Phase formation and microstructure in ultra-thin epoxy films on metals

摘要

The curing reaction and the resulting network structure in 2-part epoxy adhesives strongly depend on the curing temperature and on the mixing ratio. Besides these obvious influences further factors affect the kinetics and the structure of network formation. Especially in ultra-thin films (100 nm range) the curing process differs from the behaviour in the bulk state and the final microstructure in the adhesive is depending on film thickness. Many factors are responsible for these structural variations in the interphase region of adhesive joints. The most important reasons must be seen in the influence of the adjacent substrate surface which besides its chemical influence and besides its potentially catalytic effects can induce phase separations or may lead to an immobilisation of one adhesive component by preferential adsorption. Furthermore, it is well-known that because of the dimensional constraints the molecular dynamics in ultra-thin polymer films is significantly altered as compared to the bulk behaviour. For a reacting system this change in mobility has an additional effect on the developing molecular structure of the network during adhesive cure. Together, all these effects lead to laterally inhomogeneous topologies and contribute to structure gradients in the thin films. Since exactly the interphase region predominantly determines the overall performance and the bond strength for adhesive systems which show near-interface failure, e.g. like many aluminium-epoxy bonds, it is necessary to study how the curing process and the resulting adhesive microstructure vary for ultra-thin adhesive films (100 ― 200 nm) and how they are changed compared to the bulk adhesive.