Polyester resin is ubiquitous in modern life. It is used to make many items, including wall and ceiling panels, car engine covers and circuit breakers for electrical appliances. Cobalt-based products are added to the polyester resin to ensure proper curing or hardening to a solid state.

Polyester Resin

The building blocks of polyester resin are called monomers. The chemical structures of these monomers are different, depending on the type of resin needed. The monomers form polymer chains. A reactive diluent then bonds the polymer chains. This bonding, or cross linking, process is known as free radical copolymerization. A resin manufacturer can make many types of polyester resins to suit the many different uses.

Curing the Resin

During the curing process, the reactive diluent, which is usually styrene, begins the cross-linking process that bonds the polymer chains until the resin forms a gel and then hardens. Cross-link density determines the strength of the finished product. An initiator, which is usually a peroxide, begins the process by decomposing into highly reactive molecule fractions that start a chain reaction. Accelerators activate the initiator, promoting the decomposition of the initiator as the resin cures at room temperature. The accelerators, or promoters, are almost always a form of cobalt metal salt (cobalt naphthenate, cobalt octoate, or cobalt neodecanoate). Usually the cobalt accelerator is added at about 0.01 phr (parts per hundred resin), although the amount varies depending on the finished product. But small amounts of cobalt can make the difference between a strong, usable finished product and one that is cracked or flawed.


For thousands of years people have used cobalt to make a blue dye. However it wasn’t until 1735 that a Swedish chemist isolated and named the metal. At the time of publication, 39 percent of the cobalt in use comes from Africa — especially the Democratic Republic of Congo and Zambia — where it is a byproduct of copper mining. Cobalt is also a byproduct in the mining of nickel, silver, lead and iron ores, and it is found in meteorites. Cobalt has many uses today. For example, it enhances the performance of rechargeable batteries. It also enhances the high-temperature strength of alloys used in just engines.

Alternatives to Cobalt Accelerators

While the accelerators used in polyester resin manufacture are almost always cobalt products, a company in the Netherlands is developing alternatives that use no cobalt or a smaller amount of cobalt. The company, AkzoNobel, launched the first of its alternative accelerators late in 2010 and plans to launch additional cobalt-free alternatives in 2011.