Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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polyethylene ( polythene )

A waxy, translucent, somewhat flexible thermoplastic, prepared by polymerizing ethylene at high pressure (1,000 to 4,000 atm) and high temperature (180 to 190° C.) in the presence of a trace of oxygen. It is one of the lightest of the plastics, having a specific gravity of 0.92 to 0.93. Below 60° C., polyethylene is insoluble in all solvents and is resistant to the action o f most reagents, other than strong oxidizing acids. Above 115° C., the polymer changes from a clear solid to a relatively low-viscosity melt. At this temperature and above, exposure to air causes relatively extensive oxidative degradation, unless antioxidants are included with the polymer.

Polyethylene is widely used as a film by itself or as a hot extrusion onto paper to provide additional strength and moisture-resistant characteristics. It is also applied to printing papers to provide finish and strength. The material is also made in sheets for use as a facing to prevent materials from sticking to a surface in operations requiring the application of pressure. The film which does not adhere permanently to waxes and many plastics in the unhardened state, is easily peeled off when the operation is completed. In sheet form, it is used in conservation work, in lieu of cellulose acetate lamination, to protect brittle paper, in which case the paper is placed between two sheets of the film, which is then sealed with double-sided adhesive tape around the edges. It may also be sealed by means of plastic welding. (81 , 233 )




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