Thickening Agents

In a different class to filters but in some ways having the same effect are thickening agents, soluble substances added to increase viscosity. These also are used mainly in the hot setting plywood glues. One effect of increasing the viscosity is to enable more water to be added, so making the glue cheaper, similar to adding fillers. Indeed, it is obvious that both methods can be used together; and there are probably some applications where a little of each is desirable.

Among the earliest thickening agents were the soluble and dextrins. In more recent years boric acid, polyvinyl alcohol, and methyl cellulose and polyethylene glycol have been used or suggested. Quite small amounts only are required, usually less than two percent. Less than one percent of a hydroxy-substituted alkyl cellulose is recommended, usually hydroxyethyl cellulose since it has better water solubility than methyl cellulose. The disadvantage of methyl cellulose are an inverse water solubility temperature curve, and a liability to be salted out by the strong alkali in the modern medium temperature plywood adhesives. Not quite in the same class as either the filter or thickening agent is the addition of sugar (sucrose); this probably has certain advantages, although being soluble it would be expected to reduce water resistance more than an insoluble substance if used in substantial amounts.

Over the years a large amount of chemical research has been done on phenolic resins generally, but in the field of adhesives no revolutionary advance has been made. Many users of phenolic adhesives have no doubt speculated on the advantages, particularly in plywood manufacture, to grained by adding urea-resins. This can only be done in certain cases, and the results are usually disappointing. The urea resins do not make very satisfactory polymers when cured under alkaline conditions, they may a more useful place a resinous adulterants in the acid setting phenolic glues; and there is also the question of incompatibility.