Some Industrial Outlets for Seed Flax

Howard M. Teeter.

The flax plants from which we get fibers are tall, little-branched, and early maturing. Their seeds are small, and the fibers in the stems are long.

On the other hand, seed-flax plants are short and branched. They are selected for high yield of seed. Processed by conventional methods, they produce short fibers of little value for making cloth.

The seed of the fiber-flax plant yields an oil of good quality. Seed not required for replanting therefore is usually sold in the oilseed market.

Nearly 5 million acres of seed flax is grown each year in the United States. Our acreage of fiber flax has varied from 4 thousand to 18 thousand since 1940. Most of the fiber-flax acreage is in Europe.

How long seed flax will continue to be used almost exclusively for oil is problematical. Considerable research has been done with the object of making satisfactory cloth from the fiber of seed flax.

A continuous process for producing yarn from the straw of ripe seed flax was announced by the University of Minnesota in 1948. The straw is separated into fibers 12 to 18 inches long by chemical and mechanical means instead of by the traditional retting, which depends on fermentation and produces, from seed-flax straw, short fibers of little value. A continuous method of roving, followed by wet spinning, yields a yarn that can be woven into cloth. Linen crash woven from this yarn is said to have the same properties as linen crash made from fiber flax. Commercial success of the process should greatly increase the value of the seed-flax crop.

THE FIRST STEP in utilization of linseed oil is its isolation from flaxseed. Traditionally, this has been accomplished by squeezing the oil from the ground seed (meal) in presses. In early presses, the meal was placed between plates, which were then forced together by driven wedges. Later, screws and finally hydraulic forces came into use for applying pressure to the plates. The most modern method employs screw presses, which operate continuously.

Now, the solvent-extraction process, which has been so successful for soybeans, is not satisfactory for flaxseed. In that process, the seed is rolled into flakes (linseed flakes), which are treated with a petroleum hydrocarbon solvent, such as hexane, to dissolve the oil (linseed oil). But when flaxseed is flaked, and then extracted, the solvent tends to dissolve the connective material that holds the flakes together. Linseed flakes, therefore, tend to disintegrate, become powdery, and cake. The solvent can then run through cracks in the caked material, and the efficiency of the process is lost.

Success has been achieved by utilizing a combination of pressing and solvent extraction. Linseed meal is pressed to an oil content of 10 to 15 percent. The residual oil in the cake is then extracted with solvents. The pressing operation imparts a more rigid structure, which withstands the action of the solvent. The first plant to use this process was built near Minneapolis and put into operation in 1949. It has a capacity of 12,000 bushels a day.

Another possibility, which is being investigated by a commercial processor, is solvent extraction of linseed meal directly in a slow-speed centrifuge. Although the meal still tends to disintegrate under those conditions, the centrifugal action effects separation of the fine particles from the solution of oil obtained.

The continuous screw-press method works thus : Upon receipt at the mill, the flaxseed is cleaned of weed seeds, dirt, particles of metal, and other undesirable materials. It is then crushed or ground to a suitable particle size, after which it is passed through cookers. In the cooker, which is operated continuously, the meal is exposed to heat and live steam. The temperature is gradually raised, as the meal approaches the bottom of the cooker, to 190 to 200 F. The temperature and moisture content of the meal are carefully regulated at this point to assure economical recovery of the oil. The preliminary grinding and cooking operations destroy in part the cellular structure of the seed, so that removal of the oil is easier and more complete.

From the cooker, the meal passes to the continuous screw press, which is essentially a hardened steel block with a tapered bore. By means of an arrangement of screws, meal is forced through the bore, resulting in compression of the meal. The oil released in this operation passes out through fine grooves and perforations in the press. The meal emerging through the outlet of the press contains about 4 percent of oil. After further grinding to a suitable size, the meal is sold as feed for animals.

The freshly expressed oil is warm-170 to 180 F. In that state it is filtered to remove any fine particles of meal. It is then stored until cool. During cooling, waxes and phosphatides separate and are removed by a second filtration to leave the ordinary double-filtered raw linseed oil of commerce.

For certain uses further refining is required. The refining agent may be acid or alkali. Acid refining is conducted by treating the oil with sulfuric acid in a lead tank. The strong acid chars many of the undesired contaminants in the oil and produces a precipitate, which is removed by filtration. Excess acid is eliminated by washing the refined oil with water. Acid-refined oils are used in grinding pigments, for example, in preparing white lead pastes and in making paints of a high content of lead.

In the alkali-refining process, a sludge of impurities is formed by heating the oil at 60 F. with a small amount of caustic soda solution. After the sludge is removed, the oil is washed, dried, and heated with bleaching clay to remove undesirable color. Alkali-refined oils are important because of their low free acidity. They are used in making varnishes, enamel vehicles ( particularly for light-colored enamels) , and printing inks.

Linseed oil often is subjected to still further processing. One common operation is polymerization, or heat bodying. Polymerization is accomplished by heating the oil at high temperatures in order to thicken it. By control of time and temperature the body, or viscosity, of the oil can be varied. Polymerized oil is frequently referred to as boiled oil.

Another common treatment of linseed oil is blowing. In that process, air is blown through the heated oil. As a result, the oil is thickened and its acidity is increased. Blown oils generally improve the leveling properties of paint and increase the ability of a paint oil to wet the pigment.

Sometimes linseed oil is heated with excess caustic to form a soap. Acidification of the soap liberates the linseed fatty acids, which are used in the manufacture of resins and various synthetic drying oils.

THE LARGEST SINGLE USE of linseed oil is in paint, varnish, enamel, and similar products. That accounts for about 70 percent of all linseed oil used in the United States.

Any paint is essentially a suspension of a pigment in a liquid, the vehicle. Its properties depend in a complicated fashion on the kind of materials selected for pigment and vehicle. Most properties can be varied by changes in pigment, in vehicle, or in both a factor that greatly complicates paint research and tends to prolong the period of testing before new paint formulations are placed on the market.

For many years people believed that the best exterior white paint should consist of white lead and pure linseed oil. Modern research has shown this idea to be wrong. Pigments other than white lead have been found to impart superior weathering properties with freedom from cracking and checking and with controlled chalking, which leave the surface in excellent condition to receive future coats of paint. One excellent pigment in use today is a mixture of titanium dioxide, asbestine, and leaded zinc oxide. Some satisfactory formulations contain no lead in any form.

Linseed oil, basically the best available oil for the vehicle, is seldom used in unmodified form. The modern vehicle consists of a mixture of linseed oil and heat-bodied linseed oil with enough thinner to make the final thickness about the same as that of linseed oil itself. Its use makes a paint that contains a smaller proportion of oil but that has, nevertheless, better leveling, brushing, and flowing properties, better gloss, and better resistance to penetration on new work. That vehicle was originally proposed and used during the Second World War as a replacement oil to conserve stocks of linseed oil.

THE MANUFACTURE of a paint requires two steps.

The first is to grind the pigment into a paste with a small amount of oil. The oil may be the same as the one to be used as 'vehicle for the paint. It is often advantageous, however, to use a special grinding oil. Such an oil usually contains free fatty acid, which helps the oil wet the dry powdered pigment.

The second step is to dilute, or reduce, the paste with the vehicle to the desired consistency. Driers (soluble compounds of cobalt, lead, or manganese) are also added to promote combination of the vehicle with atmospheric oxygen, to form the dry paint film.