Phenol formaldehyde resin

Phenol formaldehyde resins (PF), also known as phenolic resins or phenoplasts, are synthetic polymers created by combining phenol (or a substituted phenol) with formaldehyde. They formed the basis of Bakelite and were the first synthetic resins to be sold commercially. These resins have been used extensively to make molded items such as billiard balls and laboratory countertops, as well as coatings and adhesives. They were once the main material for circuit boards but have largely been replaced by epoxy resins and fiberglass cloth, like in fire-resistant FR-4 boards.

Two main production methods exist. In one, an acid catalyst is used with an excess of phenol, producing a prepolymer called novolac. This can be molded and then hardened by adding more formaldehyde and heat. In the other method, formaldehyde is in excess and a base catalyst is used, forming a prepolymer called resole. Many variations in both production and raw materials yield a wide range of specialized resins.

The formation of phenol-formaldehyde resins involves a step-growth polymerization that can be catalyzed by either acid or base. Because formaldehyde in solution exists as a dynamic mix of methylene glycol oligomers, the amount of reactive formaldehyde depends on temperature and pH. Phenol reacts with formaldehyde at its ortho and para positions (sites 2, 4, and 6), allowing up to three formaldehyde units to attach to the ring. The first step always forms a hydroxymethyl phenol. This hydroxymethyl group can then react with another free ortho or para site, creating a methylene bridge, or with another hydroxymethyl group, forming an ether bridge. The resulting diphenol, bisphenol F, is an important monomer for epoxy resins and can link further into tri-, tetra-, and higher phenol oligomers.

Novolaks (or novolacs) are PF resins with a formaldehyde-to-phenol molar ratio of less than one. They are often made using cresols instead of phenol. The polymerization is completed with an acid catalyst like sulfuric, oxalic, or hydrochloric acid. The phenolic units are linked mainly by methylene or ether groups, and the molecular weights are in the low thousands, corresponding to about 10–20 phenol units. These polymers are thermoplastic and need a curing agent to become a thermoset. Hexamethylenetetramine is a common hardener; above 90 °C, it forms methylene and dimethylene amino bridges. Resoles can also cure novolacs. In both cases, the curing agent provides formaldehyde to create bridges between novolac chains, fully crosslinking the system. Novolacs are used as tire tackifiers, high-temperature resins, binders for carbon-bonded refractories and carbon brakes, photoresists, and curing agents for epoxy resins.

Resoles are base-catalyzed PF resins with a formaldehyde-to-phenol ratio greater than one, typically around 1.5. Phenol, formaldehyde, water, and catalyst are mixed and heated. The first stage, around 70 °C, produces a thick, reddish-brown, tacky material rich in hydroxymethyl and benzylic ether groups. The base-catalyzed reaction rate increases with pH, peaking at about pH 10. The reactive species is the phenoxide anion, which activates the ortho and para sites for reaction with formaldehyde. As thermosets, hydroxymethyl phenols crosslink when heated to about 120 °C, forming methylene and methyl ether bridges and releasing water. This creates a three-dimensional network, giving the resin hardness, thermal stability, and chemical resistance. Resoles are called "one-step" resins because they cure without a crosslinker, unlike novolacs, which are "two-step" resins. They are widely used for gluing and bonding building materials, such as exterior plywood, oriented strand boards (OSB), and engineered high-pressure laminates.

When the molar ratio of formaldehyde to phenol reaches one, every phenol is theoretically linked via methylene bridges into a single, fully crosslinked molecule. This explains why novolacs (F:P < 1) need a crosslinking agent to harden, while resoles (F:P > 1) do not.

Phenolic resins appear in countless industrial products. Phenolic laminates are made by saturating layers of materials like paper, fiberglass, or cotton with phenolic resin and then laminating them under heat and pressure. The resin fully cures during this process, forming a thermoset polymer matrix. The choice of base material depends on the intended use. Paper phenolics are used for electrical components like punch-through boards, household laminates, and paper composite panels. Glass phenolics are used for specific applications.