Type 1 photoinitiators

Secondary Reactions of Type 1 Photoinitiators Model Studies with Polystyrene... 

Those that undergo intramolecular bond cleavage, known as homo-lytic fragmentation type or Type 1 photoinitiators (a-cleavage type). Examples are benzoin ethers, substituted acetophenone derivatives, acyloxime esters, benzil ketals, and cyclic benzoin and benzils. Representative examples are shown in Figure 5.15. 

Various bifunctional resins are based on acrylic epoxide monomers. Such systems can photopolymerize by the radical and/or cationic mechanism. With iron arene photoinitiators in the presence of an oxidant, radical as well as cationic photopolymerization of these monomers is possible . Onium -type photoinitiators form radical species upon photolysis, as shown in Figs. 3 and 4. The local radical concentration is, however, too low to permit the polymerization of such systems... 

UvlmerTM 530 and the other members of the UvlmerTM series may be cured with benzoin ether type photoinitiators and with diethoxy acetophenone. Table III presents data on the cure speed of UvlmerTM 530 formulated with butyl benzoin ether. These data were obtained in air without the use of Inert blanketing. Films of 3 mil thickness were irradiated on a moving belt using a 5000 watt Addalux uv lamp (Berkey Photo, Inc.) focussed on one linear Inch, This Is a low pressure lamp Incorporated In an experimental unit. Faster cure speeds (up to 200 ft. per mlnute/lamp) were obtained in later studies with a unit employing two medium pressure, 200 watts/llnear inch Hanovia lamps. 

SCHEME 13.9 Homogeneous synthesis of graft copolymers by a-cleavage type photoinitiation. 

A similar study has been performed on EPI blends in which the vinyl ether was replaced by an acrylate monomer (HDDA) to produce, by different mechanisms, two interpenetrating polymer networks. With the onium salt as sole photoinitiator, the cationic polymerization of the EPI epoxy groups occurred as fast in the formulation containing 20% of HDDA by weight as in the EPI/DVE-3 blend, to reach nearly 100% conversion within 0.6 s (Fig. 11). The polymerization quantum yield was found to be similar to that measured in the EPI/vinyl ether blend Op 650 mol E. By contrast, the acrylate double bonds were found to polymerize at a much slower pace, most probably because of the low reactivity of the free radicals generated by the cationic-type photoinitiator. 

The polymerization rate of both functional groups was markedly increase when the HDDA content was rised from 20 to 60% (Fig. 11), as expected from the resulting increase in the molecular mobility. The addition of a phosphine oxide radical-type photoinitiator (2 wt % of TPO) leads to a drastic increase of the acrylate polymerization rate (75% conversion after 0.1 s), as shown in Figure 12 for an equimolar blend of ELNR-70 and HDDA exposed to UV-radiation. The early formation of the acrylate polymer network will reduce the molecular mobility of the reactive species and is therefore responsible for the observed decrease of both the rate of polymerization and the final conversion of the epoxy groups. A most remarkable feature of this... 

All UV curable systems have four basic components which must be included in order to develop a successful coating. They are the photoinitiator(s), oligomer(s), monomer(s), and additive(s). Table II lists the properties of each component which make it essential to the UV curable formulation. In the next few pages, we summarize each of the components and their properties. The photoinitiator section covers both radical and cationic type photoinitiators while the oligomer and monomer sections are restricted to components used in free radical systems. 

The abstraction type photoinitiator acts by abstraction of a l drogen atom from a donor compound, the most common of which are aliphatic amines such as triethylamine, methyl diethanol amine (MDEA), or dimetl l ethanol amine. The radical produced (R) is the initiator of the free radical polymerization process. The semi-pinacol radical is unreactive and simply couples to give the pinacol. The abstraction process is, however, highly dependent on the amine co-initiator. Examples of abstraction type photoinitiators are given below ... 

The second interesting feature of the results is the clear distinction between the cationic and anionic type photoinitiators. 

It should finally be emphasized that the use of DPB is not restricted to acrylic compounds since it proved to be also a very efficient photoinitiator for the polymerization of vinyl monomers, like N-vinylpyrrolidone (NVP). In addition, DPB appeared to be particularly well-suited to photo-cure systems that need hydrogen abstraction type photoinitiators, like the thiol-polyenes resins (2), since it is then to be compared to the poor-performing benzophenone. 

Crivello s pioneering work on onium salt-type photoinitiators (sulfonium and io-donium salts) gave great impetus to investigations of cationic polymerizations [5, 6]. A common feature of mechanisms proposed in relation to onium salt-type initiators of the general structure is the generation of... 

Figure 5 Influence of the radical-type photoinitiator on the light-induced polymerization of a polyurethane-acrylate resin. Decay curve of the phosphine oxide photoinitiator...

Endo and coworkers, investigated nonsalt type latent initiators, like phenols. Phenols initiate polymerizations of epoxides with difficulty, even at elevated temperatures, probably due to insufficient acidity and relatively higher nucleophilicity of the dissociated phenoxide ion that acts as a polymerization termination. This, however, can be overcome by the substituent on the benzene ring. Hino and Endo report that some substituted phenols, like difluorophenol do act as nonsalt type photoinitiators. 204 They propose the following mechanism to explain the mechanism of the reaction ... 

Matsubara et al., applied reactive pyrolysis gas chromatography in the presence of organic alkali to study the network structure of a UV.-cured acrylic ester prepared from polyethylene glycol diacrylate. An a-amino-aUcylphenone type photoinitiator was used. The network formation can be illustrated as follows... 

A combination of benzophenone and 1,3-dioxane is a convenient hydrogen abstraction-type photoinitiator system for the free radical photopolymerization of methyl methacrylate, styrene and other monomers. As an advantage, this system does not require an additional hydrogen donor as other conventional initiators. In a similar way, mixtures of thioxanthone derivatives and fluorenes can be used as visible light absorbing oil- and water-soluble photoinitiators for free radical polymerization of methyl methacrylate, ethyl 2-(2-phosphonoethoxymethyl)acrylate and trimethylolpropane triacrylate. Photopolymerization and laser flash photolysis studies reveal that initiation occurs by intra- and intermolecular hydrogen abstraction by the thioxanthone-like triplet excited state. 

Most of the radical-type photoinitiators used in UV-curable adhesives consist of aromatic ketones which are known to generate free radicals upon UV-exposure, either hy homolytical cleavage of C-C bonds, or by hydrogen abstraction from a H-donor molecule [11] ... 

Chart 1 Chemical formula of radical-type photoinitiators. 

The photoinduced addition of a thiol (RSH) to an olefinic double bond has been used to produce polymer networks by taking multi-functional monomers [37-44]. The thiol-ene polymerisation proceeds by a step growth addition mechanism which is propagated by a free radical, chain transfer reaction involving the thiyl radical (RS ). The initial thiyl radicals can be readily generated by UV-irradiation of a thiol in the presence of a radical-type photoinitiator. The overall reaction process can be schematically represented as follows ... 

The photosensitive resin consists either of a liquid functionalised oligomer, usually with acrylate groups, or a reactive polymer which will be applied as hot melt. There are two main classes of reactive polymers acrylic hot melts which may contain a tackifying monomer, and thermoplastic rubbers which predominantly block copolymers with polybutadiene or polyisoprene chains [3]. A radical-type photoinitiator is always needed to produce, upon UV-exposure, the free radicals that will initiate the polymerisation or cross-linking process. 

Michler s ketone, 4,4 -bis(dimethylamino)benzophenone, is another efficient hydrogen-abstraction-type photoinitiator that possesses both chromophoric aromatic ketone and tertiary amine groups in its structure. It absorbs much... 

Good reviews of the synthesis, performance, and general research into various radical type photoinitiators can be found in [3, 4, 104]. 

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). 

Ultraviolet Photoinitiators. Photoinitiators are used in increasing volume for a multitude of appHcations. The most important of these are in the formulation of uv-curable inks and in the production of coatings on vinyl flooring, wood, and electronics components (28,29). The most common types of photoinitiators are phenone derivatives, for example, acetophenones and hen 7ophen ones (30). 

However, Ciba-Geigy has introduced (31,32) a type of phosphine-based photoinitiator. In general, the compound can be described as a bis(acyl)phosphine oxide and is prepared by the reaction of a monoaLkylphosphine with a substituted ben2oyl chloride (33). The composition of the first commercial product is proprietary. 

The second type of photoinitiators, ie, those that undergo electron transfer followed by proton transfer to give free-radical species, proceed as follows, where is the rate constant for intersystem crossing. 

---