EffectofChemicalTreatmentonMechanicaland Water ...downloads.hindawi.com/archive/2012/134683.pdf · 2 ISRN Materials Science the large moisture sorption of natural ﬁbers adversely

International Scholarly Research NetworkISRN Materials ScienceVolume 2012, Article ID 134683, 8 pagesdoi:10.5402/2012/134683

Research Article

Effect of Chemical Treatment on Mechanical andWater-Sorption Properties Coconut Fiber-Unsaturated Polyesterfrom Recycled PET

Nurul Munirah Abdullah and Ishak Ahmad

School of Chemical Science and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia,43000 Bangi, Malaysia

Correspondence should be addressed to Ishak Ahmad, [email protected]

Received 4 January 2012; Accepted 16 February 2012

Academic Editors: E. Collar, A. V. Dos Santos, and H. Yoshihara

Copyright © 2012 N. Munirah Abdullah and I. Ahmad. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Coconut fibers were used as reinforcement for unsaturated polyester resin from recycled PET that has been prepared usingglycolysis and polyesterification reaction. Various concentrations of alkali, silane, and silane on alkalized fiber were applied andthe optimum concentration of treatments was determined. Morphological and mechanical properties of the composite have alsobeen investigated to study the effect of fiber surface treatment. The influence of water uptake on the sorption characteristicsof composites was studied via immersion in distilled water at room temperature. Surface treatment of coconut fiber caused asignificant increase in the tensile properties with the optimum treatment of 0.5% silane on the 5% alkalized coconut fiber/polyestercomposites. For water absorption study, it was observed that the treated fiber composites showed lower water absorption propertiesin comparison to those of untreated fiber composites. SEM investigation showed that the surface modification of fiber has betterfiber-matrix interaction. Thus, chemical treatments on the fiber improve fiber/matrix adhesion especially for silane on alkalizedfiber treatment.

1. Introduction

The increases in the amount of plastics and agriculturalwastes have contributed to various environmental problems.This has attracted the attention of researches to find newideas to solve the problem. Polyethylene terephthalate (PET)is one of the versatile engineering plastics that are used tomanufacture films and bottle for packaging. PET does notcreate any direct hazard to the environment, but due to theincreasing in the plastics waste and its high resistance to theatmosphere and biological agents, it could be considered asa pollutant materials. Therefore, recycling of PET will notonly serve as a partial solution to the solid waste problembut also contribute to the conservation of raw petrochemicalproducts and energy. In other words, product made fromrecycled plastics can results in half energy saving [1]. The useof UPR from recycled PET as a binder is important for theconstruction industry and also helps in the long-term utiliza-tion of PET waste in an effective and economical manner [2].

Source from agricultural waste such as natural fibers haverecently become an excellent reinforcement in compositematerials. Furthermore, these fibers can also be used asreplacement to man-made fiber such as carbon, fiber glass,Kevlar, nylon, and asbestos to make a good ecologic and eco-nomic composite material. Because of their biodegradablenature, natural fibers have been increasingly adopted toreplace synthetic polymers in the industrial applications [3].Natural fibers are very attractive for composite materialsbecause of their low cost, eco-friendly, available in highquantities, renewable, biodegradable, excellent mechanicalproperties, and low density [4]. However, the use of naturalfibers as reinforcement agent, particularly in hydrophobicpolymeric matrices, has some drawbacks such as poor adhe-sion in the fiber/matrix interfacial region. This is because thenatural fibers are highly polar due to the high polysaccharidecontent (hemicelluloses and cellulose), making the lignocel-lulosic fibers to also be highly hydrophilic [5]. Moreover,

2 ISRN Materials Science

the large moisture sorption of natural fibers adversely affectsadhesion with hydrophobic matrices that may cause materialdegradation and loss of strength [6].

The aim of this research is to use coconut fiber (CF) asfibrous reinforcement in unsaturated polyester resin (UPR)synthesized from glycolyzed product of waste PET. CF isobtained from the extraction external layer of the exocarpand from the endocarp of the fruit [7]. CF is one of the manyagricultural wastes due to the use of and demand for coconutpulp. So, waste CF could be used as a useful advantage for theproduction of composites in the material science area. How-ever, as other natural fibers, coconut fiber is also found to bea poor reinforcement due to the large and variable diameter,high microfibril angle, and high lignin and hemicellulosecontent [8]. Despite the fact that the natural fibers have somedisadvantages, it can be solved via various fiber-matrix inter-face modifications that can improve performance of theresulting composite [9]. The objective of this study is to ana-lyze the three most used treatments on natural fibers: alkali,silane, and silane on alkalized CF treatment as a function ofthe variation concentration of the treatment. Despite the factthat a lot previous work was done studying the same treat-ments, most of the work did not take into account the varia-tion of concentration of the treatment. Therefore, this inves-tigation seeks to find the optimum concentration of fibersurface treatment of CF and to study the effect of the treat-ment on the mechanical, water sorption, and morphologyUPR/CF composites based on recycled PET.

2. Experimental

2.1. Material. Drinking bottles of PET were collected andthen grounded into fine pieces. Glycolysis of PET was donetogether with ethylene glycol and zinc acetate. Both chemi-cals were supplied by MERCH and Riedel-de Haen. Maleikanhydride, hydroquinone, and styrene monomer fromMERCK were used for the synthesis of unsaturated polyesterresin. Curing process of UPR was done by using Methyl ethylketone peroxide (MEKPO) as accelerator and Cobalt octoateas an initiator, respectively, supplied by Revertex Sdn. Bhd.and Aldrich Co. The coconut fibers (CF) that act as a rein-forcing agent were obtained from Fibromat (M) Sdn. Bhd.To modify the surface of CF by chemical treatment, Sodiumhydroxide (SYSTERM) and silane (supplied by Dow Corn-ing) were used.

2.2. Glycolysis of Waste PET. Fine pieces of PET were put intoa four-necked flat bottom flask with the capacity of 1 L. Eth-ylene glycol was added into the flask at the ratio of PET : EG =1 : 4 (w/w) together with 0.5% zinc acetate based on weight ofPET as a catalyst. Then, magnetic stirrer was put into the flaskto ensure the mixture occurs uniformly. This was carriedout under reflux in nitrogen atmosphere with temperatureat 190◦C for 8 h by using heated silicon oil.

2.3. Preparation of Unsaturated Polyester Resin. Unsaturatedpolyester resin (UPR) was prepared by using polyesterifica-tion reaction. Glycolysed product from glycolysis reaction

(a)

(b)

Figure 1: Aspect of the coconut fibers: (a) tangled fibers and (b)pressed mat.

was reacted with maleic anhydride at a fixed ratio of 1 : 1 forthe hydroxyl to carboxyl groups. The polyesterification wascarried out in a 1 L four-necked round bottom flask connect-ed to a reflux condenser under nitrogen gas at 200◦C. Theexperiment was done by heating the reactant in an oil bath.The acid value was monitored during the reaction and det-ermined by the titration of 0.1 N NaOH into the weighedresin in 100 mL acetone according to ASTM D 1639-90method. The reaction was stopped when the acid value re-ached 29–34 mg of KOH/g. The liquid resin was then cooledto 120◦C and 0.45% hydroquinone based on weight PET wasadded to prevent the precuring of the resin. Finally, the liquidresin was dissolved in styrene monomer to achieve a 40%w/w styrene in the resin that equals to commercial UPR [10].

2.4. Fiber Treatments. Modification on the CF has been doneto get a uniformed thickness of fiber mat. CF in the tangledform (Figure 1(a)) was vacuumed and compressed with athickness around ∼1-2 mm. CF in the form of unwoven matwas cut according to the size wanted. Before specific treat-ments were carried out, the fiber was first rinsed with distill-ed water to remove the dirt on the fiber surface. Washed CFwas left to dry at room temperature and finally dried in ovenfor 2 hours at 60◦C. These fiber mats were used for the pre-paration of untreated and treated fiber composites as shownin Figure 1(b).

2.5. Alkali Treatments. CF has been treated with various con-centrations of alkali between 2.5% to 10% (w/w). The fiber

ISRN Materials Science 3

Table 1: Percent concentration of the chemical treatment of eachsample.

SampleConcentrations (%)

Alkali SilaneSilane on

Alkalized CF

A 2.50 0.25 2.50 + 0.25

B 5.00 0.50 5.00 + 0.50

C 7.50 0.75 7.50 + 0.75

D 10.00 1.00 10.00 + 1.00

was immersed in alkaline solution for 2 hours then neutrali-zed with acetic acid and washed with distilled water repeat-edly until all sodium hydroxide was eliminated. Finally, thefiber was washed with distilled water and dried at room tem-perature for 48 h [11].

2.6. Silane Treatments. Various concentrations of silane solu-tion were used in fiber treatment, ranging between 0.25% to1% w/w. For each concentration, silane was dilute with disti-lled water and stirred using a glass rod for 15 minutes to forman aqueous solution. The silane solution was then addedwith acetic acid until pH of solution becomes 4 in order tooptimize the performance of reinforcing material. CF wasimmersed in the solution for an hour. Finally, the fiber waswashed with distilled water and dried at room temperaturefor 2 days.

2.7. Silane Treatment on the Alkalized Fiber. CF that has beenpretreated with alkali solution was immersed in dilute silanesolution using the same method as for silane treatment of CF.Percentage of different concentrations used for each sampleis shown in Table 1.

2.8. Preparation of the Composites. In order to prepare CFmat/polyester composites, hand lay-up method was applied.A stainless steel mould with the dimensions of (235 × 235 ×235) mm3 and 3 mm of thickness was used. The curing of thepolyester resin was done by the incorporation of 0.75 wt.%cobalt octoate and 1.5 wt.% methyl ethyl ketone peroxide(MEKPO) as a initiator and accelerator, respectively. Themixture was then poured on the mat form of CF. The volumefraction of CF in the composite, Vf , was approximately 0.30with the thickness of specimens was 3 mm. The compositewas cured at room temperature for 24 h before removing itfrom the mold. Finally, the composite was post-cured in theoven at 60◦C for 6 h.

2.9. Analysis and Testing. Tensile test on composites was de-termined according to ASTM D-638-91 standard. The mea-surements were done using Instron Universal Tensile Machi-ne (Model 5567) at a speed of 5 mm/min at room tempera-ture.

The impact test was carried out using Universal Fracto-scope (CEAST) 6546/000 impact tester according to ASTMD256-88 at room temperature. It was used to test the strengthof composite samples when the load was suddenly applied.

0

5

10

15

20

25

Untreated A B C D

Ten

sile

str

engt

h (

MP

a)

Untreated

Alkali

Silane

Silane on alkalized CF

Figure 2: Tensile strength graph for the various treated CF/UPRcomposites.

Fiber O−-Na+ +

HOFiber O− Na+ + H2N CH2CH2CH2 Si

NH2 + NaOH

HO

HO

Fiber O Si

HO

HO

CH2CH2CH2

Fiber OH + NaOH H2O

Figure 3: Possible reaction mechanism during the silane treatmenton the alkalized CF.

At least 7 samples were tested for each mechanical measure-ment.

Rectangular specimens with dimensions of (25.4 × 76.2)mm2 were cut from each sample. 7 replicate specimens weretested and the results were presented as an average of thetested specimens. The samples were dried in an oven at 60◦Cfor 24 h, cooled in desiccators, and immediately weighed tothe nearest 0.001 g. In order to measure the water absorptionof the composites, all samples were immersed in water forabout 24 h at room temperature as described in ASTM D570-99 procedure. Excess water on the surface of the sampleswas removed before weighing. Weight change was deter-mined by means of a microbalance calculated at 10 mg [12].

The scanning electron micrograph of fracture surfacesfollowing the tensile test of composite materials was takenusing scanning electron microscope (SEM), Philips XL30.The samples were first dried in oven to remove air moistureand then coated with a thin layer of gold by using SputterCoater machine. Examination of microscopy has been doneat 500x magnification with the highest voltage of 20 V.

3. Results and Discussion

3.1. Tensile Test. Mechanical properties of composites werestrongly influenced by the adhesion between the matrix, thefibers, and interphase properties. Due to the presence of hy-droxyl groups in natural fibers, this makes the fiber to be of


0

0.5

1

1.5

2

2.5

3

Ten

sile

str

ain

(%

)

Untreated A B C D

Untreated

Alkali

Silane


Figure 4: Tensile strain graph for the various treated CF/UPRcomposites.

high moisture absorption and leads to the poor wettabilityand weak interfacial bonding between fibers and hydropho-bic matrices. Therefore, in order to produce a composite withbetter mechanical strength, a modification on the fiber sur-face by using chemical treatments is needed to make thefibers more hydrophobic [13]. Figure 2 shows the analysisresults of tensile strength for untreated CF and treated CFcomposites. It can be seen that the tensile strength increasedwith the treatment. This increase in tensile strength indica-tes that modifying the surface of CF improved the interfacialadhesion between CF and polyester matrix. On the otherhand, untreated fiber composite has the lowest tensile streng-th which is due to the weak compatibility between fiber andmatrix. Figure 2 also shows that optimum concentration isobtained for sample B for all treatments. In sample B, the ten-sile strength increased 34% for alkali treatment in compari-son to the untreated CF composites. This is an indication thatalkali treatment improves the fiber-matrix adhesion becauseof the removal of natural and artificial impurities. Moreover,during the alkali treatment, NaOH reacts with hydroxylgroups of the cementing material hemicelluloses and it leadsto the destruction of the cellular structure and, therefore, thefibers splits into filaments. This phenomenon is called fib-rillation, which breaks the untreated fiber bundle down intosmaller fibrils by the dissolution of the hemicelluloses. Thisfibrillation then increases the effective surface area availablefor contact with the matrix. In addition, alkali treatment re-duces fiber diameter and thereby increases the aspect ratio,improving mechanical characteristics of the resulting com-posites [14].

Silane treatment on the surface of CF also increases thetensile strength of the composite with the improvement inthe tensile strength is 28%. Silane is used as coupling agentsto allow natural fiber to adhere to a polymer matrix, thus sta-bilizing the composite material. Silane coupling agents mayreduce the number of cellulose hydroxyl groups in the fib-er-matrix interface. Silane can be hydrolyzed into silanolwhen it reacts with moisture in the fiber. This silanol groupsreacts with the hydroxyl groups on the surface of fiber, form-ing a stable covalent bond on the cell wall. Meanwhile, the

0

1

2

3

4

5

6

7

Impa

ct s

tren

gth

(kJ

/m2)

Untreated A B C D

Untreated

Alkali

Silane


Figure 5: Impact strength graph for the various treated CF/UPRcomposites.

Wat

er u

ptak

e (%

)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Untreated A B C D

Untreated

Alkali

Silane


Figure 6: Water uptake graph for the various treated CF/UPR com-posites.

remaining silanol groups are adequate of hydrogen bondingor condensing with adjacent silanol groups (Si–O–Si). There-fore, the hydrocarbon chain of polymerized silane can adhereto the matrix mainly because of the van der Waals type of at-tractive forces. As a result, silane-coupling agent forms a bri-dge at the interface and improves the adhesion between fiberand matrix [15].

However, chemical modification of fiber surface withonly alkali treatment was not enough to increase the strengthof composite materials. Further treatment with a silane cou-pling agent solution to provide improvement adhesion be-tween fiber and matrix by chemical interaction was neces-sary. The improvement of tensile strength for silane treatedon alkalized CF composites is higher than that untreatedCF composites with a 41% increase. The increase in tensilestrength of the silane treatment on alkalized CF compositesis due to the improvement of adhesion between the CF andthermoset matrix. Moreover, due to the intra- and interfib-rillar swelling of alkali treatment during first step of pretreat-ment and the present of covalent bonds between the fiberand matrix after the silane treatment, composites treatedwith silane on alkalized CF have the highest improvement


(a) (b)

(c) (d)

Figure 7: SEM micrographs of fiber surface for the (a) untreated CF, (b) alkali treated CF, (c) silane treated CF, and (d) silane treated onalkalized CF.

in the mechanical performance [16]. The proposed chemicalreaction during alkali treated fiber followed by silane treat-ment is as shown in Figure 3.

The tensile strain of CF/UPR composite as a function ofvariation fiber surface treatment is shown in Figure 4. It canbe seen that untreated CF composite has the lowest tensilestrain in comparison to all treated CF composites. Based onFigure 4, silane treated on alkalized CF composite for sampleB gave the highest value of tensile strain with the optimumconcentration. It can be noted that composites reinforced bysilane on alkalized CF were characterized by a little impro-vement in their resistance (higher tensile strength). Duringthe reaction of silane treatment on alkalized CF, silanol form-ing stable covalent bond grafted between the microfibrils andleading to a high increase in the tensile strain. It is suggestedthat by providing additional sites of mechanical interlocking,silane treatment on alkalized CF leads to the improvement ofthe CF/UPR composites [17].

3.2. Impact Test. The impact strength of a composite is influ-enced by many factors, including the toughness properties ofthe reinforcement, the nature of the interfacial region, andfrictional work involved in pulling out the fiber from thematrix. The nature of the interface region is the most impor-tant and is directly related to the toughness of the composite.The impact test is to determine the amount of energy absorb-ed by a material during fracture [18]. Figure 5 shows theimpact strength of CF/UPR composites with variation of fib-

er surface treatment. Relative to the untreated CF composite,the values of treated CF composites are higher. These resultssuggest that the treatment had improved adhesion betweenthe CF and UPR matrix and can act to prevent the prop-agation of cracks generated during the impact test. The fi-ber/matrix adhesion mainly determines the strength of com-posites. Improved adhesion after treatment can lead to per-fect bonding and thus the failure of composites can occurat relatively high impact strength. Composite having betterinterfacial bonding need more energy absorbing capacitywhen the crack propagation occurs along the fiber/matrixinterface and this leads to a significant increase of impactstrength [19]. The highest value of impact strength is forsilane treated on alkalized CF composites with sample B is theoptimal treatment and these are similar to the tensile analysisresults. Further treatment with silane coupling agent on alka-lized CF has induced chemical interaction between the fiberand the matrix. Hydroxyl group on the cellulose fibers couldinteract better with the silane due to the availability of pos-sible reaction sites in large numbers. Hence, chemical treat-ment with silane on alkalized CF has increased the mechani-cal properties of composite from the aspects of tensile andimpact strength.

3.3. Water Absorption. The water absorption behaviors ofuntreated and surface-modified CF polyester composites areshown in Figure 6. The extent of water absorption decreasesconsiderably on surface modification of CF. Treated CF


(a) (b)

(c) (d)

Figure 8: SEM micrographs of tensile fracture surface for the (a) untreated CF composite, (b) alkali treated CF composite, (c) silane treatedCF composite, and (d) silane treated on alkalized CF composite.

shows considerable reduction in water absorption, whichagain confirms a better adhesion between treated CF withpolyester resin. In case of untreated CF/UPR composites thepoor wettability and adhesion between untreated CF towardspolyester resin are due to hydrophilic nature of CF. This hy-drophilicity is responsible for the higher percentage wateruptake in untreated CF composites. Meanwhile, in case ofsurface modified CF composites, the fibers get masked withthe polyester resin in the laminate with a stronger adhesion,resulting in greater hydrophobicity and lesser water absorp-tion [20]. As it can be observed in Figure 6, silane treatedalkalized CF composite shows lesser water absorption in allsample. The order of decreasing value of water absorptioncomposites is as follows: untreated < silane < alkali < silaneon alkalized CF. After treatment, the first layer of fiber sorp-tion has been modified by creation of a water resistant resin.It can be observed that the silane treatment on alkalized CF,which is the most efficient in terms of moisture resistance,is characterized by the lowest polarity parameter. Therefore,silane treatment on alkalized fiber had enhanced the fiber-matrix adhesion and led to a most hydrophobic surface [21].

From the Figure 6 for sample B, the water uptake of alkalitreated CF composite (1.87%) showed close values to silanetreated on alkalized CF composite (1.90%). Other than theremoval of impurities such as waxes, pectins, and mineralsalts as well as treatment with NaOH, pretreatment withalkali promote the activation of hydroxyl groups of cellulose

unit by breaking the hydrogen bond [22]. Thus, after goingthrough the silane treatment, a stable covalent bond has beenaccepted between fiber and matrix made the surface topologyof fiber vary and the fiber become more hydrophobic innature leading to good fiber/matrix interaction.

3.4. Scanning Electron Microscopy. Scanning electron micro-scopic analysis examined the surface morphology of treatedand untreated CF. The removal of surface impurities on plantfibers is advantageous for fiber-matrix adhesion as it facili-ties both mechanical interlocking and the bonding reaction[23]. Figure 7 shows the SEM images of CF surfaces beforeand after modification with alkali, silane, and silane on alka-lized CF. After the alkali treatment (Figure 7(b)), the surfaceof the fiber becomes rougher and micropores were morevisible compared to untreated fiber. Alkali-treated CF sur-face showed rough surface topography due to the removalof hemicelluloses, lignin, and amorphous waxy layer.Figure 7(d) shows the SEM micrograph of silane treatmenton alkalized CF surface structure. The surface of thefiber becomes rougher in comparison to untreated fiberand micropores were not so apparent in comparison toalkali treatment and silane treatment. Cellulose is generallyunreactive to many chemicals and the OH groups of themicrofibrils have very low accessibility. Based on this fact, anoptional step is to activate the alkoxysilane by hydrolyzingthe alkoxy groups, thereby forming a more reactive silanol


groups. As a result, the silanol may react with the hydroxylgroups of fibers and facilitate the enhancement of interfacialadhesion of treated fibers and polymer matrices [24].

Interface study of untreated and surface-modifiedCF/UPR composites was also carried out to investigate the fi-ber-matrix interaction by scanning electron microscopy.Figures 8(a)–8(d) display the SEM micrographs of the un-treated, alkali-treated fiber, silane-treated fiber, and silanetreated on alkalized CF composites fracture surface respec-tively. In other words, it appears that the fibers are extractedfrom the matrix. These clearly indicate that the interfacialadhesion between the fiber and the matrix is poor for un-treated composites.

However, treated fiber composites (Figure 8(b) to 8(d))have a different fracture surface in which the matrix debond-ing cannot be properly seen. It also can be seen that polyesterresin adhered to the fiber surfaces of treated fiber composite.This may support good adhesion between treated fiber andpolyester matrix. In the case of silane treated on alkalizedCF composite, a large amount of polyester resin was adheredmore to the fiber surfaces clearly and this indicates silanetreatment on alkalized CF leads to better adhesion betweenthe fiber and the polyester resin. From SEM observations, itcan be concluded that modification of alkalized CF with si-lane coupling agent improved the adhesion between the fiberand the matrix. Therefore, the silane treated on alkalized CFcomposites has the highest tensile and impact strength incomparison to untreated and other treated fiber composites.

4. Conclusion

Coconut fiber has been treated with different chemicals; suchas alkali, silane, and silane on alkalized coconut fiber at dif-ferent concentrations. It may be pointed out that silane treat-ment on alkalized coconut fiber results in an increase in ten-sile strength as well as tensile strain. Moreover, from impactresults, silane treatment on alkalized coconut fiber leadsto the increase in coconut fiber/unsaturated polyester resincomposites, which can be related to the effect of improvedinterfacial adhesion between coconut fiber and polyestermatrix. The water absorption studies showed that treatmentdecreases the water uptake of the composites, which supportsa better fiber/matrix interaction. The SEM of the treatedfibers revealed that the fiber became porous and fibrillat-ed by alkali, silane, and silane on alkalized CF. The micro-scopic investigation of the fracture surfaces of compositesalso revealed enhanced bonding between the matrix and thefiber.

Acknowledgments

The authors would like to extend their utmost gratitude tothe Ministry of Higher Education (MOHE), Malaysia, forfunding the project under FRGS UKM-ST-07-SF0153-2010and for awarding a National Science Fellowship Scholarshipto N. Munirah. The authors would also like to acknowledgethe Polymer Research Centre (PORCE) of Universiti Keban-

gsaan Malaysia (UKM) for their support in providing theinfrastructure for the project.

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