Monomers radiation-curable systems

Figure 11. Effect of pigment level on skinniness ratings of polyester/cotton, 50/50 fabrics printed with radiation-curable system (2.5% Graphtol Blue 6825, 15% acrylo-urethane oligomer (MW 6000), 79% monomer mixture EP-4 (80%) and v-pyrol (20%), 3% Cab-O-Sil, and 3% vicure 10. Then required amount of...

Monomers. In radiation-curable systems, the solvent is replaced by reactive diluents (monomers) which are incorporated in the network during cross-linking. These monomers have two important functions they reduce the viscosity of the mixture and they strongly influence the physical and chemical properties of the final coating. Monomers can be divided into three groups ... 

The contributions of physics to adhesion are further manifested by the extension of uv- and electron-beam curing of adhesives to low temperatures. Radiation-curable adhesives were originally intended to eliminate solvents in the systems. However, the use of certain toxic acrylate monomers presented some problems. So far, radiation-curable systems have become technologically challenging, but economically less attractive than most thermally cured systems. Since one of the earliest papers on photoadhesives by Nakano (35) many patents have been issued on this subject. Two reviews have been published by Stueben (36). ... 

Free-radical, radiation-curable systems generally consist of monomers, oligomers, photoactivators, other resins, and fillers or tackifiers. An ultraviolet source or electron-beam generator is used to cure the systems. As an illustration Stueben (37) describes a typical UV-cured PSA system containing acrylates and polyvinyl ether. McGinniss (38) discusses formulation design related to UV-curable systems. The cationic photoinitiators are shown in Fig. 1. 

More commonly, acrylic polymers have been utilized as the sole or major components of radiation-curable adhesive systems. These acrylics have included a homopolymer and many examples of copolymers. One UV-cured acrylic ester copolymer system cited was claimed to have 180° peel strength of 4.8 to 5.5 pounds/inch. Radiation-curable systems based on copolymers of vinyl acetate and 2-ethylhexyl acrylate have been reported more frequently than other compositions and sometimes have been applied by hot melt techniques. These pressure-sensitive adhesives have displayed 180° peel strengths ranging from about 1.5 to 3 pounds/inch when cured by either UV or EB radiation. Not surprisingly, solutions of acrylic polymers in acrylic and vinyl monomers have also been used as radiation-curable pressure-sensitive adhesives. 

Photopolymer systems are photocurable resins incorporating reactive liquid monomers, photoinitiators, chemical modihers and hhers. Typically stereolithography utilizes UV radiation, so UV-curable systems are used. Free-radical-photopolymerizable acrylate systems were originally used however, newer cationic epoxy-resin and vinyl ether systems (based on iodinium- or sulfonium-salt cationic initiators) are now being utilized. 

Chem. Descrip. Neopentyl glycol propoxylate diacrylate Uses Monomer for pigmented systems, inks, flexible substrates, metals, radiation-curable applies. 

Chem. Descrip. Polyether triol containing primary hydroxyl groups Uses Flow aid, leveling agent, abrasion resist, aid for radiation-curable monomers, urethane and polyester resins, high-solids coatings, polyester resin synthesis reactive modifier for UV cationic cure systems Features Good compat. with resin systems Properties Lt. yel. clear liq. m.w. 240 sp. gr. 1.1 dens. 9.2 Ib/gal vise. 800-900 cps hyd. no. 680-710 pH 6-8 (2%) 0.5% moisture... 

Uses Flow aid. leveling agent, abrasion resist, aid for radiation-curable monomers, urethane and polyester resins, high-solids coatings, polyester resin synthesis reactive modifier for UV cationic cure systems Trade Names Photonol PHO-7149... 

Epoxy systems can also be used for radiation curables, and although their use is more limited presently because of patent conflicts more of them will be seen in the future. Radiation curable resins, oligomers and monomers that are designed specifically for their adhesive properties will not become readily available to formulators until the markets grow much larger. 

All of these UV-curable systems have very different chemistries than those used in solvent-based systems. Most contain a dispersant, a plasticizer (not necessary in all cases), a photopolymerizable binder, and an initiator to activate the UV curing. A typical binder/initiator system found in the literature is a polyester acrylate binder and an initiator such as 2-hydroxy-2methyl-l-phenyl-propan-l-one. The binder is a liquid low-viscosity monomer diluted into hydroxy-ethyl methacrylate. Most of the UV-curable systems contain an acrylate monomer of some sort, since they have relatively low viscosities (100 mPa s or cP) and excellent reactivity with the UV radiation. The low viscosity allows the preparation of ceramic slurries with a relatively high solids loading, and the resulting high-density green sheets have excellent mechanical properties. 

---