SELF-ASSEMBLING MONOLAYER SILANE FILMS AS ADHESION PROMOTERS

The structure of adsorbed silane films formed by the adsorption of tri chlorosilane molecules containing long alkyl chains from solution onto aluminium substrates has been investigated. The structure of the adso rbed films is dependent on the alkyl chain length. When the chain leng th is equal to or greater than 18, the molecules form a film in which the hydrocarbon chains are densely packed together and orientated away from the substrate. As the chain length is reduced the films become p rogressively more disorientated. When a terminal vinylic group is pres ent on the molecules, the films can be activated after adsorption to y ield a hydroxyl group that is available for further reaction with a po lyurethane resin. The ability of 10-undecenyltrichlorosilane and 18-no nadecenyltrichlorosilane to act as adhesion promoters has been investi gated using the blister test. Both these silanes yielded adhesive frac ture energies of 18 J m-2 under dry conditions. When water was present , only the joints pretreated with 18-nonadecenyltrichlorosilane exhibi ted a high resistance to moisture attack. In these joints the dense pa cking of the hydrocarbon chains impeded the diffusion of water to the interface. In the case of the adsorbed 10-undecenyltrichlorosilane fil ms, the disorientated nature of these films allowed the water to reach the interface more easily, resulting in rapid joint failure.