Synthesis And Properties Of Latent Hardeners Used In Epoxy Resins

Epoxy resins, one of the most important industrial thermosetting resins, own manyexcellent properties including chemical resistance, good adhesion to many substrates,mechanical properties, and thus find extensive use in various industrial areas. As is wellknown, nearly all epoxy end-use products need reaction with a hardener, and the use ofdifferent hardeners may have different effects on the terminal properties of epoxy resins. Theapperance of latent hardeners enhanced storage stability of epoxy resins containing hardenersunder ambient conditions and extended the limits of application for epoxy resins. Amongvarious latent hardeners, boron-containing compounds, such as BF3-amine complexes andamine-containing borate esters, show not only good latent curing properties, but also animprovement in the thermal stability for epoxy resins.Firstly, a series of short chain amine-containing borates were synthesized using boricacid (BA), N,N-dimethylethanolamine (DMEA) and different di-hydroxyl chemicals byone-step method. Their curing activity and latent properties were studied and compared. Theresults showed that the short chain amine-containing borate (named as NBD) synthesized byneopentyl glycol (NPG), boric acid (BA) and N,N-dimethylethanolamine (DMEA) exhibitedexcellent curing activity and latent properties, its triggered temperature of curing activity was150oC and could be kept stably when mixing with epoxy resins at60oC for54days; DSCcharacterization indicated that one-pot epoxy resin system using NBD as a hardener stillpossessed curing reactivity after being set under ambient conditions for more than315days,for an exothermic peak related with curing reaction was found at200oC on DSC curve.Secondly, a series of long chain amine-containing borates were synthesized usingdi-ethylene glycol, boric acid (BA), ethylene glycol (or NPG), oxalic acid (or hexanedioicacid) and N,N-dimethylethanolamine (DMEA) by two-step method. Their curing activity andlatent properties were also studied and compared. The results exhibited that curingtemperature of long chain amine-containing borates were lower10oC than that of shortamine-containing borate, because the former presented higher curing activity than the latter.Among the long chain amine-containing borates, the product synthesized by NPG, BA,DMEA, glycol and oxalic acid presented high curing activity and good latent properties,whose triggered temperature of curing activity was140oC and pot life at60oC was6dayswhen mixing with epoxy resins; and DSC results suggested that the one-pot epoxy resins stillpossessed curing reactivity after being set at60oC for5days. Thirdly, curing kinetics and curing process of epoxy resins using NBD as a hardenerwere investigated. Isothermal differential scanning calorimetry (DSC) results and fouriertransform infrared (FTIR) spectroscopy indicated that boron-nitrogen coordination in NBDstructure blocked the curing activity of tertiary amine group below140oC, and thecoordination disassociated above150oC. The curing processes of bisphenol A diglycidyl etherepoxy resins (E51) using NBD as a latent hardener in comparison with a common hardener,DMEA, were studied using Avrami-Arrhenius method as well as Horowitz-Metzger methodbased on DSC results. It could be found that NBD showed nearly no curing activity below140oC and exhibited evident curing reactivity when temperature exceeded160oC; and thecuring reactivity of one-pot epoxy resin E51-NBD became higher than E51-DMEA once thetriggering temperature was reached. The apparent activation energy associated with curereaction (Ea) for E51-NBD and E51-DMEA were calculated by Avrami-Arrhenius method orHorowitz-Metzger method. It could be noticed that whatever the method adopted, EaofE51-NBD was larger than that of E51-DMEA, suggesting the cure reaction of the formersystem was more difficult to start than that of the latter. Transition of Avrami exponent n wasconsidered to be related with significant changes of cure mechanism. n value of E51-NBDshowed the first transition at170oC, which had some relationship with its gel point, and thesecond transition occurred at210oC, which was related with the advent of end stage of curingreaction. Avrami-Arrhenius methods could reveal the transition associated with gel point, andHorowitz-Metzger method could disclose the transition associated with the end of curingreaction.Fourthly, effects of different hardeners (NBD and DMEA) on thermal stability of curedepoxy resins (marked as E51/NBD and E51/DMEA respectively) were discussed, and theapparent activation energy associated with thermal degradation (Ea’) was calculated byHorowitz-Metzger method. It could be found the introduction of borate by NBD enhancedthermal stability of cured epoxy resins. Comparing with cured epoxy resins E51/DMEA, T5(the temperature at which weight loss is5%) of E51/NBD increased from356.5oC to405.4oC,and char yield at900oC of E51/NBD was14.8%, much higher than that of E51/DMEA(3.0%). It was also noticed that the content of NBD in cured epoxy resin also influenced itsthermal stability. When mass ratio of E51to NBD was lower than100:12, thermal stability ofE51/NBD was enhanced with NBD content increasing. However, when mass ratio of E51toNBD was higher than100:12, few differences could be found in thermal stability of E51/NBDwith NBD content increasing, and the increase of NBD content may induce the reduction ofthermal stability of E51/NBD. E51/NBD（100:12）possessed good thermal stability, whose T5 was423.7oC, Ea’ was350.3kJ/mol, and char yield at900oC was17.4%. SEM resultsindicated that E51/NBD after thermal decomposition at900oC presented porous structure,and the enclosed porous structure was helpful to prevent the contact between epoxy resins andoxygen and inhibit the release of flammable gas.Finally,9,10-dihydro-9-oxa-10-phosphahenanthrene-10-oxide (DOPO) structure wasintroduced into amine containing borate to synthesize one novel non-halogen flameretardance hardener (DBD), and its curing activity, latent property, flame resistance andthermal stability were investigated. The results displayed that the introduction of DOPOstructure led the formation of a sterically hindered structure, which blocked the curing activityof tertiary amine and the formation of boron-nitrogen coordination, thus induced loss ofcuring activity and latent property. E51/DBD(100:20), whose phosphorus content is1.29%,possessed good flame retardance (V-0), LOI value was35.3%, and could be kept stably at60oC for9days when mixing with E51. It could also be found that thermal stability of E51/DBDdecreased, yet char yield of E51/DBD increased as DBD content increasing. When mass ratioof E51to DBD increased from100:10to100:25, T5of cured epoxy resin E51/DBD decreasedfrom367.1o C to349.6oC, Tmaxdecreased from435.1oC to398.8oC, however char yield at900oC increased from14.3%to19.6%.