Coating curing

Curing is the process from the beginning of painting to the formation of a dry hard film. There are four curing methods:

-Solvent evaporation and drying

-Air reactive drying

-Cure by thermal or chemical reaction

-Radiation (molecular fusion) curing

1. Solvent evaporation and drying

  The solvent evaporates through the coating surface and the solid material is released. This coating can be self-drying at room temperature or can be dried by heating (called forced air drying). Usually the coating is thermoplastic, and the formed film can be redissolved with a suitable solvent. Acrylic resin paints and nitrocellulose paints are similar to typical solvent-volatile coatings. There are only physical changes and no chemical changes during the curing process.

2, air reaction drying

  Oxygen in the air reacts with the coating to form a coating film. This kind of cross-linking reaction usually occurs in high viscosity dry oils (such as linseed oil). For example, the paint used as the exterior wall decoration of the house must first add some solvents to improve its flow characteristics. After coating, the solvent in the paint film quickly evaporates, but the crosslinking reaction between the paint and oxygen may take several weeks to reach the paint. The film is completely cured (dried). The difference between solvent volatilization curing and air reaction curing is: solvent volatilization curing is "curing in air", air reaction curing is "using air" to cure.

3.Cure by thermal or chemical reaction

  This type of coating produces a chemical cross-linking reaction under the action of heat (generally above 140 ° C), light or catalyst. Various base ingredients in the coating are fused with each other and cross-linked to form a three-dimensional network structure. After it will not be dissolved or softened by heat, it is also called thermosetting coating. During the heating process, the solvent evaporates and the paint film shrinks with it. Porcelain paint and alkyd paint are typical examples of chemical reaction coatings.

The chemical composition and structure of these coatings changed during the curing process.

4.Radiation curing

  Radiation curing is further divided into two types: ultraviolet curing and electron beam curing. Electron beam curing has not been widely used due to the disadvantages of high equipment prices, strict requirements, large irradiation blind spots, and poor bending curing effects. Compared with UV curing, it has the advantage of high energy and can be used for curing opaque coating films.

  The light wave used for ultraviolet curing is near-ultraviolet light having a wavelength between 300-450 nm. After being irradiated, a coating film containing a photoinitiator generates free radicals. Free radicals initiate polymerization reactions between the unsaturated units and the resin to achieve cross-linking and curing of the coating film. This process is very short and usually completes in a few minutes. It is generally used for coatings that require high coating quality and are not easy to bake, such as certain wood, paper, and communication fiber.

  Drying time is related to coating film thickness, UV light intensity, and irradiation distance. The stronger the light, the closer the irradiation distance, the thinner the coating film, the shorter the drying time; otherwise, the drying time will sing. Ultraviolet light sources used in industry generally include high-pressure mercury lamps, arc lamps, xenon lamps, fluorescent lamps, etc. Among them, high-pressure mercury lamps are most commonly used.