Ultraviolet-light-curing adhesives

II. ULTRAVIOLET LIGHT CURING - AEROBIC ACRYLIC ADHESIVES... 

The advantages of "aerobic" acrylic adhesives are retained, and in a few cases, improved upon in their ultraviolet light curing analogues. 

ASTM D3658-01 Standard practice for determining the torque strength of ultraviolet light-cured glass/metal adhesive joints. 

Another bonding method which allows rapid assembly and gradual development of bond strength hinges on a combination of anaerobic and ultraviolet light curing.In the attachment of small electronic components to printed circuit boards, drops of adhesive are dispensed onto the board. Each component is then placed on top of the drops so as to form a fillet around... 

Nearly 42% of the demand for all adhesives comes from the packaging sector. Radiation-curable adhesives are used primarily for packaging, with paper and paperboard the dominant materials used in the packaging. Radiation-curable adhesives can be used on glass, metal and some plastic materials. Other applications for radiation-curable adhesives are in healthcare, electronics, communications, pressure-sensitive tape and consumer applications. Ultraviolet (UV)-curable adhesives are best suited to small-scale applications, while electron beam (EB)-curable adhesives are more appropriate in high-volume applications (an EB system has a higher installation cost). One additional characteristic of EB-curable adhesives is that they can cure the area between two substrates. UV light-cured adhesives can also be applied on heat-sensitive substrates and are not affected by ambient temperature or humidity. 

Until recently, only long wave ultraviolet light curing systems (320 - 400 nm) were used. The first of these was a pit and fissure sealant which was reported in 1970 (1). This was followed by a composite restorative in 1973 (2). Shortly thereafter, this material was slightly modified for use as an orthodontic bracket adhesive. 

Binders and Resins. The choice of binder is the most important ingredient choice in the formulation process because the binder affects the performance properties of a paint more than any other single ingredient (3). The physical properties of binders required for paints include the abiHty to dry or cure under various ambient conditions, good adhesion to various substrates, abrasion resistance, washabiHty, flexibiHty, water resistance, and ultraviolet light resistance. The balance of these required properties is mosdy dependent on whether the paint is being developed for interior or exterior appHcations. 

Epoxy adhesives that cure via radiation (ultraviolet light or electron beam energy)... 

The main sources of energy for curing epoxy adhesives by radiation are electron beam (EB) and ultraviolet light (uv). Both provide instantaneous curing of resins that polymerize from a liquid to a solid when irradiated. The uv systems account for approximately 85 percent of the market for radiant cured adhesives, EB systems account for about 10 percent, and the remainder are chiefly adhesives that can cure by exposure to both visible and infrared light. 

One very interesting new application of Lewis acids in curing epoxy adhesives has appeared within the last 25 years. The Lewis acid initiator for the cationic polymerization is formed by the heat or ultraviolet light-induced decomposition of Lewis acid Lewis base salts. Several patents by Crivello and coworkers43 47 describe compounds containing... 

Cyanoacrylates are not appropriate for the bonding of the steel parts of an automobile, because of the environments that the car will be exposed to. Those environments include such things as rain, variations in temperatnre, exposure to solvents (such as gasoline, oil, and windshield washer solntion), ozone, acid rain, salt spray, and ultraviolet light from the Sun. (A more appropriate adhesive for car parts would be an epoxy-based adhesive.) Another example of a special adhesive would be the one used to attach a new rearview mirror in an antomobile. Because the cured adhesive in this case will be exposed to wide variations in temperature and to an extremely large amonnt of nltraviolet fight from the Snn for prolonged periods of time, an adhesive formnlated specifically for these conditions should be used. 

One of the newer developments in adhesives is the growing use of ultraviolet light or electron beam radiation to cure adhesives. Adhesives designed for UV- or E-beam curing are usually pressure sensitive or hot-melt systems based on acrylates, functional rubbers, or epoxidized rubbers, and use special UV or EB lamps to provide the cure. These systems can provide greatiy improved heat resistance compared to hot melts, and avoid the soivent emission problems of some of the solvent-based systems with which they compete. 

The papers presented in the following chapters represent advances in pressure sensitive adhesives (ultraviolet light activated acrylate monomer - low Tg polyether formulations) photoinitiated cationic polymerization (light activated aryliodonium and arylsulfonium salts of lewis acids in epoxy resin formulations) polymer and formulation design criteria for radiation curable adhesives radiation curable composites (dynamic thermal analysis characterization of electron beam cured... 

Aerobic acrylic adhesive technology has proven itself remarkably well suited to cures effected by ultraviolet light. Aerobic acrylics cured by UV light have the potential for vastly broadening the market for acrylic adhesives. 

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