Curing photochemical

Photochemical curing has found many industrial applications, not only for the protection of metal surfaces but also for printing and decoration on a variety of materials. It should be added that optical and mechanical properties of surfaces can also be modified by the coating, for example a porous material can be made impermeable by the thin coat of solid organic substance. 

Benzoin derivatives are used as initiators for the photochemical curing of printing inks, lacquers and other surface coatings, since the intermediate radicals in a reaction such as (4.7) can be diverted to initiate the polymerization of vinyl monomers. The use of an unsvm-metrical ketone (4.8) also shows that discrete radicals are produced in the cleavage reaction, since the ratio of hydrocarbon products is close to that expected tor a random combination of separated radicals. Esters that give rise to similar stabilized radicals undergo loss of carbon dioxide (decarboxylation) by a closely related mechanism, and this has proved useful in making quite strained cyclic systems bv irradiation of readily prepared cyclic diesters (4.9). 

Liquid crystals exhibit a partially ordered state (anisotropic) which falls in-between the completely ordered solid state and completely disordered liquid state. It is sometimes referred to as the fourth state of matter . In recent years, interest in liquid crystalline thermosets (especially liquid crystalline epoxy) has increased tremendously [33-44]. If the liquid crystal epoxy is cured in the mesophase, the liquid crystalline superstructure is fixed permanently in the polymer network, even at higher temperature. Liquid crystal epoxies are prepared using a liquid crystal monomer [33-38] or by chemical modification of epoxy resin [43] which incorporates liquid crystal unit in the epoxy structure. Liquid crystalline epoxy resins with different types of mesogen such as benzaldehyde azine [33], binaphthyl ether [34, 35], phenyl ester [36, 37] and azomethine ethers [38, 39] have been reported. Depending on the chemical nature of the mesogen, the related epoxies display a wide range of thermomechanical properties. The resins can be cured chemically with an acid or amine [40, 41] or by photochemical curing in the presence of a photo-initiator [3]. Broer and co-workers [42] demonstrated the fabrication of uniaxially oriented nematic networks from a diepoxy monomer in the presence of a photo-initiator. 

Z.K. Hu, L.M. Pitet, M. A. Hillmyer, J.M. DeSimone, High modulus, low surface energy, photochemically cured materials from liquid precursors. Macromolecules 43 (December 28, 2010) 10397-10405. 

ED paints may be thermally or photochemically cured for improved performance. None the less, it was some time before serious attempts were made to use ED photocurable films as resists for metal patterning. It had been foreseen that dry-film photoresists, which have been the mainstay of the printed circuit board inner-layer fabrication process for the last two decades, would soon reach their resolution limit and that a process that coated much thinner layers of resist would take over. ED resists that were capable of coating layers up to five times thinner than dry film seemed the natural successors. In 1986 the Rohm and Haas Co. [2] issued a patent describing a photoresist composition for cataphoretic deposition onto copper during the process of forming a printed circuit board. Many other patents in this field, describing both cataphoretic and anaphoretic deposition of a wide variety of resins, have been issued since then. 

The photochemically cured systems are similar in composition to those which are chemically cured. They employ the same, or similar, monomer systems, i.e. mono- and dimethacrylates the filler (if used) are similar, i.e. various forms of glass and they polymerize via free radical mechanisms. The final physical properties of all of these systems, when they are properly cured, are similar and are clinically acceptable for their intended applications. 

Photochemistry has found a place in the field of dental materials for a variety of applications. The use of photochemically cured systems requires a curing light which is not needed in the amine-peroxide system. However, this is offset by the advantage that the dentist is afforded as much working time as he desires. Although the physical evaluation of dental materials is greatly dependent upon the test methods and test parameters employed, the final physical properties of these materials, when they are properly cured, are clinically acceptable for the intended applications. 

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