BURNING OF POLYMERS

Rods of 'low-density' polyethylene, 'high-density' polyethylene and polymethylmethacrylate were burned in a candle-like manner in air. Directly above the melt there was a thin non-luminous gap, and the flame consisted of a roughly cylindrical blue zone on top of which was a sooty yellow cone. The surface temperature was about 400°C and the maximum flame temperature about 700°C. At the base of the flames, at least 60 per cent of the gas mixture was nitrogen. Very little oxygen was detected and oxides of carbon and water were present in greater amounts than polymer degradation products (lower hydrocarbons, methyleethacrylate). The latter and carbon monoxide decreased in concentration up through the flame, while the amounts of carbon dioxide and water increased. Normally, the concentrations of nitrogen, oxygen, carbon dioxide and water increased towards the flame edges while those of the hydrocarbons decreased. Increasing the concentration of oxygen in the ambient atmosphere increased the surface temperature, but gain very little oxygen was present in the flame gases. Replacing nitrogen by helium gave a less stable flame. The burning of n-eicosane (C20H24) in air was also investigated, the results being very similar to those with the polymers. It is suggested that heat is conducted from the flame to the polymer (or n-eicosane) surface at which thermal degradation, oxidative degradation and complete oxidation occur as the ambient atmosphere is drawn over it. In the highly dilute flame the fuels undergo further oxidative cracking and some oxidation to give carbon dioxide, water and soot. © 1969.