Hydroxypropylcelluloses

Hydroxypropylcelluloses (HP) (2,4-6) are cellulose ethers prepared by reacting cellulose with propylene oxide. The products are characterized in terms of moles of substitution (MS) rather than DS (see "Carboxymethylcelluloses"). MS is used because the reaction of a propylene oxide molecule with cellulose leads to the formation of a new hy-droxyl group with which another alkylene oxide molecule can react to form an oligomeric side chain. Therefore, there is no limit to the moles of substituent that can be added to each d-glucopyranosyl unit. MS denotes the average number of moles of alkylene oxide that has reacted per d-glucopyranosyl unit (Fig. 4.)

In general, the MS controls the solubility of a hydrox-ypropylcellulose. Commercially available hydroxypropylcelluloses are insoluble in hot water, soluble in cold water, and compatible with several oils. HP is produced in a wide range of viscosity grades and forms clear, smooth, uniform solutions with pseudoplastic rheology similar to that of other cellulosics (see "Carboxymethylcelluloses").

Because they are nonionic gums, hydroxypropylcelluloses are unaffected by pH. Because of their ability to reduce surface and interfacial tension, they are used as emulsifying agents, emulsion stabilizers, and whipping aids. Flexible, nontacky, heat-sealable packaging films and sheets can be produced from hydroxypropylcelluloses by conventional extrusion techniques. Because hydroxypropylcelluloses form strong, edible films that provide a barrier to oxygen and water vapor, they are used to coat nuts and confections.

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