2025 The Complete Guide To Principles of free radical photoinitiator application
Photoinitiator as one of the key raw materials in the photopolymerization formula, there are some common principles to pay attention to when formulating applications, such as: matching the principle with the light source, matching the principle with the pigment, matching the principle with the coating thickness, dosage principle, other principles (solubility principle, combination principle, safety principle, price principle) and so on. No matter what kind of matching principle, the ultimate goal is the same: that is, to design a cost-effective formulation of the product. Different formulations of the design of the photoinitiator requirements are also very different, the specific photoinitiator selection, dosage and combination of specific experiments to determine, especially now more and more personalized products, different performance of the formulation of the product requires the corresponding photoinitiator corresponding to it.
Matching principles with the light source: the current light source of the photopolymerization industry is mainly mercury lamps, the main spectral intensity of conventional medium-pressure mercury lamps as shown in Table 3, Figure 10 is the UV emission spectrum of medium-pressure mercury lamps, from Table 3 and Figure 10 can be seen in the mercury lamp in 220nm-1300nm are different intensities of emission light waves. Metal halide lamps are a class of mercury lamps that can enhance the intensity of specific wavelengths by adding different metals to the mercury lamp to change the lamp’s emission wavelengths. In practice, they are often used in conjunction with conventional medium-pressure mercury lamps. Therefore, when designing a photopolymerization formula, we should first consider the type of light source, and choose the photoinitiator with the corresponding wavelength for different light sources to maximize the efficiency of photoinitiator utilization. For example, the α-hydroxy ketone photoinitiator light absorption wavelength itself is short, with conventional medium-pressure mercury lamp can meet production needs, but the acyl phosphine oxygen photoinitiator and thioxanthrone photoinitiator light absorption wavelength is longer, can reach 370nm-400nm, if the choice of iron lamp (specific enhancement of 370nm-390nm band), compared to the conventional medium-pressure mercury lamp If the iron lamp is selected (specific enhancement of 370nm-390nm band), the polymerization effect can be obtained relatively well compared with the conventional medium pressure mercury lamp.
Nowadays, UV-LED light source technology is becoming more and more mature, especially the commercialization cost of 365nm, 385nm, 395nm, 405nm band light source is getting lower and lower, and it has many advantages compared with mercury lamp light source, such as: energy saving, environmental protection, high efficiency, health, long life, etc., which makes people increase the investment in the formulation of UV-LED light source. As UV-LED light sources are single-wavelength light sources, compared with mercury lamps, UV-LED light sources for the selectivity of the photoinitiator is greatly reduced. Therefore, for UV-LED light source photoinitiator selection needs to pay more attention to match the problem, in the case of UV-LED light source photopolymerization formula design is not perfect, the use of UV-LED light source + mercury lamp light source combination can also be different degrees to achieve the purpose of energy saving and environmental protection.
Matching the color principle: the principle of matching the photoinitiator with the color mainly refers to the UV absorption peak of the photoinitiator and the color transmission window match, the so-called transmission window refers to the pigment / dye light absorption is relatively weak light wave band, this band is conducive to the transmission of UV light, so more action on the photoinitiator. If the UV absorption peak of the photoinitiator is not well matched 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, coupled with the impact of oxygen blocking aggregation, which can seriously lead to no polymerization of the product at all. In addition, in practice, the choice of photoinitiator also needs to match the pigment coverage, dosage, particle size, etc., such as: strong coverage of the pigment is relatively strong absorption of light, so the photoinitiator needs to use some of the same concentration of products with high absorbance, but also appropriate to increase the amount of photoinitiator; pigment dosage corresponding to the amount of initiator also needs to be increased appropriately; pigment particle size is not conducive to The large particle size of pigment is not conducive to the penetration of light, so the initiator should be selected from products with high absorbance at the same concentration, or the amount of initiator should be increased appropriately.
Matching principle with the thickness of the coating: inevitably encounter the problem of coating thickness in the practical application, photoinitiator for thick coatings is to ensure that the principle of the selection of deep taking into account the surface layer, the use of long wavelength photoinitiator and relatively short wavelength photoinitiator combination, the amount of combined initiator also need to make corresponding adjustments according to the thickness of the final product. For thin coatings should pay special attention to the issue of oxygen blocking, in the selection of photoinitiators can be considered preferable to have a certain anti-oxidation blocking effect of hydrogen capture type photoinitiator and cracking type photoinitiator used in conjunction with the appropriate increase in the amount added, the more typical combination of 184 + BP, but the amount added should not be too much, too much is prone to the phenomenon of light shielding.
Dosage principle: whether it is a mercury lamp light source or UV-LED light source, photoinitiator in the actual application process in addition to consider the match with the light source, but also need to consider the impact of absorbance, the amount of additive and other factors. The addition amount to meet the polymerization needs as a basic principle, high activity photoinitiator can be added to reduce the amount of appropriate, low activity photoinitiator can increase the amount of appropriate, but also high activity photoinitiator and low activity photoinitiator can be used in conjunction, that is, to meet the polymerization needs and balance the cost of the formula. Increase the amount of photoinitiator can indeed improve the curing speed, but not add more is better, add too much will bring many problems, such as: the occurrence of light shielding phenomenon, the degree of free radical coupling increased, the instantaneous polymerization temperature is too high resulting in heat-sensitive substrate deformation, polymerization speed is too fast on the adhesion of the product has a negative impact, volume shrinkage increased product deformation, the final product molecular weight reduction, the overall Mechanical properties decline, raw material costs increase, aging resistance decline, aggravate the yellowing of the final product, etc.; reduce the amount of photoinitiator may bring the direct problems of insufficient polymerization, increased energy consumption, the final product performance failure, etc..
Bernhard Steyrer et al. used a 405 nm 3D printer (DLP) to compare the UV absorbance spectra of Ivocerin (Bis (4-methoxybenzoyl) diethylgermanium, BAPO (819) and TPO-L (the UV absorbance spectra of three photoinitiators are shown in Figure 11, under the same conditions Ivocerin and BAPO have higher absorbance compared to TPOL under the same conditions). and BAPO showed high photoinitiator activity at low concentrations, and when the photoinitiator addition was increased, Ivocerin and BAPO showed more obvious light shielding, which adversely affected the performance of the final product.
Other principles (solubility principle, combination principle, safety principle, price principle).
Solubility principle, different monomer resins have different solubility to photoinitiators, different photoinitiators have different solubility in the same resin or monomer, and the solubility of the same initiator in the same resin or monomer may also be different in different seasons. The solubility of photoinitiators can often be solved by adjusting the type of resin and monomer as well as the amount of photoinitiator added. At present, the solubility of the conventional commercial free radical photoinitiators are relatively poor varieties: 369, 819, PBZ, etc..
Combination principle, each photoinitiator has its unique advantages and shortcomings, such as the widely used 1173, although the photoinitiating activity is high, cheap, and good compatibility with resin monomer, but its light absorption wavelength is short, thick coating bottom dry deficient, odor, easy to volatilize. In a full understanding of the advantages and disadvantages of each photoinitiator and then effectively combined with the use of the results can often be obtained 1 + 1 > 2. Combination with the use of the general principle of complementary wavelengths, types of complementary, types of streamlined, common classical combinations are: 184 + BP, TPO + 184, 819 + 1173, ITX + 907, BP + EMK, etc..
Safety principles, the current commercial photoinitiators are more or less harmful to humans, in the use of the process should try to avoid the use of odor, volatile, easy to sublimate the product, in addition to the debris generated after exposure residues and migration issues should also be considered when designing the formulation, especially the final application in food packaging, cosmetic packaging, pharmaceutical packaging and other products in close contact with the human body. Compared to traditional small molecule photoinitiators, large molecule photoinitiators and polymerizable photoinitiators are relatively much safer and can be considered for use in some industries that are sensitive to safety requirements. At present, commercial small molecule photoinitiators have a relatively high safety 2959 and CQ (camphor quinone).
Price principle, in recent years, with the frequent emergence of environmental protection policies, various chemical raw materials have shown varying degrees of shortage, photoinitiator industry in 2017, there are individual products available at the price of the situation, so in the formulation design should always pay attention to the price changes in the market and prepare a backup plan. Although the maximization of product profits is the pursuit of people, but sometimes it is not the cheaper the price the higher the profit, to ensure product quality under the premise of trying to choose low-cost photoinitiator to design a cost-effective products recognized by everyone.
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