Study On The Synthesis Of Biomimetic Sulphur-containing Polymer-based Flame Retardant Coatings And The Applications Of Their Hybrid Phytic Acid In Flame Retardant Rigid Polyurethane Foams

Rigid polyurethane foam(PU)features low density,good compression-and corrosion-resistance,excellent water-proof,sound-and heat-insulation properties,and thus has been widely used in the areas of energy-saving building materials,chemical and light.With increasing market demand,China has become the world's largest producer of polyurethane and one of the largest polyurethane markets.However,the porous structure and organic composition of polyurethane rigid foam make it extremely flammable,as reflected by a very low limited oxygen index(LOI)of about 18 vol%,and high calorific value,fast spreading speed and the release of a large amount of smoke during burning,which thus has significantly restricted its widespread real-world applications in industries.Therefore,it is particularly urgent to create flame retardant PU foam materials to mitigate its fire threats to lives and properties.To this end,many flame retardant methods have been proposed to improve its flame retardancy while minimizing undesirable impact its original excellent properties,such as thermal insulation ability.In this work,several fire-retardant PU systems with high fire-retardant efficiency,smoke suppression and toxicity reduction,and water-proof properties have been constructed.The effects of the structure and composition/components of the flame-retardant coatings on the properties of coated PUs were thoroughly studied,and the fabrication of the coating was also optimized to maximize flame-retardant efficiency.(1)Inspired by the adhesive behavior of snails and frogs in nature,a new class of biomimetic sulfur-containing copolymers,poly(VS-co-HEA),with phase-separated micro/nano-structure have been synthesized by a facile radical polymerization of hydroxyethyl acrylate(HEA)and sodium vinyl sulfonate(VS).The molecular structure was analyzed by IR,~1H-NMR,elemental analysis and gel permeation chromatography.The unique phase separation structure analogous to the foot pad of tree frog was observed by scanning electron microscope and transmission radio microscope.The copolymers have desired mechanical properties that matches with PU and exhibit high adhesion to the PU surface and other flammable substates due to strong interfacial hydrogen-bonding and mechanical interlocking.Thanks to the high carbonization ability of the coating,the flame retardancy of coated PUs can be significantly improved.For example,the coated PUs with a thickness of 600?m has a high LOI of 35.5 vol%,good self-extinguishing property,low heat release,and can pass the UL-94 V-0 rating during vertical combustion tests.In addition,the coated PUs feature hydrophobic without significantly compromising its good thermal insulation performances after the hollow glass beads were added.(2)In order to further improves the flame retardant efficiency of coatings while reducing the required coating thickness,ammoniated few layer BN was prepared by dry milling hexagonal boron nitride and urea,and then blended with poly(VS-Co-HEA)coating together with low melting point glass powder(GP)to prepare hybridized flame retardant coatings.Thermogravimetric analysis and microcalorimetric analysis show that the thermal stability and charring property of the hybridized coating can be effectively improved by the combination of aminated BN and GP.Coated PUs can pass a UL-94 V-0 rating at a coating thickness of 200?m,showing an LOI as high as 35.8 vol%,and significantly reduced heat release and smoke gas release.The carbon residue analyse reveal that BN and GP have good high temperature stability,and play a"barrier effect"role at high temperatures by isolating heat and mass transfer and maintaining the compactness and integrity of the carbon layer.In addition,the coated PUs can maintain its original thermal insulation performance very well because of the thin coating.(3)A three-layer“sandwich-like”flame retardant coating was constructed on the PU surface using phytic acid,copolymer poly(VS-co-HEA),graphene oxide(GO),carbon nanotubes(CNTs)and BN as building blocks.Due to the participation of phytic acid,a biomass flame retardant,the flame retardant efficiency of the composite coating is further improved,which can be reflected by a higher carbon residue and lower heat release.With the increase of BN content in the composite coating,the flammability of coated PUs decreases and the LOI reaches up to 58.0 vol%,leading to a UL-94 V-0 rating oduring vertical burning tests.Meanwhile,the release of CO also decreases greatly,showing a more stable carbon layer structure after combustion.However,due to the presence of highly thermally conductive BN,the thermal insulation ability of PU is slightly reduced.In addition,the coated PUs can reduce its electrical resistance by several orders of magnitude in a short time under high temperature environment,and thus shows an early fire-warning performance.The low resistance property of the alarm system can still be well maintained during the continuous combustion process to achieve a continuous alarm effect in a fire.This work has successfully developed a new class of sulfur-containing copolymer flame-retardant coatings along with two kinds of hybridized coatings,all of which have demonstrated outstanding flame retardancy efficiency on PU foam while preserving the good thermal insulation properties.This work offers an innovative strategies for the development fire-safe PU foam which is expected to find more practical applications as thermal insulation materials in industries.