Structural Phenolic Adhesives

Phenolic resins are supplied as liquids and powders and quite often they are compounded into adhesives by the addition of extenders, solvents, and catalysts just prior to use. Compounded phenolic adhesives are also supplied, as supported or unsupported films, one-part catalyzed pastes, or two-part room temperature curing pastes. 

The main use of phenolic adhesives is for structural bonding applications, since they excel in strength and heat, moisture, and chemical resistance. 

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. 

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Many adhesives cure by step polymerization. Structural phenolic adhesives are based on resoles, which are made by the step polymerization of phenol with excess formaldehyde under basic conditions (see Phenolic adhesives single-stage resoles). They contain... 

Fiberboard -for food packaging [FOOD PACKAGING] (Vol 11) -medium-density [WOOD-BASED COMPOSITES AND LAMINATES] (Supplement) -phenolic adhesive [PHENOLIC RESINS] (Vol 18) -sampling standards for [SAMPLING] (Vol 21) -structural [WOOD-BASED COMPOSITES AND LAMINATES] (Supplement)... 

Phenolic/ adhesives adhesives / sealants / gaskets abrasives and break pad binders printed circuit boards structural composites/laminates Metal/glass binders metal oxide binders retroreflective coatings glass optical coatings Hard copy printing inks toners... 

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. 

The adhesive properties of lignin, its reactivity with formaldehyde, and its structural similarity with phenolic adhesives invited investigation of the applicability of lignin in adhesive resin systems. Therefore, during the past several years, numerous attempts have been made to replace the expensive petrochemical resins totally or partially with the renewable raw material lignin (i). 

Phenolic adhesives are structural adhesives with specific applications, e.g. where wide gap bonding is required and where large structures need to be bonded. Phenolic resins are the product of a special reaction ratio of a phenol and formaldehyde in the presence of an organic catalyst. There are two main types of phenolic resins phenol or methylol terminated. The phenol terminated are called novalacs, while the methylol terminated are called resoles (one step resins). Modem phenolic resins are prepared in the presence of metal carboxylates and these resins contain a large number of benzylic ether linkages and have open para positions which have good temperature stability and are usually of low viscosity. 

Furfural has replaced formaldehyde in phenol-resorcinol-formaldehyde resins.These formaldehyde-containing resins have been used as cold-set, exterior grade structural wood adhesives for almost half a century. The use of furfural to prepare these resins has several advantages. In addition to longer resin shelf life, when furfural is used, the emission of formaldehyde is lowered. To enhance the reaction rate when furfural is used, a small amount of formaldehyde is added to the furfural system. Hence, the furfural system is not formaldehyde-free, but nearly so. Finally, furfural has a higher molecular weight than formaldehyde. This results in a smaller amount of the expensive resorcinol being required in the system. 

Waferboard is a structural board made of wood wafers cut to predetermined dimensions randomly distributed and bonded with phenolic adhesives." It is estimated that by 1981, 80 million pounds of adhesives on a solid basis will be used in waferboard and other types of particleboard bonding. Currently either solid or liquid phenolic resins are being used in these applications. Formaldehyde and phenol fumes and phenol formaldehyde powder dust,... 

Fig. 13.7 The TEM image of the nanophase-separated structural epoxy adhesive containing nitrile rubber/phenolic polyurethane liquid rubber blends (cure with dicyandiamide at 180°C).

A large number of commercially important condensation polymers are employed as homopolymers. These include those polymers that depend on crystallinity for their major applications, such as rylons and fiber-forming polyesters, and the bulk of such important thermosetting materials like phenolics and urea-formaldehyde resins. In many applications, condensation polymers are used as copolymers. For example, fast-setting phenolic adhesives are resorcinol-modified, while melamine has sometimes been incorporated into the urea-formaldehyde resin structure to enhance its stability. Copolyesters find application in a fairly broad spectrum of end uses. 

For glulam beams and load bearing timber structures, standards are numerous and stringent. A significant distinction is made between adhesives types, taking into account their chemical and mechanical properties. In this way, it is necessary to consider EN 301 (2006) [6] for aminoplastic and phenolic adhesive and EN 15425 (2008) [13] for polyurethane adhesives. Associated test methods are described to assess the critical properties of these adhesives and to evaluate the differences between them in more or less the same conditions. 

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. 

The traditional Structural adhesives capable of operating at temperatures in excess of 150 °C for both short- and long-term applications can be divided into three classes, namely (1) phenolics, (2) polybenzimidazoles, (3) condensation polyimides (see Phenolic adhesives single-stage resoles. Phenolic adhesives two-stage novolacs, Polyhenzimida-zoles and Polyimide adhesives). 

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... 

The cure mechanisms of structural adhesives depends on the resin chemistry and can be based on either Chain polymerization or Step polymerization reactions. The condensation cure mechanism is the most important for phenolic adhesives. Epoxide adhesives and Polyurethane adhesives, which require stoichiometric or balanced combination ratios as well as thorough mixing for maximum performance. Off-ratio mixing or incomplete mixing can alter significantly the final adhesive structure and effect the resulting properties. 

Phenolic adhesives single-stage resoles J A BISHOPP Resoles structure modifications uses... 

Automotive trim and clips have been bonded with nitrile rubber/phenolic adhesives and polyamide has been bonded to rubbei and to pavement in a road marking application. Plastic/paper laminates for printing blankets have been prepared. Finally, grit tiles have been bonded to paperboard and other substrates to produce prefabricated tile structures which are stable on application to buildings. 

However, as many of the types of bonded structures are very similar to those already discussed under the section on the application of phenolic adhesives, examples are limited to a representative cross-section, which cover bonding of airframe structures, floors and specific components for civil aircraft. 

Thirdly, the adhesive type does appear to influence the environmental stability of the interfacial regions. For example, especially noteworthy is the often-superior joint durability shown, in the case of bonding aluminium alloys, by some of the older phenolic-based structural adhesives, compared to the more modern epoxy-based adhesives. This does not arise from the phenolics possessing lower diffusion or solubility coefficients with respect to water than the epoxies [153]. Indeed, many such phenolic adhesives are more permeable to moisture. However, it may arise from the ability of the phenolics to establish stronger, more stable intrinsic interfacial forces, as discussed later. Alternatively, it may arise from the pH of residues from phenolic-based adhesives being acid and therefore not attacking and weakening the aluminium oxide, unlike the more alkaline extracts from some of the epoxies. It is believed... 

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