Effect Of Ammonium Polyphosphate/The Melamine Resins Modified With Polyols On The Combustion Behavior Of PP And Its Mechanism

The melamine resins modified with polyols（modified resin for short）are a new type of macromolecular triazine charring agent.The results of limiting oxygen index measurement and vertical combustion test show that the charring agent and ammonium polyphosphate（APP）have an exceptional synergistic flame retardancy in polypropylene（PP）.Cone calorimetry test（CCT）is the most advanced and comprehensive evaluation method for flame retardant property at present,and its results have good correlation with actual fire,so it is widely used to evaluate the combustion behavior of polymer materials.In addition,the flame retardant mechanism of the above modified resin has not been studied systematically,but the action mechanism has an important guiding role for the development of more efficient charring agents.Therefore,it is of great theoretical and practical significance to use CCT to evaluate the effect of the modified resin on combustion behavior of materials and to study its flame retardant mechanism.The effect of the modified resin on combustion behavior of PP was evaluated by CCT.The results show that the modified resins have obvious synergistic effects with APP on inhibiting the cracking of PP,flame propagation during combustion and reducing the fire intensity of combustion.The type of the modified resin has a marked influence on the above synergistic effect.The synergistic effect of the resin modified with pentaerythritol（PER）is the best,followed by the resin modified with propylene glycol and the resin modified with ethylene glycol,and that of the resin modified with glycerol is the worst.The synergy of the above modified resins are all better than PER.In addition,they also have a significant synergistic effect with APP on smoke generation during PP combustion,and the synergistic effect of the resins modified with PER and propylene glycol is the best.The flame retardant mechanism of the modified resin/APP was studied through TG and TG-MS analysis of flame retardant PP,the composition and structure analysis of CCT residues and the effect analysis of heating treatment on the composition and structure of flame retardant PP.The results show that the modified resin/APP imparts a flame retardancy through condensed phase,and the mechanism was as follows:in the combustion process,the modified resin was first cracked into polyols,while APP was cracked to polyphosphoric acid,which was esterified with the modified resin and the above polyols to form phosphoric ester and a small amount of phosphoramide.When the phosphoric ester,phosphamide and phosphoric acid compounds are in the melting state,the inert gases such as N₂,NH₃ and water released by the pyrolysis of the modified resin and APP make them foaming;When the temperature continues to increase,and polyphosphoric acid compounds are dehydrated to polymetaphosphoric acid with the strong dehydration,which dehydrates phosphoric ester and phosphoramide into char.The char has a protective effect on polymetaphosphoric acid,which provented further decomposition of polymetaphosphoric acid,and was interwoven with polymetaphosphoric acid to form an intumescent layer with a dense structure and good strength,which brings about the suppression on pyrolysis of the PP composites by the isolation of oxygen and heat,and preventing volatilization of pyrolysis products,which results in a flame retardant effect and inhibition on the flame propagation and the fire intensity. the degree of graphitization of the char has a significant effect on the above effects.The higher the degree of graphitization,the better the thermal stability and mechanical strength of the char layer,and thus the better the flame retardancy and the inhibition of flame propagation.The type of modified resin has a significant effect on the formation of phosphate ester,phosphoramide and C=C structure,which in turn affects the amount of char residues and the stability of the char layer,and ultimately its synergistic flame retardancy.The residues of the resins modified with PER/APP/PP and the resin modified with propylene glycol/APP/PP contain obvious P-O-P and P-O-C structure and high graphitized char,so the residues are more stable,while the flame retardant effect is the best;The residues of the resin modified with ethylene glycol contain relatively less P-O-P,P-O-C structure and graphitized char,so the flame retardant effect is poor;In contrast,the residues of the resin modified with glycerol contain almost no P-O-P and P-O-C structure,and the graphitization degree is the lowest,thus the residual is the least,and the char layer is unstable,thus the flame retardant effect is the worst.