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DryAdd for Phenol-Formaldehyde Resins

The reaction of phenol with formaldehyde shows a complex range of chemistry, depending on the pH of the system, the presence of catalyst, and on the relative amounts of each material. Textbooks differentiate between resols, which are highly-branched molecules formed under alkaline conditions, with a large excess of formaldehyde. Novolacs, on the other hand, are frequently formed in acidic conditions, with excess phenol. Novolacs are generally linear systems, and require addition of a crosslinker before they will form a network.

The following examples show representative cases for resols and novolacs, and discuss the case where a 'mixed' chemistry is used, when a phenolic resin is reacted to give epoxy groups, which can then crosslink. DryAdd's ability to form 'prepolymers' is a useful feature in such multistep simulations.

Resols are formed in alkaline conditions, with an excess of formaldehyde. The active ortho and para sites on the phenol (denoted P in the screenshots) react with formaldehyde (denoted by F) to give methylol (-CH₂OH) sidegroups. The methylolphenol is even more reactive than phenol itself towards reaction with formaldehyde - this can be modelled with DryAdd's substitution effects, and gives rise to branched molecules.

The methylol groups are set up as 'phantom groups' on the formaldehyde molecule. Phantom groups are formed once the reaction starts, and they can participate in later reactions - they are ideal for cases in which a reactive site is formed in one of the reactions, as happens here. Two group types - ortho and para- are set up, to allow for differences in reactivity.

For a system with so few materials, the reaction scheme is very complex, as shown in the Figure above. There are three main types of reactions: phenol with formaldehyde, to form methylol groups; reaction of methylol sites with ortho or para sites on phenol, and consequent loss of water to form methylene bridges; and reaction of two methylol groups to give ether linkages, with loss of water. Kinetics have been taken from published literature.

Reaction can be monitored from the A-stage resol, through the resitol and resit, to look at ultimate crosslinking density and mechanical properties.

Novolacs are formed under acidic conditions, usually with a molar excess of phenol. In acid conditions, the methylol group is unstable; it decomposes to leave an active methylene group which reacts with phenol to give methylene bridges. Therefore, ether bridges, found in resols, do not exist for novolacs. Since methyol-methylol condensations are not present, the reaction scheme is relatively simple. The kinetics are not as well established as they are for resoles, but reaction of a substituted phenol with another phenol is known to be 5 to 10 times as fast as the initial reaction of formaldehyde with phenol. These have been used in setting up the relative reaction rates.

In resols, the initial reaction to give methylol increases the tendency of the unreacted o or p sites to react. In the acid conditions found for novolacs, these sites are inhibited from further reaction. Again, this can be modelled by DryAdd's substitution effects. The result for novolac materials is the formation of fairly short linear chains.

The exact molecular weight distribution depends on the phenol-formaldehyde ratio. Generally for novolacs, P:F is in the ratio of 1:0.7 to 1:0.85. Two molecular weight distributions, obtained at these extremes, are shown in the screenshots - these compare favourably with published results, which show a higher molecular weight 'tail' when there is more formaldehyde.

Novolacs can be reacted with epichlorohydrin to give epoxy-novolac systems. The OH from the phenol forms an epoxide group, so there are generally between 3 and 6 epoxide groups per molecule. Formation of such epoxy-novolacs can be modelled with DryAdd, as can their subsequent crosslinking - our Epoxy Application notes give more information about how reactions of epoxy groups with tertiary or primary amines can be set up within DryAdd.

Crosslinking with materials like HMTA (Hexamethylenetetramine) poses a bigger challenge within DryAdd, because the reaction scheme is exceptionally complex, and kinetics are not readily available. Depending on temperature, either the HMTA can decompose to formaldehyde and ammonia, and ammonia will catalyse further reaction, or else the amine will be retained in the final resin. Good kinetic data provide the key.

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