Moisture adsorption processes carried out in successive steps at three increasing levels of RH (45, 75, 85%) at 20°C for Sitka spruce (Picea sitchensis Carr.) were studied. Moisture content and dimensional changes in radial and tangential directions of the specimens were measured and it was found that moisture changes were slower than dimensional. The modeling on this moisture-dimensional relationship, based on the idea of dividing sorbed water into two components having different effects on dimensional changes, not only shows a good agreement with experimental results, but also presents a new understanding of the mechanism of hygroexpansion of wood

Kawamura, Susumu

Ma, Erni

Nakao, Tetsuya

Ohata, Hiroshi

Zhao, Guangjie

English

Wood and Fiber Science (E-Journal)

RELATION BETWEEN MOISTURE SORPTION ANDHYGROEXPANSION OF SITKA SPRUCE DURINGADSORPTION PROCESSESErni Ma*PhD StudentTetsuya NakaoProfessorDepartment of Natural Resources Process EngineeringShimane UniversityMatsue, Shimane, 690-8504, JapanGuangjie ZhaoProfessorCollege of Material Science and TechnologyBeijing Forestry UniversityBeijing 100083, ChinaHiroshi OhataSection ChiefSusumu KawamuraChief ResearcherDepartment of Material TechnologyShimane Institute for Industrial TechnologyMatsue, Shimane, 690-0816, Japan(Received November 2009)Abstract. Moisture adsorption processes carried out in successive steps at three increasing levels of RH(45, 75, 85%) at 20C for Sitka spruce (Picea sitchensis Carr.) were studied. Moisture content and dimen-sional changes in radial and tangential directions of the specimens were measured and it was found thatmoisture changes were slower than dimensional. The modeling on this moisture-dimensional relationship,based on the idea of dividing sorbed water into two components having different effects on dimensionalchanges, not only shows a good agreement with experimental results, but also presents a new understandingof the mechanism of hygroexpansion of wood.Keywords: Adsorption, dimensional change, moisture change.INTRODUCTIONWood as a hygroscopic material, has characteris-tics such as mechanical properties and dimen-sional stability that are affected by moisturecontent (MC). Because environmental conditions(RH and temperature) are rarely constant, wood iscontinually subject to moisture sorption processesand at the same time producing correspondingdimensional changes. As a result, it is importantto have a good understanding of the moisturesorption of wood as well as its relation withdimensional change.Various theories have been developed to describemoisture sorption in wood. Many are based onFick’s diffusion law (Stamm 1959; Droin-Josserand et al 1988a, 1988b; Liu et al 2001).* Corresponding author: maerniya@yahoo.com.cnWood and Fiber Science, 42(3), 2010, pp. 304-309# 2010 by the Society of Wood Science and TechnologyHowever, the relation between moisture sorptionand time cannot be easily determined because ofthe complex solution of the Fickian differentialequation. In addition, some studies have shownthat the equation does not adequately describemoisture movement in small wood specimensand that other processes operate to limit the rateof moisture change in wood (Christensen andKelsey 1959; Kelly and Hart 1969; Skaar et al1970; Nakano 1994a, 1994b; Zhang et al 2007).Although much information is available in theliterature on the kinetics of moisture sorption,there have been few tests and theoretical modelsrelated to dimensional changes during the sorp-tion process. This has been partly because it isdifficult to measure dimensional changes duringsorption and the focus on sorption. Nakano(1994a, 1994b) proposed a nonsteady state adsorp-tion equation for wood, in which the process wasregarded as an autocatalyzed sorption reaction.The mathematical model made a great break-through in that it can characterize either moistureor dimensional changes during adsorption.The work reported here was conducted to inves-tigate the relation between moisture and dimen-sional changes during sorption process and to tryto model and give a tentative explanation of thisrelationship.MATERIALS AND METHODSWood specimens were prepared from Sitkaspruce (Picea sitchensis Carr.) with dimensionsof 4 mm along the grain and 20 mm in tangen-tial and radial directions. The specimens werefirst oven-dried at 105C, after which theiroven-dry weight and dimensions were mea-sured. They were placed in an environmentalchamber maintained at 20  0.2C throughoutthe experiment.The specimens were subjected to successivemoisture sorption by increasing RH inside thechamber following a discontinuous stepwiseprocess (45, 75, 85  1.0% RH) according to apredetermined program. Each step was 48 h,which was determined by pre-experiments sothat quasiequilibrium could be attained. Tem-perature and RH transmitters were placed nearthe specimens to monitor the conditions. Weightand radial and tangential dimensional changeswere recorded by an electronic balance andthree CCD laser displacement sensors, as shownin Fig 1. The precision of measurement was0.1 mg and 1.0 mm.In addition, a group of three end-matched spec-imens was used during the test: one for dimen-sion measurement and the other two for weight.Successive adsorption was repeated three timeswith oven-dry specimens. The dimensional mea-surement specimens were rotated so that each ofthe three specimens was measured for dimen-sion once and weight twice. In this way, theeffect of variability among specimens could bereduced considerably. Average values of thethree measurements were taken as the finalresults for analysis.Figure 1. Diagram showing the instrumentation for (a) the entire assembly (b) detecting the dimensional changes in thespecimen, in which laser head 1 is for the radial measurement and laser heads 2 and 3 are for the tangential.Ma et al—RELATION BETWEEN MOISTURE SORPTION AND HYGROEXPANSION 305RESULTS AND DISCUSSIONGeneral Adsorption BehaviorA general behavior of moisture uptake andhygroexpansion of Sitka spruce during succes-sive adsorption is shown in Fig 2 in which theMC and swelling are calculated based on oven-dry conditions. Both moisture and dimensionalchanges follow the same trend, that is, the sorp-tion and swelling rate of the specimens are highat the initial stage during adsorption and deceasegradually until equilibrium is reached. In addi-tion, tangential swelling is much greater thanradial, especially in the high RH range.Figure 3 gives detailed information on a com-parison between MC and radial swelling foradsorption from oven-dry to 45% RH. The fig-ure indicates that dimensional change reaches anequilibrium state first followed by MC, meaningthat moisture changes are slower than dimen-sional. This agrees with Hunt’s (1990) researchin which longitudinal dimensional changes ofScots pine were found to act quickly than mois-ture changes during adsorption and desorptionprocesses. The phenomenon is particularly inter-esting because moisture change is supposed to befaster than, or at least simultaneous with, dimen-sional change, because it is believed that swellingand shrinkage are caused by moisture moving inand out of wood (Yin 1996; Bowyer et al 2003).Therefore, this unusual behavior suggests theneed for further study on the mechanism of hy-groexpansion of wood. Because the relationbetween sorption and swelling rate is not as obvi-ous for the other two RH ranges, Nakano’sadsorption theory was applied to investigate this.Theoretical Analysis by Nakano’sAdsorption TheoryMa et al (2009) made a modification to theadsorption equation proposed by Nakano(1994a, 1994b) so that it could be applied toadsorption from a specific initial MC. The mod-ified equation can be written asm ¼ ðme  m0Þ=½1þ expðrðlog t aÞÞ þ m0ð1Þor in the case of dimensional changes,l ¼ ðle  l0Þ=½1þ expðrðlog t aÞÞ þ l0 ð2Þwhere m and l are MC (%) and swelling (%),respectively, at time t (s). The subscripts o ande refer to initial and equilibrium states, respec-Figure 2. Plots of moisture content (MC) (a) and swelling(b) against time during adsorption processes of Sitka spruce.Figure 3. Normalized curves of moisture content (MC)and radial swelling against time for the adsorption of Sitkaspruce at 45% RH.306 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)tively, and a and r are constants. Figure 4 showsthe MC and moisture sorption rate curves for theadsorption of Sitka spruce at 45% RH. Thevalue of a indicates the time taken for wood toreach a maximum sorption rate and, accord-ingly, 2a is assumed to be the time needed toapproach the equilibrium state.Figure 5 shows a comparison of theoreticalcurves with experimental results for MC andradial and tangential swelling at various RH con-ditions. The plots suggest that theoretical curvesagree satisfactorily with the corresponding exper-imental results. The values of a for moisture sorp-tion and radial and tangential swelling are listedin Table 1 as well as the average a value of radialand tangential swelling for each RH condition. Itis clear that a increases with an increase in RH,which means that the higher the RH, the longertime needed for wood to reach equilibrium, asshown in Fig 5. In addition, a is greater for mois-ture sorption than for swelling at a given RH.This suggests that moisture changes are slowerthan dimensional changes. Furthermore, the datain Table 1 also show that radial a value is alwaysseveral percent higher than tangential value, illus-trating that hygroexpansion is slower in the radialthan the tangential direction, which is in accor-dance with Chomcharn and Skaar (1983).Modeling the Relation between Moisture andDimensional ChangesAbundant studies have been presented to showthat chemical treatment can reduce hygroexpan-sion and improve dimensional stability of wood(Skaar 1988). In the Hailwood-Horrobin sorp-tion theory (Hailwood and Horrobin 1946),water is presumed to exist in two forms. TheseFigure 4. Plots of moisture content (MC) (m) and sorptionrate (dm/d[logt]) against logt for the adsorption of Sitkaspruce at 45% RH.Figure 5. Comparison of theoretical curves with experi-mental results of moisture content (MC) (a), radial swelling(b) and tangential swelling, and (c) for the adsorption ofSitka spruce.Ma et al—RELATION BETWEEN MOISTURE SORPTION AND HYGROEXPANSION 307are “hydrated water,” which forms a hydratewith the wood substance, and “dissolved water”that forms a solid solution in the cell wall, bond-ing only with other water molecules. Yasudaet al (1994) applied the Hailwood-Horrobinsorption theory to their adsorption data of chem-ically modified wood and found that the amountof dissolved water decreased considerably,whereas hydrated water suffered only from aslight or almost no reduction in MC by thechemical treatment. This indicates that dimen-sional changes of wood are almost caused by achange in dissolved water.Therefore, assume hydrated water has little orno relation to dimensional changes and dissolvedwater is strongly associated with dimensionalchanges. The Hailwood-Horrobin sorption equa-tion (Hailwood and Horrobin 1946) was thenused to calculate the two water components andthe results are given in Table 2. To quantifyhydrated water and dissolved water duringadsorption, Eq 1 was applied separately to thesecomponents, and the sum of the two parts wastaken as the theoretical total MC that was com-pared with experimental data.Moreover, according to the characteristics ofhydrated and dissolved water described previ-ously, it is convenient to consider that1. Moisture contents of hydrated and dissolvedwater in Table 2 are supposed to be their cor-respondingme at the current RH andm0 for thesubsequent RH condition, respectively, and2. The average a value of radial and tangentialswelling in Table 1 is taken as the a value fordissolved water and that of moisture sorptionis adopted as the one for hydrated water ateach RH.Therefore, during adsorption, dissolved waterdevelops rapidly, causing dimensional changesof wood to take place quite quickly and reachequilibrium first followed by much slowerchanges in MC because hydrated water in-creases slowly with little or no further dimen-sional changes, as shown in Fig 3.The calculated MC of hydrated and dissolvedwater and their total amount during adsorptionin comparison with experimental data are shownin Fig 6. It is evident that the theoretical totalMC is in good accordance with experimentalresults for each RH condition. However, the ini-tial MC of hydrated water for adsorption at 45%RH (Fig 6a) is about 1%, which is not anexpected value because the specimens wereoven-dried. This is perhaps because wood usu-ally has a MC of about 0.5% under oven-dryconditions (Stamm 1964). In addition, the RHerror caused by opening the conditioning cham-ber when inserting specimens at the beginningof the experiment could also be responsible forslightly higher initial MC of hydrated water inthe modeling.CONCLUSIONSDimensional changes of wood are suggested totake place more quickly than moisture changesduring adsorption in this study. The modeling ofthis relationship, based on the idea of dividingadsorbed water into two components having dif-ferent effects on dimensional changes as well asbeing adsorbed in different modes depending onthe RH, not only shows good agreement withexperimental results, but also presents a newand interesting viewpoint for the interpretationof this unexpected behavior. It is hoped that theidea could enable one to gain a fuller insight intothe moisture sorption process and contribute to aTable 2. Calculated moisture contents of hydrated anddissolved water with experimental results.RH (%)Moisture content (%)Experimental Hydrated water Dissolved water45 6.45 3.27 3.1875 11.32 4.39 6.9385 13.42 4.67 8.74Table 1. Values of constant a for moisture sorption andradial and tangential swelling.RH range (%)MoisturesorptionRadialswellingTangentialswellingAverage of radialand tangentialswellingOven-dry-45 3.3953 3.3269 3.2969 3.311945-75 3.6111 3.5471 3.5171 3.532175-85 4.7016 4.2782 4.1806 4.2294308 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)deeper understanding of the mechanism of hygro-expansion of wood. In future work, moisturedesorption processes and longitudinal dimen-sional changes will be investigated.REFERENCESBowyer JL, Shmulsky R, Haygreen JG (2003) Forest prod-ucts and wood science: An introduction. 4th ed. IowaState Press, Ames, IA. 554 pp.Chomcharn A, Skaar C (1983) Moisture and transversedimensional changes during air-drying of small greenhardwood wafers. Wood Sci Technol 17(3):227-240.Christensen GN, Kelsey KE (1959) The rate of sorption ofwater vapor by wood. Holz Roh Werkst 17(5):178-188[in German with summary in English].Droin-Josserand A, Taverdet JL, Vergnaud JM (1988a)Modeling the kinetics of moisture adsorption by wood.Wood Sci Technol 22(1):11-20.Droin-Josserand A, Taverdet JL, Vergnaud JM (1988b)Modelling the absorption and desorption of moistureby wood in an atmosphere of constant and program-med relative humidity. Wood Sci Technol 22(4):299-310.Hailwood AJ, Horrobin S (1946) Absorption of water bypolymers: Analysis in terms of a simple model. TransFaraday Soc 42B:84-102.Hunt DG (1990) Longitudinal shrinkage–moisture relationsin softwood. J Mater Sci 25(8):3671-3676.Kelly MW, Hart CA (1969) Water vapour sorption rates bywood cell walls. Wood Sci 1(4):270-282.Liu JY, Simpson WT, Verrill SP (2001) An inversemoisture diffusion algorithm for the determination ofdiffusion coefficient. Drying Technol 19(8):1555-1568.Ma EN, Nakao T, Zhao GJ (2009) Adsorption rate of woodduring moisture sorption processes. Wood Res-Slovakia54(3):13-22.Nakano T (1994a) Non-steady state water adsorption ofwood, Part 1: A formulation for water adsorption. WoodSci Technol 28(5):359-363.Nakano T (1994b) Non-steady state water adsorption ofwood, Part 2: Validity of the theoretical equation of wateradsorption. Wood Sci Technol 28(6):450-456.Skaar C (1988) Wood-water relations. Springer-Verlag,Berlin, Germany. 283 pp.Skaar C, Prichananda C, Davidson RW (1970) Someaspects of moisture sorption dynamics in wood. WoodSci 2(3):179-185.Stamm AJ (1959) Bound-water diffusion into wood in thefiber direction. Forest Prod J 9(1):27-32.Stamm AJ (1964) Wood and cellulose science. The RonaldPress Company, New York, NY. 549 pp.Yasuda R, Minato K, Norimoto M (1994) Chemical modi-fication of wood by non-formaldehyde cross-linkingreagents, Part 2: Moisture adsorption and creep proper-ties. Wood Sci Technol 28(3):209-218.Yin SC (1996) Wood science. China Forestry PublishingHouse, Beijing, China. 272 pp [in Chinese].Zhang MH, Cazo R, Cassens D, Xie J (2007) Water vaporadsorption in kiln-dried red oak. Wood Fiber Sci 39(3):397-403.Figure 6. Theoretical moisture content (MC) for hydratedwater and dissolved water and their total amounts in com-parison with experimental data during adsorption at (a) 45,(b) 75, and (c) 85% RH.Ma et al—RELATION BETWEEN MOISTURE SORPTION AND HYGROEXPANSION 309

Relation Between Moisture Sorption and Hygroexpansion of Sitka Spruce During Adsorption Processes

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Relation Between Moisture Sorption and Hygroexpansion of Sitka Spruce During Adsorption Processes

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