New Goods From Wood

by ALFRED J. STAMM and G. H. CHIDESTER

PRACTICALLY everything was scarce the first year or two of the war, and we tried to make wood a substitute for steel, aluminum, rubber, and many other materials. Then wood became scarce, so we worked to make wood products as serviceable as possible in the uses to which they were most applicable. Our research in both periods had the same technical objective: The improvement of wood service.

From the search came an array of materials with strange names. Many of them will doubtless find their way into the production lines of the Nation's furniture, farm machinery, building, and other industries. The new products include woods specially treated to fortify them against shrinking, swelling, and the inroads of decay organisms, and other ills that raw wood is heir to; low-cost wood plastics made of the wastes ordinarily burned or otherwise disposed of by sawmills, paper mills, and similar wood-using plants; and paper and wood-pulp materials strengthened and made more durable for exposure to rain, heat, cold, and other rigorous conditions. Many of the materials are adapted to large-scale Production methods.

We worked on three principal types of materials. One was the so-called "modified" woods, consisting principally of wood in the form of thin veneer sheets that were given various treatments to decrease their natural tendency to swell, and that were sometimes compressed or molded under heat and pressure to form panels or molded products. From these treatments came impreg, compreg, and staypak.

The second principal type of wood materials extensively developed during the war can be broadly called "plastic" wood-base materials, or "lignin plastics." The basic production process consists of a chemical treatment of hardwood sawdust or wood chips that removes part of the cellulose and leaves a residue of stable cellulose and lignin. When a plasticizer like phenol-formaldehyde is added to the resulting powder or pulp, it can be molded under heat and pressure to form various boards and other articles.

The third general type includes various paper-base laminates and wood-pulp plastic materials that can be molded. Most important among these products during the war was papreg, the basic material of which is a special paper chemically treated so that it can be compressed under heat to form a multilayered sheet suitable for many uses. Thin papreg can also be molded to the faces of plywood to provide a water-resistant surface that hides plywood defects, minimizes checking, and can be painted readily. Among molded pulp materials is a long-fibered pulp preform that can be compressed to various shapes in a mold.

Impreg (from "impregnated") is wood treated with phenolic resin-forming chemicals according to a method developed at the Forest Products Laboratory, in which the chemicals enter and bond to the cell-wall structure, and the resin is dried and cured within the structure. When resin is thus made an integral part of the wood, the tendency of the wood to swell and shrink is permanently reduced. Phenolic resin-forming systems have proved to be the most effective in stabilizing wood dimensionally. It is possible to reduce swelling and shrinking -to 30 percent of normal.

The stabilizing of wood by a resin treatment differs from preservative and fire-retardant treatments in that it must be much more complete. The resin must be uniformly distributed throughout the entire cell-wall structure to be fully effective. For this reason the treating of lumber and the treating of freshly felled logs have not met with the success that some investigators have claimed. Veneer of practically any species can be adequately treated. Practically none of the woods can be properly treated in lumber thicknesses and lengths. Even if lumber could be adequately treated, the increase in cost would make the material prohibitively expensive for the majority of proposed uses.

For these reasons, development work on impreg and its commercial production have been confined to the use of resin-treated veneer. These treated plies have proved suitable as facing for solid wood or untreated plywood. The chief advantage of such resin-treated facings is that they practically eliminate face checking and greatly reduce grain raising, even when exposed outdoors without a surface finish. Even the checking of fancy crotch veneer used in furniture can be nearly eliminated.

The treatment also imparts to the panels considerable resistance against decay and attacks by termites and marine-borers. Panels consisting of two resin-treated face plies with a single untreated core ply were inserted in the ground for a year in a field in Mississippi where termite action is severe. The termites tried the faces but found them not to their liking.

Like soldiers who failed in a frontal thrust, they tried a flank attack and, finding the untreated core just what they wanted, proceeded to clean it out. Similar material that has had the edges protected with a preservative treatment and all the plies treated is frequently sound after 5 years.

The resin treatment cuts down the passage of water vapor through the panels to a marked extent and greatly increases the electrical resistance and the resistance to most chemicals, except strong alkalies. Resin treatment has a negligible effect upon fire resistance. Fire-resistant salts, however, when incorporated in the wood with the treating resin, are fixed in the structure and give the wood good fire-retardant properties.

Only a few of the strength properties of wood are significantly increased by a resin treatment. Toughness is lowered, but hardness and compressive strength are increased.

Impreg was manufactured during the war only for military use. The most interesting use was for facing laminated redwood aircraft carrier decking to obtain a wear- and splinter-resistant decking equivalent to teak but lighter than Douglas fir. Service tests on one carrier indicated wearing qualities far superior to Douglas fir. Another wartime application of impreg was as housing for electrical control equipment, in which its superior electrical properties proved especially advantageous. Impreg shows the greatest promise as resin-treated faces for ordinary plywood. Such panels might be used as siding for houses, trailers, and boxcars, flooring, and paneling. It remains to be proved, however, that the improved properties warrant the increased cost of the treatment.