U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WIS. In Cooporation with the University of Wisconsin
U.S.D.A. FOREST SERVICE RESEARCH NOTE FPL-0203 FEBRUARY 1969
LONGITUDINAL SHRINKAGE IN SEVEN SPECIES OF WOOD
Abstract
Measurements were made of longitudinal shrinkage occurring in seven species of wood dried from a green condition to equilibrium at five relative humidities. The relationship of shrinkage to relative humidity and the within-board variability shrinkage of clear, straight-grained lumber are reported. Results of interest are the amount of longitudinal expansion that occurred during drying and the significant variability in longitudinal shrinkage and expansion within a board.
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LONGITUDINAL SHRINKAGE IN SEVEN SPECIES OF WOOD
By
1R. A. HANN, Forest Products Technologist
Forest Products Laboratory,2 Forest Service U.S. Department of Agriculture
During recent years, the Forest Products Laboratory has received inquiries on longitudinal shrinkage and swelling of wood. These inquiries have reflected that engineers and architects are designing wood products and structures more precisely and that the use of composite wood-base panel products such as plywood and particleboard is increasing. Examples of how longitudinal shrinkage values are used are given in two recent reports. In 1963, Heebink (6) discussed the importance of knowing the unrestrained longitudinal movement of various types of panel products to properly engineer balanced composite panels. In 1964, Heebink, Kuenzi, and Maki (8) pointed out therelationship of longitudinal shrinking and swelling of wood to the dimensional stability of wood-base panel products such as plywood and particleboard.
For practical purposes, the conclusion that "the longitudinal shrinkage or swelling of normal wood with moisture changes is so small that it may be safely ignored for most purposes" was adequate as recently as the 1950's. Today more extensive information on longitudinal shrinking and swelling of wood is needed for designing furniture, doors, wall sections, and other products.
This paper presents information on the longitudinal shrinkage of seven species of wood; emphasis is on the relation of shrinkage to moisture content, on negative shrinkage, and on within-board variation in shrinkage.
1 Acknowledgment Is made to Henry Zingg, Technician, Forest Products Laboratory, for the shrinkage measurements and to Dale Turner, Director of Research, Dierks Forests, Inc., Hot Springs, Ark., for aid in obtaining the material used in this investigation.
2 Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
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Past Work
In the United States, the commonly accepted information on longitudinal shrinkage was summarized in 1931 by Koehler (12): "The longitudinal shrinkage of normal wood ranges from 0.1 to 0.3 percent. In drying to an average air-dry condition of about 12 percent, the shrinkage would be about one-half as much." He showed an interesting phenomenon--some specimens of normal wood were longer at 12 percent moisture content than they were when green.
He also reported that low-density wood shrank more longitudinally than did high-density wood of the same species. However, low-density species did not shrink more in the longitudinal direction than did the high-density species. He explained this effect of density to be the result of the relative amounts of different types of cells, and not the result of the amount of wood substance.
In 1932, Welch (5) measured the longitudinal shrinkage of 1- by 4- by 10-inch specimens of 62 species from the green to air-dry condition. Apparent longitudinal movement was often a swelling rather than a shrinking. In a second article (14) two years later, he examined the influence of the extent of drying and density on longitudinal movement. In general, during the initial period of drying (until average moisture content was at fiber saturation point) low-density species showed a tendency to swell, and high-density species tended to remain stationary. Below the fiber saturation point, low-density species tended to shrink whereas high-density species tended to remain stationary or to swell. Frequently, longitudinal movement was irregular, with expansion and contraction occurring several times during drying. This indicated internal stresses may be partly responsible for the longitudinal movement during drying. Welch did not dry his samples to the ovendry condition.
In 1943, Cockrell (3) presented data on longitudinal shrinkage of ponderosa pine and concluded "The percentages of green to air-dry and green to ovendry shrinkage for identical blocks did not indicate any uniform relationship between moisture content and longitudinal shrinkage. In the case of those blocks that remained constant or increased in length to the air-dry condition and then shrank with further drying to the ovendry condition, this disparity was quite marked."
Cockrell (4) later attempted to explain longitudinal movement of wood by assuming that the wood cell wall structure behaved like a folding gate; a decrease in one dimension would cause an increase in the other dimension. Therefore,
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longitudinal expansion would result from transverse shrinkage. Also, as transverse shrinkage continued, the longitudinal expansion would become proportionately less. This phenomenon when combined with the inclination of the fibril angle from the longitudinal axis could explain the longitudinal movement of wood. Calculations based on this theory gave reasonable results.
The review of information on longitudinal shrinkage by Kelsey (10) in 1963 includes a rather complex theory that is analogous to Cockrell's theory.
Kelsey (11) reported results from measuring longitudinal and transverse movement of Pinus radiata, Agathis macrophylla, and Eucalyptus regnans. The relationships of longitudinal shrinkage to moisture content were quite different from those of transverse shrinkage to moisture content. Between green and 8 percent moisture content, a small expansion or contraction took place, and, on further drying, appreciable contraction usually occurred. Large variations in longitudinal shrinkage were observed. Longitudinal shrinkage did not correlate well with micellar angle.
Ivanov (9) explained the early elongation of wood during sorption from the dry condition to be the result of the normal uptake of water caused by a longitudinal component of a transverse expansion. The subsequent observed shrinkage, he explained, is a result of an extreme stretching of the transverse distance between chains as more water is absorbed. This is essentially the same explanation that Cockrell uses.
Beyer (2) in his early work measured the longitudinal shrinkage (through 10 cycles) of specimens (five hardwood and one softwood species) from soaked to ovendry conditions. In all species, there was a decrease in shrinkage after the first cycle, but during subsequent cycles the shrinkage was erratic. These results are similar to those of Ivanov and tend to support his hypothesis that the formation and breaking of hydrogen bonds lead to nonreproducible longitudinal shrinkage and swelling of wood during repeated sorption cycles.
Harris and Meylan (5) investigated the relationship between microfibril angle and longitudinal shrinkage and in particular how this relationship compared with the predictions of the reinforced matrix theory (Barber and Meylan (1)). The relationship between microfibril angle and longitudinal shrinkage was curvilinear with minimum values occurring at a microfibril angle of 20° to 25°. Harris and Meylan suggest that observed deviations from the theory can be explained by the effect of the components of the cell wall other than the S2 layer
or by changes in the ratio between the elastic moduli of the amorphous matrix of the S2
layer and of the reinforcing microfibrils (the E per S ratio).
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The reinforced matrix theory of Barber and Meylan is especially interesting because it predicts negative longitudinal shrinkage during drying for wood with a rather large microfibril angle and a high E per S ratio.
Methods
Measuring Longitudinal Movement
Three methods of measuring the small changes associated with the longitudinal movement of wood were examined: By linear displacement transformers, by a traveling microscope, and by micrometers. The primary purpose of this study was to obtain practical data: the sliding-bar micrometer method developed by Heebink (Heebink and Hann (7)) is rapid and quite accurate; it appeared to be the best. Change in length is determined by measuring the distance between two grommet-filled holes drilled approximately 10 inches apart in each specimen, The distance is measured by a dial micrometer on a slide that contains one fixed point and an adjustable point. The reproducibility, indicated by several readings of the same specimen, was within 0.001 inch when the dial micrometer was accurate to 0.0001 inch. This system, therefore, gave longitudinal shrinkage values accurate to ±0.01 percent.
The specimens were first measured in the green condition and then dried. They were dried slowly by conditioning to apparent equilibrium in rooms with a series of controlled temperatures and humidities in the following sequence:
90 percent relative humidity, 80° ±2° F. 80 percent relative humidity, 80° ±2° F. 65 percent relative humidity, 80° ±2° F. 30 percent relative humidity, 80° ±2° F. 12 percent relative humidity, 75° ±5° F. Ovendried overnight at approximately 212° F.
Specimen Selection and Preparation
Specimens were selected from boards cut from logs of the following seven species: Post oak (Quercus stellata Wangenh.), white oak (Q. alba L.), rock elm (Ulmus thomasii Sarg.), b l a c k g u m (Nyssa sylvatica Marsh.), sweetgum (Liquidambar styraciflua L.), white ash (Fraxinus americana L.), and baldcypress (Taxodium distichum Rich.).
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Specimens were sawed from the green material and scribed along the grain to ensure that each specimen was as straight grained as possible and that measurements were taken in the longitudinal direction. Specimens were 12 inches along the grain, about 1 inch wide, and as thick as the original board (rough 4/4 or 6/4 lumber). Holes were drilled 1 inch from each end, and grommets were pressed into each hole to provide fixed points for measuring.
Specimens were examined at each stage of drying, and any specimens that showed visible warping were discarded.
Results and Discussion
The percentages of longitudinal shrinkage for the boards of various species are listed in table 1, Figures 1 through 7 show the curves for shrinkage plotted over relative humidity. The most and the least stable specimen (maximum and minimum) from each board are shown to demonstrate variability.
The extreme variability and the negative shrinkage that extends, in some cases, to low relative humidities are the two outstanding characteristics of these results. Because these specimens were selected as representative of clear straight-grained material of the seven species, the variation is representative of what might be found within a single board of the species. The variation between boards would presumably be as great or greater, and, of course, significant amounts of reaction wood or cross-grain would greatly increase the shrinkage.
Correlation within boards between longitudinal shrinkage and distance from the pith appeared good as shown in figure 8. However, the change in longitudinal shrinkage at increasing distances from the pith was sometimes positive and sometimes negative. Therefore it seems unlikely that longitudinal shrinkage can be predicted on a practical basis from position in the tree.
The density of the specimens was measured, and a regression analysis of the relation of longitudinal shrinkage to density was made. The results are shown in table 2. They indicate that, although the correlation coefficients within a board are frequently high, the slopes range from -3.22 to a +6.13. This conflicts with Koehler’s observations (12); it seems unlikely that density can be used as a practical indicator of longitudinal shrinkage.
Based on the work of Harris and Meylan (5) it does seem likely that a major cause of the observed variation in longitudinal shrinkage is varying microfibril
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Table 2.--Coefflcients of regression equation Y = A t BX for predicting longitudinal shrinkage (Y) as a function of density (X) for the boards in this study
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Figure 1.--Relation of longitudinal shrinkage to relative humidity in
post oak. M 136 044
Figure 2.--Reiation of longitudinal shrinkage to relative humidity in least and most stable specimen from boards 1 and 3 of white oak.
FPL-0203 -8- M 136 041
Figure 3. --ReIation of longitudinal shrinkage to relative humidity in least and most stable specimen from boards 1 and 2 of crypress.
M 136 043
Figure 4.--Relation of longitudinal shrinkage to relative humidity in least and most stable specimen from boards 2 and 3 of rock elm.
M 136 042FPL-0203 -9-
Figure 5.--ReIation of longitudinal shrinkage to relative humidity in least and most stable specimen from boards 2 and 3 of blackgum.
M 136 039
Figure 6.--Relation of longitudinal shrinkage to relative humidity in sweetgum. M 136 037
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Figure 7.--Relation of longitudinal shrinkage to relative humidity in least and most stable specimen from boards 1 and 2 of ash. M 136 040
Figure 8.--Relation of distance from the pith to percent of longitudinal
shrinkage in boards of four species. M 136 038
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angle. The reinforced matrix theory of Barber and Meylan (1) provides a reasonable qualitative explanation for the negative longitudinal shrinkage frequently observed during drying to equilibrium at 30 or 65 percent relative humidity. According to their theory, negative shrinkage would be expected when the microfibril angle is relatively large and the E per S ratio is 20 or more. (The elastic modulus of the microfibril is 20 or more times greater than the elastic modulus of the amorphous region of the S2 layer of the cell wall in the theory.)
When the wood is green, it is logical to expect that the E per S ratio is high because the water in the amorphous region reduces the elastic modulus of that region. As the wood dries, the amorphous region would presumably become more rigid and the E per S ratio decrease, and cause a change from negative to positive shrinkage.
Further investigation to more fully evaluate the reinforced matrix theory as an explanation of the unique longitudinal shrinkage behavior of wood could be interesting, although it is difficult to visualize how this information could be used to assist the designer who needs to calculate precisely the longitudinal movement of wood products as they undergo a moisture change.
Conclusions
The data obtained here show that longitudinal shrinkage of wood is variable within a single board of clear straight-grained material. Longitudinal expansion during drying was a common occurrence, especially during drying from green to equilibrium at 30 or 65 percent relative humidity. The amount of longitudinal shrinkage when specimens were ovendried frequently correlated well either with distance from the pith or with density for a single board, but the slopes of both plots varied from positive to negative among the different boards. This indicates some other phenomenon, perhaps microfibril angle, is the primary influence in determining longitudinal shrinkage.
Literature Cited
(1) Barber, N. F., and Meylan, B. A. 1964. The anisotropic shrinkage of wood, A theoretic model, Holzforschung
18(5):146-156. (2) Beyer, Frank Kemp
1930. Inherent causes of variation in the longitudinal shrinkage of wood. M.S. thesis, Univ. of Wis.
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( 3) Cockrell, R. A. 1943. Some observations on density and shrinkage of ponderosa pine wood.
Amer. Soc. Mech. Eng. Trans. 65:729-739. ( 4)
1947. Explanation of longitudinal shrinkage of wood based on interconnected chain-molecule concept of cell -wall structure. Amer. Soc. Mech. Eng. Trans. 69: 931-935.
( 5) Harris, J. M., and Meylan, B. A. 1965. The influence of microfibril angle on longitudinal and tangential
shrinkage in Pinus radiata. Holzforschung 19(5):144-153. ( 6) Heebink, B. G.
1963. Importance of balanced construction in plastic-faced wood panels. U.S. Forest Serv. Res. Note FPL-021, Forest Prod. Lab., Madison, Wis.
( 7) , and Hann, R A. 1959. How wax and particle shape affect stability and strength of oak
particle boards. Forest Prod. J. (9): 197-203. ( 8) , Kuenzi, E. W., and Maki, A. C.
1964. Linear movement of plywood and flakeboards as related to the longitudinal movement of wood. U.S. Forest Serv. Res. Note FPL-073, Forest Prod. Lab., Madison, Wis.
( 9) Ivanov, Yu. M. 1962. (Elongation and longitudinal shrinkage of timber during swelling.)
Akad. Nauk SSR Ind. Lesa, Truday 51:107-119. (10) Kelsey, Kathleen E.
1963. A critical review of the relationship between the shrinkage and structure of wood. Australia. C.S.I. R.O. Div. Forest Prod. Technol. Pap. No. 28, 35 pp., Melbourne,
(11) 1963. The shrinkage-moisture content relationship for wood with special
reference to longitudinal shrinkage. Australia. C.S.I. RO. Div. Forest Prod. Proj. T.P. 8, Prog. Rep. No. 2, 18 pp., Melbourne.
(12) Koehler, Arthur 1931. Longitudinal shrinkage of wood, Amer. SOC. Mech. Eng. Trans.
53:17-20, April. (13) Welch, M, B.
1932. The longitudinal variation of timber during seasoning. J. and Proc. Royal SOC. New South Wales 66:492-497.
(14) 1934. The longitudinal variation of timber during seasoning. Pt. II. J. and
Proc. Royal SOC. New South Wales 68:249-254.
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