Gas permeability of loblolly pine sapwood was determined for samples from six plantation-grown trees. Longitudinal permeability measurements were made on wood from five heights in the trees, juvenile and mature wood, and earlywood and latewood. Samples were either solvent-dried to obtain a green-equivalent state or air-dried. Permeability was also measured in the tangential and radial directions. Longitudinal permeability was significantly less in the lower part of the tree than in the upper part. Mature wood was more permeable than juvenile wood for both green-equivalent and air-dried wood as well as earlywood and latewood. Permeability did not appear to be solely a function of specific gravity

Mianowski, Theodore

Milota, Michael R.

Tschernitz, John L.

Verrill, Steve P.

English

Wood and Fiber Science (E-Journal)

GAS PERMEABILITY OF PLANTATION LOBLOLLY PINE Michael R. Milota Associate Professor Department of Forest Products Oregon State University Cowallis, OR 97331-7402 John L. Tschernitz Chemical Engineer Steve P. Verrill Mathematical Statistician and Theodore Mianowski Technician USDA Forest Service Forest Products Laboratug ' One (;)ford Ptnchot l > n w  Madison, W1 53705-2398 (Received February 1994) ABSTRACT Gas permeability of loblolly pine sapwood was determined for samples from six plantation-grown trees. Longitudinal permeability measurements were made on wood from five heights in the trees, juvenile and mature wood, and earlywood and latewood. Samples were either solvent-dried to obtain a green-equivalent state or air-dried. Permeability was also measured in the tangential and radial directions. Longitudinal permeability was significantly less in the lower part of the tree than in the upper part. Mature wood was more permeable than juvenile wood for both green-equivalent and air- dried wood as well as earlywood and latewood. Permeability did not appear to be solely a function of specific gravity. Keywords: Permeability, loblolly pine, solvent-dry, air-dry, juvenile wood, mature wood. INTRODUCXION 1.013 darcys. The permeability of a wood spe- permeability is the property of a cies is one factor that determines how easily that indicates how freely fluids flow in response it can be dried and treated. Wood exhibiting to a pressure gradient. 11 is commonly reported high permeability, such as southem pine sap- in darcys, a mixed unit. ~hroughout this paper, wood, is easy to dry and treat; that with low the SI unit fim2 is used, which is equivalent to permeability, such as Douglas-fir heartwood, is not easy to dry and treat. The obiective of this work was to comvare the gas permeability of loblolly pine juvenile ' The Forest Products Laboratory is maintained in co- and mature as a function ofposition operation with the University of Wisconsin. This article . was written and nreoared bv U.S. Governmentemnlovees In the tree. Samples were selected that dif- . . . . on oficial time, and it is therefore in the public domain ference~ in permeability could be compared for and not subject to copyright. earlywood and latewood and permeability in W w d o n d f i k r  Sciewe. 2711). 1995. PP. 34-40 19 1995 by ~ h c  Society of Wood kienee and Technolopi Milolo el oi.-GAS PERMEABILITY OF PLANTATION LOBLOLLY PINE 3 5 the longitudinal, radial, and tangential direc- * -- ---~ -~ tions. 1/ 4 ,  31". 1 811. 7 '  BACKGROUND L- -- ~ 34 It. The factors affecting permeability were dis- cussed in general by Cbt6 (1963), and for 1 southern pine, by Erickson (1970). Comstock (1965, 1967, 1968, and 1970) and Comstock and Cbt6 (1968) extensively investigated both gas and liquid permeability. Gas permeability of a softwood is affected by the anatomical characteristics of the species, pit aspiration : ' ,f& during drying, and encmstation and aspiration during heartwood formation. The character and quantity of extractives can also affect heart-  I wood permeability (Comstock 1970). Report- ed gas permeability values for southern pine - .  - sapwood at 14% moisture content in the lon- FIG. I. Cutting diagram for (A) 7.62-cm (3-in.) disks gitudinal, tangential, and *adia1 directions are from 10, 1-rn ( 3 3 4  logs (not to scale) and (B) blocks from 2.12, 0.0003, and O.ol l 2  pm2, disks; P indicates block was used for permeability cores; (Comstock 1970). SG indicates s~ecific eravitv. ' Comstock (1967) demonstrated that wood permeability measurements are independent of the fluid type (liquid or gas) if the liquid is nonswelling or a correction is made for slip flow of the gas through the small pore spaces. Gas permeability is simpler to measure than is liquid permeability, because gas permeabil- ity has no problems with air blockages or sus- pended particles. A gas can be used to measure the permeability ofgreen wood after removing the water by a method that does not aspirate the pits, such as solvent- or critical-point dry- ing (Liese 195 1). METHODS Six, 28-year-old, plantation-grown, loblolly pine (Pinus taeda L.) trees ranging from 34.5- to 42.4-cm- (13.6- to 16.7-in.-) diameter breast height were selected for study. This material was also used in a concurrent drying study (Milota and Tschemitz 1990). The initial planting density was 1,850 trees/ha (750 trees/ acre) until thinnings at ages 14 and 22 when the stand density was reduced to 703 and 345 trees ha (285 and 140 trees/acre), respectively. At age 22, the stand was fertilized. Five disks, 7.62 cm (3 in.) thick, were cut from each tree at 2.5 14-m (8-ft 3-in.) intervals, starting approximately 0.914 m (3 ft) above the ground (Fig. 1A). Two, 5.08- by 5.08- by 5.08-cm (2- by 2- by 2-in.) blocks were cut 1.27 cm (0.5 in.) from either transverse face of each disk (Fig. 1B). One block was taken close to the pith within the juvenile wood, and the other was taken on the same radius close to the bark within the mature wood. This set of blocks was frozen under water to eliminate deterioration. A matching set of 60 blocks was taken from the opposite radial side ofeach disk to obtain an estimate of specific gravity. The green specific gravity was determined by mea- suring the green block dimensions and weigh- ing the oven-dried blocks. Core preparation Two cylindrical sample cores, 1.27 cm (0.5 in.) along the grain with a 1.27-cm (0.5-in.) diameter, were cut from each of the 60, 5.08- cm (2-in.) blocks so that the annual ring boundary crossed the sample (Fig. 2). The per- centage ofearlywood and latewood in the cores was selected to be representative of that in the 36 WOOD AND FIBER SCIENCE, JANUARY 1995. V. 27(1) Ro. 2. Cut out view of block showing how 1.27- and 0.64-cm (0.5- and 0.25-in.) cores were cut. A indicates air-drying, S indicates solvent-drying. block. After solvent-drying, these 120 cores were used to determine the longitudinal green- equivalent permeability as a function of po- sition in the tree. Additional cores, 1.27 cm (0.5 in.) long by 0.64 cm (0.25 in.) in diameter (half the di- ameter of the original 120 cores), were also cut from a 10-block subset of the 60 blocks (Fig. 2). The effects of grain direction, ring position, wood type (juvenile or mature), and drying method on permeability were compared. Two blocks were from each of the five disks, taken from a different height in five trees. This con- founded the tree effect with the height effect, but still allowed a comparison of the effects in which we were interested. Permeability as a function of grain orien- tation was determined using 10 pairs of cores cut so that the longitudinal axes of the cores corresponded to either the tangential or radial direction of the wood. Each pair of cores was matched along the grain (Fig. 2) to one of the 1.27-cm- (0.5-in.-) diameter longitudinal cores to allow a comparison of permeability mea- surements in the three directions. These cores were solvent-dried. The effects of moisture state, ring position, and wood type on the permeability were de- termined using four sample cores from each of 10 blocks. The axes of the cores correspond- ed to the longitudinal direction of the wood. Each pair of earlywood and latewood cores was end-matched along the grain (Fig. 2). One core of each pair was solvent-dried, the other was air-dried. Air- and solvent-drying The green cores were solvent-dried in batch- es of approximately 20 cores by placing them in sealed jars with 150 cm3 (5 oz) of absolute ethyl alcohol. The alcohol was replaced three times at 1-week intervals. Finally, the alcohol was drained, the mouths of the jars were loose- ly plugged with a paper towel to retard evap- oration, and the alcohol was allowed to slowly evaporate in an oven at 49 C (120 F) for 1 week. The cores were heated to 107 C (225 F) for 24 h and then stored in sealed jars in a desiccator. Air-drying was accomplished in an oven at 49 C (120 F), again in jars with the mouths plugged with a paper towel to retard drying and prevent checking. After 1 week in this con- dition, the cores were dried at 107 C (225 F) for 24 h and stored in sealed jars in a desic- cator. In both air- and solvent-drying, few end- checks were observed. The radial cleavage surfaces of eight cores 6.4 mm (0.25 in.) were examined using scan- ning electron microscopy (SEM). The cores came from earlywood and latewood and ju- venile and mature wood that had been air- and solvent-dried. Observations confirmed that bordered pits in the longitudinal tracheids were completely aspirated after air-drying and un- aspirated in the green-equivalent state. The size and number of bordered pits were greater in the earlywood; however, these values were not quantified. Permeability measurements A core was placed in a rubber stopper that had been drilled along its longitudinal axis to the diameter of the core. The stopper and core were placed in a test jig so that flow could occur only parallel to the long axis ofthe core (Com- Milola et a/.-GAS PERMEABILITY OF PUNTATION LOBLOLLY PINE 37 stock 1967). The jig slightly compressed the rubber stopper to seal the cylindrical surfaces ofthe core. Nitrogen gas was reduced to a gauge pressure of approximately 0.138 MPa (20 Ib/ in.2) and applied to the upstream side of the sample. The nitrogen cylinder was changed when the gauge pressure reached 3.45 MPa (500 Ib/in.'), to minimize the partial pressure of water vapor in the gas stream and maintain the dry state of the core. One of three rota- meters was used to measure the flow on the downstream side of the sample at atmospheric pressure. The flow ranged from 400 to 1,100 cm3/min (24 to 67 in.'/min). The pressure drop across the sample ranged from 3.39 to 135 kPa (I to 400 in. water) and was measured with either a mercury- or an oil-filled manometer. The diameter and length of each core were measured to the nearest 0.0254 mm (0.001 ysis of variance (ANOVA) on tree and height factors. Scheffe and Bonferroni's multiple comparison procedures were performed at a 0.05-significance level. (2.) A paired t-test was then done on this same data set to compare the permeability of the juvenile and mature wood. Because of a lack of full independence, it was not appropriate to expand the ANOVA to in- clude this factor. (3.) The effect of grain orientation was inves- tigated using a paired t-test on data from the radial and tangential cores. Linear re- gression analyses were performed to de- termine the relationship between the lon- gitudinal and crossgrain permeability. RESULTS AND DISCUSSION in.). Position in tree ANALYSIS The permeability was calculated from the following (Comstock 1967): where K, is gas permeability, pm2 A specimen area, cm2 L specimen length, cm Q gas flow rate, cmVsec q gas viscosity, Pa.sec AP pressure drop, MPa p pressure at rotameter, MPa p' pressure in sample, MPa As indicated by the Bonferroni groupings in Table 1. ~ermeabilitv was not statisticallv dif- . . ferent from tree to tree, but it did depend upon height within the tree. The permeability in- creased significantly with height between the first and second disks, then slightly, if at all. This trend occurred in both juvenile and ma- ture wood and in every tree (Table 1). This indicates that wood from the base of the tree may dry more slowly and be more difficult to treat than wood from farther up the stem. Sim- ilarly, Comstock (1965) observed increasing permeability with height for green eastern hemlock. Juvenile wood had lower permeability than did mature wood, but juvenile wood also had a lower specific gravity (Table 1). In contrast, by tree, permeability was lowest in the tree The last term in the equation corrects for the gas pressure in the sample. with the highest specific gravity. The tree height The following three comparisons were per- ANOVAs were rerun with specific gravity as formed on solvent-dried cores: a covariate. The height effect did not vanish. From the ANOVA, it is clear that s~ecificgrav- (1.) The effect of position in the tree on Ion- ity explains part of the height effect, but not gitudinal permeability was determined by all of it (Table 2). Perhaps this is why the lit- averaging the values for juvenile and ma- erature disagrees regarding the effect of specific ture wood permeability from the 1.27-cm gravity on gas permeability (Choong and Fogg (0.5-in.) cores and using a two-way anal- 1968; Benvenuti 1968). 38 WOOD AND FIBER SCIENCE, JANUARY 1995, V. 27(1) TABLE 1. Effect ofposition in tree on l~ngitudinalpermmbility.~ Meisturn Ring Nvmbcr of Spccific Pcrmeabi1it)r R o n f e ~ n i  Paircd I mnfcntb 1%) covnwcmh mmnle~ navitvi lum? rnuocns rnuoine Tree number 5 149 1.42 20 0.41 10.5 A 6 121 1.34 12 0.46 8.4 A 13 116 1.42 12 0.42 9.0 A 19 156 1.14 16 0.40 10.6 A 24 141 1.46 20 0.41 9.8 A 27 144 1.26 20 0.41 10.0 A Disk TOP 156 1.22 20 0.39 11.4 B Fourth 146 1.34 20 0.40 10.3 B Third 145 1.42 24 0.40 10.9 B Second 134 1.54 16 0.43 10.2 B Bottom 105 1.14 20 0.48 5.7 A Wood type Juvenile 154 0.98 50 0.38 8.4 A Mature 122 1.69 50 0.45 11.3 B Combined 138 1.35 100 0.42 9.9 'Sir ofthc 120 1.27.m (0.5-in.) solventdried cons were excluded fmm Ulc analysis a. outlim, four with visible checks. Mcasurcd on aample blocks aa received: 10 for !re<. 12 for dirk. and 30 cach forjuvcnilc and rnaturc. 'Least wluam mcanr (Sarlc ct al. 1980). Directional permeability The average permeability of the green- equivalent samples was 0.073 pm2 in the tan- gential direction and 0.20 pm2 in the radial direction (Table 3). The paired t-test showed this difference to be significant at the 95% con- fidence level (P = 0.0053). The difference be- tween the radial and tangential permeability has been reported to be a factor of 20 for dry wood. However, the results presented here in- TABLE 2. Effect of adding specific gravity and height to the mde l .  Root mom woad Vm Prcdinoe R2 wl-ermr Juvenile tree + SG 0.71 1.40 tree + height 0.87 1.04 tree + SG + height 0.88 1.02 Mature tree + SG 0.49 2.25 tree + height 0.79 1.57 tree + SG + height 0.79 1.62 Combined tree + SG 0.79 1.21 tree + height 0.88 1.00 tree + SG-+ height 0.88 1.03 S O  is spcific p v i t y .  dicate that for green wood the difference may be less. This might be explained by a greater number of unaspirated pits in the radial walls while the wood is green (or solvent-dried), con- tributing to tangential flow while radial flow is predominately through the ray structure. Nei- ther the radial nor tangential permeability was significantly different between the juvenile and mature wood. Linear regression analyses indicated no strong relationships between permeability in the longitudinal and radial directions (r = -0.74), longitudinal and tangential directions (r = -0.01), or the radial and tangential di- rections (r = 0.60). A strong relationship was not necessarily expected because of the aniso- tropic nature of the wood structure. Moisture state, ring position, and wood type The longitudinal permeability of green- equivalent cores was 8 to 29 times greater than that of air-dried cores, demonstrating the dif- ference in moisture state on pit aspiration (Ta- ble 3). The difference was greatest in the ear- lywood, where the size and number ofpits were Milola el aL-GAS PERMEABILITY OF PLANTATION LOBLOLLY PINE 39 TABLE 3. Effect of direction and wood type on perme- ability.8 Permeability ma) Dirrnion Juvenilc Mature Combined Longitudinal 7.8 11.3 9.5 Radial 0.21 0.19 0.20 Tangential 0.062 0.084 0.073 Each valve for juvenile and mature wood mpments the awlage of 10 core mcaaumrncntr. Lonsltudinal and cmss win measuremenrs were done on I 2 7 s m  (0.5-in.) and 0.63-cm (0.25-in.) cores, rrspmivrly. greater. The average longitudinal permeability for the 0.064-cm (0.25411.) green-equivalent cores was 10.1 pm2. This agrees well with the value of 9.7 pmZ obtained using the 1.27-cm (0.5-in.) cores, indicating that our measure- ments were independent of sample size. Permeability was less for the air-dried ear- lywood samples than that for the latewood samples; however, the reverse was true for the green-equivalent samples. This is in agreement with Erickson (1970), who states that the ear- lywood of pine sapwood seems to have greater permeability in the green condition, whereas the results are more variable and permeability can often be greater in the latewood for the seasoned condition. The mature wood consistently had 50% to 100°/o greater permeability than did the juve- nile wood for both the 0.064-cm (0.25411.) cores (Table 4) and the 1.27-cm (0.5-in.) cores (Ta- ble 1). This was true for both earlywood and TABLE 4. Permeability as a function of wood type, ring position, and drying method. Permeability kt",') Early- Late- wood type ~ o i s t u r c  star wood wood ~ v c r a g c  Juvenile Air-dried 0.34 0.81 0.58 Green-equivalent 10.0 6.3 8.0 Mature Air-dried 0.96 1.14 1.05 Green-equivalent 14.4 9.5 11.9 Combined Air-dried 0.65 0.98 0.81 Green-eauivalent 12.4 8.0 10.1 latewood and the air-dried and green-equiv- alent samples. Mature wood also had a greater specific gravity than did juvenile wood; how- ever, the permeability difference was probably attributable to wood type rather than specific gravity. Within our data, the effect of specific gravity could be interpreted differently, de- pending on which set of data was used. For example, based on the differences in perme- ability by tree or disk (Table I), one could conclude an inverse relationship between spe- cific gravity and permeability. It is experi- mentally difficult not to confound specific gravity and other wood effects. CONCLUSIONS Loblolly pine sapwood from the lower part of the tree has a lower permeability and may not dry or treat as well as wood from higher in the tree. Also, permeability ofjuvenile wood is less than that of mature wood. Specific grav- ity is not a good indicator of permeability. It is also likely that factors such as presence of juvenile or mature wood and between-tree variability have a greater effect on permeabil- ity than does specific gravity. REFERENCES BENVENWI, R. R. 1968. An investigation of methods of increasing the permeability of loblolly pine. Unpub- lished M.S. thesis, Depanment of Forestry, Nonh Car- olina State College, Raleigh, NC. CHWNG, E. T., AND P. J. FOGG. 1968. Moisture move- ment in six wood species. Forest Prod. 1. 18(5):66-70. COMSTOCK, GILBERT L. 1965. Longitudinal permeability of green eastern hemlock. Forest Prod. J. 15(10):441- 449. -. 1967. Longitudinal permeability ofwood to gas- es and nonswelling liquids. Forest Prod. J. 17(10):41- 46. -. 1968 Relationqhip between prmeabilit) ofgreen and d n  eastern hemlock. Forest Prod. J. 18iR)?O-23. . 1970. Directional permeability of softwoods. Wood Fiber 1(4):283-289. -, AND W. A. C6rf. 1968. Factors affecting per- meability and pit aspiration in coniferous sapwood. Wood Sci. Technol. 2(4):279-291. CirrB, WILFRED A. 1963. S t ~ c t u r a l  factors affecting the permeability ofwood. J. Polymer Sci. Pan C, Polymer Symposia N. 2:231-242. ERICKSON, HARVEY D. 1970. Permeability of southern pine wood-a review. Wood Science 2(3):149-158. 40 WOOD AND FIBER SCIENCE, JANUARY 1995, V. 27(1) LIESE, W. 1951. UberdieEindringungvonoligen Schutz- S m ,  S. R., F. M. SPEED, rn G. A. MILLIKEN. 1980. mitteln in Fichtenholz. Holz Roh Werkst. 9374-378, Population marginal means in the linear model: An al- M ~ A ,  MICHAEL R., AND JOHN L. T S C H E ~ Z .  1990. ternative to least squares means. The American Stat- Correlation of lohlolly pine drying rates at high tem- istician 34(4):216-221. perature. Wood Fiber Sci. 22(3):298-313. 

Gas Permeability of Plantation Loblolly Pine

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Gas Permeability of Plantation Loblolly Pine

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