Effect of surface cleanliness of aluminium substrates on silicone rubber adhesion

Abstract

The aim of this work was to determine the minimum surface cleanliness of aluminium substrates required for good and reproducible silicone rubber adhesion. Aluminium substrates were prepared, ranging from ‘contaminated’ to different degrees of ‘cleaned’. The surface energy of the substrates was determined by contact angle measurements. The surfaces were also compared using simplified methods, such as a wettability test or by the use of inks with known surface tension. Silicone rubber was then compression moulded onto the cleaned and primed substrates. The silicone rubber adhesion was then evaluated by lap-shear testing, before and after ageing. The ageing step consisted of immersion of samples in boiling water for 100 h to evaluate the hydrolytic stability of the interfaces. The failure modes after lap-shear testing were determined using optical microscopy and scanning electron microscopy and were divided into three different categories: cohesive failure, adhesive failure or a mixture thereof. Energy dispersive x-ray mapping was useful in clarifying the failure modes by determining the position of the primer, which contained Ti. It was concluded that in order to obtain a strong and stable interface, exhibiting mainly cohesive failure between the aluminium substrate and silicone rubber, the surface energy of the substrate before priming should be >45 mJ m−2, including a polar component of >10 mJ m−2. This corresponded to a hydrophobicity class of the substrate of ⩾6, according to IEC 62073.