Epoxy adhesives Phenolic

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. 

Epoxy-phenolic Epoxy-phenolic adhesives are generally used in aerospace applications requiring high shear strength at temperatures in excess of 150°C. Usually die phenolics are a resole-type, and often the epoxy is a minor component. These adhesives are relatively brittle and have low peel and impact strengths. 

Epoxy-phenolic adhesives are made by blending epoxy resins with phenolic resins to improve the high-temperature capabilities of the standard epoxy resins. Developed in the early 1950s, they were the first high-temperature epoxy adhesives to become commercially available.2,3... 

Epoxy-phenolic adhesives were developed primarily for bonding metal joints in high-temperature applications. Their first major application was to join major aircraft components. They are also commonly used for bonding glass, ceramics, and phenolic composites. Because of their relatively good flow properties, epoxy phenolics are also used for bonding honeycomb sandwich composites. 

TABLE 7.4 Epoxy -Phenolic Adhesive Compositions of Commercially Available Types5... 

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. 

FIGURE 15.3 The effect of 260°C aging in air and nitrogen on an epoxy-phenolic adhesive.10... 

TABLE 15.5 Effect of Several Chelating Agents on the Resistance of an Epoxy-Phenolic Adhesive to Thermal Aging13... 

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. 

TABLE 15.6 First Commercial Epoxy-Phenolic Adhesive Systems17... 

Epoxy-phenolic adhesive compositions of commercially available types Epoxy-nylon adhesive composition... 

First commercial epoxy-phenolic adhesive systems... 

According to one author, epoxy-phenolic adhesives for high-temperature applications were developed during World War II at Forest Product Laboratories in Madison, Wisconsin, and nitrile phenolic adhesives shortly after World War II [9, pp. 153, 156]. A patent for epoxy resins was applied for in Germany in 1934, and the inventor disclosed that it could be hardened with equivalent amounts of amines, diamines, or polyamines and that it showed strong adhesion [14, p. 8]. Epoxy resins are believed to have been commercialized in the United States first by the former Jones Dabney Company sometime after 1942. 

Epoxy-phenolic adhesives exhibit good adhesive properties at both elevated and low temperatures. Polyurethane and epoxy-nylon systems exhibit outstanding cryogenic properties. 

Exceptions to this generahzation are the epoxy-phenolic adhesives, which are composed of two thermosetting adhesives. 

PI adhesives require higher cure temperatures than epoxy-phenolic adhesives. Curing at 250°C is usually adequate when service temperatures do not exceed die cure temperature. Volatiles are released during the cure of PI adhesives. The best results are therefore obtained when the volatiles can freely escape (e.g., honeycomb or perforated-core structures). For longterm aging at temperatures in the range of 204-316°C, Pis are superior to PBI and epoxy-phenolic adhesives. 

Epoxy phenolic adhesives are polyepoxy compounds which possess epoxy reactivity and phenolic resin heat resistance. These are prepared by condensing epichlorohydrin and novolac phenolics (phenol/formaldehyde ratio > 1). A recent Japanese patent issued to Mitsubishi Petrochemical Co. describes the preparation of epoxy resins from epichlorohydrin condensation with phenolic resins prepared from phenol and substituted aldehydes, e.g., vanillin. These resins (23) have high epoxy contents (epoxy equivalent weights >200) and relatively high softening points (85-95°C). 

The best epoxy or epoxy-phenolic adhesives employed in the aerospace industry show initial lap shear strength values often higher than those of the polyimides but the maximum operating temperatures are in the range of 150-170°C. For example, the adhesion strength of Al/Al assemblies bonded with a high temperature epoxy resin is of the order of 30 MPa at 150°C and remains virtually constant when aged at that temperature for 5000 h. The adhesive film 3M Scotchweld... 

AF 131, based on epoxy-phenolic resins, provides Al/Al bonds with lap shear strengths of 25.5, 20 and 10.3 MPa at 180, 200 and 260°C, respectively. Solventless epoxy and epoxy-phenolic adhesives provide bond lines with a low level of porosity. This is also the case with poly(ether-imide) Ultem 1000 and poly(imide-siloxanes), which are generally processed at temperatures 50-100°C above their Tg. In contrast, high porosity is observed with most condensation... 

It has been previously outlined that the thermal stability of organic materials is a time-temperature dependent concept. Thus, heat-resistant adhesives are those that can be safely used for thousands of hours between 200 and 300°C. This definition excludes the epoxy and epoxy-phenolic adhesives which are, however, the predominant classes of polymers used in aeronautics. Epoxy adhesives based on... 

Epoxy-phenolic adhesives are exceptional in that they have good adhesive properties at both elevated and low temperatures. Vinyl-phenolic adhesives maintain fair shear and peel strength at -423 °F, but strength decreases with decreasing temperature. Nitrile-phenolic adhesives do not have high strength at low service temperatures, because of rigidity. 

Figure 8 Chemical formulae of phenolic resins used to prepare epoxy-phenolic adhesives based on Gunei Kagaku phenol novolac 61, Dow Chemical OCN ortho-cxQsoX novolac 62, Mitsui Toatsu para-xylene-modified phenol novolac 63, phenolic ortho-ortho-vtsoX of para-cresol 64, and poly (para-hydroxy styrene) 65.

Figure 105 (Wake, 1982) illustrates the temperature limitations of epoxy resins based on bis-phenol A. The two component polyamine cure shows relatively poor performance but these figures refer to torsional shear and the two one-component epoxies here exhibit strengths which would cause gross distortion of lap-shear test pieces of the usual thickness. None is usable above 120°C. By contrast, Fig. 106 (Wake, 1982) shows the very different behaviour of an epoxy-phenolic adhesive supported on a glass cloth carrier. There are two features to be reckoned with when considering the use of adhesives at elevated temperatures. There is the temperature at which they can be used continuously and at which the joint will retain adequate strength...

Fig. 106. Strength-temperature profile of an epoxy-phenolic adhesive on stainless steel (Hidux 1197A, Ciba-Geigy) (Wake, 1982).

---