| In recent years,due to the advantages in high flame retardancy,less smoke emission and non-toxic environmental protection,intumescent flame retardants have become one of the most typical representatives of halogen-free flame retardants instead of highly toxic halogen-containing flame retardants.It is currently the most promising flame retardants for development and application.Generally speaking,the chemical intumescent flame retardant system is composed of acid source,char-forming agent and gas source.Among them,the most commonly used char-forming agent is pentaerythritol and its derivatives,but most of these char-forming agent are derived from petroleum-based products.With the increasing consumption of petrochemical resource,petroleum-based char-forming agent becomes more expensive and non-renewable.Therefore,exploiting clean,cheap and sustainable charforming agent has become a new challenge and research hotspot in the field of flame retardants.Similar to the structure of pentaerythritol,polyhydroxy biomass materials are candidates for replacing petrochemical resource-based char-forming agents.As renewable and abundant natural carbohydrate polymers,soluble starch(SS)is composed of ?-Dglucopyranose units with polyhydroxy structure.By modifying the starch structure,constructing sustainable and green char-forming agent has become an effective way to get rid of the dependence on petrochemical resource in the traditional intumescent flame retardant system.This paper have addressed the flammability of polypropylene and the unsustainability of traditional char-forming agent.Through chemical reaction,the acid source and gas source have been combined with starch,in order to prepare a three-source integrated,starch-based,halogen-free and intumescent flame retardant.At the same time,by fabricating the hydrophobic surface for the starch-based flame retardant,the compatibility and flame retardant efficiency of flame-retarded polypropylene are further improved.In addition,the synergistic intumescent flame retardant system have been created by combining modified starch-based flame retardant with expandable graphite,and the properties and mechanism of flame-retarded polypropylene system have been investigated.The main research contents of this paper are summarized as follows:1.First,a three-source integrated starch-based flame retardant(SSPM)was prepared by hydrothermal method and neutralization method,and introduced into polypropylene by melt blending.SSPM has excellent char formation ability and he char-formation temperature is concentrated at 350-400 °C.Compared with pure PP and PP/30 phr SS,the flame retardancy of PP/30 phr SSPM is significantly enhanced.The limiting oxygen index(LOI)and vertical combustion rating of PP/30 phr SSPM is 26.4% and UL-94 V-1.This shows that the introduction of SSPM can effectively improve the flame retardancy and resistance to melt dripping of PP.In addition,the combustion residues of PP composites were systematically analyzed.The results show that the combustion residue is a dense and continuous char layer with a P-O-C crosslinked structure,which has become the important reason for the improved flame retardancy of PP.Based on these facts,the condensed flame retardant mechanism of starch-based flame retardants in PP was further proposed.2.In order to improve the dispersion and compatibility of SSPM in PP and reduce the agglomeration and migration problems of SSPM,silicone resin was used to enhance the surface hydrophobicity of SSPM.The results show that MSSPM particles have the rough surface,the narrow particle size distribution with an average diameter of only 1.9 ?m and hydrophobic surface with WCA value of 128°.The water solubility of MSSPM is greatly reduced and changes little with increasing temperature.The results of TGA show that the residual rate of MSSPM is as high as 37.5% at 700 ?.Through the surface modification process,the silicone resin can reduce the dissolution problem of SSPM,and at the same time greatly improves its hydrophobic performance.Based on these studies,the feasibility for surface modification of SSPM by silicone resin have been confirmed.The flame retardant tests that the LOI value of PP/30 phr MSSPM can reach 27.8%,and the vertical combustion test can obtain UL-94 V-1 rating.The chemical-physical synergistic intumescent flameretardant PP system have been built via combining MSSPM with expandable graphite(EG).The results show that when the total addition is 30 phr and the ratio of MSSPM/EG is 1:3,the flame retardancy of PP composites is optimal.The LOI value and vertical combustion test can reach 33.5% and UL-94 V-0 rating.It is shown that the synergistic effect of MSSPM and EG can not only greatly improve the flame retardancy of PP composites,but also effectively overcome the melt-dripping problem during combustion.TGA results show that PP/MSSPM/EG 1-3 system has excellent thermal stability at high temperature,and the residual rate at 700 °C is as high as 25.9%.By comparing the fitted curve with the experimental curve,the synergistic char-formation action of MSSPM and EG in PP composites was further confirmed.Based on the systematic analysis of combustion residues,the synergistic and complementary mechanism of MSSPM/EG system in PP is deduced.In this paper,a three-source integrated halogen-free flame retardant have been prepared based on starch.Besides,a chemical-physical synergistic flame retardant system have been constructed for the purpose of effectively improving the flame retardancy of PP.Furthermore,the application range of PP materials has been widened,and the experimental foundation and theoretical basis have been established for the research and development of biomassbased intumescent flame retardant system. |
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