The Effect of Solvents on the Chemical Composition. Of Archaeological Wood

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1 The Effect of Solvents on the Chemical Composition Of Archaeological Wood S.S. Darwish and N.M.N. El Hadidi Conservation Dept. - Faculty of Archaeology - Cairo University Summary Solvents are widely used in wood conservation for either dissolving polymers for consolidation purposes or for removing dirt or foreign matter, which was applied during previous conservation. Solvents may cause temporary swelling of the wood, and when they evaporate wood returns back to its original size. They may form complexes with wood components when unlimited swelling arises as a consequence of breaking the adjacent bonds. These complexes have been shown to be stable for long periods of time at elevated temperatures or under high vacuum, while they are not stable with regard to moisture. Wood absorbs organic solvents, which are retained in the wood cells for short periods of time only in normal conditions. During the short presence of solvents inside the wood, wood components are slightly affected, and after evaporation of the solvent the chemical composition of wood may undergo some changes. Using FTIR, the molecular bonds in wood samples that were treated with three different organic solvents were closely studied. Keywords: solvents effect; archaeological wood; FTIR Introduction Different types of dirt such as grease, old varnish, paints, glue and mildew stains are often found on archaeological wooden artifacts. It may often be difficult to choose a solvent which is both effective and safe. However, wood has giant molecules with either primary or strong secondary bonds linking them together. These are more difficult to 1

2 dissolve than the small dirt molecules. It has always been believed that the consequence for cleaning non deteriorated wood is that organic solvents, in general, are likely to be innocuous as far as any risk of dissolving the main structural materials of wood such as cellulose and lignin. One can be rather less sure about the supplementary materials of an object such as varnishes, colours and pigments ( Moncrieff and Weaver, 1994). In cases where wood is deteriorated, cleaning may become a very difficult matter, because major wood polymers may have deteriorated due to different factors. Usual cleaning solvents do not only dissolve dirt or other foreign matters, moreover they can also remove some of the deteriorated outer layers of the wood. Solvents are used for many purposes in conservation including cleaning and the application or removal of coatings, consolidants and adhesives. Making the most of any solvent requires familiarity with the basic principles of molecular bonding and an understanding of how the structure of solvents affects their physical and chemical properties. Chemical cleaning of wooden surfaces involves the use of reagents, which are chemicals that break primary molecular bonds, converting dirt, varnish or other unwanted material to a different form in order to remove it from the surface. After solvent cleaning, original material cannot be recovered in the same way as it was, when the archaeological object was first made in the past (Rivers and Umney, 2003). The aim of this study is to find out how the two major wood components are affected by solvents commonly used in cleaning archaeological wood. Materials and Methods Ancient deteriorated samples taken from the Mashrabieh of Bazarah (Ottoman period), which had been previously identified as oak wood (El Hadidi, 2003) were used for studying the effect of three solvents commonly used for either cleaning wood or during 2

3 wood consolidation in Egypt at the present time. The three solvents chosen were: Ethanol, Acetone and Toluene. Three wood samples were immersed in every solvent separately for one hour. Samples were then removed and left to dry out in normal room temperature and humidity conditions. Samples were then divided into three groups, each group consisting of 4 samples; i.e. archaeological sample before immersion in solvent and archaeological sample after immersion in one of the three solvents. The first group was studied by FTIR spectroscopy (JASCO FTIR plus 460). The second group was aged using UV radiation (Spectroline UV A lamp, wave length 365 nm) for 100 hours in normal room temperature at a distance of 15cm and the third group was aged using heat for 100 hours at105+3 C. Samples from the second and third groups were also studied by FTIR spectroscopy, so as to study and compare the changes that had occurred in the chemical bonds of both wood cellulose and lignin before and after ageing. Results and Discussions I- Effect of Heat and U.V. ageing on Wood Samples I.1- Heat Ageing: Heat ageing of wood is expected to undergo hydrolysis of glycosidic bonds of cellulose and oxidation of functional groups of glucopyranose rings. Lojewska, et al. (2005b) found that oxidation and hydrolysis of cellulose are supposed to proceed at temperatures not higher than 100 o C. At higher temperatures the reaction scheme would have to include dehydration, condensation or transglycosidation reactions. I.1.1- Hydrolysis: is indicated by splitting of the hemiacetal bond between the two glucopyranose rings C 1 & C 4. The terminal rings giving rise to the cleavage of C 1 -O-C 5 bond in the same ring. On opening the ring, the C 5 -OH formed becomes available for oxidation and the formation of CHO group. 3

4 I.1.2- Oxidation: Carbon atom occupying various positions in the ring C 1, C 2..C 6 gradually transforms by oxidation into various carbonyl groups, therefore intensities of C-O bands decrease. Oxidation was detected as fairly broad and overlapping bands in FTIR spectroscopy. Shafizadeh & Chin (1977) and Englund & Nussbaum (2000) studied some minor changes that can occur in wood at temperatures above 50 C, such as elimination of water and release of volatile components (i.e. monoterpenes). Hancock (1963) and Nuopponen, et al. (2003) observed migration of wood resin onto the surface of wood at temperatures between C. Heat ageing also caused some modifications in lignin structure including depolymerisation and condensation reactions. The first thermal changes in lignin can be detected at temperatures above 150 C (Fengel and Przyklenk (1970); Faix (1988) & Nuopponen (2005)). Molecular weight of lignin has been reported to decrease extensively at temperatures above 180 C in various thermal treatments as a result of breaking down of aryl-ether interunit linkages (Westermark, 1977). The amount of methoxyl groups in lignin diminished when wood was heated at temperatures higher than 180 C. At elevated temperatures (> 200 C) structure of lignin becomes more condensed (Wikberg and Maunu, 2004). Nuopponen (2005) found that of the structural components, hemicelluloses are the most vulnerable to thermal degradation. Degradation rates of hemicelluloses have been reported to be four times higher at 150 C than that of wood or α-cellulose. Sundqvist (2004) revealed that acetic and formic acids liberated from wood during thermal treatment enhanced hydrolysis of hemicelluloses and cellulose. Noticeable decrease in the content of polysaccharides occurs at temperatures above150 C. Fengel and Wegener 4

5 (1989) found that hydrolyzed sugars are further dehydrated and great varieties of volatile compounds are formed, such as furfural and hydroxymethyl furfural. The results in figures (1,2,3,4&5) clarified the effect of both heat and U.V. ageing on the archaeological wood samples. The results showed that the band at 1645 cm -1 due to bending modes of water molecules disappeared, which is evidence of complete water desorption. These findings were noted in the spectrum of the wood samples that were heated for 100 hours at C. Similar results were recorded in previous research in the field of degradation of wood components (Lojewska, et al. (2005b); Englund and Nussbaum (2000); Hatakeyama, et al. (1976); Zhou, et al. (2001) & Lojewska, et al. (2005a)); where they had noted the disappearance of adsorbed water by the vanishing of the 1640 cm -1 bond from the samples that were being monitored, and at elevated temperatures water desorbs from wood and does not reabsorb again. We may summarize the changes in our samples due to heat ageing as follows: - In the archaeological sample sharp OH stretching band appeared at 3782 cm -1 due to free OH; i.e. heat broke H-bonds. A carbonyl band at 1621 cm -1 appeared in the sample as a result of natural ageing of the sample. This band was due to conjugated carbonyl groups resulting from partial oxidation of C-OH groups at C 2 and C 3 in the glucopyranose ring. - Heat ageing caused sharp decrease in C-O stretching band intensities at 1112 cm -1 and 1056 cm -1 and the disappearance of the band at 1033 cm -1. These findings are attributed to the oxidation of CH-OH groups to carbonyl groups which appeared at 1635 cm -1 leading to broadening of the band. - Lignin bands at 1509 cm -1 (aromatic >C=C< stretching) and 1269 cm -1 (C-O-R stretching) decreased as a result of its degradation. This result agreed with previously published research (Westermark, et. al., 1997) which noted that degradation of lignin 5

6 results from breaking down of aryl-ether interunit linkages, so the amount of methoxyl groups in lignin diminishes. - The intensity of various bands observed in the OH bending and CH deformation zone (between 1400 cm cm -1 ) also decreased. Similar studies were done by other researchers (Nuopponen, 2005) who had examined the thermally induced changes in pine wood with Fourier transform infrared (FT-IR) and UV resonance Raman (UVRR) spectroscopy. The Spectroscopic data revealed that the structure of thermally treated wood was extensively modified at temperatures above 200 o C. These modifications included the depolymerisation and condensation of lignin, degradation of hemicelluloses as well as the removal and/or decomposition of the wood resin components. I.2- U.V. ageing: U.V ageing showed a slight variation compared to heat ageing. Hydrolysis and oxidation occurred in limited cases. Exposure of the sample to 100 hours of U.V. radiation caused complete water desorption and disappearance of the water band at 1645 cm -1. U.V. radiation had a similar effect as heat on the H-C-OH groups that were oxidized to carbonyl groups, leading to the decrease in C-O intensity at 1117 cm -1 and its disappearance at 1033 cm -1 in addition to the broadening of the carbonyl band. A decrease in the lignin band (aromatic C=C) at 1509 cm -1 was noticeable. II- Effect of solvent treatment on archaeological wood samples Figures (6,7,8,9&10) showed the effect of two polar solvents, ethanol and acetone, and a moderately polar solvent like toluene on the stability of archaeological wood samples. Wächter (1974) suggested that the treatment of paper with an organic solvent during the conservation treatment might result in the formation of a permanent cellulose-solvent complex. The formation of such a complex might increase the reactivity of the paper and accelerate its rate of aging. Other researchers (Wiertelak and Garbaczowna (1935); Staudinger (1953); Wade and Creely (1974) & Arney and Pollack (1980)) have 6

7 demonstrated that these complexes have shown some stability for long periods of time at elevated temperatures or under high vacuum, while they are not stable with regard to moisture. Horvath (2006) suggested that the swelling of cellulose in organic solvent is related to the swelling of wood. The swelling of cellulose appears to be intercrystalline (the solvent enters into the amorphous areas) and intracrystalline (the solvent penetrates in the crystalline regions). The solvent forms complexes with cellulose when unlimited swelling arises as a consequence of breaking the adjacent bonds. The extent of swelling depends on the solvent as well as on the nature of the cellulose sample. The resulting separation of the polymer chains indicates the beginning of the solubility. The dissolving ability entails formation of a complex with the two secondary hydroxyl groups in cellulose and with breaking of hydrogen bonds. The swelling and solubility of lignin is greater with hydroxylated solvents (swelling solvents), e.g., methanol, ethanol, phenol, and water than non polar solvents (non swelling solvents) like benzene and toluene. The hydrogenbonding capacities of various solvents are proportional to the shift in wave length of the infrared region of the spectrum. II.1- Unaged alcohol treated sample: The results in figures (6,7,8,9&10) showed that asymmetric and symmetric stretching modes of water molecules at 3534 & 3406 cm -1 disappeared and typical OH stretching resulted due to the formation of wood- alcohol complex; i.e. alcohol displaces water molecules. The results agreed with that of Horvath (2006) who proved the formation of this complex in case of swelling solvents (polar solvents, e.g. ethanol, acetone,.). The formation of wood-alcohol complex accelerated the rate of wood ageing. Intensity of C-O stretching bands at 1112, 1057 and 1032 cm -1 decreased (in comparison to the untreated sample) due to the oxidation process on C 2 7

8 OH, C 3 OH and C 6 OH and the formation of enolic group-carbonyl group tautomor at 1646 cm -1 (Mosini, et al.(1990); Ali, et al.(2001) & Calvini and Gorassini (2002)). A new additional band appeared at 1160 cm -1 (in comparison to the untreated sample). This band may be due to opening of the terminal rings and cleavage of C 1 -O-C 5 bond and formation of C 5 -OH group. All the bands between cm -1 decreased. These bands were due to OH bending, CH deformation, aromatic >C=C< stretching of lignin and C-O-R stretching in lignin. II.2- Unaged acetone treated sample: Acetone accelerates wood oxidation leading to decrease of C-O stretching band at 1119 cm -1 and disappearance of C-O bands at 1056 cm -1 and 1033 cm -1 due to complete oxidation of C-OH groups. Asymmetric and symmetric stretching modes at 3534 & 3406 cm -1 and bending modes at 1646 cm -1 of water molecules slightly decreased as some acetone replaced water and formed woodacetone complex. A complex vibrational pattern of various carbonyl groups due to partial cellulose oxidation products appeared at 1682, 1646 and 1621 cm -1. Lignin bands decreased as a result of lignin degradation. II.3- Unaged toluene treated sample: Formation of toluene wood complex is limited due to its lower polarity. So, its effect on wood reactivity and on the ageing of wood components is small compared with that of alcohol and acetone.. Toluene may accelerate hydrolysis of both cellulose and lignin and whose effect on lignin bands was more than that of cellulose. Asymmetric and symmetric stretching modes of hydroxyl groups as well as C-O stretching bands slightly increased. These findings may result from opening some of the glucopyranose rings or/and splitting few of the hemiacetal bonds between the two glucopyranose rings C 1 & C 4 and formation of C 1 -OH, C 4 -OH and C 5 -OH groups. Lignin bands at 1509 and 1267 cm -1 decreased due to its degradation. 8

9 III- Effect of heat ageing on wood treated samples Figures (11,12,13,14&15) clarified the heat effect on the wood treated samples that were heated for 100 hours at C III.1- Heat aged alcohol treated sample: New bands due to free OH stretching appeared in the region between 3710 and 3565 cm -1. These bands were formed as a result of heat breaking down of intermolecular hydrogen bonding. Various carbonyl groups bands (CO-CHO-COOH groups) were formed due to partial oxidation occurring predominantly on C-OH groups in glucopyranose rings. These bands were reasonably broad and overlapping, and their intensities increased in comparison to similar bands in the heated untreated samples. The band at 1732 cm -1 is presumably from the ester groups which may arise at this position of the spectrum and may form in the reaction of the carboxylic groups with unreacted alcoholic group or with residual ethyl alcohol. Similar results were obtained by Inari, et al.(2007) who stated that compared to lignin, holocellulose exhibits important infrared absorptions of about 1,730 cm 1, characteristic of ester or urethane linkages. This hypothesis is confirmed by the presence of additional newly formed C-O-C vibration from ester at 1155 cm -1. The band at 1716 cm -1 may represent carboxylic groups (the final oxidation step of C-OH groups). Rutherford, et al.(2004) recorded the same band in the case of charred lignin spectra. Bands at 1844cm -1 and 1771cm -1 fit the pattern observed in five-member ring cyclic anhydrides (Colthup, et al., 1990). Band at 1683 cm -1 could be attributed to ß-diketones similar to quinone-type vibrations described by Agarwal (1998). Lignin degraded faster when treated with alcohol i.e. band at 1268 cm -1 disappeared and at 1509 cm -1 decreased compared to heat untreated sample. Intensity of C-O band at 1112 cm -1 increased as a result of hydrolysis of the hemiacetal bond between two glucopyranose rings C 1 -O-C 4. The terminal rings may open giving rise to the cleavage of C 1 -O-C 5 band in the same ring and to the 9

10 formation of CHO groups. On opening the ring, the C 5 -OH groups were formed and this increased the C-O intensity. The above results showed that oxidation accompanied hydrolysis, because residual oxygen is always present in wood, and conversely, hydrolysis cannot be avoided during oxidation because residual water is present in wood structure and also because water is a product that occurs during wood oxidation. The same results were obtained by Lojewska, et al. (2005b) who proved that oxidation accompanied hydrolysis during cellulose degradation. III.2- Heat aged acetone treated sample: The results obtained were nearly the same as in case of alcohol. New band at 1156 cm -1 appeared (in comparison to the untreated sample). It may be due to C-O group of ester formation or new C-OH groups resulting from opening of the pyranose ring. More carbonyl groups were formed as a result of further oxidation of C-OH groups of cellulose molecules. III.3- Heat aged toluene treated sample: Heat ageing of toluene treated samples showed slight variations compared to heat aged untreated ones. More decrease in C-O stretching bands, lignin bands at 1509 &1267cm -1 and increase in C=O bands. While, in comparison to unaged toluene treated samples, heat increased the effect of toluene on wood components, i.e. more oxidation and hydrolysis. Sharp decrease in C-O and lignin bands; and an increase and broadening in C=O bands occurred. IV. Effect of U.V. ageing on wood treated samples Exposure of wood treated samples to U.V. radiation for 100 hours had a great effect on wood components as shown in figures (16,17,18,19&20). IV.1- U.V. aged alcohol treated sample: C-O band intensity at 1121cm -1 increased in comparison to U.V. aged untreated sample and to the alcohol treated sample. This 11

11 increase is due to the combination of new C-O of C 5 -OH resulting from the opening of the pyranose ring and C-O of residual unoxidized C 2 -OH and C 3 -OH. Carbonyl group due to C 2 and C 3 oxidation had the same intensity compared to U.V. untreated sample. Lignin bands at 1509 cm -1 decreased and at 1268 cm -1 disappeared, i.e. alcohol increases the effect of U.V. ageing on wood. IV.2- U.V. aged acetone treated sample: As in case of alcohol, intensity of C-O stretching band increased due to the combination of new C-O of C 5 -OH resulting from the opening of the pyranose ring and C-O of residual unoxidized C 2 -OH and C 3 -OH. Hydration band at 1646 cm -1 appeared again as some water molecules replaced solvent in wood-acetone complex. i.e. U.V ageing occurred in normal (humid) conditions. Intensity of carbonyl groups band at 1623 cm -1 increased as a result of oxidation process. Lignin bands decreased as a result of lignin degradation. IV.3- U.V. aged toluene treated sample: Like heat ageing, U.V. increased the effect of toluene on wood components. Absorption of C-O groups decreased while that of C=O increased (compared to both unaged treated & U.V. aged untreated samples) as a result of conversion of C-O-H to C=O by oxidation. Also, intensities of lignin bands at 1509 & 1267 cm -1 decreased. Conclusion It was found that the chemical composition of the two major wood components is affected by organic solvents commonly used in cleaning archaeological wood. Ethyl alcohol and acetone accelerated oxidation and hydrolysis of both cellulose and lignin. Toluene showed a slight change compared to the other solvents used. So, it can be used safely in conservation treatment. More over, it is an environmentally friendly compound. 11

12 (Fig.1): Effect of heat and U.V. ageing on wood sample All cellulose and lignin bands decreased as a result of heat and U.V ageing. Some changes in position or shape of bands are noticeable. Some bands disappeared and new bands were formed. (Fig.2): C-O stretching zone (Fig.3) : OH bending and CH deformation zone Heat ageing; accelerated hydrolysis and oxidation processes. New band was formed at 1158 cm -1 (in comparison to the untreated sample) as a result of ring cleavage i.e. C 5 -OH was formed. The intensities of all C-O bands decreased. (Fig.4): C=O stretching zone The intensity of various bands observed in the OH bending and CH deformation zone (between 1400 cm cm -1 ) decreased. (Fig.5): O-H stretching zone Heat & U.V. ageing increased the intensity of carbonyl groups as a result of partial oxidation of various C-OH groups to C=O groups. Water bending modes band at 1645 cm -1 was also removed. A decrease in the lignin band (aromatic C=C) at 1509 cm -1 was noticeable. A carbonyl band at 1621 cm -1 appeared in the sample as a result of natural ageing of the archaeological sample. Asymmetric & symmetric water stretching modes were present in both archaeological sample & U.V. aged sample. As a result of heat ageing water stretching bands were removed and typical O-H stretching band was formed. 12

13 (Fig.6): Effect of solvent treatment on unaged archaeological wood samples Solvents displaced water molecules forming solvent-wood complexes and accelerated the rate of wood ageing. Formation of these complexes depends on (Fig.7): C-O stretching zone (Fig.8): OH bending and CH deformation zone Ethanol treatment decreased the intensity of C-O stretching bands at 1112, 1057 and 1032 cm -1. A new additional band (in comparison to the untreated sample) appeared at 1160 cm- 1. Acetone treatment caused disappearance of C-O bands at 1056 cm -1 and 1033 cm- 1 due to complete oxidation of C-OH groups. Lignin bands at 1269 cm -1 decreased as a result of its degradation. The intensity of various bands observed in this zone also decreased. (Fig. 9) C=O stretching zone (Fig.10) O-H stretching zone Hydration band at 1646 cm -1 decreased due to formation of wood-solvent complex. Various carbonyl groups appeared at 1682, 1646 and 1621 cm -1 as a result of acetone treatment. Lignin bands at 1509 cm -1 decreased as a result of lignin degradation. Intensities of water stretching modes decreased due to the formation of wood- solvent complexes. 13

14 (Fig.11): Effect of heat ageing on wood treated samples Heat ageing accelerated oxidation and hydrolysis of wood treated samples compared to untreated ones. i.e. more new bands and more carbonyl groups were formed. (Fig.12): C-O stretching zone (Fig.13): OH bending and CH deformation zone New band (in comparison to the untreated sample) appeared at 1156 cm -1. This band may be due to C-O group of ester formation or opening of the pyranose ring. Lignin degraded faster when treated with alcohol & acetone i.e. band at 1268 cm -1 disappeared and at 1509 cm -1 decreased compared to heat untreated sample. (Fig.14): C=O stretching zone (Fig.15): O-H stretching zone Various carbonyl groups bands were formed in case of heat ageing of alcohol and acetone treated samples. New bands due to free OH stretching appeared in the region between 3710 and 3565 cm -1 in case of heat ageing of alcohol treated sample. These bands were formed as a result of heat breaking down of intermolecular hydrogen bonding. 14

15 (Fig.16): Effect of U.V ageing on wood treated samples U.V. ageing of wood treated samples increased wood oxidation compared to wood untreated ones. (Fig.17): C-O stretching zone (Fig.18): OH bending and CH deformation zone C-O band intensity at 1121cm -1 increased in comparison to U.V aged untreated sample and to the alcohol and acetone treated samples. Lignin bands at 1509 cm -1 decreased and at 1268 cm -1 disappeared, i.e. alcohol increases the effect of U.V. ageing of wood. (Fig.19): C=O stretching zone (Fig.20): O-H stretching zone Intensity of carbonyl groups bands of acetone and alcohol at 1623 cm -1 increased as a result of oxidation process. Asymmetric and symmetric water stretching bands were formed as water replaced solvents. U.V ageing occurred in normal (humid) conditions. 15

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