Ind. Eng. Chem. Res. 2006, 45, 3991-4000
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Physical, Mechanical, and Macromolecular Properties of Starch Acetate during Extrusion Foaming Transformation† Junjie Guan and Milford A. Hanna* Industrial Agricultural Products Center, Department of Biological Systems Engineering, UniVersity of NebraskasLincoln, 208 L. W. Chase Hall, Lincoln, Nebraska 68583-0730
Due to the transformation of starch acetate during extrusion, the functional properties of extruded starch acetate-based foams were significantly affected by the extrusion process parameters. To obtain the optimum functional properties as packaging materials, it was necessary to analyze the functional proprieties of extruded starch acetate foams based on the morphological properties after extrusion transformation. Corn starch acetate with degree of substitution (DS) of 2.30 and potato starch acetate with DS of 1.09 were extruded with 10, 15, and 20% ethanol in a twin screw extruder using screw speeds of 110, 130, and 150 rpm and barrel temperatures of 130, 150, and 170 °C. A response surface design was applied to analyze the effects of ethanol content, screw speed, and barrel temperature on the physical properties (radial expansion ratio, unit and bulk densities, and water absorption index), mechanical properties (unit and bulk spring indices and compressibility), and macromolecular properties (scanning electron micrographs) of the extruded starch acetate foams. Ethanol content, barrel temperature, and screw speed had significant effects on the functional properties of extruded starch acetate foams. Because of the differences in molecular structure degradation in the corn and potato starch acetates, the functional properties of extruded corn starch acetate foams were higher than those of extruded potato starch foams. This was substantiated with significant macromolecular structural differences. 1. Introduction Starch, as a major renewable biomass, has received much attention as a raw ingredient for biodegradable packaging materials. However, due to their natural hydrophilicity, starch granules are degraded by thermomechanical processes, such as extrusion, in the presence of a plasticizer (water). Hydrophobicity is preferred in packaging materials so the packaging will maintain its functional properties when exposed to a highhumidity environment. Starch is a mixture of amylose and amylopectin. Amylose is a linear polymer of R-D-glucopyranosyl units linked by 1,4-Dglucosidic linear linkages. Amylopectin is a branched polymer of R-D-glucopyranosyl united by 1,4-D-glucosidic linear linkages and 1,6-D-glycosidic linkages at the branch points. Starch can be melt-processed with water or other hydrophilic plasticizers in extruders in much the way as conventional polymers.1 The major drawback of using thermoplastic starch-based polymers to produce biodegradable plastics is their hydrophilic characteristic.1 Starch-based thermoplastics are produced using processes involving heat and shear, e.g. extrusion. Thermomechanical treatment induces changes such as melting of crystallites, disruption of granules, and molecular breakdown of amylopectin.2 Therefore, when immersed in water, thermoplastic starch will absorb moisture rapidly and lose most of its functional properties.1 One way to minimize this problem is to chemically modify the starch backbone by converting the hydrophilic hydroxyl groups to esters via reaction with carboxylic acids.3 Fringant et al.4 reported that acetylation resulting in a degree of substitution (DS) greater than 1.7 was an efficient way to produce thermoplastic starch-based materials with enhanced hydrophobicity. By substituting hydrophobic acetyl groups for * To whom correspondence should be addressed. Tel.: 1-402-4721634. Fax: 1-402-472-6338. E-mail:
[email protected]. † A contribution of the University of Nebraska Agricultural Research Division, Lincoln, NE. Journal Series No. 14959. This study was conducted at the Industrial Agricultural Products Center.
Figure 1. Water absorption isotherms of selected extruded corn and potato starch acetate foams (five replicates were tested to obtain each point).
hydrophilic hydroxyl groups from glucopyranosyl rings, the water solubility of starch acetate decreases as DS increases. Starch acetates with DS ) 0.02-0.5 are hydrophilic and generally used in food applications. Highly substituted starch (Figure 1) (DS > 1.5) is hydrophobic and used predominately in industrial and pharmaceutical applications.5 Even though hydrophobicity is improved by acetylation, the starch acetates are still susceptible to degradation during extrusion.6 Loss of DS and molecular weight (MW) reduction were observed after extrusion, suggesting decreased hydrophobicity and poor functional properties of the extruded foams. Because of the relationship between molecular characteristics and functional properties, research has been conducted on the effects of molecular characteristics changes on the properties of starch acetate-based materials prepared from various processing. Shogren7 extruded foams from high-amylose starch acetate. Fringant et al.4 studied the physical properties of starch acetatebased materials and correlated them to their molecular characteristics. Miladinov and Hanna8,9 studied the effects of blowing
10.1021/ie050766d CCC: $33.50 © 2006 American Chemical Society Published on Web 05/03/2006
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Table 1. Experimental Design of the Split Plota on Corn Starch Acetate (SA) Used for Statistical Analysis of the Treatment Combinations coded factorsc treatmentb 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
actual factors
SA type
X1
X2
X3
X1
X2
X3
corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn
-1 -1 -1 -1 -1 -1 -1 -1 -1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 -R A 0 0 0 0 0 0 0
-1 -1 -1 0 0 0 1 1 1 -1 -1 -1 0 0 0 1 1 1 -1 -1 -1 0 0 0 1 1 1 0 0 -R A 0 0 0 0 0
-1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 0 0 0 0 -R A 0 0 0
10 10 10 10 10 10 10 10 10 15 15 15 15 15 15 15 15 15 20 20 20 20 20 20 20 20 20 7.93 22.07 15 15 15 15 15 15 15
110 110 110 130 130 130 150 150 150 110 110 110 130 130 130 150 150 150 110 110 110 130 130 130 150 150 150 130 130 121.72 178.28 130 130 130 130 130
130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 150 150 150 150 101.72 158.28 150 150 150
a The other half of the split plot (potato starch acetate) was the same. b Treatments 1-27 were the complete randomized design (CRD) data points; 28-33 and 34-36 were additional six axial points (R ) 1.414) and central points for response surface design, respectively. c X1 ) ethanol content, X2 ) extruder barrel temperature, and X3 ) extruder screw speed.
agents and extruder barrel temperature on physical and molecular properties of extruded starch acetates. They concluded that starch acetates had good thermoplastic properties and maintained good functional properties after thermomechanical processing. A series of studies1,6,10-19 conducted on extruded blends of starch acetates found starch acetates had good compatibility with other biopolymers, including biodegradable plastics, natural fibers, and native starches. However, the effects of starch acetate transformation on the functional properties after extrusion processing have not been studied in detail. It has been shown that morphological properties, including DS, MW, thermal properties, and crystallinity, were affected significantly by extrusion parameters (screw speed, barrel temperature, and ethanol content).6 These morphological properties were direct indices for the analyses of functional properties. Recent studies have indicated that the molecular weight (distribution) of starch determines product properties to a large extent.20 Thermomechanical breakdown has been known to affect the MW distribution significantly.21 Recent advances in understanding the process parameters show potential for optimization of thermomechanical processes to obtain the desired product properties.22,23 The objectives of this study were to study the effects of ethanol content, extruder screw speed, and extruder barrel temperature on physical, mechanical, and macromolecular characteristics of extruded starch acetates and to analyze the relationships between morphological properties and functional properties.
2. Experimentation 2.1. Materials. High amylose corn starch (Hylon VII), with 70% amylase, was purchased from American Maize Products Co. (Hammond, IN). Potato starch with 20% amylose was purchased from Penford Products Co. (Englewood, CO). Chemicals used for starch modification (acetic anhydride and sodium hydroxide) were purchased from Aldrich Chemical Co. (Milwaukee, WI). Talc (magnesium silicate) was purchased from Barrett Minerals, Inc. (Dillon, MT). Denatured ethanol was purchased from Fisher Scientific, Inc. (Fair Lawn, NJ). 2.2. Starch Acetates Preparation. Starch was dried at 50 °C for 48 h before acetylation. First 110 kg of acetic anhydride was placed in an agitated steam-jacketed reactor. Then 45.5 kg of starch was added to the reactor and mixed for 5 min. Finally, 5 kg of 50% aqueous solution of sodium hydroxide was added as the catalyst. The temperature of the reactor jacket was maintained at 123 °C (approximately 137.9 kPa). After 3 h, the reaction was stopped by quickly adding 200 L of cold water to the reactor. The pH value was adjusted to 5.0 by washing repeatedly with water before drying at 50 °C. 2.3. Blend Preparation. The starch acetate was dried in a precision mechanical convection oven (GCA Corp., Chicago, IL) at 105 °C for 1 h before being used in sample preparation.24 Talc was added to all samples at a 5% level (w.b.). Talc functioned as a nucleating agent to ensure the uniformity of extruded, expanded cell structure. Ethanol (10, 15, and 20%) were added to the starch and mixed in a Hobart mixer (Model
Ind. Eng. Chem. Res., Vol. 45, No. 11, 2006 3993 Table 2. Equilibrium Moisture Contents (%) of Native Starch-Starch Acetate Foams with Different Saturated Salt Solutions starch acetate
ethanol content (%)
barrel temp (°C)
screw speed (rpm)
LiCl (11.3%)
K2CO3 (43.2%)
NaCl (75.3%)
KCl (84.3%)
BaCl2 (90.0%)
K2SO4 (97.3%)
corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn corn potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato potato
10 10 10 10 10 10 10 10 10 15 15 15 15 15 15 15 15 15 20 20 20 20 20 20 20 20 20 10 10 10 10 10 10 10 10 10 15 15 15 15 15 15 15 15 15 20 20 20 20 20 20 20 20 20
110 110 110 130 130 130 150 150 150 110 110 110 130 130 130 150 150 150 110 110 110 130 130 130 150 150 150 110 110 110 130 130 130 150 150 150 110 110 110 130 130 130 150 150 150 110 110 110 130 130 130 150 150 150
130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170 130 150 170
2.11 2.62 3.26 2.52 2.97 3.55 2.93 3.28 4.12 1.77 2.12 2.36 1.96 2.32 2.68 2.12 2.53 3.15 1.42 1.97 2.32 1.98 2.52 3.25 2.43 2.96 3.52 5.36 5.94 6.32 5.96 6.32 7.21 6.21 6.84 7.21 4.96 5.21 5.91 5.12 5.62 6.12 5.95 6.23 6.89 4.11 4.95 5.36 4.56 5.63 6.22 5.32 6.11 6.98
3.53 3.99 4.16 3.81 4.29 4.97 4.22 4.53 5.23 3.16 3.51 4.00 3.59 4.12 4.39 3.96 4.33 4.76 3.00 3.48 3.86 3.45 3.63 4.25 3.86 4.13 4.54 7.95 8.26 8.94 8.26 8.96 9.65 8.99 9.56 10.12 6.23 6.94 7.11 6.59 7.12 7.99 7.11 7.85 8.36 5.46 6.23 6.84 6.32 6.89 7.26 6.98 7.56 8.12
5.64 5.93 6.35 6.19 6.42 6.95 6.39 6.83 7.15 5.00 5.42 5.87 5.32 5.87 6.40 5.83 6.15 6.50 4.64 5.13 5.52 4.81 5.20 5.82 5.25 5.69 6.12 10.12 11.23 12.11 11.56 12.65 13.11 12.11 12.95 13.62 8.94 9.11 10.02 9.15 9.56 10.00 9.88 10.11 11.26 7.56 8.55 9.32 8.15 8.96 9.11 8.95 9.36 10.23
7.62 8.13 8.67 8.19 8.52 8.95 8.39 8.70 9.11 6.43 6.81 7.14 6.58 6.93 7.34 6.81 7.27 7.69 5.83 6.20 6.53 6.45 6.90 7.12 6.96 7.23 7.80 13.23 13.99 14.26 13.95 14.56 15.11 14.16 14.95 15.65 12.54 13.15 13.82 13.22 13.85 14.56 13.89 14.56 15.11 11.86 12.56 13.21 12.23 12.98 13.66 13.26 14.22 14.98
9.66 10.33 11.24 10.18 10.60 11.43 10.66 11.10 11.91 8.17 8.64 9.22 8.57 8.99 9.33 9.12 9.52 9.97 7.28 8.14 8.67 8.32 8.90 9.12 8.98 9.50 10.32 16.32 17.23 17.99 17.11 17.82 18.26 17.83 18.23 18.94 15.45 16.10 16.84 16.11 16.59 17.15 16.72 17.12 18.01 14.26 15.11 15.83 14.89 15.26 16.36 15.32 16.12 17.23
12.30 13.52 14.64 13.22 13.97 14.90 13.62 14.35 15.60 10.23 11.56 12.10 11.14 12.21 12.96 12.35 13.52 13.94 9.94 10.23 10.98 10.53 11.23 12.10 11.33 11.98 12.56 18.23 18.99 19.65 19.03 19.91 20.36 19.99 20.56 21.23 17.65 18.23 18.99 18.23 18.69 19.21 19.09 19.65 20.23 16.22 17.06 17.94 17.23 18.02 19.11 17.91 18.56 19.65
C-100, Hobart Corp., Troy, OH) for 5 min. Then, samples were stored in plastic containers at room temperature for 24 h before they were extruded. 2.4. Extrusion. A twin-screw extruder with co-rotating mixing screws (Model CTSE-V, C. W. Brabender, Inc., S. Hackensack, NJ) was used. 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 flights were cut away. The mixing section enhanced the mixing action and also increased the residence time of the sample in the barrel. Screw speeds of 110, 130, and 150 rpm and barrel temperatures of 130, 150, and 170 °C were used. The feeding section of the barrel was unheated. A 3 mm diameter die nozzle was used to produce cylindrical, ropelike, extrudates. The samples were large enough to guarantee equilibrium conditions within the extruder before the extrudates to be used
for analyses were collected. A marker was again added at the end of each sample, followed by polyethylene. The extrudates were placed in a convection dryer at 45 °C for 24 h to evaporate the residual ethanol. The products were sealed in plastic bags and stored at room conditions until analyzed further. 2.5. Water Absorption Isotherms. Six saturated aqueous salt solutions were prepared (LiCl, 11.3%; K2CO3, 43.2%; NaCl, 75.3%; KCl, 84.3%; BaCl2, 90.0%; K2SO4, 97.3%) and placed in desiccators to obtain the required relative humidity levels. Extruded samples were cut into 20 mm lengths, and their weights were recorded. Samples were then placed in the desiccators at 25 °C. After 24 h of equilibration, the samples were weighed, dried at 40 °C for 24 h in a vacuum oven, and then reweighed. The equilibrium moisture content (d.b.) was determined for each sample group on the basis of the weight changes. 2.6. Physical Properties. The radial expansion ratio (RER) was calculated by dividing the mean cross-sectional area of an
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Figure 2. Effects of ethanol content, extruder barrel temperature, and extruder screw speed on radial expansion ratio of extruded potato starch acetate foams.
extrudate by the cross-sectional area of the die nozzle. Each mean value was the average of 10 measurements. Unit density (Fu) was determined using a glass bead displacement method.25
Samples were cut 20 mm long and weighed (PR 5002 Mettler Toledo, Swizerland). Glass beads (0.1 mm diameter), as the displacement media, were used to replace the void space of the 500 mL measuring cylinder after putting the extrudate pieces in it to determine the volume of the foam. Unit density was obtained by dividing the mass by the volume of the extrudate being measured. Five replications were measured for each sample. Extrudate bulk density (Fb) was measured using a cylindrical Plexiglas container.24 The container had a diameter of 160 mm and a height of 160 mm. A funnel, having an opening of 160 mm at the top and an opening of 64 mm at the bottom, was mounted at a height of 160 mm above the container. Bulk densities (kg/m3) of the extrudates were calculated from the mass of the as-compacted sample divided by the volume of the container. Five replications were measured for each sample. 2.7. Mechanical Properties. Spring index refers to the ability of a material to recover its original shape after it has been deformed, reflecting its elasticity and resilience. Samples were compressed using an Instron 5566 universal testing machine (Instron Co., Canton, MA) to achieve a deformation of 80% of their original dimensions at a loading rate of 30 mm/min.26 For each unit spring index (SIu) run, five pieces approximately 20 mm long, of each sample, were used. The force required to initially compress a sample and the force required to recompress the same sample 1 min after releasing the initial load were recorded.27 Bulk spring index (SIb) was tested on bulk samples using the same Instron universal testing machine. A cylindrical aluminum container with a volume of 365 cm3 (6.93 cm in diameter and 9.68 cm in depth) was used to hold the bulk samples. The forces required compressing the samples to 80% of their original volume, and the forces required to recompress the same samples 1 min after releasing the initial load, were recorded. Unit and bulk spring indices were calculated as the ratio of the recompression force to the value of the initial compression force. Five replicate tests were performed on each sample. Compressibilities of the (foamed) extruded starch acetates were tested using the same Instron universal testing machine. The 20 mm long extrudates were placed on a flat plate, carefully aligning the cut surfaces so that edges were perpendicular to the axis of the extrudate sample (direction of extrusion). Then, the force required to compress an extrudate to 80% of its original diameter, at a loading rate of 1 cm/min, was measured. The force (kN) divided by the sample density (kg‚m-3) was reported as compressibility (kN‚kg-1‚m3). The compressibility of each sample was measured five times and reported as an average of the five readings. 2.8. Scanning Electron Microscopy. Micrographs of selected extrudates were obtained using a scanning electron microscope (SEM; Hitachi S-3000N, Tokyo, Japan). The extrudate samples were dried in a vacuum oven at 40 °C for 24 h and cooled in desiccators to minimize reabsorption of moisture. The sample cross-sections were sliced horizontally, mounted on SEM stubs using silver colloidal paste, and sputter-coated with goldpalladium to a thickness of 10 nm. 2.9. Experimental Design. The experimental design was a split plot, with the type of starch acetate as the main plot factor and the three independent variables (ethanol content, extruder barrel temperature, and extruder screw speed) as the split-plot factors. The whole plot used a completely randomized design (CRD) with three blocks (blocked by type of starch acetate) and another CRD, which were applied in subplots (Table 1). Each experimental unit was replicated. The subplot CRD was
Ind. Eng. Chem. Res., Vol. 45, No. 11, 2006 3995 Table 3. Regression Equation Coefficientsa of Second-Order Polynomialsb for Specific Mechanical Energy Requirement and Selected Physical and Mechanical Properties of Extruded Starch Acetates Foams coeff b0 linear b1 b2 b3 cross-product b12 b13 b23 quadratic b11 b22 b33 R2 probability of F
radial expansion ratio
unit density (g/cm3)
bulk density (kg/m3)
compressibility (kN‚kg-1‚m3)
unit spring index (%)
bulk spring index (%)
-115.62
237.58
125.16
-79293.83
-119.22
-18.95
3.21*** 0.93** 0.58**
-2.31** -2.26** -1.09**
-1.15* -1.06** -0.45*
-6714.73*** 2468.23*** -50.21***
0.061** 1.16* 1.42**
0.85** 1.10* 0.28***
9.89 × 10-3 -6.77 × 10-3 4.43 × 10-3
5.24 × 10-3 -2.45 × 10-3 3.42 × 10-3
8.76 × 10-4 -4.45 × 10-4 1.04 × 10-3
24.77*** 34.02*** 0.33
0.013 -1.75 -5.15**
8.28 × 10-3 0.013* -2.54 × 10-3
-0.12*** -5.57 × 10-3* ** -4.18 × 10-3* ** 0.9332
