US2500954A - Method involving the use of chemicals for increasing the drying rate of wood - Google Patents

Description

4 content.

I movement in wood.

' I We have discovered that if wood is treated with Patented "Mar. 21, 1950 T I' FF E INVOLVINGTHE USE OF CHEMI- i CALS-FOR INCREASING THE DItYlN G EATE 0F wool) I William Karl Loughborough and Leif Dedrick Espenas, Madison, Wis., a'ssign'ors to the United v States of America as re tary of Agriculture presented by the Secre- No Drawing. Application March 4, 1947,

Serial No.732,368 I 5 Claims. (CI. 34-95) (Granted under the act of March 3, 1883, as

amended April 30, i928; 370 0. G. 757) This application is made under the act of March.3, 1883, as amended by the act of April 30, 1928, and the invention herein described, if patented, may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to us of any royalty thereon.

Our invention relates to a method for increasing the drying rate of wood by the use of chemicals.

The object of ourinvention is to provide a means for increasing the drying rate of wood and thereby provide a means for decreasing the time required to dry green wood to a desired moisture A further object of our invention is 'to provide a means for eliminating or minimizing the water pockets found in wood and at the same time provide a means for reducing the tendency for the wood to collapse in drying. 7

Our invention is based upon the facts that certain chemicals decompose within the ordinary range of drying kiln temperatures to give gaseous products, or react with constituents in the sap of wood to give gaseous products, and that certain chemicals or the decomposition products of these chemicals have solvent properties with respect to the extractives found in wood., Hence these chemicals tend to open up the capillary structure of the wood and promote certain chemicals, for example, ammonium carbonate (WHO-200a) or a mixture of urea (CO(NH2) 2) and sodium bicarbonate (NaHCOa), it will dry faster than similar untreated wood under identical dryin conditions, and the tendency for water pockets. is reduced. By an empirical experiment, we have discovered that the water in wood treated with these chemicals actually 'diifuses faster than that in similar untreated wood and therefore the chemicalsmake .possible more rapid drying of wood.

We are aware'of the use of chemicals to season wood in a process ordinarily referred to as chemical seasoning. However, the function of the chemical and the results obtained are different from our invention. In ordinary chemical seasoning the vapor pressure characteristics of the aqueous solution of the chemical and the anti the rate of moisture shrink properties of the chemical in the wood are of paramount importance;- whereas in our invention the ability ofthe chemical to decompose or react with constituents in the sap in the wood to give gaseous products and the ability of the chemical or its products to dissolve extractives in the wood are the most important properties.

- In ordinary chemical seasoning, the primary purpose of the chemical is to prevent surface checking by preventin the surface of thewood .from shrinking either by keeping the surface moist or by the anti-shrink efie'ct of the chemical. In practice, the surface layers of the wood are impregnated with the chemical by one means or another after which the wood is dried. Because ofthe comparatively low relative vapor pressure of the chemical solution in the surface layers of the wood, the surface fibers are kept moist during drying providing the relative humidity of the atmosphere in which the wood isbeing dried remains equal to or greater than that' in equilibrium with the chemical solution. In kiln drying, the atmospheric conditions may be controlled so that the relative humidity of the air is in equilibrium with the chemical solution, but in air drying, this is not possible. Hence in air drying, the anti-shrink properties of the chemical become of more importance, since it is necessary to prevent the surface of the wood from shrinking during the periods of low relative humidity by some means other than keeping the surface moist. Since the surface of the wood does not shrink, surface checking is prevented.

More rapid drying is sometimes possible under the practice of ordinary chemical .seasonin 'because the use of the chemical may permit more severe drying conditions to be used without harmful results to the wood. Under the same drying conditions, however, the treated wood will not dry at a faster rate than untreated wood since nothing in the wood has been altered to promote diilusion of the water. Many woods can be dried satisfactorily in atmospheric conditions that are as severe or even more severe than those that would be used in ordinary chemical seasoning.

. Consequently, ordinary chemical seasoning is ad vantageous as far'as drying time is concerned only for those woods that require comparatively mild drying conditions.

In the drying of wood, the rate at which the wood dries is dependent upon the rate at which moisturecsn move through 'the wood which in turn is dependent, among other things, upon the presence or absence of air within the wood cells and the size and number of the minute capillaries within the cell wall.

If the cell is completely filled with water, no water can escape from the cell until the cell wall collapses, or, if the cell wall is .suiliciently strong, until the liquid tension of the water is exceeded by the pull of the meniscus in the largest capillary in the cell wall, thus forming a bubble in the cell cavity which on subsequent drying can readily expand. or until sufllcient tension is developed to pull back the meniscusin the largest capillary into he cell, thus permitting the entry of air into the cell cavity. Even under severe'drying conditions, these reactions take place slowly with the result that a long time is required to dry the wood to a satisfactory moisture content condition. Frequently, even though a rather low average moisture content is indicated by the wood as a whole. localized regions on the inside of the piece may have a very high moisture content. Thes regions are commonly called water pockets and are composed of a large proportion of cells completely filled with water.

Since water within a cell cavity must leave the cell by passage through the cell wall or through the openingsbetween the cells, such as the pores in the pit membranes, it follows that the water can leave faster if the capillaries in the cell wall are either more numerous or larger in size. In sapwood, diffusion of liquids is relatively rapid when compared to the heartwood of the same species, yet the heartwood and sapwood are structurally identical... The reason for the diil'erence in the rates is that when sapwood is converted into heartwood in the natural development of the tree, various substances are deposited which tend to obstruct the capillaries, thereby reducing the number and size of capillaries.

In our invention the. chemical is used to promote a greater diffusion of the water in the wood. At the start of drying, the water soluble chemical diffuses into the cell cavity where decomposition takes place as a result of temperature or conventional manner. The success of our invention is primarily dependent upon the proper selection of the chemical or mixture of chemicals. A knowledge of the properties of the chemical or mixture of chemicals, including the behavior with respect to the particular species of wood in question, is, therefore, of paramount importance. The following general properties of the chemical or mixture of chemicals should be considered: (1) temperature of decomposition, (2) products of decomposition, (3) solubility in water of the chemical or mixture of chemicals and oi the decomposition products, (4) molecular weight as an indication of the diffusion rate of the chemicals. In use with a particular species of wood, the following characteristics should be considered: (1) reaction with constituents in the wood to givegaseous products, (2) solubility of gaseous reaction products, (3) reaction with constituents in the wood to give precipitates which which may defeat the purpose of the treatment by plugging the capillaries, (4) solvent properties with respect to naturally deposited substances in the wood.

The chemical may be applied to the green wood in one of several ways, namely; (1) by piling the wood with the solid chemical between layers until the chemical has disappeared into the wood, (2) soaking the wood in solutions of the chemicals until a suitable amount of chemical has difiused into the wood. (3) painting, dipping, brushing, or otherwise applying a suitable amount of the chemical to the wood in the form of a solution thickened by a thickening agent such as starch, (4) by pressure treatment. In the case of use of solid piling, or of the thickened solution, if a chemical is used having a high.

' period following chemical application is necesreaction with constituents'in the wood, thereby forming a slightly soluble gas. The gas forms a bubble which is easily expanded and thus per-' mits water to leave the cell. The chemical or the decomposition products may also have a solvent action on the deposited substances in the wood and thus further promote diffusion by opening up the capillary structure. The gaseous products produced and surplus chemical diffuse into the wood, where similar reaction takes place. The diffusion of the gas is more rapid than that of the chemical since it is not dependent solely upon a continuous path of water, but can diffuse through the air spaces that are present as well. The presence of even a small amount of gas in solution in a water-filled cell cavity is beneficial to the diffusion of water, since under reduced pressure, such as results in drying a full cell cavity, bubbles form more easily because of a decreased surface tension. Thus, greater diffusion and consequently faster drying results even in the event that drying prematurely arrests the diffusion of the chemical. Also, water pockets are largely eliminated.

The procedure to be followed in carrying out sary since the chemicalwill continue to diffuse into the wood during the drying process. particularly during the initial stages when a major portion of the water columns in the cell structure are continuous from the surface to the transverse center of the wood.

During the chemical treatment, other chemicals may be added or incorporated which may impart such desirable properties to the wood as resistance to fire and attack by insects and deup the capillaries of the wood and thus decrease our invention consists simply of the chemical treatment 01' green wood followed by drying in a that usually exhibit slow drying rates,. butit is also useful in drying wood having ordinary in; characteristics. v

The following examples illustrate a..." eflectiveness and practice of theinvention but do not limit the invention to the cited examples.

Six vertically grained pieces and six 'fiatgrained pieces of so-called "sinker", redwood stock each measuring 3 inches by 8 inches by winches were end-coated to prevent end-drying and solid piled with sodium bicarbonate between layers. beneath the bottom layer, and on topoi the-top" layer. The sodium-bicarbonete was applied at therate of 40 pounds per 1,000 board feet. The pile was covered with a canvas to minimize dry ing. The specimens were left in the solid pile for 12 days during which periodwater was occasionally sprinkled on and in the pile to maintain moist surfaces. On the vtwelfthday the specimens were unpiled and replied in a like manner with urea applied at the rate of 100 pounds per 1,000 board feet. Thespecimens were left in the pile with themes. for 3 daysat which time no chemical was visible on the surface of the conditions and moisture content of the diiierent groups of specimens after various ofdrying.

It is evident from the data that the 'rate of" drying with the bicarbonate treatment is decidedly increased over that of untreated material and that obtained by using a conventional chemical seasoning agent, namely urea alone.

material was dried to a moisture content of about 18 to 20 percent since this is the usual ilnal moisture content range for material of the type used in the experiment. The following table (Table 11) shows the results obtained.

Table I! Kiln Moisture content at which schedule changes were mm I made and total time required-to dry to given moisture content v I Temper Controls Urea and sodium Ammonium ature (untreated) bicarbonate carbonate D I Moisture Moisture Moisture gi come! .Tima mum Time Time content FLAT-GRAIN s'rocx 1 Before chemical treatment. I Alter chemical treatment.

Table I l Moisture content or chemical treatments by grain types Dr bulb Wot bulb No cbemical treat- Urea plus sodium Time ten iperatempera- Egan; ment (controls) Urea bicarbonate ture 'ture I I Flat Vertical Flat Vertical Flat Vertical grain grain grain grain grain grain Dal/a F. F. Permit Percent Percent Percent Percent Percent Percent I93. 0 168. 0 104. 8 170. 7 i 203. 8 182. 9 101. 8 1 70. 2 0 115 105 71 196.0 171. 7. a 213. 8 182. 9 101. 8 170. 2 4 142. 0 136. 3 142.5 133.8 134.8 125. 8 l4 110. 7 114. 8 108. 3 U0. 3 07. 2 27. 5 29 79.7 96.3 76.7 88.2 02.2 72.7 46 56.0 77.1 62.6 72.0 40.4 55.7

1 Boioretreatment. After treatment.

i The gain in moisture content during chemical treatment is at the surface of the wood. Hence this gained moisture is readily lost in the first few days oi drying.

EXAMPLE II a like number of matched untreatedpieces. All

groups were subsequently dried by virtually the same moisture content-temperature schedule. Schedule changes were made by removing the group from theoriginal kiln when it reached the desired moisture content and placing it in a difierent kiln having the desired kiln condition. The,

It is apparent from the data that the use of the chemical has made possible a considerable saving in drying time.

EXAMPLEIII The diffusivity factor. the percentage of moisture per hour flowing transversely with respect to the fiber direction when the moisture gradient is one percent per inch in the direction of flow (similar to diflusivity as used in heat flow), for treated and untreated redwood was determined from matched specimens '1 inch by 1 inch by 30 inches. Two treatments were used as follows:

i. Soak 7 days in an aqueous solution made according to the proportions of 7.4 grams sodium bicarbonate, 18.5 grams urea, and 74.1 grams water.

2. Soak 7 days in an aqueous solution made according to the proportions of 27.6 grams ammonium carbonate and 72.4 grams of water.

The diffusivity factor for redwood treated by each of the above treatments was found to be 0.0014 percent per hour per inch at 120,l'..whil e that of untreated materialwas found to be 0.0011

percentper hour' per inch. The drying time varies inversely as the diffusivity factor: hence the treated material should dry to a given moisture 3 content in roughly to the time required for untreated material. These values checkthe suits given in Example 11. v

Among the'chemicals whichmay be useful for increasing the diffusion of water in wood and,

therefore helpfulin reducing the time required to dry wood are the carbonates, bicarbonates. thiocarbonates, thionates,-su lfldes. formates'and acetates of ammonia, sodium, potassium, calcium,

'magnesium, lead. zinc-and other metals; .-Be-

,cause of their solvent properties, organic solvents, thatare miscible .in water. such as the alcohols.

ketones, aidehydes and acids, preferably those of,

low molecular weight and boiling point and'those I that swell wood, mayalso be useful.

Having thus described our'invention', we claim:

1-. A process of drying wood which comprises impregnating with and diffusing into the subsurface of wood a material that generates a gas within the cellular structure of the wood at the temperature of the subsequent drying operation, the wood being maintained in contact with the material for a period suiiicient to cause penetra tion into the sub-surface of the wood, the material impregnated into the wood being sodium hi- Number carbonate and urea, and-then subjecting the wood to kiln-drying.

2. The process of drying wood desc'ribed in claim 1 comprising impregnating the wood with sodium bicarbonate and subsequently impregnating the wood with urea and thereafter subjecting the impregnated wood to kiln-drying.

" 3. 1'he process of wood described in claim i'comprising impregnating the wood with bicarbonate by. piling the wood with sodium bicarbomte between layers of wood while nfaintaining the surfaces of the wood moist, then impregnating with urea byrepiling the wood with urea between layers ofwood, and subjecting the woodto kiln-drying. e

:4. A- processof dryingwood comprising piling the-wood for several days with sodium bicarbonate between layers'of wood while maintaining the sprfacesmoist'to cause impregnation of the sodium bicarbonate into and diffusion into the subsurface-of the wood, repiling the wood for several days with urea between layers of wood,

causing impregnation'and diffusion of the .urea

REFERENCES crrnn' jrhe following references are of record in the file of this patenti UNITED STATES PATENTS Name Date 746,678 Cowles Dec. 15, 1903 Smith Apr. 25, 1944 a OTHER REFERENCES Che'micaiBeasoning, Progress Report #1, published by 1 revised October 1945.

West Coast I umbermens Association.

Claims (1)

1. A PROCESS OF DRYING WOOD WHICH COMPRISES IMPREGNATING WITH AND DIFFUSING INTO THE SUBSURFACE OF WOOD A MATERIAL THAT GENERATES A GAS WITHIN THE CELLULAR STRUCTURE OF THE WOOD AT THE TEMPERATURE OF THE SUBSEQUENT DRYING OPERATION, THE WOOD BEING MAINTAINED IN CONTACT WITH THE MATERIAL FOR A PERIOD SUFFICIENT TO CAUSE PENETRATION INTO THE SUB-SURFACE OF THE WOOD, THE MATERIAL IMPREGNATED INTO THE WOOD BEING SODIUM BICARBONATE AND UREA, AND THEN SUBJECTING THE WOOD TO KILN-DRYING.