Research on the application of UV-cured coatings in the field of metal corrosion prevention

China Coatings Online News Information
Shen Xuefeng, Gou Xiaoqing, Gu Bin (CNOOC Changzhou Coatings and Chemical Research Institute, Changzhou, Jiangsu 213016)
( Summary): A brief overview of the development of UV-cured coatings at home and abroad. The problems and solutions in the application of UV curing coatings in the field of metal corrosion protection are analyzed. This paper discusses the ways and means to solve the adhesion of UV curing coating to metal substrate and to improve the curing degree of UV curing paint.
: UV-cured coatings; anti-corrosion coatings; adhesion; curing degree
(middle classification number): TQ630.7 document identification code: A Article No. 1009-1696 (2013) 12-12-1 0031-06
1. Overview of the development of ultraviolet curing coatings at home and abroad The radiation curing market has made great progress in recent years, radiation curing has been applied in many fields, such as photoelectronics, coatings, inks and other industries, to replace the traditional heat treatment solvent-based coatings, inks and adhesives. The focus of this paper is on ultraviolet light (UV) curing coatings in radiation curing products, which is the most important application area of radiation curing technology.
most important advantages of
UV curing coating technology are higher productivity and lower energy consumption, fast curing speed, operation at room temperature, as low as possible VOC (volatile organic compounds), high-quality end products. For its final application, the largest application in the United States is printing, including printing inks and overprint inks, followed by industrial coatings, photoelectronics and adhesives. The largest applications in Europe are wood coatings and furniture coatings, followed by printing inks. In Japan, the largest applications are electronics and fiber optics, followed by printing inks and adhesives.
as early as the early 1970s, China began to study UV curing coatings. Shanghai and Beijing set up several furniture UV curing paint production lines, but due to the lack of raw materials, light sources and curing equipment does not match and get off the horse. The real development of UV cured coatings in China began in the 1990s, more than 20 years, the research and product varieties in this field have been developed by great length, has formed an industrial scale. In 1993, China officially set up a radiation curing branch, in Asia is second only to Japan has this association of the country, now has more than 200 member units. China has become the second largest country after the United States and Japan in the production and application of UV curing products.
In the past few years, China's UV curing coatings in fiber optic coatings, CD coatings / DVD adhesives, credit cards, wood, beverage cans, food packaging, magazine covers, medical devices and the automotive industry have been very rapid development. However, there is very little research on UV-cured anti-corrosion coatings, only Wang Defeng and others reported on the development of UV-cured anti-corrosion coatings. According to incomplete statistics, 30% of the world's steel produced each year is corroded, of which 10% will be turned into scrap iron. The economic losses caused by metal corrosion amount to hundreds of millions of pounds a year in the United Kingdom, about 4.2% of GDP in the United States and about 4% of gdp in our country, more than the total losses caused by fires, wind and earthquakes. UV curing technology, because of its environmental protection, low energy consumption, fast curing speed and so on, it is applied to the field of metal corrosion protection, has great practical significance, can greatly improve the production efficiency of corrosion-resistant products, reduce costs, improve environmental pollution and so on
2. UV-cured coatings in the metal anti-corrosion applications In the current market, the use of UV-cured coatings in metal anti-corrosion is still less, but in some areas of light anti-corrosion heavy decoration, such as metal sign decoration, can processing, metal trim plate manufacturing, aluminum alloy door and window protection and steel pipe temporary coating protection, UV curing coating coating has a wide range of applications.
UV-cured coatings face the first problem in metal corrosion-proof applications is adhesion. General UV curing coating on the metal adhesion is not good, if not added special functions of adhesion promoter, conventional UV curing coating is difficult to obtain a better adhesion. The second is the curing of the coating. This is due to the anti-corrosion coating system with more pigments, they have a strong absorption and reflection effect, many of the light trigger effective ultraviolet rays are shielded out, resulting in a system curing rate greatly reduced, or even not cured, to achieve a high cover force is difficult. This paper will focus on the above two points of analysis, put forward a reasonable solution, hoping to promote the UV curing coating in the field of metal corrosion prevention application research.
2.1UV curing coating adhesion problem on metal substrate
Coating adhesion formation mechanism is generally expressed by the following theories: first, wetting and surface energy; It can also be simply expressed that when two materials that are not similar reach "close" contact, the two free surfaces in the air disappear, forming a new interface. The nature of the interface interaction determines the strength of the bond between the coating and the substrate, the degree of this interaction is basically determined by the wetting of one phase by the other phase, when using liquid coating, the fluidity of the liquid phase is also very helpful, so wetting can be regarded as close contact between the coating and substrate. In order to maintain the adhesion of the coating and substrate, in addition to ensuring the initial wetting, it is important to keep the bonding condition unchanged after the film is completely moisturized and cured.
UV-cured coating has the advantages of fast curing, less pollution, energy saving and suitable for pipeline production compared with traditional thermo-cured or self-drying coatings, but its attachment to metal substrates is more difficult. This is because: on the one hand, UV curing coating curing speed, and acrylic monolith polymerization when the volume shrinks too large, so that the internal stress produced by the paint film when curing is too late to release; In general, the adhesion between the coating film and the substrate is improved by adding various additives, but for the more specific metal substrate, a combination of factors affecting adhesion is required.
2.1.1 body resin selection
With the wide application of UV curing products, the types of photoactive polymers are also emerging, mainly:
unsaturated polyester (unsaturated polyester) lyester, UPE): This is the first low-polymer used in photo-curing materials, due to its slow photo-curing speed, poor surface dry performance, coating is not soft enough, polyester main chain a large number of ester-based acid resistance and alkalinity, is now rarely used.
epoxy acrylates (epoxycrylate, EA): This is currently the most widely used and most dosed light-cured low-polypolymer, according to the structure type, can be divided into bisphenol A epoxy acrylates, phenolic acrylates, acrylic oxide and modified epoxy acrylates.
polyurethane acrylate (PUA): This is a low polymer that can be set by molecular design and has better comprehensive properties. Because polyurethane acrylic molecules contain urethane bonds, can form a variety of hydrogen bonds between polymer chains, so that the curing film has excellent wear resistance and flexibility, high fracture elongation, at the same time has good chemical resistance and high, low temperature performance, better impact resistance.
polyester acrylates (polyesterracrylate, PEA): Its biggest feature is its low price and viscosity, neither as a low-concentration nor as an active thinner. In addition, polyester acrylics are mostly low odor, low irritation, better flexibility and pigment wetting, suitable for color paint and ink.
polyetheracrylate and pure acrylates: generally low mechanical strength and hardness, poor acidity and alkalinity resistance. Therefore, in practical application, these two resins are not used as main resins, just to improve some properties of UV curing coatings, such as improving yellow resistance, improving the adhesion to the substrate and inter-coating adhesion and so on.
New photoactive compounds: With the continuous development of UV-cured coatings, some low viscosity, low volatilization, low toxicity, high-speed curing, low-level curing, with special functions of low-concentrations, the main aerobic silicone photoactive adhesives, water-based compounds mixed together, superchip polypolymers and so on.
I have used fatty polyurethane acrylates (PUA), polyester acrylates (PEA), polyurethane acrylates mixed with polyester acrylates (PUA/PEA), epoxy acrylates and polyester acrylates mixed The adhesion test results on the steel substrate were shown in Figure 1 when the collation (EA/PEA), the acrylates of the ring oxide oil and polyurethane acrylates (EA-1/PUA), and the phenolic ehyoxypropylate (EA-2) were tested.
Table 1
is visible from Figure 1: Phenol ethylene oxide acrylates (EA-2), epoxy acrylates and polyester acrylates mixed (EA/PEA) have better adhesion to the metal substrate, wherein polyester acrylates (PEA) just cured when the adhesion is better, cooling adhesion slightly worse, mainly due to polyester resin curing contraction stress concentration, resulting in a decrease in adhesion. Due to the large viscosity of phenolic epoxy acrylates themselves, and the dispersion, wetting and heat resistance of low viscosity polyester acrylates and epoxy oil acrylates to pigments are better, and the above test results, in the final resin selection, consider the phenolic epoxy acrylates with low viscosity polyester acrylates and epoxy acrylates as a mixture, should be a better choice.
2.1.2 Active thinner selection
Active diluent is an organic small molecule containing polymeric energy group, in UV curing coating not only soluble, dilute lymer, adjust the viscosity of the system, but also participate in the photo-curing process, affecting the curing speed of the coating and the various properties of the curing film. Most of the active thinners used at present are acrylic monoliths, according to the number of double bonds in the molecule, the active diluent can generally be divided into: single-function, dual-function and multi-function 3 categories;
active thinners for UV-cured coatings are shown in Table 1.Table
Table
Active thinners should be selected with lower shrinkage, low viscosity, small surface pressure varieties, which is conducive to improving the adhesion of the coating on the surface of the substrate, and the wetting and dispersion of pigments is also better, but excessive use of low-level diluents will lead to the curing performance of the coating affected. Proper selection of a portion of the multi-functional active thinner to be used is necessary, and will have a significant effect on the hardness, wear resistance and chemical resistance of the coating film. Multi-official energy active thinner each molecule contains 3 or more active groups, relative to the molecular mass, high viscosity, shrinkage is also large. Acetylene or propylene-based acetone thinners can effectively improve their shrinkage defects, and the viscosity is also relatively reduced.
2.1.3 adhesion promoter
adhesion promoter can act as a bridge between substrate and coating. Due to the fast curing speed of UV curing coatings, some adhesion promoter molecules often migrate to the substrate surface, thus losing the role of enhanced adhesion, so the choice of adhesion promoters is particularly harsh. At present, more silicone oxane and phosphate adhesion promoters are used on the market.
Silicone oxane adhesion promoter in the metal surface, through the swallowtail wedge effect and interaction with the metal to complete the penetration to the metal surface, silicone oxygen can directly react with the metal surface oxide to form a co-priced bond and mating bond.
There are two main methods for introducing phosphate groups into UV curing systems: one is to modifie acrylic monosomes with phosphates or phosphorus-containing compounds, but these products tend to have higher acid values and light curing systems. Poor compatible and very expensive, the other is phosphorus-containing PUA, which is synthesized from diphosphate and biphenol A-shrinking glycelycerides, and isocyanate and acrylic hydroxylate, which are synthesized in different structures. It can participate in cross-linking polymerization and enter the cross-linking network, which has a significant effect on the adhesion of coating on metal substrates.
there may be a series of complex chemical reactions between the various parts of the coating and the adhesion promoter, so it is important to pay attention to the matching of the group between the various compositions and the adhesion promoter in the coating when selecting the adhesion promoter. If the adhesion promoter participates in the reaction to increase the internal stress of the paint film too much, it is not conducive to the attachment of the coating.
2.2UV curing coating in the metal anti-corrosion system coating curing problem
Because pigment is an indispensable component of anti-corrosion coating system, which seriously affects the UV curing coating in this field of popularization and application. Because of the strong absorbent and reflective effect of pigments, many UV rays that are effective against light emitters are shielded, resulting in a significant reduction in the curing rate of the system, or even non-curing.
the choice of 2.2.1 light
The light trigger commonly used in UV curing coatings is divided into two categories: one is free-form light trigger, the other is cation-type light trigger, in which free-form light trigger can be divided into lysate type and hydrogen-grabbing type.
ultraviolet rays are divided into: short-wave ultraviolet (UV-C), wavelength 200 to 290nm, medium-wave ultraviolet (UV-B), wavelength 290 to 320nm, long-wave ultraviolet (UV-A), wavelength 320 to 400nm, visible light (Visible light), wavelength 400 to 760nm. Industrial production generally uses long-wave UV (UV-A), color paint formula using the traditional 1173 and 184 triggers in this wavelength absorption range will be blocked without being absorbed trigger, so it is necessary to choose different light triggers according to different pigments. In the colored coating system, the selection of monoimil-based phosphorus oxide and diamyl-based phosphorus oxide agents have a very obvious effect on deep curing. For example, TPO, 819 triggers are the representative products of the commercialization of these triggers, its maximum absorption wavelength is 380nm, in the visible light 430nm area also has absorption, so it is particularly suitable for the curing of colored coating systems.
2.2.2 pigments on the coating film curing speed
different pigments have different absorption rates for different wavelengths of light, the absorption rate of ultraviolet light is generally: black >violet> blue >green>green>> red. However, it was found in the experiment that the above UV absorption order does not represent the UV curing velocity order of the various color coatings. In real formulations, red and yellow coatings generally have poor curing properties. Figure 2 shows the laws of the effect of pigment phase, content and particle size on curing speed.
Table 2
Table 2
inorgetable pigment system