The Influence of Secondary Chromophores on the Light Induced Oxidation of Paper
Part II: The Influence of Light on Groundwood Paper by VLADIMÍR BUKOVSKÝ &
MÁRIA TRNKOVÁ
INTRODUCTION
Nowadays there is a great deal of information on the influence of radiation on lignin, an element,
which appears in a significant quantity in high-yield pulps and groundwood papers and
considerably reduces their quality1-5. Understanding of the course of light induced oxidation of
lignin6 indicates that the mechanism is very complicated and that, besides UV radiation, visible
light (VIS) radiation also has an impact on the destruction of lignin7. The chromophoric structures
of lignin (primary chromophores) absorb UV-A radiation in a broad range of wavelengths (from
300 nm up to 390 nm) which leads to the formation of coloured degradation products. This process
has been described in detail8 and is known as yellowing. Bleaching is a process brought about by
the radiation from the VIS spectrum area at wavelengths ca 420-470 nm, and during this process
yellow chromophores are changed into colourless agents - leukochromophores, which are able,
under certain circumstances, to reabsorb the UV radiation. These reactions can alternate
(photocycling, photochroming) in an experimental configuration according to the type of light used,
for instance such as a redox cycling system quinone-hydroquinone. Using multivalent light, these
reactions occur simultaneously in a certain dynamic balance. Some of the coloured as well as the
colourless agents cease to participate in the photocyclic processes and are considered final degradation products of the light induced oxidation reactions, and they gradually reduce the
concentration of the original chromophores8.
Degradation products of oxidation, or light induced oxidation of groundwood paper, rely on the
secondary chromophores for their colour and the ability to absorb light in the visible daylight area.
These agents are particularly products of oxidative degradation reactions (disintegration) of lignin
and they occur in two forms. The absorbtion of light in certain chemical structures takes place in the
lignin molecule and these chromophoric groups are changed by the radical reac-
tions in the presence of oxygen. This initial phase of oxidation is connected to an increase in
carbonyl creation, but the splitting of the lignin molecule does not occur at this point. Another
group of secondary chromophores consists of the agents of low-molecular mass formed by a
complicated process of oxidation degradation and splitting-off of the lignin molecule1' 9, 10. We can
extract these agents from paper and try to analyse their structure. As shown in Part 1 of this report'
the extracts of these agents create an intricate mixture of agents and not all the degradation products
are chromophores, i.e. agents absorbing in the VIS spectrum area. There are differences in the
degradation products of every type of paper. In principle, reactions forming these agents are known,
but there is less information on how these agents influence the life of a paper, in which they accumulate during natural or accelerated ageing. We found that these agents in the process of
accelerated ageing - light induced oxidation simulating the oxidation processes - were further
decomposed into colourless agents, particularly under the influence of the VIS part of radiation7.
This can result in the bleaching of the paper containing them, which is usually accompanied by a
reduction in the acidity to a varying degree. The analyses using a UV filter showed that besides VIS
radiation UV radiation also, to a lesser extent, participates in the degradation of secondary, i.e.
extractable, chromophores. We assume that the balance between various types of light induced
reactions initialized by different wavelengths of light takes place as it does in chromophores bound
to the molecule of lignin8. Further analysis of these processes should bring more clarity to this
problem. On the other hand, even the cellulose component of paper suffers oxidative or photooxidative destruction11. The extent of this destruction is, compared to the degradation of lignin, very
small. The light induced reactions of degradation products of groundwood paper in wood-free paper
even influence the degradation of cellulose, at least in the initial phase of oxidation. It is possible
that both processes, i.e. the light induced degradation of chromophores and initiation of the
cellulose oxidation, are connected12 and can occur in groundwood paper as well.
The aim of this report is to consider the share of the short-term action multi-valent daylight with
partly reduced UV radiation has on the oxidation/light induced oxidation degradation of
groundwood paper in the presence of various amounts of secondary chromophores, formed in the
paper itself. Based on previous observations we assume that it will be possible to suggest an
effective way of preserving groundwood paper when it is exposed to daylight, or recommend the
type of lighting13 that minimizes adverse reactions on paper such as reducing UV radiation,
reducing light intensity, light sources, etc. We also tried to consider the efficiency of UV filters for
the preservation of groundwood paper documents during long-term storage, and particularly of
those documents after deacidification.
Table 1: Measurement data.
Fig. 1: Carbonyl content.
MATERIAL AND METHODS
For Part II of our work we used the same materials and methods as for Part I7: see pp. 18-35 of this
volume. The papers soaked in the chromophore solutions were, of course, not pure cellulose paper
Whatman no. 1, but newsprint Nl and N2. For the results see Table 1.
RESULTS AND DISCUSSION: UNDEACIDIFIED SAMPLES
Carbonyls (Fig. 1)
The number of carbonyls in N1 and N2 were relatively high, obviously a result of natural oxidation
ageing14. There was a difference between Nl and N2, the latter being of higher quality, i.e. more
resistant to natural ageing. The quantity of carbonyls was virtually unchanged with the addition of
each papers' own extracts of chromophores (ch l to Nl, ch2 to N2).
Irradiation by light caused a sharp increase of the amount of carbonyls in both papers: +173% in Nl
and +165% in N2. Looking at paper Nl, soaking in the relevant chromophore solution ch 1 of
concentration a had no impact on the amount of carbonyls created during irradiation, but it seems
that at the higher concentra-
Fig. 2: Increase of the amount of carbonyls in paper N2 and in paper N2 soaked in its own
chromophore extract of the higher concentration (ch2b) during irradiation by window pane filtered
light (irradiation B).
tion (ch1b) a certain inhibition of the light induced oxidation might have occurred. The same was
true with paper N2 for both chromophore concentrations (ch2a and ch2b). This inhibition was
measurable from the very beginning of irradiation and lasted even after approximately 30-35 days
of irradiation, i.e. after a certain carbonyl creation; cf. Fig. 2. The amount of carbonyls measured
after the irradiation of paper with the various chromophore content was the result of UV-radiation
action (yellowing), VlS-radiation action (bleaching) and the influence of these processes on the
carrier, for instance cellulose7,12.
The light passing through the UV filter (irradiation C) had almost no influence on the amount of
carbonyls in Nl. This meant that their creation was caused by VIS radiation only. With N2 the
situation is a little different. The UV filter (irradiation C) reduced the creation of carbonyls
(oxidation) by about 11 %, compared to irradiation B. Possibly, UV radiation had an influence on
the light induced oxidation of this paper. The amount of chromophores created by irradiation C (UV
filtered) as well as B of papers Nl and N2 was so great that adding chromophores before irradiation
could not significantly influence it. The fact that the carbonyl content of the samples irradiated by
direct light through a window (B samples) was not very different from those irradiated through the
UV filter (C samples), indicated that VIS radiation played a crucial role in the first stage of
oxidation, i.e. carbonyl creation. This phenomenon was the same for both types of paper.
Acidity of paper (Fig. 3)
The initial acidity of both papers Nl and N2 already highlighted the differences between them. Nl
was an acid paper: pH 4.3 (cf. p. 20 Footnote a), its acidity was considerably lessened, but was not
completely eliminated by methanolic washing (pH 6.05), which indicated the presence of acid
groups tightly bound to
Fig. 3: Acidity and alkalinity.
the structures of lignin, cellulose or sizes and fillers. Adding chl measurably reduced the pH of Nl,
i.e. increases its acidity. N2 was a neutral paper and neutral chromophore ch2 changed its pH only
very slightly.
After irradiation B the pH of both papers significantly decreased: in Nl by 26.8% to 4.43 and in N2
by 35.3% to pH 4.48. This links to the fact that the lignin molecule is considerably degraded by
light induced oxidation, and extractable acid degradation products are created. Adding the papers'
own chromophores to Nl and N2 before their irradiation had no influence on the course of light induced oxidation of the paper (B samples). It seems that their amount, compared to the amount
created by light induced oxidation, was very small and did not significantly influence the final pH
of the irradiated papers. The amount and structure of acid agents in paper after short-term
irradiation5 is a result of two simultaneously occuring degradation reactions of chromophores,
where their re-creation significantly predominates over their consequent light induced oxidation
degradation into acid leukochromophores in the process of bleaching7,8.
UV filter (irradiation C) had a different effect on the light induced oxidation of the two papers. In
Nl the pH did not change in comparison to the B samples, whereas in N2 less acid degradation
products were created and the pH was higher than in sample B. This effect was also seen in almost
the same amount in the control and in the chromophore containing samples in both concentrations
(Nl/2a and Nl/2b).This indicated that chl and ch2 had no significant influence on the creation of
further degradation products.
Fig. 4: Brightness, irradiated side.
Brightness (Figs. 4 & 5)
By washing the paper samples in methanol the degradation products formed by natural ageing were
eliminated12. Adding the relevant chromophores in both concentrations slightly reduced the
brightness of both papers.
The brightness of both pure papers was considerably reduced by irradiation, in Nl by 21% and in
N2 by 15%. This corresponded to the sharp decrease of pH and increase in the amount carbonyls
reported above and to the increase of ex-tractable degradation products (Fig. 6). The amount of
original chromophores in the papers, before adding chl or ch2, had no influence on a decrease of
brightness during irradiation. As stated in out previous report pure cellulose paper Wl containing
chromophores chla/b and ch2a/b was bleached by irradiation (cf. p. 27 sq.); this was not the case
with groundwood papers Nl and N2. We assumed that the change of coloured agents in ch 1 and
ch2 into colourless agents was the same in all three papers; in Nl and N2, however, it was
overlapped by the significant creation of other, "new" coloured degradation products.
Use of a UV filter had a favourable influence on the light induced oxidation of paper and at least
partly prevented it from yellowing. There was little or no difference in brightness after irradiation
between the samples having been impregnated with the lower (a-samples) and those samples with
the higher chromophore concentration (b-samples); this meant that the paper under the UV filter
yellowed less irrespective of the amount and quality of chromophores occurring in it.
Fig. 5: Brightness, reverse side.
The brightness of the verso of Nl and N2, with or without added chromophores, was similar to the
brightness of the recto sides. After irradiation B the yellowing occurred even on the non-irradiated
side, but to a considerably lesser extent. The presence of chromophores in the paper had in fact no
influence on a change in brightness during irradiation. This was not influenced even by the UV
filter.
Water extractable chromophoric agents (Fig. 6)
An essential part of extractable agents created in the papers by natural ageing was extracted with
methanol during sample preparation. This was why we could distinguish the amount of the agents
resulting from sample preparation from those created by the light induced oxidation during
irradiation.
Soaking paper Nl in chromophore extract chl obviously increased the content of the water
extractable chromophoric agents: concentration a increased by 0.022, concentration b by 0.038, i.e.
1.7 times more. For N2 the relevant numbers were 0.012 (a), 0.024 (b), ratio 1:2, which is exactly in
accordance with the relationship between chromophoric extracts (cf. p. 20).
The higher amount of added chromophores did not influence their further creation during irradiation
considerably: for Nl the numbers for aB and bB and
Fig. 6: Optical density of aqueous extract at 457 nm.
the numbers for aC and for bC were exactly the same, for N2 the b-numbers were a little higher.
Modifying the light with a UV filter provided at least partial information on the type of reactions
prevailing during the 35 days of our experiment. We calculated that irradiation C (VIS only)
increased the content of water extractable chromophoric agents in paper Nl without added
chromophores, i.e. the control, by 0.099; for irradiation B (VIS + UV) the increase was 0.183; the
difference, i.e. 0.084, must be attributed to UV alone. Table 2 gives the relevant relationship for all
samples. The different behaviour towards UV and VIS of N2 compared to N2 was quite
remarkable: with Nl the share was nearly equal, with N2 the share of VIS was more than two thirds.
Even if the UV component was eliminated (irradiation C) both Nl and N2 suffered a high degree of
yellowing compared to the samples which were not irradiated, which was quite surprising, because
generally yellowing is related to UV radiation. According to Fig. 1, VIS radiation had a significant
influence not only on the initial stage of oxidation, but also on the creation of coloured degradation
products and their further changes. It seems that dissimilarities in structure and quality of Nl and N2
resulted in significant differences in the effect of the VIS radiation , which can even lead to
differences in the light induced oxidation mechanism.
Table. 2: Changes in water extractable chromophoric agents caused by visible and by UV light
irradiation.
DEACIDIFIED SAMPLES
Carbonyls (Fig. 1)
Deacidification had little influence on the carbonyl content and its further creation by irradiation in
Nl and N2; in other words: pH and an alkaline reserve had little influence on this quality, whether
there are chromophores or not. The increase of carbonyls caused by irradiation B was +106% in Nl
and +141% in N2, i.e. somewhat smaller than with the non-deacidified samples (p. xy). The inhibition effect of Mg on the light induced oxidation of groundwood paper - for reasons of correctness it
must be added: if MMMC is used as deacidificating agent -is proved again. The inhibition effect in
N2 (141% vs. 165%) was lower than in Nl (106% vs. 173%). From Table 3 it can be seen that 40%
of the total carbonyl increase in Nl was caused by UV; in N2 the relevant share was 32%. There
might have been a different mechanism of the light induced oxidation in the alkaline medium. We
assumed that the Mg bond on the phenolic group of lignin inhibited the creation of free radicals
formed by UV radiation action and thus also inhibited further oxidation of some lignin structures. It
is interesting that even after the elimination of UV radiation the oxidation (creation of carbonyls)
was not stopped and continued under the influence of VIS radiation.
Table 3: Changes in carbonyl content caused by visible and by UV light irradiation.
Alkalinity
Deacidification shifted the pH of paper Nl from being very acidic to becoming neutral or slightly
alkaline, depending on the concentration of the neutralizing agent13. N2, which was neutral, reached
a significantly higher pH. Adding chromo-phores into alkaline papers during the preparation of
samples did not change their original acidity or alkalinity considerably.
Irradiating deacidified Nl paper with daylight (irradiation B) led to a slight decrease of pH, which is
to be understood as being the result of light induced oxidation degradation of lignin in this paper.
This corresponded to a high increase of water soluble degradation products (Fig. 6) and reduced
brightness (Fig. 4), i.e. yellowing. N2 behaved comparably, to a lesser extent. It seems that
deacidification with 2% MMMC of a very acid paper such as Nl did not provide enough alkaline
reserve to neutralize the acid agents produced during irradiation.
Using a UV filter (irradiation C) reduced the decrease of pH. The final pH of both papers modified
by chromophores was the same as in papers without chromophores.
Brightness (Figs. 4 & 5)
Deacidification of groundwood papers with MMMC led to binding Mg to lignin molecules 5,15,16
and is almost always related to a decrease of brightness. In Nl
and N2 is the decrease was 7% or nearly 8% respectively. The addition of chromo-phores to paper
led to a further decrease of brightness and was independent from their concentration. In comparison
to Wl paper (cf. p. 30), the decrease was less significant because the papers Nl and N2 are of
relatively low brightness.
A significant decrease of brightness in both papers (8°/o—13°/o) related to the daylight irradiation
of the deacidified papers. This decrease was relatively smaller in the chromophore samples
containing chromophores, regardless of their amount. It is important to note that UV filtering
reduced yellowing of all samples, N1 and N2 with and without chromophores in both
concentrations, although not to a very high degree. This was because the increase of colour
degradation products was a result of both UV and VIS radiation.
Water extractabh chromophoric agents (Fig. 6)
Both the untreated and the chromophore treated deacidified samples of paper Nl contain many more
water extractable compounds. Obviously, the changed environment from this initially acidic paper
(p. 20: pH 4.3 before methanolic washing) to an alkaline one resulted in many chromophoric agents
being retained during methanolic washing and during impregnation with the relevant chromophore
extract. With the initially neutral paper N2 (pH 6.8 before methanolic washing) the opposite was
true, al least for the control and for the low-concentration treated samples.
From Table 1 it can be seen that daylight irradiation increased the amount of extractable agents of
acid paper Nl by the factor 9 (from 0.023 to 0.206) and that of neutral N2 by much less (factor 4.9).
With the deacidified samples the relation between the two papers was the reverse, the rate of
increase for Nl (7.5) was smaller than that of N2 (9.9). With the pure cellulose paper Wl there was
no such difference between the deacidified and those samples not deacidified (see p. 21 Table 1 and
p. 31). The alkaline environment brought about by MMMC de-acidification significantly supported
the degradation of lignin and the creation of extractable agents, and with a primarily neutral paper
this happened to a quite dramatic degree.
The effect of using UV filter significantly reduceing the creation of extractable agents of the nondeacidified newsprints reported above was even more true for the deacidified samples. Again it
became evident that the share of UV radiation on the creation of degradation products was quite
high; it was further promoted by MMMC deacidification in some of the samples. The relevant
numbers are given in Table 2.
The amount of added chromophores (chl/2 a vs chl/2 b) had no influence on the share of UV created
degradation products. More detailed research on the simultaneous formation of coloured
degradation products (yellowing) and their degradation (bleaching) under the influence of VIS
radiation in groundwood papers and their analysis, will provide more information on the course of
light induced oxidation.
CONCLUSIONS
• VIS radiation has a crucial influence on the creation of carbonyls in ground-wood paper, which is
to be seen as the first stage of light induced oxidation.
• UV radiation influences the creation of extractable degradation products in groundwood paper,
i.e. the second stage of light induced oxidation.
• Both UV radiation and VIS radiation have an influence on the yellowing of groundwood paper.
• A UV filter does not stop the initiation of light induced oxidation, where VIS has a crucial
influence, but it significantly prevents or at least decreases the disintegration of lignin, the creation
of acid and of coloured degradation products.
• A UV filter considerably reduces the creation of coloured degradation products significantly
slows down the yellowing of paper and the creation of acidity.
• The creation of carbonyls (first stage of oxidation) in deacidified groundwood paper is
considerably more influenced by UV radiation than in non-deacidified groundwood paper. A UV
filter only partly inhibits this process, probably by reducing the creation of radicals.
• Mg bound to lignin structures during deacidification plays a part in the decrease of paper
brightness. It significantly inhibits light induced oxidation of groundwood paper in an alkaline
medium.
• Mg bound to lignin structures as a result of MMMC deacidification plays a part in the decrease
of paper brightness. It significantly inhibits the formation of carbonyls, but contributes to reduced
brightness (yellowing) and to the creation of extractable agents.
• The alkaline environment brought about by MMMC deacidification significantly supports the
degradation of lignin and the creation of extractable agents, and in the case of a primarily neutral
paper this happens quite dramatically.
• For long-term storage of deacidified groundwood papers (MMMC application) it is very
important to eliminate both radiation components from illumination.
• For long-term storage of groundwood papers it is particularly necessary to eliminate VIS
radiation, which initiates light induced oxidation, but also UV radiation, which causes further
disintegration.
SUMMARIES
The Influence of Secondary Chromophores on the Light Induced Oxidation of Paper. Part II: The
Influence of Light on Groundwood Paper
The samples in our research were different kinds of paper, to which chromophores, i.e. oxidative
degradation products of groundwood papers were added. In the first part several parameters of pure
cellulose paper were checked, for the second one papers which already contained those degradation
products, i.e. just the very groundwood papers from which the added degradation products were
extracted were studied. We were mainly interested in the changes provoked by light with both high
and reduced levels of UV. Finally, some recommendations are given as to how to preserve and
exhibit groundwood papers, including those which have been deacidified using a solution of
methanolic methyl magnesium carbonate.
L'influence des chromophores secondaires sur I'oxydation du papier induite par la lumiere. 2°™
partie: I'influence de la lumiere sur lepapier a pate mecanique
Ont ete etudies differents echantillons de papier auxquels on a ajoute des chromophores, prc-duits
de degradation oxydative du papier a base de pate mecanique. Dans un premier temps plu-sieurs
parametres du papier de cellulose pure ont ete testes. Ensuite on a analyse des echantillons de papier
qui contenaient deja ces produits de degradation, c'est-a-dire les papiers a pate mecanique dont on
avail extrait les produits de degradation ajoutes. L'interet de cette etude portait sur-tout sur les
changements des parametres provoques par Peffet de Pintensite faible ou forte des rayons ultraviolets de la lumiere. Finalement certains conseils sont donnes pour conserver et ex-poser les
papiers a base de pate mecanique, y compris ceux qui ont ete desacidifies au moyen d'une solution
methylique au carbonate de magnesium.
Der Einflufi van sekundaren Chromophoren auf die lichtinduzierte Oxidation von Papier. Teil II::
Der EinflujS von Licht auf Hol&chlijfpapier
Gegenstand der Untersuchungen ist Papier, dem Abbauprodukte zugefiigt wurden, wie sie durch
Oxidation in Holzschliffpapier gebildet werden. Fur den ersten Teil wurde reines Zellstoffpapier
entsprechend behandelt, fur den hier referierten zweiten Papiere, die solche Abbauprodukte be-reits
enthalten, namlich eben die Holzschliffpapiere, von denen die zusatzlich hinzugefugten Abbauprodukte gewonnen worden waren. Die Untersuchungen galten vor allem der Veranderung, die
bestimmte MeBparameter unter dem EinfluB von Licht mil hohen und mil reduzierten UV-Anteil
erfahren. Als Ergebnis werden Ratschlage zur Aufbewahrung zur Beleuchtung von Holzschliffpapieren gegeben, auch solchen, die eine Entsauerungsbehandlung mit methanolischer Losung von Methylmagnesiumcarbonat erfahren haben.
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Vladimír Bukovský, Mária Trnková
Slovak National Library
Preservation Service Branch
Nam. J.C. Hronskeho 1
036 01 Martin
Slovak Republic
E-mail: [email protected] - [email protected]