Epoxy-phenolic system

A classic example is Cytec BR-127, which is an epoxy-phenolic system dissolved in a solvents blend. Corrosion inhibition results from a significant strontium chromate loading - present as a fairly unstable suspension. 

Fig. 55. Dynamic cure of a brominated epoxy-phenolic system at a heating rate of 7 C/min (125) superimposed on the dielectric loss factor plot are djmamic viscosity data taken with a rheometrics rheometer. 1 Pa s = 10 P.

The intracoordination compounds with the O-ligands were applied in the epoxy-phenol systems [297,298] as well as in the compositions involving... 

Unsaturated hydrocarbon resins are able to react with unsaturated polyesters with epoxy-phenolic systems [255]. 

Alcohol-soluble non-heat-reactive types are low MW novolacs derived from phenol or cresols that are used in the preparation of novolac epoxy resins, epoxy curing agents, epoxy-phenolic systems, and powder coatings. 

Until recently the classic example of this type of primer has been Cy tec-Fiberite s BR-127. This comprises an epoxy-phenolic system dissolved in a blend of solvents. Corrosion inhibition is achieved through the addition of a significant loading of... 

The strengths of stainless steel lap-joints bonded with the epoxy-phenolic system (Fig. 5) remained essentially the same throughout the range of test temperatures and gave the highest values obtained for any type of adhesive tested... 

In addition to electrical uses, epoxy casting resins are utilized in the manufacture of tools, ie, contact and match molds, stretch blocks, vacuum-forrning tools, and foundry patterns, as weU as bench tops and kitchen sinks. Systems consist of a gel-coat formulation designed to form a thin coating over the pattern which provides a perfect reproduction of the pattern detail. This is backed by a heavily filled epoxy system which also incorporates fiber reinforcements to give the tool its strength. For moderate temperature service, a Hquid bisphenol A epoxy resin with an aHphatic amine is used. For higher temperature service, a modified system based on an epoxy phenol novolak and an aromatic diamine hardener may be used. 

The most common aerospace adhesives include nitrile epoxy systems, epoxy polyamides, epoxy phenolics, and various unmodified epoxies [18]. Polyimides... 

In addition to epoxy-phenolic adhesives three-part epoxy-phenolic-nitrile rubber systems are used in metal-metal edge joints and honeycomb constructions [208], These add toughness not available in most EP systems and improve peel strengths. When used on honeycomb, the NR-P is normally applied to the aluminum skin and the EP to the honeycomb for assembly. Service temperature limitations are those imposed by the NR-P part. 

Epoxies - The term "reactive oligomer" is relatively new but the concept is fairly old. This concept has been used for many years with systems such as epoxies, phenolics, unsaturated esters, cyanates, isocyanates and many other crosslinked systems. An example of a 177°C curing epo y system (Narmco s 5208) which was introduced into the marketplace about 1971 is shown in Eq. 1. 

The solution of the precursors of the thermosetting polymer (mixture of monomers or oligomers with or without initiators, catalysts and different additives) is usually a liquid at room temperature e.g., unsaturated polyester styrene, some epoxy anhydride and epoxy amine formulations, cya-nate esters, one-stage phenolics, etc. Cooling any of these solutions below room temperature leads to a glass. The temperature at which the glass-liquid transition of the initial formulation takes place is denoted as Tg0. Some other particular formulations, such as two-stage phenolics (novo-lac-hexa mixtures), some epoxy-amine systems, etc., exhibit a Tg0 above room temperature. 

The inherent thermal stability of the phenol formaldehyde chemistry is preserved but with the crosslinking characteristics of the epoxy groups. However, epoxy novolacs also form very rigid and brittle polymers when fully cured because of their high crosslink density. For this reason, they are often used as modifiers in epoxy adhesive systems rather than as the base polymer. 

Whereas most room temperature curing epoxy adhesives are cured with aliphatic amines, polyamides, or amidoamines, most elevated-temperature curing epoxy adhesives are cured with aromatic amines, modified aliphatic amines, alcoholic and phenolic hydroxyls, acid anhydrides, Lewis acids, and a host of other curatives. Latent curing agents, such as dicyan-diamide and imidazoles, are typically used in one-component epoxy adhesives systems. 

Typically tape or film epoxy adhesives are modified with synthetic thermoplastic polymers to improve flexibility in the uncured film and toughness in the cured adhesive. Epoxy resins can also be blended with phenolic resins for higher heat resistance. The most common hybrid systems include epoxy-phenolics, epoxy-nylon, epoxy-nitrile, and epoxy-vinyl hybrids. These hybrid film adhesives are summarized in Table 13.2, and structural properties are shown in Table 13.3. 

Several hybrid epoxy emulsions have been commercially prepared. An epoxy emulsion blended with waterborne aliphatic urethanes exhibited peel strength on aluminum of 10 lb/in—1.5 times greater than with the polyurethane itself. The optimum concentration of urethane in the final emulsion was about 50 percent by weight.13 Epoxy-phenolic dispersions have also been developed to provide waterborne adhesive systems with high glass transition temperature and chemical resistance. 

Epoxy coreactants which will increase the temperature resistance of the system (e.g., epoxy-phenolic adhesives)... 

Epoxy Coreactants. One of the most successful epoxy coreactant systems developed thus far is an epoxy-phenolic alloy. The excellent thermal stability of the phenolic resins is coupled with the valuable adhesion properties of epoxies to provide an adhesive capable of 371°C short-term operation and continuous use at 175°C. The heat resistance and thermal-aging properties of an epoxy phenolic adhesive are compared with those of other high-temperature adhesives in Fig. 15.5. Epoxy-phenolic adhesives are generally preferred over other high-temperature adhesives, such as the polyimides and polybenzimidazoles, because of their lower cost and ease of processing. 

Structural adhesives that are commonly used for composites are supplied in two basic forms semisolid B-stage film and thixotropic pastes. The film adhesives are cast or extruded onto carrier fabrics or films and partially cured to a semisolid. They can easily be handled, cut, and applied to the joint area. There is no need for mixing, metering, or dispensing of liquid components. In use, these adhesive systems are activated by heat and pressure. The semisolid B-stage film liquefies briefly on application of heat and then cures to an insoluble state. Epoxy, polyimides, epoxy-nylons, epoxy-phenolic, and nitrile-phenolic adhesives are available as B-stage film. 

First commercial epoxy-phenolic adhesive systems... 

The acid resistance of epoxy phenolics to continual immersion conditions would generally be poor to fair. Special formulations are available to provide resistance to dilute sulfuric, hydrochloric, nonoxidizing mineral acids, and fatty acids. Many systems are available for resistance to spillage and fumes from most acids with the exception of nitric, formic, chromic or hydrofluoric. 

Combining an excess of epichlorohydrin with the fo/s-phenol A (4, 4 -isopropylidenediphenol) gives the required prepolymer component of epoxy resin systems (Eq. 21.26). 

Phenolic-epoxy adhesive systems for structural bonding use similar formulations but are frequently modified with a linear polymer having reactive end groups (carboxyl, amino) for toughness and vibration resistance. 

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