The attainment of controlled adhesion by incorporation of low level additives in a PDMS-based adhesive
Stearates are commonly used in polymer technology as slip additives and lubricants. The release properties of stearic acid and its metallic derivatives are extensively manipulated in rubber processing industries where they are used as mold release agents. Previous unrelated studies have shown that increased additions of stearic acid consistently reduced mold sticking. Despite their widespread usage, however, few studies have been directed to study their mechanism of action. In a preliminary study conducted by AWE and Loughborough University, the attainment of low levels of adhesion was studied and in particular the use of stearic acid as an adhesion modifier was studied. It was found that adding minute amounts of stearic acid resulted in dramatic reductions to the strength of aluminium bonded with a PDMS-based adhesive. The cause of this fall-off in adhesion (as measured by T-peel test) was initially attributed to the migration of stearic acid to the surface forming a weak boundary layer. The possibility that stearic acid acts as an inhibitor for the cure reaction was also considered. Surface analysis predominantly with static secondary ion mass spectrometry (SSIMS) and X-ray photoelectron spectroscopy (XPS) was carried out to ascertain locus of failure and the likelihood of stearic acid at the interface. Failure was identified as having taken place in a cohesive, and potentially interphasial, layer near the interface of the joint. No evidence was given to support the WBL theory proposed by most researchers, thus findings from the present study contradicted existing literature. Fourier Transform infrared (FT-IR) spectroscopy in collaboration with differential scanning calorimetry (DSC) was used to investigate the effect of stearic acid on the cure reaction. These studies disclosed a catalytic effect on the curing process of the adhesivp... It was suggested that the resultant increase in modulus of the stearic acid modified material in the region of failurew as the cause of prematurejo int failure at low level loads.