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poly(butylene adipate-co-terephthalate) blends
Materials Science and Engineering C 30 (2010) 255–262
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Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Microcellular extrusion foaming of poly(lactide)/poly(butylene adipate-co-terephthalate) blends Srikanth Pilla a, Seong G. Kim b, George K. Auer a, Shaoqin Gong a,c,⁎, Chul B. Park b a b c
Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, Wisconsin-53211, USA Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, Canada M5S 3G8 Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin-53706, USA
a r t i c l e
i n f o
Article history: Received 11 August 2009 Received in revised form 12 October 2009 Accepted 25 October 2009 Available online 30 October 2009 Keywords: Microcellular extrusion foaming PLA PBAT Compatibilization
a b s t r a c t Foamed poly(lactide) (PLA)/poly(butylene adipate-co-terephthalate) (PBAT) blends were processed via the microcellular extrusion process using CO2 as a blowing agent. Talc has been added to promote heterogeneous nucleation. Two types of PLA/PBAT blend systems were investigated: Ecovio, which is a commercially available compatibilized PLA/PBAT blend; and a non-compatibilized PLA/PBAT blend at the same PLA/PBAT ratio (i.e., 45:55 by weight percent). Six different formulations were investigated: pure PLA, PLA-talc, Ecovio, Ecovio-talc, non-compatibilized PLA/PBAT blend, and non-compatibilized PLA/PBAT-talc. The effects of various processing parameters such as die temperature, talc and compatibilization on various foaming properties such as cell morphology, volume expansion ratio (VER), open cell content (OCC) and crystallinity were investigated. As per the DSC thermograms, it was observed that compatibilization has merged the two distinctive melting peaks of PLA and PBAT into a single peak while lowering the peak temperature. In general, the addition of talc has decreased the average cell size and VER and increased the cell density and crystallinity; however, it has varying effects on the open cell content. Compatibilization has reduced the average cell size and volume expansion but increased the cell density and had varying and no effects on the OCC and crystallinity, respectively. Similar to compatibilization, the die temperature was found to have varying and no effects on the OCC and crystallinity, respectively. Except for PLA and noncompatibilized PLA/PBAT blend, the cell size and VER of all other formulations did not vary much throughout the entire temperature range (130–150 °C). The cell density was found to be insensitive to die temperatures except for Ecovio and Ecovio-talc. © 2009 Elsevier B.V. All rights reserved.
1. Introduction 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 a 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(ε-caprolactone) (PCL) [8–12] and many other non-biodegradable polymers [13–19]. Though the blended polymers exhibited certain improved mechanical properties compared to virgin 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.
⁎ Corresponding author. E-mail addresses: [email protected] (S. Gong), [email protected] (C.B. Park). 0928-4931/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2009.10.010
Foamed plastics are used in a variety of applications such as insulation, packaging, cushions, and structural components [20,21]; especially, microcellular foaming is capable of producing foamed plastics with less 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 (CO2) and nitrogen (N2) in their supercritical state [23]. Microcellular process also improves the cell morphology with typical cell sizes in tens of microns and cell density on the order of 109 cells/cm3 [23]. Additionally, compared to solid part 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
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extrusion foaming. Mihai et al. [31] have investigated the foaming ability of PLA blended with starch using microcellular extrusion. Reignier et al. [32] have studied extrusion foaming of amorphous PLA using CO2; however, due to a 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 CO2 as a blowing agent. Two types of blend systems were investigated: (1) Ecovio, which is a commercially available compatibilized PLA/PBAT blend; (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 compatibilization, talc and die temperature on the cell morphology, volume expansion, crystallinity and open cell content were evaluated.
cell size and cell density was performed using an image analysis tool (UTHSCSA ImageTool). The cell density was calculated using the following formula [34]:
Cell Density =
3 = 2 N M L2
ð1Þ
where N is the number of cells, L is the linear length of the area, and M is a unit conversion resulting in the number of cells per cm3. The volume expansion ratio (ϕ) (VER) was calculated by taking the ratio of the bulk density of pure PLA material (ρp) to the bulk density of foam sample (ρf) as follows [34]:
2. Experimental 2.1. Materials PLA (PLA 3001D) in a pellet form was obtained from NatureWorks® LLC, Minnetonka, MN, USA. Its specific gravity was 1.24 and had a melt flow index around 15 g/10 min (190 °C/2.16 kg). PLA 3001D was synthesized from approximately 92% L-lactide and 8% meso-lactide [33]. Ecovio (compatibilized blend of PLA/PBAT at 45/55 ratio) was obtained from BASF (melt viscosity b 2.5–4.5 ml/10 min at 190 °C). For the non-compatibilized blend, PBAT was supplied by BASF under the trade mark of Ecoflex (density: 1.25–1.27 g/cm3; melt viscosity: 2.5– 4.5 ml/10 min at 190 °C; and melting point: 110–120 °C). Talc was obtained from Luzenac (grade-JetFil700C) with a density of 2.8 g/cm3 and an average particle size of 1.5 μm.
ϕ=
ρp : ρf
The bulk density of the microcellular samples was calculated by measuring the weight and dividing it by the volume of the microcellular sample which was determined by the water displacement method (ASTM D792). Differential scanning calorimetry (TA instruments, Auto Q-2000) was used to analyze the crystallization property of the foamed samples. The samples were first heated from 40 °C to 180 °C, kept isothermal for three minutes, cooled to 0 °C, and finally reheated to 200 °C. The ramp speed in all the heating and cooling processes was 10 °C/min.
2.2. Foaming process
3. Results and discussion
A single screw extruder (Brabender, 05-25-000) with a mixing screw (Brabender, 05-00-144) of 30:1 L/D ratio and 3/4″ diameter was used to process the materials. First, the materials (any formulation) were fed into the barrel through the hopper and were completely melted by the screw rotation and shear before the gas injection port. A metered amount of physical blowing agent (4% SCF CO2) was injected into the extrusion barrel at the gas injection port by the positive displacement pump and was mixed intensively with the polymer melt stream. The shear fields generated by the screw via mixing facilitated the formation of a single phase polymer–gas solution. Also a static mixer (omega, FMX-84441-S) was installed for enhancing diffusion at the extruder to improve the dissolution of the physical blowing agent in the polymer melt. The resulting single phase polymer/gas solution went through the gear pump (Zenith, PEP-II 1.2 cc/rev), where the volumetric displacement was controlled by the motor, and then through the heat exchanger (consisting of homogenizing static mixers viz. Labcore Model H-0466912) where it was cooled to a designated temperature. The cooled polymer/gas solution entered the die (L/D: 0.89″/0.05″) and foaming occurred subsequently at the die exit. The heat exchanger and die temperatures were synchronized for simplicity. While optimizing all the parameters, the heat exchanger and die temperatures were lowered gradually and samples were randomly collected at each designated temperature when the variation in the pressures was approximately ±30 psi. Six different formulations were investigated: PLA, PLA−0.5% Talc, Ecovio, Ecovio−0.5% Talc, PLA−55%PBAT, and PLA−55%PBAT−0.5% Talc. A nucleating agent, talc, was added to all the formulations to assist in the heterogeneous nucleation process.
3.1. Effect of die pressure The foaming experiment was conducted with 4% CO2. The variation of exit die pressure with exit die temperature is shown in Fig. 1. It can be noted that the pressure increased as the temperature decreased. This is due to the increase in viscosity at lower temperatures. Thus, the formulations could not be processed at lower temperatures (below 130 °C) as the pressure increased beyond safe limits. Also, it can be observed that the exit die pressure for all the samples is much higher than the 4% CO2 solubility pressure in PLA, which implies that a polymer–gas single phase solution was formed. The CO2 dissolved in the PLA melt serves as a plasticizer [35], thereby reducing the overall viscosity and thus leading to reduced temperature required to process the polymer. This is very desirable for temperature-sensitive polymers such as PLA because lower processing temperatures can minimize PLA degradation.
2.3. Characterization of foams The foamed materials were characterized using SEM (JEOL-6060) and gas pycnometer (Quantachrome Co.) for cell morphology and open cell content, respectively. A quantitative analysis of the average
ð2Þ
Fig. 1. Die pressure vs temperature profile for 4% CO2 for all formulations.
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As shown in Table 1, two crystallization peaks exists for pure PLA and non-compatibilized PLA/PBAT blends. The first peak is termed as cold crystallization peak and the second one recrystallization. With the addition of talc, these two peaks vanished for PLA. The disappearance of the cold crystallization peak indicates an enhanced crystallinity for PLA produced during the cooling cycle, which is confirmed by the data shown in Table 1. In addition, the cold crystallization temperatures of non-compatibilized PLA/PBAT blends, shifted to the left by a few degrees due to the nucleating effect of the talc [7]. Crystallization is an important property that needs to be investigated in foaming as it affects the cell growth [37]. In fact, the cell growth stops at the moment of crystallization [38]. Fig. 3 shows the degree of crystallinity of all formulations measured from the second heating cycle of the DSC thermograms and Table 1 shows the numerical values of the characteristic temperatures and enthalpies and crystallinity obtained from the second heating cycle. The crystallinity of PLA is computed using Eq. (3) [39]: χc ð% CrystallinityÞ =
ΔHm 100 × 0 w ΔHm
ð3Þ
where, ΔH0m — 93.7 J/g w — weight fraction of PLA in the sample. Fig. 2. Melting curves of (A) Ecovio, (B) Ecovio−0.5% Talc, (C) PLA−55%PBAT, and (D) PLA−55%PBAT−0.5% Talc processed at 130 °C.
3.2. Thermal properties DSC was used to study the thermal characteristics (melting peaks and crystallization) of the foamed samples. Fig. 2 shows the melting peaks of all the non-filled and talc filled PLA/PBAT blends processed at 130 °C. As can be observed in the figure, the non-compatibilized blends exhibited two distinct melting peaks, at around 127 °C and 168 °C, representing PBAT and PLA respectively. Also the PBAT melting peak was observed to be broad. The broadness and peak temperature of PBAT are consistent with results reported in [36]. On the other hand, the compatibilized blends showed only one melting peak at 151 °C. This shows that the distinctive melting peaks observed in non-compatibilized blends merged into a single melting peak in compatibilized blends and also the peak temperature decreased. This illustrates the effect of compatibilization on the blending of PLA and PBAT.
ΔHm is the enthalpy for melting, ΔH0m is the enthalpy of melting for a 100% crystalline PLA sample, and w is the weight fraction of PLA in the sample. To determine the crystallinity of the sample, the extra heat absorbed by the crystallites formed during heating (i.e., cold crystallization) had to be subtracted from the total endothermic heat flow due to the melting of the whole crystallites [40]. Thus, the modified equation can be written as follows: χc ð% CrystallinityÞ =
ΔHm −ΔHcc 100 × 0 w ΔHm
ð4Þ
where, ΔHcc: cold — crystallization enthalpy. 3.2.1. Effects of talc, compatibilization and die temperature on degree of crystallinity As shown in Fig. 3a, the addition of talc has increased the degree of crystallinity (χ) for all the samples. This shows that talc has acted as a nucleating agent during the crystallization process and thereby
Table 1 Thermal characteristics of PLA and its blends. Formulation
Die temperature (°C)
Tcc (°C)
ΔHc (J/g)
Tr (°C)
ΔHr (J/g)
PLA PLA PLA PLA−0.5% Talc PLA−0.5% Talc PLA−0.5% Talc Ecovio Ecovio Ecovio Ecovio−0.5% Talc Ecovio−0.5% Talc Ecovio−0.5% Talc PLA/55% PBAT PLA/55% PBAT PLA/55% PBAT PLA/55% PBAT−0.5% Talc PLA/55% PBAT−0.5% Talc PLA/55% PBAT−0.5% Talc
130 140 150 130 140 150 130 140 150 130 140 150 130 140 150 130 140 150
103.1 102.9 102.9 – – – 132.9 133.6 134.9 119.1 106.3 119.7 101.9 102.1 102.5 98.4 97.9 99.5
29.5 27.9 26.3 – – – 2.6 2.7 2.8 4.1 3.8 3.6 12.6 12.1 11.6 6.6 6.6 7.9
155.8 155.7 156.1 – – – – – – – – – 155.2 154.6 155.6 154.5 154.1 154.2
1.4 1.4 1.4 – – – – – – – – – 0.9 0.5 0.7 0.4 0.4 0.6
Tm (°C) 168.8 168.7 168.9 169.6 169.4 169.3 151.4 151.8 151.5 150.2 149.7 150.1 127.2 127.4 127.9 127.4 127.6 127.2
167.8 167.7 167.9 168.1 167.7 167.6
ΔHm (J/g)
χc (%)
38.9 37.6 34.9 41.8 38.3 38.4 6.5 6.4 6.2 8.9 9.7 9.6 16.8 15.9 15.4 16.3 15.7 17.2
9 9 8 45 41 41 9 9 8 13 16 16 8 8 7 25 23 23
Tcc — cold crystallization temperature; ΔHc — cold crystallization enthalpy; Tr — recrystallization temperature; ΔHr — recrystallization enthalpy; Tm — temperature of melting peak (values in bold represent that of PBAT); ΔHm — melting enthalpy; χc — degree of crystallinity.
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3.3.1. Effects of talc, compatibilization and die temperature on the average cell size From Fig. 5a, it can be noted that the addition of talc has decreased the average cell size significantly for pure PLA and for noncompatibilized PLA/PBAT blend and meagerly for Ecovio. This shows that talc has acted as a nucleating agent thereby reducing the average cell size. Thus, as more cells started to nucleate, due to excess nucleation sites provided by talc, there was a 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 [41]. For Ecovio, there was a slight reduction in the average cell size, which can be considered insignificant. Of all the six formulations processed, the average cell size is found to be the lowest for Ecovio-talc sample processed at 130 °C (~ 10 µm). From Fig. 5a, it can also 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/PBAT-talc). The average cell size of non-compatibilized blends is at least twice the average cell size of compatibilized ones. In fact at 130 °C, the average cell size of the non-compatibilized PLA/PBAT blend is approximately 7.3 times of the average cell size of Ecovio. 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 [42]. The cell wall stability increases with the melt strength [43], thereby reducing the average cell size. Fig. 5b shows the variation of average cell size with temperature. In general the cell size remained consistent throughout the temperature range, i.e., 130 °C to 150 °C except for PLA and non-compatibilized PLA/PBAT blend without talc. For the latter two, the cell size decreased as the temperature increased. This is consistent with our earlier published results [28] and it might be due to higher amount of gas loss at elevated temperatures.
Fig. 3. Variation of degree of crystallinity (a) with formulation and (b) with temperature.
increased the χ [37]. The highest crystallinity was observed for PLAtalc (41–45%) which is four folds when compared to its base material, i.e., pure PLA. For Ecovio-talc and non-compatibilized PLA/PBAT-talc, increment in crystallinity, in comparison to their respective base materials, is one and two folds, respectively. Also the addition of talc did not affect the melting temperature of the base materials (i.e., pure PLA, Ecovio and non-compatibilized PLA/PBAT blend). It is interesting to note that the degree of crystallinity of all the non-filled formulations i.e. pure PLA, Ecovio and non-compatibilized PLA/PBAT blend, have coincided at all the temperatures, i.e., they have the same degree of crystallinity (Fig. 3b). This shows that neither blending nor compatibilization had any notable effect on the crystallization of PLA. This is in agreement with the findings from the literature [7] that adding PBAT did not increase the final crystallinity of PLA in the blends. Also, the crystallinity of all the samples did not vary much with the die temperature indicating that die temperature has no notable effect on the crystallinity.
3.3. Foam morphology Representative SEM images of the cell structures of different formulations are shown in Fig. 4. A quantitative study performed on the SEM images yielded cell size and cell density that are shown in Figs. 5 and 6, respectively. In the text that follows, the effects of talc, compatibilization and die temperature on the foam morphology (cell size and density) are discussed.
3.3.2. Effects of talc, compatibilization and die temperature on the cell density Fig. 6a shows the variation of cell density with formulation. In general the addition of talc has increased the cell density because of the heterogeneous nucleation. For example, the addition of talc has increased the cell density of PLA by approximately one order of magnitude. 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 [44]. For the non-compatibilized PLA/PBAT blend, the addition of talc has increased the cell density by two and one order of magnitude at 130 °C and 140 °C, respectively but decreased the cell density by approximately 47% at 150 °C. Similarly, for Ecovio, the addition of talc increased the cell density at 130 °C and 140 °C but at 150 °C a slight reduction (~6%) in cell density was observed. This reduction in cell density at higher temperature viz. 150 °C might be due to cell coalescence. Overall, it can be said that the addition of talc had a much significant effect on the cell density of PLA in comparison to the blended polymers. While comparing the compatibilized and non-compatibilized samples, it can be observed that the cell density is the much higher for Ecovio samples (i.e. both Ecovio and Ecovio-talc) (Fig. 6b). Among all the six formulations processed, the highest cell density measured was approximately 2 × 107 cells/cm3 at 150 °C for Ecovio. This is one order higher than non-compatibilized PLA/PBAT blend (~ 2 × 106) and two orders of magnitude higher than the base polymer i.e. pure PLA (~2 × 105), all processed at same temperature, i.e., 150 °C. 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 [45]. Fig. 6b shows the variation of cell density with temperature. In general, it can be said that cell density was insensitive to the die
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Fig. 4. Representative SEM images of various formulations.
temperatures except for Ecovio and Ecovio-talc. For these two compatibilized blends, the cell density increased with temperatures. This is in agreement to results published by Kaewmesri et al. [46]. 3.3.3. Effects of talc, compatibilization and die temperature on volume expansion ratio (VER) Fig. 7a and b presents the volume expansion ratio of the foamed materials with respect to formulations and temperature, respectively. The effects of talc, compatibilization and die temperature on the VER are discussed below. The addition of talc has decreased the VERs of PLA and noncompatibilized PLA/PBAT blend. This is due to increase in stiffness and strength of the polymer melt. The highest and lowest reductions were approximately 24% and 4% for non-compatibilized PLA/PBAT blend at 140 °C and 150 °C, respectively. 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 noncompatibilized PLA/PBAT blends possess 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 [47]. Fig. 7b shows the variation of VER with die temperature. A mountain shaped curve, as reported in [37], could be observed for non-compatibilized PLA/PBAT blend. For all the other samples except pure PLA, the VER did not vary much over the entire temperature range (130 to 150 °C). For non-compatibilized PLA/PBAT blend, the reduction in VER at higher temperatures is governed by the gas loss because of high gas diffusivity. At higher melt temperatures, the gas escaped through the hot skin layer during the initial process of expansion thus leaving a smaller amount of gas inside the cells. This resulted in insufficient gas pressure for foam expansion and thus a
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Fig. 6. Variation of cell density (a) with formulation and (b) with temperature. Fig. 5. Variation of average cell size (a) with formulation and (b) with temperature.
lower VER is observed at higher temperatures. On the contrary, at lower temperatures, the expansion process was governed by stiffness of the polymer. As the exit die temperature decreased, the viscosity increased and so the melt strength of the polymer is higher than that of the gas pressure inside the cells. Thus, the cells could not grow due to stiffened cell walls thereby resulting in lower VER [37]. Hence based on the above discussion, for non-compatibilized blend, a maximum VER of 1.72 was obtained at an optimum temperature of 140 °C. For the PLA sample, within the temperature range investigated (130–150 °C), a reduction in the VER was observed with increasing temperatures. This is again due to gas loss at higher temperatures as discussed. 3.3.4. Effects of talc, compatibilization and die temperature on open cell content (OCC) Fig. 8a and b shows the variation of open cell content (OCC) with formulation and temperature respectively. In general the open cell content is governed by cell wall thickness [47]. As per the cell opening strategies discussed in [47], 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. Below we discuss the effects of talc, compatibilization and die temperature on the OCC of PLA and its blends. With the addition of talc, the OCC decreased for all samples made of PLA and non-compatibilized PLA/PBAT blend (Fig. 8a). The largest
reduction in OCC was from 54% to 19% for non-compatibilized blend processed at 130 °C. The reduction in OCC observed with the PLA and non-compatibilized PLA/PBAT blend material systems due to the addition of talc 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. The largest increase in OCC was from 10% to 23% with the addition of talc processed at 130 °C. Thus, talc had a varying effect on the OCC of PLA and its blends (compatibilized and noncompatibilized). 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. The OCC of pure PLA, PLA-talc and non-compatibilized PLA/PBAT blend decreased as the temperature was increased. This is due to the difficulty in achieving thinner cells at higher temperatures because of the high rate of gas loss (gas diffusivity) through the hot skin layer of the foam [47]. On the other hand, non-filled and talc filled Ecovio exhibited contrasting trends of increase in OCC with die temperature though the increment was prominent in non-filled Ecovio. This shows that in compatibilized blends, the gas loss was less at higher temperatures thereby increasing the OCC. Also, the OCC of talc filled non-compatibilized PLA/PBAT blend remained constant throughout the entire temperature range. Among all the formulations, the highest OCC of approximately 54% was observed for PLA and non-compatibilized PLA/PBAT blend at 130 °C.
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Fig. 8. Variation of open cell content (a) with formulation and (b) with temperature. Fig. 7. Variation of volume expansion ratio (a) with formulation and (b) with temperature.
4. Conclusions In this study, biodegradable PLA/PBAT foams have been successfully produced using CO2 as a blowing agent. Two types of blends systems have been investigated, compatibilized and non-compatibilized. The effect of compatibilization was illustrated using DSC thermograms and the effects of talc, compatibilization and die temperature have been studied on different foam properties such as cell morphology, volume expansion, open cell content and crystallinity. The following are the key findings from the experiments: 1. Generally, addition of talc has decreased the average cell size and VER and increased the cell density but it had varying effect on the OCC of the foamed samples. The addition of talc has decreased the OCC of all the samples except Ecovio. Finally it was found that, with the addition of talc, the degree of crystallinity increased significantly but the melting temperature did not change. 2. Compatibilization has reduced the average cell size and volume expansion ratio and increased the cell density but had varying and no effect on the OCC and crystallinity, respectively. 3. In general the cell size remained same throughout the entire temperature range, except for PLA and non-compatibilized PLA/ PBAT blend without talc. For the latter two, the cell size decreased as the temperature increased. Also, the cell density was found to be insensitive to the die temperatures except for Ecovio and Ecoviotalc. For these two the cell density increased with temperature. No variation was observed for VER of all the samples over the entire
temperature range except pure PLA and non-compatibilized PLA/ PBAT blend. Among these two, the non-compatibilized PLA/PBAT blend showed a mountain shaped curve while the VER of pure PLA decreased with temperature. OCC showed varied effects with respect to temperature for all the samples. OCC of PLA, PLA-talc and non-compatibilized PLA/PBAT decreased with temperature but that of non-filled and talc filled Ecovio increased with temperature. Finally, it was found that temperature had no effect on the crystallinity of the formulations. Acknowledgements The financial support from National Science Foundation (CMMI0734881) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Microcellular extrusion foaming of poly(lactide)/poly(butylene adipate-co-terephthalate) blends Srikanth Pilla a, Seong G. Kim b, George K. Auer a, Shaoqin Gong a,c,⁎, Chul B. Park b a b c
Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, Wisconsin-53211, USA Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, Canada M5S 3G8 Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin-53706, USA
a r t i c l e
i n f o
Article history: Received 11 August 2009 Received in revised form 12 October 2009 Accepted 25 October 2009 Available online 30 October 2009 Keywords: Microcellular extrusion foaming PLA PBAT Compatibilization
a b s t r a c t Foamed poly(lactide) (PLA)/poly(butylene adipate-co-terephthalate) (PBAT) blends were processed via the microcellular extrusion process using CO2 as a blowing agent. Talc has been added to promote heterogeneous nucleation. Two types of PLA/PBAT blend systems were investigated: Ecovio, which is a commercially available compatibilized PLA/PBAT blend; and a non-compatibilized PLA/PBAT blend at the same PLA/PBAT ratio (i.e., 45:55 by weight percent). Six different formulations were investigated: pure PLA, PLA-talc, Ecovio, Ecovio-talc, non-compatibilized PLA/PBAT blend, and non-compatibilized PLA/PBAT-talc. The effects of various processing parameters such as die temperature, talc and compatibilization on various foaming properties such as cell morphology, volume expansion ratio (VER), open cell content (OCC) and crystallinity were investigated. As per the DSC thermograms, it was observed that compatibilization has merged the two distinctive melting peaks of PLA and PBAT into a single peak while lowering the peak temperature. In general, the addition of talc has decreased the average cell size and VER and increased the cell density and crystallinity; however, it has varying effects on the open cell content. Compatibilization has reduced the average cell size and volume expansion but increased the cell density and had varying and no effects on the OCC and crystallinity, respectively. Similar to compatibilization, the die temperature was found to have varying and no effects on the OCC and crystallinity, respectively. Except for PLA and noncompatibilized PLA/PBAT blend, the cell size and VER of all other formulations did not vary much throughout the entire temperature range (130–150 °C). The cell density was found to be insensitive to die temperatures except for Ecovio and Ecovio-talc. © 2009 Elsevier B.V. All rights reserved.
1. Introduction 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 a 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(ε-caprolactone) (PCL) [8–12] and many other non-biodegradable polymers [13–19]. Though the blended polymers exhibited certain improved mechanical properties compared to virgin 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.
⁎ Corresponding author. E-mail addresses: [email protected] (S. Gong), [email protected] (C.B. Park). 0928-4931/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2009.10.010
Foamed plastics are used in a variety of applications such as insulation, packaging, cushions, and structural components [20,21]; especially, microcellular foaming is capable of producing foamed plastics with less 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 (CO2) and nitrogen (N2) in their supercritical state [23]. Microcellular process also improves the cell morphology with typical cell sizes in tens of microns and cell density on the order of 109 cells/cm3 [23]. Additionally, compared to solid part 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
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extrusion foaming. Mihai et al. [31] have investigated the foaming ability of PLA blended with starch using microcellular extrusion. Reignier et al. [32] have studied extrusion foaming of amorphous PLA using CO2; however, due to a 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 CO2 as a blowing agent. Two types of blend systems were investigated: (1) Ecovio, which is a commercially available compatibilized PLA/PBAT blend; (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 compatibilization, talc and die temperature on the cell morphology, volume expansion, crystallinity and open cell content were evaluated.
cell size and cell density was performed using an image analysis tool (UTHSCSA ImageTool). The cell density was calculated using the following formula [34]:
Cell Density =
3 = 2 N M L2
ð1Þ
where N is the number of cells, L is the linear length of the area, and M is a unit conversion resulting in the number of cells per cm3. The volume expansion ratio (ϕ) (VER) was calculated by taking the ratio of the bulk density of pure PLA material (ρp) to the bulk density of foam sample (ρf) as follows [34]:
2. Experimental 2.1. Materials PLA (PLA 3001D) in a pellet form was obtained from NatureWorks® LLC, Minnetonka, MN, USA. Its specific gravity was 1.24 and had a melt flow index around 15 g/10 min (190 °C/2.16 kg). PLA 3001D was synthesized from approximately 92% L-lactide and 8% meso-lactide [33]. Ecovio (compatibilized blend of PLA/PBAT at 45/55 ratio) was obtained from BASF (melt viscosity b 2.5–4.5 ml/10 min at 190 °C). For the non-compatibilized blend, PBAT was supplied by BASF under the trade mark of Ecoflex (density: 1.25–1.27 g/cm3; melt viscosity: 2.5– 4.5 ml/10 min at 190 °C; and melting point: 110–120 °C). Talc was obtained from Luzenac (grade-JetFil700C) with a density of 2.8 g/cm3 and an average particle size of 1.5 μm.
ϕ=
ρp : ρf
The bulk density of the microcellular samples was calculated by measuring the weight and dividing it by the volume of the microcellular sample which was determined by the water displacement method (ASTM D792). Differential scanning calorimetry (TA instruments, Auto Q-2000) was used to analyze the crystallization property of the foamed samples. The samples were first heated from 40 °C to 180 °C, kept isothermal for three minutes, cooled to 0 °C, and finally reheated to 200 °C. The ramp speed in all the heating and cooling processes was 10 °C/min.
2.2. Foaming process
3. Results and discussion
A single screw extruder (Brabender, 05-25-000) with a mixing screw (Brabender, 05-00-144) of 30:1 L/D ratio and 3/4″ diameter was used to process the materials. First, the materials (any formulation) were fed into the barrel through the hopper and were completely melted by the screw rotation and shear before the gas injection port. A metered amount of physical blowing agent (4% SCF CO2) was injected into the extrusion barrel at the gas injection port by the positive displacement pump and was mixed intensively with the polymer melt stream. The shear fields generated by the screw via mixing facilitated the formation of a single phase polymer–gas solution. Also a static mixer (omega, FMX-84441-S) was installed for enhancing diffusion at the extruder to improve the dissolution of the physical blowing agent in the polymer melt. The resulting single phase polymer/gas solution went through the gear pump (Zenith, PEP-II 1.2 cc/rev), where the volumetric displacement was controlled by the motor, and then through the heat exchanger (consisting of homogenizing static mixers viz. Labcore Model H-0466912) where it was cooled to a designated temperature. The cooled polymer/gas solution entered the die (L/D: 0.89″/0.05″) and foaming occurred subsequently at the die exit. The heat exchanger and die temperatures were synchronized for simplicity. While optimizing all the parameters, the heat exchanger and die temperatures were lowered gradually and samples were randomly collected at each designated temperature when the variation in the pressures was approximately ±30 psi. Six different formulations were investigated: PLA, PLA−0.5% Talc, Ecovio, Ecovio−0.5% Talc, PLA−55%PBAT, and PLA−55%PBAT−0.5% Talc. A nucleating agent, talc, was added to all the formulations to assist in the heterogeneous nucleation process.
3.1. Effect of die pressure The foaming experiment was conducted with 4% CO2. The variation of exit die pressure with exit die temperature is shown in Fig. 1. It can be noted that the pressure increased as the temperature decreased. This is due to the increase in viscosity at lower temperatures. Thus, the formulations could not be processed at lower temperatures (below 130 °C) as the pressure increased beyond safe limits. Also, it can be observed that the exit die pressure for all the samples is much higher than the 4% CO2 solubility pressure in PLA, which implies that a polymer–gas single phase solution was formed. The CO2 dissolved in the PLA melt serves as a plasticizer [35], thereby reducing the overall viscosity and thus leading to reduced temperature required to process the polymer. This is very desirable for temperature-sensitive polymers such as PLA because lower processing temperatures can minimize PLA degradation.
2.3. Characterization of foams The foamed materials were characterized using SEM (JEOL-6060) and gas pycnometer (Quantachrome Co.) for cell morphology and open cell content, respectively. A quantitative analysis of the average
ð2Þ
Fig. 1. Die pressure vs temperature profile for 4% CO2 for all formulations.
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As shown in Table 1, two crystallization peaks exists for pure PLA and non-compatibilized PLA/PBAT blends. The first peak is termed as cold crystallization peak and the second one recrystallization. With the addition of talc, these two peaks vanished for PLA. The disappearance of the cold crystallization peak indicates an enhanced crystallinity for PLA produced during the cooling cycle, which is confirmed by the data shown in Table 1. In addition, the cold crystallization temperatures of non-compatibilized PLA/PBAT blends, shifted to the left by a few degrees due to the nucleating effect of the talc [7]. Crystallization is an important property that needs to be investigated in foaming as it affects the cell growth [37]. In fact, the cell growth stops at the moment of crystallization [38]. Fig. 3 shows the degree of crystallinity of all formulations measured from the second heating cycle of the DSC thermograms and Table 1 shows the numerical values of the characteristic temperatures and enthalpies and crystallinity obtained from the second heating cycle. The crystallinity of PLA is computed using Eq. (3) [39]: χc ð% CrystallinityÞ =
ΔHm 100 × 0 w ΔHm
ð3Þ
where, ΔH0m — 93.7 J/g w — weight fraction of PLA in the sample. Fig. 2. Melting curves of (A) Ecovio, (B) Ecovio−0.5% Talc, (C) PLA−55%PBAT, and (D) PLA−55%PBAT−0.5% Talc processed at 130 °C.
3.2. Thermal properties DSC was used to study the thermal characteristics (melting peaks and crystallization) of the foamed samples. Fig. 2 shows the melting peaks of all the non-filled and talc filled PLA/PBAT blends processed at 130 °C. As can be observed in the figure, the non-compatibilized blends exhibited two distinct melting peaks, at around 127 °C and 168 °C, representing PBAT and PLA respectively. Also the PBAT melting peak was observed to be broad. The broadness and peak temperature of PBAT are consistent with results reported in [36]. On the other hand, the compatibilized blends showed only one melting peak at 151 °C. This shows that the distinctive melting peaks observed in non-compatibilized blends merged into a single melting peak in compatibilized blends and also the peak temperature decreased. This illustrates the effect of compatibilization on the blending of PLA and PBAT.
ΔHm is the enthalpy for melting, ΔH0m is the enthalpy of melting for a 100% crystalline PLA sample, and w is the weight fraction of PLA in the sample. To determine the crystallinity of the sample, the extra heat absorbed by the crystallites formed during heating (i.e., cold crystallization) had to be subtracted from the total endothermic heat flow due to the melting of the whole crystallites [40]. Thus, the modified equation can be written as follows: χc ð% CrystallinityÞ =
ΔHm −ΔHcc 100 × 0 w ΔHm
ð4Þ
where, ΔHcc: cold — crystallization enthalpy. 3.2.1. Effects of talc, compatibilization and die temperature on degree of crystallinity As shown in Fig. 3a, the addition of talc has increased the degree of crystallinity (χ) for all the samples. This shows that talc has acted as a nucleating agent during the crystallization process and thereby
Table 1 Thermal characteristics of PLA and its blends. Formulation
Die temperature (°C)
Tcc (°C)
ΔHc (J/g)
Tr (°C)
ΔHr (J/g)
PLA PLA PLA PLA−0.5% Talc PLA−0.5% Talc PLA−0.5% Talc Ecovio Ecovio Ecovio Ecovio−0.5% Talc Ecovio−0.5% Talc Ecovio−0.5% Talc PLA/55% PBAT PLA/55% PBAT PLA/55% PBAT PLA/55% PBAT−0.5% Talc PLA/55% PBAT−0.5% Talc PLA/55% PBAT−0.5% Talc
130 140 150 130 140 150 130 140 150 130 140 150 130 140 150 130 140 150
103.1 102.9 102.9 – – – 132.9 133.6 134.9 119.1 106.3 119.7 101.9 102.1 102.5 98.4 97.9 99.5
29.5 27.9 26.3 – – – 2.6 2.7 2.8 4.1 3.8 3.6 12.6 12.1 11.6 6.6 6.6 7.9
155.8 155.7 156.1 – – – – – – – – – 155.2 154.6 155.6 154.5 154.1 154.2
1.4 1.4 1.4 – – – – – – – – – 0.9 0.5 0.7 0.4 0.4 0.6
Tm (°C) 168.8 168.7 168.9 169.6 169.4 169.3 151.4 151.8 151.5 150.2 149.7 150.1 127.2 127.4 127.9 127.4 127.6 127.2
167.8 167.7 167.9 168.1 167.7 167.6
ΔHm (J/g)
χc (%)
38.9 37.6 34.9 41.8 38.3 38.4 6.5 6.4 6.2 8.9 9.7 9.6 16.8 15.9 15.4 16.3 15.7 17.2
9 9 8 45 41 41 9 9 8 13 16 16 8 8 7 25 23 23
Tcc — cold crystallization temperature; ΔHc — cold crystallization enthalpy; Tr — recrystallization temperature; ΔHr — recrystallization enthalpy; Tm — temperature of melting peak (values in bold represent that of PBAT); ΔHm — melting enthalpy; χc — degree of crystallinity.
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3.3.1. Effects of talc, compatibilization and die temperature on the average cell size From Fig. 5a, it can be noted that the addition of talc has decreased the average cell size significantly for pure PLA and for noncompatibilized PLA/PBAT blend and meagerly for Ecovio. This shows that talc has acted as a nucleating agent thereby reducing the average cell size. Thus, as more cells started to nucleate, due to excess nucleation sites provided by talc, there was a 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 [41]. For Ecovio, there was a slight reduction in the average cell size, which can be considered insignificant. Of all the six formulations processed, the average cell size is found to be the lowest for Ecovio-talc sample processed at 130 °C (~ 10 µm). From Fig. 5a, it can also 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/PBAT-talc). The average cell size of non-compatibilized blends is at least twice the average cell size of compatibilized ones. In fact at 130 °C, the average cell size of the non-compatibilized PLA/PBAT blend is approximately 7.3 times of the average cell size of Ecovio. 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 [42]. The cell wall stability increases with the melt strength [43], thereby reducing the average cell size. Fig. 5b shows the variation of average cell size with temperature. In general the cell size remained consistent throughout the temperature range, i.e., 130 °C to 150 °C except for PLA and non-compatibilized PLA/PBAT blend without talc. For the latter two, the cell size decreased as the temperature increased. This is consistent with our earlier published results [28] and it might be due to higher amount of gas loss at elevated temperatures.
Fig. 3. Variation of degree of crystallinity (a) with formulation and (b) with temperature.
increased the χ [37]. The highest crystallinity was observed for PLAtalc (41–45%) which is four folds when compared to its base material, i.e., pure PLA. For Ecovio-talc and non-compatibilized PLA/PBAT-talc, increment in crystallinity, in comparison to their respective base materials, is one and two folds, respectively. Also the addition of talc did not affect the melting temperature of the base materials (i.e., pure PLA, Ecovio and non-compatibilized PLA/PBAT blend). It is interesting to note that the degree of crystallinity of all the non-filled formulations i.e. pure PLA, Ecovio and non-compatibilized PLA/PBAT blend, have coincided at all the temperatures, i.e., they have the same degree of crystallinity (Fig. 3b). This shows that neither blending nor compatibilization had any notable effect on the crystallization of PLA. This is in agreement with the findings from the literature [7] that adding PBAT did not increase the final crystallinity of PLA in the blends. Also, the crystallinity of all the samples did not vary much with the die temperature indicating that die temperature has no notable effect on the crystallinity.
3.3. Foam morphology Representative SEM images of the cell structures of different formulations are shown in Fig. 4. A quantitative study performed on the SEM images yielded cell size and cell density that are shown in Figs. 5 and 6, respectively. In the text that follows, the effects of talc, compatibilization and die temperature on the foam morphology (cell size and density) are discussed.
3.3.2. Effects of talc, compatibilization and die temperature on the cell density Fig. 6a shows the variation of cell density with formulation. In general the addition of talc has increased the cell density because of the heterogeneous nucleation. For example, the addition of talc has increased the cell density of PLA by approximately one order of magnitude. 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 [44]. For the non-compatibilized PLA/PBAT blend, the addition of talc has increased the cell density by two and one order of magnitude at 130 °C and 140 °C, respectively but decreased the cell density by approximately 47% at 150 °C. Similarly, for Ecovio, the addition of talc increased the cell density at 130 °C and 140 °C but at 150 °C a slight reduction (~6%) in cell density was observed. This reduction in cell density at higher temperature viz. 150 °C might be due to cell coalescence. Overall, it can be said that the addition of talc had a much significant effect on the cell density of PLA in comparison to the blended polymers. While comparing the compatibilized and non-compatibilized samples, it can be observed that the cell density is the much higher for Ecovio samples (i.e. both Ecovio and Ecovio-talc) (Fig. 6b). Among all the six formulations processed, the highest cell density measured was approximately 2 × 107 cells/cm3 at 150 °C for Ecovio. This is one order higher than non-compatibilized PLA/PBAT blend (~ 2 × 106) and two orders of magnitude higher than the base polymer i.e. pure PLA (~2 × 105), all processed at same temperature, i.e., 150 °C. 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 [45]. Fig. 6b shows the variation of cell density with temperature. In general, it can be said that cell density was insensitive to the die
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Fig. 4. Representative SEM images of various formulations.
temperatures except for Ecovio and Ecovio-talc. For these two compatibilized blends, the cell density increased with temperatures. This is in agreement to results published by Kaewmesri et al. [46]. 3.3.3. Effects of talc, compatibilization and die temperature on volume expansion ratio (VER) Fig. 7a and b presents the volume expansion ratio of the foamed materials with respect to formulations and temperature, respectively. The effects of talc, compatibilization and die temperature on the VER are discussed below. The addition of talc has decreased the VERs of PLA and noncompatibilized PLA/PBAT blend. This is due to increase in stiffness and strength of the polymer melt. The highest and lowest reductions were approximately 24% and 4% for non-compatibilized PLA/PBAT blend at 140 °C and 150 °C, respectively. 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 noncompatibilized PLA/PBAT blends possess 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 [47]. Fig. 7b shows the variation of VER with die temperature. A mountain shaped curve, as reported in [37], could be observed for non-compatibilized PLA/PBAT blend. For all the other samples except pure PLA, the VER did not vary much over the entire temperature range (130 to 150 °C). For non-compatibilized PLA/PBAT blend, the reduction in VER at higher temperatures is governed by the gas loss because of high gas diffusivity. At higher melt temperatures, the gas escaped through the hot skin layer during the initial process of expansion thus leaving a smaller amount of gas inside the cells. This resulted in insufficient gas pressure for foam expansion and thus a
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Fig. 6. Variation of cell density (a) with formulation and (b) with temperature. Fig. 5. Variation of average cell size (a) with formulation and (b) with temperature.
lower VER is observed at higher temperatures. On the contrary, at lower temperatures, the expansion process was governed by stiffness of the polymer. As the exit die temperature decreased, the viscosity increased and so the melt strength of the polymer is higher than that of the gas pressure inside the cells. Thus, the cells could not grow due to stiffened cell walls thereby resulting in lower VER [37]. Hence based on the above discussion, for non-compatibilized blend, a maximum VER of 1.72 was obtained at an optimum temperature of 140 °C. For the PLA sample, within the temperature range investigated (130–150 °C), a reduction in the VER was observed with increasing temperatures. This is again due to gas loss at higher temperatures as discussed. 3.3.4. Effects of talc, compatibilization and die temperature on open cell content (OCC) Fig. 8a and b shows the variation of open cell content (OCC) with formulation and temperature respectively. In general the open cell content is governed by cell wall thickness [47]. As per the cell opening strategies discussed in [47], 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. Below we discuss the effects of talc, compatibilization and die temperature on the OCC of PLA and its blends. With the addition of talc, the OCC decreased for all samples made of PLA and non-compatibilized PLA/PBAT blend (Fig. 8a). The largest
reduction in OCC was from 54% to 19% for non-compatibilized blend processed at 130 °C. The reduction in OCC observed with the PLA and non-compatibilized PLA/PBAT blend material systems due to the addition of talc 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. The largest increase in OCC was from 10% to 23% with the addition of talc processed at 130 °C. Thus, talc had a varying effect on the OCC of PLA and its blends (compatibilized and noncompatibilized). 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. The OCC of pure PLA, PLA-talc and non-compatibilized PLA/PBAT blend decreased as the temperature was increased. This is due to the difficulty in achieving thinner cells at higher temperatures because of the high rate of gas loss (gas diffusivity) through the hot skin layer of the foam [47]. On the other hand, non-filled and talc filled Ecovio exhibited contrasting trends of increase in OCC with die temperature though the increment was prominent in non-filled Ecovio. This shows that in compatibilized blends, the gas loss was less at higher temperatures thereby increasing the OCC. Also, the OCC of talc filled non-compatibilized PLA/PBAT blend remained constant throughout the entire temperature range. Among all the formulations, the highest OCC of approximately 54% was observed for PLA and non-compatibilized PLA/PBAT blend at 130 °C.
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Fig. 8. Variation of open cell content (a) with formulation and (b) with temperature. Fig. 7. Variation of volume expansion ratio (a) with formulation and (b) with temperature.
4. Conclusions In this study, biodegradable PLA/PBAT foams have been successfully produced using CO2 as a blowing agent. Two types of blends systems have been investigated, compatibilized and non-compatibilized. The effect of compatibilization was illustrated using DSC thermograms and the effects of talc, compatibilization and die temperature have been studied on different foam properties such as cell morphology, volume expansion, open cell content and crystallinity. The following are the key findings from the experiments: 1. Generally, addition of talc has decreased the average cell size and VER and increased the cell density but it had varying effect on the OCC of the foamed samples. The addition of talc has decreased the OCC of all the samples except Ecovio. Finally it was found that, with the addition of talc, the degree of crystallinity increased significantly but the melting temperature did not change. 2. Compatibilization has reduced the average cell size and volume expansion ratio and increased the cell density but had varying and no effect on the OCC and crystallinity, respectively. 3. In general the cell size remained same throughout the entire temperature range, except for PLA and non-compatibilized PLA/ PBAT blend without talc. For the latter two, the cell size decreased as the temperature increased. Also, the cell density was found to be insensitive to the die temperatures except for Ecovio and Ecoviotalc. For these two the cell density increased with temperature. No variation was observed for VER of all the samples over the entire
temperature range except pure PLA and non-compatibilized PLA/ PBAT blend. Among these two, the non-compatibilized PLA/PBAT blend showed a mountain shaped curve while the VER of pure PLA decreased with temperature. OCC showed varied effects with respect to temperature for all the samples. OCC of PLA, PLA-talc and non-compatibilized PLA/PBAT decreased with temperature but that of non-filled and talc filled Ecovio increased with temperature. Finally, it was found that temperature had no effect on the crystallinity of the formulations. Acknowledgements The financial support from National Science Foundation (CMMI0734881) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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