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Selected Morphological and Functional Properties of Extruded Acetylated Starch-Polylactic Acid Foams Junjie Guan and Milford A. Hanna* University of NebraskasLincoln, Industrial Agricultural Products Center, 208 L. W. Chase Hall, Lincoln, Nebraska 68583-0730
Native starch and polylactic acid (PLA) are biodegradable polymers that can be used in packaging materials. Their hydrophilic properties hinder their widespread use. Hydrophobicity can be improved significantly by acetylating native starch. This study was conducted to determine the morphological and functional properties of acetylated starch-PLA foams. Acetylated corn starch, with a degree of substitution (DS) of 2.3, and acetylated potato starch, with a DS of 1.07, were extruded with 5, 10, or 15% PLA in a twin screw co-rotating extruder at 150, 160, or 170 °C barrel temperatures and 130, 150, or 170 rpm screw speeds. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) were used to analyze the morphological properties of the extruded foams. A central composite response surface design was used to analyze the effects of acetylated starch type, PLA content, barrel temperature, and screw speed on the specific mechanical energy requirements of preparing extruded foams and the radial expansion ratios and compressibilities of the extruded foams. DSC showed that glass transition temperatures (Tg’s) and melting temperatures (Tm’s) of DS 2.3 corn starch-PLA foams were between the Tg’s and Tm’s of DS 2.3 corn starch and PLA. The Tg’s of DS 1.07 potato starch-PLA foams were higher than those of acetylated potato starch and PLA, while the Tm’s were closer to that of the PLA, when acetylated potato starch was the predominant phase in the blends. XRD showed that both acetylated starch and PLA lost crystallinity during extrusion. The X-ray pattern of the DS 1.07 potato starch-PLA foam was similar to those of DS 1.07 potato starch and PLA. FTIR spectroscopy confirmed no new bonds were formed in either DS 2.3 corn starch-PLA or DS 1.07 potato starch acetate-PLA foams. The type of acetylated starch, PLA content, barrel temperature, and screw speed had significant effects on the specific mechanical energy requirements, radial expansion ratios, and compressibilities of the acetylated starch foams. 1. Introduction Biodegradable expanded foams, predominantly prepared from biodegradable polymers, have been studied widely.1-17 Most commercially available biodegradable plastics are starch-based polymers.10 Different native starches (corn, wheat, and potato) have been extruded to prepare expanded foams.4-5,15-20 However, the major drawback of biodegradable polymers is their hydrophilicity. Biodegradable plastics need to be hydrophilic so as to allow microorganisms to attack their backbones to degrade them. Native starches are inherently hydrophilic, and their hydrophilicity increases with thermal processes such as extrusion.21-22 However, for packaging materials, high hydrophobicity is desirable. One possible way to accomplish this is to modify the starches. The hydrophilicity of extruded starch is due to the hydrogen bonds among the hydroxyl groups of the glucopyranosyl units in starch molecules. By substituting the hydroxyl groups with larger, less polar groups, such as acetyl groups, covalent bonds are formed instead of hydrogen bonds. These substitutions significantly increase the hydrophobicity of modified starches. A starch acetylation method was developed by Whistler and Hilbert.23 Later, Caldwell and Hills24 and Smith and Tuschhoff25 esterified starches and hydroxyl compounds with organic acid anhydride and vinyl esters, respectively. The * Corresponding author. Tel.: (402) 472-1634. Fax: (402) 472-6338. E-mail:
[email protected].
average number of hydroxyl groups substituted by acetyl groups is represented as the degree of substitution (DS). The DS varies with the source of starch, amylose and amylopectin fraction, chemical amounts, and reaction time. High DS (DS > 1.0) starches are hydrophobic and are the favored replacement for native starches in preparing water-resistant loose-fill packaging materials. Recent research projects have shown that the hydrophobicities of extruded acetylated starch-based foams were significantly higher than those of nativestarch-based extruded foams, while other functional properties were the same or better.8-17,26-28 Recently developed and commercially available polylactic acid (PLA) has created opportunities for developing 100% biodegradable/renewable loose-fill packaging materials. Produced by fermentation from corn starch, lactic acid is polymerized to form polymers with different molecular weights and degrees of crystallinity. This biodegradable thermoplastic polyester has functional properties comparable to those of many petroleum-based plastics.29-32 PLA has high mechanical strength, thermal plasticity, fabricability, biodegradability, and biocompatibility. It has been proposed as a renewable degradable plastic for uses in service ware, grocery waste and composting bags, mulch films, and controlled release matrixes for fertilizers, pesticides, and herbicides.33 Starch-PLA blends have good functional properties.3-6,19-20,34 However, hydrophilicity is a major
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drawback. Extruded native starch is highly hydrophilic, and PLA is more hydrophilic than many polyesters such as poly(glycolic acids) (PGA), polycarprolactone (PCL), polyhydroxybutyrate (PHB), and poly(hydroxybutyrateco-hydroxyvalerate) (PHBV). Functional properties significantly decrease when they absorb moisture. Exhaustive research has been done on compactibility of starch acetates and biodegradable polymers, in terms of thermal, physical, and mechanical properties.7-13 However, unlike these macroproperties, the morphological (micro) properties have not been studied. The objectives of this study were to analyze the morphological properties of two extruded acetylated starch-PLA foams and to evaluate the effects of PLA content, barrel temperature, and screw speed on the functional properties and specific mechanical energy requirements of the foams during extrusion. 2. Experimentation 2.1. Materials. High (70%) amylose corn and potato (27% amylose) starches were purchased from National Starch Co. (Hammond, IN). PLA resin with a numberaverage molecular weight of 85 000 was purchased from Cargill, Inc. (Minneapolis, MN). It contained about 93% L-lactide, 2% D-lactide, and 5% mesolactide. Talc (magnesium silicate) was purchased from Barret Minerals, Inc. (Dillon, MT). Denatured ethanol was purchased from Fisher Scientific, Inc. (Fair Lawn, NJ). Acetic anhydride was purchased from Vopak, Inc. (Dallas, TX). Sodium hydroxide (50% solution) was purchased from Harcros Chemicals, Inc. (Kansas City, KS). 2.2. Starch Acetylation. The corn and potato starches were dried at 50 °C for 48 h. To begin the acetylation process, 110 kg of acetic anhydride was placed in a steam-jacketed reactor with a rotating, self-wiping paddle. Then, 45.5 kg of starch was added into the reactor with 5 min of continuous mixing. Finally, 5 kg of 50% NaOH solution was added while mixing. The temperature of the reactor jacket was maintained at 123 °C. After 3 h, the reaction was stopped by quickly adding 200 L of cold water to the reactor. The pH value was adjusted to 5 by washing with tap water before drying at 50 °C to a moisture content of 4% (w.b.). The starches were ground in a standard model no. 3 Wiley mill (Arthur H. Thomas Co., Philadelphia, PA) to pass through a 5 mm opening sieve. 2.3. Blend Preparation. The acetylated starches and PLA were dried at 105 °C for 1 h and then cooled in a desiccator for 1 h to ensure they were moisturefree before being used in sample preparation.5 Talc was added to all samples at a 5% level (w/w). Talc functioned as a nucleating agent to enhance the uniformity of the cells.2 Different amounts of the prescribed PLA (5, 10, and 15%) and 13% (w.b.) ethanol were added to the starches and mixed in a Hobart mixer (model C-100, Hobart Corp., Troy, OH) for 5 min. Samples were then stored in plastic containers for 24 h at room temperature (25 °C) to allow the ethanol to be absorbed by the blends. 2.4. Extrusion. A twin-screw extruder (DR-2027K13, C. W. Brabender, Inc., S. Hackensack, NJ) with manufacturer predesigned co-rotating mixing screws (model CTSE-V, C. W. Brabender, Inc., S. Hackensack, NJ) was used to conduct extrusions. The conical screws had diameters decreasing from 43 to 28 mm along their length of 365 mm from the feed end to the exit end. On each screw, there was a mixing section, in which small portions of the screw flight were cut away. The mixing
section enhanced the mixing action and also increased the residence time of the sample in the barrel. The temperature at the feeding section of the barrel was maintained at room temperature (∼25 °C) while the other two barrel sections and the die were set at 150, 160, or 170 °C. Screw speeds were 130, 150, or 170 rpm. A 3 mm diameter die nozzle was used to produce cylindrical extrudates. The extruder was controlled by a Plasti-Corder (type FE 2000, C. W. Brabender, Inc., S. Hackensack, NJ). An adjustable-speed rotating knife, located right next to the nozzle, was used to cut the extrudates into 20 mm lengths. Extrusion data were recorded for subsequent analyses. 2.5. Experimental Design. The experimental design was a split-plot, with DS starch type as the whole-plot factor. PLA content, barrel temperature, and screw speed were the split-plot factors. The whole-plot used a completely randomized design with two blocks (blocked by starch type). Response surfaces were applied to splitplots. Response surface methodology (RSM) was used to determine the effects of PLA content, barrel temperature, and screw speed on the specific mechanical energy requirements, radial expansion ratios, and compressibilities of the acetylated starch foams. A central composite experimental design, described by Lee and Han35 for three variables and three levels of each variable, was used. The independent variable levels were selected based on preliminary experiments. All treatments were performed in random order, and data were analyzed using a response surface regression procedure. The generalized regression model was
Y ) b0 + b1X1 + b2X2 + b3X3 + b12X1X2 + b13X1X3 + b23X2X3 + b11X12 + b22X22 + b33X32 where Y ) response, X1 ) PLA content, X2 ) barrel temperature, X3 ) screw speed, b0 ) intercept, and bn ) regression coefficient. For each response, three-dimensional plots were produced from regression equations by holding two variables fixed. Design-Expert version 6 (Stat-Ease Co., Minneapolis, MN) was used to conduct the statistical analyses and surface plotting. 2.6. Morphological Properties of Starch-PLA Foams. 2.6.1. Differential Scanning Calorimetry (DSC). DSC analyses were conducted on all raw materials and selected extrudates to study the thermal properties of foams, including Tg and Tm. A PerkinElmer DSC 7 differential calorimeter (Perkin-Elmer, Wilton, CT) was used to analyze the thermal properties of DS starches, PLA, and extruded blends. The instrument was first calibrated with indium and purged with nitrogen gas at 80 mL/min. About 10 mg of sample was sealed in an aluminum pan, allowed to equilibrate to 25 °C, scanned from 25 to 220 °C at a constant heating rate of 10 °C/min, and then slowly cooled and rescanned up to 220 °C at 10 °C/min. Three separate scans were run on each sample. 2.6.2. X-ray Diffraction (XRD). Crystallinity determines the flexibility of the molecular chains of polymers. Polymers with higher crystallinity have better mechanical strengths. XRD was used to observe the crystallinity of both the raw materials and the extruded blends. DS starches, PLA, and extruded blends were dried at 40 °C to a constant moisture content in a vacuum oven prior to X-ray scanning with a Rigaku model D/Max-B X-ray diffractometer (Brandt Instruments, Inc., Slidell,
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LA) with a Bragg-Brentano parafocusing geometry, a diffracted beam monochromator, and a conventional copper target X-ray tube set to 40 kV and 30 mA. The X-ray source was Cu KR radiation composed of Cu KR1 ) 1.5405 Å and Cu KR2 ) 1.5443 Å. The weighted average of the two was Cu KRavg ) 1.54184 Å. Data were collected from 2θ ) 4° to 2θ ) 35° (θ being the angle of diffraction) with a step width of 0.02° and a step time of 0.4 s. The value of 2θ for each identifiable peak on the diffractograms was estimated, and crystal d-spacings were calculated using Bragg’s law. 2.6.3. Fourier Transform Infrared (FTIR). A Nicolet Avatar 360 (Thermo Electron Co., Woburn, MA) with an Analect diffuse/specula reflectance apparatus was used. The acquisition parameters were 128 scans at 4 cm-1 resolution. To prepare the samples, 1 mg of each of the samples (extruded DS starch-cellulose foams, DS starches, and cellulose) combined with 20 mg of KBr was ground to a fine powder. The powders were placed in the oven at 130 °C for 1 h to remove any moisture and stored overnight in desiccators at room temperature (25 °C). Before running the samples, a background spectrum for ground, pure KBr was collected. Then, small amounts of each dried powder were placed in the diffuse reflectance sample holder, and data were collected. 2.6.4. Specific Mechanical Energy Requirement (SME). SME is defined as the total input of mechanical energy per unit dry weight of extrudate. SME was determined as described by Bhatnagar and Hanna.36 Extruded materials were collected for 30 s and dried. SME (W h/kg) was calculated as
SME ) [2π(n/60)τ]/MFR where n ) screw speed (rpm), τ ) torque (N m), and MFR ) mass flow rate (kg/h). 2.7. Functional Properties of Starch-PLA Foams. Radial expansion ratio (RER) was calculated by dividing the mean cross-sectional area of an extrudate by the cross-sectional area of the die nozzle. Each mean value was the average of 10 measurements. An Instron universal testing machine (model 5566, Instron Engineering Corp., Canton, MA) was used to measure the compressibilities of foamed extrudates. The 20 mm long extrudates were placed on a flat plate with careful alignment of cut surfaces so that the edges were perpendicular to the axis of the extrudate sample (direction of extrusion). Then the extrudates were compressed once to 80% of their original diameter at a loading rate of 1 cm/min using another flat plate. The force (kN) divided by the sample density (kg/m3) was reported as compressibility (kN kg-1 m3). Compressibility for each sample was measured five times and reported as an average. 3. Results and Discussion 3.1. Morphological Properties of Starch-PLA Foams. 3.1.1. Differential Scanning Calorimetry (DSC). Thermal properties are the most important macroproperties of thermal plastics. They are direct responses of the results of thermoprocesses. Tg and Tm are indices that correspond to the amorphous and crystalline regions of the materials, respectively. The molecules of solids are distributed as lattices. In singlephase solids, molecules have relatively stable oscillations among the molecules. However, when solids with
Figure 1. DSC thermal graphs of raw materials: (A) DS 2.3 cornstarch acetate and DS 1.07 potato starch acetate; (B) PLA.
two or more phases are heated, the molecular vibrations are significantly different because of intermolecular forces among the different phases. Therefore, Tg and Tm may function as indices of the degree of mixing (homogeneous or heterogeneous) of multiphase polymeric systems. Well-mixed or homogeneous polymeric systems tend to have higher Tg’s and Tm’s than heterogeneous systems. The thermal and mechanical energies associated with the extrusion process have significant effects on longchain polymers. Thermal energy melts the crystalline regions and mechanical energy depolymerizes the longchain molecules further, resulting in an amorphous dough of short-chained molecules in the barrel. Upon exiting the die nozzle and experiencing sudden pressure and temperature drops, these short-chain molecules reassociate. The corresponding Tg and Tm will be significantly lower than those for the unextruded materials. For multiphase mixed polymer doughs, depolymerization favors a more homogeneous dough. Intermolecular forces can form organized crystalline regions in the multiphase homogeneous dough so as to maintain or even increase Tg and Tm. Thermograms of raw materials and selected extruded foams are presented in Figures 1-3, and Tg and Tm are summarized in Table 1. The Tg and Tm of the DS 2.3 corn starch were 107.86 and 184.63 °C, respectively. They were significantly higher than the Tg (77.82 °C) and Tm (127.46 °C) of the DS 1.07 potato starch. This was due to the higher intermolecular restrictions in the higher DS acetylated starch. When heated, these re-
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Figure 2. DSC thermal graphs of selected extruded starch acetate-PLA foams.
Figure 3. X-ray diffractograms of raw materials: (A) PLA; (B) DS 2.3 cornstarch acetate; (C) DS 1.07 potato starch acetate.
stricted chains needed to absorb more thermal energy to overcome the intermolecular forces. PLA had a Tg of 65.04 °C and a Tm of 154.86 °C. For the DS 2.3 corn starch-PLA foams, as more PLA was blended in, both Tg and Tm decreased. With the same PLA content, Tg and Tm increased as barrel temperature and screw speed increased. At higher extruder screw speeds and barrel temperatures, the differences in the molecular weights of the depolymerized PLA and acetylated starches were less than those of the unprocessed polymers. This provided a better chance of forming a homogeneous mixture with more organized crystalline structure, resulting in a higher Tg and Tm. But after extrusion, the Tg’s and Tm’s of the DS 2.3 corn starch foams were between the Tg’s and Tm’s of PLA and DS 2.3 corn starch. Similar trends were recorded for DS 1.07 potato starch-PLA foams when PLA content, barrel temperature, and screw speed increased. How-
ever, the Tg’s of the DS 1.07 potato starch-PLA foams were higher than those of PLA and DS 1.07 potato starch, and the Tm’s were significantly higher than that of DS 1.07 potato starch, even when acetylated potato starch was the predominant component (>85%, w/w) in the blends; this suggests more orderly amorphous and crystalline regions were formed. This probably was due to the hydrophilic properties of PLA and acetylated potato starch. DS 1.07 potato starch more readily absorbed moisture, similar to PLA, as compared to DS 2.3 corn starch. Therefore, the phase separation tendency was smaller, resulting in better heat resistance (higher Tg and Tm) of the blends. Another possibility would be that there was some kind of chemical reaction between the PLA and the acetylated starch backbones. When starch was acetylated, it became more polarized, which favored a reaction with polarized PLA molecules. However, when the DS increased, intermolecular forces became predominant, limiting the chemical reaction between acetylated starch and PLA. 3.1.2. X-ray Diffraction. The X-ray diffractograms of PLA, corn starch, and potato starch are presented in Figure 3. PLA had narrow and sharp peaks at 16° and 19°. Acetylated corn and potato starches had similar peaks at 9°. Potato acetate had another higher intensity broad peak at 22°. Since potato starch had a lower DS value, it maintained an X-ray pattern closer to that of native potato starch. Because of more substituted glucopyranosyls, intermolecular forces were the predominant force in the acetylated starch system. With the additional thermal degradation during the starch acetylation reaction, starch granular structures were destroyed and a more amorphous and homogeneous system was formed. Therefore, it was expected that the higher the DS of the acetylated starch, the less chance of having sharp peaks in the X-ray diffractograms. Figure 4 shows the X-ray diffractograms of selected DS 2.3 corn starch-PLA foams. All the foams had similar patterns with narrow sharp peaks at 10°, 29°, and 33° and a broad peak at 22°. These peaks were significantly different than those for PLA and DS 2.3 corn starch. As mentioned previously, the thermal and mechanical energies associated with the extrusion process changed the crystallinity of the thermal plastics. PLA content, barrel temperature, and screw speed had significant effects on foam crystallinity. In general, as PLA content increased, the intensity of the sharp peaks decreased, indicating thermal and mechanical energies had greater effects on the PLA than on the acetylated starch in the barrel. This was due to the lower Tg and Tm of PLA (Table 1). As barrel temperature and screw speed increased, the crystalline intensities increased, suggesting that higher thermal and mechanical energies favor the formation of new crystalline structures. Figure 5 shows the X-ray diffractograms of DS 1.07 potato starch-PLA foams. Similar to the case of DS 2.3 corn starch-PLA foams, sharp peaks were observed at 10°, 28°, and 33° and a broad peak was observed at 22°. However, one peak also was noticed at about 16°, which was similar to the location of PLA’s sharp peak. Also, the overall X-ray pattern was close to the X-ray pattern of DS 1.07 potato starch. As mentioned previously, compared to DS 2.3 corn starch, DS 1.07 potato starch was closer in hydrophilicity to PLA. This was substantiated by the lower DS value. Due to this phase similarity, it was highly possible for the blends to maintain similar
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Table 1. Thermal Properties of Selected DS 2.3 Corn Starch-PLA and DS 1.07 Potato Starch-PLA Foams DS 2.3 corn starch-PLA foamsa
b,d
Tg Tmc,d
DS 1.07 potato starch-PLA foamsa
PLA
DS 2.3 corn starch
DS 1.07 potato starch
15%, 170 °C, 130 rpm
5%, 150 °C, 130 rpm
10%, 160 °C, 150 rpm
5%, 170 °C, 170 rpm
15%, 170 °C, 130 rpm
5%, 150 °C, 130 rpm
10%, 160 °C, 150 rpm
5%, 170 °C, 170 rpm
65.04 154.86
107.86 184.63
77.82 127.46
92.60 147.21
99.52 162.35
100.40 163.76
102.35 167.78
96.55 137.49
103.85 148.23
98.42 139.16
102.42 140.99
a Selected acetylated starch foams with formulation of PLA content, barrel temperature, and screw speed, respectively. b Glass transition temperature, °C. c Melting temperature,°C. d All the Tg’s and Tm’s were significantly different (P < 0.05) in a standard t-test.
Figure 4. X-ray diffractograms of DS 2.3 corn starch-PLA foams: (A) 15% PLA, 170 °C barrel temperature, 130 rpm screw speed; (B) 5% PLA, 150 °C barrel temperature, 130 rpm screw speed; (C) 10% PLA, 160 °C barrel temperature, 150 rpm screw speed; (D) 5% PLA, 170 °C barrel temperature, 170 rpm screw speed.
crystallinities. As PLA content increased, the intensities of both sharp and broad peaks tended to decrease due to a more homogeneous crystallinity. As barrel temperature and screw speed increased, peaks became more apparent, again suggesting that higher thermal and mechanical energies favored the formation of new crystalline structures. Similar X-ray diffractogram patterns were observed (Figures 4 and 5) when different acetylated starches were extruded. As mentioned previously, the crystalline characteristics of the final extrudates depended upon the alignment and formation of blended molecules when exiting the die nozzle. These were controlled by extrusion conditions, which were the predominant factors in molecular degradation. There were no differences in the extruder setups, including screw configuration, feed rate, or die nozzle opening. Therefore, the similar patterns corresponded to the specific extrusion setup. The new peaks, shown after extrusion, may have been due to limited dispersibility of the PLA chains in the polymers’ melt after exiting the die nozzle. Because of
Figure 5. X-ray diffractograms of DS 1.07 potato starch-PLA foams: (A) 15% PLA, 170 °C barrel temperature, 130 rpm screw speed; (B) 5% PLA, 150 °C barrel temperature, 130 rpm screw speed; (C) 10% PLA, 160 °C barrel temperature, 150 rpm screw speed; (D) 5% PLA, 170 °C barrel temperature, 170 rpm screw speed.
its limited mobility, the PLA formed specific crystalline regions with acetylated starch after solidification. There is not enough information to fully address this phenomenon. 3.1.3. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectrograms for DS starches, PLA, DS 2.3 corn starch-PLA foams, and DS 1.07 potato starch-PLA foams are shown in Figures 6-8. The peaks of corresponding characteristic groups are noted in Tables 2 and 3. PLA, DS 2.3 corn starch, and DS 1.07 potato starch had similar characteristic group stretchings. The most important group stretching was the Cd O carbonyl stretch. The carbonyl stretching showed up at 1761, 1739, and 1734 cm-1 for PLA, DS corn starch, and DS potato starch, respectively. After extrusion, the CdO carbonyl stretchings of the DS corn starch-PLA foams shifted to the higher wavenumbers which were closer to the PLA stretching wavenumber. These shifts seemed to indicate that a depolymerization of DS corn starch and PLA led to a diminution of hydrogen bondings in unextruded DS corn starch and PLA. When PLA content increased, the carbonyl stretchings shifted to
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Figure 6. FTIR spectroscopy of PLA, DS 2.3 corn starch, and DS 1.07 potato starch.
Figure 7. FTIR spectroscopy of DS 2.3 corn starch-PLA foams: (A) 15% PLA, 170 °C barrel temperature, 130 rpm screw speed; (B) 5% PLA, 150 °C barrel temperature, 130 rpm screw speed; (C) 10% PLA, 160 °C barrel temperature, 150 rpm screw speed; (D) 5% PLA, 170 °C barrel temperature, 170 rpm screw speed.
lower wavenumbers. When barrel temperature and screw speed increased, CdO stretching shifted to a higher wavenumber. These suggested that a higher degree of depolymerization increased the bonding tendency, which favored the formation of a homogeneous two-phase polymer system. However, for the DS potato
Figure 8. FTIR spectroscopy of DS 1.07 potato starch-PLA foams: (A) 15% PLA, 170 °C barrel temperature, 130 rpm screw speed; (B) 5% PLA, 150 °C barrel temperature, 130 rpm screw speed; (C) 10% PLA, 160 °C barrel temperature, 150 rpm screw speed; (D) 5% PLA, 170 °C barrel temperature, 170 rpm screw speed.
starch-PLA foams, CdO stretching shifted to the higher wavenumber but not closer to the CdO stretching wavenumber of either PLA or DS potato starch. The carbonyl stretching shifted to a higher wavenumber when PLA content increased. Barrel temperature and screw speed did not have significant effects on shifting the CdO stretching. Overall, most of the functional groups had wavenumbers closer to those of the DS starches rather than to those of the PLA, indicating the DS starch-PLA two-phase polymer system was dominated by the DS starch. 3.1.4. Specific Mechanical Energy Requirement (SME). The SME response surface graphs of DS 2.3 corn starch-PLA foams are shown in Figure 9. PLA content, barrel temperature, and screw speed had significant effects (P < 0.01, Table 4) on the SME of DS 2.3 corn starch-PLA and DS 1.07 potato starch-PLA foams. As PLA content and screw speed increased, SME increased. Conversely, SME decreased as barrel temperature increased. When more PLA was blended in, the previously single-phase-dominated (DS starch) system became less homogeneous. Therefore, more mechanical energy was required. Thermal energy (barrel temperature) melted both phases of polymers, which reduced the viscosity of the mixed dough. This reduced the torque requirement of the extrusion. As screw speed increased, it was apparent that SME increased. Screw speed had a more significant effect on the SME at low PLA content, because mechanical energy (screw speed) was applied primarily to polymer depolymerization while, at high PLA content, mechanical energy was consumed in mixing and melting the polymers. This suggested that mechanical energy had a more direct effect on the single-phase system than on the two-phase system. Overall, the SME of the DS 1.07 potato starch-
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Table 2. Data Obtained from FTIR Spectrogram for PLA, DS 2.3 Corn Starch Acetate, and DS 2.3 Corn Starch-PLA Foams peak position, cm-1 DS corn starch-PLA foamsa assignment
PLA
corn DS
A
B
C
D
-OH stretch (free) -CH- stretch -CdO carbonyl stretch -CH3 bend -CH- deformation bend -C-O- stretch -OH bend -C-C- stretch
3646, 3499 2997 1761 1457 1359 1179, 1092 1047 874
3553, 3303 2921 1739 1429 1380 1239 1043 890
3510, 3319 2943 1750 1440 1364 1228 1032 890
3542, 3276 2943 1751 1429 1369 1228 1032 901
3493 2949 1751 1429 1369 1228 1037 901
3564, 3325 2949 1756 1435 1369 1228 1015 896
a A ) 15% PLA, 170 °C barrel temperature, 130 rpm screw speed. B ) 5% PLA, 150 °C barrel temperature, 130 rpm screw speed. C ) 10% PLA, 160 °C barrel temperature, 150 rpm screw speed. D ) 5% PLA, 170 °C barrel temperature, 170 rpm screw speed.
Table 3. Data Obtained from FTIR Spectrograms for PLA, DS 1.07 Potato Starch Acetate, and DS 1.07 Potato Starch-PLA Foams peak position, cm-1 DS potato starch-PLA foamsa assignment
PLA
potato DS
A
B
C
D
-OH stretch (free) -CH- stretch -CdO carbonyl stretch -CH3 bend -CH- deformation bend -C-O- stretch -OH bend -C-C- stretch
3646, 3499 2997 1761 1457 1359 1179, 1092 1047 874
3526, 3292 2954 1734 1429 1364 1217 1032 895
3559, 3248 2938 1756 1462 1359 1217 1075
3521, 3314 2943 1751 1424 1375 1233 1032 896
3510, 3303 2943 1745 1440 1369 1228 1031 866
3510, 3319 2943 1745 1435 1369 1228 1032 896
a A ) 15% PLA, 170 °C barrel temperature, 130 rpm screw speed. B ) 5% PLA, 150 °C barrel temperature, 130 rpm screw speed. C )10% PLA, 160 °C barrel temperature, 150 rpm screw speed. D ) 5% PLA, 170 °C barrel temperature, 170 rpm screw speed.
Table 4. Regression Equation Coefficientsa of Second Order Polynomialsb for Two Response Variables SMEc (W‚h/kg) 2.3f
COMPe (kN‚kg-1‚m3)
RERd 1.07f
corn 2.3f
potato 1.07f
-527.30
-1.54 × 106
-1.55 × 106
1.07*** 6.78*** -0.28**
1.24*** 6.80*** -0.30***
3153.80*** 14336.57*** 5766.39***
2871.37*** 14321.75*** 5698.60***
0.30*** 0.049* 5.90 × 10-3
-0.18*** -0.026*** -5.60 × 10-3***
-0.18*** -0.026*** -5.60 × 10-3***
24.23 -33.64*** -9.46***
24.23 -33.64*** -9.46***
0.086 -0.12** -0.014 0.9451
