On the use of ASTM closed vessel tests in accelerated ageing research

Available online at

www.sciencedirect.com Journal of Cultural Heritage 9 (2008) 401e411

Original article

On the use of ASTM closed vessel tests in accelerated ageing research Tomasz Sawoszczuk a,1, Andrzej Baran´ski a,b,*, Janusz Marek qagan b,2, Tomasz qojewski a,1, Katarzyna Zi˛eba a,1 b

a Jagiellonian University, Department of Chemistry, Ingardena 3, 30-060 Krako´w, Poland Jagiellonian University, Regional Laboratory for Physico-Chemical Analyses and Structural Research, Ingardena 3, 30-060 Krako´w, Poland

Received 17 July 2007; accepted 17 October 2007

Abstract The ASTM D6819-02e3 standard for testing the accelerated ageing of paper, published in 2002, recommends using closed glass vials in order to keep the degradation products in contact with the paper and thus permitting a better simulation of the natural ageing conditions inside closed books. In the present study, the actual conditions and their stability inside closed vessels have been evaluated. The necessity of assuring a very high sealing performance (tightness) of the systems in order to avoid leakage of water vapour is the main drawback of this ageing method. Systematic studies presented in this publication tried to monitor this tightness and have provided data that helped to answer the question of what circumstances could lead to achieving its improvement. Both gravimetric monitoring of water content in vials and ‘‘in situ’’ IR measurements of the aged paper humidity have been applied for this purpose. As a result, better sealing materials (gaskets and caps) than those recommended by the ASTM standard, have been found. Additionally, application of a dynamometric spanner for closing the vials is recommended, as this is a guarantee of reasonably high and recurring tightness of the systems used in tests. Nevertheless, the systematic, linear with time, loss of moisture for all used vials was observed. A 9% loss of the initial moisture content in samples of the aged paper was observed for the tested conditions (14 days at 90  C). Therefore, it seems that including some kind of gravimetric control for ageing tests performed in closed vessels is of essential importance. Kinetic studies of accelerated ageing in both closed and open systems, as monitored by DP and breaking length measurements, unexpectedly show that no statistically meaningful difference of degradation rates can be observed. On the other hand, the pH and whiteness index values reveal a meaningful difference between the rates of ageing in these systems, thus confirming the basic assumption of the ASTM test concerning the interaction of paper degradation products with the paper itself. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Accelerated ageing of paper; ASTM D6819 standard; Closed vessel ageing; Model cellulose paper; Acidic hydrolysis of cellulose; Kinetics of cellulose degradation

1. Introduction and research aims

* Corresponding author. Jagiellonian University, Department of Chemistry, Ingardena 3, 30-060 Krako´w, Poland. Tel.: þ48 12 6632220; fax: þ48 12 6340515. E-mail addresses: [email protected] (T. Sawoszczuk), [email protected] (A. Baran´ski), [email protected] (J.M. qagan), [email protected] (T. qojewski), [email protected]. edu.pl (K. Zi˛eba). 1 Tel.: þ48 12 6632087. 2 Tel.: þ48 12 6632918. 1296-2074/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2007.10.010

Accelerated ageing is a common practice in materials science. In the case of paper, the possibilities and limitations of the accelerated ageing have been reviewed by Porck [1]. A new ASTM standard for the accelerated ageing of paper in closed vials has been published in 2002 [2]. The standard was based on the research carried out by the Library of Congress (LOC) and the Canadian Conservation Institute (CCI) within the ASTM Paper Ageing Research Program [3,4]. As pointed out by the program leaders e Chandru Shahani and Paul Be´gin e the accelerated ageing in airtight vials is economically more

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feasible than ageing in a climatic chamber. Additionally, ageing in vials better simulates the natural ageing process at room temperature because it facilitates the interaction of the paper degradation products with the paper itself. The first of these statements seems obvious, and the second has been convincingly documented by Shahani et al. ([4], p. 50). The accelerated ageing of paper in closed vessels, at 90 and 100  C, has been carried out recently by several research groups [5e7], and it seems that this kind of test also becomes an object of attention for other researchers. This is why the phenomena occurring inside vials during the tests became the main object of interest in our recent studies. They are summarized in the present paper. The above-outlined approach comes in line with the call by reviewers of the ASTM program ([4], p. 79). 1.1. General assumptions a) Temperature of the test, recommended by the ASTM standard, is equal to 100  C. The equivalency of a 5-day test at 100  C and a 14-day test at 90  C has been indicated by Kaminska and Be´gin [8] and Shahani ([4], pp. 6 and 22). The latter temperature has been chosen for the present study, as a higher level of experimental errors is expected during studies conducted at 100  C, when the system is not too far from the boiling point of water ([4], p. 82). b) We intended to use vials and seals identical with those recommended by ASTM. The commercially available parts are, however, somewhat different from the ones recommended in the ASTM standard. Detailed information can be found in the Section 2.2. c) Basic assumptions of the ASTM standard, concerning conditions of ageing, are valid only when the vials employed are perfectly leak-proof. Only then can an equilibrium state be attained during the tests. However, in the reports of the ASTM program we have found some indications of lack of tightness, as the data obtained in identical experiments in LOC and CCI sometimes differ considerably. The quality of sealing materials and the fact that the systems were closed ‘‘by hand e as tight as possible’’ ([3], p. 34) could be the sources of these differences ([3], p. 38). In the present research, during the accelerated ageing tests, tightness was controlled by a gravimetric method. The loss of mass of the inadequately tightened vials may be due to the water vapour and/or air escaping from the vials at elevated temperature, during the accelerated ageing tests. This assumption implicates the necessity for monitoring the mass of the vials e both the ones filled with paper as well as those containing only air (they will be called ‘‘reference vials’’ in the text below). 2. Experimental 2.1. Materials Model paper ‘‘Paper 1’’ (P1), supplied by the Center for Paper and Board Research (TNO, Delft, Netherlands) [9], of

grammage equal to 78 g/m2, was used. This paper, especially ordered and manufactured on a production paper machine in a commercial paper mill, contains 99% of softwood cellulose, and its ash content is 0.45%. No lignin is present in the paper but traces of hemicellulose can be observed by the IR method. No sizing and no fillers were used during the production. The mean moisture content of paper P1, stored at 23  C and RH ¼ 50%, is 7.18  0.36%; its pH determined by the cold extraction method is equal to 6.3. 2.2. Equipment and procedures 1. Glass vials: a) LabLine vial (ca. 150 ml), original silicon-Teflon gasket and cap (GL45) made of polybutyleneterephthalate supplied by the vial manufacturer (assembly L), the Teflon layer adheres to the vial edge after the whole system is tightened. The ASTM standard recommends usage of polypropylene caps and TFE gaskets, but this combination is not commercially available now; b) Kontes or Pyrex vials, polyphenylsiloxane (BOLA) caps and customized gaskets made of VitonÒ of hardness 80 in Shore’s scale (assembly KP); this system has been selected after many preliminary experiments. eThe vials of all three manufacturers, mentioned in points a) and b), were made of Pyrex glass. The vials were tightened prior to the experiments with a dynamometric spanner with a force moment of 20 Nm. This was the maximal force that did not result in destruction of the vials. 2. Tightness of the studied systems was estimated gravimetrically. The procedure was embedded into the protocol of accelerating ageing tests, but for the sake of clarity, the information in question will be separately presented below. The masses of vials were recorded at room temperature before and after the tests. The cooling period at that temperature was fixed at 24 h. All gravimetric measurements were done with the accuracy of 0.1 mg on the analytical balance (WAA 220/C/1, RADWAG, Poland). The ageing experiments were carried out simultaneously for the vials containing samples of model paper P1 and the reference assemblies containing only air. It can be assumed that the values of moisture content reduction, corresponding exclusively to the lack of tightness of a reactor containing paper, are obtained when the mean mass reduction of the reference assembly (containing only air) is subtracted from the mean mass reduction of the assembly with paper. All further results of gravimetric measurements for the systems containing paper presented in this publication (save only for the exemplary results presented in Fig. 2) are already corrected for the mass reduction of reference systems. 3. Stainless-steel autoclaves (of capacity ca. 160 ml) with gaskets made of NBR 85 rubber were also used in the kinetic experiments. The autoclaves were tightened with the force moment of 50 Nm. Tightness of these systems

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could not be measured either gravimetrically or with the infrared moisture meter. It was believed that the loss of mass was negligible. 4. Gravimetric measurements in the climatic chamber were done with a moisture balance (model WPS 110S, produced by RADWAG, Poland). The heating program used in the moisture balance was based on TAPPI standard T412 om-94. 5. Direct measurements of humidity of paper placed in a closed KP assembly were carried out with the MCA 1410 infrared moisture content analyzer from FIBRO system ab, Sweden. This measuring device (further abbreviated as MCA) allows for the determination of the amount of water in paper by measuring the absorption of near infrared radiation. The main advantage of this equipment is the possibility of making the measurement through the glass wall of the vial without opening the latter. During the measurements of moisture content, carried out at 90  C, a vial with paper was placed in the holder with the attached side tube used for placement of the optical fiber probe e an element of the MCA meter (Fig. 1). As the values given by the meter itself are expressed in millivolts, it was necessary to calibrate the MCA meter prior to experiments. 6. The acidity of paper samples was found by the cold extraction method, according to the TAPPI T509om-96 standard. The pH measurements were done with a classic combination electrode WTW Sentix and the pH-meter manufactured by Hanna Instruments. 7. The accelerated ageing tests of P1 paper samples were performed in the closed systems at 90  C according to the ASTM D6819-02e3 method. Ageing in the closed system was performed: e in the KP assemblies, regarded as the optimal ones after the tests of tightness; e in stainless-steel autoclaves e for comparison purposes. eSamples of paper P1 prepared for the tests had the shape of paper strips of size allowing for cutting-off (after the

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ageing process was finished) samples for tests of the chosen mechanical properties. The strips of paper were conditioned for at least 24 h at 23  C and at RH ¼ 50% prior to the experiment and after its conclusion. eThe strips were rolled and placed inside a vial or autoclave. 1 g of the dry paper mass was contained in 36 ml of the reactor volume. This corresponds to the paper ‘‘packing density’’ of 0.278 g/ml. After the intended time of ageing has been attained, the vials (or autoclaves) were removed from the dryers (VENTICELL, BMT, Brno), cooled for 24 h and opened afterwards. 8. Additionally, ageing in the climatic chamber (NCC 0130, NEMA INDUSTRIETECHNIK) was also conducted, at 90  C and RH ¼ 59%. This value of relative humidity was established in the present study for sealed vials containing P1 paper at 90  C. The strips of paper were loosely hung in the climatic chamber, and they were spaced so as to make their mutual contact impossible during the ageing. 9. The degree of cellulose polymerization was measured by the viscometric method according to the Scandinavian standard [10]. In order to obtain a cellulose solution necessary for viscosity measurements, the samples of paper were treated with a cupriethylenediamine solution. Measurements were performed with a glass viscometer (Werner Glass & Instrument AB, Sweden). The intrinsic viscosity [h] calculated from these experiments was a basis for calculating the degree of polymerization (DP) values of cellulose chains in the studied samples e according to the formula given by Immergut [11]: ½h ¼ Q  DPa

ð1Þ

where Q ¼ 1.33 ml/g; a ¼ 0.905. 10. The dry zero-span breaking length of aged samples was measured according to the TAPPI T231 om-85 test method on the Pulmac Troubleshooter Z-100. 11. Colour measurements of aged paper samples were done with the SpectroEye spectrophotometer (geometry 45/0) manufactured by GretagMacbeth.

Fig. 1. Measurement of paper moisture with the MCA meter. a) MCA 1410 meter with optical fiber probe; b) glass vial in the holder: 1 e cross-section of the holder; 2 e side tube; 3 e probe of the MCA meter; 4 e vial; 5 e cap; 6 e paper P1; distance a ¼ 1.5 cm (according to the manufacturer’s suggestion), angle b ¼ 70 ; the vial strictly adheres to the wall of the holder.

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3. Results and discussion

Line a b c d

3.1. Tightness of vials during the accelerated ageing tests

Regression equation Δm = 0,0042 t Δm = 0,0023 t + 0,0016 Δm = 0,0018 t - 0,0001 Δm = 0,0005 t + 0,0017

Two experimental variables were modified when checking tightness of the L and KP assemblies: a) a method used for tightening of the assembly e manually (as tight as possible) or with a dynamometric spanner; b) type of the sealing material used in the assembly. The influence that the tightening force used on a reacting system has on its tightness was investigated by making a series of gravimetric measurements for the L and KP assemblies. Results of these measurements are presented in Table 1. The assemblies tightened only by hand are much less leakproof than those tightened with the dynamometric spanner. The loss of mass of assemblies L and KP tightened with the spanner is, respectively, 30% and 40% lower than the loss of mass of the systems tightened by hand. In this context, the phenomena described in the section ‘‘Repeatability’’ of ASTM report ([3], p. 37) are worth the reader’s attention. A small dispersion of the results obtained in the systems tightened with the spanner is a proof of better tightness but it also implicates a better repeatability of the conditions existing during the measurements applying the ASTM method. In all further experiments, the vials were tightened with the dynamometric spanner. Influence of the applied sealing material on the tightness of systems was studied for the L and KP assemblies containing paper and air, and the results, based on the gravimetric measurements, are shown in Fig. 2. The linear regression equations shown in the table inserted into Fig. 2 describe the dependence of the loss of mass of the systems on the heating time. As the ordinates of intersection of regression lines with the y-axis are sufficiently small in these equations, the lines in the drawing were forced to pass through the beginning of the coordinate system. The directional coefficients of the regression lines are regarded as the 24-h losses of mass (D) expressed in [grams/day]. These values are a convenient measure of lack of tightness of the systems, and they will be used further in the discussion. One can see from Fig. 2 that the sealing system used in the KP assembly is more effective (independently of the

Fig. 2. Mean loss of mass (MLM) of vials containing paper during heating at 90  C as a function of the type of applied sealing material. a e L assemblies containing paper corrected for MLM of L assemblies containing only air; b e KP assemblies containing paper; c e KP assemblies containing paper corrected for MLM of KP assemblies containing only air (c ¼ b  d); d e KP assemblies containing only air.

tightening method) than the one in the L assembly e as evidenced by the data shown in Table 1. The gravimetric results for systems containing only air, shown in Fig. 2, indicate a necessity of making experiments both for vials containing paper and for reference vials containing only air. The loss of mass for these systems (0.0005 g/ day) is due to the air escaping from the vials as well as to the degassing effect of their components. This loss of mass could be observed even in systems previously heated for 24 h at elevated temperature. The small value of D confirms the effectiveness of the applied sealing system. If a large leakage existed in this system, then the pressure of air at 90  C (1.2 atm inside the vial) would be quickly lowered to the pressure of 1 atm. As seen in Fig. 2, the loss of mass after 14 days amounts to only 0.01 g. Taking into account the mass of the air components, one can calculate that the maximum loss of mass would be equal to 0.03 g under these circumstances. The observed total loss of mass never exceeded this value. If a high repeatability of tightness is assured, then it can be assumed that the 24-h loss of mass D (corrected for the leakage of air) is proportional to the relative humidity inside the vial with the specific type of sealing. Therefore, if these losses of mass are determined for two vials e one containing only water, and a second one with paper only e one may roughly assess the RH value over paper from the relation:

Table 1 Decrease in mass (in grams) of paper-containing vials after 14 days of heating at 90  C.

RHpap ¼

Type of assembly Type of tightening Dynamometric spanner By hand (the strength of tightening 20 Nm) (as tight as possible) L KP a

0.0585 0.0579 0.0247 0.0254 Mean value for two vials.

a

0.0582

0.0251a

0.0964 0.0694 0.0424 0.0388

a

0.0829

0.0406a

Dpap Dpap  RHwater ¼  100 Dwater Dwater

ð2Þ

Eq. (2) practically applies the principles of the Knudsen method used for determination of vapour pressures over liquids or solids. This method is based on the assumption that ‘‘. if the vapour pressure of a sample is p, and it is enclosed in a cavity with a small hole, then the rate of loss of mass from the container is proportional to p.’’ [12].

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Three values of RHpap were calculated for the experimental data obtained in the most tightly closed vials (45%, 45%, and 67%). Their mean value, equal to 52%, is close to the value of 59% based on the measurements performed with the infrared moisture meter (see Section 3.2). 3.2. Changes of paper humidity during the test of accelerated ageing in a vial In this study, the paper moisture content was monitored by: (i) recording the loss of mass in the vials; (ii) recording the moisture content of paper inside the vials using MCA; (iii) employing the records obtained from the moisture balance for paper samples aged in the climatic chamber. These records will be discussed separately at first, as each of them bears a specific information. When the initial moisture content of P1 paper is known, and assuming that the mass change of the KP assembly with paper (corrected for the mass change of the reference vial) at time t is totally due to the water vapour escaping from its interior, then the paper humidity may be calculated from the following formula: wcal ½% ¼

m0H2 O  Dm
0 mH2 O  Dm þ mp

 100

ð3Þ

The symbol m0H2 O in this equation stands for the initial amount of water in paper [g], Dm is loss of the vial mass after time t, and mp is the dry mass of paper in the vial [g]. The results of these calculations are shown in Fig. 3 (line a). Measurements performed with the MCA meter for samples of paper closed in the KP assembly at 23  C and RH ¼ 50%, and subsequently placed in the drier at 90  C, have shown that the moisture content in these samples is equal to 6.63% after attaining the quasi-equilibrium state (3 h of heating). Based on the amount of water present in the air contained in the system closed at 23  C and RH ¼ 50%, and taking into account the P1 paper humidity estimated at these conditions (7.18%), the total mass of water in the system at 23  C has been calculated. The same balance has been subsequently done for the 90  C temperature based on the paper humidity measured with the

b a c

Fig. 3. Analysis of paper humidity changes as a function of ageing time. a) Humidity of paper P1 calculated from the loss of vial mass (assembly KP); b) humidity of paper aged in the climatic chamber; c) humidity of paper measured with the MCA meter (assembly KP).

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MCA meter (6.63%) and on the humidity of air inside the system (59%).3 These balance calculations give consistent results. The numbers corresponding to the total mass of water (grams) in a vial at two temperatures, 23  C and 90  C, differ only by 2.4%. The MCA meter was also used for recording changes in the humidity of P1 paper during the accelerated ageing tests performed in the KP assemblies. The results obtained are presented in Fig. 3 (line c). The loss of paper humidity during the ageing process may also be due to the decrease in the number of amorphic regions accessible to water along with the simultaneous increase in the number of hydrophobic crystallic regions [13]. This phenomenon is a result of changes in the cellulose structure that are observed during its degradation. Studies on the correlation between the crystallinity index and the moisture of cellulose samples at varying RH (11e75%) have shown that the moisture sorption was lower for the materials with a higher crystallinity index [14]. The scale of this effect has been estimated in our studies by measuring the humidity of nonaged P1 paper samples and that of samples aged in the climatic chamber throughout various time periods at 90  C and RH ¼ 59%. After ageing in the chamber, the samples were pre-conditioned for 24 h at 23  C and RH ¼ 50%, and, subsequently, the measurements of their moisture content were performed in a moisture balance. Results of these measurements have been shown as line b in Fig. 3. A similar effect of lowering the moisture content in paper during ageing has been observed by Tse et al. [15]. Let us summarize the previously discussed information. After inspecting Fig. 3, one can see that the moisture content of paper in a closed vial is lowered with increasing time of ageing. Lines drawn in Fig. 3 correspond to various effects described below. Line b describes the change in hydrophobic properties of paper samples while line a is characteristic of a sample drying due to a vial not being leak-proof enough. Simultaneous action of both of these factors, augmented by an effect of migration of some water molecules from the paper sample to the ambient atmosphere inside a vial e when the latter is heated from 23  C to 90  C at the beginning of the accelerated ageing test e can be ascribed to line c. The statement just formulated may be supported by elementary calculations. By adding the directional coefficients of regression lines shown in Fig. 3 (Dwm þ Dw) and calculating the error of this sum (supposing the worst case of accumulation of the individual errors), the value (e0.0594  0.0045) %/day is obtained. The value of directional coefficient for regression line c is DwIR ¼ 0.0667  0.0072%/day. Obviously, these values are in agreement within the limits of error, thus confirming the assumption made beforehand. Similarly, by calculating the difference between the ordinates of regression

3

Our unpublished results.

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equations for lines a and c, as well as their maximal cumulated error, one arrives at the expression Dw ¼ (7.0991  6.4979)  (0.0272 þ 0.0667) z 0.60  0.09. This difference falls inside the range of 0.51e0.69%, just like the 0.55% difference between the initial moisture content of paper (7.18%) and the moisture content measured with the MCA meter (6.63%). Therefore, the statement that the mutual shift of lines a and c in the drawing is due to the description of different effects, becomes plausible. The IR measurement (with MCA meter) records the total withdrawal of water from paper. The gravimetric measurement can record only this part of water that was removed from the paper and also that which leaked out of the vial. Assuming, in accordance with the conclusion expressed above, that:  the loss of the moisture content of paper due to the increasing hydrophobic character of cellulose is denoted by the factor Dw ¼ 0.0106%/day,  the loss of the moisture content of paper due to water escaping from the vial is denoted as Dwm ¼ 0.0488%/ day one can calculate the relative importance of these effects: Dw 0:0106 ¼ ¼ 0:178 Dw þ Dwm 0:0106 þ 0:0488 This means that the 18% loss of moisture content of the paper is due to changes occurring in its structure, and the system leaks are responsible for 82% of moisture loss. The fundamental drawback of the ASTM method is the problem with obtaining the proper tightness of vials. The Dwm coefficient is the best quantitative measure of this effect. During the fortnight test, the total decrease of moisture content in paper amounts to 0.683%, i.e. ca. 9% of the initial amount of water placed into the vial at the beginning of the test. This statement and calculations are, of course, valid only for the results obtained in our study for the model paper P1. Additional gravimetric measurements carried out for the sealing system used in the KP assemblies have shown that the maximal amount of water absorbed during heating in the gasket structure is equal to 3% of the total mass of water in the system. There is also another reason for the changes in paper moisture content. The number of water molecules contained in a closed system drops as they are consumed in the acid hydrolysis reaction of cellulose. Elementary stoichiometric calculations give an estimation of this loss of water. After 14 days of ageing, the value obtained for the loss of water is approximately 0.05% of the initial content of water in the vials, therefore it can be neglected in other calculations. 3.3. Acidity of P1 paper samples aged in various systems The acidity (pH) of paper samples was one of the properties used to differentiate them after ageing in closed (KP vials and autoclaves) and open (climatic chamber) systems. The

regression equations given in Table 2 show the changes of pH with time of ageing in the specific systems. The ordinates of the equations shown in Table 2, corresponding to the pH of non-aged samples, fall within the range of 6.22e6.25. These values are in agreement with the initial pH values of paper P1 (6.3) obtained at TNO [16]. When comparing the directional coefficients of the regression equations (DpH) for changes in pH of the paper samples aged in various systems, one can see that: DpHA > DpHKP > DpHCh and the 24-h decrease in pH of the samples aged in closed systems ðDpHA andDpHKP Þ is markedly higher than that observed for the DpHCh system. Let us try to find explanation for these inequalities. During ageing in the open system (chamber), volatile products of cellulose degradation (including those of acidic nature) are continuously removed from the paper, and, thus, its acidity is lower than that found for the samples aged in closed vessels. In the closed systems, the volatile products of degradation remain in contact with paper until the end of the ageing process. It is to be expected that at the end of the ageing experiment, when the temperature is lowered to room temperature, some volatile compounds with lower boiling points (including those of acidic nature) are absorbed within the paper structure. The pH of the paper is measured from water extracts, which also contain these products. Similar observations have been made in other related studies [17e19]. 3.4. Kinetic runs in vials, autoclaves and the climatic chamber 3.4.1. Introduction One could expect that the kinetic curves of paper degradation, obtained in two somewhat different closed systems (vials and autoclaves) according to the ASTM method and in the open system (climatic chamber), would be different [5,18]. However, the results of measurements of DP for samples of P1 paper are ambiguous. They are presented in Fig. 4 together with the lines drawn on the basis of linear regression calculations carried out in the coordinate system (1/DP  1/DP0) vs. time (see Figs. 5e7). This picture aroused a suspicion that, due to large irregularities in the individual curves, the kinetic runs performed in different conditions in fact were not differing very much among themselves. Therefore, a more thorough statistical analysis of the results obtained is obviously needed.

Table 2 Values of pH of paper as a function of ageing time (days). Type of reactor

Regression equation

Autoclave Vial (KP) Chamber

pHA ¼ 0.0090(0.0012)  t þ 6.2267(0.0252) pHKP ¼ 0.0071(0.0011)  t þ 6.2479(0.0246)a pHCh ¼ 0.0049(0.0002)  t þ 6.2205(0.0057)

a

Results averaged for the measurements in two vials.

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Fig. 4. Experimental points and lines calculated from linear regression in the coordinate system (1/DP  1/DP0) vs. time. Each point represents a mean value of DP calculated from several (2e4) measurements. For the sake of clarity, errors of the individual points have been omitted from the drawing.

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Fig. 6. Regression line and 95% limits of confidence interval for the linear regression calculated on the basis of kinetic data obtained in autoclaves.

3.4.2. Errors of individual kinetic points It must be stressed here that this kind of analysis was not the primary goal of the current research. Therefore, the recurrence of the DP values was checked in various ways for every type of kinetic run. Generally, two different vials were used for the DP measurement at a given time of degradation, and one viscometric analysis was done for each of them. In the climatic chamber, two DP measurements were performed for one sample degraded for a given number of days. The most complex system was used for autoclaves. For some points, only one autoclave was used, and for others two autoclaves were employed (these were the points corresponding to 2, 8, 14, 25 and 30 days of degradation). In every case, two viscometric DP measurements were performed. This situation is very inconvenient for statistical analysis. Sources of random experimental errors are somewhat different in every case. However, due to the long time necessary for repeating some measurements, it was decided to use the current data set for statistical analysis. Providing all pertinent kinetic data along with their errors would be too space-consuming for this article. The reader may, however, get a feeling of the magnitude of errors by studying Table 3.

3.4.3. Statistical analysis When looking at Fig. 4, one may assume that no statistically significant difference exists between the kinetic runs obtained in different reactors, and the kinetic curves presented in the figure belong to the same general population. Remembering that the obtained kinetic data are not statistically harmonized (different sources of random statistical errors), one can at least try to make an assessment of the validity of this hypothesis. This can be done by analyzing the confidence intervals for the regression lines obtained after transforming the original system of coordinates (DP vs. time) into the linearized system (1/DP  1/DP0) vs. time. When making such transformation, one obviously needs also to transform the experimental errors of DP measurements into the new system, because the linear regression analysis must take statistical weights into account. This can be achieved by using the classic law of error propagation. Figs. 5e7 show the results obtained from regression analysis e separately for each type of reactor. In every drawing, the regression line for the current reactor, as well as regression lines for two remaining reactors, are presented simultaneously with the limits of 95% confidence interval for the regression results.

Fig. 5. Regression line and 95% limits of confidence interval for the linear regression calculated on the basis of kinetic data obtained in vials.

Fig. 7. Regression line and 95% limits of confidence interval for the linear regression calculated on the basis of kinetic data obtained in climatic chamber.

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Table 3 Relative errors (DDP/DP) of measurements for kinetic runs performed in vials, autoclaves and the climatic chamber. Type of reactor

Number of exp. Total number Minimal Maximal Average points shown of DP rel. error rel. error rel. error in Fig. 4 measurements [%] [%] %]

Vials 12 Autoclaves 12 Climatic 12 chamber

26 34 24

0.08 0.02 0.02

2.74 4.79 1.66

0.75 0.84 0.37

For every one of these diagrams, the regression lines corresponding to the reactor types other than the current one are situated within the 95% confidence limits calculated for the current type of reactor. There may be only one conclusion drawn from this fact e there is no reason to discard the investigated hypothesis that no statistically significant difference exists between the kinetic runs obtained in different reactors. This is a strong indication that e from a statistical point of view e kinetic data obtained in all types of reactors belong to the same general population. During a rigorous statistical reasoning, no significance test can confirm the null hypothesis. It is possible however to make an assessment of probability of error connected with discarding this hypothesis. In our case, this assessment may be done by finding the significance level which would result in finding at least part of only one regression line situated beyond the calculated confidence limits. This critical probability is equal to about 78% for the confidence limits calculated for vial data, 26% for the autoclave data and 66% for data obtained for samples aged in the climatic chamber. It is to be concluded, therefore, that even if the kinetic curve obtained for autoclaves is taken as a reference (probability of error equal to 26%), then the error connected with rejecting the hypothesis about affiliation of all kinetic curves to a common general population would be unacceptable. The analysis presented above allows for putting forward a working hypothesis that kinetic data obtained in the closed systems are not significantly different from the kinetic curves obtained in a more traditional way, in the open system. Of course, the uncertainties connected with the statistical nonuniformity of the experimental data used for the described analysis should be cleared by making more thoroughly planned experiments. Finally, it is important to note that the difference between kinetic curves of paper degradation, as evidenced by the DP data, is poorly documented in the ASTM research report. In the CCI part of the report [3] there are no DP results for paper degraded in vials. In the LOC part of the report [4], a comparison of different ageing configurations, as expressed by DP data, is limited only to Fig. 165 (no tabularized data are given). This figure has been plotted in a semi-logarithmic scale (log(DP) vs. time), polynomial trends of 2-nd order have been applied to experimental data quite arbitrarily, and the implications of this way of presentation are very momentous indeed. The authors claim that this figure shows faster rates of degradation within vials. The trend lines compel the reader to

believe this statement to be true. This belief is further increased by the lack of error bars in the figure (and the authors clearly declare on p. 43 that ‘‘A constant drawback in this work has been the large standard deviation values attached to all the property measurements due to the poor formation characteristics of the papers’’). But if the data were plotted in the natural scale (DP vs. time), this difference in degradation rates would not be so obvious. Therefore, in order to prove or disprove the authors’ claims, a rigorous statistical treatment should be given to the data. 3.4.4. Evolution of mechanical properties with time Determinations of zero-span breaking length have been made for the samples of paper taken from the same kinetic runs that provided DP data. The results were plotted vs. the ageing time in Fig. 8. As previously, the points represent only mean values for individual degradation times, as plotting the error bars would make this drawing too complex and unclear. The errors are given in Table 4, and they were calculated from standard deviations for 30e40 individual measurements done for every degradation time. Even without any statistical calculations, it is obvious that the data are too scattered to be useful when discerning the kinetic curves obtained in reactors of differing type. Other measured mechanical properties (number of double folds and tearing index) give a similar picture, and they are not presented in this work. 3.4.5. Changes of colour in the aged samples Measurements of paper yellowing were carried out for the samples of paper taken from the same kinetic runs that provided the DP data. The results of these analyses, expressed as Whiteness Index (WI) were plotted as a function of ageing time in Fig. 9. The points in Fig. 9 represent mean values obtained from 5 measurements made after each individual degradation time. Due to better separation of points, their standard deviations have also been shown in this picture. It can be easily observed that changes in WI of the samples aged in closed systems (vials and autoclaves) differ only

Fig. 8. Dependence of zero-span breaking length on the degradation time in various reactors.

T. Sawoszczuk et al. / Journal of Cultural Heritage 9 (2008) 401e411 Table 4 Relative errors of breaking length values for kinetic runs performed in vials, autoclaves and the climatic chamber. Type of reactor

Minimal rel. error [%]

Maximal rel. error [%]

Average rel. error [%]

Vials Autoclaves Climatic chamber

2.91 2.57 2.82

5.67 3.96 6.36

3.87 3.37 3.78

slightly (at least up to the 20 days degradation time), and they are situated markedly lower than points corresponding to ageing in the climatic chamber. Statistical tests (identical with those applied for the DP kinetic data) confirmed the hypothesis that the regression line obtained for climatic chamber belongs to a general population different from the one corresponding to data obtained in the closed systems, at the significance level of 0.05. These observations are in accordance with the conclusions drawn from the pH values obtained for these samples, and they can be explained in a similar way: eIn the closed systems, the volatile organic products of cellulose degradation are re-absorbed in the structure of paper after the ageing process has been finished. This assumption is supported by previous research on this topic: Refs. [6,20]. The authors of these papers have found, by conducting SPME/GS/MS analysis, that some of the cellulose degradation products observed in the mixture of volatile organic compounds were emitted from aged paper samples, but they could also be found, by capillary zone electrophoresis (CZE) measurements, in the extracts taken from samples of the aged paper. Chromophore compounds are formed through the interactions of degradation products with oxidized groups in cellulose chains [21]. Even extremely low concentrations of these chromophores (in ppb scale) visibly change the appearance of aged paper samples due to their high extinction coefficients [22]. eOne can make some speculations based on literature data. A number of degradation products occurring in the aged paper are supposed to belong to the same groups of chemicals as the chromophores (e.g. benzoquinones,

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naphthalene and phenanthrene derivatives) that are regarded to be responsible for discoloration of paper [21,23]. On the other hand, formation of chromophore compounds is possible through interactions of paper degradation products with the carbonyl groups in cellulose chains [24]. The higher the concentration of degradation products in paper, the bigger is the amount of chromophores formed. This situation is found during ageing in a closed system, where products of cellulose degradation are absorbed in the paper after ageing. The role of the carbonyl group in the formation of chromophores is crucial. This conclusion can be derived from the statement presented in Ref. [24]: ‘‘condensation reactions underlying thermal ageing phenomena and chromophore generation require free carbonyls’’,4 and also from the results of Adorjan et al. [22], who established that a linear correlation exists between the concentration of carbonyls and the rate of formation of chromophores in cellulose. eIn the open system (climatic chamber), organic products of cellulose degradation are continuously and easily removed from the paper structure during the accelerated ageing tests. Obviously, they are more volatile under the test conditions. Therefore, they do not influence the colour of paper so much because their lower concentration causes the formation of chromophores to be minor.

4. Conclusions The primary objective of the research presented in this publication was to know what conditions really do exist inside the vials during the accelerated ageing tests ([4], p. 79). The lack of tightness of the vials became the pivotal problem. Water and air were leaking out of the vials. None of the vials employed was completely tight, though this lack of tightness was not considerably high. The ageing tests may be done in vials if one is ready to accept a ca. 10% loss of water after 14 days of ageing at the temperature of 90  C. Similarity of results obtained in vials and autoclaves shows that the assemblies used in our study could be regarded as sufficiently tight. Unfortunately, no direct experimental proof for the tightness of autoclaves exists. The preceding ideas give rise to two useful implications: e The gravimetric inspection of mass should be an important element of ageing tests performed in vials. Various kinds of closed vials are used in different laboratories [6,7], and it is possible that some researchers work with tight systems. It would be advantageous for other researchers working in this field to be informed about such tight systems, as seeking for systems more leak-proof than those recommended by the ASTM standard would not be necessary. 4

Fig. 9. Dependence of whiteness index on the degradation time in various reactors.

The term ‘‘free carbonyls’’ describes in this situation the carbonyl groups occurring in the form C]O and not in hemiacetal/hemiketal or hydrate form because, at elevated temperature, some water is removed from cellulose [24].

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e Statements that the recurrence of experimental results proves the tightness of closed systems are not convincing ([4], p. 79, point c). Recurring outcome of experiments is only an indication that no major leaks exist in any vial found among the tested ones. The degree of ‘‘non-tightness’’ can be reduced and kept at acceptably constant level if a dynamometric spanner is used, this being a noteworthy element of novelty in the presented study. Authors of the ASTM report made an assessment of the relative humidity in vials containing paper. In the equilibrium state this value is estimated to be equal to ca. 93% at 100  C ([3], p. 42). It is true that our experiments were performed at 90  C e another temperature value recommended by Be´gin and Kaminska [8]. However, this difference of temperature is probably too small to explain the difference between the value of 93% and our value of 59% obtained during the Moisture Content Meter calibration5 and the value of 52% obtained during the approximate calculations based on the analysis of the lack of tightness in vials. The reader should keep in mind the fact that the measurement done with the MCA meter does not require the vials to be opened, whereas Be´gin had to open his vials in order to determine the moisture content gravimetrically (p. 40 of Ref. [3]). Therefore the RH values given in the ASTM report could be overestimated. A strong support for this conclusion may be found in Ref. [3] on p. 42: ‘‘It is possible that some of the water may have been lost in the process of determining its moisture content, so that the amount of water released by the paper was overestimated.’’ The vials recommended in the ASTM standard have been employed in kinetic studies of cellulose degradation. Degree of polymerization (DP) was the primary property of cellulose monitored in our experiments. The DP measurements are highly sensitive and more precise than measurements of mechanical properties ([4], p. 22). It has been found, unexpectedly, that the kinetic curves obtained for vials, autoclaves and the climatic chamber could not be regarded as statistically different. One cannot reject a supposition that they belong to the same general population. In this context, let us note that the concentration of acidic products of degradation inside vials and autoclaves is small at the beginning of the test. As pointed out by Zervos and Moropoulou [18], in the very early stages of ageing the models that do and do not take into account the increasing acidity of paper must give similar results. For the increasing time of ageing, the lack of difference between kinetic results obtained for vials, autoclaves and climatic chamber may result from a depletion of water in the closed systems due to drying (in vials) and recrystallization (vials and autoclaves). The significance of the decrease in humidity at the final stage of ageing tests has already been noticed by Shahani ([4], pp. 21e22). One should note that the difference between the kinetic curves of paper degradation, as evidenced by the DP data, is

5

Our unpublished results.

poorly documented in the ASTM research report (see Section 3.4.3). However, it should be stressed that the effect of the interaction between the acidic products of degradation and paper e so closely related to the origins of the ASTM program e is confirmed by: (a) results obtained for degradation of paper stacks in vials, (b) pH measurements, (c) statistically different values of whiteness index, as measured in closed vessels and the climatic chamber (see Section 3.4.5). ad a) Our unpublished experiments, not reported in the present study, consisting in ageing samples of paper P1 according to the ASTM standard inside vials e but in stacks instead of loose rolls of paper e have shown that small, but statistically significant, differences in polymerization degree of samples aged for 14 days at 90  C existed. The mean DP value for the samples taken from the stack was smaller than the mean DP measured for the samples aged in rolls. A direct and close contact of sheets of paper during ageing might lead to a degradation that better imitates the effects observed during natural ageing. The kinetics of paper ageing in closed vessels, when the paper is loosely rolled, very much resembles the results obtained for samples aged in the climatic chamber. ad b) The pH data given in Table 2 for samples aged in open and closed systems are statistically different at the significance level a ¼ 0.05. ad c) The products formed at elevated temperatures, believed to be responsible for the yellowing of paper, could be released from the paper matrix in the open environment and thus did not influence the colour of aged paper as much as in a closed system (be it a closed book or an airtight vial). Further studies building their conclusions on colour changes of the samples of aged paper should take into account the described difference in results obtained for open and closed systems. It should also be noted here that a pure-cellulose paper was used in the present study, whereas the effect of colour changes could be more pronounced for lignin-containing papers. The function of volatile products of paper degradation, especially acidic products, can be estimated and explained by investigating the mechanisms of acid hydrolysis and oxidation observed in model papers (consisting mainly of cellulose) as well as in commercially available ones e especially those containing lignin. This kind of research was accomplished at the Library of Congress [17], the CRCC in Paris [6,20,23], the University of Stuttgart [25], and the National Library and University in Lubljana [26]. Mass balances play an important role in such research if it is to be conducted quantitatively and give reliable and repeatable results. The credibility of such balances obviously depends on the tightness of vials.

T. Sawoszczuk et al. / Journal of Cultural Heritage 9 (2008) 401e411

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