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Wednesday, April 7, 2010

Cellulose Ethers

The hydroxyl groups of cellulose may also be etherified. The commonest ethers are the methyl, sodium, carboxyl methyl, ethyl, hydroxyethyl and benzyl. In each case the starting material is alkali cellulose; which is prepared by digesting a fairly purse form of cellulose with concentratrated sodium hydroxide. This, in the manufacture of the alkyl ethers is reached at elevated temperature with the appropriate alkyl (or alkyl) chloride. Hydroxyethyl cellulose is prepared with ethylene oxide, and carboxy methyl cellulose with sodium chloroacetate.

The properties of the cellulose ethers depend on both the molecular weight of the polymer and on the degree of etherification. The mechanical properties of ethyl cellulose are reported to be influenced more by the former; whereas other physical properties such as solubility and water absorption are influenced more by the latter. Thus different grades of cellulose ethers can be prepared, including some that are soluble in organic solvents and some that are soluble in water. Benzyl cellulose at all degrees of substitutions is insoluble in water. Methyl cellulose is soluble in cold water but has an inverse solubility/temperature gradient.

Ethyl and Benzyl, but not methyl cellulose, can be used as hot melt adhesives. Methyl cellulose is a tough mateial. completely non toxic, tasteless and odorless, which makes it valuable adhesive for food packages. The phenomenon of gelling on heating (as well as on cooling) at certain concentrations is a properly of some grades of methyl cellulose, and is specially valuable for adhesive that are required to "set-up" on heating. Methyl cellulose is capable of forming high viscosity solutions at very low concentrations, and it is therefore a widely used thickening agent in water soluble adhesive. Being non ionic, its solutions are stable over a wide pH range. Hydroxyethyl cellulose and carboxyethyl cellulose and sodium carboxymethyl cellulose can also be used as thickeners in water soluble adhesives. All these water soluble ethers are used mainly in gluing paper and textile.

The cellulose ethers have fair to good resistance to dry heat; water resistance varies from excellent for benzyl to poor for methyl cellulose.