Adhesive Extender

The most useful extenders used with urea glues, especially in veneering work, are farinaceous substances such as cereal or root flours. These are essentially starches, and water causes them to swell. If the temperature is high enough the swelling continues until the granules burst and form a colloidal dispersion. Although the gelatinising behaviour of all starches is not the same, the process conveniently takes place within the normal curing temperature of a hot setting glue, flour, at one time a popular extender in central Europehas a low gelatinisation temperature; this is of undoubted advantage if gluing is done at only a moderately high temperature such as in rubber bag presses

Wheat flour is probably the most widely used flour today, presumably because it is plentiful and cheap, but maize, potato and tapioca are all used in various parts of the world. If the curing temperature of the glue is below the gelatinisation point, undegraded starch behaves predominantly as an inert filler and contributes little towards adhesive properties, whereas if the starch gelatinises, it assists in preserving the viscosity of the glue as the temperature rises and contributes also to adhesion. For room temperature curing, the chemically degraded could water soluble adhesives starches are useful.

Gelatinous temperature of five types of starch that have been used in different parts of the world as extenders.

Starch Type	Rupture observed, oC	Gelatinisation begins, oC	Gelatinisation complete, oC
Rye	45	50	55
Maize	50	55	62.5
Potato	46.2	58.7	62.5
Tapioca	50	62.5	68.5
Wheat	50	65	67