The Degrading Action of Iron and Copper on Paper

The Degrading Action of Iron and Copper on Paper

A FTIR-Deconvolution Analysis

by P. CALVINI & A. GORASSINI

introduction

It is well known that iron and copper salts cause damage to paper, depolymeriz-ing cellulose chains and increasing discoloration of paper containing cellulose1-5. Generally, depolymerization is ascribed to the acidity, and colour formation depends on the oxidation and dehydration of the glucopyranose ring.

While the degradation of unoxidized paper can be monitored by the decrease of intrinsic viscosity6 ' in Cupriethylendiamine (CUEN), the presence of aldehydes or ketones in oxidized paper causes an anomalous decay of intrinsic viscosity, because the strong alkalinity of the CUEN solution causes a p-alkoxy fragmentation8. The determination of carbonyl and carboxyl groups is therefore of primary importance in the field of paper conservation science. The chemical determination of CO groups is generally performed by the hydrazine9 or triphenylformazan8 methods, while the amount of COOH groups can be determined by means of the Methylene Blue procedure10.

A quick and easy method of determining both the hydrolytic scissions and the amount of carbonyls along the chains, recently proposed in this Journal8, relies on the viscometric analysis of oxidized paper before and after reduction of the CO groups, to restore the initial alcoholic function. These analyses detect the overall amount of oxidized groups, but do not tell us anything of their positions in the pyranose ring or of the presence of conjugated structures, responsible of discoloration11- 12.

Some efforts have been made to detect, at least qualitatively, the oxidized functions by means of Infrared Spectroscopy13' 14, but results are generally unsatisfactory. The main problem is that aldehydes show a reduced IR absorbance in the normal position of CO vibration (>1700 cm"1), possibly owing to their hy-dration to gemdiols or to the formation of other structures11' !(i. COOH groups generally absorb at ~ 1740 cm"1, but an alkaline pH shifts the absorbance of car-boxylates10' !/ to ~ 1600 cm"1. Even if the pH is accurately buffered to a low acidic

The Degrading Action of Iron and Copper on Paper

range, so that COOH groups absorb in the normal region, and the sample is heated enough to show the CHO absorbance, IR analysis reveals, at first sight, only the unconjugated groups, which are rarely responsible for discoloration. The conjugation of these carbonyl groups causes a low-frequency shift, in a region of the spectrum where the broad IR band of the adsorbed water generally masks the underlying information12' 18' 19.

It was shown in a previous paper12 that the FTIR-deconvolution technique can detect some of these conjugated structures. We present here the results of this technique applied to the analysis of the iron- and copper-induced degradation of paper.

experimental

Preparation of samples

0.1M aqueous solutions of analytical grade FeCl3 and CuCl2 respectively, were

added to 2 g of Whatman filter paper no.l (chromatographic grade), in a volume (1 ml) sufficient to wet the paper samples completely. Application was done slowly with the aid of a hair dryer on a warm setting, so that the solids remained deposited in the paper and the water evaporated. This procedure resulted in an average of 1 metal ion for every 124 glucose units.

Ageing

Having been conditioned for 7 days at 23°C and 50% RH untreated and treated samples (0.6 to 0.7 g) were aged in 25 ml sealed glass tubes at 90±10°C.. According to a rough evaluation20, relative humidity inside the tube was ca. 60%. Ageing was performed until discoloration became visible (Table 1).

Measurements

Degree of polymerization (DPv) was measured at 20.°C, according to the French Standard AFNOR T12-005.

pH was measured according to TAPPI standard method T435m-52 (cold extraction).

For infrared analysis a Perkin-Elmer 1710 FTIR was used to obtain 4 cm ! resolution spectra (absorbance mode) in the 4200-450 cm ~' region, scanned ten times.

Table 1: Properties of the samples before and after sealed tube ageing

* No colour change caused by subsequent alkaline washing (pH 10)

Details about sample preparation and the deconvolution procedure have been published elsewhere12.

Comparison of absorbance data was performed12 by means of a sum of peaks shaped as Lorentzian curves (Y):

where H is the peak height in absorbance units, w the full width at half height, xo the centre of the peak and x the frequency; the unit of w, xo and x are cm-1units.

characteristics of the samples

Table 1 gives some chemical and physical data of unaged and aged samples. From DPv of the unaged Fe(III) and Cu(II) treated samples it can be seen that these metal ions promoted an immediate decrease of the cellulose chain length. The initial degradation of Fe(III) was higher than that of Cu(II), as already found by other authors1. After ageing, both Fe(III) and Cu(II) treated samples showed a very strong depolymerization and little pH variation. All the aged samples showed a more or less pronounced brown discoloration, which does not disappeared after alkaline washing (pH 10).

infrared analysis

For a better understanding of the deconvolution bands in the 1600-1700 cm-1 region, Table 2 gives the FTIR bands of some model compounds21, whose structural formula are shown in Fig.l.

The FTIR absorption at ~ 1670 cm"' of the cx-diketone in Fig.la indicates that this compound is fully enolized, since un-enolized diketones absorb in the normal position1' of CO groups (>1700 cm"1). Similarly, ap-unsaturated groups (Figs.lb

The Degrading Action of Iron and Copper on Paper

Table 2: FTIR bands of some conjugated carbonyl compounds in the range of 1600-1700 cm '

Fig.1. Structural formula of some models of oxidized groups in cellulose, a) 1,2-Cyclohexanedione (enol form); b) 1-Acetyl-1-cyclohexene; c) Shikimic acid; d) keto-enol tautomery and chelate resonance form of 2-Acetyl-cyclohexanone.

and Ic) absorb in the 1670-1680 cm1 region, with a weak band at -1640 cm1, generally masked by adsorbed water in oxidized cellulose. A particular ab-sorbance in the 1600-1620 cm ' region is shown by p-diketones when their steri-cal conformation allows for a resonance-stabilised hydrogen bond in the enol structure (chelate form) as in Fig.Id.

From the FTIR-deconvolution data of metal-treated cellulose it appears that copper and iron have different mechanisms of degradation; so their properties will be discussed separately, together with the behaviour of the reference samples.

Table 3: FTIR-deconvolution bands of Whatman no.l reference paper

• For peak identification see text and Ref.12.

Fig. 2: FTIR absorbance of unaged (a) and sealed tube aged (b) Whatman no.1 reference paper in the range of 1250 to 1850 cm-1.

The degradation of reference samples

Fig. 2 shows the FTIR spectra of unaged and aged reference samples, Table 3 the results of deconvolution.

The unaged cellulose was the same as used for the experiment reported in our previous paper12; it shows a faint trace of oxidation due to its storage without particular precautions in the laboratory. 15 days ageing caused a decrease of DPv, with a clear increase of unconjugated CO groups, and some unconjugated COOH. A rather strong deconvolution band at 1658 cm"1 revealed the formation of a-diketones (enol tautomer structure, as in Fig.la) or of some ap-unsaturated car-bonyls (aldehyde or carboxyl groups as in Fig.lc). A clear cut association to the 1658 cm ! band to these groups cannot be done without further chemical analyses.

The Degrading Action of Iron and Copper on Paper

Fig. 3: Accelerated ageing of cellulose paper (sealed tube technique). Oxycelluloses a, c and e account for the increase of unconjugated CO groups (1716 and 1742 cm-1); oxycelluloses b, d and f account for the formation of the 1658 cm-1 signal. Structures a and b are in the keto and enol form respectively.

Taking into account the FTIR-deconvolution data, a reasonable model for the accelerated ageing of the reference paper involves (Fig. 3)

• some oxidation at C2 and C3 position, as well as a C6 oxidation to CHO and COOH;

• a slow acidic hydrolysis with splitting of the C4-bonded cellulose side chain (which accounts for the decrease of DPv) and formation of a C4-OH group;

• dehydration at the C4-C5 position, with formation of a double bond; together with the second mechanism, this step leads to the formation of an α-un-saturated aldehyde or acid, like that illustrated in Fig.3 d and f.

The first mechanism accounts for the oxidation bands at 1716 and 1742 cm"1, and the second and the third for the FTIR-deconvolution band at -1658 cm"1.

Table 4: FTIR-deconvolution bands of the iron-treated sample

For peak identification see text and Ref. 12.

tooth = small and sharp signal above the large OH absorption; mrs= more resolved structure (both characteristic of acidic degradation22; spk = small, but clearly visible peak, characteristic of C=C double bond12.

Fig. 4: FTIR absorbance of unaged (a) and aged (b) Fe-treated paper in the range of 1250 to 1850 cm-1. Curve c was obtained after pH10 washing of sample (b).

The degradation of iron-treated sample

Fig.4 shows the FTIR spectra of unaged and aged iron-treated samples, and Table 4 the results of deconvolution. The FTIR-deconvolution spectra of this sample

The Degrading Action of Iron and Copper on Paper

strongly follows a similar pattern to that of the accelerated degradation of the reference paper, with an important difference: the degradation processes that require 15 days for the reference sample occur after 1 day or even less with the Fe-treated samples. The DPv of the unaged sample is approximately three times lower than that of the unaged reference, although the FTIR oxidation bands are largely the same. This means that the initial reduction of DPv is due to the acidity of the iron solution rather than to a (3-alcoxy fragmentation in CUEN. After ageing, the 1660 cm' band reveals the formation of some α conjugations, while unconjugated CO groups at >1700 cm ' do not increase substantially.

The full FTIR spectrum of the aged sample (not given in Fig.4) showed a more resolved structure in the 1000-1200 cm~lrange, which indicated, together with a "tooth" at 3340 cm"1, a strong acidic degradation12- 22. A small, but clearly visible peak at 800 cm"1 revealed some kind of unsaturated C=C group12.

The colour changes from yellowish to brown during ageing and is not reversed after alkaline washing. This means that the discoloration was not due to low-molecular weight compounds, if there were any. It might have been due to an iron-cellulose complex, as the deconvolution"'band in the 1533-1576 cm~lregion, which did not disappear during ageing and alkaline washing, seems to indicate17.

It maybe mentioned that alkaline washing caused a partial decrease of the unconjugated COOH band, and the height of the conjugated CO band at - 1660 cm4 was reduced to approximately one third of the original value. The formation of carboxylate ion along the cellulose chains was responsible both for these band reductions and for the new band1' at 1594 cm"1. A C6 oxidation (partly to CHO and partly to COOH) is then highly probable.

The degradation of copper-treated sample

Fig. 5 shows the FTIR spectra of unaged and aged copper-treated samples, and Table 5 the results of deconvolution. After ageing the FTIR-deconvolution spectrum showed an increase of the COOH groups with a small, low-frequency shift: from 1745 cm~lof the reference paper to 1734 cm"1; this was possibly due to intramolecular hydrogen-bonding. Another band at 1622 cm"1 suggested the presence of a fi-conjugated diketone (enol tautomer) stabilised by internal resonance1', like that of Fig.Id.

The full FTIR spectrum of the aged sample (not shown in Fig.5) appeared more resolved in the 1000-1200 cm ; range, which indicated, together with a "tooth" at 3340 cm"1, an acidic degradation22. Nevertheless, the C=C signal at -800 cm"1 was absent. As in the case of iron, a signal in the 1541-1547 cm"1 region seemed to indicate a copper-cellulose complex, responsible for discolora-

Table 5: FTIR-deconvolution bands of the copper-treated sample

For peak identification see text and Ref. 12.

tooth = small and sharp signal above the large OH absorption; mrs= more resolved structure; both characteristic of acidic degradation22.

Fig. 5: FTIR absorbance of imaged (a) and aged (b) Cu-treated paper in the range of 1250 to 1850 cm-1. Curve (c) was obtained after pH10 washing of sample (b)

tion1'. This brown discoloration did not change with alkaline washing, so that low-molecular-weight compounds seemed to be absent. Nevertheless, the pH 10 washing gave rise to the carboxylate form of COOH, inferred by the presence of the 1601 cm"1 band, with partial and full disappearance of the 1734 and 1622 cm' bands respectively.

The Degrading Action of Iron and Copper on Paper

Fig. 6: FTIR absorbance of Fe (a) and Cu (b) treated paper in the range of 1250 to 1850 cm-1. Both samples have been treated with a concentration of metals ten times higher than those of Figs.4 and 5, and aged 1 day at 90°C (sealed tube technique).

THE MECHANISM OF IRON AND COPPER DEGRADATION OF PAPER

In a paper published previously in this journal, related to the dry ageing of iron-and copper-treated paper at 80°C and 65% RH, M. Bicchieri & S. Pepa1 monitored the DPv and the formation of COOH groups at these conditions. In that experiment both the deionised paper and the iron(III)-treated sample showed an increase of COOH after ageing, with an S-shaped kinetic, while the copper(II)-treated sample showed an exponential-like kinetic. In order to evaluate the rate of formation of COOH groups, the authors interpolated their experimental data with an arbitrarily chosen three-parameters non-linear function, which is a good empirical method, but does not clarify the underlying chemical mechanism.

Bearing in mind that the formation of carboxyl groups in cellulose is a two-stage consecutive process (C-OH • CHO • COOH), we can define two constants: k1, i.e. formation of aldehyde groups from -OH or from >CO, and k2, i.e. formation of COOH. The actual shape of the kinetic plot depends on the relative value of ki and k2. The fully-developed kinetic law is shown in the Appendix. By means of a non-linear curve fitting procedure, also illustrated in the Appendix, the actual values of ki and ka can be obtained, as shown in Table 6.

Referring both to the deionised and iron-treated data, the rate of carbonyl formation approximately equalled the rate of carboxyl production, so that it is likely that only the C6 position of cellulose was involved in the process.

Referring to the copper-treated data, ki was lower than k2, so that C2 and C3 oxidation is highly probable, where the opening of the pyranose ring, with formation of CHO groups, is the rate-determining step.

Table 6: Non-linear fit of Bicchieri & Pepa1 COOH data

k1 rate constant of CHO formation

k2 rate constant of COOH formation

For the meaning of Z, M and N parameters see Eq. 1 in the Appendix.

A further confirmation of the different mechanism of iron and copper degradation is shown in Fig. 6, where the concentration of metal ions deposited on the fibres was ten time higher and the treated samples were subjected to 1 day's ageing at 90°C using the sealed tube technique.

Both iron- and copper-treated items showed a broad, strong band in the 1500-1750 cm' region, thus revealing a large amount of oxidation and conjugations. Nevertheless, the iron-treated sample (Fig. 6a) showed higher signals in the region of conjugations (1500 to 1700 cm"1), while the copper-treated sample (Fig. 6b) showed a higher signal in the region of unconjugated CO groups (>1700 cm~1). Since the fragmentation of the pyranose ring reduced the possibilities of conjugation, it appeared that iron gave rise mainly to a C6 oxidation, while copper preferred to attack at the C2 and C3 positions, opening the glucose monomers of cellulose, in accordance with the Bicchieri & Pepa1 kinetic data of COOH formation.

conclusions

The FTIR-deconvolution analysis shown in this paper highlights the different pathway of iron and copper degradation.

Iron(III) treatment in an acidic medium caused, after artificial ageing, the following reactions (Fig. 7):

• Strong hydrolysis of the cellulose chains induced by acidity, with a random attack (decrease of the DPv and absence of low-molecular-weight compounds);

• dehydration of the C4-C5 bond, with formation of a C=C group (small peak at 800 cm1);

• oxidation of the C6 hydroxyl to CHO and then to COOH (Fig. 7a,b). Together with the second step, this mechanism resulted in the formation of α-

The Degrading Action of Iron and Copper on Paper

Fig.7. Accelerated ageing (sealed tube technique).of iron-treated paper. Oxycellulose a accounts for the relatively high direct hydrolysis and, together with oxycellulose b, for the formation of the 1660 cm-1 signal; oxycellulose c causes the -alcoxy fragmentation in CUEN found in Ref.1

unsaturated aldehydes C=C-CHO or acids C=C-COOH, inferred by the presence of the 1662 cm' band. As indicated by the FTIR-deconvolution analysis of the pH 10 washed sample and by the ki and ka values of Table 6, both unsaturated aldehydes and acids should have been present at the same time, and only the latter were ionised by alkalinity, with a low-frequency shift to 1594 cm-1;

• oxidation of the C6 hydroxyl without hydrolysis (Fig. 7,11).

Copper(II) treatment in an acidic medium caused, after accelerated ageing, the following reactions, where oxidation prevails over hydrolysis, as illustrated in Fig.8:

• oxidation to aldehyde of the pyranose ring, with breaking of the C2-C3 bond (although this was the rate-determining step, the ki value was higher than that of the iron-treated sample, indicating a stronger oxidising power of copper), quickly followed by the formation of COOH groups (Fig. 8,1);

• random hydrolysis of the cellulose chains (induced by acidity), with an immediate oxidation of the new OH group in the C4 position;

• oxidation to COOH in the C6 position. Together with the previous step, this mechanism accounted for the 1622 cm • band by the formation of a -keto-

acid, enol form C(OH)=C-COOH (Fig.8,11), the internal resonance of which disappeared with alkaline washing, when the carboxyl group became a car-boxylate.

The relative importance of mechanisms I and II illustrated in Figs. 7 and 8 cannot be inferred from FTIR analysis alone, since the relationship between the actual concentration and the infrared absorbance of CO, CHO and COOH is unknown. Moreover, the absence of a well defined signal of unconjugated aldehydes11' I6 makes it difficult to detect such an important group, therefore, further studies are in progress to better determine the characteristics of oxidized groups.

acknowledgements

This work has been supported by the Italian C.N.R., "Progetto Finalizzato Beni Culturali" - Operative Unity Antonio Zappala.

appendix

The formation of COOH groups in cellulose can be seen as a multistep process:

The Degrading Action of Iron and Copper on Paper