Epoxy resin paste adhesive

The major uses of BPA are in the production of polycarbonate resins (63%) and epoxy resins (27%). Polycarbonates have major outlets in automotive parts, compact discs, eyeglasses, and sheet and glazing applications, and have caused bisphenol A consumption to more than double during the past decade. Epoxy resins are two-component adhesives for very strong bonding. Miscellaneous uses include flame retardants (mostly tetrabromobisphenol A) and other polymer manufacture. Polycarbonate grade bisphenol A is >99% p,p isomer. The epoxy grade is 95% p,p. The p,p and o,p isomers can be separated by a combination of distillation and crystallization. 

Over the past several decades, significant advances have been made in developing epoxy-based adhesives having improved performance over these early adhesive systems. These improvements were made possible by (1) the incorporation of toughening additives into epoxy resin formulations and (2) the use of multifunctional epoxy resins primarily for high-temperature applications. These innovations are discussed in later chapters. 

These blends can take a number of different forms. The added resin may be reacted with the epoxy resin, or it may be included as an unreacted modifier. The modifier may be blended into a continuous phase with the epoxy resin (epoxy alloys) or precipitated out as a discrete phase within the epoxy resin matrix (as is generally done in the case of toughening modifiers). Epoxy hybrid adhesives are often used as film (supported and unsupported) or tape because of the ease with which formulated systems can be dissolved into solvent and applied to a carrier or deposited as a freestanding film. Some systems, notably epoxyurethanes and epoxy-poly sulfides, can be employed as a liquid or paste formulation because of the low-viscosity characteristics of the components. 

Solid shapes or powders are formulated from solid or liquid (when used as a B stage) epoxy resins and curing agent. Fillers, additives, and other modifiers are often used as they are with liquid or paste epoxy adhesive formulations. However, consideration must be given to the flow properties of the adhesive when heated as well as the application properties. 

Covering of reinforcing bars with resin, apphed as an anti-corrosion protection, can lower their adhesion to cement paste. Worsening of adhesion occurs in the case of vinyl polychloride, but it does not occirr if the epoxy resins are used. However, some epoxy resins reveal the creep effect [56]. 

The use of synthetic adhesives in the past twenty-five years (1) has grown/ particularly the use of eight classes of polymers polyvinyl acetate/ polyolefins/ styrenic block copolymerS/ acrylicS/ cyanoacrylates/ anaerobicS/ polyurethanes/ and epoxy resins. Some of these polymers are Still in high demand as specialty adhesives (2). During the last several yearS/ however / other polymers have been added to this list/ e.g. / polyamides/ polyimideS/ and polyesters. Today/ synthetic adhesives account for 75% of the adhesives produced and 85% of the sales/ while the market share of natural products has steadily declined. 

Adhesive KH-225 is a paste structural adhesive based on CTBN-toughened epoxy resin. It can be cured with EMI at a temperature such as 60 to 80 C under contact pressure. 

The use of adhesives/sealants in both the industrial and consumer spheres has increased dramatically in the past 20 years. In the industrial segment, both reactive and nonreactive systems are used in a wide variety of applications. The increasing use of reactive systems has, however, tended to overshadow that of the nonreactive systems. Most prominent amongst the reactive systems favored in industrial applications are anaerobic sealants (methacrylate ester based), instant adhesives (alkyl cyanoacrylate ester based), acrylic (toughened) adhesives, epoxy resin adhesives, polyurethane/isocyanate-based adhesives, silicone adhesives/sealants, and phenolic resin adhesives. 

A typical thermally conductive epoxy system used as an adhesive, as well as for other purposes, has a thermal conductivity of 0.0026 cal/cm/sec/°C and a volume resistivity of 1.5 x 10 ohm.cm (1.5 x 10 ohm.m). Fillers include alumina (aluminum oxide), beryllia (beryllium oxide), other unspecified inorganic oxides, boron nitride, and silica. Boron nitride is an excellent choice as a thermally conductive filler except that its content reaches a maximum at about 40% by weight in epoxy resins. The resultant products are always thixotropic pastes. BerylUa powder has excellent thermal conductivity by itself, but when mixed with a resin binder its conductivity drops drastically. It is also highly toxic and high in cost. Alumina is a commonly used filler to impart thermal conductivity in resins. ... 

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. ... 

These adhesives are products of reachon between an epoxy resin and liquid polysulfide polymer, usually catalyzed by an additional tertiary amine.They are available as two-part liquids or pastes that are usually cured at room or higher temperatures to rubbery solids that provide bonds with excellent flexibility and chemical resistance. Epoxy-polysulfide adhesive forms satisfactory bonds to different substrates. Shear strength and elevated-temperature properties are low, but resistance to peel and low temperature is acceptable. ... 

The benefit of such an approach is that a dispersion of the precipitated polymer can be made in liquid epoxy resin, which can then be used as a formulating ingredient in its own right in, for instance, paste adhesives. 

Reactive liquid polymers (RLPs) The Uquid rubbers most often used in epoxy adhesives are those based on acrylonitrile-butadiene copolymers or on long-chain polyethers. To act as effective tougheners, however, they cannot be directly added to the formulation but have to be prereacted with some of the epoxy resin, or sometimes with the amino hardeners in two component paste adhesives (adduct fonnatiou). To achieve this, only RLPs with reactive eud groups are used by the formulator. 

---