In addition to the hydrocolloids, there is another group of elastic impression materials. They are soft and rubber like and far more stronger and stable than the hydrocolloids. They are known as elastomers or synthetic rubbers. The ADA Sp. No. 19 refers to them as nonaqueous elastomeric dental impression materials. Elastomeric materials contain large molecules with weak interaction between them. They are tied together at certain points to form a three-dimensional network. On stretching the chains uncoil, and on removal of the stress they snap back to their relaxed entangled state. They are liquid polymers which can be converted to solid rubber at room temperature. By mixing with a suitable catalyst, they undergo polymerization and/or crosslinking reaction to produce a firm elastic solid. These materials are the most accurate and dimensionally stable impression materials available in dentistry. Synonyms Initially they were called rubber-base or rubber impression materials. Currently, they referred to as elastomeric impression materials.