Solvent-based coatings contain solvents, which emit a large amount of VOC (Volatile Organic Compounds) during the production and construction process, endangering human health and causing environmental pollution. According to statistics, the global annual VOC emissions to the atmosphere are up to several million tons.
VOC is a major pollutant in the paint shop, including benzene, styrene, formaldehyde and acetaldehyde, which are harmful to human body and even have the risk of cancer. Therefore, it is necessary to wear labor protection supplies, especially breathing apparatus, when entering the painting workshop.
Many companies have tall chimneys erected in the painting workshop, which is the symbol of the painting workshop. These chimneys are used to emit harmful gases, and with the increasingly strict environmental regulations on VOC emission standards, reducing VOC emissions has become the most important thing for companies. So where do VOC emissions mainly come from?
According to the process of VOC emission, it is not difficult to find that source control and end-of-pipe treatment are the more appropriate ways to control VOC, and many domestic enterprises reflect a high level of VOC control. In fact, the workshops of these enterprises also adopt the source + end method to control VOC, and water-based painting is the best way to control VOC emission from the source. For water-based paint, companies have developed water-based coating process, which reduces VOC generation during the process. And in the production line of water-based coating, the following systems are included.
Conveyor and paint mixing system.
Since water is a corrosive medium, it will cause metal corrosion, cause equipment damage and produce metal ions, which will not only yellow the color of the paint film, but also reduce the anti-corrosion ability of the paint film. For this reason, the paint storage container and transport pipeline of water-based paint should be made of stainless steel and should be treated by chemical passivation. The paint mixing system, mixer and back pressure regulating valve in the painting line should be of low shear type. The connection of paint pipeline should avoid dead ends, and the conveying system should be better with two lines, and the pipeline should be enlarged to increase the pressure and flow of circulating material pump. The circulating speed of the mixer should be 0.15m/s. The temperature between paint mixing should be controlled at (3±1)℃ lower than the paint. The storage stability condition of water-based paint: 5~30℃/6 months.
Spraying system is roughly divided into: 1, high-pressure airless spraying; 2, air spraying; 3, electrostatic spraying.
1、High-pressure airless spraying
The principle of high-pressure airless spraying is the use of high-pressure plunger pump to continuously transport paint to the closed paint pipeline, so as to pressurize the paint to 210 kg / cm2 in a closed space to form a high pressure, and then release the gun trigger connected to the end of the paint tube, so that the high-pressure paint flow forced through the extremely small nozzle and the formation of atomization, so as to shoot the surface of the object to be coated. Its advantages are: 1, suitable for spraying high viscosity paint; 2, spraying efficiency is about 2 times higher than ordinary spraying; 3, excellent surface quality: it is the paint pressure spraying into tiny particles, so that it is evenly distributed on the surface of the wall, so that the paint in the workpiece surface to form a smooth, smooth, dense coating, which is incomparable to the original method of brushing, rolling, etc.; 4, improve the adhesion of the paint and extend the life of the coating. High-pressure spray atomization makes the paint particles gain strong kinetic energy, and the paint particles use this kinetic energy to reach the pores, thus making the coating more dense, enhancing the bite force with the workpiece, improving the adhesion, and effectively extending the coating life.
Air spraying is a kind of coating process which is widely used in coating construction. Air spraying is the use of compressed air flow, flowing through the gun nozzle hole to form a negative pressure, negative pressure so that the paint from the suction tube inhalation, sprayed by the nozzle, the formation of paint mist, paint mist sprayed to the surface of the painted parts to form a uniform paint film. Air spraying can produce uniform paint, coating fine and smooth; its advantages are: easy to operate, for the parts of the more hidden parts (such as crevices, concave and convex), can also be uniformly sprayed. But in the construction efficiency and out of the paint is slightly worse than high-pressure airless spraying.
Electrostatic spraying refers to the use of corona discharge principle to make atomized paint in the high-voltage DC electric field under the action of the negative charge, and adsorbed in the positively charged substrate surface discharge coating method. Electrostatic spraying equipment is composed of spray gun, spray cup and electrostatic spraying high voltage power supply. Water-based paint resistance is less than 0.1M, there is a high-voltage electrical leakage, should use special electrostatic spraying equipment, usually using an external electric paint delivery system, the advantages of the paint delivery system and color change and the traditional way the same. There are spraying companies develop and use the cup type electrostatic spray gun with electricity inside, the advantage is to improve the utilization of paint. Robot magazine-type spinning cup spraying system, the transit paint container into an independent paint container (called magazines), can be quickly combined with the spinning cup, pressure feeding paint. The advantages are: it saves cleaning solvent when changing colors and can be applied to both solvent-based and water-based coatings for electrostatic spraying. The advantages of electrostatic spraying compared with other spraying methods are: 1. High utilization rate of paint, when spraying in high voltage electrostatic field, the paint particles with negative charge are coated to the surface of the workpiece along the direction of the power line, therefore, basically there is no paint jet rebound and paint mist scattering phenomenon, paint mist loss is very small, the utilization rate of paint can reach 85% to 90%. 2. The effect of electrostatic field, paint particles dispersion is high, and the distribution in the jet is more uniform, so the coating formed on the surface of the workpiece to be decorated is also more flat and uniform, and the gloss and adhesion of the paint film are higher.
Drying system is roughly divided into: 1、 Hot air drying; 2、 Infrared drying; 3、 Microwave drying.
1、Hot air drying
Hot air drying is a heating and drying method that adopts convection principle and takes hot air with temperature of 50～90℃ as the heat carrier to transfer heat energy to the coating on the surface of workpiece, and the coating absorbs energy and cures into film. Electricity or steam is commonly used as the heat source to heat the air first, and the heat is transferred from the hot air to the surface of the coating through convection form to make the coating get dried quickly. Using hot air drying process can obviously accelerate the drying speed of the coating and has the characteristics of strong adaptability, so it is a more widely used form of drying.
Infrared curing can be used to dry the coating of water-based wood coatings. The so-called infrared curing actually refers to the painted furniture panels and their coatings under the irradiation of infrared light, absorbing radiation energy and being converted into heat energy, so as to achieve the curing of the coating. Infrared ray is a kind of invisible ray, between visible light and microwave, the wavelength is 0.72-1000 um. According to the wavelength range, it can be divided into “near”, “medium”, “far “infrared, commonly used far infrared drying coating. Infrared curing has the advantages of fast curing speed, rapid temperature rise, good curing quality, etc. However, when drying the coating with infrared heating, the coating has an obvious temperature gradient, and its drying is extended from the surface to the inside, which makes it unsuitable for drying thicker coating film; infrared drying can only heat the area that can be irradiated by infrared light, and cannot be used to dry three-dimensional objects.
Microwave refers to the wavelength of 1mm-1m, frequency of 300MHz-300GHz, with penetrating electromagnetic waves, commonly used microwave frequency of 915-2450MHz. microwave heating uses the principle of dielectric loss, and the dielectric constant of water is much larger than dry matter, the vast majority of the energy released by the electromagnetic field is absorbed by the water in the coating. The microwave field periodically changes the direction of the applied electric field at a high speed of several hundred million times per second, causing the water molecules to swing rapidly and producing a significant thermal effect, thus causing the temperature of the paint interior and surface to rise rapidly at the same time. The advantages of microwave heating are that the drying speed is particularly fast, and different substances have selective absorption for microwaves; there is no shape requirement for the object to be dried; the heating for the coating film is very uniform, and there is no temperature gradient, which can dry thick films.
With the continuous and rapid economic growth, the construction of a large number of infrastructure, the gradual increase of people’s awareness of environmental protection, coatings will certainly go to the road of health, environmental protection, water-based, and water-based coatings industry will usher in a broader space for development.