rubberlike products made from aqueous dispersions of natural rubbers, or latices.
The industrial process for the manufacture of latex products includes the following steps: (1) preparation of the latex mixture, which contains (in addition to the usual ingredients of a rubber stock) surface-active materials (used to facilitate the dispersion of the ingredients in the latex and to impart storage stability to the mixture), thickeners, antiseptics, and antifoaming agents; (2) production of the semifinished latex product, or gel; and (3) contraction (syneresis) of the gel, drying, and vulcanization of the latex product.
Most methods for the production of gels are based on the interaction of latex globules with astabilizing additives, such as electrolytes, which lower the stability of the colloidal system and thereby accelerate the coagulation of the latex. In the production of dipped latex products, the gel may be produced by immersing the mold, or former, which has the shape of the product, first in an electrolyte solution (for example, CaCl2) and then in the latex mixture (ionic deposition). Another method involves immersing the former, preheated to 60°-100°C, in a latex mixture containing an additive that astabilizes the system at elevated temperatures (for example, polyvinyl methyl ether); this process is called thermal sensitization. A third method involves the multiple immersion of the former in the latex mixture, drying each layer of gel between immersions (multiple dip).
The ionic deposition and thermal sensitization methods show high productivity. For example, a semifinished product with a wall thickness of about 1 mm may be produced in several minutes by the first method and in less than 1 min by the second. Dipped latex products include radiosonde and weather balloon cover envelopes, surgical and household gloves, sanitary products, toys, and football bladders. The wall thickness of these products as a rule does not exceed 2–3 mm.
Products with irregular shapes and thick walls, such as the working elements, or fingers, of tea harvesters and light sponge-rubber, or foam rubber, articles (for example, automobile seats), are produced by a method based on gelation of the latex mixture. A mixture containing a gelating agent, such as sodium silicofluoride, charged into a mold; this quickly astabilizes the entire system (for foam rubber, the mixture is foamed before gelation). Certain products, such as latex threads, are produced by extruding the latex mixture through spinnerets into baths filled with an astabilizing agent, such as acetic acid solution.
Syneresis, which is required not only to contract the gel (that is, for “squeezing out” the aqueous phase, or serum) but also to accelerate the subsequent drying of the latex product, is performed in water at room temperature. The latex products are dried in air chambers for as long as 15 hours at 40°-80°C. The drying time may be significantly reduced by combined heating with industrial-frequency currents and infrared radiation. The products are usually vulcanized in hot-air ovens at 100°-140°C.
The possibility for the great variety of latex products has to do with the relatively simple production technology, the high productivity of the process, and the possibility of mechanizing and automating all stages of the process. In addition to articles that can be produced only from latices, such as foam rubber and thin-walled seamless meteorological casings, there are articles that can be produced either from solutions of solid natural rubbers (rubber adhesives) or from latices (gloves and threads). Latex technology precludes the use of toxic and flammable solvents.
V. V. CHERNAIA and M. I. SHEPELEV