The dielectric constant, loss tangent, and AC resistivity of 60 Douglas-fir (Psendotsuga menziesii (Mirb.) Franco) heartwood specimens were determined at 100 Hz, 1 kHz, 50 kHz, and 100 kHz before and after treatment with copper sulphate solutions. The copper retentions, based on oven-dry weight before treatment, ranged from 1 to 7%.Of the three electrical properties, AC resistivity changed most as a result of treatment. This change, a factor of 7, occurred at 100 Hz and with specimens at an estimated 20% moisture content. A statistical analysis showed even changes of this magnitude are insufficient to form a basis for a nondestructive method for estimating copper retention in wood

Knezevic, Olaf R.

Lin, R. T.

English

Wood and Fiber Science (E-Journal)

COPPER IN DOUGLAS-FIR AND ASSOCIATED DIELECTRIC CHANGES1 Oluf R. Knexeuic Research Assistant of Forest Products S(~hool of Forc%stry, Or(3goi1 State I!ni\re~..sit),, Corvallis, OH 97331 R. T .  Lin' rZssistant l'rofrssor of Wood l'liysics School of Fore.;try, Oregon Stat<% University, C o r ~ ~ l l i s ,  OK 97331 ABSTRACT The clielcctric consta~it, loss tangent, and h C  resistivity of 60 Douglas-fir ( P . ~ e l t d o t s ~ ~ g [ ~  tnetrziesii (Mirl).) Franco) l~eart\vood speci~iicns \\.ere determind at 100 IIz, 1 kHz, 50 kHz, and 100 kIIz before and after trc.atnient Ivitli copper sulphate solutions. Thc copper retel~tiolis, 11asrd on oven-dry \\#t,igl~t I~rforc treat~ilcnt, ranged from 1 to 7%,. Of the three electrical proprvties, AC resistivity changed most as a 1.rsu1t of treatment. This cllange, a factor of 7, occrll.rec1 at 100 Hz and with specilricns at an  esti~nated 20% 111oist11re contt~rrt. A statistiral analysis s l~o \ \~ed  v<,il changes of this magnitude artL insllf- ficit~nt to for111 a I)a.;is for a nondestnlctivct method for estimating coppcr retenti011 in \\.OOd. Atltlilio~~nl kc!lzoovtls: Psc,rr(/olvrrgc~ trrolzicsii, dic*lectric constant, loss tangrrrt, AC resistiv- ity, \vood prc~servation. INTRODUCTION 13ecauscl of the hcavy demands on pre- scrvati17c. trcatcd \vood products and their str:atlily increasing costs, ilew methods for quality control n~us t  bc considered and their feasibility investigated. Ideally, these tilc~thocls or variations thc.rc.of could bc uscd in the nrill and on the product in sc'rvicc. A useful tool for the nlootl preservation intlustry would be a method by which chcm~ical rc>tcntio~l coulcl hc dcternlinecl quickly and nondestructivcly both inilnc- cliatt~ly nftvr trcatmcut and also after years of scarvicc. Preliminary work i~~dicatcd thc tliclrctric parameters at thc lower frc- cluct~~cies wcrcb strongly affc,ctcltl by salt ' 'Tl~e projvet tvas supported by hlcIntire/ Stc~lnis Fluncls. Paptsr 980, Forest Researell Lab- oratory, School of Fo~.cstry, Oregon State I!ni- content. The authors thought these changes perhaps could be used for estimating the salt contc~lt. This study, thereforc, is ai~ncd at nleasuring the dielectric paramc- ters of copper sulphatc trcatcd and un- treated wood alld the11 to see whether a dielcctric method could be developed to estimatc nondc5tructively the coppcr re- tention. Thc dielectric propcrtics cousidcred in this investigation are dielectric constant ( K ), loss tangent ( D) and alter~latillg current rcsistivity ( R \, ) . Thcsc propertics lTary, among other physical parameters, with density, frequency, and moisturc. Because little work has been done at low trcquei~cies and because inoisture contents belo\\? 20% arc usually the inorc important ior wood in use, this investigation was limited to frequencies between 100 IIz and 100 kHz and moisture contents below 20%. vrrsity. hfATEKIALS AND PROCEDURES ' No\v: Senior Wood. Technologist, hlK-KI)A Inc., l)ortlalld, OR. Present addr($ss: P.O. Sixty specimens of cdgc-grain Douglas-fir 204'ili, l'ol.tland, OH $17220. heartwood, 0.2 em thick by 10 em square t V 0 0 1 1  A h l )  FIBER 270 WINTER 1975, V. F ( 4 )  DIELECTRICITY IN I)OU(:LAS-FIR 271 COPPER, % COPPER, % FIG. 1. Effcct of copper retention on loss FIG. 2. Effect of copper rctention on A C  re- tangent ( D )  at  100 Hz for l lo~~glas-fir  heartwood sistivity at  100 Hz of Douglas-fir lieart\vood at  at 207& MC. 0 represents the average value of D 20% MC. 0 represents the average value of and tht. Imrs represent the rstent of tlic 955% InH\ ,  alld the I~ars  repl.esent thc cbxtent of tlic, co~~fidts~rce intrr\.al aro~und 1111). 95% confidvncr i~lterval a r o ~ ~ n d  11rR\~.. ( the longituclinal-ratlial plane formed the 10 cnl square face), wcrc preparcd and clivitlrd ranclomly into five groups. All thc samples werca conditioned at conditions for 0, 7, and 15% EMC. The dielectric mea- surt~mcnts were madr when EMC was attained. Each group then was saturated with tli~tillecl water or various nqucous solution\ containing 1, 5, 10, or 20% copper sulphate. Thc treated specilllens were re- conditioned to equilibrium at the previous conditionr for EMC of 0, 7, ancl 15% for ~nltrcatc~d wood, and dielectric measure- ments wcw repcatecl. Each spc.ci111en was reducc~d to ash and its rctention of copper was determined. Electrical mcasuremcnts consisted of sandwiching cach specimen bctween two 1)rass electrodes 0.5 cm thick by 9.75 cm scluarc and clamping the sandwich in a teflon-insulated jig. The loss tangent and capacitance w e r ~  measured on a capaci- tance bridge at 100 Hz, 1 kHz, 50 kHz, and 100 kIlz. All series capacitance values were converted to ccluivalcnt parallcl val- ucls nccorcling to: where Cp is thc equivalent parallel capaci- tance (farads), Cs is the series capacitance (farads) and D is the loss tangent. The dielectric constant was calculated from Cp in the standard manner for a thin parallel- plate capacitor as: where Cp is the capacitance, A is the area of the plates ( c~n ' )  and d is the distance between the plates (cm).  R A ( 7  was calcu- lated according to definition (Skaar 1949) as : where f is freclucncy ( H z )  of the imping- ing clcctrical field. Copper retentioil was determined gravi- illctrically ancl photoinctrically. Gravimct- rically, the chemical pickup was considercd as the difference in oven-dry weight before anci aftcr chemical treatment. Photornctric clctc~rrninations consistchcl of dissolving the 272 OLAF R. KNEZEVIC AND R.  T. LIN COPPER, % 171~:. 3 .  Effect of copper retention o11 AC re- sistivity at 1 klIz of Ilouglas-fir hcartwood at  20% kfC. O represents the valuc<s of InR.\I- and tl~c. I~ars  represent the extent of thv 95% confi- drnc.v intr~.vnl arolind InK.,,.. ashcd remains in a concentrated mixture of nitric ant1 sulphuric acid and through use, of 13ec.r'~ law (Ewing 1969). hloisturc. content in each sample was calc~ilatecl from the oven-dry weight before and aftcr treating. A statistical analysis was performed to determine which combination of dielectric variables, frequency, and moisture content showed the highest sensitivity to copper sulphate treatment, and to deterrnine whcxther that combination was sufficiently sc~nsitivc. to discriminate between different alnounts of copper sulphate in wood. I11 this analysis, wood density, moisture con- tent, ancl frequency of thc applied clectric ficld were treated as independent variables, 2nd thri elc.ctrica1 propertics wcrc dc- ~xnc1c.nt variables. RESULTS Coppcr rctcntion in trcatcd wood ranged fro111 al,out 1 to 7%, compared to 1.6% copper chrome arscnate ( CCA ) salt found in conlnlercial use (James 1966). The largest dielectric changes attributable to copper sulphatc, treatlncnt occurred at the COPPER,  % Frc. 4. The dielectric constant ( K ) ,  loss tan- gent ( D ) ,  and AC resistivity (Rnl,)  at 100 IIz for Ilonglas-fir heartwood at 7% MC and va r io~~s  copper rcxtentions. Copper retention is based on oven-dry untreated \vc>od. highest moisturc Ievcl (ranging from 15 to 20F ) and lowest frequency ( 100 Hz) .  Chemical treatment and moisture in wood are interacting variables. A first step in statistically separating these vari- ables consisted of finding an acccptal~lc regression model. Thc following model: was tested. Herc, Y is the dependent vari- able such as R A ( , ,  K, or D; Bo is the I' intercept; B, is the slopc of the regression line; and E is an error term. The model's validity was established by an analysis of the rcgrcssion residuals ( Draper and Sinith 1966). By this analysis, Eq. 4 may be a valid   nod el when residuals of thc regres- sion show a normal distribution and arc. uncorrelatecl with MC. The behavior of the residuals met the criteria. Further analysis showed that density when con- sidercd aftcr rnoisture content was not a statistically significant variable. If, how- ever, density was considcrcd bcfore mois- ture content, then density became signifi- cant and accounted for about 30% of the variability in Y. Density being significant in onc instance and not in another indicates density and moisturc content are not sta- tistically independent. As density of wood varies with moisture content, these results arc consistc~nt. Thc)rcxforc, provitled ~nois- DIELECTRICITY IN DOUGLAS-FIR 273 Tp causc changes in RA(q at  100 Hz and RAG 4r R A r c J 7 4 ~ 1 0 7 6 0  at 1 kHz by factors as high as 7 and 5, - 0 L O  0 2 4 6 J 2 O n  C O P P E R ,  % PI(:. 5 .  The dielectric corrstant ( K ) ,  loss tan- gent ( I ) ) ,  and AC resistivity ( R I ~ . )  at 100 IIz of Douglas-fir heartwood at 15% MC and various copper retentions. turcb content changcls, Eq. 4 is statistically valid and physically reasonable. With Eq. 4 as a model, regressions of lily then provided estimates and 95% con- fidence intervals of three variables for estimated moisture contents of 0, 5, and 15%, interpolatcd for 10% and extrapolated to 20% for all five specimen groups before and after chemical treatment. These esti-  nates of InY and 95% confidencc intervals were plotted against moisturc content. Ilihen the confidence intervals of these lcast squares lines do not overlap, 1nY diwrirninatcq between treatments. Thc ad- vantag' of this analysis is that discrimina- tion sc,nsitivity call be seen physicalIy on plots as the distance between confidcncc intclrvals. The greater this distance, the gwatcr is the sensitivity. The highest moisture leveI offcred the best separation of confidence interval and thcrefore sensitivity for all dielectric vari- ables. Loss tangent at 100 Hz and RA,T at 100 and 1 kHz when observed at 20% MC wcrc dielectric variables showing large change because of treatment. These re- sults at  20% R4C are plotted in Figs. 1 through 3. In Fig. 1, copper retention changes the avcrage value of 1nD by 1.6 at 20% h4C and 100 Hz, or D changes by a factor of ahnost 5. Si~nilarly from Figs. 2 and 3, coppcr retention can be seen to - respectively. Despite these appareiltly large changes, in Fig. 2 for example, the confidence in- terval representing specimens with no cop- per overlaps those of specimens with 1.03, 3.5, and 6.6% copper retention and is just separated from specimens with 2.1% cop- per. In Figs. 1 through 3, the vertical distance between confidence intervals, when it exists, is small and therefore we havc poor sensitivity. Even though large changcs exist in the means of the dielcctric properties, for practical purposes, the di- electric methods presented here cannot be used to readily separate specimens con- taining various amounts of copper. As copper retention increases, RAG should decrease. Figures 2 and 3 are consistent with this expectation below copper retention of 1.03%. At higher retentions, RAG increases. This incrcase in RAC seems contrary to the concept that number of potential charge carriers increases as concentration of inor- ganic salt increases and therefore electrical resistivity should decrease. We theorize that lnoisturc in the wood- salt aggregate can be divided into moisture associated with wood and that associated with salt as watcr of hydration. Experi- mentally, we found the water of hydration adds little to the dielectric properties. As treated wood was oven-dried, we assume copper sulphate in wood lost between three and four moles of water. On being exposed to humidity, this salt competes with \vood for available water. Uecausc water in wood above 5 to 6% moisture content (multilayer sorption) is not as tightly held as water that composes the monolayer ( 0  to 6%), wood below 5 to 6% moisture content may, compared to partially anhydrous copper sulphate, pref- erentially adsorb water. TWO modcl reac- tions arc considered, one the takeup of moisture by wood and two, the takeup of moisture by the salt, i.e. Wood (oven-dry) + H20 -+ Wood (H,O) Reaction1 374 OLAF I<. KNEZl\'IC AND R. T. L I S  CuSO, (31 1,O) + 2H,O + CuS0,  (5H,O) Reaction 2. For reactions ono and two to procced to coniplction, watcr needs to diffusc through thc salt as well ac wood. All else being (~qual, th(. salt, being both on thc inside ant1 outsidc of the cell wall, should have thc, grclatcst opportunity to takc up water. Further, it thc kinetics in reactions I and 2 arcb such that both reaction5 go easily to completion, the, change in Gibbs frce cn- erg? may offcr an cstimCite of thc tc>ntlcncy to procc,ctl spontaneously. Undcr isothermal condition\, AGO for rcwtion 1, according to Kclscy ( 1956), is -2.4:3 ant1 Sta~nm (1934) ic  -2.88 ken1 per mole ot water adcor1)ed onto oven-dry wood. The IZG" of wood at 7 and 15% moisture, contcnt adsorbing water, drops to al)out -0.36 and -0.036 kcal/molc (Stanml 1934). Thc handbook value for th(. change in Gibbs frce energy for rcac- tion 2 i5 -7.43' kcal per molt. of watcr t'tkcn up. Therefore, from a strictly ther- moclynainical point of view, both reactions arc) expected to proceed spontaneously, but reaction 2 would have a greater tendency to procced. \T7atcr a\railable for sorption in wood I~eyond the moriolayer, having AG even Icsc ~~cgat ivc  than that for the monolayc-r, could be even more likely to be taken up by the C>uSO,(3N,O) as water of hydra- tion. As a rcsult, the actual moisture con- tent of \!rood as moisture increases beyond 7% could 1)~ .  substantially lcss than the calculated or apparcnt moisturc~ contcnt. The greater the amount of salt in mood, the lower thc nioisturc~ contctnt of thc wood substances. The lower the moisture con- tent thc highcr thc resistivity, all clsc cyual. Thrrctore, AC resistivity apparently may increase as degree of treatment in- crcasos beyond a critical level. This critical level ic the level of treat- ment at  which the incrc,asc in chargc car- '111 th15 elipe~inlcnt, ~cactlon 2 cor~ld ju\t a\ well ha\? been with C I I S ~ ~ ( I I - O ) .  IIerc, AGO would ht, -11.0 !,~al/mole Tl~et~fore,  reaction 2, as dlonn In teut, gives a mlnlmunl value f o ~  pc1. ricw is offsct bv a clecreasc in water as- sociated with wood, and the net result is an increase in ~eesistance rather than the cxpected decrease. This critical level will vary with moisture content. At lower moisture contents, for example 7% as in Fig. 4, Rzc. decreases with treatment and no critical retention is evident. As thc moisturcl contcwt incrc,asc.s to 15% (Fig. 5) ,  hotvever, R A P  decreases with retentions up to 1.03%. Beyond this critical level R A r :  incrcascs. We bclieve the amount of water associated with wood at higher moisture levels diminishes as copper retention in- creases. Therefore, at  high salt concentra- tions the expccted decrease in R A c  because of increase in salt ( charge carriers) could be cancellcd bv loss of mobilc water, with the net result of an unexpected increase in 11.4~. CONCLUSIONS The AC resistivity of Douglas-fir heart- wood at 20% rnoisturc content when mea- sured at 100 Hz is the combination of cliclcctric property and frcquency that shows the greatest response to changes in copper sulphate concentration. Even these changes in AC resistivity, however, arc insufficient to serve as a quantitative basis for nondestructively segregating heartwood sasnplcs with copper retentions ranging from 0 to 7%. REFERENCES ~ > I < A I > E ~ ,  N. R., AN11 I i .  S~XITH. 1966. Applied regression analysis. John Wiley Sons, Inc., Ne\v York. 307 pp. E\vt~c:, G. W. 1969. Instnnnental inethods of chen~ical analysis. McGraw-Hill, New York. 627 pp. JAIIES, W. L. 1966. Effects of wood prcserva- tivcs on electric moisture meter readings. U.S. Ucp. Agric., For. Sew., For. Prod. Lab., Res. Note FPL-0106. Madison, WI. Rev. A ~ ~ g u s t  1966. 12 pp. KELSEY, K. E., AND L. N. CLARK. 1956. The, heat of sorption of water by wood. Aust. J. Appl. Sci. 7 ( 2 )  : 160-175. SKAAR, C. 1949. The dielectric properties of \oood at several radio frequencies. N.Y. State Coll. For., Tech. Publ. No. 69. 36 pp. STAIM, A. J., AND W. K. LAUGHBOROUGH. 1934. Themlodynamics of the swelling of wood. J. Phys. Chcm. 39( 1 )  :121-132. 

Copper In Douglas-Fir and Associated Dielectric Changes

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Copper In Douglas-Fir and Associated Dielectric Changes

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Wood and Fiber Science (E-Journal)