a comparison of the mechanical properties of phenol formaldehyde composites reinforced with banana...

A comparison of the mechanical properties of phenol formaldehydecomposites reinforced with banana fibres and glass fibres

Seena Josepha, M.S. Sreekalab, Z. Oommena, P. Koshyc, Sabu Thomasb,*aDepartment of Chemistry, CMS College, Kottayam, Kerala, India

bSchool of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills (PO), Kottayam , Kerala, IndiacRegional Research Laboratory, Thiruvananthapuram, Kerala, India

Received 23 October 2001; received in revised form 27 May 2002; accepted 28 May 2002

Abstract

Composites were fabricated using banana fibre and glass fibre with varying fibre length and fibre loading. The analysis of tensile,flexural and impact properties of these composites revealed that the optimum length of fibre required for banana fibre and glassfibre are different in phenol formaldehyde resole matrix. Both banana fibre and glass fibre reinforced composites show a regular

trend of increase in properties with fibre loading. Interfacial shear strength values obtained from single fibre pull out test reveal thatthe interlocking between banana fibre and phenol formaldehyde resin is much higher than that between glass and phenol formal-dehyde resin. SEM studies were carried out to evaluate fibre/matrix interactions. Finally the experimental tensile strength were

compared with the theoretical predictions. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: A. Polymer/matrix composites; A. Short fibre composites; B. Fibre/matrix bond; B. Interfacial strength; B. Mechanical properties

1. Introduction

The interest in using natural fibres such as differentplant fibres and wood fibres as reinforcement in plasticshas increased dramatically during last few years. Withregard to the environmental aspects it would be veryinteresting if natural fibres could be used instead ofglass fibres as reinforcement in some structural applica-tions. Natural fibres have many advantages comparedto glass fibres, for example they have low density, theyare recyclable and biodegradable. Additionally they arerenewable raw materials and have relatively highstrength and stiffness [1–6]. Their low-density valuesallow producing composites that combine goodmechanical properties with a low specific mass. In tro-pical countries fibrous plants are available in abundanceand some of them like banana are agricultural crops.Banana fibre at present is a waste product of bananacultivation. Hence without any additional cost input

banana fibre can be obtained for industrial purposes.Banana fibre is found to be a good reinforcement inpolyester resin [7]. Resole type phenolics possess excep-tional adhesive properties and have high rigidity,dimensional stability and exceptional heat and fireresistance due to a highly crosslinked aromatic struc-ture. The modification of phenolic resins by inclusion offibres, particulate fillers or elastomeric materials [8–10]enables them to overcome the high brittleness and cureshrinkage, the major drawbacks that prevent the wide-spread application of resins. Phenolic resin generateschemical bonding with lignocellulosic reinforcement,leading to strong forces between fibre and resin. Thus ahigh compatibility in the system between vegetable fibreand polymer is achieved. It is reported that oil palmfibre is a potential reinforcement for phenolic resoles[11]. The properties of the composites are stronglyinfluenced by the fibre length. The strength, modulus,mode of failure and fracture toughness of a composite isnot only dependent on the properties of the fibre andmatrix, fibre volume fraction and fibre orientation butalso on the interfacial parameters of the composite. Aweak interface drastically reduces the off axis strength, theflexural strength [12] and the compression strength [13].

0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0266-3538(02 )00098-2

Composites Science and Technology 62 (2002) 1857–1868

www.elsevier.com/locate/compscitech

* Corresponding author. Tel.: +91-481-730003/731036; fax: +91-

481-561190.

E-mail addresses: [email protected] or [email protected]

(S. Thomas).

An increase in interfacial strength leads to a substantialincrease in tensile strength and modulus of a short fibrecomposite [14]. The single fibre pull out is a very effec-tive method for determining the interfacial bondstrength.In the present work a detailed investigation has been

carried out on banana/PF composites especially on theeffect of varying fibre length, and fibre loading. Tensileflexural and impact performance of these composites arecompared with those of glass fibre composites. Inter-facial shear strength of banana fibre and glass fibre inphenol formaldehyde resin is also calculated from theload displacement curves of pull out tests.

2. Experimental

2.1. Materials

Banana fibre obtained from Sheeba Fibers and Han-dicrafts, Poovancode, Thamilnadu was used in thisstudy. Phenol formaldehyde resole type resin obtainedfrom M/S West Coast Polymers Pvt. Ltd., Kannur,Kerala was used as matrix. Important characteristics ofbanana fibre and phenol formaldehyde resins are givenin Tables 1 and 2. The glass fibre used was E-glass rovingobtained from Hitech Fibre Corporation, Bangalore.

2.2. Preparation of composites

Prepreg route was followed for the preparation ofcomposites. Hand lay-up method followed by compres-sion molding was adopted for composite fabrication.Mats of uniform thickness were prepared from choppedbanana fibres and glass fibers of particular fibre length.The mats were impregnated in PF resin and the prepregwas kept at room temperature up to semicured stage. Itwas then pressed at 100 �C in a mould having dimensions150 mm�150 mm�3 mm to get a three dimensionallycrosslinked network. Different composite sheets areprepared varying the fibre length (keeping fibre volume

fraction constant) and volume fractions (keeping thefibre length constant).

2.3. Mechanical tests

Single fibre pull out specimen is prepared as shown in theFig. 1. Here a single fibre is taken and partially embedded inresin kept in a box like container. The embedded length, thefibre length between the matrix and fibre is kept as 5 mmand fibre free length as 20 mm. After curing, the uprightfibre is pulled out from the block of resin using universaltesting machine at a crosshead speed of 10 mm/min. Theload and displacement are recorded during the test.Tensile testing of banana fibre and glass fibre were

carried out in a FIE universal testing machine at a crosshead speed of 10 mm/min. Specimens were prepared bymounting single fibres on a stiff card board piece with a50 mm window. The ends of fibres were fixed on thecardboard. The diameters of the fibre specimens weremeasured microscopically and average of six readingswas taken for diameter determination.Test specimens were cut from composite sheets. Ten-

sile testing was carried out in a FIE universal tensiletesting machine TNE-500 according to ASTM D 638–76. The three point flexural properties were determinedby same machine according to ASTM D 790. The loaddisplacement curves were obtained and flexural strengthand modulus were calculated. Izod Impact test wasdone on notched specimen with an impact speed of 3.46m/s and incident energy of 2.75 joules according toASTM D 256. Minimum of four samples were tested ineach case and the average value is reported.The surface of the fractured specimens under tensile

and impact tests were examined using a Jeol scanningelectron microscope.

3. Results and discussion

3.1. Adhesion between fibre and matrix

The strength of interfacial bond between the fibres andmatrix is a key parameter in controlling the behavior of

Table 1

Chemical characteristics of banana fibre

Cellulose 63–64%

Hemi cellulose 19%

Lignin 5%

Moisture 10–11%

Table 2

Characteristics of phenol formaldehyde resole

Appearance Deep brown colour

Viscosity (CPS) 18–22

Water tolerance 1:18

Solid content% 50Fig. 1. Single fibre pull out test specimen.

1858 S. Joseph et al. / Composites Science and Technology 62 (2002) 1857–1868

composites. One of the methods used to assess andinvestigate the interfacial bond is the single fibre pull-out test. The shape of the force displacement curves inthe pull-out test depends ultimately on the intrinsiccharacteristics of the interface and on the dynamics ofthe test. However, as pointed out by Laws [15] andChua and Piggot [16] there is a sudden release of thestrain energy stored in the free path of the fibreunder tension when the interface fails. The elasticcontraction of the fibre may result in its full extrac-tion from resin.The force displacement curves obtained from pull out

tests in banana fibre and glass fibre embedded in PFresin matrix is shown in the Figs. 2 and 3. In bananafibre/PF system due to strongly bonded interface, thestrain energy stored in the free length of the fibre is sohigh that immediate extraction follows the interfacefailure. Therefore the load displacement curve of thepull out test of banana fibre embedded in PF matrixshows only the maximum load. In the load displacementcurves of pull out test of glass fibre from PF matrix,another type of behavior as observed by Penn et al. [17]is found. Here first peek corresponds to completedebonding; the later ones originate in the damagecaused by friction when non-embedded part of the fibreis pulled up through the matrix. The shear strength ofthe fibre/resin interface is a key property when investi-gating the micromechanical behavior of compositessince it is a measure of the integrity of the interface. Inglass/PF composites the load displacement curve ishaving different regions. The debonding crack initiatesaround the half waypoint and propagates in region 1.Region 2 is a transition regime where the maximum

force for complete debonding, Fd, reduces to a valuerequired for extracting the debonded fibre from thematrix. A relaxation in strain energy in matrix andfibre accompanies this reduction in force. The oscil-lations in region 2 are due to stick and slip betweenthe fibre and matrix during the transition from crackdebonding phase to pull out phase, which occurswhile the specimen is being stained in the testingmachine.From load displacement curves the debonding shear

strength (�d) is calculated from the equation

�d ¼Fd

D�lcð1Þ

where Fd is the debonding force, D fibre diameter mea-sured microscopically, and lc, the embedded length ofthe fibre.The � values obtained from the above equation are

given in Table 4. The interfacial shear strength value ismuch higher in banana/PF composites than for glass/PF composite. This is due to the hydrophilic nature ofcellulose and PF resin. Hydrophilicity of fibre arisesfrom the hydroxyl groups of lignin and cellulose, whichcan easily form hydrogen bonds with methylol andphenolic hydroxyl groups of the resole resulting in astrong interlocking between the two. Fig. 4 shows thechemical interaction between banana fibre and phenolformaldehyde resin. But in glass/PF composites such aninteraction is not present and the interfacial shearstrength value is very small as compared to that ofbanana fibre composites.

Fig. 2. Force displacement curves obtained from single fibre pull out

test of banana fibre.

Fig.3. Force displacement curves obtained for single fibre pull out test

of glass fibre.

S. Joseph et al. / Composites Science and Technology 62 (2002) 1857–1868 1859

3.2. Tensile properties

3.2.1. Intrinsic properties of fibresTensile properties of banana fibre and glass fibre are

given in Table 3. Tensile strength and modulus valuesare found to be much higher in glass fibres comparedto banana fibre. The elongation at break value is foundto be higher in banana fibre. The cellulose fibre isfound to have higher extensibility compared to glassfibre.

3.2.2. Effect of fibre lengthThe properties of the composites are strongly influ-

enced by the fibre length. The effect of fibre ends playsan important role in the fracture of short fibre compo-sites. Inorder to achieve the maximum level of stress inthe fibre, the fibre length lf must be at least equal tocritical fibre length lc, the minimum length of fibrerequired for the stress to reach the fracture stress offibre. It is reported that the composites with fibre lengthlf<5 lc has strength significantly lower than that of acontinuos fibre composite with same volume fraction offibre. However, the strength of a short fibre compositeincreases with length of fibre for a given Vf and forlength above 10lc, the difference between the strength ofthe two composites becomes equal [18]. Fig. 5 shows aschematic representation of the situation when thefibres are having length below lc, above lc, and at lc.Thus it is very important to optimize the fibre lengthfor a particular matrix/fibre system so that maximumproperties can be achieved. The uniaxial stress-strainbehavior of banana/PF and glass/PF composites withdifferent fibre lengths are shown in the Figs. 6 and 7.The stress value is found to increase linearly withstrain at low elongation. A non-linear behavior isobserved at high elongation. This behavior isobserved in all lengths of banana/PF composites.

Table 4

Interfacial shear strength values obtained from single fibre pull out

tests

Fibre type ISS values (MPa)

Banana fibre/PF 44

Glass fibre/PF 7

Table 3

Tensile properties of banana fibre and glass fibre

Fibre Tensile

strength (Gpa)

Initial

modulus (GPa)

Elongation

at break (%)

Banana fibre 0.5 12 7

Glass fibre 3 65 3

Fig. 4. Schematic model showing the interface of banana fibre and phenol formaldehyde.


From the figure it is clear that for a given strainlevel stress increases with fibre length up to 30 mmand then decreases indicating the optimum fibrelength of 30 mm for banana/PF composites. In glassfibre composites almost linear increase of stress withincrease of strain is observed in all lengths of fibres.The stress value is found to be the highest for 40mmfibre and the value then decreases on increasing thefibre lengths. This lowering of stress value at higherfibre lengths can be attributed to the fibre entangle-ments formed at higher lengths.The influence of fibre length on tensile strength and

young’s modulus of banana/PF composites are obtainedfrom Table 5. In banana/PF composites the tensilestrength and young’s modulus values are increased on

increasing the fibre length to 30 mm and then a decreaseis found at a higher fibre length. The elongation value isnot found to have any considerable variation with fibrelength. From Table 6 it is clear that in glass/PF com-posites, the maximum tensile properties are shown bycomposites of 40 mm and hence 40 mm is the optimumfibre length for effective stress transfer between the glassfibre and PF resin.From the tensile behavior of resole composites fibre

length of 30 mm was found to be the optimum fibrelength for banana/PF composites and 40 mm was foundto be the optimum length for glass fibres. In the case offibres shorter than this optimum length, the fibres willdebond from the matrix resulting in failure of compositeunder low strain.

Fig. 5. Diagrams of tensile stress applied at different fibre lengths.

Fig. 6. Stress–strain behavior of banana fibre reinforced phenolics at

different fibre lengths (fibre loading 30%).

Fig. 7. Stress–strain behavior of glass fibre reinforced phenolics at

different fibre lengths (fibre loading 30%).


3.2.3. Effect of fibre loadingThe stress strain behavior of banana/PF and glass/PF

composites at varying fibre loading is shown in Figs. 8and 9. From stress strain curves of composites it isfound that stress strain curves of pure PF is similer tothat of brittle materials. The behavior is elastic in nat-ure. However addition of fibers makes the matrix duc-tile. This is evident from the high elongation at breakvalue of composites.

In Banana/PF composites the tensile strength andyoung’s modulus is found to increase with increase infibre loading (Table 7). The tensile strength and young’smodulus are found to have 400 and 320% increase withincreasing banana fibre loading to 48% when comparedwith neat resin. The percentage elongation at break isvery low in pure PF. The brittle nature of PF resindecreases with the addition of banana fibre and thereforeelongation value increases with fibre loading. It is inter-esting to note that the failure elongation of the compositeis much higher than that of the individual components athigher fibre loading indicating a synergic effect.Similar behavior is observed in the glass/PF compo-

sites as shown in Table 8. Tensile strength and modulusvalues increase with increasing glass fibre loading. Theelongation value is higher compared to banana fibrecomposites. This is because in banana/PF composites thepredominant failure mechanism is fibre fracture sincethere is a strong interaction between vegetable fibers andphenolic resin, due to hydrophilic nature of cellulose andPF resin. So the debonding of fibre from matrix is difficultand fibre pull out is less in banana/PFsystem. But in glass/PF composites fibre pull out is possible due to weak inter-facial shear strength and composite have more elongationcompared to banana/PF composites.The tensile fractographs of composites glass/PF and

banana/PF composites containing 40% fibre loadingare shown in the Figs. 10 and 11, respectively. In glass/PF composites (Fig. 10a and b) there are fibre pull outand fibre debonding. The wetting of the glass fibre bythe matrix is very poor. But in the case of banana fibrecomposites (Fig. 11a and b), the interlocking betweenthe fibre and matrix is very strong and the fibre fracture

Table 5

Tensile properties of banana/PF composites at different fibre lengths

(fibre loading 45%)

Fibre

length (mm)

Tensile

strength (MPa)

Youngs

modulus (MPa)

Elongation

at break (%)

0 7 175 3

10 12 268 8

20 20 291 8

30 26 556 7

40 23 398 9

Table 6

Tensile properties of glass/PF composites at different fibre lengths

(fibre loading 25%)

Fibre

length (mm)

Tensile

strength (MPa)

Young’s

modulus (MPa)

Elongation

at break (%)

0 7 175 3

20 11 256 6

30 13 284 11

40 17 304 8

50 15 316 7

Fig. 8. Stress–strain behavior of banana fibre reinforced phenolics at

different fibre loadings (fibre length 30 mm).

Fig. 9. Stress–strain behavior of glass fibre reinforced phenolics at

different fibre loadings (fibre length 40 mm).


occurs by the tensile force. In glass fibre compositesfibre fracture is very small and predominant fracturemechanism is fibre pull out. Fig. 11b is the magnifiedview of a banana fibre from the tensile fracture surface ofbanana fibre composites. It shows the interface failure,which results in loss of surface smoothness and defi-brillation of the fibre. Amount of resin adhering to thebanana fibre is higher compared to that in glass fibre.In both banana fibre and glass fibre composites the

specific strength and modulus values increase withincreasing fibre content. Due to the higher density ofglass fibres compared banana fibres, the glass fibrereinforced composites are having higher density com-pared to banana fibre reinforced composites. The spe-cific strength and specific modulus values of glass fibrereinforced composites are much lower than that of thetensile strength and modulus values. A slight decrease inthe specific strength and modulus values is observed athigher fibre loading. But in banana fibre composites thespecific strength is almost same as tensile strength andmodulus values. In terms of the specific modulus andspecific strength, the properties of banana fibre compo-sites are comparable to glass fibre composites.

3.3. Flexural behavior

By the application of flexural force, the upper andlower surface of the specimen under three point bending

Table 7

Tensile properties of banana/PF composites at different fibre loading

(fibre length 30 mm)

Fibre

loading

(wt.%)

Tensile

strength

(MPa)

Specific

strength

(MPa/g/cc)

Tensile

modulus

(MPa)

Specific

modulus

(MPa/g/cc)

Elongation

at break

(%)

0 7 5 175 135 3

16 6 5 197 163 7

27 16 13 370 354 9

32 20 19 375 357 10

41 26 25 440 427 11

48 28 26 560 552 11

Table 8

Tensile properties of glass/PF composites at different fibre loadings

(fibre length 30 mm)

Fibre

loading

(wt.%)

Tensile

strength

(MPa)

Specific

strength

(MPa/g/cc)

Tensile

modulus

(MPa)

Specific

modulus

(MPa/g/cc)

Elongation

at break

(%)

0 7 5 175 135 3

18 7 5 256 179 3

28 17 13 304 227 9

35 37 27 551 408 16

40 42 26 547 344 14

Fig. 10. Tensile fracture surface of glass fibre/PF composites (fibre loading 40%).

Fig. 11. Tensile fracture surface of banana fibre/PF composites (fibre loading 40%).


load is subjected to compression and tension and axi-symmetric plane is subjected to shear stress. So there aretwo failure modes in the materials; bending and shearfailure. The specimen fails when bending or shear stressreaches the corresponding critical value. The modes offailure of the composites under three point bending canbe obtained from the force deflection curves [19]. Whena specimen fails such that the slope of force deflectioncurve decreases to zero shear failure takes place. If acurve is practically linear due to abrupt failure this isdue to flexural failure. Fig. 12 shows the stress–strainbehavior of banana fibre and glass fibre compositesunder flexural loading. The glass fibre composites showhigh elasticity and lower extensibility. The high flexuralstrength of glass fibre is due to inherent property ofglass fibre. Banana fibre introduces plasticising effect onPF matrix. So the banana /PF composites are havinghigher toughness. Due to the high extensibility of thefibre banana fibre can withstand the stress applied andwill prevent the catastrophic failure of the composite.The dependence of flexural properties of the compo-

sites on fibre length in banana fibre and glass fibre rein-forced resoles is given in Tables 9 and 10. In glass-reinforced composites flexural strength values increasewith increasing fibre length and maximum value isobtained for 50mm long fibre composites. In bananafibre composites the flexural strength and modulusvalues are found to be very much lower compared withthe values at higher lengths and maximum flexuralstrength and modulus values are obtained for 40 mmfibres.

The variation of flexural strength and flexural mod-ulus values with fibre loading in banana fibre and glassfibre composites are shown in Tables 11 and 12. Specificstrength and modulus values also increase by incorpor-ating glass fibres. Studies of unidirectional compositesformed of sisal/epoxy [20] show that there is a linearrelationship between the flexural strength and fibreloading and the flexural strength of composite exceedsthat of resin. In banana fibre polyester composites it hasbeen shown that the flexural strength is lower than theneat resin [7]. In oil palm fibre reinforced resoles the

Fig.12. Flexural stress-strain curves of banana fibre and glass fibre

reinforced composites (fibre loading 40%).

Table 9

Flexural properties of banana/PF composites at different fibre lengths

Fibre

length (mm)

Flexural

strength (MPa)

Flexural

modulus (MPa)

10 25 516

20 34 593

30 50 2283

40 50 2481

Table 10

Flexural properties of glass/PF composites at different fibre lengths

Fibre

length (mm)

Flexural

strength (MPa)

Flexural

modulus (MPa)

20 38 4120

30 50 5309

40 51 2989

50 55 3781

Table 11

Flexural properties of banana/PF composites at different fibre load-

ings

Fibre

loading

(wt.%)

Flexural

strength

(MPa)

Specific

flexural

strength

(MPa/g/cc)

Flaxural

modulus

(MPa)

Specific

flexural

modulus

(MPa/g/cc)

0 10 8 1973 1517

16 34 28 572 475

27 36 34 588 563

32 45 43 1120 1079

45 50 49 2400 2330

Table 12

Flexural properties of glass/PF composites at different fibre loadings

Fibre

loading

(wt.%)

Flexural

strength

(MPa)

Specific

flexural

strength

(MPa/g/cc)

Flexural

modulus

(MPa)

Specific

flexural

modulus

(MPa/g/cc)

0 10 8 1973 1517

18 25 17 4110 2876

28 52 38 4381 3195

35 63 45 5094 3773

40 73 46 6454 4059


flexural strength increased linearly up to a maximum of38% fibre loading [11]. In the case of polyester resin theflexural strength is 50 MPa compared with that ofresoles which is about 11 MPa. In glass fibre reinforcedresoles both flexural strength and modulus values arefound to increase with increasing fibre loading. About600% increase in flexural strength is obtained on incor-porating 40% glass fibre in neat phenolic matrix. At lowloading of banana fibre the flexural modulus of rein-forced composites is found to be lower than that of theneat resin. But on increasing the fibre loading to 45%the flexural modulus is increased to about 25%. Theflexural strength also shows very good enhancement onincreasing fibre loading. This improvement in propertiesis more pronounced in glass fibre composites than thatof banana fibre composites. The specific strength andmodulus values of both banana fibre and glass fibrecomposites also increase with fibre loading. The specificflexural strength of banana fibre composite is compar-able to glass fibre composites.

3.4. Impact behavior

The impact performance of fibre-reinforced compo-sites depends on many factors including the nature ofthe constituent, fibre/matrix interface, the constructionand geometry of the composite and test conditions. Theimpact failure of a composite occurs by factors likematrix fracture, fibre/matrix debonding and fibre pullout. Even though, fibre pull out is found to be animportant energy dissipation mechanism in fibre rein-forced composites [21]. The applied load transferred byshear to fibres may exceed the fibre/matrix interfacialbond strength and debonding occurs. When the stresslevel exceeds the fibre strength, fibre fracture occurs.The fractured fibres may be pulled out of the matrix,which involves energy dissipation [22]. For banana fibrereinforced composites and glass fibre reinforced com-posites, the fibre length was varied in order to examinethe influence of this parameter on impact strength asshown in Fig. 13. In both the composites the impactstrength was found to increase with increasing fibrelength. Long fibres have a larger absorption capacity,and distribution of impact energy occurs at high speed.Also as the length of fibre increases, lower is the numberof fibre ends and the number of defects that those pointscan generate in the composite [23]. However, after anoptimum length of fibre only a small proportion of thefibre will be pulled out of matrix compared to shorterlengths where fibre pull out is the active fracture mechan-ism, thus leading to a small decrease in impact strength asobserved by Uma Devi et al. [24]. In accordance with theabove expectations, in banana/PF composites the impactstrength increases with fibre length up to 30 mm followedby a decrease. For glass fibre reinforced resoles the impactstrength decreases after 40 mm fibre length.

The relationship between fibre loading and impactstrength is shown in Fig. 14. In both banana fibre com-posites and glass fibre composites the impact strength ofthe composite is found to increase with weight fractionof fibre. Sanadi et al. reported that the impact resistanceof unidirectional sunhemp/polyester composites shows alinear increase with fibre loading [3]. The impactstrength of 40% glass fibre reinforced resole is found tobe 700% higher than pure resole. In the case of glassfibre reinforced composites, frictional losses as fibre is

Fig. 13. Variation of impact strength with fibre length for banana

fibre and glass fibre reinforced composites.

Fig. 14. Variation of impact strength with fibre loading for banana

fibre and glass fibre reinforced composites.


pulled out of the matrix are a major contributor to theobserved toughness of the composite. This is due to thesurface smoothness and regular cross section of theseglass fibres. In the case of natural fibres such amechanism is not favored due to the mechanical inter-locking between fibres and matrix. In banana fibrecomposites a small decrease in impact strength, com-pared to neat resin occurs at lower fibre loading and agood increase in impact strength is observed with a fur-ther increase in fibre loading. SEM of the fracture sur-face of banana/PF composites containing 30% fibre isshown in Figs. 15 and 16. Fig. 15 shows the poor inter-action between glass and PF resole and in Fig. 16, thePF resin is penetrated into the natural fibre surface. It isclear from figures that the banana fibres adhere well tothe PF matrix and undergo breaking and delaminationduring fracture, where as glass fibres are easily pulledout from the matrix during the impact failure.

3.5. Theoretical modelling

Several theories have been proposed to model thetensile properties of composite material in terms of dif-ferent parameters. For determining the properties ofrandomly oriented fibres in a rigid matrix, series andHirsch’s model are useful. According to these modelstensile strength is calculated using the followingequations

Series Model

Tc ¼TmTf

TmVf þ TfVmð2Þ

Hirsch’s Model

Tc ¼ xðTmVm þ TfVf Þ þ ð1� xÞTmTf

TmVf þ TfVmð3Þ

where Tc, Tm, and Tf are the tensile strength of thecomposite, matrix and the fibre respectively. Vf and Vm

are the volume fraction of fibre and matrix, and x is aparameter between 0 and 1. It is reported that theparameter ‘x’ in the above equation determines thestress transfer between the fibre and matrix [25]. Forcalculations, the value of x was varied to obtain best-fitvalues with experimental results. In banana/PF compo-sites the value of x is found to be very much highercompared to glass/PF composites indicating effectivestress transfer between banana fibre and PF, due to thestrong interaction between the two.Theoretical values of tensile strengths were calculated

using the above models and is compared with theexperimental values as in Figs. 17 and 18. In the case ofseries and Hirsch’s models it is found that the tensilestrength increases regularly with increase in the volumefraction of fibres. The agreement is better for banana

Fig. 15. Impact fracture surface of glass fibre/PF composites (fibre loading 30%).

Fig. 16. Impact-fracture surface of banana fibre/PF composites (fibre loading 30%).


fiber composites. In banana/PF and glass/PF systemsthe tensile strength values are found to be much differ-ent from those predicted by the series model. Thedeviation is higher in glass/PF systems. The deviationsarise because the interaction between the components isnot taken into consideration in the series model.According to this model the fibre and matrix exist astwo components without any adhesion. But in an actualcomposite system there is a chance of interaction

between the components depending on the chemicalnature of the constituents. The interaction is higher in abanana/PF system as is evident from interfacial shearstrength values of pull out tests.A good agreement is observed between the values

obtained from Hirsh’s model and experimental values.Here a parameter x, which determines the stress transferbetween the fibre and matrix, is introduced and so anagreement is observed between the theoretical andexperimental values.

4. Conclusions

The results of the present study reveal that compositeswith good strength could be successfully developedusing banana fibre as the reinforcing agent. From singlefibre pull out tests, the interfacial shear strength (ISS)values are calculated for banana fibre and glass fibre.These values show that ISS is higher in banana fibreembedded in PF than for glass fibre in PF indicating astrong adhesion between the lignocellulosic banana fibreand PF resoles. The tensile stress–strain behaviorreveals that the neat PF is brittle. Addition of fibresmakes the matrix more ductile. The tensile, flexural andimpact properties of the composites are found to bedependent on fibre length and optimum length of fibrerequired to obtain banana/PF composites of maximumproperties is found to be 30 mm for banana/PF com-posites and 40 mm for Glass/PF composites due to thevariation in interfacial properties. Both banana fibreand glass fibre composites are found to have an increasein tensile, flexural and impact properties with increasingfibre loading. The banana/PF composites exhibit super-ior mechanical properties, which can be compared wellwith synthetic fibres like glass in terms of specific prop-erties and could be used as structural material. Theexperimentally obtained tensile strength values ofbanana fibre composites are found to be comparablewith Hirsh’s theoretical predictions

References

[1] Sanadi AR, Prasad SV, Rohatgi PK. Journal of Scientific and

Industrial Research 1985;44:437.

[2] Roe PJ, Ansell MP. Jute reinforced polyester composites. J Mater

Sci 1985;20:4015.

[3] Sanadi AR, Prasad SV, Rohatgi PK. J Mater Sci 1986;21:81.

[4] Heijenrath R, Peijs T. Advanced Compos Lett 1996;5(3):81.

[5] Marchovich N, Reboredo M, Aranguren M. J Appl Poly Sci

1996;61:119.

[6] Zarate CN, Aranguren MI, Reboredo MM. J Appl Poly Sci

2000;77:1832.

[7] Pothen LA, Thomas S, Neelakandan NR. J Reinforced Plast

Compos 1997;16:744.

[8] Bledzki AK, Reihmaine S, Gassan J. J Appl Polym Sci 1996;

59:1329.

Fig. 17. Comparison of experimental and theoretical tensile strengths

of glass fibre reinforced phenolics.

Fig. 18. Comparison of experimental and theoretical tensile strengths

of banana fibre reinforced phenolics.


[9] Achari PS, Ramaswamy R. J Appl Polym Sci 1998;69:1187.

[10] Harikumar KR, Joseph K, Thomas S. J Reinforced Plast Com-

pos 1999;18(4):346.

[11] Sreekala MS, Thomas S, Neelakantan NR. J Polym Eng 1997;

16:265.

[12] Broutman LJ. Polym Eng Sci 1996;6:1.

[13] Hancox NL. J Mater Sci 1975;10:234.

[14] Sanadi AR, Piggott MR. J Mater Sci 1985;20:421.

[15] Laws V. Compos 1982;13:145.

[16] Chua PS, Piggott MR. Compos Sci Tech 1985;22:33.

[17] Penn L, BystryKarpW, Lee S. In: Molecular characterization of

composite interfaces. Ishida H, Kumar G, editors. Plenum, New

York: 1985, p. 93–109.

[18] Matthews FL, Rawlings RD. Composite materials, engineering

and science. Chapman & Hall: 1994 [chapter 10].

[19] Zang M, Zeng H, Zang L, Lin G, Li RKY. Poly Polym Compos

1993;1(5):357.

[20] Bisanda ET, Ansell MP. Compos Sci Technol 1991;41:165.

[21] Wells JK, Beaumont PW. J Mater Sci 1985;20:1275.

[22] Thomason JL, Vlung MA. Composites Part A 1997;28A:277.

[23] Elias HG. Macromolecules. 2nd ed. New York, USA: Plenum

Press; 1984 [vol 2, p. 1173]..

[24] Uma Devi L, Bhagavan S, Thomas S. J Appl Polym Sci 1997;

21:1739.

[25] Kalaprasad G, Joseph K, Thomas S. J Mater Sci 1997;

32:4261.


a comparison of the mechanical properties of phenol formaldehyde composites reinforced with banana...

Documents