This invention relates to the formation of pellets from metal soap powders. Metal soaps, such as calcium stearate, aluminum stearate, zinc stearate, and lead stearate are useful industrial chemicals. For instance, these metal soaps are used in the plastics industry as processing aids to assist in the manufacture of plastics and plastic products.
One disadvantage of such metal soaps is that they are typically manufactured in ways that result in a finely divided powder which is easily suspended in the air. Since many of these materials can form explosive mixtures with air, and since many are toxic upon inhalation, it is necessary to control the dust generated by such powders.
One way of limiting the dust of metal soaps is to convert the soap into a paste by the addition of an inert solvent or diluent. However, pastes are difficult to work with and cannot be measured out accurately in small quantities. They also are difficult to blend uniformly in the resin because of their cohesive tendency and their slow flow rate, even under the pressure and stress of mixing.
U.S. Pat. No. 4,235,794 (Rieber; Chemische Werke Munchen Otto Barlocher; 1980) teaches the manufacture of metal soaps directly into granulates without going through a powder stage. This is done by reacting (1) an aqueous solution or suspension of a metal oxide, metal hydroxide, or metal carbonate with (2) a water-insoluble C.sub.8 to C.sub.32 fatty acid having a melting point of 0.degree. C. to 100.degree. C., under constant agitation at a temperature below the melting point of the acid until a structurally stable crust of metal soap has formed, and thereafter continuing the reaction at a temperature above the melting point of the acid. While such a process may produce relatively non-dusting granules, the process unfortunately requires investment in specialized mixing, filtration, and drying equipment, and may require specially trained operators. Furthermore, because of the frangible nature of the granules produced by this method, the drying process itself may cause the generation of fines and dust, creating a considerable explosion hazard.
U.S. Pat. No. 4,368,139 (Pollock; Argus Chemical; 1983) discloses absorbing a liquid epoxy fatty acid ester onto the metal soap to render it non-dusting, while maintaining its flowability. The powder of this method, however, lacks the handling advantages of pellets, such as the ability to use the material in a conventional screw feeder. Moreover, the presence of a liquid epoxy fatty acid ester renders the pellets produced by this method unsuitable many applications.
U.S. Pat. No. 5,028,486 (Dunski; Mallinckrodt Specialty Chemicals Company; 1991) teaches a tableting method in which a metal soap is mixed with a binder such as mineral oil, low melting wax, fats, fatty acids, and silicone oil, and compressing the mixture into pellets. While this method does indeed produce stable pellets, it is unsuitable for end use applications in which the presence of the binder is unacceptable.
It would be desirable to produce metal soap pellets by compression of the metal soap without the use of a binder. However, in the absence of a binder, metal soaps will not compress to form stable pellets. Such pellets, if formed at all, are extremely friable and cause considerable dusting.