Effect of Wettability of Wood on Glue-Line Behavior of Two Urea Resins

The purpose of this study was to examine more closely the effect of the wettability of wood on glue-line behavior. The general approach was to glue blocks of several species selected to cover a wide range in wettability and to vary the gluing procedure only from the standpoint of employing a series of closed assembly times with each of two urea-resin adhesives. For each assembly time tests were made to determine glue-line solids content and glue-line viscosity immediately prior to the application of gluing pressure. Following completion of adhesive cure, block-shear tests were made to determine glue-joint strength. The six species used were almendro, bulletwood, guayacan, teak, muira-juba, and timbauba. Wettability was determined using the contact-angle method previously described by Freeman. During the periods of closed assembly prior to the application of gluing pressure, glue-line solids content and viscosity increase more rapidly in woods of high wettability. Slip load (a measure of glue-line viscosity) at the time of pressure application has, in turn, been shown to be related to ultimate glue-joint quality. Although attained over a wide range of closed assembly times with two urea-resin adhesives, maximum glue-joint strength was found to be associated with a relatively narrow range of slip loads.

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