Preparation And Application Of Phosphorous Copolymerized Flame Retardant Waterborne Polyurethane

Waterborne polyurethane(WPU)is a new type of polyurethane system with water as solvent,which is widely used in painting,ink printing,adhesive,coating agent,fabric finishing agent and other fields.Among these application,WPU is conventional adhesive of ultra-high molecular weight polyethylene(UHMWPE)fiber uni-direction cloth(UD).UHMWPE fiber has the characteristics of high strength,high modulus,low density and excellent impact resistance,making UHMWPE fiber become one of the most commonly used bullet-proof substrates.But its heat resistance and flame retardant performance is poor,so it is of great significance to improve the flame retardant of WPU adhesive used for UHMWPE fiber UD.In this study,small molecular phosphorous-based flame retardant 10-(2,5-dihydroxyphenyl)-10-hydrogen-9-oxa-10-phosphaphenanthrene-10-oxide(ODOPB)were added into the main chain of WPU,and isophorondiamine(IPDA),6-methyl-1,3,5-triazine-2,4-diyldiamine(AG)and 2,4-diamino-6-phenyl-1,3,5-triazine(BG)were used as post-chain extender,respectively.The phosphorus-containing flame retardant modified WPU emulsion was synthesized by prepolymer method.The effects of the addition of AG,BG and ODOPB on emulsion storage stability,flame retardancy,heat resistance,char formation,mechanical properties and bond properties of WPU were investigated by otational rheometer,dynamic laser scattering(DLS),zeta potential analysis,limiting oxygen index(LOI)test,cone calorimeter analysis(CCT),vertical burning test,thermogravimetric analysis(TGA),thermogravimetric infrared analysis(TG-IR),raman microscopic spectrum analysis,scanning electron microscope(SEM),mechanical analysis and180°peel test after bonding UHMWPE fiber sheets.The flame retardancy mechanism and difference of different components were also studied.Firstly,the phosphorus-based flame retardant ODOPB was used to modify the WPU,and IPDA was used as the chain extender.A new type of WPU(FR-WPU)was prepared by prepolymer method.The effect of the content of ODOPB on the structural performance of WPU was investigated.It was found that with the increase of ODOPB content,the particle size increased,and viscosity of FR-WPU emulsion decreased gradually.There is no demulsification phenomenon occurred after standing for 1 month.The decomposition temperatures of FR-WPU's soft and hard chains were 42?and 20?higher than those of unmodified WPU,respectively.The limiting oxygen index(LOI)of FR-WPU films increased first and then decreased,reaching28.7%,and the heat release rate and total heat release rate decreased obviously with the increase of ODOPB content.The fire rating of FR-WPU films could reach V-0 level.After pyrolysis,the carbon layer on FR-WPU surface became dense and appeared obvious bubbles with the increase of ODOPB content.ODOPB mainly acts as a condensing phase flame retardant.The tensile strength increased and elongation at break decreased separately with the increase of ODOPB content.At the same time,the peel strength and peak load of FR-WPU adhesive on UHMWPE fiber sheets decreased with the increase of ODOPB,which still meets the industrial requirements of UHMWPE fiber UD.Secondly,in order to further improve the flame retardant performance of WPU,the AG/ODOPB co-modified WPU emulsion(FR-AG-WPU)was synthesized using AG as the chain extender instead of IPDA on the basis of ODOPB modified WPU.The effects of AG and ODOPB content on modified WPU performance were investigated.It was found that using AG as extender making AG-WPU emulsion's particle size larger,and viscosity decreased obviously.There is no demulsification phenomenon at the same time.The influence of ODOPB content on AG-WPU emulsion is same as FR-WPU emulsion,which average particle size grew and viscosity reduced further.Meanwhile,the addition of AG and ODOPB delayed the thermal decomposition process of hard and soft sections of WPU by 30?and 41?,respectively.LOI value increased with the addition of AG and the increase of ODOPB content,UL-94 rating of modified WPU could reach V-0 grade,heat release rate increased first and then decreased,and total heat release decreased with the increase of AG and ODOPB content.The influence of ODOPB content on mechanical properties and peel strength of UHMWPE fiber sheets was the same as FR-WPU,which tensile strength grew and elongation of break as well as peel strength decreased,but still meets the industrial requirements.Apart of that,AG and ODOPB can promote the graphitization degree of carbon layer on the surface during WPU combustion,and the carbon layer is spongy.During the pyrolysis process,there were NH₃,NO₂,phenyl compound and vapor appeared.Combined with thermal stability,condensed phase and gas phase analysis,it is shown that after adding AG the flame retardant mechanism of FR-AG-WPU is changed into the combination of vapor phase and condensed phase flame retardant,and the gas phase released by combustion plays a critical role.Finally,BG was used to replace AG as the chain extension,and ODOPB was used to modify WPU macromolecular chain,by which BG/WPU co-modified flame retardant WPU(FR-BG-WPU)was synthesized.The effects of BG and ODOPB content on WPU performance were investigated.It was found that FR-BG-WPU emulsion particle size grew,while viscosity decreased.There was also no demulsification phenomenon.Compared with pure WPU,the decomposition temperatures of hard and soft segments of FR-BG-WPU/10 increased by 29?and38?respectively.There was apparently more CO₂ in gaseous phase as well as some other nonflammable gases.After the pyrolysis,graphitization degree of carbon residue was also increased,the same spongy structure like FR-AG-WPU's residual carbon layer appeared,making higher coverage.With the increase of BG and ODOPB content,the LOI value of modified WPU was up to 27.1%,and UL-94 rating grew to V-0 grade.Compared with the pure WPU,the heat release rate and total heat release of unmodified WPU decreased with the increase of BG and ODOPB content.The tensile strength increased with the increase of ODOPB content as well as elongation at break and peel strength after bonding UHMWPE fiber sheets decreased gradually.BG modification makes FR-BG-WPU films show a condensation phase and gas phase synergistic flame retardant mechanism,because the benzene ring structure of BG can form more aromatic condensation phase,the carbon residue is more than FR-AG-WPU's,showing a better flame retardant effect.