Wednesday, April 9, 2008

Addition of Polyvinyl Acetate to Urea Formaldehyde Adhesives

Many people concerned with gluing wood and other cellulose materials and having at hand both urea glues and polyvinyl acetate glues have mixed them together and used them in this way. A large number of test with such mixtures have no doubt been made in laboratories throughout the world, but with the exception of the report of Raknes on work done in Norway, little has been published, Raknes investigation was extensive but brought to light no unexpected effect, the properties of the adhesives being governed largely by the ratio of the thermosetting to the thermoplastic component.

Water soluble U.F. glues are compatible with aqueous dispersion of polyvinyl acetate, although mixture of the two may not be stable over long periods of time. The acidity of PVA dispersion may reduce the pot-life of UF adhesive, and in extreme cases the addition of conventional hardener may not be required. Where the amount of U.F. component is for example PVA would be further reduced, but where the U.F. component is for example only 10 % of the total, there may little or no advantage in adding a hardener.

With each component having advantages or disadvantages not possessed by the other, mixtures can be formulated that, as it were, upgrade one component while inevitably downgrading the other. But desirable intermediate properties can be obtained, for example increase impact strength compare with a straight U.F. adhesive, and reduced creep under stress compared with a straight PVA adhesive. Other effects created by the PVA component include an increase in tack or grab, and a reduction of wet strength and heat resistance.

Although there is little evidence of recommendation by adhesive manufacturers it is clear that certain desirable properties can be obtained with mixtures varying from about 20 % of one component to 20 % of the other.

That there is some interest among adhesive manufactures is, however, shown by the granting of a patent on the spray-drying of a mixture of a U.F. resin and a PVA dispersion, from which patent specification it may inferred that the dried mixture can be easily re-dissolved or re-dispersed in water.

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