Preparation Of Novel Phosphorus/nitrogen/sulfur Based Flame Retardants And Their Application In Bio-based Polymers

The conventional halogen based flame retardants(FRs)are gradually withdrawn from the market because of the increasing pressure from strict legislation and environmental concerns.However,the introduction of most non-halogen FRs(such as phosphorus,nitrogen,sulfur based FRs)cannot meet the industrial standards because of their relative low efficiency and poor compatibility with the substrate.Therefore,the synergistic effect between different elements,such as P-N,N-S,P-N-S synergism,becomes the promising research topic in the field of fire science.Synthetic bio-based fiber/fabric has been played an important role in our life duo to its rich source,sustainability and eco-friendliness.Polylactic acid(PLA)and polyamide 11(PA 11)are two type of these bio-based polymers.PLA is obtained from corn,and PA 11 comes from castor.They both have wide applications due to their excellent properties.However,most bio-based synthetic polymers are flammable,which limits their application in many cases.Therefore,it is of great interest to develop novel flame retardant PLA and PA 11 composites and investigate their burning mechanisms.Based on the ideas of synergistic effects between P-N,N-S and P-N-S,this study synthesized five novel FRs and designed a new expandable graphite(EG)/melamine formulation.These FRs/formulation were introduced into polymer matrix by molten blending approach in order to improve the flame retardancy of both PLA and PA 11.The fire properties,thermal stability and mechanical properties of PLA and PA 11 were fully discussed.The flame retardant mechanisms were proposed.The main research contents/outcomes include six parts:1.Based on the ionic exchange reactions of ammonium polyphosphate(APP),a novel P-N-S intumescent FR(A-A)has been successfully produced.The chemical structure of A-A is characterized.The PLA/A-A composites are prepared by molten blending.The flammability of PLA composites are tested by limiting oxygen index(LOI),vertical burning tests(UL-94)and cone calorimetry(CONE).The results show that the presnce of 15%A-A in PLA sample improves the LOI value to 30.5%,upgrades the UL rating to V-0 and decreases the heat release.It is suggested that the flame retardant PLA can release non-flammable gases while constructing thermal stable intumescent char rapidly.2.Based on the self-assembly reactions of melamine(MEL),a new P-N contained intumescent FR(DT-M)has been successfully synthesized.Moreover,based on the strong acidity of phytic acid(PA),a new layer double hydroxide(LDH)has been prepared by the intercalation of PA into LDH layers(PA-LDH).The chemical structures of DT-M and PA-LDH are also discussed.PLA composites are prepared through molten blending.Fire tests results demonstrate that the presence of 14%DT-M/1%PA-LDH in PLA improves the LOI value from 19.8%to 38.9%,upgrades the UL rating from no rating to V-0 and significantly decreases the peak heat release rate(pHRR)and total heat release(THR)value.Combined the ingredient analysis in both condensed and gas phase,it is therefore can be concluded that(I)DT-M is responsible for the char construction,while PA-LDH provides more cross-linking sites,leading to the formation of continuous and dense char residues;(II)the release of inert gases from DT-M and PA-LDH also gives rise to the improvement of the fire resistance of PLA composites.3.Based on the interactions between casein(CS)and PA,a bio-based polyelectrolyte(PC)has been prepared and used as a shell structure to perform microencapsulated APP(PC@APP).The chemical structure,especially for the morphology change after the microencapsulation,is fully measured.PLA composites are manufactured through physical molten blending.The dispersion of PC@APP is observed by electron probe micro analysis(EPMA)and scanning electron microscopy(SEM).The results indicate that PC@APP is uniformly dispersed within the PLA matrix.Moreover,the 5%loading of PC@APP in PLA composite can enhance the LOI value up to 28.3%,increase the UL rating to V-0 and decrease the pHRR value.Besides the fire resistance,the toughness of PLA composites is also significantly improved.It is assumed that the incorporation of PC@APP can promote the thermal degradation of PLA matrix,accelerate the formation of molten drippings which can remove the fuel source from flame zone,thus enhance the fire resistances of PLA composites.These drippings will self-extinguish rapidly due to the presence of PC@APP,which can avoid the spread of the fire.4.Based on the self-assembly of MEL and sulfamic acid(SA),an N-S contained FR has been synthesized(SA-M).The chemical structure of SA-M is fully characterized.Combined with Al2O3,flame retardant PA 11 composites are performed by physical blending approach.The observation from EPMA and SEM shows that SA-M is uniformly dispersed within PA 11 because of the similar polarity between SA-M and PA 11 matrix.The fire properties of PA 11 samples are tested by LOI,UL-94 and CONE.The results reveal that the presence of 17.5%SA-M/2.5%Al2O3 in PA 11 composite sample improves the LOI value to 30.9%,upgrades the UL-94 rating to V-0 and decreases pHRR and THR values.Gas phase analysis by thermogravimetric analysis combined with Fourier transform infrared spectra(TGA-FTIR)suggests that the interactions between SA-M and PA 11 can release non-flammable gases and radical scavengers.In the condensed phase,SA-M can accelerate the charring rate of PA 11 composites,while Al2O3 also participates into this charring process,leading to the formation of physical barrier,and thus postpone the degradation of PA 11 composites.5.A novel macromolecular precursor(AM)has been successfully prepared by the MEL self-assembly reactions,and then a P-N based intumescent FR(AM-APP)has been synthesized by the ionic exchange reactions between APP and AM.The chemical structures of AM and AM-APP are thoroughly characterized.Combined with Ti02,AM-APP is incorporated into PA 11 matrix by melt blending to prepare flame retardant PA 11 composites.The fire performance tests results demonstrate that the loading of 22%AM-APP and 3%TiO2 in PA 11 composite sample increases the LOI value up to 29.2%,improves the UL-94 rating to V-0 and decreases the pHRR and THR values to 177 kW/m2 and 44 MJ/m2 respectively.The mechanism analysis suggests that the incorporation of AM-APP alter the degradation pathway of PA 11 matrix,leading to the quick release of inert gases.In the condensed phase,AM-APP is the major contribution for the formation of cross-linking char residues,while Ti02 can consolidate this barrier through both physical and chemical way.6.The particle size effects of EG on the fire resistance and mechanical properties of PA 11 composites are carefully discussed.It is found that with the incorporation of 100 mush EG,the PA 11 composites possess good flame retardancy and acceptable mechanical properties.Then,an effective halogen free flame retardant formulation has been designed by the combination of EG(100 mush)and MEL.The results indicate that the addition of 15%EG and 5%MEL to PA 11 sample increases the LOI value to 32.3%,improves UL-94 ratings to V-0 and decreases the pHRR and THR values.It is suggested that the incorporation of MEL can reduece the hydrogen bonding force in PA 11 matrix,leading to the early degradation.Some decomposition products of PA 11 are then filled into char layers formed by EG,resulting to the formation of uniform and dense barriers,and thus improving fire resistance of PA 11.