Gluing Characteristics of Softwood Veneers and Secondary Western Hardwood

Most adhesives developed for the softwood plywood industry were designed for use with Douglas-fir (Pseudotsuga menziesii). With the interest in utilizing other western softwoods and hardwoods in plywood, a review of typical gluing characteristics of several of these species is made. The western soft pines (Pinus ponderosa, P. lambertiana, and P. monticola) have been successfully glued with most of the common plywood adhesives. Their more absorptive character requires heavier glue spreads and staining through face veneers is a problem. Pitch may require certain modifications in protein adhesives, but phenolic formulations appear satisfactory. Lower press temperatures and longer times are recommended relative to Douglas-fir pressing. Redwood (Sequoia sempervirens) bonds well with cold- press proteins, hot-press proteins, and exterior phenolics, but stains due to a chemical reaction with the glue. This requires careful spreading and control of veneer quality. Compressibility of veneer suggests the need for lower pressures and the lower density requires greater glue spreads. Western redcedar (Thuja plicata) requires lower than normal temperatures to avoid blistering and pressure as low as 100 psi to avoid over-compression. Western larch (Larix occidentalis) glues well with standard adhesives except where galactan is present. Successful gluing of veneer with galactan requires modification of the adhesive. Sitka spruce (Picea sitchensis) requires heavier glue spreads because of its absorptiveness and when rough veneers are used. Rocky Mountain Douglas-fir glues well when veneers are properly dried. The true firs, including Abies concolor, A. procera, A. grandis, and A. amabilis, bond well with most glues but require drying to low moisture contents because of wet spots. This necessitates heavier glue spreads. Western hemlock (Tsuga heterophylla) requires heavy spreads where rough veneer is used and where the veneer is dried to low moisture content to avoid wet spots. The western hardwoods considered are red alder (Alnus rubra), cottonwood (Populus spp) Pacific madrone (Arbutus menziesii) and tanoak (Lithocarpus densiflora). Little work has been done with most of these species, but they appear to glue well. Cottonwood requires high viscosity glue and heavy spreads but no unusual difficulties were found with the other hardwoods.

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