UV coatings are used in many surface coating fields for their excellent surface properties: high strength, high hardness, high abrasion resistance, high gloss, high solvent resistance; UV curing technology has become an environmentally friendly green technology with its fast curing speed, low pollution and energy saving. UV coatings account for about 98% of the radiation cured coatings. From interior coatings such as floor coatings and wood furniture coatings to industrial coatings such as plastic coatings, anti-corrosion coatings, motorcycle coatings and automotive coatings, more and more applications confirm Harbourne’s idea of “ubiquitous radiation curing technology”. As UV coatings are used outdoors, weatherability has become an issue of concern. This paper is a preliminary discussion on the weatherability of UV topcoats.
1. The basic concept of weathering resistance
The weatherability of paint mainly refers to the mechanical properties such as modulus, strength, adhesion and optical properties (such as color and light retention), as well as changes in chemical properties (such as embrittlement, chalking and corrosion) when the paint is exposed to outdoor environmental conditions.
Under the action of light, air and water (acid rain), the outdoor degradation process of coatings mainly includes photo-induced oxidative degradation, water degradation, thermal degradation and high-energy radiation degradation. 2.
2. The special characteristics of UV cured coatings
From the theory we know that due to photo-induced oxidative degradation, hydrolysis, thermal degradation and high-energy radiation degradation are the factors that cause the degradation of weathering resistance of the coating surface. To improve the weathering resistance, the following three factors should be excluded as much as possible from the composition of the coating: (1) absorption of wavelengths above 290 nm, (2) resins that are susceptible to hydrogen atom capture, and (3) functional groups that are susceptible to hydrolysis.
However, the composition of UV coatings has at least two of the above points: the photoinitiator absorbs wavelengths in the range of 200~400nm; the photoinitiator generates free radicals with active hydrogen atoms (from resin or additives). Therefore, UV cured coatings have the problem of weathering resistance from the very beginning.
The weathering problem of UV coatings is mainly photo-aging. Its special characteristics are: it needs UV light to cure, and the long time exposure to UV light will lead to the deterioration of the film quality. Sunlight contains UVA and UVB long-term irradiation curing coating is easy to make the cross-linked network of carbonyl, aryl and other light-absorbing groups, as well as residual photoinitiator, photoinitiator promoter (photosensitizer) and other light-absorbing impurities to UVB or even UVA absorption and chemical bond rearrangement and aging deterioration. Under outdoor oxygen conditions, molecular oxygen can be photosensitized to produce highly reactive single-linear oxygen to form oxidation products and photodegradation of macromolecular polymers, and can also form peroxyl radicals and hydrogen capture, cleavage, cross-linking, rearrangement and other reactions. As a result, the modulus becomes smaller, the yellowing increases, and the coating film becomes brittle and the weatherability deteriorates. Therefore, it is extremely important to understand the weathering properties of UV cured coatings from their composition.
3 Factors affecting the weatherability of W topcoat
3.1 Composition of resin
UV cured coatings from the development of the resin system from unsaturated polyester system transition to acrylate system, from the acrylic structure can be analyzed, if it is a pure acrylate coating, its weatherability should be excellent, but due to the cost and the need for modification and the introduction of other functional groups, which changes the structural properties. At present, the most used in the world are still epoxy acrylate and urethane acrylate resins. The following experiments allow us to understand the weathering properties of some categories of resins.
3.1.1 Raw materials and formulations
Sinomer® EATM (standard bisphenol A epoxy acrylate resin), UVU6609 (aliphatic urethane acrylate resin), UVP9200 (polyester acrylate resin), UVA1000 (pure acrylate resin), all of which are products of Longchang chemical company; UVU6200 (aromatic polyether urethane UVU6200 (aromatic polyether polyurethane acrylate resin), Longchang chemical; 1173 (2-hydroxy-2-methyl-1-phenylacetone), Longchang chemical TPGDA (tripropyleneglycol diacrylate), Longchang chemical company.
3.1.3 Discussion of the results
(1) Bisphenol A epoxy acrylic resin is the most abundant in UV applications, and its advantages are reflected in the fast curing speed, high gloss and good hardness. Since the Ar-O-R in the resin can absorb UV light above 290 nm and undergo photocracking to produce free radicals and participate in oxidative degradation, the yellowing is more serious in the strong UV light at the beginning, but not significant under natural conditions.
(2) Polyurethane acrylic resins can be classified into aromatic and aliphatic based on the structure of the – NCO group involved in the reaction. The aromatic carbamates (Ar-NH-COOR) can also absorb UV light at 290 nm and directly cleave to quinone structures.
In addition, the ether bonds of polyether polyurethanes with ether bonds are also highly susceptible to photodegradation.
Aliphatic polyurethanes have a slight discoloration when first cured, but show excellent weathering resistance under natural conditions, and because of the straight chain structure, cross-linked films are slightly less alkali resistant.
(3) Pure acrylate resins have superior structural weathering resistance. Although acrylate polymers have excellent aging resistance, there are many drawbacks if used as the main resin of coatings: mainly, the cured film has poor acid and alkali resistance, solvent resistance, and boiling in 10% KOH solution for 15min after film formation, the film will blister and peel off due to hydrolysis of the polymer.
(4) Polyester acrylate resin, due to the branched chain structure to strengthen the cross-linkage, tight structure, better strength, solvent resistance is also strong. But the polyester synthesis part will affect its yellowing due to the number and position of benzene ring and heteroatoms.
In UV cured coatings, photoinitiators are free radical initiators. According to the structural characteristics, they can be divided into: carbonyl compounds, dyes, metal organic, halogen-containing compounds, azo compounds and peroxy compounds. According to the mechanism of free radical generation can be divided into cleavage type and hydrogen extraction type.
At present, the global industrial application is still mainly free radical type initiator, other classes only a very small amount in use, and even individual classes are still only used in the laboratory. In China, there are mainly 1173, 184(1-Hydroxycyclohexyl phenyl ketone, Longchang chemical), TPO(2,4,6-trimethylbenzoyl-diphenyl phosphine oxide, Longchang chemical) and other cleavage type and BP (benzophenone, Longchang chemical), ITX (isopropylthioanthrone, Longchang chemical), CTX (2,4-dichlorothioxanthrone, Longchang chemical) and other hydrogen extraction types.
3.2.1 Cleavage type
The cleavage type photoinitiator is applied in acrylate system, which is not easy to produce yellowing or has a small yellowing coefficient. The main reason is that the red-shift wavelength of the substituted benzyl is small, and it is not easy to produce resonance discoloration. However, its unpleasant odor becomes an application obstacle.
3.2.2 Hydrogen extraction type
This type of photoinitiator needs to be together with compounds containing active hydrogen in order to produce bimolecular reactions and generate free radicals to promote the reaction. The compounds that provide active hydrogen (also called photosensitizers) are mainly tertiary amines, triethanolamine, active amines, and experiments using different photosensitizers can show their relationship with coating film yellowing.
The presence of photosensitizers is also likely to be due to the presence of color-emitting groups: carbonyl groups conjugated to amino or aromatic rings, which intensify the yellowing and degradation reaction. Another reason is that the photoinitiator in UV coatings will remain 1~2% in the system to react, this part of the photoinitiator in natural light absorption of ultraviolet light caused by the residual double bond deep cross-linking, resulting in discoloration, cracking or wrinkling of the coating film.
Different monomers have different reaction rates in the curing process, the faster the reaction, the more residual double bonds in the monomer. The presence of functional groups with ether bonds is prone to photodegradation, so for TPGDA, DPGDA (dipropylene glycol diacrylate), (EO)TMPTA (ethoxytrimethylolpropane triacrylate), (PO)TMPTA (propoxytrimethylolpropane triacrylate) and other monomers containing alcohol-condensed ether structure is more prone to photodegradation reaction, it is reported that the ethoxy than propylene oxide structure is less stable than the light. The light stability of the ethoxy structure is poorer than that of the propoxy structure, and the order of light stability among several conventional monomers is.
TMPTA > NPGDA (neopentyl glycol diacrylate) > HDDA > TPGDA > (EO)TMPTA ≈ (P0)TMPTA
4 Measures to improve the weather resistance of UV topcoat
4.1 Selection of resin
In order to improve the weather resistance of outdoor UV finishes, it is necessary to select resins that are resistant to yellowing, change in coating film modulus to adapt to environmental changes, high hardness, good flexibility, and scratch resistance. The most suitable resin for these requirements is an aliphatic acrylate polyurethane resin with multiple functional groups. In order to reduce the cost, we can also choose a part of modified epoxy acrylate resin and aliphatic urethane acrylate or pure acrylic resin combination.
4.2 Choice of monomer
There is a contradiction in the use of monomers: from the skin irritation should choose alkoxylated acrylate monomers, from the weatherability should not choose alkoxylated acrylate monomers. The author suggests that the best choice of monomers for weathering topcoats is as follows: TMPTA, HDDA, TPGDA, which can reduce photodegradation.
4.3 Selection of photoinitiators
The hydrogen-raising photoinitiator yellowing coefficient is larger, generally should choose the cracking type to help reduce yellowing, usually choose 1173, 184, TPO and other types in the clear coat or colored system applications.
Photoinitiators Same series products
|Ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate
|1-Hydroxycyclohexyl phenyl ketone
|Benzene, (1-methylethenyl)-, homopolymer,
|Difunctional alpha hydroxy ketone
|Benzeneacetic acid, alpha-oxo-, Oxydi-2,1-ethanediyl ester
|(4-Methylphenyl) [4-(2-methylpropyl)phenyl] iodonium
4.4 Selection of other additives
(1) UV absorber
In other types of coatings, UV absorbers are usually used to reduce the UV absorption of polymers or to add simple excited state trapping agents to eliminate free radicals in order to improve the weathering resistance. UV cured coatings require maximum absorption of UV light to produce more radicals during curing, so adding UV absorbers will more or less shield the photoinitiator in the system, resulting in lower curing rate and lower polymer conversion. The best addition amount <0.1 % has been tested, and the addition of formamidine is better than benzotriazole and aromatic ester compounds.
(2) Hindered Amine Light Stabilizers (HALS) and Antioxidants
The main feature of HALS is the photo-oxidation into nitrone radicals (R2NO – ), R: NO – in the polymer radicals and the termination of the reaction by the discordance reaction or coupling reaction, respectively, to generate hydroxyl amines and ethers hydroxyl amines and ethers, and then decomposed into hydroperoxides and subsequently generated R: NO -and so on, which greatly reduces the possibility of resin degradation. Some experts believe that the addition of HALS will be beneficial to the weatherability of UV coatings.
The same principle as UV absorber, adding HALS will also interfere with the reaction of free radicals in the light curing process. Adding 0.1% to 0.05%, in matte coating system (gloss <50 at 60°), there is a phenomenon that the surface does not dry at all. Therefore, it is recommended not to add HALS.
Antioxidants are divided into pre-oxidants and chain-breaking antioxidants. Since it mainly blocks the reaction of peroxide by redox reaction, it has less effect on UV curing, so it is optional, and it is usually better to choose triphenyl phosphite.
(3) Choice of partially transparent dyes
Dark or solid color coatings are still a problem for UV coatings, but in thin coatings can be added < 3% transparent dyes to strengthen the shielding of natural light in UVA, especially the absorption or reflection of sunlight in the 330 ~ 400nm ultraviolet light, so as to reduce the light aging of the coating.
Light Stabilizers Same series products
70624-18-9 / 71878-19-8
(4) Selection of fluorescent whitening agent
Fluorescent brighteners are used to absorb UV light, and the coating will be blue or purple, which means the coating will be “blue light” to eliminate the yellow color. After using fluorescent whitening agent, there is a problem that the whitening agent competes with the photoinitiator to absorb light, so it is required to choose the photoinitiator with high initiation efficiency.
(1) The structure of resin, photoinitiator and monomer were analyzed from the composition of UV topcoat to influence the weatherability.
(2) The choice of aliphatic polyurethane acrylate resin and cracking photoinitiator and several monomers without alkoxylation can improve the weathering resistance of the coating film.
(3) From the selection of additives, it is proposed that adding a small amount of UV absorbers, antioxidants and dyes can shield UV light from natural light to reduce photoaging; fluorescent whitening agents are used to complement blue to eliminate yellowing.