Epoxy adhesives preforms

First use of electrically conductive epoxies Premixed, frozen adhesives introduced by Ablestik Laboratories, Epoxy Technology, and Amicon Reduced ion content epoxies and polyimides Introduction of epoxy preforms First high-purity RTV silicone adhesives from G.E. and Dow Coming... 

Similarly prepared solid epoxy films can be ground into powder. The powder can be pressed into preformed pellets and applied directly to the adhesive joint as annular rings or... 

There are several common forms of solid epoxy adhesives. These include film, tape, powder, and preformed shapes. Certain formulations are better suited for specific forms. For example, casting of tape or film adhesive from solvent solutions lends itself to working with multicomponent hybrid systems, where each resin can be solubilized and blended together in a universal solvent. B-staged systems are generally more brittle and better suited for powders or preformed adhesives. 

Solid epoxy adhesives may also be fused or compacted into various shapes or preforms (see Fig. 13.2) such as sticks, rings, and beads.5 They can then be easily applied to a specifically shaped part. Similar to thermoplastic hot-melt adhesives, these thermosetting epoxy adhesives flow on the substrate by heating. However, the product will cure to an infusible thermoset stage with continued heating. Once cured, the product demonstrates properties similar to those of other structural epoxy adhesives. These solid epoxy adhesives are sometimes referred to as thermosetting hot melts. 

Preformed sticks or tubes of solid epoxy may also be applied much as a solder is.6 To apply, the area to be bonded is heated until sufficiently hot to cause the stick solder to flow when drawn across the surface. After application, the adhesive is cured by conventional thermal techniques. If two substrates are to be bonded, both must be preheated prior to... 

Solid epoxy adhesives, such as powder, preforms, or film, are the least toxic because they have very low vapor pressures. However, they still can present a hazard when they are heated to a liquid and more volatile state to cure. 

One-part epoxy adhesives include solvent-free liquid resins, solutions in solvent, liquid resin pastes, fusible powders, sticks, pellets and paste, supported and unsupported films, and preformed shapes to fit a particular joint. Two-part epoxy adhesives are usually comprised of the resin and the curing agent, which are mixed just prior to use. The components may be liquids, putties, or liquid and hardener powder. They may also contain plasticizers, reactive diluents, fillers, and resinous modifiers. The processing conditions are determined by the curing agent employed. In general, two-part systems are mixed, applied within the recommended pot life (a few minutes to several hours), and cured at room temperature for up to 24 hours, or at elevated temperatures to reduce the cure time. Typical cure conditions range from 3 hours at 60°C to 20 minutes at 100 C. ... 

Chapter 7. A preform can be initially impregnated with an epoxy to provide some interfilament adhesion and then impregnated with a pitch [59]. 

Preformed particles are incorporated into the epoxy matrix by simple mechanical mixing. The dispersibility of the particles can be improved by 1) introducing crosslinking into the shell or 2) using comonomer-like acrylonitrile or GMA, which increases the interfacial adhesion by polar or chemical interaction [96, 97]. Quan and co-workers [98] reported that for poly (butadiene-co-styrene) core poly (methyl methacrylate) (PMMA) shell particles, the cluster size reduces from 3-5 pm to 1-3 pm as a result of using 5 wt% crosslinker (divinyl benzene). They also found that the cluster size could be further reduced to 1-2 pm by using a methyl methacrylate-acrylonitrile (MMA-AN) or methyl methacrylate-glycidyl methacrylate (MMA-GMA) copolymer shell composition. 

Elastomer-modified epoxy resins are used in composites and structural adhesives, coatings, and electronic applications. Similar approach to toughen epoxy vinyl esters using other elastomeric materials has been reported (204). Other elastomer-modified epoxies include epoxy-terminated urethane prepolymers, epoxy-terminated polysulfide, epoxy-acrylated urethane, and epoxidized polybutadiene. Preformed dispersions of epoxy-insoluble elastomers have been developed and reported to achieve toughening without Tg reduction (205,206). 

Polyimides became popular for die-bonding adhesives because they are cleaner (in terms of ionic contaminants) than equivalent epoxy products. The market at present, however, is dominated by high-purity epoxy adhesives. Polyimides are always applied from solvent solution and require higher curing temperatures than epoxies. They are stable to higher temperatures. Solvent-borne thermoplastics pastes (or cast-film preforms) have been used for some lower reliability die-attachment applications. 

Another way of automating assembly is to use the epoxy adhesive in the front of a silver filled or oxide filled tape. These tapes are made by impregnating the epoxy adhesive into a glass mesh support, B-staging to form a tack-free film, and then slitting the tape into rolls or diecutting the tape into custom-made preforms. In production, the user places a preform under the part to be bonded, or uses a machine to chop or punch preforms from a roll of tape. In either case, the tape provides precise control of bond thickness, eliminates voids, and avoids an excessive fillet. The tapes can be cured very quickly, at temperatures over 160°C, and the controlled thickness of the bond reduces thermal stresses due to thermal expansion mismatch between bonded part and the substrate. 

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