Polyolefin Flame Retardant Cable Materials - Effect of Additives and Copolymer Structure
Fire safety of cables is becoming increasingly important as the use of electrical equipment steadily increases. Due to their excellent dielectric properties, polyolefin materials are widely used as insulating materials for cables. However, polyolefins are inherently flammable and need to be protected to meet the requirements on flame retardancy. The work presented in this thesis covers the study of a halogen-free flame retardant material system based on polyethylene copolymers together with calcium carbonate and silicone elastomer. The flame retardant mechanism of this system is a combination of dilution by the calcium carbonate, formation of a stable foam, and formation of a protective silica layer on the surface. This layer will act as a barrier for mass transport of both oxygen into the insulation, and volatile decomposition products out of the material.
The effect of different copolymers on the formation of the foamed structure and burning characteristics of the system has been investigated. Poly(ethylene-co-butyl acrylate) (EBA) and poly(ethylene-co¬-methacrylic acid) (EMAA) have been compared. Acrylic acid, formed at elevated temperatures from EBA and present in virgin EMAA, reacts with the calcium carbonate to form ionomeric cross-links that increase the melt viscosity. This effect is more pronounced when EMAA is used as the copolymer and leads to less intense burning and lower dripping.
Several different additives have been evaluated as components in the system described above. Small amounts of clay, with at least one dimension in the nanometre scale, reduce the dripping and heat release rate. Different metal oxides have been studied with various results. Some additives show promising effects in certain tests but fail to improve the overall flame retardant behaviour. The results show, that rather small changes in formulation have large impact on the performance in a fire situation.
Furthermore, significant differences in flame retardancy can be seen when comparing different grades of calcium carbonate, e.g. chalk and limestone based systems.