Preparation Of UV-Curable Flame-Retardant Coating And Study On Its Flame-Retardant Natural Fiber Composites

Nature fiber composites have many advantages,such as low density,high strength,low cost,good sustainability and recyclability,which can be widely used in automotive indestry,aerospace and other fields.Flame-retardant coatings containing P/N and P/N/S were prepared on the surface of fiber composites by ultraviolet(UV)curing technology according to the requirements of these fields for the flame retardant performance of fiber composites.This coating has high adhesion,good flexibility,fast reaction speed,and no harmful gases,so it meets the development requirements of green flame retardant.In this paper,the curing and flame-retardant mechanism of the UV-curable coating are clarified by analyzing the curing process and flame retardant properties of the UV-curable flame-retardant coating on the surface of the composites.The main content of this paper is devided into the following two parts:(1)Bisphenol A epoxy acrylate(BAEA),polyurethane acrylate(PUA)and 2-hydroxyethyl methacrylate phosphate were used to prepare flame-retardant coatings.The PM-2 and PUA improved the flame retardancy and the adhesion of the coatings.Isopropyl thioxanthone(ITX)and triethanolamine(TEOA)were selected as co-photoinitiators,together with phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide(BAPO)to form a photoinitiator system.The photoinitiator system could supress the effect of oxygen inhibition and improve the double bond conversion rate during the curing process.The results showed that the peak heat release rate(PHRR)and smoke generation(TSP)of the coated composites are significantly lower than that of the uncoated composites.The limiting oxygen index(LOI)could reach 26.1%.The continuous dense carbon layer could be seen after the horizontal combustion test.At the same time,the higher the PM-2 content,the slower the combustion rate.This showed that the coating could effectively suppress the spread of flames.In addition,the flexural strength of the coated composites was increased by 90%compared to the uncoated sample.However,as the content of PM-2 in the formulation increases,the flame retardancy of the coating increased and the double conversion rate of the coating decreased.(2)In order to increase the double bond conversion rate of the coating and reduce the oxygen inhibition effect during the UV-curing process,a new type of the composite flame-retardant coating was prepared by the thiol-ene click reaction in the second part of this article.The UV-cured flame-retardant coating containing N/P/S was prepared on the surface of the fiber composites by changing the relative molar content of pentaerythritol tetrakis(3-mercaptopropionate)(PETMP),triallyl cyanurate(TAC)and PM-2.The synergistic effect of N,P,S elements in the coatings was investigated by characterizing the flame retardant properties and thermal degradation behavior.The results showed that when the molar ratio of N and P was 3:2 and the thiol to double bond molar ratio was 4:6(both N and P were provided by the prepolymer containing double bond),the sample S₄(N₃P₂)₆ had the highest LOI(up to 28.6%)and the lowest PHRR.In addition,thermogravimetric analysis(TG)showed that phosphate groups could decompose at low temperatures,promoting the formation of the char layer,reducing the pyrolysis rate and delaying the decomposition process of the composites.Scanning electron microscopy(SEM)tests showed that a honeycomb-like porous carbon layer was formed on the surface of the composites,which indicated that N,S elements could produce non-flammable gases(CO₂,SO₂)and the gases could dilute the flammable gases produced in the combustion process.In addition,the double bond conversion rate of the coating was more than 90%,which showed good film forming performance.The transparency of the coating was about 90%and the relative change of the prepolymer content didn't affect the transparency of these coatings.To sum up,this coating has obvious practical value and can effectively prevent fire in industrial production.