Polyamide-enhanced flame retardancy of ammonium polyphosphate on epoxy resin

An attractive intumescent flame retardant epoxy system was prepared from epoxy resin (diglycidyl ether of bisphenol A), low molecular weight polyamide (cure agent, LWPA), and ammonium polyphosphate (APP). The cured epoxy resin was served as carbonization agent as well as blowing agent itself in the intumescent flame retardant formulation. Flammability and thermal stability of the cured epoxy resins with different contents of APP and LWPA were investigated by limited oxygen index (LOI), UL-94 test, and thermogravimetric analysis (TGA). The results of LOI and UL-94 indicate that APP can improve the flame retardancy of LWPA-cured epoxy resins. Only 5 wt% of APP can increase the LOI value of epoxy resins from 19.6 to 27.1, and improve the UL-94 ratings, reaching V-0 rating from no rating when the mass ratio of epoxy resin to LWPA is 100/40. It is much interesting that LOI values of flame retardant cured epoxy resins (FR-CEP) increase with decreasing LWPA. The results of TGA, FUR, and X-ray photoelectron spectroscopy (XPS) indicate that the process of thermal degradation of FR-CEP consists of two main stages: the first stage is that a phosphorus rich char is formed on the surface of the material under 500 degrees C, and then a compact char yields over 500 degrees C; the second stage is that the char residue layer can give more effective protection for the materials than the char formed at the first stage do. The flame retardant mechanism also has been discussed according to the results of TGA, FTIR, and XPS for FR-CEP. (c) 2008 Wiley Periodicals, Inc.