| 摘要 |
A thixotropic plaster mix is produced by mixing the following:- (i) burnt gypsum, e.g. between 3 and 40%; (ii) one or more other powdered inorganic materials such as quartz sand, e.g. between 15 and 50% of graded sand having a grain size between 0,01 and 1 mm, crushed marble, e.g. 5-12% (smallest particles at least 0,1 mm larger than the largest particles in the remainder), chalk, e.g. between 5 and 30% precipitated chalk, and/or pumice; (iii) a binder in the form of a water-soluble cellulose derivative such as methyl and/or ethyl cellulose or cellulose glycolate; and (iv) water; with or without dextrin, casein or other vegetable or animal glue, mechanical or chemical cellulose pulp e.g. between 0,1 and 1%, emulsions (water-in-oil or oil-in-water type) of alkali-resistant binders such as of acrylate, alkyds, chlorinated rubber, P.V.A., P.V.C., or co-polymers thereof e.g. between 0,2 and 7% of anhydrous components, bentonite, silicic acid, fuller's earth, kaolin, borax and/or boracic acid. The plaster of paris may be effectively mixed with an aqueous solution of retarders, such as the above glues and/or borax with or without methanol, prior to its inclusion with the other ingredients; besides the plaster of paris, ground gypsum and/or anhydrite may be present. The above mixture is mechanically treated and kneaded for such a period that it has lost its ability to set when stored so that water is prevented from evaporating, e.g. by packing in containers. |
