Green Coating Technology for Construction Machinery: A Guide to Reducing VOC Emissions
With the development of society and the improvement of people’s aesthetic awareness, users have increasingly higher requirements for the appearance quality of engineering machinery products. As an important process in the manufacturing of construction machinery, coating has an important impact on the appearance quality and anti-corrosion performance of the product, but it has also become an important source of VOCs. To this end, the national and local governments are committed to “green mountains, green water, blue sky” and air pollution prevention and control, increasing the control of VOCs in coatings, guiding enterprises to use low-VOCs coatings, adopting efficient spraying technology, and requiring waste gas treatment Facilities treat painting waste gas, and it can be discharged only after it meets standards. Although some solvent-based coatings used in construction machinery have good construction performance, the emission of organic solvents seriously exceeds the standard, and there are many unorganized emissions of coating exhaust gas. Therefore, it is of practical significance to promote the application of green coating technology in the field of construction machinery .
1
General overview
The most efficient and environmentally friendly coating process in the coating system of construction machinery is to learn from the concept of automobile coating and change the coating process route from the main machine coating to the component coating. That is, after the parts are painted, the whole machine after careful assembly and debugging is only repaired and painted (or in serious cases, sprayed with a top coat) before leaving the factory, and the spray-free process of the whole machine is implemented. By introducing new coatings, new processes, new technologies, and new equipment, parts painting can maximize energy saving and consumption reduction in engineering machinery painting.
2
Pre-treatment process
Steel structural parts need to be cleaned and degreased before shot blasting. Solvent-based cleaning agents are used for cleaning in the construction machinery industry. Most cleaning stations have no chambers, and there is a problem of fugitive emissions. The existing solvent cleaning process can be replaced by applying high-pressure steam cleaning to parts coating. After the shot blasting treatment, no phosphating or silane treatment is required, and a high-quality anti-rust primer can be directly sprayed on the shot blasting surface. The degreasing silane treatment technology is used to replace the degreasing phosphating process for powder spraying of thin plates and small parts.
2.1
High pressure steam cleaning
High-pressure steam cleaning is to use the high temperature and high pressure of saturated steam to dissolve, strip, vaporize and evaporate the oil on the surface of the parts, so as to achieve the purpose of cleaning the surface of the workpiece. Tap water is used as the medium, and high-pressure steam (10-30 bar, 160-185 ℃) can be ejected by heating with diesel oil or natural gas for 1-2 minutes to clean oil and dust on the surface of parts without pollution. After high-pressure steam cleaning, compressed air can be used to blow dry or dry.
2.2
Defatted silane treatment
Degreasing silane treatment is an environmentally friendly pretreatment process. The silane treatment solution does not contain chromium and metal phosphates. During the pretreatment process, no acidic metal waste will be generated. Silane treatment is widely used, the process is simple, and no surface adjustment is required. Under the current situation of increasing environmental protection pressure in the coating industry, silane pretreatment process will gradually replace traditional iron-based and zinc-based phosphating processes. The general phosphating process flow is: pre-degreasing → degreasing → water washing 1 → water washing 2 → surface conditioning → phosphating → water washing 3 → water washing 4 → pure water washing → drying → powder spraying. The silane treatment process is: pre-degreasing → degreasing → washing 1 → washing 2 → pure water washing → silane treatment → washing 3 → washing 4 → pure water washing → drying → powder spraying. If the original phosphating treatment line in the coating shop can be directly converted into a silane treatment line. The corresponding surface adjustment process is adjusted to a pure water washing process, and the phosphating process is adjusted to a silane process.
3
coating
At present, most construction machinery manufacturers use solvent-based coatings, which have high VOC content and cannot meet the environmental protection requirements of low-VOC coatings. The existing solvent-based coatings can be gradually replaced by high-solid and low-viscosity coatings and water-based coatings in stages, and solvent-free coatings can be popularized for small structural parts. type powder coating.
3.1
High solid and low viscosity coating
High-solid and low-viscosity coatings reduce the viscosity of the coating and increase the solid content of the coating by adding a resin with a narrower molecular weight. When the high-solid and low-viscosity coating has a high solid content, the consumption of organic solvents will be reduced, and the purpose of reducing VOC can be achieved by reducing the consumption of solvents. When the construction solid content is increased from 50% to 70%, the solvent consumption will be reduced by more than 50%. The application of high-solid and low-viscosity coatings on the existing coating line does not need to be changed except that the drying temperature is recommended to be raised, and the coating equipment and process are basically not changed.
3.2
Water-based paint
The dispersion medium of water-based paint is water. After the paint is applied, water and a small amount of additives evaporate from the paint film, which has environmental protection and safety properties. However, water-based coatings have higher requirements on the quality of pre-treatment. During the construction process, the parts that have not been cleaned completely have oily defects after painting, and the spraying process is prone to sagging. After spraying, there will be prickly heat if it is not flash-dried. Lower the temperature and bake Dry tempo extension and other issues. The construction window of water-based coatings is narrower than that of solvent-based coatings. The construction of water-based coatings generally requires construction above 5°C, and the relative humidity requirements are also stricter. Under the environmental conditions of temperature and humidity (15-35°C/ 30-70%RH), a certain brand of water-based paint can be applied according to the existing solvent-based paint coating process in the construction machinery industry. Mechanical industry coating appearance quality requirements.
4
Spraying process
The coating in the construction machinery industry is all manual spraying, the utilization rate of paint is low, there is cost waste, and there is a lot of paint residue. At the same time, due to the high labor intensity and harsh environment, there is a large gap in personnel. It is recommended to use an environmentally friendly spraying process to achieve the goal of saving energy and reducing consumption. Spraying methods with high coating efficiency, such as electrostatic spraying, high-pressure mixed gas spraying, robot automatic spraying, etc., can be preferentially selected according to the adaptability of the workpiece. Both electrostatic spraying and high-pressure mixed gas spraying can improve the utilization rate of coatings, but the cost of equipment is also high. It is recommended to improve the application effect after improving the level of on-site management.
5
Drying process
The coating of construction machinery is generally a two-coat/three-coat coating system, and the workpiece needs to be dried and cooled several times; the drying process is one of the high-energy-consuming processes, and reducing the drying temperature or reducing the number of drying times can reduce the The energy consumption of the drying process is reduced to achieve energy saving and emission reduction. Low-temperature curing coatings can be selected to reduce the drying temperature. The “wet-on-wet” green process technology for intermediate coating and top coating has been obtained in industrial applications. This technology simplifies the curing process of the coating film and reduces the number of drying times. 1 tumble dry.
The heating method is preferably the gas catalyst infrared heating process. The combustible gas can generate mid-wave infrared radiation through the catalyst plate, and the generated mid-wave only heats the carboxyl and hydroxyl molecules in the paint, that is, only heats the coating, and has no obvious heating effect on uncoated metal and air. This process can achieve rapid curing of only the paint film of the workpiece, reducing the drying cycle and achieving energy saving effects.
6
Waste treatment process
Coating exhaust gas refers to the volatilized paint mist and solvent volatilized vapor of spray paint. It mainly comes from the exhaust of the spray booth, the volatilized vapor of the solvent during leveling, and the exhaust of the drying room. These exhaust gases need to be treated effectively. The current process technologies applicable to waste gas treatment in the construction machinery industry include zeolite runner concentrated waste gas treatment technology, activated carbon adsorption and desorption technology, regenerative catalytic combustion treatment technology, and direct combustion technology. Each of these treatment methods has advantages and disadvantages and can be used according to different working conditions and Combined application of different needs.
6.1
Zeolite runner concentrated waste gas treatment technology
The zeolite runner device uses the adsorption property of zeolite to allow the zeolite to adsorb waste gas, and then converts low-concentration waste gas into high-concentration waste gas through the runner device, which facilitates the centralized treatment of waste gas. The zeolite runner treatment device includes pretreatment equipment, zeolite runner, exhaust gas purification equipment and control system, etc. The purification device can purify the high-concentration exhaust gas and then convert it into standard gas discharge. The advantage of this process is that the setting of the zeolite runner device is simple and the daily maintenance is relatively convenient. The disadvantage is that the cost of purchasing the zeolite runner device is relatively high.
6.2
Activated carbon adsorption and desorption waste gas treatment technology
The activated carbon adsorption device uses activated carbon as a filter to adsorb harmful substances on the surface of activated carbon to purify the exhaust gas. The equipment used in activated carbon adsorption and desorption waste gas treatment technology includes pretreatment equipment, activated carbon adsorption box, waste gas purification equipment and control system, etc. The process is: the organic solvent volatile gas is filtered and sent to the activated carbon adsorption box, and is absorbed by the activated carbon layer to saturation. Then treat the saturated activated carbon with exhaust gas at a certain temperature to resolve the adsorbed solvent gas. Then the analyzed waste gas is purified and discharged up to the standard. The advantage of this method is that the operating cost of purifying low-concentration exhaust gas is relatively low. The disadvantage is that the treatment efficiency of activated carbon will gradually decrease, and the activated carbon needs to be replaced at a certain period. The replaced activated carbon is hazardous waste and needs to be disposed of.
6.3
Regenerative catalytic combustion treatment technology
This method is to heat the gas containing organic solvent to 200-400 ℃, through the catalyst. carry out the oxidation reaction. This can burn at a lower temperature. Heat energy consumption is less. Its advantages are: small device and low fuel cost. The disadvantages are: the catalyst needs to be replaced regularly, and a pretreatment device is required for direct use, and the treatment efficiency is lower than that of the direct combustion method.
6.1
direct combustion method
The direct combustion method is to heat the gas containing an organic solvent to 600-800°C to burn it directly, carry out an oxidation reaction, and decompose it into CO2 and water. Its advantages are: simple operation and easy maintenance; no pretreatment is required, organic matter can be completely burned, and it is beneficial to purify high-concentration gases. The disadvantage is that when the high-volume low-concentration exhaust gas is directly burned, the treatment cost is relatively high.
7
Conveyor Technology
There are many conveying methods used in engineering machinery coating production lines. The more commonly used ones include accumulation chain conveying, bicycle conveying, heavy-duty skid conveying, etc., which are suitable for different types of workpieces.
Small parts in the construction machinery industry can use the product chain conveyor line. It can realize automatic reloading, variable pitch transportation, fast travel and accumulation storage, etc., which reduces the floor space of the coating line and makes production and transportation more flexible.
Large parts coating lines generally use hoist conveyor lines. Single-track or double-track hoist conveyor lines are selected according to the design weight. Circular conveyor lines or translational transfer conveyor lines are selected according to the length or spatial position of the workpiece (the length of the line body can be shortened). At the same time, through the intelligent control technology of PLC bus communication, the efficient linkage of workpieces in the production line can be realized, which can improve the conveying efficiency by more than 10%. The improvement of conveying efficiency can save energy and reduce consumption of the coating line equipment as a whole.
8
Conclusion
After the coating used in the parts coating process design is determined, the process needs to be reviewed from the aspects of advancement, reliability, high quality and high yield, economy, environmental protection, energy saving and emission reduction, etc., and the design and selection of suitable coating equipment such as steam cleaning equipment, Degreasing silane spraying equipment, automatic spraying robot and its spray booth, gas infrared drying equipment and high-efficiency conveying methods.
The whole machine implements a spray-free process, and only repairs and paints the paint film bumped during the process of transfer, assembly, and commissioning. Generally, low-VOC paints (such as water-based paints, high-solid paints) are used for local repairing by low-pressure air spraying. According to the requirements of environmental protection and the painting cycle of the whole machine, the drying process should be designed after the whole machine is repaired. It is recommended to use activated carbon adsorption and desorption + catalytic combustion treatment for the exhaust gas treatment of the complete machine coating with large output.
As the country pays more and more attention to environmental protection and energy saving and emission reduction, VOCs emission standards will become more and more stringent. Construction machinery companies must reduce VOCs emissions through corresponding technical transformation and the selection of advanced coating materials and processes. The application scenarios of new green coating processes will become more and more extensive.
