[Technology] Research and Development of Self-cleaning Powder Coatings
HC Coatings Network News: This article focuses on research and development of a powder coating with oleophobic, water-repellent, self-cleaning, and anti-graffiti features. The new functional paint is based on a silicon-modified acrylic resin, which has the properties of oleophobic, hydrophobic, antimony dust, and self-cleaning properties. Its coating film has a water contact angle of 85-105°, and the surface is smooth and sticky. With small appendix, easy to clean, not easily contaminated, with self-lubricating properties and low friction properties, but also has a certain scrub resistance and other properties, but the price is relatively expensive. By controlling the addition amount of the silicon-modified acrylic resin and adopting a dry blending process, a relatively good self-cleaning effect can be achieved while reducing the content of the silicon-modified acrylic resin, and the formula cost is greatly reduced.
As a typical representative of environmentally friendly coatings, powder coatings have become more widely used in all walks of life. In recent years, the application of home appliances and IT in the furniture industry is very popular with the market. Small to the necessary kitchen hood, large outdoor base stations in the communications industry, powder coatings as their outer surface plays a protective role. However, the ever-increasing pursuit of film performance in various industries and industries continues to make new demands on powder coatings. For example, the working environment of the hood is rather harsh, and it is often eroded by high-temperature oil and gas oils. Over time, oil deposits are deposited on the hood surface. Conventional powder coatings generally have a certain degree of oleophilicity. After the oil droplets come into contact with the surface of the coating film, they will immediately Spread and adhere to the surface, ordinary wipe does not completely remove the oil, oil will become more and more difficult to clean over time, affect both health and appearance. This requires the coating film of the shell to have oleophobic, easy-to-clean, scrub-resistant, detergent-resistant properties. The cabinets of base stations and communication equipment in the communications industry also need to have good easy-to-clean performance on the outer surface, and also require the coating film to have good weatherability. For this reason, the development of a new powder coating with self-cleaning function has great practical value and broad market prospects.
1Hydroxy-functional silicon-modified acrylic resin system
The functional group-containing silicon-modified acrylic resins referred to in the text fall into two categories. One is a silicon-modified acrylic resin with a hydroxyl group, and the other is an epoxy group-containing functional group, respectively applied to different matrix resins. system. Both types of resin have a basic structure, as shown in Figure 1:
Figure 1: Silicon-modified acrylic resin with hydroxyl functional groups and its curing reaction
First, the skeleton structure of this type of resin is long-chain acrylic, which is also the main structure of the resin, and then both sides of the main chain are functional silicon segments and reactive functional groups, and the silicon segment plays a role in achieving the self-cleaning function. The reactive functional group can react with an appropriate curing agent to make the resin self-curing or cross-linking into the matrix powder coating system, so that the functional silicon can maximize the effect, and after chemical crosslinking, the coating can also make The service life is greatly extended.
The following describes the hydroxyl-containing functional silicon-modified acrylic resin, and this self-cleaning acrylic resin is used as a main body, and the self-cleaning acrylic resin is used as a main body. Although the cost is high, the effective component of the self-cleaning functional group is used. The highest self-cleaning effect is also best.
Table 1: Basic Formulation of a Silicon-Modified Acrylic Resin System Containing Hydroxy Functional Groups
Curing conditions: 15min@200°C
This system has very excellent self-cleaning performance. It is difficult for normal water-based pens and oil-based pens to write on the coating film. After writing, the handwriting will automatically shrink into small droplets, which can be wiped cleanly with ordinary cloth or napkin. Go (Fig. 2). After the oily pen is easily evaporated completely, you can still erase all the marks with less effort. And this anti-graffiti effect can be maintained for quite some time. After repeated writing and wiping 100 times, the film still has a fairly good anti-graffiti effect. The water contact angle test showed that the water contact angle of the coating film of this system reached 101.17° (Fig. 3). With a relatively high hydrophobicity, the water droplets would automatically shrink into a circle on the coating film surface, and the fluidity would be quite good. Similar to the effect of lotus leaf surface. The oleophobicity of this system is also very good. After edible oil drops on the coating surface, it will quickly shrink into a round shape. By slightly tilting the coating at an angle, the oil drop will fall to the edge of the coating and flow away due to gravity. The paint film will not adhere to grease at all.
Figure 2: The anti-graffiti properties of the hydroxy-functional silicone-modified acrylic system on the oil-based pen, before wiping (left) and after wiping (right)
Figure 3: Water contact angle of hydroxyl-functional silicon-modified acrylic system coating before aging (left) and after UVB200 aging (right)
However, the hydroxyl-functional silicon-modified acrylic system also has the common problem of polyurethane systems. The high hardness of the coating results in poor mechanical properties. Although the curing density and the catalyst can be used to adjust the crosslinking density, the mechanical properties are improved. After all, the mechanical performance is hard to increase a lot. From the standpoint of weatherability, the polyurethane system is not too good. After 200 hours of UVB aging, the water contact angle of the coating film has declined to 84.54° (Fig. 3), the self-cleaning performance has been greatly reduced, and the color difference and retention rate data are not That's great. Moreover, the production stability of this system is poor, so it is subject to certain restrictions in practical applications. 2 Silicon-modified acrylic resin system containing epoxy functional groups
As mentioned earlier, the basic structure of another type of silicon-modified acrylic resin with epoxy functional groups is shown in Figure 4:
Figure 4: Schematic diagram of epoxy-functional silicon modified acrylic resin and its curing reaction
Compared isocyanate (polyurethane) systems, epoxy - - hydroxy acid system and having better weather, better mechanical properties, the advantages of fewer pinholes.
Table 2: Basic Formula of Silicon Modified Acrylic Resin System with Epoxy Functional Group
Curing conditions: 15min@200°C
The self-cleaning performance of this system is also very good. Ordinary water-based pens and oil-based pens are not easy to write on the coating film. After writing, the handwriting will automatically shrink into small droplets. With ordinary cloth or napkins, it can be done completely without effort. Wipe off (Fig. 5). After the oily pen is easily evaporated completely, it can still wipe off all the marks with less effort. And this anti-graffiti effect can be maintained for quite some time. After repeated writing and wiping 100 times, the film still has a fairly good anti-graffiti effect. The water contact angle test showed that the water contact angle of this coating film reached 99.72° (Figure 6). Other oleophobic hydrophobic properties are also very close to the hydroxyl functional system.
Figure 5: Anti-graffiti properties of epoxy-functional silicone-modified acrylic systems for oil-based pens. The left picture shows the paper towel before wiping. The right picture shows the paper towel after wiping.
Figure 6: Water contact angle of epoxy-functionalized silicon-modified acrylic system coating. Left picture is before aging. Right picture is after UVB 200-hour aging.
The mechanical properties of the epoxy-diacid system are significantly better than those of the polyurethane system. Although the hardness is slightly lower, all other mechanical properties are better than the polyurethane system, and the leveling performance and appearance of the coating film are also better (Table 3). From the viewpoint of weatherability, the epoxy-diacid system is also better than the polyurethane system. After 200 hours of UVB aging, the contact angle of the coating-film water of the epoxy-diacid system can still reach 89.99° (Fig. 6). The system still retains A certain degree of self-cleaning performance, which is much better than the polyurethane system, its color change and gloss retention are also better than the polyurethane system (Table 4). Although the anti-graffiti properties of the epoxy-diacid system are slightly weaker than those of the polyurethane system (this is due to the fact that the high density properties of the polyurethane system itself determine the overall resistance of the polyurethane system to ink penetration resistance of the marker pens compared to other systems. Good), but overall overall performance is better than polyurethane systems. Of course, whether it is an epoxy-diacid system or a polyurethane system, if a silicon modified acrylic resin is used as the main resin (more than 60% by mass), the cost is relatively high, which limits its large-scale application.
Table 3: Comparison of comprehensive properties of hydroxy system/epoxy system coatings
Table 4: Coating color difference and gloss retention after 200h weathering test