Hard rubber is a plastic. It is a resinous material mixed with a polymerizing or curing agent and fillers, and can be formed under heat and pressure to practically any desired shape.
The bulk of today’s hard rubber is made with SBR synthetic rubber. Other types of synthetic rubbers, such as butyl or nitrite or, in rare cases, silicone or polyacrylic, can also be used.
Once it has gone through the process of heat and pressure, hard rubber cannot be returned to its original state and therefore falls into the class of thermosetting plastics, i.e., those that undergo chemical change under heat and pressure. It differs, however, from other commercial thermosetting plastics such as the phenolics and the ureas in that after it has gone through the thermosetting process it will still soften somewhat under heat. In this characteristic it most resembles the thermoplastic acetates, polystyrenes, and vinyls. It differs from all others in that it is available in pliable sheet form before vulcanization and is therefore adaptable to many shapes for which molds and presses are not necessary. Because of this feature and because it can be softened again after vulcanization, it falls into a class by itself in the field of plastics.
The term hard rubber is self-descriptive. The hardness is measured on the Shore D scale, which is several orders of magnitude higher than the Shore A scale used for conventional rubbers and elastomers. Similar in composition to soft rubber, it contains a much higher percentage of sulfur, up to a saturation point of 47% of the weight of the rubber in the compound. If sulfur is present in rubber compounds in amounts over 18% of the weight of rubber in the compound when the material is completely vulcanized, the product will be generally known as hard rubber.
Properties and Fabrication
The most important properties of hard rubber are the combination of relatively high tensile strength, low elongation, and extremely low water absorption.
Hard rubber may be compression-, transfer-, or injection-molded. In sheet form it can be hand-fabricated into many shapes. Its machining qualities are comparable to brass, and it may be drilled and tapped. The material lends itself readily to permanent or temporary sealing with hot or cold cements and sealing compounds.
The size and shape of a hard rubber part is dependent only upon the size of press equipment and vulcanizers available.
Uses
Perhaps the largest application for hard rubber is in the manufacture of battery boxes. The water-meter industry is also a large user. Hard rubber linings and coatings either molded or hand laid-up account for large amounts of material. In the electrical industry, hard rubber is used for terminal blocks, insulating materials, and connector protectors. The chemical, electroplating, and photographic industries use large quantities of hard rubber for acid-handling devices.
