Price: 10.00
The absence of solvents and the possibility of complete elimination of VOC emissions belong to the main advantages of UV curable coating materials. Additionally, the overall expenditure of radiation curing technology is lower due to reduced requirements with respect to processing time, place, and energy needed for curing as well as for solvent removal before film drying. On the other hand, certain technical demands sometimes can lead to a break in this concept of 100 % crosslinkable formulations. This, e.g., may be caused by viscosity reduction necessary for spray-application at ambient temperatures or by a certain finish desired which is difficult to achieve in full-solids. In such cases, water offers ecological advantages compared to organic solvents despite the fact that higher evaporation enthalpy has to be summoned up before UV irradiation. Moreover, the use of water instead of acrylic monomers which are frequently used for dilution gives less problems from the physiological point of view. There are three different basic possibilities of water-containing UV curable formulations 1-18]: Aqueous emulsions: These materials are prepared from water-free unsaturated polyester or acrylate resins. Special types with internal self-emulsifying groups bound on the oligomer molecules as well as selected oligomers which need additional emulsifiers are in use. Aqueous dispersions: Compared with emulsions the dispersions are characterised by lower particle sizes and by higher molecular weights. The latter property enables them to form dry, tack-free films already after water evaporation (i.e., before UV curing). UV irradiation, however, causes a more intense crosslinkage.
o Water thinnable oligomers: These resins can be diluted with limited amounts of water (for the most part, less than 30 wt.%). Oversaturation with water causes turbid mixtures which often can not be handled. Frequently, the products can be combined with each other but, sometimes, incompatibilities are found. Furthermore, blends with pure physically drying dispersions based on polyacrylate or relating copolymers, polyurethane, or nitro-cellulose are known 16, 19].
However, specific problems of aqueous UV curable materials can result from the volatility of the
photoinitiators. Thus, at higher temperatures they are partially evaporated from the coating together with water before UV irradiation can take place 16, 18, 20, 21]. To maintain sufficient reactivity it can be necessary to use pre-drying conditions (which cause higher time and space requirements for this technological step) and/or less volatile initiators. On the other hand, certain commercially available photoinitiators are non-volatile but solid or waxy. They are frequently considered to give only limited possibilities of incorporation into aqueous formulations. This, e.g., may be connected with their more or less hydrophobic character. Therefore several attempts have been made to create photoinitiators with reduced volatility as well as hydrophilic or water soluble
initiator types. This should enable optimum curing results even at low initiator concentrations 20-261.
