Curable adhesive

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

One key consideration in developing radiation curable adhesive systems is the thermal stability and volatility of any photoinitiators used. These chemicals are designed for liquid systems where these issues do not arise. Few of the commercial photoinitiators have adequate thermal stability at the highest hot melt temperatures (180-200°C) and many are too volatile. Reduced application temperatures and special antioxidant packages are often required. 

Over the years there have been several studies examining electron-curable adhesives [9-12]. Off-the-shelf acrylate adhesives were the primary focus of the studies. These adhesives, that have potential use for the repair of advanced composites using high-energy electron accelerators, offer several advantages over conventional repair systems, including [6] ... 

Table 3 lists the selected properties [16] that we have measured for several commercially available acrylate resins manufactured by the Sartomer Company and the Rohm and Haas Company. The resins were cured in an AECL Gammacell Model 240. The temperature rise was measured for an 8-g sample using Acsion s (formerly AECL Radiation Applications Branch) Gamma Calorimetry method [17]. All of this information is being used to evaluate the applicability of EB-cured acrylate adhesives for repairing composite structures. Combinations of these adhesives can be used to create electron-curable adhesives suitable for composite repair. 

Loctite has developed several acrylate adhesives that can be used as EB-curable adhesives [12]. Some of the resins were cured and tested for EB curing properties... 

The second observation is that the EB-curable adhesive resins show no change in adhesive properties from liquid helium temperatures up to temperatures Just below their service temperatures. This can be seen for the adhesives, EB2000 and 1 1L. At this time there is no explanation for this behavior. 

Of the commercially available EB-curable adhesives [9-12], the resins fall within one of two categories based on their curing mechanisms. The majority of EB-curable resins are based on (meth)acrylate-functionalized oligomers involving a free-radical curing mechanism. The second category is the epoxy resins that cure by a cationic mechanism. 

All of the types of repairs described can be accomplished using electron/X-ray curing and suitable electron-curable adhesive systems. The advantages ol using an electron accelerator are faster curing cycles, short turn-around time, and higher-temperature-resistant bonds, cured at ambient temperatures. 

For most electron-curable adhesives, a dose of 100 kGy is required to cure. Fig. 4 shows the speed of curing various widths of material as a function of accelerator power. A reasonably controllable speed for moving parts through an accelerator would be approximately 10 m per minute. Based on this criterion, the lower wattage limit for an accelerator curing an adhesive strip 5 cm wide would be 5 kW. For curing very large parts a 50 kW accelerator would be more suitable. 

Lopata, V.J., Chung, M., McDougall, TE. and Weinberg, V.A., Electron-curable adhesives for high-performance structures. 39 Int. SAMPE Symp., Apr. 11-14, 1994. 

Woods, J.G., Radiation-curable adhesives. In Pappas, S.P. (Ed.), Radiation Curing, Science and Technology. Plenum Press, New York, 1992. 

Fig. 8.5 Epoxy curing through resistive heating of nanocarbons dispersed in the matrix (a) shows a schematic representation of the process (b) experimentally obtained curing cycle and (c) repair of a structural composite panel using the conductive epoxy as resistively curable adhesive [36]. With kind permission from Elsevier (2013).

There are several interesting applications reported in the literature, such as insulated wire and cable, UV cross-linking of drawn fibers, and tapes from ultra-high-molecular-weight polyethylene. Semi-interpenetrating networks (IPNs) from acrylates and polyurethanes are suitable as UV curable adhesives with high elasticity, good impact resistance, and excellent adhesion to a variety of substrates. ... 

As pointed out above, radiation or energy curable adhesives can be essentially hot melts or liquids at the room temperature. For this technology, a liquid curable PSA formulation is comprised of four essential components ... 

UV curable adhesives have been used in electronics and electrical manufacturing operations for many years. There are many uses, such as bonding of coils, bonding of loudspeaker membranes, bonding and sealing of wires in ducfs, bonding of liquid crystal displays, and bonding of membrane swifches. ... 

In the medical field, UV curable adhesives are used in fhe assembly of medical devices, such as syringes, valves, manifolds for filtering equipmenf, and arteriographs. In these applications, many dissimilar substrates, such as stainless steel, aluminum, glass, polycarbonate, polymethyl methacrylate (PMMA), PVC, and other thermoplastics, are bonded. ... 

Since the optical fibers are placed info locations fhat are difficult to access, the coatings are expected to last over 20 years. Tests indicated that UV cured protective coatings would be mechanically stable up to 100 years under ambient conditions. Radiation curable adhesives can also be used for end-to-end splicing, termination of bundles, construction of optical sensors, and other areas in the optical field. ... 

The development work also yielded UV curable adhesives for aluminum to aluminum, graphite to aluminum, and graphite to graphite. 

In 1995, U.S. consumption of radiation cured products was 77 million lb, valued at 450 million.1 Growth is forecast to average about 7 percent per year—a rate about twice that of conventional thermal cured products. Although radiation curable adhesives comprise a relatively small segment of the overall adhesive market (13 percent) and epoxy adhesives represent an even smaller component, epoxy systems are a fast-growing part of the market. 

Market penetration is expected to increase further due to stricter environmental regulations and the availability of a greater variety of products. The first radiation curable adhesives were limited to acrylate and epoxy resins. Today, many different types of radiation curable adhesive systems are commercially available. 

The two most prominent liquid radiation curable adhesives are free radical polymerization epoxy acrylates and cationic polymerization epoxies. Such adhesives are generally used as polymerizable syrups. A wide range of prepolymers can be acrylated including epoxies, urethanes, polyesters, polyethers, and rubbers. Elastomer-tackifying resin blends are often used in these formulations. 

Epoxy acrylates are dominant oligomers in the radiation curable adhesives market. A bisphenol A epoxy resin is reacted with acrylic acid or methacrylate acid to provide unsaturated terminal reactive groups. The acrylic acid-epoxy reaction to make bisphenol A diacrylate destroys any free ingredients such as epichlorohydrin used to make the DGEBA epoxy starting raw material. 

Some of the uv curable adhesives contain a combination of uv and infrared (ir) initiators to take advantage of the ir output that many uv lamps generate. At times a photoactive crosslinking agent is used to improve cohesive strength without affecting tack and peel. 

Pigments generally inhibit uv curing to some degree since the pigments absorb and/or scatter uv radiation. This interferes with the ability of the photoinitiator to absorb the light energy required to initiate the polymerization reactions. Thus, the majority of commercial radiation curable adhesives are clear or contain silica. 

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