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Certain adhesives are loaded with fillers, whereas others are without any filler whatsoever. Certain adhesive makers claim their products are filler free and hence more stronger. Do fillers reduce the cost only or they impart any benefit to the adhesives other than giving any cost advantage. Do fillers reduce the strength of glues even if added in small quantities.
I am not an industrial chemist so this is largely intuition, but I take "fillers" to be particulates that take up space and do not shrink, thus aiding in gap-filling when gluing imperfectly machined joints, much like sand in sanded tile grout.
I would think about epoxies as an example. Not so much in the woodworking sense as more often than not we need a clear glue, hence no fillers.
In many epoxy applications such as fiberglass work and wooden boat work, fillers are necessary to control viscosity, hardness, sandability, and adhesion(microfibers soak up resin and prevent it from soaking into a substrate).
In general the fillers are added to other adhesives to control viscosity. They do slightly reduce adhesion, but it is a negligible amount.
Please give a example of a filler free product.
Adding fillers in theory does reduce absorption into the wood. Wipe the wood surface with some of the unfilled epoxy first then apply the filled epoxy which does more than bond with the unfilled layer- it mixes. Best of both worlds.