Brief description of hydrogen-capturing photoinitiators and their two main categories
Hydrogen-capturing photoinitiators, also known as type II photoinitiators, are generally dominated by aromatic ketone structures and also include certain thick-ringed aromatic hydrocarbons. They have certain light-absorbing properties, and the matching co-initiator, i.e. hydrogen donor, itself has no absorption in the long-wave UV range. Hydrogen-capturing photoinitiators absorb UV energy and bimolecularly interact with the co-initiator in the excited state to produce reactive radicals. Tertiary amines are commonly used as co-initiators for pairing with hydrogen-trapping photoinitiators. The following diagram takes benzophenone photoinitiator as an example to describe its action process.
First, benzophenone + tertiary amine photoinitiator system
Benzophenone (BP) photoinitiator is generally colorless or slightly yellow crystals, solubility in common solvents is relatively good, the maximum absorption wavelength of about 340nm, and the medium-pressure mercury lamp emission wavelength match. Here we should pay attention to the difference between benzophenone photoinitiator and benzophenone UV absorber, their structure is relatively similar, the maximum absorption wavelength of benzophenone UV absorber is generally around 330 nm. BP synthesis is simple, is a low-cost photoinitiator, but the photoinitiating activity is generally not as good as HMPP, HCPK and other commonly used cracking photoinitiators. The curing rate of BP-type photoinitiators is relatively slow, and it is easy to cause yellowing of the cured coating, and the yellowing will be aggravated by the use of large amounts of tertiary amine co-initiators.
As a hydrogen capture photoinitiator, BP also has its advantages. First of all, its low cost and low price can be used in some formulations with low added value and low quality requirements. Such as decorative snap coatings and colored substrate varnish coatings. In order to balance the cost, yellowing, curing rate and other factors, BP is often used in combination with other cracking photoinitiators, BP and active amine combination application, active amine has the function of antioxidant polymerization, so the surface of BP + active amine system antioxidant polymerization effect is better. However, it should be noted that when the amount of BP is large, it is easy to lead to the bottom layer of light shielding.
BP has many substituted derivatives are effective photoinitiators, the most important derivative is Michler’s ketone (MK), which is a 4,4-bis (dialkylamino) substituent of BP, the common structure is shown in the left figure.
Michler’s ketone relative to BP, absorbing light wavelength red-shifted tens of nanometers, has a strong absorption of 365nm ultraviolet light. Because it contains tertiary amine structure, so michanone can also be used as a photoinitiator alone, but the efficiency is not fully developed. Such as MK and BP used in conjunction with the photopolymerization of acrylates, found that the initiation activity is much higher than MK / tertiary amine system and BP / tertiary amine system, the polymerization rate is about 10 times the latter two.
Second, thioxanthrone + tertiary amine photoinitiating system
Thioxanthone is also used as a hydrogen grabbing photoinitiator, and its maximum absorption wavelength can reach 380~420nm, and the extinction coefficient is also higher, about 102 orders of magnitude, which can make full use of the light wave energy of 365nm and 405nm of the light source, which is much more effective than benzophenone photoinitiator. In terms of initiation mechanism, thioxanthrone photoinitiator system is similar to benzophenone system. The structural formulae of thioxanthone (TX) and its various derivatives are shown below.
Thioxanthone is a light yellow powder with very poor solubility in most solvents, so it is difficult to be dispersed in resin systems. Most of them have good solubility and dispersion properties, and the absorbance and photochemical activity can be improved. The common substituted TX include 2-chlorothianthrone (CTX), CPTX, isopropylthioanthrone (ITX) and 2,4-diethylthioanthrone (DETX), etc. CTX is still not satisfactory in solubility and has been gradually replaced by the latter two.
The thianthrone substituents must be paired with appropriate active amines to achieve efficient photoinitiating activity. It was found that ethyl 4-dimethylaminobenzoate (EDAB) is the most suitable reactive amine co-initiator for use with thianthrone, which is not only highly active but also has less severe yellowing. ITX has been widely accepted by the market because of its relatively good cost performance.
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