Preparation And Mechanism Of Halogen-free Flame-retardant High-performance Polycarbonates

Bisphenol A polycarbonate（PC）is renowned as an engineering plastic,which is due to its lightweight design,remarkable strength,resilience,transparency,and dimensional stability.These attributes have established its indispensable role in both the electronics and aerospace sectors.However,despite its array of advantages,the commonly used PC falls short in terms of flame retardancy,only achieving a UL-94 vertical burning rating of V-2 and a limiting oxygen index（LOI）value of 25.0%.This deficiency fails to meet market demands,prompting a pressing need to enhance the flame retardancy of PC.Traditional flame-retardant PC products often use halogen-based and sulfonic acid salt-based flame retardants.However,halogen-containing flame-retardant PC generates a large amount of toxic smoke during fires.Due to their bio-toxicity and bioaccumulation,the application of halogen-based flame retardants has been increasingly restricted by regulations.Additionally,the Stockholm Convention has listed fluorosulfonic acid salt derivatives as persistent organic pollutants to be banned.Therefore,it is of great practical significance to develop a series of halogen-free,low-smoke,low-toxicity,and efficient flame retardants suitable for PC,in line with the requirements for green and environmentally friendly flame retardants.PC with environmentally friendly flame retardants,such as organophosphate and organosilicon still suffered from some issues including low flame-retardant efficiency,high addition and poor compatibility,which seriously decreased the mechanical properties,transparency and heat resistance of PC.Therefore,it always has been a great concern in academia and industry to reduce the impact of flame retardants on PC,balance the flame retardancy with other properties,and prepare the flame-retardant PC with good comprehensive performance.In this work,oriented by the requirements of practical applications,a series of flame-retardant PC with high performance was prepared,including flame-retardancy and high-toughness（Chapters 2 and 3）,flame-retardancy,transparency,and scratch-resistance（Chapters 4 and 5）,as well as flame-retardant and phase-change PC supporting skeleton（Chapter 6）,from the synthesis of halogen-free flame-retardants and the construction of surface coatings.This study delved into the interaction mechanisms among various properties and effectively addressed the conflict between flame retardancy and other material properties.This not only resolved inherent contradictions but also broadened the scope for multi-functionalization of PC.The primary research findings are outlined below:（1）Polyborosiloxane elastomer（PBASi）was synthesized through the dehydration condensation reaction between boric acid（BA）and hydroxyl terminated polydimethylsiloxane（HO-PDMS）.The flame-retardant and toughened PC/PBASi composites were prepared by reactive blending with the introduction of PBASi（Chapter 2）.It was found that PBASi improved the flame retardancy of PC by increasing the melt viscosity and promoting the formation of a ceramic char layer.With the addition of 2.0 wt.%PBASi,the PC/PBASi composites passed the UL-94 V-0 rating with a limiting oxygen index（LOI）value of 30.4%.It was found that PBASi formed a micro-crosslinked structure with PC through ester exchange reaction during the reaction blending,which acted as rubber particles to toughen PC.PBASi effectively eliminated the notch sensitivity of PC,and the notched impact strength was increased by 502.1%.（2）Styrene-maleic anhydride copolymer（SMA）was grafted on the surface of silicon dioxide nanoparticles（Si O₂）to prepare amphiphilic SMA-grafted Si O₂（SMA-Si O₂）.SMA-Si O₂ was then introduced into the PC/ABS via reaction blending to simultaneously improve the compatibility and flame retardancy of PC/ABS composites（Chapter 3）.It was found that SMA-Si O₂formed a Janus structure through molecular chain entanglements and esterification during the reactive blending,which made SMA-Si O₂ tend to distribute at the phase interface between PC and ABS.This selectively distributed SMA-Si O₂ improved the compatibility of PC and ABS and showed a synergy effect with bisphenol-A bis（diphenyl phosphate）（BDP）in improving the flame retardancy.The addition of 1.0 wt.%SMA-Si O₂ significantly improved the flame retardancy and mechanical property of PC/ABS/BDP composites.The peak heat release rate（p HRR）and total smoke production（TSP）were reduced by 18.0%and 12.4%,respectively.The tensile strength and elongation at break were improved by 8.4%and 51.1%,respectively.（3）A colorless,transparent,and liquid flame retardant,phosphonamide（PPA）,was synthesized and then combined with（3-chloropropyl）trimethoxysilane（CPTMS）via nucleophilic substitution reaction,resulting in the creation of an organic/inorganic hybrid transparent flame-retardant coating（PPA-Si）.This coating exhibited excellent hardness and flexibility.Subsequently,the PPA-Si coating was applied to the surface of PC using a casting method to enhance both the flame retardancy and scratch resistance of PC,while maintaining high transparency（Chapter 4）.It was observed that PPA enhanced the flexibility of the coating and served as an acid source during combustion,while CPTMS contributed hardness and curing ability to the coating and acted as a carbon source during combustion.During combustion,the PPA-Si coating swiftly swelled to form a robust and continuous protective char layer,preserving the inherent transparency and appearance of the PC substrates.When the weight gain of PPA-Si coating reached 12.0 wt.%,the PC achieved a UL-94 V-0 rating,with reductions of 56.4%and 18.3%in p HRR and TSP,respectively.Moreover,the high transparency and flexibility of PPA-Si coating were maintained even after treatment with liquid nitrogen.（4）To enhance the solvent resistance of the flame-retardant and transparent coatings,a flame-retardant curing agent（DA）was synthesized by combining 1-（3-aminopropyl）imidazole（AI）and diphenylphosphinyl chloride（DPC）.Subsequently,a transparent flame-retardant coating（DA-AE）was developed by facilitating the anionic polymerization of epoxy-siloxane oligomers（AEOS）to form a crosslinked network in the presence of DA.These DA-AE coatings were then applied to PC films to augment flame retardancy,scratch resistance,and solvent resistance（Chapter 5）.It was found that when the molar ratio of DA to AEOS was 1:2 with a weight gain of 12.0 wt.%,PC/DA-AE film was able to self-extinguish within 1.0 s.The shedding areas was 0%in the cross-cut test.The surface hardness was increased from 2B to 6H.The coatings did not show any breakage or peeling after wiping with high/low polarity solvents（silicone oil,ethanol,and water）and 100 cycles of folding.（5）A polycarbonate/expandable graphite（PC-EG）porous skeleton was prepared using non-solvent induced phase separation（NIPS）and innovatively applied as the encapsulation for the phase change materials paraffin wax（PW）（Chapter 6）.The flame-retardant and phase change PC composites were prepared by introducing triphenyl phosphate（TPP）into the skeleton as flame retardants.The flammability and leakage issue were solved due to the char-forming ability and shape stability of PC.It was found that there existed the self-expansion of the PC-EG skeleton and the synergistic flame-retardant effect with TPP.When the weight ratio of PW and TPP was 3:1,the PC-EG/PW/TPP composites self-extinguished within 4.0 s and the p HRR and total heat release（THR）were decreased by 58.8%and 39.1%,respectively.Meanwhile,due to the capillary force of the PC-EG porous skeleton,PW was tightly adsorbed inside the skeleton.The leakage rate of PW was only 0.3%after 10 times of heating/cooling cycles.The latent heat remained constant after 50 cycles of heating and cooling.The flame-retardant and phase change PC composites decreased the maximal operating temperature of the lithium pouch cells by 11.2^oC,improving the operating stability.