Staypak and Staybwood

Resin-treated wood in both the uncompressed and compressed forms is more brittle than the original wood. To meet the demand for a tougher compressed product than compreg, a compressed wood containing no resin was developed at our laboratory. It will not lose its compression under swelling conditions as will ordinary compressed wood. The material, named staypak, is made by modifying the compressing conditions so as to cause the lignin-cementing material between the cellulose fibers to flow sufficiently to eliminate the internal stresses.

Staypak is not so water-resistant as compreg, but it is twice as tough and has higher tensile and flexural properties. The natural finish of staypak is almost as good as that of compreg. Under weathering conditions, however, it is definitely inferior to compreg.

For outdoor use staypak should have a good synthetic-resin varnish or paint finish. Staypak can be used in the same way as compreg where extremely high water resistance is not needed. It shows promise for use in propellers, tool handles, forming dies, and connector plates where high impact strength is required.

The cheapest and simplest method of imparting dimensional stability to wood thus far found is to heat the wood under conditions that just avoid charring. This can be done with a minimum loss in strength properties by our method of heating the wood under molten metal for a few minutes. The wood becomes dark brown in color and loses about one-half of its original toughness, and a moderate amount of other strength properties. Equilibrium swelling and shrinking can be reduced to 60 percent of normal and an appreciable decay resistance is imparted to the wood by this treatment. Staybwood may find some use in places where dimensional stability and moderate decay resistance are more important than strength.

Lignin Plastics

Lignin, which in a sense is nature's cementing material between the cellulose fibers, can be freed from the cellulose by a mild acid hydrolysis and subsequently used as a semiplastic to bond the structure together again.

Besides breaking the cellulose-lignin bond, the mild hydrolysis converts the hemicelluloses to sugars while the stable cellulose remains with the lignin to serve as a plastic reinforcing material. The removed sugars can be either fermented or used for the growing of yeast. The residue is dried and then ground to a powder. Although this hydrolyzed residue does have some plastic properties, it does not make a good plastic when used alone, because of the extremely high temperature necessary to cause the lignin to flow even moderately and the relatively low water resistance of the product. For this reason it is used preferably in conjunction with other plastics, such as phenol-formaldehyde, which improve both the flow and the water resistance.

Under these conditions, a plastic quite similar in appearance, water resistance, and electrical properties to common black phenol-formaldehyde plastics can be made by using 75 percent of hydrolyzed wood and 25 percent of phenolic resin. This mixture is in contrast with the mixture of 50 percent of wood flour and 50 percent of phenolic resin used in making the phenol-formaldehyde molded products. The strength properties notably toughness of the lignin plastic are slightly lower than those of the normal phenol-formaldehyde molded products. Mold flow is also inferior, but the acid resistance of the ligning product is better.

A commercially developed modification of the acid-hydrolysis process, in which the wood is hydrolyzed with an alkaline medium that becomes slightly acid at the end of the cook, gives a similar molding powder with strength properties superior to those of the acid-hydrolyzed product. This material, when used with only 25 percent of phenolic resin, still lacks the rapid and more extensive flow of the ordinary phenol-formaldehyde molding powders. Although the addition of more resin improves the flow, it reduces the price advantage. It is this lack of flow that has held back the commercial use of hydrolyzed-wood plastics. In large objects with limited need for flow, hydrolyzed-wood plastics may be used to advantage, however, because of their lower cost.

Unfortunately, none of the molding compositions shows promise of utilizing very large quantities of wood waste. For example, if. all the present phenol-formaldehyde molded products were to be replaced by the hydrolyzed-wood plastics, three moderate-sized lumber mills could furnish all the raw material needed in the country. As board materials show promise of larger volume consumption, we have focused considerable attention upon such materials.

The hydrolyzed-wood molding powders are not suitable for making board materials with adequate strength properties, notably toughness. The strength properties can be greatly improved by having the cellulose reinforcing material in longer-fibered form. This can be accomplished by using hardwood chips in place of sawdust and abrading the washed hydrolyzed chips while still wet to a pulp rather than grinding them to a powder. This pulp can be made into paper on a paper machine. After 10 to 15 percent of phenolic resin has been added, the sheets can be pressed at high temperatures and a pressure of about 2,000 pounds to the square inch into a high-density board with good strength properties and water resistance. The board cannot be nailed but can be drilled. This fact, together with its high density and molding cost, makes it unsuitable for general housing applications. It appears suitable for electrical paneling and for such purposes as shower-bath walls.

Pulp boards have been made in the laboratory from the hydrolyzed chip fiber by forming thick pulp mats that are pressed wet under a pressure of 100 pounds to the square inch or less without the addition of any phenolic resin. The boards have properties comparable to those of untempered commercial hardboards. They can be nailed. They can be made from softwoods as well as hardwoods, but the strength properties and water resistance of the softwood product are somewhat inferior to those of the hardwood product.

Although these and other similar hardboards show promise for use as a sheathing material for houses and in other ways that wood is used, they are far from being synthetic lumber. Their use in housing should nevertheless expand.

Cooperative research with a paper mill has demonstrated the possibility of using soda-pulp lignin in laminated plastics. Soda-pulp lignin is the simplest to isolate and has the best plastic properties of the various forms of lignin waste. It can be incorporated with the pulp in the beater to form a laminating sheet which requires no auxiliary resin to produce a dense plastic material with good properties. This is potentially one of the cheapest plastic laminates now known and should find considerable use. We have learned that soda-pulp lignin in can be used to dilute phenolic resin and the solution can be applied to paper or fabric. It can replace 50 percent of the phenolic resin ordinarily used without significantly affecting the properties of the resulting laminate.