Solid dispersions starch gelatinization

Figure 4-30 shows the dynamic properties, storage modulus (G, Pa) and loss modulus G", Pa) versus frequency o> (rad s ) of gelatinized cross-linked waxy maize (CLW) starch dispersions of 3, 4, and 5% solids. In all dispersions, G was much higher than G", that is, the dispersions exhibited gel-like behavior over the studied range of frequencies. Both G and G" increased with concentration and showed a weak dependence on frequency at low values. This dependency was more pronounced in dispersions of higher concentration. 

Figure 4-30 The Dynamic Pr< )erties, Storage Modulus (O, Pa) and Loss Modulus (G", Pa), versus Frequency (rad s ) of Gelatinized Cross-Linked Waxy Maize Starch Dispersions of 3,4, and 5% Solids (da Silva et al., 1997).

Any formal definition of a gel has been avoided, because its formulation would be difficult on the basis of external properties. By any criterion, a 1 % aqueous solution of agar does form a gel when it is cooled from 95 to 20°, whereas a 1 % solution of sucrose does not but properties may be so continuous between these extremes that any dividing line would be arbitrary. Most of this Chapter is concerned with fairly permanent network structures formed from polymer solutions. As a definition of gels, this description is incomplete. Some cellulose gels, and pastes of gelatinized, starch granules, would be excluded, because they are formed by limited dispersion of solids. In other words, an arbitrary choice will be made to focus on those gels that are dilute with respect to polymer, because the information available about them at the molecular level is more precise than for others. 

Many glues, especially those commonly used in conservation work, harden primarily by an increase in concentration of the glue s polymer solids as the carrier (water or solvent) is lost into the substrates and surrounding air. Glues based on water-soluble or dispersible polymers (e.g., starch, casein, animal hide gelatin, and cellulose esters and ethers) would be in this category. 

Water-based dispersions or emulsions such as polyvinyl acetate, acrylics, polyvinyl chloride and polyvinyl alcohol with plasticizers and tackifiers. In addition, this range can include urea formaldehyde and phenolic adhesives, resins, natural adhesives produced from starch, dextrin, casein, animal glues (see Polyvinyl alcohol in adhesives, Phenolic adhesives single-stage resoles. Phenolic adhesives two-stage novolacs. Animal glues and technical gelatins) and rubber latex (see Emulsion and dispersion adhesives). Solvent-free 100% solids such as polyurethane. Hot melt adhesives include Ethylene-vinyl acetate copolymers, polyolefins, polyamides, polyesters with tackifiers and waxes. More recent additions include cross-linkable systems. 

Graft polymerization may be carried out with either gelatinized (dispersed) or granular starch however, most work has been done with granular starch, because one can work at higher solids levels without developing high viscosities. 

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