Abstract
Organic coatings are commonly applied on metals to protect the substrates from their environment. Their applications are varied, but the expectation is that their protective action lasts for many years. The conventional means of achieving a high level of adhesion is by the use of a chemical specific pretreatment. This is costly as the application of a primer is an additional step in the application process and may eventually be harmful to the environment. The enhancement of the adhesion of organic coatings through their formulation would therefore provide ecological and economical benefits to the coating industry. In the current work, the use of an organosilane as an adhesion promoter between a polyamide powder coating and a steel substrate is explored. Powder coatings are formed through the application of a powder on the metal substrate, which melts to form a continuous film. The liquid silane is introduced in the powder stock prior to the application on steel. To optimise the use of the silane, it is desirable to understand the mode of action of the silane. A study has been undertaken on a polyamide 11 based powder coating applied to a steel substrate. Powders have been prepared, based on a reference commercial product with addition of 3-aminopropyltriethoxysilane (APS) at various concentrations. The extent of delamination after salt spray testing has been documented. For all silane-modified coatings, the extent of delamination is reduced. The failure becomes more cohesive after the introduction of the silane. The presence of silane in the coating does not affect the mechanism of corrosion, which is a cathodic delamination but reduces down its kinetics. The presence of APS in the bulk of the coating is limited. Most of the introduced APS has segregated towards the coating outer surface and interface with the substrate. An interphase of APS and polyamide is formed close to the steel. This interphase is of micrometre thickness and contains crosslinked APS. The bond between APS and steel is covalent. Combined with the physical entanglement of APS with the polyamide matrix, this leads to an increasing durability of the coating. The performance obtained for the silane-modified coatings demonstrates the efficiency of the silane as coupling agent. By optimising the choice of the silane and the process, new and environmentally friendly coating formulations are within reach.