Processing of Epoxy Adhesives

Epoxy adhesives have achieved their commercial success due in no small way to their processing capabilities. Epoxy chemistry is compatible with (1) a variety of formulating techniques and processes accessible to the adhesive manufacturer and (2) a variety of application and curing methods accessible to the end user. 

Epoxy resins are not ideal adhesives in their natural form so they are mixed with materials to improve and enhance their properties and, thereby, make them more useful in a variety of applications. This process is called compounding or formulating. Compounding is the combining of a base epoxy resin with curing agents, modifiers, additives, reinforcement, fillers, and other polymers to make the base polymer perform better, cost less, and process more easily. 

Under this broad definition there are two groups that perform compounding (1) the suppliers of the base resins or adhesives and (2) the end users of the adhesive system. 

Most often the compounding is done by the formulators who are specialists in compounding methods and equipment. They can marry a set of specifications to a formula and provide the product at lowest cost. These independent compounders can be subdivided into two main groups the proprietary compounder and the custom compounder. Proprietary compounders compound and sell their own special formulations, and custom compounders do compounding for someone else. 

Adhesive compounding also occurs at the end-user level. This is generally done when the end user cannot find a product to meet his or her individual needs, to reduce cost associated with a middleman, or to make use of other materials (e.g., scrap resin, fillers, etc.) that are generated during the course of business. Epoxy adhesives are more conducive to the end user or in-house processor than other polymeric adhesives because of the ease with which the liquid resins can be mixed with other ingredients. Many of these operators buy raw materials from various suppliers and do their own blending and formulating. 

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The silicon substrate is removed while the temporary substrate provides mechanical support. Next, the substrate is bonded to a final carrier substrate 54 with a layer 52 of epoxy adhesive. The bonding layer 48 is removed, which also removes the temporary carrier substrate 50. The final carrier substrate is chosen according to the same criteria as described in the first embodiment. Processing continues by forming indium interconnects 34b for later hybridization of the read-out integrated circuit assembly with a detector array. 

The properties of epoxy adhesives in their uncured condition will determine primarily how easily the adhesive can be processed, applied, and cured. They will also determine, to some extent, the performance characteristics of the cured joint. The properties of the individual components as well as that for the mixed formulation are important. 

Diluents are higher-MW components than solvents that are also added to the epoxy adhesive formulation to lower the viscosity and modify processing conditions. The primary function of a diluent in an epoxy resin formulation is to reduce its viscosity to make it easier to compound with fillers, to improve filler loading capacity, or to improve application properties. Solvents, certain curing agents, and flexibilized epoxy resins can also lower the viscosity of epoxy adhesive formulations, but this is not their primary function. The effect of various diluents on the initial viscosity of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin is illustrated in Fig. 6.3. 

The moisture resistance properties of filled molding compounds are enhanced by the treatment of the fillers with silane adhesion promoters prior to compounding. Silane promoters on wollastonite fillers in thermoplastic polyester molding compounds (50 percent filled) will improve the flexural strength after 16 h in 50°C water by as much as 40 percent. Silane-treated silica fillers have been found to significantly increase the moisture resistance of epoxy adhesives used in the electronics industry for chip, surface-mounted, and printed-circuit processes.8... 

The primary disadvantage of these processes is that they are relatively inefficient, mainly because the entire joint must be heated to cure only several mils of epoxy adhesives. The energy consumed, the time to get up to temperature, and the time to cool down to a safe handling temperature can be prohibitive in many production applications. 

Optimal joints can generally be fabricated by the correct combination of epoxy adhesive formulation (i.e., type of resin, curing agent, fillers, modifiers) and surface treatment process. 

This chapter certainly does not consider all possible substrates. However, the guidance that is offered should be sufficient for the user to select candidate joining processes and epoxy adhesive materials, no matter what substrate or combination of substrates is involved. 

The following section looks at the processes and equipment commonly used in the compounding of epoxy adhesive systems. Since they can be utilized by either the formulator or the end user, there will be no strict division of the discussion by user. Hazards and safety issues related to these materials and processes are discussed somewhat in this chapter, but a more thorough discussion is found in Chap. 18. Test methods, quality control processes, and standards that are commonly used at the formulation level are addressed in Chaps. 19 and 20. 

Just as there are many formulations, there are many ways to compound and process the requisite raw materials into an adhesive formulation. The methods chosen and how they are used also influence the final physical properties of the adhesive. The important processes to be considered in compounding and use of epoxy adhesives can be listed as follows ... 

Several of the more common processes that occur with the formulation and end use of epoxy adhesives are described in Table 18.3. Potential exposure risks for these processes are characterized as dermal, inhalation, or ingestion. Comments regarding the effect of the process on potential exposure are also included. 

TABLE 18.3 Exposure Potential of Epoxy Adhesive Production and Application Processes... 

Chapters 17 through 20 describe the various processes and equipment employed in the formulation or end use of epoxy adhesives. Health and safety issues regarding the use of these materials are discussed. The importance of quality control methodologies and specification preparation is also noted. Finally, test methods that are commonly used by both the formulator and the end user are identified. 

Pike, R. A., Lamm, F. P., Pinto, J. P. Factors affecting the processing of epoxy film adhesives 1. Room temperature aging, J, Adhesion, 12, 143 (1981)... 

Consequently, this kind of reaction is not based on two monomer molecules A and B of completely different structure, as in the case of epoxy resin and polyurethane adhesives, but on monomers of the same kind or, at least as far as the C=C double bonds are concerned, of similar monomers. The double bond is therefore the precondition for the curing process of acrylate adhesives. 

Analytical methods to probe adhesion have also advanced. Dynamic mechanical analysis methods have been found especially useful in investigating the cure process of epoxy resins and in research on pressure-sensitive formulations. Widely used is torsional braid analysis Surface investigation avails itself of electron spectroscopy for chemical analysis (ESCA). Other investigative approaches to following the cure of epoxies include dielectric spectroscopy and viscosity-dependent fluorescent probe. 

Titanium alkoxides are used for the hardening and cross-linking of epoxy, siUcon, urea, melamine, and terephthalate resins in the manufacture of noncorrodable, high temperature lacquers in the sol-gel process as water repellents and adhesive agents (especially with foils) to improve glass surfaces as catalyst in olefin polymeri2ation, and for condensation and esterification. 

The thermal stabiUty of epoxy phenol—novolak resins is useful in adhesives, stmctural and electrical laminates, coatings, castings, and encapsulations for elevated temperature service (Table 3). Filament-wound pipe and storage tanks, liners for pumps and other chemical process equipment, and corrosion-resistant coatings are typical appHcations using the chemically resistant properties of epoxy novolak resins. 

The kinetics of reaction of free radical chain reactions are complicated compared to the second-order kinetics of epoxy and urethane adhesives. Many of these complications offer practical advantages to the process of using acrylic adhesives. 

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