2 августа, 2015 
Malyar Like cellulose, starch is a naturally occurring polymer of glucose. It differs from cellulose in two significant aspects: the glucose rings are in the a-D configuration rather than the (3-d configuration, and starch can be differentiated into two types of polymers. One polymer, amylose consists of a-D-anhydroglucopyranose monomeric units combined linearly through 1-4 linkages with little or no branching. The other polymer, amylopectin, is linked through 1-4 linkages but also has branches that form at the primary alcohol group on C-6 (Fig. 3). Careful analyses of various starches have shown that there is also an intermediate fraction that is thought to be an infrequently branched amylopectin [6].
The amount of amylose and amylopectin in a starch depends on the source of the starch. Most starches contain 20 to 30% by weight of amylose, although certain hybrids can contain more than 80% amylose. The most commonly available industrial starches are waxy cornstarch, regular cornstarch, high-amylose cornstarch type V, and high-amylose cornstarch type VII, with amylose concentrations of 0, 28, 55, and 70%, respectively [7].
Starch alone suspended in cold water is essentially unable to act as an adhesive because the starch is so tightly bound in granules. The granules consist of crystalline regions where straight-chain molecules and straight sections of branched molecules are aligned. The crystallite regions are linked together by more amorphous areas where the molecules are not aligned. Within the starch granule, the molecules and crystallites are
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Figure 3 Molecular structures of a-D-glucose and the two major molecules that make up starch, amylose, and amylopectin. |
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Figure 4 Outer surface of starch granule showing radial arrangement of crystallites. Crystallites are shown as thicker lines. [Adapted from K. H. Meyer, Adv. Colloid Sci. 1: 168-169 (1942).] |
arranged radially in concentric layers (Fig. 4). It is these granules that must be opened to obtain adhesive bonding.
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