Emerging Technologies for Materials and ... - ACS Publications

Chapter 13

Emerging Polymeric Materials Based on Starch William M . Doane, Charles L. Swanson, and George F. Fanta

Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch013

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 o i l 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 o i l , 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

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 i s starch? Where i s i t found? What are i t s properties? Hew i s it/can i t be used as an industrial material? These and other related questions have been af^rpssprt quite thoroughly i n several publications (1-4). For the purpose of this report, which i s to consider starch as a source of new polymeric materials, we w i l l review only briefly some of the fundamental information on starch. Starch i s the name given to the major food reserve polysaccharide produced by photosynthetic plants. I t occurs i n various plant tissues as discrete granules with a size and shape characteristic of the source. The granules may vary i n size, depending on the source, from a few microns to f i f t y or more microns. Insolubility of the starch granule i n cold water facilitates i t s recovery from plant tissue i n rather pure form. Although starch occurs i n many plant tissues, cxmiiercially i t mostly i s 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 i n excess of 300 b i l l i o n 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 i s 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 i n structure and molecular dispersity. Starch i s 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 i n 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. Mthouçî starch i s hicîly hydrophilic, the granules do not dissolve i n 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 d i f f i c u l t when granules are disrupted and starch molecules become more soluble. When granules are heated i n 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


Emerging Polymeric Materials Based on Starch


13. DOANE ET AL.


199


200


temperature by treatment with alkali or other reagents to disrupt the hydrogen bonds that give rise to the crystallinity. Solubil ization 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 b i l l i o n pounds of starch or flour are provided for industrial (non-food) uses. This does not include the starch i n 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 i s i n paper making applications. About three-and-a-half b i l l i o n pounds are used i n 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 χ 10 ) (wet end 0.4 χ 10 )

9

1.6 χ 10 l b

9

9

9

Pigment Bonding

0.6 χ 10 l b

Corrugating Board

0.9 χ 10 l b

Textiles

0.15 χ 10 l b

Miscellaneous (bags, cartons, labels, envelopes, briquettes)

0.3 χ 10 l b

9

9

9

Smaller but significant amounts are used i n other operations, such as the textile, o i l recovery and mining industries. In these applications, starch i s used i n 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 i n the popular press, on the need to develop biodegradable polymers to replace plastics. I t was perceived (often stated) that use of such biodégradables would



13.

DOANE ET AL.


201

greatly lessen, i f not solve, the solid waste disposal problem i n landfills. These various reports were followed by considerable legislative activity from local to national fronts and resulted i n 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 i n landfills, i s primarily responsible for the activity towards plastics from natural materials that would biodegrade. In the U.S., about 58 b i l l i o n pounds of petroleum derived plastics were produced i n 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 i s 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 i n thermoplastic materials (10). Replacement of petrochemically based plastics by biologically derived plastics, where feasible, would reduce petroleum usage. I t would also slow introduction of fossil fuel derived OO2 into the atmosphere since incineration or biological digestion of annually renewablebiomass derived polymers simply recycles OO2 to maintain the ambient level. Litter from such plastics would disappear into i t s 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 i n the 1970s by a scientist i n the United States and one i n the United Kingdom, many of the articles appearing i n the press beginning i n the mid-1980s and many of the legislative proposals singled out starch as an additive to plastics to impart biodegradability. Otey, i n the U.S., was studying starch-synthetic polymer films for use as biodegradable agricultural mulch, while Griffin, i n the U.K., was developing polyethylene films œntaining 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 w i l l be discussed later i n this paper. What has been generated by the legislative actions and numerous articles on biodegradable plastics, i n addition to much continuing debate, i s a heightened awareness of the need for more scientific data addressing this biodegradability issue. Scientists i n industry, academia and the public sector are responding to this need, and today many scientific articles are appearing i n the literature and are being presented at scientific meetings at the local, state, national and intenational levels.


202



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 f i a t requires use of more expensive materials. In this presentation we w i l l f i r s t 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 w i l l discuss other starch-based polymers for areas of application other than plastics. Biopolymer Plastics v i a 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 f i r s t 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 i s more efficient and the polymer has better properties i f the lactic acid i s f i r 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 i n 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. Workers at Battelle, Columbus, Ohio have done considerable research and development of lactic acid based polymers during the last two decades. A review article by Lipinsky and Sinclair (12) discusses the properties and market opportunities for these polymers and problems that need to be overcome. They project the potential of multihundred million pound markets i n cxmnodity plastics and controlled release agrochemical formulations. In such applications, the environmentally benign poly (lactic acid), or copolymers with glycolic acid or caprolactone, would biodegrade i n the environment to natural products. The wide range of physical properties achievable on processing these copolymers of various compositions confirms the excellent potential for their use i n place of many of the current commercial thermoplastics. While properties of the polymers are excellent, f u l l realization of their potential w i l l depend on improved preparation and recovery of the basic fermentation chemicals. Improved biotechnology that leads to higher solids fermentation to produce lactic acid i n pure state i s needed. Enhanced recovery technology i s required to readily recover lactic acid from the fermentation broth. Direct



13. DOANE ET AL.


203

fermentation of starch, rather than conversion f i r s t to glucose, has been reported and could assist i n improving the economics of lactic acid production (13). Although fermentation of sugars to produce polyhydroxybutyrate (FHB) has been known for decades, several shortcomings of the polymer have prevented i t s use on a significant cxxrmercial scale. Copolymers of hydroxybutyrate with hydroxyvalerate (FHBV) overcome many of these shortcomings, and such copolymers are now i n commercial use (Figure 3). The bacterium Alcaligenes eutrophus ferments sugars to FHBV. The British Company I d has developed a range of FHBVs with up to 30% hydroxyvalerate (HV). Whereas FHB i s a rather b r i t t l e polymer, FHBV, with 25% hydroxyvalerate, i s quite flexible. ICE now offers FHB and FHBV with up to 25% HV for a variety of applications. A 1988 product bulletin of ICI Americas Inc. l i s t s several applications for FHBV with varying HV content i n such areas as medical implants, injection molding, extrusion/injection blow molding for packaging materials, and slow release delivery of medicinals. The biodegradability of FHB and FHBV polymers has drawn attention to these natural polymers, as interest i n replacements for nonbiodegradable polymers has grown. The polymers, which have good shelf stability, undergo microbial degradation when buried i n s o i l . ICI has announced that i t s Biopol (FHBV) i s now being used i n Europe in blow molded shampoo bottles, and ICE i s actively seeking other markets for i t s biodegradable polymers. The price of the polymers i s expected to drop from the i n i t i a l $15 per pound to around $2 to $4 per pound by the mid-1990s. Granular Starch As F i l l e r In Plastics Plastic materials can be made from starch i n numerous ways. Starch hydroxyl groups can be esterified or etherified with hydrophobic groups to reduce hydrophilicity and rétrogradation. Derivatization of a majority of the hydroxyl groups produces thermoplastic materials (14). However, due primarily to the cost of such derivatives, l i t t l e cxitinercial interest has been shown i n them. Rather, a major interest has evolved i n plastics i n which granular starch i s incorporated as a f i l l e r . Granular starch has been used as a f i l l e r i n numerous composite plastic compositions having utility as packaging films and containers. Griffin (15,16) demonstrated cxxrçxxinding of natural granular starch with PE (8-23% starch, w/w) to produce films that became embrittled i n less than three months i n moist compost and thus were said to be biodegradable. Entorittlement of the petroleum-based polymers was assisted by incorporation of about 5% unsaturated fatty acid or fatty acid ester i n the formulations. Soil-born transition metal salts catalyzed oxidation of the unsaturated additives to peroxides, the active degradative agents for the synthetic polymers. Later formulations included metal salts (17). Starch was predried to less than 1% moisture to avoid steam-generated defects i n the plastics. Alternatively, desiccants such as CaO (18) i n the formulations trapped moisture from the starch. Esterification of the hydrophilic hydroxyl groups at the surfaces of the starch granules with alkyl siliconate (19) or




Ο II

OH I 2Chb-CH—GOgH Lactic Acid

Δ —

CH -CH I Ο

Ο I HC-CH

3

II ο Lactide

\ M+

O II O - C H - C - -OH I CH .

Ο Ο Il II O - C H - C - O - C H - C - O - < C H ^ -C+OH 5

CH

3

CH

o|

3

3

3

Figure 2. Polymers from lactic acid.

ÇH CH I CH

0

II C

\

/

CH

2

3

CH

0

2

II C

\

CH

\

0\

3

1

/

CH

\ ,

2

0

Figure 3. Poly (hydrc«ybutyrate-oo-h



13.

DOANE ET AL.


205

cx±enyl-suœinate (20) groups improved adhesion to the surrounding hydrophobic plastic matrix. For some applications, mixing of the dried starch with paraffin wax was sufficient surface treatment (21). Jane et al. (22) produced sub-granular sized starch particles (average diameters of 2.2, 3.8, and 8 microns) for plastic f i l l e r applications i n thin films, while Griffin proposed use of small wheat starch granules, and Maddever (27) suggested use of rice starch. Jane et al. (23) demonstrated the use of oxidized FE as a bridging agent between granular starch and FE. Duan et al. (24) showed that the physico- mechanical properties of starch-FE films were improved by added graft copolymer or polymer emulsion compatibilizers. Emulsions of soft polymers, such as poly (butyl ac^late-œ-ethylene-cxwinyl acetate), improved elongation at the expense of tensile strength, while emulsions of hard polymers, such as poly(aoylonitri2e-cx>-styrene), gave higher tensile strengths with lower elongation. FE films containing granular starch are now being produced directly by, or under license from, Ooloroll Ltd i n England, St. Lawrence Starch in Canada, and Archer Daniel Midland Company in the U.S., for use as garbage bags, grocery shopping bags, and over-wraps for mail. Other markets are being developed. Common plastics other than FE that have been compounded with granular starch include: PS, Poly (vinylidene c±iloride-cx>-vinylidene acetate) (PVDC/A), and PVC. A l l of these composites support microbial growth and lose strength when buried near the surface. PVDC/A spray dried with 90% granular starch or PVC compounded on rubber r o l l s with 50% granular starch produced resins suitable for molding meat display trays and transplanting pots (19). Smoke production of burning PVC was reduced by incorporating 30 percent granular starch that had been permeated with 12% ammonium molybdate and spray dried (22). Griffin (26) produced molding sheets of PS œntaining 9-50% granular starch that were suitable for thermoforming into thin walled containers and drinking cups. Compatibility between starch and various synthetic polymers was improved by admixture of copolymers of the synthetic polymer with monomers that contained carboxyl groups (25). Various techniques were investigated by Westhoff et al. (26) for incorporating large amounts of starch as a f i l l e r i n PVC plastics. Starch-PVC films were prepared, and their properties were measured i n Weather^K^neter and outdoor exposure tests. By varying the composition, films were obtained that lasted from 40 to 900 hr. in the Weather-Cmeter and from 30 to more than 120 days i n the s o i l . A l l samples tested under standard conditions with common s o i l mioroorganisms showed microbial growth, with the greatest amount of growth recorded for samples containing the highest amount of starch. Otey et al. (27) used cornstarch, wheat starch, dialdehyde cornstarch, cornstarch graft copolymers, and dialdehyde cornstarch graft copolymers at levels of 15 to 55% as inert f i l l e r s i n PVC. The starch or starch graft copolymers were dry blended with PVC and dioctyl phthalate on a rubber mill and were then pressed into films. Tensile strength of many of the molded starch-PVC products, even with 50% starch i n the plastic were on the order of 3000 psi


206


(20.7 MPa) or higher; however elongation decreased rapidly as the starch level increased. Clarity of the plastics was good, except for plastics made by dry blending of unmodified starch. Plastics made from dialdehyde starch grafted with a mixture of polyacrylonitrile and poly (methyl methacrylate) had outstanding tensile strength.


Gftlatinizpri Starch As A Component In Plastics Considerable work has been reported where starch i s gelatinized and thus may form a cxînuous phase with the synthetic polymer rather than merely being present as a particulate f i l l e r . Starch-PVA films, investigated by Vfesthoff et al. (28) represent such a œritinuous phase system and may have application as a degradable agricultural mulch. A composition containing 60 to 65% starch, 16% PVA, 16-22% glycerol, 1 to 3% formaldehyde, and 2% ammonium chloride was combined with water to give a mixture containing 13% solids and was then heated at 95° C for 1 hr. The hot mixture was then cast and dried at 130°C to form a clear film. Films were passed through a solution of FVC or Saran to give the film a water-repellent coating, since uncoated films had poor wet strength. Coated films retained good strength even after water soaking, and Weather-Qweter tests suggested that film with 15 to 20% coating (by weight) might last 3 to 4 months i n outdoor exposure. Nwufo and Griffin (29) found poor adhesion between granular starch and FVA i n calendered film. Unooated starch-FVA films are used commercially to produce a water-soluble laundry bag for use by hospitals to store soiled or contami nated clothing prior to washing. The bag and i t s contents are placed directly into the washing machine, where the bag dissolves. To provide enhanced solubility, a slightly derivatized starch i s used for this application. Such water-soluble bags are also being suggested for packaging agricultural chemical pesticides to improve safety during handling. Otey et al. (30,31) prepared films from various combinations of starch and EAA that have potential application i n biodegradable mulch, packaging, and other products. EAA contains about 20% copolymerized acrylic acid and i s dispersible i n aqueous ammonium hydroxide. Films were prepared by solution casting and oven drying heated aqueous dispersions of starch and EAA or by fluxing dry mixtures of starch and EAA on a rubber mill. Cast films œntaijiing 30, 40, 50, 70, and 90% starch were exposed to outdoor s o i l contact, with the ends buried i n the s o i l , to observe their resistance to sunshine, rain, and s o i l micaôorganisms. Films with more than 40% starch deteriorated within 7 days, but those œnt^LUiing 30 to 40% starch remained flexible and provided mulch protection for at least 70 days. Other tests revealed that starch-EAA films have sufficient strength, f l e x i b i l i t y , water resistance, and heat scalability for a variety of mulch and packaging applications. In further research on blending starch with synthetic polymers, Otey developed a semi-dry process to extrude thermoplastic starchEAA mixtures cxxitaining up to 60% starch and 2-10% moisture (3.3-18.3% on a starch basis at the 60% starch level). Films were


13. DOANE ET AL.

207


blown at a die temperature of 125-145° C (32), and film properties are shown i n Table II. Otey also produced transparent starch-EAA films with semipermeability properties (33,34) by using aqueous sodium hydroxide to gelatinize the starch i n the extruder at temperatures near 100° C. Alternatively, ammonium hydroxide-urea solution was used to déstructure the starch granules 35), and films prepared i n this manner were blown on a cxxnmeroial film blowing apparatus. IDFE was incorporated as a partial replacement for EAA, which further reduced film cost and i n (some instances improved properties. Swanson et al. (36) showed that shelf l i f e of these films can be extended by low level


Table H .

Starch-EAA Film Properties

Composition of Thermoplastic Starch EAA IDFE Urea Water Base (%) (%) (%) (%) (PPh) (PPh)

Elongation Extrusion Tensile Temperature Strength (MN/m ) (°C) (%)

a

7

25.0

60

40

0

0

5-10

7.5N

110

34

51

0

15

5-8

8.5A

125-145

6.2

120

40

45

0

15

5-8

8.5A

125-145

11.0

90

40

50 10

0

5-8

3.6A

125-145

25.8

80

40

40 10

10

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