CHEMICALS FROM WOOD

ALFRED J. STAMM.

Chemical processing of wood, up to the present time, has been limited to (1) destructive distillation, whereby charcoal, wood alcohol, acetic acid, turpentine, and tars are produced, and (2) extraction processes with water or petroleum solvents that remove the extraneous materials, such as tannins, turpentine, rosin, and essential oils.

The destructive-distillation process in recent years has not proved to be very profitable, because many of the products formerly produced exclusively by the process are now made more cheaply by synthetic methods.

The extraction process, although highly profitable when applied to a few species, is not suitable for all woods and actually utilizes only from 1 to 20 percent of the weight of the wood.

Two rather new processing methods, hydrolysis and hydrogenation, show promise of broader application. Hydrolysis changes the cellulose and other carbohydrate material into sugars. Hydrogenation causes hydrogen gas to react with the wood components at high temperatures and pressure to form liquid products.

The new procedures, together with the old, provide a practical approach to the chemical utilization of wood residues.

Wood residues are especially adapted for chemical processing because wood of any form or size or quality can be used. Sawdust, shavings, slabs, trimming, cordwood, and cull logs are all suitable. Further, the presence in the residue of relatively large amounts of knots, bark, and even wood in the early stages of decay does not interfere with most of the processes, although it may reduce the yield of chemical products. Even though it would usually pay to process the hardwoods (broadleaved species) separately from the softwoods (cone-bearing species) for the reason that the products and yields from those two general classes of wood differ, it is not necessary generally to separate them.

The amounts of wood residues available indicate the possible magnitude of a chemical industry based on their full utilization. Naturally, the first wood residues to be considered for such an industry would be those that occur at sawmills, veneer mills, and secondary manufacturing plants, because the material is already at hand and a large part (in the form of sawdust and shavings) is already reduced sufficiently in size for use. Some 16 million tons of such material now remain unused each year. An additional 27 million tons are burned to generate steam for plant operations. As soon as its chemical-processing value becomes greater than its fuel value, which at present averages about $4 a ton, this material, too, will be available for chemical processing.

The total mill residue is equal in weight to one-fifth of the national petroleum production. Left unused in the woods each year, because their removal is considered unprofitable, are 44 million tons more of cut wood, chiefly crooked, split, and partly decayed material unsuitable for lumber but admirably suited for chemical use. Still another 23 million tons of standing timber are killed by fire, lightning, or insects each year and left in the woods; a large part of that would also be suitable. Altogether, those residues equal in weight about half the present petroleum production. Enough wood residues are available, then, to supply a great new chemical industry.

Such a large industry would not be warranted unless its products were in sufficient demand at a price for which they could be profitably produced. The nature and uses of the products obtained by the various methods of chemically processing wood can be a measure of this demand and value.

EXTRACTION differs from the other chemical-processing methods in that it is highly dependent upon species and alters the wood substance only slightly. The only extensive wood-extraction industry is the naval stores industry of the South, which extracts turpentine and rosin from old stumps of longleaf and slash pines from which the sapwood has decayed. Only the heartwood stumps of those species are used because of their high extractive content. The industry processes about 6,000 tons of stump wood daily to obtain 12,500,000 gallons of turpentine and 750,000 drums (520 pounds to the drum) of rosin a year. Turpentine is used chiefly as a paint thinner, a medicinal, and a raw material for making synthetic camphor and other valuable synthetic products. The rosin is used chiefly in soaps, paper size, paints, varnishes, sealing waxes, cements, and plastics. Large amounts of cymene and a rosin residue are also obtained. The latter is used in plastics and as a binder for sand in foundry cores.

Chestnut wood chips and hemlock bark are extracted to obtain tannin for tanning leather. In no case is the tannin content of wood sufficient to make extraction profitable for it alone. In the case of chestnut, the extracted chips have been used to form pulp for paper making. The chip residue might also be used for further chemical processing.

Years ago a small industry existed in the Northwestern States in which the butt logs of western larch were extracted with water to remove the large amount of water-soluble gum that they contain. The gum was chemically converted to mucic acid, which is used as the gas-liberating acid in some brands of baking powder. The process was not a financial success because the large amount of chip residue was unused.

A number of small plants scattered about the country extract essential oils, medicinals, and flavoring materials from needles, bark, roots, or wood of various species. Those plants, like all other extraction plants, could profit by chemical refining of their residues.

DESTRUCTIVE DISTILLATION is by far the oldest wood-chemical-processing industry. For years the charcoal residue was the only product sought. Charcoal is used as a domestic and picnic fuel; in smelting and reducing various ores; in making such chemicals as carbon bisulfide, which, in turn, is used in making viscose, rayon, and cellophane, and also sodium cyanide, a powerful disinfectant.

Now a number of valuable volatile products are also obtained by condensing the vapors from destructive distillation. In the case of hardwoods, methyl alcohol ( wood alcohol) , acetone, and acetic acid are obtained as a water-soluble distillate, together with the water-insoluble tars and pitches. Methyl alcohol is used as an antifreeze agent in the radiators of automobiles, for denaturing grain alcohol, as a solvent in many industries, and for making formaldehyde, which, in turn, is used as a disinfectant and in making plastics. Acetone is used as a solvent in the rayon and plastic industries, and acetic acid in making white lead paint and acetate rayon and films. Yields of methyl alcohol, acetone, and acetic acid are lower from softwoods than from hardwoods.

The pines, however, yield considerable turpentine and softwood tar. Softwood tar is used in the compounding of rubber, to some extent in manufacturing oakum for calking ships, in cordage, and also in medicinals. Both softwood and hardwood tars find use as flotation oils in mineral separation and as gasoline gum inhibitors. The heavier fractions are used as preservatives, disinfectants, and stains. The pitch finds use as a waterproofing and insulating agent and as a binder for briquets.

Although the products obtained by the two older wood-processing methods named are of considerable industrial importance and could perhaps be used in larger amounts than are now produced, a large expansion in their production does not now seem warranted. Any real increase in the chemical utilization of wood will thus have to be by the processes which produce products that are in greater demand.