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A study of mechanical properties of papers exposed to various methods of accelerated ageing. Part I. The effect of heat and humidity on original wood-pulp papers
Journal of Cultural Heritage 10 (2009) 222–231
Original article
A study of mechanical properties of papers exposed to various methods of accelerated ageing. Part I. The effect of heat and humidity on original wood-pulp papers Bohuslava Havlínová a , Svetozár Katuˇscˇ ák a , Miroslava Petroviˇcová a , Alena Maková b , Vlasta Brezová c,∗ a Institute of Polymer Materials, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 81237 Bratislava, Slovak Republic b Slovak National Library, Námestie J. C. Hronského 1, 03601 Martin, Slovak Republic c Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 81237 Bratislava, Slovak Republic
Received 9 January 2008; accepted 17 July 2008
Abstract The damage to historical documents and books caused by the acidic character of paper is often manifested as a complete loss of their mechanical properties. Deacidification and restoration of archived paper objects require knowledge of the long-term behaviour of paper before and after repair actions. Our study was focused on the investigation of mechanical properties (tensile strength, stretch, tensile index, zero-span tensile strength, folding endurance) of original papers (one alkaline and three different acidic samples) exposed to five methods of dry-heat and moist-heat accelerated ageing. The degree of paper deterioration upon ageing was significantly influenced by the temperature and relative humidity, along with the intrinsic chemistry of the individual paper samples. The correlation matrix evaluated at a 95% confidence level for tensile strength, stretch, tensile index and zero-span showed linear correlations between these mechanical properties for all the paper samples. However, a linear dependence of folding endurance on zero-span tensile strength was found only for alkaline paper, which revealed the highest resistance to the accelerated ageing tests. In addition, the concentration of paramagnetic semiquinone species in the acidic lignin-containing paper samples was monitored by Electron paramagnetic resonance spectroscopy. © 2009 Elsevier Masson SAS. All rights reserved. Keywords: Paper; Mechanical properties; Accelerated ageing tests; Paper degradation; EPR spectroscopy
Abbreviations
1. Introduction
Bpp DPPH EPR p r RH ROS
Deterioration in the physicomechanical, chemical and optical properties of historic paper documents and artefacts stored in libraries and archives is responsible for an enormous loss of cultural heritage [1–5]. The consequences of acidic paper production after 1850, and the utilization of additives such as aluminium sulphate have been ignored for many years, but nowadays a great deal of attention is focused on the processes of mass deacidification in order to preserve huge volumes of historic objects [6–12]. The quality and long-term stability of paper materials produced from cellulosic fibers with various additives are determined by the extent of oxidative and hydrolytic reactions taking place upon ageing. These processes may considerably reduce the physicomechanical and chemical properties
∗
peak-to-peak line width 1,1-diphenyl-2-picrylhydrazyl Electron paramagnetic resonance p-value (probability) correlation coefficient Relative humidity Reactive oxygen species
Corresponding author. Tel.: +421 2 59325666; fax: +421 2 52926032. E-mail address: [email protected] (V. Brezová).
1296-2074/$ – see front matter © 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2008.07.009
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of paper, resulting in damage to paper structures, an increase of brittleness, and finally, a total loss of material quality. The hydrolytic and oxidative processes are related to each other, and are affected by various intrinsic factors (sizing, filling, adhesives, the presence of acid groups, metal ions, lignin, degradation products, etc.) and extrinsic factors (storage conditions, temperature, RH, the presence of oxygen, light and environmental impurities) [2,13–17]. The process of natural paper ageing is too slow to permit observing changes in a reasonable time frame. Thus, methods of accelerated ageing under dry-heat or moist-heat treatment at higher temperatures have often been used to speed up the chemical processes in paper. However, the relation between accelerated and natural ageing represents a serious problem, as changes monitored upon accelerated ageing procedures must be correctly extrapolated to the ambient conditions [18–22]. Our study is focused on the investigations of mechanical properties (tensile strength, stretch, folding endurance, tensile index and zero-span tensile strength) of four different paper samples exposed to five different dry-heat and moist-heat methods of accelerated ageing. The results obtained serve as a basis to predict the behaviour of the investigated paper samples during long-term storage in archives before deacidification and stabilization treatments.
2. Experimental 2.1. Materials The characteristic properties of four different paper samples investigated are summarized in Table 1. Paper A represents a modern alkaline paper with sufficient alkaline reserve, suitable for long-term storage according to ISO 9706 [23]. On the other hand, the acidic papers B–D were chosen in order to model the deterioration processes of documents and records which were printed on various acidic papers during the 19th and 20th centuries.
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Sodium hydroxide and hydrochloric acid were purchased from Lachema (Czech Republic); deionized water was used for the preparation of solutions. 2.2. Measurements of mechanical properties of paper All paper samples were conditioned according to ISO 187 [24] before measurements at a temperature of 23 ◦ C and a RH of 50% over 24 hours. The experiments, which were focused on the mechanical properties of paper, were performed in compliance with ISO standards [25–29], using a device for thickness measurement (Lorentzen & Wettre, Sweden); a universal instrument for mechanical tests (INSTRON 1011, England); an apparatus for measuring the folding endurance according to Shopper (DFP, VEB Werkstoffprüfmaschinen, Germany). The mean values of tensile strength, stretch, tensile index and zero-span tensile strength were calculated from 10 measurements with a precision in the ± 10% range, while the mean values of folding endurance were evaluated from 20 measurements within the ± 15% range. Relative losses (%) of the monitored properties were calculated for the individual ageing periods. The changes in the mechanical properties of paper samples A–D (tensile strength, stretch, tensile index, zero-span tensile strength, folding endurance) were measured at individual ageing times (0, 0.33, 1, 3, 7, 14 and 28 days), and the experimental data were analyzed by a suitable model of chemical kinetics, e.g., first-order kinetics [30]. The pH values of aqueous extracts prepared with cold water and determined as specified in ISO 6588 [31] were measured at 25 ◦ C by pH-meter OP-208/1 (Radelkis, Hungary) using a combined glass electrode. The alkali reserve of paper A was measured and evaluated according to ISO 10716 [32]. 2.3. Methods of accelerated ageing In order to model the long-term degradation processes in an appropriate time scale, the paper samples A–D were exposed to accelerated-ageing procedures. The dry-heat ageing at 120 ◦ C and 105 ◦ C, respectively, was performed according
Table 1 Characteristics of investigated paper samples. Paper
Grammage (g m−2 )
Characteristics
Producer
pH of cold aqueous extract
Alkali reserve ( mol kg−1 )
A
80
Chemical pulp paper, lignin-free, alkaline, offset paper
9.6
4.3
B
50
5.9
–
C
80
–
60
Slavoˇsovce Paper-mills, Slavoˇsovce, Slovak Republic South-Czech Paper-mills, Vˇetˇrní, Czech Republic
4.4
D
Voluminous, softwood pulp, mechanical pulp content 65%, unbleached chemical sulphite pulp 35%, slightly polished, unsized print paper Mechanical and chemical hardwood pulp paper, acid, writing Softwood pulp, mechanical pulp 54%, chemical unbleached sulphite pulp 18%, chemical bleached sulphite/sulphate pulp 13%, kaolin 15%, sized print paper, machine-glazed
North-Slovakian Pulp and Mill in Ruˇzomberok, Ruˇzomberok, Slovak Republic South-Czech Paper-mills, Vˇetˇrní, Czech Republic
6.2
–
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to ISO 5630-1 [33,34] in a laboratory drier WSU 100 (VEB MLW Laboratortechnik, Illmenau, Germany) for 0.33, 1, 3, 7, 14 and 28 days. The effect of RH on paper was monitored during moist-heat procedures at 80 ◦ C/65% RH (climate chamber Feutron, GmbH Greis, Germany in accordance with ISO 5630-3 [35]), and 80 ◦ C/45% or 80 ◦ C/25% RH (climate chamber Challenge 160, 9286 Angelantoni Industrie, Italy; analogously to ISO 5630-2 [36]) for 0.33, 1, 3, 7, 14 and 28 days.
2.4. EPR measurements Lignin-containing paper samples (∼100 mg; sample column height 70 ± 2 mm) were placed in thin-wall quartz EPR tubes (internal diameter: 3 mm; length: 150 mm; wall thickness: about 0.1 mm). The EPR tube was then inserted into a standard TE102 (ER 4102 ST) rectangular cavity of an EMX X-band EPR spectrometer (Bruker, Germany) and the EPR spectrum was recorded at 25 ◦ C under air. Temperature control was achieved using a Bruker temperature control unit ER 4111 VT. EPR spectrometer settings were as follows: microwave frequency, 9.42 GHz; microwave power, 0.63 mW; center field, 334.5 mT; sweep width, 20 mT; gain, 5 × 105 ; modulation amplitude, 0.2 mT; modulation frequency, 100 kHz; sweep time, 42 s; time constant, 40.96 ms, number of scans, 20. The g-values were determined with an uncertainty of ± 0.0005 by simultaneous measurement with a reference sample containing the stable free radical DPPH (Sigma Chemicals) filled in a precise capillary. The experimental EPR spectra processing and simulation was carried out using the WIN EPR and SimFonia programs (Bruker, Germany). The EPR instrument settings for quantitative evaluation were examined by DPPH standards calibration. The integral intensities of the EPR signals were obtained by double integration of the experimental spectrum, and the spin concentration was recalculated per 1 g of paper sample.
3. Results and discussion 3.1. The effect of accelerated ageing conditions on mechanical properties of paper samples Paper represents a very complex system consisting of cellulose and a variety of non-cellulosic materials. The mechanical properties of different paper samples are substantially influenced by the individual characteristics of cellulose fibers, by the nature, concentration and chemical properties of fillers and additives, as well as by the network structure of the paper [37–39]. The different properties and characters of the investigated papers A–D (Table 1) induced variations in the extent of paper damage upon accelerated ageing. In order to accelerate and to model the degradation processes of paper materials, the dry-heat treatment at 120 or 105 ◦ C serves as a relatively simple experimental technique. In accordance with data in the literature, three days of accelerated ageing at 105 ◦ C correspond to 25 years of natural ageing [20,21,40], although the mechanism of paper dry-heat degradation is not fully compatible with the natural ageing processes, which are influenced by the presence of environmental humidity [18]. Consequently, the moist-heat experiments at 80 ◦ C and 65%, as well as 45 and 25% RH were also performed with the aim of monitoring the effect of RH on the deterioration of papers. In order to display the experimental data obtained in a comprehensible form, we evaluated the percent loss of mechanical characteristics caused by the ageing procedure on samples A–D after 3 and 28 days, respectively (Figs. 1–4a and b). Values of tensile strength reflect the detailed structure of the paper and the properties of its individual fibers, i.e., the dimension and strength of fibers, their arrangements, and interfiber bonding [38,40]. The effects of accelerated ageing processes on paper are interpreted in terms of cellulose chain scission producing weaker fibers, and covalent crosslinking by the additional hydrogen bonds leading to increased brittleness [38,40,41].
Fig. 1. Percent loss of tensile strength monitored for paper samples A–D exposed to various accelerated ageing conditions for: a: 3 days; b: 28 days.
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Fig. 2. Percent loss of stretch monitored for paper samples A–D exposed to various accelerated ageing conditions for: a: 3 days; b: 28 days.
Fig. 1 shows the percent loss in tensile strength after 3 days (Fig. 1a) or 28 days (Fig. 1b) of accelerated ageing under different experimental conditions. These results confirmed the strong impact of the paper’s properties on the loss of tensile strength upon accelerated ageing. The application of dry heat at 120 ◦ C caused a noticeable decrease of tensile strength of acidic paper C (34% after 3 days and 48% after 28 days of ageing, respectively). The influence of RH on the loss of tensile strength of the paper samples is evident generally for longer ageing periods, as moist heat at 80 ◦ C/65% RH yielded similar changes as the dry heat did at 105 ◦ C (Fig. 1b). It should be noted here that alkaline paper sample A demonstrated good stability, since after 28 days the monitored decrease in tensile strength was lower than 20% for all ageing conditions (Fig. 1b). Stretch can be related to the paper’s ability to conform and maintain conformance to a particular contour [37], and is also regarded as one of the most important criteria for the satisfactory behaviour of paper in applications [42]. Fig. 2 shows the percent loss of stretch for samples A–D initiated by different ageing conditions after 3 days and 28 days, respectively. Substantial decrease of stretch was observed after dry-heat ageing for acidic paper C, e.g., 50% after 3 days, and 80% after 28 days at 120 ◦ C (Fig. 2a and b). Only a minor effect of RH on stretch was observed for samples A–D aged during 3 days at 80 ◦ C (Fig. 2a). However, 28 days of ageing at 80 ◦ C/65% RH caused a loss in stretch of 12% for alkaline paper A, and about 30% for acidic papers B–D. A decrease of RH to 25% resulted in an analogous loss of stretch for all investigated samples, not exceeding 15% (Fig. 2b). The measurement of zero-span tensile strength of the paper materials serves as an indicator of the individual cellulose fiber’s strength [38,40,43], consequently, the zero-span changes initiated by accelerated ageing methods has been used to demonstrate the damage to cellulose chains caused by the oxidation and hydrolytic processes [43]. Previously, it has been confirmed that
zero-span represents a more sensitive tool to monitor the effects of ageing on paper than tensile strength, and upon dry-heat ageing, the changes in zero-span tensile strength reflected mainly the chemistry of the samples [44]. Fig. 3 depicts the percent loss in zero-span tensile strength evaluated for paper samples A–D upon ageing under different experimental conditions over 3 and 28 days, respectively. The results confirmed the loss of zero-span tensile strength for acidic paper C, proportionally to the temperature and RH applied, reaching 48% and 100% after 3 or 28 days of dry-heat treatment at 120 ◦ C (Fig. 3a and b). Alkaline paper A showed the best resistance to the dry heat or moist heat but the monitored effect of raised temperature and humidity exhibited more complex behaviour compared to acidic paper C (Fig. 3a and b). The values of tensile strength, stretch, tensile index and zero-span tensile strength monitored upon accelerated ageing procedures are related to the extent of oxidative and hydrolytic damage to cellulosic fibers [20,21,38,40,41,45]. In order to find the relationships between these mechanical properties, we performed a linear correlation analysis at a 95% confidence level using all experimental data measured in accelerated ageing tests for the individual paper samples A–D. Table 2 summarizes the calculated correlation coefficients, r, between the couples of mechanical properties for paper samples A–D, evidencing their significant linear dependencies (r ∈ 0.7615–0.9688; p < 0.0001), reflecting thus the predominant role of cellulose fibers in the damage to paper materials caused by heat and humidity. Folding endurance represents the most sensitive indicator of paper breakage upon ageing, as the difference in folding endurance is observable long before the changes in tensile strength, stretch or zero-span tensile strength are measurable. On the other hand, the measurement of folding endurance is coupled with lower precision, as well as the necessity of precise conditioning during experiments [9,38].
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Fig. 3. Percent loss of zero-span tensile strength monitored for paper samples A–D exposed to various accelerated ageing conditions for: a: 3 days; b: 28 days.
Fig. 4a and b show the percent loss in folding endurance of paper samples A–D monitored under different experimental conditions after 3 and 28 days, respectively. The results confirmed a high sensitivity of folding endurance to the effects of dry-heat and moist-heat procedures. Acidic paper C showed a complete loss of folding endurance after only 3 days of ageing at 120 ◦ C. For this unstable paper sample, the loss of folding endurance after 3 days is proportional to the temperature and RH (Fig. 4a). The highest resistance to ageing was found for alkaline paper A, as 3 days of ageing at 105 ◦ C, corresponding to 25 years of natural ageing [19,40], caused only a 20% loss in folding endurance, and the decrease of folding endurance after 28 days at 105 ◦ C reached 50%. However, all the investigated papers A–D exhibited 100% loss in folding endurance after 28 days of ageing at
120 ◦ C. After 28 days of ageing the samples of acidic papers B–D showed a significant decrease in folding endurance from 80% at 80 ◦ C/25% RH, up to 100% at 105 ◦ C (Fig. 4b). The decrease in the number of double folds monitored upon different accelerated ageings of paper samples was described by exponential functions corresponding to the formal firstorder kinetic model [30]. The experimental time dependencies as shown in Fig. 5 for sample C were fitted by a non-linear least-squares method (Scientist, MicroMath), and the formal half-lives of paper samples exposed to the different accelerated ageing conditions was calculated. The evaluated values of formal half-lives for folding endurance summarized in Fig. 6 unambiguously demonstrated the highest resistance of alkaline paper A to accelerated ageing, as its formal half-life values are
Fig. 4. Percent loss of folding endurance monitored for paper samples A–D exposed to various accelerated ageing procedures for: a: 3 days; b: 28 days.
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Table 2 Correlation matrix between mechanical properties of paper samples A–D. (The values represent the linear correlation coefficients calculated at a 95% confidence level; p < 0.0001). Tensile strength
Stretch
Tensile index
Zero-span tensile strength
Tensile strength
1
0.8746 (A) 0.9688 (B) 0.9204 (C) 0.9395 (D)
0.9287 (A) 0.9187 (B) 0.9583 (C) 0.9490 (D)
0.8769 (A) 0.9461 (B) 0.9299 (C) 0.9309 (D)
Stretch
0.8746 (A) 0.9688 (B) 0.9204 (C) 0.9395 (D)
1
0.8333 (A) 0.8796 (B) 0.8776 (C) 0.9169 (D)
0.9424 (A) 0.8837 (B) 0.9603 (C) 0.9133 (D)
Tensile index
0.9287 (A) 0.9187 (B) 0.9583 (C) 0.9490 (D)
0.8333 (A) 0.8796 (B) 0.8776 (C) 0.9169 (D)
1
0.7615 (A) 0.9458 (B) 0.9049 (C) 0.9372 (D)
Zero-span tensile strength
0.8769 (A) 0.9461 (B) 0.9299 (C) 0.9309 (D)
0.9424 (A) 0.8837 (B) 0.9603 (C) 0.9133 (D)
0.7615 (A) 0.9458 (B) 0.9049 (C) 0.9372 (D)
1
the highest for all ageing procedures; for acidic papers B–D, the best results were obtained for paper B. Despite comparable pH values of papers B and D (5.9 and 6.2, respectively, Table 1), the formal folding endurance half-lives demonstrated significant differences (Fig. 6). Most likely the hydrolytic and oxidation processes induced by accelerated ageing were also affected by various additives present in paper matrices. The monitored effect of accelerated ageing on tensile strength, stretch and tensile index for all papers and types of ageing was not as significant as the effect of accelerated ageing on folding endurance and zero-span tensile strength. Previously, zero-span and folding endurance were suggested as the suitable
Fig. 5. Decrease of folding endurance on accelerated ageing period measured for paper C during various accelerated ageing conditions: : 120 ◦ C; : 105 ◦ C; : 80 ◦ C/65% RH; : 80 ◦ C/45% RH; : 80 ◦ C/25% RH. The marked symbols represent the experimental data, and dashed lines, their mathematical simulations using least squares analysis.
indicators of paper deterioration caused by natural or accelerated ageing [38,40,41]. Despite the lowered precision, folding endurance represents the most sensitive test of damage to the paper [8,38]. Fig. 7 shows the dependence of folding endurance on zerospan tensile strength evaluated for the investigated paper samples A–D exposed to the various accelerated ageing conditions. The results obtained clearly demonstrated that the number of double folds is proportional to the zero-span values higher than 3.5 kN m−1 , and the values of zero-span below 3.5 kN−1 were in our study associated with complete loss of folding endurance
Fig. 6. The values of folding endurance formal half-life evaluated using a firstorder kinetic model for paper samples A–D exposed to various accelerated ageing procedures.
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Fig. 7. Dependence of folding endurance on zero-span tensile strength evaluated from data sets obtained during accelerated ageing procedures: a: paper sample A; the symbols represent the experimental data, and the solid line, the linear dependence, with the 95% confidence intervals (dotted lines); b: paper sample B; c: paper sample C; d: paper sample D.
for the investigated paper samples (Fig. 7). It should be noted here, that for alkaline paper A the experimental data of folding endurance and zero-span tensile strength were fitted by a linear dependence at a 95% confidence level reaching the correlation coefficient r = 0.8292 and p < 0.0001 (Fig 7a). 3.2. EPR spectra of paper samples exposed to accelerated ageing Oxidative degradation of cellulosic materials upon natural or accelerated ageing is coupled with the formation of radical species and hydroperoxides [46,13,47]. Additionally, the presence of transition metal ions in paper can play a significant role in the decomposition of hydroperoxides, producing ROS [48]. EPR spectroscopy represents a unique and sensitive tool for the study of structures containing free radical species and paramagnetic
transition metal ions in the cellulosic materials [49–51]. The presence of lignin in wood-pulp papers substantially influences their deterioration processes. The light-sensitivity of lignin also plays a specific role, causing the low photostability and material discolouration [52]. It has been previously shown that the chemical reactivity of lignin resulted in the loss of mechanical properties of lignin-containing papers. The degradation process was mainly affected by pH value, since the damage to papers with sufficient alkaline reserve was not influenced by the presence of lignin [53]. On the other hand, due to the structures of lignin compounds, corresponding to hindered phenols, lignins can act as effective antioxidants capable of scavenging reactive radical intermediates [54–57]. The X-band EPR spectrum monitored in lignin-containing paper samples was characterized by an asymmetric singlet line with geff = 2.0054 and Bpp = 0.83 mT (inset in Fig. 8), which
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stretch, tensile index and zero-span tensile strength were evaluated at a 95% confidence level, with correlation coefficients from 0.7615 up to 0.9688, reflecting thus the dominant role of cellulose fiber characteristics in the mechanical behaviour of paper. The changes in concentration of semiquinone paramagnetic species in acidic lignin-containing papers upon ageing tests were demonstrated by EPR spectroscopy. The results confirmed the significant role of alkaline reserve in paper for slowing the degradation processes induced by different accelerated methods. The inhibition of hydrolytic and oxidation processes may be attained by the addition of compounds enhancing paper durability, as well as inhibiting oxidation processes. The influence of accelerated ageing tests on the mechanical properties of deacidified and/or stabilized [58] paper samples A–D will be shown in the second part of our investigations. Acknowledgements Fig. 8. Dependence of spins in 1 g of paper sample C on the accelerated ageing period: : 120 ◦ C; : 80 ◦ C/45% RH. The inset represents the experimental EPR spectrum of paper C (magnetic field sweep width 20 mT).
was assigned to stable semiquinone radical species produced by one-electron oxidation of lignin phenolic groups [49–51]. For illustration, the concentration of paramagnetic species in the original paper sample C measured under air was 7 × 1015 spins g−1 , and it increased upon accelerated ageing as is shown in Fig. 8. The concentration of paramagnetic species was influenced by the accelerated ageing method applied; the highest values were measured for samples exposed to dry heat at 120 ◦ C. The decline of spin concentration monitored after 28 days of accelerated ageing was assigned to the transformations of paramagnetic semiquinone species to quinone and quinone-like structures responsible also for colour changes in the paper [52]. A relationship between increased concentration of paramagnetic species measured in lignin-containing papers, and changes of mechanical properties, was not confirmed. 4. Conclusions The mechanical properties (tensile strength, stretch, tensile index, zero-span tensile strength, folding endurance) of original paper samples were monitored during dry-heat and moist-heat accelerated ageing tests under various experimental conditions (120 ◦ C, 105 ◦ C, 80 ◦ C/65% RH, 80 ◦ C/45% RH and 80 ◦ C/25% RH). Our results showed that folding endurance is a highly sensitive means for monitoring changes in paper structure. Our experiments confirmed that alkaline paper A exhibits resistance to ageing in accordance with ISO 9706 [23], as shown by high values of double folds number, with a significant decrease only upon prolonged dry-heat ageing at 120 ◦ C. The increased temperature and RH caused a significant loss of folding endurance, especially for acidic papers B–D. However, the application of accelerated ageing test performed at mild conditions (80 ◦ C/45% RH and 80 ◦ C/25% RH) make it possible to compare the durability of different paper samples A–D even after 28-day period. The linear correlations between tensile strength,
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[36] ISO 5630-2:1985, Paper and board – Accelerated ageing – Part 2: Moist heat treatment at 90 ◦ C and 25% relative humidity, International Organization for Standardization, Geneva, Switzerland, 1985. [37] Properties of paper. Available at http://www.paperonweb.com/paperpro. htm (May 08, 2008). [38] D.F. Caulfield, D.E. Gunderson, Paper testing and strength characteristics, in: TAPPI Proceedings of the 1988 Paper Preservation Symposium, Paper Preservation (1988) 31–40, Available on http://www.fpl.fs.fed.us/documnts/pdf1988/caulf88b.pdf (May 08, 2008). [39] P. Luner, Evaluation of paper permanence, Wood Science Technology 22 (1988) 81–97. [40] K.L. Kato, R.E. Cameron, A review of the relation between thermallyaccelerated ageing of paper and hornification, Cellulose 6 (1999) 23–40. [41] X. Zou, N. Gurnagul, T. Uesaka, J. Bouchard, Accelerated aging of papers of pure cellulose: mechanism of cellulose degradation and paper embrittlement, Polymer Degradation and Stability 43 (1994) 393–402. [42] H.G. Higgins, J. de Yong, The beating process: Primary effects and their influence on pulp and paper properties, in: F. Bolam (Ed.), in: The formation and structure of paper: Transactions of the symposium held at Oxford, September 1961, vol. 2, Technical Section, British Paper and Board Makers’ Association, London, 1962, pp. 651–697. [43] R. Hägglund, P.A. Gradin, D. Tarakameh, Some aspects on the zero-span tensile test, Experimental Mechanics 44 (2004) 365–374. [44] P. Krkoˇska, J. Hanus, Description of paper ageing by zerospan tensile strength, Cellular Chemical Technology 22 (1988) 633–645. [45] D.H. Page, A theory for the tensile strength of paper, Tappi Journal 52 (1969) 674–681. [46] M. Strliˇc, J. Kolar, Evaluating and enhancing paper stability – needs and recent trends. Protection and treatment of paper, leather and parchment, in: R. Kozłowski (Ed.), Proceedings of 5th EC Conference on Cultural Heritage Research, Cracow, Poland, 16–18 May 2002, 2003, pp. 79–86, Available at http://www.heritage.xtd.pl/pdf/full strlic.pdf (May 08, 2008). [47] J. Kolar, M. Strlic, B. Pihlar, A new colourimetric method for determination of hydroxyl radicals during ageing of cellulose, Analytica Chimica Acta 431 (2001) 313–319. [48] M. Strlic, J. Kolar, B. Pihlar, The effect of metal ion, pH and temperature on the yield of oxidising species in a Fenton-like system determined by aromatic hydroxylation, Acta Chimica Slovenica 46 (1999) 555–566. [49] F. Czechowski, I. Golonka, A. Jezierski, Organic matter transformation in the environment investigated by quantitative electron paramagnetic resonance (EPR) spectroscopy: studies on lignins, Spectrochimica Acta Part A: Molecular Biomolecular Spectroscopy 60 (2004) 1387–1394. [50] S. Zanini, C. Riccardi, C. Canevali, M. Orlandi, L. Zoia, E.L. Tolppa, Modifications of lignocellulosic fibers by Ar plasma treatments in comparison with biological treatments, Surface Coating Technology 200 (2005) 556–560. ˇ [51] M. Humar, A. Straˇze, M. Sentjurc, F. Pohleven, Influence of wood moisture content on the intensity of free radicals EPR signal, Holz als Roh-und Werkstoff 64 (2006) 515–516. [52] U.P. Agarwal, J.D. McSweeny, Photoyellowing of thermomechanical pulps: Looking beyond ␣-carbonyl and ethylenic groups as the initiating structures, Journal of Wood Chemical Technology 17 (1997) 1–26. [53] X. Zou, N. Gurnagul, The role of lignin in the mechanical permanence of paper. Part II. Effect of acid groups, Journal of Wood Chemical Technology 15 (1995) 247–262. [54] J.A. Schmidt, C.S. Rye, N. Gurnagul, Lignin inhibits autoxidative degradation of cellulose, Polymer Degradation and Stability 49 (1995) 291–297. [55] C. Pouteau, P. Dole, B. Cathala, L. Averous, N. Boquillon, Antioxidant properties of lignin in polypropylene, Polymer Degradation and Stability 81 (2003) 9–18.
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ˇ [58] B. Havlínová, A. Maková, S. Katuˇscˇ ák, M. Ceppan, M. Mikula, M. ˇ Reháková, K. Cepcová, Method of treating of archival paper carrier. Patent SK 286412 B6, Faculty of Chemical and Food Technology, Bratislava, Slovak Republic, 2008, ISBN PP 109-2005.
Original article
A study of mechanical properties of papers exposed to various methods of accelerated ageing. Part I. The effect of heat and humidity on original wood-pulp papers Bohuslava Havlínová a , Svetozár Katuˇscˇ ák a , Miroslava Petroviˇcová a , Alena Maková b , Vlasta Brezová c,∗ a Institute of Polymer Materials, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 81237 Bratislava, Slovak Republic b Slovak National Library, Námestie J. C. Hronského 1, 03601 Martin, Slovak Republic c Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 81237 Bratislava, Slovak Republic
Received 9 January 2008; accepted 17 July 2008
Abstract The damage to historical documents and books caused by the acidic character of paper is often manifested as a complete loss of their mechanical properties. Deacidification and restoration of archived paper objects require knowledge of the long-term behaviour of paper before and after repair actions. Our study was focused on the investigation of mechanical properties (tensile strength, stretch, tensile index, zero-span tensile strength, folding endurance) of original papers (one alkaline and three different acidic samples) exposed to five methods of dry-heat and moist-heat accelerated ageing. The degree of paper deterioration upon ageing was significantly influenced by the temperature and relative humidity, along with the intrinsic chemistry of the individual paper samples. The correlation matrix evaluated at a 95% confidence level for tensile strength, stretch, tensile index and zero-span showed linear correlations between these mechanical properties for all the paper samples. However, a linear dependence of folding endurance on zero-span tensile strength was found only for alkaline paper, which revealed the highest resistance to the accelerated ageing tests. In addition, the concentration of paramagnetic semiquinone species in the acidic lignin-containing paper samples was monitored by Electron paramagnetic resonance spectroscopy. © 2009 Elsevier Masson SAS. All rights reserved. Keywords: Paper; Mechanical properties; Accelerated ageing tests; Paper degradation; EPR spectroscopy
Abbreviations
1. Introduction
Bpp DPPH EPR p r RH ROS
Deterioration in the physicomechanical, chemical and optical properties of historic paper documents and artefacts stored in libraries and archives is responsible for an enormous loss of cultural heritage [1–5]. The consequences of acidic paper production after 1850, and the utilization of additives such as aluminium sulphate have been ignored for many years, but nowadays a great deal of attention is focused on the processes of mass deacidification in order to preserve huge volumes of historic objects [6–12]. The quality and long-term stability of paper materials produced from cellulosic fibers with various additives are determined by the extent of oxidative and hydrolytic reactions taking place upon ageing. These processes may considerably reduce the physicomechanical and chemical properties
∗
peak-to-peak line width 1,1-diphenyl-2-picrylhydrazyl Electron paramagnetic resonance p-value (probability) correlation coefficient Relative humidity Reactive oxygen species
Corresponding author. Tel.: +421 2 59325666; fax: +421 2 52926032. E-mail address: [email protected] (V. Brezová).
1296-2074/$ – see front matter © 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2008.07.009
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of paper, resulting in damage to paper structures, an increase of brittleness, and finally, a total loss of material quality. The hydrolytic and oxidative processes are related to each other, and are affected by various intrinsic factors (sizing, filling, adhesives, the presence of acid groups, metal ions, lignin, degradation products, etc.) and extrinsic factors (storage conditions, temperature, RH, the presence of oxygen, light and environmental impurities) [2,13–17]. The process of natural paper ageing is too slow to permit observing changes in a reasonable time frame. Thus, methods of accelerated ageing under dry-heat or moist-heat treatment at higher temperatures have often been used to speed up the chemical processes in paper. However, the relation between accelerated and natural ageing represents a serious problem, as changes monitored upon accelerated ageing procedures must be correctly extrapolated to the ambient conditions [18–22]. Our study is focused on the investigations of mechanical properties (tensile strength, stretch, folding endurance, tensile index and zero-span tensile strength) of four different paper samples exposed to five different dry-heat and moist-heat methods of accelerated ageing. The results obtained serve as a basis to predict the behaviour of the investigated paper samples during long-term storage in archives before deacidification and stabilization treatments.
2. Experimental 2.1. Materials The characteristic properties of four different paper samples investigated are summarized in Table 1. Paper A represents a modern alkaline paper with sufficient alkaline reserve, suitable for long-term storage according to ISO 9706 [23]. On the other hand, the acidic papers B–D were chosen in order to model the deterioration processes of documents and records which were printed on various acidic papers during the 19th and 20th centuries.
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Sodium hydroxide and hydrochloric acid were purchased from Lachema (Czech Republic); deionized water was used for the preparation of solutions. 2.2. Measurements of mechanical properties of paper All paper samples were conditioned according to ISO 187 [24] before measurements at a temperature of 23 ◦ C and a RH of 50% over 24 hours. The experiments, which were focused on the mechanical properties of paper, were performed in compliance with ISO standards [25–29], using a device for thickness measurement (Lorentzen & Wettre, Sweden); a universal instrument for mechanical tests (INSTRON 1011, England); an apparatus for measuring the folding endurance according to Shopper (DFP, VEB Werkstoffprüfmaschinen, Germany). The mean values of tensile strength, stretch, tensile index and zero-span tensile strength were calculated from 10 measurements with a precision in the ± 10% range, while the mean values of folding endurance were evaluated from 20 measurements within the ± 15% range. Relative losses (%) of the monitored properties were calculated for the individual ageing periods. The changes in the mechanical properties of paper samples A–D (tensile strength, stretch, tensile index, zero-span tensile strength, folding endurance) were measured at individual ageing times (0, 0.33, 1, 3, 7, 14 and 28 days), and the experimental data were analyzed by a suitable model of chemical kinetics, e.g., first-order kinetics [30]. The pH values of aqueous extracts prepared with cold water and determined as specified in ISO 6588 [31] were measured at 25 ◦ C by pH-meter OP-208/1 (Radelkis, Hungary) using a combined glass electrode. The alkali reserve of paper A was measured and evaluated according to ISO 10716 [32]. 2.3. Methods of accelerated ageing In order to model the long-term degradation processes in an appropriate time scale, the paper samples A–D were exposed to accelerated-ageing procedures. The dry-heat ageing at 120 ◦ C and 105 ◦ C, respectively, was performed according
Table 1 Characteristics of investigated paper samples. Paper
Grammage (g m−2 )
Characteristics
Producer
pH of cold aqueous extract
Alkali reserve ( mol kg−1 )
A
80
Chemical pulp paper, lignin-free, alkaline, offset paper
9.6
4.3
B
50
5.9
–
C
80
–
60
Slavoˇsovce Paper-mills, Slavoˇsovce, Slovak Republic South-Czech Paper-mills, Vˇetˇrní, Czech Republic
4.4
D
Voluminous, softwood pulp, mechanical pulp content 65%, unbleached chemical sulphite pulp 35%, slightly polished, unsized print paper Mechanical and chemical hardwood pulp paper, acid, writing Softwood pulp, mechanical pulp 54%, chemical unbleached sulphite pulp 18%, chemical bleached sulphite/sulphate pulp 13%, kaolin 15%, sized print paper, machine-glazed
North-Slovakian Pulp and Mill in Ruˇzomberok, Ruˇzomberok, Slovak Republic South-Czech Paper-mills, Vˇetˇrní, Czech Republic
6.2
–
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to ISO 5630-1 [33,34] in a laboratory drier WSU 100 (VEB MLW Laboratortechnik, Illmenau, Germany) for 0.33, 1, 3, 7, 14 and 28 days. The effect of RH on paper was monitored during moist-heat procedures at 80 ◦ C/65% RH (climate chamber Feutron, GmbH Greis, Germany in accordance with ISO 5630-3 [35]), and 80 ◦ C/45% or 80 ◦ C/25% RH (climate chamber Challenge 160, 9286 Angelantoni Industrie, Italy; analogously to ISO 5630-2 [36]) for 0.33, 1, 3, 7, 14 and 28 days.
2.4. EPR measurements Lignin-containing paper samples (∼100 mg; sample column height 70 ± 2 mm) were placed in thin-wall quartz EPR tubes (internal diameter: 3 mm; length: 150 mm; wall thickness: about 0.1 mm). The EPR tube was then inserted into a standard TE102 (ER 4102 ST) rectangular cavity of an EMX X-band EPR spectrometer (Bruker, Germany) and the EPR spectrum was recorded at 25 ◦ C under air. Temperature control was achieved using a Bruker temperature control unit ER 4111 VT. EPR spectrometer settings were as follows: microwave frequency, 9.42 GHz; microwave power, 0.63 mW; center field, 334.5 mT; sweep width, 20 mT; gain, 5 × 105 ; modulation amplitude, 0.2 mT; modulation frequency, 100 kHz; sweep time, 42 s; time constant, 40.96 ms, number of scans, 20. The g-values were determined with an uncertainty of ± 0.0005 by simultaneous measurement with a reference sample containing the stable free radical DPPH (Sigma Chemicals) filled in a precise capillary. The experimental EPR spectra processing and simulation was carried out using the WIN EPR and SimFonia programs (Bruker, Germany). The EPR instrument settings for quantitative evaluation were examined by DPPH standards calibration. The integral intensities of the EPR signals were obtained by double integration of the experimental spectrum, and the spin concentration was recalculated per 1 g of paper sample.
3. Results and discussion 3.1. The effect of accelerated ageing conditions on mechanical properties of paper samples Paper represents a very complex system consisting of cellulose and a variety of non-cellulosic materials. The mechanical properties of different paper samples are substantially influenced by the individual characteristics of cellulose fibers, by the nature, concentration and chemical properties of fillers and additives, as well as by the network structure of the paper [37–39]. The different properties and characters of the investigated papers A–D (Table 1) induced variations in the extent of paper damage upon accelerated ageing. In order to accelerate and to model the degradation processes of paper materials, the dry-heat treatment at 120 or 105 ◦ C serves as a relatively simple experimental technique. In accordance with data in the literature, three days of accelerated ageing at 105 ◦ C correspond to 25 years of natural ageing [20,21,40], although the mechanism of paper dry-heat degradation is not fully compatible with the natural ageing processes, which are influenced by the presence of environmental humidity [18]. Consequently, the moist-heat experiments at 80 ◦ C and 65%, as well as 45 and 25% RH were also performed with the aim of monitoring the effect of RH on the deterioration of papers. In order to display the experimental data obtained in a comprehensible form, we evaluated the percent loss of mechanical characteristics caused by the ageing procedure on samples A–D after 3 and 28 days, respectively (Figs. 1–4a and b). Values of tensile strength reflect the detailed structure of the paper and the properties of its individual fibers, i.e., the dimension and strength of fibers, their arrangements, and interfiber bonding [38,40]. The effects of accelerated ageing processes on paper are interpreted in terms of cellulose chain scission producing weaker fibers, and covalent crosslinking by the additional hydrogen bonds leading to increased brittleness [38,40,41].
Fig. 1. Percent loss of tensile strength monitored for paper samples A–D exposed to various accelerated ageing conditions for: a: 3 days; b: 28 days.
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Fig. 2. Percent loss of stretch monitored for paper samples A–D exposed to various accelerated ageing conditions for: a: 3 days; b: 28 days.
Fig. 1 shows the percent loss in tensile strength after 3 days (Fig. 1a) or 28 days (Fig. 1b) of accelerated ageing under different experimental conditions. These results confirmed the strong impact of the paper’s properties on the loss of tensile strength upon accelerated ageing. The application of dry heat at 120 ◦ C caused a noticeable decrease of tensile strength of acidic paper C (34% after 3 days and 48% after 28 days of ageing, respectively). The influence of RH on the loss of tensile strength of the paper samples is evident generally for longer ageing periods, as moist heat at 80 ◦ C/65% RH yielded similar changes as the dry heat did at 105 ◦ C (Fig. 1b). It should be noted here that alkaline paper sample A demonstrated good stability, since after 28 days the monitored decrease in tensile strength was lower than 20% for all ageing conditions (Fig. 1b). Stretch can be related to the paper’s ability to conform and maintain conformance to a particular contour [37], and is also regarded as one of the most important criteria for the satisfactory behaviour of paper in applications [42]. Fig. 2 shows the percent loss of stretch for samples A–D initiated by different ageing conditions after 3 days and 28 days, respectively. Substantial decrease of stretch was observed after dry-heat ageing for acidic paper C, e.g., 50% after 3 days, and 80% after 28 days at 120 ◦ C (Fig. 2a and b). Only a minor effect of RH on stretch was observed for samples A–D aged during 3 days at 80 ◦ C (Fig. 2a). However, 28 days of ageing at 80 ◦ C/65% RH caused a loss in stretch of 12% for alkaline paper A, and about 30% for acidic papers B–D. A decrease of RH to 25% resulted in an analogous loss of stretch for all investigated samples, not exceeding 15% (Fig. 2b). The measurement of zero-span tensile strength of the paper materials serves as an indicator of the individual cellulose fiber’s strength [38,40,43], consequently, the zero-span changes initiated by accelerated ageing methods has been used to demonstrate the damage to cellulose chains caused by the oxidation and hydrolytic processes [43]. Previously, it has been confirmed that
zero-span represents a more sensitive tool to monitor the effects of ageing on paper than tensile strength, and upon dry-heat ageing, the changes in zero-span tensile strength reflected mainly the chemistry of the samples [44]. Fig. 3 depicts the percent loss in zero-span tensile strength evaluated for paper samples A–D upon ageing under different experimental conditions over 3 and 28 days, respectively. The results confirmed the loss of zero-span tensile strength for acidic paper C, proportionally to the temperature and RH applied, reaching 48% and 100% after 3 or 28 days of dry-heat treatment at 120 ◦ C (Fig. 3a and b). Alkaline paper A showed the best resistance to the dry heat or moist heat but the monitored effect of raised temperature and humidity exhibited more complex behaviour compared to acidic paper C (Fig. 3a and b). The values of tensile strength, stretch, tensile index and zero-span tensile strength monitored upon accelerated ageing procedures are related to the extent of oxidative and hydrolytic damage to cellulosic fibers [20,21,38,40,41,45]. In order to find the relationships between these mechanical properties, we performed a linear correlation analysis at a 95% confidence level using all experimental data measured in accelerated ageing tests for the individual paper samples A–D. Table 2 summarizes the calculated correlation coefficients, r, between the couples of mechanical properties for paper samples A–D, evidencing their significant linear dependencies (r ∈ 0.7615–0.9688; p < 0.0001), reflecting thus the predominant role of cellulose fibers in the damage to paper materials caused by heat and humidity. Folding endurance represents the most sensitive indicator of paper breakage upon ageing, as the difference in folding endurance is observable long before the changes in tensile strength, stretch or zero-span tensile strength are measurable. On the other hand, the measurement of folding endurance is coupled with lower precision, as well as the necessity of precise conditioning during experiments [9,38].
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Fig. 3. Percent loss of zero-span tensile strength monitored for paper samples A–D exposed to various accelerated ageing conditions for: a: 3 days; b: 28 days.
Fig. 4a and b show the percent loss in folding endurance of paper samples A–D monitored under different experimental conditions after 3 and 28 days, respectively. The results confirmed a high sensitivity of folding endurance to the effects of dry-heat and moist-heat procedures. Acidic paper C showed a complete loss of folding endurance after only 3 days of ageing at 120 ◦ C. For this unstable paper sample, the loss of folding endurance after 3 days is proportional to the temperature and RH (Fig. 4a). The highest resistance to ageing was found for alkaline paper A, as 3 days of ageing at 105 ◦ C, corresponding to 25 years of natural ageing [19,40], caused only a 20% loss in folding endurance, and the decrease of folding endurance after 28 days at 105 ◦ C reached 50%. However, all the investigated papers A–D exhibited 100% loss in folding endurance after 28 days of ageing at
120 ◦ C. After 28 days of ageing the samples of acidic papers B–D showed a significant decrease in folding endurance from 80% at 80 ◦ C/25% RH, up to 100% at 105 ◦ C (Fig. 4b). The decrease in the number of double folds monitored upon different accelerated ageings of paper samples was described by exponential functions corresponding to the formal firstorder kinetic model [30]. The experimental time dependencies as shown in Fig. 5 for sample C were fitted by a non-linear least-squares method (Scientist, MicroMath), and the formal half-lives of paper samples exposed to the different accelerated ageing conditions was calculated. The evaluated values of formal half-lives for folding endurance summarized in Fig. 6 unambiguously demonstrated the highest resistance of alkaline paper A to accelerated ageing, as its formal half-life values are
Fig. 4. Percent loss of folding endurance monitored for paper samples A–D exposed to various accelerated ageing procedures for: a: 3 days; b: 28 days.
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Table 2 Correlation matrix between mechanical properties of paper samples A–D. (The values represent the linear correlation coefficients calculated at a 95% confidence level; p < 0.0001). Tensile strength
Stretch
Tensile index
Zero-span tensile strength
Tensile strength
1
0.8746 (A) 0.9688 (B) 0.9204 (C) 0.9395 (D)
0.9287 (A) 0.9187 (B) 0.9583 (C) 0.9490 (D)
0.8769 (A) 0.9461 (B) 0.9299 (C) 0.9309 (D)
Stretch
0.8746 (A) 0.9688 (B) 0.9204 (C) 0.9395 (D)
1
0.8333 (A) 0.8796 (B) 0.8776 (C) 0.9169 (D)
0.9424 (A) 0.8837 (B) 0.9603 (C) 0.9133 (D)
Tensile index
0.9287 (A) 0.9187 (B) 0.9583 (C) 0.9490 (D)
0.8333 (A) 0.8796 (B) 0.8776 (C) 0.9169 (D)
1
0.7615 (A) 0.9458 (B) 0.9049 (C) 0.9372 (D)
Zero-span tensile strength
0.8769 (A) 0.9461 (B) 0.9299 (C) 0.9309 (D)
0.9424 (A) 0.8837 (B) 0.9603 (C) 0.9133 (D)
0.7615 (A) 0.9458 (B) 0.9049 (C) 0.9372 (D)
1
the highest for all ageing procedures; for acidic papers B–D, the best results were obtained for paper B. Despite comparable pH values of papers B and D (5.9 and 6.2, respectively, Table 1), the formal folding endurance half-lives demonstrated significant differences (Fig. 6). Most likely the hydrolytic and oxidation processes induced by accelerated ageing were also affected by various additives present in paper matrices. The monitored effect of accelerated ageing on tensile strength, stretch and tensile index for all papers and types of ageing was not as significant as the effect of accelerated ageing on folding endurance and zero-span tensile strength. Previously, zero-span and folding endurance were suggested as the suitable
Fig. 5. Decrease of folding endurance on accelerated ageing period measured for paper C during various accelerated ageing conditions: : 120 ◦ C; : 105 ◦ C; : 80 ◦ C/65% RH; : 80 ◦ C/45% RH; : 80 ◦ C/25% RH. The marked symbols represent the experimental data, and dashed lines, their mathematical simulations using least squares analysis.
indicators of paper deterioration caused by natural or accelerated ageing [38,40,41]. Despite the lowered precision, folding endurance represents the most sensitive test of damage to the paper [8,38]. Fig. 7 shows the dependence of folding endurance on zerospan tensile strength evaluated for the investigated paper samples A–D exposed to the various accelerated ageing conditions. The results obtained clearly demonstrated that the number of double folds is proportional to the zero-span values higher than 3.5 kN m−1 , and the values of zero-span below 3.5 kN−1 were in our study associated with complete loss of folding endurance
Fig. 6. The values of folding endurance formal half-life evaluated using a firstorder kinetic model for paper samples A–D exposed to various accelerated ageing procedures.
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Fig. 7. Dependence of folding endurance on zero-span tensile strength evaluated from data sets obtained during accelerated ageing procedures: a: paper sample A; the symbols represent the experimental data, and the solid line, the linear dependence, with the 95% confidence intervals (dotted lines); b: paper sample B; c: paper sample C; d: paper sample D.
for the investigated paper samples (Fig. 7). It should be noted here, that for alkaline paper A the experimental data of folding endurance and zero-span tensile strength were fitted by a linear dependence at a 95% confidence level reaching the correlation coefficient r = 0.8292 and p < 0.0001 (Fig 7a). 3.2. EPR spectra of paper samples exposed to accelerated ageing Oxidative degradation of cellulosic materials upon natural or accelerated ageing is coupled with the formation of radical species and hydroperoxides [46,13,47]. Additionally, the presence of transition metal ions in paper can play a significant role in the decomposition of hydroperoxides, producing ROS [48]. EPR spectroscopy represents a unique and sensitive tool for the study of structures containing free radical species and paramagnetic
transition metal ions in the cellulosic materials [49–51]. The presence of lignin in wood-pulp papers substantially influences their deterioration processes. The light-sensitivity of lignin also plays a specific role, causing the low photostability and material discolouration [52]. It has been previously shown that the chemical reactivity of lignin resulted in the loss of mechanical properties of lignin-containing papers. The degradation process was mainly affected by pH value, since the damage to papers with sufficient alkaline reserve was not influenced by the presence of lignin [53]. On the other hand, due to the structures of lignin compounds, corresponding to hindered phenols, lignins can act as effective antioxidants capable of scavenging reactive radical intermediates [54–57]. The X-band EPR spectrum monitored in lignin-containing paper samples was characterized by an asymmetric singlet line with geff = 2.0054 and Bpp = 0.83 mT (inset in Fig. 8), which
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stretch, tensile index and zero-span tensile strength were evaluated at a 95% confidence level, with correlation coefficients from 0.7615 up to 0.9688, reflecting thus the dominant role of cellulose fiber characteristics in the mechanical behaviour of paper. The changes in concentration of semiquinone paramagnetic species in acidic lignin-containing papers upon ageing tests were demonstrated by EPR spectroscopy. The results confirmed the significant role of alkaline reserve in paper for slowing the degradation processes induced by different accelerated methods. The inhibition of hydrolytic and oxidation processes may be attained by the addition of compounds enhancing paper durability, as well as inhibiting oxidation processes. The influence of accelerated ageing tests on the mechanical properties of deacidified and/or stabilized [58] paper samples A–D will be shown in the second part of our investigations. Acknowledgements Fig. 8. Dependence of spins in 1 g of paper sample C on the accelerated ageing period: : 120 ◦ C; : 80 ◦ C/45% RH. The inset represents the experimental EPR spectrum of paper C (magnetic field sweep width 20 mT).
was assigned to stable semiquinone radical species produced by one-electron oxidation of lignin phenolic groups [49–51]. For illustration, the concentration of paramagnetic species in the original paper sample C measured under air was 7 × 1015 spins g−1 , and it increased upon accelerated ageing as is shown in Fig. 8. The concentration of paramagnetic species was influenced by the accelerated ageing method applied; the highest values were measured for samples exposed to dry heat at 120 ◦ C. The decline of spin concentration monitored after 28 days of accelerated ageing was assigned to the transformations of paramagnetic semiquinone species to quinone and quinone-like structures responsible also for colour changes in the paper [52]. A relationship between increased concentration of paramagnetic species measured in lignin-containing papers, and changes of mechanical properties, was not confirmed. 4. Conclusions The mechanical properties (tensile strength, stretch, tensile index, zero-span tensile strength, folding endurance) of original paper samples were monitored during dry-heat and moist-heat accelerated ageing tests under various experimental conditions (120 ◦ C, 105 ◦ C, 80 ◦ C/65% RH, 80 ◦ C/45% RH and 80 ◦ C/25% RH). Our results showed that folding endurance is a highly sensitive means for monitoring changes in paper structure. Our experiments confirmed that alkaline paper A exhibits resistance to ageing in accordance with ISO 9706 [23], as shown by high values of double folds number, with a significant decrease only upon prolonged dry-heat ageing at 120 ◦ C. The increased temperature and RH caused a significant loss of folding endurance, especially for acidic papers B–D. However, the application of accelerated ageing test performed at mild conditions (80 ◦ C/45% RH and 80 ◦ C/25% RH) make it possible to compare the durability of different paper samples A–D even after 28-day period. The linear correlations between tensile strength,
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