Biotechnol. hog. 1992, 8, 51-57
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Effect of Starch Granule Size on Physical Properties of Starch-Filled Polyethylene Film+ Seung-taik Lim and Jay-lin Jane* Department of Food Science and Human Nutrition, Center for Crops Utilization Research, Iowa State University, Ames, Iowa 50011
Shyamala Rajagopalan and Paul A. Seib Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas 66506
Physical properties of blown films (25-60-pm thickness) from compounded mixtures of linear low-density polyethylene (LLDPE) and starch were investigated. As starch content increased, tensile strength, percent elongation, and light transmittance decreased and film thickness increased. Among the tested films, small-particle corn starch (2-pm average diameter) film had the highest elongation rate and tensile and yield strength (560%, 3.15 kg/mm2, and 1.07 kg/mm2, respectively, at 15% starch content). Potato starch (35-pm average diameter) film had the lowest values (508%, 1.52 kg/mm2, and 0.55 kg/mm2,respectively, at 15% starch content). Potato starch-LLDPE film had the highest light transmittance and film thickness; small-particle corn starch had the lowest. Tensile and yield strength of the films had strong negative correlations with average starch granule diameter ( R = -0.99 and -0.94, respectively). Film thickness and light transmittance were linearly correlated with starch granule size ( R = 0.93 and 0.87, respectively). Using small-particle corn starch substantially increased incorporated starch level in the film while maintaining the film quality.
Introduction With growing concern about environmental pollution, the accumulation of plastic waste needs immediate resolution. Biodegradable plastics have been intensively studied in recent years (e.g., Evangelistaet al., 1991;Gage, 1990; Otey et al., 1987) and have been commercialized into various products such as garbage bags, composting yard-waste bags, grocery bags, and agricultural mulches. Commercial biodegradable films are generally manufactured from low-density polyethylene with degradative additives such as starch and prooxidants (Gage, 1990). Griffin (1974) has suggested naturally available starch as a biodegradable filler satisfying thermal stability and minimum interference with melt-flow properties of most manufacturing applications. He reported that, among a number of different starch sources (including rice, maize, arrowroot, wheat, bean, and potato), polyhedral starches such as rice and corn starch, preferably a mixture of the two, were suitable as a dry filler in plastic films (Griffin, 1977a,b). Additionally, he proposed using unsaturated fatty acids and their derivatives to enhance the degradative oxidation of plastic films. Starch incorporation produces a plastic film with a porous structure, which enhances the accessibility of the plastic molecules to oxygen and microorganisms (Griffin, 1974). Degradation of starch-filled polyethylene films has been reported by many researchers (Lee et al., 1991;Gould et al., 1990; Cole, 1990; Gage, 1990). Lee et al. (1991) t Journal Paper No. 5-14512 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA. Project No. 2863. Presented at the 201st National Meeting of the American Chemical Society in Atlanta, GA. * Corresponding author.
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Table I. Polyethylene and Starches of Different Sizes. abbreviation average granule compound used in the text diameter, pm linear low-density polyethyleneb LLDPE corn starchC cs 14.3d small-particle corn starche SPC 2.d wheat starch8 ws 16.5d large granule wheat starch LWS 22.04 small granule wheat starch sws 6.5h rice starch’ RS 5.5d potato starch’ PS 35.P a All starches were vacuum-dried at 100 O C to Y v
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Figure 1. Tensile strength (a) and percent elongation (b) of LLDPE films containing corn starch at various concentrations (w/w). Data were averages of 10 replicates.
Figure 2. Film thickness (a) and light transmittance (b) of LLDPE films containingcorn starch at variousconcentrations. Data were averages of 14 and 6 replicates, respectively, for film thickness and light transmittance tests.
Materials and Methods Both large and small wheat starch granules were isolated according to a modified procedure of Bathgate and Palmer (1972). A nylon screen (10-pm mesh diameter) was used to sieve small granules from an aqueous wheat starch slurry. The mesh was attached to the end of a glass column (40 X 6 cm) inside a 2-L beaker. A magnetic stir bar was placed on the mesh inside the column. Wheat starch slurry (25 g in 400 mL of distilled water) was transferred into the column and stirred continuously to promote granule passage through the mesh. Distilled water was added until the passing filtrate cleared. The filtrate was centrifuged (5875g,6 min), and the precipitated small wheat starch granules were dried in an oven (40-45 "C, 24 h). The residue collected on the mesh was suspended in water (1.5 L), and the large starch granules were allowed to settle (25 "C, 15min). The upper water layer containing small granules was discarded. This process was repeated nine times. Final sediment was collected by centrifugation (5875g, 6 min) and dried (40-45 "C, 24 h). Small-particle corn starch was prepared by an acid hydrolysis of corn starch followed by a ball-milling, according to the procedure of Jane et al. (1991). Film Preparation. Corn starch was added to LLDPE a t 0,4,7,10,15, and 20% by weight to examine the effects of starch content on film properties. For comparison among the starches, 7 and 15% starch was incorporated into LLDPE films. Starch and LLDPE (total weight of 200 g) were compounded with a Brabender PL 2000 Plasti-Corder drive (Hackensack,NJ) and a Brabender twin screw mixer
Table 11. Tensile Strength of LLDPE Films Filled with Various Starches at 7% and 15%. starch content starch variety
7% 2.89 f 0.20 3.29 0.37 2.90 f 0.25 3.64 f 0.43 3.76 f 0.32 2.03 f 0.21 3.91 f 0.20
15% 2.46 i 0.27 ws 2.48 i 0.16 LWS 2.21 i 0.10 sws 2.81 f 0.11 RS 2.89 f 0.22 PS 1.52 i 0.10 SPC 3.15 i 0.28 Data reported were averages of 10 replicates. LLDPE films without starch had an average tensile strength of 4.70 i 0.12 kg/ mmz.
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(Model 15-02-000). The mixer was equipped with two counterrotating screws (42-mm diameter X 9.6-mm lead) having an interrupted mixing zone (Model 15-02-001)and with a pelletizing die (Model 15-20-000). The extruder barrel was thermocontrolled at three consecutive heating zones (175,185, and 190 OC, in the direction toward the die). The die was maintained at 195 "C. Two strands of compounded starch-plastic mixture were extruded through the die nozzles (3.2-mm diameter) at 20 rpm, air-cooled in a cooling trough (152 cm X 12.7 em X 12.7 em), and pelletized (Brabender pelletizer, Model 12-72-000). The mixture of 20% starch was compounded twice to achieve uniform starch-LLDPE mixing. The compounded material was stored in a sealed container within a desiccator to prevent atmospheric moisture absorption. Film-blowing was conducted with a 3.2-cm Brabender single-screw extruder (Model 125-25 HC) and a side-
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Figure 3. Scanning electron micrographs of isolated small B-granules of wheat starch contaminated with A-granules (left) and large A-granules of wheat starch (right). Scale bars represent 10 pm.
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Figure 4. Tensile strength of starch-LLDPE films (a) and linear correlation with the average granule diameters of starch (b). Data were averages of 10 replicates.
Figure 5. Yield strength of starch-LLDPE films (a) and linear correlation with the average granule diameters of starch (b). Data were averages of 10 replicates.
feeding vertical blown film die (Model 05-74-000). The barrel temperature was set at four zones (170, 180, 185, and 190 "C, in the direction toward the die) and the die temperature a t 200 "C. The screw speed was 15rpm, with a compression ratio of 3:l. The blow-up ratio of the film was 4.0 f 0.1. Physical Property Tests. An Instron Model 4502 testing system (Park Ridge, IL) was used to test tensile strength at break, yield strength, and percent elongation at break in the machine direction according to ASTM D882-83. Ten specimens were tested for each treatment (n = 10). Film thickness was measured randomly at 14
points (n= 14) with a Fowler Digitrix Mark 2 micrometer (Chicago, IL). Light transmittance of the starch-LLDPE films was measured with a Beckman Model DU-50 spectrophotometer (Fullerton, CA) a t 650 nm according to a modification of ASTM D1003. The film was placed perpendicular to the light path, and the average transmittance was obtained from six measurements (n = 6). Regression analyses of the analytical data with average granule size and incorporation percentage were obtained with SigmaPlot (IBMPC and compatibles version 4.0, December 1989).
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Figure 6. Percent elongation of starch-LLDPE films (a) and linear correlation with the average granule diameters of starch (b). Data were averages of 10 replicates.
Results and Discussion 1. Effect of Starch Content on Film Properties. The control LLDPE film had an average tensile strength of 4.7 kg/mm2. A 4% corn starch incorporation caused a dramatic reduction to 3.5 kg/mm2 (Figure la). The strength reduction became less substantial at high starch concentrations. Regression analysis revealed that the relation between starch concentration (0-20% ) and tensile strength of the film fitted a second-order equation (R2 = 0.956, Figure la). Percent elongation of the film also decreased with starch concentration, showing a strong negative linear correlation (R2 = 0.980, Figure lb). In starch incorporation of greater than 20 % ,the film tended to possess air bubbles, and the blown extrusion became difficult to control. Reduction in tensile strength and in percent elongation of the film by incorporating starch has been reported by several researchers (Griffin, 1977a,b; Gage, 1990; Evangelista et al., 1991). Because covalent linkage between starch and polyethylene was not likely formed during the processing, starch incorporation produced discontinuity in the film matrix. As starch concentration and discontinuity increased, it became difficult to blow intact thin films. Starch also made the film thicker (Figure 2a). Film thickness corresponded with a second-order equation (R2 = 0.979). Corn starch granular diameter ranged between 5 and 30 pm (Snyder, 1984). When the starch was incorporated into pure LLDPE film (22 pm), the thickness increased in the area of pure LLDPE, as well as where starch granules were accommodated. As the film starch
Figure 7. Light micrographs of LLDPE films containing 15% corn starch (top) or small-particlecorn starch (bottom)stretched by tensile force.
load and distribution increased, the number of bumped granules increased, as did the measured thickness. Light transmittance of LLDPE films decreased as starch concentration increased (Figure 2b) because starch granules shielded the light. Pure LLDPE film had 69% light transmittance. Light transmittance decreased inversely and exponentially as starch concentration increased (R2 = 0.990). 2. Effect of Starch Granule Size on the Physical Properties of Film. Starch Granule Size. Percent yields for large and small starch granulesfrom native wheat starch were 29.6% and 15.8% (w/w),respectively. Scanning electron micrographs showed that the isolated small spherical B-granules were contaminated with small diskshaped A-granules (Figure 3, left), whereas the isolated large A-granules were free of B-granules (Figure 3,right). Average diameters of small and of large wheat starch granules were 6.5 and 22.0 pm, respectively (Table I). The average diameter of the small-particlecorn starch was determined to be 2.0 pm by an image analyzer (Table I). Tensile Strength at Break. Tensile strength of starch-LLDPE films differed among starch types (Table I1 and Figure 4a). The film containingwheat starch (WS) showed slightly greater tensile strength than did that containing corn starch (CS). When three wheat starches (WS, LWS, and SWS) with different average granule sizes were compared, tensile strength of the films exhibited an inverse relation to average granule size (Figure4b). Potato starch (PS), with the greatest average diameter among all the tested starches (35pm, Table I), produced films with the least tensile strength (2.0 and 1.5 kg/mm2,respectively,
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Biotechnol. Prog,, 1992, Vol. 8, No. 1
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Figure 8. Light transmittance of starch-LLDPE films (a) and linear correlation with the average granule diameters of starch (b). Data were averages of 6 replicates.
Figure 9. Film thickness of starch-LLDPE films (a) and linear correlationwith the averagegranule diametersof starch (b).Data were averages of 14 replicates.
a t 7 and 15% starch contents, Figure 4a). In contrast, small-particlecorn starch (SPC),with the smallest average diameter (2 pm, Table I),produced films with the greatest tensile strength (3.9 and 3.2 kg/mm2). Among the unmodified starches (CS, WS, RS, and PS), rice starch (RS) produced the film having the highest tensile strength. There was a negative linear correlation between starch granule diameter and film tensile strength (Figure 4b, R = -0.99). Film containing 15% SPC exhibited greater tensile strength (3.2 kg/mm2) than did that containing 7 % CS (2.9 kg/mm2) (Figure 4a). This difference suggested that using small-particle corn starch instead of native corn starch may raise the incorporated starch content while maintaining the film strength. Yield Strength. Yield strength of LLDPE films containing starches of different sizes (Figure 5a) followed a similar trend as tensile strength (Figure 4a). At 15% starch content, SPC film exhibited the greatest yield strength (1.07 kg/mm2),whereas PS film showed the least (0.55 kg/mm2)(Figure 5a). When starch content was raised from 7% to 15% in the film, small-particle corn starch (SPC) showed a negligible decrease in yield strength (0.01 kg/mm2),whereas other starches showed significantdrops (0.05-0.17 kg/mm2). Yield strengths of the 15% starch films were negatively correlated with the average starch granule diameters (R = -0.94, Figure 5b). Percent Elongation at Break. Reduction in percent elongation is another undesirable aspect of starch incorporation in plastic films. Standard deviations for each treatment (Figure 6a) were greater than those for the
strength tests (Figures 4a and 5a). Corn starch ((2s)LLDPE f i b s had smaller percent elongations than did WS-LLDPE films. Percent elongation of the 15% starch films was also negatively correlated with the average size of starch granules although the R value was relatively small (-0.69). Elongation of the starch-LLDPE films may relate not only to starch granule size but also to granule shape. According to Khan and Prud'homme (19871, shape and orientation of filler particles showed significant effects on the melt rheology of filled thermoplastics. Griffin (1990) demonstrated that wheat starch granules may possess inherent advantages as a filler in plastic films. Both large and small wheat starch granules were readily accommodated inside a thin film, because the disk-shaped large granules oriented with the long axis in the machine direction of the extruded film. Among the films containing 15% starch, SPC film displayed the greatest average percent elongation (560% ), which was also greater than the corresponding value for 7 % CS film (544% 1 (Figure 6a). Using SPC instead of CS would allow the film to incorporate more starch without reducing the film's tensile properties. The starch-LLDPE films produced voids (air spaces) on the starch granule surface duringtensile testing (Figure 7). These voids indicated that there was no linkage between starch and polyethylene produced during the processing and that discontinuity in the film existed at the interface of both materials. When LLDPE films containing 15% CS and those containing 15% SPC were compared under a light microscope, the CS film exhibited
Bbtechnol. Prog., 1992, Vol. 8, No. 1
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Figure 10. Scanning electron micrographs of LLDPE films containing 15%corn (top left), wheat (top right), potato (bottom left), or small-particle corn (bottom right) starch. Scale bars represent 20 pm.
obvious, large voids (Figure 7,top), whereas the SPC film exhibited much smaller voids uniformly distributed throughout the film (Figure 7, bottom). It is plausible that the uniformly distributed small particles caused less severe discontinuity of the LLDPE matrix. Consequently, the loss of tensile properties was less significant. Light Transmittance. Pure LLDPE film had a light transmittance of 69 % . Film with 15%PS had the highest light transmittance (4276 ) among the starch-LLDPE films, whereas 15% SPC film had the lowest (18%)(Figure 8). Films containing wheat starches (WS, LWS, and SWS) showed slightly higher transmittance than did those containing CS or RS. Light transmittance of the films showed a positive correlation with the average starch granule diameter (R = 0.87, Figure 8b), indicating that a film containing larger starch granules was more transparent. A plausible explanation of the differences in light transmittance is that a small granular starch sample at the same weight basis consisted of more particles than a large granular starch. Although each individual large starch granule has a greater area of cross section than a
small granule, the larger number of small granules (when evenly distributed) would provide a greater total area of cross section in the film. Therefore, more light was shielded by the SPC film, and the lowest light transmittance was observed. Film Thickness. Thickness of the starch-LLDPE films prepared under the same conditions ranged from 25 to 60 pm (Figure 9a), whereas that of pure LLDPE film was 22 pm. The thinnest film was produced with SPC (25 and 26 pm, respectively, at 7 and 15% starch contents); the thickest was PS (51and 60 pm). Small-particle corn starch (SPC)showed a negligible difference (1pm) in film thickness between 7 and 15% incorporation, whereas other starch-LLDPE films differed from 3 to 9 pm (Figure 9a). A strong correlation between starch granule size and film thickness was observed (R = 0.93, Figure 9b). Scanning electron micrographs of starch-LLDPE films containing CS, WS, PS, and SPC are shown in Figure 10, top left, top right, bottom left, and bottom right, respectively. The micrographs indicated substantial protrusion of potato starch granules on the surface of the film (Figure 10, bottom left) compared with other starches.
Biotechnol. mug., 1992, Vol. 8, No. 1
In the film cross sections, embraced starch granules and the holes in which starch granules had been lost during cutting were evident (Figure 10, top panels). The loosely filled starch granules readily fell apart at the cut edge, leaving the holes visible in the micrographs. The film cross section containing WS (Figure 10,top right) revealed flat holes in the direction of the film orientation; this direction indicates that disk-shaped granules had located in that orientation. This finding agrees with Griffin's (1990)observation that WS readily oriented in the machine direction. The shape and the orientation of the WS granules are believed to contribute to the smooth surface of the film.
Acknowledgment We thank the Iowa Corn Promotion Board and the Iowa Department of Economic Development for financial support, American Maize-Products Co. and Midwest Grain Products, Inc., for supplying starch samples, and D. J. Burden for reviewing the manuscript. Literature Cited Bathgate, G. N.; Palmer, G. H. A Reassessment of the Chemical Structure of Barley and Wheat Starch Granules. Staerlze 1972, 24, 336-341. Cole, M. A. In Agricultural and Synthetic Polymers, Biodegradability and Utilization;Glass,J. E., Swift, G., Ed.; ACS Symposium Series 433; American Chemical Society: Washingto-n, DC, 1990, pp 76-95. Evangelista, R. L.; Nikolov, Z. L.; Sung, W.; Jane, J.; Gelina, R. J. . Effect of . ComDoundine and Starch Modification on ProDerties of Starch-killed Low Density Polyethylene. Ind. En'g. Chem. Res. 1991,30,1841-1846. Gage, P. Degradable Polyethylene Film-the Fact. Tappi J.1990, IO, 161-169.
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Gould, J. M.; Gordon, S. H.; Dexter, L. B.; Swanson, C. L. In Agricultural and Synthetic Polymers, Biodegradability and Utilization;Glass, J. E., Swift,G., Eds; ACS SymposiumSeries 433; American Chemical Society: Washington, DC, 1990, pp 65-75. Griffin, G. J. L. Biodegradable Fillers in Thermoplastics. Adu. Chem. Ser. 1974,134, 159-170. Griffin, G. J. L. Biodegradable Synthetic Resin Sheet Material Containing Starch and Fatty Material. U.S. Patent 4,021,388,1977a. Griffin, G. J. L. Synthetic Resin Sheet Material. US. Patent 4,021,388, 1977b. Griffi,G. J.L. In Wheat is Unique;Pomeranz,Y.,Ed.; American Association of Cereal Chemists: St. Paul, MN, 1990, pp 695706. Jane, J.; Evangelista, R. L.; Wang, L.; Ramrattan, S.; Moore, J. A.; Gelina, R. J. Use of Modified Starches in Degradable Plastics. Corn Utilization Conference 3Proceedings 1990,4, 1-5. Jane, J.; Shen, L.; Wang, L.; Maningat, C. C. Preparation and Properties of Small Particle Corn Starch. Cereal Chem. 1991, in press. Khan, S. A.; Prud'homme, R. K. Melt Rheology of Filled Thermoplastics. Rev. Chem. Eng. 1987,4,205-269. Lee, B.; Pometto, A. L., 111; Fratzke, A.; Bailey, T. B., Jr. Biodegradation of Degradable Plastic Polyethylene by Phanerochaete and Streptomyces Species. Appl. Enuiron. Microbiol. 1991, 57 (3), 678-685. Otey, F. H.; Westhoff, R. P.; Doane, W. M. Starch-Based Blown Films. 2. Ind. Eng. Chem. Res. 1987,26,1659-1663. Snyder, E. M. In Starch Chemistry and Technology; Whistler, R. L., BeMiller, J. N., Paschall, E. F., Eds.; Academic Press Inc.: Orlando, FL, 1984; pp 661-673. Accepted October 14,1991. Registry No. PE2045, 26221-73-8; starch, 9005-25-8.
