The Application Of Boron-based High-efficiency Flame Retardant Formulations In Polypropylene And Polylactic Acid

Polypropylene（PP）is one of the widely used general plastics with good tensile properties,good impact resistance and stable weather resistance,however,its fire resistance is very poor with heavy dripping during burning.Polylactic acid（PLA）is a typical bio-based thermoplastic resin,which has attracted more and more attention.However,PLA has a series of shortcomings such as poor thermal stability,poor toughness,and flammability.The continuous improvement of the flame retardancy of PP and PLA with the balance of other properties is conducive to their better application in various occasions.Boron-based flame retardant is a halogen-free flame retardant.Compared with phosphorus and nitrogen flame retardants,there are fewer research reports.The halogen-free boron-based flame retardant can assist the formation of dense vitrified char layers in the condensed phase to protect the unburned polymeric materials.In this paper,three paths were used to construct boron-based high-efficient flame retardant formulations:1.Selecting zinc borate（ZB）,ammonium polyphosphate（APP）and a new type of polyhydroxytriazine-based char-forming agent（TTPC）for compounding as a system.2.Ion exchange method to modify APP with boron.3.Synthesizing phenylboronic functionalized chitosan（B-CH）.Subsequently,the three boron-based high-efficiency flame-retardant system are blended into the resin matrix as additive flame retardants to prepare flame-retardant PP and flame-retardant PLA composites,and then flame retardancy,mechanical properties and char structures are studied.The main results are as follows:1.Blending ZB with APP and TTPC into PP,and the experiment reveals that the optimal addition amount of the system in PP is 21%（the ratio of APP:TTPC:ZB=16:4:1）,and the limiting oxygen index（LOI）of the composite reaches the maximum of 29.6%,at the same time,the vertical combustion rating（UL-94）reaches the V-0 rating,and the peak heat release rate（PHRR）drops from 1547 kW·m^-2 to 230 kW·m^-2.While the flame-retardant synergistic effect of ZB plays a role in suppressing droplets and heat release in this system,meanwhile,ZB can promote char formation in the high-temperature process of PP composites.2.Preparation of phenylboronic-acid-modified ammonium polyphosphate（B-APP）by modifying APP with 3-amino-phenylboronic acid via ion exchange method,and then introducing B-APP into the PLA matrix to prepare flame-retardant PLA composites.Through combustion and mechanical properties,electron microscope observation and analysis,etc.,the following results are obtained:PLA composites containing 5%B-APP have excellent flame-retardant properties with a LOI value of 29.4%（an increase of 10.2%compared with neat PLA）,passing the UL-94 V-0 test,and the total heat release（THR）drops from 80 MJ·m^-2 to 63 MJ·m^-2.B-APP is more effective than APP in improving the mechanical properties of PLA,and has an obvious toughening effect.The PLA sample with 5%B-APP changes from brittle fracture to ductile fracture,the elongation at break increases from 9.2%to 19.6%,and the impact strength soars from 0.66 kJ/m² to 1.07 kJ/m².3.Using chitosan（CH）and 4-formyl-phenylboronic acid to synthesize bio-based B-CH（integrating acid source,gas source and carbon source）via Mannich and reductive amination two-step reaction.The results show that B-CH has a good flame-retardant effect on PLA.Adding 7%of B-CH can increase the LOI of PLA composites to 26.2%and pass the V-0 level.The PHRR value of PLA-7%B-CH material declines from 564 kW m^-2 to 432 kW·m^-2,and the THR decreases from 81 MJ·m^-2 to 63 MJ·m^-2.