Effects of rubber particles on mechanical properties of epoxy resin

Abstract

This research aims at investigating the effect of solid and liquid rubber on the mechanical and thermal properties of epoxy resin. The solid rubber in this present study is nitrile butadiene rubber (NBR) and the liquid one is carboxyl-terminated acrylonitrile-co-butadiene rubber (CTBN) with an acrylonitrile (AN) content of 15% and 30%. They were added to epoxy resin by 5 to 30% concentration by volume, except the CTBN with 30% AN which was limited to 10% by volume only due to its exceedingly high viscosity. The NBR particles were applied untreated and treated with a titanate coupling agent. Within the range of concentration studied, all types of rubber were found to effectively reduce the modulus of elasticity and the yield stress as the amount of rubber was raised. Significant enhancement of the fracture energy by upto 33%, 188% and 625% was achieved when the fracture energy of the rubber-modified epoxy systems was determined under Izod impact, falling weight and double torsion respectively. The sequence of the rubber found to increase the toughness of epoxy is NBR < treated NBR < CTBN with 15% AN < CTBN with 30% AN respectively. The main mechanism for the effective toughening induced by the CTBN is the interfacial bonding arising from the reaction between the carboxy in the CTBN and and epoxide group in the epoxy resin. The finer particles in the CTBN with 30% AN, as was evident in the scanning electron microscope (SEM) photomicrographs, mean the there was greater surface area for interfacial bonding sites in the system with 30% AN than that in the 15% AN system. Consequently, greater amount of energy is required to overcome these abundant intervacial bonding sites for fracture to take place. Moreover, the dynamic mechanical thermal analysis test also reveals a decrease in the glass transition temperature (T[subscript g]) of the epoxy resin in the CTBN-modified system towards that of the CTBN’s. This clearly supports the proposed mechanism of the interfacial bonding between the CTBN and the epoxy resin and verifies that they are partially compatible. In the case of NBR, the less effective toughening imparted by the NBR is due to its non-reactive surface property. With additional surface treatment by the titanate coupling agent, the NBR surface becomes more reactive and hence a better toughening effect was observed in the treated NBR-epoxy system. However, the larger particle size of the NBR, which was found to be 100-200 µm in the microscopic observation, renders less surface area for reactive sites than the finer particles in the CTBN-modified epoxy systems.