If the A-stage resin is heated, it passes to the "Resitol" or "B-Stage. In the B-stage the resin is swollen by solvents and instead of melting becomes rubbery on being heated, due to molecular entanglement by branching and some cross linking. On further heating the resitol is converted into the more completely cross linked "Resite" or C-stage resin, which is substantially insoluble and infusible.
An A-stage resol that is made by reacting a novolak with additional formaldehyde (two-stage resin) follows a similar route to the C-stage.
The thermosetting reaction normally takes place rapidly at elevated temperature but extremely slowly at room temperature. However, by addition of strong acid catalyst the C-stage can be reached rapidly at room temperature and this is the method used to cure cold-setting phenolic adhesive.
Since an alkaline catalyst favours the formation of the highly reactive methylol compounds and an acid catalyst the formation of the relatively unreactive phenol-methylene chains, the reaction of phenol and formaldehyde in equimolecular proportions can produce either a resol or a novolak, depending on the catalyst used.
Types of resin used as adhesives
Completely novolak forms of phenolic resins are not themselves important as adhesives. On the other hand the two stage resin (the novolak that has been converted to a resol by addition of formaldehyde) and also the single stage resin, which is potentially thermosetting from the outset, form the two basic chemical types of phenolic resins used as adhesives.
When used as a hot setting adhesives in, for example, plywood manufacture, the resin is made, stored and finally applied in its A stage, and in the hot press passes through the B stage to the C stage.
The average user may not be greatly concerned due to:
Ø Hot setting phenolic resins which are either strongly alkaline and water soluble and commonly used as adhesive for plywood, or weekly alkaline and alcohol-soluble as used in the Two-Polymer adhesives.
Ø Resins which are cold setting on addition of strong acids and therefore of importance as assembly glues.