As one of the key raw materials in photopolymerization formulations, photoinitiators have some common principles to pay attention to when designing and applying formulations, such as the principle of matching with light sources, pigments, coating thickness, dosage, and other principles (solubility, combination, safety, and price). Regardless of the matching principle, the ultimate goal is the same: to design formula products with high cost-effectiveness. The demand for photoinitiators varies greatly depending on the design of different formulas. The specific selection, dosage, and combination of photoinitiators still need to be determined through specific experiments, especially with the increasing number of personalized customized products. Different performance formula products require corresponding photoinitiators product.
Basic principle
1. Principle of matching with light sources: Currently, mercury lamps are the main light sources in the photopolymerization industry. The main spectral line intensities of conventional medium pressure mercury lamps are shown in Table 3. Figure 10 shows the ultraviolet emission spectrum of medium pressure mercury lamps. From Tables 3 and 10, it can be seen that mercury lamps emit light waves of different intensities at 220nm-1300nm. Metal halide lamps are a type of mercury lamp that can enhance specific wavelength intensity by adding different metals to the lamp to change its emission spectrum. In practical operation, it is often used in conjunction with conventional medium pressure mercury lamps. Therefore, when designing the photopolymerization formula, the first thing to consider is the type of light source, and different photoinitiators with corresponding wavelengths should be selected for different light sources to maximize the utilization efficiency of photoinitiators. such as α－ The absorption wavelength of hydroxyketone photoinitiators is relatively short, which can be met by conventional medium pressure mercury lamps for production needs. However, acyl phosphine oxide photoinitiators and thioanthrone photoinitiators have longer absorption wavelengths, reaching 370nm-400nm. If iron lamps (specifically enhanced in the 370nm-390nm wavelength range) are used, relatively better polymerization effects can be achieved compared to conventional medium pressure mercury lamps.
2.The principle of color matching: The principle of matching photoinitiators with colors mainly refers to the matching of the UV absorption peak of the photoinitiator with the transmittance window of the color. The so-called transmittance window refers to the light wave band where the pigment/dye has relatively weak absorption, which is conducive to the transmission of ultraviolet light and thus acts more on the photoinitiator. If the UV absorption peak of the photoinitiator does not match well with the transmittance window of the pigment/dye, the pigment/dye will compete with the photoinitiator to absorb the corresponding wavelength of UV light, resulting in a decrease in photoinitiator efficiency. In addition, the impact of oxygen inhibition can seriously lead to the product not polymerizing at all. In addition, the selection of photoinitiators in practical applications also needs to be matched with the covering power, dosage, particle size, and other aspects of the pigment. For example, pigments with strong covering power have relatively strong absorption of light. Therefore, photoinitiators need to choose products with high absorbance at the same concentration, and the dosage of photoinitiators can also be appropriately increased; The amount of pigment used should also be appropriately increased, corresponding to the amount of initiator used; The large particle size of pigments is not conducive to light penetration. When selecting initiators, it is necessary to choose products with high absorbance at the same concentration, or increase the dosage of initiators appropriately.
3.Matching principle with coating thickness: In practical applications, it is inevitable to encounter the problem of coating thickness. The selection principle of photoinitiators for thick coatings is to ensure that the deep layer takes into account the surface layer, and to use a combination of long wavelength photoinitiators and relatively short wavelength photoinitiators. The amount of composite initiators also needs to be adjusted according to the thickness of the final product. For thin coatings, special attention should be paid to the issue of oxygen inhibition. When selecting photoinitiators, it is advisable to choose hydrogen withdrawing photoinitiators with certain anti oxygen inhibition effects in combination with cracking photoinitiators, and increase the dosage appropriately. The typical combination is 184+BP, but the dosage should not be too much, as excessive dosage can easily lead to light shielding.
4.Dosage principle: Whether it is a mercury lamp light source or a UV-LED light source, in the actual application process, the photoinitiator should not only consider the compatibility with the light source, but also the influence of factors such as absorbance and dosage. The basic principle of adding amount is to meet the polymerization needs. High activity photoinitiators can be appropriately reduced, while low activity photoinitiators can be appropriately increased. High activity photoinitiators can also be used in combination with low activity photoinitiators, which not only meets the polymerization needs but also balances the formula cost. Increasing the amount of photoinitiator can indeed improve the curing speed, but the more it is added, the better. Excessive addition can bring many problems, such as the occurrence of light shielding phenomenon, increased degree of free radical coupling, deformation of thermosensitive substrate caused by high temperature at the moment of polymerization, adverse effects on product adhesion caused by too fast polymerization speed, increased product deformation due to volume shrinkage, decreased molecular weight of the final product, decreased overall mechanical properties, increased raw material costs, decreased aging resistance, and aggravated yellowing of the final product; The direct problems that may arise from reducing the dosage of photoinitiators include insufficient polymerization, increased energy consumption, and substandard final product performance
Other principles (solubility principle, combination principle, safety principle, price principle):
The principle of solubility is that different monomer resins have different solubility for photoinitiators, and the solubility of different photoinitiators in the same resin or monomer is also different. The solubility of the same initiator in the same resin or monomer may also be different in different seasons. By adjusting the types of resins, monomers, and the amount of photoinitiators added, the solubility problem of photoinitiators can often be effectively solved. At present, the varieties with relatively poor solubility in conventional commercial free radical photoinitiators include 369, 819, PBZ, etc.
The principle of combination is that each type of photoinitiator has its unique advantages and disadvantages, such as 1173, which has been widely used. Although photoinitiators have high activity, low cost, and good compatibility with resin monomers, their absorption wavelength is short, thick coatings have insufficient bottom drying, strong odor, and are prone to volatilization. After fully understanding the advantages and disadvantages of each photoinitiator, effective combination and use can often yield results of 1+1>2. The general principles for combination and usage are wavelength complementarity, type complementarity, and type simplification. Common classic combinations include: 184+BP, TPO+184, 819+1173, ITX+907, BP+EMK, etc.
The safety principle is that currently commercialized photo initiators pose some harm to the human body. During use, products with strong odors, volatile substances, and easy sublimation should be avoided as much as possible. In addition, the residual fragments and migration issues generated after exposure should also be considered in the design of the formula, especially when applied to products that come into close contact with the human body, such as food packaging, cosmetics packaging, pharmaceutical packaging, etc. Compared to traditional small molecule photoinitiators, the safety of macromolecular photoinitiators and polymerizable photoinitiators is relatively improved. They can be considered for use in industries that are sensitive to safety requirements. At present, 2959 and CQ (camphor quinone) have relatively high safety among commercial small molecule photoinitiators.
Price principle: In recent years, with the frequent emergence of environmental protection policies, various chemical raw materials have shown varying degrees of shortage. In 2017, the photoinitiator industry also experienced a situation where some products were priced but not in stock. Therefore, when designing formulas, it is necessary to always pay attention to market price changes and prepare backup plans. Although maximizing product profits is a pursuit of people, sometimes it is not the case that the cheaper the price, the higher the profit. It is important to choose low-cost photoinitiators while ensuring product quality in order to design products with high cost-effectiveness that are recognized by everyone.
