Fighting flames

PVC could be on the verge of becoming more fire retardant and environmentally friendly, thanks to the work of researchers at the College of William and Mary in Williamsburg, Virginia.

One of the most widely used plastics in the world – PVC – could be on the verge of becoming more fire retardant and environmentally friendly, thanks to the work of researchers at the College of William and Mary in Williamsburg, Virginia.

Polymer chemists developed additives to make PVC (poly(vinyl chloride)) more resistant to fire shortly after its commercialisation in the 1930s, but some of these additives contain heavy metals that can be toxic to humans and the environment.

Organic chemist William H. Starnes, Ph.D., of the College of William and Mary recently described new, more benign additives at the 228th national meeting of the American Chemical Society.

Starnes and fellow chemistry professor Robert D. Pike, Ph.D., have replaced heavy metals the industry currently uses, such as molybdenum, with copper, which is essentially non-toxic. The new additives appear to work in part like many traditional fire retardants: In the presence of heat, they create chemical crosslinks within the polymer to form an inert char on its surface that helps block flammable gases inside from escaping.

The Starnes-Pike additives appear to improve the way fire retardants make the char, however. Current technology uses electron-seeking compounds called Lewis acids, which in very intense temperatures can start attacking the crosslinks of char, defeating its own mechanism to reduce smoke and fire. The new additives rely instead on copper atoms to stitch together the crosslinks.

Test conducted within the PVC industry “suggest our additives are actually better,” says Starnes. “That’s why we’re so excited about them.”

The hurdle likely to remain is colour. “Cuprous copper [Cu(1+)] tends to actually give us white, which is desirable,” noted Starnes. “The problem, though, is that it oxidises fairly easily to cupric copper [Cu(2+)], which carries an aqua colour” that can tint the final product.

The bottom line for industry is cost. The synthesis of the copper-based fire retardants and smoke suppressors is quite easy, says Starnes. “If we can solve the colour problem, the economics would be favourable, at least in a preliminary way.”

These more benign additives would not be Starnes’s first. His group is also developing alternatives to make PVC less vulnerable to heat, and which are even closer to practical use in industry.

“PVC intrinsically is somewhat unstable. In the presence of heat, it will fairly easily lose hydrogen chloride and gradually turn yellow and, eventually, black,” he explained. “For at least 50 years chemists have tried to develop organic heat stabilisers to replace those containing heavy metals. And we’re now to the point where some companies are interested in licensing ours.”