It is suitable for white-drying paints such as volatile paints, air-drying paints, and curing agent-curing paints. They are dried into films by solvent volatilization, oxidative polymerization, or curing agent curing in normal temperature and atmospheric environments. The drying speed is affected by environmental conditions. It has a great impact, requiring good ventilation and less dust, which is beneficial to solvent volatilization and the safety of the work site, and reduces the adhesion of dust. When the ambient humidity is high, it inhibits solvent volatilization, slow drying, and causes defects such as whitening of the coating film. The humidity of the working environment should be low rather than high. When the temperature is high, the solvent volatilizes quickly, the curing reaction is fast, and the drying is also fast. This is beneficial to reduce dust adhesion, but it may make the leveling property worse. The thinner should be replaced to make the surface dry at a moderate speed .
Drying is divided into low temperature drying, medium temperature drying and high temperature drying. Below 100°C is called low-temperature drying. It is mainly to implement forced drying of self-drying paint or drying of coating film on the surface of materials with poor heat resistance. The drying temperature is usually 60~80°C, which greatly shortens the drying time to meet the needs of industrialized production lines.
For example: it takes 1.5 hours for nitro paint to dry at room temperature, but only 10-30 minutes at 60-80°C; the drying time for two-component polyurethane paint at room temperature is 12 hours, 30 minutes at 60°C, and 15 minutes at 80°C.
Medium temperature drying is below 150°C, mainly used for drying and filming of topcoats. When it exceeds 150°C, the coating film will turn yellow and become brittle, so it is usually baked at 120~140°C.
The suitable drying temperature for various coatings is as follows:
Acrylic baking paint 120~140℃, 30~60min
Amino baking varnish 120~140℃, 30~60min
Alkyd paint 100~120℃, 20~30min
Water-based amino 140℃, 20~40min
Epoxy baking paint (acid catalysis) 130~150℃, 20~40min
Synthetic resin surfacer 120~140℃, 20~30min
Amino oil at 100~120℃, 30~60min
Zinc yellow primer 120~140℃, 20~40min
Light-colored paint generally adopts a lower drying temperature (such as 120°C) to cure the coating film for a longer time to avoid yellowing; dark-colored paint adopts a higher drying temperature to shorten the drying time and increase productivity.
Above 150 ℃ belongs to high temperature drying. For epoxy phenolic primers, water-based phenolic primers, cathodic electrophoretic paints, etc., the coating film is fully cross-linked and cured at a high temperature of 180~200°C to improve the anti-corrosion performance of the coating film. Since primers only require protective properties and have no requirements on the color of the coating film, they all adopt high-temperature drying methods.
In order to prevent defects such as pinholes and orange peels in the coating film during the drying process, the wet coating film should be pre-dried for 3 to 8 minutes according to the thickness of the coating film before drying.
Radiation curing is the use of ultraviolet light or electron beams, so that the unsaturated resin paint is quickly initiated, polymerized, and hardened quickly. Ultraviolet rays can only cure varnishes, and the hardening time generally does not exceed 3 minutes; electron beam radiation can be used for rapid hardening of colored paints because of its high energy and strong penetrating power, and the hardening time only takes a few seconds. Electron beam radiation curing equipment has a large investment, strict safety management, and less use; while ultraviolet curing is widely used for curing coatings on flat surfaces such as wood, plastic, paper, and leather.
In addition to the above three main drying methods, there are also electric induction drying and microwave drying, but they are mainly used for rapid curing of adhesives. Electric induction drying is also called high-frequency heating. When a metal workpiece is put into the coil, the coil passes through 3oo~4ooHz/s alternating current, and a magnetic field is generated around it to heat the workpiece. The maximum temperature can reach 250~280℃. It can be adjusted according to the current intensity. Because the energy is directly applied to the workpiece, the resin film is heated and dried from the inside out, the solvent can be quickly and effectively released and the coating film is cured, the bonding strength is very high, and it is better used in the bonding field.
Microwave drying is the vibration of specific substance molecules under the action of microwaves (1mm~lm) to obtain energy and produce thermal effects. Microwave drying is limited to coating films on the surface of non-metallic substrates, which is just the opposite of high-frequency heating. Microwave drying is selective to the object to be dried, and the equipment investment is large, but its drying is uniform and fast, and the drying time is only 1/10~1/100 of the conventional method.