Effects of Air Pollutants on the Accelerated Ageing of
Cellulose-based Materials
by JOHN HAVERMANS
INTRODUCTION
Paper, one of the most important information-bearing and storage media in use, was invented in
China before the year 105 AD. Fibres from rice, stalks, hemp, and bark were used. Today, 19
centuries later, nearly all paper is produced from wood pulp. Although the paper-making process is
nowadays of a high quality, an important part of our cultural heritage consisting of cellulosecontaining paper is in danger. It is threatened to be lost due to deterioration caused by internal and
external effects. At present, paper made between 1850 and 1880 has yellowed and embrittled to
such an extent that books printed on this paper can no longer be used. The deterioration of modern
paper can be accelerated by the uptake of air pollutants.
Aim
Co-funded by the European Union, a research project has been realized the objectives of which
have been to determine effects, especially synergistic effects, of air pollutants and climate on the
stability of cellulose materials, in particular paper used and stored in museums, archives and
libraries.
To reach these objectives, a number of paper grades were selected and reference papers were
manufactured. The investigation was not limited to newly manufactured materials; old and naturally
aged papers were also used. At present many deterioration effects are attributed to the acidification
of the materials and therefore a (mass) deacidification has been recommended. However, the effects
of the deacidification process and especially the effects of air pollutants on deacidi-fied materials
are unknown.
Participants
This project summary presents the results, conclusions and recommendations of the European
research project, which was carried out by four European research institutes:
TNO Centre for Paper and Board Research, Delft, The Netherlands;
Centre de Recherches sur la Conservation des Documents Graphiques, Paris,
France;
Swedish Pulp and Paper Research Institute, Stockholm, Sweden; and
Chalmers University of Technology and Goteborg University, Sweden.
Philosophy
The approach used in this European research project was to study
a) the changes occurring in normal ageing, but here accelerated by the use of a higher temperature
(thermal ageing);
b) the tendency for the materials to absorb air pollutants (deposition studies), especially synergistic
effects of SO2 and NOX;
c) the deterioration resulting from exposure to air pollutants;
d) the changes during thermal ageing after exposure to pollutants;
e) the effects of different deacidification processes; and
f) the sensitivity of paper to pollutants after deacidification.
By splitting the project into 5 tasks, each cooperating partner had a responsible role in each task
using their speciality and expertise. Briefly these tasks were:
• Task 1, materials and available test methods were selected and evaluated. The materials were
thermally aged to find their basic characteristics.
• Task 2, freshly manufactured materials were exposed to air pollutants. The synergistic effects of
air pollutants were studied by examining the deposition of the pollutants on these materials.
• Task 3, old (and naturally aged) archive files and books were used to investigate the effects of air
pollutants on the stability of these materials.
• Task 4, selected old materials and acid materials were deacidified and then exposed to air
pollutants to examine the effects of (mass) deacidification processes.
• Task 5, finally, the results of tasks 1—4 were screened, combined, and evaluated.
• For the present, this research project has dealt only with SO2 and NOX in
combination with moisture and thermal ageing processes. The conclusions and recommendations
drawn from the results of this European project are fully presented in this project summary.
MATERIALS USED
For the whole project there was a need for a range of reference materials, so that the influences of
air pollutants on various paper grades could be examined. One of the important differences between
the materials chosen was their composition. Since it is difficult to know exactly what is the
composition of a commercial paper, some paper grades were made by special request, without
sizing, fillers or other additives.
Another important factor to be varied was the alkaline reserve. Nowadays, many factors promote
the use of alkaline papers instead of acid ones, but the effect of this material change on the
durability was unknown. A paper grade was also obtained with and without an optical brightener.
Finally we had to search for an acid paper to fill up the material matrix.
These papers are listed in Table 1, PAPER-1, -2, -3 and -7 have a pH cold extract below pH 7 and
the alkaline reserve of these papers is therefore zero. PAPER-4, -5 and -6 have a pH of 9. The
alkaline reserve of PAPER-4 is approximately 3% CaCO3 and PAPER-5 and -6 have an alkaline
reserve of approximately 6% CaCO3.
Table 1. The new materials
The project also needed some old, naturally aged papers to establish a link with the real situation. In
general, no library or archive will freely deliver sufficient homogeneous ancient material for
research purposes, especially for this project, since we required large quantities to look for
degradation effects. However, after contacting the Dutch State Archives Department, the Royal
Library in The Hague, the Netherlands, and the Bibliotheque Nationale in France, ancient materials
were found which could be used in the project. These materials and their composition are listed in
Table 2.
During the project, three of the selected papers were deleted from the programme due to the
inhomogeneity of their composition (PAPER-11,-12 and -13).
Quantities of all the materials used in this project are still in stock and are being stored in the dark at
23°C and 50% RH. Therefore these materials could and should also be used in any future research
programme, in order to compare and expand the results.
EXPERIMENTAL
Deacidification
The following mass-deacidification methods were chosen:
The DEZ (diethyl zinc) process. This is a gas diffusion process run by AKZO Chemicals, Texas.
Table 2. The ancient materials
A methyl magnesium carbonate (MMC) solvent-based process, the so called Sable process. This is
a modified Wei T'o process used at the Bibliotheque Na-tionale in Sable, France.
The magnesium butoxytriglycolate solvent-based process at the FMC Corporation (MG3LITHCO).
To evaluate these processes, a special sampling procedure was developed for new and ancient acid
materials. The evaluation was carried out by the CRCDG with the help of the participating partners.
Artificial pollution and artificial ageing of the deacidified materials was carried out by all the
participating institutes.
Artificial pollution and artificial ageing
The experimental set-up for the EC/STEP project consisted of two parts: the first concerned the
levels of the air pollutants to be used and the second the development of the artificial pollution
equipment and conditions to be used. The equipment and conditions can be further divided into two
essential parts: the accelerated or thermal ageing and the equipment for exposing the paper to air
pollutants.
In this joint project on the effects of air pollutants on the ageing of paper, we found it necessary to
use an accelerated-ageing method. The method was required to yield useful results in terms of paper
degradation. Therefore two conditions for accelerated ageing were first tested: 90°C/50% RH and
90°C/65% RH. Two paper grades were tested under these conditions, an archival paper meeting the
proposed requirements of ISO/TC46/SC10, and a rosin-sized copying paper. The archival paper
contained an alkaline reserve corresponding to at least 0.4 mol/kg acid, whereas the acid copying
paper contained no alkaline reserve at all. After preliminary trials it was decided that the thermal
ageing should be carried out for 12 days at 90° C and 50% RH.
In order to expose the papers to the air pollutants, three exposure chambers were used:
1. For model studies at ambient levels, an atmospheric corrosion test was developed at the
Chalmers University of Technology and Goteborg University.
2. For the artificial pollution and artificial ageing experiments, two similar exposure chambers were
used at the TNO Centre for Paper and Board Research, Delft, and at the Centre de Recherches sur la
Conservation des Documents Graphiques, Paris.
Three major deterioration processes were involved. First, the thermal ageing process for 12 days at
90° C and 50% RH was involved to deteriorate the paper
using high temperature and moisture. Secondly, the artificial pollution process. Although the
pollutant level was the same at TNO and at CRCDG, there was some difference in the active
pollution observed. The artificial pollution was therefore carried out at TNO for 4 days and at
CRCDG for 12 days using a ratio of SO2:NOX= 1:2 at 90°C and 50% RH on the basis of the
available literature on pollution in Europe in indoor and outdoor situations. The SO2 and NOX levels
of 10 ppm and 20 ppm, a magnification of 1000, were used. Thirdly, samples were exposed to a
combination of the artificial pollution followed by the thermal ageing.
Deposition studies, which indicate, for example, the reactivity of the papers with the air pollutants,
were carried out using specimens obtained from the material matrix. The deposition studies were
carried out using ambient levels of SO2 (65, 120 and 500 ppb) alone and mixed with NO2 (120 and
350 ppb). The pollution was carried out for a maximum of 20 hours at 22°C and 50, 65, and 85%
RH.
The schedule of the material matrix obtained from the artificial pollution and ageing is given in Fig.
1.
RESULTS AND DISCUSSION
This document describes and discusses only some of the major results. The detailed results are
given in the final report to be published later.
The deacidification
In general, none of the selected materials that were deacidified showed visually any salt deposition.
However, all the materials had a slight odour. The odour of the FMC treatment was described as a
burned wood odour, while the Sable
Fig. 1. The material matrix obtained from the artificial deterioration and ageing experiments.
(Therm aged=thermally aged; Art pol=artificially polluted)
Table 3. Minimum and maximum concentration of metals, determined in deacidified papers
The alkaline reserve of the Sable-deacidified papers was one third of that in the FMC-treated
papers.
and the DEZ odours were described as being a sweet odour. None of the mass-deacidification
treatments showed a significant homogeneous distribution of the active compounds, although there
is no description and/or definition for the distribution. The resulting concentration levels of metals
(metal ions) of active alkaline compounds in two different papers are given in Table 3.
Fig. 2 presents a summary of the change in pH of five deacidified samples, the sulphite (PAPER-1),
the acid mechanical (PAPER-3), the acid copy (PAPER-7), the newspaper (PAPER-9) and the PTT
log (PAPER-10). Deacidification increased the alkalinity and therefore the pH. The FMC and the
Sable methods gave the highest pH. The overall value after the DEZ treatment is equal to the
theoretical pH of ZnO. Sable gave the greatest variation in pH. This could be ascribed to the
dissolution of the sizing of the papers or of the book-binding glue.
Fig. 3 shows the alkaline reserve. To interpret the obtained results, it should be known that 0.4
mol/kg alkaline reserve is equivalent to 2% CaCO3, but CaCO3 is not used alone to create the
alkaline reserve.
The solvent-based processes showed some deterioration of inks while the gas-phase DEZ process
showed in the middle of the paper sheets so called 'Newton-
Fig. 2. The pH of five deacidified papers.
Fig. 3. The alkaline reserve.
rings', due to the zinc distribution in the paper caused by a poorly executed drying stage in the
treatment.
After the mass-deacidification treatment, the papers used showed a good mechanical performance.
No deterioration was showed in the mechanical data only, but the chemical data showed that all the
methods caused a slight deterioration, especially when the deacidified papers were subsequendy
thermally aged. This was evident, for example, in the slight decrease in degree of polymerization of
the wood-free papers used.
The deposition of air pollutants on paper
Initially, the uptake was almost the same for all paper qualities, but after 10 h of exposure, the
deposition rate on the sulphite, linters and acid copy papers was less than 1 μg per hour and gram
paper. The rate of decline is probably due to the papers' capacity to buffer the adsorbed SO2. The
papers containing CaCO3 show the greatest SO2 uptake throughout. The acid mechanical paper
(containing 70% groundwood) also showed a large SO2 uptake. The high deposition of SO2 was
possible due to the sulphonation of the lignin and the morphological structure of this material.
The CaCO3 content in the archive and alkaline copy paper explains the higher SO2 absorption of
these papers. The fact that the two (alkaline) papers reacted differently could be due to the
difference in CaCO3 content and/or origin. The deposition of SO2 declines with increasing acidity
of the paper. When NO2 was introduced into the system, the SO2 uptake was greater in all the
papers, especially when they contained CaCO3. The results of the exposure indicate that the
deposition of acid pollutants on papers is dependent on the papers' capacity to buffer the acid
species. In general, after the thermal ageing, the pH value
Fig. 4. Correlation between CRCDG and TNO.
dropped for all the papers, but it seems that the decrease in pH is too small to influence the
deposition rate on the papers. The decrease in the SO2 deposition rate on the acid mechanical paper
after the thermal ageing was remarkable.
An increase in the RH led to an enormous increase in the deposition rate on the papers, greater than
the effect of NO2, but even at 50% RH, NO2 led to a significant increase in the uptake of SO2.
Artificial pollution and artificial ageing
Although as mentioned previously, the artificial pollution at TNO and at CRCDG showed some
difference, the chemical analyses showed a good correlation, as can be seen in Fig. 4. All the results
obtained from these experiments could therefore be used for interpretation and in the subsequent
analyses. The 158 samples and about 60 test methods finally yielded more than 9480 data points.
Only a small selection of these results are given in this summary.
Fig. 5 shows the pH of the papers before and after thermal ageing or exposure and the combination
of exposure and ageing. It is evident that the acidity increases slightly during the thermal ageing.
The pH drops greatly on exposure to pollutants and there is no extra effect if the exposure is
followed by the thermal ageing.
Another interesting result is the change in the copper number or the content of reducing groups, as
shown in Fig. 6. Each cellulose chain has a terminal hemi-acetal group and the reducing power is
attributed mainly to these groups. All the treatments (the thermal ageing, the artificial pollution and
the artificial pollution
Fig. 5. pH after several treatments of two cotton and two bleached softwood papers.
followed by the thermal ageing) generally result in an increase in the copper number, but the
increase is less in the alkaline papers (PAPER-3 and -5) than in the papers with no fillers, additives
etc. (PAPER-1 and -2). The combination of artificial pollution and thermal ageing results in a large
increase in the number of reducing groups, especially for the archival paper. The data indicate that
cotton paper deteriorates faster into cellulose compounds with reducing groups than the bleached
softwood-containing papers.
Data relating to the alkali-extractable fraction (AEF) and the total extractable organic compounds
(TOC) of the four papers are shown in Figs. 7 and 8.
The AEF shows the amount of medium long cellulose chains and other low-molecular-weight
carbohydrates present in the materials. These compounds can be related to the amount of
degradation that has taken place. As expected, the amount of AEF is lowest in the cotton reference
paper. After exposure all the samples show the same amount of AEF and the content of the AEF in
the alkaline copy paper remains almost constant. The combination of the artificial exposing
Fig. 6. The copper number of two cotton and two bleached softwood papers.
Fig. 7. The alkali-extractable fraction of two cotton and two bleached softwood papers.
Fig. 8. The total organic compounds of the water extract of two cotton and two bleached softwood
papers.
and thermal ageing led to an increase in the degradation compounds. The alkaline papers degraded
less than the reference papers.
The TOC represents the water-extractable organic degradation compounds in the papers (ppm/g).
As in Fig. 8, a low value was obtained for the cotton reference paper, indicating the purity of this
cellulose paper. The behaviour during the accelerated or thermal treatment also became very clear.
The thermal ageing results in a large amount of water-extractable compounds. The exposure to the
pollutants results in less of these water-extractable compounds, and we can draw the conclusion that
the two treatments give different degradation patterns.
Results of a mechanical test are presented in Fig. 9.
The figure shows the zero-span strength of the papers in the machine direction for the different
treatments. Accelerated ageing alone shows only a minor effect. The deterioration may already be
seen in chemical data, though not yet enough to affect the (deterioration of) mechanical properties.
Exposure of the papers to
Fig. 9. The zero span strength (machine direction) of two cotton and two bleached softwood papers.
pollutants gives a significant strength loss, and the combination of artificial exposure and thermal
ageing leads to an even greater deterioration. From this we may conclude that the air pollutants
affect the paper strength negatively, especially as this result indicates that the cellulose chains are
getting shorter, in agreement with the previously shown chemical data. Although the alkaline papers
also react, it was shown than the reaction is slower and that some strength remains in these alkaline
papers.
From the results it was shown that after CRCDG pollution, the sulphite reference paper (PAPER-1)
had a higher degree of polymerization than after TNO pollution, which is in agreement with the
AEF and conductivity data (the TNO-exposure gave the higher values). Acid-hydrolysis of paper by
air pollutants is thus a fact. The alkaline copy paper (PAPER-5) and the alkaline archival cotton
paper (PAPER-4) showed no significant difference in degree of polymerization after pollution at
TNO and CRCDG. After TNO pollution the conductivity was higher. Using infrared spectroscopy it
was established that nitrates were present after the TNO exposure, and this may explain the high
conductivity. Comparing these results with those of the acid copy paper (PAPER-7), it can be
concluded that air pollutants attack this kind of paper, but that this attack is very dependent on the
concentration. The alkaline wood-free papers also deteriorate less rapidly than the acid wood-free or
groundwood-containing papers during exposure to air pollutants.
The old papers, the newspaper, PAPER-9, and the PTT-log, PAPER-10, which are not further
discussed here, showed large changes in conductivity and in the amount of extractable organic
carbons. The degree of polymerization of the old cotton paper was not affected, because these
papers were already very degraded.
Fig. 10. The deterioration level indicated by the DPv and alkali-extractable fraction values.
DISCUSSION
Although there are several possible interpretations of the data obtained, attention is here given to the
reactivities of the different papers. Using the bleached sulphite cellulose paper as an internal
standard and using the results obtained from the deteriorated of degree of polymerization (DPv) and
alkali-extractable fraction AEF), the following deterioration sequence was obtained from the lowest
to the highest deterioration: the reference (r), thermal aged (a), artificially polluted at CRCDG (c),
artificially polluted at TNO (t), artificially aged at CRCDG (ca) and artificially aged at TNO (ta), as
shown in Fig. 10.
Using this approach, the degradation can also be followed with the AEF data, and not only using the
DPv. This is an improvement, since the DPv could not be used for wood-containing paper, while
the AEF can. The acid catalysed hydrolysis of cellulose can be considered as a first order reaction,
and it is thus possible to determine the rate of deterioration. Because of time restrictions (each
experiment has a limited duration), it is not possible to calculate the rate coefficient k. However,
with the available data, it is possible to calculate the factor k.t, the relative deterioration rate, using
the data for the alkali-extractable fraction. The following formula has therefore been used:
where Po is for example the AEF at t=0 or the reference (Pr), and Pt is then the
Fig. 11. The relative deterioration rates of cotton paper compared to that of the chosen internal
standard paper.
AEF after each experiment. The sequence of deterioration levels gives data sets for each paper that
can be compared with the internal standard paper used.
For the wood-free papers, for example, it was shown that the alkaline papers have a slower relative
deterioration rate than the acid papers. This is shown in the Fig. 11, where the calculated reactivity
or relative deterioration rate is presented for the cotton papers.
The reactivity of the acid mechanical paper was comparable to that of the acid wood-free copy
paper, as is shown in Fig. 12.
This approach can also be used after deacidification, e.g., to compare the three deacidification
methods. In the case of the bleached standard sulphite paper, which contained only softwood
cellulose, it was shown that the three methods gave different protection towards deterioration by air
pollutants, as shown in Fig. 13. However, using commercial paper, the protection against air
pollutants is not only effective but also comparable for the three deacidification methods used (see
Fig. 14).
The acid mechanical and the acid wood-free copy paper were compared before
Fig. 12. Acid mechanical (PAPER-3) and acid copy (PAPER-7) compared.
Fig. 13. Three processes compared using the internal standard paper only.
Fig. 14. Three deacidification processes compared using acid-wood-free copy paper.
and after deacidification. After treatment with DEZ and FMC, the relative deterioration rate
decreased dramatically, and the relative deterioration rate for the wood-containing paper was then
comparable with that of the wood-free paper. After the treatment using the Sable method however,
no decrease in reactivity was observed for the groundwood-containing paper. This can indicate that
the different active deacidification compounds used can behave differendy, depending on the paper
grade used, and that care has to be taken in the case of the lignin-containing papers.
CONCLUSIONS AND RECOMMENDATIONS
This section contains the main conclusions of our research project on the syner-gistic effects of air
pollutants on the accelerated deterioration of paper and is similar to chapters 9 and 10 in the final
report (PART 2). First conclusions are given on paper-deacidification including the stability of
deacidified papers against acidification. Secondly, conclusions will be given of the SO2 deposition
using ambient concentrations of SO2 and NOX and using different levels of relative humidity.
Thirdly the conclusion derived from the artificial pollution and ageing experiments using high
levels of air pollutants are presented.
Main conclusions concerning deacidification
Three mass deacidification processes were chosen, the diethyl zinc, the FMC and the Sable
processes. Comparable acid papers were deacidified, artificially polluted and aged, and the
performance of the papers was then evaluated. The following conclusions were drawn.
• The permanence of the deacidified papers was increased by all the three deacidification methods
used.
• Deacidification leads not only to an increase in pH and an alkaline reserve, but also to an
increase in the amount of water-extractable organic compounds present in the paper. These
compounds are introduced by the active deacidification compound used and/or by the solvent.
• To protect paper against air pollutants, a total homogeneous distribution of the active compounds
is not needed, but to prevent papers against their internal acid, the distribution of the alkaline
reserve has to be as homogeneous as possible.
• Neither the DEZ - nor the Sable - nor the FMC-deacidification gave a homogeneous distribution
of the active compounds in the papers tested.
• The deposition studies using deacidified paper indicated that reactivity of these papers was
proportional to the SO2 concentration.
• The alkaline compounds introduced into paper by any of the DEZ, FMC or Sable methods were
reactive with SO2, thus shielding the paper cellulose.
• The three mass-deacidification methods used all have a similar reactivities towards the air
pollutants used at 50% RH.
• In the presence of air pollutants, the alkaline compounds were consumed faster at high RH than
at low RH.
• After exposure to pollution and ageing, the reactivity of the deacidified papers towards the air
pollutants decreased to zero. However, the DEZ-treated papers showed a somewhat strange effect,
in that the reactivity recovered. This suggests that the zinc has reacted to zinc sulphite and not to
zinc sulphate during the artificial pollution process.
• Deacidification improved the durability of the paper, and the deacidified papers will have a good
protection against acid attack under the recommended archive conditions (50% RH and 23°C).
• Modern acid papers benefited more (better protection against acid attack) against deacidification
than acidic naturally aged papers.
• After deacidification, the deterioration process continued, but the rate of deterioration was lower.
• The relative rate of deterioration of the deacidified groundwood paper was comparable with that
of the deacidified acidic copy paper.
Deacidification provides protection against acid deterioration by the air pollutants used, and the
factor mainly responsible for the protection is the amount of alkaline reserve introduced into the
paper.
At higher RH, not only was the uptake of air pollutants increased but also a difference in reactivities
among the three methods used was found. Considering the model using the first order reaction
kinetics, the relative deterioration rate was highest for the Sable-deacidified papers. This could be
an effect of the different alkaline reserves introduced into the paper. From the first order reaction
kinetic model, the deacidified groundwood papers had a higher relative deterioration rate than the
deacidified wood-free papers. Therefore care has to be taken when using lignin-containing
materials.
Conclusions derived from the deposition studies
Reactivity of S02
• SO2 only, i.e. in the absence of other pollutants, does not react with pure cellulose. No reaction
was found even at concentrations up to 0.5 ppm at 85% RH.
• The deposition of SO2 on a paper depends on the amount (concentration) of additives and/or
fillers present in the paper.
• The reactivity of the papers towards air pollutants increased with increasing atmospheric relative
humidity (RH).
• There is a two stage mechanism: an initial absorption, kinetically controlled and depending on
the pollutant concentration, and then a slower chemical reaction.
• The adsorption of SO2 onto paper fibres was kinetically controlled and the adsorption was
proportional to the SO2 concentration.
• SO2 reacted more rapidly with GaCO3 than with cellulose fibres; therefore CaCO3 can be seen as
a protective agent against acid attack on the fibres.
• The reaction between CaCO3 fillers and SO2 was dependent on the relative humidity.
• The reactivity of the paper determines the durability of the alkaline reserve in the paper sheets.
The pH of the cold water extract was therefore of less importance.
• The protection of paper towards acid attack depends not only on the amount of alkaline
components but also on their type. Even calcium carbonate cannot always be seen as a general
protective alkaline filler. Chalk behaves, in higher humidities, differendy from ground limestone,
which had a higher reactivity with SO2 compared to the former.
• The presence of lignin caused a higher degree of absorption of SO2 on acid
mechanical paper than on acid copy paper.
• The reaction of SO2 with groundwood cellulose fibres was mainly diffusion
controlled, since it was very little dependent on the SO2 concentration.
• From the studies of the reactivity of sulphur dioxide in the presence of NO2 it
was concluded that in none of the cases was NO2 absorbed by the papers
tested.
• There is a synergistic effect of the presence of moisture and NO2 when the
uptake of SO2 is considered. At high relative humidities, there was a strong
synergistic effect between SO2 and NO2, which was shown by the increase in
the reactivity of SO2 with cellulose fibres.
Deposition of air pollutants on aged papers
Three types of aged papers were used: artificially (thermally) aged, artificially polluted and
naturally aged paper.
• There was a synergistic effect of NO2 on the uptake of SO2.
• Cotton fibres reacted with SO2 in old paper but not in newly produced paper. This difference
may be an effect of ageing of cotton cellulose fibres and/or of the additives used and/or of the
ageing of the additives in the old papers used.
• The groundwood paper changed significantly during the ageing procedure, resulting in
passivation towards the strong SO2+NO2 synergistic effect that occurred with unaged paper.
• After artificial pollution by treatment with 10 ppm SO2 and 20 ppm NO2 under the TNO
conditions, most papers lost their reactivity towards SO2. This means that all reactive sites have
been consumed, including the alkaline reserve. Other chemical and mechanical tests showed that
the paper performance was reduced after this type of artificial pollution.
• The SO2 adsorption capacity of naturally aged paper was reduced compared to that of
comparable thermally aged new paper.
• No significant difference was found between the reactivity of the artificially polluted naturally
aged papers and that of the new artificially polluted unaged papers.
• Using the corrosion apparatus developed by Chalmers University of Technology, it seems that a
12 day thermal ageing at 90° C and 50% RH can be a reasonable way to simulate paper storage in a
good archive for about 50-100 years.
Conclusions derived from the artificial pollution and ageing experiments
Experimental pollution set-up
• Different mechanisms were involved in the artificial pollution experiments: at CRCDG a dry
deposition was obtained, while at TNO a wet deposition was obtained.
• The total deterioration process was accelerated dramatically when the artificial pollution was
followed by thermal ageing (12 days storage at 90°C and 50% RH).
• The acidity of the artificially polluted papers decreased after the thermal ageing, indicated that
after the artificial pollution experiments all the papers contained free acid compounds. These
compounds were not only removed by the thermal ageing process but they also were responsible for
the subsequent deterioration, 'the acid-catalysed hydrolyses of cellulose'.
Correlations
Although it was not our main target to correlate institutes and methods, the following can be
concluded:
• The artificial pollution methods used as well as the artificial ageing processes at CRCDG, France
and TNO the Netherlands, correlated well with each other.
• The deterioration of groundwood-containing papers can be followed using the alkali-extractable
fraction. This was shown using data obtained from all the experiments and the choice of pure pulp
reference paper. A correlation between the degree of polymerization, DPv, and the alkaline
extractable fraction, AEF was established.
• A good correlation was established between the DPv and the copper number.
• No general correlation was found between the fold number and the pH of the cold-water extracts
of the papers due to the deterioration by the air pollutants used (acidification).
Deterioration process based on the chemical, physical and mechanical properties
• Pollution of paper leads to acidification, but the acidification was less in the alkaline papers.
• Acidification and the artificial ageing process led to a deterioration of all the papers, but the
alkaline papers deteriorated more slowly than the acid papers.
• Pollution led to a levelling-off of the pH, i.e., the deterioration continued even
though the pH of the cold water extracts of the papers did not change significantly.
• A first-order reaction kinetics may be used to model the degradation products as measured with
the alkali-extractable fraction.
• The acid groundwood-containing papers showed a relative deterioration rate comparable to that
of the acid wood-free papers.
• After pollution, the copper number increased for the groundwood-containing papers more than for
the wood-free papers. Pollution has created an increasing number of accessible carbonyl groups in
lignin-containing papers. These groups could easily be transformed to carboxyl groups, which
could accelerate the deterioration.
• A copper number in paper of 2.5 or higher means danger. This conclusion was derived from the
correlation between the DPv and the copper number, and from the measured paper performance. It
was shown that all the polluted papers became brittle at a measured copper number of 2.5 and
higher.
• The old papers were already strongly deteriorated at the beginning of the project. This
deterioration not only continued but it also increased in the presence of air pollutants.
• In the case of the new papers, it was concluded that there is first a chemical deterioration before
the mechanical properties will be changed. This is mainly caused by the additives/fillers etc. It was
concluded that chemical analysis is essential to follow the ageing process and predict the ageing
behaviour of paper.
• The air pollutants used had a synergistic effect on the hornification of paper. After thermal
ageing, pollution and the combination of the two, there was an irreversible loss of swelling capacity.
This effect was greater for the thermally aged polluted papers.
• Alkaline papers were more stable against thermal ageing alone than acid papers. This was shown
for example using the zero-span tensile index.
• The alkaline papers were more stable against the acid attack of the artificial pollution used.
• The artificial ageing, i.e., the combination of artificial pollution followed by thermal ageing,
made all the papers brittle except the alkaline copy paper. It can be concluded that the air pollutants
accelerated the (normal) ageing of paper. Acid hydrolysis occurred and all the fibres were damaged
after the thermal ageing at 90°C and 50% RH.
General recommendations and storage of paper
• It is recommended to use alkaline paper rather than acid paper, since the alkaline reserve extends
the lifetime of paper.
• It is recommended to deacidify old acid papers. The new alkaline reserve created will extend the
lifetime of the paper
• It is recommended that also other techniques for conservation purposes than deacidification be
developed because deacidification does not reverse the natural degradation, nor does it totally stop
the deterioration, although it lowers the rate of deterioration.
• The reactivity, as determined by the deposition test method developed by the Chalmers
University of Technology, is recommended for the estimation of the durability of the alkaline
reserve in a paper under different conditions.
• It is recommended that papers be stored at a relative humidity (RH) not higher than 50%, at room
temperature in order to decrease the uptake of air pollutants.
• It will be recommended to use temperatures as low as possible for paper storage to decrease the
natural deterioration rate. This recommendation was derived from the results of the different
artificial deterioration techniques used, although low temperatures were not studied.
• A range of RH from 40—50% is recommended for paper storage; although pure papers can be
stored at a very low RH, care has to be taken at low RH not to damage other materials like leather
bindings and glueing.
• Although it is a high cost solution, a combination of deacidification and a clean air environment
for paper storage is recommended and preferable to reduce the deterioration of paper by air
pollutants and therefore extend the lifetime of documents, books and other relevant materials. A less
expensive solution is to store the paper in alkaline archive boxes and/or in alkaline envelopes or
materials that contain specific absorbers. More specific research on these storage types is needed.
• New acid-absorbers need to be developed in order to protect paper against acid attack.
Suggestions for future research
Several items for future research projects have been identified. To assist researchers in formulating
project proposals, the following recommendations can be of use.
• In general, the European institutes cooperating within this project concluded that there was a
synergetic effect of working together. Therefore we recommend for future research on paper ageing
that institutes cooperate to prevent overlaps and to produce a research synergism.
• To achieve a greater research effort, European researchers and also politicians
must be brought together with more (non-European) researchers and politicians. Funds have to be
established to continue research on paper ageing and preservation.
• Both experimental types, the deposition experiments using the Chalmers corrosion test apparatus,
and the artificial pollution experiments used at CRCDG, France and/or at TNO, The Netherlands,
are needed for specific research on the synergistic influence of the deterioration of paper by air
pollutants.
• It is recommended to artificially pollute paper using normal storage conditions (23°C and 50%
RH).
Deacidification
• All the mass-deacidification methods used in this project work chemically, but the way of
processing the mass-deacidification has to be optimized. After the project was carried out the three
mass-deacidification methods have all been changed in one way or another. The most important
change is that AKZO stopped running the DEZ-deacidification method. The chemical composition
of the FMC method has been modified, therefore the obtained results are no longer valid for the
modified FMC method. The Sable method will be modified as well. For example, the Freon (CFC)containing solvent will be exchanged, which may influence the quality of paper after deacidification
with this method.
• Research has to be undertaken for the further understanding of the modified deacidification
processes used in this project.
• There is a need for scientific research into the merits of the recently developed deacidification
processes like the Battelle process in Germany and the Bookkeeper process in the USA.
Storage conditions
• Storage has to be performed at low relative humidities and at low levels of air pollution to reduce
the uptake of air pollutants by the papers. Therefore the energy consumption related to air
purification systems must be an item for further research.
• The artificial accelerated deterioration of paper has to be further validated in relation to natural
ageing processes.
New paper materials
To prevent the mistake made in about 1850 we have to take care that paper of good quality is stored
in archives or is used in books. Therefore we recommend the following.
• The identification of the important chemical and physical mechanisms of the deterioration of
new fibre sources, additives and fillers related to the stability and durability of paper.
• Normal thermal ageing processes do not tell us everything concerning the durability of paper,
and even the new ISO standard on paper permanence, which describes only some paper
characteristics, has to be reconsidered carefully. Therefore credible, reliable test methods for
determining the durability of paper have to be developed.
• The use of different alkaline compounds on the durability of paper must be investigated. In our
study, indications were found that the amount of alkaline reserve was not the only important factor
to extend the lifetime of the paper, but that the type of alkaline compound was also important. •
The upper and lower limits for the amounts of alkaline compounds forming the alkaline reserve,
related to the most acceptable pH, must be established. • It is recommended to use artificial
pollution experiments as a supplement to thermal ageing, especially when deacidification and/or
added alkaline inorganic compounds are to be evaluated.
Environment
• This type of research must be expanded to include the effects of air pollutants on the accelerated
ageing of paper using oxidative air pollutants such as ozone, and/or cross-linking pollutants such as
formaldehyde.
• The environmental effects of the present as well as newly developed deacidification processes
must be studied.
Harmonization
• In order to characterize new paper with regard to its durability as well as to interpret results from
future research projects, it is recommended that artificial pollution processes and their limits be
standardized as soon as possible. It is recommended that mild exposure conditions be used and that
a time effect be included.
• In future research on the ageing of paper, it is recommended that at least two reference papers of
the present EC-STEP project be used. This will not only-increase the amount of results, but make it
possible to compare the different experimental types. These reference papers can be obtained from
TNO Centre for Paper and Board Research, Delft, The Netherlands.
ACKNOWLEDGEMENTS
As stated previously, this project was carried out by four European institutes. In this way I want to
thank all the researchers for their help and especially the project leaders of this project for the
fruitful cooperation and finishing our final report. Francoise Flieder from Centre de Recherches sur
la Conservation des Documents Grafiques, Paris, France; Petter Kolseth from Swedish Pulp and
Paper Research Institute, Stockholm, Sweden and Oliver Lindqvist from Chalmers University of
Technology and Goteborg University; Department of Inorganic Chemistry, Goteborg, Sweden.
Finally we want to thank our financial contributors:
• The European Commission, Directorate General XII for Science, Research and Development:
STEP Area 7,
• The Dutch Ministry of Welfare, Health and Cultural Affairs; and
• The Swedish R&D Project for Paper Preservation.
SUMMARIES
Effects of Air Pollutants on the Accelerated Ageing of Cellulose-based Materials
A report is given on a research project of the behaviour of some papers, different in fibre, filler, pH
and age, artificially treated to imitate air pollution, deacidified according to three different methods
and artificially treated to imitate ageing. Exhaustive conclusions and recommendations regarding library and archives conservation, regarding further research and regarding research methodology are
given.
Effet des polluants sur le vieillissement accelere des materiaux cellulosiques
Un projet de recherches a ete realise pour etudier le comportement de quclques papiers differents du
point de vue de leur composition fibreuse, de leurs charges, de leur pH et de leur age. Ges papiers
ont etc traites artificiellement pour imiter la pollution, desacidifies selon trois methodes differentes
et vieillis artificiellement. On tire des conclusions exhaustives ainsi que des recommandations
concernant la rcstauration des livres et des archives, les recherches futures et la methodologie de ces
recherches.
Über die Wirkung von Umweltgasen auf die beschleunigte Alterung von Material aus Cellulose
Es wird ein Forschungsprojekt über das Verhalten von verschiedenen Papieren bcrichtet,
verschieden in ihrer Faser- und Füllstoffzusammensetzung, im pH und im Alter. Die Papiere
wurden einer Behand-lung zur Imitation der von einer Luftverschmutzung ausgehcndcn
Veränderung, drei verschiedenen Behandlung zur Neutralisierung und einer beschleunigten
Alterung ausgesetzt. Es werden ausführliche
Schlüsse und Empfehlungen ausgesprochen in Bezug auf die Bestandserhaltung in Bibliothek und
Ar-chiv sowie auf ktlnftige Forschungen und deren Methodologie.
REFERENCES
1. Havermans, J.B.G.A., Van Deventer, J.P., Van Dongen, R., Flieder, F., Daniel, F., Kolseth, P.,
Iversen, T., Lennholm, H., Lindqvist, O. &Johansson, A.S.: The effect of air pollutants on the
accelerated ageing of cellulose containing materials -paper. EC/DGXII/STEP Project CT 90-0100,
BU3.94/1068/ JH. TNO, Delft, The Netherlands, 1994.
John Havermans
TNO Centre for Paper and Board Research
P.O. Box 6034
NL-2600JA Delft
The Netherlands