| http://www.w3.org/ns/prov#value | - Face piece 12 may be made of any of a variety of materials, including flexible materials such as silicone, rubber, or thermoplastic elastomers, such as those commercially available under the trade designations ???Kraton??? from Shell Oil of Houston, Tex., ???Monprene??? from Teknor Apex of Pawtucket, Rhode Island, and ???Santoprene??? from Advanced Elastomer Systems of Akron, Ohio.
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