Plastics are characterized by high strength-to-density ratios, excellent thermal and electrical insulation properties, and good resistance to acids, alkalis, and solvents. The giant molecules of which they consist may be linear, branched, or cross-linked, depending on the plastic. Linear and branched molecules are thermoplastic (soften when heated), whereas cross-linked molecules are thermosetting (harden when heated).
The development of plastics began about 1860, after Phelan and Collander, a United States firm manufacturing billiard and pool balls, offered a prize of $10,000 for a satisfactory substitute for natural ivory. One of those who tried to win this prize was U.S. inventor John Wesley Hyatt. Hyatt developed a method of pressure-working pyroxylin, a cellulose nitrate of low nitration that had been plasticized with camphor and a minimum of alcohol solvent. Although Hyatt did not win the prize, his product, patented under the trademark Celluloid, was used in the manufacture of objects ranging from dental plates to men's collars. Despite its flammability and liability to deterioration when exposed to light, Celluloid achieved a notable commercial success.
Other plastics were introduced gradually over the next few decades. Among them were the first totally synthetic plastics: the family of phenol-formaldehyde resins developed by the Belgian-American chemist Leo Hendrik Baekeland about 1906 and sold under the trademark Bakelite. Other plastics introduced during this period include modified natural polymers such as rayon, made from cellulose products.