Rubber in Mold

Mulden-blei, n, pig lead, -heizung,/. (Rubber) cure in molds. 

To date, with the exception of vehicle tires, TPEs have been replacing TS rubbers in virtually all applications. Unlike natural TS rubbers, most TPEs can be reground and reused, thereby reducing overall cost. There are types where the need to vulcanize them is eliminated, reducing cycle times, and products can be molded to tighter tolerances. Most TPEs can be colored, whereas natural rubber is available only in black. TPEs also weigh 10 to 40% less than natural rubber (166). 

Addition of scrap rubber in the form of either ground waste vulcanizates or reclaim in rubber compounds gives economic as well as processing advantages. In addition to lowering the cost of rubber compounds, the use of cross-linked rubber particles has beneficial effects such as faster extrusion rate, reduced die swell, and better molding characteristics. 

When a thermoplastic polyurethane elastomer is heated above the melting point of its hard blocks, the chains can flow and the polymer can be molded to a new shape. When the polymer cools, new hard blocks form, recreating the physical crosslinks. We take advantage of these properties to mold elastomeric items that do not need to be cured like conventional rubbers. Scrap moldings, sprues, etc. can be recycled directly back to the extruder, which increases the efficiency of this process. In contrast, chemically crosslinked elastomers, which are thermosetting polymers, cannot be reprocessed after they have been cured. 

Most polystyrene products are not homopolystyrene since the latter is relatively brittle with low impact and solvent resistance (Secs. 3-14b, 6-la). Various combinations of copolymerization and blending are used to improve the properties of polystyrene [Moore, 1989]. Copolymerization of styrene with 1,3-butadiene imparts sufficient flexibility to yield elastomeric products [styrene-1,3-butadiene rubbers (SBR)]. Most SBR rubbers (trade names Buna, GR-S, Philprene) are about 25% styrene-75% 1,3-butadiene copolymer produced by emulsion polymerization some are produced by anionic polymerization. About 2 billion pounds per year are produced in the United States. SBR is similar to natural rubber in tensile strength, has somewhat better ozone resistance and weatherability but has poorer resilience and greater heat buildup. SBR can be blended with oil (referred to as oil-extended SBR) to lower raw material costs without excessive loss of physical properties. SBR is also blended with other polymers to combine properties. The major use for SBR is in tires. Other uses include belting, hose, molded and extruded goods, flooring, shoe soles, coated fabrics, and electrical insulation. 

Crumb Rubber in Rubber and Plastic Products. Crumb rubber may be incorporated into rubber sheet and molded products such as floor mats, vehicle mud guards, and carpet padding or into plastic products, including plastic floor mats and adhesives. Additional uses that have contributed to the expansion of this market over the last three years are rubber play surfaces, tracks and athletic surfaces, and garbage cans. In 1987 about 2.3 million tires (1 percent) were utilized in this manner. 1990 estimates have risen to 8.6 million tires per year, or 3 percent of the scrap tires generated that year. 

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