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Fats. Fatty acids . Fibers . Hardwoods. Heat

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REfRj..~ I ' : :i>~O Conwnonweelth of Australia COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH DRGANIZATION RePlinted from: Tappi Vol. 52, No. 11, ~2149-2155, November 19f5 R. W. HEMINGWAY The surface wettabiJity and fats of yellow bird1wood were examined in an attempt to illustrate how heat-induced d1anges in wood fats might be related to d1anges in surface wettability. A marked reduction of surface wettability accompan;ed heat- ing of yellow birchwood. The degree of water repellency imparted to the waod was highly dependent upon heating temperature and time. Acetone extraction of wood prior to heating to 105°C prevented a d1ange in wettability and increased the surface wettability of wood heated at higher temperatures. Examination of the fats after heating indicated little hydrolysis and ~iderable oxidation of the unsaturated fatty acids and esters. The amounts of free fatty acids present in fresh, air-dried, or heated wood were far too low to approod1 amounts considered necessary to influence surface wettability. The preponderance of linoleic acid ester and its rapid oxidation suggest that oxidation products from this ester might be responsible for the observed d1anges in wettability. Keywords Betula lurea . Cell structure . Hardwoods. Heat . . Surface ~ttability tests . . Esters . Fats. Fatty acids . Fibers Hydrolysis. Oxidation. Surface properties Thermalstability. Wettability EXPERIMENT At . Wood Source and Storage Yellow birchwood bolts approximately 12 in. long and 8-14 in. in diameter were cut from the lower stems of three trees felled during the course of the study to insure that the wood was fresh. The first tree was cut in March 1966, near Ann Arbor, Mich.; the second in December 1966, near PeJ1ston, Mich.; and the third in July 1967, near Ann Arbor. Particular care was taken to freeze the bolts as soon as possible after felling the trees. Blocks 1 in. tan~tially, 11/1 in. radially, and 5-6 in. long were cut from the periphery of the frozen bolts and were stored at -4 °C until prepared further for analysis. MANYproblems encountered in the manu- facture and use of wood products center around the nature of their interactions with water. A reduction of surface wet- tability of wood and paper after heating has been well documented (/-5). Studies of wood wettability after heating have shown that at least some of the changeis attributed to acetone- or ether-soluble wood components (/-5). The mechanism involved has, however, not been well established. Swansonand co-workers and Hancock have emphasized the adsorption of long chain length saturated fatty acids on wood and paper surfaces (2-4). Bu- chananet al., however,havesuggested that glyceride esters playa significant role in the self-sizingof paper (I). Since fatty acids in fresh wood are present primarily as triglycerides (6) and are highly localized in ray and longitudinal parenchyma (5), changes in the location and possibly the chemistry of the fats must occur prior to their influencing wood wettability. If saturated fatty acids of long chain length are responsible for changes in wettability after heating, it would appear that hydrolysis of glyceride esters must take place during heating. Mutton has shown that considerable hydrolysis occurs during long-term air drying of wood bolts (7). Investigation of birchwood fats during air seasoning of logs by Assarsson and Croon showed that, besides hydrolysis, autoxidation of unsaturated fatty acids and esters was considerableafter 1 month under summer conditions (8). There is little literature on the quantities of individual fatty acids and esters present in wood, and none dealing with the re- sponses of fatty acids and esters to heat treatments associated with changes in sur- face wettability. It would be helpful to know the responses of wood fats to these conditions before predicting a self-sizing mechanism. This study explores some of the properties of heat-induced loss of wettability and shows the response of fatty acids and esters to heat conditions which produce self-sizing of yellow birch- wood. Sample Preparation Wood strips 0.10 in. thick, 1 in. wide and 5-6 in. long were prepared on a band saw from frozen blocks. With the bark against the fence, two strips 0.10 in. thick were cut and discarded. Three additional strips were cut from the frozen blocks and used as samples. The sawdust on the faces was scraped away and then rinsed off with distilled water. The strips were then dried at room temperature under vacuum for 8-10 hr, at which point they reached a moisture content of 6-8 %. Care was taken to insure the strips presenteda tanaential face. ._II':fI-::~:~~~-IT.I~ R. W. HEMINGWAY.Resea~h Scientist. Division of Forest Products, CSlRO. South Melbourne, Australia. PrePCJrotion 0' Extraded Wood Str~ The vacuum-dried wood strips were acetone extracted for 48 hr in a large soxhlet. After air drying for 1 hr, the strips were soaked for 3 hr in distilled water and then dried under vacuum overnight at room temperature. Ninety percent of the diethyl ether solubles were removed from the strips by the 48-hr acetone extraction, as determined by grinding and extracting the previously acetone-extracted strips. Air Drying One set of six strips was randomly selected and was further subdivided into two sets of three strips to represent two samples of fresh wood. These sample blocks bad been stored, frozen, about 1 month prior to samplina. The strips were vacuum-dried for 10 hr and then ground on a Wiley mill to pass a 4O-mesh screen. Ground wood from each sample was divided into three parts, extracted, and the amounts of individual free and esteri- fied fatty acids present were determined. A similar set of strips was allowed to air dry at room temperature (68-70°F) for 1 month. The air-dried strips were sub- divided into two sets of three strips and three samples of groundwood obtained for each set as described above. Heat Treatment. The heating chamber was a Pyrex glass 2149 Topp; I November J969 Vol. 52, No. J J &
Transcript

REfRj..~ I ' :

:i>~O

Conwnonweelth of AustraliaCOMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH DRGANIZATION

RePlinted from: TappiVol. 52, No. 11, ~2149-2155, November 19f5

R. W. HEMINGWAY

The surface wettabiJity and fats of yellow bird1wood were examined in an attemptto illustrate how heat-induced d1anges in wood fats might be related to d1anges insurface wettability. A marked reduction of surface wettability accompan;ed heat-ing of yellow birchwood. The degree of water repellency imparted to the waod washighly dependent upon heating temperature and time. Acetone extraction of woodprior to heating to 105°C prevented a d1ange in wettability and increased thesurface wettability of wood heated at higher temperatures. Examination ofthe fats after heating indicated little hydrolysis and ~iderable oxidation of theunsaturated fatty acids and esters. The amounts of free fatty acids present in fresh,air-dried, or heated wood were far too low to approod1 amounts considerednecessary to influence surface wettability. The preponderance of linoleic acidester and its rapid oxidation suggest that oxidation products from this ester might beresponsible for the observed d1anges in wettability.

Keywords Betula lurea . Cell structure. Hardwoods. Heat .. Surface ~ttability tests .

. Esters . Fats. Fatty acids . FibersHydrolysis. Oxidation. Surface propertiesThermal stability. Wettability

EXPERIMENT At .Wood Source and Storage

Yellow birchwood bolts approximately12 in. long and 8-14 in. in diameter werecut from the lower stems of three treesfelled during the course of the study toinsure that the wood was fresh. The firsttree was cut in March 1966, near AnnArbor, Mich.; the second in December1966, near PeJ1ston, Mich.; and the thirdin July 1967, near Ann Arbor. Particularcare was taken to freeze the bolts as soonas possible after felling the trees. Blocks1 in. tan~tially, 11/1 in. radially, and5-6 in. long were cut from the peripheryof the frozen bolts and were stored at-4 °C until prepared further for analysis.

MANY problems encountered in the manu-facture and use of wood products centeraround the nature of their interactionswith water. A reduction of surface wet-tability of wood and paper after heatinghas been well documented (/-5). Studiesof wood wettability after heating haveshown that at least some of the change isattributed to acetone- or ether-solublewood components (/-5). The mechanisminvolved has, however, not been wellestablished. Swanson and co-workers andHancock have emphasized the adsorptionof long chain length saturated fatty acidson wood and paper surfaces (2-4). Bu-chanan et al., however, have suggested thatglyceride esters playa significant role inthe self-sizing of paper (I).

Since fatty acids in fresh wood arepresent primarily as triglycerides (6) andare highly localized in ray and longitudinalparenchyma (5), changes in the locationand possibly the chemistry of the fats mustoccur prior to their influencing woodwettability. If saturated fatty acids of longchain length are responsible for changes inwettability after heating, it would appearthat hydrolysis of glyceride esters musttake place during heating. Mutton hasshown that considerable hydrolysis occursduring long-term air drying of wood bolts(7). Investigation of birchwood fats duringair seasoning of logs by Assarsson andCroon showed that, besides hydrolysis,autoxidation of unsaturated fatty acidsand esters was considerable after 1 monthunder summer conditions (8).

There is little literature on the quantitiesof individual fatty acids and esters presentin wood, and none dealing with the re-sponses of fatty acids and esters to heattreatments associated with changes in sur-face wettability. It would be helpful toknow the responses of wood fats to theseconditions before predicting a self-sizingmechanism. This study explores some ofthe properties of heat-induced loss ofwettability and shows the response offatty acids and esters to heat conditionswhich produce self-sizing of yellow birch-wood.

Sample Preparation

Wood strips 0.10 in. thick, 1 in. wideand 5-6 in. long were prepared on a bandsaw from frozen blocks. With the barkagainst the fence, two strips 0.10 in. thickwere cut and discarded. Three additionalstrips were cut from the frozen blocks andused as samples. The sawdust on the faceswas scraped away and then rinsed off withdistilled water. The strips were then driedat room temperature under vacuum for8-10 hr, at which point they reached amoisture content of 6-8 %. Care was takento insure the strips presented a tanaentialface.

._II':fI-::~:~~~-IT.I~

R. W. HEMINGWAY. Resea~h Scientist. Divisionof Forest Products, CSlRO. South Melbourne,Australia.

PrePCJrotion 0' Extraded Wood Str~

The vacuum-dried wood strips wereacetone extracted for 48 hr in a largesoxhlet. After air drying for 1 hr, the stripswere soaked for 3 hr in distilled water andthen dried under vacuum overnight atroom temperature. Ninety percent of thediethyl ether solubles were removed fromthe strips by the 48-hr acetone extraction,as determined by grinding and extractingthe previously acetone-extracted strips.

Air Drying

One set of six strips was randomlyselected and was further subdivided intotwo sets of three strips to represent twosamples of fresh wood. These sampleblocks bad been stored, frozen, about 1month prior to samplina. The strips werevacuum-dried for 10 hr and then groundon a Wiley mill to pass a 4O-mesh screen.Ground wood from each sample wasdivided into three parts, extracted, andthe amounts of individual free and esteri-fied fatty acids present were determined.A similar set of strips was allowed to airdry at room temperature (68-70°F) for1 month. The air-dried strips were sub-divided into two sets of three strips andthree samples of groundwood obtainedfor each set as described above.

Heat Treatment.

The heating chamber was a Pyrex glass

2149Topp; I November J969 Vol. 52, No. J J&

six wood .~rips were ground to pass a 40-mesh ~n and 8 g of ground wood wereweighed into each of two thimbles and ex-tracted for 10 hr with acetone. About 2 gwere used to determine the moisture con-tent. The acetone extract was evaporated.covered with diethyl ether. and 1 g ofsodium sulfate was added to it; it was thenallowed to stand overnight.--The ethersolution was filtered, evaporated undervacuum, and stored under nitrogen notmore than 3 br prior to separation ofacidic from neutral components.

Separation of Free FoHy Acid.

Free fatty acids were separated fromthe ether-soluble extract on a DEAESephadex A-25 anion exchange columnprepared as described by Zinkie and Rowe(9). Three grams of DEAE Scpbadex werepacked in an 11 X 300 mm chromatog-raphy tube, and the neutrals were elutedwith 150 ml of a solvent containing 89parts by volume of diethyl ether, 10 partsof methanol, and 1 part of water. Theacidic fraction was eluted with 200 mI of asolvent containing 90 parts of diethylether, 10 parts of methanol, and 4 parts offorntic acid by volume.

Methyl Esterification

The free fatty acid and saponifiablefmctions were coUected and methylatedwith 5mI of BFa-methanol for 3 min on ahot water bath (/0). After cooling, themethylated extracts were separated fromwater with diethyl ether, and the diethylether solubles were dried overnight withsodium sulfate. The ether solubles wereevaporated to about 10 mi. and 30 mI ofpurified hexane was added. The extractwas again evaporated to 10 ml and theextract diluted to 40 mI. A yeUow to brownprecipitate was filtered from the hexanesolubles and discarded. The hexane solu-bles were evaporated under vacuum andthe sample diluted to 2 mI prior to gaschromatography.

GaI-Liquid Chromatogrophy

Th~ fatty acid methyl esters were chro-matographed on a 6-ft 4-mrn ID glasscolumn packed with 7.1 8 of a 6% Lac728 diethylene slycolsuccinate liquidphase on 8O-1OO-mesh Diatoport S. Thecolumn ternpemture was 170°C and thehelium carrier gas flow rate was 40 milmin. Injection port and detector tempem-tures were 2SO and 275°C respectively.

Standard solutions of individual methylesters in concentrations from 0.5 to 0.001g per 100 mI of hexane were preparedfrom pure compounds obtained fromApplied Science Laboratory. State Col-lege. Pa. Measurement of standard peakheights for each methyl ester gave linearcalibration curves whose variability wasinsignificant, compared to the variationof individual wood samples.

tube 5 cm in diameter and 59 cm long.Six iron-constantan thermocouples wereplaced opposite each other at three posi.tions along the fength of the tube, about1/. in. from the tube wall. The beating tubewas wrapped with a 384-w beating tape,glass wool, and two layers of asbestostape. Water-pumped dry air was set at aflow rate of 500 ml/min, thus exchangingthe air about every 2 min. The air passedthrough the heating tube to a cold trap inice water which also served as a referencefor the pyrometer. After temperaturecalibration, thermocouples were removedto insure that volatiles from the thermo-couples were not influencing wettability.

There was a time lag for the tempera-ture at the wood surface to equilibratewith the heating temperature. Thermo-couples were stapled to the surface of thewood strips, insuring tbat the junction wasagainst the wood surface. FortY minutesat 105°C, 12 at 160°C, and 7 min at220°C were required for reaching equi-borium temperatures.

Wood strips were held in position withholders fashioned from wire screen suchthat three strips were held parallel to andabout II. in. from each other in the centerof the tube. No systematic difference inthe change of surface wettability withrespect to position in the sample bolderwas observed. After each set of three heattreatments, the interior of the heating tubeand sample holders were swabbed thor-oughly with ethanol and then baked at250°C for 2 hr.

Separation 0' Esterified fotty Acid.

Neutrals were evaporated to about 30ml and reftuxed for 4 hr with 100 rnI of2N methanolic KOH with 10% wateradded. The solvent was t.-vaporated andthen diluted to 250 rnI with distilled waterand acidified with sulfuric acid. Afterextracting twice into diethyl ether, theether solubles were washed three timeswith distilled water, combined, and driedovernight with sodium sulfate. The saponi-fied extract was concentrated' and storedunder nitrogen not more than 3 hr priorto separation of saponifiables from neu-trals on DEAE Sephadex as descn"bedabove (9).

Meoalrement of Weffability

A most sensitive method of measure-ment of changes in wettability was foundto be timing the interval from placementof a 5-1£1 water droplet on the wood sur-face until the droplet disappeared. Theapplicator was a lO-IAI Hamilton gaschromatography syringe with the needlesurface coated with silicon grease to limitwetting of the needle and to help main-tain an intact droplet when removing thesyringe. With the needle just touching thesurface, the plunger was slowly depressedand then carefully lifted away, with caretaken not to displace the droplct. Thedroplet was judged to have disappearedwhen light reflection was no longer visible.A total of six observations (three alongthe length of each face) was made on eachstrip. Fresh, unheated wood required onlyabout 5 sec to lose gloss, while the upperlimits of no wetting required about 20 min.The intact water droplet could be movedon the wood surface by gentle teasing withthe syringe needle when more than 20min were required to lOse gloss, suggestingno wetting. Care was necessary to obtaintangential surfaces, as grain angle resultedin more rapid water absorption.

.. -

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Fig. I. Change in surface wettability of thinyellow birchwood strips after heating at105°C, effect of overnight equilibration andsanding surfaces. 0 Surface wettabilitymeasured after specimens cooled; 0 Speci-mens allowed to equilibrate with roomatmosphere overnight prior to mealUrincsurface wettability; ~ Specimens landedwith fine sandpaper prior to measuring sur-face wettability;. Specimens sanded withcoarse landpaper prior to measuring sur-face wettability.

RESULTS

Chonge of Wettobility Induced by Heating

After heating the wood strips at 105 °Cfor 6 hr, there is a 14-fold increase in thetime required for absorption of a S-Jl.1water droplet (Fig. I). If the surfaces areallowed to stand overnight to equilibratewith the room atmosphere, there is nofurther change in wettability. Sanding thesurface with both fine and coarse sand-paper after heating restores it to its orig-inal wettability. The changes in surfaceroughness of the sample do not appear tobe significant because the penetration timereturns to nearly the same level whetherthe wood is sanded with fine or coarsesandpaper. These results suggest that thechange in wettability induced by heatingis appreciable, is a permanent change, andis a surface effect.

Acetone extraction of the wood stripsprior to heating markedly reduces thechange in surface wettability induced bythe heat treatments (Figs. 2-4).

Some acetone-extracted samples thathad been soaked in distilled water andvacuum dried had a surface coating thatbecame yellow after heating. This materialwas especially prevalent in samples usedfor 105 and 220°C heat treatments and

Extrodion

After the appropriate heat treatment

2150 Vol. 52, No, November J969 I Toppi

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HEATING TIME AT IO5°C,tV'

Fig. 2. Change in surface wettability ofthin yellow birchwood strips after heatingat 105°C, effect of prior acetone extractionand sanding treatment. 0 Specimens notextracted or sanded prior to heating;0 Specimens not extracted but sandedprior to heating; l:.. Specimens extractedbut not sanded prior to heating; . Speci-mens both extracted and sanded prior toheating.

Fig. 3. Change in surface wettability ofthin yellow birchwood strips after heatingat 160°C, effect of prior acetone extractionand sanding treatment. 0 Specimens notextracted or sanded prior to heating j0 Specimens not extracted but sandedprior to heating j ~ Specimens extractedbut not sanded prior to heating j . Speci-mens both extracted and sanded prior toheating.

Fig. 4. Change in surface wettability ofthin yellow birchwood strips after heatingat 220°C, effect of prior acetone and sandingtreabnent. 0 Specimens not extracted orsanded prior to heating; 0 Specimens notextracted but sanded prior to heating;A Specimens extracted but not sandedprior to heating; . Specimens both ex-tracted and sanded prior to heating.

water absorbency, it should result in anoverall decrease in wettability rather thana surface effect. It is evident ftom theabove results that acetone-soluble com-ponents of yellow birchwood are re-sponsible for changes in wettability of thiswood, particularly at the lower tem'pera-tures studied.

Free FaHy Acids of Fresh Yellow Sirchwood

The avemge amounts of free fatty acidsin wood from the three trees sampledare shown in Table I. The individual freefatty acids are present at about one tenththeir concentmtion as esters for each fattyacid studied. The unsaponifiables amountto approximately 40 % of the lipid extmct.Thus, the individual free fatty acids areuniformly distributed in proportion totheir amounts present as esters, and thetotal free fatty acid fmction amounts toabout 5% of the lipid extract. Wood whichwas obtained at three vastly differentphysiological times (December, March,and July) contains a remarkably similarfree fatty acid composition. The totalamount of satumted free fatty acid in thethree samples studied amounted to only?0-40 ppm of the ovendry wood weight.

was related to the dumtion of water soak-ing. Strips were heated both with andwithout prior sanding in an attempt toisolate the effect of the yellow substanceand acetone-soluble wood components onthe surface wettability of heated wood.The surface wettability of acetone-ex-tracted wood was significantly greater forwood strips sanded prior to heating at105 and 22Q°C than those heated directlyafter drying (Figs. 2, 4). Sanding unex-tracted wood strips prior to heating didnot increase the wettability of wood afterheating. The yellow substance appears tobe related to water soaking, to which un-extmcted wood was not subjected, and theyellow material does decrease surfacewettability after heating.

The change of wettability attributableto fatty materials is more accurately evi-denced by comparing the effect of acetoneextraction on the wettability of wood thatis sanded prior to heating (Figs. 2-4).Acetone-extmcted wood retains its un-heated wettability after 6 hr of heating at105 °C, while there is a marked reductionof wettability in the corresponding un-extmcted wood. When wood is heated at160 and 220°C, there is a reduction in thewettability of acetone-extmcted wood,but it is significantly less than that of theunextracted wood (Figs. 3,4). Changes inwettability at these higher tempemturesmay be caused, in part, by the modifica-tion of pentosans, since they are unstableat these tempemtures (II). Oxidized hemi-celluloses retain a high affinity for water(9) and even if polymerization does reduce

ences shown could be due to variationsin the wood supply. The saturated fattyacids both increase (stearic acid) and de-crease (palmitic acid). The decrease in theamounts of unsaturated fatty acids sug-gests oxidation of the linoleic and linolenicacids.

Effect of Heoting in Air on the Fat, ofYellow 8irchwood Saturated Free FattyAcid,

The concentration of saturated freefatty acids generally increases with increas-ing heating periods at the three tempera-tures studied (Table III). The longer chainlength saturated fatty acids increase inconcentration during heating more rapidlythan the shorter fatty acids. After 1 hrand 30 min of heating at 160°C, palmiticacid increases from 14 to 20 ppm, stearicacid from 4.8 to 12.6 ppm, and arachicacid from 0.4 to 6.4 ppm. An increase insaturated free fatty acid concentration isnot observed between 11/2 and 3 hr ofheating at 160°C. The increase in satu-rated free fatty acid that occurred afterheating at 105 °C also appears to approacha plateau.

Wood which could be sampled from asmall area, thus minimizing the samplevariation, was heated at 160°C for 1, 2,and 4 hr. The results from these samplesverified that further increases in saturatedfree fatty acids are not developed after 1hr at 160°C (Fig. 5). The heat treatmentsat 220°C also show an increase in thesaturated free fatty acid concentrationsafter heating.

Response of Yellow Sirchwood FreeFoHy Acid to J Month of Air Drying

One month of air drying of thin stripsof yellow birchwood does not result in asignificant increase in the concentrationof free fatty acids (fable II). The differ-

2151Tapp; I November 1969 Vol. 52, No.1

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2439313142

4746344725

11971040708930494

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110.50

after 7 hr of heating at 10SoC and 3 hr at160°C. Only 3 % of the originallinolenateremains after 7 hr at 10SOC and 3 hr at160°C. Twenty-five percent of the lin-olenate is present after 20 min at 220°C.

Saturated Fatty Acid Esters

The saturated fatty acid esters showlittle response to the heat treatmentsstudied (Table IV). There is a decrease inthe concentration of the esters after 3 hrof heating at 10SoC and 1 hr at lW°c.which corresponds to the heating tiIne$where increases in free fatty acids wereevident. Palmitate decreases from 112 to97 ppm, stearate from 71 to 47 ppm,arachidate from 3S to 24 ppm. andbehenate from 24 to 17 ppm after 3 hr ofheating at 160°C. The decreases of esteri-fied fatty acids are higher than the ob-served increase in the free fatty acids dis-cussed above. Approximately half of thedecrease in saturated ester appears as anincrease in the corresponding free fattyacid.

DISCUSSION

Although small amounts of free fattyacids have been found in the middlelamella (/3), the majority of the fats inwood are located in the ray and longi-tudinal parenchyma (.5) and are largelytrigtycerides (6). If these fats are to influ-ence the surface wettability of wood, itwould appear that they must be dispersedfrom the rays to the fibers at the surface ofthe wood. The structure of the simple andhalf.bordered pit membranes thus be-comes extremely important to a considera-tion of the relevant pool of fats that con-tribute the changes in surface wettability.The few studies that have been made ofthese pit membranes show a relativelythick (from 0.5 to 1 p) and imperforatestructure (14, 1.5). Krahmer states thatwhile these dense imperforate membranesmay readily allow liquid transport bydiffusion, the rate of gas phase transfermust be greatly restricted (1.5). The lowpermeability to gas transfer coupled withthe low vapor pressure of the free fattyacids under the conditions studied (5.5 X10-1 mm Hg for stearic acid at 105°C)must mean that fatty acid transfer fromray cells in the interior of the wood inthe vapor state to the external surface ofthe wood is highly unlikely. If vaporizedfatty acids did penetrate the pit mem-branes, data obtained by Stamm andMillett on selective adsorption of stearicacid from benzene by lumen surfaceswould indicate that internal adsorptionwould not allow concentration at thewood surface (16). Although Huffmanobserved a concentration of resins on the

surface of kiln-dried pine wood, a similarconcentration of fats to the surface ofbirchwood would not be expected becausethe resins of pine are apparently forcedthrough the resin canals (/7).

A similar exudation of fats from raycells would not appear to be effective dueto the void space in the ray cells andrestraint of the pit membranes. A transferof the fatty acids with water would beunlikely because of low solubility and theminimal time that liquid water is at thesurface, as evidenced by the rapid initia-tion of the falling rate drying curve. Itwould appear, then, that only those fatslocated near the surface of the piece cancontribute to changes in wettability ob-served after heating.

Swanson and Cordingiy showed thatthe surface wettability of paper changedwhen from 0.14 to 0.3 g of stearic acid per100 g of ~per was adsorbed from thevapor state (3), while there was no furtherchange above this level of stearic acid.Assuming that an oriented stearic acidmolecule has an area of 21 A 2 per mole-cule, this would correspond to a mono-molecular layer on 150 m2 of surfacearea. A surface area of I.S m2jg of paperis very similar to actually measured areasand it would appear, then, that the limitof wettability change per gram of stearicacid adsorbed does correspond to thecompletion of a monoroolecular layer.Projecting from the results of Swansonand Cordingiy, it would appear that ad-sorption of stearic acid would alter thewettability of wood when from one thirdto a complete monomolecular layer wasdeveloped.

To estimate the amount of surface areaaccessible to fatty acids in birch wood it isnecessary to make an approximation fromthe data of Stamm and Millett (/6). Theseauthors determined the unswollen surfacearea of sugar pine wood (P. /ambertiana)by selective adsorption of stearic acidfrom a benzene solution. They found asurface area of 2500 cm2jg using thismethod, which has been confirmed byother methods of measurement. The sur-face area per gram of birch wood wasapproximated from this data by adjustingthe data according to the average specificgravity by the ratio

~ z.~-g-=s;

Unsaturated Fatty Acid Est The unsaturated fatty acid esters are

extensively oxidized under the heat treat-ments studied (Table IV). After 7 hr orheating at 105°C, 3 hr at 160°C, and 20min at 220°C, the amount or oleate inthe wood is reduced by about 50% (47to 24 ppm). The oleic acid was postulatedto increase by 9 ppm through hydrolysis,which would account for a portion of theloss, and about 3.5 % reduction of oleatewould then be attributed to oxidation.

Linoleate is severely oxidized since onlyII % of the unheated concentration re-mains arter 7 hr at 10.5°C, 6% after 3 hr at160°C, and 42% after 20 min at 220°C:This corresponds to decreases from 1197ppm to about 100 ppm, by far the mostimportant reaction from the standpoint ofquantities of material altered. It is postu-lated that about 200 ppm of the loss oflinoleate corresponds to hydrolysis at amaximum; thus about 900 ppm or 80 %of the linoleate is lost through oxidation

where~, ~ effective surface area of yeUow.

birchwood~1 = effective surface area of white pine

(2.soo cm'/g),. - spec:ific gravity of ceU waU sumtaoce

(1.53)51 = specific gravity of white pine wood

(0.35)51 = specifK: sravity of yellow birch-

wood (0.63)

A surface area of about 1900 cro'la is thusobtained. The amount of stearic acid

Vol. 52, No. 2153Toppi I November J969

Table I. Free Fatty Acids of Fresh Yellow Birchwood(ppm methyl ester on ovendry wood weight)

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Souree and date of lampiing --Ann Arbor Pel/.rton Ann A 'bor

March 1966 Decemberl966 July 1967

2~ATING TIME AT 16O-C,hl

Fig. 5. Effect of heating at 160°C on theamounts of saturated free fatty acids in thinyellow birch strips.

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9915

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7.43.7

130

26

Fatty acid

Palmitic CI6Stearic CISOleic CIS1Linoleic CIS'Linoleic CIS'Arachic C2OBehenic C22

TotalTotal saturated

Table II. Response of Individual Free Fatty Adds in Thin Yellow Birchwood Stripsto 1 Month of Air Drying

(ppm methyl ester on ovendry wood weight)

CIII ~J.t3.t3..13.13.83.'

.3.3

4.14.44.33.8...

CIa Cl81 CUiCII

Refrigerated1

2

Avg.Air-dried 1 month

1

-4..1Ava.

Table III. Effect of Heating in Air an the Amounts of Free Fatty Acids in Thin YellowBird1wood Strips

(ppm methyl ester on ovendry wood weight)- ~ ~Ct. ClIt Claa C.'CtfC.1

105°C2442~155

107

3.4'.23.'4..83.33.63.3

1m170114128385334

3.34.43.1'.37.37.29.2

0.440.350.370.732.62.13.9

Unheated avg.3Omin1 hr2hr3hr4hr1hr

14161417181825

1600C2433154.57.1

3.45.65.34.03.3

1001SO853133

3.34.87.1

1311

0.440.412.56.44,5

Unheated avg.15 min45 miD90 min3hr

1424232022

wood do not change markedly in thethree temperatures studied. There is ageneral increase in oleic acid early in theheating period and a decrease to theorigina1level. Although it is not known ifthe amount of oleic acid generated byhydrolysis is proportional to stearic acid,the assumption is made to obtain an esti-mation of the amount of oleic acid oxida-tion. Considering the amount of oxidationof the free fatty acid in this way yieldsan oxidation rate for oleic acid which issomewhat greater than the oxidation ofthe oleic acid ester. Swern states that theoxidation rates of free fatty acids exceedthose of their glycerides (I2).

It appears, then, that the two responsesof hydrolysis and oxidation of oleic acidresult in a nearly constant concentrationof oleic acid in the heated wood. Acceptingthe difference between the rel3tive re-sponse of stearic acid and the unsaturatedacid as oxidation, the oleic acid is 64%oxidized after 7 hr at 105°C, 71 % oxidizedafter 3 hr at 160°C, and 56% oxidizedafter 20 min at 220°C.

Linoleic acid is 90% oxidized after 7 hrat 105°C, 94% after 2 hr at 160°C, and90% oxidized after 20 min at 220°C. Interms of actual quantities of linoleic acid,the unheated average of seven samples was109 ppm, which increases to as high as170 ppm after 30 min and then decreasesto 34 ppm after 7 hr of heating at 105°C.Similar responses are found for the heattreatments at 160°C, where linoleic acidconcentration decreases to 31 ppm after11/2 hr, and at 220°C, where the concen-tration is reduced to 23 ppm after 20 minof heating.

Linolenic acid is rapidly oxidized underthe heat treatments studied. Because ofthe small amount of linolenic acid remain-ing after short heating periods, and due tothe presence of a small contaminating peakwhich showed a shoulder on the back side-of the methyllinolenate peak, the linolenicacid in wood is probably more extensivelyoxidized than is shown. With these limita-tions in mind, linolenic acid is 93 %oxidized after 7 hr at 105 DC, 95 % oxidizedafter 2 hr at 160°C, and 93% oxidizedafter 20 min at 220°C, assuming thatlinolenic acid is generated similarly tostearic acid during. the heat treatments.

220°CUnheated ava. 14 3.3 0.44 3.4 100 142 min 19 3.8 0.37 4.1 76 157 min 19 3.9 O.~ 4.0 67 1220 min 16 6.2 2.0, 2.8 23 4

Unsaturated Free FaHy AcidIt is evident from all three tempemturetreatments that there is an inductionperiod when little change in the saturatedfree fatty acids occurs. This time periodcorresponds well to the time period neces-sary for the wood to reach equilibrium in,heating tempemture, and mpid loss ofwater is apparent.

As would be expected from studies ofrelative oxidation rates on model fattyacids and esters, the oxidation of the CISunsaturated fatty acids is highly dependenton the degree of unsaturation (fable III).The amounts of oleic acid in the heated

~~ Vol. 52, No. J J November J969 I Tappi2152

,4;.

44

necessary to coat this surface with a mono-molecular layer can then be approximatedas

z aNWm/M

where2: = effective area of a~rbenta = effective cross-sectional area of

adsorbate moleculeN = Avogadro's number (6.02 X 1011)Wm = weight of adsorbateM ~ molecular weight of adsorbate

or about 400 ppm. It should be noted thatthe surface area per gram of wood is notappreciably dependent on the geometry ofthe specimen. Assuming smooth surfacepreparation, the effect of changing from acube to a sheet is that cut ceU waUs assumea greater proportion of the total surfacearea with a corresponding decrease in thelumen surface area. Surface roughness ismore important as the geometry ap-proaches a thinner sheet, and the estimateof 1900 cm2/g is a minimum surface area.Accepting 1900 cm2/g as the surface areaand the previously discussed restrictionagainst movement of fatty acids from thecenter of the sample to the external sur-face, it is necessary to have a fatty acidconcentration of about 400 ppm toachieve a monomolecular layer and about130 ppm to begin to influence the surfacewettability of the wood.

The amount of saturated free fatty aciddid not exceed 40 ppm during any of thethree heat treatments studied. The dis-crepancy between 40 and 400 ppm withthe accompanying large change in surfacewettability does not appear to allow anexplanation of reduced wettability as ad-sorption of long chain length free fattyacids.

If the fatty constituents of wood do playa role in the heat-induced loss of waterabsorbancy, as would appear from thedata obtained in this study, and if satu-rated long chain length free fatty acidsare not present in sufficient quantities toexplain this observed reduction in wetta-bility, it would appear that the reducedwettability might be associated with thesevere oxidation of the unsatUrated fattyacids and esters. In terms of quantities ofmaterials altered, the oxidation of linoleicacid and its ester are by far the mostsignificant.

Although purely speculative, an exami-nation of the oxidation products of thesecompounds might provide an answer towhere to look for a reason for the observedchanges in wettability. Heating paper at105°C for 8 hr reduced the critical surfacetension of wetting from a value of morethan 60 dynes/cm for unheated paper tobetween 25-30 dynes/cm (2). A criticalsurface tension of wetting as low as 25-30dynes/cm places definite restrictions onthe chemical nature of the componentsresponsible, and from data collected byZisman, dictates an aliphatic nature (24).

The oxidation of linoleic acid and

chain length of adsorbed fatty acid on thepercent wood failure of strip shear speci-mens (4). Buchanan found linoleic acid tobe more effective in reducing the wetta-bility of paper than either stearic or oleicacid (I). Glycerol esters were also highlyeffective water repellents. Although pureglycerides were not available for analysis,fractions of the neutral lipids from paperbirch containing mixed glycerides with themajor acid linoleic were very effective.The data obtained in this work substan-tiate this proposition. The role of oxida-tion in this process seems to be significant.Attention should be focused on the inter-action of free radicals of the glycerides,formed by chain cleavages of unsatu-rated fatty acid esters, especially linoleate.

CONCLUSIONSThere is a marked reduction of surface

wettability of yellow birchwood afterheating in air. The change in wettabilityis a surface effect and related to theacetone-soluble constituents of wood, es-pecially at the lower heating temperatures.

The fresh wood contains only about 40ppm saturated free fatty acids, which is toosmall an amount to cause changes insurface wettability. Air drying for 1month does not significantly increase theconcentration of free fatty acids. Heatingwood strips in air at temperatures between105 and 220°C does not increase the con--centration of free fatty acids sufficientlyto allow an explanation of the reducedsurface wettability. The unsaturated fattyacids and esters undergo considerable oxi-dation under heat conditions which pro-duce water repellency. It would appearthat the reduced wettability might berelated to the oxidation of the linoleicacids and esters. '

linoleic acid esters would proceed alongthe same routes, except that the free acidshould be more rapidly oxidized than theester. The first step in the oxidation routeshould be formation of hydroperoxidederivatives under the conditions studied.(See reaction illustrated in top box above.)The hydroperoxides are 90% conjugatedand there is isomerization fcom the cis-cisconfiguration to cis-trans under the milderconditions below 150°C and a shift to thetrans-trans configuration under the moreextreme oxidation conditions (19). Thehydroperoxides are somewhat stable be-low 100°C and they tend to accumulatewithout significant chain cleavage (12). Ifthe fats are then subjected to higher tem-peratures (150°C), chain cleavage initiatesa geometric increase in the oxidation rateand the hydroperoxide concentrationrapidly decreases (22).

Chain cleavage of hydroperoxide Ishown above leads to four products shownin bottom box above. At temperatures be-low 100°C, Crossley and co-workers ob-tained the aldehyde cleavage products,while above 100°C the aldehydes wererapidly oxidized to their correspondi:ngmono and dibasic acids (23). Free radi-cals such as CHa(CH2)"cHt. readilyadsorb more oxygen to form a seriesof aldehydes and/or acids (2/). A polarend group is necessary to have thecompound act as an efficient adhesive(18). The above compounds satisfy re-quirements for a polar adsorption site tothe substrate, while exposing an aliphaticstructure to the air interface. The chainlength of these products is short, however,and Brockway and Jones have demon-strated the importance of long chainlength on rough surfaces to form a con-tinuous film (25). Hancock observed asimilar effect when examining the effect of

~Vol. 52, No.1 November 1969 I Tappi2154

12. Swern, D., In "Fatty Acids" (K. S.Markley,.~Ed.), 2nd edn., New York,Interscielie, 1967, Part 2, p. 1387.

13. PeriJa, O. and Manner, P., Pllprrl ja P""38 (10): 499 (1965).

14. Harada, H., In "Cellular Ultrastructureof Woody Plants" (W. A. COte, Jr., Ed.),Syracuse. N. Y., Syracuse UniversityPress, 1966, p. 244.

15. Krahmer, R. L. and C6te, W. A., Jr.,Tappi 46 (I): 42 (1963).

16. Stamm, A J. and Millett. M. A, J.Phy.J. Chern. 4.5 (1): 43 (1941).

17. Huffman, J. D., ForF.Jt Prod. J. 5 (4):135 (1953).

18. ~man, W. A., In "Adlx:sion and C0-hesion" (P. Weiu, Ed.), New York,Elsevier, 1962, p. 176.

19. Privett, O. S., Lundberg, W.O., Kahn,N. A., Tol~, W. E., and Wheeler,H. D., J. Am. Oil Chern. Soc. 30 (2):61 (1953).

20. Endra, J. G., DhaJerao, U. R., andKu~, F. A.,J. Am. Oil Chern. Soc.39 (2): 118 (1962).

21. Esterbauer, H. and Schauenstein, E.,Fette, Seifen, Anstrich. 68 (I): 7 (1966).

22. Ney, K. H., F~tte, S~ifen, Anstrich. 67(3): 190 (1965).

23. Cr~ley, A., He~, T. D., and Hu<hon,B. J., J. Am. Oil Chem. Soc. 39 (1): 9(1962)

24. Zisman, W. A., In ""Contact Angle,Wettability and Adhesion" (F. M.Fowkes, Ed.), Washington, D. C.. Am.Chern. S<x:., Advances in ChemistrySeries 43,1964, p. 1.

25. Brockway, L. O. and Jones, R. L., In"Contact Angle, Wettability and Ad-hesion" (F. M. Fowk~, Ed.), Washing-ton, D.C., Am. Chern. S<x:., Adval-=esin Chemistry Series 43, 1964, p. 275.

RECEIVED FOR REVIEW April 7, 1969.ACCfPnD July 13, 1969.

This work was conducted at the Universityof Michipn, Ann Arbor, Mich., as part orPh.D. requirements, under the direction of Dr.A. A. Marra. Dr. Marra'. helpful criticismand encouragement are greatly appreciated.

LITERATURE CITED

I. Buchanan, M. A., Burson, S. L., andSprinaer, C. H., Tappi 44 (8): 516 (1961).

2. Swanson, l. W. and Becher, l. l., Tappi49 (S): 198 (196S).

3. Swanson, l. W. and Cordingly, S., Tappi42 (10): 812 (1959).

4. Halx:<x:k, W. V., Ph.D. Thesis, Univer-sity of British Columbia, 1963.

5. Back, E., S~nsk Papperstid. 63 (22):793 (1960).

6. Buchanan, M. A., Sinnett, l. A., andlappe, l. A., Tappi 42 (7): S78 (19S9).

7. Mutton, D. B., Tappl 41 (II): 632~ (19S 8)

8. Assarsson. A. and Croon, I., SomskPap~rstid. 66 (21): 876 (1963).

9. ZinkJe, D. P. and Rowe, l. W., Anal.ClJem.36(6): 1160(1964).

10. Metcalf, C. D. and Schmitz, A. A., Anal.Chern. 33 (3): 363 (t96I).

11. Mitchell, R. L, Seborg, R. M., andMiUctt, M. A., F~st Prrxl. J. 3 (1):38 (1953).

Tapp; I November J969 Vol. 52, No. J J 2155


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