Accepted Manuscript
Mechanical and physical stability of polyhydroxyalkanoate (PHA)-based wood plasticcomposites (WPCs) under natural weathering
Clement Matthew Chan, Steven Pratt, Peter Halley, Desmond Richardson, AlanWerker, Bronwyn Laycock, Luigi-Jules Vandi
PII: S0142-9418(18)31597-6
DOI: https://doi.org/10.1016/j.polymertesting.2018.11.028
Reference: POTE 5704
To appear in: Polymer Testing
Received Date: 28 September 2018
Accepted Date: 19 November 2018
Please cite this article as: C.M. Chan, S. Pratt, P. Halley, D. Richardson, A. Werker, B. Laycock,L.-J. Vandi, Mechanical and physical stability of polyhydroxyalkanoate (PHA)-based wood plasticcomposites (WPCs) under natural weathering, Polymer Testing (2018), doi: https://doi.org/10.1016/j.polymertesting.2018.11.028.
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Material Properties
Mechanical and physical stability of polyhydroxyalkanoate (PHA)-
based wood plastic composites (WPCs) under natural weathering
Clement Matthew Chana, Steven Pratta, Peter Halleya, Desmond Richardsonb, Alan Werkera,c,
Bronwyn Laycocka, Luigi-Jules Vandia
a School of Chemical Engineering, The University of Queensland, St Lucia, QLD, Australia.
b Norske Skog Paper Mills (Aust) Ltd, Boyer, TAS, Australia
c Promiko AB, Sweden
Keywords:
Wood flour; Polyhydroxyalkanoate (PHA); Biocomposites; Mechanical properties; UV stability;
Weatherability
ABSTRACT
This study investigated the effects of natural weathering on the physical and mechanical
properties of biodegradable composites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate)
(PHBV) and wood flour (WF). Composite samples with three different wood contents (0, 20 and
50 wt%), as well as polylactic acid (PLA) and polyethylene (PE) composites containing 50 wt%
of WF, were placed on an outdoor inclined rack for 12 months. Neat PHBV and the 20% WF
samples showed minimal mould growth and little loss in mechanical properties over the 12-
month period. By contrast, the tensile strength of the composites (of all polymer types) made
with 50% WF dropped gradually. This decrease was interpreted to be due to the evolution of
surface/bulk defects brought about by the degradation of wood particles from moisture-induced
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fungal (mould) attack. Overall, the wood content and the accessibility of wood particles inside
the matrix control the stability of the composites under natural weathering.
1. Introduction
In recent years, the applications of bio-derived and biodegradable polymeric materials have
expanded to address concerns over the accumulation of plastic waste in landfill and in the
environment, particularly the ocean. The use of such polymers as the matrix in wood plastic
composites (WPCs) is one of the growing areas of interest because of the expanding WPC
market [1] and the cost competitiveness achieved by taking advantage of cheap, abundant and
biodegradable wood fibres as structural fillers [2].
Among all biodegradable polymers, polyhydroxyalkanoates (PHAs) have been of particular
interest, as these polymers can be synthesised intracellularly by a range of bacteria and archaea
from renewable/waste resources, and are biodegradable under ambient conditions [3]. PHAs are
a family of polyesters which have been shown to have similar physical and mechanical
properties to polypropylene [4]. PHAs also have a lower melt viscosity than traditional
polyolefins, which can be an advantage in the processing of composites [5].
Extensive research is being conducted to understand the physical and mechanical behaviour of
PHA/wood composites. Among all the PHAs, poly(3-hydroxybutyrate) (PHB) and its copolymer
poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) are the most common polymer matrices
used in WPCs. Several successful attempts have also been made to improve the composite
mechanical properties through compatibilisation techniques including maleic anhydride grafting
[6, 7] and wood pretreatment [6-8]. Although such studies have shown the great potential of
PHA-based WPCs, a current lack of understanding of the material stability while in service
remains an obstacle to suitable applications and commercial developments.
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Given the market growth in exterior applications, the durability of WPCs such as moisture
resistance, weatherability and resistance to fungal attack need to be explicitly assessed. Therefore,
numerous studies have been targeted at understanding the weatherability of WPCs through
natural and accelerated weathering [9-13].
It is well known that the hygroscopic nature of wood causes deterioration of mechanical
properties of WPCs through moisture absorption. For example, studies on PP-based WPCs [14,
15] have shown that a higher wood content leads to higher total water absorption and faster
mechanical deterioration under water immersion. This deterioration is due to the incomplete
encapsulation of wood filler by the polymer matrix at such higher wood contents [14]. The
decolouration of PP/wood flour composites (from brown to chalky white) was also observed
during accelerated weathering [9, 12] and attributed to photo bleaching of lignin components in
these WPCs [12]. Loss in mechanical properties [11], a higher degree of surface roughness [9]
and surface cracking [10] on UV exposure were also reported. The type of polymer matrix [10]
and processing methods [13] also play a role in the weatherability of the resulting WPCs.
To date, however, very little research has been conducted on the durability of prototype
PHA/wood composites. Christian et al. [16] and Srubar et al. [17] investigated the moisture-
induced mechanical deterioration of PHBV/oak wood flour composites and found that the
addition of wood flour accelerated the deterioration of mechanical properties due to a higher
degree of wood swelling, which induced macroscopic surface cracking. However, there have
been no studies on the effect of outdoor (non-soil) weathering on the durability of PHA-based
WPCs [18].
Given the growing interest in fully biodegradable PHA-based WPCs, a more thorough
understanding of the physical and mechanical stability of PHBV/wood composites under outdoor
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weathering is, therefore, required. In this work, composite samples based on PHBV and Pinus
radiata wood flour with three different wood content levels (0, 20 and 50 wt%) were placed on
an outdoor inclined rack at a fully monitored field site in sub-tropical Queensland (27.5o S,
152.9o E). Polylactic acid (PLA) and polyethylene (PE) composites containing 50 wt% WF were
also included as reference materials. The objectives for the material environmental testing was to
evaluate the effect of outdoor (natural, non-soil) exposure on the material properties of the
composites over an extend period of one year, and to understand the factors that control the
stability of the composites.
2. Experimental
2.1. Materials
PHBV with 1 mol% HV content in powder form was supplied by TianAn Biopolymer, China
under the trade name of ENMAT Y1000. PLA (IngeoTM 2003D) and HDPE (EL-Lene H5818J)
pellets were obtained from NatureWorks and SCG Chemicals respectively. Pinus radiata wood
flour (WF) sieved to below 300 µm in particle size was obtained from Micromilling, Australia.
2.2. Composite extrusion
The composite materials were prepared by a two-step process, consisting of dry mixing and melt
compounding. The formulations with associated sample identification are summarised in Table
1. PHBV and WF were first dried separately at 105oC in a vacuum oven at a gauge pressure
of -80 kPa for 24 h. The dried PHBV and WF were then premixed according to the formulations
shown in Table 1 by manual stirring using a Homemaker double-blade kitchen stick blender at
200 rpm for 2 mins (150 g per batch). The pre-mixed formulations were then fed to a co-rotating
twin screw extruder (Eurolab 16 (ThermoScientific), diameter: 16 mm, L/D ratio: 40:1). The
optimised extrusion parameters were determined based on a preliminary study [19]. Flood
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feeding and a decreasing temperature profile with a maximum barrel temperature of 180oC and a
die temperature of 160oC were implemented. The screw profile consisted of forward conveying
elements only, with no mixing zones. The screw speed was maintained at 100 rpm, which
resulted in a residence time of around 1 min. A slit die with cross sectional dimension of 25 by 2
mm was placed at the die to yield rectangular strips. The rectangular strips were then melt-
pressed at 180oC with 5 tonnes of force for 2 mins to produce consistent flat sheets with a
thickness of 1.5 mm.
The extruder set-up was modified for extrusion of the dried PLA/dried wood flour and PE/dried
wood flour mixes, due to their different melt temperatures and viscosities. Two gravimetric
spring feeders were calibrated to deliver a 50/50 polymer to wood ratio by weight into the screw.
A screw speed of 100 rpm, a maximum barrel temperature of 210oC and a die temperature of
160oC were employed for both PLA and PE composites. A kneading section that consists of five
30o forward, three 60o forward and four 90o forward elements was included to facilitate mixing
between the polymer pellets and wood flour. The rectangular strips were then melt-pressed at
200oC with 5 tonnes of force for 2 mins to produce consistent flat sheets with a thickness of 1.5
mm.
Table 1 Composite formulations with associated samples identifications
Samples PHBV (wt%) PLA (wt%) HDPE (wt%) Wood flour (wt%)
PHA0W 100 -- -- --
PHA20W 80 -- -- 20
PHA50W 50 -- -- 50
PLA50W -- 50 -- 50
PE50W -- -- 50 50
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2.3. Experimental set-up
The melt-pressed sheets were laser cut to produce 70 x 15 mm rectangular specimens for outdoor
exposure. The specimens were clipped, as shown in Figure 1, onto an unbacked 45o inclined
aluminium rack according to ASTM G7, with one surface facing the sun and the other in the
shade. The rack was placed facing the equator at a fully monitored field trial site in sub-tropical
Queensland (27.5o S, 152.9o E). On-site weather data including temperature range, total rainfall
and solar radiation were recorded. Specimens were collected after 1, 2, 3.5, 6 and 12 months for
physical and mechanical characterisations.
Figure 1 Unbacked 45o inclined aluminium exposure rack used in this study (with samples
attached)
2.4. Colour measurement
The colour analysis of all samples was performed using Adobe Photoshop software. The samples
were scanned using a document scanner with a white background. The colour of the scanned
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images was determined by using the eyedropper tool with a 31 by 31 average across the sample
according to the CIE 1976 L*, a*, b* colour space. L* (from 0 to +100) represents lightness: an
increase in L* indicates a lighter colour. a* and b* (from -300 to +300) represent colour
components from green to red and from blue to yellow respectively. The mean values were
obtained from the analysis of 5 areas on each of 3 replicate samples.
2.5. Scanning electron microscopy (SEM)
Samples were imaged by JOEL 6460 and JOEL 6610 scanning electron microscopes (the 6460
broke during the study) under secondary electron mode. The top and bottom surfaces were
exposed for imaging. They were coated with ~30 nm of Iridium and vacuum dried at room
temperature for at least 24 hours before imaging. Image acquisition was done on the Iridium-
coated samples at 5 kV accelerating voltage and approximately 10 mm working distance.
2.6. Bio-sequencing
The samples after 6 months outdoor exposure were subjected to bio-sequencing 16S rRNA gene
sequencing. Samples were cut into small pieces of less than 0.5 mm in length. They were then
placed into a PowerBead tube and secured horizontally using a PowerLyzer® 24 Bench Top
Bead-Based Homogenizer and vortexed at maximum speed for 45 seconds. DNA extraction was
performed using the Fast DNA®SPIN Kit for Soil (MO BIO Laboratories, Inc.). Extracted DNA
was sent to the Australian Centre for Ecogenomics (ACE) sequencing facility for Illumina iTag
sequencing and data processing. Returned 16S rRNA gene sequences that met the quality
threshold were clustered using a 95% cut-off and used for community composition analyses.
2.7. Differential scanning calorimetry (DSC)
A differential scanning calorimeter Q2000 (TA Instruments) under a constant nitrogen flow of
50 mL/min was used to determine the thermal properties of the polymer composites. Samples of
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2 to 3 mg were placed in a sealed aluminium pan and analysed using standard DSC heating and
cooling scans. Each sample was heated from 25oC to 185oC at 10oC/min and kept isothermal for
0.1 min, and then cooled to -70oC at 10oC/min. The melting temperature, Tm, and enthalpy of
fusion, ∆Hm, were determined from the first heating cycle. The crystallisation temperature, Tc,
was determined from the first cooling cycle. Two replicates were assessed to confirm the
reproducibility of the thermal properties.
2.8. Mechanical testing
Prior to any characterisation, the samples were dried in a vacuum oven at 60oC and -80 kPa
gauge for 24 hours to remove any absorbed moisture. The final drying before mechanical testing
provided a common basis and conditions from which to compare the influence of environmental
exposure on material properties. Tensile mechanical tests were performed according to ASTM
D638 on an Intron 5584 with a 1-kN electronic load cell. Sections of extruded rectangular strips
were laser cut into Type V dumbbell-shaped specimens. Tests were performed at a rate of 2%
strain/min until fracture. An extensometer was used to obtain an accurate strain value across the
narrow region. Five replicas were performed for each sample.
3. Results and Discussion
3.1. Physical appearance
Figure 2 shows the visual appearances of both surfaces (FRONT: facing the sun and BACK: in
the shade) of the unaged and weathered composite samples at five different exposure times (1, 2,
3.5, 6 and 12 months). Whitening was clearly observed on the surface exposed to UV sunlight
for all samples including neat PHBV (Figure 2 top row). Such a bleaching effect on all samples
suggests that both PHBV and wood flour contain chromophores that can be modified through
UV radiation.
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The colour changes through UV sunlight exposure were quantified using the CIE Lab colour
scale, with Figure 3 showing the changes in colour properties (∆L*, ∆a*and ∆b*) of the surface
facing the sun for all samples. The changes were calculated by subtracting the colour values of
the weathered samples from those of the unaged samples.
As can be seen in Figure 3a, the ∆L* values increased for all composites after 1 month but then
decreased afterwards, except for the sample containing 20 wt% wood flour (PHA20W), which
showed a gradual increase in lightness throughout the 12-month period. Unlike PP, which
showed no changes in colour under UV exposure [9, 12], neat PHBV showed an increase in the
∆L* value (lighter in colour) after 1 month and then stabilised. The ∆a* values (Figure 3b)
decreased (i.e. the redness was reduced) gradually throughout the first 2 months for all samples
and then stabilised. There was less of a change for neat PHBV than for the composites.
In terms of yellowness, all samples including neat PHBV showed steep decreases in yellowness
over the first 3.5 months of UV exposure, with limited differences between samples (Figure 3c).
The UV-activated reduction reaction of the quinone structures in lignin has been proposed to
result in a decrease of yellowness, and thus a decrease in b* values [20], which could explain the
observed decrease in yellowness in the wood composite samples. A similar trend of decrease in
b* values for neat PHBV suggests that similar types of chromophores could be present in both
the PHBV and wood flour. The photo-bleaching reaction could also be similar; however, a
deeper study has to be conducted to fully explain the yellowish colour in processed PHBV,
which may be related to residual biomass components or to by-products formed during extrusion
[9, 12].
The colour values for all samples remained steady after 2 months of UV sunlight exposure i.e.
the photo-bleaching reactions were complete. The weather data showed a total accumulated solar
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radiation of 1040 MJ/m2 was applied to all samples over this time. It could be inferred that a
post-manufacture UV-treatment of samples by exposure to 1040 MJ/m2 irradiation could be used
for achieving a whiter colour of PHBV and its composites with wood. However, this does not
take into account any damage caused using such a process.
The physical appearances of the composites were affected by aging (Figure 2 bottom row), with
black regions associated with mould growth being found on the shaded surface of all of the
composites containing 50 wt% of WF within 2 months. All samples had experienced natural wet-
dry cycles through rainfall (accumulated rainfall of 890 mm throughout the period) and sunlight
(day/night) cycles. More moisture was naturally retained on the shaded surface leading to
increased susceptibility to mould growth on this side. On the other hand, very few signs of mould
growth were observed on the PHA0W and PHA20W samples after 12 months of weathering.
Since PHBV is a hydrophobic material, the limited mould growth on PHBV supports a proposal
that the growth of mould requires a moist environment and the supply of substrate and/or niche
microenvironments such as found in the unprotected wood phase [21]. The absence of evident
mould on the PHA20W composites surface implies that, at the lower wood flour content, the
PHBV matrix sufficiently encapsulates the wood particles acting as something of a barrier to the
fungi [14].
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Figure 2 Visual appearances of all unaged and weathered samples at five different exposure times (1, 2, 3.5 and 6 months) FRONT
(top row): the surface facing the sun; BACK (bottom row): the surface in the shade
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Figure 3 Changes in CIE Lab colour parameters of the surface facing the sun of all composites
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3.2. Scanning electron microscopy (SEM)
Figure 4 shows the SEM micrographs of the shaded (mouldy) surfaces of all samples before and
after 12 months of weathering. No observable micro-organisms were evident on the surface of
neat PHBV (PHA0W) after 12 months of natural weathering. When comparing the lower
magnification images between PHA20W and PHA50W, a denser network of fungal hyphae cells
was observed for the PHA composites with higher wood content. A network of long fungal
hyphae cells with an average diameter of around 5 µm, including a colony of ~3 µm-diameter
spores, was observed on the surfaces of all composites containing 50 wt% WF. Such a network
of long, filamentous fungal hyphae and spherical spores is commonly seen in decayed woody
materials [21, 22].
Surface roughness with visible divots was observed before weathering for the composites
containing 50 wt% WF due to incomplete mixing or imperfect wetting of the WF during
processing. It can be seen from the SEM images that the fungal cells attacked the accessible
wood particles created by processing and created deeper divots on the surfaces. Thus, the fungal
cells could access deeper wood flour particles inside the composite matrix and create voids.
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Figure 4 SEM micrographs of the surface in the shade of all samples (in rows) before weathering
(left column) and after 12 months at 100x magnification (middle column) and after 12 months at
higher 1000x magnification (right column), showing the cell structure of the as-colonised fungal
hyphae.
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3.3. Bio-sequencing
The organisms present on the samples after 6 months of natural weathering were identified using
16S rRNA gene sequencing, and the relative abundance profiles (Order level) of all the samples
are presented in Figure 5. As expected, fungi had the highest relative abundance in all
composites. PHA composites, both 20W and 50W, showed a lower relative abundance of fungi
(70%) when compared to PLA50W and PE50W, which had 87% and 81% abundance,
respectively. Instead, PHA composites had a higher relative abundance of Rhizobiales, which are
nitrogen-fixing bacteria that are symbiotic on plant roots. Although the abundance profiles of
PHA20W and PHA50W were similar, it was observed that the absolute amount of rRNA
obtained from PHA20W is lower than for PHA50W.
By contrast, neat PHBV had a much lower relative abundance of fungi (21%) alongside a mix of
different bacteria (including Bacillales and Rhizobiales), where the total bacteria shared a higher
proportion of the RNA than fungi. The lower relative abundance of fungi for all PHBV
containing samples suggests that the PHBV matrix may show potential for fungal resistance and,
in the present work, the neat PHBV was relatively more susceptible to bacterial enzymatic
attack.
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Figure 5 Abundance profiles of all samples after 6 months of natural weathering (order level).
Orders with relative abundance less than 1% were combined into “others”.
3.4. Weight loss and Molecular weight
Based on the visual evidence of fungal network on the surface of the samples, it is hypothesised
that the fungal cells could attack the wood flour and promote wood decay. To test this
hypothesis, the dry weight loss values of the samples after 12 months of weathering were
determined using the following equation:
%������ℎ�� �� =�����������ℎ������12� ��ℎ� − �������������ℎ�
�������������ℎ�× 100%
The % dry weight loss values are given in Table 2. Minimal weight loss was observed from neat
PHBV. The weight loss increased with increasing wood content, which correlates with the
observed denser fungal cell network in samples with higher wood content. When comparing the
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composites with 50 wt% WF, higher weight loss values were found in PLA50W and PE50W
composites when compared to PHA50W. This indicates that more wood particles were decayed
by fungal attack for the PLA50W and PE50W samples, which is consistent with the observed
higher relative abundance of fungi in the bio-sequencing results.
Table 2 Dry weight loss of all samples after 12 months of natural weathering (data presented as
mean value ± 95% confidence interval)
Sample Dry weight loss after 12 months (%)
PHA0W 0.22 ± 0.05
PHA20W 3.3 ± 0.3
PHA50W 4.9 ± 0.2
PLA50W 6.5 ± 0.2
PE50W 6.6 ± 0.1
To assess whether the natural weathering caused degradation of remaining PHBV chains, the
molecular weight values were characterised. The molecular weight values of PLA50W were also
determined as reference. The number-average molecular weight (������), weight-average molecular
weight (�������) and the polydispersity (PDI) are shown in Figure 6(a), 6(b) and 6(c), respectively.
Both the PHA0W and PHA20W showed slight decreases in both ������ and ��
����� over time. These
results indicate that the combination of UV exposure, rainfall cycle and fungal growth led to a
higher degree of polymer chain scissions (either at the surface or throughout). On the other hand,
while the ������ values for the PHA50W samples also decreased slightly throughout the period, the
������� values remained roughly the same, with ��
���� being more sensitive to changes to the
proportion of low molecular weight chains in the distribution, and hence indicating the possible
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loss of some of this fraction. PHA50W had a lower ������� initially when compared to PHA0W and
PHA20W, possibly due to the the presence of moisture in the wood, which could facilitate
hydrolytic chain scissioning of the PHA during processing [23]. The ������ and ��
����� values of PLA
in PLA50W were lower than for the PHA50W initially but also showed slight decreases in ������
and ������� over time, meaning that chain scissions also occurred in PLA composites. No obvious
trends were observed from the PDI values due to the complexity and randomness of the chain
scission.
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Figure 6 The (a) number-average molecular weight (������), (b) weight-average molecular weight (��
�����) and (c) polydispersity (PDI) of
all unaged and weathered samples against exposure times
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3.5. Thermal properties
The melting and crystallisation behaviour of PHBV and its composites were analysed in order to
fully understand the effect of weathering/UV exposure on the polymer matrix. The peak melting
temperature (Tm) values of all samples are presented in Figure 7(a). The peak melting
temperatures of PHBV and its composites remained relatively constant throughout the 12-month
exposure. These trends suggest that the bulk crystal structures of the PHBV matrix were not
significantly influenced by the exposure to natural UV during weathering. It is proposed that any
surface changes induced by cross-linking may affect the mechanical properties but the bulk
properties will remain unchanged. It has already been shown that the bulk properties of PHBV
such as melting point are stable under UV exposure, even with an observed decrease in
molecular weight from 350 to 130 kDa [24], unless the polymer was exposed to high energy
sources such as gamma [25] or electron irradiation [26]. The UV exposure to sunlight may not be
sufficient to influence the lamellar structure of PHBV, and thus the peak melting temperature
[24]. Under stronger energy sources, PHBV could undergo chain scission, accompanied by
cross-linking mainly in the amorphous region, which leads to change in the bulk crystal structure
and lowers the peak melting temperature [26].
The enthalpy of melting (∆Hm) values from the 1st heating scan of all samples are shown in
Figure 7(b). No clear trends were observed over the exposure time. ∆Hm has been shown to be
directly proportional to the ‘equivalent weight’ crystallinity of PHBV, which can be calculated
by dividing the experimentally measured ∆Hm by the ∆Hm of the 100% crystalline crystal [27].
Therefore, the result indicates that the bulk crystallinity of the PHBV was not affected during the
period of natural weathering.
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The melt crystallisation temperatures (Figure 7(c)) of all PHBV samples, both neat and
composite, increased after 1 month of weathering and then stabilised. In other words, a
nucleating effect was introduced to the polymer in the melt after 1 month of natural weathering.
Exposure to UV could have a nucleating effect on PHBV, with trace cross-linked particles acting
as seeds to promote polymer crystallisation. Such a nucleation effect of in-situ cross-linked
particles has been reported in a study with isotactic PP [28]. Higher chain mobility through
random chain-scission reactions could also explain the phenomenon.
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Figure 7 The (a) peak melting temperature, (b) enthalpy change during melting and (c) melt crystallisation temperature values of all
unaged and weathered PHBV and PHBV/WF composite samples at different exposure times.
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3.6. Tensile properties
The tensile strength and modulus, and strain at break values for the composites over time are
presented in Figure 8(a), 8(b) and 8(c), respectively. Although photo-bleaching of the PHBV was
observed, the mechanical properties were maintained over the course of natural environmental
exposure (non-soil sunlight and rainfall) for a 12-month period. This agrees with the literature,
which suggested that the biodegradation, and thus mechanical deterioration, of PHBV requires
the presence of an active microbial community such as found in soil, landfills, and marine
environments [29]. Interestingly, the tensile modulus of neat PHBV increased and its strain at
break decreased during the first 2 months of weathering. Similar mechanical changes were also
reported in an accelerated weathering study on PHBV under long-wave irradiation (340 nm) [24].
The UV-induced cross-linking was usually associated with the above mechanical changes for PE
[30, 31]. However, aliphatic polyester such as PHA and PLA have been shown to undergo both
chain scission and chain recombination rather than cross-linking [32, 33]. The constant tensile
strength suggests that the chain recombination mechanism is more dominant than chain scission
throughout the exposure time period.
On the other hand, the mechanical strength (Figure 8a) and stiffness (Figure 8b) of the
composites containing 50 wt% wood decreased throughout the 12-month weathering period. It
was observed that PLA50W suffered a more substantial drop in the tensile strength throughout
the exposure time when compared with PHA50W and PE50W.
Figure 9 shows a schematic diagram illustrating the underlying factors that contribute to the
worsened mechanical properties. The mechanical deteriorations observed from the composites
containing 50 wt% wood are considered to be due to the introduction of surface/bulk defects
brought about by both the swelling and shrinkage of wood during the wet-and-dry cycles and the
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observed mould growth. Such defects could act as initiation points for crack propagation through
the bulk, leading to earlier onset of mechanical failure. In addition, the mechanical properties of
WPCs are well-known to be governed by the strength of the polymer-wood interface [2]. On
weathering, the as-colonised fungal hyphae cells, which were presumably associated with the
wood particles, could loosen the wood-polymer interfaces through enzymatic erosion or
mechanical action. Thus, the load in this case would not be effectively transferred within the
matrix and this would explain the observed deterioration of mechanical properties with time.
In contrast, the mechanical strength and stiffness of PHA20W were maintained for the first 3.5
months but then decreased slightly, and less substantially than for PHA50W, over the period
from 3.5 to 12 months. These observations support the hypothesis that, at low wood content, the
wood particles are sufficiently sealed by the PHBV matrix and, in this way, shielded from fungal
colonisation [14].
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Figure 8 The (a) tensile strength, (b) tensile modulus and (c) tensile strain at break of all unaged and weathered samples at different
exposure times showing the mean values with error bars representing 95% confidence interval (PE50W samples at the 3.5 and 6
month time point were lost, therefore not shown in the above graphs).
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Figure 9 Schematic diagram showing the natural weathering of the composites with 50 wt% wood flour
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4. Conclusions
The effect of natural weathering on neat PHBV and PHBV/WF composites was studied, using
PLA/WF and PE/WF composites as reference materials. Real-time sunlight UV exposure
introduced a bleaching effect to all samples, including neat PHBV, but had no significant effect
on mechanical properties. On the other hand, mould grew slowly on the shaded surface of all of
the composites containing 50 wt% WF, independent of the polymer type. The trapped moisture
and the bulk supply of substrate (the unprotected wood phase) contributed to the mould growth,
which led to weight loss from fungal attack. Neat PHBV showed no sign of mould growth and
negligible dry weight loss. A less dense fungal cell network and lower dry weight loss values
were observed on the PHA20W composite when compared to PHA50W. At lower wood content,
the PHBV matrix acted as a barrier, partially encapsulating the wood particles. Interestingly,
lower relative abundance of fungi was shown in all PHBV containing samples, suggesting that
PHBV does not promote fungal colonization relative to woody materials. PHA depolymerase is
expressed by bacteria, and so the polymer is more susceptible to bacterial enzymatic attack. Neat
PHBV and PHBV/20 wt% WF plaques showed stable mechanical properties over the 12-month
period. However, the tensile strength and modulus of the composites (all polymer types) made
with 50 wt% dropped after 12 months of weathering. Such decreases were considered to be due
to the introduction of surface/bulk defects and an anticipated worsened wood-polymer interface
brought about by fungal attack of wood. Overall, it is concluded that wood content and the
accessibility of wood particles inside the matrix control the stability of the composites under
natural weathering. Future work will be directed to investigate the presumed anti-fungal
properties of PHA and the effect of PHA coating on the stability of the mechanical properties of
PHA WPCs following outdoor aging.
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5. Acknowledgement
The authors thank the Australian Research Council for funding through Linkage Grant
LP140100596. CC thanks the University of Queensland for the tuition waiver and the Research
Training Program from the Australian Government for scholarship during this study. The authors
also thank: the Centre of Translational Polymer Research group for lab support; Dr. Paul Evans
and Ms. Pawarisa Luangthongkam for aid with bio-sequencing work; the facilities, and the
scientific and technical assistance of the Australian Microscopy & Microanalysis Research
Facility at the Centre for Microscopy and Microanalysis, the University of Queensland for their
expertise in SEM imaging.
6. Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the
data also forms part of an ongoing study.
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Highlights
• UV exposure resulted in surface whitening of PHA and their composites.
• Neat PHA was physically and mechanically stable upon natural weathering.
• PHA composites behaved similarly to PLA and PE composites upon natural
weathering.
• Wood is the species responsible for mechanical deterioration due to mould growth.
• Partial sealing of wood flour by hydrophobic PHA provided protection from mould.