Light absorption-photoinitiators

Peroxides are used most commonly either as thermal initiators or as a component in a redox system. While peroxides are photochemically labile, they seldom find use as photoinitiators other than in laboratory studies because of their poor light absorption characteristics. They generally have low extinction coefficients and absorb in the same region as monomer. Kinetic parameters for decomposition of some important peroxides are given in Table 3.5,... 

Photochemical or photoinitiated polymerizations occur when radicals are produced by ultraviolet and visible light irradiation of a reaction system [Oster and Yang, 1968 Pappas, 1988]. In general, light absorption results in radical production by either of two pathways ... 

In the UV curing process, photons from the UV source are absorbed by a chromophoric site of a molecule in a single event. The chromophore is a part of the photoinitiator. The light absorption by the photoinitiator requires that an emission light from the light source overlap with an absorption band of the photoinitiator. 

The peroxide and azo thermal initiators also are photochemically unstable and have been used as radical sources at well below their normal thermal decomposition temperatures. However, their industrial use as photoinitiators has been limited because their light-absorption characteristics frequently are unsuitable and because of the obvious potential complication owing to their slow thermal decomposition, which leads to poor shelf-life and nonreproducible photoactivity in given formulations (88). Further information on photoinitiators can be found in the literature (92). 

The catalytic reaction involving light absorption by a substrate and catalyst species is called photocatalysis [29], At the end of the reaction cycle, the photocatalyst is regenerated to its original state. Sometimes, however, the agent that initiates certain chemical transformations under the action of light is itself consumed in the reaction or process then it is called a photoinitiator [127-129]. 

Alternatively, light is consumed and the reaction progress is possible only under continuous light absorption this option, called catalyzed photolysis, includes photoassisted generation of a reactive form of substrate or photocatalyst. In the former the process is called catalyzed photochemical reaction, whereas in the latter either catalyst activation may lead to formation of catalyst or photoinitiator, which initiates chemical transformations but is consumed within a reaction cycle, or the catalyst reacts with substrate in its excited state (photosensitization) in both cases... 

In all previous examples discussed, the monomer molecules participate in the excited states reactions, which give the initiating species X. Another principle for the utilization of PET in photopolymer systems consists of the use of donors and acceptors that are independent of the monomer molecule. Then, that combination of both compounds is a true photoinitiator system, and it supplies the initiating species X after irradiation. This variant is more widely usable, and possesses a greater variability with respect to light absorption. 

Iron arene photoinitiators have excellent light absorption properties in the ultraviolet and visible parts of the spectrum. As shown in Fig. 18, the absorption can be varied over a wide range by structural changes in the ligands. Iron arene salts can be sensitized, for example with anthracene derivatives... 

Hydroxytelechelic polymers can be synthesized via a photoinitiated radical process 49,50 76 77). This reaction resembles that of the redox system because an electron transfer mechanism is operative and the synthesis is carried out in aqueous solution. The reactive species is a complex ion such as Fe3+, X (OH-, Cl-, N". ..). The light absorption (hv) by the ionic species results in an electron transfer reducing the cation oxidation of the anion leads to a free radical X which initiates the polymerization. 

Irradiations were conducted at 366 nm (Corning 7-83 filter combination) in a merry-go-round apparatus in which the samples rotated about a stationary 450-Watt Hanovia medium pressure lamp for constant light exposure. The samples consisted of degassed 5 ml solutions of the photoinitiators in neat MMA contained in Pyrex tubes. Two initiator concentrations were utilized 1.05 x 10and 4.11 x 10 which corresponded to 16 and > 99% light absorption, respectively. Samples were irradiated to about 7% monomer conversions which required 15 min for the optically dense solutions and 30 min for the tubes with low initiator concentration. The resulting poly-... 

In each of these cases, the thickness of the coating film is very important (inks 2 to 10 microns versus a fluid coating of 0.5 to 2 mils) on the amount of light energy absorbed by the photoinitiator. The amount of pigment, dispersion, and reflectance of the paint film are also Important for light absorption by the photocatalyst. 

Even if direct light absorption as above does not occur, polymerization can still be initiated if photosensitizers are present that produce free radicals when they absorb ultraviolet or visible light. The same substances that are used for thermal initiation are often used for photosensitization. For example, azo compounds and peroxides are photosensitizers, and the photoinitiation reaction is the same as is the thermal initiation process, described earlier in this chapter. However, the photoinitiation can take place at much lower temperatures than in the thermal initiation by the same initiators. Moreover, many initiators can be used as photosensitizers even though they do not dissociate thermally at convenient rates or temperatures to be useful as thermal initiators. For example, azoisopropane does not dissociate sufficiently rapidly below 180°C to be useful thermal initiator. However, it photodissociates even at low temperatures when irradiated with near-ultraviolet light ... 

As reported in a recent review [88], difunctional photoinitiators containing two cleavable moieties exhibit a practical efficiency sometimes better than that of the mono derivatives. In the case of the compound shown in (10.40) (where X = CPU or O), the photochemical reactivity is almost not affected [89]. A better light absorption is noted this is due to a large change of the molecular orbitals. 

For curing with UVA is, the first step is light absorption by the photoinitiator Rj hv... 

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