Low-temperature Curing And Thermal Degradation Kinetics And Toughening Modification Of Epoxy Adhesives

Epoxy resin has been widely used for its excellent fluidity, chemical resistance, low shrinkage on cure, superior electrical and mechanical properties, et al. Nowadays, more and more researches have been focused on low temperature performance and thermal degradation of epoxy resin due to the application requirments.This thesis consists of four parts:Part One, synthesis of thiourea-modified DETA as epoxy resin low-temperature curing agent. The synthesis conditions were optimized by three aspects, there were the monomer mole ratio, synthesizing time and synthesizing temperature. The curing agent TU-DETA was successfully prepared with its structure characterized by fourier transform infrared (FTIR) spectrum and nuclear magnetic resonance (C-NMR) spectrum. The polymerization degree (n) of TU-DETA was confirmed according to liquid-chromatography-mass spectrometry (LC-MS) analysis. The discussions were made of effects of the synthesizing time, temperature and monomer mole ratio on the curing agent performance by the contact angle and differential scanning calorimetry analysis (DSC). The investigation on optimum addition ratio of curing agent to epoxy resin was further made by DSC. The results indicated that when the curing agent synthesized with mole ratio of DETA to thiourea as1.6at130℃for3h and added to epoxy resin at1:5by weight, the contact angle of curing agent TU-DETA was minimum and reaction heat was maximum that were the optimum synthesis conditions.Part Two, kinetic researches on low-temperature cure of epoxy adhesives. A kinetic research on low-temperature cure of epoxy adhesives was experimentally and analytically investigated by means of DSC on both isothermal and dynamic scanning. The experimental rusults showed that:(1)For dynamic curing section, in order to consider apparent activation energy Ea varied with the variation of curing degree a, an advanced isoconversional method was taken into account for computing the minimum Ea value for each value of a lying in between0.05and0.95with a step of0.01. The correlation of invariant Ea and pre-exponential factor were expressed by "compensation parameters" equation.(2)Curing did take place at temperatures of10-15℃but it needed time to reach complete reaction owing to the effects of material vitrification and the associated diffusion controlled in the late isothermal curing stages in isothermal experimental. The cure reaction orders m and n increased with the increment of temperature overall. Similarly, the rate constant k increased with the increment of temperature. Comparing the diffusion factor to that of previously reported epoxy adhesives cured at higher temperature, we found that the diffusion factor was similar. Diffusion-controlled only modified the later curing stages and the modification effect was same.(3)Existing dynamic and isothermal curing phenomenological autocatalytic models developed for hot-curing epoxy adhesives proved applicable to simulate the curing behavior of the cold-curing adhesives even at low temperatures. However, a heating rate-dependent pre-exponential factor and diffusion control had to be taken into account. The modified modeling with diffusion controlled was much more accurate simulate to the experimental data by low-temperature isothermal curing adhesives, similarly, the modified modeling with heating rate-specific pre-exponential factor agreed well with experimental data.(4)Analysis of nonlinear regression was carried out on the isothermal modeling. Results showed that the nonlinear least squares fitting which carried out on linear approximation by Taylor formula had a satisfactory effect.Part Three, non-isothermal kinetics of thermal degradation of DGEBA/TU-DETA epoxy system. The three methods (Flynn-Wall-Ozawa method, Coats-Redfern method and Phadnis-Deshpande method) of kinetics were stated in details. The thermal degradation behavior of the epoxy system cured by TU-DETA was observed by thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). The results revealed that the thermal degradation of epoxy adhesives was only a single weight-loss step. It was the decomposition of the main chain of polymeric substance. The average Ea calculated with the Flynn-Wall-Ozawa method was140.4KJ/mol. Combined with the Coats-Redfern method and the Phadnis-Deshpande method, the most probable mechanism of degradation process of the cured epoxy adhesives was Fl (Random nucleation with one nucleus on individual particle) deceleration type.Part Four, researches on low-temperature toughning modification of epoxy adhesives. The epoxy system can be cured totally in8days at low temperature-5℃ according to the result of FTIR analysis. The formula of epoxy adhesives was optimized by orthogonal experimental method and mechanical property testing. Epoxy resin was modified by toughening agent polyether polyols (BE). Scanning electron microscopy (SEM)、dynamic mechanical thermal analysis (DMTA) and mechanical property analysis demostrated the effect of the toughness of that BE modified epoxy resin was increased markedly at low temperature. The polymeric substance exhibited better thermal ability after the toughness of that BE modified epoxy system. And when the addition amount of BE was9%, the polymeric substance had a better thermal stability and toughening effect.