The Impact of Silane Coupling Agent on Adhesion of Coatings

Organic silicon coupling agents, such as X3Si(CH2)nY, can significantly improve the cohesion and adhesion to metal substrates of coatings. The X-group is a hydrolysable group that can react with the hydroxyl or water on the metal substrate surface, while the Y-group is an organic functional group that can react with the resin. For the coupling agent, the Y-group is an epoxy-functional group. The clean surface of the steel substrate is not iron but hydrated iron oxide. The reactive silicon coupling agent's X-group and the hydroxyl (or surface water layer) on the metal substrate surface form hydrogen bonds, which then condense into an oxapropane ring. This results in high adhesive strength of the organic material to the inorganic substrate in wet conditions.

Interaction between silicon coupling agent and metal substrate

When using silicon coupling agent KH-560, the amount of silane coupling agents used affects the pull-off adhesion of the epoxy anticorrosive coating to the substrate. With increasing silicon coupling agent KH-560 usage, the adhesion of the coating to the substrate gradually increases. The adhesion reaches its maximum value at 1.0% usage, then stabilizes. At this point, the adhesion strength of the coating increases from 12.2 MPa to 16.3 MPa. This is because, in addition to the chemical bonding between silicon coupling agent KH-560 coupling agent and the metal substrate surface, its epoxy functional group also undergoes limited copolymerization reaction with the amine curing agent, forming an interpenetrating polymer network structure with the epoxy resin curing system. This greatly enhances the cohesion of the paint film. As the silicon coupling agent KH-560 coupling agent usage increases, the IPN structure of the interface also increases, and the cohesion of the coating film increases with it. However, after reaching a certain amount, the number of hydroxyl groups on the inorganic metal substrate surface is limited. Therefore, the hydrogen bonds and oxapropane rings formed between the silicon coupling agent KH-560 coupling agent and the metal substrate also reach their maximum value, resulting in a stabilizing of the pull-off strength at its maximum value.

Impact of silicon coupling agents on the salt spray corrosion resistance of coatings

Under conditions where silicon coupling agent KH-560 usage is 1.0%, the salt spray corrosion resistance of the epoxy anticorrosive coating before and after the addition of the coupling agent was studied. The cold-rolled steel plate coated with the epoxy anticorrosive coating showed severe rusting after 600 hours of neutral salt spray test without KH-560 coupling agent added. However, with the addition of KH-560 coupling agent, only slight rusting occurred. 

The main reason for this is twofold. First, during the salt spray test, the plasticization effect caused by water absorption on the coating film at a certain temperature led to relaxation, and the adhesion points of the coating did not move due to the coating relaxation when silicon coupling agent KH-560 coupling agent was added. This was due to the improved adhesion between the coating film and the substrate. Second, the cross-linking reaction between the epoxy functional group on the silicon coupling agent KH-560 coupling agent and the amine curing agent forms an IPN structure in the epoxy coating system, further improving the coating's compactness, greatly reducing the rate of penetration of water, oxygen, and inorganic ions and effectively slowing down the corrosion of the substrate by corrosion factors. Therefore, the salt spray corrosion resistance of the epoxy anticorrosive coating was significantly improved by the addition of silicon coupling agent KH-560 coupling agent.

Silicon coupling agents have higher humidity sensitivity and are hydrolyzed to form silanol, which self-condenses to form low-molecular-weight silicone polymers, resulting in loss of activity. Therefore, when applied to solvent-based protective coatings, the paint components must be dry, and the choice of solvent must also be carefully selected to avoid water content. Most industrial-grade organic solvents contain a small amount of water, and inorganic fillers can also contain a certain amount of water due to moisture absorption. In this study, the effect of water content in the coating on its performance was studied by dehydrating the raw materials used. KH-560 coupling agent can significantly improve the adhesion and salt spray corrosion resistance of the epoxy anticorrosive coating to the substrate. When the raw materials are dehydrated and the KH-560 coupling agent usage is 1.0%, the pull-off adhesion of the coating film to the substrate is 16.3 MPa, and the salt spray corrosion resistance time is 720 hours.