Because the shellac primer has a good sealing effect, people are accustomed to using shellac paint as a primer to make woodware for a long time, but it has been proved by practice that when using shellac paint as a primer for polyurethane paint finishes, bottoming, The adhesion between the top coats is poor, and even a large area of peeling may occur. What is the reason for this?
The main component of shellac is lac resin, is a kind of low molecular weight resin, and average molecular weight is about 1000, and on average each molecule contains at least one free radical, one aldehyde group and five carbonyl groups. This chemical composition determines that shellac resin has a strong hydrogen bond structure, so it has strong cohesion and adhesion. Although the molecular weight is not high, it can also form a strong connecting film, and synthetic polymers with a molecular weight of less than 1oooo can form There are not many paint films. However, the free radicals in the shellac resin have become an obstacle to its use with polycool paint.
In the process of using polyurethane lacquer, avoid contact with water, because the water can react with the isocyanate group in the first component of polyurethane lacquer to release carbon dioxide gas, forming pinholes in the paint film (i.e. bubbles). Similarly, organic compounds containing bonding groups can also produce the same effect as water, and the generated carbon dioxide gas will inevitably weaken or even destroy the adhesion between the interfaces, resulting in peeling off of the paint film.
