Study on green coating process of construction machinery

Industrial coatings

introduction

With the development of society and the improvement of people's aesthetic awareness, users have higher requirements for the appearance quality of construction machinery products. As an important process in the production and manufacturing of construction machinery, coating has an important impact on the appearance quality and anti-corrosion performance of products, 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 the key work of air pollution prevention and control, strengthen the control of VOCs in coating, guide enterprises to adopt low VOCs coating, adopt efficient spraying technology, and require the use of waste gas treatment facilities to treat the coating waste gas, and discharge it after reaching the standard. While some solvent-based coatings used in construction machinery have good construction performance, the emission of organic solvents is seriously beyond the standard, and there are many phenomena of unorganized emission of coating waste gas. Therefore, it is of practical significance to promote the application of green coating technology in the field of construction machinery.

one

General overview

The most efficient and environmentally friendly coating process of the construction machinery coating system is to change the coating process route by using the automobile coating concept for reference, and change the coating process from the whole machine coating to the parts coating. That is, after the parts are coated, the whole machine after careful assembly and commissioning will only be repaired and coated before leaving the factory (or in serious cases, a coat of finish paint will be sprayed), and the whole machine will be sprayed free. The painting of parts and components can maximize energy saving and consumption reduction by introducing new coatings, new processes, new technologies and new equipment.

two

Pretreatment 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 of the cleaning stations have no chambers, and there is a problem of unorganized discharge. The existing solvent cleaning process can be replaced by high-pressure steam cleaning in the coating of parts. After shot blasting, phosphating or silane treatment can be stopped, and high-quality antirust primer can be directly sprayed on the shot blasting surface. The degreasing silane treatment technology is used to replace the degreasing phosphating process for the pretreatment of thin and small pieces before powder spraying.

two point one

High-pressure steam cleaning

High-pressure steam cleaning is to use the high temperature and high pressure of saturated steam to dissolve, peel, vaporize and evaporate the oil on the surface of parts, so as to achieve the purpose of cleaning the surface of parts. With tap water as the medium, high-pressure steam (10-30 bar, 160-185 ℃) can be sprayed out after heating with diesel oil or natural gas for 1-2 minutes to clean the oil and dust on the surface of parts without pollution. Compressed air can be used for drying or drying after high-pressure steam cleaning. See Fig. 1 and Fig. 2 for the effect before and after high-pressure steam cleaning of the workpiece.

Fig. 1 Before high-pressure steam cleaning of workpiece

Fig. 2 Workpiece after high-pressure steam cleaning

two point two

Degreasing silane treatment

Degreasing silane treatment is an environmentally friendly pre-treatment process. The silane treatment fluid does not contain chromium and metal phosphates. During the pre-treatment process, acidic metal waste will not be generated. Silane treatment is widely used, with simple process flow and no need for surface adjustment. Under the current situation of increasing environmental pressure in the coating industry, silane pretreatment process will gradually replace the traditional iron and zinc series phosphating process. 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 flow is: pre-degreasing → degreasing → water washing 1 → water washing 2 → pure water washing → silane treatment → water washing 3 → water washing 4 → pure water washing → drying → powder spraying. If the original phosphating line in the coating workshop can be directly converted to silane treatment line. Adjust the corresponding surface adjustment process to the pure water washing process and the phosphating process to the silane process.

three

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". They can gradually replace the existing solvent-based coatings by stages through high-solid low-viscosity coatings and water-based coatings, and promote the application of solvent-free powder coatings for small structural parts.

three point one

High-solid low-viscosity coating

High solid and low viscosity coatings aim to reduce the viscosity and increase the solid content of coatings by adding a narrow molecular weight resin. The consumption of organic solvent will be reduced when the solid content of high solid and low viscosity coating is high. VOC can be reduced by reducing the consumption of solvent. If the construction solid content is increased from 50% to 70%, the solvent consumption will be reduced by more than 50%. In addition to the recommended increase in drying temperature, the coating equipment and process basically need not be changed when applying high solid and low viscosity coating on the existing coating line.

three point two

Waterborne paint

The dispersion medium of water-based coating is water. After the coating is applied, water and a small amount of additives are volatilized from the coating film, which has environmental protection, safety and other properties. However, water-based coatings have higher requirements on the quality of pretreatment. During the construction process, there are many problems such as oil flashing defects in the parts with incomplete oil stain after painting, sagging easily in the spraying process, prickly heat after spraying without flash drying, reduced temperature and prolonged drying time. The construction window of water-based coating is narrower than that of solvent-based coating. The construction of water-based coating is generally required to be above 5 ℃, and the relative humidity is also strict. Under the temperature and humidity (15-35 ℃/30-70% RH) environment, the water-based coating of a certain brand can be constructed according to the existing solvent-based coating process flow in the construction machinery industry. The coating is well matched, and the appearance quality of the paint film meets the requirements of the coating appearance quality in the construction machinery industry.

four

Spraying process

The coating in the construction machinery industry is manual spraying, with low utilization rate of coating, cost waste, and much paint slag. At the same time, there is a large gap in personnel due to the high labor intensity and bad environment. It is recommended to use environmentally friendly spraying process to achieve the goal of energy conservation and consumption reduction. According to the adaptability of the workpiece, electrostatic spraying, high-pressure air mixing spraying, robot automatic spraying and other spraying methods with high coating efficiency can be selected preferentially. Both electrostatic spraying and high-pressure air-mixing spraying can improve the coating utilization rate, but the equipment cost is also high. It is suggested that the application effect is better after improving the on-site management level.

five

Drying process

Construction machinery coating 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 processes with high energy consumption. Reducing the drying temperature or drying times can reduce the energy consumption of the drying process and achieve energy conservation and emission reduction. Low-temperature curing coatings can be selected to reduce the drying temperature. The green process technology of "wet touch wet" for intermediate and top coats has been obtained. This technology simplifies the film curing process, reduces the drying times, and combines two drying to one drying.

The gas catalyst infrared heating process is preferred as the heating method. The combustible gas can generate medium-wave infrared radiation through the catalyst plate. The medium-wave generated only heats the carboxyl and hydroxyl molecules in the coating, that is, only the coating, and has no obvious heating effect on the uncoated metal and air. This process can only realize the rapid curing of the paint film of the workpiece, reduce the drying time, and achieve the energy-saving effect.

six

Waste treatment process

Paint exhaust gas refers to the paint mist and solvent vapor volatilized by paint spraying. It mainly comes from the exhaust gas from the paint spraying room, the vapor from the solvent volatilization in the flow, and the exhaust gas from the drying room. These waste gases need to be effectively treated. At present, the process technologies applicable to waste gas treatment in the construction machinery industry include zeolite runner concentration waste gas treatment technology, activated carbon adsorption and desorption technology, regenerative catalytic combustion treatment technology, and direct combustion technology. These treatment methods have advantages and disadvantages, and can be combined according to different working conditions and different requirements.

six point one

Treatment technology of waste gas from zeolite runner concentration

The zeolite runner device uses the adsorption of zeolite to make zeolite absorb waste gas, and then converts low concentration waste gas into high concentration waste gas through the runner device to facilitate the centralized treatment of waste gas. The zeolite runner treatment device includes pretreatment equipment, zeolite runner, waste gas purification equipment and control system. The purification device can purify the high concentration waste gas, and then convert it into the standard gas emission. The advantages of this process are that the setting of the zeolite runner device is simple and the daily maintenance is convenient. The disadvantage is that the cost of purchasing zeolite runner device is relatively high.

six point two

Activated carbon adsorption and desorption waste gas treatment technology

Activated carbon adsorption device uses activated carbon as filter to adsorb harmful substances on the surface of activated carbon to purify waste gas. The equipment used in the activated carbon adsorption and desorption waste gas treatment technology includes pretreatment equipment, activated carbon adsorption tank, waste gas purification equipment and control system. The process is as follows: the volatile gas of organic solvent is filtered and sent into the activated carbon adsorption tank, which is adsorbed to saturation by the activated carbon layer. Then the saturated activated carbon is treated with waste gas at a certain temperature to resolve the adsorbed solvent gas. Then the resolved waste gas is purified and treated to meet the standard. The advantage of this method is that the operating cost of purifying low concentration waste gas is low. The disadvantage is that the treatment efficiency of activated carbon will gradually decrease, and the activated carbon needs to be replaced periodically. The replaced activated carbon belongs to hazardous waste and needs to be treated.

six point three

Regenerative catalytic combustion treatment technology

This method is to heat the gas containing organic solvent to 200-400 ℃ and conduct oxidation reaction through the catalyst, so that it can burn at a lower temperature with less heat consumption. Its advantages are: small device and low fuel cost. The disadvantage is that the catalyst needs to be replaced regularly, and the pretreatment device needs to be equipped when it is used directly, and the treatment efficiency is lower than that of the direct combustion method.

six point one

Direct combustion method

The direct combustion method is to heat the gas containing organic solvent to 600-800 ℃ to make it burn directly, conduct oxidation reaction, and decompose it into CO2 and water. Its advantages are: simple operation and easy maintenance; Without pretreatment, organic matter can be completely burned, which is conducive to purification of high concentration gas. The disadvantage is that when the exhaust gas with large air volume and low concentration is directly burned, the treatment cost is high.

seven

Conveying technology

There are many conveying methods used in the coating production line of construction machinery. The more commonly used ones are chain conveyor, self-propelled trolley conveyor, heavy-load sled conveyor, etc., which are suitable for different workpiece types.

Small parts in the construction machinery industry can use the chain conveyor line. It can realize automatic transfer, variable pitch transportation, fast walking and storage, reduce the floor area of the coating line, and make the production and transportation more flexible.

The large parts coating line generally adopts the hoist conveyor line. The monorail or double-rail hoist conveyor line is selected according to the design weight, and the circular conveyor line or the translational transfer conveyor line is selected according to the length or space position of the workpiece (the length of the line body can be shortened). At the same time, through the PLC bus communication intelligent control technology, the efficient linkage of the whole production line workpiece can be achieved, which can improve the conveying efficiency by more than 10%. The improvement of conveying efficiency can make the coating line equipment realize energy saving and consumption reduction as a whole.

eight

Conclusion

After the coating used in the coating process design of parts and components is determined, it is necessary to carry out process review from the aspects of advanced, reliable, high-quality and high-yield, economy, environmental protection, energy conservation and emission reduction, and design and select appropriate coating equipment, such as steam cleaning equipment, degreasing silane spray equipment, automatic spraying robot and its spray booth, gas infrared drying equipment and efficient transmission mode.

The whole machine adopts the spray-free process, and only the paint film that is bumped during transportation, assembly and commissioning is repaired and coated. Generally, low VOC type coatings (such as water-based coatings and high-solid coatings) are used for local repair by low pressure air spraying. According to the environmental protection requirements and the painting rhythm of the whole machine, the drying process shall be designed after the whole machine is repaired. It is recommended to use activated carbon adsorption and desorption+catalytic combustion treatment for the treatment of coating waste gas of the complete machine with large output.

As the country pays more and more attention to environmental protection, energy conservation and emission reduction, VOCs emission standards will also become more and more strict. Construction machinery enterprises will inevitably reduce VOCs emissions through corresponding technical transformation and selection of advanced coating materials and processes, and the application scenarios of new green coating processes will become more and more extensive.