Epoxies/adhesives control system

Computer controlled systems have been developed that enable investigation of the spatial distribution of impedance within resins and in turn the identification of flaws (62). Water trapped within the composite will cause dielectric permittivity to increase, thus revealing its presence. Detection of voids, variation in the cross section of the adhesive layer, ingress of moisture into a joint structure, and characterization of cure in epoxy resin can be achieved with high frequency dielectric spectroscopy (10" -10 Hz) (63). 

Epoxy-polyamide adhesives can be made serviceable at very low temperatures by the addition of appropriate fillers to control thermal expansion. But the epoxy-based systems are not as attractive as some others because of brittleness and corresponding low peel and impact strength at cryogenic temperatures. 

Since epoxy formulations are generally good thermal insulators, the exotherm will depend on the mass of the system. A high rate of exotherm is needed with some epoxy adhesive systems to achieve practical curing rates. However, excessively high exothermic temperatures can result in bubble formation, thermal degradation, and even a potentially hazardous situation. Control of the exotherm is, therefore, a very important factor in formulating epoxy adhesives. 

Table 12.14 shows a comparison of dicyandiamide cured epoxy adhesives formulated with and without a CTBN adduct. When compared to the control epoxy, the toughened formulation exhibits significantly higher peel strength and moderately higher tensile shear strength. CTBN modified epoxy adhesives are generally one-part systems, cured with dicyandiamide at elevated temperature. 

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. 

In the epoxy adhesive industry, fillers are generally added as a batch process rather than a continuous process. Batch systems are especially suited to short-run operation in which additives and materials are often changed. They allow for greater control of residence time, shear, and temperature, and they can accommodate a feed rate that is not free-flowing. 

Potential exposure to the chemicals used in epoxy adhesive formulations varies with the type of process or task. Closed systems with engineering controls are used to prevent workers from overexposure. However, occasionally open areas with limited controls are encountered, and the potential for exposure increases. 

This chapter discusses quality control and the writing and use of specifications that apply to epoxy adhesive systems. These subjects are of interest to both the formulator and the end user. 

Quality control testing processes that are commonly used with epoxy adhesives are discussed in Chap. 20. Tests included are those used by the formulator as well as those employed by the end user of the adhesive system. 

Abbey, K. J., Pressley, M. W., and Durso, S. R., Controlled Cure of Thiol-Epoxy Systems, in Proceedings of the 22nd Annual Meeting of The Adhesion Society, Panama City Beach, FL, February 21-14, 1999, David R. Speth (Ed.). 

Fillers are often useful in epoxy-polyamide adhesives (41). Fillers aid flow control—particularly at elevated temperatures. Other properties contributed by fillers are exhibited in the cured system. For example, they can increase the modulus of elasticity, modify the coefficient of thermal expansion, increase heat resistance, and even affect bond strength. Of course, the excellent wetting properties of polyamides Increase the dispersion efficiency of these fillers. The type of filler selected and the amount used also have a pronounced effect on adhesive quality. A few of these effects are illustrated in Tables VI and VII (37). (See also Table VIII.)... 

Although epoxy resin and hardener may be used in unmodified form in adhesive systems, most systems will consist of components that have been modified by incorporation of various additives to achieve specific effects. Formulators will add catalysts or blend hardeners to obtain a specific usable life of the mix and to control the curing temperature. Reactive diluents may be added to modify viscosity or flexibility. Fillers impart improved compression strength and reduce shrinkage and cost. Solvents may be used to reduce viscosity or improve adhesion. Various additives may be added, usually at a low percentage, to reduce aeration, improve adhesion to difficult surfaces, or minimize settlement of fillers. Depending on the application, particular properties such as flame retardency. 

The 120 C cur ing epoxy paste adhesives on aluminum are covered in Fig. 10. In all cases, the sustained load is substantially less than 50% of the control strength and most of the adhesives show excellent durability by sustaining loads for close to 8 years. One interesting phenomenon is noted. The toughened system, EC-3455 shows much improved durability over the less crack resistant EC-2214 HD. Three... 

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