Effect of Chemical Treatment on Wood Permeability ALFREDJ. S T A ; CForest ~ ~ , Products Laboratory, AMadison,W i s .
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XE of the primary objects in developing the previously reported physical methods for studying the capillary structure of wood (4, 5, 6, 7 , 8) h a s b e e n to obtain a means of quantitatively investigating the effect of chemical and physical treatment on its permeability. Data obtained from such measurements should be of value in developing means of increasing the permeability, which, in turn, might be of value in the impregnation of resistant woods with preservatives and in the impregnation of c h i p s w i t h c h e m i c a l s in chemical pulping processes,
DOUGLAS FIR specimens I foot (30.48 cm.) long and thin Sitka spruce sections were treated with chlorine gas, followed in some instances by a treatment with ammonia gas or ammonia water. Permeability measurements, using the preciously dereloped differential pressure-drop hydrostaticjlow apparatus, and measurements of the effective opening diameters, using the method for ocercoming the effect of surface tension, show that the treatment opens the capillary structure, presumably as a result of solzient action on the lignin of the pit membranes that constitute the major resistance to jlow. Permeability increases ranging from twofold to one hundred and thirly fold were obtained under i w y i n g conditions.
RELATEDINVESTIGATIOSS Other investigators have become interested in the same problem. Johnson and PtIaass ( I ) found that treatment with pulping liquors in general increases the subsequent permeability of wood to water. Scarth and Spier ( 3 ) ,measuring the permeability of wood, found that untreated heartwood sections of red spruce (it was not stated whether these were seasoned or unseasoned) gave a flow of only 1 cc. of water in 22 hours, under the conditions selected. K h e n the sections were boiled in water the permeability increased, the flow becoming 20 cc. for the same length of time. Treatment with lignin qolvents gave a flow of 10 to SO cc. in the same time. These investigators state that the increase in rate of flow caused by boiling the Tvood in water is due to the removal of hemicelluloses. This, however, seems highly improbable. Even with unseasoned wood, in which the air content is relatively small, it is still sufficient to affect the permeability markedly (5, 6). The boiling process, which removes a large percentage of the residual air, can readily account for the twenty fold increase in permeability. The rates of flow obtained with t,he chemically-treated sections, from which some air undoubtedly is removed during treatment, should be compared with the rate of flow obtained with the water-boiled specimens rather than with the unboiled. The largest increase in permeability obtained on this basis with the lignin solvents used was ahout fourfold. As different amounts of air may be removed in different solvents, quantitative comparisons of the increase in permeability are impossible. Seither the simple experimental method used by Scarth and Spier nor the much more complicated method of Johnson and Maass, which was designed to duplicate as nearly as possible commercial pulping conditions, is adequate for obtaining data that can be used in calculating the dimensions of the effective capillaries and the changes in these dimensions. Such measurements can be made, however, by using methods previously reported ( 5 , 6 , 7 , 8 ) . This paper, supplementing earlier ones, is a preliminary report in which the possibilities of increasing the permeability of wood are demonstrated and some of the theoretical inferencec: arr ~ x ~ i n t ecut. d
FINEC.4PILLhRY
STRUCTURE O F
WOOD
A brief description of the fine capillary structure of wood will help t o w a r d an understanding of the effect of chemical treatment on permeability. The fiber cavities that make up the major part of the void v o l u m e of wood are closed a t both ends, the only communication from fiber cavity to fiber cavity being through the pores i n t h e m e m b r a n e s of t h e bordered pits. It is the size, as well as the number, of these fine openings in the pit membranes t h a t c o n t r o l s the permeability of wood to hydrostatic flow. Each softwood fiber with an average length of about 0.3 cm. and a diameter of approximately 0.003 cm. has from 30 to 300 pits connecting it with adjoining fibers, and there are from 50,000 to 100,000 such fibers in a square centimeter of cross section. Figure 1 shows a photomicrographic section of a typical softwood cut across the fibers. A bordered pit connecting two adjacent fiber cavities is s h o w in cross section. The openings in the adjacent cell walls, which are almost circular, are separated by a membrane that is a continuation of the middle lamella, the cementing material between the fibers. The pit membranes, particularly those of softwoods, have a slight central thickening called the torus. Sometimes the pits are aspirated, that is, the torus is held against one of the seats of the cell wall opening, making the pit act like a closed valve. The permeability of a pit depends primarily upon its condition. If the pit is not aspirated, the permeability is a function of the porosity, the area, and the thickness of the membrane. If it is aspirated, the permeability further depends upon how close the valve seat fits and whether or not it is cemented in a closed position with resinous material. The greater Permeability of sapwood over that of heartwood ( 7 , 8) is probably due to far less aspiration of the pits and clogging of the membranes with resin in sapwood than in heartwood.
EXPERIMENTAL PROCEDURE AXD RESULTS The fact that practically the entire resistance to the flow of liquids through wood is in the pit membranes, which are composed of lignin ( 2 ) , immediately suggests that the permeability of wood might be increased through the use of lignin solvents. Since it would be corninercially desirable to increase the permeability of resistant woods only, it is important to use a means of solvent treatment that is as effective as possible in completely penetrating the wood. -4gaseous treatment of the partially seasoned wood seemed the most promising method. Chlorine reacts with lignin to form a compound that is soluble i n dilute alkali or ammonium hydro\-ide. The procedure ad-ipted n-as to paaq
cliliiriiie gas, wliicii hi l>ecn hirmidified by b~i1,biingtliniugh atteinpts to cause ruptnre of the pit meinbraiies mechaniwater, longitudinally tlirungh the sperimca. Glass adapters cally will be made, using high pressures, freezing, and perhaps a with hard rubber llanges for clamping ti^ the ends of the combination of freezing and high pressure. men s e n d to concinct the gas. The disctiarge tube was Chloriire treatment of the long specimens of Douglas fir, dipped iuto a beaker CJS oil so that the rate of passago of the followed by soaking in ammonia water, was less effective in qas could be obsemed fnirrr the rate of bubbling. Witli thc iiniformly opening the structure than it was with the thin type of coonection used, leakngc~made it iinpossiiile to apply Sitka spruce sections. The permeability increase for the chlorine pressures greater tiran R few atinrq)heres. Improve- center section was appreciably less than that for the section ment of the ciainping device may pcrmit cut from the intake end, slrowing that the effeotivoness of the The chlorine-gns tretrtineiit w:w in si~irice, treatment falls off toward the middle of the specimen. When similar treatincnt with oiniiionia gas :mil this, in i,wn, i ~ yti ammonia gas was pa,ssed through the specimen, instead of treatment with steam. soaking it in ammonia water, the opening of the capillary structure wns m o r e BI e a s u r e m e n t s I I ea rl y c o m p 1 e t c were f i s t made upon throughout tlie specitransverso sections of m e n . Witli b o t h Sitka s&Jruce 1 to 2 forms of treatment, em. t h i c k (3 tu ti radial p e n e t r a t i o n film lengths). Tire and tangential pcnechlorine and the a n t r a t i o n were negliinonia gas passed gi ble. Longitudinal t h r o u g h these seepenetrati
