Migration of Volatile Compounds from

Paper Stacks during Accelerated Ageing

A thesis submitted to the Department of Art

In conformity with the requirements for the

Degree of Master in Art Conservation

Queen's University

Kingston, Ontario, Canada

September, 1999

Copyright O Anna Elisabeth Bülow National Library Bibliothèque nationale ($1 of Canada du Canada Acquisitions and Acquisitions et Bibliographic Services services bibliographiques 395 Wellington Street 395. rue Wellington Ottawa ON KI A ON4 Ottawa ON K! A ON4 Canada Canada

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In order to develop improved artificial ageing methods for paper, several research projects conducted by different laboratories throughout North America have shown that paper sheets, aged in an enclosed environment or arranged in stacks, deteriorate faster than single sheets aged under the same conditions. This was found to be especially true for acidic paper. The objectives of this project are: to investigate the changes in paper properties from the top of the stack (open to environment) to the centre. to determine if they are effected by possible migration of volatile products from the paper. and to find out whether or not the diffusion of acidic compounds have the same effect on the stability of therrnally aged paper and naturally aged paper. An alurn- rosin sized fully bleached kraft paper was aged in stacks, at three different temperature and humidity conditions and four different intervals. The stacks were open at the top to allow diffusion of volatile products to occur. Test methods included grammage,

moisture content, brightness, yellowness, L*a*b*, zero-span tensile strength, pH and

the degree of polymerization. These tests showed a decrease in paper properties from

the top of the stack to the centre. This indicates a diffusion effect where volatile

compounds escape from the paper at the top of the stack, but remain trapped towards

the centre, where more severe degradation occurs. It is not entirely clear if this

diffusion effect is the same at ambient conditions as it is at the elevated temperatures

used in this study. The research project was carried out in collaboration with the

Canadian Conservation Institute. .. . III

Acknowledgements

I would like to thank:

Paul Bégin, Canadian Conservation lnstitute (CCI), Ottawa, Ontario, Canada, for

his invaluable advice. assistance and guidance during my research project. In

addition. I would like to thank Paul Bégin and Joe lraci from CC1 West for the

"Pizza-Lunches".

Thea Burns, Art Conservation Program. Department of Art at Queen's University,

Kingston, Ontario. Canada, for her supervision. advice and interest throughout the

project.

The Canadian Conservation Institute, for providing laboratory equiprnent and

Elizabeth Kaminska for proofreading and assisting with the occasions I rescuing of

samples during the determination of the degree of polymerization. Also1, l would like

to thank Season Tse for her support.

Dr. David Grattan, Manager of Conservation Processes and Materials Research at

CC1 for his advice and support during this research work.

The School of Graduate Studies and Research, Queen's University, for its financial

support of this thesis. Contents

A bstract II

., . Acknowledgements III

Contents iv

List of Figures

List of Tables vii

1 Introduction

2 Historical Ovetview

2.1 Important Developrnents in Paperrnakinç

2.2 Permanence of Paper

3 Current Research in Accelerated Ageing Procedures

4 Experirnental

4.1 Samples

4.2 Accelerated Ageing

4.3 Test Methods

5 Results

6 Discussion

7 Conclusion References

Appendix 1 (Figures)

Appendix 2 (Tables)

Appendix 3 (Materials and Equiprnent)

Vita List of Figures

Figure 1 Distribution of Test Specimens

Figure 2 Brightness vs. Page

Figure 3 Yellowness vs. Page

Figure 4 b' vs. Page

Figure 5 Stightness ILS. ogeiq Tirne, pages #i and ?#5

Figure 6-1 b* vs. Ageing Time, 70°C 1 61% RH

Figure 6-2 b* vs. Ageing Time, 80°C 1 65% RH

Figure 6-3 b* vs. Ageing Time, 90°C 169% RH

Figure 7-1 Yellowness vs. Ageing Time, 70°C 1 61 O/O RH

Figure 7-2 Yellowness vs. Ageing Time, 80°C 1 65% RH

Figure 7-3 Yellowness vs. Ageing Time, 90°C 169% RH

Figure 8 pH vs. Page

Figure 9-1 pH vs. Ageing Time, 70°C / 61% RH

Figure 9-2 pH vs. Ageing Time, 80°C 1 65% RH

Figure 9-3 pH vs. Ageing Time, 90°C 1 69% RH

Figure 10 Zero-span Breaking Length vs. Page

Figure 11 Degree of Polymerization vs. Ageing Tirne

Figure 12 b* vs. Ageing Time, Pages #Il#3 and #15

Figure 13 pH vs. Ageing Time, incl. single sheet and stack sheet 70

Figure 14 Rate of Degradation (Zero-span) 71

Figure 15 Rate of Degradation (DP) 72

Figure 16 Zero-span vs. Number of Broken Bonds 73 vii

List of Tables

Table 1 Differences in the Amoun! of Ageing

Table 2 Grammage

Table 3 Moisture Content

Table 4 Brightness

Table 5 ve!lowness

Table 6 L'

Table 7 a*

Table 8 b*

Table 9 PH

Table 10 Zero-span Tensile Strength

Table 11 Degree of Polymerization 1 Introduction

Over the last decades there has been an increasing awareness of the problem of paper deterioration on the part of both the public and sc~entistsinvolved either in paper manufacture or in its preservation. This in turn has led to many different research studies concerning this issue.

The following project was carried out in the context of extensive research studies currently being perforrned at the Canadian Conservation Institute, Ottawa. These studies focus on the development of an improved ageing method in order to predict the permanence of paper (Bégin et al. 1998 and 1999). Their results suggest that generally

more attention should be paid not only to the ageing conditions thernselves, but also to

the arrangement of paper in the ageing chamber during artificial ageing.

Based on these experiments, it was found that volatile products, inherent in paper, are

able to escape from free hung sheets, whereas they remain trapped if the paper sheets

are arranged as a stack, when aged at elevated temperatures. Furthermore, it was

shown that paper sheets aged in an open stack age to a different extent, depending on

their placement in the stack (Carter 1996).

The present study, therefore, focuses on the determination of the extent to which

diffusion influences the ageing properties of paper in an open stack. lt also examines

the extent to which this diffusion influences the properties of paper at different ageing

conditions. The results of this study could help elucidate the question of whether or not artificial ageing methods are able to reproduce the same ageing mechanisms as

natural ageing.

In order to better understand issues involved in paper permanence, it is necessary to

investigate the history of paper permanence from an historical point of view. The main

focus, therefore, will not be the deterioration itself but the perception of paper

permanence and the growiiig awareness of its importance now and in the future. The

most important changes in papermaking will be outlined in the first half of this section.

The second half will focus cn the causes of deterioration and their detection during the

last hundred years as well as the development of artificial ageing procedures.

The background section will explain recent developrnents in accelerated ageing

studies, which indicate a difference in the results of ageing between free hung sheets

and sheets aged as a stack. The outline and objectives of the project will be presented.

The choice of sarnples, ageing conditions and test methods used, are illustrated in the

first part of the experimental section. Following this, the test methods will be further

explained. Results and discussion are presented separately in order to keep actual

data and their interpretation apart. Conclusion and recommendations for future

research will be given in the last section. Historical Overview

The history of paper has been described many times. However, it seems necessary to recall the most important changes in papermaking in order to understand the history of paper-related science.

The principal of papermaking as it was invented in China has undergone only little change. Its history has been thoroughly described, e.g. by Hunter (1978). Therefore. this ovewiew only focuses on issues relating to paper stability and permanence.

The following section briefly outlines the historical developments in papermaking since the beginning of paper manufacture in Europe in the Middle Ages. The history of the science concerned with paper deterioration is focused on in part two of this section.

2.1 lmoortant Developments in Paoermakinq

Early Papermaking Processes

Paper is defined as a web consisting mainly of vegetable fibres, traditionally produced by dipping a mould into a fibre suspension. In addition, paper may be sized to prevent the feathering of ink, and contain fillers and colouring matters.

It is commonly recognized that the art of papermaking was invented in China about two

thousand years ago. Paper, as it was manufactured in Europe since the rniddle of the

121h century, consisted of fibres derived from rags, whose principal constituent is cellulose. Pure cellulose is known to be very stable; however, it seldom occurs in the pure state in nature. The processes of rernoving undesirable impurities in papermaking can cause a lack of stability by destabilizing the fibres physically and chemically as well as starting an autocatalytic degradation of the cellulose (Rydholm 1985: 857-860).

The use of rags as the main source of fibres, a common practice until the rniddle of the

19" century, enabled the papermaker to produce paper of great stability Rags consisted mainly of linen (flax) as well as hemp and, later. cotton. These fibres have a rather small content of impurities with respect to pure cellulose, which is one of the main reasons for the stability of papers made from rags. In addition, the quality of water at the time as well as the use of lime as an aid in the fermentation of rags contributed to paper stability (Justi 1762).

Also of great importance are the types of sizing that were used before the 19'~century.

Sizing of paper is necessary in order to make it irnpervious to ink and to enhance its

strength. After the paper was dry. it was dipped into a size, which the papermaker

usually made from the parings of hides'. Rag papers sized with this animal glue were

found to be very stable. There is, however, evidence that in the late lithcentury, alum

was added in order to harden the size. This is one of the prirnary causes of

deterioration of paper made after the mid-17'~century. (Barrow 1967: 16)

' The refuse of tanning works and scraps of parchment were most valued for this purpose, since they could be used without further preparation. However, tanned leather could be used, employing a special process to remove tannic acids, thus allowing the hide to dissolve in water. (Dawidowsky 1884: 42-60) The invention of movable types during the 1 5'h century and the fast increase in literacy during the 18'~century caused a greater demand for paper than ever before. This, in turn, resulted in a scarcity of rags, the main source of fibres. In addition, papermaking needed to be made more economical.

Technological Changes since 1700

As a result of the rapidly increasing need for paper that could not be satisfied with the methods of papermaking employed until then, papermakers had to find new fibre sources and pulping processes to fulfil those needs. During the 18'"entury several people were concerned with finding a new source of fibres2.

The first one who made paper from vegetable sources on a commercial scale was

Matthias Koops. He used straw as the sole fibre source for his book published in

1800~.However, the immediate need for fibre sources other than rag was deferred by

two inventions towards the end of the 18'~century.

In 1774 Karl Wilhelm Scheele discovered chlorine. Chlorine was soon found to be

capable of bleaching dyed and discoloured rags. Thus, coloured rags that had to be

discarded previously were now usable. The other important invention relating to paper

fibres was the cotton gin in 1793; this is a machine that was able to separate cotton

' In 1719 René Antoine Ferchault de Réaumur suggested the use of wood as a papermaking material and Jean Ét~enneGuettard advocated several forms of vegetation in his publication in 1768 (Recherches sur les matières qui peuvent servir a fam du papier. In: Mémoires sur différentes parties des Sciences et Arts. Paris 1768 (Hunter 1978: 316)). Extensive work on this subject was also done by Jacob Christ~an Schaffer He published six volumes behveen 1765 and 1771, in which he used different specimens of paper in order to demonstrate the use of different fibre sources (Versuche und Muster ohne alle Lumpen oder doch mit einem geringen Zusalze derselben Papier zu machen. Regensburg 1765-1771 (Hunter 1978: 317)). Historical Account of the Substances Which have been Used to Descnbe Events and to Convey Ideas. from the Earliest Date to the invention of Paper. Printed by T Burton, London (Hunter 1978: 332). seeds from their seed hairs. This invention doubled cotton production and was increasingly used in the manufacture of fabrics, resulting in a higher cotton content of the rags used for papermaking. Cotton is the purest naturally occurring cellulose fibre. requiring less harsh processing procedures than flax, which had been the dominant fibre up to that time. Cotton has therefore the potential to make very strong paper.

(Barrow 1967: 14115)

The increasing need for paper also led to the development of a papermaking machine.

In France, in 1798. Louis Nicolas Robert invented a paper machine that was able to produce long sheets of paper. which could be used for wallpaper Shortly after that. in

1804. Bryan Donkin, then working in England for Henry and Sealy Fourdrinier, inventod the first continuous paper machine (André 1996: 84/85). The patent for this machine was given to Henry Fourdrinier in 1806. The principle of this "Fourdrinier machine" has

undergone comparatively little change, and it is still in use in today's paper industry.

One of the most important changes in papermaking was Moritz Friedrich Illig's

invention of alum-rosin size in 1807~.Alum-rosin sizing had the principal advantage

that it could be added directly to the beater. This reduced manufacturing costs by

eliminating a separate step of sizing. Alum-rosin size gradually displaced animal glue

as a sizing. It is known to have been introduced to the United States in about 1830, but

it seems to have been used infrequently until about 1870 when it became the universal

sizing agent. (Barrow 1967: 17)

The commonly used resin is colophony which 1s insoluble in water It is therefore boiled together with sodium carbonate to form a resinate. The solution is filtered and added to the beater. The addition of alum (aluminum sulphate) causes a precipitation of the dissolved rosin ont0 the fibres (Witham 1920). The use of fillers as a loading material for paper was adopted in 1733 by William

Cookworthy in England but did not become general practice until the l9Ih century

(Barrow 1967: 18). Before the invention of effective bleaching processes, fillers were used to improve the colour of the rags. With the introduction of the Fourdiinier machine, paper was sold by weight instead of by sheet or ream. This led the papermaker to the extensive use of loading material, since fillers were both heavier and cheaper than fibres. Eariy fiilers were barium suifate and calcium sulfate (gypsum).

Later, more expensive fillers such as zinc oxide, zinc sulphide and titanium dioxide were used in accordance with the purpose of the paper (Grant 1937: 20121). Calcium carbonate fillers could not be introduced into papermaking before the 1950's. since their use requires an alkaline environment, which could not be accomplished in combination with alurn-rosin as the comrnon sizing (Espy 1990: 28/29).

The increasing demand for paper in the lgthcentury led. in 1840. to the invention of the groundwood process by Friedrich Gottlob Keller. It allowed for the econornical use of wood as a principal fibre source. Voelter and Voith further developed the process in

1846, which started the mechanical wood pulping industry. Since paper containing

mechanical wood pulp was found to exhibit low strength properties and tended to turn

yellow in sunlight and in storage, chernical wood pulping processes were developed5.

The process of pulping wood under pressure in sodium hydroxide was patented in

1853. The first commercial miIl for alkaline or soda pulping was erected in the United

States in 1854. Car1 Dahl further modified this process for economical reasons in 1879

by using sodium sulphate, which could be recovered from the systern and reused. This

-- - The low strength, rapid weakening and yellowing of the mechanical pulp was for a long time attributed to the presence of lignin. process produced a very strong paper and is therefore called kraft pulping ("Kraft" is the German word for "strength"). (Rydholm 1985: 2781279)

As a result of experiments in an American soda mill, the sulphite process was invented in 1866. This process treated wood under pressure with solutions of bisulphites and sulphurous acid. It was first used commercially when, in 1874. it was found that the addition of magnesium bisulphite and the excess of sulphurous acid considerably

improved the pulp. (Rydholm 1985: 279)

The paper industry from then on did not undergo major changes until 1955 when

Aquapel. an alkaline sizing agent, was introduced (Espy 1990: 28). This enabled the

papermaker to produce alkaline paper, which in turn made the use of calcium

carbonate as a filler possible.

2.2 Permanence of Parier

Permanence of paper was recognized as a problem in the early 19" century Often,

inventions in paperrnaking were found to be disadvantageous to the paper soon after

their introduction, which in turn led to new developments. However, it was not before

the end of the lgthcentury that scientists undertook systematic research on the causes

of paper deterioration.

In order to carry out research on the permanence of paper, it was necessary to have a

tool for the prediction of the life expectancy of paper. This tool was found in different artificial ageing methods. The historical development of ageing rnethods will be addressed in the last part of this section.

Causes of Paper Deterioration

John Murray was the first person whose concerns regarding the permanence of paper

are generally known ta us. His book, Obsen~ationsand Expements on the Bad

Composition of Modern Paper, with the Description of a Permanent Wfiting Ink which

cannot be Discharged, was published in 1824. He stated that the chief causes of paper

deterioration were excessive acidity and over-bleaching (Grant 1937: i65). Murray, in

1829, criticized the then newly invented Fourdrinier machine, because it made it

possible to use shorter fibres, resulting in a weaker paper (Barrow 1967: 16). This

issue has never been addressed in particular again6.

Feichtinger. in 1882, first found the alurn-rosin sized papers to be acidic. He carried out

experiments, which showed sulphuric acid as well as traces of chlorine to be the

possible reason for acidity. He also proved that acidity causes brittleness of paper

(Feichtinger 1882: 1751176). However, the first systematic approach to investigating

the causes of deterioration was undertaken during the 1880's by the German

Govemment in Berlin. Herzberg, who called for extensive experiments on naturally

aged papers, started a large research project by testing certain papers and storing big

quantities of the sarne paper for future investigations.

- - 6 Even though the possible use of shorter fibres as well as a certain alignment of fibres during manufacture on the Fourdrinier machine does have an impact on the strength properties of paper, it does not directly affect its permanence. These investigations were carried out between 1895 and 1911. In his first paper from

1895 he determined chlorine in combination with alum-rosin sizing to be the main cause for acidity, but emphasized that thorough research projects would have to be carried out to prove his supposition. Nevertheless, he was not able to draw any conclusions from his subsequent experiments carried out in 1905, 1907 and 1911, although he noticed that rag papers retained strength properties better than papers, which contamed wood. (Herzberg 1895 (a) and (b), 1905, 1907 (a) and 1911) The assumption that acidic compounds in paper are consumed and hence are no longer harmful, as was stated by Herzberg in 1895, was refuted in 1903 (Winkler 1903: 27).

The increasing uncertainty regarding the permanence of paper led several newspaper

companies in the following years to print special editions on rag paper for archival

purposes7 (Walton 1929).

One of the most important research projects in the 1920's was carried out by Hall. He

was the first to define the difference between durability and permanence of paper by

characterizing durability as a resistance to mechanical violence and permanence as a

capacity to preserve original properties8.

' The BrooWyn Eagle which had to discontinue this practice the same year because of costs, initiated this practice in 1913. However, the London Times in 1917, the New York Times in 1926 and the United States Daiiy followed the practice. a The dependence of permanence on strength characteristics was first recognized by Korschilgen in 1905, and two years later by Herzberg. However, neither of them distinguished between durability and permanence by means of a definition (Korschilgen 1905: 267012571; Herzberg 1907 (b): 21 13). Outstanding results of his research also included the acknowledgement of the need for a form of artificial ageing as well as the isolation of different destructive factors by basing his research on self-prepared laboratory paper sheets. (Hall 1926)

Considerable progress concerning paper permanence was made in the early 1930's.

Important research was conducted ai Brown Co. in Berlin (New Hampshire, USA) by iiasch and Ricnter. They 00th focused their researcn on the use of punfied wooa fibres as a papermaking material. Rasch directed his research towards different aspects such as the role fibres, fillers and sizing play during ageing. In addition. he investigated different ageing methods. Richter carried out a thorough study of physical and chernical properties of wood fibres.

The most important results from Rasch's studies were that degradation products of cellulose could act catalytically to promote further deterioration (Rasch 1929). Rasch noted a strong correlation between the a-content of cellulose and the retention of strength properties. He found that alum-rosin size was not harmful as long as it was not present in excess (Rasch et al. 1931). Richter's study, on the other hand, focussing

more on the fibres themselves. found that the strength properties of paper are not

directly related to the permanence. He was among the first to suggest the harm caused

by atmospheric pollutants (Richter 1931 (a) and (b)). During this time, researchers

became more aware that moisture present during ageing played an important role in

the outcome of accelerated ageing tests. (Richter 1931 (c); Minor 1932).

An interesting çtudy was conducted in 1938 by Shaw (Shaw/OYLeary 1938). His

research proved that acidity is an important factor in deterioration and showed that the

addition of calcium carbonate would have a protective effect on the stability of paper. At the sarne time, his experiments suggested fillers lower the strength properties of pape?.

During the following two decades research on paper permanence seemed to stagnate, probably a consequence of the Second World War. The study that had been started by

Rasch in 1929 was continued in 1955 by Wilson et al. (Wilson et al. 1955). Wilson stated that oniy qualitarive conclusions cm be drawn from the earlier experiments, since the manufacture and testing of paper had not progressed far enough in the

1920's to permit the control of variables. Although he was among the first to employ statisticai analysis upon test data, his research did not uncover any new crucial aspects to the permanence of paper.

William Barrow began a very thorough study in 1957 by investigating naturally aged book papers dating from 1507 to 1949 (Barrow 1964, 1967 and 1974). His rnost important contributions to the topic were the conscious use of calcium and magnesium carbonates as fillers in order to enhance the stability of paper. This was made possible

by the invention of an alkaline sizing agent in the late 1950's'~(Clapp 1972).

Since researchers had illustrated a relationship between a-cellulose content and

strength properties, the emphasis that had previously been laid on the purity of pulp

itself (i.ehigh a-cellulose content), shifted in the following years increasingly towards

manufacturing conditions (Burton 1931). However, it was not until 1981 that fully

Advantages and disadvantages of fillers had been discussed before. however, no systematic research on the subject has been undertaken (Franke 1908: 141). 'O 'Aquapel-Kymene' was commercially produced by Hercules Powder Company since the late 50's (Espy 1990: 28). bleached wood pulp fibres were considered to be as stable as cotton or linen"

(Gurnagul et al. 1994: 242).

Recent experiments have confirmed that acidity of paper is the most significant factor affecting paper stability. These studies suggested that papers containing lignin can be very stable as long as the paper is buffered with a sufficient amount of calcium carbonate. However, the optical properties are negatively influenced by the presence of lignin. (Bégin et a1.1999: 1501151)

Accelerated Ageing

Ageing processes under ambient temperatures are extremely slow. For the scientific investigation of paper permanence it is therefore necessary to accelerate the ageing processes. This approach is based on the fact that the rate of chemical reactions increases as the temperature iacreases. Accelerated ageing as a technique in paper related research was first recognized in 1888 by Wurster (Wurster 1888: 411).

However, from research in the same year, revealing only insignificant changes after 14 months of natural ageing. It was concluded that paper might retain its properties indefinitely (Frank 1888: 894).

Different studies in 1895 mention both the use of elevated temperatures for

accelerated ageing (Anon. 1895: 2834) and the need for extensive studies on naturally

11 The stabtiity of wood pulp fibres compared to rag fibres had been discussed in the literature as early as 1895 (Herzberg 1895 (b): 160). aged paper (Herzberg 1895 (a): 479). Henberg started to promote ageing tests under elevated temperatures in 18W1* (Herzberg 7899).

Hall undertook the earliest systematic investigation of different artificial ageing methods

in 1926 (Hall 1926). His experiments included both Sun and heat ageing, and while he

found thern to produce comparable results, he favoured the heat ageing, using 100°C

for 20 hours, because of its convenience. Rasch, too, investigated four different

methods of ageing, using combinations of dry and humid environment. light and

atmospheric pressure with essentially the same result: using the dry heat appeared to

produce the most suitable and at the same time reproducible results (Rasch 1929).

During the following decades most researchers ernployed the "heat test" (100°C for 72

hours). Their main concern was the ranking of papers in terms of their stability, i.e. the

art~ficialageing should rank the permanence of different papers in the same order as

natural ageing would (e.g. Rasch et al. 1933; Scribner 1939).

Even though the crucial effect of moisture upon ageing and the need to adjust the

relative humidity while ageing was recognized in 1932 (Minor 1932), it was not until

1968 that researchers made more effort to maintain a constant moisture content of the

paper during ageing. This was achieved by either ventilating moist air through the oven

or sealing the samples in tubes (BrowningNVink 1968).

Through the last decades several attempts have been made to correlate artificial

ageing with natural ageing in a way that would allow the prediction of the life-

'2 Experiments investigating different ageing methods such as light and elevated temperatures in order to accelerate yellowing of paper, ied ta the conc!usion that heat aged samples reproduce natural yellowing expectancy of paper (e.g. Wilson et al. 1955; BrowningNVink 1968; Baerllndictor 1977;

Zou et al. 1996 (b)). However, the question of whether or not certain accelerated ageing methods can be used to predict the relative life-expectancy still remains

(Strofer-Hua 1990).

The literature review reveals that the problem of paper permanency was recognized as early as the beginning of the 19"' century Acidity was soon found to be one of the main causes, but its source remained unclear. Early researchers believed alum or residues

remaining from bleaching to be the main cause, while later. sulphuric acid was thought

to be the main reason. There seemed to be no doubt about the negative impact of

alum-rosin size; however. it was used for econornic reasons until an alkaline sizing

agent was invented in the late 1950's. This invention not only made it possible to

produce alkaline paper but also to use calcium carbonate fillers as a buffer.

Another reason for the stability of paper was believed to lie in a high a-cellulose

content. For a few years, papermakers therefore went back to using all-rag pulps for

archival papers, but they soon realized that sizing, too, played a major role in the

permanence of paper.

In conclusion, it seems that the main focus of permanence-related experiments until

the 1960's was the stabiiity of paper itself. Later investigations put more emphasis on

artificial ageing techniques. Contemporary researchers are still trying to find a reliable

method of predicting the life-expectancy of paper.

best (Zschokke 1913: 3168). 16

3 Current Research in Accelerated Ageing Procedures

Accelerated ageing has become one of the most important tools for the evaluation of the relative permanence of paper. However, artificial ageing remains a very controversial issue in paper conservation research, because there seems to be a significant difference between the characteristics of naturally and artificially aged papers. Since the only alternative to this method is to wait until measurable changes occur, it has been the objective of many researchers to improve the methods of accelerated ageing in order to obtain a reliable method to predict the permanence of paper. (e.g. Baer/lndictor 1977; Strofer-Hua 1990; Zou et al. 1996 (a) and (b))

A number of studies have been completed or are in process relating to this topic. At the

moment, several laboratories in North America are conducting research to develop

improved ageing methods for paper using light. pollutants. elevated temperatures and

humiaity. This work was initiated in a workshop held in 1994 and is managed by the

American Society for Testing and Materials i lnstitute for Standards Research

(ASTMIISR 1994 (a)). The idea behind these projects is that a composition-based set

of requirements for permanent paper is always less efficient and more vulnerable than

a performance-based standard (Shahani 1994: 121).

The purpose of these studies is outlined by ASTMllSR as follows (ASTMIISR 1994

(b): 3):

The chosen test methods have to correlate the physical and chemical changes in

paper properties, produced in the artificial ageing procedure, with natural ageing experiences. The ageing procedure should not produce changes in properties other then those that will occur during natural ageing under reasonable conditions of storage.

At the same tirne the ageing procedure should be applicable to a wide range of printing

and writing papers with reasonable combinations of composition variables. Further-

more, the research is to be conducted in a way that measures severat different

elevated temperatures so that Arrhenius plots can be developed. In this respect. the

ASTMilSR recommends the use of low temperatures in order to be able to extrapolate

results to room temperature with some certainty. The developed test method will utilize

the most aggressive possible conditions that adequately replicate the physical and

chemical changes that occur in paper during natural ageing so that the time to perforrn

the accelerated ageing test will be kept to a minimum. The overall aim is that end-users

of measured papers will have a reliable prediction of the time that will elapse until the

papers have reached allowable limits of strength and other physical performance

variables.

Studies in this context (Shahani 1994: Bégin et al. 1996) have shown that paper

sheets, aged in an enclosed environment or arranged in stacks, deteriorate faster than

single sheets aged under the same ageing conditions. These results indicated that

more attention should be paid not only to the ageing conditions themselves, but also to

the arrangement of paper while ageing. This issue is of particular interest, since most

naturally ageing paper is arranged in stacks (Le. books).

Recent studies seem to confirm that both stack and single sheet aged paper degrade

in a sirnilar way but do so at different rates (Hendriks 1994; Bégin et al. 1996). Shahani

believes that degradation products form during the ageing process, accumulate inside

the stack of paper, and accelerate its degradation, a phenornenon which is not seen with single sheets in immediate contact with the air and moisture of the hurnid ageing oven". Acidity in the stacked paper was found to be about half a pH unit lower than that of single sheets. From this observation it was suggested that at least some of these accumulated degradation products must be acidic. Furthermore, the concentration of acidic compounds increases with length of ageing, therefore accelerating the acid hydrolysis of cellulose (Shahani 1994: 125).

Using a fully bleached kraft paper aged at 90°C and 69% relative humidity (RH) Carter

(1996) showed that volatile degradation products are able to diffuse from the top sheets of a stack during accelerated ageing, while there is little or no diffusion from the centre of the stack. The results also revealed that paper deterioration gradually increases towards the centre of the stack. as a consequence of their decreasing access to the environment. This was especially the case for properties such as CIE

L'a*b* colour coordinates and pH.

The present study was laid out based on Carter's results using two additional lowet temperatures (70aC and 80°C) while keeping a constant relative humidity. The main

objective was to find out whether or not the diffusion of acidic degradation products has

the same effect on the stability of thermally aged paper and naturally aged paper.

In order to meet the objective outlined above, the project focused on the determination

of the extent to which the diffusion of volatile products affects the paper properties frorn

the top to the centre of the stack as the ageing temperature is decreased. In addition, it

focused on whether or not the same mechanisms seen at high temperatures occur at

" Studies by Buchbauer in 1995 have characterized the volatile products responsible for the odour of old books (Buchbauer et al. 1995). However, these experiments did not focus on the possible detrimental ambient temperatures by using three different ageing conditions between 70°C and

90°C.

Since this project involved only one test paper, aged at three different temperatures, the correlatioq between zero-span tensile strength as a measure of fibre strength and the degree of polymerization in order to find a simplified method for the determinatior: of cellulose degradation was investigated.

4 Experimental

In order to simulate the paper that is found in books and to allow for significant changes in a relatively short artificial ageing time, an acidic paper was chosen for this

project. The paper was aged in stacks, at four different intervals, under three different

conditions.

Test methods performed were colour (including brightness, yellowness, L'a'b*),

moisture content. grammage, zero-span tensile strength, pH and the degree of

polymerization (DP).

effect of those compounds. 4.1 Samples

The paper used for this study was made from fully bleached kraft pulp, containing 50% hardwood and 50% softwood pulp. It was alum-rosin sized, and contained 4% clay as a filler. This paper was chosen for its similarity with paper that is often found in books. It was also well characterized and tested in a previous study (Bégin et al. 1998 and

1999)

The samples, arranged in 13 stacks of 50 sheets each, were numbered from the top to

the bottom. AH sheets were aged using three different conditions and four different

intervals. For the actual tests, ten sheets were taken out of each stack (these were

sheet nos. 1, 2, 3, 4, 5, 6, 10, 15, 20, and 25). The corresponding sheets of each stack

were tested. This setup resulted in a total of 130 different samples to be tested,

including the unaged control.

For the DP measurements the sheets no. 16 were used. This sheet was chosen after

al1 other tests had been carried out, and showed that the rate of degradation essentially

did not change anymore in this part of the stack. Using sheet no. 16 out of each stack

resulted in 13 different samples on which to perform the measurements, and allowed

for a calculation of the different rates of degradation under the three different ageing

conditions. 4.2 Accelerated Aqeing

The choice of ageing conditions was based upon experiments conducted at the

Canadian Conservation Institute, where the equilibrium moisture content of paper was determined at various temperature and humidity combinations. Those studies were performed in order to develop moisture profiles that can be used to determine the temperature and relative humidity combinations that would keep the moisture content of the samples the same during accelerated ageing as they would be during natural ageing at 23O C and 50% RH (CC1 1997: 2). This had been found to be crucial because water is essential in acid hydrolysis. Since the moisture content in paper is not very high, the amount of water available for hydrolysis rnay be the controlling factor.

Therefore. relative humidity during ageing plays a significant role. (Zou et al. 1996 (a):

2 53)

According to the results of this study the following ageing conditions and intervals were

chosen:

90° C at 69% RH; the stacks were removed after 5, 9, 16, and 30 days.

80° C at 65% RH; the stacks were removed after 7, 15, 28, and 50 days

70° C at 61% RH; the stacks were removed after 12, 32,61, and 110 days

The four stacks of each ageing condition were always put in the same oven at the

same time, and were removed by opening the oven and taking them out without having

ramped down the conditions in the oven. Since the removal of a stack could be done quickly, this procedure was found to be the least disturbing for the samples remaining in the ovens.

The stacks, 50 sheets of 8%" x 7 l",were placed on glass plates. In order to keep the sheets in place, due to the airflow in the ovens, the stacks were covered with plexiglass frarnes, whose sides were about 1" wide. This allowed the stacks to be open at the top

(surface area of about 7%"x IO") and to interact with the environment through their centre.

The samples were preconditioned for at least 48 hours at constant temperature and humidity to allow for equilibration of the moisture content at 23O C and 50% RH, as outlined by the Canadian Pulp and Paper Association (CPPA) in CPPA standard (A.4) for conditioning of paper and paperboard for paper testing, before being placed in the oven. The ovens were then set at 23O C and 50% RH and the samples left overnight at these set conditions. The ageing program was started the following day, gradually raising the temperature and humidity over two hours to the desired conditions.

The control stack was kept in the dark at 23 I2' C and 50 k 2% RH during the accelerated ageing.

All accelerated ageing was carried out using ESPEC PRA-SGP controlled temperature and hurnidity chambers. These ovens are equipped with microprocessors that allow for automatic, gradua1 control to set conditions.

After accelerated ageing, the sarnples were again conditioned at 23O C and 50% RH for

at least 24 hours before being tested. 4.3 Test Methods

Due to the nature of the experirnent, sorne of the test methods perforrned had to be modified with respect to their standard. This was necessary because the amount of

sample per specimen could not be adjusted to the amount needed for the tests to be

perfoned, as is normally done. In this project, each sheet of paper was of particular

interest in order to examine the effect of the diffusion of volatile products out of a stack.

Test methods that had to be modified were the determination of moisture content and

pH measurernents, respectively. In both cases only half the amount of sample could be

used.

The edges that had been covered by a plexiglass frame during ageing, were trimmed

off to ensure that only the centre parts of each sheet were used for testing. Ali sheets

were cut up in the same way. Figure 1 shows the distribution of test specimens on a

sheet for the different methods used.

Colour

Colour depends on the spectral characteristics of the illuminant. the geometrical

conditions of illuminating, the spectral reflectance of the sample, and the

charaderistics of the detector. The reflectance of light in the case of paper is a function

of the scattering and absorption characteristics of paper. its thickness and the reflecting

characteristics of the background. (Casey 1981: 1839) The colour measurements were carried ou? using the CIE L*a*b* system since measured differences in colours using CIE L*a*b* usually correlate well with visual estimates. As described by the Technical Association of the Pulp and Paper lndustry

(TAPPI) the symbols Y, a*, b* are used to designate colour values: L* represents lightness from zero for black to 100 for perfect white; a* represents a colour change from red when positive to green when negative, and b* represents a change in colour from yellow when positive to bhe when negative (.TAPPI T 524 om-86 1986: 1).

All colour measurements were carried out according to CPPA standard (E.1) using an

ELREPHO 2000 instrument. This instrument is equipped with two pulsed xenon flash lamps. The sample was placed on a support. which pressed it softly onto a measuring diaphragm (18 mm in diameter). It was cornpared with a reference area in the photorneter sphere using 16 measuring values in the range of visible radiation (400-

700 nm) (Datacolor International 1992: 8-9). Brightness, yellowness as well as L*a*bh were recorded using six replicates per page. The measurements were taken using the integrated UV filters in order to minimize the UV port~onof the flash lamps. The filters filtered out radiation below 395 nm.

Brightness refers to the reflectance factor of a sample with respect to blue Iight of specific spectral and geometric characteristics (TAPPI T 452 om-92 1992: 1). A wavelength of 457 nm was chosen by the lnstitute of Paper Chernistry in order to

provide a method for evaluating the degree of bleaching of pulp. It is considered as the

wavelength most sensitive to colour changes in paper. (Casey 1981: A828-1829) Yellowness is a parameter calculated from the tristimulus values X, Y, 214 according to the Deutsches Institut für Normung (DIN 6167 1980). It represents the yellowness of near white samples before and after yellowing.

Moisture Content

Moisture content is a very important paper specification since it affects al1 other properties, e.g., weight, strength and dimensional stability. As a hygroscopic material, paper can absorb moisture from the air as well as lose moisture to the air. The rate of absorption and desorption is dependent on the amount of moisture in the air and its temperature. It is also known that the equilibrium moisture content is affected by the composition of paper. Unbleached wood fibres. for exarnple, tend to absorb more moisture than bleached fibres. In addition, the amount of Mers and the kind of sizing

influence the moisture content. (Casey 1981: 1895-1900)

The measurement of moisture content was needed in order to calculate the dry weight

of the paper. This in turn was necessary to determine the zero-span tensile strength

(breaking length) as well as the degree of polymerization.

To determine the moisture content, the conditioned paper was cut into pieces of about

0.5 cm x 0.5 cm. The sample was then weighed into a weighing bottle under cuntrolled

temperature and humidity (23"C and 50% RH). The measurements were performed

according to CPPA standard (G.3) using three replicates per page and 0.5 g per

replicate instead of using 1.O g, which is utilized in the standard procedure. The sample

was then dried in an air-circulated oven at 105O C until constant weight was obtained.

14 Tristimulus vatues X, Y, Z are norm values that stand for the colour perception of a norm detector The loss in weight after drying was reported as a percentage of the original weight and was assurned to represent the original moisture content.

Grarnrnage is the most cornmon paper specification and can be expressed either as basis weight, which is the weight of a paper conditioned according to CPPA standards

(A.4), or it is reported as oven dry grammage. Oven dry gramrnage of paper is the weight after subtraction of rnoisture content. The dry weight has to be considered in calculating results for some physical test rnethods.

The measurernent of grammage was carried out at standard conditions of 23 + 2O C and 50 k 2% RH. Due to the Iimited sample size, grammage could not be weighed in replicates. Instead, every sheet of paper was weighed and measured individually after trirnrning off the edges that had been covered by the plexiglass frames during ageing.

The oven dry gramrnage was then calculated for each sheet.

Zero-span Tensile Strength

Zero-span tensile strength is a special type of tensile strength measurement. It was

first developed by Hoffrnan Jacobsen in 1925 as a method of determining an index of

fibre çtrength (Hoffman Jacobsen 1925: 53) as opposed to other specifics such as e.g.

inter-fibre bonding, elongation, etc., measured by other types of strength tests. Fibre

strength is an important parameter that affects paper failure. (Gumagul 1994: 144) The zero-span tensile strength of paper is measured between two jaws that are set up against each other with zero separation. The effective gauge length is assumed to be zero so that fibre strength is measured without major interference by inter-fibre bonding. This test was performed using the Pulrnac Zero-span Tensile Tester.

Since the moisture content of paper affects ail physical properties of paper such as weight, strength and dimensional stability, the test specimens were conditioned according to CPPA standard (A.4) at 23 + 2O C and 50 I 2% RH. The paper was cut into strips of 1" x 4", and the test was performed in the machine direction under standardized conditions. The measurements were carried out using 18 replicates per

specimen. In order to overcome the tendency of this test to give scattered results, the

paper was weighed prior tu cutting the strips and the individual weight of every sheet

included in the calculation of results.

Acidity is known to affect paper stability more than any other factor. It is determined by

measuring the amount of hydrogen ions of a water extract of paper (pH). It is thought

that these hydrogen ions can be dissociated in the presence of moisture, catalyzing a

hydrolysis reaction. Acid hydrolysis is known to be the most significant reaction during

ageing of paper. (Zou et al. 1996 (a): 255-257)

The determination of pH was carried out in triplicates using the cold extraction method

according to CPPA standard (G.25P). The standard method was modified by using

only 0.5 g instead of 1.0 g to carry out the measurements (the amount of water was

adjusted accordingly from 70 ml to 35 ml to keep the concentration the same). The sarnples were cut in pieces of about 0.5 cm x 0.5 cm and slightly macerated with a stirring rod in 10 ml of deionized water. The water had been boiled for at least 20 min prior to use in order to expel dissolved carbon dioxide. 25 ml of deionized, boiled-out water was then added and the sample left to stand for one hour at room temperature.

The mixture was stirred during the last 15 min before taking measurements. The pH readings were recorded afier nitrogen had been bubbled through the suspension to remove any carbon dioxide that it could have picked up from the air during extraction.

Degree of Polymerization

The degree of polymerization (DP) is one of the most sensitive indicators of cellulose degradation. It is a measure of the average length of molecules. As the cellulose

molecules break down upon degradation, the DP decreases correspondingly. The DP

is not measured directly but is estimated by measuring the viscosity of a cellulose

solution since its viscosity is related to the length of the molecules. (Browning

1967: 520; Kaminska 1997: 45)

Different cellulose solvents. such as cuprammonium hydroxide (cuam), cupriethylene-

diamine (CED) and cadmiumethylenediamine (cadoxen), can be used to dissolve

cellulose. They belong to the class of solvents in which cellulose forms a coordinated

complex with the metal ion. The mechanism of dissolution depends partly on the highly

alkaline nature of these systems. Both hydroxide ion and ethylenediamine act as

swelling agents for cellulose. This disrupts intermolecular hydrogen bonding and

thereby facilitates molecular chain separation allowing thorough penetration of the

solvent to occur, which in turn enhances complex formation with cellulose glycol

groups. The complex thus formed, is sufficiently stable to prevent reaggregation of chains as well as precipitation. (Johnson 1985: 183) However, other suggestions have been made in the literature on how cellulose dissolves in cadoxen and the question has not been clearly solved (BikaleslSegal 1971: 391 -392).

Different methods are used to determine the degree of polymerization depending on which solvent is selected. The method used in this project was based on the use of cadoxen, chosen because of its advantages over other solvents; it is odourless, colourless and more stable than the others are. In addition, the cellulose in cadoxen solution is less sensitive to degradation by atmospheric oxygen than in the other solvents mentioned above. (Browning 1969: 63-64) The cadoxen was prepared following the method described by Burgess (1979).

Since cellulose molecules are not of uniform size. the sample can comprise a range of different molecular weights. The calculated DP value thus gives a mean of the molecules' length. Therefore. the values cannot be compared with values derived from different papers. nor can values determined by different methods be compared without conversion. DP measurements are most useful in evaluating the effect of ageing or various treatments on the same paper. (Browning 1967: 519; Kaminska 1997: 50)

Prior to analysis, the paper samples were treated with sodium borohydride. This

stabilized oxidizing groups and prevented possible solvent-induced degradation of

cellulose chains (Burgess 1990: 447). Following this pretreatment, the sarnples were

washed with deionized water until the water was neutral (as indicated by using litmus

paper strips), and then air dried. The moisture content of these pretreated sarnples was

determined in order to calculate their DP. The measurements were perfoned in triplicates according to the method described by

Kaminska (1997). The paper sample was cut into uniforrnly small pieces (ca.

2 mm x 2 mm) and dissolved in cadoxen. The weight of sample had to be determined individually for each paper to ensure that the efflux time for each of the cellulose solutions was 2-3 min. The efflux time depends on several factors. such as the degree of degradation of cellulose, the viscometer used and the viscosity of the solvent itself.

Once the sarnple was dissolved in cadoxen. the solution was diluted with an equal

volume of distilled water to stop possible solvent induced degradation. This gave a

stock solution (100%) from which two dilutions (67% and 50%) were prepared for the

viscosity measurements,

The viscosity was measured using a Cannon-Fenske viscometer. The efflux time of the

stock solution (100%) and two dilutions (67% and 50%). was measured at 30 + 0.l0C.

The measurements were repeated until three readings were within 101100 sec. Specific

viscosities (qspc)of each of the three concentrations were calculated from their efflux

times. The average value of DP was calculated by finding the intercept of a linear

regression line of the three specific viscosities (q,,) versus the concentration of the

cellulose solution (c). The value of the intercept is equal to the intrinsic viscosity (ri.) of

the cellulose sample (Kaminska 1997: 48). The average DP of cellulose was calculated

from 11, using the following equation (Brown 1967: 459): 5 Results

Colour

Colour as measured by L'a'b', yellowness and brightness, showed similar effects for al1 mentioned properties. Among these. brightness, yellowness and b* can best be used to demonstrate the changes which occurred during ageing. L' and a* follow generally the same trends and are therefore not particularly discussed.

There is a significant change in brightness as well as an increase in yellowness from the top of the stack to the middle. i.e., from page no. 1 to page no. 25 (Figures 2 and

3"). Both properties change most in between the top three sheets (no. 1 to no. 3).

These sheets are distindly different from each other The following three sheets (no. 4 to no. 6) still exhibit a pronounced loss of brightness and increase in yellowness and b*

(Figure 4). compared to the control. The effect seems to level off towards the centre of

the stack. For all colour properties, no distinct differences can be rneasured from sheet

no. 10 to sheet no. 25. Therefore. sheet no. 15 can be seen as representative of

sheets frorn the centre of a stack. The phenornenon described can De detected with al1

three ageing conditions, but is most obvious for the sarnples aged at the highest

temperature, 90°C and 69% RH. In addition. it occurs at an early stage of ageing. The

same effect is seen for L* and a', respectively: there is a marked decline in these

properties through the first six sheets after which the effect levels off.

The loss of brightness and increase in yellowness is significantly accelerated with an

increase in temperature during the artificial ageing, i.e. the rate of degradation is rnuch

l5For ail figures (2-16) the control value represents an average of the ten sheets tested. higher at 90°C than it is at 80°C and 70°C. This is tnie for every sheet of the stack. It is demonstrated with brightness in Figure 5 for sheets no. 1 and no. 5. respectively.

A slight difference can be seen in the amount of ageing for a particular sheet in the

stack under the three different conditions. The difference in b' and yellowness between

the top sheet and sheet no. 3 is smaller at the lower temperature than at 80°C and

90'C. respectively. This difference can also be seen between the top sheet and a

sheet from the centre of the stack (no 15) (Figures 6-1 to 6-3 and 7-1 to 7-3). This

shows that, the higher the temperature during ageing, the more differentiation cm be

observed between sheets from the top of the stack and those from the centre

(Table 1). It is further clarified in Figure 12, using b' as an example. Since no diffusion

takes place in the middle of the stack, an intercept was taken for sheet no. 15 after

ageing for 110 days at 70°C (8.05). A second intercept was then taken to measure the

differences in the amount of ageing between this sheet and sheets no. 3 and no. 1,

respectively. By intercepting the lines that demonstrate ageing at 80°C and 90°C at the

exact same point (8.05). the differences in ageing at the different temperatures can be

seen. This procedure was followed in order to get the values of the different properties

shown in Table 1.

However, differences are not seen clearly with al1 colour properties (brightness. for

example, is an exception) and are generally very small (Table 1).

In general, the pH shows the same trend as the colour properties: there is a significant

loss of the initial pH, i.e. an increase in acidity, from the top of the stack towards the centre after ageing (Figure 8). As with colour, the change between the pH of the top sheet and the pH in the centre of the stack is largest between the sheets no. 1 and no. 6. The nearer a sheet was placed to the centre of the stack, the less of a difference is measured between the particular sheet and a sheet from the centre. However, unlike colour properties, small differences in pH can still be observed between sheets no. 6 and no. 15. Below the sheet no. 15, the acidity level stays the same within experimental srror.

The increase in acidity is accelerated with an increase in ageing temperature, which indicates that there is a higher degradation rate at elevated temperatures. This effect is more pronounced for pH than for the colour properties.

Differences in pH can be measured for part~cularsheets in the stack under the different ageing conditions (Figures 9-1 to 9-3). The difference in pH between the top sheet and sheet no. 3 is smaller after ageing at 70°C and 61% RH than the difference at the same point after ageing at 80°C and 65% RH. The corresponding point measured on the sample aged at 90°C and 69% RH shows an even larger difference than is obsewed on papers aged at lower temperatures (Table 1). These differences are even more pronounced between the sheet no. 1 and sheet no. 15, which represents a sheet from the middle of the stack.

Zero-span Tensile Strength

Although every single sheet used for zero-span tensile strength had been weighed

prior to analysis, as described in the experimental section, this test gave comparatively

widely scattered results, as is typical. Therefore, for a better graphical demonstration an average number was taken for the control and the changes of the different sheets are shown as trendlines.

Breaking length as measured by zero-span tensile strength decreases from the top of the stack towards its centre at al1 three ageing conditions (Figure 10). This pheno- menon levels off when sheet no. 15 is reached. However, these differences cm not be distinguished as easily as rs possible for the other properties measured. This 1s probably due to the low precision exhibited by this test.

As is true for colour properties and for pH, the loss of initial breaking length is markedly accelerated with increasing ageing temperatures.

The difference within particular sheets with respect to the amount of ageing under different ageing conditions is very small, but shows the same trend as pH and some of the colour properties. The contrast between the top sheet and a sheet from the middle of the stack is smaller at lower ageing conditions than at high temperatures (Table 1).

Degree of Polymerization

Since each of the ten sheets tested was completely used for the determination of pH

and moisture content, the degree of polyrnerization could not be determined on those

sheets. Therefore, another sheet from the centre of the stack (no. 16) was used.

Consequently, not al1 phenomena that can be seen with other properties such as

colour, pH and breaking length, can be observed. However, as with the other properties, the loss of DP is considerably accelerated with increasing ageing temperatures (Figure Il).

After 30 days of artificial ageing at 90°C and 69% RH the cellulose of this paper has reached a DP of about 250. This is considered as the limiting value, i.e the bulk of the cellulose is broken down to the length of cellulose crystallites, whereas this point is not reached at lower ageing temperatures in the given time.

6 Discussion

The significant change of initial colour properties through the top six sheets and the

gradua1 leveling off through the sheets below, confirrns eariier studies, indicating that

volatile products, inherent in paper, accelerate cellulose degradation (Hendriks 1994;

Shahani 1994;Begin et al. 1996). These products are able to escape from the top of a

paper stack, whereas they remain trapped towards the centre of the stack, thereby

accelerating the ageing of paper in the middle of the stack.

As can be seen from Figures 2, 3 and 4. interaction between environment and paper

sarnple with respect to colour mostly takes place in the first six sheets of a stack. This

is true for al1 three ageing conditions. However, very small differences can be

measured regarding the difference in ageing for particular sheets at different temperatures (Table 1 and Figure 12). The fact that measurable differences between

the top sheet and sheets no. 3 and no. 15, respectively, are slightly srnaller at lower

temperatures than at higher temperatures suggests that volatile products diffuse from

the centre of the stack before they can react with the paper. This in turn implies that

diffusion might not play as significant a role at room temperature as has previously

been thought (Carter 1996). However, since not al1 colour properties followed this

trend, no conclusions can be drawn from the colour measurements alone.

Since chemical reactions are known to proceed more quickly at high temperatures than

at lower ones, it is not surprising that the change in colour was largest after ageing at

90°C and smallest at 70°C, after a certain amount of time. This indicates a more rapid

change in brightness at 90°C than at lower temperatures In addition, Figure 5 shows

slower changes in brightness for the top sheets than for sheet no. 5. This, too, shows

that volatile products accelerate degradation processes if trapped inside the paper

during artificial ageing. To a different degree, this can be observed at each ageing

condition.

Since acidity is the most significant factor that affects paper stability. measuring pH has

been more commonly used for assessing paper degradation than colour properties. In

this study. the pH shows generally the same trend as colour but, while colour

properties are only able to distinguish clearly between the top three sheets. pH

demonstrates a clear difference up to the sixth sheet (Figures 8 and 9-1 to 9-3). In

addition, the determination of pH not only shows the changes in pH of a paper in a

particular stack, but also suggests some of the volatile products trapped inside a stack

could be acidic. As with the colour properties, the change in pH increases as the

ageing temperature is increased. The difference in pH at different temperatures measured between the top sheet and

sheets no. 3 and no. 15, respectively, is more obvious than for colour (Table 1). The

lower the ageing temperature, the smaller is the difference in pH between the top sheet

and a sheet from the middle of the stack. Thus, the pH indicates that, the higher the

ageing temperature, the more pronounced is the stack effect, Le. the more a particular

sheet will be influenced by its position in a stack.

This fact can be further clarified by comparing the results of this study with pH

measurements on the same paper used in another study carried out at the Canadian

Conservation lnstitute (Kaminska et al. 1999). This study compared the difference in

ageing between paper aged as single sheets and paper arranged as a stack, after

ageing at 80°C and 65% RH. Direct comparison reveals a marked difference in pH

between a sheet, aged individually by hanging it on clamps to avoid contact with

adjacent sheets, and a sheet that has been part of a stack, even if open on top. The

sheet aged as a single sheet is considerably less degraded than the top sheet of a

stack (Figure 13). Therefore, the diffusion of volatile products from a stack does

influence every sheet in question, even the top sheet that was able to interact at least

partially with the environment in the ageing chamber.

The stack aged samples from the above-mentioned study gave similar results to the

samples from the middle of a stack in this study at ageing conditions of 80°C and

65% RH (Figure 13). In both cases, measurements were performed on sheets that had

been aged in the centre of a stack.

From the results given by zero-span tensile strength measurements, no clear

conclusions can be drawn with respect to possible diffusion through the stack. This is due to the fact that breaking length, as measured by zero-span tensile strength, is most sensitive for DP's below 700 to 800, i.e it demonstrates distinct changes only after the sample is considerably degraded (ChamberlaidPriest 1998: 37). Obviously, the artificial ageing at 70°C and 80°C, respectively, did not cause enough changes in the given time to be clearly identified with this test. Nevertheless, measured differences between sheet no. 1 and sheets no. 3 and no. 15. respectively, seem to indicate a greater rate of diffusion at 90°C than at 70°C (Table 1).

Zero-span tensile strength also shows an increased rate of degradation with increased temperature in the ageing chamber and follows. in this respect, the same trend as the other properties discussed (Figure 14).

The degree of polymerization (DP) as determined on one sheet from the centre of each stack (no. 16), i.e. one sheet had been measured at every ageing condition and every interval, including the control stack, confirm phenornena previously seen with other properties such as colour. pH and zero-span tensile strength.

There is a signifiant difference in the rate of degradation between the different ageing

conditions. Degradation rates are higher at higher temperatures than at lower

temperatures. Moreover, the degradation of cellulose over time takes place gradually

at al1 chosen ageing conditions (Figure 15). This is due to the fact that the degree of

polymerization is the most sensitive indicator of cellulose degradation and therefore

gives more accurate rate constants than any mechanical property (Zou et al. 1996 (a): 250). Since only one sheet per stack was tested, no certain conclusi~nscan be drawn from the DP data as to the diffusion rate throughout the stacks at the different ageing temperatures employed. However, since both, physical and chemical tests can be used to determine the rate of deterioration, different studies in the past tried to correlate data obtained with zero-span tensile strength as a measure of fibre strength. and those obtained from viscosity measurements as a mean of cellulose degradation (Staudinger

1938; AgarwalIGustafson A 995; Zou et al. 1996 (a); ChamberlainIPriest 1998: 37).

The correlation between fibre strength and degree of polymerization was first discovered by Staudinger in 1938 (Staudinger 1938: 482). In his experiments

Staudinger found that the strength properties of cellulose do not increase proportionally to an increasing degree of polymerization. From this correlation he concluded that strength properties of cellulose could only be improved up to a certain extent.

The potential to use these relationships to simplify the determination of paper degradation was discovered in the 1990's. The delay in determining this was the fact that mechanical properties of paper are not solely functions of fibre strength (Gurnagul et al. 1992: 153; Zou et al. 1996 (a): 259). Previous experirnents showed that changes in viscosity and strength properties do not follow the same trend and the determination of the degree of polymerization therefore has limitations as an indicator of strength

(Agarwal/Gustafson 1995: 99).

However, recent experiments do reveal a relationship between the fibre strength as

measured by zero-span tensile strength and the degree of polymerization, since the

average chain length of the cellulose molecules has a significant impact on the fibre

strength as it ages. Extensive depolymerization of cellulose can produce significant losses in fibre strength (Zou et al. 1994: 396; Gurnagui 1994: 144). Further experiments by Zou et al. (1996 (a): 257) confirm that the loss of fibre strength for a particular paper is uniquely determined by the extent of cellulose degradation.

It has been found that viscosimetric determinations are more sensitive during the initial stage of ageing but as the degradation proceeds tensile measurements become more sensitive. Zero-span tensile tests are therefore more sensitive in lower than in high DP regions (ChamberlainlPriest 1998: 37). However. a linear relationship between fibre strength and DP can be found by plotting zero-span tensile strength against the number of broken cellulose bonds (n),which is calculated as:

where DP? is the initial degree of polymerization, and DP! is the degree of

polymerization at time t (Bégin et al. 1999: 5).

This relationship is shown in Figure 16. As can be seen from this graph, there is a good

correlation between zero-span tensile strength and the number of broken bonds for the

paper examined. However, the DP after 30 days of ageing at 90'C and 69% RH (264)

was not included in the graph because this value is too close to the limiting value for

DP. It has generally been accepted that DP values between 150 and 250 represent the

length of the crystallites present in cellulose. Once this value is reached it remains

roughly constant. Paper with a DP around this limiting value has lost al1 its mechanical

strength (Zeronian 1985: 164; Zou et al. 1996 (a): 259). Therefore, a good correlation

between those two properties can only be found above this limiting value. Although the experiment did show a correlation between breaking length and the number of broken bonds it is questionable if the degree of polymerization can be inferred from zero-span tensile strength. This cm be attributed to two main reasons.

The loss of mechanical properties is a consequence but not a direct indication of chemical degradation of cellulose (Zou et al. 1996 (a): 245). Furthermore, zero-span tensile strength as a measure of fibre strength involves a larger experimental error than the determination of the degree of polymerization.

However, in an experimental setup such as the one in this study, where only one paper is examined under different conditions, a baseline could be determined on one sample, correlating degree of polymerization and zero-span tensile strength. From this, DP values for every other sample in the experiment could be estimated using zero-span tensile strength data.

Conclusion

This project examined the ageing behaviour of acidic, fully bleached kraft paper. The paper was aged in stacks, which were open on top. at three different ageing conditions

using controlled elevated temperatures and relative humidity. The results showed clearly that volatile products are able to escape from the top of the stack, whereas they remain trapped towards the centre, therefore accelerating the ageing of papers in the middle of the stack. This was found to be particularly true for the first six sheets of a stack, regardless of the temperature / humidity conditions.

Furthermore, a measurable difference between the sheets is apparent at an eariy stage of ageing.

The test methods employed in this project detected different states of deterioration under different ageing conditions, mainly among the top six sheets. Below those, the study showed this paper to age similarly to papers aged in closed stacks. However, compared to papers aged as single sheets (Kaminska et al. 1999), the top sheets here showed a marked difference, indicating a significant stack effect during artificial ageing,

i.e. the diffusion of volatile products through the top influences every sheet of the stack.

In addition, the experiment indicated that rnechanisms seen at high temperatures are

not as pronounced at lower temperatures, therefore suggesting that the diffusion of

volatile products plays a minor role in natural ageing. At ambient ternperatures diffusion

might take place before acidic compounds are able to react with the paper.

Furthermore, this project investigated the correlation between the degree of

polymerization and zero-span tensile strength as a measure of fibre strength in order to

find a simplified method for the determination of fibre degradation by means of a

physical testing method. The results revealed a good correlation between breaking

length and degree of polymerization. For experiments such as this: where only one

paper was used at different ageing conditions and a good baseline has been

determined for the degree of polymerization versus zero-span tensile strength, it is possible to estimate the degree of polymerization for al1 samples, once the correlation has been found for a particular paper. However, it should be kept in mind that, due to the low precision of zero-span tensile strength measurements, this method can only give estirnates of cellulose degradation.

Future research should involve the determination of the degree of polyrnerization in a sirnilar experimental setup for evety sheet of a stack to clarify whether or not differences seen between sheets at different ageing conditions are significant. The study should also include paper aged as single sheets in order to determine how much the top sheet of a stack is influenced by the diffusion of volatile products from the centre of the stack.

Further research is also desirable regarding the nature of these volatile compounds.

The research should be directed towards a determination of whether those products

are inherent in paper or represent actual degradation products evolving from natural or

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I I ! I Moisture I ! I I content i i ; Moisture i pH pH pH pH pH pH Content l l I ! 1 1 Zero-span ' Zero-span Zero-span Zero-span 1 zero-span Zero-span 1 I l l 1 I 1 Colour ! Colour i Colour Colour ; Colour Colour !

Measure- 1 Measure- I Measure- ' Measure- 1 Measure- i Measure- i j ment ! ment ment ment i ment , ment

1 Zero-span Zero-span zero-span 1 i PH PH PH

.--.-----.--*--* -----.---. - - - . - - . - - - . .---..------l

1 Movàture Moisture Moisture Moisture Moisture 1 MC",',%: 1 Content Content Content Content Content , Zero-span Zero-span Zero-span

Fig. 1 Brightness vs Page

* A * w v w

R1m m 800 m.M LY a - +control +12days * 432days +6! days aJ u 5 g 600 . +110 days 6 2 Ey550 - 500.

800 .

u U u +7 days A n n 415 days w rn wm wm al U X +28 days tg 600. 6 L +#- 50 days :1550*

-t- control 43- 5 days +9 days +16 days +30 days

Fig. 2 Yellowness vs Page

70" C 1 61% RH

+control 412 days 432 days w w w - rn c. X61days

V V n n Ym X +ltOdays A A A Y Y U A rnd nY m u a A A A w v w 4

+control

w YIm wIi. 43-7 days 4 15 days u Y C1 n X28days

A * La A +50 days 0

250 +control

Fig. 3 b* vs Page

70" C 1 61% RH

w w w m m m -4-control +1 days u U U n A A +1 5 days A A Y Y +29 days m m Q .a Y El +50 days

-t- control +5 days 49 days +16 days +30 days

Fig. 4

b* vs Ageing Time, 70" C 161% RH 14.0 -

+Page #1 +Page #2 +Page 83 +Page #4 +Page #S +Page #6 +Page #10 -Pëge #15 -Page #2C1 -ô- Page #?5

O 10 20 30 40 50 60 70 80 90 100 110 days of ageing

Fig. 6-1 b* vs Ageing Time, 80" C 165% RH

+Page #1 +Page d2 4Page #3 -Jt Page #3 +Page #5 +Page #6 +Page #IO -Page #15 -Page #20 -0- Page #25

O 5 10 15 20 25 30 35 40 45 50 days of ageir,~

Fig. 6-2 b* vs Ageing Time, 90" C 169% RH

+Page #1 -4- Page #2 -A- Page #3 +Page #4 +Page #5 +Page #6 +Page #IO -Page #15 -Page #20 4 Page #25

O 5 1O 15 20 25 30 days of ageing

Fig. 6-3 Yellowness vs Ageing Time, ?O0 C 16I0/o RH 35.0 -

+Page #1 +Page #2 -t+ Page #3 +Page #il +Page #S +Page #6 +Page #IO -Page #15 -Page #?O +Page #25

0.0 --- O IO 20 30 40 50 60 70 80 90 100 110 days of ageing

Fig. 7-1 Yellowness vs Ageing Time, 80" C 165% RH 35.0 -

+Page #1 +Page #2 +Page #3 Jt Page #4 +Page #5 +Page #6 +Page #IO -Puge #15 -Page #20 +Page #25

0.0 - O 5 10 15 20 25 30 35 40 45 50 days of ageing

Fig. 7-2 Yellowness vs Ageing Time, 90" C 169% RH 35.0 -

4 Page Ut +Page #2 -i+Page #3 -B+ Page $3 +Paçe tS +Page #6 +Page #10 -Page #15 -Page #20 4Page #25

0.0 -a O 5 1O 15 20 25 30 days of ageing

Fig. 7-3 pH vs Page

52 -

+ control El 12 days A 32 days X 6 1 days X11Odays

+ control EJ 7 days A 15 days X 28 days X 50 days

4 control PJ 5 days A 9 days X 16 days X 30 days

Fig. 8 pH vs Ageing Tirne, 70" C 161% RH

+Page #1 +Page #2 4Page #3 +Page #3 +Page #5 -9- Page #6 +Page #IO -Page #15 -Page #?O +Page #25

O IO 20 30 40 50 60 70 80 90 100110 days of ageing

Fig. 9-1 pH vs Ageing Tirne, 80" C 165% RH 5.2 -

4 Page ltl +Page #2 +Page #3 +Page #4 +Page #5 +Page #6 +Page #10 -Page #15 -Page #20 4Page #25

O 5 10 15 20 25 30 35 40 45 50 days of ageing

Fig. 9-2 pH vs Ageing Time, 90" C 169% RH 5.2 -

-E+ Page #1 +Page #2 4Page #3 +Page #4 +Page #5 +Page #6 +Page #IO -Page #15 -Page #2O 4 Page #25

O 5 1O 15 20 25 30 days of ageing

Fig. 9-3 Zero-span Breaking Length vs Page

4 controi Q 12 days A 32 days X61 days X 110 days

+ controt Q 7 days A 15 days X 28 days X 50 days

4 cantrol Ei 5 days A 9 days X 16 days X 30 days

Fig. 10

b* vs Ageing Time, Pages #l, #3 and #15

b* vs Ageing Time, 70" C 1 61% RH ,page al +Page #3 12.0- -Page #15

O tO 20 30 40 50 60 70 80 90 100110 day s of ageing

b' vs Ageing Tirne, 80" C 1 65% RH +Page 140 . -b- Page #2 Page #15 120 . -

O 5 IO 15 20 25 30 35 40 45 50 days of ageing

b* vs Ageing Time, 90" C 169% RH -D- Page #l 4Page #3 -Page #?5

O 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 days of ageing

Fig. 12 pH vs Ageing Time, 80" C 165% RH

- P . . Page #l - O - - Page #2 A Page #3 X Page #4 - -X. - Page #S - - - Page #6 -a .Page#10 _.. . + ._.. . - - + - .- - - Page #15 ..-. Page #20 O Page k2S -single sheet -stack sheet

O 5 10 15 20 25 30 35 40 45 50 days of ageing

Fig. 13 alau cnc3U-lmmm aaa I- LI,

Appendix 2 Differences in the Amount of Ageing

1 OC / Page #15 ! Page #3 ! Page #1 Brightness (Weflectance) 70 i 69.00 1 70.50 ! 72.50 80 1 69.00 1 70.50 i 72.40 i Yellowness 7 80 1 16.20 1 14.80 13.10 90 ' 1620 1500 i 12.80 b I b* 70 ! 8 05 7.35 ' 645 A. 80 8 O5 7.25 6 35 ------90 8.05 7 15 6 20 4 pH (cold extraction) 70 4 40 4.56 4 69 . 80 4.40 1 4 56 4 72

: 90 ' 4.40 4.59 1 474 t Zero-span breaking tength (km) ' 70 i 10 00 10.80 11.O0 80! 1000 1 10.50 11.20 901 1000 10.60 1 11.20

Table I

I i j l 2 n wlw01- -10 oiw - o: pj ou,O & o., O +l!- +IF +,m T I - --

Table 6

Zero-span Tensile Strength (Breaking Length)(km)

Ageing

Table 10 Degree of Polymerization

Tempe- 1 Control rature -- O Ageing Time -1 d a~s.! Page #16

Table 31 Appendix 3 Materials and Equipment

Paper:

50% hardwood bleached kraft, 50% softwood bleached kraft (post-industrial), 4% filler (clay), alum-rosin sizing, starch

Chernicals for pretreatment and viscosity measurements:

Cadmium oxide, Baker analyzed reagent J.T. Baker 175 Hanson St. Toronto, Ontario M4C 1A7 Canada

Ethanol (absolute) J.T. Baker 175 Hanson St. Toronto, Ontario M4C 1A7 Canada

Ethylene diamine anhydrous, certified Fisher Scientific i12 Colonnade Rd. Nepean, Ontario K2E 7L6 Canada

Sodium borohydride, 99% Aldrich Chernical Company Inc. 1001 West Saint Paul Ave. Milwaukee, Wisconsin WI 53233 USA

Sodium hydroxide, A.C.S. reagent J.T. Baker 175 Hanson St. Toronto, Ontario M4C 1A7 Canada Equipment

Accelerated Ageing:

ESPEC PRA-3GP ESPEC Corp. 425 Gordon lndustrial Ct., SN Grand Rapids, Michigan MI 49509-9506 USA

Colour Measurements:

ELREPHO 2000, Type -l2OO-O477 Datacolor lnternational 5 Princess Road Lawrenceville, New Jersey NJ 08648 USA

Zero-span Tensile Strength:

PU LMAC Troubleshooter PULMAC Instruments International Middlesex Star Route Montpelier, Vermont VT 05602 USA

ORION Research rnicroprocessor pH 1 ORION Research, distributed by millivolt meter 81 1 Fisher Scientific 112 Colonnade Road Nepean, Ontario K2E 7L6 Canada

Degree of Polymerization:

Cannon-Fenske Viscometer 100/1528 Fisher Sc~entific 112 Colonnade Road Nepean, Ontario K2E 7L6 Canada