Effect of Heating in Water on the Strength Properties of Wood 1

Transcription

1 AMERICAN WOOD-PRESERVERS ASSOCIATION 1954 Effect of Heating in Water on the Strength Properties of Wood 1 Engineer Emeritus, Forest Products Laboratory, 2 J. D. MacLean Forest Service, U. S. Department of Agriculture The results obtained in studies of the effect of different water temperatures and different heating periods on the strength properties of small matched specimens of representative softwoods and hardwoods are discussed. Data were obtained on the following strength properties listed in order beginning with the one most affected: (1) work to maximum load, (2) modulus of rupture, (3) fiber stress at proportional limit, and (4) modulus of elasticity. Introduction Liquids such as water and preservative oils are widely used to heat wood when applying preservative and to condition timbers for various purposes. For example, hot water is commonly used for heating logs that are to be cut into veneer, and wood tanks and vats are employed under service conditions where they are in contact with hot liquids for long periods. Wood-stave pipes are also used in some installations for carrying hot water. Since it is common practice to condition green timbers by means of the Boulton process and to apply preservative treatment when the wood has a moisture content considerably higher than that of air-seasoned material, heating timbers in preservative oils under such conditions can be expected to have much the same effect on the strength properties as heating in water at the same temperature and for the same duration. Purpose of Study Several years ago experiments were started at the Forest Products Laboratory to study the effect of different temperatures, different heating periods, and different heating mediums on the strength properties of representative hard- 'All strength tests were made by the Forest Products Laboratory, Division of Timber Mechanics. Acknowledgement is made to R. P. A. Johnson, Chief, Division of Timber Mechanics, and J. T. Drow, Engineer, for very helpful suggestions and advice in connection with 'this work and also for reviewing this paper. 2 Maintained at Madison, Wisconsin, in co-operation with the University of Wisconsin. wood and softwood species. Four heating mediums were used in these experiments which were steam, water, the hot press, and oven. Results obtained in the steaming experiments were discussed in a paper presented at the meeting of this association in 1953 (1). 3 Species Used Species heated in water included Coast-type Douglas fir, Sitka spruce, yellow birch, and yellow poplar. Data obtained in the steaming experiments, which were made on Douglas fir, Sitka spruce, yellow birch, southern pine, and hard maple, indicated that there was little difference in the results obtained in steaming the three softwood species under the same conditions. Application of Data to Other Species Since the Douglas fir and Sitka spruce specimens were similarly affected when heated in steam and in water, it seems reasonable to assume that the results obtained in heating these two softwoods in water would apply in general to other softwood species. Steam and water at the same temperature and applied for the same length of time also appeared to affect the strength properties of the hardwood species in a similar manner but to a greater extent than in the case of the softwoods. Water Temperatures and Heating Periods Table 1 lists the water temperatures and heating periods used for each species. The temperatures employed were 150, 200, 210, 3 Numbers in parentheses refer to Literature Cited at end of this report.

2 2 AMERICAN WOOD-PRESERVERS ASSOCIATION Table 1.-Species Heated in Water at Different Temperatures and for Different Periods 250, 300, and 350 F. Specimens heated in Although the temperature range of 250 to water at temperatures of , and at the 350 F. is higher than that of liquids in which boiling temperature (about 210 F. at Madi- wood is normally heated, these higher temperason, Wis.) were heated under controlled tem- tures were used to study the general relation perature conditions in special equipment. Those between temperature and heating period and to heated at temperatures of 250, 300, and compare the results with those obtained in the 350 F. were placed in a small tank of water steaming experiments. enclosed in a pressure cylinder. The water was then heated to the desired temperature by steam Method of Cutting and Matching maintained at a suitable pressure. In using the Specimens lower temperatures of 150 and 200 F., dif- In most cases the specimens were cut from ferent groups of the specimens were heated for sticks about 4.5 to 5 inches square and 50 to various periods ranging up to 660 days (over 60 inches long. These square sections were 1.8 years) at 150 F. and up to 375 days or sawed into two smaller pieces as shown in over one year at 200 F. Figure 1. Each stick was designated by a letter Heating periods ranged from 1 to 32 hours such as A, B, C, etc. After the square sections at temperatures of 250 and 300 F. Because were sawed into two pieces, the four sides of of the rapid disintegration of the wood at 350' each piece were surfaced in the planer. One F., when this temperature was used, the heating piece from stick A, for example, was marked period was limited to 8 hours. The latter three A, and the second piece was marked A,. Pieces water temperatures were the same as those used from a second stick would be marked B, and in the steaming experiments. and B,, and so on for other sticks. Figure 1 With the exception of the Douglas fir and shows how the individual test specimens were yellow birch specimens heated at 210 F., all cut and marked when they were prepared. of the Douglas fir, Sitka spruce, and yellow After the specimen strips were sawed from birch specimens heated in water were matched sticks A, B, C, the wide faces were surfaced in with those heated in steam at 250, 300, and the planer so that both the control specimens 350 F. (1). and those subsequently used in the heat treat-

3 AMERICAN WOOD-PRESERVERS ASSOCIATION 3 Figure 1.-Methodof cutting and marking specimens in pieces cut from sticks. ments had a uniform thickness before heating. inch in width, and 12.5 to 13 inches in All specimens with the exception of the Sitka length. After seasoning for test, the control spruce were prepared in the green condition specimens had a moisture equilibrium of 12 to and were kept green until used, by storing them 13 percent while the equilibrium moisture conin a low-temperature high-humidity room until tent of the heated specimens varied from about they were heated. It was found that the Sitka 8 to 12 percent, the lower values being obtained spruce specimens could not be planed smooth when the higher water temperatures were used. when green and they were therefore seasoned As mentioned in the paper discussing results to a moisture content of about 12 percent obtained in the steaming experiments (1), before surfacing to the desired thickness. there are several important advantages in using Most of the specimens were prepared from small-dimension specimens. For example, it is heartwood material. possible to get better matched specimens, a larger number of variables can be studied because of the greater number of matched speci- Specimen Dimensions mens available, there is less possibility of cross As shown by Figure 1, the specimens con- grain and variability in density affecting the resisted of edge-grained material. Both the con- sults, and the time required to heat the specitrol specimens and those heated in water were mens is so short that it does not need to be conditioned for test in a humidity room main- taken into account when different heating tained at 65 percent relative humidity and 80 periods are employed. The results obtained F. The original specimen dimensions were under the different heating conditions are about 0.15 to 0.17 inch in thickness, 2.0 to 2.1 therefore comparable.

4 4 AMERICAN WOOD-PRESERVERS ASSOCIATION Method of Test and Computation of Strength Factors After the specimens had reached moisture equilibrium in the humidity room, they were tested in bending with the load applied on one of the wide faces. Data obtained in the bending tests were used to compute the following strength factors, which are named in order beginning with the property most affected : (1) Work to maximum load (2) Modulus of rupture (3) Fiber stress at proportional limit (4) Modulus of elasticity Specific gravity and moisture content determinations were made for each specimen at the time of test. Adjustments for Moisture Content and Shrinkage Differences Since the equilibrium moisture content of the wood heated in water was generally lower than that of the unheated wood of the control groups, the computed strength values of the heated groups were adjusted to the corresponding moisture content of the control specimens. This was done by using the formula given in U. S. D. A. Technical Bulletin No. 479, Strength and Related Properties of Woods Grown in the United States. Calculations of all four strength properties of both the softwood and hardwood specimens were based on the cross-sectional dimensions at time of test and the data are shown plotted in Figures 2 to 7 inclusive. There was generally little shrinkage or collapse of the softwood specimens that would have a significant effect on these calculated strength values. As was noted in the steaming experiments, however, the hardwood specimens showed a considerable tendency to warp or shrink and to develop more or less collapse when conditioned after heating. This was particularly evident in the case of yellow birch which was used in most of the experiments made with the different heating mediums. This shrinkage and collapse often decreased the cross-sectional dimensions even when the heating conditions were fairly mild as when using the lower temperatures or the shorter. heating periods at some of the higher temperatures. Because of this, the computed strength values based on the specimen dimensions at the time of test were sometimes as high as or even somewhat higher than the corresponding values for the control specimens. Shrinkage or collapse was usually indicated by a marked increase in specific gravity. For example, yellow Figure 2.-Mechanicalproperties of specimens heated in water at 150 F. A-Douglasfir; B-Sitka spruce.

5 AMERICAN WOOD-PRESERVERS ASSOCIATION 5 Figure 3.-Mechanicalproperties of specimens heated in water at 200 F. A-Douglasfir; B-Sitka spruce; C-Douglas fir; D-Sitka spruce; E-Yellow birch; F-Yellow birch.

6 6 AMERICAN WOOD-PRESERVERS ASSOCIATION Figure 4.-Mechanicalproperties of specimens heated in boiling water at about 210 F. B-Yellow birch; C-Yellow poplar. A-Douglasfir;

7 AMERICAN WOOD-PRESERVERS ASSOCIATION 7 Figure 5.-Mechanical properties of specimens heated in water at 250 F. A-Douglas fir; B-Sitka spruce; C-Yellow birch (data based on specimen dimensions after heating in water); D-Yellowbirch (data for work to maximum load and modulus of rupture based on specimen dimensions before heating in water).

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9 AMERICAN WOOD-PRESERVERS ASSOCIATION 9 Figure 7.-Mechanicalproperties of specimens heated in water at 350 F. A-Douglas fir; B-Sitka spruce; C-Yellowbirch (data based on specimen dimensions after heating in water); D-Yellowbirch (data for work to maximum load and modulus of rupture based on specimen dimensions before heating in water).

10 10 AMERICAN WOOD-PRESERVERS ASSOCIATION Figure 8.-Douglasfir and Sitka spruce specimens heated in water at 150 F. A-Datafor modulus of rupture; B-Data for work to maximum load.

11 AMERICAN WOOD -PRESERVERS ASSOCIATION 11 birch specimens heated in water at 150 F. showed very little reduction in the various strength properties even when heated at this temperature for 458 days. Specimens heated for this length of time, however, had an average specific gravity of 0.83 after heating and conditioning for test compared with a specific gravity of 0.61 for the control specimens. In order to help show the effect of collapse and shrinkage of the hardwood specimens, the values of work to maximum load and modulus of rupture were also calculated for the yellow birch specimens heated at 200, 250, 300, and 350 F. by using the dimensions before heating. These dimensions were the same as those of the control specimens. The recalculated data for these two strength properties of this wood are shown plotted in Figures 3F, 5D, 6D, and 7D, for the four temperatures. It may be noted that the recalculated values plotted in these figures are in general considerably lower than when the same strength properties were calculated using the cross-sectional dimensions at time of test which are shown in Figures 3E, 5C, 6C, and 7C. Preparation of Data and Results of Tests The computed data for the individual specimens were averaged for each temperature and heating period and these average values have been plotted in the accompanying figures. About 15 to 16 specimens of a given species, each specimen being taken from a different piece or stick, were used for each heating period. The Douglas fir, Sitka spruce, and yellow birch specimens, for which data are plotted for water temperatures of 150, 200, 250, 300, and 350 F., were matched for the respective species. Relative Effects on the Four Strength Properties Figures 2A, and 2B show the percent of control value plotted against heating period, for Douglas fir and Sitka spruce specimens heated at 150 F. The heating periods used with this temperature varied from 50 up to 660 days or over 1.8 years. Data were not plotted for yellow birch specimens heated to 150 because of the large amount of collapse and shrinkage that occurred during the longer heating periods of this wood. These dimension changes obscured much of the effect of heating the birch specimens at this lower temperature. Figures 3A and 3B show the percent of control value of Douglas fir and Sitka spruce specimens heated in water at 200 F. for heating periods varying from 16 to 720 hours, and Figures 3C to 3E inclusive show similar data for Douglas fir, Sitka spruce, and yellow birch specimens heated in water at the same temperature for the longer heating periods ranging from 10 days up to 375 days. Figure 3F shows data for work to maximum load and modulus of rupture values of yellow birch based on dimensions of the specimens before heating for comparison with data plotted in Figure 3E which are based on dimensions after heating. Figures 4A to 4C inclusive show the percent of control value plotted against heating period for Douglas fir, yellow birch, and yellow poplar specimens heated in boiling water (about 210 F.) for heating periods ranging from 4 to 64 hours. The Douglas fir and yellow birch specimens heated at the boiling temperature, however, were not matched with specimens of these woods heated at 150, 200, 250, 300, and 350' F. The plotted data in Figures 5A to 7C inclusive show the percent of control value, after different heating periods, for Douglas fir, Sitka spruce, and yellow birch specimens heated in water at 250, 300, and 350 F. The average specific gravity of the specimens in each group tested is plotted at the bottom of Figures 2A to 7C. For convenience in comparing the data for modulus of rupture and work to maximum load for the different species these two strength properties are shown plotted separately in Figures 8 to 12 inclusive. These were the two strength properties that were most affected regardless of the heating medium used. Work to maximum load, like toughness tests, is a measure of shock resistance. Modulus of rupture values are a measure of the bending strength and are used to determine the safe working stresses for structural timbers.

12 12 AMERICAN WOOD-PRESERVERS' ASSOCIATION Figure 9.-Douglasfir, Sitka spruce, and yellow birch specimens heated in water at 200 F. A-Datafor modulus of rupture; B-Data for work to maximum load.

13 AMERICAN WOOD-PRESERVERS' ASSOCIATION 13 Figure 10.-Douglasfir, Sitka spruce, and yellow birch specimens heated in water at 250 F. for modulus of rupture; B-Datafor work to maximum load. A-Data

14 14 AMERICAN WOOD-PRESERVERS ASSOCIATION Figure 11.-Douglasfir, Sitka spruce, and yellow birch specimens heated in water at 300 F. A-Data for modulus of rupture; B-Data for work to maximum load.

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16 16 AMERICAN WOOD-PRESERVERS ASSOCIATION Effect of Different Heating Periods Figures 13 and 14 show the logarithm of work to maximum load and modulus of rupture plotted against the logarithm of the heating period for specimens heated at 150 and 200 F. while Figure 15 shows the modulus of rupture data for Douglas fir, yellow birch, and yellow poplar heated in boiling water (about 210 F. at the laboratory elevation). Figures 16 and 17 show work to maximum load and modulus of rupture data plotted for Douglas fir, Sitka spruce, and yellow birch specimens heated at 250, 300, and 350 F. Effect of Different Temperatures Figures 16C and 17B show the relation of temperature and percent reduction in modulus of rupture for Douglas fir, Sitka spruce, and yellow birch specimens heated at the three higher temperatures. In the two figures the modulus of rupture values are plotted against the logarithm of temperature as was done for specimens of these woods when steamed at the three temperatures named. As stated earlier, specimens heated in steam were matched with those of the same species heated in water. The data for percent loss in modulus of rupture of the two softwoods are shown for different temperatures in Figure 16D. These values are for heating periods of 1, 2, 4, 8, 16, and 32 hours. Comparison with Results Obtained when Heating in Steam In order to show the relative effects obtained when heating in steam and in water, the average values for modulus of rupture of Douglas fir and Sitka spruce were plotted against heating periods for the temperatures of 250, 300, and 350 F. as shown in Figure 18. These curves are similar to those plotted in Figure 16A. The strength values plotted in the latter figure are the same as those shown in Figure 18 for specimens heated in water. Comparison of Effect of Heat on Ovendry Weight and on Modulus of Rupture Figures 19 and 20 were prepared so that a comparison could be made of the rate of reduction in ovendry weight of small softwood specimens heated in water under the same conditions, with the rate of reduction in modulus of rupture of the conifers used in these experiments. Figure 21 shows similar data for hardwoods. Experiments made to study the effect of heat on the rate of disintegration of wood, as indicated by the loss in ovendry weight, were discussed in another paper (2). In this earlier paper the data were plotted using rectangular coordinates. Figures 19, 20, and 21, however, have been prepared to show how the loss in ovendry weight follows the same general relations with respect to temperature and heating period, as were found to apply in the case of loss in mechanical strength. When the data are plotted as shown in these figures it is possible to determine the results to be expected over a fairly wide range of temperature. Analysis of Data Obtained Relative Effect of Changes in Temperature and in Heating Period, on Reduction in Strength In the paper discussing the effect of steaming on the strength properties of wood (1) it was shown that when the percentage strength of the heated wood (based on that of the control specimens) was plotted against the logarithm of the temperature when the heating period was constant, the relation of temperature and strength was closely represented by a straight line drawn through the plotted data. It was further noted that when the logarithm of the percent strength of the heated wood was plotted against the logarithm of the heating period, for any fixed temperature, the plotted points also approximated a straight line relat ion. In other words, the relation of temperature and strength was represented by an exponential function, while the relation of heating period and strength was represented by a power function. The same general relations that were found to apply when steam was used as a heating medium were also found to apply when water was used. These relations are shown for specimens heated in water, in Figures 13 to 17 inclusive. As previously mentioned, the same general relations were found to hold for the effect of temperature and heating period on the rate of loss in ovendry weight. This is shown in

17 AMERICAN WOOD-PRESERVERS' ASSOCIATION 17 Figure 13.-Effectof different heating periods on values of work to maximum load and modulus of rupture when specimens were heated in water. A-Averagevalues for Coast Douglas fir and Sitka spruce heated at 200 F.; B-Average values for Coast Douglas fir and Sitka spruce heated at 150 F. and 200 F.

18 18 AMERICAN WOOD-PRESERVERS ASSOCIATION Figure 15.-Effectof different heating periods on values of modulus of rupture when Douglas fir, yellow birch, and yellow poplar specimens were heated in boiling water (about 210 F.).

19 AM ERIC AN WOOD-PRESERVERS ASSOCIATION 19 Figure 16.-Effectof different heating periods on work to maximum load and modulus of rupture when Douglas fir and Sitka spruce were heated in water at 250, 300, and 350 F. Data averaged for Douglas fir and Sitka spruce. A-Logarithm of control value (percent) plotted against logarithm of heating period; B-Modulusof rupture values used in A as plotted in rectangular coordinates; C- Modulus of rupture (percent) plotted against logarithm of temperature for heating periods of 2, 4, 8, 16, and 32 hours (Data averaged for Douglas fir and Sitka spruce); D-Lossin modulus of rupture (percent) plotted against logarithm of temperature for heating periods of 1, 2, 4, 8, 16, and 32 hours. (Based on data in A)

20 20 AMERICAN WOOD-PRESERVERS' ASSOCIATION Figure 17.-Effectof different heating periods on values of work to maximum load and modulus of rupture when yellow birch specimens were heated in water at 250, 300, and 350 F. A-Logarithm of control value (percent) plotted against logarithm of heating period; B-Modulusof rupture (percent) based on control value, plotted against logarithm of temperature for heating periods of 2, 4, and 8 hours. Figure 18.-Comparisonof average values of modulus of rupture (percent) for Douglas fir and Sitka spruce specimens heated in water and in steam at 250, 300, and 350 F.

21 AMERICAN WOOD-PRESERVERS ASSOCIATION 21 Figures 19 to 21 inclusive. Since small size specimens were used both in studies of the rate of loss in ovendry weight and in these experiments made to study the effect of heat on the strength properties, results obtained in the two series of experiments can be compared. Results Obtained in Heating at 150, 200, and 210 F. A comparison of the data for Douglas fir and Sitka spruce plotted in Figures 2 and 3 shows that in heating at 150 F. the results were more variable than when the water temperature was 200 F. This is also indicated in Figure 13B in which the averaged data for these two species are shown with the logarithm of percent strength plotted against the logarithm of heating period. The data plotted in Figures 8 to 12 inclusive show that in general both the values for modulus of rupture and work to maximum load were similar for Douglas fir and Sitka spruce when heated under the same conditions. The corresponding data for these two woods were therefore averaged in preparing Figures 13A, 13B, 16A, 16C, and 16D which show the relation of the two strength properties to temperature and heating period. Figure 13A shows the average values of work to maximum load and average values of modulus of rupture for Douglas fir and Sitka spruce specimens heated at 200 for various heating periods which ranged from 16 to 720 hours while Figure 13B shows these strength values plotted for another group of matched specimens of the same species heated at 200 F. for longer periods ranging from 10 days to 375 days or a maximum of more than a year. If the heating periods given in hours (Figure 13A) are reduced to days, it will be found that the values correspond with the data plotted in Figure 13B for similar periods expressed in days. The data plotted in Figures 2 and 13B show that in heating the two softwoods at 150 F. there was little loss in either work to maximum load or modulus of rupture values until after heating for 25 to 30 days or more. These figures also show that a heating period of around 550 to 600 days or about 1 1 / 2 to 1 5 / 8 years was needed to reduce the strength values as much as 19 percent when the water temperature was 150 F. When the softwood specimens were heated at 200 F. Figure 13A indicates that some reduction in work to maximum load started after heating about 8 to 9 hours while the modulus of rupture values began to show a slight reduction after heating for about 15 to 20 hours. This figure also shows that when the temperature was 200 F. the work to maximum load was reduced about 10 percent in from 25 to 30 hours or in about 1 to 1 1 / 4 days, while the modulus of rupture was reduced about 10 percent in approximately 10 to 12 days. Figures 3F and 14 show similar data plotted for yellow birch heated at 200 F. After heating the yellow birch specimens for 160 days at 150 F., the value of work to maximum load was 99 percent and that of modulus of rupture was 96 percent of the control value. In longer heating periods at this temperature, however, these values actually showed some increase because of shrinkage and collapse and there was also a large increase in the specific gravity of this wood. The data for yellow birch, plotted in Figure 14, are based on the specimen dimensions before heating, since shrinkage decreased the cross-sectional dimensions and this affected the computed values when based on the dimensions after heating. This figure shows that a reduction in the strength values of yellow birch started after heating at 200 F. for about 6 to 7 days. It may be noted, however, that the rate of loss in strength was faster after starting than in the softwoods, even when the computed values for the yellow birch were based on the cross-sectional dimensions after heating. The latter values are shown plotted in Figure 3E. When specimens of the various species were heated in water the values for work to maximum load were somewhat more variable than they were when heating in steam. This was the case even when the higher temperatures of 250, 300, and 350 F. were used. The values, however, were not so variable at these higher temperatures as when the specimens were heated at 150, 200 and 210 F. Work to maximum load is one of the more variable strength properties observed in

22 22 AMERICAN WOOD-PRESERVERS ASSOCIATION strength tests of wood. This variability is principally due to the fact that a sustained load may remain nearly uniform over a wide change in deformation and the maximum value may develop at any point in that range. Differences of this kind will naturally cause more or less variability in the computed work values. Data for specimens of Douglas fir, yellow birch, and yellow popular specimens heated in boiling water (about 210 F.) are plotted in Figures 4 and 15. The effect of shrinkage and collapse that occurred in specimens of the hardwood species is shown by the more variable modulus of rupture values of these two woods, compared with the data for Douglas fir. Figures 4B and 4C show that in the shorter heating periods at 210 F. most of the computed strength properties of the hardwoods were even higher than those of the control specimens. Although there was little change in the specific gravity of the Douglas fir specimens, there was a general increase in the specific gravity of the yellow birch and yellow poplar specimens up to the longest heating period of 64 hours, as shown in Figures 4B and 4C. In addition to the tendency of the hardwoods to develop increased shrinkage after heating, they also had a tendency to warp and twist when heated or when seasoned after heating. This is partly responsible for the more erratic results obtained in all the strength tests on hardwoods. Since there was only a small change in the values for work to maximum load in the heating periods used, and because the results were more or less variable, only the data for modulus of rupture were plotted in Figure 15. The data plotted for Douglas fir heated at 210 F. show that the modulus of rupture values started to decrease after heating for about 3 to 4 hours, compared with about 16 hours when heating at 200 F. In heating at 210 F., the modulus of rupture of the Douglas fir specimens was reduced about 10 percent after heating for about 100 hours, or nearly 4.2 days, in comparison with about 22 days that were required for the same reduction in this strength property when heating at 200 F. Figure 19.-Effectof different heating periods on oven-dry weight of softwood specimens heated in water at 200, 250, 300, and 350 F. Data are average values for Douglas fir, Sitka spruce, southern pine, and white pine specimens 1 by 1 by 6 inches in size. A-Logarithmof percent oven-dry weight plotted against logarithm of heating period. B-Datain Figure 19-A as plotted on rectangular coordinates. C-Average percentage loss in oven-dry weight plotted against logarithm of temperature for various heating periods. Based on data in A.

23 AMERICAN WOOD-PRESERVERS ASSOCIATION 23 Figure 20.-The percentage change in oven-dry weight and corresponding percentage change in modulus of rupture values for softwoods heated in water for various periods at (a) 200 ; (b) 250 ; (c) 300 ; and (d) 350 F. Logarithm of percent change in oven-dry weight and in modulus of rupture plotted against logarithm of heating period.

24 24 AMERICAN WOOD-PRESERVERS ASSOCIATION Figure 21.-Effect of different heating periods on oven-dry weight of hardwood specimens heated in water at 200, 250, 300, and 350 F. Data are average values for basswood, hard maple, sweetgum, tangile, white oak, yellow birch, and yellow-poplar specimens 1 by 1 by 6 inches in size. A-Logarithmof percent oven-dry weight plotted against logarithm of heating period. B-Average percent loss in oven-dry weight plotted against logarithm of temperature for various

25 AMERICAN WOOD-PRESERVERS' ASSOCIATION 25 Results Obtained in Heating at the Higher Temperatures of 250, 300, and 350 F. Figure 16A shows the average data for Douglas fir and Sitka spruce specimens heated at the higher temperatures of 250, 300, and 350 F., which were the temperatures used in the steaming experiments. The corresponding data obtained in heating yellow birch specimens at these temperatures are shown plotted in Figure 17A. Specific gravity data for the yellow birch specimens, plotted in Figures 5C, 6C, and 7C, show that there was a considerable increase in specific gravity which was caused by shrinkage and collapse. In the longer heating periods, however, especially when heating at 300 and 350 F., disintegration of the yellow birch reduced the specific gravity below the maximum noted in the shorter heating periods. As mentioned earlier, for the purpose of comparison, values of work to maximum load and modulus of rupture were also calculated using the dimensions of the yellow birch specimens before heating. Those values are shown plotted separately in Figures 5D, 6D, and 7D for the respective water temperatures of 250, 300, and 350 F. The data plotted in Figure 17A, which show the logarithm of the percent strength plotted against that logarithm of the heating period, are the same as the data plotted for yellow birch in Figures 5D, 6D, and 7D. Figures 16C and 16D show the average data for modulus of rupture values of Douglas fir and Sitka spruce plotted against the logarithm of temperature for different heating periods. Data similar to that in Figure 16C for the softwoods are plotted in Figure 17B for yellow birch. The straight lines drawn through the plotted points for modulus of rupture values in Figure 16A were taken from the straight lines passing through the plotted points in Figure 16C. For example, the straight line in Figure 16C, representing the average percent modulus of rupture after heating at the different temperatures when the heating period was 4 hours, passes through 96 percent at 250, 75 percent at 300, and 56 percent for 350 F. When the heating period was 8 hours the straight line in Figure 16C passes through about 90 percent at 250, 69 percent at 300, and 51 percent at 350 F. and so on for the other heating periods. These percentages were plotted on Figure 16A for the corresponding heating periods and temperatures. The straight lines in Figure l6a representing average values for each temperature, were then drawn through these plotted values taken from Figure 16C. Lines shown on the latter figure for heating periods of 16 to 32 hours were obtained by plotting the modulus of rupture values from Figure 16A where the projected straight lines crossed the vertical lines for these two heating periods. Curves similar to those shown in Figure 16C were not prepared for values of work to maximum load as was done for the data obtained in the steaming experiments, since the corresponding values for this strength property were more variable when water was used for heating. This is indicated by the plotted data for work to maximum load shown in Figure 16A. An examination of the plotted data in the various figures shows that normally the reduction in the different strength properties was much more rapid in the earlier than in the later heating periods. This is shown by the data for modulus of rupture and work to maximum load values plotted in Figures 8 to 17 inclusive, In order to illustrate how this rate of change at the higher temperatures of 250, 300, and 350 F. is affected by the time of heating, Figure 16B was prepared by plotting the modulus of rupture values using rectangular coordinates. The smooth curves are plotted from points read from the straight lines representing average values for the plotted data shown in Figure 16A. Points representing the experimental data, which are connected with straight lines drawn between each point in Figure 16A, are shown plotted in Figure 16B for each of the heating periods used with the three temperatures. It may be seen from this figure that the most rapid rate of loss in modulus of rupture was in the first 8 to 10 hours of heating and there was a difference of approximately 20 percent reduction in this strength property, for each 50 degree increase in temperature. The higher rate of loss in strength in the earlier heating periods was evident for all species heated at the different temperatures except

26 26 AMERICAN WOOD-PRESERVERS' ASSOCIATION where shrinkage or collapse had a significant effect on the results. In preparing Figure 16D, values for percent loss in modulus of rupture were determined from the straight lines (which represent average values of the data plotted in Figure 16A) for the three temperatures of 250, 300, and 350 F. Figure 16D can be used in estimating the percent loss in modulus of rupture for intermediate temperatures since the plotted points for different temperatures fall on the straight lines showing the loss in strength after different heating periods. This figure, like Figure 16C, shows that in the range of the temperatures for which the data are plotted, increasing the temperature about 12 to 18 degrees reduced the time required to cause the same percent reduction in modulus of rupture by about one-half. Values of work to maximum load would be similarly affected by the same increases in temperature. Figure 18, which gives a comparison of the average values of modulus of rupture for Douglas fir and Sitka spruce when heated in steam and in water at temperatures of 250, 300, and 350 F., shows that after any given heating period there was about a 3 to 4 percent difference in the modulus of rupture values for the two heating mediums when the temperature was 250 F., about 5 to 6 percent difference when the temperature was 300 F. and about 7 to 8 percent difference when the temperature was 350 F. This indicates that when steam or water temperatures are below 250 F. there would probably be only a small difference in the results obtained. On the other hand, at temperatures above 250 F. the difference in the rate of reduction in modulus of rupture is appreciably greater when the wood is steamed, and this difference increases with an increase in temperature. The greater effect of steam on the strength properties may possibly be due to a higher percentage of acetic acid in the steamed wood. Comparison of Rates of Reduction in Strength, Ovendry Weight, Under the Same Heating Conditions Since small specimens 1 by 1 by 6 inches in size were used when experiments were made to study the rate of disintegration of wood (determined from the loss in ovendry weight after various periods of heating) these specimens were quickly heated to the maximum temperature as were those used for strength tests. The rate of loss in ovendry weight should therefore provide an interesting comparison with the rate of loss in strength. Figure 19A shows the logarithm of the average percent loss in ovendry weight of four conifers, Douglas fir, Sitka spruce, southern yellow pine, and white pine plotted against the logarithm of the heating period for water temperatures of 200, 250, 300, and 350 F. These data were obtained in an earlier study of the rate of deterioration of wood when heated (2). It may be noted that in this figure the plotted points fall very near or on the straight lines drawn through those plotted points. Similar data obtained in the strength tests are plotted for the average modulus of rupture values of Douglas fir and Sitka spruce in Figures 13A and 13B for heating in water at 200 F. and in Figure 16A for water temperatures of 250, 300, and 350 F. Figure 19B, which was prepared from the curves plotted in Figure 19A, shows that the rate of loss in ovendry weight is most rapid in the earlier heating periods. Similar results are shown in Figure 16B in which data are plotted for values of modulus of rupture. Under the same heating conditions there was very little difference in the rate of loss in ovendry weight of the four conifers named. The average values of percent reduction in ovendry weight for these four species, which are shown plotted in Figures 19A and 19C can therefore be compared with the averaged strength test data obtained on specimens of the two conifers used in this study. Figures 21A and 21B are similar to Figures 19A and 19C respectively and show the average ovendry weight data for seven hardwood species after heating in water at 200, 250, 300, and 350' F. In Figure 20 the broken straight lines show the average percent reduction in ovendry weight of the conifers, as plotted in Figure 19A for water temperatures of 200, 250, 300, and 350 F., while the full lines underneath show the percent reduction in modulus of rupture of the softwoods for the corresponding temperatures and heating periods. Figure 20

27 AMERICAN WOOD-PRESERVERS ASSOCIATION 27 shows that for each of the higher temperatures the percent reduction in ovendry weight, after a given heating period, was less than the percent reduction in modulus of rupture, and that for each temperature this difference did not vary much for the different heating periods. When the water temperature was 200 F., there was only a small difference of about 2 to 3 percent in the loss in ovendry weight and loss in modulus of rupture. At 250 F. the difference was about 10 to 11 percent, at 300 F. it was about 15 to 16 percent and at 350 F. it was about 24 to 25 percent. It should be borne in mind that when either the softwood or hardwood specimens heated in water had lost from 35 to 45 percent of the original ovendry weight, they were thoroughly charred and easily broken in the hand. For this reason the plotted data in Figures 19C and 21B are shown with broken lines above 50 percent loss in ovendry weight. The water-heated specimens showed conspicuous charring when they had lost between 20 and 25 percent of their ovendry weight. With this percent weight loss deterioration could be considered well advanced. Figures 19C and 21B, that show the relation between percent loss in ovendry weight and temperature for different heating periods, will be useful in comparing the relative effect of other temperatures between 200 F. and 350 F. (the range used in these experiments) and also for comparing the effect of temperatures somewhat below 200 F. In these figures, plotted points for any given heating period would lie practically on the straight line intersecting the vertical lines representing different temperatures It should be reasonable to assume that for the conifers the difference in percent reduction of ovendry weight and percent reduction in modulus of rupture values are in approximately the proportion indicated by Figure. 20 for the temperature nearest to that underconsideration. For example, at 220 the difference would be about 6 percent, at 230 about 8 percent, at 240 about 10 percent, and at 260 about 12 percent. Table 2 was prepared to help visualize therelative effect of different water temperatures. on the rate of loss in bending strength and the corresponding effect on the rate of loss in ovendry weight. The data in this table show that temperatures of 150 and 200 can be applied for much longer periods without causing an important loss in strength than when temperatures in the range of 250 F. and higher are used. This indicates that temperatures such as. 200 F. to 220 F., which are commonly specified for use in the conditioning and treatment of sawed and round timbers of Douglas fir and similar species, should not have an important effect on the strength properties under normal treating operations. When higher temperatures, particularly in the range of 250 or more, are. used the heating period should be carefully controlled to avoid an objectionable reduction in strength. The curves shown in Figures 16A to 16D inclusive will be found helpful in comparing the effect of different temperatures and heating periods and in selecting suitable heating periods when using some of the highertemperatures. Figure 19C would be useful in estimating the loss in ovendry weight of softwoods after any given heating period, and for estimating the corresponding effect on the modulus of rupture as indicated by Figure 20. Table 2.-ApproximateHeating Periods at Which the Modulus of Rupture and the Ovendry Weight of Softwoods Started to Decrease and Time Required to Cause Reductions of 10 and 20 Percent When Different Water Temperatures Were Used 1

28 28 AMERICAN WOOD-PRESERVERS ASSOCIATION Application of Data to Commercial- Size Timbers Although these strength tests were made on small-size specimens, the reduction in the strength properties shown for the various heating conditions used can be assumed to represent the minimum effect when relatively green wood is heated in hot liquids such as water or preservative oils. Seasoned material would probably show a smaller reduction in strength, depending on the moisture content, since the effect of hydrolysis would be reduced. In conditioning timbers that have a high moisture content and from which the moisture can be evaporated fairly rapidly, as in the case of sapwood, the evaporation of moisture will help cool the wood and the temperature of the timbers may remain considerably below that of the heating medium for a large part of the conditioning period. In heating resistant heartwood material, however, such as sawed timbers of Douglas fir and various other conifers, or in heating timbers of hardwood species in which the pores are filled with tyloses or gums, water evaporation will usually be too slow to have an appreciable retarding effect on the rate of temperature rise. A discussion of the application of the strength data to commercial-size timbers was given in the paper describing the results obtained when steam was used as the heating medium (1). The same general method can be used in applying data obtained in these experiments in which the wood was heated in water. Figures showing the rate of temperature change in both round and sawed timbers of different dimensions are given in reference (3). This publication also explains how to determine the temperature to be expected at any point in timbers of different species when heated in different mediums. 220 F. can be applied for much longer periods, without causing a significant loss in strength, than when higher temperatures in the range of 250 F. or more are applied. A comparison of the results obtained when heating in water and in steam showed that the loss in strength was somewhat greater when heating in steam. The modulus of rupture values of the softwoods, for example, were about 3 to 4 percent lower when heating in steam at 250 F. for the same period; at 300 F. the values for steam heating were about 5 to 6 percent lower, and at 350 F. the values were about 7 to 8 percent lower. Under the same heating conditions the hardwoods showed a greater reduction in strength than did the softwoods. The hardwoods also had a greater tendency to develop shrinkage and collapse. The results obtained in these experiments showed that, as in the steaming experiments, the relation of strength and temperature, with heating period constant, was represented by an exponential function, while the relation of strength and heating period, with temperature constant, was represented by a power function. In an earlier study of the rate of deterioration of wood heated in different mediums (2) similar relations were found to represent the loss in ovendry weight when different temperatures and different heating periods were used. Literature Cited Conclusions. Results obtained in these experiments showed that temperatures in the range of 2000 to AMERICAN WOOD-PRESERVERS ASSOCIATION 839 Seventeenth Street, N. W., Washington 6, D. C. PREPARED FOR THE annual meeting of April 26, 27, 28, 1954, The Chalfonte-Haddon Hall Hotels, Atlantic City, N. J. THIS ASSOCIATION is not responsible for any statement made or opinion expressed in its publications.