The relationship between pH and gelation time is important because, although the position and shape of the curve may differ with different resins, once the curve is established the information can be used to predict the change in pot-life that can be affected 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 of several hours can generally be obtained. The relationship, with regard to a pure resorcinol novolak and a resorcinol/phenol novolak, each with a para-formaldehyde hardener, is shown the graphic below.
Formaldehyde may be used in any form in which it can be made available to the resin under the curing conditions. In practice the polymeric solid form of formaldehyde known as para-formaldehyde is most frequently used as the hardening or curing agent. Aqueous solution of formaldehyde in an immediately available form, thereby coverting the novolak to resite and effecting complete cure of the resin at a lower temperature than is possible when using para-fromaldehyde. The rate of depolymerisation temperatures, and is further accelerated by acids and alkalis. The curing reaction may be exothermic depending on the novolak and the form in which the formaldehyde is added.
Formaldehyde denote as phenolic resin can be used to cure resorcinol glue, and moreover such a hardener can be used by separate application. There are certain advantages in using a phenolic resin a hardener, such as a reduction in the rate of drying out and the ability to cure at temperatures as low as 5oC. Unfortunately we impossible make an inherently liquid resorcinol resin.