In 1917 five companies, all related to the electric arc furnace or acetylene industry, merged to form the Union Carbide and Carbon Corporation, later the Union Carbide Corporation. In 1920, Union Carbide purchased a small refinery at Clendenin and began developing the processes for making ethylene and related chemicals. Ethylene is made from natural gas or petroleum, and the new processes gave birth to the modern petrochemical industry.

In 1925, Union Carbide purchased an obsolete chemical plant in South Charleston to build facilities to produce the olefins ethylene and propylene. Soon chemical units covered the company acreage on the South Charleston mainland and most of the 80-acre Blaine Island in the nearby Kanawha River. Starting in the early 1930s, Union Carbide built similar plants at other locations in the U.S. and Puerto Rico, including units that supplied 100 percent of the world's ethylene.

In 1947, Union Carbide expanded to the Institute plant, west of Charleston, originally built as a part of the government's synthetic rubber program during World War II. In 1949, "Carbide," as it was called locally, built a new Technical Center in South Charleston to house research, development, and engineering facilities on an expansive campus.

The number of chemicals and plastics produced by the company increased from about 80 in 1934 to more than 400 in the early 1960s, the majority developed in South Charleston. Most of the chemicals were used by industrial customers, but some, such as Prestone antifreeze, Eveready batteries, and Sevin and Temik insecticides, were consumer products. Five basic types of plastics were produced, including phenolics (Carbide had bought the Bakelite company), epoxy, polystyrene, vinyls, and polyethylene.

Between 1946 and 1982, revenues increased from about $415 million to more than $10 billion, placing Union Carbide in the top 10 companies in the United States. Worldwide employment was near 80,000, with about 12,000 in West Virginia. The company was West Virginia's largest employer for several years. But, starting in the 1950s, competition in chemicals and plastics increased substantially, while poor economic conditions resulted in decreased prices for most chemicals. Carbide's headquarters were moved from New York City to Danbury, Connecticut, in 1981.

On December 3, 1984, an incident at the Union Carbide insecticide plant in Bhopal, India, released a large cloud of MIC, methyl isocyanate gas, which killed an estimated 3,000 people and injured thousands more. The tragedy led to new concerns about safety at chemical plants, including a Union Carbide methyl isocyanate plant in the Kanawha Valley. In 1989, the Indian government and Union Carbide reached a settlement for $470 million in the incident. Union Carbide dropped a counter-suit which argued that sabotage caused the tragic incident.

Bhopal substantially weakened the company. To avoid a hostile takeover attempt by the GAF Company, Carbide sold almost half its productive businesses. The agricultural business, which included most of the Institute plant, was sold to Rhone-Poulenc. The Consumer Products Division was sold, as was the carbon electrodes plant near Clarksburg. The electro-metallurgical plant at Alloy, whose associated hydroelectric tunnel project had claimed the lives of a multitude of workers during the construction of Hawks Nest tunnel in the early 1930s, had been sold in 1981 to the Elkem Company. In 1992, the Linde Division was split off to form a separate company, Praxair. A silicones products plant built near Sistersville in 1956 was sold in 1994.

Finally on February 6, 2001, the Dow Chemical Company acquired Union Carbide, and the company ceased to exist as a separate corporation. The Union Carbide Foundation is a contributor to The West Virginia Encyclopedia.