For hardening, resins are sometimes also used by weak organic acids (sorrel, citric, milk, etc.). However, all these are expensive products and their use is not justified economically. To slow down the hardening process, an ammonium oxide hydrate (NH4OH), urea or urotropin is added to the resin. Ammonia water and urea slightly reduce the release of formaldehyde during hot pressing.
The English company, which put in the USSR, the equipment for continuous pressing, recommended the following recipes of hardeners for its resin.
To reduce the viscosity of the resins, various solvents are usually added. For carbamide resins, such a solvent is water, sometimes with a certain amount of NH4OH (2-3%solution). It should be borne in mind that the water is still introduced into the resin along with a hardener solution and with paraffin emulsion. Therefore, before diluting the resin with water, it is necessary to establish to what value the viscosity of the resin will decrease when these components are added.
The aging process is influenced by the temperature conditions of storage of resins. Resins are recommended to be stored at a temperature of +5 °. At temperatures above 20 °, the aging process of adhesives proceeds very quickly.
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