The Preparation And Properties Of Halogen-free Flame Retardant Silicone Rubber

Silicone rubber has good stability at elevated temperatures because of its inherentstrength of the siloxane(Si-O) bond. What more, silicone rubber has comparatively low heatrelease rates, slow burning rate without a flaming drip and when pure, no emissions of toxicsmokes. Based on these fire properties, silicone rubber has been widely used in the field ofelectrical wires and cables as a kind of novel fire-retardant material which is expected toreplace traditional fire-retardant materials such as mica, magnesium oxide and halogenmaterials. However, one weak aspect of silicone rubber is that it can smolder long before itburning. And it will restrict its use at high temperature and high pressure conditions.The purpose of this paper is to improve the flame retardancy of silicone rubber by usingsome flame retardant compounds.The research content and the main results are as follows:(1) The flame retardancy and mechanical properties of silicone rubber based onaluminum hydroxide(ATH) with zinc borate(ZB) have been studied. And cone calorimetertest(CCT), thermo gravimetric(TG), and SEM analysis methods were used in this work.Results show that the flame-resistance of silicone rubber is improved when ATH and ZB areused alone but with mechanical properties decreased. The presence of ZB(10 phr) and ATH(20 phr) in silicone rubber results in higher limiting oxygen index(LOI), which is 35.5%, andmeet the V-1 rating in the UL-94 test. And it leads to a better flame retardancy in CCT. TGcurves show that SR/ATH/ZB samples have high thermal oxidative stability in the early stageof combustion in air. SEM observations of residues reveal a significant char/hard residueformation due to the presence of the ATH/ZB flame retardant compound.(2) The flame retardancy and mechanical properties of silicone rubber based onammonium polyphosphate(APP) with aluminum hydroxide(ATH) have been studied bymechanical properties test, LOI, UL-94 test,TG and CCT. Results show that, when APP andATH are used alone, the flame-resistance of silicone rubber is improved with the increasing oftheir loading level, but the mechanical properties of silicone rubber are decreased. Whenusing APP/ATH flame retardant compound, silicone rubber behaves lower flame retardancy inLOI, UL-94 test and CCT test. TG curves show that the thermal stability and thermaloxidative stability of silicone rubber are decreased after filled with flame retardants. Thepictures of silicone rubber fire residues reveal that a reaction between APP and ATH happensin silicone rubber combustion, and a granular fire residues is formed, which decreases theeffectiveness of the surface protection provided by the surface coating formed on heating APPin silicone rubber.(3) The flame retardancy and mechanical properties of silicone rubber based onammonium polyphosphate(APP) with melamine(MEL) have been studied. And CCT, TGand SEM analysis methods were used to discuss their mechanism of flame retardancy. Resultsshow that LOI value of 30 phr APP sample is 31.3%. When 0-10 phr MEL filled in siliconerubber with 30 phr APP, LOI value increases slightly, and always meets the V-0 rating in theUL-94 test. However mechanical properties of silicone rubber go down overly. CCT analysisshows that APP/MEL increases the combustion performance of silicone rubber.TG curvesreveal two-step flame-retardant mechanism. First, APP/MEL decomposes endothermicallywith the gas release in the 320~380oC temperature range and MEL is in favour of the gasphase flame retardant of APP. Second, APP/MEL promotes the formation of compact charredlayers slowly in the condensed phase in the 380~540oC temperature range, but MEL is not infavour of the carbon formation of APP/SR. Overall APP/MEL flame retardant compoundreduces silicone rubber thermal stability in TG analysis.SEM observations of residues reveal aporousness residue formation due to the presence of the APP/MEL flame retardant compound.