May, 1941
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Literature Cited (1) Barton-Wright, E. C., “General Plant Physiolow”, PP. 294-5, Philadelphia, P. Blakiston’s Son h Co., 1938. (2) Besse, R. S.,Oregon Bull. 359 (1938). (3) Cook, R. L., Mich. Agr. Expt. Sta., personal communication. (4) Crane, F. H., Univ. Ill., M.S. thesis, 1926. (5) Eckerson, 8. H., Contrib. BoyceThompson Inst., 3 , 197-217 (1931). (6) Gassner, G., and Strait, W., Angew. Botan., 19,225-45 (1937). (7) Hoffer, G. N.,IND.ENO.CHIM., 30,885-9 (1938). (8) Koehler, B.,Phytopathology, 29, 817-20 (1939).
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(9) Lang, A. L., Dept. Agronomy, Univ. Ill., Mimeographed Leaflet, 1940. (10) McMurtrey, J. E.,U. S. Dept. Agr., Tech. BUZZ. 612 (1938). (11) Onslow, M. W.,“Anthocyanin Pigments of Plants”, 2nd ed., London, Cambridge Univ. Press, 1925. (12) Snider, H. J., Univ. Ill., personal communication. (13) True, R. H., J. Am. SOC.Agron., 13, 91-107 (1921); Science, 55, 1-6 (1922). P R ~ B ~ N before T B D the Division of Fertilizer Chemistry at the 100th Meeting of tbe Ameriosn Chemiosl Society, Detroit, Mioh.
Chemical Seasoning of Wood Hygroscopic and Antishrink Values of Chemicals EDWARD C. PECK Forest Products Laboratory, Madison, Wis.
chemical, the moisture loss from O N S I D E R A * B L Ew o r k Of the chemicals and mixtures studied, the the surface layers ceases but has been done a t this ones that showed most promise for use in continues from the interior porlaboratory during recent the chemical seasoning of wood, both from tions which have not become years on the seasoning of wood the standpoint of relative humidity and impregnated. Unless the surwith chemicals. The process antishrink, and of lacking undesirablepropfaces are dressed off before the consists in soaking green wood wood is put in service, a chemiin an aqueous solution (usually erties, were urea, invert sugar, and a mixcal of this nature will also cause saturated) of a chemical or mixture of invert sugar and urea. Some of the trouble through condensation of ture of chemicals and in subchemicals studied are too expensive; others moisture from the surrounding sequent air-drying, kiln-drying, possess undesirable properties, such as coratmosphere. or seasoning in service. Somerosiveness or increased flammability. When green wood is immersed times a dry salting method is in a chemical solution, it underused in which the green lumber is Since in commercial practice it is difficult bulk-piled with alternate layers goes certain physical changes. to maintain a solution in any form but a Some of the moisture in the of the chemical. I n the course of saturated one, some chemicals produce too these studies a number of chemiwood will pass into the chemilow a relative humidity with consequent cals and mixtures of chemicals cal solution because of the difdanger of surface checking in the seasoning ferencein vapor pressure between have been tried in an attempt to the green wood and the chemifind the most satisfactory. bath. The safe relative humidity for most cal solution. Some of the A chemical must possess lumber items is probably between 70 and 85 chemical will diffuse into the certain characteristics in order per cent. wood through the water conto be suitable for use in chemitained in the wood, the extent of cal seasoninn. The aaueous this diffusion depending on the nature of the chemical, the solution must cause a considerable reduction in relative vapor duration of the soaking, the available moisture in the wood, pressure or a pronounced antishrink effect when absorbed by wood, or both. It must be cheap and available and readily and the morphological characteristics of the wood. Within soluble in water. It must not be dangerous to handle or corthe penetrated zone, chemical is present in the cell cavities rosive to metals. It may have additional desirable characterand also in the finer wood structure. If the chemical is posiistics, such as decay, insect, or fire resistance. The possession of tively adsorbed within the finer wood structure, swelling some of these properties to an unlimited extent is not always beyond the green dimension takes place; if negatively adsorbed, no swelling takes place, the adsorbed chemical merely wholly beneficial. The solution of a chemical which causes a large reduction in relative humidity is advantageous as far replacing an equivalent volume of water. That which is in the cell cavities affects the vapor pressure within the cells as final seasoning is concerned but disadvantageous from the and acts as a reservoir of chemical for further adsorption standpoint of checking in the chemical bath. When green within the finer wood structure or for further penetration wood is placed in an aqueous solution of a chemical which possesses a low vapor pressure, moisture is lost from the surdeeper into the wood. After soaking in a chemical solution face of the wood because of the vapor pressure difference. a piece of wood possesses an outer zone which contaias a When the surface layers have become impregnated with the gradient of chemical concentration and, therefore, a vapor
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 33, No. 5
The antishrink effect was measured by completely impregnating strips of wood with the chemicals and comparing their shrinkage with that of normal wood, throughout a controlled Relative Humidity, % drying process, Adams and Perry and Present The first step in the analysis of the data was a comparison Chemical Stamma Merz (1) Duus (3) expts. .. 68 of the results obtained with this method and apparatus with 63 67 NHdNOi &.N~;,hSO+ .. 81 80 83 those of other experimenters (Table I). 35 .. 32 32 After this method and apparatus had produced reasonably Ca(N0a)s .. 55 .. 59 .. g; consistent results, many experiments were conducted with MgCb 376 3 NaCl 7..s .... 76 solutions of various concentrations and mixtures of chemicals. NaNOi 76 77 CO(NHdsb .. 80 79 A mixture usuallv consisted of two chemicals, a saturated a Calculated from International Critical Tables. a base with various Of One wGch was used b J. F. T . Berliner found arelative humidity of 78% for a saturated solution of urea. trations of the other. The , concentration is expressed by the weight Of the anhydrous HUMIDITY IN EQUILIBRIUM WITH DIFFERENT CONCENTRATIONS OF AQUEOUS TABLE 11. RELATIVE SOLUTIONS AT 68" F. chemical per 100 grams of water in the solution, regard% Relative Humidity, Using Following Grams of Chemical per 100 Q. Water*: Chemical 10 20 30 40 60 60 70 80 90 100 110 120 120+ less of whether, as in the Invert sugar 99 9 8 9 8 97 96 96 95 (67) ... . . . . . . . . . . . . . case of a mixture, the water Diethylene glycol 99 98 97 96 94 93 92 90 89 87 ... already contains a dissolved Calcium nitrate 99 98 97 95 93 90 84 77 71 66 63 Ai 59'(129) Urea 98 96 94 92 90 88 86 84 82 80 79 (105) . . . . . chemical. Ammoniumsulfate 98 94 9 3 91 89 86 84 83(75) 7 e ; ~ ) . . . . . . . . . . Sodium nitrate 97 96 94 93 90 86 82 78 .. ,.. The relative humidities a t Ammonium nitrete 97 94 91 88 86 83 81 79 76 74 ii s s . ( i b z ) 68" F. which were found over Sodium rhloride 9.9 89 82 78 (36) . . . . . ... . . . . . . . . . . . . . Calcium chloride 90 80 70 61 52 44 36 32(76) the solutions of single chemiMagnesiumchloride 86 72 6 0 48 37 32 ( 5 6 ) ... ... .. .. .. .. .. .. .. .. .. .. . . . . . . cals are given in Tables I1 Figures in parentheses are concentration values which do not fall on the even intervals used in the table. and 111. The last relative humidity value given in each case 18 that over the saturated solution, except for invert sugar and TAB- I. RELATIVE HUMIDITIES OVER ~~ATURATED CHEMICAL SOLUTIONS AT 68" F.
Q
diethylene glycol.
pressure gradient. There is also a difference in vapor pressure between the moisture in the untreated part of the wood and in the impregnated zone. (It is assumed and has been generally demonstrated by experience that wood impregnated with a chemical reacts like the chemical solution.) These differences in vapor pressure establish a potential for moisture transfusion which is independent of the normal moisture content gradient potential. If this piece of wood is placed in a drying atmosphere, the vapor pressure of which is identical with that of the chemically impregnated outer skin of the wood, no immediate drying will take place. Soon, however, the moisture moving from the interior of the wood to the surface dilutes the chemical solution and thus raises the vapor pressure which, in turn, permits drying to commence. The moisture evaporated from the outer surface is continually being replaced by moisture moving from the interior parts; thus it is possible, by manipulation of the drying conditions during the early stages of drying, to maintain the initial moisture content of the impregnated surface zones. This, in conjunction with the antishrink effect of the chemical, greatly lessens the formation of tension on the surfaces and makes the formation of surface checks less likely. THERE is a vast amount of information on the properties of simple chemical solutions but little on the properties of solutions of chemical mixtures that might be used in the chemical seasoning of wood. Where two or more chemicals are mixed in solution, the effect on vapor pressure or relative humidity a t a given temperature is apparently unknown at the relatively high concentrations that have to be used for chemical seasoning. The purpose of the experiments here described was to determine the relative humidity a t 68" F. of chemical solutions of various concentrations and mixtures of chemicals in solution, and to measure the antishrink properties imparted to the impregnated wood. The relative humidities over the various solutions were measured by determining the equilibrium-moisture-content values of wood specimens suspended over the solutions in closed containers. Provision was made for a positive and continuous circulation of air within the containers and for a gentle stirring of the solution.
ACCORDING to Adams and Merz ( I ) , when two chemicals chemically inert to each other are mixed in a saturated solution, the vapor pressure of the mixture is less than that of either constituent. In certain instances, however, the two chemicals may form a compound which gives a greater vapor pressure than that of the constituent with the lesser vapor pressure. The vapor pressure in the latter case will also depend on whether the compound alone is present or whether i t is accompanied by an excess of either chemical. I n this series of experiments mixtures of chemicals in solution were employed; in most cases one solution, usually a saturated one, was used as a base and various amounts of the other chemical were added, up to a point in some instances where the available water was saturated by both chemicals, and in others where an excess of the second chemical was present.
TABLE 111. RELATIVE HUMIDITY IN EQUILIBRIUM WITH DIFFERENT PERCENTAGES OF SATURATION OF AQUEOUS SOLUTIONS AT 68" F.
r0Relative Humidity at Saturation of: Chemical Ammonium sulfate Urea Sodium chloride Sodium nitrate Ammonium nitrate Calcium nitrate Calcium chloride Magnesium chloride Invert Bugar Diethylene glycol a
20%
96 96 96 96 89
40% 93
80%
100%
..
.. ~.
2 35
92
92 94
97
80 92
.. ..
..
85 85
60%
70 69 D.
1 .
0"
Approximate value for anhydrous chemical.
Although these experiments were conducted at 68" F. (20" C.), two special tests were made a t 86" F. (30" C.) to check the results obtained by other investigators. A given quantity of water was saturated with urea and also with ammonium nitrate and ammonium sulfate, respectively, and the relative humidities were determined. The results are given in Table IV. The values obtained do not agree with those of other experimenters, but no reason can be given for the discrepancy.
I N D U S T R I A L A N D E N GI I N E E R I N G C H E M I S T R Y
May, 1941
TABLEIV. RELATIVEHUMIDITIESIN EQUILIBRIUM WITH SATURATED SOLUTIONS OF MIXTURES OF CHEMICALS AT 86" F. Relative Humidity, % Adams and Present Mer5 (I) expts. 18.1 49.0 56.4 62.0
Mixt. of Urea with: Ammonium nitrate Ammonium sulfate
The relative humidities measured over the mixtures of chemicals in solution are summarized in Table V. TABLB V. RELATIVE HUMIDITY IN EQUILIBRIUM WITH SATURATED CHEMICAL SOLUTIONS AND SATURATED SOLUTIONS OF MIXTURES AT 68" F.o Relative Humidity of Mixt.'. % 95 78 73 95 79 79 95 32 80 95 78 570 70 32 NaCl 78 44 32 78 79 68 58 Urea 65 83 79 73 73' 79 79 NH~H~POI 80 75 0 In the mixtures the water was assumed to satisfy all chemicals. b Not a saturated solution 67 rams per 100 grams of water. Mixture includes urea 0$79'# relative humidity. d Not a saturated sol$ion. 100 grams per 100 grams of water. 6 Stamm's value. Chemioal Invert sugar 40'%b
Relative Humidity,
%
Chemical NaCl Urea MgCb NaCl MgClz CaClz
Relative Humidity,
%
The relative humidities over the solutions of the mixtures were as low as, or lower than, that over the solution of either constituent except where calcium chloride or magnesium chloride was involved. With these two chlorides the relative humidities of the mixtures fell between those of the two constituents. The greatest reduction in the relative humidity of the binary mixtures occurred with urea and ammonium nitrate, while the greatest reduction occurred with the trinary mixture. The relative humidity of the mixture was compared with the average relative humidity of the constituent chemicals to obtain a measure of the reduction in relative humidity. Where magnesium chloride was used in a mixture, the resultant relative humidity was greater than the average for the constituents. Although not shown in Table V, in some instances the chemicals added to the first solution were added in excess of the amount required to saturate the water present in the solution. In the mixtures of sodium chloride with calcium or magnesium chloride, respectively, excess amounts of the latter two were added. This was also done with the invert sugar-magnesium chloride mixture. Apparently in some instances the addition of excessive amounts of the second chemical caused the first chemical to crystahze from solution and give a saturated solution of the second chemical with the first present in the solid phase.
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When 100 grams of calcium chloride and 300 grams of magnesium chloride, respectively, were added to a saturated solution of sodium chloride containing 100 grams of water, the relative humidity over the resultant solution was approximately that of the saturated solution of the second chemicali. e., calcium or magnesium chloride. One hundred grams of magnesium chloride added to a 40 per cent invert sugar sohtion containing 100 grams of water produced a relative humidity of 66 per cent compared with a relative humidity of 80 per cent when just enough (54.5 grams per 100) of magnesium chloride to saturate the water in the solution was added. I n all these mixtures, after the second chemical had been added in sufficient amount to cause saturation, an increase in the amount of the excess of the second chemical produced a progressively decreasing relative humidity in the final solution. A n important criterion in estimating the value of a chemical for the chemical seasoning of wood is the magnitude of its antishrink effect. The reduction in shrinkage of wood impregnated with chemicals is due to two causes-the bulking volume of the chemical within the fine wood structure or the modification of the hygroscopic properties of the wood. The latter causes the wood to retain more moisture when exposed to a given relative humidity. Whatever the reason, the value of a chemical in the chemical seasoning of wood can be gaged by the shrinkage of the impregnated wood a t equilibrium with a definite relative humidity. Table V I summarizes the antishrink effect obtained on strips of black gum impregnated with various chemicals and mixtures. TABLEVI. TANGENTIAL SHRINKAGE AT EQUILIBRIUM (100' F. AND 30 PER CENTRELATIVE HUMIDITY) OF STRIPSOF BLACK GTJMWOODIMPREGNATED WITH SOLUTIONSO OF CHEMICALS AND OF MIXTURES Chemicals Calcium nitrate Qmmonium nitrate and urea Invert sugar (50%) Ammonium nitrate Sodium chloride and urea Invert sugar (40%) and urea Ammonium sulfate and urea Invert sugar (400j0), sodium chloride, and urea Tlrra
ih'% suqar (40%) Sodium nitrate Ammonium sulfate 5
b
Shrinkage, yo of That of Control 7.7 11.6 14.0b 16.4 16.9 17.9 20.5 37.2 44.7 47.4 56.5 65.4
Saturated unless otherwise stated. Data from Stamm.
Wood impregnated with a saturated solution of calcium nitrate shrank only 7.7 per cent as much as normal wood a t an equilibrium with 100" F. and 30 per cent relative humidity.
Literature Cited (1) Adams and Mers, IND.ENQ.CHEM.,21, 305-7 (1929). (2) Perry and Duus, Chew. & Met. Eng., 41, 127-9 (1934).
