Epoxy adhesives Phenol Novolac resins

The resins, solvents and curing agents used in modem conductive epoxy adhesives are chosen to minimize residual vapors which can be released after cure. Modem epoxy adhesives generally consist of high purity Bis-A epoxies cured with a phenolic novolac resin. These mixtures do not contain boron or fluorides, and generate less ammonia or other corrosive vapors than epoxies available before 1982. 

Novolac resins are broadly used in electronics because their functionality higher than two increases the crossimkmg density and yields cured resins exhibiting enhanced chemical and physical properties. Mixtures of epoxy resins and phenol novolacs 61 are excellent structural adhesives in the aerospace industry. However, the phenolic hydroxyl groups are not very reachve at moderate temperatures and most systems include catalysts or accelerators. Classical adhesive compositions are prepared by mixing a solid epoxy resin, t5rpicahy an epoxidized phenol novolac resin (60 parts), a phenol novolac resin (40 parts), a solvent such as 2-butoxyethanol or butylcehosolve acetate, an imidazole catalyst, and silver flakes. 

The neat novolac resins are also the precursors in the manufacture of a range of (see Epoxide adhesives), such as epoxy phenol novolacs and epoxy cresol novolacs spoxy resins. 

The increased functionality yields cured adhesives with higher crosslink densities resulting in higher temperature performance and increased chemical resistance. The functionality of commercially available, phenol based epoxy novolac resins varies from 2.3 to 6.0. Epoxy novolac resins can also be produced from substituted phenols like creosol and po-lyhydioxy phenols such as resorcinol. In the United States, Dow Chemical and Ciba-Geigy supply both phenol and cresol based epoxy novolac resins. 

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.

Compounds such as 2-methylimidazole 41, 2-undecylimidazole, and 2-ethyl-4-methylimidazole 42 are highly reactive materials. It has been stated that soluble imidazoles are too reactive to permit their use in one-part adhesive systems stable at ambient temperature. Among the other compounds, the less reactive 4-phenylimidazole 43 would be a convenient curing agent at elevated temperature, although the reaction still occurs over a few days of storage at 25 °C. 4-Methyl-2-phenylimidazole is cited in different patents allowing a 3-day pot life at room temperature for silver-filled epoxy resins based on bisphenol-A, bisphenol-F, and phenol novolac hardener. Imidazole 44 mixed with bisphenol-F... 

Novolac resins are broadly used in electronics because their functionality higher than two increases the cross-linking density and yields cured resins exhibiting enhanced chemical and physical properties. Mixtures of epoxy resins and phenol novolacs 61 are excellent structural adhesives in the aerospace industry. However, the phenolic hydroxyl groups are... 

This class of compounds is one of the most important adhesive groups with applications ranging from consumer to aerospace markets. Epoxies are thermosets and are cross-linked during the cure cycle. The chemical stmcmre for a simple epoxy (ethylene oxide) in its unhardened state is shown in Figure 5.2. All epoxy compounds contain two or more of these groups. Epoxy resins form adducts with vinyl, acrylic, and polyester resins producing compounds such as phenol novolac, cresol novolac, bis-[4(2,3-epoxy propyoxy) phenyl] methane, and phenol hydrocarbon novalac [53]. 

The novolac phenolic resins described above can also be used to generate materials known as novolac epoxies by the reaction of the novolac with epichlorohydr in. Th ese materials use latent catalyst systems based on aromatic amines. The reactions which take place will be discussed below. Table 1 also contains information about the physical properties of adhesive bonds made with adhesives based upon novolac epoxies, AF-143 and AP-147. These adhesives have similar chemistries, but AF-147 is a more flexible version of AF-143. This is, of course, to be ascertained by the higher peel strength of AF-147. The high strength values at... 

Most phenolic adhesives are based on resoles, novolacs (or resorcinol-modified novolacs), and phenolic alloys. The last mentioned class includes the important blends of phenolic resins with epoxies, vinyl, polyamide, and nitrile rubber. Phenolic-based adhesives perform well on various types of surfaces the more common substrates are shown in Table III. 

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). 

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