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Minimizing Wood Shrinkage and Swelling The hygroscopicity and subsequent swelling and shrinking of dry wood are decreased by heating in various gases above thermal decomposition temperain hygroGreater reductions tures. scopicity are obtained in an oxidizing than in a reducing atmosphere for the same heating conditions; but, by increasing the temperature, equal reductions in hygroscopicity can be obtained in reducing atmospheres. The darkening of the wood on heating appears to vary directly with the resulting reduction in hygroscopicity, regardless of heating conditions. Soaking in water after heating has but a slight tendency to restore the original hygroscopicity. Heating wood in water-saturated atmospheres has no permanent effect upon the swelling and shrinking.
Effect of Heating in Various Gases1 ALFRED J. STAMM and L. A. HANSEN Forest Products Laboratory, Madison, Wis.
in the toughness of 50 to 75 per cent. Data of Greenhill (S) on the relation between maximum strength of beech in tension perpendicular to the grain and the moisture content at different temperatures indicate that the loss of strength on heating of the wood decreases with a decrease in moisture content to a negligible value for very dry wood. Although the temperature range covered by these data is below the temperatures required to obtain appreciable antishrink efficiencies, they indicate the possibility that less loss in strength will occur if the wood is adequately seasoned at normal kiln temperatures before being subjected to the higher temperatures required to reduce its subsequent swelling and shrinking.
Heating Experiments
long time it has been recognized that excessive heating of wood reduces its hygroscopicity. Tiemann {10) found that heating air-dry wood in superheated steam to about 150° C. for 4 hours reduced the subsequent moisture absorption by 10 to 25 per cent with but relatively small reductions of the strength, except for red oak which showed a reduction in crushing strength and modulus of rupture of about 60 per cent. An unpublished Forest Products Laboratory report {1) shows that heating black gum in dry air at 205° C. for 6 hours reduces the subsequent hygroscopicity to almost half of its original value with only a slight accompanying dein its strength. crease Koehler and Pillow {5) and Pillow {6) heated air-dry Sitka spruce and ash to 138° C. for 1 to 8 days and obtained reductions in the equilibrium moisture content at several different relative humidities for the longer time of heating of 30 to 40 per cent with accompanying reductions in the crushing strength of 15 to 25 per cent and reductions a
The purpose of this preliminary research was to confirm this loss of hygroscopicity of wood which occurs on heating, and to determine its permanence and how it is affected by the medium in which heating occurs. Sections of white pine 9 cm. long in the tangential direction (the direction of maximum swelling), 2 cm. radially, and 0.6 cm. in the fiber direction, were used. The short dimension in the fiber direction ensured rapid attainment of moisture equilibrium and made possible the cutting of a number of adjacent sections from the wood with a minimum variation of structure from sec-
FOR
1 Other papers in this series appeared in April, 1935, page 401, in December, 1935, page 1480, and in October, 1936, page 1164.
*
tion to section, Four specimens were suspended in a small steel bomb that was heated on the outside by an electrical resistance coil and lagged with asbestos insulation. The temperature was determined by a thermocouple inserted into a well in the wall of the bomb. The
temperature
manually
was
controlled
to about 2° C. When the specimens were heated in gases other than air, the bomb was evacuated and
of Chabactebistic Shbinxage and Distobtion Squabes, and Rounds as Affected by the Dibection Annual Gbowth Rings
831
Flats,
of
the
the gas admitted several times to ensure the elimination of air. The moisture content of wood used in the tests was about 6 per cent. When the heating was carried out in presence of water vapor, a large excess of water over that necessary to saturate the specimens was placed in the bottom of the bomb.
INDUSTRIAL AND ENGINEERING CHEMISTRY
832
Effect of Gases
still
VOL. 29, NO.
7
more so after the time was increased to 6 hours. In each of these cases the darkening obtained by heating in the different gases increased in the following order: hydrogen, illuminating gas, air, and oxygen. The specimens heated to 260° C. in hydrogen, however, were as dark as those heated for 6 hours at 205° C. in oxygen. The darkest specimens were about the color of unfinished walnut. The antishrink efficiency appears to parallel the darkening of the wood, irrespective of the temperature and the gas used. Although part of the antishrink efficiency may have been due to oxidation in the cases where the wood was heated in air and oxygen, it is hard to imagine that this was a major factor since equal efficiencies can be obtained by heating in hydrogen at a slightly elevated temperature. The phenomenon can best be explained on the Efficiency of Heating Dry Wood Table I. Effect upon Antishrink basis of thermal decomposition. Loss of water in Various Gases and Subsequent Soaking in Water for 5 Days of constitution is the first thermal reaction. If -Antishrink Efficiency· .—Before Soaking0—'-—After Soaking^-. this loss were due to the formation of an ether Tangential Tangential of dimension Weight dimension Weight linkage between two adjacent cellulose chains basis basis Gas Temp, Heating through adjacent hydroxyl groups, the loss in C. Hr, % % % % hygroseopicity could be readily explained. Not 0.25 5.9 6.3 1.8 165 2.8 Hydrogen 17.0 11.4 205 2.00 16.0 11,5 only would the hygroseopicity be reduced be2,00 32.0 31.2 260 32.0 31.8 cause of the substitution of the less hygroscopic 0.25 8.5 6.3 4.8 165 8.2 Illuminating gas 205 2.(X) 18.0 19.0 14.0 13.2 ether group for the more hygroscopic hydroxyl 6.00 19.0 17,6 205 20.6 19.9 groups, but also because of the parallel bonding 0.25 6.4 4.4 165 8.3 4.9 Air 2.00 19.0 12.3 205 17.5 14.0 of the cellulose chains. Staudinger (9) showed 21.2 205 6.00 23.2 22.0 21.0 that the formation of such bridges between the 165 0.25 10.0 12,0 7.0 6.1 Oxygen 2.00 21.0 15.4 13.6 205 20.7 chains in polystyrene resins with p-divinylben205 6,(to 28.0 30.0 28,7 27.2 zene, cuts down the swelling tremendously even * In terms of retardation of the dimension and weight changes, for the average of four when only enough p-divinylbenzene is used to specimens, per unit change of the untreated controla when alternately brought to equilibrium with 30 and 90 per cent relative humidity. form a single bridge for several thousand fr Based upon the second humidity change cycle as the first is appreciably affected by hysteresis (Ú). molecules of monomeric styrene. Just an ................ occasional cross link evidently cuts down appreciably the tendency for water to be taken up between structural chains. The formation of ether linkages are referred to the corresponding change for the control. The between the hygroscopic hydroxyl group not only explains the sections heated in dry atmospheres were soaked in water for decreased hygroseopicity of wood heated in dry atmospheres 5 days after the humidity cycles were complete and then subbut also the fact that heating in the presence of a large excess jected again to the humidity change cycles. Only the second of water vapor causes no change in hygroseopicity. The pressubsequent humidity cycle was used in the calculations, since ence of an excess of water vapor would depress the thermal the first would involve a higher desorption curve due to the reaction in which water is evolved according to the principle soaking and thus give results affected by the sorption hystereof LeChatelier and thus markedly reduce the tendency to sis (11). The same is true for sections heated in the presence form the ether bridges. If the change in hygroseopicity were of water. The first cycle gave appreciable negative efficiencies because of tfiis hysteresis effect. merely a physical change, such as that postulated by Urquhart (11) to explain hysteresis, soaking of the specimens in Heating the sections for as short a time as 15 minutes at water should largely restore their original hygroseopicity. This 165° C., a temperature at which thermal decomposition wrs investigator believes that the free hydroxyl groups of cellujust becoming appreciable, gave definite antishrink efficiencies in all the dry gases. Increasing the temperature and the lose, which are originally satisfied to a large extent by water, draw closer together on drying and finally mutually satisfy time of heating increased the antishrink efficiency. In each case the efficiency was greater in an oxidizing than in a reducone another. Although these bonds are only partially broken on rehumidification they should be largely broken on soaking atmosphere. Subsequent soaking of the sections in water reduced the antishrink efficiency by a relatively constant amount, regardless of the heating conditions. Sections heated in the presence of water vapor gave very small uncorrelatable antishrink efficiencies after the first cycle, part of the values being positive and part negative (average antishrink efficiency 0.25 per cent, mean deviation 1.1 per cent). These values for all the temperatures and times may be considered within experimental error of being zero; that is, heating in water vapor has no effect upon the antishrink efficiency after the first humidit y change cycle. The sections heated at 165° C. were but slightly darkened without a perceptible difference in appearance between the sections heated in the various gases. The darkening was more apParquetry Flooring Offers Possibilities of Chemical Treatment to Minimize Swelling and Shrinking preciable after heating for 2 flours at 205° C. and
Table I gives the effect of heating the white pine sections in different dry atmospheres upon the subsequent swelling and shrinking. Measurements were made of both the tangential dimension change and the weight change occurring when the specimens were alternately brought to equilibrium with 30 and 90 per cent relative humidity in humidity rooms held at 26.7° C. The specimens were exposed 2 weeks at each relative humidity, which proved adequate for the attainment of equilibrium. The antishrink efficiencies are calculated on the basis of the reduction in dimensional change or weight change between 90 and 30 per cent relative humidity. These changes
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•
JULY,
1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
mg in water and the active groups again satisfied by water. The reversible part of the antishrink efficiency—that is, the difference between the antishrink efficiency obtained directly after heating and that subsequent to soaking in water—is practically constant, regardless of heating conditions. This part is undoubtedly due to a physical effect such as that given by Urquhart (11), The physical mutual satisfaction of hydroxyl groups evidently increases until the free water is rather completely removed but does not increase on further heating. These preliminary results indicate that the antishrink efficiency resulting from the excessive heating of dry wood in several common gases is sufficiently great and permanent to warrant a more extensive investigation in which the strength properties are simultaneously studied. Although this method of minimizing the swelling and shrinking of wood does not
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appear to be so effective as methods previously described (£, 4, 7, 8), its possible value rests on the fact that it would be relatively inexpensive.
Literature Cited (1) (2) (3) (4) (5)
Bette, N. D., Forest Products Lab., File No. 2B468 (1916). Browne, F. L., Ind. Eis'O. Chem., 25, 835 (1933). Greenhill, W. L., J. Council Sci. Ind. Research, 9, 265 (1936). Hunt, G. M„ IT. S. Dept. Agr., Circ. 128 (1930), Koehier, A., and Pillow, . Y,, Southern Lumberman, Dec. 19,
1925 219. (6) Pillow,’µ. Y., Wood Working Ind., Oot., 1929, 8. (7) Stamm, A. J., and Hausen, L. A,, Ind. Bnq. Chem,, 27, 1480 (1935). (8) Stamm, A. J., and Sei.org, R. M., Ibid., 28, 1164 (1936). (9) Stmidiriger, H., Trans. Faraday Soc., 32, 323 (1936). (10) Tiemami, H. D„ Lumber World· See., 28, No. 7, 10 (1915). (11) Urquhart, A, R., J. Textile Inst., 20, 125T (1932).
Treatment with Sucrose and Invert Sugar ALFRED J. STAMM
and The treatment of wood with sucrose sugar solutions greatly reduces the subsequent shrinkage. Shrinkage takes place when the relative vapor pressure under which the specimens are dried is less than the relative vapor pressure o£ the treating solution at the concentration attained when evaporation has proceeded to the fiber-saturation point. The large reduction in shrinkage to the ovendry condition is due to the deposition of sugar This reduction within the swelling structure. invert
treatment of wood with sugar solutions dates back
to the Powell patent of 1904 (7). Powell did not THE sider the stabilization of the dimensions of the wood but con-
rather interested in the prevention of decay. The following year Tiemann showed that sugar materially reduces the shrinkage of wood as a result of the retention of the solution. Further measurements made by him in 1928 are summarized by Hunt (4). Antishrink efficiencies (the reduction was
can be calculated from the partial specific volume of sugar in the concentration attained within the swollen structure on the basis that this concentration becomes equal to the corresponding bulk concentration. Invert sugar reduces the dimension changes of wood to a greater extent than sucrose and should serve as a good antishrink agent under conditions that will not be too conducive to the leaching of the sugar from the wood.
in shrinkage
of the dimension changes of the treated specimens per unit dimension change of the controls) of better than 70 per cent were obtained between the saturated and the air-dry condition for specimens treated with concentrated sugar solutions. This research was undertaken to determine more definitely the effect of sugar treatment upon the shrinking of wood and to find out if the shrinkage is governed by the same principles as were found to hold for salt treatment (!)). It was also desirable to determine the dimension stabilization of wood treated with invert sugar compared to that treated with sucrose, since invert sugar had been shown by Dittmar (.3) to be considerably more hygroscopic than sucrose, and by Leete (5) and Pike (6) to increase the moisture retention of paper when used in only moderate concentrations.
Experimental Procedure Thin sections of northern white pine,
Untreated Wood Should Be Dried to the Moisture Content Prevailing in Service to Prevent Subsequent Shrinkage or Swelling Le/t, wood put in service too wet? right, too dry.
2 mm. in the fiber direction and 4.4 cm. in the other two· directions, were used for these meastiremente. Oven-dry sections were weighed, and the tangential and radial dimensions determined. The sections were then soaked in water or sugar solutions, with intermittent applications of suction to remove the air. After soaking for 3 days to permit diffusion of the sugar into the fine structure, the sections were brought to equilibrium with the decreasing relative vapor of pressures
