US2060902A - Method for simultaneously seasoning and treating water-swollen fibrous materials - Google Patents

Description

Patented Nov. 17, 1936 2,060,902 METHOD FOR SIMULTANEOUSLY SEASON- INGAND TREATING WATEB- SWOLLEN FIBROUS MATERIALS Alfred Joaquin Stamm, Madison, Wis. Dedicated to the free use of the Public No Drawing. Application May 9, 1934, Serial No. 724,834

2 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3'10 0. G. 757) This application is made under the act approved March 3, 1883, as amended by the act of April 30, 1928, and the invention herein described, it patented, may be manufactured and used by or 5 for the Government, for governmental purposes, without payment to me of any royalty thereon.

I hereby dedicate the invention herein described to the free use of the public, to take effect upon the granting of a patent to me.

Before describing this new process for the combined seasoning and treating of fibrous materials, the principles involved in the processes as ordinarily applied will be briefly described so as to give a background which will help in making clear the difference between this new process and methods in present use.

All fibrous materials hold water intimately on the fiber surface and in the fine capillary structure within the fiber walls and also mechanically in the coarse capillary structure between the fibers and within the fiber cavities. The removal of the mechanically held water has no effect upon the properties of the material. The removal of the intimately held water is accompanied by shrinkage as well as other physical changes. The transition point between the intimately held and mechanically held water is known as the fibersaturation point. The removal of the intimately held water from green wood, that is, wood which has never been dried below the fiber-saturation point, is difiicult if not impossible to accomplish without detrimental eiiects being introduced during the process. Under all seasoning conditions a drying moisture gradient, that is, a decrease in moisture content with the distance in from the surface to the center of the material, is set up. The surface always dries below the fiber-saturation point before the inner part of the wood reaches this stage, thus causing stresses to be set up which may result in surface checking. If the surface can withstand this stress, it sets in tension and when the center finally dries and tries to shrink stresses are again set up which may result in honeycombing. When the wood is subjected to less drastic humidity conditions the resulting moisture gradient is less steep and, consequently, checking, honeycombing, and warping are reduced but not eliminated and the drying time is greatly increased. Almost prohibitively slow drying schedules are required for the proper seasoning of some refractory woods and others can not be satisfactorily dried under any process in use at the present time.

In the course of drying fibrous materials the water intimately held between the fine structural units of which the fibers are composed is removed and these structural units are drawn closer'together to eliminate the void space that would result if water were removed without shrinkage. Fibrous materials can again be swollen with water 5 and water-soluble materials which have similar properties. Water-insoluble materials will, however, not cause swelling. They can be made to penetrate only the structure previously occupied by mechanically held water whether they are 10 forced into the fibrous material in the completely dry condition or when it is just below the fibersaturation point. The direct treatment of wood with water-insoluble materials, hence, can have no efiect upon the shrinking and swelling other 15 than retarding the rate of moisture absorption.

Under present seasoning practice it is thus evident that seasoning can not be carried out without the accompanying shrinkage and other detrimental efiects, and treating can not be carried 20 out so as to cause water-insoluble materials to enter the structure within which swelling takes place. Further nothing is gained in combining the two processes except by the new novel method covered in this patent.

This invention consists in brief of a combined diffusion and distillation or a modified extraction and distillation method for replacing the intimately held water in the fine swollen structure of wood, paper, fabric, or other fibrous materials with a water-insoluble material in such a way that the fibrous material more completely retains the water-swollen dimensions than is possible by other methods, thus providing a simultaneous seasoning and impregnation process for the fibrous materials which are difficult to dry without introducing the detrimental effects caused by shrinkage, and also providing a means of distributing the treating material more intimately throughout the structure of all of the fibrous materials than can be accomplished by ordinary impregnation methods, thus ofiering an improved means of permanently reducing swell-' ing and shrinking and the permeability to gases and liquids, of water proofing, of maintaining the low electrical resistance of the dry fibrous materials, of increasing the efiiciency and permanence of preservative and other impregnation treatments, and of modifying the physical properties in general. I

The basic fact upon which the operation of this process depends is that one liquid can replace another in the swollen structure of a swelling material only if the two liquids are completely miscible in each other. This principle is here applied for the first time in a practical way in the replacing of water in the swollen structure of a fibrous material, first with a completely water-soluble solvent which in turn approaches complete miscibility with the water-insoluble treating material, and then replacing the solvent in a similar manner with the treating material. Many desirable water-insoluble treating materials can be made to impregnate the swollen structure of fibrous materials with the use of but one intermediate replacement step. Some treating materials may require additional intermediate steps, however. The salient feature of the process is that if the water is replaced by a nonvolatile water-insoluble material both the seasoning and the treating can be accomplished without an appreciable shrinkage taking place and the subsequent swelling and shrinking is greatly minimized.

In order to make possible the use of the simple distillation method for removal of the replaced liquid, the miscible solvent must not only be miscible in all proportions with water and practically so with the treating material, but it should also have a boiling point somewhat above that of water and below that of the treating material. The process, can, however, be carried out with the use of a solvent boiling below the boiling point of water by a modified procedure.

The treating material may be either a solid or a liquid. It is preferable if it is a liquid at temperatures somewhat below 100 C. and miscible in all proportions with the solvent, but a solid with a high 'degree of solubility in the solvent can be used. It is also desirable that the treating material be made up of as small molecules as possible, thus increasing the speed of diffusion and increasing the accessibility to the finest structure.

I In order to assure permanence of the treatment it is also desirous that the treating material be positively adsorbed on the fiber surface rather than being merely held mechanically and that it also sets to a plastic nonvolatile solid at ordinary temperatures.

The procedure used for carrying out the replacements will vary considerably with the amount, dimensions, capillary structure, and physical nature of the fibrous material to be treated, with the nature of the miscible solvent and whether it boils above or below the boiling point of .water, with the nature of the treating material used, and with the requirements of completeness of moisture removal and of replacement with the treating material desired.

The replacements can be carried out in a vacuum still when the solvent used boils at a higher temperature than water. This apparatus will of necessity vary in design with the amount and nature of the fibrous material to be treated. The still should, however, be provided with an adequate fractionating column, a drain for removing the miscible liquid, an inlet through which the treating material can be added, and a weighted screen case to hold the fibrous material beneath the surface of the liquid in the still.

The water-swollen fibrous material to be treated, with a moisture content preferably just above the fiber-saturation point, is placed in the distillation apparatus and covered with the watermisclble solvent. A high vacuum is intermittently applied and released, or, still better, alternate vacuum and pressure to remove air from the coarse capillary structure of the fibrous material and allow it to be replaced by the water-miscible solvent by capillary action or forced penetration. The time required for this operation will vary considerably with the dimensions and capillary structure of the fibrous material. Heat is then applied and a partial vacuum maintained so that refluxing rather than distillation takes place. This step is to allow the water in the fine swollen 5 structure to be replaced by the water-miscible solvent by diffusion aided by thermal convection currents. The duration of this step will also vary with the dimensions and capillary structure of the fibrous material. The water is then slowly disl0 tilled off, preferably under a slightly reduced pressure so as to lower the distillation tempera-' ture somewhat below the boiling point of water and thus prevent degradation of the fibrous material. when distillatiomof water ceases the apl5 plied vacuum can then be somewhat increased for a short period of time, allowing a small amount of the water-miscible solvent to distill over to insure complete removal of the water. This step is of importance only where the most exacting 20 removal of water is required; for example, in preparing electrical insulators. The excess of watermisclble solvent is then drawn off from the still in case the treating material is a liquid at temperatures below 100' C., and the impregnating 25 liquid is added in excess in the molten form so as to completely immerse the fibrous material. In case the treating material is a solid the excess of miscible solvent has to be retained in the still to act as solvent and the solid is added in a considerable excess over the amount that will enter the fibrous material. The still is again operated under refluxing conditions for a period of time depending on the dimensions and capillary structure of the fibrous material and then the miscible solvent is slowly distilled off under reduced pressure at a temperature of preferably less than 100 C. The fibrous material is then removed from the still and drained and cooled in a dry atmosphere.

The replacement of water with a solvent boiling below the boiling point of water requires a somewhat modified procedure. The specimens are placed in a weighted screen cage in the extraction chamber of a special extraction apparatus. The overflow of the extraction chamber must be near the top so as to keep the fibrous material continuously submerged. Fresh dry solvent is supplied to the extractionchamber from a still with a condenser extending to the bottom of the extraction chamber. The fresh solvent thus enters at the bottom of the extraction chamber, extracts water from the fibrous material, and is returned from the top of the chamber to the still. A suitable drying agent is added to the still to prevent water from distilling off with the solvent. The time required for the extraction will nat vary with the size and structure of the fibrous material. In general, it will take considerably longer than the removal of water by the direct distillation method. The replacement of the solvent by the water-insoluble material is then carried out in an ordinary vacuum still as previously described.

A number of diii'erent miscible solvents may be used for the process. Ethylene glycol monoethyl ether known as "cellosolve and diacetone alcohol have been successfully used. They have the advantages of being relatively cheap, boiling at temperatures conveniently above that of water, and being completely miscible with water in all proportions as well as being excellent solvents for many water-insoluble materials that it would be desirous to inject into the fibrous material. Alcohol and acetone have also been successfully used as intermediate solvents. They boil below the boiling point of water and require the modified procedure for the replacement of water.

The nature of the treating material will, of course, vary, depending upon the property which it is most desirous to impart to the fibrous material. Antishrink, water-proofing, and electrical insulating properties can be imparted by a large variety of waxes, resins, resin-forming materials, drying oils, fats, pitches, asphaltic materials, and paraffin. Many of these have the advantage of melting at temperatures below 100 C. and dissolving in practically all proportions in such solvents as cellosolve and diacetone alcohol. After distilling off the solvent and removing the fibrous materials from the molten excess of treating material, these injected materials will solidify in both the fine and coarse capillary structure of the fibrous material as a plastic solid. Resinforming materials such as indene will require a subsequent heat and pressure treatment to form the resin throughout the structure. Any number of solid and liquid water-insoluble preservatives can be used as the treating material, making possible a combination of the desirable properties of uniformity of distribution in both the fine and coarse capillary structure and nonleachability.

Some of the more specific uses of this general method for replacing the water in the swollen structure of fibrous materials with a water-insoluble material with a minimum of shrinkage taking place will be given to illustrate the general applicability of the method.

The method is especially valuable for both the seasoning and treating of wood. In this particular case the seasoning phaseof the method is of as great importance as the treating part for many species of wood which are difficult if not impossible to season according to present seasoning practice without obtaining the characteristic seasoning defects of warping, checking, and honeycombing, can be properly seasoned by this new method without introducing these defects because of the fact that only a slight shrinkage accompanies the removal of water. As a minimum of stresses is set up in the wood during thisv new process it can be carried out as rapidly as the replacement will allow which in all cases should be more rapid than the rate which can be safely used in normal seasoning practice. The treating phase of the method using waxes or paraffin, etc., also makes possible the minimizing of subsequent swelling and shrinking and permeability to liquids and gases, permanence of electrical insulating properties, etc., by filling practically all of the capillary structure both coarse and fine with a nonhygroscopic plastic solid.

The application of the method to other fibrous materials than wood would be of value primarily from the standpoint of treating for drying of such materials as paper and fabrics can be done by ordinary methods without detrimental effect. The method should be of considerable value nevertheless because of the fact that the treating material becomes dispersed in the fine structure which is not accessible by ordinary treating methods, thus providing an improved method for water proofing, preparing electrical insulators, etc.

The following specific procedure for the seasoning and treating of small blocks of white pine wood 3 inches long in the radial direction, 1 inch tangentially, and inch in the fiber direction was found to be quite satisfactory. The blocks containing approximately 40 percent water were immersed in ethylene glycol monoethyl ether and subjected to a reduced pressure of 2 centimeters of mercury for 10 minutes. The pressure was then released and. after 2 minutes the pressure was again reduced as before. This was repeated 10 times after which the blocks sank in the ethylene glycol monoethyl ether. They were then left in the cellosolve over night to permit diffusion of the ethylene glycol monoethyl ether into the fine structure and to allow water to diffuse out. They were then placed in a glass distillation apparatus and the water slowly distilled off under a reduced pressure of 10 centimeters of mercury at a temperature of about 50 C. This required about 2 hours. The practically complete removal of the water was indicated by the liquid no longerboiling. The wood specimens were then removed and measured. The volumetric shrinkage was about 0.5 percent as compared to 10 percent when drying in air. The specimens were again placed in the distillation apparatus and molten stearic acid added. The temperature was maintained at about 50 C. for a day without the application of a vacuum in order to permit diffusion into the fine structure. A reduced pressure of about 10 centimeters of mercury was applied and the temperature raised until ethylene glycol monoethyl ether distilled over slowly. This occurred at about C. When distillation ceased the blocks were removed from the still and allowed to drain and cool. The total volumetric shrinkage from the original green condition was about 1.0 percent.

I claim:

1. A process for drying and impregnating green wood consisting of:

(a) Heating the green wood in ethylene glycol monoethyl ether for 24 hours at a temperature of 50 C. to replace the water by ethylene glycol monoethyl ether and then applying-a reduced pressure of 10 centimeters of mercury to distill off the water I (1)) Heating the ethylene glycol monoethyl ether saturated wood in molten paraffin for 24 hours to replace the ethylene glycol monoethyl ether by paraffin and then applying a reduced pressure of 10 centimeters of mercury and heating to 80 C. to distill off the ethylene glycol monoethyl ether.

2. The process of impregnating a water containing fibrous material with a water-insoluble substance which comprises immersing the fibrous material in a solvent completely misciblewith water and the water-insoluble substance and which has a boiling point above that of water. alternately applying a vacuum and releasing to replace the air present by the solvent, slowly distilling off the water under reduced pressure, immersing the fibrous material in the waterinsoluble substance in molten form for a sufficient period to permit diffusion into the fine structure of the fibrous material and then slowly distilling off the solvent under reduced pressure.

ALFRED JOAQUIN STAMM.