Epoxy adhesives energy

Our experience indicates that fluorination generally results in decreased adhesion strength compared to oxyfluorination (see Tables 16.6 and 16.8). This is in contrast to reported findings in the literature where epoxy adhesives were used.7,29 This discrepancy can be attributed to the surface energy implications of the increased acyl fluoride to the C—H—F functionalkation ratio (compare Figures 16.3 and 16.4). However, as indicated by our data, certain resin formula-... 

Consider the case of an adhesive joint created by bonding two poly(vinyl chloride) (PVC, component A) sheets together with an epoxy adhesive (component B). The snrface energies for the PVC solid snrface, Uqnid epoxy, and PVC/epoxy interface are 47 erg/cm, 41.7 erg/cm, and 4.0 erg/cm, respectively. 

Weaver, F. Epoxy Adhesive Surface Energies Via The Pendant Drop Method, Air Force Materials Laboratory Report, AFWAL-TR-82-4179 (1982)... 

Water is sometimes used as a solvent for water-soluble resins. Certain epoxy adhesives are available as water-based emulsion or latex formulations. In the early 1970s, during the time of the petroleum crisis, water-based adhesives were thought of as a possible replacement for solvent-based adhesives systems. However, water-based adhesives never met the lofty expectations primarily because of the time and energy required to remove water from the bond line, the corrosion that the water causes in drying ovens, and the poor moisture resistance of cured water-based adhesives. 

Most common adhesive liquids readily wet clean metal surfaces, ceramic surfaces, and many high-energy polymeric surfaces. However, epoxy adhesives do not wet low-energy surfaces such as polyethylene and fluorocarbons. The fact that good wetting requires the adhesive to have a lower surface tension than the substrate explains why organic adhesives, such as epoxies, have excellent adhesion to metals, but offer weak adhesion on many untreated polymeric substrates, such as polyethylene, polypropylene, and the fluorocarbons. 

Since slower-curing epoxy adhesives systems flow over and wet high-energy surfaces very well, there is little chance for air to become trapped at the interface. As a result, mechanical abrasion is often recommended as a substrate surface treatment prior to application of the epoxy adhesive. The added surface area and the mechanical bonding provided by the additional peaks and valleys on the surface will enhance adhesive strength. If the adhesive does not wet the substrate surface well, such as in the case of epoxy resin on polyethylene, mechanical abrasion is not recommended since it will only encourage the probability of gas voids being trapped at the interface. 

The base epoxy resin can be either liquid or solid. As molecular weight increases, the epoxy equivalent weight and the number of hydroxyl groups available for reaction increase. Waterborne epoxy adhesives provide excellent adhesion to metals and other high-energy substrates. Modified waterborne epoxy adhesives can also provide good adhesion to substrates such as vinyl and flexible plastic film. Characteristics of these epoxy dispersions are summarized in Table 4.6. 

Epoxy adhesives that cure via radiation (ultraviolet light or electron beam energy)... 

The main sources of energy for curing epoxy adhesives by radiation are electron beam (EB) and ultraviolet light (uv). Both provide instantaneous curing of resins that polymerize from a liquid to a solid when irradiated. The uv systems account for approximately 85 percent of the market for radiant cured adhesives, EB systems account for about 10 percent, and the remainder are chiefly adhesives that can cure by exposure to both visible and infrared light. 

One of the major disadvantages of using structural epoxy adhesives is the cost and time required for long cure cycles. The number of fixtures, energy use, and production times (either at room temperature or in an oven) required have discouraged many from exploring epoxy adhesives as an alternative to mechanical fastening. 

These processes have an advantage in that the heat penetrates deeply into the joint and into the epoxy material itself. With conventional thermal energy processes, the heat must be conducted into the mass of the epoxy adhesive from outside the joint. This is hindered by the presence of the substrates, the substrate geometry, and the relatively low thermal conductivity of the epoxy itself. 

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. 

Several studies have shown that a microwave cure cycle can be developed that provides equivalent performance properties to a thermal cure cycle. Table 14.9 shows the processing and performance characteristics of three commercial one-component epoxy adhesives cured via microwave and conventional thermal energy. Certain commercial epoxy adhesives could contain a large number of bubbles due to volatiles present during the cure cycle and the fast rate of cure. Therefore, specifically formulated adhesives for microwave curing may be necessary to optimize performance. 

This chapter identifies and discusses various epoxy adhesives and the processes that have been used to successfully bond or seal specific substrates. There are only a few materials that epoxy adhesives will not bond well. These uncooperative substrates are most notably low-surface-energy plastics, such as the polyolefins, fluorocarbons, and silicones. However, even these materials can be bonded effectively with epoxy adhesives if a prebond surface treating process is used to change the nature of the substrate surface. Of the other substrate materials, there are some that epoxy adhesives will bond more effectively than others. Table 16.1 lists substrates that generally provide excellent epoxy adhesive joints. 

Formulation details are then presented in Chapters 11 through 14 for the various possible forms of epoxy adhesive systems room temperature and elevated-temperature curing liquids, pastes, and solids. The more or less unconventional forms of epoxy adhesives are also identified and discussed, since these are now achieving prominence in industry. These include uv and electron beam radiation curable, waterborne systems, and epoxy adhesives capable of curing via the indirect application of heat or energy. 

Epoxy adhesives get their name from the portion of the adhesive containing 1,2-epoxy, epoxide, or oxirane ring. This three-member ring consists of two carbon atoms joined to an oxygen atom. The highly strained geometry of this moiety with a strain energy of 114 kJ/mole accounts for its reactivity with many nucleophilic or electrophilic compounds. Typical... 

Certain aspects of the adsorption theory of adhesion are developed more fully than has been done previously. The consequences of nonreciprocity of spreading are pointed out, and are used to develop a more general practical point of view with respect to the adhesive bonding of materials of low-surface free energy. The system epoxy adhesive-(nonsurface-treated) polyethylene, normally considered nonadherent, is investigated experimentally in some detail. It is shown how this system, without material modification, can be made adherent. An area of study for possible adhesives for materials of low-surface free energy is suggested. 

These results indicate that a layer of undercured polymer can be formed when epoxy adhesives are used on high-energy substrates. When cementing surfaces with low surface energy, no such layer was observed, but in this case the achievement of high adhesion strength is hindered by poor substrate wetting by the adhesive. 

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