Synthesis Of Boron-amine Complexes As Latent Curing Agents And Study On Properties Of Epoxy Resin System

In this paper, three novel thermally latent catalysts:2-(2-dimethylaminoethoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane(PDB),2-(2-dimethylaminoethoxy)-1,3,2-benzodioxaborinine(SDB) and2-(2-dimethylaminoethoxy-1,3,2-benzodioxaborole (CDB), were synthesized andthe chemical structures of the boron-amine complexes characterized by FT-IR,1H NMR,13CNMR, and MS analysis.1H NMR experiments were conducted to study the effect of theelectronegativity of substitution on the B-N dative bond. It was found that the increase ofpositive charge on the boron atom causes a shift to lower frequencies in the1H NMR spectra.The lewis acidity of boron atom in these three complexes and the strength of B-N dative bondwere in the order: CDB>SDB>PDB. The effect of the substituents on the activity of thesecuring agents was investigated by isothermal and non-isothermal differential scanningcalorimetry (DSC). The results showed that the boron atom with electron withdrawing groupwill have higher lewis acidity and display lower curing activity and better latency tobisphenol A diglycidyl ether epoxy resin (E51).Kissinger, Ozawa and Flynn-Wall-Ozawa methods were utilized to determine the kineticparameters and showed that the kinetic reaction of E51/boron-amine complexes was anautocatalytic curing process. According to research in recent literatures on B-N dative bondof boron-amine complexes, we proposed a reasonable curing mechanism of E51/boron-aminecomplexes systems. As a first step, ROH was deprotonated by tertiary amine and the alkoxyanions was coordinated with the boronate to afford a zwitterionic complex. The complexshowed latency due to the weak curing activity of oxide anion and proton to epoxy resin atroom temperature. After heated to a definite temperature, the alkoxy anions and proton willconcertedly attack and cleave the epoxy ring to generate activated chain ends, and thencatalyze the chain growth reaction continuously.In our study, the effects of different curing agents on the thermal stability of cured epoxyresin was investigated by Thermogravimetric Analysis (TGA). It demonstrated that theboron-amine complexes can significantly increase the initial degradation temperature andchar yield of cured resins. Compared with E51-DMEA system, the5wt%weight loss temperature of E51/boron-amine complexes system increased from372.0℃to400.3℃and the ultimate char yield also increased by more than6.5wt%at800℃when E51resinsand the curing agents were at the ratio of100:9. The char yield of the cured resins improvedwith boron contents increasing by adding6wt%~20wt%curing agents. When the dosage ofSDB was18wt%, the ultimate char yield was up to20.5wt%at800℃. In addition, fromDynamic Mechanical Analysis(DMA), we drew a conclusion that the complexes containingB-N dative bond can effectively promote the cross linking density of the resins, resulting inincreasing storage modulus (E’) and glass transition temperature (Tg). However, to bothE51-SDB and E51-PDB systems, the cross linking density decreased as curing agentsincreased gradually when the dosage was6wt%~18wt%, lowing Tgand rubber state E’ ofthe cured resins.