DIMENSIONAL CHANGES IN KILN-DRIED SOFTWOOD LUMBER AFTER SURFACING AND DURING STORAGE

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1 U. S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY In Cooperation with the University of Wisconsin MADISON, WIS. U. S. FOREST SERVICE RESEARCH NOTE FPL-0144 SEPTEMBER 1966 DIMENSIONAL CHANGES IN KILN-DRIED SOFTWOOD LUMBER AFTER SURFACING AND DURING STORAGE

2 Abstract Dimensional changes after kiln drying caused by surfacing and storage at constant moisture content were measured on 2- by 6-inch specimens of Douglasfir and loblolly pine. Two test procedures were used for observing the dimensional changes. In procedure I, the effect on width only of surfacing immediately after drying and storing was determined. In procedure II, the effects on both width and thickness of storage before surfacing were included as well as the effects after surfacing. Small dimensional changes were observed, but these are of little or no practical significance, since they are much smaller than the dimensional changes due to normal moisture fluctuations.

3 DIMENSIONAL CHANGES IN KILN-DRIED SOFTWOOD LUMBER AFTER SURFACING AND DURING STORAGE By G.L. COMSTOCK, Technologist Forest Products Laboratory, 1 Forest Service U.S. Department of Agriculture Introduction The question has been asked by some lumbermen, Does a delayed shrinkage occur in softwood lumber that has been rapidly kiln-dried? It is hypothesized that delayed shrinkage occurs in lumber after it has been rapidly dried irrespective of any changes in moisture content. This delayed shrinkage would presumably be the result of some mechanism, such as relaxation of stresses that remain after the drying process is completed. The purpose of this study was to determine whether delayed shrinkage actually occurs, and if it does, to obtain measurements of the magnitude in relation to the time factors involved. The study, based on a plan developed by E. C. Peck at the U.S. Forest Products Laboratory, was limited to two species, Douglas-fir and loblolly pine. Both were dried using kiln schedules for rapid-drying that were closely related to those used in industry. Since these experiments were intended to determine whether important dimensional changes occur in lumber after drying irrespective of changes in moisture content, an attempt was made to include the effect of variables in processing which might occur in commercial practices. Thus the variables 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. FPL-0144

4 studied were effects of storage and surfacing. Two procedures were used on flat-, bastard-, and edge-grain boards to study the effect of these variables on dimensional stability. Materials Douglas-fir (Pseudotsuga menziesii (Mirb.) F.) and loblolly pine (Pinus taeda L.) were used for the investigation. Three Douglas-fir logs were obtained by the Pacific Northwest Forest and Range Experiment Station from the Estacada Ranger District in the Mount Hood National Forest. Three loblolly pine logs were obtained by the Southern Forest Experiment Station from the Crossett Experimental Forest, Crossett, Ark. All logs were 16 feet long. The specimens were cut to simulate commercially produced lumber: they were sized a constant 2 by 6 inches in cross section and 33 inches in length. The logs were sawed to obtain two flat-grain, two edge-grain, and four bastardgrain planks from each log. Figure 1 shows a diagram of the pattern for cutting and the system for numbering. An arbitrary N direction was marked on each log and the quadrant from which planks came was identified in relation to it. The first digit designates the log from which the material came. The planks were sawed, slightly oversize, and each was cut into five specimens 33 inches long. Sections 1 inch long, taken at each end and between each 33-inch specimen, were used for estimating the moisture content when green. An additional specimen digit, 1 to 5, was added to designate the order in which the specimens were sawed from the plank. Two specimens were randomly selected from each plank, one was assigned to procedure I and the other to procedure II. Thus the specimens for the two procedures were matched as closely as possible. A total of 18 specimens per species was used for each procedure. The 33-inch specimens were then surfaced to a thickness of 2 inches and jointed to a width of 6 inches. Between machining operations they were kept covered to prevent loss of moisture and were weighed immediately after the final jointing operation. They were then end coated with two coats of aluminum end coating to prevent end drying. FPL

5 After end coating, they were wrapped in polyethylene bags and stored at 40 F. until the experiment began. Methods of Investigation Drying of Specimens Specimens were dried in a small experimental kiln. The intention was to simulate the rapid kiln drying used for the two species by industry. The actual schedule used for the loblolly pine (table 1) was derived from time schedules used by industry as determined by J. M. McMillen at the Forest Products Laboratory. Time intervals and total time were shortened slightly to compensate for the faster drying in the small experimental kiln rather than in a large commercial kiln. The conditions for the first 48 hours of drying the Douglas-fir provided for equilibrium moisture content conditions milder than typical industrial schedules to eliminate surface checking such as developed in some pine specimens. For both species, final equilibrium moisture content conditions were low and temperatures high to simulate the most rapid final drying rate used in some commercial drying. Schedules are given in table 1. The only major deviation from these schedules was at the start of the Douglasfir run. The wet-bulb temperature was considerably below the prescribed 142 F. for the first 6 hours of the run because of a sticky valve which did not open the steam-spray line. A few flat-grain boards checked during this period. In each drying run, only three planks were weighed and measured each day to follow the moisture content of the kiln loads. These values for moisture content were used as a guide. When they approached the target value of 12 percent moisture content, all specimens were weighed on a regular basis. An attempt was made to remove individual specimens when they reached 12 percent moisture content. The results show that this was not achieved in very many cases. Many pieces of Douglas-fir came out drier than 12 percent, whereas a majority of the pieces of loblolly pine were over 12 percent when removed. FPL

6 Demensional Measurements Thickness of the 18 specimens for each procedure was measured at three points across the width with an apparatus similar in design to that used in a previous study at the Oregon Forest Products Laboratory (fig. 2). The thickness values were measured to inch. Preliminary checks showed thickness measurements to be reproducible to about ± inch. Width was measured at two points on the boards. Figure 3 shows the apparatus used for measuring the width of specimens. To improve the reproducibility of readings, 1/16-inch holes were drilled into the edges of the boards at midthickness so that the two holes were exactly opposite each other. The probes of the width-measuring apparatus had needle extensions in their exact center which fit into these holes, insuring that the measurements were always made exactly between the same points. This setup improved the overall reproducibility of measurement; however, some inaccuracies were introduced. For example, slight indentation on the edges of the boards was observed in some cases, and some needle holes were made in the wood by failure to always fit the needle exactly into the hole the first time. The widths were measured to inch, but the accuracy was not as good as that for thickness measurements. Inaccuracies due to surface indentation and needle punctures amounted to to inch on some boards. However, this is not of any great significance, since it would produce an error of less than 0.1 percent. After the final storage period, two 1-inch sections were cut from each specimen. These sections, cut to include the points for width measurements, were used to determine the values for actual moisture content shown in table 2 and for the shrinkage values in table 4. In procedure I, the matched boards were dried to the predetermined moisture content of 12 percent. After drying, they were allowed to cool in a room at 90 F. and 65 percent relative humidity for 1 hour, after which they were weighed and the dimensions were measured. They were then surfaced to 1.5 inches thickness; several passes through the single-head planer were made to remove approximately equal amounts of wood from each face. Width and thickness were then remeasured, and the change in width due to removing the surface layer was calculated. 2 Espenas, L.D., Snodgrass, J.D., and Kozlik, C.J. Shrinkage of 2- by 8-inch Douglas-fir lumber. Preliminary Report, Forest Research Laboratory, Oregon State University, Corvallis, FPL

7 Table 3.--Maximum change in dimension resulting from two procedures

8 Table 4.--Shrinkage from green to ovendry in kiln-dried 2 by 6's of loblolly pine and Douglas-fir

9 Figure 8.--Effect of moisture content values on change in width of loblolly pine specimens caused by surfacing immediately after drying. M

10 Figure 9.--Relation of changes in dimension to time. M Figure 10.--Relation of changes in dimension to time. M

11 Figure 4.--Relation of corrected moisture content values to time. M Figure 5.--Relation of corrected moisture content values to time. M

12 Figure 6.--Relation of corrected changes in dimension to time. M Figure 7.--Relation of corrected changes in dimension to time. M

13 Figure I.--Diagram showing how logs were broken down to obtain varied grain orientation and how test planks were numbered.

14 M Figure 2.--Apparatus used for measuring thickness. M Figure 3.--Apparatus used for measuring width.

15 Table 1.--Time- temperature schedules used for kiln- drying

16 Table 2.--Moisture content for boards of procedure II (target--12 percent)

17 After surfacing and remeasuring, all boards were stored in separate polyethylene bags in a kiln controlled at 90 F. and 65 percent relative humidity. The purpose was to try to maintain the specimens at the constant moisture content of approximately 12 percent. The boards were then measured periodically to determine change in dimensions as related to time of storage. Procedure II consisted of storing the group of matched dried specimens at constant moisture content until they showed no change in dimension before surfacing. After constant dimension was reached, the boards were surfaced and the change in width of the boards determined. They were then stored again to determine whether subsequent changes in dimension would occur. The loblolly pine boards were stored 45 days, surfaced, and stored for an additional 92 days. Douglas-fir boards were stored 55 days, surfaced, and stored for an additional 77 days. Storage conditions were the same as those used in procedure I. Drying and Final Moisture Content Results Curves showing the relation of moisture content to time for test boards of loblolly pine and Douglas-fir are shown in figures 4 and 5. Loblolly pine reached the 12 percent moisture level much faster than Douglas-fir, even though it started at a much higher moisture content. Drying time to 12 percent moisture content was about 90 to 100 hours for loblolly pine and 140 to 160 hours for Douglas-fir. Some defects developed during the drying process. In particular, the flatgrain boards of loblolly pine showed a considerable amount of surface checking. Apparently, the drying conditions were too severe. No surface checking was observed on the faces of the edge- or bastard-grain boards. As mentioned earlier, some of the flat-grain Douglas-fir boards developed surface checks because of severe conditions at the start of drying as a result of a mechanical failure in the kiln control mechanism. It would have been desirable to remove every board at exactly 12 percent moisture content, but this was not possible for a number of reasons. The estimate of moisture content of specimens when green was not exact. This was true particularly of loblolly pine, where initial moisture content was very high, FPL

18 about 100 percent, and there were substantial variations along the length of pieces. The estimated final moisture content values for loblolly pine averaged 3.1 percent lower than the actual moisture content of the boards. The actual average at time of removal was 15.4 percent. The discrepancies were probably due to errors in the procedure for estimating the moisture content when green or to changes in the board during preparation. It was not possible, however, to isolate the error. The Douglas-fir estimates and actual moisture content values came out much closer. The actual average at time of removal was 11.8 percent, 0.4 percent less than the estimate. Table 2 shows the estimated and actual moisture content values of procedure II boards at end of drying. The standard deviation of the actual moisture content values was 2.17 for loblolly pine and 0.90 for Douglas-fir. It was not possible to determine the actual moisture content at time of removal for specimens from procedure I, because these specimens were surfaced immediately after drying. Since there was a moisture gradient in the boards at that time, removal of the drier surfaces resulted in an increase in the average moisture content of the boards. The average moisture content of the boards for procedure I immediately after surfacing was 16.4 for loblolly pine and 12.7 for Douglas-fir. Because many of the boards were not at the desired 12 percent moisture content at the end of drying, there were some changes in moisture content during storage in the kiln at 90 F. and 65 percent relative humidity. The specimens were stored in polyethylene bags which minimized the rate of change, but there were changes up to 2 to 3 percent moisture content over a 4-month period. This change was, of course, considerably greater with the loblolly pine than with Douglas-fir because of the loblolly pine's higher moisture content. Dimensional Changes In analyzing the data, it was of primary interest to investigate the effect of storage only on dimension without the effect of changes in moisture content. Thus it was necessary to apply a correction for the change in moisture content. The method used was to determine the change in dimension associated with normal moisture changes and subtract the changes associated with moisture content from the total change. The net result was then taken as the change that would have occurred with no change in moisture content. This method, of course, has some drawbacks. Because of the large number of measurements, it was necessary to deal with averages rather than specific FPL

19 values. Also, the assumption was made that the normal shrinkage and the relaxational changes were independent. This was probably not true, particularly in view of recent findings that relaxation and creep in wood are closely related to changes in moisture content. 3 However, there appeared to be no other reasonable method, so this was used and as such must be considered in interpreting the results. Shrinkage data used as the basis for the change in dimension with change in moisture content were the measurements of widths after ovendrying and at the end of the experiment. The difference in thousandths of an inch per percent change in moisture content was determined and averaged for each species and direction of grain. The width values were then corrected to the median value of moisture content. The thickness corrections were made using the width shrinkage values for the appropriate grain direction and multiplying by the ratio of thickness to width of the specimens. This method has two inaccuracies in addition to those mentioned previously. First, the relationship of shrinkage-moisture content is not linear to ovendry moisture content, and the corrections were probably somewhat low because of this. Second, the ring-angle orientations were not perfect, so applying width shrinkage to thickness in the appropriate grain direction was somewhat inaccurate. Procedure I (surfacing before storage).--figures 6 and 7 show the relation of the corrected change in dimension to time for procedure I specimens for both the loblolly pine and the Douglas-fir. The changes in width were based on the dimension immediately before surfacing at the end of the drying process. The changes in thickness were based on the dimension immediately after surfacing. The value at zero time for width is thus the change in the width dimension occurring as a result of surfacing. The reductions in width due to surfacing were fairly uniform, being greatest for the flat-grain and least for edge-grain boards as would be expected on the basis of shrinkage in the different directions. The reduction in width with surfacing would, of course, be expected because at this stage of drying the stresses have reversed and the outer portion of the wood is under a compressive stress and the interior under a tensile stress. After surfacing there is some reduction in dimension due to storage only. This appears to level off at about 20 to 30 days for width, but the inherent 3 Christensen, G.N. The use of small specimens for studying the effect of moisture content changes on the deformation of wood under load. Australian Journal of Applied Science 13(4): , FPL

20 inaccuracies mentioned earlier do not allow any conclusions as to the exact shape of the curve. As can be seen from the graphs, there is a substantial amount of variation. The maximum total change in width due to surfacing and storage for procedure I appears to be about inch for loblolly pine and inch for Douglas-fir. These would amount to shrinkage percentages of 0.27 percent and 0.23 percent, and they do not appear to be of any practical significance. The change in thickness with time, on the other hand, does not appear to level off very quickly. There is a rapid decrease in dimension the first 10 days after which there appears to be a very slight reduction in thickness over long periods. The changes for Douglas-fir after 14 days are slight and within the measurement errors. The changes in loblolly pine are larger and greater than could be accounted for on the basis of inaccuracy in measuring. The changes after 74 days, however, are very slight and could be due to inaccuracies in measuring. The maximum total decrease in thickness after 137 days for loblolly pine was inch, approximately 0.75 percent of the dressed thickness. The corresponding decrease for Douglas-fir was inch, which corresponds to a shrinkage of 0.38 percent. Figure 8 represents graphically the change in width due to surfacing plotted as a function of the values of moisture content for the specimens after surfacing. A least-squares linear regression was calculated and is shown on the graph. This indicates that for the range of values for moisture content observed, the reduction in width occurring when the surfaces are machined increases as the moisture content goes down. This would be expected, since the tensile strain on the interior increases as moisture content goes down in this region. The moisture content at which no change in dimension occurred was approximately 22 percent. This is close to the value of 18 percent which McMillen 4 indicates is the point of stress reversal in some softwoods. Procedure II (surfacing after storage).--figures 9 and 10 are plots of the relation of change in dimension to time for loblolly pine and Douglas-fir specimens subjected to procedure II. The loblolly pine boards were stored 45 days, surfaced, and stored for an additional 92 days. Douglas-fir boards were stored 55 days, surfaced, and stored an additional 77 days. The break in the curves at 45 and 55 days with time was because the specimens were then surfaced. The 4 McMiIIen, J.M. Stresses in wood during drying. Forest Products Laboratory Report No. 1652, 33 pp., iiius., FPL

21 width changes after surfacing are shown in relation to original width. The thickness changes, however, are shown in relation to the thickness after surfacing, because part of the thickness was removed by surfacing. Figure 9 shows the effect of storage on dimensions of loblolly pine. The maximum width reduction after 45 days was inch or 0.28 percent of the dimension when green. The maximum thickness reduction was inch or 0.80 percent. The corresponding values for Douglas-fir from figure 10 are inch in width and inch in thickness, or percentage reductions of 0.20 percent and 0.40 percent. In all cases it appears that the percentage of reduction in thickness is about two times that for width. Surfacing resulted in a decrease in width in all specimens as would be expected. For loblolly pine, the reductions in width were 0.009, 0.010, and inch for flat-, bastard-, and edge-grain boards. For Douglas-fir, the corresponding values were 0.004, 0.009, and inch. After surfacing, the changes in thickness with time were very small for both species, the maximum change being inch. After surfacing there appears to be little or no effect from storage on the width of either species. Table 3 is a summary table showing the maximum decrease in dimension occurring from surfacing and storage with procedures I and II. The total changes in width are slightly greater when the storage period precedes surfacing. However, neither of these changes, due to storage and to surfacing, nor the thickness change appears to be of any practical consideration. A point of interest is that both the thickness and width decrease in dimension. Intuitively it would seem that if one dimension decreased at constant moisture content, the other would increase, since the total stress in the system must be zero. A possible explanation for the decrease in both thickness and width is that width was measured at midthickness. As the moisture gradient equalizes in the wood, the surfaces would tend to swell and the center shrink. These two forces oppose one another with a resultant decrease in thickness. The edges of the wood in width would probably increase in size due to the gain of moisture, but these measurements were made only at midthickness, which tends to shrink because of the loss of moisture. Total Shrinkage The total shrinkage in width for all of the specimens used is given in table 4. Neither treatment seemed to have a consistent effect on the amount of total FPL

22 shrinkage, The total shrinkage values are similar to the values given by Peck 5 for the radial and tangential shrinkage of the two species dried under moderate conditions. His values for loblolly and shortleaf pine sapwood of 6.5 percent tangentially and 4.5 percent radially compare to 6.9 and 5.3 percent in this study. For Douglas-fir, Peck s values were 6.5 and 4.8 percent compared to 7.1 and 4.5 percent found in this study. Although the values here are somewhat higher, they are well within the inherent variations in shrinkage within species 6 and lower, generally, than the values for small sections in the Wood Handbook. 7 It appears, therefore, that the drying conditions employed in this investigation had no large effect on the shrinkage of the boards during drying and that delayed shrinkage was of no practical significance. Conclusions Dimension lumber of Douglas-fir and loblolly pine appears to undergo slight reductions in both width and thickness when stored at constant moisture content after kiln drying. Surfacing kiln-dried dimension lumber of Douglas-fir and loblolly pine, which has not had stresses relieved, results in a slight decrease in the width of boards. When kiln-dried dimension lumber is stored for days before surfacing, little or no subsequent change in dimension occurs after surfacing when stored at constant moisture content. Changes in dimension at constant moisture content, subsequent to drying, are generally quite small and are of little or no practical significance. 5 Peck, Edward C. Shrinkage of boards of Douglas-fir, western yellow pine, and southern pine. American Lumberman, 2774:52-54, illus., Cornstock, G.L. Shrinkage of coast-type Douglas-fir and bid-growth redwood boards. U.S. Forest Service Research Paper FPL 30, 11 pp., illus., U.S. Forest Products Laboratory. Wood Handbook. U.S. Department of Agriculture, Agriculture Handbook No. 72, 528 pp., Illus., FPL

23 PUBLICATION LISTS ISSUED BY THE FOREST PRODUCTS LABORATORY The following lists of publications deal with investigative projects of the Forest Products Laboratory or relate to special interest groups and are available upon request: Architects, Builders, Engineers, and Retail Lumbermen Box, Crate, and Packaging Data Chemistry of Wood Drying of Wood Fire Protection Fungus and Insect Defects in Forest Products Furniture Manufacturers, Woodworkers, and Teachers of Woodshop Practice Glue and Plywood Growth, Structure, and Identification of Wood Logging, Milling, and Utilization of Timber Products Mechanical Properties of Timber Structural Sandwich, Plastic Laminates, and Wood-Base Components Thermal Properties of Wood Wood Fiber Products Wood Finishing Subjects Wood Preservation - Since Forest Products Laboratory publications are so varied in subject Note: matter, no single catalog of titles is issued. Instead, a listing is made for each area of Laboratory research. Twice a year, January 1 and July 1, a list is compiled showing new reports for the previous 6 months. This is the only item sent regularly to the Laboratory s mailing roster, and it serves to keep current the various subject matter listings. Names may be added to the mailing roster upon request.