Synthesis of hydrogel with benzophenone/polyethyleneimine photoinitiator system

Photopolymerization is is environmentally sensitive polymerization technique which occurs by means of reactive species formed by the illumination of photoactive molecules. It can be applied in many industrial applications, such as varnishes, paints, adhesives, various coatings, printing inks, encapsulations of electronics components, graphic arts, printing plates, stereolithography, photoresist, lazer direct imaging, computer-to-plate imaging technology. In comparison with thermally initiated polymerizations; the photopolymerization provides several advantages including low energy requirements, low operating temperatures and the release of non-volatile organic compounds (solvent-free systems). In addition, this technique has been found to be interesting and preferred method in accordance with a green technology due to numerous advantages such as the ability to easily control the properties that affect the performance of the material such as adhesion, hardness, colour, electrical conductivity and permeability. The key factor in controlling of photopolymerization process is the photoinitiator that produces active radical for initiation step. Upon UV exposure, they are readily converted into active radicals to initiate the free radical polymerization. Generally, there are two type photoinitiator systems; (i) cleavage "Type 1" and hydrogen abstraction "Type II''. In Type I system, the photoiniator is directly decomposed into two active radicals under light irradiation. Whereas, the Type 2 photoinitiator are based on bicomponent compounds, (photosensitizer and hydrogen donor) that can produce active radicals by means of hydrogen abstraction reaction or electron/proton(for example, from amines) transfer. In generally, Type II photoinitiators are slower than type I photoinitiators. However, recent research areas have concentrated on Type II photoinitiators due to their best optical absorption properties in the near UV spectrum region. Benzophenone is the most commonly used conventional Type II photoinitiators to date. The continuing interest for benzophenone and its derivatives have been resulting from diverse reasons. One of these reasons; it has capability to efficiently absorbs both long wavelength (UV-A, 400-315 nm) and the medium wavelength (UV-B, 315-280 nm). Amines, thiols and alcohols are well-known hydrogen donor compounds. Among them, the amines are more preferable due to their oxygen inhibition properties. Recently, the use of high molecular weight amines, polymeric amines, is developped to eliminate yellowing properties in the product. In this study, type II photopolymerization of meth(acrylate) monomers was accompolished by using benzophenone/polyethylenimine photoinitiating system. The effects of molecular weights of polyethylenimines were also investigated. Thus, two major limitations of photopolymerization, oxygen inhibition and landslide event known as yellowing effect after polymerization, were eliminated.