UV Adhesives

Adhesive systems which use UV or visibie iight to begin the curing are intrinsicaiiy very attractive systems to the manufacturing engineer. They aiiow assembiy of components and then curing on command by exposure to UV irradiation. UV adhesives have been marketed for 30 years, with many successfui appiications in severai market segments. 

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Free Radical Cure UV Adhesives. As with any adhesive, formulation variables are critical to the processing and performance characteristics. Variables such as oligomer selection, modifiers and additives, monomer structure, molecular weight, and glass transition temperature directly affect application and performance properties. 

A typical cationic uv adhesive formulation contains an epoxy resin, a cure-accelerating resin, a diluent (which may or may not be reactive), and a photoinitiator. The initiation step results in the formation of a positively charged center through which an addition polymerization reaction occurs. There is no inherent termination, which may allow a significant postcure. Once the reaction is started, it continues until all the epoxy chemistry is consumed and complete cure of the resin has been achieved. Thus, these systems have been termed living polymers. 

The disadvantages of UV adhesives are that one transparent substrate is normally required they suffer from oxygen inhibition and only a limited depth of cure can be achieved. The latter problem has been tackled by the development of dual-cure adhesives. In these systems, two independent curing mechanisms are incorporated into a single system. Thus the adhesive is cured first to a chemically stable form by UV irradiation and subsequently led to fiiU cure by a second means, for example, thermal cure. 

Figure 8 (Top) Optical region of the electromagnetic spectrum is defined into ultraviolet, a narrow band of visible light, and a belt of infrared radiation. (Bottom) An exploded view of the LTV spectrum with absorbance curves for Dymax UV adhesives compared to typical UV inks and coatings. X Maximum = that point at which maximum absorption of UV energy occurs.

An electronics manufacturer attaches a plastic base to a 1-in. square ceramic circuit holder with a UV adhesive. Cure time is 10 seconds, which is about the same as the previously used cyanoacrylate. The lower cost, higher performance, and worker safety features justified the change in adhesive. 

Manufacturers have begun using UV adhesives because of the very large differences in the cost of energy used for cure and the increase in productivity associated with faster cure cycles. Specially designed UV-aerobic acrylics cure rapidly, adhere to borosilicate glass, and withstand the severe environmental and vibration resistance required of automotive parts. 

In the case of radiation-curable (e.g., UV) adhesives, TLC represents a very useful screening test for the presence of several characteristic photo-initiators, e.g., benzophenone, 2,2 -diethoxyacetophenone, 1-benzoyl-cyclohexanol, and 4,4 -dimethylaminobenzophenone (Michler s ketone). 

UV hardening resin was first reported in 1968. From the report, UV hardening resins have been applied to many industrial fields, such as wood coating, electronic equipment, printing inks, and adhesive reagents. UV adhesive resin is one of the most important applications of UV hardening. The merits of UV adhesive resins are ... 

Anion Nw starts to polymerize methacrylate. Anion acts in the reaction as a nucleophilic addition. Since oxygen disturbs the radical reaction, an anion or cation initiator is more effective for UV adhesion. 

Epoxyacrylate is a typical UV adhesive polymer. The structure of epoxyacrylate is shown in Figure 5.8. 

Linevol, Plasticizer alcohol. Shell Chemicals Lio Dispersions, Single pigment dispersions in PE wax carrier for polyolefins, Toyo Ink America LioClean>A, Nonabrasive purge compound, LioChem Inc. Toyo Ink America LioFeeder, Color feeder, LioChem Inc. Liquiphant, Level switch, Endress + Hauser Liquistik, Level Unfit switch, Endress + Hauser Lite-Tak, UV adhesives, Loctite Cotp., North American Group... 

Fig. 4.3 Suction method applied for the filling of liquid crystal measuring cells, a Syringe attached to the cell via heat shrinkable tubings, b top view of the cell showing the enclosure with UV adhesive, and c side view of the cell...

Carter, J.W., Lamb, K.T., Cationic UV-adhesives laminate opaque fihn via dark cure mechanism (pp. 9 9099), Adhesives and Sealants Industry August 1999. Cationic UV-cure of Cyracure cycloaliphatic epoxides (pp. 1-40), Technical brochure from Union Carbide Corporation (now Dow Chemicals), 1999. 

Linevol Plasticizer alcohol. Shell Lite-Tak UV adhesives, Loctite Lo-Vol Silica flatting agents, PPG Lamod Copolyester elastomer, GE Lucite Methyl methacrylate, GE Lupersol Liquid organic peroxide. Elf Atochem... 

The cure time of UV adhesives is usually less than 60 s and depends on ... 

The curing time of EB adhesives is comparable to that of UV adhesives. In general, EB radiation allows adhesive curing to be achieved at greater depths than is possible with UV radiation. Electrons can pass through substrates that are opaque to UV light. In addition, the area of exposure and the depth of penetration can be controlled by means of the EB conditions. 

Many componnds have been studied over the last 30 years but the commonest photoinitiators in UV adhesives and coatings are ... 

Lamination applications are attractive new markets for UV adhesives, and lamination of flexible packaging films and DVDs are forecast to be significant growth areas. 

Ultraviolet (UV) adhesives are used if situations demand time to adjust parts followed by rapid curing. Applications include bonding plastic and metal trim to glass shelves in refrigerators. 

Wire-tacking adhesives - cyanoacrylates and UV acrylic adhesives can be used to tack wires in place. Cyanoacrylates are applied to components and then a topical activator is applied by brush or spray to cure the adhesive instantly. UV adhesives achieve the same result and give more heat-resistant bonds. 

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