Resorcinol Modified Novolacs

The resorcinol resins used for structural bonding applications are cured at room temperature, or slightly elevated temperatures, and have a 

Pure resorcinol novolacs cured with paraformaldehyde. 

According to Pizzi, only type 2 adhesives are used extensively, because they are cheaper than types 1 or 3 and are easier to handle. 

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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. 

The second is a two-component adhesive " component A is a m-aminophenol modified novolac at a pH of 5 (about 63% solids) and component is a formaldehyde donor used as hardener. The adhesive has gap-filling capability, impact resistance, and is thixotropic. A phenol-resorcinol laminating resin can be used as a primer, with good results. 

While phenol is the most common monomer for novolac manufacture, it is far more common to see incorporation of other phenolic materials with novolacs than with resoles. Cresols, xylenols, resorcinol, catechols, bisphenols, and a variety of phenols with longer alkyl side chains are often used. While most resoles are made with a single phenolic monomer, two or more phenolic materials are often seen in novolac formulae. These additional monomers may be needed to impart special flow characteristics under heat, change a glass transition temperature, modify cure speed, or to adjust solubility in the application process among others. 

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