Composites Part A 107 (2018) 326–333
Contents lists available at ScienceDirect
Composites Part A
journal homepage: www.elsevier.com/locate/compositesa
Impact modified PLA-hydroxyapatite composites – Thermo-mechanical
properties
T
⁎
John O. Akindoyoa,b,c, Mohammad D.H. Bega,b, , Suriati Ghazalia, Hans P. Heimc,
Maik Feldmannc
a
Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, Gambang 26300, Kuantan, Malaysia
Center of Excellence for Advanced Research in Fluid Flow, Universiti Malaysia Pahang, Lebuhraya Tun Razak, Gambang 26300, Kuantan, Pahang, Malaysia
c
Institute of Materials Engineering, Mönchebergstr. – 3, 34125 Kassel, University of Kassel, Germany
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
A. Polymer-matrix composites (PMCs)
B. Impact behaviour
Thermomechanical
E. Injection moulding
Synergistic composites from poly(lactic acid) (PLA) and hydroxyapatite (HA) hold great potential for load
bearing applications. In this study, PLA-HA composites were produced through extrusion, and test specimens
were subsequently prepared through injection molding. Impact properties of the composite was improved by
incorporating different amount (0 to 15 wt%) of Biomax® Strong 120 (BS) impact modifier. The mechanical,
thermal, morphological and dynamic mechanical properties of the resulting composite were investigated.
Likewise, chemical interactions during blending were investigated through Fourier transforms infrared spectroscopy. Incorporation of BS decreased the crystallization activities at the PLA-HA interface, but with a right
shift in crystallization temperature as the BS content increases. Also with increasing BS content, mechanical
properties of the PLA-HA composite were reduced. However there is an obvious increment in impact strength
(78%) and elongation at break (206%). The composite containing 5 wt% BS content presents the best compromise among the investigated properties.
1. Introduction
Recently, calcium orthophosphate based biomaterials have received
much attention. They are being investigated for different medical application such as bone replacements and other fixations. Specifically,
for bone replacement applications, interest in these materials is based
on their salient properties such as biocompatibility, chemical stability,
low density, and the close chemical resemblance to natural bones minerals [1,2]. Hydroxyapatite (HA) is of particular interest because HA is
the main calcium phosphate phase present in bone. Literature revealed
that HA possesses the main properties desirable for effective bone replacement applications. These include noninflammatory, bioactive,
nonimunogenic, biocompatible, nontoxic, osteointegration, and osteoconductive properties [3–5]. However, HA is inherently hard, fragile
and brittle which makes it difficult to be processed into the required
form and shape. Also, the strength of HA is low which restricts its use to
non-load bearing applications. Currently, HA is being investigated as
filler for different polymeric matrices to produce biomaterials with
potential loadbearing replacement application [6].
One of the most important and suitable polymer matrix is poly
(lactic acid) (PLA), which at the moment is majorly used in the
⁎
packaging industry [7]. Interestingly, there is much interest in PLA for
bone replacement applications. In fact PLA based medical devices are
recently becoming commercially recognized [8], based on its peculiar
properties: PLA is biodegradable, biocompatible and bioresorbable [9].
In addition, PLA is characterized by considerable level of stiffness [7],
acceptable physicomechanical properties such as rigidity, high tensile
and flexural strength [10], and the ease of processability on conventional laboratory equipment such as extrusion and injection molding
[11]. These properties make PLA suitable for different applications and
specifically, based on their versatility, PLA based materials are often
considered as a standard for most regenerative applications [12].
Therefore, by taking advantage of the individual properties of PLA and
HA, synergistic PLA-HA composites can be produced with suitable
bioactive, biocompatible, bioresorbable, osteoconductive and biodegradable properties. One major benefit of this approach is property
tunability for different application, and it can also help to overcome the
several surgical procedures associated with non-degradable biomaterials [13].
However, one major issue that need to be addressed when producing PLA-HA composite is the poor adhesion between the hydrophilic
HA and hydrophobic PLA. Another main challenge is the high rate of
Corresponding author at: Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, Gambang 26300, Kuantan, Malaysia.
E-mail addresses: dhbeg@yahoo.com, mdhbeg@ump.edu.my (M.D.H. Beg).
https://doi.org/10.1016/j.compositesa.2018.01.017
Received 31 October 2017; Received in revised form 15 January 2018; Accepted 17 January 2018
1359-835X/ © 2018 Elsevier Ltd. All rights reserved.
Composites Part A 107 (2018) 326–333
J.O. Akindoyo et al.
HA agglomeration in the PLA matrix which sometimes lead to undesirable failure at the interface and a subsequent decrease in mechanical properties. Efforts had been concerted towards modifying the
HA surface in order to increase its compatibility with hydrophobic
polymers [14,15]. In a recent study, a phosphate based modifier was
used to enhance the dispersion of HA in PLA, enhance the interfacial
adhesion, and to improve the composite properties [16]. However, the
inherent brittleness of the PLA-HA composites demands further deliberate attention. Although the impact strength of the modified HA
composites was improved compared with unmodified HA composite,
the improved impact strength is still lower than for neat PLA [16].
Toughening of PLA to overcome the inherent brittleness of its composites and increase the impact properties have been well reported in
literature [10,11,17,18]. Different plasticizers and impact modifiers
have been investigated for PLA toughening [17–19], but report showed
that there is high tendency for low molecular weight plasticizers to
migrate out as a result of their high mobility within the PLA [18].
Therefore, high molecular weight plasticizers with low mobility within
PLA such as poly(propylene glycol) (PPG), and poly(ethylene glycol)
(PEG) are more preferable [18]. However, high molecular weight
plasticizers often present undesirable decreases in mechanical and
thermal properties of the composite [20,21].
An alternative approach for improving the impact properties of PLA
based materials is the use of impact modifiers. This can help to reduce
the inherent brittleness without adversely sacrificing the mechanical
and thermal properties [20]. Among the available impact modifiers,
Biomax strong (BS) from Dupont have been reported to offer good
impact and toughness enhancement to PLA, as the material was produced with special chemistry suitable for PLA toughening [10]. Indeed
there are several reports on PLA toughening using BS, most of which are
recent [10,18,21]. However, there is no report on incorporation of BS to
modify the impact properties of PLA-HA composites. Although BS has
not been medically approved, its well proven suitable chemistry for PLA
modification makes it a good candidate to investigate the effect of
impact modifiers on PLA-HA composites. Therefore, the aim of the
study is to investigate the effect of impact modifier on mechanical,
thermal and dynamic properties of PLA-HA composites containing different wt% impact modifier content.
Table 1
Code names and composition of the composite categories.
Code
PLA
PLA + HA
PLA + HA + 5% BS
PLA + HA + 10% BS
PLA + HA + 15% BS
Sample composition (wt%)
PLA
HA
BS
100
90
85
80
75
–
10
10
10
10
–
–
5
10
15
content < 0.1%), using TR–Dry–Jet EASY 15 (TORO-systems) air
drier. Likewise, the HA was dried (moisture content < 1%), in a convectional laboratory oven. HA was fed through the side of the compounder carrying the pre-melted PLA and BS, followed by mixing and
homogenization [23]. The extruded composite strand was cooled down
on a discharge conveyer and then cut to a length of about 3 mm by a
Scheer SGS 25-E strand pelletizer. The pellets were then dried (moisture
content < 0.1%) and test specimen were prepared in an injection
molding machine (Arburg allrounder 320C golden edition). The relevant parameters for extrusion and injection molding processes are
reported in previous article [16].
2.2.2. Fourier transform infrared (FTIR) spectroscopy
A Shimadzu FTIR spectrometer (Model-IR affinity-1S) was used for
the FTIR analysis. To obtain viable information about possible reactions
within the composite, FTIR spectra of the individual components as
well as the composites were all taken. The IR spectra were obtained
over a wavelength range of 400–4000 cm−1 using the standard KBr
technique.
2.2.3. Scanning electron microscopy
Morphology of the composite fractured surfaces after impact test
was investigated with the help of a Camscan Electron Optics Scanning
Electron Microscope (SEM) (Model-MV2300). Before SEM observation,
the samples were dried to make them moisture free after which they
were coated with thin layer of gold through sputtering so as to make
them conductive.
2. Materials and methods
2.2.4. Tensile test
Tensile testing was conducted on specimens prepared according to
EN ISO 527, using a Zwick/Roell Z010 testing machine. The dry specimens were conditioned at 23 °C and 50% relative humidity before
testing at speed of 5 mm min−1. Average result of seven specimens was
recorded for tensile strength (TS) and tensile modulus (TM). In addition, elongation at break of the composites under stress was obtained as
part of the tensile results.
2.1. Materials
The matrix used for this research is NatureWorks Poly(lactic acid)
(PLA), 3052D Biopolymer. The PLA is an injection molding grade
(Mw = 139,000 g/mol), with 1.25 g/cm3 density, melt flow index of
30–40 g/10 min (190 °C/2.16 kg), and melting temperature of
160–170 °C. The HA used was produced in our laboratory and its surface was modified as described in previous articles [16,22]. The impact
modifier Biomax® Strong 120, referred to as BS in this paper was kindly
supplied by Dupont. Other chemicals used such as ethyl acetate and
ethanol are of analytical grade and were procured from Roth, Germany.
These reagents were used without additional purification. Fabulase®
361 used as surface modifier for HA was obtained from Chemische
Fabrik Budenheim KG, Germany.
2.2.5. Flexural test
Specimen for flexural test was prepared according to EN ISO 178.
Testing was conducted on a Zwick/Roell Z010 universal testing machine at a speed of 10 mm min−1. Prior to testing, the specimens were
conditioned at 23 °C and 50% relative humidity. Flexural strength (FS)
and flexural modulus (FM) were obtained as average result of seven
specimens.
2.2. Methods
2.2.6. Charpy impact test
Charpy impact test was conducted using a Zwick charpy impact
machine according to EN ISO 179-1, on samples without notch. The test
speed was 2.93 m/s, and a hammer weighing 1 J was allowed to fall
freely to break the positioned impact specimens. Impact strength (IS)
was recorded as average result of seven specimens.
2.2.1. Production of composites
Composites were produced from PLA and surface modified HA [22]
with different wt% (0–15 wt%) BS. The amount of HA in the composite
was fixed at 10 wt% based on previous research [16]. The different
categories of composites produced and their code names are listed in
Table 1. Components of the composites were mixed and compounded
using a twin screw extruder (Leistritz ZSE 18 HPE, D = 18 mm, L/
D = 40) [23]. Before the extrusion process, PLA was dried (moisture
2.2.7. Thermogravimetric analysis
Thermogravimetric analysis
327
(TGA)
and
differential
thermal
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J.O. Akindoyo et al.
gravimetry (DTG) analysis were performed using a TA analyzer (TGA
Q500 V6.4, Germany). Samples were placed in a platinum crucible and
analysis was conducted in a nitrogen atmosphere (gas flow rate:
40 mL min−1) at 10 °C/min from room temperature to 800 °C.
2.2.8. Differential scanning calorimetric analysis
Calorimetry analysis (DSC) was conducted using a TA instrument
(DSC Q1000). Samples were heated from 20 to 250 °C at a scanning rate
of 10 °C min−1. From the DSC thermogram, glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm)
were determined. In addition, crystallinity index (XDSC%) of PLA in the
composite was calculated using Eq. (2) [24].
HOCl→H+ + OCl−
Low pH High pH
%crystallinity (XDSC ) =
(1)
ΔH
× 100%
ΔHm W
(2)
where, ΔH is the heat of fusion of the samples, ΔHm is the heat of fusion
of 100% crystalline PLA, and W is the mass fraction of the matrix. The
PLA crystallinity in the composites was calculated by taking the heat of
fusion (ΔHm) of 100% crystalline PLA to be 93.6 J/g [25].
2.2.9. Dynamic mechanical analysis
Dynamic mechanical analysis (DMA) was performed using a
Dynamic Mechanical Analyzer (DMA Q800). A single cantilever mode
(deformation amplitude: 20 µm) was used, from room temperature to
130 °C, at a constant frequency (1 Hz) and heating rate of 3 °C min−1.
The cantilever has a clamp length of 35 mm, and 10 mm x 4 mm specimens were analyzed.
3. Results and discussion
3.1. Mechanical properties
The tensile and flexural properties of PLA and its composites with
HA and BS are illustrated in Fig. 1. As can be seen, the tensile and
flexural properties of the PLA-HA-BS composites are influenced by the
impact modifier content. From Fig. 1a, the tensile strength (TS) and
tensile modulus (TM) of the impact modified composites can be seen to
reduce as the BS content increases. Similar trend is observed for flexural
strength (FS) and flexural modulus (FM) as illustrated in Fig. 1b. The
mechanical properties of PLA compared with the composites revealed
that the largest reduction in tensile and flexural properties was observed for composite with the highest BS content (15 wt%). It is not out
of place to observe a reduction in tensile and flexural properties of
polymeric materials after the inclusion of softer materials. Generally, in
semicrystalline materials such as PLA, it is expected that the inherent
bulk crystallinity would enhance the tensile and flexural properties of
its composites [26]. Therefore, the reduction in tensile and flexural
properties of the impact modified composites can be accrued to possible
suppression of PLA crystallinity in the PLA-HA-BS blend [26]. In another vein, these decreases can probably be as a result of weaker interface between the BS and other components of the composite especially HA. It can also be as a result of reduced load bearing capacity of
the impact modified composites due to reduction in the actual volume
of the stiffer and more stronger load bearing phase (PLA) within the
composite [27]. However, tensile and flexural properties alone are not
sufficient for overall assessment of materials suitability for specific load
bearing applications. Specifically, a very high capacity to absorb energy
is well desirable [17]. In fact, literature reveals that one necessary
feature of fixation materials is the need for good energy absorption
capacity during fracture [28].
The impact properties of PLA and its composites with HA and BS is
illustrated in Fig. 2. Obviously, the incorporation of BS strongly
modifies the impact strength (IS) of the PLA-HA composites. In general,
Fig. 1. (a) Tensile strength (TS) and tensile modulus (TM), and (b) flexural strength (FS)
and flexural modulus (FM) of PLA and the composites.
Fig. 2. Impact strength (IS) of PLA and the composites.
when impact modifiers are incorporated into semi crystalline polymers
such as PLA, the impact modifiers would act as lubricants to lubricate
the PLA molecules, thereby reducing its inherent brittleness and enhance its flexibility. As can be seen from Fig. 2, IS of the PLA-HA
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ethylene. It is worthy of note that this band exhibit a very high intensity
in the BS spectra which is attributed to the reasonably high amount of
ethylene components in BS. The intense peak at 1750 cm−1 represents
C]O stretching vibration of acetyl, carboxylic and ester groups [24].
The peak at 1452 cm−1 and the small band around 1365 cm−1 is attributed to CH3 stretching and CeH deformation respectively. The peak
at 1083 cm−1 is a characteristic peak from CeO stretching band [31].
One peculiarity of the BS spectra is the peaks at 1641 cm−1 and
1555 cm−1 which corresponds to C]C stretching, attributed to the
acrylic groups of BS.
The FTIR spectra of the PLA-HA composite and the PLA-HA-BS
composites are illustrated in Fig. 4b. The possible sites for bond formation between the individual components include the –OH groups of
HA [16], the epoxy groups of BS [27], and the functional –OH groups as
well as the carboxylic groups in the PLA chain [32,33]. As illustrated in
Fig. 4b, the spectra of all the composites are similar but by comparing
them with the spectra of the components in Fig. 4a, it can be seen that
as the BS content increased the intensity of the band around
3050–2830 cm−1 also increases. This is attributed to the increasing wt
% BS [10,21]. The –OH groups at the higher wavelength region around
3400 cm−1 in the HA spectra was not detected in the PLA-HA composite which is an indication of hydrogen bond formation between HA and
the functional groups of PLA. Likewise, the peaks at 1641 cm−1 and
1555 cm−1 in the BS spectra were not detected in the PLA-HA-BS
composites. Literature revealed that there is high possibility for chemical reaction between BS and PLA [27]. Therefore the disappearance
of the peaks is a strong indication of chemical reactions at the molecular level between BS, HA, and PLA. Furthermore, the peaks at
1750 cm−1 and 1083 cm−1 in the PLA-HA composite spectra can be
seen to slightly shift to a lower wavelength in the spectra of the PLAHA-BS composites and the shift becomes further as the BS content increases. This gives further evidence for intermolecular interaction
within the composite.
composite was increased at the incorporation of BS which is an indication of improved toughness. Similar observation was seen for the
elongation at break of the samples (Figure not shown). Compared to
PLA, the IS of the composite with 15 wt% BS content is about 61%
higher (Fig. 2) whereas compared with the composite without impact
modifier (PLA + HA), the IS increased by about 78%. Interestingly,
after incorporation of just 5 wt% BS, the IS of the PLA + HA composite
was increased by about 42% from 17.4 kJ/m2 to 24.7 kJ/m2. Similarly,
the elongation at break of the unmodified composite was improved by
about 144% after incorporation of 5 wt% impact modifier. These improvements after the incorporation of impact modifier suggest that the
impact modifier might have acted as stress concentrators. This would
enhance the energy absorption of the impact modified composites
during fracture, thereby resulting in increased impact and elongation
properties. Increase in impact strength of PLA composite at the incorporation of BS impact modifier is also reported in literature [18,29].
Based on the results of the mechanical properties, it can be inferred that
the composite with 5 wt% BS content presents the best compromise
between impact strength, elongation, tensile and flexural properties of
impact modified PLA-HA composites. Specifically, the IS and elongation
at break were obviously improved without adversely reducing the
tensile and flexural properties.
3.2. Morphological analysis
Dispersion of HA in PLA matrix is very important for good mechanical performance of the resulting composite. Improvement of HA
dispersion in PLA is reported in previous study [16]. However, due to
the inherent brittle nature and low impact strength of the PLA-HA
composites, impact modifier (BS) has been incorporated into the PLAHA system described in previous study [16]. It is well known that aside
dispersion, the bond strength at the interfacial boundaries of the PLA,
HA and BS is of paramount importance for desirable toughness properties. To investigate this, SEM images of the sample fractured surfaces
were taken and presented in Fig. 3. As shown in Fig. 3a, neat PLA revealed smooth morphology, typical of brittle materials. Surface of the
PLA-HA composite without BS (Fig. 3b) also revealed fairly smooth
morphology, attributed to little contribution of plastic deformation
from the brittle PLA matrix, as reported in a similar research [21].
On the other hand, morphology of the BS containing composites
revealed well dispersed droplets of BS, which becomes more conspicuous as the amount of BS in the system was increased (Fig. 3c–e)
similar to what was reported in literature [11]. It is well known that
dispersion and size of BS droplets in the PLA matrix could influence the
PLA toughness [11]. Also, there is possibility for good chemical interaction between PLA and BS [26]. As stated in Section 2.2.1, HA was
side-fed into the compounder carrying the pre-melted PLA and BS.
Therefore, it is believed that as BS becomes dispersed into the PLA in
form of micro domains (Fig. 3c–e), it might modify the interfacial
boundary of PLA and HA. Interestingly, this might be responsible for
the increased impact strength of the PLA-HA-BS composites (Fig. 2),
most probably due to crack bridging, matrix shear yielding and energy
absorption as reported previously [21]. However, this might also contribute to the reduced tensile and flexural properties of the PLA-HA-BS
systems compared with the PLA-HA composite (Fig. 1). This may be
attributed to disrupted interfacial adhesion between PLA and HA, due
to possible modification of the PLA-HA interface by BS.
3.4. Thermal properties
The TGA and DTG thermograms of PLA and its composites with HA
and BS are illustrated in Fig. 5. Thermal degradation properties of the
samples are summarized in Table 2. Comparing the degradation properties of the composites with pure PLA, it can be seen that the degradation onset temperature (Tonset), and degradation temperature (Td)
of the composite are all lower than for pure PLA. This can be attributed
to higher destabilization of PLA structural framework within the composites. Among the composites, the unmodified composite (PLA + HA)
revealed the highest thermal stability and among the composites containing BS, composite with the lowest wt% BS (PLA + HA+5% BS)
revealed the least thermal stability. The strength of interfacial bonding
within a composite is known to be one of the determinants of the
composite thermal stability. Electrostatic attraction between the Ca2+
ions of HA and the carboxylate group of polymers is reported to largely
influence the interfacial interaction in HA based polymer composites
[4]. Interference with this electrostatic interaction might be responsible
for the reduced thermal stability of the composites containing BS,
compared with the unmodified composite.
On the other hand, the lower thermal stability of the composite
containing 5 wt% BS suggests inferior interfacial bonding compared to
composites containing higher BS content. Literature revealed that there
is high possibility for molecular interaction between PLA and BS when
the BS content is > 1 wt% [27]. The possible reaction between PLA and
BS is illustrated in Fig. 6. Therefore, as the BS content increases, there is
larger possibility for interaction between PLA and BS which might have
increased the thermal stability of the impact modified composites as the
BS content increases. Increase in thermal resistance of composites following the incorporation of impact modifiers was also reported elsewhere [17]. The residue after 700 °C is included in Table 2 and it can be
seen that the composites revealed higher residue content compared to
3.3. Fourier transforms infrared (FTIR) spectroscopy
The FTIR spectra of PLA, HA, and BS are illustrated in Fig. 4a. The
major peaks common to all the components include the higher wavelength vibrational stretching band of bonded –OH groups [30], obvious
around 3300–3200 cm−1 in BS spectra and 3500–3400 cm−1 in HA
spectra. The characteristic stretching band at 3050–2830 cm−1 corresponds to asymmetric and symmetric stretching of CeH of ethyl and
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Fig. 3. SEM images of fractured surfaces of (a)
pure PLA, (b) PLA + HA, (c) PLA + HA + 5% BS,
(d)
PLA + HA + 10%
BS,
and
(e)
PLA + HA + 15% BS.
higher temperature and the right shift can be seen to increase as the BS
content increases. As explained in previous research, the lower temperature shift of the PLA-HA composites is an indication of increased
crystallization rate attributed to the ability of HA to induce heterogeneous nucleation into the PLA matrix [16,34]. This also contributes
to the right shift in Tm peak of the PLA-HA composite owing to the
formation of less perfect crystals which would normally melt at higher
temperature compared with the more prefect crystals [16]. For the
composites containing BS, it can be seen that the exothermic crystallization peak became broader with decreased magnitude coupled with a
right shift as the BS content increases. It is well known that variation in
crystallization temperature of a material after incorporation of other
components would manifest in the form of either a faster or slower rate
of crystallization [35]. For the composites containing 10 and 15 wt%
BS, the crystallization peak is not obvious which can be explained as
follows: the crystallization behaviour of semi crystalline matrices such
as PLA in partially miscible blends is influenced by the nucleating activities at the surface boundaries of the two components [26]. Due to
the rubbery nature of BS, increasing the BS content could lead to
creation of increased number of droplets (confirmed through SEM
neat PLA which is attributed to the presence of HA in the composites.
3.5. Differential scanning calorimetry (DSC) analysis
The DSC thermograms of PLA and its composites with HA and BS
are illustrated in Fig. 7, and the important transitions were analyzed.
From Fig. 7, Tg of the unmodified PLA-HA composite can be seen to shift
towards the right side whereas the Tg of the composites containing BS
was shifted to the left side compared with pure PLA. Reason for the
right shift of the unmodified composites is well explained in literature
[16]. It is well know that at temperature close to the Tg of polymers, the
polymer chains are more flexible and can therefore move more freely.
One of the main reasons for incorporating impact modifier into semi
crystalline polymers such as PLA is to lubricate the polymer chains,
make them more flexible and increase their mobility. Therefore the left
shift in Tg of the BS composites suggest enhanced polymer chain mobility. The DSC parameters of PLA and the different composite categories are summarized in Table 3.
The Tc of the unmodified composite moved to a lower temperature
than pure PLA whereas Tc of the composites containing BS moved to
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Fig. 4. FTIR spectra of (a) components of the composites, and (b) PLA-HA composites
with different BS wt% content. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Fig. 5. (a) TGA and (b) DTG thermograms of PLA and the different composite categories.
(For interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
observation), leading to reduced interfacial contact area between PLA
and HA such that the crystallization activities at the interface becomes
reduced. This indicates that incorporation of impact modifier can influence the crystallization behaviour of the PLA-HA composite as shown
in Fig. 7 and Table 3.
The endothermic melting peak of PLA and PLA-HA composite reveals two distinct peaks. This is attributed to the different crystalline
structure of the beta- and alpha- forms of semi crystalline PLA [26]. On
the other hand, the BS composites showed single and reduced Tm peaks
which are due to the reduced crystallization activity at the interfacial
boundaries, due to hindrance to PLA crystallization within the composites. Similar observation was also reported in literature for PLA and
epoxidized natural rubber composites [26]. Crystallization of PLA in
the composites was calculated as described in Section 2.2.8 and the
result is included in Table 3. The result shows that the crystallinity of
PLA in the PLA-HA composite was reduced and the effect become increased as the BS content was increased. Reduced crystallinity is an
indication of higher free volume content within the composite, and an
increase in the amorphous content [27]. Increase in amorphous nature
of the composites as the BS content increases would lead to reduction in
the composite stiffness and this might have contributed to the low
tensile and flexural properties of the BS composites as illustrated in
Fig. 1. Interestingly, controllable PLA crystallinity is a positive from
medical point of view as it can facilitate controlled degradation of the
Table 2
Thermal properties of PLA and the different composite categories.
Sample code
Tonset (°C)
Td (°C)
Residue (%) at T ≥ 700 °C
PLA
PLA + HA
PLA + HA + 5% BS
PLA + HA + 10% BS
PLA + HA + 15% BS
330
314
272
276
280
369
360
324
332
334
0.33
10.29
10.17
10.14
9.95
Fig. 6. Possible reaction between PLA and BS during blending.
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Fig. 8. Storage modulus, E′ curves of PLA and the different composite categories. (For
interpretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)
Fig. 7. DSC thermograms of PLA and the different composite categories.
materials such that the active components can be made available at the
implant site at the desired time [28].
DSC result and also aligns with observation from previous research
[27].
3.6. Dynamic mechanical properties
4. Conclusions
The storage modulus (E′) curves of PLA, PLA-HA composite and
PLA-HA-BS composites are illustrated in Fig. 8. As can be seen from
Fig. 8, addition of HA led to increase in storage modulus of PLA which
is attributed to the higher modulus of HA compared with PLA [16].
Incorporation of BS into the PLA-HA composite led to decrease in the
storage modulus of the composite and the decrease becomes greater as
the BS content increases. For example, at 5 wt% BS content, there was
less than 1% decrease in storage modulus of the PLA-HA composite
whereas at 15 wt% HA content, the decrease in storage modulus is
about 37%. The storage modulus values of PLA and the composites are
included in Table 3. The decrease in storage modulus of the composites
as BS content increases can be associated with the possible reduction in
stiffness of the composite. This also aligns with what has been reported
in literature for impact modified PLA blends [27]. Generally, the storage modulus of all the samples gradually decreases with increasing
temperature but with a sharp drop around 57–63 °C. This region represents the glass transition region and the steep drop in storage
modulus is attributed to increased chain mobility due to softening and
segmental movement of the PLA molecules [27].
Furthermore, the influence of BS on the PLA-HA composite was
evaluated through the damping coefficient (tan δ). One important
feature of the tan δ curve is its ability to provide accurate measurement
of the Tg. From the tan δ curve, the Tg is determined as the temperature
at the maximum tan δ peak. The Tg obtained from the tan δ peak (not
shown) is included in Table 3 with the corresponding maximum tan δ
peak value. From Table 3, magnitude of the tan δ peak of the PLA-HA
composite can be seen to gradually reduce as the BS content increases,
accompanied by a reduction in Tg. This observation conforms to the
Composites were prepared from poly(lactic acid) (PLA) and hydroxyapatite (HA) for potential load bearing applications. To reduce the
inherent brittle nature of the composites, different wt% (0–15 wt%)
biostrong impact modifier (BS) was incorporated into PLA-HA composites containing 10 wt% HA content. Impact strength of the PLA-HA
composite was increased at the incorporation of BS which is an indication of improved toughness and enhanced energy absorption
ability. Elongation at break of the unmodified PLA-HA composite was
also enhanced by about 144% at 5 wt% BS content. The composite with
5 wt% BS content presents the best compromise between impact
strength, tensile and flexural properties. On the other hand, evidence of
intermolecular interaction between PLA and BS was confirmed through
FTIR analysis but interference with the electrostatic interaction between PLA and HA was observed to reduce the thermal stability of the
impact modified composites. Similarly, disrupted crystallization activities at the PLA-HA interface led to reduced crystallinity of the PLA-HABS composites. Understanding of the mechanical, thermal and crystallization behaviour of impact modified PLA-HA composite can help to
tailor the composite performance for desirable properties. It is evident
from this study that incorporation of impact modifier led to sacrifice of
mechanical, thermal and dynamic mechanical properties of PLA-HA
composites. However, at 5 wt% impact modifier content, the reduction
is minimal and it came alongside a remarkable improvement in impact
and elongation properties which makes it suitable for potential loadbearing applications.
Table 3
DSC and DMA parameters of PLA and the different composite categories.
Sample code
PLA
PLA + HA
PLA + HA + 5% BS
PLA + HA + 10% BS
PLA + HA + 15% BS
DSC
DMA
Tg (°C)
Tc (°C)
Tm1 (°C)
Tm2 (°C)
Xc (%)
E′ (MPa)
Tg (°C)
Tan δ peak
62.21
63.64
60.91
60.91
60.91
111.34
108.37
132.35
–
–
148.23
150.09
149.58
149.44
148.19
155.35
158.44
–
–
–
28.48
34.43
5.66
2.63
1.55
2295
3064
3048
2232
1923
65.65
66.78
65.22
65.20
64.98
2.29
1.93
1.89
1.82
1.70
332
Composites Part A 107 (2018) 326–333
J.O. Akindoyo et al.
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The authors sincerely appreciate Universiti Malaysia Pahang for the
financial assistance through short term grant (RDU 1603139). We also
thank Hessen State Ministry of Higher Education, Research and the
Arts-Initiative for development of Scientific and Economic Excellence
(LOEWE)-special research project “safer materials” for the financial
support.
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