Bending strength of spruce end-grain butt-joints using resorcinol formaldehyde-based adhesives

This paper presents a literature review of wood end-grain butt-joining and a study of butt-jointed end-grain surfaces of sawn spruce heartwood (Picea abies Karst) using different resorcinol formaldehyde (RF)-based adhesive formulations. A general discussion of end-grain joining is also presented, based on the literature and on an analysis of joined samples. The analysis concentrates on bending strengths of the butt joints from surfaces directed both tangentially and radially when loading the beams containing end-grain butt joints. In some of the trials, the RF adhesive was impregnated into fibers as carrying media (i.e., a thin adhesive-impregnated mat between the two pieces being joined) to ensure a high retention of the RF adhesive in the end-grain surface. An increase in the bending strength was achieved by increasing the amount of resorcinol in the RF adhesive formulation by raising the solids content of the polymer used. The highest three-point bending strength value obtained was approximately 15 MPa, which was 20 percent of the bending strength of the straight-grained clear spruce. Both the conventional RF adhesive formulation and the fiber-carried formulation led to lower bending strength values (approximately 3 to 9 MPa (5% to 13% of the values for the clear spruce)). The low bending strength values and the large scatter in the case of the commercial adhesive formulation was presumably induced by the filler in the adhesive hardener. This type of hardener probably prevented uniform penetration of the adhesive. The low bending strength values for the fiber-carried adhesive formulation appear to be related to a porous glueline structure due to excessive penetration by pressure followed by capillary inhibition in earlywood, and a presumable shrinkage in the glueline during the curing of the adhesive. This porous glueline structure is particularly seen in the highly filled earlywood boundary region in the end-grain surface. It was observed with different microscopy techniques (light microscopy and scanning electron microscopy) that the sawn end-grain butt-joined surfaces in this study were deformed despite the use of a circular saw that was sharpened and balanced three times. The need to develop an appropriate method or tool to obtain undamaged end-grain surfaces to ensure penetration and anchorage of the adhesive to undamaged wood end grain was mentioned in the literature 30 years ago; that need still exists.

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