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SterilTR
Industrial Technologies
SterilTR Industrial Technologies

Inks, Coatings and Adhesives

The early use of electron beam curing for coatings was stimulated by the work of Bill Burlant at the Ford Motor Company. In the early 1970s, Burlant showed that EB cured coatings on plastic components could be produced at 750 times the speed of conventional paint or coating application and drying techniques

 The early use of electron beam curing for coatings was stimulated by the work of Bill Burlant at the Ford Motor Company. In the early 1970s, Burlant showed that EB cured coatings on plastic components could be produced at 750 times the speed of conventional paint or coating application and drying techniques [69, 70, 198]. Figure 83 shows some of those automotive parts with coatings which were then cured by electron beam processing [75]. Although the interest at Ford never developed into a sustained commercial practice, EB curable coatings have found major market uses on a variety of substrates, such as paper, wood, metals and plastics.

 

An advantage to EB curing and crosslinking of coatings is that pigmentation does not interfere with the crosslinking process as it does with the use of ultraviolet radiation. Likewise, metallic pigments can be used. Figure 84 shows a pigmented metal coating that was cured with low-energy EB. Figure 85 shows a white EB cured coating that withstood 1000 hours in a salt spray test. Figure 86 shows that such coatings could tolerate subsequent bending as required for metal coil coatings [199, 200]. Figure 87 shows a pigmented coating that was vacuum coated onto tubular aluminum and cured using four low-energy EB units positioned around the rigid tube [201]. Figure 88 shows an array colors on metal test panels that were coated and then low-energy EB cured.
 
e-beam cured
 
The use of radiation curing for printing inks was pioneered by Dan Carlick of Sun Chemical (now part of Dai Nippon) [202]. Electron beams are more often used with wide-web presses for high volume production and for printed items that require outstanding graphics and color highlights. Electrons have the ability to penetrate pigments, whereas UV does not. EB ink formulations tend to be considerably less complex. Because electron beam processing is not a thermal means of energy transfer and takes place at near ambient temperatures, EB “drying” of inks can be used on heat sensitive substrates, such as plastic films, minimizing concerns over film distortion. Electrons generate free radicals in vinyl terminated monomers leading to double bond opening and polymerization. A balance of properties, especially for over-print materials, is attained by using oligomers which are terminated with acrylate functionality. Familiar ink and coating materials, such as polyesters, polyurethanes, epoxies and acrylates themselves are used in developing reactive oligomers which enhance the flexibility and other properties of the cured or crosslinked system. Since there are no extractable initiators used in EB curable inks and over-print systems, Sun Chemical has been able to develop over-print materials that are compliant with US Food & Drug Administration regulations for direct food contact [203]. Use of such systems could replace film laminates used atop printed materials to prevent leaching of extractables for compliance with direct food contact regulations.
 
The major suppliers of monomers and oligomers for inks, coatings and adhesives have addressed issues of toxicity, Clean Air Act compliance, food contact and a host of other areas of concern in contemporary industry. These have been driving factors in changing the portfolio of materials available to formulators over the past several decades. As noted above, the energy required to convert formulations of inks, coatings or adhesives using electron beams is significantly less than that to use alternative drying systems, even with what are called “high solids” content products. Besides eliminating volatile organic emissions (VOCs), EB curing also lessens potential greenhouse gas emissions. Table X compares an EB cured system with that of a high solids solvent system in which solvent incineration would be used to dispose of volatiles [200, 204].
 
 
Low-energy EB processing is used in making laminates of thin films or thin film overlays. Higher energy EB or even X-rays can be used to set the adhesive bonds between thicker substrates. Materials with vastly different coefficients of thermal expansion can be bonded with EB curable adhesives without creating the interfacial strain generated when using thermal curing. Pressure sensitive adhesives (PSAs) are also cured using EB. Beiersdorf and the 3M Company use this approach with low-energy electron beams. Formulated PSAs based on natural rubber or similar diene polymers EB crosslink at high product through-put rates [13]. Acrylic adhesives often based on butyl (C4) and 2-ethyl hexyl acrylate (C8) monomers and combinations thereof provide transparent adhesives. When using such monomers by themselves, attention must be given to dose-rate effects in order to avoid unwarranted chain termination [205, 206]. The propagation step of such in-situ polymerization/crosslinking reactions can be extended by extending the residence time under the EB unit [207].

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