Design Of DOPO Hybrid Systems And Synergistic Flame-retarded Modification On Epoxy Resins

Multi-element synergistic flame retardant has attracted extensive attention of researchers under the trend of research and development of flame retardants with characters of halogen-free,low-toxicity and green sustainable.Therefore,in the field of flame-retardant materials,considerable attentions have been focused on flame-retardants comprising both phosphorus and nitrogen elements.Metal-based flame retardants have also attracted more and more attention due to their ability to catalyze polymer char formation under hot oxygen activation.In this paper,compound synergistic and intramolecular synergistic flame retardants were designed and synthesized based on the three elements of phosphorus,nitrogen and titanium.And phosphorus/titanium,phosphorus/nitrogen and phosphorus/nitrogen/titanium flame-retardant epoxy resin systems were established.The influences of the flame retardants on the thermal properties,flame retardant performance and mechanical properties of epoxy resin were investigated.In addition,the flame retardant mechanism was studied preliminarily,which provides new references and ideas for the research and development of halogen-free flame retardant epoxy resin composites.The main research contents are as follows:Part one,using DOPO and TBT as phosphorus source and titanium source to construct binary phosphorus/titanium compounding flame retardant,adjusting the addition of DOPO and TBT to synergistically modify the epoxy resin.The TG,LOI,SEM and other analytical methods were used to characterize the moisture resistance,flame retardancy,mechanical properties and dielectric properties of EP/Ti/DOPO composites and the modification mechanism was further discussed.The experimental results showed that the moisture-resistance property of EP/Ti/DOPO was improved because TBT reacted with hydroxyl group of epoxy resins,resulting in decreasing of the hydrophilic hydroxyl numbers and increasing of the crosslinking density of epoxy resins.When the titanium content was 0.56%,the EP/Ti/DOPO system reached UL-94 V-0 rating with a LOI of 34.7%.Compared to pure epoxy resin,the PHRR,HRC and THR of EP/Ti/DOPO were decreased by 33.3%,35.1%and 16.7%,respectively.TGA showed that the introduction of TBT and DOPO could increase the amount of residual char and reduce the thermal decomposition rate,thus enhancing the thermal stability of epoxy resin.Through the analysis of the carbon residue,it was found that the carbon residue of EP/Ti/DOPO has a honeycomb inner layer structure and a continuous dense outer layer,which could be used as a physical protective layer to hinder the heat and oxygen,and ultimately weaken the combustion intensity,thereby improve the flame-retardant performance of the polymer.Dielectric test results showed that the introduction of TBT or DOPO could reduce the dielectric constant and dielectric loss of epoxy resin.However,when the simultaneous introduction of TBT and DOPO,the dielectric loss of epoxy resin possessed better stability with the frequency changes.In this chapter,EP/Ti/DOPO modified by DOPO/TBT has excellent moisture resistance,high thermal stability,flame retardancy and mechanical properties and is expected to be better applied to the field of microelectronics.Part two,three novel flame retardants(TAT-triDOPO,TAT-hexaDOPO and TAT-enneaDOPO)were designed and synthesized via classical Kabachnik-Fields reaction by the combination of 2,4,6-tris(p-aminoaniline)-1,3,5-triazine(TAT),DOPO and POM,which correspond to the ratio of the number of triazine and DOPO units in the molecule 1:3,1:6 and 1:9.And the influences of adding 5%TAT-DOPO flame retardants on the thermal stability,flame retardancy and mechanical properties of epoxy resin were investigated and the possible synergistic mechanism of phosphorus and nitrogen was suggested.The experimental results showed that the thermal stability and flame retardant properties of the three kinds of modified EP were all improved by adding 5%TAT-DOPO flame retardants.The proportion of phosphorus and nitrogen in TAT-triDOPO could make the phosphorus and nitrogen elements play an effective synergistic effect.That is among three flame retardants,TAT-triDOPO possessed the best thermal stability;The LOI value of EP/TAT-triDOPO composite increased from 26.2%(pure EP)to 34.0%and reached UL-94 V-0 rating.The mechanical performance test results showed that it could increase flexural strength and elastic modulus of epoxy resin at the same time,which may due to the structural advantage of TAT-triDOPO.Finally,it was speculated and verified that the flame retardancy of TAT-triDOPO in EP composite was the combination of condensed phase and gas phase flame retardant mechanisms.During the combustion process of EP/TAT-triDOPO,free radicals containing phosphorus and nitrogen and inert gases were released,which not only diluted the concentration of combustible gases,but also promoted to form expanded and honeycomb structural carbon layer.On the other hand,generating polyphosphoric acid and compounds with triazine structural covered the surface of substrate to form a protective layer with excellent thermal stability,which could block the transmission of heat and oxygen and prevent further ablation of substrate.The self-extinguishing effect of high-concentration free radicals,the generation of non-flammable gases and the stable carbon layer formed by the combination of phosphorus and nitrogen effectively improved the flame retardancy of the epoxy resin.Part three,TAT-triDOPO and amino-functionalized nano-titanium dioxide(NH2-TiO2)were used as phosphorus/nitrogen source and titanium source to obtain a tri-phosphorus/nitrogen/titanium synergistic flame retardant,and a ternary synergistic flame retardant epoxy system with combination of compound synergy and intramolecular synergy was further established.The mechanical properties,moisture resistance,dielectric properties,thermal stability and flame retardancy of epoxy composites were characterized.The experimental results showed that the incorporation of NH2-TiO2 and TAT-triDOPO not only could effectively improve the moisture resistance,but also could improve the toughness and flexural strength of EP composites at the same time,and maintain a high elastic modulus.The results declared that the composite has excellent mechanical behavior.Meanwhile,the introduction of NH2-TiO2 and TAT-triDOPO made EP exhibit lower dielectric constant and dielectric loss.However,it was found that the compatibility of modified titanium dioxide and epoxy matrix was still not very good,and the modified EP was completely opaque due to the defect in the amination of TiO2.Especially the modified epoxy resin occurred agglomerates inside with 5%NH2-TiO2,which reduced thermal stability of EP/NH2-TiO2.Flame retardancy test results showed that NH2-TiO2 did not have flame retardant effect.However,compounding NH2-TiO2 with TAT-triDOPO,the LOI value of EP/TAT-triDOPO/NH2-TiO2 increased to 28.9%with UL-94 V-1 rating,and the PHRR and THR were significantly reduced,which all indicated that the simultaneous introduction of NH2-TiO2 and TAT-triDOPO could improve the flame retardancy of the epoxy resin but the increase was not significant.SEM test results showed that modified epoxy resin by compounding NH2-TiO2 and TAT-triDOPO formed a hard carbon layer with a small amount of cracks on the surface.This kind of carbon layer could not effectively block transmission of external heat and oxygen,thus flame retardancy of EP/TAT-triDOPO/NH2-TiO2 was not particularly improved.This part of the work needs to be further improved.