1-s2.0-S1359835X1000312X-main

oFe

E. Injection moulding

opeweodbrech atrea

tent. The mechanical properties of the PLA/ALK composites were increased further due to alignment of

from retroleucausell as coe rst

forced PLA composites by extrusion and compression moulding.They found that tensile strength of the composites increasedslightly at 30 wt.% bre content (53 MPa) compared to PLA onlysamples (50 MPa). However, the tensile strength of 40 wt.% bresamples decreased to 44 MPa. This decrease in tensile strength at

creased about 10% compared to unreinforced PLA samples, whichwas inconsistent with other works [6,7]. This could be due to thedifference in reinforcement type and processing method. However,for the same composites, Youngs modulus increased about 71%,which was consistent with other reports [6,7]. They also found thatimpact strength of notched samples increased from 7.4 to 12.2 J/mwith increased bre content (030 wt.%) but decreased in the caseof unnotched samples (from 76 to 52 J/m). This trend in impactstrength of the unnotched composite samples was in generalagreement with other researchers [7]. Serizawa et al. [9] also used

Corresponding author. Present address: Composite Materials Research, PultronComposites Ltd., 342 Lytton Road, Gisborne 4040, New Zealand. Tel.: +64 6 8678582; fax: +64 6 867 8542.

Composites: Part A 42 (2011) 310319

Contents lists availab

Composite

evE-mail address: [email protected] (M.A. Sawpan).duced from annually renewable resources. For a long time, PLA wasmainly used in biomedical applications because of its high produc-tion costs. However, recent developments in the manufacture of itsmonomer (i.e. lactic acid) economically from agricultural products(e.g. corn, potato and cane sugar) have placed this material at theforefront of the emerging plastic industries [25]. Due to the com-mercial potential for natural bre reinforced polymer compositesin automotive applications and building construction as well as de-mands for environmentally friendly materials, the development ofPLA based composites for many applications is an interesting areaof research. For instance, Oksman et al. [6] produced ax bre rein-

and injection moulding. It was found that tensile strength of thecomposites increased from 44.5 to 54.1 MPa and Youngs modulusincreased from 3.1 to 6.31 GPa as the bre content increased from0 to 30 wt.%. The above ndings were fairly consistent with otherresearch work [6]. It was also observed that impact strength(unnotched samples) of the composites at all bre contents waslower than the PLA only samples.

Garcia et al. [8] fabricated PLA composites reinforced with kenafbre using extrusion and injection moulding. They also added mal-eated-PLA in the composites as compatibiliser. It was found thattensile strength of the composites reinforced with 30 wt.% bre de-E. Compression moulding

1. Introduction

In recent years, polymer matricesgaining ground over conventional ppolyethylene, polypropylene, etc.) belems related to their disposal as weavailability [1]. Polylactide (PLA) is th1359-835X/$ - see front matter 2010 Elsevier Ltd.doi:10.1016/j.compositesa.2010.12.004long bres. 2010 Elsevier Ltd. All rights reserved.

enewable resources arem based matrices (e.g.of environmental prob-ncerns over petroleumcommodity plastic pro-

higher bre content may be due to: (i) inadequate amounts of ma-trix to wet the bres and (ii) reduction of bre length during pro-cessing. Youngs modulus at 30 and 40 wt.% bre content wasfound to be 8.3 and 7.3 GPa, respectively, which was signicantlyhigher than that of PLA only samples (3.4 GPa). In another study,Bax and Mussig [7] also used ax bres to reinforce PLA (PLAwas in the form of bre) by hot-pressing followed by pelletisingKeywords:A. Polymermatrix compositesB. Mechanical properties

75.5 MPa, Youngs modulus of 8.18 GPa and impact strength of 2.64 kJ/m2 was found to be the best. How-ever, plane-strain fracture toughness and strain energy release rate decreased with increased bre con-Improvement of mechanical performancepolylactide biocomposites

Moyeenuddin A. Sawpan a,, Kim L. Pickering a, AlanaDepartment of Engineering, University of Waikato, Hamilton, New ZealandbBiomaterials Engineering, Biopolymer Network/SCION, Rotorua, New Zealand

a r t i c l e i n f o

Article history:Received 14 August 2010Received in revised form 30 October 2010Accepted 5 December 2010Available online 10 December 2010

a b s t r a c t

In this work, mechanical prreinforced PLA compositestensile strength, Youngs mincreased with increased and impact properties whitallinity. A 30 wt.% alkali

journal homepage: www.elsAll rights reserved.f industrial hemp bre reinforced

rnyhough b

rties of chemically treated random short bre and aligned long hemp brere investigated over a range of bre content (040 wt.%). It was found thatulus and impact strength of short hemp bre reinforced PLA compositescontent. Alkali and silane bre treatments were found to improve tensileppears to be due to good bre/matrix adhesion and increased matrix crys-ted bre reinforced PLA composite (PLA/ALK) with a tensile strength of

le at ScienceDirect

s: Part A

ier .com/locate /composi tesa

ites:kenaf bres to fabricate PLA composites by extrusion and injectionmoulding. They found that impact strength of the notched samplesdecreased from 4.4 to 3.1 kJ/m2 as the bre content increased from0 to 20 wt.%, which did not agree with Garcias ndings [8]. Sugges-tion was made that instead of using a twin screw extruder, com-posite impact strength can be improved by compounding thematerials with a single-screw extruder which prevents the brefrom being ground (crushed particles) during processing.

Mathew et al. [10] used microcrystalline cellulose (MCC), woodour andwood pulp to reinforce PLA using similar processingmeth-ods to Oksman et al. [6]. They found that the tensile strength ofMCCreinforced composites decreased (from 49.6 to 36.2 MPa) with in-creased MCC content (025 wt.%) whereas Youngs modulus in-creased signicantly (from 3.6 to 5 GPa) as for Garcia et al. [8]with kenaf bre. They also observed that the tensile strength,Youngsmodulus and storagemodulus ofwood our andwood pulpreinforced composites were higher (for wood our composites ten-sile strength and Youngs modulus were 45.2 MPa and 6.3 GPa,respectively, and for wood pulp composites tensile strength andYoungsmoduluswere 45.2 MPa and6 GPa, respectively) than thoseof MCC reinforced composites at similar level of reinforcement. Inanother study, Lee et al. [11] reinforced PLA with bamboo bresusing batchmixing and compressionmoulding. They also found thatYoungs modulus increased with increased bre content(1050 wt.%) but tensile strength decreased. In addition, they foundthat tensile strength and Youngsmodulus improved at all bre con-tents when maleic anhydride treated bamboo bres (5 wt.%) wereused as a compatibiliser and dicumyl peroxide as a free radical ini-tiator. In a later report, Lee and Wang [12] applied a bio-couplingagent (lysine-based diisocyanate) as compatibiliser in the PLA/bam-boo bre composites using similar processingmethod of [11]. As forLee et al. [11], they observed that Youngs modulus of the compos-ites increased with increased bre content (050 wt.%) but tensilestrength decreased. In addition, tensile strength and Youngsmodu-lus improved at all bre contents in the presence of coupling agent.

Vila et al. [13] used eucalyptus wood bre and rice husks toreinforce PLA using extrusion and injection moulding. They didnot observe any notable increase in tensile strength for the com-posites reinforced with 30 wt.% wood bre or rice husks comparedto PLA only samples but Youngs modulus increased signicantly(57% increase for PLA/wood bre and 45% increase for PLA/ricehusks composites). Pilla et al. [14] used silane treated pine woodour (PWF) to fabricate PLA composites by kinetic mixing andcompression moulding. It was observed that tensile strength ofthe untreated bre composites was unchanged compared with thatof PLA matrix (55.5 MPa) at 20 wt.% PWF content but decreased(51.7 MPa) at 40 wt.% PWF. Youngs modulus was found to increasesignicantly with increased PWF content (0.63 GPa for PLA,0.86 GPa for 20 wt.% PWF/PLA composites and 1.18 GPa for40 wt.% PWF/PLA composites). Notable change in tensile strengthand Youngs modulus was not found for the composites reinforcedwith 20 wt.% silane treated bre compared to the untreated brecomposites. However, tensile strength of the silane treated brereinforced composites (57.1 MPa) slightly increased at 40 wt.% -bre content but with no signicant change in Youngs moduluscompared to the untreated bre composites. Iwatake et al. [15]prepared micro-brillated cellulose (10 wt.%) reinforced PLA com-posites by kneading and compression moulding. They found thattensile strength and Youngs modulus of the composites increasedby 25% and 40%, respectively, compared to PLA only samples. Theyalso observed that further addition (15 wt.%) of micro-brillatedcellulose caused a decrease in tensile strength.

Plackett et al. [16] fabricated aligned long jute bre mat

M.A. Sawpan et al. / Compos(40 wt.%) reinforced PLA composites, which were rst pressedand consolidated under vacuum at different temperatures (180220 C) then compression moulded. They found that tensilestrength and Youngs modulus of the composites increased signif-icantly compared to PLA only samples (tensile strength 55 MPa andYoungs modulus 3.5 GPa) at all processing temperatures. Compos-ites processed at 210 C had the highest tensile strength(100.5 MPa) and Youngs modulus (9.4 GPa). This signicant in-crease in tensile strength and Youngs modulus compared to otherndings [6,7,17] appears to be due to the alignment of bres inloading direction.

The above studies indicate that generally Youngs modulus ofPLA composites can be improved by adding bres. This is becausenatural bres are very stiff compared to PLA matrix. However, ten-sile and impact strength of composite are greatly inuenced by -bre type and processing method.

Natural bres such as hemp, sisal, ax, kenaf and jute are coveredwithwaxymaterials, thus hindering thehydroxyl groups fromreact-ingwith polymer matrices. This can lead to the formation of ineffec-tive interfaces between the bres and matrices, with consequentproblems such as debonding and voids in resulting composites.Chemical treatmentsprovidean importantandeffectivemeans to re-move non-cellulosic components in cellulose bres and add func-tional groups to enable better bonding in polymer composites. Inthis study, industrial hemp bres were subjected to different chem-ical treatments, namely alkali, silane and acetic anhydride, in an at-tempt to produce high strength and stiff PLA composites.

2. Materials and methods

2.1. Materials

NatureWorks PLA (polylactide) polymer 4042D, from Nature-Works LLC, USA was used as a thermoplastic matrix. The industrialhemp bres were supplied by Hemcore Ltd., UK. [3-(2-Aminoethylamino)propyl]trimethoxy silane was purchased from Aldrich andSigma, respectively. All other chemicals used were of analyticalgrade obtained from local commercial sources.

2.2. Methods

2.2.1. Fibre treatmentPrior to treatment, untreated bres (FB) were washed with hot

water (50 C) to remove dirt. Afterwards, bres were dried in anoven at 80 C for 48 h.

2.2.1.1. Alkali treatment. Pre-dried bres were soaked in 5% sodiumhydroxide solution at ambient temperature for 30 min. After treat-ment, bres were copiously washed with water to remove anytraces of alkali on the bre surface and subsequently neutralisedwith 1% acetic acid solution. The treated bres (ALK) were thendried in an oven at 80 C for 48 h.

2.2.1.2. Silane treatment. A solution of 0.5 wt.% silane couplingagent [3-(2-aminoethyl amino)propyl trimethoxy silane] was pre-pared in acetone. The pH of the solution was adjusted to 3.5 withacetic acid and stirred continuously for 5 min. Fibres (67 wt.%moisture content) were then immersed in the solution for45 min. After treatment, bres were removed from the solutionand dried in oven at 65 C for 12 h. Finally, the bres (SIL) werethoroughly washed with water to remove chemical residues untila pH of 7 was obtained and then dried in an oven at 80 C for48 h. Similar silane treatment procedures also employed for bresthat were previously alkali treated (ALKSIL).

Part A 42 (2011) 310319 3112.2.2. Fabrication of compositesChopped dried short bres (average length 4.9 mm) and PLA

pellets were compounded (10, 20 and 30 wt.% bre) by using a

fore testing. Four samples were evaluated for each batch ofsamples.

2.2.5. Scanning electron microscope (SEM)Composite fracture surface morphology was studied using a

Hitachi S-4000 and a S-4700 eld emission scanning electronmicroscopes. Hitachi S-4000 was operated at 5 kV and Hitachi S-4700 was operated between 5 and 20 kV. All samples were ionsputter-coated with platinum and palladium to provide enhancedconductivity. Samples were mounted with carbon tape on alumin-ium stubs and then sputter-coated with platinum and palladium tomake them conductive prior to SEM observation.

3. Results and discussion

3.1. PLA crystallinity in composites

In Fig. 1, it is apparent that the crystallinity of PLA in compositesincreased with increased bre content which could be due to the

3.2. Tensile properties

5

(%

ites: Part A 42 (2011) 310319ThermoPrism TSE-16-TC twin screw extruder for good mixing of -bre and polymer. The extruded composite material was pelletisedand dried at 80 C for 24 h and then injection moulded using aBOY15-S injection moulding machine. No processing aids or otheradditives were used.

PLA and aligned long bre (average length 65 mm) compositeswere produced by compression moulding using lm-stacking atthree different bre contents (30, 35 and 40 wt.%). Dried long breswere aligned using a hand carding machine from Ashford Handi-crafts Limited, New Zealand. PLA lms (0.5 mm thick) were pro-duced from dry pellets, using an extruder equipped with a coathanger die. PLA sheets and bres were weighed prior to compositefabrication to determine the weight percentage of bres and ma-trix of the resulting composites. Stacks of PLA lms and bres wereprepared by placing alternately PLA lms and aligned bre mats ina parallel array. Before pressing, these were placed between twoTeon sheets in a stainless steel matched-die mould (220 150 3.5 mm3). The stacks of PLA and bres were pre-pressed at185 C for 5 min keeping a constant pressure of 2 MPa using ahot press machine and afterwards compacted at elevated pressureof 5 MPa for 3 min. The assembly was consolidated under a pres-sure of 5 MPa until the mould was naturally cooled down to ambi-ent temperature. Composite plaques were cut to desired shapesusing a computer numerical controlled (CNC) mill.

2.2.3. Measurement of PLA crystallinityCrystallinity of PLA in the composites was measured using a

DSC 2920-TA Instruments machine. All DSC scans were carriedout at a scan rate of 10 C/min from room temperature to 200 Cin the presence of air using samples of about 10 mg.

The percent crystallinity (XDSC) of PLA was calculated by usingthe following equation [14]:

XDSC% fDHf DHcc 100g=DHof w 1

where DHof = 93 J/g for 100% crystalline PLA, DHf is the enthalpy ofmelting, DHcc is the cold crystallisation enthalpy and w is theweight fraction of PLA in the composite.

2.2.4. Mechanical properties measurement2.2.4.1. Tensile testing. Tensile testing was carried out according tothe ASTM D 638-03 Standard Test Method for Tensile Properties ofPlastics [18]. Five samples of each type were tested by an Instron4042 tensile test machine. The cross-head speed was 5 mm/min.

2.2.4.2. Impact testing. The impact testing was carried out accord-ing to the EN ISO 179 Plastics Determination of Charpy impactstrength [19] using a Ray-Ran Pendulum Charpy Impact System.The impact velocity was 2.9 m/s and the hammer weight of0.475 kg. Dimensions of the samples were 80 8 3.5 mm3 witha single notch of 0.25 mm. Five replicates were evaluated for eachbatch of samples.

2.2.4.3. Fracture toughness testing. Mode I fracture toughness test-ing was carried out using single-edge-notched-bend (SENB) speci-men according to the ASTM D 5045-99 Standard Test Methods forPlane-Strain Fracture Toughness and Strain Energy Release Rate ofPlastic Materials [20]. LLOYD LR 100 K universal testing machinewas used for this purpose. The Length (L), Span length (S), Width(W) and Thickness (B) of the specimens were 126, 56, 12.7(0.03) and 3.3 (0.03) mm respectively, which satises the condi-tion 2B

3.5

wt% wt% wt%

ites:0.0 0.5 1.0 1.5 2.0 2.5 3.00

10

20

30

40

50

60

70

A: PLAB: PLA/10C: PLA/20D: PLA/30

Stre

ss (M

Pa)

Strain (%)

B

CD

(a)

M.A. Sawpan et al. / ComposThe average tensile strength and Youngs modulus of the un-treated and treated hemp bre/PLA composites are depicted inFigs. 3 and 4, respectively. As can be seen, the tensile strengthand Youngs modulus of the PLA/ALK, PLA/ALKSIL and PLA/SIL com-posites increased compared with those of the PLA/FB composites.This could be attributed to good adhesion in between treated bresand PLA as shown by decreased bre pull-out (see Fig. 5) and in-creased PLA crystallinity in the treated bre reinforced PLA com-posites. Among all the samples, tensile strength and Youngsmodulus of the PLA/ALK composites were found to be the highest.The tensile strength and Youngs modulus of the PLA/ALK compos-ites were 75.5 MPa and 8.2 GPa, respectively, about 10.5% and 8.2%higher than those of the PLA/FB composites. These results werebetter than any reported tensile properties for the short natural -bre/PLA composites [8,14,24].

As can be seen in the fracture surface of PLA/ALK composite (seeFig. 6), ALK bres are tightly connected with the PLA matrix. It canalso be seen that some bres were broken and/or torn. The brilla-tion of bre was another feature, which was most probably causedby regions of the bre, well bonded to the matrix, being torn from

Fig. 2. (a) Typical stress versus strain curves for tensile testing of PLA and compositesreferences to color in this gure legend, the reader is referred to the web version of thi

0 10 20 3045

50

55

60

65

70

75

80

Tens

ile s

treng

th (M

Pa)

Fibre content (wt%)

PLA/FB PLA/ALK PLA/SIL PLA/ALKSIL

Fig. 3. Tensile strength of untreated and treated hemp bre reinforced PLAcomposites as a function of bre content. (For interpretation of the references tocolor in this gure legend, the reader is referred to the web version of this article.)4.0

FB FB FB

A

(b)

Part A 42 (2011) 310319 313underlying layers of the bres. All these observations supportedstrong bonding in the PLA/ALK composites.

As can also be noted in Fig. 3, the relationship between tensilestrength and bre content was not linear, which indicated that athigher bre content, the benet to composite strength by addingbre was somewhat decreased. It is well known that bre shorten-ing inevitably occurs during extrusion and injection moulding ofthe composites containing both natural [25,26] and synthetic -bres [27], due to the strong shear stresses that act in the viscousmolten polymer. So, in the present case, as the bre content in-creased, the probability of the bre/bre interaction and bre/equipment wall also increased, resulting in an increase of short -bre (i.e. bres below the critical length) populations in the compos-ites. This is evident by the fact that bre pull-out increased withincreased bre content as can be seen in Fig. 7. Thus, the non-linearrelationship at higher bre content could be explained by the in-crease of population of the shorter bres.

As for untreated bre/PLA composites, the failure strain of thetreated bre/PLA composites also decreased with increased brecontent as can be seen in Fig. 8. This behaviour could be due to

(PLA/FB), (b) photograph of the specimens after testing. (For interpretation of thes article.)

0 10 20 30

3

4

5

6

7

8

9

Youn

g's

mod

ulus

(GPa

)

Fibre content (%)


Fig. 4. Youngs modulus of untreated and treated hemp bre reinforced PLAcomposites as a function of bre content. (For interpretation of the references tocolor in this gure legend, the reader is referred to the web version of this article.)

ites:.

Poor fibre/matrix adhesion

314 M.A. Sawpan et al. / Composthe lower failure strain of the bres compared to that of PLA. Stressconcentrations brought about at the broken bre ends could pro-mote fracture of the matrix, leading to overall failure of the com-posites at strains below that of the unreinforced PLA itself. It isevident that the variation of failure strain for different compositesdid not follow a similar order as the bre content increased from10 to 30 wt.%. It is also found that failure strain of all the samplesvaried insignicantly (less than 1%) for 1030 wt.% reinforcements.It must be accepted that experimental error as well as inconsistentbre dispersion will have inuenced variability.

As the alkali treated short hemp bre/PLA composites had thebest tensile strength and Youngs modulus, the investigation wasexpanded to produce high strength alkali treated hemp bre/PLA

(a) PLA/FB

(c) PLA/SIL

Good fibre/matrix adhesion

Fig. 5. SEM micrographs of tensile fracture surface of untrea

Fibrils

20 wt% fibre

Fig. 6. SEM micrographs of tensile fracture surface of PLA/ALK composites.(b) PLA/ALK

(d) PLA/ALKSIL



ted and treated hemp bre reinforced PLA composites.

Part A 42 (2011) 310319composites using aligned long bres. The composites were fabri-cated according to the method described in Section 2.2.2. The aver-age tensile strength, Youngs modulus and failure strain of thealkali treated aligned long bre/PLA composites as a function of -bre content are shown in Fig. 9. Tensile properties of alkali treatedshort bre/PLA composites are presented for comparison. As can beseen, the average tensile strength and Youngs modulus of the longbre composites (30 wt.% bre) were higher than those of the shortcomposites, which would be expected due to higher reinforcementefciency for the aligned long bres. In short bre composites, ahigh amount of composite fracture near the bre-end positionsof the short bres might occur which could reduce the effective -bre length and hence the tensile strength and Youngs modulus[28].

It is also apparent that the tensile strength and Youngs modu-lus of the long bre PLA composites increased with increased brecontent up to 35 wt.% and further increment of bres (40 wt.%)caused a decrease in the tensile strength and Youngs modulus.At 35 wt.% bre content, the average tensile strength and Youngsmodulus of the long bre/PLA composites was 85.4 MPa and12.6 GPa, respectively. This tensile strength of the long hemp -bre/PLA composites was found to be lower (about 14 MPa) thanthe reported tensile strength of aligned jute bre mat/PLA compos-ites [16]. This could be due to some misalignment of hemp bres inthe composites. The hemp bres used in this work were in bales oftangled and twisted strips. Despite the fact that the bres werecarded, they appeared to be somewhat crimped due to spring-backduring composite processing. However, Youngs modulus of thelong hemp bre/PLA composites was found to be better than anyreported long bre/PLA composites [16].

In the case of 40 wt.% bre content (see Fig. 10), bres were notthoroughly wetted due to the insufcient amounts of matrix beingavailable to cover the bres. As a result, bres were not wellconnected with the matrix, and some gaps between the bres

(a) 10 wt% fibre (b) 20 wt% fibre (c) 30 wt% fibre Fig. 7. SEM micrograph of the tensile fracture surface of PLA/FB composites at different bre contents (scale bar = 100 lm).

2.4

2.8

3.2

train

(%)


M.A. Sawpan et al. / Composites: Part A 42 (2011) 310319 3151.6

2.0

Failu

re sand matrix were evident. In this situation, load on the compositeswas not distributed evenly from the bre to bre through the ma-trix, and catastrophic failure of the composites was observed be-cause of poor wetting of the bres.

0 10 20 301.2

Fibre content (%)

Fig. 8. Failure strain of untreated and treated hemp bre reinforced PLA compositesas a function of bre content. (For interpretation of the references to color in thisgure legend, the reader is referred to the web version of this article.)

50

60

70

80

90

0 10 20 30 40 0 10

3

6

9

12

15

Long fibre Short fibre

Tens

ile s

treng

th (M

Pa)

Fibre loading (wt%)

Youn

g's

mod

ulus

(GPa

)

Fibre lo

Fig. 9. Tensile properties of alkali treated aligned long bre and random short bre reinlegend, the reader is referred to the web version of this article.)3.3. Impact strength

The average impact strength of short hemp bre (untreated andtreated) reinforced PLA composites as a function of bre content isdepicted in Fig. 11. As can be seen, impact strength of all compos-

0.5

1.0

1.5

2.0

2.5

3.0

0 10 20 30 4020 30 40


Failu

re s

train

(%)

Fibre loading (wt%)


ading (wt%)

forced PLA composites. (For interpretation of the references to color in this gure

Fig. 10. SEM micrograph of the tensile fracture surface of long aligned bre ALK/PLA composites (40 wt.% bre).

2.1

2.4

2.7

pact

stre

ngth

(kJ/

m2 )

4

6

8


act s

treng

th (k

J/m

2 )

Fig. 13. Impact strength of long and short bre reinforced PLA composites (PLA/ALK) as a function of bre content. (For interpretation of the references to color inthis gure legend, the reader is referred to the web version of this article.)

316 M.A. Sawpan et al. / Composites: Part A 42 (2011) 310319ites increased with increased bre content. This was because as thebre content increased, more interfaces exist on the crack path,and more energy was consumed. In fact, the concentration of shortbres would have increased with increased bre content, whichcould lead to increased pull-out and also increased impactstrength. Similar to the tensile strength and Youngs modulus, itcan be observed that the alkali and silane treatments enhancedthe impact strength of composites. This nding is in agreementwith other studies [26,29,30]. The PLA/ALK composites with30 wt.% bres had the highest impact strength (2.64 kJ/m2), whichwas approximately 12% higher than that of the PLA/FB composites(2.34 kJ/m2).

Fibrillation of bres, which is associated with high energyabsorption [31], can be observed in the impact tested fracture sur-faces of alkali and silane treated bre reinforced PLA composites(see Fig. 12). The increased PLA crystallinity of the alkali and silanetreated bre composites compared to untreated bre composites

0 10 20 301.5

1.8Im

Fibre content (wt%)


Fig. 11. Impact strength of untreated and treated hemp bre reinforced PLAcomposites as a function of bre content. (For interpretation of the references tocolor in this gure legend, the reader is referred to the web version of this article.)could be another factor leading to increased impact strength. In-deed, impact strength of the composites increased in the followingorder: PLA/FB < PLA/SIL < PLA/ALKSIL < PLA/ALK which is consis-tent with the order of PLA crystallinity in composites. Peregoet al. [32] and Todo et al. [33] also showed that the impact strengthof PLA increased with increased crystallinity of PLA.

Fig. 13 presents the average impact strength as a function ofbre content, for alkali treated long bre/PLA composites. Impactstrength of the alkali treated short bre/PLA composites arepresented for comparison. Similar to tensile strength of the long -bre composites, it may be observed that the impact strength of

(a) PLA/ALK (b) PLA/

..

Fig. 12. SEM micrographs showing brils (indicated by arrow) in the0 10 20 30 40

2

Imp

Fibre loading (wt%)long bre composites increased as the bre content increasedand attained the maximum value (7.4 kJ/m2) at 35 wt.% bre con-tent. Further increment of bres caused a decrease in impactstrength of the composites. For an equivalent amount of bres(30 wt.%), the impact strength of the long bre composites was101% higher than that of the short bre composites.

SIL (c) PLA/ALKSIL impact fracture surface of PLA composite samples (30 wt.% bre).

Fig. 14. Photograph of the impact tested long hemp bre reinforced PLA composites(35 wt.% bre). (For interpretation of the references to color in this gure legend,the reader is referred to the web version of this article.)

rfac

ites:(a)

Fibre pull-out

Fibre fracture

Fig. 15. SEM micrographs of the impact fracture su

2.0

2.2

M.A. Sawpan et al. / ComposA photograph of impact tested long bre PLA composites is pre-sented in Fig. 14. As can be seen, samples were not completely sep-arated into two pieces but bres bridged the gap to hold thesample together. This mode of failure was associated with high en-ergy absorption [34]. In addition, examination of the impact frac-ture surfaces showed bre pull-out due to the fracture of longbre during impact loading (see Fig. 15a). Fibrillation was also evi-dent in the impact fracture surface (see Fig. 15b), which was con-sistent with the short bre composites.

3.4. Fracture toughness

Fig. 16 presents the average KIc and GIc of short hemp bre (un-treated and treated) reinforced PLA composites as a function of -bre content. As can be seen, KIc and GIc of all the compositesdecreased with increased bre content. This could again be dueto increased stress concentrations (bre ends) and PLA crystallinityof the composites compared to PLA only samples. ALK, SIL and ALK-SIL bres were found to decrease the KIc and GIc of the compositescompared to the FB bre composites. This could be attributed tothe greater PLA crystallinity and improved interfacial adhesion inthe PLA/ALK, PLA/SIL and PLA/ALKSIL composites compared withthat of the PLA/FB composites [35].

1.0

1.2

1.4

1.6

1.8

0 10 20 30

KIc (M

Pa-m

1/2 )

Fibre content (wt%)


(a)Fig. 16. KIc and GIc of untreated and treated hemp bre reinforced PLA composites as alegend, the reader is referred to the web version of this article.)(b)

Fibrils

e of long bre PLA/ALK composites (35 wt.% bre).

6

Part A 42 (2011) 310319 317Fig. 17 shows the average KIc and GIc of the alkali treated longand short bre composites as a function of bre content. As canbe observed, KIc and GIc of the long bre composites was higherthan those of the short bre composites. This was because in thelong bre composites the bres were oriented perpendicular tothe loading direction thus had greater resistance to crack propaga-tion. Fig. 18 indicated that crack propagation was suppressedsomewhat due to the bre bridging. Like short bre composites,KIc and GIc of the long bre composites decreased with increased -bre content. This could again be due to the crystalline interface ofPLA/hemp bre composites through which cracks can propagateeasily. At 30 wt.% reinforcement, KIc and GIc of the long bre com-posites were 36.4% and 25.1%, respectively, higher than those ofthe short bre composites.

4. Conclusions

Tensile strength, Youngs modulus and impact strength of shorthemp bre reinforced PLA composites were found to be increasedwith increased bre content (1030 wt.%). It was found that PLAcould be reinforced with a maximum of 30 wt.% bres using con-ventional injection moulding, but could not be processed at higherbre contents due to poor melt ow of the compounded materials.

0 10 20 30

2

3

4

5

GIc (k

J/m

2 )

Fibre content (wt%)


(b)function of bre content. (For interpretation of the references to color in this gure

2.0 Long fibre

ites:0.8

1.2

1.6

Short fibre

KIc (M

Pa-m

1/2 )318 M.A. Sawpan et al. / ComposKIc and GIc of the composites decreased with increased bre con-tent which could be due to the increase of stress concentration(number of bre ends) and crystallinity of PLA in composites. Ten-sile properties and impact strength of the composites were in-creased further with bre treatments (e.g. alkali and silane)which could be due to improved bre/matrix adhesion and in-creased PLA crystallinity. Alignment of long bres was found tobe an effective technique to improve the mechanical propertiesof PLA/hemp bre composites compared to those of short hemp -bre/PLA composites. The highest mechanical properties were ob-tained with a 35 wt.% aligned long alkali treated bre compositeswith tensile strength of 85.4 MPa, Youngs modulus of 12.6 GPaand impact strength of 7.4 kJ/m2.

Acknowledgement

The nancial support from Biopolymer Network Ltd., New Zea-land for this work is greatly acknowledged.

0.40 10 20 30 40

Fibre loading (wt%)

Fig. 17. KIc and GIc of long and short hemp bre reinforced PLA composites (PLA/ALK) as alegend, the reader is referred to the web version of this article.)

Fibre bridge

Fig. 18. SEM micrograph of the long bre PLA/ALK composites showing the brebridging in SENB tested sample (30 wt.% bre).References

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M.A. Sawpan et al. / Composites: Part A 42 (2011) 310319 319

Improvement of mechanical performance of industrial hemp fibre reinforced polylactide biocompositesIntroductionMaterials and methodsMaterialsMethodsFibre treatmentAlkali treatmentSilane treatment

Fabrication of compositesMeasurement of PLA crystallinityMechanical properties measurementTensile testingImpact testingFracture toughness testing

Scanning electron microscope (SEM)

Results and discussionPLA crystallinity in compositesTensile propertiesImpact strengthFracture toughness

ConclusionsAcknowledgementReferences

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