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Groundwood

( Originally Published 1920 )



THE first producer of groundwood, as well as the first papermaker, was the wasp. He obtained his raw material by chewing off the weathered surface of old timbers or logs and in doing this he moistened it with saliva, which acted as a sizing agent. This made the nest which he constructed sufficiently water resistant so that it would not disintegrate during heavy rains. He is still using the same methods which his ancestors used thousands of years ago, and, so far as is known, he is the only papermaker who is not bothered by the increasing cost of materials, obsolete machinery, labor troubles, impossible specifications and the demand of the mill owner for greater speed and more product.

The lesson of the wasp was observed by Reaumur, who in 1719 remarked that the study of making paper from wood should not be neglected. The publication of Schaeffer (1765 ), already mentioned in a preceding chapter, included samples of paper made from beech, willow, aspen, mulberry and spruce, and the book published in England in 1800 by Mathias Koops contained an appendix of five leaves on paper made from wood alone. Koops claimed it to be the first practical paper made from wood, but did not describe how it was made. In spite of this promising start it was not until 1844 that the groundwood process really came into being, when Chas. Fenerty, of Halifax, N. S., submitted a sheet of paper to prove that wood could be reduced by a "chafing ma-chine," and manufactured into paper. It was also in 1844 that Keller in Germany patented a wood-pulp grinding machine; this was sold to Henry Voelter who improved it in 1847, making possible the production of wood-pulp for newspapers. Two of these Voelter grinders, imported by the Pagenstecher brothers and set up in Curtisville (now Interlaken), Mass., in 1867, formed the first commercial groundwood mill in the United States. Their first sale of pulp was to the Smith Paper Co., of Lee, Mass., at eight cents per pound. Since that time the use of groundwood has increased enormously until now millions of tons are produced annually.

The present methods of manufacture do not differ in principle from that of 1867, though the size, capacity and form of the grinders have undergone much change. In all equipment the logs of wood are pressed against the face of a rapidly revolving grind-stone in such a way that the length of the log is parallel to the shaft holding the stone. In the older grinders, logs two feet long were placed by hand in pockets attached to the grinder frame, and were forced against the stone by pressure plates operated hydraulically. Usually there were three pockets on each stone so that one could be opened, filled with wood, and put back into operation without shutting down the entire grinder.

Modern grinder installations are very different in appearance from these old ones, and operate much more efficiently. Continuous magazine grinders have been developed, in which the logs are fed into the grinder on one floor and dropped down through the magazine to the pockets of the grinder on the floor below. Such grinders are usually installed in pairs driven by a motor which may rate as high as 4000 horse power. These grinders still require the logs to be placed in the magazine by hand, but others have been developed which can take 4 foot logs, and which may be fed by conveyors. These are similar in output and power requirements to the magazine grinders.

The stones originally used were natural sandstones which had to be carefully quarried, seasoned and shaped. As pressures and speeds increased, it became more and more difficult to find such stones sufficiently free from flaws and of uniform grit characteristics. Artificial monolithic stones of cement with any desired degree of bond and grit were then developed, and finally segmental artificial stones with vitrified bond were produced. Either of the latter types, if the grit is properly selected, will produce pulp comparable in quality to that from natural stones, and the cost of the stone per ton of pulp is lower. European practice favors the monolithic cast stones, while the vitrified type is preferred in America.

With any type of stone the quality of the pulp produced depends largely on the condition of the stone's surface, which includes the size and sharpness of the individual grit particles, the ease with which the binding material is worn away and the manner of "dressing" the stone. Dressing is done by working across the face of the stone under pressure a small, hard, steel roll with a design on its surface, which leaves a corresponding design on the stone, and roughens, or sharpens, its surface. This operation is the most important single factor in making groundwood; other conditions on which its quality depends are the pressure applied to the wood, the temperature of grinding and the quality of the wood used.

The peripheral speed of grinder stones is high, sometimes as much as 4500 feet per minute. This, together with the pressure against the wood, causes the development of much heat, and to absorb this, as well as to carry the pulp away from the surface of the stone, a flow of water is always supplied during grinding. The amount of water applied determines whether the pulp is "cold ground" or "hot ground." Cold ground pulp is finer and more uniform in character, but hot ground, being coarser, parts with water more freely and is better for use on high-speed newspaper machines where it finds its greatest use.

The pulp coming away from the stone collects in a pit under the grinder and from there goes to coarse screens which take out slabs, knots or large splinters, and then to fine screens which re-move smaller, undesirable particles that are yet coarse enough to cause unsightly or defective paper. Before this final screening it is desirable to pass the diluted fibers over a sand settler to remove particles of sand which may have been broken from the stone. The screened pulp is partially separated from the large amount of water which accompanies it, and is collected in tanks at about 4 per cent concentration, from which it is drawn for use.

Practically any kind of wood can be made into groundwood pulp, but, because much of the fiber is used without bleaching, woods which give the lightest colored fiber are preferred. Spruce and balsam were the choice in the early days, but because of their present scarcity many other kinds have had to be used—including jack pine, hemlock, white pine, southern pines, gumwoods, birch and poplar. These woods lose little of their non-cellulose constituents on grinding, so the chemical properties, as well as the physical characteristics of the groundwood, will vary with the kind of wood employed.

Unlike the chemical processes, where there is a fiber yield of less than half of the original dry weight of the wood, groundwood process gives a commercial yield of about 88 per cent of the wood used. About 2 to 7 per cent is lost as screenings and other mechanical losses, and the remaining 5 to 10 per cent is lost as soluble organic or inorganic material. The fiber obtained per 100 cubic feet of solid wood will vary with the different species and with their specific gravity. The various kinds of fir will give about 1900 to 2000 pounds; the spruces may go as high as 2400 pounds; hardwoods—though not often used—may yield 2200 to 2900 pounds.

The microscopic appearance of groundwood shows how greatly it differs from chemically cooked fibers from the same wood. The chemical fibers are individuals, and relatively few are broken; groundwood shows few individual fibers, but many which are torn off with ragged ends or sides, and which remain adhering to one another. It is the size of these associated groups of fibers which largely determines the quality of the groundwood and the uses to which it can be put.

Groundwood is not rendered sterile during its production and so is subject to serious deterioration from decay if stored for any length of time. Slime growths in the groundwood system itself also contribute to the trouble, but there this can be reduced, though not entirely cured, by the use of materials which are toxic to the bacteria and algae present. However, at best such corrections can-not do away with the necessity for scrupulous cleanliness.

The tests which are used to control the operation of the grinders are almost wholly designed to show the physical character of the fibers. The two outstanding tests are known as the freeness, and the blue glass tests. A freeness tester is an instrument to measure the rate at which water drains away from the fibers under standard conditions; it therefore measures the fineness of groundwood. The blue glass test is a method for examining a small amount of the groundwood suspended in water. If this is placed in a tray with a blue glass bottom the light-colored fiber stands out prominently against the blue background and an experienced observer can form an excellent opinion of the quality of the product, and can judge whether the grinder stone needs attention. To be of value this test requires much experience on the part of the observer.

As already mentioned the chief use of groundwood is in making newsprint, of which it forms about 80 per cent of the total weight, but groundwood is also used in a variety of other products such as tissues, wall papers, cheap writing and book papers, wall and insulating boards, molded objects such as egg containers, etc. The use of groundwood is generally restricted to papers which are of temporary value, for it becomes yellow when exposed to light, and papers of which it forms a considerable part are not strong enough to stand any great amount of handling. However, if groundwood is used in the base paper over which a coating of mineral matter with an adhesive is applied it seems to be much less affected by the weathering influence of light and air, and such papers have a fair degree of permanence.

Another, and quite different, grade of groundwood is made by steaming the logs, or boiling them in water, before grinding. This treatment removes much of the soluble material in the wood, and forms acetic and formic acids from some of the wood constituents. The fiber produced is longer and stronger than that from unsteamed wood, but in order to gain these advantages to the fullest extent the steaming treatment must be severe enough to cause a distinct darkening of the fiber. Thoroughly steamed groundwood, in contrast to ordinary grades, responds to the beating action in much the same way as chemical pulp. It is used in an appreciable tonnage of box boards, bogus manila wrappings, etc.

A process which yields a fiber somewhat similar to ground-wood, in that it includes practically all the constituents of the wood, is the so-called "explosion" process. Steam "guns" for the production of pulp from cane and bamboo were employed experimentally in 1868 by the American Fiber Disintegrating Co., of Brooklyn, N.Y. This mill was destroyed by fire that same year and apparently nothing more was done along this line until 1926, when W. H. Mason developed apparatus for the same purpose in order to utilize pine sawmill waste. The process has proved so successful that there is no longer enough such waste available and so cordwood, including hardwoods, is now being used.

The wood is "hogged" or chipped to about the size ordinarily used in the chemical processes, and about 200 pounds are then charged into "guns" which are about 20 inches in diameter and 5 feet high. Steam is turned on to heat the chips to about 375°F. in 30—40 seconds; then steam at 1000 pounds per square inch is admitted and after about 5 seconds a discharge valve is opened, and the chips are forced through a small port in the bottom of the gun, where they explode immediately. The fibers pass to a cyclone separator, which separates the steam from the fibers, and the latter are mixed with hot water and refined by rod mills, or other refining apparatus, before being used.

Fiber produced by this process is usually slightly darkened in color. It is somewhat coarse, and has not found much use in paper, but it serves excellently for giving the loose, puffy texture desired in insulating boards. A very hard, dense and stiff board can also be made from it by drying the wet boards in hydraulic presses with steam heated platens.

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