U. S. FOREST SERVICERESEARCH PAPERFPL 60MAY 1966U N I T E D S T A T E S D E P A R T M E N T O F A G R I C U L T U R EFOREST SERVICEF O R E S T P R O D U C T S L A B O R A T O R YMADISON. WISCONSIN

SORPTIONand SWELLINGCHARACTERISTICSof SALT-TREATEDWOOD

ABSTRACTSeven inorganic salts, commonly used in the

impregnation of wood, were investigated in termsof their effect on the sorption and swelling char-acteristics of ponderosa pine and red oak in arange of relative humidities normally encounteredin service.

Swelling was, without exception, greater inchemically treated wood than in matching speci-mens of untreated wood. Only sodium chlorideshowed an appreciable effect on the dimensionalstability of the two species as measured by volu-metric shrinkage and swelling. However, treat-ment with the various chemicals increased theanisotropic swelling characteristics of bothspecies. Equilibrium moisture content valueswere substantially increased by treatments withammonium sulfate, sodium chloride, and zincchloride. For these salts, equilibrium moisturecontent was a consistent linear function of thesalt-wood weight ratio. Treatment with two phos-phate salts had no apparent effect on equilibriummoisture content, while treatment with borax andboric acid caused increases in moisture contentthat are not considered permanent.

SORPTION and SWELLINGCHARACTERISTICS

of SALT-TREATED WOOD1�

by B. A. Bendtsen, Forest Products Technologist

FOREST PRODUCTS LABORATORY2�FOREST SERVICE

U. S. DEPARTMENT OF AGRICULTURE

INTRODUCTIONIn recent years the demand for chemically

impregnated wood has increased significantly asbuilding code requirements, building specifica-tions, promotion by the manufacturers of chemi-cals, and better premium rates offered by insur-ance companies have encouraged wider use ofsuch treatment. These treatments are commonlyformulated from inorganic salts. There is someconcern about the use of these treatments, sinceit is known that certain inorganic salts or com-binations of salts may affect the strength andrelated properties of wood. Some of the saltsthemselves are hygroscopic and in the presenceof wood may increase its apparent moisture con-tent, An abnormal amount of swelling may alsoaccompany these treatments because of increasedmoisture content. Furthermore, increases in thedry dimensions may be expected due to chemicalbulking or restraint of shrinkage upon drying(2, 3, 4, 8).3�

Wood moisture-swe11ing relationships fre-quently assume great importance. especially instructural uses of wood. All strength properties,with the possible exception of toughness, decreaseas wood adsorbs moisture in the hygroscopic

range. Important properties such as modulus ofrupture and compressive strength parallel to grammay decrease up to 4 and 6 percent, respectively,for each 1 percent increase in moisture content(9). In addition to this intimate moisture-strengthrelationship, the integrity of a structural assemblysuch as a truss may be affected by the dimensionalstability of its components. The ability to �stayin place,� a term frequently applied to wood havinggood dimensional stability, is also important inboth interior and exterior wall coverings made ofwood, in millwork, and for all other decorativepurposes. Increased hygroscopicity may alsoinfluence the gluability, paintability, and the per-manence of the preservatives themselves.

This research evaluates the effects of severalinorganic salts, commonly used in formulatingproprietary treatments, on certain physical prop-erties of wood, particularly upon the sorptionand swelling characteristics. It must be empha-sized that the research deals with individualchemical effects only. The interaction effects ofseveral chemicals in a single formulation werenot studied. Therefore, the results of the studyare not meant to imply that treatment with pro-prietary formulations will have similar effects orthat the effects of a formulation can be easily

1�T h i s r e s e a r c h w a s p e r f o r m e d i n c o o p e r a t i o n w i t h t h e N a t i o n a l F o r e s t P r o d u c t s A s s o c i a t i o n , t h eA m e r i c a n W o o d P r e s e r v e r s I n s t i t u t e , a n d t h e A m e r i c a n I n s t i t u t e o f T i m b e r C o n s t r u c t i o n .

2� Maintained at Madison, Wis., in cooperation with the University of Wisconsin.3� Underlined numbers in parentheses refer to Literature Cited at the end of this report.

predicted based upon a knowledge of its compo-nents. However, it is hoped that further researchwill substantiate how such predictions can bemade.

Specifically, the study encompasses the sorptionand swelling characteristics of ponderosa pineand red oak when treated with seven inorganicsalts--diammonium phosphate, monoammoniumphosphate, borax, boric acid, ammonium sulfate,sodium chloride, and zinc chloride. A typicalhardwood and a typical softwood were selectedfor their treatability and because they differedsubstantially in specific gravity. Chemical treat-ments were made to three levels of retention,0.5, 1.5, and 4.5 pounds per cubic foot. Water-treated controls were also prepared; these hadno salt content. Observations of weight and dimen-sion changes were limited to the 30 to 80 percentrelative humidity range, since it is this part ofthe overall hygroscopic range that is commonlyencountered by wood in service.

PROCEDURE

Wafers, 1/4 inch along the grain, were cutfrom a ponderosa pine and a red oak stick approxi-mately 2 by 2 inches in cross section and 10 feetlong. In order that true tangential and radialcharacteristics might be measured, the stickswere machined so that one face was tangent to theannual rings. The oak specimens were approxi-mately 2 by 2 inches in cross section, while thepine specimens were approximately 1-1/2 by1-1/2 inches. Ten replicate specimens wereprovided for each combination of the seven chemi-cals, the three nominal levels of salt retention,and the water-treated controls for a total of280 specimens of each species. They werearranged and identified in an end-matching schemethat permits principal comparison between chemi-cals and secondary comparison between retentionlevels. All matching was good, however, as onecomplement of all chemicals and retention levelsrequired only about 9 inches along the grain. Atest for growth stresses was conducted by measur-ing the transverse dimensions of several extraspecimens before and after heating in water. Nochange in the relative radial and tangential dimen-sions during heating indicated a freedom fromsuch stresses.

Before treatment, the specimens were care-fully air dried to approximately 6 percent mois-

ture content and then further dried under vacuumand slightly elevated temperatures to essentiallyovendry condition. This technique was selected inpreference to ovendrying to minimize the effectof drying on subsequent measurements for determining sorption and swelling characteristics.Measurements of weight and dimensions in thedry condition before treating were used as abasefor computing all moisture content, swelling, andspecific gravity results.

Treating was also done in such a manner tominimize processing effects on the sorption andswelling characteristics. Salt was impregnatedinto the wood by initially pulling a vacuum overthe specimens, followed by soaking at atmosphericpressure after solution flow-in, Slightly elevatedtemperatures were required for the borax andboric acid treatments because of their low watersolubility. The concentration of treating solutionsrequired to obtain the nominal retention levelswas computed from solution pickup data obtainedduring the processing of the water-treated con-trols.

After treatment, the specimens were cycledthrough an initial desorption cycle, an adsorptioncycle, and a final desorption cycle. The cycleswere limited to the 30 to 80 percent relativehumidity range. During each cycle, measurementsof weight and dimensions were made on eachspecimen while at equilibrium moisture conditionin atmospheres controlled at 80° F. and 80, 65.50, and 30 percent relative humidity. Followingthe initial desorption cycle, the specimens werepartially air dried over vacuum, so that equili-brium would be approached during the adsorptioncycle at 30 percent relative humidity.

RESULTS

Salt Retention

In astudy of this nature, it is essential that theamount of salt in a specimen be determined asaccurately as possible. If the amount of salt isoverestimated, equilibrium moisture content willbe too low; if underestimated, the equilibriummoisture content will be too high. Furthermore,any attempt to establish relationships betweenretention levels and the effects of salts on physi-cal properties of wood will be in error if the saltretentions themselves are inaccurate.

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It is common to calculate salt retentions byusing solution pickup data and the concentrationof the treating solutions. However, selectiveadsorption of either the chemical or the watercan cause an error in this estimate. Anothermethod is the ovendrying technique where thedifference between the dry weights before andafter treating is interpreted as being the weightof the salt present. Some salts, however, are notstable at temperatures required to drive off allthe water and, consequently, retentions deter-mined by this method are too low. In borax,another similar source of error in the dryingtechnique would be that associated with the lossof an indeterminate number of molecules of waterof crystallization. Another alternative is chemicalanalysis. Appendix A shows the results of esti-mates of salt retention by each of these methods:the method of chemical analysis for each chemi-cal is also given. The results obtained by chemi-cal analysis were selected as being most accu-rate, and all of the results in this report arebased upon retentions obtained by this method.

Effect of Salts on Moisture Content

In a study of this type, moisture content isoften expressed as a percent of the combined dryweight of the salt and wood. However, in thisresearch, moisture content is based upon theovendry weight of the untreated wood. This per-mits better comparisons between chemical effectsand between treated specimens and controls,because the base for computing moisture is con-sistent in all cases. The equation for computingmoisture must, however, consider the salt in thespecimen and is:

(1)

where M = moisture content in percentWT= weight of specimen (grams) at the

equilibrium moisture content underconsideration

WS = weight (grams) of salt in the specimen

WD

= untreated dry weight (grams) of thespecimen

Moisture content values computed by this meth-

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od for treated oak and pine specimens and forwater-treated controls are presented in Appen-dixes B and C respectively.

Trends such as apparent increases in moisturecontent due to certain salts, increasing moisturecontent with increasing salt retention, and themore pronounced effects in pine than in oak arereadily seen by examination of the data in Appen-dixes B and C. However, because the data arevoluminous, graphic analysis has been used toelucidate the trends.

Initial desorption--Figures 1 and 2 show theinitial desorption curves for treated oak and pine.respectively, and for the water-treated controlsfor each chemical. These curves are plots ofdata obtained upon drying from the saturatedcondition after treatment.

The effect of the seven chemicals is varied inmagnitude and nature, yet all the curves retainthe typical sigmoid shape which describes thenormal relationship between moisture content andrelative humidity. These normal relationships arerepresented here by the curves for the water-treated controls.

It appears from these curves that the chemicalscan be divided into two groups based upon theirapparent hygroscopic effect. One group, repre-sented by diammonium phosphate and monoam-monium phosphate, is characterized by little over-all effect on moisture content but typically showsa slight decrease in moisture content at 30 percentrelative humidity, which grades to a slight in-crease at 80 percent relative humidity. This effectincreases with increasing salt retention.

The other group, which includes borax, boricacid, ammonium sulfate, zinc chloride, and sodiumchloride, shows a characteristic increase in mois-ture content throughout the 30 to 80 percentrelative humidity range considered: this effectalso increases with increasing salt retention.

There are several exceptions to the abovegrouping. The borax treatments of oak show effectssimilar to those chemicals in the first group,while the 4.5-pound diammonium phosphate treat-ment of pine shows a considerable decrease inmoisture content throughout the humidity range.This apparent decrease in moisture content mayreflect a loss of ammonium from the chemicalrather than an actual decrease in moisture content,as there is a tendency for this chemical to decom-pose into monoammonium phosphate, particularlyat high relative humidity. Also, the 0.5-poundtreatments of boric acid, ammonium sulfate, and

MOlSTURE CONTENT (PERCENT)

F i g u r e 1 . - - O a k : I n i t i a l d e s o r p t i o n c u r v e s f o r s a l t - t r e a t e d w o o d a n d f o r t h e w a t e r - t r e a t e dc o n t r o I s .

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F i g u r e 2 . - - P i n e : I n i t i a l d e s o r p t i o n c u r v e s f o r s a l t - t r e a t e d w o o d a n d f o r t h e w a t e r - t r e a t e dc o n t r o l s .

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Figure 3. --Oak: Sorption curves for untreated wood and wood treated to a nominal salt retentionof 4 .5 pounds per cubic foot .

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F igure 4 . - -P ine: Sorpt ion curves for unt reated wood and wood t reated to a nominal sa l t retent ionof 4 .5 pounds per cubic foot .

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zinc chloride in oak showed little or no effect;in fact, at certain humidities the moisture contentactually decreased at this level of salt retention

The effects observed on moisture content at alllevels of treatment are generally greater in pinethan in the corresponding treatments in oak. Onewould expect these effects to be more prominentin pine because there is approximately twice asmuch salt in the pine as in the oak on a weightbasis (weight of salt per unit weight of wood).This effect will be discussed later in this paperwhere it is shown that the difference in moisturecontent between the two species is proportionalto the difference in specific gravity.

It is interesting to note that in both groups,regardless of whether the chemical effect resultsin an increase or decrease in the moisture con-tent, the change of moisture content from 30 to80 percent relative humidity was greater in allchemically treated specimens than in the water-treated controls. This, in effect, means that theslope of the curves of treated material is probablyeverywhere greater than that of untreated materi-al. This is important from the standpoint ofstrength properties, because it means thatstrength properties of treated material will de-crease more rapidly than those of untreatedmaterial as relative humidity increases: thestrength-moisture relationship for treated woodis assumed to be similar to that for untreatedwood. Dimensional stability may also be affected.

Adsorption-desorption cycle.--When wood isdried in successive steps from an initial green orsaturated condition, the equilibrium moisture con-tent measured will be higher than during anysubsequent desorption or adsorption cycle at thesame atmospheric conditions. Repeateddesorption-adsorption cycling results in sorptioncurves forming closed hysteresis loops whichessentially coincide except for the first desorp-tion curve (1).

In service, when seasonal atmospheric changesoccur, small hysteresis loops will be formed ifthe moisture content of the wood is plotted as afunction of the relative humidity of the atmosphere.The location of the small loops relative to themain hysteresis loop will depend upon the pre-vious exposure of the particular piece involved.If the piece was placed in service at a moisturecontent above or near that of the equilibriummoisture conditions of the atmosphere, the smallloops would be formed near the desorption side

of the main hysteresis loop. lf placed in serviceat a moisture content substantially below theequilibrium moisture content of the atmosphere,they will be formed near the adsorption side ofthe main hysteresis loop (5). For these reasons,it is important that subsequent adsorption anddesorption cycles be considered in addition tothe initial desorption cycle already discussed.

Figures 3 and 4 show sorption curves, whichinclude the initial desorption cycle, a subsequentadsorption cycle, and a final desorption cycle,for oak and pine treated to the nominal 4.5-poundretention level with the various chemicals con-sidered. Similar curves for water-treated con-trols are shown in the upper left-hand corner ofeach figure.

Except for the borax and boric acid treatments,the curves for treated material, in general, re-semble those of the water-treated controls. How-ever, a displacement of the curves on the moisturecontent axis, which reflects the hygroscopicity orthe nonhygroscopicity of the chemical, and anincrease in the slope of the curves are stillevident. Essentially no difference is apparent ineither the oak or pine curves representing treat-ments with the phosphate compounds. The ammo-nium sulfate and sodium chloride curves resemblethe shape of the control curves in the 30 to 65 per-cent relative humidity range but thereafter deviateupward quite abruptly. The zinc chloride curvesfor oak are similar to the controls, but in pinethe hysteresis effect was somewhat reduced,particularly at low levels of humidity.

Perhaps of most significance in the sorptioncurves is that they indicate whether the effectsnoted are of a permanent nature. It is reasonablycertain that additional desorption-adsorption cy-cling with diammonium phosphate, monoammo-nium phosphate, zinc chloride, ammonium sulfate,and sodium chloride would result in curves coin-cidental with the adsorption and final desorptioncurves already obtained. This premise is basedon the observation that the second desorption to30 percent relative humidity yields essentiallythe same equilibrium moisture content as thefirst desorption

The sorption curves for boric acid and borax-treated pine, however, differ considerably fromthose of the other chemicals and from the con-trols. A much larger hysteresis effect occurredbetween the initial desorption and the adsorptioncurve and, in addition, the moisture content

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obtained in the final desorption cycle at lowrelative humidity was lower than that previouslyobtained under adsorbing conditions. It appearsthat the moisture data were obtained before thespecimens reached true equilibrium; however,check weights before actual measurement at eachcondition indicated that the specimens were oscil-lating in weight with fluctuating control conditions.This does not necessarily establish that equili-brium was reached, as it is possible that thegross weight of the wood and chemical couldoscillate with the control conditions due to thehygroscopicity of the wood, masking a much slowerloss of water from the chemical itself. This isunlikely, however, since aqueous solutions of boricacid quickly became anhydrous when placed inatmospheres in the 30 to 80 percent relativehumidity range. Therefore, repeated cycling wouldprobably show a continued trend toward a lowermoisture content which would eventually resultin coincidental sorption curves, and the equili-brium moisture content would be comparable,or nearly so, to that of untreated wood or to woodtreated with chemicals of the nonhygroscopicgroup.

After the completion of the final desorptioncycle, the borax- and boric acid-treated pinespecimens were ovendried and again exposedsuccessively to atmospheres of 80° F. and 30, 50,65, and 80 percent relative humidity. The dataobtained offer further evidence that the increasedmoisture content shown previously for borax andboric acid is not a permanent effect, because theequilibrium moisture content was approximatelyequal to or less than that for untreated wood.These data are included in the graphs for boraxand boric acid in figures 3 and 4. The loss of thetypical sigmoid shape of the curves for the boraxtreatments perhaps reflects a regaining of thewater of hydration to the borax molecule lostduring ovendrying.

It is also recognized that exposing wood toelevated temperatures may reduce its hygro-scopicity by converting the hygroscopic hemi-cellulose to polymers that are much less hygro-scopic (7). The rate at which these reactionsoccur is greatly accelerated by certain inorganicsalts. The time required to ovendry these speci-mens was sufficient for a reaction of this natureto occur, and undoubtedly accounts for a part ofthe reduced equilibrium moisture content ob-tained after ovendrying.

9

Because of these considerations, boric acidwill not be considered in further discussionregarding hygroscopic effects. The same rea-soning can be applied to the 4.5-pound boraxtreatment of pine. Regarding permanent effectson moisture content, boric acid and borax shouldtherefore be grouped along with diammoniumphosphate and monoammonium phosphate.

Relative chemical hygroscopicity.--A compari-son of the relative hygroscopicity of wood treatedto the same nominal retention level with ammo-nium sulfate, zinc chloride, or sodium chloride,is not realistic because in certain cases theactual retention level varies considerably fromthat of the nominal level. However, comparisonscan be made by using figure 5, which shows therelationship between salt retention and the mois-ture content of pine and oak when treated withthe hygroscopic chemicals. Selecting a commonsalt retention from these plots for any species-chemical-humidity combination and reading thecorresponding moisture content permits directcomparisons of the relative hygroscopicity of thethree chemicals.

It is readily apparent that wood treated withsodium chloride is considerably more hygroscopicat any level of humidity and salt retention thanwood treated with either zinc chloride or ammo-nium sulfate. For instance, sodium chloride-treated pine has a moisture content of 42 percentand similarly treated oak has a moisture contentof 26 percent at the same retention level of 2.5pounds per cubic foot and 80 percent relativehumidity. For the same conditions the moisturecontent of zinc chloride-treated pine is 23 per-cent and of oak is 17 percent. The ammoniumsulfate treatment results in a 31 and 20 percentmoisture content for pine and oak respectively.However, at other levels of humidity, zinc chlor-ide and ammonium sulfate produce nearly thesame moisture content conditions.

Probably most obvious in figure 5 is the muchgreater effect of the three chemicals on the mois-ture content of pine than on oak. However, thisdoes not necessarily indicate a greater chemicaleffect, since absolute increases in water wouldreflect a larger increase in pine than in oak byan amount in proportion to the difference in theirspecific gravity. By definition, the water in salt-treated wood may be considered in two parts--that representing the normal equilibrium mois-ture content of untreated wood, and the additional

F igure 5 . - - Re lat ionsh ip between sa l t retent ion and mois ture content of p ine and oak at severa llevels of re lat ive humidi ty when t reated wi th three hygroscopic chemicals .

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moisture due to the influence of the salt. Symboli-cally,

(2)

where Ww,u = weight of water at equilibrium foruntreated wood,

Ww,s = weight of water associated withthe salt.

Other symbols are as previously defined.It is recognized that, at a given temperature

and relative humidity, the equflibrium moisturecontent is essentially constant if the wood isrelatively free of extractives (9). That is, theweight of adsorbed water is proportional to theweight of the wood or

(3)

where 4 is the proportionality constant. If equa-tion (3) is true for wood (a complex hygroscopicmaterial), it is reasonable to suppose that ananalogous relationship exists for any of the salts(relatively simple hygroscopic chemicals) con-sidered in this study when they are in the pres-ence of wood. Then

(4)

where B is the proportionality constant. Equa-tions (3) and (4) may be substituted into (2) toeliminate Ww,u andcomes

Ww,s and the equation be-

(5)

where or the ratio of the weight of salt

to the weight of dry wood.For those concerned with the use of these

chemicals in treating solutions, equation (5) canbe used for predicting an increase in moisturecontent due to treatment to any level of retention,regardless of the species treated. To test thevalidity of equation (5), M was plotted as a functionof x for ammonium sulfate, zinc chloride, andsodium chloride (figures 6, 7, and 8). The plotsare practically independent of the two speciesevaluated, and appear to be essentially straight

lines for salt-wood weight ratios up to 0.10.Since proprietary solutions are always made ofcombinations of salts, it is doubtful if individualsalt retentions would ever exceed this value, asa salt-wood weight ratio of 0.10 is approximatelyequivalent to a retention of 4.0 and 2.0 poundsper cubic foot in the oak and pine, respectively,used in this study.

The dashed lines in figures 6, 7, and 8 arefreehand curves fitted to these data Curvilinearrelationships are indicated by these curves forall chemicals at the 80 percent relative humiditylevel with the slope of the curve increasing withincreasing salt-wood weight ratio. At lowerhumidities, a similar effect was noted for zincchloride, although the rate of increase appears todecrease as relative humidity increases. At lowerhumidities in sodium chloride-treated materialthe relationships were also curvilinear but therate of increase in moisture decreased with anincreasing salt-wood weight ratio. The data forammonium sulfate at lower humidities, however,appear to fit a straight line.

The solid lines are linear regressions of mois-ture content on the salt-wood weight ratio in the0 to 0.10 range. Individual data were used in theregression analysis, while the plotted pointsrepresent the averages of 10 values.

Table 1 gives the regression coefficients (A andB), coefficients of correlation, and the number ofspecimens for each of the regression analyses.Correlations are extremely good, ranging from0.88 for material treated with zinc chloride andexposed to 30 percent relative humidity condi-tions to 0.99 for sodium chloride at both the30 and 80 percent relative humidity exposures.These correlations indicate the validity of equa-tion (5).

The use of the regression results to predictincreases in moisture due to treatment might bemore accurate if the equilibrium moisture con-tent of the species or preferably of the untreatedmaterial involved were substituted in place ofthe regression coefficient A in the equation.This would be particularly true in species suchas redwood, in which the equilibrium moisturecontent values differ substantially from those ofa wood containing a more normal amount ofextractives or from those of the species involvedin these regression analyses (6, pp. 155-158).In figures 6, 7, and 8 the equilibrium moisturecontent of oak is almost consistently lower than

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F i g u r e 6 . - -Re lat ionsh ip between mois ture content and the sa l t -wood weight rat io for ammoniumsu l fate. So l id symbols represent oak and the open symbols represent p ine.

that of pine at comparable levels of the salt-wood weight ratio. This difference is probablycharacteristic of the particular material involvedin the study rather than a species characteristic.

As indicated by the regression coefficient B,the slope of the regression line, the hygroscopicityof sodium chloride-treated material is consider-ably greater at all levels of humidity than thattreated with either ammonium sulfate or zincchloride. At the 80 percent level of humidity,ammonium sulfate-treated material was approxi-mately twice as hygroscopic as that treated withzinc chloride, whereas at lower humidities thereis little difference between the two. Withoutexception. the hygroscopicity of the treated mate-

rial increased with each increase in relativehumidity.

Interaction of wood-chemical hygroscopicity.--From equation (4), B is the hygroscopicity of thesalt considered by itself, when in the presenceof wood. It is of interest to know if this is anydifferent from the hygroscopicity of the free salt(not in wood); i.e., can the superposition of effectsimplied by equation (2) be treated as independenteffects of wood and salt, or does each mutuallyaffect the hygroscopicity of the other? To answerthis question, increases in weight by the additionof water were observed when anhydrous ammo-nium sulfate, zinc chloride, and sodium chloridewere exposed to an atmosphere controlled at

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F igure 7 . - - Re lat ionsh ip between mois ture content and the sa l t -wood weight rat io for z incchlor ide. So l id symbols represent oak and the open symbols represent p ine.

80° F. and 80 percent relative humidity. Thehygroscopicities of the three chemicals defined byequation (4), but now not in wood, were deter-mined to be 1.350 for ammonium sulfate, 1.700for zinc chloride, and 3.454 for sodium chloride.Note that ammonium sulfate and zinc chlorideare reversed from the ranking found in materialtreated with the three chemicals as indicated bythe regression coefficient B for 80 percentrelative humidity in table 1. These hygroscopi-cities for the salts, and the hygroscopicities foruntreated wood from Appendixes B and C weresubstituted into equation (5) to give predictedmoisture content values for specimens exposedunder adsorbing conditions at the high relativehumidity. The predicted moisture content is com-pared with measured moisture content in table 2.

The amount by which the estimated moisturecontent differs from the measured moisture con-tent is indicated in column (12) of table 2. A

negative value indicates that the estimated mois-ture content is greater than the measured, whilepositive values indicate that the actual moisturecontent is higher.

The results for the two species parallel eachother reasonably well. Where differences do exist,they can probably be attributed to the differencein specific gravity, and to the inability to exactlydetermine salt content in the wood. A thirdpossibility for a difference is the tacit assumptionthat the hygroscopic properties of a salt in woodare the same as those of the pure, separatecrystal. Actually, the salt in the cell wall isextremely subdivided and one would expect asomewhat different hygroscopic property. Thedifferences between comparable values for pineand oak in column (8) of table 2 is, in part, anindex of the magnitude of the specific gravityeffect. With two exceptions, the nominal 4.5-poundretention of ammonium sulfate- and sodium

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F igure 8 . - -Re lat ionsh ip between mois ture content and the sa l t -wood weight rat io for sodiumchlor ide. So l id symbols represent oak and the open symbols represent p ine.

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F i g u r e 9 . - - O a k : V o l u m e t r i c s w e l l i n g - r e l a t i v e h u m i d i t y r e l a t i o n s h i p s f o r s a l t - t r e a t e d w o o d a n df o r u n t r e a t e d c o n t r o l s . S w e l l i n g i s b a s e d o n u n t r e a t e d d r y v o l u m e . E x c e p t w h e r e i n d i c a t e dt h e f o u r c u r v e s i n e a c h g r o u p r e p r e s e n t , f r o m t o p t o b o t t o m , n o m i n a l s a l t r e t e n t i o n s o f4 . 5 , 1 . 5 , a n d 0 . 5 p o u n d s p e r c u b i c f o o t , a n d t h e w a t e r - t r e a t e d c o n t r o l s .

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F i g u r e 1 0 . - - P i n e : V o l u m e t r i c s w e l l i n g - r e l a t i v e h u m i d i t y r e l a t i o n s h i p s f o r s a l t - t r e a t e d w o o d a n dfor unt reated cont ro l s . Swel l ing i s based on unt reated dry vo lume. Except where ind icatedthe four curves in each group represent , f rom top to bottom, nominal sa l t retent ions of 4 .5 ,1 .5 , and 0 .5 pounds per cubic foot , and the water - t reated cont ro I s .

17

F i g u r e 1 1 . - - O a k : V o l u m e t r i c s w e l l i n g - r e l a t i v e h u m i d i t y r e l a t i o n s h i p s f o r w o o d t r e a t e d t o an o m i n a l r e t e n t i o n o f 4 . 5 p o u n d s p e r c u b i c f o o t . S w e l l i n g i s b a s e d o n u n t r e a t e d d r y v o l u m e .

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F i g u r e 1 2 . - - P i n e : V o l u m e t r i c s w e l l i n g - r e l a t i v e h u m i d i t y r e l a t i o n s h i p s f o r w o o d t r e a t e d t o anominal retent ion of 4 .5 pounds per cubic foot . Swel l ing i s based on unt reated dry vo lume.

19

chloride-treated pine, the estimated moisturecontent is greater than the measured moisturecontent. These negative interactions perhaps sug-gest a mutual sharing of water by the wood andchemical or that the chemical reacts with cellu-lose to satisfy a portion of the adsorption sites.This mutual sharing or chemical reaction effectwas much greater in zinc chloride at the 4.5-poundretention than either sodium chloride or ammo-nium sulfate. In the 4.5-pound zinc chloride treat-ment of pine it appears that there was mutualsharing of water or chemical satisfaction of allthe wood sorption sites since the interaction effectwas greater than the normal equilibrium moisturecontent of untreated wood. In sodium chloride andammonium sulfate the effect appeared to maxi-mize at or near the 1.5-pound retention. There-after it remained relatively the same in ammo-nium sulfate but decreased sharply in sodiumchloride to the 4.5-pound retention.

Based upon the individual hygroscopicity of thechemical and the wood, certain generalizationscan be made about the predicted equilibriummoisture of wood treated with the hygroscopicchemicals. In an atmosphere controlled at 80 per-cent relative humidity, ammonium sulfate-treatedmaterial will equilibrate at approximately themoisture content expected; zinc chloride-treatedmaterial will be somewhat lower than expected;and at high levels of retention sodium chloride-treated material will be approximately that esti-mated but perhaps somewhat less than expectedat low retention levels.

Effect of Salts on Volumetric Swelling

Volumetric swelling data are presented inAppendixes D and E for oak and pine, respec-tively. Included are data collected during an initialdesorption cycle and subsequent adsorption anddesorption cycles for material treated with sevenchemicals to three levels of retention, and thewater-treated controls. Swelling in all cases hasbeen based upon the untreated dry dimensionsand includes swelling due to moisture and due tochemical bulking or the restraint of shrinkageupon drying.

Volumetric swelling versus relative humidity.--Figures 9 and 10 show the relationship betweenvolumetric swelling and relative humidity over the30 to 80 percent relative humidity range evaluatedfor treated oak and pine and the water-treated

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controls. These curves represent data obtainedunder adsorbing conditions.

Increased swelling occurred in all treatedspecimens, and this effect generally increasedwith increasing salt retention. The swelling isconsiderably greater in specimens treated withthe three chemicals considered as having a per-manent hygroscopic effect-ammonium sulfate,sodium chloride and zinc chloride--than in thosetreated with the nonhygroscopic chemicals. Thisreflects the fact that the swelling by the hygro-scopic group is due to the combined effect ofincreased moisture content and to chemicalbulking. Swelling by the nonhygroscopic group islargely a bulking effect only, with perhaps a slighteffect due to increased moisture. In sodiumchloride the increase in swelling due to treatmentis minimum at the 30 percent humidity level andincreases uniformly to a maximum at the 80 per-cent level. In all other treatments the increasedswelling appears to be constant throughout thehumidity range considered. In some instances,swelling reaches a maximum at or near the 1.5-pound retention level, with little additionalswelling at the 4.5-pound level. In the 4.5-poundmonoammonium phosphate treatment of pine andthe boric acid treatment of oak the swelling wasnot as great as in the 1.5-pound treatments.

Figures 11 and 12 show volumetric swelling-relative humidity relationships for salt-treatedand water-treated controls for the initial desorp-tion cycle and subsequent adsorption and desorp-tion cycles. With a few exceptions, the curvesfor treated material closely resemble those ofthe controls. The most obvious differences occurwith the hygroscopic chemicals. In the sodiumchloride-treated oak and pine, swelling wasgreater during the second desorption cycle thanin the first, which is contrary to that of allother treated material, including the water-treated controls. Furthermore, some of the curvesfor the hygroscopic chemicals do not have thecharacteristic sigmoid shape. However, this isprobably because at some point below 80 percentrelative humidity the specimens reached the fibersaturation point because of the hygroscopicity ofthe chemical, and thereafter the swelling did notincrease as the relative humidity increased to80 percent. Although additional data points wouldbe required between 65 and 80 percent relativehumidity to show whether or not this is true, itseems very probable because the saturationvapor pressure of sodium chloride is about 75 per-

20

cent relative humidity.It is interesting to note that essentially no

hysteresis effect occurred in zinc chloride-treated pine: practically the same swelling valueswere obtained during both desorption cycles andthe adsorption cycle.

These figures are particularly interesting whenconsidered in retrospect with corresponding fig-ures 3 and 4 that show the relationship betweenmoisture content and relative humidity. Highequilibrium moisture content values were obtainedfor boric acid-treated pine and oak and borax-treated pine during the initial desorption cycle,but the effect was not considered permanent be-cause equilibrium moisture content values ob-tained during a later adsorption-desorption cyclewere considerably lower. Generally, it seems theswelling curves would show an effect comparableto that of the moisture content-relative humiditycurves. However, the swelling curves for boraxand boric acid generally show an effect similarto that of other treated material and to the water-treated controls. Only the borax treatment of oakshows an effect comparable to the moisturecontent curves. This inconsistency indicates thatswelling was probably internal and progressedinto the cell cavity.

Chemical bulking.--Figures 13 and 14 show themoisture-swelling relationships for treated oakand pine obtained under adsorbing conditions.The swelling of treated material is greater thanthat of untreated material at any selected mois-ture content because of chemical bulking of thefibers. However, in those chemicals which arehygroscopic, this does not necessarily reflectall the bulking that occurred. It is possible withthese chemicals that water normally in the cellwall for untreated wood is associated with saltin the cell cavities. If the water is held by thesalt it will not cause the increase in volume ofthe wood normally associated with such a mois-ture content. If this is true, the bulking effect isgreater than that shown by these curves.

The bulking effect characteristically increasedwith increasing retention to the l.5-pound levelin all but the boric acid treatment of pine. Whenincreasing the retentions to the 4.5-pound level,however, several species-chemical combinationsshowed no further increase. In fact, in some in-stances a negative bulking is apparent at the4.5-pound 1evel. This negative bulking probablyreflects the effect discussed in the preceding

paragraph regarding the location of salt in thecell cavities. It is also possible that the cell isin a fully distended condition and additionalswelling may progress internally into the cellcavity.

The hygroscopic character of some of the saltsmakes them a poor means of dimensionally stabl-lizing wood; additional swelling is caused byadditional moisture adsorbed by the salt. How-ever, even if the salts were not hygroscopic, atthese levels of treatment, one would not expectmuch stabilization because of the relatively highdensity of the salts. Consider, for instance, a20 percent treatment, that is, 20 grams of saltper 100 grams of wood. If the treatment is withmaterial with a density of unity, bulking amountsto 20 cubic centimeters. If the treatment is witha material of a density of 2.0 or 3.0, only 10 or7 cubic centimeters of bulking results.

The uniformity of the slopes of the moisture-swelling curves for treated material and for thoseof the controls shown in figures 13 and 14 indicatesthat bulking in general (except the 115-poundtreatment of ammonium sulfate) is not an indica-tion of improved dimensional stability with re-spect to changing moisture content. In fact, sinceall treated material showed a slightly increasedrate of change in moisture content with respectto relative humidity, the dimensional stability canbe expected to decrease by a correspondingamount. From the swelling-relative humiditycurves in figures 9 and 10 it can be seen, how-ever, that only in the case of sodium chloride isthis effect of any significance. In specimenstreated with this chemical to the 4.5-pound level,the total swelling or shrinkage over the 30 to 80percent relative humidity range was approxi-mately double that of the controls in both pine andoak.

The lower slope of the moisture content-swelling curves of the 4.5-pound ammoniumsulfate treatment (figure 13) indicates consider-able dimensional stabilization with respect tomoisture content changes. However, with respectto changing relative. humidity, this is at leastpartially offset by the increased rate of changeof moisture content with respect to relativehumidity changes. Figures 9 and 10 show that thenet effect results essentially in no change in thedimensional stability of oak and only a slightdecrease in pine.

Excess swelling.--Excess swelling, or the dif-

21

F igure 13 . - -Oak: Moi s tu re-swel l ing re lat ionsh ips fo r t reated wood under adsorb ing condi t ions .Swel l ing i s based on unt reated dry vo lume. Except where ind icated the four curves ineach group represent , f rom top to bottom, nominal sa l t retent ions of 4 .5 , 1 .5 , and 0.5pounds per cubic foot , and the water - t reated cont ro l s .

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F i g u r e 1 4 . - - P i n e : M o i s t u r e - s w e l l i n g r e l a t i o n s h i p s f o r t r e a t e d w o o d u n d e r a d s o r b i n g c o n d i t i o n s .Swel l ing i s based on unt reated dry vo lume. Numbers on l ines ind icate nominal sa l t reten-t ion in pounds per cubic foot .

23

ference in swelling between treated and untreatedspecimens, is plotted as a function of salt reten-tion in figure 15; data were obtained underadsorbing conditions. Although these curves rep-resent swelling measurements obtained at 65 per-cent relative humidity, they should apply rea-sonably well at other levels of humidity in the30 to 80 percent range, since in figures 9 and 10it was shown that the increase in swelling dueto treatment was generally constant in the rangeof humidities investigated. Sodium chloride-treated material is an exception, and the use ofthe curves in figure 15 must be limited to 65 per-cent relative humidity, as in this instance theexcess swelling due to treatment increased withincreasing relative humidity.

For a given chemical, the swelling-salt reten-tion relationship for the two species is quitesimilar and generally appears to be linear in the0- to 1.5-pound nominal retention range. Therate of swelling decreases quite sharply whenthe salt retention increases from the 1.5-poundto the nominal 4.5-pound level and in some in-stances is maximum at the 1.5-pound level. Inzinc chloride, the relationship is linear throughoutthe retention range considered for oak, whereasin pine the rate of swelling increases throughoutthe range.

In terms of the relative excess swelling causedby the various chemicals, it is apparent fromthe curves in figure 15 that the swelling causedby the hygroscopic chemicals ammonium sulfate,sodium chloride, and zinc chloride is considerablygreater than that of the nonhygroscopic group.In the hygroscopic group, the excess swelling isgreatest in the zinc chloride treatments, leastin ammonium sulfate, and intermediate in sodiumchloride. Excess swelling of approximately 6 per-cent for pine and 4 percent for oak was noted forthe nominal 4.5-pound zinc chloride treatments,while at the same nominal retention level theexcess swelling was approximately 3 and 3-1/2percent for the sodium chloride treatments ofpine and oak and approximately 1-1/2 and 2-1/2percent in the ammonium sulfate treatments.

Effect of Salts on the Anisotropic Swelling-Shrinkage Properties

It is well known that wood shrinks more in thetangential direction than in the radial direction

This differential transverse shrinkage or aniso-tropy of wood is responsible for the distortionthat occurs in a piece of wood as it dries. As thedegree of anisotropy increases, so does thedistortion. Also, for a given amount of volumetricswelling, it is likely that the perpendicular-to-grain stresses developed during drying willincrease with increasing anisotropy, making itmore difficult to dry wood without degrade. Itis important, therefore, that the transverseswelling and shrinkage also be considered.

Tangential and radial swelling data for treatedoak and water-treated controls are presented inAppendixes F and G, respectively. Comparabledata for pine are presented in Appendixes H and I.

In figures 16 and 17 for oak and pine, respec-tively, the transverse swelling-relative humidityrelationships are shown for the water-treatedcontrols and for specimens treated with the var-ious chemicals to the three nominal retentionlevels. These curves are quite similar to thoseshown in figures 9 and 10 for the volumetricswelling. As in figures 9 and 10, these curvesreflect total swelling due to increased moisturecontent, where this is an effect, and to chemicalbulking. It is apparent that the increase in swellingdue to treatment, as measured by the verticaldistance between the curves representing treatedmaterial and those of the appropriate controls,is greater in the tangential direction than in theradial. As would be expected from previous dis-cussion of volumetric swelling, the increase intransverse swelling was greatest in specimenstreated with the hygroscopic chemicals ammo-mum sulfate, sodium chloride, and zinc chloride.

The greater overall tangential swelling and thegreater slope of the tangential swelling curvescharacterizes the anisotropic properties of thetreated and untreated material, but it is difficultto analyze this effect from these curves. Thiswill be discussed later in conjunction with otherfigures.

In figures 18 and 19, for oak and pine, respec-tively, the excess transverse swelling due totreatment (in excess of the water-treated con-trols) is plotted as a function of salt retention,For all combinations of chemicals and species,it is apparent that the excess tangential swellingexceeds that of the radial at all levels of reten-tion. This was expected in specimens treatedwith the hygroscopic chemicals, since at leastpart of the excess swelling was due to increased

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F i g u r e 1 5 . - - E x c e s s s w e l l i n g d u e t o t r e a t m e n t a t 6 5 p e r c e n t r e l a t i v e h u m i d i t y ( a d s o r b i n gc o n d i t i o n s ) . S w e l l i n g i s b a s e d o n u n t r e a t e d d r y v o l u m e .

25

F i g u r e 1 6 . - - O a k : T r a n s v e r s e s w e l l i n g - r e l a t i v e h u m i d i t y r e l a t i o n s h i p s f o r t r e a t e d w o o d a n df o r u n t r e a t e d c o n t r o l s ( a d s o r b i n g c o n d i t i o n s ) . S w e l l i n g i s b a s e d o n u n t r e a t e d d r yvolume. Except where ind icated the four curves in each group represent , f rom top tobottom, nominal sa l t retent ions of 4 .5 , 1 .5 , and 0 .5 pounds per cubic foot , and thew a t e r - t r e a t e d c o n t r o I s .

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F i g u r e 1 7 . - - P i n e : T r a n s v e r s e s w e l l i n g - r e l a t i v e h u m i d i t y r e l a t i o n s h i p s f o r t r e a t e d w o o d a n df o r u n t r e a t e d c o n t r o l s ( a d s o r b i n g c o n d i t i o n s ) . S w e l l i n g i s b a s e d o n u n t r e a t e d d r yvolume. Except where ind icated the four curves in each group represent , f rom top tobottom, nominal sa l t retent ions of 4 .5 , 1 .5 , and 0 .5 pounds per cubic foot , and thew a t e r - t r e a t e d c o n t r o l s .

27

F igure 18. - -Oak: Excess t ransverse swel l ing in wood at 65 percent re lat ive humid i ty due tot reatment (adsorb ing condi t ions) . Swel l ing i s based on unt reated dry vo lume.

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F i g u r e 1 9 . - - P i n e : E x c e s s t r a n s v e r s e s w e l l i n g i n w o o d a t 6 5 p e r c e n t r e l a t i v e h u m i d i t y d u e t ot reatment (adsorb ing condi t ions) . Swel l ing i s based on unt reated dry vo lume.

29

moisture pontent. It is interesting to note thatthe excess swelling due to bulking, as indicatedby those specimens treated with nonhygroscopicchemicals, is greater in the tangential directionthan in the radial. It appears that this aniso-tropic phenomenon always occurs with respectto dimensional changes whether it is because ofswelling by water, thermal expansion straincaused by external loading, or other factors.

The degree of anisotropy in wood is commonlyexpressed as the T/R ratio or the ratio of thetangential shrinkage to the radial shrinkage. Itis usually determined on the total shrinkage fromthe green to the dry condition and is generallyassumed to be constant at ail intermediate mois-ture conditions. Ratios of T/R calculated at var-ious equilibrium moisture conditions in the 30 to80 percent relative humidity range for treated anduntreated material are shown in figures 20 and 21for oak and pine, respectively. These ratios arecomputed from swelling measurements that arebased upon dry dimensions; however, the onlyeffect resulting from this technique is a slightincrease in the T/R value because of the dif-ference in bases involved in computing swellingor shrinkage. Relative differences between humid-ity levels are not affected.

It is interesting to note that the T/R ratiosare not constant at different moisture levels.Pine control groups generally show an overalldecrease in the ratio going from 30 to 80 percentrelative humidity and appear to resemble quiteclosely the mirror image of the sigmold-shapedsorption curves. In oak the various controlgroups generally show an increased T/R ratiogoing from 30 to 80 percent relative humidity.

In treated oak specimens the ratio increasedwith increasing chemical retention for all treat-ments. The effect of treatment was especiallypronounced at 30 percent relative humidity, wherethe average change for the seven chemicals wasfrom approximately 1.7 for the controls to 2.2for the 4.5-pound nominal retention level. Thisincrease in anisotropy of oak due to treatmentmakes drying without checking more difficult.

The T/R curves for treated pine are not asconsistent between chemicals and between treat-ment levels as those for oak Similar to thecontrols, however, there is an overall tendencyfor the ratio to increase with decreasing relativehumidity. The T/R ratio of pine when treatedwith the nonhygroscopic chemicals generally in-

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creased with increasing retention, which is simi-lar to, but not as great as that noted for oak.In treatments by the hygroscopic chemicals,however, the tendency is toward a decreasing T/Rratio at higher retentions, particularly as dryingprogressed to the 30 percent relative humiditylevel.

Distribution of Salt in the Wood

The distribution of salt between the cell wailsand the cell cavities may be important in under-standing the nature of the effect of chemicals onthe strength and related properties of treatedwood. The data obtained make it possible toestimate the relative amounts of the total im-pregnated salt located in the cell walls and inthe cell cavities at the various equilibrium condi-tions studied. Required in the calculations are:the untreated dry weight and volume of the speci-men, the moisture content and volume of thespecimen at the equilibrium moisture conditionunder consideration, the weight of the impregnatedsalt, and the density of the salt involved. Theequation for computing the salt distribution is:

(6)

where S = amount of salt located in the cell wall(percent of impregnated salt byweight)

VT = volume of specimen at equilibriummoisture content considered (cubiccentimeter)

VD = untreated dry volume (cubic centi-meter)

M = moisture content at same equilibriummoisture content as VT (percentuntreated dry weight)

WD = untreated dry weight of the specimen(gram)

D = density of the saltW S = total weight of salt in the specimen

(gram).In this equation. VT - VD represents the total

change in volume because of chemical bulkingand swelling with water from the untreated drycondition to the equlilbrium moisture condition

3 0

being considered, and represents the in-

crease in volume because of swelling with wateralone (cgs system only). The difference betweenthe total increase in volume and the increase due

to water, represents the

increase in volume because of chemical bulking.Multiplying the calculated volume increase dueto bulking by the density of the salt gives theweight of the salt required to do the bulking.This also represents the weight of salt in the cellwalls.

In equation (6) it is assumed that, (a) 1 gramof water swells wood by 1 cubic centimeter,(b) the salt exists in the same state or at leastat the same density in the specimen as when thedensity measurement was made, and (c) lumendiameter is not changed by treatment. It lsknown that the first assumption is not true be-cause a part of the adsorbed water is compressedby adsorption forces. This results in slightlyless than a cubic centimeter of swelling for anincrease of 1 gram of water. This effect is mostpronounced at lower equilibrium conditions be-cause the first water adsorbed is compressed tothe greatest extent. By including a factor, K, thiseffect can be corrected for and equation (6)becomes

(7)

(8)

The symbols for equation (8) are the same asfor (6) except they represent values for theappropriate untreated controls, as indicated bythe subscript c. Since K varies with temperatureand relative humidity, it was necessary to deter-mine K for each atmospheric condition considered.

Table 3 shows the percent by weight of thetotal impregnated salt that is located in the cellwall when calculated by equation (7). The datawere obtained under adsorbing conditions andrepresent measurements for three levels of saltretention for each combination of two speciesand seven chemicals.

All values in the table would be expected torange between 0 and 100 percent: however, thereare numerous exceptions at each extreme of thisrange. This can be attributed, at least partially,to experimental error, as the measurements are,admittedly, somewhat crude for this type ofcalculation. Nevertheless they are sufficientlyaccurate to indicate differences between species,chemicals, and retention levels. A possible sourceof error in these data may be the water that isadsorbed to salt present in the cell cavities,thus not affecting swelling. The negative valuesin the table can be attributed to this effect asthese values are limited to those chemicalswhich were previously shown to be either tem-porarily or permanently hygroscopic. The valuesgreater than 100 percent suggest an interactioneffect and this is entirely realistic as certaininorganic salts are known to be cellulose solventswhich disperse the wood fibers or cause the dia-meter of the lumen to increase (6 pp. 73-74).The data for the nonhygroscopic phosphate com-pounds are probably the most accurate. Thereare no negative values which is consistent withthe previous discussion in that there should beno water adsorbed to the salt in the cell cavities.The values for these chemicals greater than100 percent are probably indicative of the magni-tude of the error involved in the methods used.

Generally speaking, there is a larger percent-age of salt located in the cell walls in oak thanin pine, and in both species this percentagedecreases with increasing retention level. Thegreater percentage in oak reflects the differencein specific gravity, because there is more cellwall substance per unit of gross wood volume.The decrease in percentage with increasing reten-tion does not necessarily indicate a decrease inthe absolute amount of salt in the cell wall butthat the cell walls are approaching a saturatedcondition. The addition of more salt results inpractically all of it being situated in the cellcavities.

Of ail the chemicals, zinc chloride providedthe largest percentage of salt in the cell walls.Even at the 4.5-pound retention level, particu-larly at the lower humidity levels, the data showthat all or nearly ail of the salt is located in thecell walls. This reflects, as was previouslyshown, that the excess swelling due to treatmentis greatest with zinc chloride. It is likely thattreatments to higher levels of retention with

31

F i g u r e 2 0 . - - O a k : R e l a t i o n s h i p b e t w e e n t h e t a n g e n t i a l / r a d i a l s w e l l i n g r a t i o a n d r e l a t i v ehumid i ty for t reated wood and for unt reated cont ro l s (adsorb ing condi t ions) . The fourcurves in each group represent , f rom top to bottom, nominal sa l t retent ions of 4 .5 , 1 .5 ,and 0 .5 pounds per cubic foot , and the water - t reated cont ro l s .

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F i g u r e 2 1 . - - P i n e : R e l a t i o n s h i p b e t w e e n t h e t a n g e n t i a l / r a d i a l s w e l l i n g r a t i o a n d r e l a t i v ehumid i ty for t reated wood and for unt reated cont ro l s (adsorb ing condi t ions) . Numbersa s s o c i a t e d w i t h l i n e s i n d i c a t e n o m i n a l s a l t r e t e n t i o n l e v e l i n p o u n d s p e r c u b i c f o o t .

33

this chemical would result in even larger amountsof salt located in the cell walls because of thehigh water solubility of this chemical. Of theseven chemicals, it appears that the relativeamount of salt located in the cell walls is mini-mum in the borax and boric acid treatmentswhich is commensurate with their low solubility.

SUMMARY and CONCLUSIONS

This research considers the effect of severalinorganic salts, commonly used in commerciallyformulated treating solutions, on the sorption andswelling properties of ponderosa pine and redoak Salts evaluated were diammonium and mono-ammonium phosphate, borax, boric acid, sodiumchloride, zinc chloride, and ammonium sulfate.Treatments were conducted to three nominallevels of retention--4.5, 1.5, and 0.5 pounds ofsalt per cubic foot of wood, and the water-treated controls. Observations necessary todetermine equilibrium moisture content and

shrinkage and swelling data were made duringdrying from a saturated condition following treat-ment (initial desorption cycle), during a subse-quent adsorption cycle, and during a final desorp-tion cycle, all at a constant temperature of 80° F.These cycles were limited to the 30 to 80 percentrelative humidity range, humidities which arecomparable to conditions normally encounteredin service,

In terms of their effect on the apparent mois-ture content of wood, the seven chemicals con-sidered can be separated into three groups. Onegroup, which includes the phosphate treatmentsonly, showed no significant change in equilibriummoisture content values. A second group, rep-resented by borax and boric acid, was charac-terized by a substantial increase in moisturecontent throughout the 30 to 80 percent relativehumidity range during the initial desorptioncycle. However, this effect was not as pronouncedduring subsequent adsorption and desorption cy-cling. Also, after the specimens of this groupwere ovendried, they were again subjected to anadditional adsorption cycle and the equilibrium

FPL 60 34

moisture content values obtained were nearlycomparable to those of the controls. In view ofthese considerations and the fact that these chemi-cals themselves are not hygroscopic, it is be-lieved that the increased moisture content notedfor borax and boric acid treatments is not apermanent effect. Kiln drying to a desired mois-ture content or sufficient air-drying time aftertreatment will probably reduce the increasedmoisture observed during the initial desorptioncycle to a moisture content more nearly normal.

A third group included sodium chloride, zincchloride, and ammonium sulfate. This group wascharacterized by a substantially increased mois-ture content throughout the relative humidityrange considered. However, the increases inmoisture content due to treatment with thesethree chemicals were generally not as great aswould be expected based upon the individual hy-groscopicity of the chemical and the wood. Thiseffect was particularly pronounced in the zincchloride treatments. Also, the relative increasesin moisture content due to treatment with thethree chemicals were not as would be expected.The treated wood, ranked in order of decreasingmoisture content, contained sodium chloride,ammonium sulfate, and zinc chloride; the chemi-cals when ranked in order of decreasing hygro-scopicity are sodium chloride, zinc chloride, andammonium sulfate.

The increases in moisture content as a resultof treatment with chemicals in the third groupwere considerably greater in pine than in oakwhen expressed as a percent of the ovendryweight of the specimen. However, it was alsoobserved that this moisture difference was in-versely proportional to the difference in specificgravity of the two species. When moisture con-tent values at various levels of relative humiditywere plotted as a function of salt retention withretentions expressed in terms of a salt-woodweight ratio (grams of salt per 100 grams of drywood), it was found that the data for one chemicaltreatment of both species could be combined.Although in some instances these relationshipswere curvilinear, it was observed that in allinstances a straight line could be used to describethe relationships at levels of retentions commonlyencountered in practice. Straight-line regressionanalyses were made on the data at each of fourlevels of relative humidity for each chemical.Excellent correlations, ranging from 0.88 to 0.99,

were found between moisture content and thesalt-wood weight ratio for the various combina-tions of chemicals and humidity levels. The slopesof the regression lines indicate that the sodiumchloride treatment caused a considerably greaterincrease in the apparent equilibrium moisturecontent than the other two chemicals and that,without exception, the effects of the treatmentincreased at increasing levels of humidity.

It is felt that the equilibrium moisture contentof a specimen of any species of wood when treatedwith one of these three chemicals can be pre-dicted by the straight-line regression equation

where A = the equilibrium moisture content ofuntreated wood of the species in-volved at the atmospheric conditionbeing considered,

B = the slope of the regression curves(table 1), and

x = the salt-wood weight ratio (weight ofsalt per unit weight of dry wood).

Increased volumetric swelling was found inboth species with all treatments when swellingwas based upon the untreated dry volume of thespecimen. This effect was considerably greaterin treatments with chemicals that are themselveshygroscopic. In treatments with nonhygrcscopicchemicals, the swelling is attributed to chemicalbulking of the fibers, while in the hygroscopicgroup the swelling is due to the combined effectof chemical bulking and swelling as moisturecontent increases.

The largest increase in swelling occurred inzinc chloride treatments where excess swelling asa result of treatment at the 4.5-pound level wasmeasured at approximately 6 percent in pine and4 percent in oak This is important from a stand-point of other properties: for example, a cor-responding 6 and 4 percent reduction can beexpected in specific gravity.

Generally speaking, there was no significantchange in the dimensional stability because oftreatment as measured by volumetric changeswith respect to humidity changes. There wereinstances of increased dimensional stability withrespect to changes in moisture content. However,with respect to changes in relative humidity thiswas offset by the increased rate of change in themoisture content with respect to relative humiditychanges.

35

In addition to the increase in volume due to the controls to 2.2 in the 4.5-pound treatments.treatment, the anisotropic swelling characteris- This effect was somewhat less pronounced attics of both species were also increased. This higher humidities and also in the ponderosa pineproperty is commonly measured as the ratio of treatments. It is expected that, with this increasedtangential shrinkage to radial shrinkage. At the anisotropy, somewhat more caution needs to be30 percent relative humidity level the T/R ratio exercised in drying treated material, particularlyof oak was increased on an average from 1.7 in in oak.

LITERATURE CITED

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37

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39

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41

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43

FPL 60 44 2.-45GPO 825�680�2