Study On Preparation And Performance Of Modified Melamine-formaldehyde Resins Based Flame-retardant Polyurethane Foams

The polyurethane foams are widely used in aerospace,pipeline transportation,furniture and automobile and building facades due to their excellent properties such as shock absorption and shock resistance,pressure resistance,heat insulation and heat preservation.As an important part of insulated materials in buildings and furnitures,the application of polyurethane foams is limited by their flammability.Melamine is widely used in polyurethane foams as a nitrogen-rich flame retardant.However,its compatibility with foams is inferior.Thus,the mechanical properties of the foams would reduce to some extent.Melamine is mostly added to foam by physical blending.In this case,the flame retardant elements are prone to migration,so that the flame-retardant efficiency is indurable.And the flame-retardant efficiency is unfavorable with single use of melamine.Therefore,a modification method of melamine is proposed in this thesis,by which the glycerol modified melamine-formaldehyde resins was prepared with melamine,paraformaldehyde and glycerol under anhydrous system.This resins improved the disadvantages of melamine-formaldehyde resins,such as easy cross-linking and curing and high hardness,brittleness and water content.Furthermore,it was compatible with foams and maintained a clear and transparent liquid state after one and a half years stable storage.Based on the best process conditions of the resins,the sodium alginate and lignin modified resinss were prepared sequentially to realize the high-value utilization of biomass resources.The above three resinss were used as flame-retardants to prepare polyurethane foams.The thermomechanical properties and flame-retardant properties of the foams were characterized.The results showed that theμ-values of GMF-PPU,SGMF-PPU and LGMF-PPU were increased from 4.08,2.81 and 3.81 to5.60,4.41 and 5.70,respectively,compared with MA-PPU,SA-PPU and LS-PPU,all of which were greater than 3.03 of PPU.Meanwhile,the foam volume was large and the density decreased.The pore wall of the foam pores became thicker and the closed pore ratio increased.The thermal stability of SGMF-PPU and LGMF-PPU decreased in the initial pyrolysis stage.However,the T_50%increased from 374°C and 373°C to 408°C and 398°C,respectively.The LOI values of GMF-PPU,SGMF-PPU and LGMF-PPU increased from 18.2%to 23.0%,25.8%and 25.2%,and the flame-retardant efficiency was significantly enhanced.The heat and smoke release performance results showed that the pk-HRR and THR of MA-PPU and GMF-PPU were 214.78 k W/m²,53.53MJ/m² and 257.54 k W/m²,44.66 MJ/m²,respectively,where the modified resins GMF had comparable flame-retardant performance with MA.The pk-HRR and THR of SGMF-PPU and LGMF-PPU had substantial decreaset and the SGMF-PPU showed more prefarable flame-retardant efficiency.Melamine,sodium alginate and lignin could reduce the amount of smoke generation to different degree during the combustion of the foams,in which the melamine and sodium alginate had more particularly remarkable performance.In order to further improve the flame-retardant efficiency of the modified resins foams,another phosphorus-nitrogen flame-retardant BHAPE was synthesized and it was compounded with GMF,SGMF and LGMF to prepare a phosphorus-nitrogen synergistically modified melamine-formaldehyde resins-based polyurethane foams.The modified resins and BHAPE showed favorable compounding effect and could effectively improve the mechanical properties of the foams.The T_5%of GBPU,SGBPU and LGBPU were only 254℃,246℃and 256℃.And the initial thermal degradation temperature was further reduced compared with the unmodified flame-retardant foams,especially the T_10%decreased from 319℃to 275℃,270℃and 277℃.In addition,GBPU,SGBPU and LGBPU effectively improved the temperature at the maximum thermal weight loss rate of modified resins foam.Compared with 346℃,363℃and 335℃of GMF-PPU,the temperature of SGMF-PPU and LGMF-PPU rised to 377℃,377℃and 380℃,respectively.The char layers of GBPU,SGBPU and LGBPU after combustion became thicker and more uniform and compact.Both BPU and GBPU showed self-extinction,and GBPU had more outstanding combustion behavior with short burn time,less damage in foam matrix and well preservation for the main structure of the foam.Although SGBPU and LGBPU did not show self-extinction,they gradually formed an expanded and dense char layer during combustion.The LOI values of GBPU,SGBPU and LGBPU improved to 29.6%,28.2%and 28.6%,respectively.The lowest pk-HRR is only 258.92 k W/m² for GBPU and the lowest THR is 27.09 MJ/m² for SGBPU.The smoke emission rate of BPU is much larger than that of other foams,and the TSP of GBPU,SGBPU and LGBPU decreased from 16.05 m² to 4.83 m²,3.07 m²and 3.17 m²,respectively,which greatly improved the defect of large smoke emission of BPU.The CO generation of the compounded foams declined significantly,even to0.0005 kg/kg for GBPU and 0.28 kg/kg for CO₂ generation,which are the lowest among all flame-retardant foams.The addition of sodium alginate and lignin enhanced the phosphorus-nitrogen synergistic flame-retardant efficiency,and made outstanding contributions in reducing the smoke and toxic gas release of the foams and improving the char formation ability of the foams.