Improved method for evaluating the quality of phenolic resin bonds of Douglas fir
Creators: name: Northcott, Philip Lachlan
Publisher: name: University of British Columbia
Year: 2012

description: The purpose and history of glue bond testing have been reviewed. Certain deficiencies of standard test methods have been elaborated, and the need for a more accurate test procedure stated. The objective of this particular research was to develop a test that would meet such a need. Experimentation was limited to Douglas fir veneers bonded with hot-press phenolic resin adhesives. The following requirements of an ideal method of estimating glue bond quality were used as a guide in selecting new designs. The foremost requirement is reproducibility of test results. Other essential features include a universally acceptable unit of measurement and a test specimen which is simple and economical to prepare. Other desirable features are that a maximum number of specimens be obtainable from a given sample, and that the method be adaptable to both research and production testing. The Glueline-Cleavage Test developed through this research meets all of the above requirements. The principle is to measure the force required to split or cleave a one inch square plywood specimen along the glue line by means of a "knife" or wedge. The action is similar to the splitting of any wood with a wedge except that the knife is particularly positioned along the glue line. Specimens for this purpose may be either cross-banded or laminated. If the data are to be directly comparable every detail of the specimen, manufacture, testing machine, and test procedure must be standardized. This requirement is common to all methods of test. For quality control purposes test specimens would be out from the plywood at an angle of forty-five degrees to the grain. Every glue line of the specimen may be tested. Special two-ply specimens have been designed for research purposes where accuracy is of the utmost importance. In this type, the material is cut so that the cells intersect the veneer surface at an angle of ten degrees. This small angle, plus the relative weakness of wood in tension perpendicular to the grain, tends to concentrate the stress in the glue line. This insures a glue line failure when the knife is applied in the correct direction. If the knife is not applied in the correct direction the specimen tends to split along the grain of the wood away from the glue line rather than toward it as intended. These research specimens are prepared from two edge-grain veneers glued with the springwood-summerwood bands crossed. Any cross-banding angle up to ninety degrees (common for commercial plywood) may be used. The strengths of matched phenolic-bonded Douglas fir glue lines were compared by five methods, four being mechanical and one relying on wood failure. The mechanical methods were the Block Shear, Glueline-Cleavage, Tension Normal to the Glue Line, and Tension Shear Tests. The Per Cent Wood Failure Method employed wood failure readings from the Tension Shear specimens as estimates of bond quality. The Glueline-Cleavage and Tension Shear Methods included several test specimen designs. The above-mentioned comparisons yielded the following information. (1) mechanical methods proved more accurate than those based upon wood failure estimations, (2) for quality-control purposes the Glueline-Cleavage Test was shown to be equal or superior to the other mechanical methods, and (3) the Glueline-Cleavage Test, when used with specimens designed for research purposes, proved of superior accuracy to all others tested. Additional advantages of the Glueline-Cleavage Method include: (1) the simplest possible test specimen shape and therefore simple and inexpensive manufacture, (2) a maximum yield of specimens from a given plywood sample (a valuable feature with experimental designs requiring large numbers of matched specimens), (3) a lower time requirement per test, (4) much less expensive machinery is required to perform the test, and (5) the exposure of every glue line for inspection purposes when research-type specimens are used. Although the Glueline-Cleavage Test is believed to be the most accurate method yet developed, imperfections remain and several further methods have been proposed for increasing its accuracy.