Issue 01/2021

Automotive
www.engr.wisc.edu
bioplastics MAGAZINE [01/11] Vol. 6 37
10
Years ago
Foam
tinyurl.com/pbatfoam2011
Published in
bioplastics MAGAZINE
In January 2021, Srikanth Pilla,
now Clemson University,
Greenville, South Carolina, USA said:
“The study, conceived about 10 years ago,
was timely back then when the need for
biobased and biodegradable/compostable
packaging materials including foams was in
high demand. Today they almost became certain.
With the emergence of circularity, plastics
being biobased and/or biodegradable has become
a necessity that their presence is more
authenticated now. While a commercial potential
is yet to be realized, my own lab has started
to constitute more advancements in this field that is
much closer to commercialization.”
www.clemson.edu
Volume Expansion Ratio
Open Cell content (%)
Article contributed by
Srikanth Pilla, George K. Auer, Shaoqin Gong
University of Wisconsin, USA
Seong G. Kim, Chul B. Park,
University of Toronto, CA
Figure 2: Volume Expansion Ratio vs Temperature
1.8
1.6
1.4
1.2
1
60
50
40
30
20
10
0
PLA
Ecovio
PLA+55%PBAT
Figure 3: Open Cell Content vs Temperature
PLA
Ecovio
PLA+55%PBAT
125 130 135 140 145 150 155
36 bioplastics MAGAZINE [01/11] Vol. 6
PLA+0.5%Talc
Ecovio+0.5%&Talc
PLA+55%PBAT+0.5%Talc
130 140 150
Die Temperature (°C)
PLA+0.5%Talc
Ecovio+0.5%&Talc
PLA+55%PBAT+0.5%Talc
Die Temperature (°C)
Biodegradable
PLA/PBAT Foams
I
Foam
investigated the foaming ability of PLA blended with starch
using microcellular extrusion. Reignier et al. [32] have studied
extrusion foaming of amorphous PLA using CO 2 ; however,
due to very narrow processing window of the unmodified PLA,
a reasonable expansion ratio could not be achieved.
In this study, PLA/PBAT blends have been foamed by the
microcellular extrusion process using CO 2 as a blowing agent.
Two types of blend systems were investigated: (1) Ecovio ® ,
which is a commercially available compatibilized PLA/PBAT
blend (BASF); (2) A non-compatibilized PLA/PBAT blend at the
same PLA/PBAT ratio (i.e., 45:55 by weight percent) as Ecovio.
The effects of talc,compatibilization and die temperature on
the cell size, cell density, volume expansion and open cell
content were evaluated.
n this study, a unique processing technology viz. microcellular
extrusion foaming, was used to produce biodegradable foams
that could potentially replace existing synthetic foams thereby
reducing carbon footprint and contributing towards a sustainable
society.
Introduction
Effects on Cell Size and Cell Density
Representative SEM images of the cell morphology of
different formulations are shown in Figure 1. From the figure,
it can be noted that the addition of
talc has decreased the cell size.
This shows that talc has acted as a
nucleating agent thereby reducing
the cell size. Thus, as more cells
started to nucleate, due to excess
nucleation sites provided by talc, there
was less amount of gas available for
their growth that lead to reduction
in cell size. Also, the addition of
talc significantly increased the melt
viscosity, which made it difficult for
the cells to grow, leading to smaller
cell sizes [33]. Also, from Figure 1
it can be observed that the cell size
of the compatibilized blends (both
Ecovio and Ecovio-talc) is much less
than that of the non-compatibilized
ones (PLA/PBAT and PLA/PBATtalc).
Thus it can be concluded that
compatibilization has reduced the cell
size. This might be due to increase in
the melt strength of the blend as a
result of the compatibilization [34].
In general, as shown in Figure 1,
the addition of talc has increased
the cell density because of the
heterogeneous nucleation. In a
heterogeneous nucleation scheme,
the activation energy barrier to
nucleation is sharply reduced in the
presence of a filler (talc in this case)
thus increasing the nucleation rate
and thereby the number of cells [35].
While comparing the compatibilized
and non-compatibilized samples, it
can be observed that the cell density
As a biodegradable and biobased polymer, polylactide (PLA)
has attracted much interest among researchers world-wide in
recent times; however, its commercial application is still limited
due to certain inferior properties such as brittleness, relatively
high cost, and narrow processing window. Certain drawbacks
can be overcome by copolymerizing lactide with different
monomers such as ε-caprolactone [1-4], trimethylene carbonate
[5] and DL-β-methyl-δ-valerolactone [6] and by blending PLA
with poly(butylene adipate-co-terephthalate) (PBAT) [7], poly(εcaprolactone)
(PCL) [8-12] and many other non-biodegradable
polymers [13-19]. Though the blended polymers exhibited certain
improved mechanical properties compared to non-blended parts,
immiscible polymer blends may lead to less desirable properties
that were anticipated from blending. Thus, compatibilizers are
often used to improve the miscibility between the immiscible
polymer blend.
is the much higher for Ecovio samples (i.e. both Ecovio and
Ecovio-talc). Thus as seen in cell size, compatibilization had
positive effect on the cell morphology of the foamed materials,
i.e., increasing the cell density. This is in agreement with the
published literature [36].
Effects on Volume Expansion Ratio (VER)
Volume expansion ratio denotes the amount of volume
that has proportionately expanded as a result of foaming.
Figure 2 presents the volume expansion ratio with respect
to temperature. The addition of talc has decreased the VERs
of PLA and non-compatibilized PLA/PBAT blend. This is due
to increase in stiffness and strength of the polymer melt. For
Ecovio, the addition of talc had no significant effect on VER.
While comparing the non-filled and talc filled compatibilized
and non-compatibilized PLA/PBAT blends, it can be inferred
that non-compatibilized PLA/PBAT blends possesses
higher VER in comparison to compatibilized blends. Thus,
compatibilization had a negative effect on the VER which could
be due to increase in the melt strength of the compatibilized
blends [37].
PLA
Effects on Open Cell Content (OCC)
The open cell content illustrates the interconnectivity
between various cells. A highly open cell structured foam can
be used in numerous industrial applications such as filters,
separation membranes, diapers, tissue engineering etc.
Figure 3 shows the variation of open cell content (OCC) with
temperature. In general, the open cell content is governed
by cell wall thickness [37]. As per the cell opening strategies
discussed in [37], higher cell density, higher expansion
ratios, creating structural inhomogeneity by using polymer
blends or adding cross-linker and using a secondary blowing
agent, all decrease the cell wall thickness thereby increasing
the OCC. Some of them work in conjunction with the other.
With the addition of talc, the OCC decreased for PLA and noncompatibilized
PLA/PBAT blend which might be attributed to
an increase in stiffness and strength of the talc filled samples.
For Ecovio, the OCC increased with the addition of talc. Thus,
talc had a varying effect on the OCC of PLA and its blends
(compatibilized and non-compatibilized). In the analysis of
OCC for compatibilized and non-compatibilized blends, it
can be inferred that compatibilization has reduced the OCC
significantly among non-filled blends but increased the OCC
slightly among talc filled blends. Further investigation is
required to study the varied effects of compatibilization on
the OCC of blends.
In summary, biodegradable PLA/PBAT foams have been
successfully produced using CO 2 as a blowing agent. Two types
of blends systems have been investigated, compatibilized and
non-compatibilized. The effects of talc and compatibilization
have been studied on different foam properties such as cell
morphology, volume expansion, and open cell content.
The financial support from National Science Foundation
(CMMI-0734881) is gratefully acknowledged.
PLA
+
0.5% Talc
Ecovio
Ecovio
+
0.5%Talc
PLA
+
55% PBAT
PLA
+
55% PBAT
+
0.5%Talc
500 μm
Foamed plastics are used in a variety of applications such
as insulation, packaging, furniture, automobile and structural
components [20-21]; especially, microcellular foaming is capable
of producing foamed plastics with less used material and energy,
and potentially improved material properties such as impact
strength and fatigue life [22]. Also compared to conventional
foaming, microcellular foaming process uses environmentally
benign blowing agents such as carbon dioxide (CO 2 ) and nitrogen
(N 2 ) in their supercritical state [23]. Microcellular process also
improves the cell morphology with typical cell sizes of tens
of microns and cell density in the order of 109 cells/cm 3 [23].
Additionally, compared to conventional extrusion, the microcellular
extrusion process allows the material to be processed at lower
temperatures, due to the use of supercritical fluids (SCF), making
it suitable for temperature- and moisture-sensitive biobased
plastics such as PLA. Solid PLA components processed by
various conventional techniques such as compression molding,
extrusion and injection molding have been investigated by
many researchers [24-25]; however, foamed PLA produced via
microcellular technology has been a recent development. Pilla
et al. [26-29] and Kramschuster et al. [30] have investigated the
properties of PLA based composites processed via microcellular
injection molding and extrusion foaming. Mihai et al. [31] have
Figure 1: Representative SEM Images of Various Formulations
Temperature Increase
130°C 140°C 150°C
Foam
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