Materials and Design 59 (2014) 279–286

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Materials and Design

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Mechanical, thermal and morphological properties of durian skin fibrereinforced PLA biocomposites

http://dx.doi.org/10.1016/j.matdes.2014.02.0620261-3069/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +60 361965752; fax: +60 361964477.E-mail address: hazleen@iium.edu.my (H. Anuar).

M.R. Manshor a,b, H. Anuar a,⇑, M.N. Nur Aimi a, M.I. Ahmad Fitrie a, W.B. Wan Nazri b, S.M. Sapuan c,Y.A. El-Shekeil c, M.U. Wahit d

a Department of Manufacturing and Materials Engineering, Faculty of Engineering, International Islamic University Malaysia, Jalan Gombak, 53100 Kuala Lumpur, Malaysiab Food Technology Research Center, Malaysian Agriculture Research and Development Institute, 43400 Serdang, Malaysiac Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysiad Center for Composites, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 October 2013Accepted 28 February 2014Available online 12 March 2014

Keywords:Durian skin fibrePolylactic acidPre-treatmentThermogravimetryDifferential scanning calorimetry

Durian skin waste generated by durian fruit or Durio zibethinus Murray show potential as a new reinforce-ment based-natural fibre. Similar to other lignocellulosic fibre, durian skin fibre (DSF) is capable inreinforcing polylactic acid (PLA) through extrusion and injection moulding processes for various applica-tions. In current study, the effects of fibre content and pre-treatment using 4% sodium hydroxide (NaOH)on DSF were investigated on impact and thermal properties of PLA biocomposites. Treated DSFsignificantly enhanced the properties of PLA biocomposites as compared to untreated biocomposite.PLA can be replaced by 30 wt% DSF for similar impact performance. Thermogravimetry analysis (TGA)demonstrated that pre-treated DSF improved the thermal stability of PLA biocomposite. Differential scan-ning calorimetry (DSC) showed the presence of pre-treated DSF minimally enhanced the glass transitiontemperature (Tg), crystallization temperature (Tc) and melting temperature (Tm) relative to untreated DSFwhich suggests on better reinforcement with NaOH pre-treatment.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In an effort to supplant the petroleum-based polymers andreduce waste-related environmental problems, biopolymers arethe best candidate due to their renewability, biodegradabilityand commercial viability. Polylactic acid (PLA) is among thecommodity biopolymers that are comparable in terms of proper-ties and performances to polyolefin. Life cycle assessment of PLAreported that it is possible to reduce green house gas emission,yet it is subjected to the raw materials of PLA, process and sourceof energy used [1].

Development of polylactic acid filled with natural fibre obtainedfrom biomass will produce biocomposite while maintaining trulygreen characteristics, compensating for the expensive cost of PLA.Durian skin fibre (DSF) is a natural fibre extracted from crops ofDurio zibethinus Murray, or durian, which is obtainable in SoutheastAsian countries such as Thailand, Indonesia and Malaysia.According to the Malaysia Ministry of Agriculture and Agro-BasedIndustry statistical crop data, the production of durian fruit in

Malaysia in 2013 is estimated to be about 320,164 MT (metricton). The production in 2013 is about 20,000 MT higher than in2012. From each durian, only 50–65% flesh is consumed as food.The rests which is about 45–55% can be described as waste whichincludes skin, seed, etc. Out of 250,000 MT, 60–70% which is equiv-alent to 85,000 MT of waste, is actually collected. In general, from1 kg of durian skin waste, approximately 40% of durian skin fibremay be generated.

This waste can be potentially turned to benefit biomass throughincorporation as a filler or reinforcement into polymer matrix. As areinforcing agent, the presence of natural fibre in polymeric mate-rial should be able to enhance the performance of virgin materialsapart from reducing the density and lowering the cost of endproducts. Like other natural fibres, durian skin fibre consists ofcellulose, hemicelluloses, lignin and extraneous components.Durian fruit and its cross-sectional parts are shown in Fig. 1.

Regardless of the type of natural fibre chosen as reinforcementin PLA matrix, the properties of the resultant composite are mainlyinfluenced by the interaction between the hydrophilic natural fi-bres and hydrophobic matrix [2,3]. Compatibility of the combinedconstituents is the key issue in ensuring the performance of thedeveloped biocomposite. Surface modification of natural fibre istherefore needed to avoid incompatibility and promote bonding

Fig. 1. Durian fruit and its cross-section.

280 M.R. Manshor et al. / Materials and Design 59 (2014) 279–286

during compounding of natural fibre and biopolymer. Incompati-bility problems may arise due to the presence of highly polarnatural fibre and non-polar PLA matrix. As a consequence it willlead to difficulties in the dispersion of fibres and poor interfacialadhesion between fibre and matrix [4]. Surface treatment alsocleans the fibre surface, chemically modifies the surface of naturalfibre, stops moisture uptake and increase the surface roughness ofnatural fibre [5].

Although polymer composite is easily found in open literature,research on durian skin fibre reinforced polymer specifically poly-lactic acid has never been explored. This paper presents a novelidea of utilising durian skin waste as reinforcing fibre for polylacticacid. The effect of surface modification via alkali treatment and theeffect of fibre loading on the impact strength and thermal proper-ties of PLA–DSF biocomposites are discussed.

2. Experimental details

2.1. Materials

Polylactic acid (PLA) (grade 3051D) produced by NatureWorks�,China was used in this study. The density, melting temperature andglass transition temperature of PLA are 0.998 g/cm3, 152.3 �C, and57–61 �C, respectively. Durian skin waste was collected from localmarket and processed in our laboratory before used as reinforce-ment. Preparation of durian skin fibre involved washing, chopping,

grinding, drying and sieving to get appropriate range of fibre size.Details of fibre preparation are shown in Fig. 2.

2.2. Durian skin fibre characterization

2.2.1. Moisture contentThe moisture content of durian skin fibre (DSF) was determined

according to ASTM: D4442-07. Durian skin fibre (DSF) wasweighed and dried in an oven at 105 �C for 30 min. The moisturecontent of the test specimen was expressed as a percentage lossin weight of the final oven-dry weight using Eq. (1):

Moisture content ¼ W0 �W1

W1

� �� 100% ð1Þ

where W0 is weight of fibre before dried in an oven and W1 isweight of fibre after dried in an oven.

2.2.2. Fibre length, diameter and fibre aspect ratioDimension of fibre was determined by using scanning electron

microscope (SEM) model Quanta 200. About 50 DSF length anddiameter were measured. The average diameter and length werecalculated by using Eqs. (2) and (3):

Diameter of fibre ¼ D1 þ D2 þ . . . Dn

nð2Þ

Fig. 2. Preparation of durian skin fibre.

M.R. Manshor et al. / Materials and Design 59 (2014) 279–286 281

Length of fibre ¼ 11 þ 12 þ . . . 1n

nð3Þ

where D is the fibre diameter, n is the total number of fibres thatwere measured and l is the fibre length.

The aspect ratio (ratio between length and diameter) was thencalculated because it is one of the most important parameters indetermining reinforcing capability of the fibres. Aspect ratio offibres was determined using the relationship in Eq. (4):

Aspect ratio of fibres ¼ LengthDiameter

ð4Þ

2.2.3. Fibre densityDensity of DSF was determined by using Archimedes principle.

The test was conducted according to ASTM: D3800M-11. DSF waspelletized after dried at 60 �C for 72 h using uniaxial pellet press.Then, densitometer was used to measure the density of durian skinfibre. Thus, the density of the fibre obtained was estimated usingEq. (5):

Density; q ¼ Mass of ample; mVolume of sample; V

ð5Þ

2.2.4. Durian skin fibre pre-treatmentThe dried DSF was immersed in a solution containing 4 wt%

NaOH, where the ratio of fibre:solution was 1:10 (by wt%) at roomtemperature. The mixture was then stirred for 1 h.

2.3. Biocomposite preparation

PLA–DSF composite was extruded using Haake 600P twin screwextruder at 190 �C with screw speed of 250 rpm. The compositionof composite prepared varied from 0 to 40 wt% DSF content. Afterextrusion, PLA–DSF biocomposites were injection moulded forimpact and flexural specimens as per ASTM standard usingBATTENFIELD HM 600/850 injection moulding machine at 800 barspressure, 20 s cooling time at 170 mm/s1 speed. UTC is referring tountreated PLA–DSF, while TC is referring to treated PLA–DSFbiocomposites.

2.4. Biocomposites characterization

2.4.1. Impact testCharpy impact test was conducted according to ASTM: D256-10

to characterize the impact property of PLA–DSF composite usingDynisco Polymer Test, Sismatic OP7 machine. The velocity andweight of the hammer were 3.67 m/s and 0.898 kg, respectively.

2.4.2. Flexural testFlexural test was performed on Lloyd (Ametec) universal testing

machine according to ASTM: D790-10. The load cell was 10 kN andthe span length was 100 mm.

2.4.3. Scanning electron microscopeScanning electron microscope (JEOL, JSM-5600) was used to ob-

serve the fracture surface after the impact test for both treated anduntreated PLA–DSF biocomposite. The fracture ends of the samples

0 10 20 30 40

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Impa

ct S

treng

th (k

J/m

2 )

DSF Content (wt%)

UTC TC

Fig. 3. The effect of DSF content and pre-treatment on the impact strength of PLA–DSF biocomposite.

282 M.R. Manshor et al. / Materials and Design 59 (2014) 279–286

were mounted on an aluminium stub and coated with a thin layerof gold to avoid electrostatic charging during examination.

2.4.4. Thermogravimetry analysis (TGA)Thermogravimetry analysis (TGA) was conducted using TA

Instruments TGA Q500 analyzer based on ASTM D3850. About8–10 mg biocomposites were used for TGA analysis. The sampleswere heated from 30 �C to 1000 �C at a heating rate of 10 �C/min.

2.4.5. Differential scanning calorimeter (DSC) analysisDSC of PLA biocomposites was performed using TA Instruments

DSC Q1000 analyzer. About 7–9 mg biocomposites were used forDSC analysis. Measurements of glass transition temperature (Tg)and crystallization temperature (Tc) were recorded as a functionof temperature in the range of 25–200 �C with heating rate of10 �C/min. The degree of crystallinity (Xc %) of biocomposites wasdetermined using the Eq. (6):

Xc % ¼ DHm � DHc

93:1� 100 ð6Þ

where DHm is enthalphy of fusion and DHc is enthalphy of crystal-lization and DHo, enthalphy of fusion of perfectly 100% crystalline istaken as 93 J/g [6].

3. Results and discussion

3.1. Durian skin fibre

Non-wood biofibres from plant can be classified into trunk/stem/frond, bast, straw, leaf, seed/fruit and grass [7]. Therefore, itis valuable to compare the characteristics of durian skin fibre with-in its group; fibre obtained from fruit. Physical properties of DSFwere determined and presented in Table 1. Table 1 also tabulatedcomparison of DSF in terms of length-to-diameter (L/D) ratio,density and moisture content with other types of fruit fibres.Generally, DSF is comparable to other fruit-based natural fibresas it shows within the range of properties as other natural cellulosefibre. Therefore DSF is a potential reinforcement for biopolymer toproduce natural fibre composites [8].

3.2. Impact strength of PLA–DSF biocomposite

The effect of durian skin fibre content from 0 to 40 wt% and pre-treatment of DSF on the impact strength of PLA composites areillustrated in Fig. 3. The presence of DSF at lower fibre content(10 and 20 wt%) reduced the impact strength for both treatedand untreated biocomposites. It is worth noting that the impactstrength for pre-treated DSF composite was slightly higher thanUTC and unreinforced PLA. By comparing at 30 wt% fibre content,the impact strength for TC is about 20% higher than UTC. Incorpo-ration of DSF from 0 to 40 wt% into PLA matrix minimally modifiedthe impact strength of PLA composite, and this agreed well withthe previous reported results on the impact strength of naturalfibres composites [9,10].

It is generally known that composite materials depend more onmatrix to absorb the impact energy. However, in the case of brittlepolymer like PLA, fracture propagation energy is smaller in

Table 1Characteristics of fruit-based natural fibres.

Natural fibre Moisturecontent (%)

Diameter(lm)

Length(mm)

Density(g/cm3)

8–9Coir 8.00 250 30 1.2010–11Empty fruit bunch 2.20–9.50 250–610 0.89–0.99 0.7–1.55Durian skin 7.92–9.10 170–447 0.84–2.38 1.24

comparison to the total energy required to initiate the fracture ofthe specimen, to propagate the fracture across the specimen andto throw the free end of the broken specimen. In reinforced com-posite, the presence of fibres is also expected to absorb the energythrough mechanical friction, fibre pull-out and debonding of the fi-bres from the matrix. In this study however, the average measuredlength as seen in Table 1 for DSF is about 1475 lm, thus DSF can beclassified as short fibre. The utilization of short fibre tends toreduce the elongation at break, thus reduce the impact absorbingenergy. Stress concentration around fibre ends or areas of pooradhesion of fibre is also potential to reduce the impact strength[11]. The finding is also in agreement with Yang et al. [12] who re-ported on the decrement of impact strength with filler content.This can be attributed to the poor interfacial bonding whichinduces micro spaces between filler and matrix thus causing mi-cro-cracks when impact occurs as seen in Fig. 4(a–c), and conse-quently inducing crack propagation easily. Apart from brittlematrix, high fibre loading and poor fibre–matrix interface, incon-sistent DSF dimension that are observed under SEM micrographsin Fig. 4(a–f) may also contribute to the low impact strength ofPLA–DSF biocomposites.

In the case of pre-treated DSF with NaOH, positive effect of pre-treatment was observed. It was reported that the alkalizationtreatment also known as mercerization, may produce high-qualityfibres which subsequently results in better fibre–matrix adhesionand increased mechanical properties [13]. Additionally, removalof the pectin and waxy materials from the surface of the DSFenhanced the interfacial strength between DSF fibres and PLAmatrix [14,15]. Recent studies on PLA/kenaf biocomposites alsoreported that maximum impact strength was achieved as kenaffibre was pre-treated with 6% NaOH [16].

3.3. Flexural properties of PLA–DSF biocomposite

Flexural strength is the ability of material to withstand bendingforces applied perpendicular to its longitudinal axis. The effect ofdurian skin fibre content on the flexural strength and modulus ofPLA–DSF biocomposite are presented in Fig. 5(a and b). Similartrend is seen for both treated and untreated PLA–DSF biocompositewhere the presence of DSF significantly dropped the flexuralstrength particularly at lower fibre content. It is generally knownthat the properties of the fibre and matrix, fibre-to-matrix ratio,

Fig. 4. Scanning electron micrographs for UTC at (a) 20 wt%, (b) 30 wt%, (c) 40 wt% and TC at (d) 20 wt%, (e) 30 wt%, (f) 40 wt% of PLA biocomposites at 500� magnification.

M.R. Manshor et al. / Materials and Design 59 (2014) 279–286 283

mode of fabrication and interfacial adhesion are the main factorsthat determined the mechanical perormance of the compositeproduced. Therefore as revealed under SEM micrographs in Fig. 4,the biocomposite is easily broken and could not resist the bendingforces due to fibre-to-fibre interaction, agglomeration of fibredispersion pattern and poor fibre orientation which may lead tothe formation of voids during composite fabrication.

It is worth noting in Fig. 5(b) that flexural modulus of treatedbiocomposite with 30 wt% DSF shows the highest value at35 GPa. The value is about 22% and 4% higher than unreinforcedPLA and untreated DSF biocomposite, respectively. Flexural modu-lus for PLA/DSF obtained is far higher than PLA/kenaf fibre [9], PLA/abaca and PLA/cellulose [17]. As the lignin is removed from thecellulose structure during pretreatment, it provides enoughflexibility to the biocomposite. The inter-fibrillar region became

Fig. 5. The effect of DSF content and pre-treatment on the (a) flexural strength and(b) flexural modulus of PLA–DSF biocomposite.

Fig. 6. TG and DTG curve

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less dense and less rigid making the fibrils capable of rearrangingthemselves along the direction of applied stress [18].

3.4. Thermogravimetry analysis (TGA) of durian skin fibre

It is known that the pyrolysis of biomass like durian skininvolves several weight loss steps in a thermogravimetry (TG)curve as a result of the removal of moisture and decompositionof the major components of the biomass such as hemicelluloses,cellulose and lignin. Besides the chemical composition, thermalanalysis also depends on the structure and degree of crystallinityof the natural fibres [19]. TG curve for DSF is shown in Fig. 6.

Three distinct decomposition peaks can be seen from the deriv-ative TG (DTG) curve in Fig. 6. The first decomposition peak atapproximately 100 �C is due to the removal of moisture from thesamples. The second peak at about 220 �C is due to the decompo-sition of hemicelluloses while the final peak at approximately330 �C is due to the decomposition of cellulose and lignin. Investi-gation on agricultural waste using TGA found that the final peak ormajor peak with a maximum decomposition temperature at336.7 �C was attributed to cellulose and lignin decomposition[20]. The ash which remained at the end of the TG analysis forthe sample is about 25% of the total sample weight. The breakdownof the durian samples constituents as analysed from the TG curve istabulated in Table 2.

3.5. Thermal stability of PLA–DSF biocomposite

The effect of temperature over the mass of PLA biocomposites isillustrated in Fig. 7. There are two degradation regions observedfrom Fig. 7. The first region is due to the thermal degradation ofcellulose, hemicelluloses and lignin in DSF [21,22]. The second re-gion at higher temperature is associated to depolymerization ofPLA [23].

It is evident from Fig. 7 that alkali treatment improved the ther-mal stability of biocomposite. The T15% and char residue at 600 �Cfor PLA biocomposites are tabulated in Table 3. The presence of

of durian skin fibre.

Table 2Composition of durian skin from TG curve.

Composition (wt%)

Step 1 Step 2 Step 3 Residue(moisture) (hemicellulose) (cellulose and lignin) (ash)

7 15 52 25

Fig. 7. TG curves for PLA biocomposites.

Table 3Thermal properties of PLA biocomposites obtained from thermogravimetry analysis.

T15% (�C) Tf (�C) Char residue (%) at 600 �C

PLA 300 476 0.61UTC 265 490 8.96TC 310 477 2.66

Fig. 8. DSC curves for biocomposites.

Table 4Thermal properties of PLA biocomposites obtained from differential scanningcalorimetry.

Tg

(�C)Tcc

(�C)Tm

(�C)DHcc

(J/g)DHm

(J/g)Percentage ofcrystallinity (%)

PLA 63.5 115.3 155.6 19.9 28.8 9.6UTC 58.3 92.7 146.9 16.4 20.2 4.1TC 63.4 113.7 155.2 19.2 23.1 4.2

M.R. Manshor et al. / Materials and Design 59 (2014) 279–286 285

30 wt% DSF drastically reduced the thermal stability of PLA bio-composite. DSF is a hydrophilic as they are derived from lignocel-luloses natural fibre which contain hydroxyl group. Incompatibilitybetween PLA and untreated DSF leads to poor interfacial adhesion,subsequently lowering the thermal stability. From Table 3, pre-treated DSF improved the T15% of the biocomposite by +10 �C ascompared to unreinforced PLA. NaOH removed the waxy materials,hemicelluloses, lignin and impurities in the fibre thus facilitatingbetter bonding between fibre and matrix. Better bonding has re-sulted in higher thermal stability as presented in Table 3. Due tothe removal of lignin and hemicelluloses in pretreated DSF, finaldecomposition temperature for TC was about 13 �C lower thanUTC. Hemicelluloses may sustain higher temperature than cellu-lose up to 290 �C and lignin up to 520 �C [24]. As cellulose isdecomposed at higher temperature, char residue is produced. FromTable 3, the percentage of char residue in UTC is higher comparedto PLA and TC due to unremoved lignin and cellulose embedded inUTC. The trend of the obtained TGA is in agreement with previousresearchers whose concluded that alkali treatment improved thethermal stability but decreased the char residue content suggest-ing a decrement in formation of carbonaceous char [16,25].

3.6. Differential scanning calorimetry (DSC) on PLA–DSF biocomposite

Incorporation of durian skin fibre alters the crystallizationbehavior of semicrystalline PLA as presented by DSC thermographsin Fig. 8. From Fig. 8, PLA and PLA–DSF biocomposites experiencedendothermic and exothermic phase transformation. Firstendothermic peak at around 53–64 �C corresponds to the glass

transition temperature (Tg). The exothermic peak at around 93–115 �C represents cold crystallization temperature (Tc) of PLAchains. As temperature is increased, the second endothermic peakis observed at around 155 �C. This signifies the melting tempera-ture (Tm) of PLA systems. The effect of pre-treated DSF on the crys-tallization temperature, Tc, melting temperature, Tm and degree ofcrystallinity, Xc of PLA biocomposites are summarized in Table 4.The degree of crystallinity is calculated using Eq. (6).

From Table 4, the workable temperature is between 60 and115 �C which is between Tg and Tm. Generally it is known that poly-mer is very stiff at Tg. Upon heating, the polymer molecules will beactivated and reduces the rigidity of material until it is soft beforemelt.

The value of DH is an important parameter since its magnitudeis directly proportional to the overall level of Xc possessed by thepolymer. Based on Fig. 8 and Table 4, glass transition (Tg), coldcrystallization (Tcc) and melting temperatures (Tm) of biocompos-ites systems are lower than the PLA matrix. However, pretreatedDSF slightly shifted the Tg, Tcc and Tm of the TC towards higher tem-perature range as compared to untreated PLA–DSF. Depression inTg, Tcc and Tm signifies that viscosity of biocomposite is higher thanPLA and DSF hinder the migration of PLA chains to the surface ofthe nucleus in the composites [26–28]. The cold crystallization(DHc) and melting enthalpies (DHm) of the TC are also increasedas compared to UTC biocomposite. The difference in enthalpies ishowever less than 10 J/g which suggested that PLA and PLA sys-tems remained amorphous.

From Table 4, degree of crystallization for unreinforced PLA isalmost 50% higher than the PLA biocomposites. The crystallinityof the PLA–DSF biocomposites are opposite to the trends on rein-forced PLA composites reported by other researchers [29–31]. Theyreported that reduction in Tg, Tcc and Tm is typically followed byincrement in crystallization. In this study, lower Xc obtained canbe associated to the inhomogenous fibre mixing and dispersionduring extrusion [32]. Reinforcement size [33] and agglomerationof filler [34] also reduce the crystallinity of PLA biocomposites.

286 M.R. Manshor et al. / Materials and Design 59 (2014) 279–286

Fig. 4(a–f) clearly shows the agglomeration of DSF in PLAbiocomposites.

Nevertheless, the presence of pre-treated DSF increased themesophase, crystalline and melting phases as compared to un-treated DSF which suggest on better reinforcement with NaOHpre-treatment. Degree of crystallinity in TC is improved by about11% as compared to UTC. This can be explained by the good inter-action between PLA and DSF as evidenced in Fig. 4(b) as lignin andhemicelluloses were removed during pre-treatment with NaOH.Good fibre–matrix will promote nucleation sites for crystallizationof PLA around the fibre [29].

4. Conclusions

Durian skin fibre minimally improved the impact strength andthermal stability of PLA. Performance of biocomposite is howeverequivalent to thermoplastic PLA. PLA–DSF biocomposite also canbe produced at a reasonable cost of manufacturing owing to lowerconsumption of expensive biodegradable PLA. About 30 wt% PLAcan be replaced by pre-treated DSF as the impact strength, flexuralmodulus and thermal properties were found at par to the unrein-forced PLA. Therefore, optimization and further development ofdurian skin composite are essential for viable application of PLA–DSF biocomposite.

Acknowledgements

The authors wish to thank Ministry of Education Malaysia forproviding funding for this project through Exploratory ResearchGrant Scheme, ERGS12-022-0022 for the financial support and toInternational Islamic University Malaysia for the facilities andequipment in making these studies a success.

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