T ECH NI C AL NOTE NU M BE R 193
UNTDSTATES DEPARTMENT OF AGRICULTURE FOREST SERVICE
FOREST PRODUCTS LABORATORY
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STARVED GLUE JOI14TS
May failures in glued wood products are caused by "starved" joints, joints in which the film of glue between the wood surfaces is not continous Strved joints are readily identified by broken joints showing liteor no wood failure or with little or no glue visible. Such joints, acoding to the Forest Products Laboratory, are not necessarily the rsult of a lack of glue spread on the wood; heavy spreads are as likely produce them under ordinary commercial conditions as light spreads. Thyare caused rather by the application of pressure to the joint while teglue is too fluid.
Stavedjoints are more likelyto occur withglues of low viscosity, such aswarm animaliglue, thanwith casein, vegetable, and other thick glues. Stavedjoints have also been observed with some of the thermosetting
snhtic resin glues when short assembly periods and heavy pressures combined with thin glue mixtures. Such thin mixtures may result frmuse of the mixed glue too soon after mixing.
Soewoods are more susceptible to the production of starved joints than ohrs. Birch, maple, red oak, and ash, which have open pores, absobglue from the spread in such considerable amounts that they often leave the joints starved. Basswood and yellow-poplar also take up a great deal of glue, but weak joints are not very noticeable in the se wood s
bcuse the woods themselves are weak. Other woods with smaller cells open to the gluing surface do not seem to be so subject to starved joints.
In some cases starved joints may result from the use of wood containing too much moisture at the time of gluing. This has been observed frequently inthe use of phenol-resin film glues and is likely to occur whenever wet wood is glued.
To avoid starved joints it is necessary to mix a glue solution thick or to allow it to thicken in the pot or on the wood before pressure is applied.
Whnit is necessary to glue under conditions that might produce starved joitsthe use of light pressure is advantageous.
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