Synthesis Of Hyperbranched Charring Agent And Application Of High-Efficient Flame Retardant Coating On Polypropylene

Polypropylene（PP）is widely used in many fields,covering buildings,automotives,electronics,and electric industries.However,a fatal weakness of PP is its relatively high flammability and severe dripping.So the flame retardancy is required for PP in some certain applications.Currently,the flame retardancy of PP is obtained by adding flame retardants.The high loading of additives can enhance the flame retardancy of PP,which inevitably weakens the intrinsic performance of PP composites.In view of this,this article applied a flame retardant coating to PP in order to achieve good flame retardant performance and maintain its intrinsic properties.The reported flame retardant coatings have been used to protect steel,wood,cloth,etc.To the best of our knowledge,however,very few studies focused on intumescent coatings onto polymeric substrates.The basic reason is that it is difficult to form a highly adhesive coating onto polymeric substrate,especially for PP.So we chose PP as a substrate,which has a relatively slippery “low energy surface” that means many common glues will not form adequate joints on it.Firstly,the PP specimens were soaked in the mixed acid solution to improve its surface energy.Secondly,9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide（DOPO）was adopted to improve the filmforming performance of the conventional film former-polyvinylformal acetal（PVFA）.The PVFA coating formed on the PP specimen is soft and easily peeled off.But the combination of PVFA and DOPO can achieve a flexible and adhesive coating.Moreover,the hyperbranched polyamide（HBPA）was synthesized as charring agent from melamine and ethylene diamine tetraacetic acid,acting as carbonization and blowing agents simultaneously.HBPA and ammonium polyphosphate（APP）were together used as flame retardant in the coating.A lot of terminal carboxyl groups of HBPA can intramolecularly and intermolecularly dehydrate to form cross-link char while HBPA decomposing.Subsequently,pyrophosphoric acid and metaphosphoric acid produced by APP as well as HPO₂ formed from DOPO interacted with cross-link char to form a well expanded char layer with a compact,smooth and continuous surface,being a barrier of oxygen and heat transfer to protect the underlying material from burning.The combustion tests show that the flame-retarded PP specimen with a coating in an average thickness of 126 μm passed V0 rating at UL-94 test,whose limiting oxygen index（LOI）value reached up to 30.5 vol%.The cone calorimeter test show heat release rate（347 kW/m²）was reduced by 71.2%comparing to pure PP（1203 k W/m²）,and the total heat release was decreased to 102.9 MJ/m² from 126.3 MJ/m²,showing good flame retardancy.The 3M tape test showed that the adhesion of coating could be rated as 5B,which decreased to 4B after water immersion at 60 °C for 8 d or after UV irradiation for 15 d.But the water immersion and UV irradiation did not influence the LOI value and the UL-94 test rating.In a word,the flame retardant coating possesses flexibility and strong adhesion,which can deform with the substrate without falling off.Meanwhile,the coating is anti-ultraviolet and water resistant.This approach will be promising for the development of intumescent flame-retardant coatings for plastics.