Preparation and characterization of sequential curing with off-stoichiometric amine-diglycidyl ether of bisphenol a /novolac epoxy blended systems

Abstract

B-stage epoxy adhesives are used in many applications such as electronic devices and prepreg composites. The most common available form is in the film form, rolls, sheets, or custom preforms. In this work, B-staged epoxy adhesives were prepared by using sequential curing with off-stoichiometric amine technique. Diethylenetriamine (DETA) was used as the curing agent for the first curing step. While dicyandiamide (DICY) was used as the latent curing agent for the second curing step and 3-(3,4-dichlorophenyl)-1,1dimethylurea (DIURON) as accelerator for reducing the curing temperature of DICY. Novolac epoxy was blended in diglycidyl ether of bisphenol A (DGEBA) with various ratio of 0 – 50 phr to enhance the glass transition temperature (Tg) and thermal stability. Non-isothermal differential scanning calorimetry (Non-iso DSC) showed simultaneous presence of two curing agents with two different curing reaction without interference. In addition, thermal stability and Tg were improved with increasing in Novolac epoxy content. However, blending ratio more than 30 phr of Novolac epoxy was practically difficult to proceed due to high viscosity, which is not suitable for film casting process at room temperature. Therefore, the weight ratio 70/30 of DGEBA/Novolac blended was then selected for further investigation owing to its acceptable Tg of 144 °C, thermal degradation at 5 % weight loss (Td5) of 355°C and suitable viscosity for use in film casting process at room temperature. Fourier-transform infrared (FTIR) spectroscopy showed that the inactivity of latent curing agent and accelerator in the B-staged. The storage stability test showed only 8.72 % change in heat of curing reaction and able to adhere with substrate after B-staged for 28 days storage at freezer (-18 °C).