The use of fillers with plywood UF adhesives has important economical
consequences and is necessary for technical reasons, because the fillers produce ‘‘body’’
in the glue solution and therefore prevent joint ‘‘starvation’’ in porous wood. Without
filler it would be difficult to prevent part of the adhesive from flowing away or flowing into
the open pores of the wood; or in the gluing of medium to thin veneers, from flowing
through them to the other side, thereby causing undesirable resin patches on the outer
veneer surfaces. As a rule, 20 to 50% filler is used for joinery and up to 100% for plywood.
The most common fillers are wheat flour, corn flour, rye flour, very fine hardwood flour,
and gypsum. If gypsum is used, it must be free of calcium hydroxide, because this
interferes with the acid curing agent.