2.2 Effect of polymerization inhibitors
An inhibitor is added during the storage and transportation of the monomer to prevent the self-polymerization of the monomer. Therefore, it is also possible to add a polymerization inhibitor to the ink system to prolong its shelf life, but the presence of the polymerization inhibitor necessarily has a negative effect on the curing rate of the ink. Therefore, the effects of the type and amount of polymerization inhibitor on the storage stability and the curing rate of the ink must be taken into consideration.
Experimental conditions: photoinitiator 6.7% 369, VP 20%, ambient temperature 25°C.
(I) Influence of the type of inhibitor
Since the same polymerization inhibitor has different polymerization inhibitory effects on different monomers, an appropriate polymerization inhibitor is selected according to the type of monomer used. The general selection principle is as follows: 1 For monomers that have electron-donating substituents, such as styrene, vinyl acetate, etc., the use of quinones, aromatic nitro compounds, or divalent metal salts as electrophilic substances can be used as a polymerization inhibitor; Monomers having electron-withdrawing substituents, such as acrylonitrile, acrylic acid, methyl acrylate, etc., may use phenols, amines, and other electron-donating substances as a polymerization inhibitor. 2 to avoid the occurrence of side reactions to avoid polymerization inhibitors and photoinitiators constitute the oxidation - reduction system and increase the reaction rate. Since the monomers and resins in the ink system are all acrylate series with electron-withdrawing substituents, the first three kinds of phenols, namely resistance 1, resistance 2, and resistance 3, were used as the polymerization inhibitors to investigate the polymerization inhibitors. When the content is 2 å„自, their respective polymerization inhibitory effects.
Table 5 Effect of different inhibitors on storage stability
Polymerization inhibitor without resistance 3 resistance 2 resistance 1 storage time /d 3 10 10 >180
The effectiveness of each polymerization inhibitor in the system is affected by many factors. For example, if the compound in the system has no substituent, the slow polymerization effect is poor; if there is a compound with multiple electron substituents, the slow polymerization effect will increase. If the compound has an electron-withdrawing substituent on the phenyl ring, the inhibitory activity also decreases. From the experimental results in Table 5, it can be seen that different polymerization inhibitors have significantly different inhibitory effects. This is determined by the structure of the polymerization inhibitors. In this ink system, the inhibitory effect of blocking 1 is the best. Therefore, resistance 1 was selected as a polymerization inhibitor in this formulation.
(B) Influence of inhibitor content
The inhibitor 1 was used as a polymerization inhibitor, and the effect of the content change on the storage stability of the ink was examined.
Table 6 Effect of the amount of resistance 1 on storage stability
The amount of resistance 1 ‰ 0 0.5 1 1.5 2 2.5 3 Storage time /d 1 16 >180 >180 >180 >180 >180
Table 6 shows that the amount of the polymerization inhibitor increases, and the storage time of the ink increases. From this point of view, the higher the content of the added polymerization inhibitor, the better. Taking into account that the role of the polymerization inhibitor is to prevent polymerization, it is disadvantageous for curing. It is also reflected from FIG. 1 that the inhibitor dose increases, and the curing time of the ink also increases. In determining the amount of inhibitor, it is important to consider this point. In order to reduce this negative effect, the polymerization inhibitor should be used in a minimal amount while meeting the storage requirements. According to the experiment, an appropriate amount of polymerization inhibitor was added to the ink. Figure 1 Relationship between inhibitor content and ink curing time
2.3 Influence of Ambient Temperature
The experimental conditions were that the photoinitiator 369 content was 5%, the VP amount was 20%, and no inhibitor was added. Table 7 Effect of ambient temperature on storage stability Temperature/°C 10 20 30 40 50 Storage time/d 1 25 18 2 1
Table 7 shows that the storage time is longest when the ambient temperature is between 20°C and 30°C.
The effects of ambient temperature can be analyzed from two perspectives: one is to consider the role of oxygen, and the other is to consider the role of photoinitiators.
On the one hand, oxygen is a strong oxidant, and its inhibition constant is the highest among known compounds. Under the general polymerization temperature, oxygen has a significant inhibitory 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 reacts with free radicals to form peroxy radicals:
Mn·+O 2 → Mn - O - O ·
Peroxy radicals can be terminated with another radical of the free radicals to form polymer peroxides:
Mn-OO·+·Mn→Mn-OO-Mn
The formation of this polymer peroxide in the system is often controlled by the diffusion process and is therefore affected by the viscosity of the solution, which in turn is affected by the temperature. Although this peroxide is stable at low temperatures, the viscosity of the solution at this time is high, oxygen is hardly diffused therein, that is, the peroxide is not easily formed. In the lower temperature range, as the temperature increases, the viscosity of the solution decreases, diffusion becomes easy, and the formation of polymer peroxide is facilitated. In the higher temperature range, contrary to the situation, polymer peroxides are prone to thermal dissociation and form active radicals. As the temperature rises, the amount of polymer peroxides decreases, and its inhibition effect decreases accordingly.
On the other hand, from the photoinitiator point of view, the storage temperature should not be too high or too low. At lower temperatures, separation and even freezing of different components in the material may result. The most obvious is that the solubility of the photoinitiator in the monomer varies with the temperature, as shown in Table 8. When the temperature is lowered, the solubility of the photoinitiator in the monomer becomes small, which easily causes the local concentration of the photoinitiator to be too high to cause gelation, thereby shortening the ink storage period. The increase in temperature will accelerate the decomposition of the photoinitiator in the system, resulting in a large number of active centers, thus initiating polymerization to produce a gel. Table 8 The solubility of 369 in VP at different temperatures g/100g Temperature/°C 12 22 32 42 52 Solubility 12.5 20 30 37.5 50 In summary, the storage environment temperature of the photopolymerization system cannot be too high or too low. The most suitable storage temperature is 20 °C ~ 30 °C.
2.4 Effect of other factors In the experiment, it was observed that the ink stored in the beaker gel cured, the surface always has a thin layer of ink uncured. A further study of this phenomenon found that the storage of inks of the same formula in different ways also had significantly different shelf life.
Method 1: 2g of ink is stored in a 100ml beaker Storage time: >30d
Method 2: 20g of ink is stored in a 50ml beaker Storage time: 1d
The oxygen content in the ink body is usually very low, and the oxygen content in the surface layer is relatively high. For the ink storage, the ink stored in the surface layer can always be in an ungelled state due to oxygen inhibition. Therefore, it is required that the container for storing the ink always leave a certain space, and the lid can be opened periodically to meet the oxygen demand of the system.
3 Conclusion
Under proper storage conditions and ambient temperature, by selecting suitable photoinitiators and polymerization inhibitors, it is possible to solve the problem that it is difficult for the UV-curable ink to be stably stored, so that it is possible to relieve the worries of widening the industrial application of the UV-curing ink.
Source: Waterborne Coatings Network