-
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 (%)
PLA/FB PLA/ALK PLA/SIL PLA/ALKSIL
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
Good fibre/matrix adhesion
Good fibre/matrix adhesion
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
(%)
PLA/FB PLA/ALK PLA/SIL PLA/ALKSIL
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
Long fibre Short fibre
Failu
re s
train
(%)
Fibre loading (wt%)
Long fibre Short fibre
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
Long fibre Short fibre
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%)
PLA/FB PLA/ALK PLA/SIL PLA/ALKSIL
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%)
PLA/FB PLA/ALK PLA/SIL PLA/ALKSIL
(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%)
PLA/FB PLA/ALK PLA/SIL PLA/ALKSIL
(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.%
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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