GB779835A - Filter bodies - Google Patents

Abstract

Particles of an organic substitution cellulose derivative, e.g cellulose acetate, are moistened in an enclosed space with a subsequently evaporated solvent so as to be formed into a porous coherent filter body for liquids which do not soften or dissolve the cellulose derivative. Cellulose acetate-propionate, cellulose butyrate, ethylcellulose or a cellulose ester of a paraffinic monocarboxylic acid containing two to four carbon atoms may be used as the organic substitution cellulose derivative, with which activated charcoal or silica gel may be mixed. The filter bodies may have densities of from 0,05 to 0,8 gram per cubic centimetre. Cellulose acetate used may have an acetyl value (expressed as acetic acid) between 51 per cent and 62,5 per cent and be plasticized with triacetin. The particles are preferably substantially uniform in size and shape, e.g. approximately spherical, but irregularly shaped particles formed by grinding cellulose acetate flake and screening it for size may be used; or spun or extruded cellulose acetate fibres, which may be crimped, may be cut up, e.g. to less than four millimetres in length, and/or dry or wet ground or may be beaten to pulp and dried. In making suitable cellulose acetate, precipitation should be carried out slowly, and methods to this end are described. Any solvents may be used in the main process which are capable of dissolving the particle surfaces at the maximum temperature reached, and they may contain up to 30 per cent of water; examples are dioxane, chloroform, methylene chloride, with or without ethanol or methanol, and acetone with or without ethanol. According to the first Provisional Specification, filter bodies may be formed merely by heating such cellulose acetate particles until melting just commences, e.g. by allowing them to fall through an upward air current in a heated space into a heated tray until they settle to a depth slightly exceeding the length of a cigarette tip, the resulting porous block having cylindrical blanks cut therefrom by a heated hollow punch device; reference is also made to blowing particles by hot air into heated split moulds and to intermittent or continuous extrusion of blanks when using solvents, including some not referred to above, to moisten the particles and, if necessary, produce porosity by sudden evaporation or evolution of gas.