Studies On Smoke Suppression Of Layered Double Hydroxides And Metal-oxygen Clusters

Aiming at the large amounts of smoke and toxic gases released during polymers combustion,the design of materials with the function of efficient smoke suppression is essential to reduce smoke and improve fire safety.As two important inorganic functional materials,layered double hydroxides(LDHs)and metal-oxygen clusters(POMs)have unique tunability of their composition and structure,and have been widely used in the fields of catalytic materials,energy storage materials and flame-retardant materials.In recent years,these two types of materials have initially shown good effects in smoke suppression,which can effectively inhibit the release of smoke from some polymers.Polymer materials with different main chain structures have diversified pyrolysis mechanisms.Therefore,it is necessary to rationally design the composition and structure of smoke suppressants for different polymers.Therefore,in order to further improve the smoke suppression performance of the two types of inorganic materials,it is necessary to comprehensively and in-depth study the mechanism of smoke suppression of LDHs and POMs materials,and develop the newtype smoke suppressants with excellent performance for exploring the application in the field of smoke suppression to provide theoretical and experimental guidance.The main results of this paper are as follows:1.Study on the smoke suppression of LDHs.The M(?)Fe-LDHs(M=Mg,Co,Ni)with different layer composition were prepared by a method involving separate nucleation and aging steps,and blended with thermoplastic polyurethane(TPU)materials.The smoke suppression effect of LDHs on TPU materials and the mechanism of promoting char formation were studied.It was found that the decomposition intermediate product of MgFe-LDH can catalyze the crosslinking and aromatization between the TPU molecular segments,and then promote the formation of char residue with a higher degree of graphitization in condensed phase.As a result,the weakened intramolecular cyclization inhibits the release of aromatic compounds(flue gas precursors)in the gas phase,so that the MgFe-LDH/TPU composite exhibits excellent smoke suppression properties in both closed and nonclosed test environments.And with the increase of the amount of MgFeLDH,the maximum smoke density and total smoke production decreased first and then increased.When the amount of LDHs was only 0.5 wt.%,the maximum smoke density decreased by 40.1%.In addition,in the actual combustion and decomposition process,MgFe-LDH will migrate to the surface of the melt under the promotion of volatile gases,which promotes the formation of char residue with multi-layer porous structure,and finally effectively prevents the outflow of smoke and toxic gases.However,due to the poor char-forming effect of CoFe-LDH and NiFe-LDH,the smoke suppression performance is not as good as that of MgFe-LDH.2.Study on the synergistic effect of smoke suppression and flame retardancy of LDHs.Faced with the problem that a large amount of flame-retardant ammonium polyphosphate(APP)filled in TPU will produce toxic gas during combustion,we have carried out a reasonable formulation design,that is,adding MgFe-LDH and APP blended in different proportions to the TPU.It was found that the alkaline metal oxide produced by the decomposition of MgFe-LDH can undergo cross-linking reactions with the acid product produced by the decomposition of APP,forming a denser carbon layer with a stable structure of-P(=O)-O-C and-P(=O)-O-M(M=Mg or Fe).It effectively prevents the release of gaseous products and improves the smoke suppression of the MgFe-LDH/APP/TPU composites.When MgFe-LDH/APP=1/4,the maximum smoke density of MgFeLDH/APP/TPU composite is reduced by 80.2%,which shows a strong smoke suppression synergistic effect between MgFe-LDH and APP.Based on in-situ TG-FTIR analysis,it was found that MgFe-LDH inhibited the release of toxic gases(CO,HCN,etc.)during the thermal decomposition of APP/TPU materials.At the same time,MgFe-LDH improves the flame retardancy and thermal stability of APP/TPU composites.Therefore,LDHs-based smoke suppressants have shown potential application prospects in phosphorus-based flame retardant systems.3.Study on the smoke suppression and crosslinking characteristics of LDHs.The method involving separate nucleation and aging steps was used to introduce the Zn element with cross-linking function into the layer of MgAl-LDH,a family of ternary ZnMgAl-LDHs was synthesized.The smoke suppression and crosslinking properties of this LDHs in thermosetting chloroprene rubber(CR)were studied.It is found that the alkaline metal oxides produced by the thermal decomposition of LDHs can absorb a large amount of acidic HCl gas produced by the decomposition of CR,thereby reducing the white smoke generated in the flameless combustion mode,and reducing the maximum smoke density from 477 to 350.In the flame combustion mode,LDHs can effectively promote the formation of a stable and dense carbon layer during the combustion of CR.The carbon layer has a good barrier effect in the condensed phase,preventing the release of smoke precursors and flammable gas.The improved smoke suppression performance is manifested in the reduction of its peak smoke production rate and peak CO emissions by 28.4%and 49.8%,respectively.As the molar ratio of Mg/Zn on LDHs laminates increases,the smoke suppression,flame retardancy and thermal stability of LDHs/CR composites are gradually improved,which are better than ZnO/CR composites filled with the same amount of ZnO.In addition,by adjusting the ratio of Mg/Zn element to change the Zn element content of the laminate,a high degree of dispersion of Zn element as a cross-linking site is realized.Therefore,the utilization efficiency of Zn element is improved.Compared with traditional ZnO/CR vulcanizates,the content of Zn in LDHs/CR is significantly reduced by 53.0-84.4%,but the maximum torque,torque difference and vulcanization rate are all improved.LDHs can be used as cross-linking agents in thermoset polymers,and then exert their smoke suppression and flame retardancy.This provides a good reference for the design and preparation of new thermoset composite with excellent fire safety.4.Study on smoke suppression and flame retardancy of metaloxygen clusters.The complex 3D porous structure makes it difficult to improve the smoke suppression and flame retardancy of flexible polyurethane foam(FPUF).By taking advantage of the advantages of POMs such as strong oxidation,water solubility,and nanostructure,as well as the redox polymerization reaction between phosphomolybdic acid and pyrrole monomers,the PMo-PPy nanoparticles rich in N,P and Mo elements are successfully deposited on the surface of the FPUF matrix.The PMo-PPy nanoparticles are uniformly dispersed on the surface of 3D matrix.It was found that the PMo-PPy coating was thermally decomposed to produce phosphoric acid and MoO3,which in turn formed-P(=O)-O-C and Mo-N-C structures in the condensed phase,which improved the stability and compactness of the carbon residue.This effectively prevents the transfer and release of smoke precursors,flammable gas and heat.Compared with FPUF,the coated FPUF@PMo-PPy composite exhibits excellent smoke suppression and flame retardancy,which is reflected in the reduction of its peak smoke production rate and maximum smoke density by 73.5%and 55.7%,respectively.At the same time,the TG-FTIR in-situ analysis showed that the PMo-PPy coating significantly inhibited the release of toxic gases(CO and isocyanate compounds),and improved the fire safety of FPUF.Compared with layer-by-layer assembly,this method is simple to operate,low in cost,and can be replicated to other porous polymer materials.