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The earliest commercial synthetic resin is based on a Phenol formaldehyde resin with the commercial name Bakelite, and is formed from a elimination reaction of phenol with formaldehyde. Phenol is reactive towards formaldehyde at the ortho and para sites (sites 2, 4 and 6) allowing up to 3 units of formaldehyde to attach to the ring. This forms a hydroxymethyl phenol, which is not usually isolated in novolacs but is found in resoles (see below). 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 catalysed. The pathway the reaction follows varies depending on the catalyst type used.
Acid catalysed 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 crosslink (polymerize) without the addition of a crosslinking agent. Novolacs are commonly used as photoresists. See also photolithography.
Base catalysed 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 goo, the resin.
The rate of the base catalysed reaction initially increases with pH, and reaches a maximum at approx. pH = 10. The reactive species is the phenolic anion formed by deprotonation of phenol. The negative charge is delocalised over the aromatic ring, activating sites 2, 4 and 6, which then react with the formaldehyde. Sort of.
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 polymerised 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 and solvation. It is also the reason they are called thermosets.
Crosslinking and the phenol/formaldehyde ratio
Phenol can react with formaldehyde at any one of three possible sites, and formaldehyde can react with up to two phenols. Thus the theoretical functionality of phenol is three and the theoretical functionality of formaldehyde is two. The actual functionality that is found in the polymer depends on the phenol:formaldehyde ratio.
Take some phenol, add a little acid catalyst (something miscible with phenol, like p-toluenesulfonic acid) and then start adding formaldehyde slowly. Under these conditions, the formaldehyde will react between two phenols to form a methylene bridge, creating a dimer. As more formaldehyde is added, more phenols will be crosslinked together generating more dimers (and as the concentration of dimers increases, there is the possibility of generating trimers, tetramers and higher oligomers). This is what occurs during the formation of a novolac. The average molecule generated depends on the ratio of formaldehyde to phenol. In novolacs this is usually around 0.8, and so, with 5 phenols for every 4 formaldehydes the average molecule is a pentamer (with respect to phenol).
When the molar ratio of formaldehyde:phenol reaches one, in theory every phenol is linked together via methylene bridges, generating one single (huge!) molecule, and the system is entirely crosslinked. This is why novolacs (F:P <1) don't harden without the addition of a crosslinking agent, and why resoles (F:P >1) will.