Phenol-Resorcinol-Formaldehyde (PRF) Adhesives
PRF adhesives are classified as thermosetting polymers and are produced by a condensation polymerization between formaldehyde, phenol and resorcinol.
PRF adhesives are cold-setting with very high weather and water resistance as well as heat resistance to be used in products for exterior, humid and interior climates. PRF adhesives can also be used in hot bonding and radio frequency curing equipment to speed up the curing process.
PRF adhesives are mainly used in the production of load-bearing timber structures where a maximum of weather resistance is required. With a proven track record such as this, these adhesives have developed a long and successful history of use in plywood laminating, boat building and wood aircraft construction. Unlike the epoxy family of adhesives, PRF’s are completely UV-resistant, outlasting epoxies in high-weather-exposure applications almost ten to one.
Representative Commercial Products:
PRF adhesives are represented by such commercial products as:
- Weldwood Resorcinol Glue (from DAP Products, popular in the wooden boat making industry)
- Aerodux-500 (popular for wood airplane construction)
- G-1131 (also called Penacolite or Cascophen, used in structural wood “glue-lam” beams)
PRF Adhesive Chemistry:
Let’s take a look at each of the three key constituents of this complex and proven adhesive system.
Phenol:
Also called benzophenol or carbolic acid, phenol is derived from the oxidation of cumene, a petroleum by-product. You will readily recognize it as the prefix of “Phenolic” as in phenolic plastics and as one of the chief constituents in epoxy.
Formaldehyde:
In its pure form, formaldehyde is a gas with a strong pungent odor and is derived through oxidation of hydrocarbons such as methanol, propane or butane in the presence of a catalyst. It is commercially offered as a 37-50% aqueous solution, called Formalin, that usually contains up to 15% methanol to inhibit polymerization.
Resorcinol:
Resorcinol (also called “Resorcin”) is the 1,3-isomer of benzenediol. Resorcinol, is produced by sulfonating benzene with fuming sulfuric acid and fusing the resulting benzenedisulfonic acid with caustic soda. Interestingly, resorcinol is often used in polyurethane resins and other coatings as a UV absorber.
Adhesive Chemistry:
The earliest commercial synthetic resin based on a Phenol formaldehyde resin is sometimes called phenolic and had the commercial name Bakelite. It is formed from a elimination reaction of phenol with formaldehyde.
Phenol is highly reactive towards formaldehyde which attaches directly to the phenol ring structure. This forms a hydroxymethyl phenol, which is not usually isolated in novolacs but is found in resoles. The hydroxymethyl group is capable of reacting with either another free ortho or para site, or with another hydroxymethyl group. The first reaction forms a methylene bridge, and the second forms an ether bridge.
Phenol formaldehyde resins, as a group, are formed by a step-growth polymerization reaction which may be either acid or base-catalyzed. The pathway the reaction follows varies depending on the type of catalyst used.
Acid-Catalyzed:
Acid-catalyzed phenol formaldehyde resins are made with a molar ratio of formaldehyde to phenol of less than one and are called novolacs. Owing to the molar ratio of formaldehyde to phenol, they will not completely polymerize without the addition of a crosslinking agent. Novolacs are commonly used as photoresists in the semiconductor industry.
Base-Catalyzed:
Base-catalyzed phenol formaldehyde resins are made with a formaldehyde to phenol ratio of greater than one (usually around 1.5). Phenol, formaldehyde, water and catalyst are mixed in the desired amount, depending on the resin to be formed, and are then heated. The first part of the reaction, at around 70 °C, forms hydroxymethyl phenols. This results in a thick reddish-brown resin.
The rate of the base-catalyzed reaction initially increases with pH and reaches a maximum at approximately a pH = 10. The reactive species is the phenolic anion formed by deprotonation of phenol. The negative charge is delocalized over the aromatic ring, activating sites 2, 4 and 6, which then react with the formaldehyde.
Formaldehyde in solution does not exist as the aldhehyde, but instead a dynamic equilibrium is formed creating a range of methylene glycol oligomers, and the concentration of the reactive form of formaldehyde depends on the exact conditions (temperature, pH) under which the reaction occurs. Thus the reaction rate law describing phenol and formaldehyde is not a simple one, and the chemical kinetics are highly complex.
Hydroxymethyl phenols will crosslink on heating to around 120 °C to form methylene and methyl ether bridges. At this point the resin is starting to crosslink, to form the highly extended 3-dimensional web of covalent bonds which is typical of polymerized phenolic resins.
It is this highly crosslinked nature of phenolics which gives them their hardness and their excellent thermal stability and which makes them impervious to most chemical attack. It is also the reason they are called thermosets.
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