Paint drying refers to the entire process of converting a liquid or viscous film into a solid coating. It is also the process of converting paint into a decorative and protective coating. It is directly related to the final effect and also an extremely important part of the entire paint application. Paint drying are generally available for physical and chemical drying process.
Physical drying process
Paint relies on solvent volatilization to form a film, such as single-component paints like perchloroethylene paint, chlorinated rubber paint, nitro paint and acrylic paint.
It has only one drying stage, the resin wraps the pigment then dissolves in the solvent. After painting, the solvent evaporates to leave a solid coating film, which belongs to the volatilization film formation of solvent-based dispersion medium.
The characteristics of this type of paint – fast drying speed, inter-layer solubility, short coating interval time and can be applied at lower temperatures. The weakness is its not resistant to solvents, usually not resistant to various types of vegetable oils and animal oils, has a low solid content, a thin dry film thickness, thus requires multiple times coatings. After drying, it still takes a long time (usually more than two weeks) to form a hard coating film and realize the protective effect.
The other type belongs to the polymer particle condensation film. This film-forming method means the paint relies on the polymer particles that serve as the film to condense with each other under certain conditions to form a continuous solid coating film, such as dispersed paints containing volatile dispersion media, water-based latex paints, non-aqueous dispersed paints, organic sols and powder paints.
Chemical drying process
Chemical drying refers to the various chemical reactions during paint drying process. Different film-forming conditions and processes are required according to different film-forming mechanisms. There are mainly two types.
1 – Room temperature curing
At room temperature, the paint relies on its main film-forming substance to react with oxygen or water vapor in the air, or to react chemically with the curing agent to form a high molecular polymer or condensate to cure into a film.
The drying process of alkyd resin, oil-modified phenolic resin, epoxy ester, natural resin is divided into two stages.
Initially is solvent evaporation, then reaction with oxygen in the air, and finally forms a network macromolecular structure through oxidative polymerization, a free radical bond polymerization reaction. The advantages of this type of paint are: single component, fast drying, easy construction and low requirements for the substrate. The disadvantage is that the film thickness is thin and the anti-corrosion performance is not high.
Moisture-curing polyurethane and inorganic silicates, the main film-forming substances react chemically with water vapor in the air. For example moisture-curing polyurethane paint, which is cross-linked by the reaction between the end groups of the prepolymer and the air to form a film. The formed coating is hard and wear-resistant, resistant to corrosion by chemicals and special lubricants, and suitable for construction in humid climates.
In room temperature curing film formation, another main film-forming method is curing and drying with a curing agent. For example, the drying of epoxy resin, polyurethane, polyester, etc., its drying speed and coating performance are determined by the type of resin, the type of curing agent and its dosage, and also the drying temperature.
This type of paint is multi-component and packed separately, onsite mixing only when use. After painting, the solvent evaporates first, and the chemical reaction accelerates along with the solvent evaporation, cross-linking to form a film. Therefore when using, we must strictly follow the requirements and proportions of the instructions and mix them evenly.
At the same time, as the construction temperature changes, the amount of curing agent should be adjusted appropriately. At minus 5 degrees Celsius, epoxy dries very slowly, and a special curing agent for winter should be selected and the dosage should be increased. In the hot summer, too much curing agent causes the reaction to release heat, and a “violent polymerization” phenomenon will occur, so the dosage should be reduced. Because it relies on chemical reaction cross-linking to form a film, this type of paint has good adhesion mechanical properties, excellent corrosion resistance, and is durable. The disadvantages are: two components, complex construction, high requirements for substrate processing, and expensive prices.
2 – Curing by heating
Thermosetting acrylic resins, some elemental organic polymers, and polyurethane resins require film-forming processes such as heating and baking. The process conditions such as temperature and time are selected according to the properties of the paint and the conditions of the objects to be coated.
The commonly used heat curing processes include steam, electricity, far-infrared heating, ultraviolet light curing processes triggered by photosensitizers, auxiliary electron beam curing processes, and ammonia vapor curing processes. With the development of new paints, special film-forming processes will continue to emerge.
