Differences in the flame retardant mechanism of melamine cyanurate in polyamide 6 and polyamide 66

It is known that the effectiveness of melamine cyanurate (MC) is higher in PA66 than in PA6. In this paper the differences in the degradation chemistries of PA6 and PA66 and the chemistry of MC with the respective degradation products are used to explain the difference in the efficiency of MC in PA6 and PA66. The chemistries of the degradation products of PA6 (caprolactam) and PA66 (cylopentanone) were determined with the aid of model compounds. At temperatures of 350-450 degreesC caprolactam reacted with MC to form primarily oligomers with different end groups (-C=N amongst others). Cyclopentanone is much more reactive at these temperatures, and the MC mixture was found to contain several cyclopentanone self-condensation products and reaction products of cylopentanone and melamine (NH3, HN = C = NH) or cyanuric acid (NH3, HN = C = 0) decomposition products. The results of the aforementioned degradation experiments using the model components were compared with those obtained for pure PA6 and PA66 in the presence of MC. It was concluded that at 400 and 450 degreesC reactions of the decomposition products of MC with decomposition products of PA6 and PA66 are comparable with the reactions of the model compounds with these polymers. It was also found that the presence of MC leads to increased formation of non-soluble degradation products in PA66 but not in PA6. The difference in activity of MC in PA66 and PA6 can be explained by a difference in the degradation mechanisms of the two polymers. The degradation products formed in PA66 (cyclopentanone) may cross-link with MC degradation products (mainly NH3) leading to less flammable high molecular weight structures. PA 6 degrades to less reactive compounds which do not cross-link. (C) 2002 Elsevier Science Ltd. All rights reserved.