Printing inks, paints, and other coating materials usually are applied as a liquid layer, followed by curing or drying to the solid state. Curing or drying can occur by physical means such as evaporation or absorption of the solvent or dispersion medium, or by chemical reaction such as polymerization or cross-linking. Curing by UV irradiation-induced chemical reaction is particularly desirable for it avoids the need for solvents, thus eliminates potential safety hazards and reduces environmental pollution; it also speeds up production.
Photocuring coating compositions are usually comprised of three components: the monomer, a photoinitiator, and additives. The monomer is an unsaturated or cyclic organic compound which can undergo polymerization. It can be a single compound or a mixture of compounds. Typical monomers employed in photocuring coating compositions include styrene-unsaturated polyesters, acrylates, thiol-enes, allyl ethers, vinyl ethers, epoxides, and the like. The monomers are preferably bi- or multi-functional, to form a highly cross-linked network providing maximum strength and stability.
The photoinitiators are photosensitive compounds which absorb UV radiation and produce activated species--typically free radicals (for styrene, acrylate and allyl monomers) or cationic in nature (for vinyl ethers and epoxides)--to initiate polymerization or cross-linking. These photoinitiators can be used as a single compound or mixtures of different compounds, to meet desired curing characteristics.
The additives include the ususal pigments, dyes, inhibitors to prevent polymerization during storage, stabilizers to reserve optimum properties and prolong service life, regulators to adjust flow characteristics, and the like.
Acrylic coatings, including those based on mono- and multi-functional acrylates, methacrylates, and acrylic oligomers derived from epoxides, polyesters, and polyurethanes combine the advantages of superior film properties (clarity, strength, adhesion, gloss, etc.); adjustable flow properties for easy application; and rapid curing. Acrylic coatings based on fluoroalkyl acrylic ester monommers (herein sometimes referred to as "F-acrylates") possess highly desirable properties such as high thermal and chemical stablitity, low surface energy, low friction, and low refractive index. Such advantageous properties are dependent on fluorine content; they tend to improve with increasing fluorine content. For example, the refractive indix of amorphous fluroracrylic polymers decreases with increasing length and number of the fluoroalkyl chains.
Unfortunately, the photoinitiators conventionally employed for polymerizing the usual acrylic monomers are not miscible with the highly fluorinated acrylate monomers, and there are no ready means for photocuring such acrylate monomers. UV curing of such fluorinated acrylate monomers without the aid of a photoinitiator, while feasible, gives less than optimum properties, is time consuming and expensive. This invention provides new photoinitiators which are compatible with highly fluorinated acrylate monomers.