Biodegradable Films Based on Partially Hydrolyzed Corn Starch or Potato Starch Herbert Sommerfeld and Rijdiger ~ l u m e ' Department of Chemical Education, University of Bielefeld,D-4800 Bielefeld 1, Germany Renewable raw materials are gaining increasing importance as starting materials for chemical syntheses (13). Applications that lead to ecologically compatible products are becoming especially interesting. The Need for Biodegradable Films An important renewable raw material is biopolymer starch. One of the basic properties of starch is its filmforming ability. In developing chemical modifications that improve upon this natural ability for industrial and household applications, attention should be paid to whether the products retain the original biodegradability (4,5).Thus, it is advantageous to preserve the native molecular structure of the starch molecule as much as possible so that decomposition by enzymes can still occur. Conventionalpolyolefm films constitute about 2% of normal household waste, and thus create a disposal problem if they are not recycled or incinerated (6).Several unsuccessful attempts have been made to improve the photooxidative decomposition of polyolefin films that do not undergo biological degradation. The development of biodegradable films has made a considerable contribution to the protection of the environment. Thus, the preparation of starch-based films is a very promising field of research (7). Demonstration of the promising developments in starch chemistry can be done with short and relevant classroom experiments. Such experiments are useful in encouraging not only the students' interest in chemistry, but also their participation in interdisciplinary discussions of related biological, toxicological, ecological, and economical problems. Improving the Quality of the Films Most corn, potato, and wheat starches comprise amylopedin at 7045% and amylose at 15-30% (8).The film-forming ability of starch is principally due to hydro-
gen bonds between the long-chain, unbranched amylose molecules. The more-globular amylopectin, which disturbs the crystalline order, improves the strength. However, the high brittleness due to amylopectin binders the commercial application of simple starch films. Nevertheless, the chemical derivation of native starch and the addition of nontoxic plasticizers can both be used to manufacture films that show satisfactory tensile strength, flexibility,and transparency for multipie applications (e.g., as packaging materials). Small molecules that form hvdroeen bonds with starch. such as water or alcohob, afread; exhibit plasticizer effects. However, water or simple alcohols can not be used satisfactorilv in this a o ~ l i cat& due to their high volatility. A better cioice is a 'definite polyol (e.g., glycerol). Since they have a high content of hydrophilic alcoholic hyclroxyl gmups, starch films (in contrast to hydrophobic polyolefin films) may be colored with water-soluble food colors without exceeding the limiting values for food additives established by WHO (9).Films consisting of starch, glycerol, and food colors are completely nontoxic. The aualitv of starch films is hiehlv dependent on the ratio ofamyiose to amylopectin a n i o n th/average molecular weight of the starch. A large percentage of amylopedin (M, = lo6 to 2 x lo7)is as &advantageous as a high content of amylose (M, = 4 x lo4 to 2 x lo5). The acid-catalyzed partial hydrolyzation of native starch can achieve two mals: decrease the amvlopedin content and decrease of the average molecular weiiht. The hydrolysis of the helical amylose molecules is slow compared to the hydrolysis of amilopectin. Tensile streneth can be imoroved bv addim comwnents that cause furtger cross-liAng. An ekample?s diaidehyde starch. which is obtained bv oxidizing starch with sodium rneta-periodate (10). Films that contgn dialdehyde starch are still biodegradable. (Continued on page A152)
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The Modern Student laboratory: Exploring the Environment Experimental Section Experiment 1: Preparation of Starch-Based Films Required time: 20 min and 1day (for desiccation). Equipment f l a s k (100 mL) heatable magnetic stirrer oil bath 'thermometer .plastic sheet (acrylic glass, about 15 x 15 cm2) 4 pipets (2 mL, 3 mL, 20 mL, and 25 mL) desiccator cabinet, if available Reagents corn starch (Merck No. 11686.0250) or potato starch (Merck No. 1259.0250) solution of glycerol in distilled water (50 vol%) distilled water solutions of food colors in water (1 g1100 mL) patent blue (Fluka No. 76270) new-coccine, also called cochineal red (Aldrich No. 19.973-7) hydrochloric acid (0.1mom) solution of sodium hydroxide in water (0.1mollL) Pmcedure The following ingredients are heated in a flask for exactly 15 min a t 1 0 5 1 1 0 'C and stirred until a homogeneous mixture is formed. 2.5 g of corn starch (or 2.5 g of potato starch) 2 mL of glycerol solution 0.2-1 mL of food color solution 3 mL of diluted hydrochloric acid '20 mL of distilled water (with potato starch, 25 mL) The hot viscous solution is neutralized by the addition of
3 mL of dilute sodium hydroxide solution. Then it is poured on a plastic sheet and dried in a desiccator cabinet a t 100110 'C for about 1.5 h. After desiccation, a colored, highquality starch film can be removed from the sheet. The drying-time dependson the thickness of the film and should be observed. Experiment2: Preparation of Dialdehyde Starch Required time: 60 min on day 1,and 20 min on day 2. Caution:Sodium meta-periodate is oxidizing and it is combustible. Handle it with care, as recommended by the manufacturer. Dialdehyde starch can be harmful to humans. Handle with care. T h e following scheme illustrates t h e formation of dialdehyde starch.
Equipment flask (100 mL) magnetic stimr dropping funnel filtering material beaker (100 mL) Reagents cam starch (MerckNo. 11686.0250) or potato starch (Merck No. 1259.0250) solution of glycerol in water (50 ~01%) distilled water sodium me&-periodate(Merck No. 6597.0050) Procedure A solution of 5.1 g sodium meta-periodate in 65 mL of distilled water is slowly added over a period of 1 h to a stirred mixture that contains 4 g of starch in 20 mL of distilled water. (When potato starch is used, 25 mL of distilled water is needed.) The mixture should be stirred for a n additional 24 h a t 20 'C and then filtered. The residue must be washed three times with water to remove ionic com~onents. The wet dialdehyde starch mixture, which weighs about 11-. e. is about 754 water. The undried oroduct will be used in Experiment 4. ~
Experiment 3: Films with Additional Dialdehyde Starch Required time: 20 min and 1day (for desiccation). Equipment Identical to that used in Experiment 1. Reagents Identical to those used in Experiment 1,with the addition of the dialdehyde starch obtained in Experiment 2. Procedure The procedure is similar that used in Experiment 1, except that the 0.5 g of starch are replaced with 2 g of wet dialdehyde starch. The resulting film exhibits improved strength. Acknowledgment We thank our collaborators Anja Bergmam and Michael Kiinsebeck for preparing several starch derivatives and the BASF AG for the friendly donation of food colors. Literature Cited 1. Noturwlss. Unfwr (Chmie): Bader,H. J.;Blume,R.,Eda.:1989.47. 2. Sommexfeld. H.:Biume, R.; Bader, H. J.Pror Nofunoh. (Chnie) 1990,2,28-33. 3. Blume. R.: Bader. H. J. Umadfchemls im Emolmmnt: Srnotor:Ran!&rt. 1989. ion, 1976. Bemiller, J. N.:Paschall, E. F. Starch--ChemlsIry and ?kchnalagy; hiatiq R. nis Ress: New York, 1984. 1'8 Eneyclopodio oflndusbial chemistry: Gerhartz, W , Ed.: Weinheim,
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Debold, Germany, 1990. 8. Carrer. C. E.: Seymour, R. B. J Chm. Educ. ISM, 64,314318. 9. Bader, H. J.; Some.rfeld. H. Pror Notunuisa. (Chamlsj 1988,3,1&20. 10. Mdhods in Corbohydmte chemistrv: Whistler, R. L., Ed.:Academic Preaa: New
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