Epoxy-phenolic adhesive formulation

Epoxy-phenolic adhesive was the first true high-temperature adhesive. It was developed in the early 1950s as a high-temperature aircraft adhesive. An example formulation is provided in Table 13.4, but these adhesives are discussed predominantly in Chap. 15. 

The outstanding performance characteristics of the resins are conveyed by the bisphenol A moiety (toughness, rigidity, and elevated temperature performance), the ether linkages (chemical resistance), and the hydroxyl and epoxy groups (adhesive properties and formulation latitude, or reactivity with a wide variety of chemical curing agents) (see also Phenolic resins). 

Properties of an epoxy system filled with glass microballoons are shown in Table 9.11. Other low-density microballoons, based on phenolic and other materials, have been developed for use in epoxy adhesive formulations. 

The degree of tensile strength improvement is often in the 50 to 100 percent range. The effect of various fillers and loading ratios on the strength properties of epoxy adhesive formulation is indicated in Fig. 9.9. The effect of different fillers loaded at a constant 100 pph is indicated in Table 9.12 for shear strength on phenolic laminate and aluminum substrates. 

A significant advantage of tape and film adhesives is the greater toughness that is available compared to other adhesive types. This is primarily due to the ease with which resinous modifiers can be added to the formulation via solvent solution. Thus, hybrid epoxy adhesives such as epoxy-nylon, epoxy-phenolic, etc., are often found in tape or film form. 

The base-catalyzed reaction of an epoxy resin with the phenolic resin produces a cross-linked polyether structure that is resistant to chemicals and heat and is a good moisture vapor barrier. Since the curing mechanism does not produce byproducts, thick sections may be obtained without voids and low shrinkage. Applications that employ the advantages of epoxy-phenolic formulations Include molding materials, laminates, coatings, and adhesives. 

A great many of outstanding adhesive formulations are based on epoxy resins. A broad spectrum of adhesive formulations with a wide range of available properties have resulted from the use of polymeric hardeners such as polyamides and polyamines, phenolics, isocyanates, alkyds, and combinations of amines with polysulfide elastomers, and the alloying of the epoxy with compatible polymeric film-formers, such as poly(vinyl acetate) and certain elastomers. 

Epoxy adhesives are generally limited to continuous applications below 300°F (149°C). However, there are epoxy formulations that can withstand short terms at 500°F (260°C) and long-term service at 300-350°F (149-177°C). A combination epoxy-phenolic resin has been developed that will provide an adhesive capability at 700°F (371°C) for short-term operation and continuous operation at 350°F (177°C). 

Epoxy resins. Most commercially available epoxy resins are based on the diglycidylethers of bisphenol-A, bisphenol-F or other phenolic compounds. A second family of epoxy resins, also used in adhesive formulations, is based on epoxidized olefin compounds such as epoxidized polybutadiene or cycloaliphatic olefins (Fig. Epoxy resins based on the... 

Epoxy-phenolic film and tape adhesives have good resistance to weathering, aging, water, weak acids, aromatic fiiels, glycols, and hydrocarbon solvents. The service-temperature range is -60°C to 200°C, but special formulations are suitable for end uses at cryogenic temperatures down to -260°C. ... 

Epoxy adhesives are generally limited to applications below 121 °C. Some epoxy adhesives tolerated short-term service at 260°C and long-term service at 149-260°C. These systems were formulated especially for thermal environments by incorporation of stable epoxy co-reactants or high-temperature curing agents into the adhesive. One of the most successful epoxy co-reactants is an epoxy-phenolic alloy. The excellent thermal stability of the phenolic resins is coupled with the adhesion properties of epoxies to provide an adhesive capable of 371 C short-term operation and continuous use at 177°C. "... 

Structural adhesives are normally categorized or subdivided into subclasses based on the resin chemistries used in their formulation, which can be in the form of solids, liquids, pastes or films. The most important categories of structural adhesives are the thermoset cured adhesives based on phenolic, epoxy and polyurethane or acrylic resins and include phenolic adhesives. Epoxide adhesives, Toughened epoxide adhesives, Polyurethane adhesives, Acrylic adhesives and Toughened acrylic adhesives. Several of the categories can be further subdivided into both one- and two-component adhesives. The one-component structural adhesives, which can be liquids, pastes or solids (films), usually require the inclusion of added energy for activation or to effect cure in the... 

Polymer blends are often used in adhesive formulations where properties associated with rigid polymers (high temperature resistance, chemical resistance, etc.) must be obtained along with properties associated with tough, elastic polymers (impact strength, high peel strength, etc.). Examples of these adhesive systems are nylon-epoxy, phenolic-nitrile, epoxy-polysulfide, epoxy-nitrile, and epoxyurethane. 

Elastomer epoxies generally contain nitrile rubber as the elastomeric component. This system is also referred to as a modified or toughened epoxy. One of the applications of widest use is in films and tapes. Elastomer epoxies cure at low pressures and low temperatures over a short time interval. This is achieved by adding a catalyst to the adhesive formulation. Bond strengths of elastomer epoxies are lower than those of nylon epoxies. However, the major advantage of elastomer epoxies is their sub-zero peel strengths, which do not decrease as fast as those of nylon epoxies. In addition, the moisture resistance of elastomer epoxies is better than that of nylon epoxies but not as good as that of vinyl-phenolics or nitrile-phenolics. Limitations to the use of elastomer epoxies include poor water immersion resistance and poor properties when exposed to marine conditions. 

The phenolic family of adhesives are very diverse in their formulations and uses. Some are filled or modified with other polymers (vinyl, nitrile or epoxy). The composition depends largely on the intended use, e.g. for temperature and chemical resistance, or for water-proof wood bonding (phenol-resorcinol-formaldehyde). The majority are heat-curing although some wood-bonding adhesives can be cured at room temperature (RT). The comparison table includes Typical Use to differentiate between the types of adhesives. Phenolic adhesives are generally poorly represented by mechanical property data. 

The first Fibrelam product was manufactured from two cross-plied glass-epoxy prepreg skins bonded to a phenolic resole coated Nomex honeycomb with a specially formulated epoxy film adhesive (Fig. 96). 

Besides the epoxies and certain phenolic adhesives, some compounds have gone beyond the stage of secret application (polybenzimidazole, polyimides (O Fig. 14.15), polyarylsulfones), but in most cases, the formulations are adapted by the users (secretly in general) to specific problems. 

Nitrile rubber is compatible with phenol-formaldehyde resins, resorcinol-formaldehyde resins, vinyl chloride resins, alkyd resins, coumarone-indene resins, chlorinated rubber, epoxies and other resins, forming compositions which can be cured providing excellent adhesives of high strength, high oil resistance and high resilience. On the other hand, NBR adhesives are compatible with polar adherends such as fibres, textiles, paper and wood. Specific formulations of NBR adhesives can be found in [12]. 

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