Extruding Foams from Corn Starch Acetate and Native Corn Starch

Biomacromolecules 2004, 5, 2329-2339

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Extruding Foams from Corn Starch Acetate and Native Corn Starch† Junjie Guan and Milford A. Hanna* University of Nebraska-Lincoln, Industrial Agricultural Products Center, 208 L.W. Chase Hall, Lincoln, Nebraska 68583-0730 Received August 18, 2004

Because of the hydrophilic characteristics of native starch foams and the cost of modifying starch, the uses of starch and modified starch foams are hindered. To decrease hydrophilicity and cost of starch foams, native corn starch was blended with starch acetate and extruded. A twin-screw mixing extruder was used to produce the foams. Native starch content, screw speed, and barrel temperature had significant effects on molecular degradation of starches during extrusion. The melting temperature of extruded starch acetate/ native starch foam was higher (216 °C) than that for starch acetate (193.4 °C). Strong peaks in the X-ray diffractograms of extruded starch acetate/native starch foam suggested new crystalline regions were formed. Optimum conditions for high radial expansion ratio, high compressibility, low specific mechanical energy requirement, and low water absorption index were 46.0% native starch content, 163 rpm screw speed, and 148 °C barrel temperature. Introduction With increasing concerns about environmental protection and garbage handling, more and more efforts are being made to develop biodegradable starch-based loose-fill packaging materials. Compared to conventional petroleum-based expanded polystyrene loose-fill foam (EPS), starch-based loosefill packaging is degraded readily in the natural environment. Starch is second only to cellulose in terms of being the most abundant renewable polymer in nature. Native starch has a granular structure and the granule is composed of a mixture of amylose and amylopectin. Amylose is a linear molecule linking R-D-glucopyranosyl units with R, 1-4 bonds. Amylopectin is a very large, highly branched molecule with R, 1-4 and R, 1-6 linked R-D-glucopyranosyl units. Native corn starch consists of approximately 25% amylose and 75% amylopectin. Genetically modified high amylose corn starch can have up to 80% amylose.1 In 2002, two million tons of starches were produced in the world.2 Because of its abundance and its renewable and environmental friendly characteristics, starch has been the feedstock of choice in numerous research efforts on development of biodegradable starch-based loose-fill foams. Altieri and Lacourse3 extruded loose-fill packaging foams with regular, waxy, and hydroxypropylated corn starches. Chinnaswamy and Hanna4 reviewed progress made in the areas of grafting starch on vinylic polymers. Wang and co-workers5 studied the physical properties of two biological cushioning materials made from wheat and corn starches blended with 3% poly(ethylene glycol). Chinnaswamy and Hanna6 hold † Journal Series No. 14568, Agricultural Research Division, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln. * To whom correspondence should be addressed. Telephone: 1.402.472. 1634. Fax: 1.402.472.6338. E-mail: [email protected].

U.S. and Australian patents on loose-fill foams made of 70% starch and 30% polystyrene. Bhatnagar and Hanna7 extruded corn, tapioca, wheat, rice, and potato starches with 30% polystyrene in a single screw extruder. They found the starchbased foams had high expansion, low density, and acceptable mechanical properties. However, the hydrophilic nature of starch hinders its use in packaging foams. During extrusion, starch is subjected to high shear, pressure, and temperature. The starch granules are ruptured by shear and the crystalline regions of the granule are melted.8-11 Amylose and amylopectin chains are depolymerized and rearranged before exiting the extruder die nozzle.9-10 In the presence of a plasticizer (water), shorter amylose and amylopectin chains are reassociated by hydrogen bonds after exiting the nozzle.12 Because the integrity of the starch granule is destroyed and the hydrogen bonds form among amylose and amylopectin chains, water attacks extruded starch, resulting in hydrophilic properties of starchbased foams. Hydrophobicity is preferred in loose-fill packaging materials. One possible approach to increase the hydrophobicity of starch is to substitute the hydroxyl groups of starch backbones with acetyl groups to prohibit hydrogen bond formation. Substituted with acetyl groups, covalent bonds are more likely formed than hydrogen bonds. Because of the covalent bonds, acetylated starch tends to have a harder texture than native starch.13 Several research projects have concentrated on preparation of acetylated starch-based foams.13-22 The hydrophobicity of extruded foams was significantly higher than native starch-based extruded foams while other functional properties were same or even better. However, the cost of preparing acetylated starch is high enough to limit its acceptance by packaging materials manufacturers. Because of structural similarities, it is possible

10.1021/bm049512m CCC: $27.50 © 2004 American Chemical Society Published on Web 10/02/2004

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to blend native corn starch with acetylated corn starch to produce packaging materials. The objectives of this research were to (1) evaluate the compatibility of native corn starch and acetylated corn starch in preparing loose-fill packaging materials, (2) analyze the effects of native starch content, extruder screw speed and barrel temperature on the specific mechanical energy input, radial expansion ratio, water absorption isotherm and compressibility of the extruded foams, and (3) estimate the optimum extrusion conditions based on the physical and mechanical properties. Experimentation Materials. Native (25%) and high amylose (70%) corn starch were purchased from National Starch, Co. (Hammond, IN). Talc (magnesium silicate) was purchased from Barrett Minerals, Inc. (Dillon, MT) and was used as a nucleating agent to ensure the uniformity of the extrudate cell void spaces.7 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). Starch Acetylation. High amylose corn starch was dried at 50 °C for 48 h. To begin the acetylation process, acetic anhydride was placed in a steam-jacketed reactor with a rotating self-wiping paddle. Then, 70% amylose starch was added into the reactor with 5 min of continuous mixing. Finally, 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 5.0 by washing with tap water before drying at 50 °C to a moisture content of 4% (w/w). The starch was ground in a Standard model No. 3 Wiley mill (Arthur H. Thomas Co., Philadelphia, PA) to pass through a 5 mm opening sieve. Blend Preparation. The acetylated starches and native corn starch were predried in a mechanical convection oven (GCA Corp., Chicago IL) at 135 °C for 2 h and then cooled and stored in a desiccator for 24 h to minimize moisture content (0.8%). Both acetylated starch and native corn starch were dried in the same oven at 105 °C for 1 h and moisture contents were measured (0.7%) before being used in sample preparation.23 The native starch contents are given in Table 1. Moisture content of the native starch was adjusted to 15% (d.b.) by adding distilled water. Ethanol was added to the acetylated starch for a final ethanol content of 20% (d.b.) Talc was added to all samples at a 5% level (w/w). Each blended sample weight was 500 g. The blends were mixed in a Hobart mixer (model C-100, Hobart Corp., Troy, OH) for 5 min and then sealed in plastic containers to equilibriate for 24 h. Extrusion. A twin-screw extruder (DR-2027-K13, C. W. Brabender, Inc., S. Hackensack, NJ) with co-rotating mixing screws (model CTSE-V, C. W. Brabender, Inc., S. Hackensack, NJ) was used to conduct extrusions. The manufacturer predesigned conical screws had diameters decreasing from 43 to 28 mm along their length of 365 mm from the feed

Guan and Hanna Table 1. Central Composite Design Used for Statistical Analysis of the Treatment Combination actual factorsa

coded factors treatment

X1

X2

X3

X1

X2

X3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

-1 1 -1 1 -1 1 -1 1 -R R 0 0 0 0 0 0 0 0 0 0

-1 1 0 0 -1 1 0 0 0 0 R 0 0 -1 0 -1 -R 1 0 1

1 1 0 0 1 -1 0 R 0 0 0 0 0 -1 -R -1 0 1 0 -1

40.00 60.00 40.00 60.00 40.00 60.00 40.00 60.00 33.18 66.82 50.00 50.00 50.00 50.00 50.00 50.00 50.00 50.00 50.00 50.00

150.00 170.00 160.00 160.00 150.00 170.00 160.00 160.00 160.00 160.00 176.82 160.00 160.00 150.00 160.00 150.00 143.18 170.00 160.00 170.00

150.00 150.00 130.00 130.00 150.00 110.00 130.00 163.64 130.00 130.00 130.00 130.00 130.00 110.00 96.36 110.00 130.00 150.00 130.00 110.00

a X ) native starch content (%), X ) screw speed (rpm), X ) barrel 1 2 3 temperature (°C) and R ) 1.682.

end to the exit end. On each screw, there was a mixing section, in which 5.3 × 3.4 mm notches were cut away every 60° the screw flight. The mixing section enhanced the mixing action and increased the residence time of the sample in the barrel. Screw speeds ranging from 143 to 177 rpm were used for the extrusions (Table 1). The temperature at the feeding section of the barrel was maintained at room temperature (∼25 °C), whereas the other two barrels sections and the die were varied from 96 to 163 °C (Table 1). The feed rate was maintained at 54 kg‚h-1 by a volumetric feeder (PW40PLUS-0, Brabender Tech. Mississauga, Ontario, Canada). A 3-mm diameter die nozzle was used to produce cylindrical extrudates. An adjustable rotating knife, positioned right next to the nozzle exit, was used to cut the extrudates into 20 mm lengths. The extruder was controlled by a Plasti-Corder (Type FE 2000, C. W. Brabender, Inc., S. Hackensack, NJ). Extrusion data were recorded for subsequent analyses. The extruded foams were stored in desiccators at room temperature (25 °C) for 24 h prior to making physical and mechanical measurements. Degree of Substitution (DS). The DS values of selected extruded foam blends and unextruded starch acetate were determined. DS indicates the average number of substitutions per anhydroglucose unit. There are three free hydroxyl groups available for modification, resulting in a maximum possible DS of 3. The DS of esterified starch was determined using the Miladinov and Hanna14 method. The 5 g samples were placed in 500 mL conical flasks and 50 mL of distilled water were added. The mixtures were then conditioned at 30 °C for 1 h in a Tecator 1024 shaking water bath (Hoganas, Sweden). The pH was measured, then 10 mL of 50% 1 N NaOH solution were added to each flask and samples were conditioned for 48 h at 50 °C to hydrolyze the substituted organic acids. Then the samples were titrated with 0.5 N HCl to the original pH. DS was calculated as

Starch Acetate/Native Starch Foams

DS )

MHCl MWAN W - MHCl (MWFA - MWH2O)

where W ) weight of the sample (g), MHCl ) moles of titrated HCl, MWFA ) molecular weight of the organic acid, MWH2O ) molecular weight of water,18 and MWAN ) molecular weight of an anhydroglucose unit (162). Size Exclusion Chromatography. Molecular degradation was traced by size exclusion chromatography. The sample (0.1000 g) was placed in a test tube, covered with 1 mL of 1 N NaOH and conditioned for 48 h in a water bath at 50 °C. To each test tube, 1 mL of 1 N HCl and 18 mL of dimethyl sulfoxide were added. The sample was conditioned for another hour under the same conditions and then filtered through a 5-µm membrane (Altech, Deerfield, IL). Then 20 µL of the solution were injected into a Shodex KS-806 HPLC column. A refractive index detector was used. The mobile phase was water and the flow rate was 1 mL/min. Differential Scanning Calorimetry (DSC). DSC analyses were conducted on all raw materials and selected extrudates to study the thermal properties of foams including glass transition and melting temperatures. A Perkin-Elmer DSC 7 differential calorimeter (Perkin-Elmer, Wilton, CT) was used to analyze the thermal properties of native starch, starch acetate, and extruded blends. Samples were dried in an oven at 60 °C for 24 h and stored in desiccators to ensure moisture free. About 10 mg of sample were sealed in an aluminum pan, allowed to equilibrate to 25 °C, and scanned from 25 to 220 °C at a constant heating rate of 10 °C/min follow by cooling at 5 °C/min and rescanning up to 220 °C at 10 °C/ min. Three replications were scanned for each sample. X-ray Diffraction. X-ray diffractograms were used to observe the crystallinity of both raw materials and synthesized composites. Native starch, acetylated starch, and extruded blends were dried at 40 °C to constant moisture (10%) in a vacuum oven prior to X-ray scanning. X-ray diffractograms were obtained from a Rigaku model D/Max-B X-ray diffractometer (Brandt Instruments, Inc., Slidell, LA) with 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 of 1.5405 Å and Cu KR2 of 1.5443 Å. The weighted average of the two was Cu KRavg of 1.54184 Å. Data were collected from 2θ of 4 to 35° (θ being the angle of diffraction) with a step width of 0.02° and step time of 0.4 s. Value of 2θ for each identifiable peak on the diffractograms was estimated and crystal d-spacings were calculated using Bragg’s law. Specific Mechanical Energy Requirement (SME). SME is defined as a total input of mechanical energy per unit dry weight of extrudate. SME was determined as described by Bhatnagar and Hanna.24 Extruded materials were collected for 30 s and dried. SME (W‚h/kg) was calculated as

Biomacromolecules, Vol. 5, No. 6, 2004 2331

the cross section area of the die nozzle. A digital reading caliper (Interface RS 50 Sylvac caliper, Crissier, Switzerland) was used to measure the dimensions of the extruded foams. Each mean value was the average of 20 measurements. 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 the bottom of desiccators to obtain the required relative humidity levels.25 Extruded samples were cut into 20 mm lengths and the weights were recorded. Samples were then placed in the desiccators at 25 °C. After 24 h equilibration, samples were weighed before and after drying at 40 °C for 24 h in a vacuum oven. The equilibrium moisture contents (d.b.) were determined based on the weight changes. Compressibility. An Instron universal testing machine (model 5566, Instron Engineering Corp., Canton, MA) was used to measure compressibility of foamed extrudates. 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 extrudate was compressed once to 80% of its original diameter at a loading rate of 1 cm/min using another flat plat. The force (kN) divided by the sample density (kg m-3) was reported as compressibility (kN kg-1 m3). Compressibility for each sample was measured five times and reported as an average of the five reading. Experimental Design and Statistical Analysis. Response surface methodology (RSM) was used to determine the effects of native starch content, barrel temperature, and screw speed on the prescribed physical and mechanical properties of native starch and acetylated starch foams and the optimized conditions based on physical and mechanical properties of the foams. A central composite experimental design, described by Lee and Han26 for three variables with three levels of each variable, was used (Table 1). The three independent variable levels used 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 ) native starch 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. If a stationary point was a saddle point, the maximum or minimum response value was obtained using a ridge analysis. Design-Expert Version 6 (Stat-Ease Co., Minneapolis, MN) was used to conduct the statistical analyses, surface plotting and optimization.

SME ) [2π(n/60)τ]/MFR where n ) screw speed (rev/min), τ ) torque (N-m), and MFR ) mass flow rate (kg/h). Radial Expansion Ratio. Radial expansion was calculated by dividing the mean cross section area of the extrudates by

Results and Discussion Degree of Substitution. Starch acetate had a DS value of 2.3. Extrusion did not significantly (P > 0.10) change the DS value. Even blended with native starch, the extrusion

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Guan and Hanna Table 3. Glass Transition Temperature (Tg) and Melting Temperature (Tm) of Extruded Starch Acetate/Native Starch Foams

Figure 1. Size exclusion chromatography of native starch and starch acetate and starch acetate native starch extrudate in tetrahydrofuran. Table 2. Peak Locations of Extruded Starch Acetate/Native Starch Foams in Size Exclusion Chromatograpy native starch (%)

screw speed (rpm)

barrel temperature (°C)

peak location (min)

40 40 50 50 50 50 60 60 60

150 160 150 160 170 170 160 170 170

150 130 110 130 110 150 130 110 150

11.4 12.6 11.0 12.3 13.1 13.5 11.8 12.1 12.7

conditions were not severe enough to cause noticeable hydrolysis of the acetic acid residues. The starch modification was carried out to impart water resistance in the starch acetate-native foam. Significant loss of the substitutes would make the starch sensitive to water. Therefore, no significant loss of the degree of substitutions was desirable. Size Exclusion Chromatography. The true molecular weight (MW) distribution patterns of the extruded samples were difficult to establish. However, it was still practical to use size exclusion chromatography to determine the molecular degradation of starches, before and after extrusion processing. After injection, similar graphs were obtained for all extruded samples. Figure 1 shows the size exclusion chromatographs of selected extruded foams (13.1 min), unextruded acetylated starch (9.5 min), and native starch (10.8 min). Significant molecular degradation occurred during extrusion. Table 2 also presents the peak locations for extruded foams in the size exclusion chromatographs.

native starch (%)

screw speed (rpm)

barrel temperature (°C)

Tg (°C)

Tm (°C)

40 40 50 50 50 50 60 60 60

150 160 150 160 170 170 160 170 170

150 130 110 130 110 150 130 110 150

150.3 ( 2.3 157.4 ( 1.1 164.5 ( 0.7 161.5 ( 1.3 156.3 ( 1.3 150.3 ( 2.5 146.5 ( 2.1 140.2 ( 2.3 133.5 ( 1.6

188.5 ( 2.2 197.5 ( 1.1 206.5 ( 2.2 216.4 ( 1.1 186.5 ( 1.7 181.4 ( 1.5 178.6 ( 0.8 169.7 ( 1.5 160.7 ( 2.5

The higher the MW was, the earlier the molecules exit the exclusion column. Native starch content, screw speed and barrel temperature significantly affected starch molecular degradation. As native starch content increased, the MW of extruded foams decreased. This was due to a higher degree of molecular degradation occurring in native starch than in starch acetate. Starch acetate had higher heat resistance than native starch. When high temperature and shear were applied to the starch mixture, native starch degraded first. Under the same temperature and shear conditions, the more native starch blended, the less chance starch acetate degraded, resulted in higher MW of the extruded sample. MW reduced significantly when barrel temperature and screw speed increased. Inside the barrel, thermal energy from barrel temperature and mechanical energy from screw speed contributed to the molecular degradation of starch mixture. It was known that higher barrel temperatures and screw speeds would result in a higher degree of starch degradation.8-10 This was substantiated by the size exclusion chromatography data. Differential Scanning Calorimetry (DSC). The DSC thermographs obtained for extruded and nonextruded samples are shown in Figure 2. Native corn starch had a glass transition temperature (Tg) of 78.4 °C. Unextruded starch acetate, with a degree of substitution (DS) of 2.3, had a melting temperature (Tm) of 193.4 °C. The melting temperatures of starch acetates with DS of 2.5 to 3.0 were reported to be in the range of 195-292 °C.27-28 When DS was lower than 2.5, it was expected that Tm would decrease because of fewer interactions among acetyl-substituted starch chains giving a less perfect crystal structure of the starch acetate. The Tg and Tm of extruded native starch/starch acetate foam (50% native starch, 160 rpm screw speed and 130 °C barrel temperature) were 161.5 and 216.4 °C, respectively, suggesting that there was an increase in crystallinity after extrusion. Similar graphs were obtained for the other extruded starch foams and the Tg and Tm were summarized in Table 3. When native starch content increased, both Tg and Tm increased initially and then decreased. This probably was because better crystalline structure was formed at the medium level of native starch content. As more native starch (lower Tg and Tm) was used in the mixture, the Tg and Tm decreased. When barrel temperature and screw speed increased, Tg and Tm decreased. This was due to mechanical



Figure 2. Differential scanning calroimetry thermograms for native (25% amylose) cornstarch, DS 2.3 acetylated starch and native cornstarch/ DS 2.3 acetylated starch extruded foam.

Figure 3. X-ray diffractograms for native (25% amylose) cornstarch, DS 2.3 acetylated starch and native cornstarch/DS 2.3 acetylated starch extruded foam.

and thermal degradation of starches in the extruder. The starch acetate molecules realigned during extrusion, facilitating crystal formation and extruded starch acetate had lower Tg and Tm than nonextruded DS 2.3 starch acetate.14 However, higher Tm was seen in the thermogram of extruded native starch-starch acetate foam than for starch acetate, suggesting a stronger crystalline structure in the extruded foams. Also, linkages may have been formed between native starch and starch acetate. X-ray Diffraction. The X-ray diffractograms of extruded and nonextruded samples are presented in Figure 3. The unextruded native starch had sharp peaks at 15°, 18°, and 23° which represent the typical pattern A of cereal starch. DS 2.3 starch had wide peaks at 8° and 20° which were significantly different from native starch. The X-ray patterns

of extruded native starch-starch acetate foams were similar and strongly followed the starch acetate X-ray pattern, with sharp peaks at 9°, 12°, 19°, and 28°. From previous thermogram, native starch had a Tg of 78.4 °C and the crystals melted quickly when extruded at 160 °C barrel temperature. DS 2.3 starch had a high melting temperature of 193.4 °C and the crystalline region was retained well at 160 °C. When the shear and heat worked together on the blends, starch acetate became soft (even below the Tm) in the presence of ethanol. Ethanol functioned as a plasticizer and a bond solubilization agent during starch acetate extrusion17 which lowered the Tm (190.5 °C). Therefore, starch polymeric chains became mobile before the crystalline regions of starch acetate and native starch were melted. The mixed and melted native starch and starch acetate formed a

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Table 4. Regression Equation Coefficientsa of Second Order Polynomialsb for Specific Mechanical Energy Requirement, Radial Expansion Ratio, and Compressibility

coefficient

b0 linear b1 b2 b3 cross product b12 b13 b23 quadratic b11 b22 b33 R2 probability of F

specific mechanical energy requirement (W h kg-1)

radial expansion ratio (unitless)

compressibility (kNkg-1m3)

-2516.94***

-856.88***

-825.25***

1.23*** 46.95** -5.89***

1.40** 7.10** 2.05***

0.27*** 10.59*** 0.54**

0.0015 0.015 -0.005

-0.0023* -0.00017 0.0023

-0.0027 -0.0012 -0.0032

-0.00926** -0.15* -0.0027 0.9068 0.0005

-0.002*** -0.021*** -0.0088*** 0.9556 < 0.0001

0.0004 -0.03*** 0.00073 0.9217 0.0002

a *, **, and *** indicate significance at p < 0.10, 0.05 and 0.01, respectively. b The models for selected functional properties (specific mechanical energy, radial expansion ratio and compressibility) as a function of X1 (native starch), X2 (screw speed), and X3 (barrel temperature) were calculated as Y ) b0 + b1X1 + b2X2 + b3X3 + b12X1X2 + b13X1X3 + b23X2X3 + b11X12 + b22X22 + b33X32 + e.

homogeneous dough. After exiting the die nozzle, the starch acetate crystalline regions were reformed and with more new crystalline regions formed in native starch (Figure 3), resulting in higher Tm than starch acetate. This also suggested that the new formed crystalline regions were from native starch-starch acetate linkages. Specific Mechanical Energy (SME). SME requirement is an easy-to-monitor, real-time indicator of the process inside the extruder.14 Mechanical energy, in the form of shear, was converted into thermal energy to melt the crystalline polymer. At higher energy levels, the long chain molecules broke to form new linkages in the mixed dough. In the two phase polymer system (hydrophilic and hydrophobic), it was important to form homogeneously mixed dough to obtain highly expanded extrudate. Also, high mechanical properties were obtained when uniform cells were formed. These required the formation of strong covalent bonds between the two phase polymers.18 During extrusion, mechanical energy changed the chemical structure of the polymer, especially depolymerization of long-chain molecules. Even low molecule weight polymers had less mechanical strength and lower melting point which limited their application, high expansion, and high mechanical properties were achievable when these depolymerized long-chain molecules reassociated and formed crystalline regions. SME requirement was affected significantly by native starch content (P < 0.01), screw speed (P < 0.01), and barrel temperature (P < 0.01; Table 4). Quadratic coefficients of native starch content and screw speed terms were significant in the SME requirement regression model (P < 0.05 and 0.1, respectively). The SME requirement of the twin-screw extruder depended on the presence of a mixing element in the screws,

barrel temperature and the viscosity of the materials. Since the mixing effect and the barrel friction were rather stable during the extrusion, the various viscosities of the materials played an important role in the final extrudate functional properties. When the starch blends were in a molten stage, viscosity depended on average molecular weight and intermolecular interaction.14 The highly substituted starch acetate molecules (DS of 2.3) interacted via hydrophobic interactions. Also, a significant amount of weak interactions still existed among the unsubstituted hydroxyl groups. However, compared to native starch, the DS 2.3 starch acetate had stronger intermolecular interactions and the starch acetate was in the form of hard agglomerates. Therefore, when native starch content increased, SME decreased (Figure 4, parts A and B) because less shear force was required to depolymerize the strongly interacted molecules. As screw speed increased, the dough viscosity decreased, causing decreases in SME (Figure 4, parts A and C). As mentioned previously, increased screw speed resulted in higher shear in the barrel. High degree of depolymerization also occurred when barrel temperature increased (Figure 4, parts B and C). Radial Expansion Ratio. Statistical analysis revealed that radial expansion ratio (RER) was affected significantly by native starch content (P < 0.05), screw speed (P < 0.05), and barrel temperature (P < 0.01). Cross products (native starch content and screw speed interaction) and all quadratic coefficients were significant in the RER regression model (Table 4). As shown in the response surface plots (Figure 5, parts A and B), RER increased as native starch content increased from 40% to 50% and decreased as native starch content was increased further. The highest RER was obtained at medium native starch content (50%). This may have been due to native starch’s inhibition of expansion when blended with starch acetate. As mentioned previously, native starch had a low Tm. During the 160 °C extrusion, the dough viscosity was contributed predominantly by starch acetate because complete molecule degradation of native starch occurred. The friction from starch acetate during extrusion also contributed to the molecule degradation of native starch. With more native starch blended in, the dough viscosity decreased significantly, resulting in less expansion of the extrudate. On the other hand, at low native starch content, starch acetate contributed in the expansion predominantly while native starch functioned as a filler. Significant shrinkage of the extruded starch acetate foams were reported by Guan and Hanna.16 Low thermal conductivity hindered the transfer of heat via the ethanol vapor. The extruded starch acetate foams remained pliable after the ethanol (blowing agent) flashed off. However, native starch became rigid quickly after exiting die nozzle when the blowing agent evaporated through the hydrophilic cell walls. Therefore, the extruded native starch-starch acetate foams became rigid quickly so that high RER was obtained. An increase was observed in RER when screw speed increased (Figure 5, parts A and C). Screw speed had a positive relation with mechanical degradation of starch molecule. Mechanical energy contributed to the melting of starch acetate even when melting temperature was not


Figure 4. Effects of native cornstarch content, screw speed and barrel temperature on specific mechanical energy requirements of extrudates.


Figure 5. Effects of native cornstarch content, screw speed, and barrel temperature on radial expansion ratio of extrudates.

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Table 5. Equilibrium Moisture Content (%) of Native Starch/Starch Acetate Foams variablesa

LiCl (11.3%)

K2CO3 (43.2%)

NaCl (75.3%)

KCl (84.3%)

BaCl2 (90.0%)

K2SO4 (97.3%)

40% NS 50% NS 60% NS 150 rpm 160 rpm 170 rpm 110 °C 130 °C 150 °C

2.26 ( 0.11 2.30 ( 0.23 2.31 ( 0.34 1.26 ( 0.23 1.89 ( 0.38 2.41 ( 0.22 1.32 ( 0.20 2.01 ( 0.42 2.75 ( 0.30

4.96 ( 0.34 4.29 ( 0.62 5.01 ( 0.52 4.45 ( 0.16 4.47 ( 0.38 5.13 ( 0.22 4.92 ( 0.28 4.72 ( 0.38 7.67 ( 0.11

12.44 ( 0.11 12.02 ( 0.32 12.51 ( 0.41 7.23 ( 0.37 7.01 ( 0.41 9.96 ( 0.33 8.04 ( 0.26 7.43 ( 0.22 11.04 ( 0.17

10.08 ( 0.28 9.87 ( 0.33 11.86 ( 0.39 9.94 ( 0.41 9.87 ( 0.24 13.02 ( 0.46 10.92 ( 0.09 10.01 ( 0.28 14.88 ( 0.29

14.63 ( 0.49 13.95 ( 0.27 15.04 ( 0.34 14.35 ( 0.41 13.76 ( 0.51 18.02 ( 0.39 14.97 ( 0.42 13.89 ( 0.49 18.00 ( 0.75

15.12 ( 0.33 14.96 ( 0.41 15.39 ( 0.50 17.78 ( 0.39 17.01 ( 0.41 23.98 ( 0.32 18.68 ( 0.49 17.73 ( 0.39 22.15 ( 0.22

a

Native starch (NS) content (%), screw speed (rpm), and barrel temperature (°C).

reached. When both hydrophilic native starch and hydrophobic starch acetate were in molten stage, homogeneously mixed dough was formed and high RER was achieved. An increase in screw speed, however, could have reduced dough viscosity and decreased RER. Therefore, RER can be changed in both directions by screw speed, depending on the relative contribution of screw speed itself and dough viscosity in the system. Because of these, RER would decrease if a higher screw speed was used. Barrel temperature had similar effects on RER (Figure 5, parts B and C). Thermal degradation of starch molecule could affect RER significantly. It has been reported that dough viscosity decreased as barrel temperature increased. A homogeneously mixed dough was formed when thermal energy (barrel temperature), combined with mechanical energy (screw speed), in the barrel. But, RER decreased with the higher level of starch degradation. Therefore, RER decreased when a higher barrel temperature was used. Water Absorption Isotherms (WAI). Water absorption properties of the starch-based foams are important when merchandise is shipped in humid and damp climates. When extruded starch foams absorb moisture, significant decreases in mechanical properties occur. Therefore, hydrophobicity (low water absorption index) is desired. As shown in Table 5, the addition of native starch significantly affected the WAI (P < 0.05). At 40% native starch content, native starch functioned as a filler in the starch acetate phase and formed certain linkages with starch acetate (Figure 3). The starch acetate agglomerates were hard compared to native starch. The combined effects of starch acetate agglomerates, shear (mechanical energy), and thermal energy significantly depolymerized the native starch. The high WAI of the foams was contributed by the highly depolymerized native starch which formed linkages with starch acetate. However, at 60% native starch content, both native starch and starch acetate were degraded by mechanical and thermal energy in the barrel. Also, when more native starch was blended in the system, the hydrophilicity increased. Therefore, WAI was even higher than that at 40% native starch content. At 50% native starch content, extruded blends had the lowest WAI. Starch acetate and native starch contents were balanced and formed mostly crystalline regions, suggesting that the best ratio of blending native starch and starch acetate to achieve lowest WAI was 50% native starch and 50% starch acetate. The screw speed had a significant effect on WAI (P < 0.01) (Table 5). At low screw speed (150 rpm), less

mechanical degradation occurred. The WAI was due mostly to the native starch and starch acetate degradation. At the medium screw speed (160 rpm), WAI was similar to the WAI at low screw speed in the low equilibrium moisture environment, and decreases were observed in the high equilibrium moisture environment. This suggested shear (mechanical energy) induced depolymerization reached the optimum level for native starch and starch acetate to form most intermolecular linkages which limited the WAI. At high screw speed (170 rpm), significantly high WAI was observed because shear depolymerization of long chain starch acetate and native starch occurred. The short chain molecules were responsible for the high WAI. Table 5 shows significant effects (P < 0.01) of barrel temperature on WAI. At low barrel temperature (110 °C), most of the melted and depolymerized long chained molecules were native starch. Because of starch acetate’s high melting temperature, it maintained good molecule integrity and hindered water absorption. At the medium barrel temperature (130 °C), a significant amount of the starch acetate melted and formed intermolecular linkages with native starch. Even thermal degradation occurred in starch acetate. The depolymerized starch acetate and native starch reached the optimum molten stage and chain lengths and formed crystalline regions which limited the WAI. At high barrel temperature (150 °C), thermal and mechanical energy greatly depolymerized both starch acetate and native starch, resulting in high WAI of the extruded foams which consisted of short-chain native starch and starch acetate segments. Compressibility. Compressibility of extruded foam is a direct measurement of the ability of material to deform under load,29 i.e., it describes its’ cushioning ability.14 Usually, foams having higher density (lower expansion ratio) tend to have thicker cell walls and, hence, resist deformation better than low density (higher expansion ratio) foams with thinner cells.29 Native starch content, screw speed and barrel temperature had significant effects on compressibility of extrudates (P < 0.01, 0.01, and 0.05, respectively; Table 4). The quadratic coefficient of screw speed was significant in the regression model (P < 0.01; Table 4). Compressibility decreased as native starch content increased (Figure 6, parts A and B). Because radial expansion ratio increased overly when native starch content increased, density decreased. Therefore, significant compression strength decreases occurred when more native starch was blended in. It has been well documented that extruded starch acetate


Figure 6. Effects of native cornstarch content, screw speed and barrel temperature on compressibility of extrudates.


foams had higher mechanical properties than native starch foams (12, 14, 15, and 31). After extrusion, native starch was melted and fully gelatinized. Starch molecules were depolymerized, forming short-chained molecules. The foam cell walls were thin and easily broken when force was applied because of the low intermolecular forces among these shortchain starch molecules (16, 20, and 21). With acetyl groups substituting hydroxyl groups in pyranosyl rings, starch acetate molecules tended to stick together by covalent forces, resulting in a strong polymeric characteristic. After a certain degree of depolymerization in the barrel, extruded starch acetate molecules realigned in the die. With more depolymerized short-chain native starch in the mixed dough, realignment of starch acetate molecules was hindered, resulting in decreased compressibility. The newly formed intermolecular forces between starch acetate and native starch, as mentioned previously, were lower than the forces among the starch acetate molecules. Therefore, decreased compressibility also was found even when optimum native starch content was used in the blend. Compressibility increased when screw speed increased from 150 to 160 rpm and then decreased as screw speed increased from 160 to 170 rpm (Figure 6, parts A and C). It has been well documented that shear (mechanical energy) results in fragmentation of the starch granule during extrusion.26 As mentioned previously, the greater degree of degradation, the thinner the cell walls and the lower the compressibility. Therefore, when screw speed was higher than 160 rpm, over depolymerization occurred in both native starch and starch acetate. However, due to the two phase system of native starch and starch acetate, it was necessary to depolymerize them to a certain degree to form a homogeneously mixed dough. At low screw speed, starch acetate was not completely melted or well mixed with native starch, resulting in phase separation, which significantly decreased the compressibility. Significant increases in compressibility were observed when barrel temperature increased (Figure 6, parts B and C). Thermal degradation is another contributor to starch degradation. Thermal energy, partially or completely, destroyed the crystalline structure of the raw starch granule, resulting in fragmented starch molecules which formed low mechanical properties foams. Usually, the depolymerization resulted from a combination of thermal and mechanical energy. If starches were extruded at a high temperature, they could be more readily melted and sheared (mechanical energy) to further depolymerize them, resulting in significant decreased compressibility. Commercial expanded polystyrene loose-fill and native starch loose-fill have compressibilities of 10.90 and 5.86 kN kg-1 m3 , respectively.32 Compressibility of starch acetatenative starch foams had higher compressibilities than the commercially available loose-fills. Optimization. Minimum specific mechanical energy requirement (423.89 W h kg-1) was achieved at 60.0% native starch content, 170 rpm screw speed and 150 °C barrel temperature. Maximum radial expansion ratio (28.5) was achieved at 51.0% native starch, 162.5 rpm screw speed

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and 137 °C barrel temperature. Maximum compressibility (58.0 kN kg-1 m3) was achieved at 40% native starch, 159.2 rpm screw speed and 110.0 °C barrel temperature. At the medium native starch content, screw speed, and barrel temperature, low water absorption index was obtained. Overall, to extrude starch acetate-native starch foam with low specific mechanical energy requirement and water absorption index, and high radial expansion ratio and compressibility, a blend containing 46.0% native starch was extruded at 163 rpm screw speed and 148 °C barrel temperature, yielding a radial expansion ratio of 26.1, a compressibility of 51.0 kN kg-1 m3 and consuming 538 W h kg-1 specific mechanical energy (Figure 7, parts A-C). Conclusions Starch acetate extruded with native starch blended well and had promising properties as loose-fill packaging materials. During extrusion, starch acetate and native starch were depolymerized by mechanical and thermal energies, forming new crystalline regions through the realignment of short chain molecules in the die in the presence of plasticizers (ethanol and water). Higher melting temperature of extruded foam was obtained. The crystallinity of extruded foam was similar to nonextruded starch acetate. Native starch content, screw speed, and barrel temperature significantly affected the specific mechanical energy requirements, radial expansion ratios, water absorption indices, and compressibilities of extruded foams. As native starch content increased from 40 to 60 %, specific mechanical energy requirement and compressibility decreased. Radial expansion ratio increased initially and then decreased while water absorption index decreased initially and then increased as native starch content increased. When higher screw speed was used, specific mechanical energy requirement and compressibility decreased, while radial expansion ratio and water absorption index increased. As barrel temperature increased, the specific mechanical energy requirement decreased while radial expansion ratio and compressibility increased. Water absorption index decreased slightly and then increased significantly when barrel temperature increased. Based on the contour plots, the optimum conditions for high radial expansion ratio and compressibility and low specific mechanical energy requirement and water absorption index were obtained with 46.0% native starch content, 163 rpm screw speed and 148 °C barrel temperature. References and Notes

Figure 7. Contour plots for optimization of response variables in preparing high radial expansion ratio and high compressibility extruded starch acetate-native starch foam with lowest specific mechanical energy requirement.

(1) Boyer, C. D.; Shannon, J. C., In Corn: Chemistry and Technology. Chapter 8: Carbohydrates of the kernel; Stanley, A., Ramstad, P. E., Eds.; AACC: St. Paul, MN, 1987. (2) Tester, R. F.; Karkalas, J. Starch. In Biopolymers: Biology, Chemisty, Biotechnology, Applications. Vol. 6, Polysaccharides II: Polysaccharides from Eukaryotes; De Baets, S., Vandamme, E. J., Steinbu¨chel, A., Eds.; Wiley-VCH: New York, 2003; pp 381-438. (3) Altieri, P. A.; Lacourse, N. L. Starch-based protective loose-fill material. Proceedings of the third corn utilization conference; National Corn Growers AssociationSt. Louis, MO, 1990; pp 159162. (4) Chinnaswamy, R.; Hanna, M. A. Extrusion-grafting starch onto vinylic polymers. Starch 1991, 43, 396.

Starch Acetate/Native Starch Foams (5) Wang, W.; Flores, R. A.; Huang, C. T. Physical properties of two biological cushioning materials from wheat and corn starches. Cereal Chem. 1995, 72, 38. (6) Chinnaswamy, R.; Hanna, M. A. Biodegradable polymers. U.S. Patent No. 5,496,895 and Australian Patent No. AU-B-63333/90. 1996. (7) Bhatnagar, S.; Hanna, M. A. Effect of talc on properties of cornstarch extrudates. Starch/Sta¨rke 1996, 48, 94. (8) Owusu-Ansah, J.; van de Voort, F. R.; Stanley, D. W. Physicochemical changes in cornstarch as a function of extrusion variables. Cereal Chem. 1983, 60, 319. (9) Davidson, V. J.; Paton, D.; Diosady, L. L.; Rubin, L. J. A model for mechanical degradation of wheat starch in a single-screw extruder. J. Food Sci. 1984, 49, 1154. (10) Davidson, V. J.; Paton, D.; Diosady, L. L.; Larocque, G. Degradation of wheat starch in a single screw extruder: characteristics of extruded starch polymers. J. Food Sci. 1984, 49, 453. (11) Gomez, M. H.; Aguilera, J. M. Changes in the starch fraction during extrusion-cooking of corn. J. Food Sci. 1983, 48, 378. (12) Guan, J. Extruded starch acetate foams. M.S. Thesis, University of Nebraska-Lincoln, Nebraska, 2002. (13) Guan, J.; Eskridge, K.; Hanna, M. A. Functional properties of extruded acetylated starch/cellulose foams. J. Polym. EnViron. 2004, 12 (3),113. (14) Miladinov, V. D.; Hanna, M. A. Physical and molecular properties of starch acetates extruded with water and ethanol. Ind. Eng. Chem. Res. 1999, 38, 3892. (15) Miladinov, V. D.; Hanna, M. A. Temperature and ethanol effects on the properties of extruded modified starch. Ind. Crops Prod. 2001, 13, 21. (16) Guan, J.; Hanna, M. A. Post-extrusion steaming on starch acetate foams. Trans. ASAE 2003, 46, 1613. (17) Guan, J.; Hanna, M. A. Functional properties of extruded foam composites of starch acetate and corn cob fiber. Ind. Crops Prod. 2004, 19, 255. (18) Guan, J.; Hanna, M. A. Macromolecular characteristics of starch acetate extruded with natural fibers. Trans. ASAE 2004, 47, 205.

Biomacromolecules, Vol. 5, No. 6, 2004 2339 (19) Guan, J.; Hanna, M. A. Morphological and functional properties of acetylated starch foams extruded with cellulose. Society of Plastic Engineering, Annual Conference Proceeding, ANTEC 2004, 62, 2452. (20) Guan, J.; Fang, Q.; Hanna, M. A. Functional properties of extruded starch acetate blends. J. Polym. EnViron. 2004, 12, 57. (21) Guan, J.; Fang, Q.; Hanna, M. A. Selected Functional Properties of Extruded Starch Acetate-Natural Fibers Foams. Cereal Chem. 2004, 82, 199. (22) Guan, J.; Eskridge, K.; Hanna, M. A. Acetylated starch-polylactic acid loose-fill packaging materials. Ind. Crops Prod. 2004, in press. (23) Fang, Q.; Hanna, M. A. Functional properties of polylactic acid starchbased loose-fill packaging foams. Cereal Chem. 2000, 77, 779. (24) Bhatnagar, S.; Hanna, M. A. Extrusion processing conditions for amylose-lipid complexing. Cereal Chem. 1994, 71, 587. (25) Standard practice relatiVe humidity by means of aqueous solutions. ASTM, American Society for Testing and Materials Standards: West Conshohocken, PA, 1992; E104-85. (26) Lee, C.; Han, O. Optimization of extrusion-process of rice-ISP-file fish mixture by response surface methodology. Food Biotechnol. 1997, 6, 97. (27) Wolff, I. A.; Olds, D. W.; Hilbert, G. E. Triesters of corn starch, amylase and amylopectin. Ind. Eng. Chem. 1951, 43, 911. (28) Burkhard, C. A.; Degering, E. F. Derivatives of starch. III Properties of starch acetate. Rayon Texture Mon. 1942, 23, 416. (29) Willett, J. L.; Shogren, R. L. Processing and properties of extruded starch/polymer foams. Polymer 2002, 43, 5935. (30) Shogren, R. L. Preparation, Thermal Properties, and Extrusion of High-amylose Starch Acetates. Carbohydr. Polym. 1996, 29, 57. (31) Wen, L. F.; Rodis, P.; Wasserman, B. P. Starch fragmentation and protein insolubilization during twin-screw extrusion of corn meal. Cereal Chem. 1990, 67, 268. (32) Bhatnagar, M. A.; Hanna, M. A. Physical, mechanical, and thermal properties of starch-based plastic foams. Trans. ASAE 1995, 38, 567.

BM049512M

Extruding Foams from Corn Starch Acetate and Native Corn Starch

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