Photoinitiating systems

J. CriveUo, in J. P. Eouiassier andj. E. Rabek, eds., Eadiation Curing in Polymer Science andTechnology (V9III) Photoinitiating Systems, Elsevier, New York, 1993, Chapt. 8, for a review on onium salts. 

Initiation of radical reactions with uv radiation is widely used in industrial processes (85). In contrast to high energy radiation processes where the energy of the radiation alone is sufficient to initiate reactions, initiation by uv irradiation usually requires the presence of a photoinitiator, ie, a chemical compound or compounds that generate initiating radicals when subjected to uv radiation. There are two types of photoinitiator systems those that produce initiator radicals by intermolecular hydrogen abstraction and those that produce initiator radicals by photocleavage (86—91). 

Examples of typical photoinitiator systems used to cure reactive coating systems are as follows (80,81). The reactive systems are primarily unsaturated acryUc acid esters of different alcohol and polymer stmctures. 

Reaction lOe is relatively slow in the Re2(CO)io initiating system, and the thermal reaction between Re(CO)6 formed in 10a and CCI4 generated CCI3 radicals thermally in the dark and so is responsible for the aftereffect. However, Mn2(CO)io reacts rapidly according to lOe and no aftereffect is observed for the Mn2(CO)io/CCl4 photoinitiating system,... 

Bamford and Mullik [23] have also investigated a new photoinitiating system composed of Mn2(CO)io or Re2(CO)io with acetylene, acetylene dicarboxylic acid, diethyl fumarate, diethyl maleate, or maleic anhydride. It was concluded that the primary radical responsible... 

IV. OTHER PHOTOINITIATING SYSTEMS BASED ON METAL COMPLEXES... 

To complete the information regarding photoinitiation of radical polymerization by metal complexes, special photoinitiation systems will be discussed and reviewed in this section. 

This photoinitiating system is also used for the polymerization of other vinyl monomers such as styrene (St), acrylonitrile (AN), and vinylacetate (VA). The efficiency of photoinitiation by this system follows the order ... 

Okimato and coworkers [51] have introduced a new multicomponent photoinitiating system composed of the metal-ion-amine CCU for the polymerization of MMA (A > 300 nm). The most active system is FeS04-7H20/... 

In another type of multicomponent photoinitiating system, Okimato and colleage [52] have investigated the Fe -salt/saccharide (SH) system for the photoinitiation... 

Iwai and coworkers [56] have introduced a novel type of multicomponent photoinitiating system for water-soluble monomer (acrylamide, acrylic acid, acrylonitrile, etc). 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

Bamford and Mullik [62] have succeeded in photografting a vinyl monomer onto a styrene-MMA copolymer using the Mn2(CO)io/C2F4 photoinitiating system in acetic acid. The following scheme was reported for this process ... 

TABLE 1. Monomers and Photoinitiator System Components Empioyed for Resin Formuiations. 

Padon, K. S. and A. B. Scranton, Recent Advances in Three Component Photoinitiator Systems, pp. 369-385 in Recent Research Developments in Polymer Science, Vol. 3, Transworld Research Network, Trivandrum, India, 1999. 

Figure 4.16 Photoinitiation system for acrylate polymerisation using an argon ion laser (488 nm).

The curing equipment will operate at the sufficient efficiency if a minimum UV dose (mj/cm ) or a minimum exposure time can be applied to obtain the desired degree of cure. This requires fhat the reactive binders and photoinitiator system match closely the emission spectrum of fhe UV source. 

As initiators, common peroxides, such as dicumyl peroxide, di-benzoylperoxide can be used, but also photo initiators have been reported. Photoinitiating systems are the combinations of benzo-phenone with amines, such as n-butylamine, trimethylamine and triethylamine (43). 

Stable hydrogels can be formed by photocrosslinking the methacrylated polysaccharides by either argon ion laser (514 nm) or ultraviolet (245 nm) in the presence of a photoinitiating system. These hydrogels appear to be candidates for the sutureless closing of wounds. For example, two-sections of porcine skin are shown in Fig. 3.6, and their end-surfaces are wetted with an adhesive and... 

Polymeric cationic curable formulations containing an d -biphenylthioxanthenium salt initiator 638 have been reported to reduce the residual odor and benzene levels associated with other -phenyl cationic photoinitiator systems <2006W02006/060281 >. The mechanism of photo-acid generation from related rS -aryl thiopyranium salts... 

Almost all of these systems employed o-Cl-HABI as photoinitiators. No serious effort was ever made to address the issue of what could be done to make HABIs even better. Could it be that the o-Cl-HABI was the perfect photoinitiator system I doubt it. It was simply a matter of expediency—this photoinitator was available and finding a better one would be difficult. 

Epoxy acrylates are also commonly used as oligomers in radiation-curing coatings and adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation. These resins are formulated with photoinitiators to cure via uv or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. The uv/EB cured epoxy formulations are discussed in Chap. 14. 

Photoinitiators are perhaps the most important component in uv cured radiation coatings. The photoinitiator is an ingredient that absorbs light and is responsible for the production of free radicals in a free radical polymerized system or cations in a cationic photoinitiated system. The photoinitiators are usually added to the reactive coating formulations in concentration ranges from less than 1 to 20 percent by weight based on the total formulation. The absorption bands of the photoinitiators should overlap the emission spectra of the various commercial light sources. 

There are two general classes of photoinitiators (1) those that undergo direct photofragmentation on exposure to uv or visible light irradiation and produce active free radical intermediates and (2) those that undergo electron transfer followed by proton transfer to form a free radical species. The choice of photoinitiator is determined by the radiation source, the film thickness, the pigmentation, and the types of base resin employed. Examples of typical photoinitiator systems used to cure reactive resins are shown in Table 14.2. Benzophenone is perhaps one of the most common photoinitiators. 

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