Behaviour of Paper Documents after Irradiation
by JIŘÍ NEUVIRT
INTRODUCTION
The main components of paper manufactured on the basis of woodpulp fibres comprise cellulose,
hemicellulose and lignin substances. Their ratios change depending on the origin of the fibres and
the technologies employed for isolation and bleaching. Because of its chemical structure, lignin is
the main source of the chro-mophores that cause colour changes when paper is irradiated1. During
the pulp production process, new chromophores are formed in the lignin structure2, and are
substantially more sensitive to the action of light than the original lignin. Pen-toses, hexoses and
hexuronic acids are formed from cellulose and hemicellulose during the pulp production process;
these substances react further and, depending on the pulping conditions, lead to the formation of
phenols, enols and furan derivatives which, in addition to lignin, are also important precursors of
photo-yellowing3. It holds in general that irradiation of paper materials causes an increase in the rate
of subsequent degradation and darkening during accelerated ageing at elevated temperatures. This is
a photo-oxidation process, which has received considerable attention in the literature4 and the
consequences of which also affect the degradation of paper during the subsequent natural ageing 5-7.
A longer period of time following completion of irradiation, a well-prepared experimental scheme
and a large number of irradiated samples are required to demonstrate the progress of the
degradation of the mechanical properties during natural ageing. On the other hand, the colour
changes caused by irradiation, which also continue after termination of the irradiation, called the
post-radiation effect
PRE), can be readily detected over an interval of days and sometimes even hours'7. Although the
colour change is not generally connected with degradation of the mechanical properties of the
paper8, it indicates an on-going reaction in the paper matrix and the intensity of this colour change
characterizes the reactivity of the medium.
From the standpoint of archive and museum work, it is desirable to monitor the effect of the source
of the radiation and the subsequent storage conditions on PRE in various kinds of paper. An
impetus for this lays in prolonged monitoring
if the reflectance of a sample of groundwood paper7, which was irradiated by
Fig; 1 : Loss of brightness of groundwood paper irradiated by source E (see Table 1) compared with
a not irradiated sample.
Fig. 2: Energy spectra of source B and D
daylight behind a double-glazed window for 10 days at the end of April 1998. It was then placed in
a covered Petri dish in the dark in a laboratory at a temperature of 23±1°C. The relative humidity in
the room was in the range 45%-60% in the summer and 35%-45°/o in the winter. Fig. 1 depicts the
change in the reflectance of the irradiated sample at a wavelength of 457 nm, compared with the
reflectance of a sample that had not been irradiated. The irradiated sample showed faster darkening
compared to that which had not been irradiated over the entire monitoring period of four years. It
can be seen that the rate of darkening was significantly greater in the summer months, when the
relative humidity in the laboratory was higher.
This work summarizes the results of a several-year study of monitoring various kinds of paper and
the post radiation effects (PRE), depending on variations in relative humidity, on the spectral
composition of the light source used to irradiate the samples and on secondary irradiation.
EXPERIMENTAL CONDITIONS
Irradiation source
Irradiation was carried out using light sources, the characteristics of which are given in Table 1.
Sources A and C had a large fraction of UVA radiation, while sources B and D were daylight
fluorescent lamps, with different fractions of the blue radiation component in the 405-500nm
wavelength region. Fig. 2. depicts
Table I: Characteristics of radiation sources.
the relative distribution of energy in the spectra of lamps B and D. Source E was sunlight filtered by
double glazing of the laboratory window.
Sample preparation
The effect of irradiation was monitored on three pulp samples and two samples of commercial
paper:
• Bleached sulphate pulp (BSAT); elementary chlorine-free bleaching (ECF) (Štětí production 1998) coniferous.
• Bleached bisulphite pulp (BSIT); totally chlorine-free bleaching (TCF) (Paskov production 1998), coniferous.
• Chemi-thermomechanical pulp (CTMP); (Norske Skog production - 1998), coniferous.
• Xerox paper; alkaline sizing, CaCO3 filler, with fluorescent brightening agent (FBA), basis
weight 89g/m2 (Jindnchov production - 1994)
• GW paper: copy-book paper containing 50% groundwood pulp; acid sizing; contains FBA, basis
weight 60 g/m2
The pulp samples were repulped and beaten at 2% consistency to 35°SR beating degree in a
laboratory beater. Sheets with a basis weight of 60 g/m2 were formed using a laboratory sheet
machine. The sheets were not sized and water from the water mains was used for dilution.
Optical properties
The brightness, reflectance factors, colour differentiation, etc. were measured using a DFC-5 Zeiss
reflectance photometer with d/80 geometry, with gloss elimination, and with a i oloiimelric filter for
measuring the X, Y and Z, colour components
and a special filter for measuring the "paper mill brightness", i.e. the reflectance factor measured
using a 457 nm narrow-band filter, Wherever brightness is mentioned in the text, this is always
„paper-mill brightness".
It is useful to employ the Kubelka-Munk function (K/S) for quantitative evaluation of the
concentration of the chromophores formed in the paper, where K is the coefficient of absorption and
S is the coefficient of scattering. As the value of S practically does not change during irradiation,
the change in the value of K/S is proportional to the change in the chromophore concentration
causing this change. The values of the K/S which are function of the reflectance factor R (%) are
calculated using the relationship:
(K/S) = (l-r)2/(2r)
where r = R/100
The R and (K/S) values are a function of the wavelength. Where not stated otherwise, a wavelength
of 457 nm is employed. The above relationship was derived under the assumption that the measured
sample consists of an infinitely thick and infinitely large homogenous plate. The size requirements
are met in practice if a change in the dimensions of the sample does not significantly affect the
measured value. In relation to homogeneity, it is important to bear in mind, when interpreting the
K/S values, that colour changes caused by irradiation are present only in the surface layer. In order
to emphasize this fact, the term KM is employed in place of the term K/S.
Our measurements involved primarily long-term measurements of the specific site on the sample
with minimal danger of affecting both surfaces of the sample during relatively frequent
manipulation. During the measurement, the sample was placed between the input opening of the
photometer and the black cavity, so that the measured site and its opposite side were never in
contact with any surface and there was no danger of their contamination.
In this work, PRE is characterized by the dependence of the relative change in K M on the time
elapsed following irradiation:
Relative change in KM =• KM/ • KM0 • 100 (%),
where ∆KM0 is the increase in KM caused by the irradiation and ∆KMt is the change in AM caused
by irradiation and subsequent storage in the dark for time t.
Sample storage following irradiation
In the first stage of the research, the irradiated samples were stored in the dark in Petri dishes in a
laboratory at a temperature of 23±1°C. The relative humidity in
Table 2: Media employed to maintain the relative humidity.
Fig. 3: Chamber with constant relative humidity.
the room was in the range 45%-60% in the summer and 35%-45% in the winter. Later, on studying
the effect of the relative humidity on PRE, the samples were stored in the dark, in hermetically
closed vessels with various relative humidities adjusted over a saturated solution of a suitable salt.
The salts employed and the corresponding relative humidities are given in Table 2 and a scheme of
the arrangement of the vessel, in which the samples were stored, is given in Fig. 39.
RESULTS
Survey of experiments
Table 3 gives a survey of the basic experiments carried out using the individual samples. The basic
experiment is considered to comprise, irradiation of the sample by the given source of radiation,
followed by its storage in the dark under the given conditions of relative humidity and the
monitoring of its reflectance over time. In some cases, the following were also monitored:
Table 3: Survey of basic experiments.
* Storage at room temperature.
** Following-up storage in closed vessels at defined RH.
•
•
•
•
The effect of the source and dose of irradiation on PRE,
the effect of varying relative humidity on PRE,
the effect of secondary irradiation on PRE,
diffusion from the irradiated surface.
The effect of the source of irradiation on PRE
To study the effect of light on various materials, both daylight and high-intensity artificial sources
were used, sources that imitated daylight or had an enriched ultraviolet component, to accelerate the
destructive effect of the light. It would also he interesting to determine the effect of the spectral
composition of the light on PRE.
Fig. 4 and Table 4 depict PRE and the absolute reflectance value respectively for sulphite and
sulphate coniferous pulp irradiated by sources A and B. It is apparent that radiation with a high
fraction of UVA radiation (source A) caused a greater decrease in the reflectance than radiation
with daylight (source B). It can be seen in Fig.4 (right diagram) that PRE was negative for
irradiation with source A, i.e. the darkening of the sample caused by the irradiation first became
lighter (KM decreased) and then slowly returned to the value existing immediately after irradiation.
In contrast, following irradiation with source B, PRE was positive (with
Fig. 4: Relative change of the Kubelka-Munk function of different pulps following irradiation by
sources A and B.
Table 4: Reflectance of several papers and pulps under irradiation (starting data of Figs. 4-8).
Fig. 5: Relative change of Kubelka-Munk function of bleached pulps, following irradiation by
light source A.
Fig. 6: Relative change of KM of xerox paper following irradiation by sources A and B.
Fig. 7: Relative change of KM of GW paper following irradiation by sources A, B, C and D.
the exception of sulphate after 4 days) and very intense. This was true of both types of pulp, where
PRE was more intense for the sulphate pulp.
During irradiation with source A, PRE was dependent on the energy of irradia-tion (Fig. 5 and
Table 4). The greater the energy, the less PRE was negative and the sooner it achieved positive
values. It is interesting that, in the range of energies employed, the sulphate pulp became saturated
and an increase in the energy did not affect the darkening immediately following irradiation. On the
other hand, the subsequent increase in PRE was dependent upon the irradiation energy and thus
must also have been caused by the darkening of the originally uncoloured compounds formed
during irradiation.
Table 5: GW paper-effect of the source of the radiation on the change in the pH of the surface.
Fig. 8: Relative change of KM of CTMP pulp following irradiation by sources A and B.
Xerox paper behaved in a similar way to the bleached pulp following irradiation by sources A and
B (Fig. 6 and Table 4). Compared to source A, source B yielded positive and very intense PRE.
It can be seen for GW paper containing groundwood (Fig. 7 and Table 4) that PRE was positive and
roughly of the same intensity following irradiation with sources A and B until the relative humidity
increased in the laboratory in the summer (after 100 days). Here it was apparent that the sample
irradiated by source B was more sensitive to the humidity and the PRE was more intense.
Comparison of pH changes following irradiation and subsequent accelerated ageing over only three
days of moist heat (80°C 65% RH), given in Table 5, indicates that the chemical reactions
following irradiation by source B were more intense than the reactions following irradiation by
source A. The radiation from sources C and D had energy in order of magnitude smaller than that
from A and B, and compareson again shows that PRE following irradiation of GW paper by UV
radiation alone was initially negative, while irradiation by visible light yielded positive PRE that
was much larger. It seems that the greater fraction of the blue component of radiation from source B
compared to source D was the reason why PRE for the sample irradiated by source B was more
sensitive to an increase in the relative humidity during storage.
For CTMP, (Fig. 8 and Table 4) irradiation led to very intense darkening through the presence of
lignin substances, formed by conversion of native lignin during
Fig. 9: Groundwood paper after irradiation by source B stored under different conditions.
the production of CTMP. Compared with native lignin present in the woodpulp, these substances
are far more sensitive to irradiation. PRE following irradiation by source A was again dependent on
the energy of the irradiation, while PRE following irradiation by source B was independentof this
energy and its intensity was much greater.
The previous experiments did not ensure constant relative humidity during storage of the irradiated
samples and some curves exhibited waviness caused by fluctuations in the relative humidity. In the
comparison, we attempted to ensure that the conditions of storage of the compared samples were
identical. In subsequent experiments, we attempted to find the dependence of PRE on the relative
humidity.
The effect of the storage conditions on PRE
Fig. 9 depicts the results of an experiment with GW paper, irradiated by source B with a dose of
1.63 MJ/m2 and then stored under various conditions. Darkening of the sample is expressed in
absolute KM values. It can be seen that the relative humidity during storage played a very
substantial role. For example, storage of an irradiated sample for 554 days at 75% RH and 23°C
had the same effect on darkening as storage of the same sample for 14 days in a dry box at 1%
humidity and 105°C. Storage for 14 days at 80°C and 65% RH led to even greater darkening.
Fig. 10 and Table 6 depict the darkening of a sample of GW paper irradiated by source F. and
stored at various RH from 0% to 75%. Between values of RH of 0% and 50%, a change in RH had
little effect on the progress of darkening; how
Table 6: Reflectance of GW paper samples. Irradiation source: E.
Fig. 10: Relative KM change of GW paper after irradiation by source E and deposition at different
relative humidities.
Fig. 12: Influence of RH on the post-radiation effect (PRE) after 400 days on ground-wood paper
(GM) and CTMP.
Table 7: Reflectance of CTMP pulp samples. Measured on a black cavity. Irradiation source: E.
Fig. 11: Relative KM change of CTMP during deposition at different relative humidities. Irradiation
source E.
Table 8: Reflectance of groundwood paper samples, measured on a black cavity. Irradiation source:
B
* 13 days in a Petri dish, then 20 days at 0% RH, then 112 days at 52%, finally at 75%.
** 13 days in a Petri dish, then 112 days at 75% RH, then 20 days at 0%, finally again at 75%.
Fig. 13: Relative KM change of GW paper during deposition at different RH. Irradiation source B.
* 13 days in a Petri dish, then 20 days at 0% RH, then 112 days at 52%, finally at 75%.
** 13 days in a Petri dish, then 20 days at 75% RH, then 112 days at 0%, finally again at 75%.
ever, a sudden increase in the rate of darkening occurred between 53% and 62%. CTMP behaved
similarly (Fig. 11 and Table 7). The dependence of PRE (darkening) for GW paper and CTMP on
the relative humidity after 400 days of storage is given in Fig. 12.
Monitoring of PRE was interesting when the RH was not constant during storage, but was adjusted
to a different value after a certain time (Fig. 13 and Table 8). The curve denoted "P-0-52-75"
corresponded to a sample of GW paper, which was stored for 13 days in a Petri dish in the
laboratory and then transferred loa dessicator containing silica gel until day 33, when it was
transferred to RH 52% until day 145 and finally moved to an environment with a RH of 75%, as
follows from the designation. The storage of the sample denoted "P-75-0-75" changed in the same
time intervals and the RH values are apparent from the designation. It can be seen that a change in
the relative humidity always appeared as a break in the time dependence of darkening of the
sample. It is important that, regardless of the history of the sample in relation to the RH value(s)
during storage, the same darkening value was attained, when il was placed in an environment with
high RH75% for a certain period of time, as that of the sample that was initially stored at RH75%.
For CTMP (Fig. 11 and Table 7), the sample denoted "0%", following almost two years of storage
at RK0%, was placed in an environment with RH 75%; it can be seen that the sensitivity to elevated
humidity was retained and it is probable that, after a certain time, the sample would have attained
the level of darkening of the "75%RH" sample. This was closely connected with the fact that, for
the sample that, following irradiation, was alternately placed in an environment with RH0% and
75%, the darkening followed the same curve as for the sample that was placed permanently in an
environment with an RH of 75%. This fact is documented by sample CTMP denoted "0-75-0" in
Fig. 11. The same behaviour was observed for GW paper, where the curve corresponding to storage
alternately at RH0% and RH75% cannot be distinguished from the curve for the sample stored at
75%RH. It thus follows that the elevated reactivity present in the damper environment continued for
a certain period of time after the sample was transferred to a drier environment. On the other hand,
following transfer from a drier environment to one with greater humidity, the rate of darkening was
greater than that corresponding to a sample that was permanently stored in a damper environment
and had the same level of darkening.
Secondary irradiation
II the sample was again irradiated after a certain period of storage in the dark ness, it became
lighter, and this increased with the increasing level of darkening caused by PRE. However, the
lightening effect was temporary and subsequent darkening soon eliminated it, particularly because
the sample had now received an overall greater dose of radiation. This can be seen in Fig. 14 and
Table Í), where the sample denoted "44%" (stored at RH44%) and the sample denoted "0-75-0"
(stored alternately at 0% and 75% RH), following 116 days of storage in the dark, were again
irradiated for two days by solar radiation through the double glazing of a window. They were then,
again, placed in a Petri dish in the dark -the relative humidity was not constant, but did not exceed
50%. It can be seen that the values for the "44%" sample were soon above the curve that they would
have followed if the sample had not been irradiated a second time. For the "0-75 0" sample, the
relative humidity during storage after the secondary radiation was lower compared to the original
storage prior to the secondary irradiation. Consequently, the darkening followed a less steep curve
than that for the original storage. 716 days after the first irradiation, both samples were placed in an
en
Table 8: Reflectance of groundwood paper samples, measured on a black cavity. Irradiation source:
B.
Fig. 14: Effect of secondary radiation on the post-radiation effect (PRE) of ground-wood paper.
Irradiation source: E.
Fig. 15: Groundwood paper reflectance during different modes of irradiation by sources C and D.
vironment with RH75°/o. It can be assumed from the darkening curve that the darkening of both
samples would reach the darkening of the "75%" sample that was not irradiated a second time. In
other words, sample "44%" and sample "0-75-0", stored at a relative humidity of 75%, would
finally exhibit greater darkening than the "75%" sample as, compared to it, they had been exposed
to a greater amount of radiation because of the secondary irradiation.
The effect of the irradiation regime on darkening of the sample
In a real environment in which paper material is present, periods of irradiation are frequently
alternated with periods of storage in the dark. The following experiment modeled this fact as
follows: GW paper was irradiated by sources C and D. For source C, the sample was alternately
irradiated for 1 hour and placed in a laboratory dry box at 60°C for 1 hour. The cycle was repeated
12 times. The experiment was compared with the case where the sample was irradiated for 12 hours
and then placed in a dry box at 60°C for 12 hours. A similar procedure was followed for source D,
but the irradiation cycle was varied somewhat, so that the decrease in reflectance in one cycle was
approximately the same as for source C.
The cycle comprised 3 hours of irradiation and 1 hour in a dry box at 60°C and was repeated 9
times. The heating in the dry box modeled a certain time of storage and the possibility of occurrence
of the PRE. Fig. 15 depicts the results of these experiments.
It can be seen that, for both sources, the interrupted irradiation caused greater yellowing of the
sample than continuous irradiation. For source C, storage in the dry box partly regenerated
brightness while, in contrast, the darkening increased for source D. This behaviour corresponded to
the PRE, following irradiation of GW paper by these sources, as can be seen in Fig. 7.
Diffusion from the irradiated surface
In connection with considerations of the effect of the water content in the paper structure on PRE,
we stated that a greater amount of water could favour diffusion of the reactive components. We
attempted to demonstrate the existence of a measurable intensity of this diffusion in the following
experiment.
We placed a sample of the same paper that had not been irradiated on both sides of a GW paper
sample that had been irradiated on one side, and pressed them together using screw clamps.
Cardboard made of bleached woodpulp without fluorescent brighteners was placed between the
pressure plates and the three sheets. Two such sets were created. Accelerated ageing with damp heat
was carried out on one set (80°C, 65% RH) for 14 days; the other set was placed in an environment
with relative humidity 75% and temperature of 22°C for one year. The papers placed on the
irradiated sample were measured as follows prior to the experiment. The reflectance was measured
on the surface placed next to the irradiated side of the sample, the surface placed next to the side of
the sample that had not been irradiated and the surface that was not in contact with the sample. The
same surfaces were measured again at the end of the experiment. The change of KM values during
experiment were calculated from the measured reflectance values. The changes of the KM values of
the surfaces placed next to the side that had not been irradiated and the side that had been irradiated
were compared with the change of the KM value of the free surface, which was taken as 100%. The
results are depicted in Fig. 16 and Table 10. Here it can be seen that the paper in contact with the
mass of the irradiated paper darkened more than the paper that had not been in contact. This was
true even when the paper was placed adjacent to the side opposite the irradiated side. This means
that diffusion exists in this system; however, we cannot state whether this is diffusion of the
coloured products or of the reactive components, or both. More detailed experiments will have to be
carried out in this respect.
Table 10: Reflectance R457 (%) of irradiated groundwood paper samples and non-irradiated ones
that have been pressed onto the the irradiated or onto the non-irradiated surfaces of the first.
Irradiation source: B.
CONCLUSIONS
The results of the experiments monitoring the effect of the irradiation source and the subsequent
storage conditions on the change of the brightness of various kinds of paper materials during
storage (PRE) indicated that:
• For comparable darkening caused by irradiation, PRE is more intense for materials irradiated by
a source of daylight compared with a high-UVA source;
• following irradiation by daylight, the dependence of PRE on the relative humidity increases
rapidly between values of 53% and 62%;
• the dependence of PRE on the relative humidity during irradiation by high-UVA light was not
monitored systematically; however, preliminary experiments indicated that it will be less marked;
• the initial phase of heat ageing copies the course of PRE (negative or positive PRE); here it
would be possible to experimentally verify the correlation between natural and accelerated ageing
under various conditions in relation to loss of brightness, reflectance spectra, etc.;
• coloured compounds or the precursors of yellowing pass from the irradiated sheet into the
neighbouring paper sheet that is in contact with the irradiated surface.
ACKNOWLEDGEMENTS
The author had the opportunity to participate in research carried out in the framework of work on
the grant "The effect of light and ultraviolet radiation on archive documents", supervised by Dr. Ing.
M. Durovic of the State Central Archives in Prague, and wishes to express his thanks for provision
of this interesting subject. A substantial part of this work was supported financially and technically
by this grant.
SUMMARIES
Behaviour of Paper Documents after Irradiation
This work summarizes the results of a several-year study of monitoring the post-radiation effect
(PRE) for various kinds of pulp and paper depending on changes in relative humidity, on the type of
the light source and dose of irradiation, and on secondary irradiation. Diffusion from the irradiated
surface was also detected.
Comportement des documents en papier après irradiation
Cette exposé résume les résultats d'une étude de plusieurs années relative aux modifications de la
blancheur de différents papiers ayant fait l'objet d'une exposition â la lumière. L'effet de postradiation (PRE) dépend de l'humidité relative de l'air et de ses variations, de la nature et de
l'intensité de la lumière ainsi que d'une éventuelle irradiation ultérieure. On a également observé
qu'une diffusion se produisait de la page irradiée vers les autres échantillons.
Das Verhalten von Papier bei einer Bestrahlung
Es warden die Ergebnisse einer mehrjährigen Untersuchung über die Helligkeitsveränderungen
verschiedener Papiere untersucht, die einer Bestrahlung mit Licht ausgesetzt waren. Der NachStrahlungseffekt (post-radition-effect: PRE) ist abhnägig von der Luftfeuchtigkeit und ihrem
Wechsel, von Art und Intensität des Lichtes und einer eventuellen späteren Bestrahlung. Es wurden
auch Diffusionen von der bestrahlten Seite des Papiers auf andere Proben beobachtet.
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Jiři Neuvirt
Ing. Chem.Tech.
Jakobiho 325
109 00
Prague 10
E-mail: neuvirt.j@volny.cz