Chemical Systems in UV Processing

Kinetics of Cationic Photopolymerization Chemical Systems in UV Processing... 

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). 

One example of the effect of such factors is UV inks used in printing, which were the snbject of a scandal early on, the positive evaluation of aqueous systems and bio paints , which were evaluated veiy positively despite their not unproblematic content of vegetable-based solvents. In addition, the comparatively speedy snbstitntion of chlorinated solvents in cleaning processes and formaldehyde separators in cooling lubricants can only in fact be explained by the pubhc-ity-effective scandals relating to chlorinated chemicals or formaldehyde. 

Hydrogen abstraction by RH02 could also participate in the process of initiating a chain of thermal oxidation reactions (pathy). In aqueous systems, cations will further react by solvolysis, and superoxide anion will readily disproportionate to yield H202 (path i). This is in contrast to the fate of superoxide anions in ozonation advanced oxidation processes (AOPs), where they react primarily with ozone to produce hydroxyl radical. This description of the chemical pathways of UV/H202 oxidation of organics illustrates that, when oxygen is present, the major paths directly or indirectly create more... 

The quantum yield 0 of a photophysical or photochemical event is a quantitative measure of the overall efficiency of this process (Braun et al., 1991). It is a unitless constant, which usually ranges from zero to one. However, some authors express 0 in units of mol einstein, which in fact is unit-less, because an einstein is defined as one mol of photons. Quantum yields greater than one indicate photo-induced chain reactions, which may involve radical species or photo-generated catalysis. Commonly used definitions of 0 are collected in Tab. 3-7. These definitions describe quantum yields of photophysical events and of photochemical reactions with regard to the reactant diminution or to the formation of the photoproduct Quantum yields may be dependent on the wavelength of the absorbed UV/VIS radiation, but many photochemical systems exist that have a constant quantum yield 0 over a defined wavelength range. Such chemical systems can be... 

Novel vinyl liquid crystalline (l.c.) polymers were synthesized with the UV-sensitive p-methoxycinnamate chromophore incorporated into the side chain of the polymers. The objective of this synthesis was to determine if a molecularly organized environment could influence the yield of a chemical reaction in the solid state. The investigation into the photochemical and physical processes of these thin films revealed that the photodimerization of the p-methoxycinnamate moieties was very sensitive to their geometrical arrangement in the polymer matrix. The relative quantum yield of cyclobutane formation increased by a factor of approximately 8 for the l.c. p-methoxycinnamate film compared to its amorphous analog. This quantum yield approaches the theoretical limit for this system. 

The UV technology alluded to above holds out the promise of even more superior labels. In this process, ultraviolet cured inks are used as the printing medium. These are cured by exposure to industrial type UV mercury vapor lamps in either a non-inert system (air environment) or in an inert (oxygen-free) system. The result is excellent adhesion to vinyls, polyesters, polycarbonates, butyrates and acetates and a cured elongation of between 20 and 40% to avoid embrittlement of the substrate. Initial testing has exhibited little sign of degradation and excellent chemical resistance after three years of exterior exposure. Such UV labels as well as other techniques should be considered whenever use demands extended label performance. 

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