Back in 1963, the heading "elastomers" described basically one type of product for joint sealants: a two component polysulfide system or some mixture of epoxies with polysulfide used primarily for vertical joints in buildings. These products start out as one or two liquids and, after mixing, cure by chemical reaction rather than by cooling or by the evaporation of a solvent. The polysulfide elastomerics are made form polysulfide polymers which cure by a linking of the molecules from end to end. The polysulfide polymer stats out as a long molecule. When many of these molecules are linked together by means of the curing agent, they become a rubber which has great elastic-like qualities. A rod of this rubber, half an inch in diameter, can be stretched some eight times its original length. And while polysulfides have been augmented with other exotic sealants, they still are capable of greater elasticity than these other products. However, since field workmanship cannot possibly meet laboratory standards, the shape of the joint and its design must be make to limit movement to a maximum of 25 percent extension and 25 percent compression. In 1964, the silicone sealants were introduced to the construction market. Silicones cure by depending upon contact with moisture in the air. They then produce- within 48 to 54 hours- a flexible rubber sealant. Among other silicone characteristics are: very low shrinkage; lack of toxicity; the ability to withstand service temperatures ranging from 200 degrees to over 500 degrees; good strength and excellent resistance to the effects of sunlight, ozone, and ultra-violet light. The latest arrival of the joint sealant scene is the polyurethane sealant. These have seen increasing popularity over the last two or three years and have many excellent characteristics to recommend them. The polyurethane sealants are generally furnished as a two part product but they are also available in one part packaging.