Radiation-cured materials, adhesive applications

The adhesive was developed for application to seal food containers with alumina foils and to seal glass plates. In this case, the vinyl polymerization step can be carried out by UV light radiation through the already mounted glass plates with a photoactive radical initiator to cure the viscous adhesive material. Another application of this material is the development of diffusion barrier containing protective coatings on medieval stained glasses (31). 

Radiation cure adhesives are beconlng Increasingly Important for structural material applications. In order to obtain optimum performance and process efficiency, It Is necessary to analyze these materials using several techniques. Thin film applications have been successfully characterized by traditional methods such as Infrared spectroscopy and thermal analysis. This Investigation Includes comparison of traditional methods and mechanical spectroscopy for characterization of structural adhesive applications. In addition, mechanical spectroscopy provides viscoelastic data dependent on structure property relationships. 

During the course of this program, experimental studies were conducted to identify a number of polymer types in formulations which can be cured via radiation for adhesive applications. Performance studies were also made to determine endurance to exposures of various environmental conditions including moisture, solvents and high temperature aging. Data have been developed on a number of good candidate materials from both commercially available products and laboratory experimental... 

Silicone adhesives cure without the application of heat or pressure to form permanently flexible silicone rubber. The rubber remains flexible despite the exposure to high or low temperatures, weather, moisture, oxygen, ozone, or UV radiation. This makes them useful for joining and sealing joints in which considerable movement can be expected, such as intermediate layers between plastics and other materials of construction (e.g., acryUc glazing). Several types of silicone adhesives/sealants are available, including one-part and two-part systems. 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

The largest application of radiation-cured adhesives is for dental procedures (see Adhesion in dentistry restorative materials). Originally UV curable, these have given way to... 

The primary limitation of UV systems is that the material to be cured must see suitable radiation. This means that, in an adhesive application, one substrate must transmit appropriate UV or visible light. Also, assemblies with shaded areas will require a secondary cure mechanism. 

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. 

In the past, electron beam radiation was applied to produce PSA exclusively however, recent improvements in UV curing technology (precise UV dose control, suitable photoinitiators) permit UV to be used to produce pressure-sensitive adhesives. PSA formulations can vary in consistency from low-viscosity liquids up to solids melting at 80°C (176°F). Therefore, applications may vary from screen printing to roll coating to melt extrusion. Coat weights for most PSA materials vary from 1 to 10 g/m. ... 

Composites Both UV and EB cures are employed for the production of wood composite materials and in fiber-reinforced composites for aircraft and aerospace applications. The EB technology has been successful in the manufacture of large structures that exceed the size of autoclaves, and in curing adhesive joints in cases where uniform radiation can be provided more easily than uniform heat. In industrial and consumer applications, multiple combinations of different reinforcing fibers can be co-cured in one cycle by EB with considerably lower residual stresses than those introduced by thermal cure.16... 

Monomers are primarily used to lower the viscosity of the uncured material to facilitate application. The monomer must be matched with the resin to give the desired set of properties with respect to adhesion to the substrate and bulk properties such as flexibility, stiffness, cure behavior, and durability. Early radiation curable monomers had problems associated with toxicity and skin sensitivity newly developed monomers have been significantly improved in this respect. 

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