LIGHT AND HEAT INDUCED DEGRADATION OF ALKALINE PAPER
J. MALEŠIČ 1 *, J. KOLAR 1 , M. STRLIČ 2
1
National and University Library, Turjaska 1, SI-1000 Ljubljana, Slovenia e-mail: jcisna.malesic@nuk.uni-lj.si
2
Faculty of Chemistry and Chemical Technology, Ljubljana, Slovenia
In the field of paper conservation it is common practice to increase the longevity of acidic paper with various alkaline reagents [1, 2]. However, it was recently demonstrated, that use of aqueous magnesium bicarbonate solution triggers extensive degradation of bleached chemical pulps during accelerated thermal ageing [3], Although some researchers reported that such behaviour is not observed with actual papers [4, 5] a general consensus was reached in the conservation community to avoid the use of magnesium containing alkalis, if possible.
In order to enable development of more efficient non-aqueous deacidification solutions, the factors leading to enhanced degradation of alkaline paper during thermal [6] and photo-ageing need to be evaluated. The effect of carbonyl groups is the focus of the present paper.
It had recently been established that auto-oxida-lion of cellulose is the .main degradative route leading to depolymerisation of alkaline cellulose during thermal accelerated ageing [7|. It had also been suggested that carbonyl groups play an important role during oxidation of paper, as demonstrated by the stabilisation of pulps achieved after carbonyl groups were reduced by sodium borohydride treatment [8]. In addition, chemiluminescence studies indicate a promoted oxidation of bleached sulphite pulp in the presence of MgCO
3
while CaCO
3
SrCO
3
and BaCO
3
containing samples exhibit better oxidative stability, which was speculated to be due to carbonyls [9|. In addition, carbonyl groups have been shown to adversely affect colour stability of pulps during heat treatment
[10], which also indicates their importance during ageing processes. Oxidation of the carbonyl in cellulose molecule in alkaline medium may proceed by reaction with molecular oxygen. The ionisation of the intermediate enediol [11] may induce a pH dependency of the reaction. During the course of the reaction, superoxide, O
2
.
-
. is formed, which may undergo disproportionation and the hydroperoxides thus formed may lead to hydroxyl radical formation via the pseudo - Fenton mechanism [12].
To verify whether carbonyl groups influence stability of cellulose, we have determined their effect on the ageing stability of seven different cellulose pulps, deacidified by immersion into aqueous solutions of Ca(HCO
3
)
2
. 0.01 mol L
-1
or Mg(HCO
3
)
2
0.04 mol L
-1
. Carbonyl content of the samples prior to a 7-day accelerated ageing experiment at 80 ºC and 65 % RH in a Votsch Climatic test
Chamber Type VC 0020, were determined in triplicate using hydrazine method f I3J. As depicted in
Figure 1, a linear correlation is obtained between the carbonyl content and the decrease of degree of polymerisation (DP) during ageing. This confirms the importance of carbonyls during ageing of alkaline cellulose, in particular for the samples with higher pH values (containing MgCO
3
).

Fig. 1. Decrease of DP of deacidified bleached chemical pulps during ageing vs. carbonyl content.
♦ deacidified using Mg(HC0
3
)
2
□ deacidified using Ca(HCO
3
)
2
.
Fig. 2: Decrease of DP of deacidified bleached chemical pulps reduced using NaBH
4
during ageing vs. carbonyl content. ♦ deacidified using Mg(HC0
3
)
2
; □ deacidified using Ca(HCO
3
)
2
.
To verify whether carbonyl groups are indeed the source of "alkali sensitivity" of pulps with higher carbonyl content, cellulose pulps were reduced using 0.01 mol L -1 aqueous solution of sodium borohydride for 24 h at room temperature, washed and deacidified using magnesium or calcium bicarbonate.
As evident from Fig. 2, the reduced samples indeed no longer exhibit pH sensitivity, as there is only a slight difference in ageing stability of the differently deacidified pulps. This suggests that carbonyl groups are likely to be responsible for a difference in pH sensitivity of papers from bleached chemical pulps and cotton ones.
Having established the detrimental effect of carbonyl groups on the stability of alkaline paper, we were interested in their role during the photo-induced degradation. Cellulose is considered to be quite stable, when exposed to light from the visible and near UV regions [ 14|. Our subsequent experiments focused on the effect of daylight on two cellulose samples - purified cotton paper and bleached sulphate chemical pulp. The untreated, calcium and magnesium carbonate containing samples were exposed to daylight from south facing window for three years and additional four years stored in darkness. The extensive degradation (up to 78 % decrease of DP for purified cotton and up to 76 % for sulphate pulp) of the exposed samples was quite surprising (Fig. 3).
Fig. 3. Degree of polymerisation of cotton (S&S) and sulphate (Sa) cellulose pulps after natural light ageing. O-untreated, C-calcium carbonate and M-magnesium carbonate containing paper.
The DP decrease during thermal ageing at 80 "C and 65 % RH during the 28 days was less than half of the one obtained during photo-oxidalive ageing. The most extensive degradation was observed

for samples containing magnesium carbonate. Interestingly, deacidification introducing either calcium or magnesium carbonate destabilised sulphate pulp suggesting a negative effect of either alkaline pH or the alkali earth metal ion.
In our previous experiments [6], decrease of brightness during thermal ageing correlated with the decrease in the degree of polymerisation. During light ageing, the situation is reverse, as the most degraded samples containing magnesium carbonate retained the highest brightness values (Fig. 4).
However, as observed during thermal ageing of cellulose, brightness correlates (R
2
= 0.864, n = 6) with the content of carbonyl groups, which is lowest in samples containing magnesium carbonate.
Fig. 4. Brightness (diffuse reflectance at 457 nm) of cotton (S&S) and sulphate (Sa) cellulose samples after natural light ageing experiment. O-untreated, C-calcium carbonate and Mmagnesium carbonate containing paper.
Results of accelerated ageing tests indicate, that high pH triggers increased degradation of samples during thermo-oxidative accelerated ageing, causing higher instability of bleached chemical pulps deacidified using magnesium bicarbonate solution. In reality, these samples are never deacidified in conservation workshops, as paper made from these pulps always contains additives. This induces a decrease in pH and increases the stability of such papers during ageing after deacidification with magnesium bicarbonate. Nevertheless, until a variety of samples are tested, caution is still required when using magnesium bicarbonate solutions for deacidification. In addition to paper degradation, other factors, such as lightening of iron gall ink when deacidified using magnesium bicarbonate should be taken into consideration, as well.
Interestingly, deacidification introducing either calcium or magnesium carbonate destabilised bleached chemical pulp during light ageing, suggesting negative effect of either alkaline pH or alkali earth metal ion on its stability. In addition, the samples deacidified with magnesium bicarbonate, were the most degraded yet exhibited the highest brightness stability.
7. ACKNOWLEDGEMENT
The authors gratefully acknowledge the support of the European Commission, Fifth Framework
Pro- 8. gramme, Key Action "City of tomorrow and cultural heritage" within the Energy,
Environment and Sustainable Development, Contract n° EVK4-CT- 9. 2000-0038,
PAPYLUM. The work is the sole responsibility of the authors and does not represent the opinion of the Community. The Community is not responsible for any use that might be made of the data appearing herein.
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