded that the percentage of binder, physical properties of the sample and
different thickness sample influence to the acoustic performance.
Putra et al., (2013) investigated the potential of sugarcane to be an alternative
acoustic material. The sugarcane fibres were cut into 1 to 3 mm length. Two different
binders were used in this research, polyurethane and polyester. There were four
different weight composition ratios of fibres and binder involved which were 90:10,
80:20, 70:30 and 60:40. The researchers revealed that the different types of binder
did not significantly affect SAC, thus, only results with different compositions were
discussed. It can be explained that as the percentage of binder increases, the flow
resistance becomes higher because the fibre arrangement inside the sample is too
close consequently causes sound wave trap in the medium and resulting in more
energy lost.
Borlea et al., (2011) conducted a comparison of different types of acoustical
material such as silencer, vibration isolators, damping treatment and absorptive
barrier. The raw material used in their study was pine sawdust with density = 0.035
g/cm3. The pine sawdust was mixed with a commercial polyurethane binder (PU:
131). There were two percentages of the binder implemented in their study, which
were 30% and 20%.
Based on results obtained, they found that the incorporation of PU binder was
influenced SAC. The sample with 30% of PU binder improved the SAC in the low
frequency range compared to the sample with 20% of PU binder. However, the
sample with 20% of PU binder showed better SAC which was 0.9 in high frequency
range at between 3150 – 6300 Hz. They also found that the optimum SAC was
obtained from the sample with 30% of PU binder. The measurements also performed
with an air gap of 20mm at the back of the samples. The result indicated that SAC
was increased at between 150 – 630 Hz and 1250 – 3150 Hz when a 20mm air gap
was implemented. They concluded that the samples can be mounted in the
conference room, amphitheaters, and also for the phonic isolation of housing.
Asdrubali et al., (2008) conducted a research with the purpose of fabricating
recycled tyre granules by mixing the rubber crumbs and PU binder of different
granule sizes, percentage, thickness and compaction ratio of the samples to enhance
the acoustical properties. The rubber granules were prepared in different sizes, which
were less than 1mm, 1mm, 3mm and 10mm, considered as powder, small size,
medium size and large size, respectively. The large size rubber granules were mixed
103
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Abdullah, Y., Putra, A., Effendy, H., Farid, W. M., & Ayob, R. (2011). Dried Paddy
Straw Fibers as an Acoustic Absorber : A Preliminary Study. Hari penyelidikan
2011, Fakulti Kejuruteraan Mekanikal, Universiti Teknikal Malaysia Melaka,
52–56.
Abidin, N. Z. (2010). Sustainable Construction in Malaysia – Developers ’
Awareness. International Journal of Human and Social Sciences, 5(2), 122–
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