The Hardening of Resorcinol Adhesives.

The novolak resin is converted to a thermosetting resin through the addition of formaldehyde by the user, and herein lies the most important feature of a resorcinol adhesive and its difference from the ordinary phenol resin. Theoretically, sufficient formaldehyde is needed to bring the molar ratio of formaldehyde to resorcinol to the region 1:1. A final ratio of slightly less than 1:1 is adequate for curing a resin that is neutral or weakly alkaline, but in practice it is usual to have formaldehyde in slight molar excess, the actual amount desirable for cross linking depending on the characteristics of the novolaks.

Although it is commonly supposed that resorcinol formaldehyde adhesives are neutral, this is not always the case unless we interpret "neutral" rather widely. The limit for commercial products are usually between about pH 6 and pH 9, a range of values that can be harmful to few adherents.

The pot-life of resorcinol adhesives is influenced by pH, the relationship between pH and gelatine time (in this context the terms gelation times and pot-life are considered to have the same meaning) for more or less strandar resorcinol formaldehyde resin. The longest gelation time (minimum reactivity) to occur between pH 3 to 4 and the shortest gelation time to occur around pH 7 – 8.

The relation between pH and gelation time is important because, although the position and shape of the curve may differ with the different resins, once the curve is established the information can be used to predict the change in pot-life that can be effected by altering the pH. The important part of the curve is between pH 6 and pH 9, and within these limits a pot-life variation to a pure resorcinol novolak and a resorcinol/phenol novolak, each with a paraformaldehyde hardener.