Chemistry for Everyone
The Chemistry of Paper Preservation Part 5. Permanent Paper Henry A. Carter Department of Chemistry, Augustana Campus, University of Alberta, Camrose, AB, Canada, T4V 2R3;
[email protected] The deterioration of paper over time has been a serious problem for libraries throughout the world, and there have been major concerns1 about the decay of large collections of books, publications, old maps, historic artifacts, and written records. It is now well established that the dominant chemical reaction that occurs during the aging of paper is the acid-catalyzed hydrolysis of cellulose in paper fibers (1). Hydrolysis results in the lowering of the degree of polymerization (DP)2 of the cellulose chain and, consequently, a loss in paper strength. However, many conservators have attributed the loss of strength in paper to the presence of lignin, which can undergo photochemical oxidation leading to the yellowing of paper. While the focus of research towards saving our written heritage has been in combatting further deterioration (through mass deacidification), recent research has involved preventing deterioration through the use of alkaline and permanent paper (1). During the last 20 years, the topic of permanent paper has been a highly debated issue in paper conservation research. This issue has not only arisen from the concerns of the cultural community (librarians, archivists, and conservators), the topic is also significant to the pulp and paper industries engaged in developing new high-yield pulps (1). In contrast to chemical pulps, the new mechanical pulps contain large amounts of lignin, yet appear to be stable when a calcium carbonate buffer is present. This contradicts past assumptions that mechanical pulp papers are more susceptible to deterioration than chemical pulp papers. Previous research in paper preservation, discussed in the first three parts of this miniseries (1–3), has examined the chemistry of preserving and stabilizing old papers through the techniques of deacidification (1), bleaching (2) and strengthening (3). The fourth part discussed the increasing use of alkaline paper in papermaking (4). However, insofar as modern papermaking is concerned, it is now important to look forward in time and try to solve the problems of the past. By taking advantage of the information available on the causes and mechanisms of paper deterioration (5–8), it should be possible to produce a paper of high quality and stability that can last for a long time. Such paper is often referred to as permanent paper, although this term must be considered relative. Through the use of permanent paper, our new records, journals, library books, art works, and all culturally and historically important documents can be preserved. Defining Paper Permanence and Durability What constitutes permanent paper and how do we evaluate it? In 1994, the Institute for Standards Research (ISR), a branch of the American Society for Testing and Materials (ASTM), conducted a workshop in Philadelphia, PA to discuss the aging of paper and paper permanence research (9). Over 100 people from 12 different countries, representing pulp and paper industries, government organizations, universities, conservation research centers, libraries, and archives, attended this workshop. While
many topics related to the permanence of paper were discussed, a great deal of interest focused on issues related to the establishment of minimum standards for paper of permanent value. Permanence in paper refers to a degree of chemical stability that allows only a very slow deterioration. Durability refers to a degree of physical strength and flexibility of paper required for extensive handling (10–14). Both permanence and durability are important qualities that must be considered in determining standards for permanent paper. Accelerated Aging of Paper Because it is not practical to wait until a paper naturally ages in order to examine its stability over time, accelerated aging methods3 are used as an approximation to determine in a short time the extent of deterioration of paper as it naturally ages over a long time (15). Typically, the sample of paper is placed in an oven under conditions of controlled relative humidity and elevated temperature. At time intervals, measured in days, some of the paper is tested while the remaining paper continues to artificially age. The chemical and physical tests used to measure the extent of paper degradation have been described elsewhere (16). Following artificial aging over a number of time intervals, testing concludes and the results are analyzed. The effect of temperature on the rate of deterioration of paper has been previously discussed in this Journal (17, 18). Numerous different conditions for heat and moisture have been employed to test paper by accelerated aging. The ISO (International Organization for Standardization) standard of 80 °C and 65% relative humidity is often used, as the moisture content of paper under these conditions is similar to that at room temperature (23 °C) and a relative humidity of 50% (8). The Lignin Debate: What Causes Loss of Paper Strength? A major component of wood is lignin, the structure of which has previously been described in this Journal (19–21). Lignin is a very complex aromatic polymer based on substituted phenylpropane units linked primarily through ether bonds and to a lesser extent by C–C bonds. It contains many chromophores with aromatic conjugated bond systems and carbonyl groups. Thus, lignin tends to interact with light in the presence of oxygen resulting in the photochemical yellowing of paper, particularly that made from mechanical pulps (22). It has been observed that the yellowing of paper often accompanies loss in strength during the aging of paper. Since lignin-containing papers tend to yellow on exposure to light, it was concluded that the presence of lignin contributed to strength loss in paper. However, the mechanisms for yellowing and strength loss are different and the chemical reactions that produce yellowing may not cause loss of strength in paper. In
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fact, there is evidence to suggest that lignin may protect cellulose from photodegradation by absorbing UV radiation and also by acting as an antioxidant by quenching free radicals formed in cellulose (23). In the past, many members of the cultural community have been sceptical about the benefits of lignin and have remained convinced that lignin plays a major role in the degradation of paper. On the other hand, the industrial community has advocated that lignin is not the culprit in the degradation of paper, but that acidity is. There have been heated debates as to whether lignin should be excluded from the composition of permanent paper (24–30). International Permanent Paper Standards A number of permanent paper standards have existed in the past (31–36). These include, among others, ASTM, ISO, ANSI∙NISO (American National Standards Institute∙National Information Standards Organization), and DIN (Deutsches Institut für Normung). In 1993, a new international standard called ISO 9706 was approved by ISO national member bodies (37–38). It was based on the 1992 revised American national standard, ANSI Z39.48-1984. Until recently, these two standards and others were based on composition rather than performance, and, generally, included the following specifications:
not fiber composition, was found to be the main cause for the degradation of paper during accelerated aging at 80 °C and 65% relative humidity. In addition, mechanical stability after aging was the same for alkaline lignin-containing paper as it was for alkaline lignin-free paper. In 1998, a draft of the Canadian Permanent Paper Standard allowing for lignin content in permanent paper was put forward to the CGSB. However, it was not approved by the CGSB because lignin-containing paper tends to yellow more and lose more brightness over a period of time compared to lignin-free paper. The advantage of using high-yield pulps in paper manufacturing is that it is less expensive, yet yields a product that is both mechanically and chemically stable. Standards that are based on composition alone, not performance, restrict permanent paper to a narrow range of expensive papers. The new Canadian standard for permanent paper was finally adopted in September 2000 (41). It offers two categories of permanence based on the research that alkaline lignincontaining papers were as stable as alkaline lignin-free papers, but not as stable optically. The first category limits the amount of lignin to less than 1% (same as existing standards) to provide maximum optical stability. The second category has no limitation on the type of fiber that can be used and, therefore, does not have limitations on the lignin content. This gives two options for buyers and users of permanent papers and more choice in the range of papers available.
• A minimum cold extract pH of 7.5.
• An alkaline reserve equivalent to 2% calcium carbonate.
ASTM’s Paper Aging Research Program
• A maximum lignin content of 1%.
As a result of the ASTM–ISR meeting of 1994, a $3.8 million research program on The Effects of Aging on Printing and Writing Paper was established to investigate test methods that could determine the life expectancy of papers (31–32). This would lead to a new standard for permanent paper based on performance4 under conditions of accelerated aging. ASTM began its work on testing procedures for permanent paper, and was particularly interested in the use of accelerated aging as an indicator of long-range stability (42). Accelerated aging test methods have been used in the past to predict the stability of papers. But how do the results of accelerated aging compare to the effects of long-term natural aging? Five research labs were chosen by ASTM to undertake a five-year research program on the aging of printing and writing papers (42) involving accelerated aging research:
Other criteria, such as tear resistance, paper stock, and resistance to oxidation were often included, depending on the individual standard. The German (DIN) standard, however, was based on performance determined through accelerated aging, and imposed no restriction on lignin content. Canadian Permanent Paper Standard In the fall of 1991, the Canadian General Standards Board (CGSB) was asked to develop a Canadian standard for permanent paper (39–40). In 1993, Canada had abstained from the vote to accept the international standard ISO 9706 for permanent paper. While it had already been established that alkaline lignin-containing papers showed good mechanical stability under conditions of accelerated aging, the cultural community felt that experimental results were needed to resolve the effect of air pollutants on lignin-containing papers. Thus, one of the main goals of the Canadian study was to examine the effect of air pollutants on paper. Meantime, the pulp and paper industries were developing high-yield pulps that contained a large amount of lignin. This was done for both economic and environmental reasons. To safeguard the development of a new standard for permanent paper, two research labs were involved: the Canadian Conservation Institute (CCI) representing the cultural community, and the Pulp and Paper Research Institute of Canada (PAPRICAN) representing industry. This joint research project called The Canadian Cooperative Permanent Paper Research Project (CCPPRP) was conducted during the years 1994–1997. Testing was done on handsheets prepared from six different pulp types and on ten different commercial papers. Acidity,
• The Canadian Conservation Institute research lab and the U.S. Library of Congress testing division both tested paper using accelerated aging by heat.
• Finland’s Pulp and Paper Research Institute lab and the U.S. Forest Products Laboratory both tested paper using accelerated aging by light.
• The Image Permanence Institute (United States) tested paper using accelerated aging by pollutant gases.
Fifteen papers were tested, including both acid and alkaline papers and samples that ranged from lignin-containing, stone ground wood to lignin-free, 100% cotton. The effect of light aging was investigated, as was pollutant aging, since atmospheric pollution was known to be a source of acidity in urban libraries (2). This study involved exposing free hung papers5 to SO2, O3, and NOx (where NOx refers to two oxides of nitrogen, NO and NO2).
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Results and Conclusions As there is some degree of overlap between the results obtained by the five labs (42), the highlights below of some of the conclusions made are based on the combined experimentation of the five research groups (42–45).
1. Using HPLC (high performance liquid chromatography), IC (ion chromatography), and GPC (gel-permeation chromatography), degradation products6 that occurred during natural aging from papers over a century old were found to be essentially the same as the degradation products from new papers that had been aged by accelerated aging methods.
2. In addition, when old papers were subjected to accelerated aging, the same chemical compounds that formed during natural aging were produced. No new chemical compounds were formed.
3. Paper stability determined after heat aging was the same for alkaline lignin-containing paper as it was for alkaline lignin-free paper.7
4. Acid papers lost a great deal of mechanical strength, while the loss of strength in alkaline papers with a calcium carbonate reserve was only marginal.
5. Providing that the paper was buffered with at least 2% calcium carbonate, the fibre composition of paper was not significant regarding its performance.
6. During light aging, the acid papers experienced more loss in optical properties than the alkaline papers.7 This effect was more pronounced with the lignin-containing papers, which endured rapid loss of brightness and increase in yellowing.
7. During pollution aging, absorption of SO2 and NO2 resulted in acidification of nonbuffered sheets whether lignin was present or not.7
8. However, if lignin were present, more brightness was lost through absorption of air pollutants than for lignin-free papers.
9. As a buffering agent calcium carbonate reduced the effect of air pollutants in both lignin-containing and lignin-free paper.
Summary The deterioration of paper is caused by the acid-catalyzed hydrolysis of cellulose. If the paper contains a reserve of calcium carbonate, the presence of lignin does not result in loss of strength of the paper. However, lignin does produce photoinduced discoloration that can be deemed acceptable if it has no significant negative effect on legibility and readability. Presently, no standard exists that specifies for optical performance. More research needs to be done to define optical properties and their requirements for permanent paper standards. Discussion—Applications to Teaching The miniseries on the chemistry of paper preservation, including this recent part on permanent paper, provides a number of useful examples that can be used in teaching chemistry. Fur
thermore, paper preservation intersects a number of disciplines, including acid–base chemistry, kinetics, air pollutants, photochemistry, bleaches, and polymers. This miniseries has been used as a resource in this Journal for Classroom Activity 39 (46) and in a resource listing on chemistry and art (47). The popular textbook on first-year chemistry by Zumdahl and Zumdahl (48) describes “self-destructing paper” in one of its Chemical Impact features found in the chapter on acids and bases. Many applications of acid–base chemistry are found in the miniseries. The use of buffer components in making permanent paper are described and go beyond the liquid buffer solutions to include solids such as oxides (e.g., zinc oxide) and carbonates (e.g., CaCO3). Without good examples, hydrolysis can be a dry topic to students. One suitable example is the hydrolysis of the Al(H2O)63+ cation shown in eq 1:
Al(H2O)63+ + H2O → Al(H2O)5OH2+ + H3O+ (1)
This equation can be totally meaningless to many students in first-year chemistry classes. However, explaining how this hydrolysis plays a large role in the degradation of old paper generates student interest. In my classes, I do a demonstration with papermaker’s alum—Al2(SO4)3 · 18H2O—using indicators to show the acidity of Al(H2O)63+. I also go on to explain that many books are now printed on permanent paper and this leads to a discussion on standards. A student pointed out to me that some of my papers published in this Journal were already starting to turn yellow. A number of experiments can be developed on the analysis of permanent paper. For example, instead of titrating juices, vinegar, and vitamin C—labs often done to teach acid–base chemistry—one could determine the buffer capacity of calcium carbonate in paper. One also could have the students do a study on the acidity of paper using pH meters equipped with surface electrodes. Notes 1. The public became aware of the seriousness of losing our printed heritage through a video made in the U.S., entitled Slow Fires, first released in 1987. See ref 49. 2. The number of β-linked anhydroglucose units found in one cellulose chain is known as the degree of polymerization. 3. Accelerated aging gives an approximation to how paper degrades naturally over a long period of time. 4. Performance refers to resistance to degradation of paper as indicated by tests following accelerated aging. 5. For a discussion on the use of single sheets versus the use of stacks of sheets during accelerated aging, see ref 50. ASTM recommends the use of hermetically (airtight) sealed glass tubes for accelerated aging in a dry oven: see ref 42. 6. For a discussion on the decomposition products of paper and the odor of old books, see ref 51. 7. The results of the ASTM research led to the development of three new ASTM Test Methods: heat aging (ASTM D6819-02e2), light aging (ASTM D6789-02e1), and the effect of pollutant gases (ASTM D6833-02e1). See ref 52. 8. The 6th edition of Cotton and Wilkinson’s internationally acclaimed textbook, Advanced Inorganic Chemistry, is printed on acid-free paper, and meets the permanent paper standard, ISO 9706, as indicated by an infinity sign in a circle, on the publication data page. See ref 53.
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