Infusible and insoluble. Alkaline catalyst, phenol, and excess formaldehyde give a low molecular weight resol which is not stable. Slight acidification and further heating give cross-linking. Acid catalyst, excess phenol, and formaldehyde give a low molecular weight novolac that can be stored. Methylolureas formed first under alkaline conditions.
Acidic conditions gives low molecular weight polymer.
Extended heating gives complex cross-linking. Epoxy Resins 1. Addition of curing agent such as ethylene diamine for cross-linking. Polyurethane Foams 1. Addition of water which reacts with some isocyanate to form CO 2 gas, the foaming agent.