Phenol formaldehyde resins (PF), also called phenolic resins or phenoplasts, are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first commercial synthetic resins. They have been widely used for the production of molded products including billiard balls and laboratory countertops, and also of coatings and adhesives. They were at one time the primary material used for the production of circuit boards but have been largely replaced with epoxy resins and fiberglass cloth, as with fire-resistant FR-4 circuit board materials.

There are two main production methods. The acid-catalyzed reaction of phenol and formaldehyde, with the phenol in excess quantity, forms a prepolymer known as novolac which can be moulded and then cured with the addition of more formaldehyde and heat. The other method has instead the formaldehyde in excess and uses a base as catalyst, forming a prepolymer known as resole. There are many variations in both production and input materials that are used to produce a wide variety of resins for special purposes.

Formation and structure

Phenol-formaldehyde resins, as a group, are formed by a step-growth polymerization reaction that can be either acid- or base-catalysed. Since formaldehyde exists predominantly in solution as a dynamic equilibrium of methylene glycol oligomers, the concentration of the reactive form of formaldehyde depends on temperature and pH.

Phenol reacts with formaldehyde at the ortho and para sites (sites 2, 4 and 6) allowing up to 3 units of formaldehyde to attach to the ring. The initial reaction in all cases involves the formation of a hydroxymethyl phenol:

: HOC<sub>6</sub>H<sub>5</sub> + CH<sub>2</sub>O → HOC<sub>6</sub>H<sub>4</sub>CH<sub>2</sub>OH

The hydroxymethyl group is capable of reacting with either another free ortho or para site, or with another hydroxymethyl group. The first reaction gives a methylene bridge, and the second forms an ether bridge:

: HOC<sub>6</sub>H<sub>4</sub>CH<sub>2</sub>OH + HOC<sub>6</sub>H<sub>5</sub> → (HOC<sub>6</sub>H<sub>4</sub>)<sub>2</sub>CH<sub>2</sub> + H<sub>2</sub>O

: 2 HOC<sub>6</sub>H<sub>4</sub>CH<sub>2</sub>OH → (HOC<sub>6</sub>H<sub>4</sub>CH<sub>2</sub>)<sub>2</sub>O + H<sub>2</sub>O

The diphenol (HOC<sub>6</sub>H<sub>4</sub>)<sub>2</sub>CH<sub>2</sub> (sometimes called a "dimer") is called bisphenol F, which is an important monomer in the production of epoxy resins. Bisphenol-F can further link generating tri- and tetra-and higher phenol oligomers.

Novolaks

thumb|class=skin-invert-image|upright=1.3|Segment of a [[novolak, illustrating the predominance of cresol subunits and presence of crosslinking.]]

Novolaks (or novolacs) are phenol-formaldehyde resins with a formaldehyde to phenol molar ratio of less than one. They are often produced using cresols (methylphenols) in place of phenol itself. The polymerization is brought to completion using acid catalysis with sulfuric acid, oxalic acid, hydrochloric acid, and rarely sulfonic acids. The phenolic units are mainly linked by methylene and/or ether groups. The molecular weights are in the low thousands, corresponding to about 10–20 phenol units. Polymers obtained from this reaction are thermoplastic and require a curing agent or hardener to form a thermoset.

Hexamethylenetetramine is a hardener added to cause crosslinking in novolac. At a temperature greater than 90&nbsp;°C, it forms methylene and dimethylene amino bridges. Resoles can also be used as a curing agent (hardener) for novolac resins. In either case, the curing agent is a source of formaldehyde which provides bridges between novolac chains, eventually completely crosslinking the system.

and Duramold.

The Dutch painting forger Han van Meegeren mixed phenol formaldehyde with his oil paints before baking the finished canvas, in order to fake the drying out of the paint over the centuries.

Atmospheric re-entry spacecraft use phenol formaldehyde resin as a key component in ablative heat shields (e.g. AVCOAT on the Apollo modules). As the heat shield skin temperature can reach 1000-2000&nbsp;°C, the resin pyrolizes due to aerodynamic heating. This reaction absorbs significant thermal energy, insulating the deeper layers of the heat shield. The outgassing of pyrolysis reaction products and the removal of charred material by friction (ablation) also contribute to vehicle insulation, by mechanically carrying away the heat absorbed in those materials.

Trade names

thumb|Tufnol (Novotext) plate

  • Bakelite was originally made from phenolic resin and wood flour.
  • Ebonol is a paper-filled phenolic resin designed as a replacement for ebony wood in stringed and woodwind instruments.
  • Novotext is cotton fibre-reinforced phenolic, using randomly oriented fibres.
  • Tufnol is a laminated plastic available as sheet and rods, which is made from layers of paper or cloth which have been soaked with phenolic resin and pressed under heat. Its high resistance to oils and solvents have made it suitable for many engineering applications.
  • Oasis Floral Foam is "an open-celled phenolic foam that readily absorbs water and is used as a base for flower arrangements."
  • Paxolin is a resin bonded paper product long used as a base material for printed circuit boards, although it is being replaced by fiberglass composites in many applications.
  • Richlite is a paper-filled phenolic resin with many uses, from tabletops and cutting-boards to guitar fingerboards.

Biodegradation

Phenol-formaldehyde is degraded by the white rot fungus Phanerochaete chrysosporium and Rhodococcus phenolicus.

See also

  • Urea-formaldehyde
  • Para tertiary butylphenol formaldehyde resin

References