Process for making dry microspheres
AssigneePierce & Stevens Corporation
InventorsGeorge E. Melber, Leon E. Wolinski, William A. Oswald
Life of this patent
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Abstract
Microsphere wet cake is mixed with a surface barrier coating effective to prevent agglomeration and surface bonding of the microspheres, and removing water by drying with continuous mixing at high shear. In a subsequent step, by the control of the application of heat and balancing temperature and low shear mixing, it is possible to also control expansion of the microspheres. The surface barrier coating in the present invention is any one of a wide diversity of materials which meet the requirements of the intended function, i.e., to prevent the agglomeration of the microspheres during the process. Suitable materials include, by way of example, dry inorganic pigments or filler materials, and the like, and related organic materials.
Claims7
The method of making free-flowing dry expanded microspheres with a coating of adherent surface barrier coating, said microspheres being substantially free of agglomerates, comprising the steps of:
- A. first mixing and drying an admixture of expandable thermoplastic microsphere wet cake and a surface barrier coating material for a time and at a temperature less than sufficient to expand said microspheres and sufficient to reduce the water content of said admixture to less than about one weight percent, said mixing at shear sufficient to disrupt substantially all microsphere agglomerates in said wet cake, said surface barrier coating material is a free-flowing particulate or fibrous solid having a softening or melting point at a temperature above that of said thermoplastic;
- B. thereafter expanding said microspheres by heating the admixture for a time and temperature sufficient to cause expansion thereof and to thermally bond said surface barrier coating to the surfaces thereof;
- and
- C. collecting free-flowing dry microspheres with an adherent coating of said surface barrier coating and substantially free of agglomerates;
- wherein said mixing is conducted in a mechanically agitated state at a Reynolds number of at least about 1,000.
Dependencies are inferred from the claim text.