The first commercial presentation of a UV curing screen ink was in Italy at FESPA Œ75 in Milan. This early demonstration was of a sepia point of sale display ink on a compressed board. Cure speed was very slow and the ink was based on materials subsequently identified as hazardous. As a point of interest, the demonstration ink was never fully commercialised or sold to the industry. UV screen ink technology first found success with printed circuits in the form of UV curing solder resists. The nature of the application was such that it accommodated the extended exposure to UV radiation required at the time. Clear UV varnishes for point of sale use, typically for overprinting offset print, and simulating film lamination, were the next step in UV screen ink technology. In the mid 1980s, the first UV curing inks were sold for blow moulded plastic containers. Instant drying and the ability to multiple-pass print plastic containers, were readily accepted by plastic container screen printers. Point of sale printers were the next to embrace UV ink technology as advances in polymer chemistry led to faster curing ink systems. In the latter 1980¹s, the use of water in UV inks was first seen. In these systems, water is used to act as a volatile component, reducing the filmweight of each printed layer. Large volumes of water thinable UV inks are now used by European point of sale printers for four colour process printing. Only in the last 10 years have UV inks for high technology applications been available. The demands of the process and sensitivity of the substrate had prevented their use for such work previously.

Very Different Demands

In the high technology business of membrane switch overlays, graphic decals for automotive clusters and in-mould decoration applications, there are very different and special demands placed on the ink formulator. Looking first at the membrane switch market, we have a requirement for a range of blending colours which when cured are compatible with subsequent processing of the part, for example with pressure sensitive adhesives. From the formulating chemist¹s point of view the need to have compatibility with a range of pressure sensitive adhesives which could be solvent based, aqueous or even UV curing, dictates the raw materials that can be used. Much greater knowledge now exists and early problems with compatibility have now been overcome. Membranes switch overlays also often include transparent windows, either clear or coloured; optical clarity and pureness in the case of coloured transparent inks is essential. However, this raises another problem. It is a widely held belief that if a pigment is optically transparent in the visible region of the electro-magnetic spectrum, ie, in a solvent based ink, it is simple case of substituting the pigment into a UV curing ink. Dyes which offer exceptional optical transmission characteristics are also widely used in traditional solvent based inks and unfortunately again there are constraints with dyes in UV ink systems, as the chemical structure interferes with the UV curing reaction. In fact, pigments that are optically transparent in visible light are quite often either semi-opaque or fully opaque in the area of the electro-magnetic spectrum in which UV ink systems spectrally respond (360-380 NM). When producing a membrane switch overlay, it is fairly common for the end customer to request a surface texture (anti- reflective) coating applied to the first surface of the part.