Compared with solvent-based coatings, water-based coatings use water as a solvent and only a small amount of alcohol ethers, alcohol esters, etc. as film-forming additives, which can greatly reduce VOC (volatile organic compounds) emissions. For example, the organic solvent content of water-based primer is 5% ~ 15%, while the organic solvent content of solvent-based primer is about 80%. Using water-based primer instead of solvent-based primer can reduce VOC emission from 45g/m2 to 7g/m2. With the enhancement of people's awareness of environmental protection and the country's requirements for cleaner production, and through the continuous upgrading of water-based coatings, in some performance and price of water-based coatings and solvent-based coatings have been basically equivalent, oil to water has become the popular trend. However, due to the construction conditions and construction technology of water-based coatings, the progress of water-based coatings, compared with developed countries, there is still a big gap .
In this paper, the characteristics of water-based coatings, according to the successful application of cases, the application and process of water-based coatings in the field of modified vehicles.
1 Modified truck water paint application status.
At present, the production market of modified vehicles in China (including fire engines, oil tankers, cement mixers, dump trucks, rescue vehicles and other special vehicles, as well as sprinklers, garbage cleaning vehicles, sweepers and other sanitation vehicles) is developing by leaps and bounds. The modified vehicle market benefits from the growth of cargo transportation volume brought about by the development of the national economy and the construction of highway infrastructure, and the industry scope and market space are constantly expanding, There are millions of mass production every year.
The modified car coatings are mostly solvent-based spray (or dip) paint, and solvent-based coatings contain solvents, which emit a large amount of VOC during production and transportation, endangering human health and causing environmental pollution. The coating materials for vehicles are developing in the direction of high solid fraction, water-based and powderization.
Although the domestic market has done water-based research on modified vehicle coatings, and has successfully carried out water-based coating in modified vehicles, agricultural vehicles, and other fields . However, it is still not widely used. In addition to price factors, the main reason is the lack of a complete set of solutions, which cannot meet the different performance and construction requirements of different parts. At present, the domestic water-based paint used for modified vehicles is less than 5%, and imported water-based paint is generally used, while the performance of water-based modified vehicle paint produced in China is still different from the international advanced level .
2 Features of water-based paint for modified vehicles and matters needing attention in construction technology
Although water-based coatings are more environmentally friendly, their performance and application are often limited due to the characteristics of water-based coatings . For example, the hot melt of water is large, which is more difficult to volatilize, and the paint film is easy to sag under high humidity conditions. The surface tension of water is large and the wettability is poor. If the paint is not adjusted, the surface quality of the finished paint film will be affected; the viscosity of water is greatly affected by shear force. When subjected to high shear, the viscosity of water-based coatings will drop a lot, which will have a great impact on the transfer paint system. Water is corrosive to metal materials and will cause corrosion to the coating equipment in contact with it. It is precisely because of the different characteristics of water and solvent, which leads to the differences between water-based coatings and solvent-based coatings in many aspects, and also leads to the differences in the construction methods and properties of water-based paints and oil-based paints. Therefore, in the construction process, special attention should be paid to the following aspects :
a The substrate shall be clean and free of oil and floating dust.
B water paint ratio to be accurate, according to the weight ratio of accurate weighing to join, strictly control the construction viscosity.
C with paint curing agent to first and paint fully stir, then add water dilution, filter after use.
D extreme environment, such as high temperature, low temperature, high humidity conditions, when using to add some special additives.
e Special attention should be paid to the corners and edges of the spraying to prevent excessive thick spraying and sagging and to avoid prickly heat.
f Primer shall be naturally dried for at least 4 hours before spraying the topcoat to ensure the best effect of the topcoat. After the topcoat surface is dry, spray operation can be carried out, and reflective tape can be pasted after 6 hours.
3 Modification of water paint equipment for modified vehicles
At present, the water-based coating and solvent-based coating production line is basically similar, the existing oil-based coating equipment does not need to carry out a major transformation can be used for water-based coating. According to the characteristics of water-based coatings, the coating production line of solvent-based coatings needs to modify and adjust some equipment and processes .
A the surface of water-based paint is fast, but the hard work is relatively slow, and the rapid drying is easy to produce bubbles. Generally, the leveling time is not less than 15 minutes, and the surface temperature of low-temperature drying workpiece is not less than 60 ℃, and the time is not less than 25 minutes;
B Water-based paint uses water as the dispersion medium, which is easy to cause ordinary carbon steel corrosion. Generally, the spraying equipment and paint supply equipment are made of stainless steel or PVC; and the carbon steel parts on the spraying site are protected to avoid corrosion;
C the drying performance of water-based paint is related to the temperature and humidity in the environment. when the temperature is high and the humidity is low, the paint film dries quickly. Low temperature bottom, high humidity, slow drying of paint film, easy to flow or sag, too thick paint film is easy to produce bubbles; Spraying room, leveling room and drying room shall control the air temperature and humidity, requiring temperature ≥ 10 ℃ and humidity ≤ 75%;
D the viscosity of water-based paint is different from that of oil-based paint. generally, the spraying viscosity is higher, and the low viscosity is easy to cause flow drop or sagging. the spraying equipment pressure is required to be air spraying, and the compressed air pressure is 0.6-0.8MPa. mixed air spraying equipment or high-pressure airless spraying equipment can also be used. for workpieces with large structure, in order to improve the spraying efficiency, the caliber of air spraying or mixed spray gun is more than 1.8.
Application of Waterborne Coatings in Refitting Car Painting
Because the modified vehicle is mainly operated outdoors, in the environment of wind and sun, high humidity and heat, large temperature difference and poor road conditions, the decoration and protection performance of the coating are required to be higher. From the use of water-based coatings in the modified vehicle industry at home and abroad, the paint used in the water-based supporting system is mainly a two-component epoxy primer, a two-component epoxy intermediate paint, a two-component polypropylene intermediate paint, and a two-component acrylic polyurethane topcoat. At present, our company has made continuous efforts to develop the water paint grade matching system: epoxy primer polyurethane topcoat, epoxy primer intermediate paint (acrylic, epoxy) polyurethane topcoat, which has reached the requirements for application on modified vehicles. And has been successfully applied to: Shenzhen Dongfeng Motor, Sany Heavy Industry, Shandong Lingong, China Machinery Mino, Shandong Jianyu, Shi Feng Group and other users.
Features of supporting products: small permeability to moisture, air and corrosive substances; Strong adsorption with the coated material, durable wear resistance; It has good rust prevention, point-to-point, insulation and heat insulation properties for metals, and has good appearance display effect. Design a complete set of coating schemes and processes according to different technical requirements of different substrates and different parts; Characteristics of rapid drying at low temperature and normal temperature; satisfy the coating of various large-scale and small-scale production equipment and parts; It does not contain harmful substances such as halogenated hydrocarbons, benzene, toluene, xylene, etc., and has low heavy metal content, which solves the problems of high VOC content, serious pollution and fire hazards of solvent-based paint, effectively simplifies and reduces the process and cost of enterprise ventilation, water treatment and other aspects, and also helps to reduce the potential probability of occupational diseases, product construction is convenient, wide application, suitable for large-scale industrial continuous production.