materials

Article
The Influence of Sub-Zero Conditions on the
Mechanical Properties of Polylactide-Based Composites
Olga Mysiukiewicz 1, * , Mateusz Barczewski 1 and Arkadiusz Kloziński 2
1 Institute of Material Technology, Faculty of Mechanical Engineering, Poznan University of Technology,
61-138 Poznan, Poland; mateusz.barczewski@put.poznan.pl
2 Institute of Chemical Technology and Engineering, Faculty of Chemical Technology,
Poznan University of Technology, 61-138 Poznan, Poland; arkadiusz.klozinski@put.poznan.pl
* Correspondence: olga.mysiukiewicz@put.poznan.pl; Tel.: +48-61-647-5951




Received: 20 November 2020; Accepted: 16 December 2020; Published: 18 December 2020


Abstract: Polylactide-based composites filled with waste fillers due to their sustainability are a subject
of many current papers, in which their structural, mechanical, and thermal properties are evaluated.
However, few studies focus on their behavior in low temperatures. In this paper, dynamic and
quasi-static mechanical properties of polylactide-based composites filled with 10 wt% of linseed cake
(a by-product of mechanical oil extraction from linseed) were evaluated at room temperature and at
−40 ◦ C by means of dynamic mechanical analysis (DMA), Charpy’s impact strength test and uniaxial
tensile test. It was found that the effect of plasticization provided by the oil contained in the filler
at room temperature is significantly reduced in sub-zero conditions due to solidification of the oil
around −18 ◦ C, as it was shown by differential scanning calorimetry (DSC) and DMA, but the overall
mechanical performance of the polylactide-based composites was sufficient to enable their use in
low-temperature applications.

Keywords: polylactide; waste filler; mechanical properties; sub-zero properties

1. Introduction
Polylactide or poly(lactic acid) (PLA) is an aliphatic polyester that can be synthesized from
renewable sources, including corn starch [1–3]. Due to its relatively good availability and satisfactory
mechanical properties, it is a common choice when an eco-friendly alternative to conventional
polymeric materials is needed [4,5]. PLA can be processed using various technologies, including
injection molding [3,6–8], extrusion [2,7,9] or fused deposition modeling 3D printing [10–13], and its
properties can be tuned by different modifiers such as plasticizers [14–17], chain extenders [18] or
nucleating agents [19–21]. Therefore, it can be successfully used in many applications, including
the automotive industry, production of packages and disposable goods, or even special medical
products [1,3,22–24]. Research works published in recent years, as well as industrial implementations,
also indicate that polylactide can also be used as a matrix of composites. Even though conventional
fillers such as glass fiber can be successfully embedded in PLA matrix [25], environmentally friendly
fillers are the most common choice, as they allow us to obtain a fully bio-based biodegradable
material. Numerous examples of polylactide-based composites reinforced with sisal fibers [26,27],
flax fibers [28–31], wood [23,32] or hemp [33] can be found in the literature, but an even more sustainable
solution is the application of the so-called waste fillers, i.e., the by-products mainly from agriculture or
food industry such as nutshells, husks, seeds, waste fibers [9,34–38] and more. The application of waste
fillers is not only in line with the idea of Circular Economy [5,39], but oftentimes improves different
properties of the resulting composites [40,41]. In our previous works, we have shown that addition of
linseed cake (i.e., the by-product of oil extraction from linseed or flax, Linum usitatissimum L.) to PLA

Materials 2020, 13, 5789; doi:10.3390/ma13245789 www.mdpi.com/journal/materials

Materials 2020, 13, 5789 2 of 12

causes an increase in crystallinity and elongation at break of the composites, which is attributed to the
plasticizing effect of linseed oil contained by the filler [42,43]. It can be concluded that polylactide and
its composites are a versatile group of environmentally friendly materials, which can be successfully
used for different applications. According to the 2020 report by European Bioplastics, PLA makes up
for 13.9% of 2.11 million tons of globally produced bioplastics and its production capacity is expected to
grow significantly in the next years [44]. Therefore, a comprehensive analysis of polylactide properties
is needed to fully benefit on its large-scale applications.
Like any other material, PLA has its disadvantages, such as brittleness and low crystallization
rate [21]. However, from the application point of view, its thermomechanical stability at elevated
temperatures is usually the main concern. Different strategies can be implemented to increase the
thermomechanical stability of PLA, including the addition of mineral fillers [45,46] or application of
nucleating agents [21,47]. A less researched area is the behavior of polylactide and its composites
at sub-zero conditions. This situation is understandable as the glass transition temperature of PLA
is around 50–80 ◦ C [1] and its properties in the glassy state are considered stable. Nevertheless,
as Kuciel et al. showed in their paper, the mechanical properties of polylactide-based composites
change in sub-zero conditions [32]—a significant increase in tensile strength was denoted at −24 ◦ C in
comparison with its +24 ◦ C value. It also needs to be realized that the addition of fillers or modifying
agents to PLA can significantly alter its low-temperature behavior. The additives can significantly
decrease the glass transition temperature of the matrix polymer or, in some cases, freeze or solidify in
low temperatures [48]. The waste fillers, such as seed cake, can be very susceptible to the temperature
changes due to the presence of the natural oil, which solidifies below 0 ◦ C [49,50].
Even though room temperature properties are crucial in most applications, the sub-zero behavior
of a material is also important, especially in the case of e.g., packages for frozen foods and other
goods stored in low temperatures as well as outdoor applications in winter in colder climates.
The temperature-induced embrittlement can result in a failure of a part, therefore the need for
evaluation of the sub-zero behavior of materials is reasonable from both scientific and economic points
of view.
The aim of this paper is a comprehensive analysis of sub-zero behavior of polylactide-based
composites filled with linseed cake (LC). The dynamic and quasi-static mechanical properties of
the composites with different LC grades were determined at room and sub-zero conditions and
analyzed in relation to the phase transitions of the natural oil contained by the filler to describe the low
temperature-induced changes of the material properties and to identify their causes. The temperature
of −40 ◦ C was chosen for the analysis, as it is slightly below the lowest temperatures denoted in Poland
in last years, which is −35 ◦ C [51]. Therefore, the paper will help to evaluate the performance of
PLA-based composites in outdoor applications in Eastern European climate.

2. Materials and Methods

2.1. Materials
A multipurpose grade of polylactide Ingeo 2500 HP by Natureworks (Minnetonka, MN, USA),
with Melt Flow Index of 8 g/10 min (210 ◦ C, 2.16 kg), a density of 1.24 g/cm3 and d-isomer content
<0.5%, was used as the matrix of the composites.
Linseed cake (LC) was obtained from a local Polish supplier (Laboratorium Biooil, Zielona Góra, Poland).
To evaluate the influence of oil content on the composite’s properties, the LC was partially defatted to
obtain 5 grades with 0.9, 4.6, 17.7, 30.4 and 39.8 wt% of natural oil, respectively. The defatting procedure
consisted of mechanical mixing of the linseed cake with acetone, filtration and drying. The fillers were
then screened through a 630 µm sieve. A more comprehensive description of the preparation of linseed
cake with various oil content can be found in our previous study [43]. The natural oil extracted from
linseed cake was also examined after removing the acetone by evaporation.
Materials 2020, 13, 5789 3 of 12

2.2. Sample Preparation

Composite samples with filler content fixed to 10 wt% were produces using a melt blending method
and specimens for evaluation of mechanical properties were injection molded. First, the components
were preliminarily mixed and dried overnight at 70 ◦ C in a cabinet drier (Memmert, Schwabach,
Germany). They were blended in a molten state using a Zamak EHD 16.2 co-rotating twin-screw
extruder (Zamak Mercator, Skawina, Poland) operating at 120 rpm and 190 ◦ C. The pelletized
composites were dried as before and injection molded in a Battenfeld PLUS-35 machine (Heilbronn,
Germany) with the following parameters: injection temperature of 210 ◦ C, mold temperature of 50 ◦ C,
the injection pressure of 72 MPa. Pure PLA was processed along with its composites. The samples
were named in reference to the used LC grade (for example, sample LC17.7 contains 10 wt% of linseed
cake containing 17.7 wt% of natural oil). A detailed description of the linseed cake preparation and
manufacturing of the composites can be found in our previous paper [43].

2.3. Methods
To evaluate the solidification of the oil extracted from the filler, its viscosity was evaluated using
an oscillatory rotational rheometer Anton Paar MCR 301 (Anton Paar, Graz, Austria) in a 25 mm
cone-plate configuration. The measurements were carried out in the temperature range from 30 to
−80 ◦ C with a cooling rate of 1 ◦ C/min. A strain of 0.5% and a frequency of 10 Hz were applied.
The thermal properties of the linseed oil were analyzed by means of differential scanning
calorimetry (DSC) using a Neztsch DSC 204 F1 apparatus (Netzsch, Selb, Germany). A sample of 10 mg
was placed in a standard aluminum crucible (Netzsch, Selb, Germany) with pierced lid and cooled
from 25 to −50 ◦ C. It was held at −50 ◦ C for 10 min and then heated to 200 ◦ C. After that, the oil was
cooled back to −50 ◦ C. The measurements were performed with a heating/cooling rate of 10 ◦ C/min in
a Nitrogen atmosphere with a flow rate of 20 mL/min.
The dynamic mechanical properties of the composites and the pure PLA were assessed by Dynamic
Mechanical Analysis (DMA, Anton Paar, Graz, Austria) in the oscillatory mode using an Anton Paar
MCR 301 apparatus. The strain was fixed to 0.01% and the frequency was 1 Hz. The measurements
were conducted in the temperature range from −80 to 25 ◦ C.
The impact strength of notched samples was determined by Charpy’s method for two temperatures:
−40 and 25 ◦ C using a Ceast 9050 tester equipped with a 5 J pendulum. At least 5 samples of each type
were tested. The differences between the mean values obtained by the composites tested in different
conditions were evaluated by a one-way analysis of variance method (α = 0.05).
The tensile strength of the composites and pure PLA was evaluated in quasi-static conditions at 25
and −40 ◦ C using a Zwick/Roell Z020 universal testing machine (Kennesaw, GA, USA). The crosshead
speed was 2 mm/min during the tensile modulus evaluation and 50 mm/min during the remaining
part of the test. At least 6 samples of each kind were tested. The differences between the mean values
measured in different conditions were evaluated by one-way analysis of variance method (α = 0.05).
Brittleness B of the samples was evaluated according to the Equation (1) proposed by
Brostow et al. [52], based on the results of the tensile test and DMA.

1
B= (1)
εG0
where: ε—elongation at break determined in the quasi-static tensile test, G0 —storage modulus
evaluated by means of DMA.

3. Results

3.1. Linseed Oil Evaluation

The changes of complex viscosity of linseed oil extracted from the filler in the function of
temperature, as well as the DSC curve registered during cooling of the oil, are presented in Figure 1.
3. Results

3.1. Linseed Oil Evaluation

The changes of complex viscosity of linseed oil extracted from the filler in the function of
temperature, as 13,
Materials 2020, well as the DSC curve registered during cooling of the oil, are presented in Figure 1.
5789 4 of 12
The DSC thermogram shows two overlapping peaks at −16 °C and −29 °C. They indicate solidification
(crystallization) of the vegetable oil [49]. The presence of the multiple crystallization peaks is typical
peaks at −16 ◦ and −29 ◦ C. They indicate solidification
for The DSC thermogram
vegetable oil, which showsusually two overlapping
shows polymorphism and C can create a hexagonal α form, an
(crystallization)
orthorhombic of the vegetable
perpendicular oil [49]. The
β’ structure andpresence of the
a triclinic multipleβ crystallization
parallel polymorph, each peaks of is typical
them for
vegetable oil,
characterized with which usually level
a different showsofpolymorphism
stability [50,53]. andThecan crystalline
create a hexagonal
form ofαthe form,
oil an orthorhombic
depends on
perpendicular
solidification β’ structure
conditions as welland a triclinic
as the amountparallel β polymorph,
of saturated each of them
and unsaturated characterized
fatty acids of differentwith a
different
length level
[50]. The of stability
phase transition [50,53]. The crystalline
of natural oil is also form
shown ofby
thethe
oilchanges
dependsinon solidification
the conditions
complex viscosity,
as well as the amount of saturated and unsaturated fatty acids
as presented in Figure 1. The η* value at room temperature is about 0.1 Pa*s, which is typicalof different length [50]. Theforphase
vegetable oil [54]. Cooling to 0 °C results in only a slight increase in viscosity, which results from less 1.
transition of natural oil is also shown by the changes in the complex viscosity, as presented in Figure
The η*movements
intensive value at room temperature
of the molecules is about
[54]. After0.1that,
Pa·s,awhich
drasticisincrease
typical for vegetableviscosity
in complex oil [54]. takes
Cooling
place, 0 ◦ C results
to which can be in only a slight
identified as the increase in viscosity,
solidification of the oil.which results rate
The highest from ofless intensive
the change movements
takes place
of temperature
in the the moleculesrange [54]. ofAfter
−11 to that,
−17a°C,
drastic
which increase
overlaps in complex viscosity
with the slope takes
of the place, which
crystallization can be
peak
identified as the solidification of the oil. The highest rate of the change
recorded during the DSC measurements. According to the literature, linseed oil solidification takes takes place in the temperature
range of −11 ◦ C, which overlaps with the slope of the crystallization peak recorded during the
−17which
place around −18to°C, is consistent with the result of our experiment [55]. After that, the η*
DSC
value measurements.
stabilizes around 40According
Pa*s and then to the literature,
increases linseed
again, whichoil may
solidification
result from takes
theplace
phasearound −18 ◦ C,
transition.
Thewhich
complex is consistent
viscosity ofwith the result
linseed of our
oil at −40 °C isexperiment [55]. After
1640 Pa*s, which that, the
is 4 orders η* value stabilizes
of magnitude higher thanaround
40 Pa·s
at room and then increases
temperature. Based on again,
thosewhich mayit result
results, can befrom the phase
predicted thattransition. The of
the influence complex
linseedviscosity
oil on of
oil at −40 ◦composites
the linseed cake-filled C is 1640 Pa·s, maywhich is 4 orders
be different in theofroom
magnitude higher temperature
and sub-zero than at roomranges.temperature.
Based on those results, it can be predicted that the influence of linseed oil on the linseed cake-filled
composites may be different in the room and sub-zero temperature ranges.

Figure 1. Changes of complex viscosity and the run of DSC curve for the linseed oil.
Figure 1. Changes of complex viscosity and the run of DSC curve for the linseed oil.
3.2. Evaluation of Polymeric Composites
3.2. 3.2.1.
Evaluation of Polymeric
Dynamic Composites
Mechanical Analysis
The run of the loss modulus of the linseed oil registered during the rheological measurements is
shown in Figure 2a. The runs of storage and loss moduli (G0 and G”, respectively) evaluated by DMA
for the composite samples and the pure PLA vs. temperature are shown in Figure 2b,c. The run of the
damping factor tanδ as a function of temperature evaluated by DMA for the composite samples is
shown in Figure 2d.
Materials 2020, 13, 5789 5 of 12
Materials 2020, 13, x FOR PEER REVIEW 6 of 12

Figure
Figure 2. 2.Changes
Changesofof(a)
(a)storage
storage and
and loss moduli
moduliofofthe
thelinseed
linseedoil,
oil,determined
determinedbyby
oscillatory rheology,
oscillatory rheology,
(b) loss modulus of the polymeric samples (c) storage modulus and (d) damping factor
(b) loss modulus of the polymeric samples (c) storage modulus and (d) damping factor of the polymeric of the
polymeric samples in function of temperature,
samples in function of temperature, determined by DMA.determined by DMA.

When the valuethe

Interestingly, G0 is higher
of melting of thethan G”be
oil can the elastic
also properties
observed on the oftanthe material
δ curve dominate,
presented and it
in Figure
can2d.beInconcluded
this case, athat behaves
deviation fromas atheviscoelastic
linear shape solid. For
of the G < G”factor
0
damping the viscous
plot canbehavior is prevalent,
be also spotted for
andthethe sample cancomposite.
PLA-LC-30.4 be considered a viscoelastic
Nevertheless, liquid.
due to smallTherefore,
values of tan theδGin0 = G”temperature
this crossover pointsrange,canthe be
associated
signal is with
noisy;the phase transition
therefore, of the oilthat
it can be decided [56].
theAsruncan
of be
theseen in Figure
G″ curve 1, below
is a more reliable ◦ C linseed
−26 indicator of oil
oilits
is in relaxation
solid form in the
andPLA-LC
above −6 ◦
composites. ◦
C it is liquid. In the range from −26 to −6 C, it can be presumed that
phase transitions between its polymorphs take place. The melting point of a fat-based substance can
also be determined if a sudden drop of complex modulus below 100 Pa takes place [57]. In the case of
3.2.2. oil,
linseed Impactthis Strength takes place around −14 ◦ C. Based on that information, it can be predicted that
behaviorEvaluation
the properties
The mean of values
the linseed oil-modified
of impact polymericincomposites
strength evaluated can change
room temperature and atdue−40to°Cphase
alongtransitions
with p-
of values
the oil.obtained
The thermomechanical properties of the linseed cake-filled composites
in one-way analysis of variance, indicating the presence of significant differences were evaluated by
DMA and the 0 and G0 vs. temperature are presented in Figure 2b,c.
curves of Gproperties
between the resulting
dynamic mechanical of the samples tested in different conditions are collected
in The
Table storage
1. The modulus of the PLA
impact strength of pureandPLAPLA-based
measured composite
at 25 °C samples
is about does
2.4 kJ/mnot2 change
and, due notably
to the in
theaddition
studied of temperature
the low-fatrange,linseed butcake,
a steady decrease(for
it decreases canthe
be observed.
PLA-LC0.9The andslope of the curve
PLA-LC4.6 is steeper
samples) or
in remains
the case at of the
the same
PLA-LC-30.4
level (in and PLA-LC-39.8
the case composites,
of PLA-LC17.7 which indicates
and PLA-LC30.4 that they
ones). This resultare more prone
is typical in
to the case of
changes of polymeric
propertiescomposites
in functioncontaining low aspect
of temperature. ratio filler,
The oil-rich whose
samples areparticles act as pointsbyofthe
also characterized
lowest 0 values, which
stressGconcentration andcanfacilitate the propagation
be associated with a lower of cracks
amount[62]. Similar
of rigid behavior was
lignocellulosic denoted[58,59]
particles by
andAndrzejewski
the fact thatetthe al. linseed
in the case of softer
oil is polycarbonate
than PLA, filled
evenwith
in biochar,
its solid whose
form. Izod The impact
analysisstrength
of G” vs.
dropped from
temperature curve650brings
J/m tomore 13 J/m [63]. However,
insight the application of the
into the thermomechanical oil-rich LC,
properties such ascake-filled
of linseed LC39.8,
causes an increase in impact strength. This result can be attributed
composites. The pure PLA and the linseed cake-based composites show a similar behavior: the to the plasticizing effect ofvalues
the
natural oil, which promotes the movements of macromolecules. Consequently,
of G” decrease slowly in the function of temperature, which is a typical result for this polymer [60]. the material can
deform during
However, the impact
the changes in the and dissipates
loss modulus more energy.
in the It canof
function betemperature
observed thatare thenot
plasticizing
the sameeffectfor all
the studied polymers. The unfilled PLA and its composites containing up to 30.4 wt% of oil in the
Materials 2020, 13, 5789 6 of 12

filler present a similar, steady decrease in G0 , whereas the loss modulus of the PLA-LC-39.4 sample
remains stable up to −25 ◦ C and then decreases rapidly. Interestingly, this sudden change of G0 takes
place in the same temperature range as the phase transition of the natural oil contained by the sample.
Therefore, it can be concluded that it is melting of the linseed oil contained by the filler, which causes a
distinct change in loss modulus of the composite. The decrease in the oil’s G” results in a lowering
of the composite’s loss modulus, which indicates that the influence of this modifying agent on the
PLA-based material is notable. The fact that the relaxation of the oil contained by the composite can be
identified in its G” plot in the same temperature range as in the case of the rheological measurements
indicates that PLA and linseed oil are not well miscible [61]. A similar result was denoted during
observations by scanning electron microscopy of the linseed cake-filled samples, as it is described in
our previous research [43].
Interestingly, the melting of the oil can be also observed on the tan δ curve presented in Figure 2d.
In this case, a deviation from the linear shape of the damping factor plot can be also spotted for the
PLA-LC-30.4 composite. Nevertheless, due to small values of tan δ in this temperature range, the signal
is noisy; therefore, it can be decided that the run of the G” curve is a more reliable indicator of oil
relaxation in the PLA-LC composites.

3.2.2. Impact Strength Evaluation

The mean values of impact strength evaluated in room temperature and at −40 ◦ C along with
p-values obtained in one-way analysis of variance, indicating the presence of significant differences
between the dynamic mechanical properties of the samples tested in different conditions are collected in
Table 1. The impact strength of pure PLA measured at 25 ◦ C is about 2.4 kJ/m2 and, due to the addition of
the low-fat linseed cake, it decreases (for the PLA-LC0.9 and PLA-LC4.6 samples) or remains at the same
level (in the case of PLA-LC17.7 and PLA-LC30.4 ones). This result is typical in the case of polymeric
composites containing low aspect ratio filler, whose particles act as points of stress concentration and
facilitate the propagation of cracks [62]. Similar behavior was denoted by Andrzejewski et al. in
the case of polycarbonate filled with biochar, whose Izod impact strength dropped from 650 J/m to
13 J/m [63]. However, the application of the oil-rich LC, such as LC39.8, causes an increase in impact
strength. This result can be attributed to the plasticizing effect of the natural oil, which promotes
the movements of macromolecules. Consequently, the material can deform during the impact and
dissipates more energy. It can be observed that the plasticizing effect of the natural oil compensates for
the decrease in impact strength due to the presence of rigid lignocellulosic particles.

Table 1. The impact strength of the samples tested at different temperatures.

Impact Strength
PLA PLA-LC0.9 PLA-LC4.6 PLA-LC17.7 PLA-LC30.4 PLA-LC39.8
[kJ/m2 ]
25 ◦ C 2.38 ± 0.2 1.83 ± 0.3 2.17 ± 0.4 2.34 ± 0.4 2.42 ± 0.4 3.12 ± 0.1
−40 ◦ C 2.24 ± 0.1 1.95 ± 0.3 2.19 ± 0.1 2.18 ± 0.1 2.63 ± 0.3 2.35 ± 0.2
p-value 0.2617 0.6078 0.9448 0.4587 0.4115 0.0002

The decrease in the testing temperature to −40 ◦ C causes a change in the impact strength for all the
tested materials, but no relationship between the low and room temperature values can be observed.
What is more, as indicated by the p-values > 0.05, the difference is statistically insignificant for all the
samples except for the PLA-LC39.8. In the case of the latter, the impact strength tested at −40 ◦ C is
0.77 kJ/m2 lower in comparison with the result of the room temperature measurement. This effect can
be explained by solidification of the natural oil, whose molecules lose the abilities to move and thus
promote the movements of PLA macromolecules. Even though the oil solidification of oil takes place
in all the linseed cake-modified samples, only in the case of PLA-LC39.8 its content is high enough to
cause a significant difference.
 enough to cause a significant difference.

Table 1. The impact strength of the samples tested at different temperatures.

Impact Strength [kJ/m2] PLA PLA-LC0.9 PLA-LC4.6 PLA-LC17.7 PLA-LC30.4 PLA-LC39.8

25 °C
Materials 2020, 13, 5789 2.38 ± 0.2 1.83 ± 0.3 2.17 ± 0.4 2.34 ± 0.4 2.42 ± 0.4 3.12 ± 0.1 7 of 12
−40 °C 2.24 ± 0.1 1.95 ± 0.3 2.19 ± 0.1 2.18 ± 0.1 2.63 ± 0.3 2.35 ± 0.2
p-value 0.2617 0.6078 0.9448 0.4587 0.4115 0.0002

3.2.3. Tensile Strength Evaluation

3.2.3. Tensile Strength Evaluation
The changes of tensile modulus E, tensile strength Rm, elongation at break ε and brittleness B of
The changes of tensile modulus E, tensile strength Rm, elongation at break ε and brittleness B of
the samples evaluated in different conditions are presented in the function of oil content in the filler in
the samples evaluated in different conditions are presented in the function of oil content in the filler
Figure 3. The difference between the corresponding samples of each kind tested at −40 ◦ C and 25 ◦ C
in Figure 3. The difference between the corresponding samples of each kind tested at −40 °C and 25
was°Cstatistically significant
was statistically (p-values
significant < 0.05).
(p-values < 0.05).

Figure 3. Tensile properties of the composites and pure PLA tested at different temperatures in function
of oil content in the fillers, (a) tensile modulus; (b) tensile strength; (c) elongation at break; (d) brittleness.
The values obtained for pure polylactide are indicated by the dashed line.

The tensile modulus of the unfilled PLA tested at room temperature is 2.26 GPa. The addition of
10% of the defatted linseed cake results in a small growth of the E value, but increasing the oil content
in the filler causes a steady decrease in Young0 s modulus, which indicates the plasticizing effect of LC
on PLA. The PLA-LC39.8 sample shows the E value of about 1 GPa lower than the neat polylactide.
The decrease in the testing temperature to −40 ◦ C results in a significant increase in the tensile modulus
for all the samples. Its value for pure PLA increases from 2.26 GPa to 2.60 GPa. This behavior is
common for the thermoplastic polymers and it can be explained by a decrease in the distance between
the molecules and a resulting increase in the binding forces [64]. Even though the Young’s modulus
of the composite samples tested at −40 ◦ C is different from the one of the unfilled polymer, only in
the case of the sample with the highest oil content, the difference is statistically significant (p < 0.05).
Therefore, the plasticizing effect of linseed cake is notably reduced. This behavior can be attributed to
the solidification of the oil (as shown by the rheological measurements), which can no longer facilitate
the movements of the macromolecules. This unwanted effect of solidification is, in fact, a common
problem among the plasticizers [48]. Nevertheless, the composite containing the highest content of
linseed oil presents a significantly lower E value. This behavior is presumably due to a replacement of
Materials 2020, 13, 5789 8 of 12

a part of the rigid polymer with considerably softer solidified linseed oil. It also needs to be noticed
that a similar situation was observed in the case of the storage modulus of the PLA and PLA-LC-39.8
samples tested at −40 ◦ C.
The tensile strength of the pure PLA at room temperature is 74.3 MPa, which is a common value for
this polymer [1]. The linseed cake-filled composites show considerably lower Rm values, which decrease
with the linseed oil content. This is typical to polylactide plasticized with non-epoxidized oils, which lack
epoxy groups capable of reacting with the polymeric chains [26]. What is more, the linseed oil does
not mix well with PLA, creating separate domains [43], so the limited interactions of the matrix and
the plasticizer also reduce the tensile strength of the composite [65,66]. The tensile strength of PLA
tested at −40 ◦ C is 110 MPa. Similarly to the increase in Young0 s modulus, the enhancement of Rm at
sub-zero temperatures can be explained by the intensification of the interactions between the polymeric
chains, which come into close proximity with one another [67]. The composites show tensile strength
−40 ◦ C lower than the pure polymer but still higher in comparison to the results achieved at room
temperature. No clear relationship between the Rm and oil content can be distinguished, which can
indicate that oil solidification improves its interactions with PLA, presumably due to the differences
in thermal expansion coefficients and mechanical interlocking of the solidified oil particles and the
polymeric matrix. Therefore, the tensile strength of the composites is mostly influenced by the filler
dispersion and structural flaws such as porosities created in the injection molding process.
The changes of elongation at the break due to the addition of oil-rich linseed cake were considered
one of the main arguments to support the hypothesis of the plasticizing influence of this filler in our
previous research [42], therefore its change should be observed due to oil solidification. The results of
the tensile test confirm this prediction. At room temperature pure PLA shows ε of 8%. The addition of
the defatted linseed cake causes a decrease in elongation to about 4.5–5.0%, which is a typical effect
of lignocellulosic particle-like filler. When the oil content in the filler exceeds 30%, the ε increases
to 45%. The linseed oil acts as an internal lubricant for PLA and enables it to deform before fracture.
Completely different behavior can be observed in the case of the LC-filled composites tested at −40 ◦ C.
The values of elongation at break of the pure PLA and its composites are lower in comparison to
the room temperature results. This result is typical for thermoplastic composites and it is commonly
explained by reduced molecular mobility of the polymer at low temperatures [67]. What is more
interesting, the elongation at break of the composites no longer depends on the oil content in the
filler—the ε value of the PLA-LC-39.8 sample decreases from 45% at 24 ◦ C to 4.8% at −40 ◦ C. Therefore,
the reduction of the plasticizing effect of the linseed cake not only influences the tensile modulus and
tensile strength of the composites but, even more notably, their elongation at break.
Brittleness, as proposed by Brostow et al., depends on both dynamic (storage modulus) and
quasi-static mechanical (elongation at break) properties of a material [52,68]. The higher its value,
the more brittle (i.e., less ductile, more prone to cracking) is a material. The PLA-based composites
tested at room temperature initially show higher brittleness, but its values decrease with oil content,
proving its plasticizing influence. In the case of the sub-zero measurements, the values denoted for
the composites containing the defatted linseed cake are almost the same as in the case of the room
temperature testing. The difference gets more and more visible as the oil content increases—the
brittleness of the oil-rich samples does not decrease as the solid oil does not have the same modifying
properties as the liquid one.

4. Conclusions
Both the sub-zero and room temperature mechanical properties of polylactide and its composites
filled with linseed cake were tested. It was found that tensile modulus and tensile strength of the
studied materials notably increase in low temperatures—the E values changed from 1.65–2.43 GPa at
25 ◦ C to 2.42–2.63 GPa at −40 ◦ C. This growth was associated with intensification of the interactions of
the macromolecules at low temperatures. The impact strength of the PLA and most of its LC-filled
composites did not change significantly except for the PLA-LC39.8 sample, whose impact strength
Materials 2020, 13, 5789 9 of 12

decreased significantly. Even though all the studied materials showed lower elongation values at
−40 ◦ C than at room temperature, the decrease was especially notable for the composites with the
highest oil content. This behavior was attributed to the solidification of the linseed oil around −18 ◦ C,
as it was shown in by DSC and DMA. It was found that even though the plasticizing effect of linseed
oil is highly reduced due to its phase transition, the LC-filled polylactide composites present good
mechanical properties at −40 ◦ C and therefore can be successfully used in sub-zero applications,
especially if strength and rigidity are needed.

Author Contributions: Conceptualization, O.M. and M.B.; methodology, O.M.; investigation, O.M., M.B. and
A.K.; resources, O.M.; writing—original draft preparation, O.M.; writing—review and editing, O.M. and M.B.;
supervision, M.B.; project administration, O.M.; funding acquisition, O.M. All authors have read and agreed to the
published version of the manuscript.
Funding: This research was funded by the Ministry of Science & Higher Education in Poland in 2020 year under
Project No 0513/SBAD/4608.
Conflicts of Interest: The authors declare no conflict of interest.

References
1. Auras, R.; Harte, B.; Selke, S. An overview of polylactides as packaging materials. Macromol. Biosci. 2004,
4, 835–864. [CrossRef] [PubMed]
2. Carrasco, F.; Pagès, P.; Gámez-Pérez, J.; Santana, O.O.; Maspoch, M.L. Processing of poly(lactic acid):
Characterization of chemical structure, thermal stability and mechanical properties. Polym. Degrad. Stab.
2010, 95, 116–125. [CrossRef]
3. Castro-Aguirre, E.; Iñiguez-Franco, F.; Samsudin, H.; Fang, X.; Auras, R. Poly(lactic acid)—Mass production,
processing, industrial applications, and end of life. Adv. Drug Deliv. Rev. 2016, 107, 333–366. [CrossRef]
4. Bałdowska-Witos, P.; Kruszelnicka, W.; Kasner, R.; Tomporowski, A.; Flizikowski, J.; Kłos, Z.; Piotrowska, K.;
Markowska, K. Application of LCA method for assessment of environmental impacts of a polylactide (PLA)
bottle shaping. Polymers 2020, 12, 388. [CrossRef]
5. Czarnecka-Komorowska, D.; Wiszumirska, K. Zrównoważone projektowanie opakowań z tworzyw
sztucznych w gospodarce cyrkularnej. Polimery 2020, 65, 8–17. [CrossRef]
6. Quiles-Carrillo, L.; Duart, S.; Montanes, N.; Torres-Giner, S.; Balart, R. Enhancement of the mechanical and
thermal properties of injection-molded polylactide parts by the addition of acrylated epoxidized soybean oil.
Mater. Des. 2018, 140, 54–63. [CrossRef]
7. Lim, L.-T.; Auras, R.; Rubino, M. Processing technologies for poly(lactic acid). Prog. Polym. Sci. 2008,
33, 820–852. [CrossRef]
8. Iozzino, V.; De Meo, A.; Pantani, R. Micromolded Polylactid Acid With Selective Degradation Rate.
Front. Mater. 2019, 6, 305. [CrossRef]
9. Quiles-Carrillo, L.; Montanes, N.; Sammon, C.; Balart, R.; Torres-Giner, S. Compatibilization of highly
sustainable polylactide/almond shell flour composites by reactive extrusion with maleinized linseed oil.
Ind. Crops Prod. 2018, 111, 878–888. [CrossRef]
10. Bulanda, K.; Oleksy, M.; Oliwa, R.; Budzik, G.; Gontarz, M. Biodegradable polymer composites based on
polylactide used in selected 3D technologies (Rapid communication). Polimery/Polymers 2020, 65, 557–562.
11. Benwood, C.; Anstey, A.; Andrzejewski, J.; Misra, M.; Mohanty, A.K. Improving the Impact Strength and
Heat Resistance of 3D Printed Models: Structure, Property, and Processing Correlationships during Fused
Deposition Modeling (FDM) of Poly(Lactic Acid). ACS Omega 2018, 3, 4400–4411. [CrossRef] [PubMed]
12. Andrzejewski, J.; Cheng, J.; Anstey, A.; Mohanty, A.K.; Misra, M. Development of Toughened Blends of
Poly(lactic acid) and Poly(butylene adipate- co -terephthalate) for 3D Printing Applications: Compatibilization
Methods and Material Performance Evaluation. ACS Sustain. Chem. Eng. 2020, 8, 6576–6589. [CrossRef]
13. Cicala, G.; Giordano, D.; Tosto, C.; Filippone, G.; Recca, A.; Blanco, I. Polylactide (PLA) filaments a biobased
solution for additive manufacturing: Correlating rheology and thermomechanical properties with printing
quality. Materials 2018, 11, 1191. [CrossRef] [PubMed]
14. Kulinski, Z.; Piorkowska, E. Crystallization, structure and properties of plasticized poly(l-lactide). Polymer
2005, 46, 10290–10300. [CrossRef]
Materials 2020, 13, 5789 10 of 12

15. Huang, H.; Chen, L.; Song, G.; Tang, G. An efficient plasticization method for poly(lactic acid) using
combination of liquid-state and solid-state plasticizers. J. Appl. Polym. Sci. 2018, 135, 46669. [CrossRef]
16. Zubrowska, A.; Piorkowska, E.; Kowalewska, A.; Cichorek, M. Novel blends of polylactide with ethylene
glycol derivatives of POSS. Colloid Polym. Sci. 2014, 293, 23–33. [CrossRef]
17. Saad, G.R.; Elsawy, M.A.; Aziz, M.S.A. Nonisothermal crystallization behavior and molecular dynamics of
poly(lactic acid) plasticized with jojoba oil. J. Therm. Anal. Calorim. 2017, 128, 211–223. [CrossRef]
18. Najafi, N.; Heuzey, M.C.; Carreau, P.J.; Wood-Adams, P.M. Control of thermal degradation of polylactide
(PLA)-clay nanocomposites using chain extenders. Polym. Degrad. Stab. 2012, 97, 554–565. [CrossRef]
19. Gui, Z.; Lu, C.; Cheng, S. Comparison of the effects of commercial nucleation agents on the crystallization
and melting behaviour of polylactide. Polym. Test. 2013, 32, 15–21. [CrossRef]
20. Courgneau, C.; Ducruet, V.; Averous, L.; Grenet, J.; Domenek, S. Nonisothermal crystallization kinetics of
poly(lactide)—Effect of plasticizers and nucleating agnet. Polym. Eng. Sci. 2012, 53, 1085–1098. [CrossRef]
21. Nagarajan, V.; Mohanty, A.K.; Misra, M. Crystallization behavior and morphology of polylactic acid (PLA)
with aromatic sulfonate derivative. J. Appl. Polym. Sci. 2016, 133, 43673. [CrossRef]
22. Singh, S.K.; Anthony, P.; Chowdhury, A. High molecular weight poly(lactic acid) synthesized with apposite
catalytic combination and longer time. Orient. J. Chem. 2018, 34, 1984–1990. [CrossRef]
23. Cicala, G.; Saccullo, G.; Blanco, I.; Samal, S.; Battiato, S.; Dattilo, S.; Saake, B. Polylactide/lignin blends: Effects
of processing conditions on structure and thermo-mechanical properties. J. Therm. Anal. Calorim. 2017,
130, 515–524. [CrossRef]
24. Blanco, I. End-life prediction of commercial pla used for food packaging through short term TGA experiments:
Real chance or low reliability. Chin. J. Polym. Sci. 2014, 32, 681–689. [CrossRef]
25. Ahmed, I.; Cronin, P.S.; Abou Neel, E.A.; Parsons, A.J.; Knowles, J.C.; Rudd, C.D. Retention of mechanical
properties and cytocompatibility of a phosphate-based glass fiber/polylactic acid composite. J. Biomed. Mater.
Res. Part B Appl. Biomater. 2009, 89B, 18–27. [CrossRef]
26. Orue, A.; Eceiza, A.; Arbelaiz, A. Preparation and characterization of poly(lactic acid) plasticized with
vegetable oils and reinforced with sisal fibers. Ind. Crops Prod. 2018, 112, 170–180. [CrossRef]
27. Duan, J.; Wu, H.; Fu, W.; Hao, M. Mechanical Properties of Hybrid Sisal/Coir Fibers Reinforced Polylactide
Biocomposites. Polym. Compos. 2018, 39, E188–E199. [CrossRef]
28. Arbelaiz, A.; Trifol, J.; Peña-Rodriguez, C.; Labidi, J.; Eceiza, A. Modification of Poly(Lactic Acid) Matrix by
Chemically Modified Flax Fiber Bundles and Poly(Ethylene Glycol) Plasticizer. In Polyethylene-Based Biocomposites
and Bionanocomposites; Scrivener Publishing: Beverly, MA, USA, 2016; pp. 429–445. ISBN 9781119038467.
29. Stepczyńska, M.; Rytlewski, P. Enzymatic degradation of flax-fibers reinforced polylactide. Int. Biodeterior. Biodegrad.
2018, 126, 160–166. [CrossRef]
30. Xia, X.; Liu, W.; Zhou, L.; Hua, Z.; Liu, H.; He, S. Modification of flax fiber surface and its compatibilization
in polylactic acid/flax composites. Iran. Polym. J. 2016, 25, 25–35. [CrossRef]
31. Andrzejewski, J.; Szostak, M. Preparation of hybrid poly(lactic acid)/flax composites by the insert overmolding
process: Evaluation of mechanical performance and thermomechanical properties. J. Appl. Polym. Sci. 2020 ,
138, 49646. [CrossRef]
32. Kuciel, S.; Mazur, K.; Hebda, M. The Influence of Wood and Basalt Fibres on Mechanical, Thermal and
Hydrothermal Properties of PLA Composites. J. Polym. Environ. 2020, 28, 1204–1215. [CrossRef]
33. Oza, S.; Ning, H.; Ferguson, I.; Lu, N. Effect of surface treatment on thermal stability of the hemp-PLA
composites: Correlation of activation energy with thermal degradation. Compos. Part B Eng. 2014, 67, 227–232.
[CrossRef]
34. Bledzki, A.K.; Mamun, A.A.; Volk, J. Barley husk and coconut shell reinforced polypropylene composites:
The effect of fibre physical, chemical and surface properties. Compos. Sci. Technol. 2010, 70, 840–846.
[CrossRef]
35. Mittal, V.; Chaudhry, A.U.; Matsko, N.B. True biocomposites with biopolyesters and date seed powder:
Mechanical, thermal, and degradation properties. J. Appl. Polym. Sci. 2014, 131, 40816. [CrossRef]
36. Mittal, V.; Luckachan, G.E.; Chernev, B.; Matsko, N.B. Bio-polyester-date seed powder composites:
Morphology and component migration. Polym. Eng. Sci. 2015, 55, 877–888. [CrossRef]
37. Balart, J.F.; Fombuena, V.; Fenollar, O.; Boronat, T.; Sánchez-Nacher, L. Processing and characterization
of high environmental efficiency composites based on PLA and hazelnut shell flour (HSF) with biobased
plasticizers derived from epoxidized linseed oil (ELO). Compos. Part B Eng. 2016, 86, 168–177. [CrossRef]
Materials 2020, 13, 5789 11 of 12

38. Sanchez-Olivares, G.; Rabe, S.; Pérez-Chávez, R.; Calderas, F.; Schartel, B. Industrial-waste agave fibres in
flame-retarded thermoplastic starch biocomposites. Compos. Part B Eng. 2019, 177, 107370. [CrossRef]
39. Geissdoerfer, M.; Savaget, P.; Bocken, N.M.P.; Hultink, E.J. The Circular Economy—A new sustainability
paradigm? J. Clean. Prod. 2017, 143, 757–768. [CrossRef]
40. Członka, S.; Strakowska,
˛ A.; Kairytė, A. Effect of walnut shells and silanized walnut shells on the mechanical
and thermal properties of rigid polyurethane foams. Polym. Test. 2020, 87, 106534. [CrossRef]
41. Członka, S.; Bertino, M.F.; Strzelec, K.; Strakowska,
˛ A.; Masłowski, M. Rigid polyurethane foams reinforced
with solid waste generated in leather industry. Polym. Test. 2018, 69, 225–237. [CrossRef]
42. Mysiukiewicz, O.; Barczewski, M. Utilization of linseed cake as a postagricultural functional filler for
poly(lactic acid) green composites. J. Appl. Polym. Sci. 2019, 136, 47152. [CrossRef]
43. Mysiukiewicz, O.; Barczewski, M.; Skórczewska, K.; Szulc, J.; Kloziński, A. Accelerated weathering of
polylactide-based composites filled with linseed cake: The influence of time and oil content within the filler.
Polymers 2019, 11, 1495. [CrossRef] [PubMed]
44. European Bioplastics. Bioplastics Market Data 2019. In Global Production Capacities of Bioplastics 2019–2024;
European Bioplastics: Berlin, Germany, 2020.
45. Tábi, T.; Zarrelli, M. Development of poly(Lactic acid) filled with basalt fibres and talc for engineering
applications. Mater. Sci. Forum 2017, 885, 303–308. [CrossRef]
46. Sinha Ray, S.; Yamada, K.; Okamoto, M.; Ogami, A.; Ueda, K. New polylactide/layered silicate nanocomposites.
3. High-performance biodegradable materials. Chem. Mater. 2003, 15, 1456–1465. [CrossRef]
47. Tang, Z.; Zhang, C.; Liu, X.; Zhu, J. The crystallization behavior and mechanical properties of polylactic acid
in the presence of a crystal nucleating agent. J. Appl. Polym. Sci. 2012, 125, 1108–1115. [CrossRef]
48. Rahman, M.; Brazel, C.S. The plasticizer market: An assessment of traditional plasticizers and research
trends to meet new challenges. Prog. Polym. Sci. 2004, 29, 1223–1248. [CrossRef]
49. Nassu, R.T.; Gonçalves, L.A.G. Determination of melting point of vegetable oils and fats by differential
scanning calorimetry (DSC) technique. Grasas y Aceites 1999, 50, 16–22. [CrossRef]
50. Sharma, M.; Lokesh, B.R. Effect of enzymatic trans- and interesterification on the thermal properties of
groundnut and linseed oils and their blends. JAOCS J. Am. Oil Chem. Soc. 2012, 89, 805–813. [CrossRef]
51. Ustrnul, Z.; Czekierda, D.; Wypych, A. Extreme values of air temperature in Poland according to different
atmospheric circulation classifications. Phys. Chem. Earth 2010, 35, 429–436. [CrossRef]
52. Brostow, W.; Hagg Lobland, H.E. Sliding wear, viscoelasticity, and brittleness of polymers. J. Mater. Res.
2006, 21, 2422–2428. [CrossRef]
53. Bayés-García, L.; Sato, K.; Ueno, S. Polymorphism of Triacylglycerols and Natural Fats. In Bailey’s Industrial
Oil and Fat Products; Wiley: Hoboken, NJ, USA, 2020; pp. 1–49. ISBN 047167849X.
54. Hashempour-Baltork, F.; Torbati, M.; Azadmard-Damirchi, S.; Savage, G.P. Chemical, rheological and
nutritional characteristics of sesame and olive oils blended with linseed oil. Adv. Pharm. Bull. 2018,
8, 107–113. [CrossRef] [PubMed]
55. Skrzyńska, E.; Matyja, M. Porównanie właściwości fizykochemicznych wybranych tłuszczy naturalnych
oraz ich estrów metylowych. Chemik 2011, 65, 923–935.
56. Bell, A.; Gordon, M.H.; Jirasubkunakorn, W.; Smith, K.W. Effects of composition on fat rheology and
crystallisation. Food Chem. 2007, 101, 799–805. [CrossRef]
57. Borwankar, R.P.; Frye, L.A.; Blaurock, A.E.; Sasevich, F.J. Rheological Characterization of Melting of
Margarines and Tablespreads. Rheol. Foods 1992, 16, 55–74.
58. Andrzejewski, J.; Barczewski, M.; Szostak, M. Injection molding of highly filled polypropylene-based
biocomposites. Buckwheat husk and wood flour filler: A comparison of agricultural and wood industry
waste utilization. Polymers 2019, 11, 1881. [CrossRef] [PubMed]
59. Salasinska, K.; Ryszkowska, J. The effect of filler chemical constitution and morphological properties on the
mechanical properties of natural fiber composites. Compos. Interfaces 2015, 22, 39–50. [CrossRef]
60. Piorkowska, E.; Kulinski, Z.; Galeski, A.; Masirek, R. Plasticization of semicrystalline poly(l-lactide) with
poly(propylene glycol). Polymer 2006, 47, 7178–7188. [CrossRef]
61. Badia, J.-D.; Santonja-Blasco, L.; Martinez-Felipe, A.; Ribes-Greus, A. Dynamic Mechanical Thermal Analysis
in Polymer Blends. In Characterization of Polymer Blends: Miscibility, Morphology and Interfaces; Wiley-VCH:
Wenheim, Germany, 2015; pp. 365–392.
Materials 2020, 13, 5789 12 of 12

62. Liang, J.Z.; Yang, Q.Q. Mechanical properties of carbon black-filled high-density polyethylene antistatic
composites. J. Reinf. Plast. Compos. 2009, 28, 295–304. [CrossRef]
63. Andrzejewski, J.; Misra, M.; Mohanty, A.K. Polycarbonate biocomposites reinforced with a hybrid filler
system of recycled carbon fiber and biocarbon: Preparation and thermomechanical characterization. J. Appl.
Polym. Sci. 2018, 135, 46449. [CrossRef]
64. Eslami-Farsani, R.; Reza Khalili, S.M.; Hedayatnasab, Z.; Soleimani, N. Influence of thermal conditions
on the tensile properties of basalt fiber reinforced polypropylene-clay nanocomposites. Mater. Des. 2014,
53, 540–549. [CrossRef]
65. Chieng, B.W.; Ibrahim, N.A.; Then, Y.Y.; Loo, Y.Y. Epoxidized vegetable oils plasticized poly(lactic acid)
biocomposites: Mechanical, thermal and morphology properties. Molecules 2014, 19, 16024–16038. [CrossRef]
66. Al-Mulla, E.A.J.; Yunus, W.M.Z.W.; Ibrahim, N.A.B.; Rahman, M.Z.A. Properties of epoxidized palm oil
plasticized polytlactic acid. J. Mater. Sci. 2010, 45, 1942–1946. [CrossRef]
67. Soleimani, N.; Khalili, S.M.; Farsani, R.E.; Nasab, Z.H. Mechanical properties of nanoclay reinforced
polypropylene composites at cryogenic temperature. J. Reinf. Plast. Compos. 2012, 31, 967–976. [CrossRef]
68. Brostow, W.; Hagg Lobland, H.E. Brittleness of materials: Implications for composites and a relation to
impact strength. J. Mater. Sci. 2010, 45, 242–250. [CrossRef]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional
affiliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).