Colloids and Surfaces, 9 (1984) 253-271 253 Elsevier Science Publishers B.V., Amsterdam – Printed in The Netherlands

APPLICATION OF ESCA TO EVALUATE WOOD AND CELLULOSE SURFACES MODIFIED BY AQUEOUS CHROMIUM TRIOXIDE TREATMENT

R.S. WILLIAMS and W.C. FEIST Forest Products Laboratory, * Forest Service, U.S. Department of Agriculture, Madison, WI 53705 (U.S.A.) (Received 8 March 1983; accepted in final form 28 October 1983)

ABSTRACT

Aqueous chromium trioxide (CrO3) is an effective surface treatment agent that inhibits the weathering of wood. The toxicity and color of CrO3 limit its usefulness, but under­ standing how chromium protects wood from weathering may lead to the development of alternative or superior treatments. Electron spectroscopy for chemical analysis (ESCA) re­ sults elucidated the surface effects of aqueous CrO3, treatment of wood and pure cellulose (Whatman No. 1 filter paper). ESCA data showed at least 75% Cr(VI) to Cr(III) reduc­ tion on all substrates. Leaching experiments confirmed that the chromium was reduced to a highly water insoluble or “fixed” chromium complex on wood (Sequoia sempervirens and Pinus sp.) and cellulose. The resulting modified cellulose and wood exhibited greatly improved water repellency and ultraviolet stability. Comparison of the C(1s) photoelec­ tron peaks for the substrates showed changes in the relative abundance of hydroxyls and hydrocarbon components. The surface concentration of hydroxyls decreased while the hydrocarbon increased. This change occurred through oxidation of primary alcohols in the cellulose to acids, followed by decarboxylation. Increased concentrations of CO2 fol­ lowing Cr(VI) treatment of filter paper were confirmed using mass spectroscopy. The similarity between treated wood and treated cellulose indicated that chromium-cellulose interactions should be included in defining the mechanism for Cr(VI) stabilized wood sur­ faces and that previously proposed chromium–wood mechanisms based solely on extrac­ tives, lignin, and/or hemicellulose are too limited.

INTRODUCTION

The photochemical degradation of all wood species on outdoor exposure occurs worldwide, and considerable work has been done to elucidate the mechanisms for this decomposition [1-8]. This degradation or weathering occurs within 75 µm of the surface and manifests itself as discoloration, checking, fading, and erosion of the wood surface [9–12]; Kalnins [1] has shown that hydrogen, carbon dioxide, methanol and carbon monoxide are evolved from softwood and hardwood during irradiation with ultraviolet (UV) light. Hardwood irradiation also produces formaldehyde [1]. Hon [2,3] and others [4,5,13,14] have characterized several free radical species

*Maintained at Madison, Wisconsin, in cooperation with the University of Wisconsin. 254 using electron spin resonance (ESR) spectroscopy, and it is generally accepted that photoinduced free radical destruction of primarily lignin brings about erosion of wood [6,7,15]. Cleavage of cellulose and hemicelluloses can also occur but at a slower rate at natural UV wavelengths [16]. Lignin protection of cellulose in wood has been demonstrated [17,18]. Highly pigmented opaque paints or solid color stains that block UV light offer the most effective protection of wood surfaces [9]. However, the pref­ erence for preserving the natural wood appearance of exterior siding has promoted considerable effort toward improvement of clear and lightly pig­ mented coatings. Traditional clear coatings such as spar varnish or finishes based on polyurethane resins provide only 1 to 2 years protection when ex­ posed directly to the weather. Modification of the coatings with UV absorb­ ers made some improvements in performance [19-21], but long-term dura­ bility has not been demonstrated. Modification of clear coatings and use of UV transparent coatings failed to stabilize the critical wood- coating inter­ face. In contrast to these, a wood finish developed at the Forest Products Labo­ ratory utilized chromate, Cr(VI)*, surface treatment of wood followed by a UV transparent coating [22,23]. This stabilization of the wood- coating in­ terface with Cr(VI) pretreatment gave 9-year protection whereas the same finish without Cr(VI) pretreatment lasted only 2 years. In addition, aqueous Cr(VI) treatments led to extended life of certain opaque paints and semi­ transparent stains, reduced erosion of uncoated wood surfaces, reduced ex­ tractive staining, and improved water repellency. Chromium, however, has distinct disadvantages; Cr(VI) compounds (the most effective valence state for treatment) are toxic and also reported to be carcinogenic [24,25] and mutagenic [26]. The reduction product, Cr(III), imparts a green color to the wood. These disadvantages limit the ultimate practical usefulness of the Cr(VI) pretreatments. A better understanding of the wood-chromium inter­ action or the mechanisms by which chromium effects improvement in wood surface properties could lead to development of possible substitutes without the toxicity and color disadvantages.

Objective

The ultimate goal of our research is the complete definition of all mechan­ isms, interactions, and/or surface modifications which effect a stable wood surface. The specific objective of research reported here was to differentiate between chromium-cellulose interactions and chromium interactions with other wood components, such as lignin and extractives. These interactions

*Cr(VI) refers to chromium in its +6 oxidation state. “Cr(VI)” is used in this paper to de­ signate the general class of chromium compounds such as Na2CrO4, Na2Cr2O7, and CrO3.

“Chromic acid” designates only aqueous solutions of chromium trioxide (CrO3). “Chromium” is used when the oxidation state can be either Cr(III) or Cr(VI) or both. 255

included the changes in surface chemistry during chromic acid treatment of wood and pure cellulose. Chromium-hemicellulose reactions were not ad­ dressed in this preliminary investigation. The critical tool used to evaluate these surface interactions was Electron Spectroscopy for Chemical Analysis (ESCA). Through this technique, subtle changes in surface composition and chemistry were detected through nondestructive, in situ analysis. Subsequent research will address other aspects of chromium-wood interactions and sur­ face modification of wood.

Background

Most previous work on the mechanism of wood-chromium interactions has been done using copper- chromium-arsenic (CCA) preservatives. Dahlgren and Hartford [27-29] measured pH variations in wood following CCA treatment and proposed a mechanism for fixing* of the preservative. In their work using low concentrations of chromium compared with the weight of wood, they reported that initial ion exchange and adsorption of the treat­ ing solution caused a rapid rise in pH. This was followed by oxidation of wood components, precipitation of copper salts and Cr(III) arsenates, and

fixation of chromium complexes [29]. Final products included CrAsO4,

Cr(OH)3, and Cr(OH)CuAsO4. In studies with CrO3 and Na2Cr2O7, they re­ ported similar ion exchange and absorption, but did not elaborate on further

reactions using CrO3; They mentioned preferential oxidation of lignin over cellulose and indicated that extractives and reducing sugars play a role in Cr(VI) reduction. Duran et al. [30] used various combinations of copper, chromium, and arsenic treatment of maple to show that chromium is the unique ingredient in this fixing process. These studies did not specifically ad­ dress the potential for different interactions with extractives, lignin, and cel­ lulose. There has been some work dealing specifically with chromium-wood in­ teractions. Belford et al. [31] and Chou et al. [32] have shown that chrom­ ium and other metal salts complex with the cellulose microfibrils primarily in the S2 layer of the cell wall. Previous work at our laboratory [33,34] has shown that Cr(VI) is reduced to Cr(III) after application to wood surfaces. Pizzi [35,36] used dilute solutions of o-methoxyphenol and glucose as model compounds for lignin and cellulose to study Cr(VI) reactions. He claimed that Cr(VI) forms an insoluble polymer with o-methoxyphenol. In other work [37], based on kinetic studies of chromic acid with glucose, o­ methoxyphenol, and cellulose, and wood, he reported that 60% of the chro­ mium was fixed to lignin as Cr(VI) and 40% reacted with cellulose in a two- step process. Chromium (VI) was first absorbed then reduced to Cr(III) forming a weakly bound complex that could be leached with water.

*Fixing is the term used to indicate that the salts are no longer water soluble, i.e., they cannot be leached from the wood with water. 256

In contrast to the incomplete information on chromium-wood interac­ tions, the polyvinyl alcohol-chromium interactions have been well defined. Duncalf and Dunn [38], Van Nice and Farlee [39], and Bravar et al. [40], report use of Cr(VI) as a crosslinking agent in films for photoengraving as long ago as 100 years. Initial formulations used colloids such as gum arabic, but these have been replaced by polyvinyl alcohol (PVA). These PVA-Cr(VI) films become insoluble in water following irradiation and therefore the por­ tion of the film protected by a photographic negative can be removed by washing. The resulting coating permits etching of the unprotected areas leaving a positive plate useful for printing. The mechanism for the formation of this insoluble film involves the co­ ordination of Cr(III) to the unoxidized PYA [39]. The initial step produces a chromate (VI) ester. This step is similar to other chromate oxidations of alcohols (Westheimer and references therein) [41]. The chromate (VI) ester is photoreduced to the chromate (V) ester which disproportionates to form chromate (VI) and chromate (IV) esters. The oxidation of PVA continues forming a ketone and Cr(III) which coordinates primarily with available hydroxyls [39], and possibly the ketones [40]. The crosslinking occurs as a result of the change in chromium coordination from tetrahedral to octahe­ dral as it is reduced from Cr(IV) to Cr(III). Because the coordination takes place with the hydroxyls, the oxidized PVA has no effect on the crosslinking except in forming the Cr(III) coordination compound [39]. Bravar et al. [40] reported coordination of Cr(III) with alcohols and the newly formed ketones. There was minor disagreement between Bravar and Van Nice et al. on the role of the ketone in Cr(III) coordination. Although Van Nice and Farlee [36,39] did not specifically mention thermal initiation, Duncalf and Dunn [38] reported that similar coordination could be induced thermally. In work with potassium dichromate, they reported that the reduction of Cr(VI) is self-limiting because of a pH increase during the reaction. In ad­ dition, the possible complexing of Cr(VI) as (Cr(H2O)6)2(CrO4)3 [38] or

(Cr(H2O)5(OH))CrO4 [40, 42, 43] may prevent complete reduction of the Cr(VI). Although the complexing and/or increased pH may explain the presence of traces of Cr(VI), the driving force for the coordination was the reduction of Cr(VI) to Cr(III). A detailed investigation by Davidson [44] showed that oxidation of cot­ ton cellulose by dichromate in the presence of sulfuric acid (2 equivalents of sulfuric acid to 1 equivalent of dichromate) caused loss of weight and strength, increase in hygroscopicity, and the formation of carbon dioxide, formic acid, and soluble products. Rapid absorption of chromium was noted and the hydroxyl content of the highly oxidized cellulose (greater that one oxygen per glucose unit) diminished by only 10%. The decrease in the num­ ber of hydroxyls took place with the first half equivalent of oxygen con­ sumption. 257

ESCA

Several surface properties of complex solids such as wood can be mea­ sured using ESCA. These analyses reveal elemental or atomic constituents of the surface, the atomic percent of these elements, and their oxidation state. Several review articles describe the use of this instrument in detail [45-48]. Dorris and Gray [49-51] used ESCA to measure the atom percent of carbon and oxygen in paper and various pulps. Oxygen/carbon (O/C) ratios based on experimental atom percent agreed with calculated values for the O/C ratio. They reported a strong indication in their spectra of three distinct carbon oxidation states which could be separated using a simple gaussian curve fitting procedure. The O/C ratio correlated with changes in the propor­ tion of various oxidation states. They were able to distinguish high and low lignin content pulps based on the O/C ratio and show that surface lignin con­ centrations of solvent extracted pulps were higher than bulk concentrations. Mjöberg reported similar results [52]. Previous work in this laboratory, using ESCA on wood [34] elucidated the change in chromium oxidation state following treatment of wood with aqueous CrO3 solutions. In this previous work, no attempt was made to quantify these observations or explore the changes occurring in the sub­ strate, particularly the oxidation of carbon. Young [53] also used ESCA to evaluate changes in carbon following HNO3 treatment of maple. Nitric acid oxidation created a wood surface rich in both carbonyl and carboxyl func­ tional groups. ESCA facilitates evaluation of chemical changes in wood and cellulose following chromic acid treatment and it should be possible to gain insight into the interactions between Cr(VI) and extractives, lignin, and cellulose in treated wood. The analysis of the lignin-chromium interactions presents a particularly difficult situation since lignin cannot be isolated without de­ gradation. The isolation of lignin involves some chemical change, therefore reactions of Cr(VI) with degraded lignin would be difficult to characterize. Model compounds have been used [35-37], but small molecules or de­ graded lignin in solution poorly represent the highly crosslinked, immobile, spatial arrangement of lignin as it exists in wood. Relating model behavior to wood seems questionable, In addition to lignin, the effect of chromium oxidation of extractives must be addressed. To avoid misrepresenting wood with model compounds or degrading the surface with chemicals, wood species having different extractives content should be compared with cellu­ lose. Differences in the surface chemistry with chromic acid treatment should reflect the importance of different wood components. ESCA compa­ rison of Cr(VI) induced surface changes in wood with those in cellulose re­ presents a new method for determining the complex chromium interaction with wood components in a nondestructive in situ manner. ESCA is not a panacea for surface problems, particularly with cellulosic materials. Complications include the following: 258

• Specimen charging • Hydrocarbon and moisture contamination • Surface roughness • Secondary effects Surface charging due to slight specimen variation in conductivity leads to different peak shapes and binding energies. Flooding surfaces with low ener­ gy electrons compensates for the charging but care must be taken. Hydrocar­ bon contamination is a problem with all surfaces. The porous nature of cellu­ lose and wood compound this problem. The hydrophilic nature of cellulose makes moisture removal difficult. If water is not removed, its contribution to the O(1s) peak must be evaluated. The rough surface eliminates angular dependence depth profile measurements and thus removes one of the most valuable uses of the instrument. Secondary effects complicate evaluation of chemical shifts and curve fit analysis. For example, in cellulose, if all hydrox­ yls are treated as equal, the C2, C3, C4, C5, and C6 are equivalent and have one

OH bound to the carbon. C1 is an acetal. Thus, only two oxidation states of carbon are considered in evaluating the C(1s) peak. If secondary effects are considered, only C4 and C5 are equivalent. All others are different. For rigor­ ous curve fittings, these secondary effects should be considered. In this pre­ liminary report the secondary effects in cellulose were not evaluated because they are minor compared to the effects due to different functional groups in lignin, extractives, and the hemicelluloses. Several different oxygen functio­ nal groups, all with their own set of secondary effects, blur the chemical shifts. In order to evaluate the trends, some oversimplification was neces­ sary.

METHODS

All specimens for a particular species were microtomed from the same springwood (earlywood) growth ring. Specimens were obtained from red­ wood (Sequoia sempervirens) heartwood and southern pine (Pinus sp.) sap­ wood. Specimens were moistened prior to cutting to prevent excessive cur­ ling. Whatman No. 1 filter paper (specimens l, 2, and 3) was taken from a freshly opened package.* Paper and wood specimens were cut to size with a 7/16-inch punch. The resulting discs had a surface area of approximately 1 cm2. These discs were placed on clean glass slides and saturated with aque­ ous CrO3. Solution concentration and amounts of solution are given in Table 1. Duplicate specimens were prepared for chromium leaching experi­ ments. The specimens were held at 100% relative humidity (RH) for 2 h to allow the treating solution to penetrate, then slowly dried overnight at 90%

*The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the U.S. Department of Agriculture of any product or service to the exclu­ sion of other that may be suitable. TABLE 1

List of specimens, treatments, stoichiometry, and water leaching results

Specimen Material Mass Treatment Stoichiometry, Mass number description (mg) cellulose/Cr(VI) Cr(VI) Volume ratio Cr(VI) added Cr(VI) leached Amount conc.a (µl) (mg) (mg × 10-4) leachedb (%)

Whatman No. 1 filter paper 1 Untreated 9.0 – – – – – – 2 Treated' 9.0 5 10 3.8 0.5 8.05 0.16 3 Treated 9.0 10 10 1.9 1.0 3.09 0.03 Redwood 4 Untreated 2.9 – – – – – – 5 Treated 2.9 2.5 8 2.5 0.2 0.23 0.01 6 Treated 2.9 5.0 8 1.25 0.4 0.14 0.003 Southern pine 7 Untreated 3.7 – – – – – – 8 Treated 3.7 5.0 8 1.6 0.4 1.08 0.03

a % indicates actual amount of ion (i.e., 2.5% Cr (VI) = 4.8% CrO3 solution). b Calculated on the amount added to specimen.

'Treated with aqueous CrO3 (chromic acid). 260

RH. This produced specimens with essentially uniform chromium concen­ tration across the disc. (Rapid drying causes the chromium to concentrate at the edges.) Specimens were heated for 25 min at 135°C [33], packaged in­ dividually in filter paper envelopes, and then sealed in aluminum foil. This packaging minimized contamination of the specimens during storage and shipment. In addition, extreme care was taken through all steps of specimen preparation to avoid contact with any uncleaned surface or impure chemi­ cals. One specimen of each type was used for ESCA analysis and the second stored at ambient conditions for leaching experiments. Each of the second set of specimens was leached with 1 ml of distilled water for 1 h on a gentle shaker. Since all treated specimens were water re­ pellent, they were broken up and stirred vigorously prior to leaching. Leach­ ing was done on the same day as the ESCA. Following filtration of the speci­ men, the chromium concentration of the water was determined using a Perkin Elmer 500 Atomic Absorption Spectrophotometer. ESCA spectra were obtained using a modified Hewlett-Packard Model 5950A ESCA spectrometer.* Specimens were held in place by a stainless steel mask having a (3 × 10) mm slit to expose the specimen surface. The stainless steel mask helped reduce surface charging. In addition, a Hewlett- Packard Model 16623A Electron Flood Gun with emission current main­ tained at 0.5 mA flooded the surface with low energy electrons. The voltage on the flood gun was varied in order to maximize the C(1s) photoelectron signal. Experimental line widths of 1.4-1.7electron volts (eV) FWHM (full width at half maximum) were obtained for the major component of the C(1s) electron emission. Curve fit analysis was done by two different meth­ ods For Method 1, the component line width was not fixed at a set value. The program used a nonlinear regression algorithm that varied all parameters to achieve the most rapid convergence of fit. For Method 2, the component line widths were fixed at the experimental­ ly determined value. A Hewlett-Packard 9825A computer was used for the analysis and gave chi square values less than 1.7%. Pieces of Whatman No. 1 filter paper weighing 26 mg were placed in four 50 ml flasks and the flasks sealed with septums. After purging each flask for 24 h with prepurified nitrogen, two of the specimens were treated with 40 µl of 4.8% aqueous CrO3 solution. After standing for 36 h, the flasks were heated at 135°C for 25 min. Each flask was fitted with a balloon via a syringe needle to equalize pressure during heating. Following heat treatment, the gas in the flask was sampled and analyzed for CO2 on a Finnigan GCMS 4500 mass spectrometer.

*Surface Science Laboratories, Palo Alto, Calif. 261

RESULTS AND DISCUSSION

In this work we focused, first, on characterizing the bulk properties of the substrate through chemical analysis; second, on showing that fixation in small specimens duplicates the fixation in large specimens; and third, on characterizing the surface of chromic acid treated specimens. Two wood species were chosen for their different type and amount of extractives. Chemical analysis established this difference and the proportions of cellu­ loses and lignin were also found. Extraction experiments proved that the chromium fixed in small wafers duplicated the modified surface of large specimens. The ESCA results showed no increase in surface carbonyl or car­ boxyl functionality following CrO3 treatment. The results with ESCA sug­ gested possible decarboxylation and the analysis of CO2 with mass spectro­ scopy confirmed this. In previous studies on chromic acid treated wood, typical specimen thick­ ness was 1/4 inch. Initial ESCA experiments evaluated specimens of this thickness. Large specimen size resulted in two problems: first, excessive out­ gassing and therefore long evacuation time in the spectrometer, and second, poorly defined surface stoichiometry because of differential absorption of the chromic acid. Microtoming treated specimens was not practical because the chromic acid treatment embrittled the wood. To avoid these problems, 100 µm thick specimens were cut prior to treatment. This led to other con­ cerns. It was necessary to establish that the small wafers duplicated the large treated specimens used previously. To approximate the treating of large specimens, the chromic acid was added to just saturate the substrate. This amount varied among the three types of substrate because of differences in absorptivity. Redwood absorbed slightly more solution than southern pine and both species absorbed more than filter paper (Table 1). Note the weight of substrate versus the volume of treating solution. The difference in absorption between wood and filter paper is probably related to the higher crystalline content of the filter paper. The crystalline cellulose portions of filter paper and wood are highly resis­ tant to moisture absorption [54]. Solution concentrations were fixed at 2.5, 5, and 10% (by weight) based on the amount of Cr(VI) ion (Table 1). The stoichiometry for the oxidation reduction reaction can be calculated for fil­ ter paper. The filter paper is composed primarily of cellulose and the nomi­ nal weight of Whatman No. 1 filter paper was 0.009 g/cm2 (Table 1). The chromic acid, a 4.8% aqueous CrO3 solution, contained 0.025 mg Cr(VI)/µl solution. Based on the equation for Cr(VI) oxidation of alcohols

+ +3 2CrO3 + 3R2CHOH + 6H 3R2C=0 + 2Cr + 6H2O, the equivalent weight for CrO3 is 33 g or 0.033 g/milliequivalent. For oxida­ tion of one hydroxyl per glucose unit, the milliequivalent weight is 0.081 g. The treatment of 0.009 g of cellulose with 10 µl of 5.0% Cr(VI) gives a 3.8:1 cellulose to Cr(VI) ratio (Table 1). The same calculation was done for the 262 two wood species (Table 1). The calculation for wood is based on cellulose and hemicelluloses and does not take into account the lignin and extractives. These components would lower the ratio since they have fewer hydroxyls. The ratios are reported only to show minimum ratios for wood and paper and that they were similar. The ratios at the surface were probably higher since the absorption into highly crystalline regions of the cellulose in both filter paper and wood are less. The main point is that sufficient chromium was added to cause a measurable difference in the chemistry of the material. Based on the ratios, roughly 10% of the hydroxyls for cellulose should be af­ fected. With preference for surface reactions, the percent would increase. Springwoods from the two wood species were analyzed to determine their composition. Springwood composition

Extractives Carbohydrates Lignin (%)

Redwood 8.8 55.0 35.8 Southern pine 4.8 66.2 27.4 Although there is a slight difference in lignin and carbohydrate percent, the largest proportional difference is in the amount of extractives. Redwood contained almost twice as much extractives as southern pine. If extractives are important in chromic acid reactions with wood, some difference should be observed between these two species. The interpretation of the surface analysis should take into account the bulk composition. In previous work with chromic acid treated wood, relatively small a­ mounts of solution were added to thick specimens. The absorption into the wood may have kept the Cr/wood ratio low. In this work with thin wafers, the danger existed of oversaturating the specimens with chromic acid solu­ tion. Too much Cr(VI) could have led to incomplete fixation. Previous work showed that the extractable chromium decreases as the complexation or fixation of chromium proceeds [33]. The aqueous extraction of the small treated wafers yielded only small amounts of leachable chromium. They ranged from 0.003% to 0.16% and were typical of previous results with thicker specimens (Table 1). Thus, small specimens seemed to represent the surface of thicker specimens. In light of previous reports on chromium fixa­ tion [ 27-29, 37-39] we were surprised to find that fixation also occurred with filter paper. The result of the aqueous extraction show that chromic acid fixes as well to pure cellulose as to wood (Table 1). In addition, the chromic acid treat­ ment gives the cellulose exceptional water repellency. On some specimens it was possible to form a bead of water which evaporated before soaking into the paper. Because the chromic acid reacts with pure cellulose, this type of reaction may also be responsible for part of the fixation with wood. The re­ ports in the literature [ 27-29, 37-39] of complexation with hemicellulose 263 and lignin may not be the whole story. An aggressive oxidizing agent and strong coordinating agent such as chromium may be rather unselective in its reaction with wood. In wood, lignin and hemicellulose are available and are no doubt oxidized and may coordinate with chromium. However, oxidation and coordination of cellulose cannot be ruled out. The reactions occurring with pure cellulose may also occur in wood. The surface composition of all specimens were readily apparent from ESCA survey spectra (Fig. 1). The spectra show the amount of electrons counted versus the binding energy in electron volts (eV) from 800 eV to 0 eV. The spectra consist of 1s electrons from carbon (C(1s)), 1s electrons from oxygen (O(1s), and 2p and 3p electrons from chromium (Cr(2p)) and (Cr(3p)). The doublet for Cr(2p) is due to spin splitting. By collecting data over a narrower range, high resolution spectra of each photoelectron peak were obtained. High resolution ESCA spectra of the electron emission spec­ tra permit calculation of atom percent, differentiation of the chemical en­ vironment, and precise measurement of relative binding energy (Fig. 2).

Fig. 1. ESCA spectrum of CrO3 treated Whatman No. 1 filter paper showing electron emissions for oxygen, carbon, and chromium.

These plots show the number of electrons counted versus binding energy. The area under the peak is proportional to the electron counts for a particu­ lar emission. The counts must be adjusted to compare peak areas of various emission and to calculate surface atom percent. This adjustment or sen­ sitivity factor accounts for photoelectron cross-sections, inelastic electron mean free path, spectrometer constants, and transmission function. The 264 empirically determined sensitivity factors for the type of instrument used were as follows: C(1s) 1.00 O(1s) 2.93 Cr(2p) 7.69 Cr(3p) 1.17 The peak areas were adjusted and the atom percent calculated [49-51] for each specimen (Table 2). The changes in atom percent are very slight and do not reflect a large surface oxidation. The amount of surface chromium also seems too small. Calculations of O/C ratio from the adjusted peak intensity shows that the surface of cellulose changed little from the theory or litera­ ture values. Redwood and southern pine have the expected O/C ratio for wood [49-52] but do not show significant surface oxidation after treat­ ment. An increase in O/C ratio should have occurred just from the added chromates. The surface for all specimens show a lack of significant surface oxidation and little accumulation of chromium. High resolution photoelectron spectra differentiate chemical shifts and splitting and permit detection of subtle chromium induced structural changes at the solid surface. In this research the C(1s) peak was of particular interest. The binding energy of C(1s) photoelectrons is dependent on the

oxidation state of carbon. The C(1s) components, arbitrarily assigned Ca, Cb,

Fig. 2. Curve fitting results (Method 1) of C(1s) and O(1s) high resolution peaks for Whatman No. 1 filter paper. 26 5

TABLE 2

Elemental surface composition and O/C ratio

Specimen Material C O Cr O/Ca number (Atom %)

Whatman No. 1 filter paper 1 Untreated 58 42 – 0.73 2 Treatedb 52 47 1.0 0.91 3 Treated 54 44 2.4 0.81 Redwood 4 Untreated 75 25 – 0.33 5 Treated 75 23 1.7 0.30 6 Treated 64 31 4.2 0.49 Southern pine 7 Untreated 73 27 – 0.37 8 Treated 70 28 2.7 0.40 Cellulosec 55 45 – 0.83 a Calculated [49-51]. b Treated with chromic acid. c Theoretical.

proportion of each oxidation state can be determined by curve fitting the C(1s) data. Different oxygen bonding can also be separated. The results of these curve fits are shown for treated and untreated filter paper (Fig. 2), red­ wood (Fig. 3), and southern pine (Fig. 4). In the C(1s) curves for filter paper, little change has occurred in the hy­ droxyl and acetal peaks with treatment. The hydrocarbon portion shows a significant increase, however. In the redwood specimens (Fig. 4), surface hydroxyl concentration decreased with chromic acid treatment while the relative proportion of the has increased drastically. The same behav­ ior was observed with southern pine. The C(1s) peaks show a decrease in oxygen after treatment, An additional constituent appears in the O(1s) peak following treatment that is probably due to added Cr-O. The loss of is compensated by the gain in Cr such that little change in the O/C ratio was observed (Table 2). The fit of the curves shown in Figs. 2-4 was obtained by method 1. High resolution C(1s) spectra were also fit by a second method and the result of both are reported (Table 3). Binding energy, full width at half maximum (FWHM) and the percent of each component shows the same trends for 266

Fig. 3. Curve fitting results (Method 1) of C(1s) and O(1s) high resolution peaks for red­ wood.

C(1s) by both methods. The components of O(1s) were found by method 1 and are also listed (Table 3). The components of the chromium peaks were estimated from the peak heights and show relative peak intensities for Cr(VI) and Cr(III) oxidation states. Both the Cr(2p) and Cr(3p) photoelectron peaks indicate a 75-100%reduction of the Cr(VI). The lack of full reduc­ tion of chromium indicates complex salt formation similar to that discussed earlier [38-40,42,43]. The photoelectron peaks must be shifted to compensate for charging ef­ fects and the adjusted binding energies in Table 3 combine data from all high resolution (C(1s), O(1s), Cr(2p3/2), and Cr(3p)) spectra. In specimens 1, 2, and 3 (cellulose), the major C(1s) emission was due to (Cb) and the maximum on the peak was adjusted to 286.2 eV, the expected absolute value for carbon bound to one oxygen. All other emission peaks on the same speci­ mens (1-3)were shifted the same amount. In the wood specimens, the high lignin and/or extractive concentration on the surface contributed to higher contributions from and oxidation state (Ca) and the maximums were corrected to 284.6 eV. This change from Cb in specimens 1, 2, and 3 to Ca in the remaining samples was supported by the atom percent and the O/C ratios. These adjustments seem very appropriate in light of the excellent 3+ 6+ 3+ agreement in binding energy of the O(1s), Cr (2p3/2), Cr (2p3/2), Cr (3p), 267

Fig. 4. Curve fitting results (Method 1) of C(1s) and O(1s) high resolution peaks for southern pine.

6+ 3+ and Cr (3p) electron emissions (Table 3). For example, the Cr (2p3/2) peak was 576.8 eV for all treated specimens. Hydrocarbon contamination of specimens is almost certain and the small component in pure cellulose is undoubtedly a result of minor conta- mination. The addition of treating chemicals may add more contamination. And in treating wood, the aqueous solution may cause the extractives’ mi- gration to the surface, Although contaminants are possible and may con- tribute to the increase in the peak, decarboxylation of the surface fol­ lowing oxidation also fits the data.

A plausible explanation for a slight increase in Ca and the lack of obser­ vable cellulose oxidation is shown in Scheme 1. Davidson [44] reported the evolution of volatile products including CO2 from acid chromate treated cellulose. TABLE 3

Binding energy, full width at half maximum, and peak component percent for high resolution spectra

Speci- Description C (1s) (Method 1) C (1s) (Method 2) O(1s)(Method 1) Cr(2p3/2) Cr (3p) men number b.ea FWHMb Pro- b.e. FWHM Pro- b.e FWHM Pro- b.e. Pro- b.e. Pro­ (eV) (eV) por- (eV) (eV) por- (eV) (eV) por- (eV) por- (eV) por­ tionc tionc tionc tiond tiond (%) (%) (%) (%) (%) Whatman No. 1 Filter paper 1 Untreated 284.6 2.1 6 284.6 1.4 -10 – – – – – – 286.2 1.3 66 286.2 1.4 70 – – – – – – 287.6 1.8 28 287.9 14 20 – – – – – – 2 Treatede 284.6 2.9 20 284.6 1.6 8 532.0 1.3 21 576.8 80 44.0 77 5% Cr(VI) 286.2 1.6 61 286.2 1.6 72 533.1 1.2 51 578.6 20 46.6 23 287.3 1.8 19 287.9 1.6 20 534.0 1.2 28 3 Treated 284.6 2.0 24 284.6 1.7 14 532.0 1.3 23 576.8 83 44.2 88 10% Cr(VI) 286.2 1.5 54 286.2 1.7 68 533.4 1.4 56 sh 17 sh 12 280.7 1.9 22 288.1 1.7 18 534.3 1.3 21

Redwood 4 Untreated 284.6 1.4 43 284.6 1.4 52 532.0 1.1 23 – – – 286.4 1.7 44 286.3 1.4 42 532.8 1.0 52 – – – 287.7 2.9 13 287.9 1.4 6 533.7 1.1 25 – – – 5 Treated – – – 283.0 1.5 3 532.0 1.7 28 576.85 85 44.3 100 2.5% Cr(VI)284.6 1.6 63 284.6 1.5 69 533.3 1.5 51 sh 15 – 286.2 2.4 33 286.3 1.5 20 534.5 1.4 21 288 7 1.4 4 288.3 1.5 8 e Treated – – – 283.5 1.5 4 532.0 1.6 28 576.8 76 44.1 79 5% Cr( VI) 284.6 1.6 70 284.6 1.5 58 533.4 1.4 46 578.9 24 47.3 21 286.2 2.1 17 286.2 1.5 25 534.7 1.4 26 288.3 2.4 13 288.2 1.5 13 Southern pine 7 Untreated 284.6 1.4 46 284.6 1.5 58 532.0 1.3 20 286.3 1.8 47 286.2 1.5 35 533.0 1.3 61 288.6 1.6 7 288.6 1.5 7 534.1 1.3 19 – – – – 8 Treated – – – 283.4 1.55 –f 582.0 1.7 29 576.8 100 44.2 77 5% Cr(VI) 284.6 1.7 70 284.6 1.55 71 533.2 1.7 52 – 47.8 23 286.3 2.2 18 286.4 1.55 17 534.6 1.5 19 288.5 1.7 12 288.3 1.55 12 a Binding energy. b Full width at half maximum. c The percent of total C(1s) peak found by curve fitting. d The percent of total peak estimated from peak height. e Treated with chromic acid. f The small contribution from 283.4 eV was combined with the 284.6 component 269

Chromic acid treatment of small pieces of filter paper in closed flasks under pure nitrogen produced five times higher levels of CO2 than the con­ trols. Compared with nitrogen, the absolute values were small and varied from a low of 0.03% for one of the controls to 0.37% for one of the treated specimens. By sampling each specimen several times and scanning over a narrow mass range (42-46 AMU), consistent ion counts were obtained. The ratio of CO2 for the treated versus untreated surfaces are ratios of these ion counts. The presence of CO2 indicates that decarboxylation could account for the increase in Ca in the C(1s) peak. In this study, the emphasis has been on surface reactions. The oxidation of the surface by chromic acid followed by decarboxylation is consistent with the ESCA data and the surface properties of treated wood and paper. The ESCA indicate a surface enriched in hydrocarbon but having only a small amount or chromium. The increased hydrocarbon (decreased hydro­ xyl) yields a more water repellent surface. In light of previous work showing that photoreduction of Cr(VI) forms a crosslinked Cr(III) structure in poly­ vinylalcohol (PVA), similar crosslinking could be applicable to cellulosic sys­ tems and could remove chromium from the surface and thus explain the small amount measured. Depth profile experiments are planned and may help confirm this tentative explanation. The partial retention of Cr(VI) as shown by ESCA may be caused by complexes similar to those discussed previously [38,40,42,43]. The coordination with lignin as suggested by Pizzi [35,37] seems possible; however, the ESCA analysis shows that the Cr(VI) has been reduced. As the fixation is accelerated through heating, this differ­ ence may be a function of the heating. In both wood and cellulose, ESCA data show that reaction of Cr(VI) to Cr(III) takes place during fixation. The reported absorption and weak attraction to cellulose reported by Pizzi [35­ 37] must be due to incomplete fixation. The ESCA data does indicate some Cr(VI) but the leaching experiments showed that this ion was strongly fixed in the substrate. The presence of complex ions seems more likely. The re­ ported reactions with PVA, in which chromium complexes were coordinated to hydroxyls, seem similar to reactions with wood. Similar coordination of chromium to the hydroxyls in cellulose and wood may cause some of the beneficial properties.

CONCLUSIONS

These experiments prove that chromic acid fixes to both wood and pure cellulose. With both materials, complete fixation of chromium resulted in a highly water repellent surface. The fixation to cellulose indicates that Cr(VI) reactions with lignin alone cannot account for the stabilized surface of treated wood. Chromium oxidation results in decarboxylation of the wood surface and occurs after oxidation with chromic acid. The chromium defi­ cient surface suggests coordination of chromium with subsurface hydroxyls but no depth profiling was done to confirm this. The coordination may be

271

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