Study on storage stability of the hottest UV curab

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Study on the storage stability of UV curing ink (Part 2)

2.2 the influence of polymerization inhibitor

polymerization inhibitor is added during the storage and transportation of monomers to prevent the self polymerization of monomers. Therefore, polymerization inhibitor can also be added to the ink system to extend its storage life, but the existence of polymerization inhibitor will inevitably have a negative impact on the curing rate of the ink. Therefore, we should comprehensively consider the influence of the type and amount of polymerization inhibitor on the storage stability and curing rate of the ink, and we can't ignore one thing and lose the other

experimental conditions: photoinitiator is 369 with 6.7%, VP is 20%, and ambient temperature is 25 ℃

(I) influence of polymerization inhibitor types

because the same polymerization inhibitor has different polymerization inhibition effects on different monomers, it is necessary to select the appropriate polymerization inhibitor according to the type of monomers used. The general selection principles are as follows: ① for monomers with electron donor substituents, such as styrene and vinyl acetate, quinones, aromatic nitrocompounds or variable valence metal salts can be used as polymerization inhibitors; For monomers with electron absorbing substituents, such as acrylonitrile, acrylic acid, methyl acrylate, etc., phenols, amines and other electron donor substances can be used as polymerization inhibitors. ② To avoid side reactions, it is necessary to avoid the oxidation-reduction system formed by polymerization inhibitor and photoinitiator, and set a 10% weight upper limit for a single stock to increase the reaction rate. Because the monomers and resins in this ink system are acrylate series with electron absorbing substituents, three phenolic electron donor substances, i.e. hindrance 1, hindrance 2 and hindrance 3, are preliminarily selected as polymerization inhibitors to investigate their polymerization inhibition effects when the content of polymerization inhibitor is 2 ‰

table 5 Effect of different polymerization inhibitors on storage stability

polymerization inhibitors have no resistance, 3 resistance, 2 resistance, 1 storage time/d 31010180

the use effect of each polymerization inhibitor in the system is affected by many factors. If the compounds in the system do not have substituents, the polymerization inhibition effect is poor; Compounds with multiple electron pushing substituents increase the retarding effect; If the benzene ring of the compound has electron withdrawing substituents, the polymerization inhibition activity also decreases. From the experimental results in Table 5, it can be seen that different polymerization inhibitors have significantly different polymerization inhibition effects, which is determined by the structure of each polymerization inhibitor. In this ink system, the polymerization inhibition effect of 1 is the best. Therefore, the inhibitor 1 is selected as the polymerization inhibitor in this formula

(II) the influence of polymerization inhibitor content

taking inhibitor 1 as polymerization inhibitor, the influence of its content change on the storage stability of ink was investigated

table 6 Effect of the amount of inhibitor 1 on storage stability

amount of inhibitor 1 ‰ 0.5 11.52 2.53 storage time/d 116 180 180 180 180 180 180

Table 6 reflects that the amount of inhibitor increases and the storage time of ink increases. From this point of view, the higher the content of inhibitor added, the better. Considering that the role of polymerization inhibitor is to prevent polymerization, it is unfavorable for curing. Figure 1 also shows that the polymerization inhibition dose increases and the curing time of the ink also increases. When determining the amount of polymerization inhibitor, we should focus on this point. In order to reduce this negative effect, polymerization inhibitor should be used at least on the premise of meeting the storage requirements. According to the experiment, add a suitable amount of polymerization inhibitor to this ink. Fig. 1 Relationship between polymerization inhibitor content and ink curing time

2.3 effect of ambient temperature

the experimental conditions are that the content of photoinitiator 369 is 5%, the amount of VP is 20%, and polymerization inhibitor is not added. Table 7 Influence of ambient temperature on storage stability temperature/℃ 1020304050 storage time/d 1251821

table 7 shows that when the ambient temperature is 20 ℃ ~ 30 ℃, the storage time is the longest

the influence of ambient temperature can be analyzed from two perspectives: one is to consider the role of oxygen Jinan new era gold assay instrument Co., Ltd. in achieving the standard of long-term cooperation with material testing manufacturers; the other is to consider the role of photoinitiator in starting the oil pump

on the one hand, oxygen is a strong oxidant, and its polymerization inhibition constant is the highest among known compounds. At general polymerization temperature, oxygen has a significant polymerization inhibition effect. From this point of view, high or low ambient temperature is not conducive to the storage of ink. Because molecular oxygen in the air can react with free radicals that affect the measurement accuracy to form peroxy radicals:

Mn · + O 2 → Mn - O - O ·

peroxy radicals can terminate with another chain of free radicals to form polymer peroxides:

mn-o-o · + · Mn → mn-o-mn

the formation of this polymer peroxide in the system is often controlled by the diffusion process, so it is affected by the viscosity of the solution, The viscosity of solution is affected by temperature. Although this peroxide is very stable at low temperature, the solution viscosity is high at this time, and it is difficult for oxygen to diffuse in it, that is, it is not easy to form this peroxide. In the lower temperature range, with the increase of temperature, the viscosity of the solution decreases, and the diffusion becomes easier, which is conducive to the formation of polymer peroxides. In a higher temperature range, on the contrary, the polymer peroxide is easy to thermally dissociate to form active free radicals. With the increase of temperature, the amount of polymer peroxide decreases, and its polymerization inhibition effect decreases accordingly

on the other hand, from the perspective of photoinitiators, the storage temperature should not be too high or too low. When the temperature is low, it may lead to the separation or even freezing of different components in the material. The most obvious is the CIS relationship between the solubility of photoinitiator in monomer and temperature, as shown in Table 8. As the temperature decreases, the solubility of photoinitiator in monomer becomes smaller, which is easy to cause the local concentration of photoinitiator to be too high and produce gel phenomenon, thus shortening the storage life of ink. The increase of temperature will accelerate the decomposition of photoinitiators in the system and produce a large number of active centers, which will trigger polymerization to produce gel. Table 8 solubility of 369 in VP at different temperatures g/100g temperature/℃ 122232452 solubility 12.5 203037.5 50 to sum up, the storage environment temperature of photosensitive polymerization system should not be too high or too low. Experiments show that generally, the more suitable storage temperature is 20 ℃ ~ 30 ℃

2.4 other factors. In the experiment, it was observed that after the ink gel stored in the beaker was cured, there was always a thin layer of ink on the surface that was not cured. Further research on this phenomenon found that the storage life of the ink with the same formula was significantly different when it was stored in different ways

mode 1:2g ink is stored in a 100ml beaker storage time: 30d

mode 2:20g ink is stored in a 50ml beaker storage time: 1D

the oxygen content in the ink body is usually very low, while the oxygen content in the surface is relatively high. For the storage of ink, the ink stored on the surface can always be in a non gel state due to the polymerization inhibition of oxygen. Therefore, it is required that the container for storing ink always has a certain space, and the cover can be opened regularly to meet the oxygen needs of the system

3 conclusion

under the appropriate storage mode and ambient temperature, by selecting the appropriate photoinitiator and polymerization inhibitor, the problem that UV curing ink is difficult to store stably can be well solved, so the worries of expanding the industrial application of UV curing ink can be relieved

information source: water based coatings

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