Research on the application of ultra-fine powder coatings in the field of automotive coating

Abstract:
the application and development of ultra-fine powder coating in the field of automotive coating. In view of its mobility problem on the automotive coating production line, a new fluidized additive technology is introduced, and the feasibility of the application of this technology in the field of automotive coating is verified by comparing the performance of modified automotive powder coating with commercial powder coating. At the same time, the technology of low temperature curing ultra-fine powder coating and its application in the coating of automotive plastic parts are
the
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with the continuous development of powder coating technology, its application in the automotive field is also expanding. In recent years, powder coatings have been widely used in the coating of hoods and body parts. However, due to its thick coating and poor appearance flatness, powder coating has not been widely used in the appearance of higher requirements of automotive external coating. In order to save costs and improve the quality of coating film, powder coating manufacturers continue to reduce the particle size of powder coating to improve the flatness of powder coating. However, the reduced particle size also leads to poor flow of powder coatings, seriously affecting the use. A new technique is therefore needed to improve the flow characteristics of ultra-fine powders, such as nano-sized additives as spacer (fluidizing aids) for the use of ultra-fine powder coatings on existing spray equipment. In addition, more and more thermal plastics or composite components are being used in the automotive industry, and the development of cryogenic cured powder coatings, especially cryogenic cured ultra-fine powder coatings, has received wider attention. Based on this, this paper first summarizes the development and challenge of ultra-fine powder coating in the field of automotive coating, and at the same time, by examining the fluid flow energy and film properties of ultra-fine powder coating after adding fluidizer, explores the feasibility of new fluidized additive technology in the field of ultra-fine powder coating in automobile, and finally introduces the application of low temperature cured ultra-fine powder coating and its application in automotive plastic parts.
1 Application of ultra-fine powder coatings in automotive coatings
1.1 ultra-fine powder coatings
ultra-fine powder coatings typically refers to powder coatings with a particle size of D50 less than 25 m. Compared with ordinary powder coatings (medium particle size D50 is greater than 30 m) and fine powder coatings (medium particle size D50 between 25 and 30 m), ultra-fine powder coatings have unique excellent performance. With the decrease of particle size, the thickness of coating film is also reduced, so that the coating film has better leveling and good decorative effect. In addition, economic costs have fallen as a result of reduced use of materials. According to the Geldart powder classification, ordinary powder coating belongs to Class A particles, easy to fluidize, but when the particle size is reduced to ultra-fine powder coating (Class C particles), the force between particles (van der Wali) increases, powder particles easily reunite, can not normally flow, resulting in airflow transport and spraying difficulties and a series of problems.
In order to improve the fluidity of the ultra-fine powder coating, the appropriate amount of fluidized additives (object particles) must be added to the coating, and these nanoscale particles adhere to the surface of the ultra-fine powder particles (main particles) in the form of small clusters, increasing the distance between the particles. Since these added object particles are lower in density and smaller in size than the ultra-fine powder particles, the inter-particle force between the adhered object particles and the main particles is significantly smaller than the inter-particle force between the main particles, thus reducing the Vanderwal force between the ultra-fine powder particles. In order to promote the dissocation of the main particles, the fluidized additive particles usually need to be evenly dispersed in the ultra-fine powder, and evenly attached to the surface of the ultra-fine powder particles, but do not need to be attached in the form of a single particle, the form of small clusters to reduce the force between the main particles is more helpful.
additives can significantly improve the fluidity of ultra-fine powder coatings, but they can also cause other problems, such as reduced gloss on the coating surface and film defects such as shrink holes and "particles". In addition, nanoscale additives themselves tend to be cohesion and tend to form sturdy, large reunions that cannot be dispersed through ordinary dry mixing processes, even with high shear mixers, so that inevitable defects such as "particles" appear in the final coating. To overcome these problems, new technologies, including specially designed additive formulations and special mixing processes, are needed to increase additive dispersion and compatible with powder coatings, ensuring excellent coating properties while reducing the force between particles and improving fluidity. The Particle Technology Research Center of the University of Western Ontario in Canada has successfully developed an efficient ultra-fine (HEUF) powder coating technology, which uses the corresponding fluidization aid formulation and optimized addition and dispersion method according to the various resin systems of the coating, ensuring excellent fluidity, sprayability and coating quality of the ultra-fine powder coating, with a minimum coating film thickness of only 20 to 30 m (e.g. Table 1). Figure 1 shows the coating surface profile (measured using Dektak contact surface profiler) from the spraying of an average particle size of 16.4 m ultra-fine powder coating and 38.8 m of ordinary powder coating, with the horizontal coordinates being the sample length and the ordinates representing the distance from the points of the profile measured in the direction of the coating to the baseline. The greater the fluctuation of the ordinate value, the coarser the coating surface.
as can be seen from Tables 1 and 1, compared with ordinary powder coatings, the high-efficiency ultra-fine HEUF powder coating significantly reduces the surface roughness of the coating film and improves the visual effect of the coating film. The reduction of coating thickness results in a significant reduction in cost, and the coating film also has the same durability, recyclability and powdering rate as ordinary powder coatings.
1.2 Overview of the application of ultra-fine powder coatings in the automotive sector
With the rapid growth of demand for high-end vehicles in the Asia-Pacific region, as well as increasing stringent environmental requirements from regulatory agencies such as china's Ministry of Environmental Protection, European REACH (Chemical Registration, Evaluation, Authorization and Limitation Regulations), and the Epa (Environmental Protection Agency of the United States), the automotive coating market is increasingly shifting from solvent-based coatings to low VOC coatings. Powder coating as a zero VOC emissions of environmental protection products, has been widely used in wheels, hoods, decorative trims, bumpers, hub covers, door handles, truck base, radiators, filters and a large number of engine components.
but there are some problems with powder coatings themselves. Common powder coatings typically have a higher film thickness, lower flatness, and a relatively poor visual appearance, which limit their application in automotive housing coating. In order to get better coating results, powder coating manufacturers have begun to reduce the thickness of the coating by reducing the particle size of the powder coating, improve the smoothness of the coating film. In recent years, fine powder coating has begun to be applied to body coating. General Motors and Chrysler have already achieved mass production of mid-paint paint in the United States and several other countries. BMW of Germany has successfully applied fine powder varnish (face paint). Although fine powder coatings have made great progress in the field of automotive coating, so far, ultra-fine powder coatings with particle sizes of less than 25 m have not been applied to automotive coatings. Powder coatings in this high-end market applications have not yet made a major technological breakthrough, so that its application in automotive body coating is still limited, more than 95% of body paint is still liquid coatings. The mobility of ultra-fine powder coatings is an important factor limiting their application in the field of automotive coating. The development of new fluidized additive technology will be the main way to solve this problem, and will also open a new door for the application of ultra-fine powder coatings in the field of automotive coating.
1.3 Performance study of ultra-fine powder coatings for mobile modified rear vehicles
High-efficiency ultra-fine (HEUF) powder coatings developed by the Particle Technology Research Center of the University of Western Ontario, Canada, are mixed with high shear force, so that nanoparticles are evenly dispersed in the coating Particle surfaces, based on calculations and experimental results, select the best nanoparticle reunification size to ensure excellent coating fluidity, in addition to the technology also includes surface changes to nanoparticles to improve the compatible of inorganic nanoparticles with organic powder coatings, to ensure coating quality.
In order to examine the application of this technology in the field of automotive coating, to test the feasibility of its scale production and performance in various evaluations, this study, in cooperation with a well-known automotive brand powder manufacturer, further refinement and particle surface change treatment of a body with fine powder coating problems in the coating production line, and spray the treated powder on the coating production line, and perform a characterization analysis of powder fluidity and coating properties. Through the optimization experiment of the pre-additive, this paper selected the fluidized additive addition amount (quality score) of 0.8%, focusing on comparing the difference between the ultra-fine powder coating after the addition of the fluidized additive and the unprocessed original commercial fine powder coating in terms of fluidity, spraying process and coating performance.
1.3.1 liquidity energy test the particle size and fluidity results of ultra-fine powders and raw commercial fine powder coatings treated after the
experiment, as shown in Table 2.
From Table 2, it can be seen that compared with commercial fine powder coatings, the university of Western Ontario prepared ultra-fine powder coatings with smaller particle size D50 and D90, D10 difference is not much, smaller D50 and D90 is conducive to the formation of smooth coating film, and no significant change in D10 can avoid a significant increase in Van der Wally. The fluidity of the powder is indicated by semi-dynamic and dynamic methods such as resting angle, avalanche angle and fluidized bed expansion ratio. As can be seen from Table 2, the modified powder coating resting angle is much lower than that of commercial powder coatings, and the lower resting angle indicates better fluidity of the powder. Typically, the smaller the powder particle size, the greater the Van der Wali, the less easily the powder is fluidized. However, under the effect of formulation additives, although the particle size of experimental powder coatings is small, it still shows better fluidity than commercial coatings in the control group. Avalanche angle, rotating fluidized bed expansion ratio, fluidized bed expansion ratio and resting angle results are similar, modified powder coating avalanche angle is smaller, rotation and ordinary fluidized bed expansion is relatively large. This is mainly due to the commercial fine powder coating particle size is small, the inter-molecular force (mainly Vanderwal force) is much larger than the gravity of the particles themselves, the particles are easy to bond with each other, poor mobility; In summary, through the comparison of various flow parameters of 2 kinds of powders, it is found that heUF powder mobility can be significantly improved, which is conducive to the fluidization of powder transmission, and is conducive to the formation of smoother and thinner coating film after spraying.
1.3.2 Spray Performance Test
In order to determine the effect of fluidizing additives on the spraying process, this experiment tested the sensitivity of powdering rate, reverse ionization characteristics and impact fusion (the phenomenon of melting blocks at curvature changes such as spray guns and bends during the transmission of powder coating airflow) during the spraying process, and the results are shown in Table 3.
As can be seen from Table 3, under 9 spray conditions (spray voltage range of 60 to 80kV, spray distance of 19 to 27cm, spray rate of about 315 to 510g/min), the average powder rate of modified powder coating is close to commercial powder, indicating that the additive has no effect on the efficiency of powder. In the reverse ionization test, under spray conditions similar to that of automotive coating production lines (spray voltage is 80kV, spray distance is 23cm, spray rate is about 315g/min), two powder coatings are repeatedly sprayed, and it is found that the experimental coating is reversed after more sprays (110 m film thickness) than commercial coatings. In the impact fusion test of the powder, no powder accumulation and collision fusion were found in the spray gun and bend during the fixed spray time, which showed that the airflow transmission performance of the experimental powder was good and the long-term continuous coating could be achieved.
1.3.3 Coating Performance Test
Coating Performance Test examines the moisture resistance, adhesion, flatness, mechanical properties, and compatibleness with substrates and transparent coatings required for medium powder coatings. For more representative results, the tests were conducted on complete coatings (including electrophoresis coatings, medium coatings, liquid coating substrates and varnish layers), where electrophoresis layers, substrates (white, black, silver or red) and varnishes were the same as the actual production line, with the medium coating being modified powder or commercial powder coatings.
first examined the attachment of the original coating and the wet air treatment (powder coating exposed to 96h in wet air with relative humidity of 99% and temperature of 23 degrees C) according to ASTMD33359-2017. The results showed that neither the original coating of the experimental powder and the coating after wet air treatment had film shedding, and the experimental powder showed the same adhesion and moisture resistance as commercial powder. The stone-resistant test was conducted on a complete coating based on ASTM D3170/D3170M-2014. As with adhesion tests, all modified paint models show the same test results as commercial powder coatings, indicating that the use of additives did not result in a decrease in the anti-stone performance of the paint film.
the first reverse ionosis and under the same film thickness (75 m) conditions. In 2 cases, the R value of the experimental powder coating (the parameters used by the cooperative automobile manufacturers in this study reflect the degree of orange peel). The higher the R value, the less orange peel, the smoother the coating film) are higher than commercial powder coating, showing a better coating appearance. The freshness (DOI) of the coating film, which also has reverse ionization, is also better than that of commercial powders, as shown in Table 4. The results show that the modified powder coating can show a clearer and smoother paint surface, which is beneficial to the application of powder in automotive exterior.
, the fluidity of the experimental powder can be significantly improved with the addition of fluidized additives. Experimental powders perform similarly or better than commercial powders in terms of spray and coating properties. The results show that this technique does not have any negative effect on the spraying process and coating performance while improving powder fluidity. This technology can be successfully applied to body coating to solve the mobility problem of ultra-fine powder coatings. At the same time, the further development of this technology is expected to continue to reduce particle size, to achieve smaller particle size powder spraying, lower coating thickness and higher coating flatness, to meet the appearance of body coating requirements, and further reduce costs.
2 Application of cryogenic curing powder coatings in automotive coatings
2.1 Introduction to cryogenic curing powder coatings
Low temperature curing powder coatings can save energy consumption and reduce production costs due to low baking temperatures, while this technology extends the application of powder coatings from conventional heat-resistant substrates to thermal substrates such as plastic products, paper sheets and wood, greatly expanding the application range of powder coatings. The development of low temperature cured powder coating is of great significance from the consideration of saving energy, reducing cost, improving efficiency and expanding the application range.
low-temperature curing of powder coatings is achieved by increasing the reaction activity of resins and curing agents. The coatings industry has been working to change thermostate resins with curing promoters