An adhesive filler or extender is a non-reactive substance (non-reactive with the adhesive) that is added to the adhesive before its use in order to have less adhesive per square foot, but still have enough adhesive with the additive to cover the surface well and get a good glue joint. If we just reduced the spread rate, we would likely get areas with very small amounts of adhesive -- a starved joint. Some commonly used additives are wood flour and flour from the shells of walnut, coconut, and pecans. Outside North America, rice husk flour is used. Occasionally, gypsum, powdered chalk, and talc can be used. The potential list of fillers and extenders is long.


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The two words, filler and extender, are often used synonymously. However, to some folks the difference is that a filler is added by the adhesive manufacturer or by the adhesive user following the manufacturer's instructions and with the manufacturer's blessing. An extender is added by the user without the adhesive manufacturer's specific knowledge or instruction.
All in all, it is a strange procedure, as wood adhesives are really quite inexpensive. A piece of furniture may have 75 percent of its cost in the wood itself, with adhesives being only a few percent. Savings by reducing wood waste and being efficient in processing have a much better opportunity for saving money in manufacturing than using a filler or extender.
Extenders and fillers are most often used with veneer where a super strong joint is not needed. That is, the normal glue joint is many times stronger than needed. Weakening the joint with an extender will not affect the quality in this case. With pva adhesives, I have seen wood flour mixed with the adhesive to get a wood-colored adhesive that is used as a wood filler or putty. Likewise, with epoxy, an extender is used to get epoxy that will not flow, so then the epoxy can be used to “fillet weld” joints, adding to the strength.


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