Reinforcing a Thermoplastic Starch/Poly(butylene adipate-co-terephthalate) Composite Foam with Polyethylene Glycol under Supercritical Carbon Dioxide

Biodegradable foams are a potential substitute for most fossil-fuel-derived polymer foams currently used in the cushion furniture-making industry. Thermoplastic starch (TPS) and poly(butylene adipate-co-terephthalate) (PBAT) are biodegradable polymers, although their poor compatibility does not support the foam-forming process. In this study, we investigated the effect of polyethylene glycol (PEG) with or without silane A (SA) on the foam density, cell structure and tensile properties of TPS/PBAT blends. The challenges in foam forming were explored through various temperature and pressure values under supercritical carbon dioxide (CO2) conditions. The obtained experimental results indicate that PEG and SA act as a plasticizer and compatibilizer, respectively. The 50% (TPS with SA + PEG)/50% PBAT blends generally produce foams that have a lower foam density and better cell structure than those of 50% (TPS with PEG)/50% PBAT blends. The tensile property of each 50% (TPS with SA + PEG)/50% PBAT foam is generally better than that of each 50% (TPS with PEG)/50% PBAT foam.


Introduction
Recently, conventional plastics prepared by the petrochemical industry have been utilized for many everyday products, such as multifunctional sensors [1,2], optoelectronic devices [3], wearable electronic devices [4], etc. However, petrochemical plastics are difficult to degrade and inconvenient to collect and store for recycling. Developing biopolymeric materials as an alternative to these materials can reduce the use of non-biodegradable polymers [5], which has become one of the world's major goals. In particular, the key advantages of biodegradable polymer-based foams are their compatibility [6], biodegradability, and renewability [7].
Starch, which can be obtained with ease, is a cost-effective biodegradable polymer. It consists of a large number of glucose components and two different structures, i.e., amylose and amylopectin. Amylose is a much smaller molecule than amylopectin, but amylose molecules in starch are more numerous than amylopectin molecules. The granular structures of starch are observed in nature and are very difficult to process directly by extrusion or injection due to the very large molecular weight. TPS consists of starch granules mixed with plasticizers, such as water or glycerin, to reduce the interaction among starch granules, and it can further be processed to obtain products. Biodegradable plastics for daily products can be an good alternative to address the difficult degradation of regular plastics, causing environmental problems. The foam products obtained from
SA contains trimethoxysilyl inorganic functional groups and one reactive glycidoxy organic functional group at the other end. The crystallinity and compatibility of TPS can be altered based on the molecular weight of PEG; therefore, the effect of the molecular weight (Mw: 1000, 2000, 3000, named PEG-10, PEG-20, and PEG-30, respectively) was studied. The PEG with different molecular weights was blended with TPS with a known amount of 10 PHR (per hundred resin) with respect to the TPS concentration. However, SA with a small amount (5 PHR) is inevitable to improve the compatibility of PEG-modified TPS with PBAT.
The composite blending ratios are displayed in Table 1. Sheets of TPS/PBAT, PEG-TPS/PBAT, and (SA/PEG-TPS)/PBAT blends were produced by a hot press at a temperature of 140-160 • C, as shown in Figure S1. The sheet area and thickness were fixed at 75 mm × 75 mm and 3 mm, respectively. These sheets were directly used for foaming under CO 2 supercritical conditions. This foam preparation experiment was repeated five times with fixed parameters to achieve a high degree of accuracy in the results, and it exhibited excellent reproducibility.

Surface and Functional Group Modification of TPS
First, the starch was uniformly plasticized into TPS commercially with the addition of available starch raw material to a binary solvent of water/glycerin in a plastic spectrometer (Barbender MIX, Kulturstraße, Germany). Then, the chemical structures of TPS blends were remodified based on our previous studies [36]. A functional group modifier-coupling agent of SA and different PEG (Mw: 1000, 2000, 3000) was added to the foam to enhance the number of porous cells, which afforded high flexibility ( Table 1). The reaction temperature was optimized in the range of 55~70 • C, and the reaction time was 30~60 min. The pelletizer was modified and pelletized to produce surface-modified PEG/TPS and SA-PEG/TPS blends. The complete reaction mechanisms are given in Figure S2.

The (SA-PEG/TPS)/PBAT Biodegradable Polyester Composite Mixing
Chemical structure modifying agents were prepared by blending pre-prepared TPS and pre-mixed SA with PEG-10, PEG-20, and PEG-30 in a 1:1 ratio. Then, they were further mixed with PBAT biodegradable polyester at a 50% weight ratio. The composite mixture was transferred into a plastic spectrometer blending machine. The SA-PEG/TPS and the biodegradable polyester were uniformly mixed and dispersed into the composite. The mixing machine plastic spectrometer temperature was set at 90 • C to 145 • C, and the granulation screw speed was 50~100 rpm. Similarly, a control sample of unmodified TPS was blended with PBAT (50% TPS/50% PBAT).

The (SA-PEG/TPS)/PBAT Composite Foam Test Piece and Supercritical Foaming Experiment
The evenly mixed (SA-PEG/TPS)/PBAT composite was placed into a hot pressing machine to foam a film. The hot pressing process took place at a temperature of 140 • C to 165 • C; the retrieved sample appeared square in shape, with an area and thickness of 75 mm × 75 mm and 3 mm, respectively.
Then, the test samples were treated with supercritical CO 2 using foaming equipment (200-ton capacity, Tainan City, Jing Day Machinery Industrial Co., Ltd., Taiwan) to achieve highly flexible, porous foam materials. The foaming process was controlled by adopting adequate parameters, namely a foaming temperature from 80 • C to 105 • C at two different pressure conditions of 17 MPa and 23.8 MPa. The impregnation chamber was maintained at a constant foaming temperature and pressure for 60 min. This time was sufficient to ensure that the CO 2 in the material reached a saturated state.

The (SA-PEG/TPS)/PBAT Composite Foam Appearance and Internal Structure SEM Analysis
A scanning electron microscope (SEM, using a Hitachi TM4000 Plus, Hitachi High-Tech Fielding Corporation, Nagano, Japan; the WD parameter was 15.6 mm, and the HV voltage was 5.0 kV) was used to examine the foam cells' structure, the size of the bubbles, and the dispersion of TPS in the composite foam samples after supercritical foaming.

FTIR Study of (P/TPS) and (SP/TPS)
The functional groups of PEG/TPS (P/TPS) prior to surface modification and after modification with SA are shown in Figure 1a Figure 1b shows the NMR spectrum of PEG/TPS (P/TPS). DMSO-d 6 was used as a solvent. Compared to the literature data [37], small signals appear at 3.62 (H-6a, H-6b, and H-5), 4.6 (H-2), 5.08 (H-4), and 5.41-5.52 (H-1 and H-3) ppm, indicating the starch. The small signals at 2.49 and 3.33 ppm correspond to the -OH (hydrogen bond and water) in DMSO and starch, respectively, and the signal from the terminal proton of the -CH 2 -OH group appears at 4.44 ppm.
The NMR of SP/TPS is revealed in Figure 1b, where SA-PEG/TPS is dissolved in the DMSO-d 6 solvent. The starch resonance is represented by a small signal at 3.6 (H-6a, H-6b, and H-5), 4.6 (H-2), 5.08 (H-4), and 5.41-5.52 (H-1 and H-3) ppm [37]. The NMR peak at 2.49 corresponds to the -OH group of DMSO and the 3.38 ppm peak is related to that of -OH (hydrogen bond and water) in starch. The terminal functional group bearing a proton resonates at 4.40 and 4.47 ppm, and the SA modifier proton of the -Si-OCH 3 functional group shows a signal at 3.49 ppm.

The Density of Foamed PBAT
In a previous study, we evaluated the density changes in PBAT foams with impregnated CO 2 at different temperatures with two different foaming pressures [36]. The foam density of PBAT was affected proportionally by the increase in temperature under the above pressure conditions. Furthermore, the foam density obtained declined from 350 to 160 Kg/m 3 .

The Composite Foam Density of (PEG/TPS)/PBAT
The effect of foaming temperatures ranging from 80 • C to 105 • C at a foam pressure of 17MPa and 23.8 MPa on the foam density of the four blends, i.e., 50% TPS/50% PBAT [N-1], 50% (TPS with 10PHR PEG-10)/50% PBAT [P-1], 50% (TPS with 10PHR PEG-20)/50% PBAT [P-2], 50% (TPS with 10PHR PEG-30)/50% PBAT [P-3], is illustrated in Figure 2. The highest foam density in the study for the four blends occurs at a temperature of 80 • C, in comparison with other foaming temperatures, under the two foaming pressures of 17 MPa (shown in Figure 2a) and 23.8 MPa (shown in Figure 2b). As the foaming temperature increases, the foam density for the four blends reduces. In essence, the foams produced at the foaming pressure of 23.8 MPa for the four blends have a lower foam density than those produced at the foaming pressure of 17 MPa under the same foam temperature. The lowest foam density under the foaming pressure of 17 MPa for the four blends is obtained around the foaming temperature of 100 • C; however, for a foaming pressure of 23.8 MPa, it is observed around the foaming temperature of 95 • C. Generally, the lowest foam density values of the three 50% (TPS with10PHR PEG)/50% PBAT blends are very close to one another under the foaming conditions. The foam density of the four blends rises again when the foaming temperature increases. The enhancement in the foam density indicates that the foam structure becomes weak and the shrinkage of the foam occurs at high temperatures. It is observed that the foam density of the [N-1] blend is higher than that of the three 50% (TPS with 5PHR SA and 5PHR PEG)/50% PBAT blends. The [SP-1] foam has the lowest foam density among the three 50% (TPS with 5PHR SA and 5PHR PEG)/50% PBAT foams. However, the [SP-2] and [SP-3] foams have a similar foam density. The lowest foam density in the study for the four foams occurs at 100 • C when the foaming pressure is at 17 MPa; when the foaming pressure is increased to 23.8 MPa, the foaming temperature shifts to 95 • C. From this, it can be seen that the four blends generate a lower foam density at the foaming ature. The lowest foam density under the foaming pressure of 17 MPa for the four blends is obtained around the foaming temperature of 100 °C; however, for a foaming pressure of 23.8 MPa, it is observed around the foaming temperature of 95 °C. Generally, the lowest foam density values of the three 50% (TPS with10PHR PEG)/50% PBAT blends are very close to one another under the foaming conditions. The foam density of the four blends rises again when the foaming temperature increases. The enhancement in the foam density indicates that the foam structure becomes weak and the shrinkage of the foam occurs at high temperatures.   Figure S4a-d show the foams (N-1), (P-1), (P-2), and (P-3), respectively, and the foams were produced at various foaming temperatures at a fixed foam pressure of 23.8 MPa. The sizes of the foams, without warpage or breakage, obtained at a foam pressure of 23.8 MPa were larger than those prepared at a foam pressure of 17 MPa under the same foaming temperatures. The foams with the (P-3) composition had a better appearance and shape than those with other compositions. A poor appearance and shape for the foams can be observed above 95 • C. Moreover, with the TGA result, we can see that the reason is that SA is an epoxy-based modifier and its structural thermal stability is better. It can also increase the compatibility of TPS and PBAT and generate more entanglement characteristics between molecules (shown in Figures S7-S9).
In general, a poor appearance and shape for the foams is observed at high foaming temperatures, such as 100 • C and 105 • C. When two foams are produced at the same foaming temperature, but at a different foam pressure of 17 MPa and 23.8 MPa, the foam produced at the higher foam pressure shows the worst appearance and structure.  Figure 3b shows the cell structure for the [P-1] foam produced at the temperature of 95 • C and pressure of 17 MPa. Numerous large (TPS/PEG-10) particles are randomly scattered in the foam observed in Figure 3b, and the size of the (TPS/PEG-10) particles in the foam is generally larger than that of the TPS particles in the [N-1] foam. A gap at the boundary between (TPS/PEG-10) particles and cells for the [P-1] foam clearly exists and is similar to that of the TPS particles and cells in the [N-1] foam. However, the cell structure for the [P-1] foam exhibits a bimodal nature in terms of the cell size. One is that (TPS/PEG-10) particles generate many large pores, for which the pore sizes can be larger than 200 µm; the other is that many small cells can be seen and the cell sizes are less than 15 µm. Figure S10b presents the cell structure for the [P-1] foam produced at the foaming temperature of 95 • C and the foaming pressure of 23.8 MPa. Many large pores and small cells are scattered in the foam produced under the above-mentioned foaming conditions; however, the mean size of cells obtained at a pressure of 23.8 MPa is smaller than that of cells obtained at 17 MPa. The cell structure is shown in Figure S10b, which is similar to that shown in Figure 3b. Therefore, TPS modified by PEG with a low molecular weight, 0.3 Kg/mole, does not improve the dispersion and compatibility of TPS in PBAT. Figure 3c is the cell structure for the [P-2] foam obtained at 95 • C and a pressure of 17 MPa. When the cell structure of the [P-2] foam is compared with that of the [P-1] foam, the [P-2] foam generally has smaller (TPS/PEG-20) particles and fewer large pores than the [P-1] foam. The result indicates that TPS with P20 can obtain a better dispersive property in PBAT than that of TPS with P10. The increase in the number of (TPS/PEG-20) particles can lead to the destruction of the bimodal structure of cells observed in Figure 3b. Figure S10c represents the cell structure for the [P-2] foam obtained at the temperature of 95 • C and pressure of 23.8 MPa. Essentially, the foam produced at the foaming pressure of 23.8 MPa has a better cell structure than that of the foam produced at the foaming pressure of 17 MPa under the same foaming temperature. A bimodal structure of the cell is observed again, probably because the [P-2] foam has smaller cells from PBAT foaming produced at the high foaming pressure of 23.8 MPa than those of the foam produced at the low foaming pressure of 17 MPa.   Cell structures for the [P-3] foam obtained at the foaming temperature of 95 • C and the two foaming pressures of 17 MPa and 23.8 MP are shown in Figures 3d and S10d, respectively. In comparison with the effect of the three PEG components with different MWs on the TPS/PBAT blend, it is found that the particle size of TPS modified by PEG with a high MW is more uniform and smaller than that of TPS modified by PEG with a low MW in the 50% (TPS with10PHR PEG)/50% PBAT foam. However, the aggregation of small (TPS/PEG-30) particles occurs and they resemble a large particle. The [P-3] foam has a better cell structure than the [P-1] and [P-2] foams. Generally, 50% (TPS with or without PEG)/50% PBAT foams produced at a high foaming pressure have a better cell structure than = foams produced at a low foaming pressure.

External and Internal Structure of (PEG/TPS)/PBAT Composite Foam
SEM images for the 50% (TPS with10PHR PEG)/50% PBAT foams reveal (TPS/PEG) particles at the boundary, without interaction with cells. PEG acts as a plasticizer in the blend. The increase in MW for PEG can lead to an enhancement in the viscosity of PEG. Therefore, TPS mixed with different types of PEG can yield different viscosity levels for (TPS/PEG), which may result in differences in dispersion for (TPS/PEG) in 50% (TPS/PEG)/50% PBAT blends.  Figure 4a. It is found that (TPS/SA + PEG-10) particles become small, and large (TPS/SA + PEG-10) particles almost disappear in the foam; the boundary between (TPS/SA + PEG-10) particles and cells becomes indistinct, and (TPS/SA + PEG-10) particles seem to be embedded in the foam. In general, the cell structure of the [SP-1] foam is more uniform than that of the [P-1] foam shown in Figure 3b. Figure S11a represents the cell structure for [SP-1] foamed at 95 • C and a pressure of 23.8 MPa. It is observed that the two foams shown in Figures 4 and S11 have a similar structure, but the foam obtained at a high foaming pressure exhibits smaller cells than the foam obtained at a low foaming pressure. Therefore, SA, ass a compatibilizer, can enhance the miscibility between (TPS/SA + PEG-10) particles and PBAT and further contribute to the good dispersion of (TPS/SA + PEG-10) particles in PBAT. is more uniform than that of the [P-1] foam shown in Figure 3b. Figure S11a represents the cell structure for [SP-1] foamed at 95 °C and a pressure of 23.8 MPa. It is observed that the two foams shown in Figures 4 and S11 have a similar structure, but the foam obtained at a high foaming pressure exhibits smaller cells than the foam obtained at a low foaming pressure. Therefore, SA, ass a compatibilizer, can enhance the miscibility between (TPS/SA + PEG-10) particles and PBAT and further contribute to the good dispersion of (TPS/SA + PEG-10) particles in PBAT.    -3] foams were achieved at the foaming pressure of 17 MPa(A), and the [SP-1] foam was also obtained at the foaming pressure of 23.8 MPa(B) under the foaming temperature of 95 • C. The tensile strength and elongation at break of the three 50% (TPS with 5PHR SA and 5PHR PEG)/50% PBAT foams are much better than those of the 50% TPS/50% PBAT foam. In addition, the tensile strength is also better than that of the 50% (TPS with 10PHR PEG)/50% PBAT foams. Distinctly, the [SP-1] foam has the best tensile strength among the three 50% (TPS with 5PHR SA and 5PHR PEG)/50% PBAT foams. However, it shows a similar elongation at break to the [SP-2] and [SP-3] foams obtained under the foaming pressure of 17 MPa. It is also observed that the [SP-1] foam produced at a foaming pressure of 23.8 MPa is slightly better in terms of tensile strength and elongation at break the [SP-1] foam produced at a foaming pressure of 17 MPa. These results for the tensile properties reflect the cell structure of the [SP-1] foam, which has small cells and also good dispersion and miscibility for (TPS/SA + PEG-10) particles with PBAT in blends, as indicated in Figures 4a and S11a.

Conclusions
Three different molecular weights of PEG, namely PEG-10, PEG-20, and PEG-30, as a plasticizer, and SA as a compatibilizer, are used to modify TPS. PEG-10 and PEG-30 have the lowest and the highest molecular weight, respectively, and PEG-20 has an intermediate molecular weight. Moreover, 50% TPS, 50% (TPS with 10 PHR different PEGs), or 50% (TPS with 5PHR SA and 5PHR different PEGs) are mixed with 50% PBAT by weight to form blends. These blends are foamed by supercritical CO 2 at foaming temperatures ranging from 80 • C to 105 • C and foaming pressures of 17 MPa and 23.8 MPa. The 50% (TPS with PEG-10)/50% PBAT foam has the lowest foam density among the three 50% (TPS with PEG)/50% PBAT foams under the foaming conditions, and the 50% (TPS with SA and PEG-10)/50% PBAT foam also shows the lowest foam density among the three 50% (TPS with SA and PEG)/50% PBAT foams. The foam density of the 50% (TPS with SA and PEG)/50% PBAT foam is generally lower than that of the 50% (TPS with PEG)/50% PBAT foam. The lowest foam density for those blends obtained at a foaming pressure of 17MPa is obtained at around the foaming temperature of 100 • C. However, at a high foaming pressure of 23.8 MPa, the lowest foam density for the blends occurs at a foaming temperature of 95 • C.
The tensile property and elongation at break for the three 50% (TPS with PEG)/50% PBAT foams are ultimately better than those of the 50% TPS/50% PBAT foam. The tensile strength of the three 50% (TPS with SA + PEG)/50% PBAT foams is much better than that of the three 50% (TPS with PEG)/50% PBAT foams; however, both foams have a similar elongation at break. The cell structure and tensile property of the 50% (TPS with SA + PEG-10) and 50% PBAT foam are better in comparison with those of other foams.

Supplementary Materials:
The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/polym15010129/s1, Figure S1: Schematic representation of foaming system, the processing system is used for CO 2 batch foaming; Figure S2: Chemical structure of foam forming process; Figure S3: Dependence of TPS/PBAT and (PEG-TPS)/PBAT foam appearance on six foaming temperatures and the foaming pressure of 17 MPa; Figure S4: Dependence of TPS/PBAT and (PEG-TPS)/PBAT foam appearance on six foaming temperatures and the foaming pressure of 23.8 MPa; Figure S5: Dependence of (PEG-TPS with SA)/ PBAT foam appearance on six foaming temperatures and the foaming pressure of 17 MPa; Figure S6: Dependence of (PEG-TPS with SA)/ PBAT foam appearance on six foaming temperatures and the foaming pressure of 23.8 MPa; Figure