Keywords: water-based varnish; UV curing; reactive diluent; photosensitizer
1 Introduction
The so-called printing glazing is to apply a layer of varnish on the surface of the replica that has completed the graphic printing process to obtain a bright coating on the surface of the printed matter. Printing and glazing can enhance the light resistance, heat resistance and moisture resistance of the ink, and play the role of protecting the imprint and beautifying the product. At present, glazing paints are mostly solvent-based, and in the context of environmental protection being greatly valued by governments of various countries, there is a tendency to be gradually replaced. UV-curable coatings mainly refer to an energy-saving and environmentally friendly coating that produces free radicals through photoinitiators under the action of ultraviolet light, so that monomers can be quickly cross-linked to form films. Compared with traditional heat-curing coatings, it has many advantages: short curing time, fast speed, simple equipment required, greatly improving production efficiency and reducing floor space; no solvent or containing a small amount of solvent, reducing atmospheric pollution Energy consumption is only 10% to 20% of traditional coatings, saving energy [1-4].
Water-based light-curing coatings combine the environmental protection advantages of water-based coatings with the energy-saving advantages of light-curing coatings, and have good development prospects.
2 Experimental part
â‘´ Raw materials: butyl acrylate (BA), styrene (St), methyl methacrylate (MMA), acrylic acid (AA), acrylamide (N-AM), acrylonitrile (AN) and hydroxyethyl acrylate (HEA) ), Are industrial grade raw materials; sodium dodecyl sulfonate (DBS) and OP-10, industrial grade emulsifier; ammonium persulfate aqueous initiator (APS), chemically pure reagent; sodium bisulfite, chemically pure reagent ; Mercaptoethanol, chemically pure reagents; sodium bicarbonate, chemically pure reagents, as the pH adjuster of the reaction system; photoinitiators 1173 and 2959, provided by Swiss Ciba Chemical Company; trimethylolpropane triacrylate (TMPTA), Neopentyl glycol diacrylate (NPGDA) and tripropylene glycol diacrylate (TPGDA), produced in Tianjin Tianjiao Chemical Plant; Benzoin butyl ether and benzophenone, chemically pure reagents.
⑵ Test instruments: electromagnetic stirrer, electric blast drying oven, Tu-4 # cup viscometer, pencil hardness tester, gloss tester.
⑶Emulsion polymer synthesis: ①Place the emulsifier and a certain amount of deionized water in a three-necked bottle, heat in a water bath, and raise the temperature to about 80 ℃, high-speed stirring to dissolve the emulsifier, the monomer and mercaptoethanol are mixed for use; ② The ammonium persulfate initiator is dissolved in water for use; ③Take 20% of the mixed monomer and 1/3 of the initiator into the flask, and the remaining monomer is added to the dropping funnel. After the reaction solution turns blue, start dropping For the remaining monomers and initiators, the reaction temperature is controlled at about 80 ° C, and the monomers and initiators are added dropwise within 2.5 ~ 3.0 h; ④After the monomers and initiators are added dropwise, keep the temperature for 1 h, and then cool the system To 65 ℃, start to add 1% sodium bisulfite solution, supplement every 20 minutes, a total of 3 times; ⑤ Finally cool the emulsion below 40 ℃ and filter out. The performance of light-curing varnish is determined according to the national standard. [next]
3 Results and discussion
3.1 Factors affecting the performance of aqueous emulsion
3.1.1 Effects of comonomer types and ratio on emulsion performance The properties such as hardness and adhesion of the coating have a great relationship with the proportion of soft and hard monomers in the polymerized monomer. The proportion of soft and hard monomers ultimately affects The glass transition temperature (T) g of the polymer. Tg can be calculated from the Gibbs-Dimarzio formula [5]. A series of tests were conducted according to the types and proportions of soft and hard monomers, and the results are shown in Table 1.
Table 1
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Tests show that the introduction of acrylic acid, acrylonitrile, and acrylamide with polar groups can make the polymer side chains with polar groups, increase the adhesion between the coating and the substrate, and improve emulsion polymerization The cross-linking properties of the substances, they are hydrophilic monomers, soluble in both water and oil phases, and can significantly inhibit the formation of aggregates. The synthesized emulsion has good mechanical stability and storage stability.
According to the paper's requirements for both high hardness and good flexibility, the No. 5 sample is ideal, that is, the comonomer ratio: m (BA): m (S) t: m (AN): m (AA): m (HEA): m (N-AM) is 24.0: 16.0: 3.0: 1.5: 2.0: 1.0, the varnish coating film has the best performance. If the Tg is too high, the hardness is too large, the surface of the coating is rough, cracked and not easy to form a film, and the toughness and adhesion are poor; if the Tg is too low, the coating is tacky, the hardness is poor, and it is not wear-resistant, which cannot meet the requirements of printing varnish Claim.
3.1.2 Influence of the amount of emulsifier
Generally, the amount of emulsifier is increased, and the aggregate is reduced. The use of anionic / nonionic composite emulsifier can give the emulsion excellent polymerization stability and storage stability. The choice of emulsifier should consider the hydrophilic affinity of various comonomers for the emulsifier. Different requirements for oil balance value (HLB). It is known from the test that DBS: OP-10 is 2: 1, and the dosage is preferably 3.0% of the total monomer.
3.1.3 Effect of reaction temperature on emulsion [6]
The temperature has a greater influence on the polymerization reaction. The higher the polymerization temperature, the faster the polymerization rate and the shorter the reaction period; when the polymerization temperature is lowered, the polymerization reaction is slow, the reaction period is longer, the emulsification efficiency is reduced, the number of micelles is reduced, and the latex particles are increased . A series of tests were conducted at different temperatures with the formula of sample No. 5, and the results are shown in Table 2. It can be seen from Table 2 that the reaction temperature is 80 ~ 83 ℃, and the reaction time is 3 h.
Table 2
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[next]
3.2 Factors affecting the performance of light-curing water-based varnish
3.2.1 Effect of photoinitiator on curing rate
The photoinitiator is a key component of the photocurable coating, and it plays a decisive role in the curing rate of the photocurable coating [7]. The selection of the UV photoinitiator should consider the following factors: â‘ The absorption spectrum of the photoinitiator should match the UV spectrum; â‘¡ The initiation efficiency is high, which has a high initiation activity, and the active intermediates produced at the same time have a high reaction Activity; â‘¢ Good compatibility in reactive diluent and oligomer; â‘£ No yellowing after light curing. According to the self-made water-based emulsion and active diluent (the composition of which is TMP-TA: TPGDA: NPGDA = 8: 1: 1), the glazing oil is prepared with the mass ratio of 2: 1, and the appropriate amount of additives is added, and then added in different ratio Photoinitiator. The test results are shown in Table 3.
table 3
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It can be seen from Table 3 that as the amount of photoinitiator increases, the photocuring rate increases. Among the three types of photoinitiators, 1173 and 2959 are better than composite photoinitiators. The best effect is 2959. Considering the cost and photocuring rate, the dosage is 3% of the total varnish.
3.2.2 The effect of reactive diluent on the curing performance of varnish and the performance of the coating film
Reactive diluents usually choose multifunctional monomers with unsaturated bonds. They are important components in photocurable coatings and participate in the photocuring process, affecting the photocuring rate and the performance of the cured film. The more functional groups in the reactive diluent that can participate in the photocuring reaction and the greater the functionality, the higher the photocuring reaction activity and the faster the photocuring rate. From the perspective of photocuring activity: multifunctional reactive diluent> bifunctional reactive diluent> monofunctional reactive diluent.
In this experiment, according to the ratio of the mass ratio of reactive diluent to oligomer is 1.0: 2.5, and add 2959 photoinitiator with a total content of 3%. The mass ratio of NPGDA and TPGDA in the combined diluent composed of three active diluents, TMPTA, NPGDA and TPGDA, is 1: 1. By changing the content of TMPTA in the compound diluent, the varnishing light curing properties and coating properties are determined The test results are shown in Table 4.
Table 4
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The test results show that when the content of TMPTA in the combined diluent is increased, the curing rate is accelerated. When all thinners were changed to TMPTA, although the curing rate was the fastest and the coating film hardness was high, the coating film was brittle and had poor adhesion. When the content of TMPTA in the compound diluent is 80%, the curing time is 6 min, the pencil hardness is 3H, but the adhesion is level 2. In order to achieve adhesion level 1, the adhesion promoter is added to the UV-curing varnish. When the amount of adhesion promoter reaches more than 0.6%, the adhesion can reach level 1. Based on the above various influencing factors, a more reasonable formula is optimized (see Table 5), and the performance test results are shown in Table 6.
Table 5, Table 6
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4 Conclusion
Through the selection of comonomer and proportion of waterborne varnish oligomer, the monomer composition with better performance of cured film was obtained. When the mass ratio of BA: St: AN: AA: HEA: N-AM is 24.0: 16.0: 3.0: 1.5: 2.0: 1.0, at 80 ~ 83 ℃, the ratio of anion / non-emulsifier is 2: 1 and the emulsifier When the content is 3% of the total monomer, the obtained emulsion coating has good performance; the amount of the photoinitiator 2959 in the aqueous glazing oil is 3%, the amount of the reactive diluent is 40% of the quality of the emulsion, and the adhesion promoter When the content is 0.6%, the photocuring time of the coating is the shortest and the coating film performance is the best.
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