Electron-beam-cured materials applications

It has been shown that in many cases for thermoset systems, there is a quantitative relationship between the chemical conversion of the thermoset and its Tg value, independent of the time-temperature cure history (111). This is very convenient from an applications standpoint because measurement of Tg is equivalent to a direct measurement of conversion. It implies that either the molecular structure at a given degree of conversion is the same, regardless of the reaction path, or that differences in the structures produced for different reaction paths do not affect Tg. While a Tg-degree of cure relationship has been found valid for many epoxy-amine and other thermoset materials, it has been observed not to hold in some cases, which include some epoxy-DICY, cyanate-ester, and phenolic systems. It has also been found not to hold for a given resin system cured by two different methods such as thermal and microwave (112) or thermal and electron beam cures (113). 

Abstract In this chapter, classification of adhesive and sealant materials is presented. For this purpose, various categories are considered depending on the polymer base (i.e., natural or synthetic), functionality in the polymer backbone (i.e., thermoplastic or thermoset), physical forms (i.e., one or multiple components, films), chemical families (i.e., epoxy, silicon), functional types (i.e., structural, hot melt, pressure sensitive, water-base, ultraviolet/ electron beam cured, conductive, etc.), and methods of application. The classification covers high-temperature adhesives, sealants, conductive adhesives, nanocomposite adhesives, primers, solvent-activated adhesives, water-activated adhesives, and hybrid adhesives. 

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. ... 

Cationic cured epoxies may also be crosslinked by electron beam radiation. A major application for this technology is the repair of composite aerospace structures. Direct benefits of EB processing include rapid cure, allowing completion of a permanent repair in the same or less time than a traditionally temporary repair, and ease of material handling. Other... 

Radiation can induce certain desired chemical reactions. It can, for example, be used in the making of plastic, or to graft plastic to other materials. Some polymers whose cross-linkage is induced by radiation can be tailored to shrink when heated—a desirable property in some packaging applications. The wood and printing industries make extensive use of electron-beam radiation to cure surface coatings. 

Cleland et al. (2003) reviewed the application of electron beams with various materials however, little process modelling has been developed due to the low industrial usage of electron-beam processing for reactive polymers. However, many researchers are examining cure models for promising materials. 

Radiation-curable coatings (RC) should enjoy a dramatic growth as the full extent of their solvent-free, rapid, and ambient-curing properties is realized and materials and application systems are perfected. UV-curable coatings are currently the most important in this category however, electron beam (EB) curable coatings should show excellent growth to 1987 and beyond. 

Electron beam induced reactions continue to grow in importance. This paper provides an introductory treatment of the equipment and materials options, including a mechanistic and kinetic view of the pertinent chemistry. Specific coverage inciudes several applications in polymer science, especially in curing of coatings. 

The effect of radiation on materials has importance in the areas of wire and cable insulation, heat-shrinkable articles, curing of elastomers, plastics, paints and inks, electron beam lithography, medical sterilization, polymer property control, and outer space applications. 

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. 

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. 

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