US2639994A - Process for manufacturing composite board - Google Patents

Description

Patented May 26, g 1953 'uNITEo sTATEsrArENT OFFICE N Drawing. Application July 26, 1948, Serial No. 40,791

1 Claim. 1

This invention relates to the manufacture of composite boards from particled ligneous material such as wood-shavings, coarse sander-dust, or mechanically-defibered wood; the customary procedure being to mix the material with a suit able binder and subject it to heat and pressure in a board-mold.

Most varieties of wood, other than those of the coniferous type having high rosin-content, require the addition of a cementitious binder for the production of a board having even only little strength or modulus of rupture; the added binder usually being a substance such as a synthetic resin which materially increases the manufac turing cost. With woods of the non-resinous variety it has heretofore been proposed to employ as a binder the lignin which constitutes the essential part of all ligneous tissue but which is inert except during the life of the tree and therefore must be liberated or reactivated before it can serve as the binder. Liberation of the lignin can conveniently be effected by the process of hydrolysis in the presence of :a catalytic agent such as an acid. However, this process, as heretofore carried out, has been costly from the standpoint of time and labor since it involved (1) prolonged heat-treatment, (2) separation of liquid, and usually (3) separation of substances added to influence the hydrolytic actionbefore the activated lignin, or the woody material containing such lignin, was available for use in the actual pressing of the composite board.

It is therefore an object of my present invention to simplify, and to render less expensive, the manufacture of composite board; this object being accomplished by the process hereinafter described and more particularly defined in the appended claim.

According to this invention :a composite board, having sufficient strength for many purposes, can be produced in the following manner: To the ligneous material, which conveniently may be in the form of shavings, coarse sander-dust, or mechanically-defibered wood, is added water to increase the normal moisture-content of the material to about 15 to 30 per cent by weight. The moist material is then sprayed with a dilute solution of an acid such as hydrochloric or sulphuric; the proportion of acid (as represented in concentrated state) to the total weight of the mixture being /3 to 2 per cent. The acidified material is then placed in a conventional board-mold and subjected to moderate heat (275-350 F.) and pressure (conveniently, 50 pounds per square inch) for about 6' to 10 minutes; the board being in finished condition when it is then removed from the mold.

The moisture content of the mixture may lie in an even narrower range than that indicated above, since it has been found by some tests that with a moisture content of 20 to 25 per cent a desirable balance is achieved between (1) the time necessary to dissipate the excess moisture in the heated mold, and (2) the beneficial effects of the moisture in softening the fibers and thereby so increasing the liberation of lignin by the acid that the final density of the board under any chosen pressure is increased.

The acid is employed in diluted form so as to more readily and fully efiect its dispersion throughout the mixture. It has been found that the best results :are obtained when the amount of acid is within the /2-130-2 per cent range mentioned; the lower the acid content the less is the danger of burning or darkening the board, while with the higher acid-content there is a greater release of lignin and resultant stronger board. Hydrochloric acid is preferred since it volatilizes readily under the heat and thus penetrates the fibers thoroughly; however, other acids, notably, sulphuric, may be employed if due precautions are taken in handling it, and if darkening of the product (as may occur due to its use) is not objectionable.

According to my invention it is not necessary to employ any binder in addition to that afforded by the lignin liberted in the process. However, to meet certain strength requirements synthetic binders may be added either during the process, or subsequent to the removal of the board from the mold, as by dipping the board in a conventional binder solution and then redrying it. If added to the acidified mixture prior to pressing, the binder may be such as a commercially-obtainable thermo-setting resin of the phenol type. A strong-er product can also be obtained by the use, in addition to the acid, of a catalytic substance such as phenol to increase the liberation of lignin. It is to be understood that the use of additional binders or catalysts are not necessary in the practice of my invention, and that their use (to meet greater strength requirements) is to be considered as a step merely additional to those of the invention as claimed.

From the foregoing it is apparent that, by my improved process, composite boards can be produced directly from the basic ligneous material merely by the addition of a small amount of water and acid, and that no equipment other than the standard board moid and press is required.

I claim:

A process for manufacturing composite board from particled dried ligneous material which consists in spraying the particied dried Iigneous material with diluted hydrochloric acid in an amount such that the-acid, asrepresented in own-- centratd state, is ih the proiuoitibE-bfibn'e half to two per cent of the total weight of the material and the moisture content of said partici'efl dried li'gneous material is brought to about twenty to twenty-five percent by weight,"

mixture in a mold and therein subjectiffi'i't 'to a temperature of 275-350 F. to eifefit -10 4 lignin by said acid, and at the same time to a. pressure suflicient to compound the material into a finished board.

WINFRED E. WILSON.

References Cited in the file of this patent UNITED STATES PATENTS UTHER, REFERENCES hemistry--W1se, 1944-Da