Acrylates radiation-curable systems

In order to improve performance and processing efficiency of radiation curable systems it has been proposed that curing should be monitored using a technique of mechanical spectroscopy SS and in relation to this other workers have found that the molecular weight and structure of the diol component of multifunctional acrylates influenced the mechanical properties of the UV cured films produced 52,193. urea group in the resin tended to enhance the modulus of the cured film whereas a vinyl caprolactam diluent reduced resin viscosity for ease of coating. 

Larmner . BA AG] Unsaturated acrylates reactive diluem or thinner fix radiation-curable systems, finishes. 

Uses Reactive diluent for radiation-curable systems (lacquers, printing inks) comonomer for crosslinked acrylic resins Trade Names Laromer DPGDA Photomer 4226... 

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. 

The use of elastomeric or flexibilizing modifiers occurred and grew with epoxy resins first. Various aspects of toughened epoxy adhesives have been covered in reviews by the present authors (2,3), where the elastomeric modifiers have essentially been carboxylic, liquid and solid butadiene/acrylonitrile polymers. There has not been a systematic review, however, of these and other reactive liquid polybutadiene/acrylonitriles in the burgeoning areas of acrylic, anaerobic and radiation-curable systems. Thus, this paper s intent. 

Another important class of modified epoxy resins, known as epoxy acrylates, are reaction products of a low MW epoxy resin and acrylic acid or methacrylic acid, which are used in radiation curable systems. The acrylate functionality is 2, but higher functionality variants can also be prepared by using epoxy phenol novolac in place of BPA epoxy resin. Radiation curable formulations from these oligomers provide a better balance of properties and cost compared to some of the alternative polymer systems. 

Pressure-sensitive adhesives based on acrylic polymers grew 3.5-fold in the last dozen years. The acrylics are mostly emulsions and solutions, plus small quantities of radiation-curable systems. In tapes, acrylic adhesives are applied mainly to non-paper substrates such as polypropylene, acetate, cellophane, and polyester. In labels, acrylics are used for making permanent paper labels and plastic labels. 

Radiation-curable systems are likely to be based on acrylated epoxy resins, aciylated polyurethane resins, aciylated epoxy-novolak resins, and aciylated isocyanurate resins. 

To be suitable for radiation-curable powder coatings, materials must contain unsaturated double bonds. They may contain acrylic or methacrylic unsaturations, but nonacrylate systems can also be used.32 The latter are based on the same chemistry... 

It has been demonstrated that the incorporation of this type of reactive fluorosurfactant into a radiation curable acrylic system provides for a high gloss coating due to wetting, flow and leveling attributes, but results also in increased stain resistance of the coating. 

Market penetration is expected to increase further due to stricter environmental regulations and the availability of a greater variety of products. The first radiation curable adhesives were limited to acrylate and epoxy resins. Today, many different types of radiation curable adhesive systems are commercially available. 

Recently, hot melt PSA systems have been introduced and radiation curable PSA systems are at the commercial development stage. High solids (50%-70% by wt.) nonaqueous dispersion acrylic PSA systems have also been reported(1). Unlike the hot melt and radiation cured systems which require new capital outlay in coating head and/or curing (drying) equipment, BFG has developed PSA systems, based on Hycar 2100R reactive acrylic liquid polymers and isocyanate terminated prepolymer, which can be processed at 80% solids (by wt.) with equipment presently used in the PSA industry, namely, the reverse roll and knife-over-roll coater. 

In 1983, radiation curable 100 % solvent-free silicone acrylates were introduced into the market [4], This system provide the opportunity to be cured by either ultraviolet light (UV) or electron beam (EB). Similar in concept to peroxide initiation, silicone acrylate systems employ photoinitiators to generate free radicals and initiate cure, which is based on the polymerization of the acrylic C=C double bond via a radical chain reaction. 

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. 

Newer adhesives of the acrylic, anaerobic or radiation-curable types must, if they are structural, have a relatively high degree of toughness and durability if they are to compete with or challenge epoxy adhesive systems. Likewise, newer radiation-curable, pressure-sensitive adhesive systems must exhibit the properties of permanence largely associated with cross-linked adhesive masses deposited from an acrylic solution polymer base. Epoxy resin structural adhesives largely define the existing area on the one hand the cross-linked acrylics deposited on plastic or metallic films the other. 

Perkins (27) published extensively on radiation curable pressure sensitive adhesives showing that permanent systems can be obtained if liquid hydroxyl-terminated polybutadiene/acrylonitrile were used with a selected acrylic interpolymer (e,g., 2-ethylhexyl-... 

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. 

Salomon, P.A., Jacobine, A.F. and Glaser, D.M., Non-acrylate photopolymer systems adhesives properties and performance of a new radiation curable thiol/norbomene system (pp. 239-259), Proc. RadTech America, 1992. 

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