High Performance Flame Retarding Cyanate Ester Resins

High performance thermosetting resins(HPTRs)are essential materials for many cutting-edge areas such as aerospace,new energy,electrical insulation and electronic information,etc.Excellent flame retardancy has become a basic performance for HPTRs.However,the conventional preparation method of HPTRs is based on heat-resistant resins.Similar with other organic materials,heat-resistant thermosetting resins have poor flame retardancy and can not meet the requirements of HPTRs.Therefore,high performance flame retarding thermosetting resins have become a hot spot of polymer researches.With the appilication widened of HPTRs,the requirements for HPTRs are becoming higher.Therefore,high performance flame retarding thermosetting resins have aroused general interest.As present,phosphorus flame retardants(PFRs)can effectively improve the flame retardancy of polymers,but always at the expense of thermal or mechanical properties of heat-resistant resins.On the other hand,with the enhancement of environmental awareness,people have gradually recognized the potential harm of PFRs to the environment.Therefore,how to reduce the use of PFRs or use environmentally friendly flame retardants to improve the flame retardancy of HPTRs while maintaining the excellent performance of HPTRs has been an important research direction of HPTRs.Cyanate ester(CE)resin is a typical representative of HPTRs which is chosen as the basic resin for studying the green flame retarding technology.The thesis contains following parts:Firstly,a novel phosphazene(E-HCCP)with self-polymerizable alkynyl groups on hexachlorocyclotriphosphazene(HCCP)was synthsized,and then E-HCCP/CE resins were fabricated,of which curing process,thermal and mechanical properties as well as flame retardancy were systematically investigated.Results show that for 5.0E-HCCP/CE(5.0wt%E-HCCP,phosphorus content is 0.56 wt%),compared with CE resin,the peak exothermic temperature(Tp)reduces 60.7-105.1 ℃,fracture toughness increases by 50%;.moreover peak heat release rate(PHRR)and total heat realease(THR)decrease 51.7%and 15.7%,respectively,the limit oxygen index(LOI)increases from 26.9 to 34.6%and the level of vertical combustion(UL-94)changes from NR to V-1.That’s because E-HCCP not only contains amino groups which can cure with CE resin,but also its alkynyl groups can self-aggregate,these change the crosslinking structure and aggregation structure of the resin.Secondly,a new type of multi-functional hybridized(FGO)with HCCP,graphene oxide(GO)and hyperbranched polysiloxane was prepared,and then,FGO/CE resins were fabricated.The effects of FGO content on curing behavior,heat resistance,mechanical properties and flame retardancy of CE resin were investigated.The integrated performances of GO/CE resins were also studied.Results show that with 1.5 wt%FGO,the resultant 1.5FGO/CE resin(its phosphorus content is as low as 0.18 wt%)has significantly improved flame retardancy,the time-to-ignition(TTI)delays 42 s,and PHRR reduces 60%;moreover,impact strength and flexural modulus are improved 48%and 58%,respectively,fully overcoming the bottlenecks of PFRs;in addition,FGO/CE resins show 19.3-70.5 ℃decreased curing temperature,but the initial degradation temperature(Tdi)increases by 0.5-18.8 ℃.These integrated results prove that FGO is a superior and multi-functional filler,which can simultaneously acts as effective flame retardant,curing catalyst,toughening agent,reinforcement while improving thermal stability of CE resin.The flame retarding mechanism of FGO/CE includes condensed phase and gas phase flame retarding,the former promotes the formation of continuous compact char layer through barrier effect of FGO,and the latter is derived from HCCP to capture free radicals and suppress gas phase reaction.Thirdly,a unique and green hybrid(GHNT)consisting of GO and halloysite nanotube(HNT)was synthesized,and then a series of GHNT/CE resins were fabricated.The effects of GHNT on heat resistance,flame retardancy and smoke suppression of GHNT/CE composites were intensively investigated,and compared with those of HNT and GO though studying the properties of HNT/CE and GO/CE.Results show that GHNT can simultaneously improve Tdi,flame retardancy and smoke suppression of CE resin.For GHNT/CE with 5.0 wt%GHNT,it not only has about 15.1 ℃ higher Tdi,but also shows 54.6%or 37.9%lower PHRR or the maximal smoke density than CE resin,clearly demonstrating that GHNT is not the simple combination of GO and HNT,instead,obviously showing synergistic effects on improving both flame retardancy and thermal resistance of CE resin.GHNT overcomes the shortcomings of low heat resistance of GO and poor flame retardancy of HNT.Its essence is that GHNT copolymerizes with CE resin through chemical bond and thus are uniformly dispersed in CE resin.Fourthly,a hyperbranched polysiloxane terminated with alkynyl groups(THSi)was synthesized through a controlled hydrolysis,and then a seires of THSi/CE resins were fabricated.The effects of THSi on the heat resistance,flame retardancy and mechanical properties of CE resin were investigated.Results show that THSi enhances the heat resistance,flame retardancy and mechanical properties of CE resin while maintaining the heat resistance of CE resin.Compared with CE resin,15THSi/CE(15 wt%THSi)has 34.6%and 12.3%increased fracture toughness and flexural strength,respectively,reduced 29.0%or 35.2%of PHRR or THR,24.6%increased LOI as well as UL-94 V-1 level.The outstanding integrated performances of THSi/CE should be attributed to the unique alkynyl structure and the effect of hyperbranched polysiloxane structure on the crosslinked structure.The flame retardant mechanism of THSi was discussed through a quantitative point of view.