Fillers and Extenders

The subject of fillers and extenders is concerned mainly with the use of phenolic glues in plywood manufacture. By applying a relatively thick layer of extended (or filled) glue, compared with a thin layer of an unextended glue, fewer glue starved areas result when coating uneven veneers in roller type glue, spreaders. This is a matter of considerable technical importance. Another is the effect of fillers in reducing absorption and penetration in the gluing of porous veneers. One of the recognized shortcomings of liquid phenolic glues cured at high temperatures is penetration. Fillers may be added to impart special rheological properties; the incorporation of filler can also facilitate spreading. Finally, there is the advantage of cheapening the flue mixture.





It has been the practice to use the terms "filler" and "extender" rather loosely and often synonymously. In considering the use and definition of these terms suggest the following: "filler" being the non-reactive component of the mixed adhesive, and usually added on the maker's instructions (presumably also by the maker), and "extender" being that added by the glue user (not necessarily with the maker's knowledge or consent). There is much to be said in favor of this distinction.

The commonly used filler or extenders are wood flour, walnut, coconut, and pecan shell flour, Soya bean flour and such cheap inorganic fillers as gypsum, powdered chalk, clays and a number of other oxides and silicates including talc. The reside from oat-hulls and corn cobs after extracting furfural is recommended, and of special interest to eastern countries is the use of ground rice husk as filler. There really is no limit to the list of insoluble substances that could be pulverized and used. One of the latest is ground plum stones.

The amount of filler that can be added -with wisdom- varies considerably it depends on density, practical size and shape; a small particle size is necessary in order to ensure a smoothly spreading mix, and a mesh size of 200 (British standard 410) is common. In the case of wood and shell flour, unlike wood flours absorb water very slowly and therefore do not cause a marked increase in viscosity during the period of the pot-life. Dried blood and starches are extenders of a different class because they are not insoluble and they are adhesives in their own right.

Objectionable as excessive additions of fillers are, the use of fillers is often technically justified –economic considerations apart. The maximum amount that may be added cannot be defined, it depends on the strength, resistance to various influences and durability required. In the manufacture of plywood, a weight of coconut shell flour to the weight of resin solids is nowadays not uncommon.