Common knowledge of coatings: factors influencing curing speed of UV curable coatings

It is suggested that UV curing technology has played an important role in many fields since it was first used in Germany in 1960s UV curable coatings have developed rapidly in the past half century, and have been widely used in many fields (such as wood, metal decoration, printing industry, optical fiber coating, leather dawn, etc.) Compared with traditional coatings, UV curable coatings do not contain any organic solvent or inert diluent, and do not need to be heated during curing It has the advantages of low environmental pollution, low energy consumption, high production efficiency, and small shrinkage Paint agent, top 10 Chinese wall paint brands Recommended introduction: interior wall paint and exterior wall paint can achieve beautiful, clean and pleasant effects, but the effects in different environments are different, so the color of the paint will be different There are ten brands of Chinese paint, ten brands of global paint, ten brands of Chinese wall paint, and ten brands of Chinese paint, Famous trademark of Chinese paint and coating In private rooms, most of us choose according to our own personality, personal hobbies and living habits The younger generation will choose the interior wall paint with rich colors and bright colors, while the older ones will have some working experience Home of coatings: UV curing technology has played an important role in many fields since it was first used in Germany in 1960s UV curable coatings have developed rapidly in the past half century, and have been widely used in many fields (such as wood, metal decoration, printing industry, optical fiber coating, leather dawn, etc.) Compared with traditional coatings, UV curable coatings do not contain any organic solvent or inert diluent, and do not need to be heated during curing, which has the characteristics of small environmental pollution, low energy consumption, high production efficiency and small shrinkage However, in addition to the above advantages, UV curable coatings also have some shortcomings, such as in some cases its curing speed has not reached satisfactory results Therefore, how to improve the curing speed of UV curable coatings has been concerned by people In this paper, the influence of photoinitiator, monomer, prepolymer and pigment on curing rate and the research direction are described one by one 1 photoinitiator 1.1 classification of photoinitiators generally can be divided into the following two categories: free radical photoinitiator and cationic photoinitiator 1.1.1 free radical photoinitiator free radical photoinitiator can be divided into split photoinitiator and hydrogen extraction photoinitiator according to its mechanism The so-called split photoinitiator is that after the absorption of UV, the C) c r bond connected with the carbonyl group breaks: c6h5co-cr3 → c6h5co • + • CR3 according to the chemical composition, most of these initiators are benzoin and its derivatives, benzoyl ketal, acetophenone derivatives and some of the carbonyl containing photoinitiators It is a kind of effective photoinitiator, especially benzoin derivative In terms of benzoin and benzoin ether, the hydrogen atom on the carbon of the benzyl ether in the molecule is relatively active and easy to be captured It can be cracked to generate free radicals Even when there is no light energy provided by the outside world, it is easy to initiate polymerization reaction Once exposed to ultraviolet light, the initiation speed is faster, so the UV curing coating can obtain faster curing speed The so-called hydrogen extraction photoinitiator means that it is in the excited state after being irradiated by UV, but it does not undergo the splitting reaction, but can extract one h from one h donor molecule to produce one carbonyl radical and one donor radical: such initiators are generally aromatic ketones, such as benzophenone and its derivatives, thioheteroketone, etc But the rate of initiation is slow As far as benzophenone is concerned, when it is used in combination with tertiary amines, although hydrogen can generate aminoalkoxy radicals under the action of light with the complexes of tertiary amines, it can react with oxygen to generate peroxides to capture hydrogen atoms, thus eliminating the polymerization inhibition of oxygen However, due to its long triplet life, it takes time to eliminate the polymerization inhibition of oxygen, so the initiation speed is slow, the curing time of the film is long, and the production efficiency is low 1.1.2 the cationic photoinitiators generally include the following categories: aromatic diazonium salt, aromatic chalcogenium salt, aromatic iodonium salt, organic aluminum complex / silane system, ferrocenyl salt, etc Taking aromatic diazonium salt as an example, although the initiation speed is relatively fast, it is gradually eliminated due to its poor storage stability, release of nitrogen during photolysis, and the formation of bubbles and pinholes in the polymer film-forming process As far as aromatic thioxanthium salt and aromatic iodonium salt are concerned, they have excellent stability, but their maximum absorption wavelength is in the far UV region, and there is no absorption in the near UV region, Their initiation rate is slow, and the curing rate of UV curing system is also slow 1.2 the influence of photoinitiator on curing speed through the discussion of the above two kinds of photoinitiator, it can be seen that generally speaking, the curing speed of free radical photoinitiator system is faster, but due to the large shrinkage of the film after curing, the adhesion is poor; on the contrary, most of the cation photoinitiator systems used now, the curing speed is slow, but due to the small shrinkage of the film, Even volume expansion will occur, so the adhesion is better Different photoinitiators have different absorptivity to the same band of UV light, so the initiation speed is different Different photoinitiators have a maximum absorption peak in the UV region At the maximum absorption peak, the initiator has the highest initiation efficiency, so the curing speed of the curing system is the fastest Therefore, different photoinitiators should be selected for different light sources For example, if the UV lamp with the maximum emission power is used at 300 cm - 1, the photoinitiator should be 907 in the system If the UV lamp with the maximum emission power is used at 320 cm - 1, the photoinitiator should be 369 in the system, so that the absorption rate of the initiator to UV light can reach the maximum, Thus the curing speed is the fastest In addition, the amount of initiator will also affect the curing speed, and the amount of initiator is generally controlled at about 3%, because if the initiator is excessive, it will also absorb UV light, so that the UV radiation reaching the bottom layer of the film is greatly reduced, resulting in the reduction of curing speed In the color system, the absorption peak of photoinitiator should be as far as possible in the wavelength range of weak UV absorption of pigment, which can also improve the curing speed of UV curing system 1.3 the research direction of photoinitiator can combine the advantages of both, and the free radical and cationic photoinitiator can be combined to synthesize hybrid system, which can improve the curing speed of the coating For example, rajamani nagavajan and others mixed BBP and MI The experimental results show that for many curing systems, such as epoxy resin, their curing speed can be improved Another method is to adopt the free radical free radical double curing system, which means that there are more than two kinds of initiation processes in the polymerization process The free radical light curing free radical thermal polymerization system is one of them Although the curing process of the light curing system is initiated by light, the light curing system also has some disadvantages as follows: (1) the curing depth is limited; (2) it is difficult to apply in the colored system; (3) the shadow part cannot be solidified, and the shape of the solidified object is limited These disadvantages also limit the application of UV curing system in some aspects Therefore, in these cases, dual curing system is adopted One stage is through UV curing reaction, while the other is through dark reaction, which includes heat curing, moisture curing, oxygen curing or anaerobic curing In this way, light curing can be used to make the system quickly finalize or reach "surface dry", while dark reaction can be used to make / shadow 0 part or bottom part complete, so as to achieve "solid dry" of the system Double curing expands the application of light curing system in opaque media, substrate with complex shape, ultra thick coating and colored coating The classification of monomer 2.1 plays an important role in radiation curing From the reaction function of monomers, monomers can be divided into active monomers and inactive monomers According to the functional degree of monomers, monomers can be divided into monomers, bifunctional monomers, trifunctional monomers and multifunctional monomers 2.1.1 monomeric monomers mainly play a role of dilution in the system, which also plays a leading role in the dissolution of photoinitiator and the flexibility of curing film Such monomers generally include vinyl monomers and acrylate, but their curing speed is more than that of functional monomers 2.1.2 bifunctional monomers bifunctional monomers are formed by the reaction of acrylic acid and diol in the ratio of 2:1 For example, TPGDA (dipropylene glycol diacrylate) and HDDA (1,6-hexanediol diacrylate) are commonly used monomers 2.1.3 the activity of these monomers is higher than that of bifunctional acrylate, and the curing rate is usually promoted by a small amount, such as TMPTA (trimethylolpropane triacrylate), peta (pentaerythritol triacrylate), etc 2.1.4 the common characteristics of multifunctional monomers are high activity and fast curing speed, such as pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate, etc 2.2 the influence of monomers on the curing speed is the main factor affecting the curing speed Ma Jiaju et al Diluted bisphenol A epoxy resin with BA (butyl acrylate), TMPTA (trimethylolpropane triacrylate) and sr295 (pentaerythritol tetracrylate) respectively The experimental results are shown in Table 1 Note: the UV curing strength is 0.40 MW / cm2, which is measured by UV-A radiometer in the photoelectric factory of Beijing Normal University The functional degrees of BA (butyl acrylate), TMPTA and sr295 are 1, 3 and 4, respectively It can be seen that with the increase of the functionality and dosage of monomer, the curing speed will be accelerated, and the same facts are illustrated by the experiments of Ju Xuecheng, Xu Maojun, etc Because with the increase of the functionality of the monomer, the activity of the monomer increases, and the curing speed increases 2.3 with the increase of monomer functionality, the curing speed of the system is accelerated, but it also brings a series of negative effects, such as large shrinkage, so the adhesion is reduced In order to overcome this deficiency, in the process of use, the multi-functional monomers and single functional monomers are generally mixed, which can not only reduce the shrinkage of the cured film, but also obtain a higher curing speed In order to obtain a higher curing speed, alkoxylation can also be used for the monomer, because alkoxylated acrylate can release hydrogen during the curing process, so in the process of combining with hydrogen extracting photoinitiator, the initiation efficiency of photoinitiator can be improved, so as to improve the curing speed of the curing system 3 prepolymer 3.1 classification of prepolymer 3.1 prepolymer for radiation curing, also known as oligomer, is a low molecular polymer containing unsaturated functional groups, most of which are acrylate terminated oligomers, There are four types of industrial acrylate prepolymers: acrylated epoxy resin, acrylated carbamate, acrylated polyester, acrylated polyacrylate, among which epoxy acrylate and polyurethane acrylate are the most important 3.2 the influence of prepolymer on curing speed generally speaking, prepolymer has high relative molecular weight and fast curing speed Different prepolymers have different curing speed The curing speed of unsaturated polyester is the slowest, and that of epoxy acrylate is the fastest.