Epoxy adhesives description

Adhesives commonly used on thermosetting materials include epoxies, urethanes, cyanoacrylates, thermosetting acrylics, and a variety of nonstructural adhesive systems. The following discussion includes a very brief description of various thermosetting substrate materials, the properties that are critical relative to epoxy adhesion, and any special processes that should be noted for the particular substrate. 

Among the various structural adhesives commercially available today, epoxies are the most widely accepted and used. A recent market study showed that epoxy structural adhesives account for approximately 41% of all structural adhesives sold, and it is expected that they will maintain this market share over at least the next several years. Numerous references exist, both in the patent and journal literature, describing every facet of these adhesives. It is not the intention to review all of the available information. Rather, this chapter will concentrate on three main areas (1) A general description of epoxy adhesives (2) A discussion of commercially available raw materials, formulations, properties, and uses of epoxy adhesives (3) A review of developments in epoxy adhesives since 1979. Earlier work has been discussed in several reviews, and will not be covered in detail in this chapter. "... 

However, it is also necessary to discuss how broadband bulk, shear and Poisson s ratio are measured. The measurement of the broadband shear modulus is easily accomplished using the time-temperature-superposition-principle (TTSP) and a torsion test. See Kenner, Knauss and Chai (1982) for a description of a simple torsiometer and the measurement of a master curve for a structural epoxy adhesive, FM-73, at 20.5° C. 

An adhesive may be defined as a material which when applied to surfaces of materials can join them together and resist separation. Adhesive is the general term and includes cement, glue, paste, etc. and these terms are all used essentially interchangeably. Various descriptive adjectives are often applied to indicate certain characteristics. For example, to indicate the physical form of the adhesive, e.g. liquid adhesive, liquid two-part adhesive, film adhesive its chemical form, e.g. epoxy adhesive, cyanoacrylate adhesive, polychloroprene adhesive to indicate the type of materials bonded, e.g. metahto-metal adhesive, paper adhesive, wood adhesive or to show the conditions of use, e.g. solvent based adhesive, cold-hardening, or -curing, adhesive, hot-melt adhesive. 

Latexes are usually copolymer systems of two or more monomers, and their total solids content, including polymers, emulsifiers, stabilizers etc. is 40-50% by mass. Most commercially available polymer latexes are based on elastomeric and thermoplastic polymers which form continuous polymer films when dried [88]. The major types of latexes include styrene-butadiene rubber (SBR), ethylene vinyl acetate (EVA), polyacrylic ester (PAE) and epoxy resin (EP) which are available both as emulsions and redispersible powders. They are widely used for bridge deck overlays and patching, as adhesives, and integral waterproofers. A brief description of the main types in current use is as follows [87]. 

The most common thermoset adhesives are epoxies, phenoUcs and thermoset polyurethanes. The most widely used thermoplastic adhesives are acrylics (including anaerobics, hot melts, cyanoacrylates) and thermoplastic polyurethanes. A brief description of some adhesives is given in the EUROCOMP Handbook, 5.3.4. 

Most of the adhesive families have either a thermoset or thermoplastic base. This is also the primary and the most traditional way of categorising adhesives, although within some adhesive families, such as polyurethanes, both thermoset and thermoplastic adhesives may be found. Thermoset adhesives form bonds that are essentially infusible and insoluble after curing and they typically have a much higher load-bearing capability than thermoplastic adhesives. Thermoplastic adhesives are fusible, soluble, soften when heated and their creep resistance is lower than that of the thermoset adhesives. The most common thermoset adhesives are epoxies, phenolics and polyurethanes, while the most widely used thermoplastic adhesives include acrylics (including anaerobics, hot melts and cyanoacrylates) and thermoplastic polyurethanes. A brief description of these adhesives (both thermoset and thermoplastic) is given below from reference 5.20 and 5.28. 

Reference has been made above to the work by Ashcroft etal. (2001) on aging joints in natural and laboratory conditions. Their program included exposing stressed joints, and their results for joints in a natural hot/wet climate after 6 years are shown in Fig. 31.9 (see Table 31.1 for further description of the adhesives used in this study). It can be seen that all stressed joints with the epoxy polyamide (E-P) and one of the modified epoxy (ME120k) adhesives are reduced to zero strength, whilst most of the others maintain a significant fraction of their strength. 

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