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Tuesday, March 30, 2010

Cellulose Ester Adhesive

Cellulose, the woody structure of plants, is a high molecular weight polymer. Although it is normally a chemically stable substance, it is capable of reaction through its hydroxyl groups to form colorless amorphous substances, usually having lower molecular weights and lacking the high tensile strength of the parent polymer but assuming other properties, for example solubility.

Historically, the first thermoplastic synthetic adhesive (only partly synthetic was cellulose ester, cellulose nitrate, often called nitrocellulose, and it is still one of the most important. Later, other esters such as the acetate were developed, as well as certain mixed esters.

Cellulose esters are made, preferebly from a pure forms of cellulose such as cotton linters, and the hydroxyl groups of cellulose. In the case of cellulose nitrate, the nitration is allowed to proceed only for enough to give on the average the dinitrate, since the trinitrate is explosive. In the case of cellulose acetate, complete acetylation takes place to form the tri-acetate, but for adhesive and plastic applications this is partially hydrolised to give a product with fewer than three acetate groups per cellulose unit; such a product is soluble in acetone, whereas the tri-acetate is not.

Cellulose esters of butyric and propionic acids have limited adhesive use; more interesting are the mixed esters such as the acetate/butyrate and acetate propionate, whic are in many ways superiors to the straight acetate in having for instance lower water absorption and greater flexibility. And cellulose caprate, having a refractive index near to that of glass and good resistance to photochemical change, is a useful hot melt optical cement.