Study of cure kinetics, chemorheological and thermal behaviour of different formulations of DGEBA epoxy

Abstract

Advanced composite industry use various methods to fabricate the composite components. The selection of fabricating method is a matter of required specific design of composite, cost effectiveness, manufacturing challenges, high volume production of the material and application. Various resin infusion techniques are being employed to meet the production challenges. Rapid resin transfer molding (RTM) is one of the vacuum assisted method for resin infusion. The resin required by RTM (Resin Transfer Molding) must have very low viscosity for fast injection into the mold cavity and into the reinforcement fabric at room temperature. Resin should also show high cure rate/short gel time when heated in the mold at its cure temperature. In this research work various formulations were developed based on DGEBA epoxy for rapid RTM (Resin Transfer Molding) process. The aim was to develop the resin formulations with low viscosity and long gel time at room temperature for quick mold filling, and very fast curing once the resin is heated inside the mold at its curing temperature. A commercially available DGEBA based epoxy resin (D.E.R.™ 331™) was used as base resin. Triethylene-tetraamine (TETA) (D.E.H.™24) an aliphatic polyamine was employed as curing agent. The viscosity of resin was varied by adding an epoxy based reactive diluent whereas the resin cure rate was enhanced by adding imidazole in the formulations. The formulations were developed by varying the mole ratios of epoxy, amine, diluent and imidazole. Several epoxy resin formulations were studied and compared with the commercially available infusion-grade epoxy system of Araldite LY 8601/Aradur 8602. DSC was employed to monitor the in-situ cure reaction by using isothermal and non-isothermal modes of measurements. Activation energy of cure reaction for each formulation was calculated using the Kissinger and the Ozawa equations. Model free analysis was used to evaluate dependence of activation energy on degree of cure. Addition of imidazole effectively reduces the activation energy of cure reaction of base epoxy from 57.5 kJ/mol to 46.5 kJ/mol. The diluent was added to lower the viscosity of resin and to enhance the resin flow behavior but it slows down the rate of reaction which is compensated by addition of 5% imidazole. Rate of cure reaction was increased as the temperature was increased which is depicted by degree of cure and glass transition temperature increase. At higher degree of cure, Tg values were utilized to vi evaluate the degree of cross linking at higher temperatures of cure. A relation between Tg and degree of cure was developed. Epoxy and amine with 10% diluent and 5% imidazole (DD/5I/10E) formulation was capable of getting maximum cured within 5 minutes at 100 ℃. Chemorheological studies were carried out to monitor the resin flow behavior. Viscosity at room temperature was measured and a time-viscosity relation was developed at various temperatures to measure viscous flow activation energy. Lowest viscosity was observed for commercially available infusion-grade epoxy system of Araldite LY 8601/Aradur 8602 at any temperature, whereas epoxy and amine with 10% diluent and 5% imidazole (DD/5I/10E) formulation showed the lowest viscosity of 1.04 Pa.s at 1 s-1 shear rate at room temperature. Fastest gelation (26s) was achieved by pure epoxy and amine formulation (DD). Thermal stability of developed formulations were calculated by TGA. All the developed formulations were found to be thermally quite stable more than 300 °C it was observed that addition of imidazole makes base resin thermally more stable. Maximum thermal stability was shown by formulation with 5% imidazole i.e. 380 °C. TGA thermograms were recorded at various heating rates and the activation energies of degradation were determined for all the formulations using Kissinger’s equation. Viscoelastic behavior of maximum cured formulations was also studied by DMA. Among all the values obtained for storage moduli (E′) the lowest values was found to be 1109 MPa associated with epoxy and amine with 10% diluent and 5% imidazole (DD/5I/10E) formulation. Among all the values of loss moduli (E″) for all the formulations the highest value, 212 MPa, was also observed for epoxy and amine with 10% diluent and 5% imidazole (DD/5I/10E) formulation. The results show that epoxy systems such as that containing 5% imidazole and 10% reactive diluent are suitable for the rapid RTM (Resin Transfer Molding) process with balanced flow and cure characteristics.