STARCH POLYMERS

Starch is one of the most exciting and promising raw materials for the production of biodegradable products. It is the major polysaccharide reserve material of photosyn­
thetic tissues and of many types of plant storage organs such as seeds and swollen stems. The primary crops used for its production consist of potatoes, corn, wheat and rice. In all of these sources, starch is produced in the form of granules, which vary in size and somewhat in composition based on the resources. Starch granule is composed of two main polysaccharides, amylose and amylopectin with some minor components such as lipids and proteins. Amylose is linear polymer of (1^4)-linked a-D-glucopyranosyl units with some slight branches by (1^-6)-a-linkages (Fig. 17.1). Amylose can have a molecular weight between 104 and 106 g mol-1, but it is soluble in boiling water.40,41 Amylopectin is a highly branched mol­ecule composed of chains of a-D-glucopyranosyl residues linked together mainly by (1^4)-linkages but with (1^6)-linkages at the branched points. Amylopectin consists of hundreds of short chains of (1^4)-linked a-D — glucopyranosyl interlinked by (1^6)-a-linkages (Fig. 17.2). It is an ex­tremely large and highly branched molecule with a molecular weights rang­ing from 106 to 108 g mol-1. Therefore, it is insoluble in boiling water, but in their use in foods, both fractions are readily hydrolyzed at the acetal link by enzymes. Amylases attack the a-(1-4)-link of starch while the a-(1-6)-link in amylopectin is by glucosidases. The crystallinity of the starch granules is attributed mainly to the amylopectin and not to amylose, which although linear, presents a conformation that hinders its regular association with other chains.42,43

image364 Подпись: CH2OH Подпись: CH2OH Подпись: O-

image276(Glucose-a(l -4 )-glucose)

image277

Starch has received significant interest during the past two decades as a biode­gradable thermoplastic polymer. Starch offers an attractive and cheap alternative in developing degradable materials. Starch is not truly thermoplastic as most synthetic polymers. However, it can be melted and made to flow at high temperatures under pressure and shear. It has been widely used as a raw material in film production be­cause of increasing prices and decreasing availability of conventional film-forming resins based on petroleum resources. Starch films possess low permeability and are thus attractive materials for food packaging. Starch is also useful for making ag­ricultural mulch films because it degrades into harmless products when placed in contact with soil microorganisms.44,45

By itself, starch is a poor alternative for any commodity plastic because, it is mostly water soluble, difficult to process, and brittle. Therefore, research on starch includes exploration of its water adsorptive capacity, the chemical modification of the molecule, its behavior under agitation and high temperature, and its resistance to thermo mechanical shear. Although starch is a polymer, its stability under stress is not high. At temperatures higher than 150 °C, the glucoside links start to break, and above 250 °C the starch grain endothermally collapses. At low temperatures, a phe­nomenon known as retrogradation is observed. This is a reorganization of the hydro­gen bonds and an aligning of the molecular chains during cooling. In extreme cases under 10° C, precipitation is observed. Thus, though starch can be dispersed into hot water and cast as films, the above phenomenon causes brittleness in the film.46

Plasticized starch is essentially starch that has been modified by the addition of plasticizers to enable processing. Thermoplastic starch is plasticized to com­pletely destroy the crystalline structure of starch to form an amorphous thermoplas­tic starch. Thermoplastic starch processing involves an irreversible order-disorder transition termed gelatinization. Starch gelatinization is the disruption of molecu­lar organization within the starch macromolecules and this process is affected by starch-water interactions. Most starch processing involves heating in the presence of water and some other additives like sugar and salt to control the gelatinization in the food industry, or glycerol as a plasticizer for biodegradable plastics applications. Most of the commercial research on thermoplastic starches has involved modified starches and or blends with additives and other appropriate polymers for its applica­tion as biodegradable plastics.47 The starch molecule has two important functional groups, the — OH group that is susceptible to substitution reactions and the C-O-C bond that is susceptible to chain breakage. The hydroxyl group of glucose has a nucleophilic character. To obtain various properties starch can be modified through its — OH group. One example is the reaction with silane to improve its dispersion in polyethylene.48 Crosslinking or bridging of the — OH groups changes the structure into a network while increasing the viscosity, reducing water retention and increas­ing its resistance to thermo mechanical shear.

One of the approaches to modify this starch is by acetylation to from starch acetate. Acetylated starch does have several advantages as a structural fiber or film­forming polymer as compared to native starch. The acetylation of starch is a well — known reaction and is a relatively easy to synthesize. Starch acetate is considerably more hydrophobic than starch and has been shown to have better retention of tensile properties in aqueous environments. Another advantage is that starch acetate has an improved solubility compared to starch and is easily cast into films from simple solvents. The degree of acetylation is easily controlled by trans esterification, al­lowing polymers to be produced with a range of hydrophobicities. Starch has been acetylated [with a high content (70%) of linear amylose] and its enzymatic degrada­tion has been studied. Apart from acetylation and esterification, some other modifi­cation of starch such as carbonilation of starch with phenyl isocyanates, addition of inorganic esters to starch to produce phosphate or nitrate starch esters, production of starch ethers, and hydroxypropylation of starches via propylene oxide modifica­tion has been performed. Generally all these modifications involve hydroxyl group substitution on the starch that will lower gelatinization temperatures, reduce retro — degradation and improve flexibility of final product.42

Starch has been used for many years as an additive to plastic for various pur­poses. Starch was added as a filler49 to various resin systems to make films that were impermeable to water but permeable to water vapor. The use of starch as a biode­gradable filler in LDPE was reported.50 A starch-filled polyethylene film was pre­pared which became porous after the extraction of the starch. This porous film could be readily invaded by microorganisms and rapidly saturated with oxygen, thereby increasing polymer degradation by biological and oxidative pathways.51 Otey et al. in a study on starch-based films, found that a starch — polyvinyl alcohol film could be coated with a thin layer of water-resistant polymer to form a degradable agricultural mulching film.47 Starch-based polyethylene films were formulated and containing up to 40% starch, urea, ammonia and various portions of low density polyethylene (LDPE) and poly(ethylene-co-acrylic acid) (EAA). The EAA acted as a compatibil — izer, forming a complex between the starch and the PE in the presence of ammonia. The resulting blend could be cast or blown into films, and had physical properties approaching to those of LDPE.52,53

Additionally, crosslinked starch may be induced by the addition of organic/inor — ganic esters, hydroxyethers, aldehydes and irradiation. Kulicke et al. examined solu­tion phase crosslinking of starch with epichlorohydrin and trisodium trimetaphos- phate.54 Jane et al. examined the crosslinking of starch/zein cast films for improving water resistance.55 Iman et al. studied the crosslinking of starch/jute composite with glutaraldehyde to improve its performance characteristics such as mechanical properties, thermal properties, flame retardancy, etc.56 The possibility of chemically combining starch or starch-derived products with commercial resins in such a man­ner that the starch would serve as both filler and a crosslinking agent may provide a possible approach for incorporating starch into plastics.

Commercial starch polymer based products are provided in Table 17.1 given below:

TABLE 17.1 Starch Polymer Based Products and Suppliers.42,57

Base

Source

Advantages

Disadvan-

Potential

Manufacturer

Poly-

Type

tages

Applications

(Product name)

mer

Starch

Renewable

Low cost, Fast

Poor

Foams, Films

Novament (Mater-

biodegradation

mechanical

and bags,

biTM), Biotec (Bio-

properties,

Molded

plast®, Bioflex®,

Hydrophi-

items,

Biopur®), National

licity

Starch-based

Starch (ECO-

composite

FOAM), Buna Sow Leuna (Sconacell), Starch Tech (ST1, ST2, ST3), Novon (Poly NOVON®)

One of the first starch-based products was developed probably by the National Starch in the brand name ECO-FOAMTM and used as packaging material. ECO — FOAMTM materials are derived from maize or tapioca starch and include modified starches. This relatively short-term, protected-environment packaging use is ideal for thermoplastic starch polymers. National Starch now has additional thermoplas­tic starch materials, blends and specialty hydrophobic thermoplastic starches for a range of applications including injection molded toys, extruded sheet and blown film applications [http://www. ecofoam. com/loosefill. asp]. Novament has been de­veloping thermoplastic starch based polymers since 1990. Mater-BiTM polymers are based on starch-blend technologies and product applications include biodegradable mulch films and bags, thermoformed packaging products, injection molded items, personal hygiene products and packaging foam [http://www. novament. com]. Simi­larly, Biotech GmbH produces Bioplast™ based on starch for a wide range of ap­plications including accessories for flower arrangements, bags, boxes, cups, cutlery, edge protectors, golf tees, horticultural films, mantling for candles, nets, packag­ing films, packaging materials for mailing, planters, planting pots, sacks, shopping bags, straws, strings, tableware, tapes, technical films, trays and wrap films [http:// www. biotech. de/engl/index_engl. htm]. Recently, Plantic Technologies Ltd. pro­duced soluble Plantic™ thermoformed trays for confectionery packaging.42

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