Rubber products are made for countless applications, using processes similar to those described for tyre manufacturing. Non-tyre products, however, use a much greater variety of polymers and chemicals to give them the properties they need (see table 1). Compounds are carefully designed to reduce hazards such as dermatitis and nitrosamines in the factory and in products like surgical supplies, respirators and baby bottle nipples that are used in contact with the body. Often processing equipment is on a smaller scale than in tyre making, with more use of mill mixing. Roofing and landfill membranes are made on the largest calenders in the world. Some companies specialize in compounding rubber to the specifications of others who process it into many different kinds of products.
Reinforced products such as drive belts, air brake diaphragms and footwear are built up from calendered rubber, coated fabric or cord on a revolving drum or stationary form. Curing is usually by compression moulding to fix the final shape, sometimes using steam pressure and a bladder or airbag as with a tyre. More synthetic polymers are used in non-tyre products. They are not as sticky as natural rubber, so more solvent is used to clean and make the built-up layers tacky. Milling, calendering and solvents or adhesives are bypassed in some cases by going directly from the mixer to a cross-head extruder to build the product.
Non-reinforced products are formed and cured by transfer or injection moulding, extruded and cured in a hot air oven or formed in a compression mould from a pre-cut slug. Sponge rubber is made by agents in the compound that release gas when heated.
Rubber hose is built by braiding, knitting or spinning reinforcing cord or wire onto an extruded tube supported by air pressure or a solid mandrel, then extruding a cover tube over it. An extruded lead cover or nylon cross-wrap is then put on the hose for compression moulding and removed after curing, or else the hose is put into the pressurized steam vulcanizer bare. Nylon cross-wrap or extruded plastic are increasingly replacing the lead. Automotive curved hose is cut and pushed onto shaped mandrels for curing; in some cases robots are taking over this strenuous manual labour. A process also exists that uses chopped fibre for reinforcement and a movable die in the extruder to shape the hose.
Cements mixed from rubber and solvent are used to coat fabric for a host of products. Toluene, ethyl acetate and cyclohexane are common solvents. Fabric is dipped in thin cement, or rubber can be built up in increments of a few micrometres by applying thicker cement under a knife-edge over a roller. Curing is done on a continuous rotational vulcanizer or in an explosion-protected hot-air oven. Latex processes are being developed for coated fabrics to replace the cements.
Rubber cements are also commonly used as adhesives. Hexane, heptane, naphtha and 1,1,1-trichloroethane are common solvents for these products, but hexane is being replaced because of toxicity.
Latex is a typically very alkaline suspension of natural or synthetic rubber in water. Forms for gloves and balloons are dipped, or the latex compound can be foamed for carpet backing, extruded into an acetic acid coagulant solution and washed to produce thread, or spread on fabric. The product is dried and cured in an oven. Natural rubber latex is widely used in medical gloves and devices. Gloves are powdered with cornstarch, or treated in a chlorine solution to de-tackify the surface. Powder-free gloves are reportedly subject to spontaneous combustion when stored in large quantity in a hot area.
Hazards and Precautions
Rubber processing hazards include exposure to hot surfaces, pressurized steam, solvents, processing aids, curing fumes and noise. Dusting agents include stearates, talc, mica and cornstarch. The organic dusts are explosive. Finishing adds a variety of hazards such as punching, cutting, grinding, printing ink solvents and alkaline or acidic surface treatment washes.
Microwave, electron beam and ultrasonic vulcanization are being developed to generate heat within the rubber instead of transferring it inefficiently from outside to inside. The industry is working hard to eliminate or find safer substitutes for lead, dusting agents and volatile organic solvents and to improve compounds for better and safer properties in processing and use.