Phenol formaldehyde resins, as a group, are formed by a step growth reaction which may be either acid or base catalysed. The pathway the reaction follows varies depending on the catalyst type used.
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2 Base catalysed 3 Crosslinking and the phenol/formaldehyde ratio |
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 between phenol and formaldehyde is not a simple one, and the 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.Acid catalysed
Base catalysed