Surface-modified ammonium polyphosphate with (3-aminopropyl) triethoxysilane, pentaerythritol and melamine dramatically improve flame retardancy and thermal stability of unsaturated polyester resin

Ammonium polyphosphate surface-modified by silane coupling agent (3-aminopropyl) triethoxysilane (KH-550), named STAPP, incorporated with pentaerythritol and melamine to prepare intumescent flame retardant unsaturated polyester resin (UPR/IFRs). Surface groups, surface elements, thermal stability and surface morphology of STAPP were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis (TG) and scanning electron microscopy (SEM). The flame retardancy of the UPR composites was studied by limit oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter test, and its thermal stability was evaluated by TG. The results showed that IFRs greatly improves flame retardancy, thermal stability and carbon formation ability of UPR. Compared with pure UPR, when the loading of IFRs was 25 mass%, the LOI was increased from 19.8 to 41.5%, the UL-94 test reached the V-0 rating from NR, peak heat release rate and total smoke production was decreased by 54.1 and 38.7%. Furthermore, the addition of IFRs delayed the product of smoke and release heat. TG results showed that 27.6 mass% chars remained in UPR/IFRs at 1073 K, showing a good char forming ability. Besides, honeycomb-type carbon layer with few cracks and holes was observed by SEM. Finally, the flame retardancy mechanism of IFRs was elaborated in detail, which was attributed to the siloxy groups and expanded carbon layer.