Synergistic Flame Retardant Effect Of Ammonium Polyphosphate/Miamine Resin Modified With Theic/Zeolite In PP

The melamine formaldehyde resin modified with THEIC（Resin modified with THEIC in brief）is a novel triazine charring agent,which has some merits such as simple synthesis process,less emission of wastes in the synthesis process,and basically no safety risks,but the flame retardant efficiency of the intumescent flame retardant（IFR）composed of it and ammonium polyphosphate（APP）（APP/Resin modified with THEIC）is lower.The use of a synergistic agent is one of the common means to improve the flame retardant efficiency of IFR.Zeolite is a green and cheap synergistic agent.In order to improve the flame retardant efficiency of APP/Resin modified with THEIC,the synergistic effect of zeolite and APP/Resin modified with THEIC in polypropylene（PP）and the synergistic mechanism were studied in this paper.The synergistic flame retardancy of zeolite and APP/Resin modified with THEIC was studied by vertical combustion test and limiting oxygen index（LOI）measurement.The results show that zeolite and APP/Resin modified with THEIC has an exceptional synergistic flame retardant effect in PP.For example,when the total loading of flame retardants is 24 wt%based on the total weight of the PP composites,the LOI and the flame retardant efficiency of the PP composites increase from 25.5%and 0.3619 to27.4%and 0.4417,respectively,the synergic efficiency is 1.20,the flame burning time for first ignition（t₁ in brief）and the flame burning time for second ignition（t₂ in brief）in the UL-94 test are reduced from 1.1 s and 4.7 s to 1.0 s and 2.1 s by combining 1.5wt%of 4A zeolite.The LOI of PP,the flame retardancy to vertical combustion,the flame retardant efficiency of flame retardants and the synergic efficiency of zeolite firstly increase and then decrease with the increasing loading of zeolite.The type of zeolite has a slight influence on the synergistic effect,that of 4A zeolite is the best,but5A zeolite is the poorest.The influence of zeolite on the combustion bevavior of the flame retardant PP was investigated by cone calorimetry test（CCT）.The results show that zeolite has obvious impact on the combustion behavior of APP/Resin modified with THEIC/PP.Compounding a small amount of zeolite can not only significantly increase the ability of APP/Resin modified with THEIC to inhibit PP cracking and flame propagation,and reduce the intensity of fire,but also significantly inhibit the generation of smoke and CO during the burning process of the flame retardant PP.For example,combining1.5wt%of 4A zeolite,the peak heat release rate,average heat release rate,total heat release,fire growth index,total smoke production,average smoke generation rate,peak smoke generation rate and peak CO generation rate decrease by 37.8%,42.1%,21.9%,49.2%,34.7%,48.4%,51.3%and 31.0%,respectively.The fire performance index increases by 87.6%.The flame retardant mechanism of APP/Resin modified with THEIC/zeolite in PP and the synergistic mechanism of zeolite were studied through TG and TG-MS analysis,the analysis of the structure and composition of the CCT residues,an the effect analysis of heating treatment on the composition of the flame retardant PP.The results reveal that APP/Resin modified with THEIC mainly imparts a flame retardancy through condensed phase,and the mechanism can be summarized as follows:In the initial stage of combustion,APP is first pyrolyzed into polyphosphoric acid and its ammonium salt,which is subsequently esterified and phosphorylated with Resin modified with THEIC and the polyhydroxyl compounds formed by the pyrolysis of the above resin to form phosphoric ester and a small amount of phosphoramide;When these products are in a molten state,the inert gases such as H₂O,CO₂,and N₂ produced by the pyrolysis of APP and Resin modified with THEIC cause them to foam and expand;With the increase of temperature,polyphosphoric acid ammonium salt are further deaminated,and dehydrated to polymetaphosphoric acid,whose strong dehydration makes phosphate ester,phosphoramide and oxidation products of PP into char,and intertwined with polymetaphosphoric acid to form a compact and strong intumescent char layer,which suppresses the release of volatile hydrocarbons within the matrix to the surface,the transfer of outside heat and oxygen to the interior of matrix,and the further pyrolysis of the matrix,so that the spread of flame is effectively blocked,which results in flame retandancy.The highly active phosphates formed through the reaction of zeolite with APP have a catalysis on the esterification of polyphosphoric acid,resulting in the generation of more ester compounds to participate in charring,thus increasing the residual in combustion.In addition,zeolite also enhanced the degree of graphitization of residual char and the strength and stability of the intumescent char layer,which in turn improves the ability of APP/Resin modified with THEIC to inhibit PP pyrolysis and flame propagation,which reducing the fire intensity.