Processing Rubber
Rubber, whether natural or synthetic, arrives at processor (fabricator) plants in large bales. Once the rubber arrives at the factory, processing goes through four steps: compounding, mixing, shaping and vulcanizing. The rubber compounding formulation and method depends on the intended outcome of the rubber fabrication process.
Compounding
Compounding adds chemicals and other additives to customize the rubber for the intended use. Natural rubber changes with temperature, becoming brittle with cold and a sticky, gooey mess with heat. Chemicals added during compounding react with the rubber during the vulcanizing process to stabilize the rubber polymers. Additional additives may include reinforcing fillers to enhance the properties of the rubber or non-reinforcing fillers to extend the rubber, which reduces the cost. The kind of filler used depends on the final product.
The most commonly used reinforcing filler is carbon black, derived from soot. Carbon black increases rubber's tensile strength and resistance to abrasion and tearing. Carbon black also improves rubber's resistance to ultraviolet degradation. Most rubber products are black because of the carbon black filler.
Depending on the planned use of the rubber, other additives used could include anhydrous aluminum silicates as reinforcing fillers, other polymers, recycled rubber (usually less than 10 percent), fatigue-reducing compounds, antioxidants, ozone-resisting chemicals, coloring pigments, plasticizers, softening oils and mold-release compounds.
Mixing
The additives must be thoroughly mixed into the rubber. The high viscosity (resistance to flow) of the rubber makes mixing difficult to accomplish without raising the temperature of the rubber high enough (up to 300 degrees Fahrenheit) to cause vulcanization. To prevent premature vulcanization, the mixing usually takes place in two stages. During the first stage, additives like carbon black are mixed into the rubber. This mixture is referred to as a masterbatch. Once the rubber has cooled, the chemicals for vulcanization are added and mixed into the rubber.
Shaping
Shaping rubber products occurs using four general techniques: extrusion, calendering, coating or molding, and casting. More than one shaping technique may be used, depending on the final product.
Extrusion consists of forcing highly plastic rubber through a series of screw extruders. Calendering passes the rubber through a series of increasingly smaller gaps between rollers. The roller-die process combines extrusion and calendering, producing a better product than either individual process.
Coating uses the calendering process to apply a coat of rubber or to force rubber into fabric or other material. Tires, waterproof cloth tents and raincoats, conveyor belts as well as inflatable rafts are made by coating materials with rubber.
Rubber products like shoe soles and heels, gaskets, seals, suction cups and bottle stops are cast using molds. Molding is also a step in making tires. The three primary methods of molding rubber are compression molding (used in making tires among other products), transfer molding and injection molding. Vulcanization of the rubber occurs during the molding process rather than as a separate step.
Vulcanization
Vulcanization completes the rubber-production process. Vulcanization creates the cross-connections between the polymers of rubber, and the process varies depending on the requirements of the final rubber product. Fewer cross-connections between the rubber polymers creates a softer, more pliable rubber. Increasing the number of cross-connections decreases the elasticity of the rubber, resulting in harder rubber. Without vulcanization, rubber would remain sticky when hot and brittle when cold, and it would rot much more quickly.
Vulcanization, originally discovered in 1839 by Charles Goodyear, required adding sulfur to rubber and heating the mixture to 280 F for about five hours. Modern vulcanization, in general, uses smaller amounts of sulfur combined with other chemicals to reduce the heating time to 15 to 20 minutes. Alternate vulcanization techniques have been developed that don't use sulfur.
