US2802403A - Hard board manufacture - Google Patents

Description

'Aug. 13, 1957 R. M. BOEHM HARD BOARD MANUFACTURE Fil'ed oct. 7, 1954 TQQ mvENToR ROBERT Medi-:HM

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ATTORNEY United States Patent f HARD BOARD MANUFACTURE Y Robert M. Boehm, Laurel, Miss., assignor to Masonite Corporation, Laurel, Miss., a corporation'of Delaware Application October 7, 1954, Serial No. 460,993

5 Claims. (Cl. 92-39) The present invention relates to the manufacture of felted lignocellulose hardboards. More particularly, the invention relates to an improvement in the method of manufacture of such hardboard products. Even more particularly, the invention relates to amethod of reducing the content of water soluble material in feltedwet laps `of lignocellulose and thus reduces sticking of ther laps durin consolidation thereof into hardboard products.

In the manufacture of lignocellulose hardb-oard products, it is common practice to first prepare an aqueous slurry of Vlignocellulose fibers land thereafter to form a continuous Wet lap therefrom. The wet lap is ordinarily carried along by a travelling wire screen and is subjected to various dewatering operations as by free drainage, application of suction, by passage of the wet lap between pressure or squeeze rolls, and finally lby heating the partially dewatered lap to remove substantially all of the remaining water. The lap is then consolidated under heat and pressure into hardboard form.

While there are various methods of obtaining lignocellulose fiber for hardboard preparation, it is common practice to subject wood, woody materials obtained from corn stalks, cane, and other vegetable growths to fibration and varying degrees of hydrolysis by treatment with steam. One Ysuch method vis described in UnitedStates Patent No. 1,824,221 to` Mason issued September 22, 1.931. In this method, relatively small wood chips are subjected to pressures of about ZOO-1200 p. s. i. at temperatures of about 200-300 C. -for approximately 30 seconds to 5 seconds, respectively, in the presence of steam. The woody material is then discharged from the treatment chamber through a constricted opening into a zone of lower pressure, thereby exploding the wood chips into comparatively finely divided fibers and bundles of fibers. This thermohydrolytic treatment renders -a' portion of the wood water soluble while other fractions thereof become dispersible or suspendible in water. Thereafter, the fibers may be subjected to refining treatment, 'felted into a continuous wet lap as described above, and then dried. The dried laps obtained from this treatment may be consolidated between two smooth pressing plates according to the disclosure of United States Patent No. 2,120,137 to Mason issued June 7, 1938 to obtain a hardboard product characterized by two smooth surfaces. In the preparation of -a continuous wet lap from an aqueous slurry of the above described hydrolyzed lignocellulose fiber, there will be present in the water contained in the lap a substantial amount of the water soluble material formed during the hydrolysis treatment. A portion of this may be removed by washing, but it is necessary to control the amount of this water soluble material in the lap within relatively narrow limits inasmuch as an excess thereof in the finished lap results in sticking of the fibers to the press plates during consolidation of the lap. The control 4of the water soluble material has heretofore presented a serious problem in the manufacture of lignocellulose hardboard products. p

It is a principal object of this invention to provide an 2,802,403 Patented Aug. 13, 1957 The invention will be described wit-h particular reference to the accompanying drawing in which:

Figure 1 is a sectional side elevation of a Fourdrinier machine showing theconventional travelling wire screen supportinga continuous fibrous wet lap.

In ya generic sense, the`invention comprises felting a continuous wet lap yof fibrous hydrolyzed lignocellulose from an aqueous slurry thereof followed by Van additional water washing treatment whereby the content of water solubles of the lap is substantially reduced.

In practicing the invention, hydrolyzed lignocellulose fiber is prepared preferably according to the procedure described in United States Patent No. 1,824,221, hereinbefore referred to,.and refined in a conventional refiner to suitable-condition such that it is desirable in the preparation of hardboard articles.V The refined fiber is formed into an aqueous slurry `and is discharged from the headbox 10 of Figure 1 onto the travelling screen 12 of a Fourdrinier type machine 14 at a consistency of from about 1% to labout 3% ber content. The moving `sheet of water and fiber passes over an area where a substantial amount of the water flows by gravity through the screen as shown by the vertical arrows of the figure. Thereafter, the partially formed lap passes over one or more suction boxes 16 where sufficient water isremoved to increase the consistency of the lap to about 22-26% of ber. The aqueous slurry is maintained, up to the .time of its discharge onto the Fourdrinier screen 12, at

elevated temperatures. This temperature maintenance, as is well known in the art, improves the drainage of water from the ylap Iboth by the application of suction and by the application of pressure thereto in that a greater quantity of water may be withdrawn from a wet lap under elevated temperature conditions than may be removed at lower temperatures.

At the point where the consistency of the wet lap 17 has been raised to about 22-26% fiber, i. e. at about point 18 of Figure 1, there is present inthe water therein dissolved solids at a concentration which ordinarily exceeds 1%, and may, in some instances, be as great at 1.5-2%. It has been found that where the water soluble concentration is appreciably in excess of 1%, sufficient thereof will adhere to the fibers -to cause them to stick to the pressing plates during consolidation of the lap thus resulting in a finished product having an imperfect surface appearance. Accordingly, in the method of the present invention, when the wet lap 17 has attained a consistency of about 22-26%, there is flowed onto the lap a volume of fresh hot water. This treatment is preferably carried outby means of the weir 20 which permits an even water flow to be obtained. The lap 17 continues to travel over additional suction boxes 16 and, as the water is withdrawn from the lap by the applied suction through suitable conduits (not shown), the fresh water substantially replaces the water soluble-containing water previously present in the lap. By displacement of the original water, therefore, the concentration of dissolved solids in the wet lap is lowered appreciably, usually to about 0.6%- 1.0%. The lap thus obtained exhibits no tendency what.-`

t soever to stick during consolidation thereof under heat and pressure. Thereafter the lap is travelled between the pressure rolls 22 where an additional proportion of the water is expressed. The lap 17, upon leaving the pressure rolls 22 ordinarily contain about 45`50% of water and 5550% of fiber on a dry weight basis. VThe lap is thereafter dried in a conventional drier to substantialV dryness and then consolidated under heat and pressure. The consolidation is carried out, according to the procedure of the heretofore mentioned United States Patent No. 2,120,137, at temperatures of 400-500 F. and under pressures of about 100G-2000 pounds per square inch.

In the normal operation of the Fourdrinier, wherein the sheet of fiber and water may be as wide as, for example, about 126 inches, the wet lap travels at speeds of from about 30 ft./min. to about 160 ft./ min. At the lower speed, the slurry flowing onto the screen consists of about 220 pounds fiber, 15,535 pounds water, and about 220 pounds of dissolved solids per minute at a ber consistency of `about 1.5%. The dissolved solids, therefore, are present at about 1.5% concentration. At the point of fresh water addition, it has been found that flowing about 100 gallons of hot water per minute onto, the lap serves to reduce the dissolved solids concentration, to about 0.8%. This represents about 9-10 pounds; of dissolved solids per minute and this small amount can be tolerated in the lap without deleterious effect to the finished hardboard product. Obviously, at greater machine speeds it is necessary to add a larger volume of Ifresh water per minute in accordance with the quantities of fiber, water, and dissolved solids present. Y

Furthermore, from the illustrative drawing it will be appreciated that there is a considerable distance between the head box and the first pressure rolls. In travelling this distance the upper portion of the lap cools to a considerable extent. For example, where theA slurry is originally maintained at 180-195 F., at the point where the wet lap passes through the first pressure rolls it has cooled to the extent that the temperature of the lap surface is about 120 F., the center thereof about 140 F., and the bottom surface about 160 F. Much of this cooling is due, of course, to the passage of air through the lap by the suction applied thereto. It is a feature of the invention that the fresh water applied to the lap by means of the weir is maintained at a temperature approaching the boiling point. The hot water addition accomplishes several things.

First, the temperature of the surface of the wet lap4 is thereby raised about 75 F., the center of the lap about 35 F., and the bottom about 15 F. on the average.

Second, due to the elevated temperature offthe lap, more water is expressed therefrom bythe pressure rolls. It has been found that, subsequent to passage through the pressure rolls, the` lap contains about 45-48% water. To the contrary, a wet-V lap to which no additional hot water has been added ordinarily contains appreciably more than 50% water. Considering the speed of the travelling lap, it will be apparent that the reduction of water in the lap greatly facilitates drying thereof. A considerable yadvantage is thereby derived from the invention.

Third, the hot Water added to the wet lap serves to reduce the dissolved solids concentration from about 1.3 1.5% to about 0.6%-0.8%. In so doing, materials not ordinarily soluble at the lower temperatures, but which are soluble above about 175 F., are also removed; The end result is a lap which is much more easily dried and which is productive of hardboard articles which are greatly improved over those previously manufactured according to prior art methods. v A

The invention, therefore is directed to a novel method of maintaining a moving felted lignocellulose wet lap at an elevated temperature while, at the same time, removing therefromV appreciable quantitiesYV of dissolved 'sol-ids material which would otherwise have a decidedly deleterious effect on the finished hardboard product.A As a result of practicing the novel method, hardboard products of improved appearance and physical properties are obtained. Additional advantages to be derived from the novel method reside in the manufacturing improvements afforded due to the removal of both greater quantities of water prior to drying of the lap and to removal of a substantial amount of the dissolved solids normally present in such wet laps.

What is claimed is:

l. In the production of felted lignocellulose hardboard, wherein a continuous lap of brous lignocellulose is felted from an aqueous fiber slurry upon a moving wire -screen and the lap is partially dewatered by passage over suction means and thereafter passed between pressure rolls, the improvement which comprises evenly flowing hot water onto the upper surface of the lap at a point subsequent to the formation of the lap and prior to the application of pressure thereto by said pressure rolls, said hot water being applied in an amount sufficient to raise the temperature of the lap about 15-75 F., thereafter applying suction to the lap to partially dewater it and to provide in the lap subsequent to its passage throughbetween said pressure rolls a water content of about 45-50%, thereafter drying the lap to substantially-bone dry condition, and then consolidating the dried lap at a temperature of about 400-500 F. and under a pressure of about 1000-2000 pounds per square inch.

2. In the production of felted lignocellulose hardboard, the steps comprising flowing onto the travelling screen of a Fourdrinier machine an aqueous slurry of lignocellulose fiber, the temperature of the slurry being about 180-195 F., travelling the slurry over suction means whereby a wet lap is formed, thereafter evenly llowing water at a temperature approaching the boiling point onto the upper surface of the lap, continuing the application of suction to the lap, then passing the lap between pressure rolls to express a portion ofthe water and provide a wet lap having a water content of about 45-50% subsequent to the application of pressure thereto, thereafter drying the lap to substantially bone dry condition, and then consolidating the dried lap at a temperature of about 40G-500 F. and under a pressure of about G-.2000 pounds per square inch.

3. A method of' decreasing both dissolved solids and water content of a wet lap of felted lignocellulose in the production of hardboard products which comprises adding to a partially dewatered and substantially cooled wet lap of fibrous lignocellulose, butprior to the application of pressure thereto, a quantity o-f hot water suicient to raise the temperature of the lap about l575 F., then applying suction to the lap, thereafter passing the lap between pressure rolls whereby there is produced a wet lapv containing a relatively small amount of dissolved solids and having a water content of about l5-50%, thereafter drying the lap to substantially bone dry condition, and then consolidating the dried lap at a temperature of about 40G-500 F. and under a pressure of about 100G-2000 pounds per square inch.

4. In the production of felted lignocellulose hardboard, a method of preventing sticking of the product to the pressing surfaces which comprises subjecting lignocellulose to hydrolysis at elevated temperature and pressure and, subsequent to deiibration Vand water washing, owing an aqueous slurry of fiber onto a travelling screen to provide a wet lap containing an excess of water soluble material, evenly flowing hoty water onto the upper surface of said wet lap at a point subsequent to the formation of the lap and prior to its passage between a series of pressure rolls, said hot water being applied in an amount sutlicient to raise the temperature of the lap about 15- 75 F., thereafter applying suction to the lap to, partially dewater it and to provide in the Vlap subsequent to its passage throughbetween said pressure rolls a water content not in excess of about 4550% and a greatly reduced water soluble content, drying the lap to substantially bone dry condition, and then consolidating the dried lap at a temperature of about 400-500 F. and under a pressure of `about 1000-2000 pounds per square inch.

5. In the production of felted lignocellulose hardboard, a method of preventing sticking of the product to the pressing surfaces which comprises subjecting liguocellulose to hydrolysis -at elevated temperature and pressure and, subsequent to deibration and Water Washing, flowing an aqueous slurry of the fiber onto a travelling screen, the temperature of said slurry being about 180- 195" F., travelling the slurry over suction means and thereby forming a wet lap containing an excess of water l and a greatly reduced water soluble content, Vdrying the lap to substantially boue dry condition, and then consolidating the dried lap at a temperature of about 400- 500 Y1F. and under a pressure of about 1000-2000 pounds per square inch.

References Cited in the le of this patent UNITED STATES PATENTS 1,824,221 Mason Sept. 22, 1931 1,844,861 Mason Feb. 9, 1932 1,947,106 Plumstead Feb. 13, 1934 2,120,137 Mason June 7, 1938 2,140,189 Mason Dec. 13, 1938 2,198,269 Linzell et al Apr. 23, 1940 2,356,285 Street Aug. 22, 1944 OTHER REFERENCES Casey: Pulp and Paper, volume I, page 205 (1952),

published by Interscience Publishers, New York, N Y.

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