[ACS Symposium Series] Emerging Technologies for Materials and Chemicals from Biomass Volume 476 || Emerging Polymeric Materials Based on Starch

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  • Chapter 13

    Emerging Polymeric Materials Based on Starch

    William M. Doane, Charles L. Swanson, and George F. Fanta

    Plant Polymer Research, National Center for Agricultural Utilization Research, U.S. Department of Agriculture, Agricultural Research Service,

    1815 North University Street, Peoria, IL 61604

    Interest in natural products as annually renewable raw materials for industry has greatly intensified, especially during the last fifteen years. Although much of this interest can be attributed to the oil embargo of the early 1970s, the increased abundance of agricultural production beyond available markets has generated an oversupply of many commodities and with it an increased interest in such comnodities as raw materials for industry to develop new and expanded markets.

    Plant and animal materials have long served beyond food and feed needs in special industrial markets. With the advent of the petrochemical industry during the last fifty years, some of the traditional markets served by materials of agricultural origin were replaced by petrochemical synthetics. This situation continues today due largely to the vast array of synthetic materials that can be produced with excellent properties and satisfactory economics.

    Now there is a perception in many countries that greater utilization of products from plants, animals and microbes from land and sea can and should play a more significant role in meeting society's needs for a broad range of industrial materials. Perceptions include improved economies through increased processing of domestically produced comnodities, reduction of imported oil , and products from natural sources that are environmentally more acceptable.

    Starch is one of the natural materials that is receiving considerable attention in this renewable resources scenario. In answer to the question: why is starch of major interest as a renewable material, one might answer that starch is one of the most abundant materials produced in nature, is easily recovered from plant organs holding it, is relatively low in cost and is readily converted chemically, physically and biologically into useful low molecular weight compounds or high molecular weight polymeries.

    The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned.

    This chapter not subject to U.S. copyright Published 1992 American Chemical Society

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    In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

  • 198 MATERIALS AND CHEMICALS FROM BIOMASS

    Starch: Occurrence, Ocxnpositicn, Properties, Oses What is starch? Where is i t found? What are its properties? Hew is it/can i t be used as an industrial material? These and other related questions have been af^rpssprt quite thoroughly in several publications (1-4). For the purpose of this report, which is to consider starch as a source of new polymeric materials, we will review only briefly some of the fundamental information on starch.

    Starch is the name given to the major food reserve poly-saccharide produced by photosynthetic plants. It occurs in various plant tissues as discrete granules with a size and shape characteristic of the source. The granules may vary in size, depending on the source, from a few microns to fifty or more microns. Insolubility of the starch granule in cold water facilitates its recovery from plant tissue in rather pure form. Although starch occurs in many plant tissues, cxmiiercially i t mostly is recovered from seeds, roots and tubers.

    In the United States, cereal grains, predominately corn, provide the major source of starch. Ine average corn crop contains in excess of 300 billion pounds of starch with only about 15% of the crop being processed to separate the starch or starch-protein (flour) matrix from the corn kernels. The corn processing industry is expanding, doubling the amount processed during the last decade, and has both the interest and capability to further expand as market opportunities increase.

    Starch granules do not contain starch as a well-defined homogeneous polymer. Rather, the granules contain starch most often as a mixture of two polysaccharides differing in structure and molecular dispersity. Starch is characterized as a mixture of a predominately linear a-(l >4)-glucan, termed amy lose, and a highly branched a-(l >4)-glucan with branch points occurring through a-(l >6) linkages, termed amylopectin (Figure 1). The amylose molecules have a molecular weight of approximately 1 million, whereas the molecular weight of the amylopectin molecules may be on the order of 10 million or more. Depending on the source, the two components are present in quite varied ratios. Some sources of starch contain almost none of the linear conponent, while others contain only small amounts of the branched polysaccharide. Typically, starches contain 20-30% of the linear amylose fraction.

    Mthou^i starch is hic^ily hydrophilic, the granules do not dissolve in ambient temperature water due to the ordered arrangement of molecules within the granule. Segments of molecules are so arranged as to give rise to apparent crystallinity within the granule. This insolubility not only allows for facile recovery from plant organs, i t also allows for chemical modification of the starch molecules without disruption of the granule, facilitating recovery of the modified product. Recovery of product becomes more difficult when granules are disrupted and starch molecules become more soluble.

    When granules are heated in water, they swell and lose their ordered arrangement, a process known as gelatinization. Rupturing of the granules releases the individual amylose and amylopectin molecules, which can become completely soluble at a temperature of 130-150 C. Gelatinization of starch can be carried out at low

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    In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

  • 13. DOANE ET AL. Emerging Polymeric Materials Based on Starch 199

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    In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

  • 200 MATERIALS AND CHEMICALS FROM BIOMASS

    temperature by treatment with alkali or other reagents to disrupt the hydrogen bonds that give rise to the crystallinity. Solubilization of the starch molecules allows for access of the entire molecule to chemical or enzymatic conversion. Also, i t allows for separation of the amylose from the amylopectin, most often accomplished through complexing of amylose with butanol or thymol, which causes precipitation of the complex.

    In the U.S., about 5 billion pounds of starch or flour are provided for industrial (non-food) uses. This does not include the starch in nearly 365 million bushels of corn converted into ethanol in 1989. Mthough a variety of industrial markets are served by starch due to i t s inherent adhesive and film forming properties, the dominant use for starch is in paper making applications. About three-and-a-half billion pounds are used in the paper, paperboard, and related industries, where starch serves a variety of adhesive functions (Table I).

    Table I. Starch Adhesive Applications

    Paper Surface Sizing

    (size press 1.2 109) (wet end 0.4 109)

    Pigment Bonding Corrugating Board Textiles Miscellaneous

    (bags, cartons, labels, envelopes, briquettes)

    Smaller but significant amounts are used in other operations, such as the textile, o i l recovery and mining industries. In these applications, starch is used in native form or after partial acid or enzyme hydrolysis, oxidation, esterification, etherification or cross-linking. Appropriate selection of chemical reagent allows for introduction of anionic or cationic charge into the starch molecules. The modification of starch, mostly by classical methods, and the properties and uses of such modified starches have been recently reviewed (5). Starch: Role In Biodegradable Plastics In about the mid-1980s there began to appear many articles and conmentaries, especially in the popular press, on the need to develop biodegradable polymers to replace plastics. It was perceived (often stated) that use of such biodgradables would

    1.6 109 lb

    0.6 109 lb 0.9 109 lb 0.15 109 lb 0.3 109 lb

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    In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

  • 13. DOANE ET AL. Emerging Polymeric Materials Based on Starch 201

    greatly lessen, i f not solve, the solid waste disposal problem in landfills. These various reports were followed by considerable legislative activity from local to national fronts and resulted in a variety of laws specifying biodegradability requirements for certain polymeric materials. Plastics, the major non-energy product of petroleum chemicals, are considered to be nonbiodegradable, or at best only slowly degradable over many years. This, coupled with the amount of plastics produced and ending up as l i t t e r or in landfills, is primarily responsible for the activity towards plastics from natural materials that would biodegrade. In the U.S., about 58 billion pounds of petroleum derived plastics were produced in 1989 (6). Municipal solid waste contains about 7% by weight (7) and 17-25% by volume (8,9) of plastic, largely from packaging materials. Traditional plastics can be altered to enable facile chemical degradation, but the toxicity of the residues is as yet undefined. Some chemically-degraded petroleum based polymer residues may biodegrade to further increase globed OO2 levels.

    Degradable plastic materials based on annually renewable biological products, such as starch, are seen by many as a solution to these problems. Roper and Koch recently reviewed the role of starch in thermoplastic materials (10). Replacement of petrochemically based plastics by biologically derived plastics, where feasible, would reduce petroleum usage. It would also slow introduction of fossil fuel derived OO2 into the atmosphere since incineration or biological digestion of annually renewable-biomass derived polymers simply recycles OO2 to maintain the ambient level. Litter from such plastics would disappear into its surroundings to leave only normal biological residues. Integrated waste management practices that include off-landfill composting of biodegradable wastes, incineration, source reduction of packaging materials, barring of toxic colorants, and recycling may bring waste disposal under control.

    Due largely to independent studies conducted in the 1970s by a scientist in the United States and one in the United Kingdom, many of the articles appearing in the press beginning in the mid-1980s and many of the legislative proposals singled out starch as an additive to plastics to impart biodegradability.

    Otey, in the U.S., was studying starch-synthetic polymer films for use as biodegradable agricultural mulch, while Griffin, in the U.K., was developing polyethylene films ntaining granular starch as a f i l l e r to impart better hand feel and printability to polyethylene shopping bags. More detail of the studies by these researchers will be discussed later in this paper.

    What has been generated by the legislative actions and numerous articles on biodegradable plastics, in addition to much continuing debate, is a heightened awareness of the need for more scientific data addressing this biodegradability issue. Scientists in industry, academia and the public sector are responding to this need, and today many scientific articles are appearing in the literature and are being presented at scientific meetings at the local, state, national and intenational levels.

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    In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

  • 202 MATERIALS AND CHEMICALS FROM BIOMASS

    Probable markets for these biodegradable materials include many single use items such as agricultural mulch films, garbage bags, shopping and produce bags, diaper linings, bottles, drums, sanitary applicators, and fast food service items. Penetration into these markets requires matching of physical properties and costs of the plastics to the needs of the applications. Costs much above those of conventional non-degradable materials may be resisted by consumers unless legislative fiat requires use of more expensive materials.

    In this presentation we will first discuss the research on starch leading to plastic materials that are more, i f not totally, biodegradable as replacements for the current plastics derived from petroleum. Following this, we will discuss other starch-based polymers for areas of application other than plastics. Biopolymer Plastics via Starch Fermentation Fermentation of starch or starch-derived sugars has long been practiced to produce a variety of alcohols, polyols, aldehydes, ketones and acids. One of the acids, lactic, has received considerable attention as the basis of biodegradable thermoplastic polymers with a host of potential industrial applications. Polymerization of lactic acid to poly(lactic acid) was first studied about 50 years ago and continues to be a topic of research today. Although lactic acid can be directly polymerized by condensation polymerization, polymerization is more efficient and the polymer has better properties i f the lactic acid is fir s t converted to the lactide, the dilactone of lactic acid (Figure 2). To improve properties of poly(lactic acid), copolymerization with glycolic acid or epsilon caprolactone has received considerable attention. A wide range of properties result on varying the ratio of comonomers in the mixture, as reported by Sinclair (21). Ccnmercial use of these polymers has been restricted mostly to the medical field, where they function as biocompatible, biodegradable, reabsorbing sutures and prosthetic devices.

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