Polymerization Degree Effects On The Photocrosslinking of Epoxyacrylate Bisphenola Oligomers

In the UV-curable formulations the most important component is very often the oligomer. Its choice exercises the control over some chemical and physical characteristics affecting the properties of the UV-cured materials, such as ,) gloss, ii) chemical resistance, iii) scratch resistance, ii,) mechanical resistance and non-yellowing 1]. These properties are often reached by a combination of different oligomers in the UV curable formulations. Since the exact chemical
composition of oligomers is seldom revealed, their selection can be made on the basis of some
criteria as the chemical composition, the number reactive functional groups and the molecular
weight. Molecular weight can be usually deducted by looking at properties like viscosity, flexibility etc. usually indicated by suppliers. The commercially available unsaturated oligomers are mainly formed by epoxy-acrylates, acrylourethanes, polyester acrylates, polyether acrytates and amine modified polyether acrylates containing at least two acrylic polymerisable groups at the chain ends. However, oligomers containing up to six acrylate groups per chains are also available for these materials. The most acrylated oligomers are usually constituted from two acrylated groups per chains and have a molecular weight ranging from 600 to 6,000 glmol 2]. The acrylated oligomers provide good film-forming properties, suitable viscosity and mechanical properties to UV cured materials 3]. There are some aromatic oligomers that after UV curing form hard rigid polymer films with high tensile strength and modulus, while aliphatic ones can generate soft, extensible coatings with high tensile elongations. Aromatic difunctional epoxy acrylates based on the bisphenol-A are one of the most important oligomers for coating applications (see scheme I). Some requirements as high viscosity and scratch resistance can be fulfilled by Bisphenol-A difunctional epoxy acrylates. These oligomers are synthesized by a polycondensation reaction between diglycidylbisphenol-A (DGBPA) and bisphenol-A (BPA). The stechiometry of the polycondensation reaction can control the chain growth providing a family of oligomers with different molecular weight (i.e viscosity) having epoxy groups as the chain ends 2]. The subsequent end capping reaction with methacrylic acid can give origin at methacrylated chain ends. These types of oligomers can have both low molecular weights (600-1500 g/mol), which gives them several attractive properties as high reactivity, high gloss, scratch resistance, or medium molecular weights (1500-6000 g/mol) with low reactivity and good flexibility 3]. This
means that these oligomers are suitable for a wide variety of UV curing coatings, ranging from
overprint varnishes for paper and board to wood coatings for furniture and parquet flooring, but
also high tech applications like compact disk coatings and optical fiber coatings 4]. The aim of this work is to study the photopolymerization reactions of three oligomers having different molecular weights. Several formulations, obtained by dilution of three oligomers with the same amount of HDDA, have been studied as a function of the commercially available photoinitiator types by using the photocalorimetric and real-time Fourier Transform techniques.