| 摘要 |
Dry polyvinyl acetate powders preponderantly of particle size less than 5 microns and reconstitutable to the original emulsion are prepared from aqueous polyvinyl acetate emulsions by atomizing and drying at a temperature such that the dried particles are maintained below their sintering temperature. The atomizing is preferably effected by a stream of air of velocity high enough to form globules containing individual particles and the inlet temperature of the air may be from 160 DEG to 250 DEG F. with an outlet temperature of 115 DEG to 125 DEG F. A suitable spray drier is described. Alternatively a freeze-stable emulsion is frozen in less than ten seconds, e.g. at or below -40 DEG F., the water is evaporated under high vacuum while maintaining the frozen state and the resultant cake is powdered. Either process may be carried out continuously instead of batchwise. The original emulsions may contain 50 to 55 per cent solids or may be diluted to 15 to 35 per cent and can be prepared as described in Specifications 466,173 and 641,653. The original emulsions are preferably free from plasticizers (such as dibutyl phthalate, butyl phthalyl butyl glycolate, dibutyl sebacate and tricresyl phosphate) as these cause a reduction in the sintering temperature, but they may be present in or added to the powder or the reconstituted emulsion, along with fillers and pigments. The products are useful as additives to polyvinyl acetate emulsion adhesives containing plasticizers, and in making paints. The powders may be sifted on to a fibrous base to give, on heating, bonded non-woven fabrics. In examples sintering temperatures given are 115 DEG and 160 DEG F. Specification 615,723 also is referred to.
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