Photoinitiator energies

Two mechanisms have been identified to describe the interaction between photosensitizers and photoinitiators energy transfer and electron transfer. In the energy transfer process, the photosensitizer absorbs the light and transfers that energy to the photoinitiator which will then go through either a imimolecular or himolecular scheme to produce initiating free radicals. 

For photoinitiation there is no activation energy for the initiator decomposition hence... 

The ptincipal commercial initiators used to generate radicals are peroxides and a2o compounds. Lesser amounts of carbon—carbon initiators and photoinitiators, and high energy ionising radiation are also employed commercially to generate radicals. 

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). 

J. Hutchison, "Photoinitiated Free Radical Chaia Reaction," Vol. 11, Energy Kesearch Abstracts Abstract No. 51555, Electricity Council Research Centre, 1986. 

Janke, C.J., Lopata, V.J., Havens, S.J., Dorsey, G.F. and Moulton, R.J., High energy electron beam curing of epoxy resin systems incorporating cationic photoinitiators, US Patent 5,877,229, 1999. 

The reaction between Cell—O radical and vinyl monomers leads to the formation of grafted cellulose. In the presence of photosensitizers generally used as photoinitiators, such as benzophenone and phenylace-tophenone derivatives, the photoinitiator absorbs the UV radiation and transforms to its singlet (S ) and then triplet (T ) states. After that it may decompose into free radicals or transfer its energy to cellulose or any other molecules in the system. Take benzophenone as an example ... 

PCSs are systems of chromophores bound into a single macromolecule. Therefore, the study of processes of electronic excitation and energy transfer, as well as the investigation of the ways of deactivation of excited states, should lay a foundation for the understanding of such properties of PCSs as reactivity in photochemical transformations, photosensitizing and photoelectric activity, photoinitiated paramagnetism, etc. 

The chemistry involved in LfV-curable resin systems has been extensively investigated and thoroughly surveyed [88-94]. LfV-radiation polymerization, is in principle, completely analogous to the conventional addition polymerization. A photoinitiator is used in UV polymerization. Its function is the same as the free-radical initiator. A conventional initiator possesses a thermally labile bond which is cleaved to form free-radical species, but the photoinitiator has a bond which breaks upon absorption of radiant energy. Benzoin ethers, benzyldialkyl ketals, benzophenone, and acetophenone derivatives are the important LfV-photoinitiators [95-99]. 

Since the pioneering work by Oster et al. in the 1950 s (3,A) where monomers were photografted with UV light onto polymers blended with photoinitiators, only few researchers have more recently followed up this work. The interest has been focused on the use of high energy radiation. 

Thus, a semilogarithmic plot of the gel time as a function of 1/T should be linear, with the slope corresponding to the apparent activation energy. We have determined the gel times for a temperature range of 25°-50° C for a thiol-ene system consisting of stoichiometrically equivalent amounts of a trifunctional thiol, trimethylolpropane tris(2-mercaptoacetate), and a trifiinctional allyl monomer, triallyl isocyanurate. In this system, we also added 0.31% by weight of hydroquinone, to prevent premature polymerization, and 1.0% by weight of a commercial photoinitiator, Esacure TZT. 

Activation energy (5-65 °C) for photoinitiated/thermal inititiated in kcal/mol 26/32 24/35... 

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