Environmental effects on the interlaminar shear strength of aramid-epoxy composites

Abstract

Resin-reinforced composites have quite good mechanical properties but are very susceptible to environmental degradation [1]. Although considerable work has been carried out on the effect of water on these composites [2-5], much less attention has been paid to other potentially aggressive substances such as gasoline and oil. In addition, the extant work mainly deals with carbon-epoxy composites [6, 7] and few data are available on the behaviour of a ramid-epoxy composites. This letter deals with the effect of immersion in gasoline and motor oil on the interlaminar shear strength (ILSS) of unidirectional (0 °) aramid-epoxy composites. The ILSS was chosen because it gives a qualitative measure of the matrix-fibre interface integrity [8]. The composites were fabricated by the pre-preg route using standard lay-up and vacuum bagging procedures. They were made with a standard epoxy resin (DER 383), cured with an aromatic polyamine (DEH 50), reinforced with Kevlar-49 aramid fibres. The cure cycle and resin-to-hardener ratio were chosen according to the manufacturer 's specifications. The final fibre volume fraction was 65% with no more than 0.5% of voids. The specimens (thickness 3.3 ram, width 10mm and length 18.5 ram) were cut from the 300ram x 300ram panels, dried at 50 °C, for 24h and subsequently cooled in a desiccator. They were then immersed in commercial gasoline or motor oil SAE 30 (Havoline HD) for 0, 500, 1000 and 2000h. The mechanical tests were performed, according to ASTM-D 2344 standard, in a hydraulic Instron machine using a span-to-thickness ratio of 5. Ten specimens were tested for each conditioning time. A plot of residual ILSS against immersion time is shown in Fig. 1. The aramid-epoxy composite shows no reduction of ILSS value up to 2000 h when immersed in motor oil. In the first 1000h gasoline reduces the ILSS by approximately 20%. Beyond 1000 h up to 2000 h no further reduction was observed. This trend agrees well with the results of Kanellopoulos et al. [6]. In their work it was shown that large organic molecules have less ability than water to enter into polymer. Therefore, it is reasonable to say that the ability of a molecule to go into the polymer and consequently to degrade its mechanical properties is inversely proportional to its size and/or atomic weight. This is in good agreement with the present results, and suggests good performance of a ramidepoxy composites when exposed to gasoline or motor oil. The behaviour for longer immersion times is now under investigation.