Paper Conservation Using Aqueous Solutions of Calcium Hydroxide/Methyl Cellulose

1. Preparation of the solution by FRANCISKA SUNDHOLM & MARIA TAHVANAINEN

introduction

A paper conservation treatment utilising calcium hydroxide together with methyl cellulose in aqueous solution was developed as a mass-deacidification process at the Austrian National Library in Vienna¹. This method was further developed and has been applied at the Centre for Microfilming and Conservation of Hel­sinki University Library since 1993². In this treatment, the paper is immersed for 30 min in a 0.01M calcium hydroxide solution containing 0.5w% of methyl cellu­lose. The aim of this treatment is to reduce the acidity of the paper and, in addi­tion, to create an alkaline reserve in the paper³-⁴''. We describe in this report, our findings from ongoing studies of this method, which show that the way in which the treatment solution is prepared has a dramatic effect on the alkaline reserve of the paper.

materials

Two paper samples were tested:

• a neutral paper, which consisted of 100% bleached sulphite softwood cellulose, without filler or sizing, prepared in 1991,

• an acidic paper from a book printed in 1903, the composition of which is 94.3% rag-cellulose fibres and 5.7% china clay and aluminium resinate sizing.

The neutral paper was provided by the TNO Centre for Paper and Board Re­search, Delft, the Netherlands. It was stored in die dark at a temperature of 23°C and 50% relative humidity in the TNO Centre. It should be noted that the name "neutral paper" indicates in our article the absence of acidity from manufacturing contents of paper such as fillers and sizers.

Paper Conservation Using Aqueous Solutions of Calcium Hydroxide/Methyl Cellulose

The chemicals which we used for the preparation of the conservation solution, were

• calcium hydroxide, Ca(OH)₂, supplied by Riedel de Haen, pro analyst,

• methyl cellulose MC-60, Zellura MC 60, supplied by Henkel Ltd.

MC 60 refers to the Brookfield viscosity (mPa-s) measured in 2% aqueous solution at room temperature².

methods

• Liquid phase method (LPM). First a lw°/o MC-60 solution was prepared: 15 g of the powder was swollen for 1 hour in 100 ml distilled water at 80°C and then diluted with 400 ml distilled water to 500 ml. The solution was cooled in an ice bath for half an hour, diluted with 1 1 of distilled water to 1.5 1 and stirred over night. Finally 1.5 1 of 0.02 M Ca(OH)₂ solution was added.

• Solid phase method (SPM). 2.23 g Ca(OH)₂ were put into 3 1 of 0.5 w°/o aque­ous MC-60 solution, which was prepared in the same way as in LPM.

In the following we shall refer to the treatment by the first solution as liquid phase method treatment or LPM, and to the treatment by the second as solid phase method treatment or SPM.

For the conservation treatment, a stack of 13 sheets of paper was placed in a plastic box 450x300x100 mm; the size of the sheets was approximately A4. The acidic paper was immersed in tap water at 40°C for 30 minutes, rinsed with tap water at 40°C for about 15 minutes and immersed in 3 1 of LPM or SPM re­spectively for 30 minutes. The neutral paper was immersed in 3 1 of the solutions without any pre-treatment. After the conservation treatment, both papers were dried at room temperature.

To estimate the permanence of the treated papers we used an accelerated age­ing procedure: 90°C, 50% RH for 12 days. The choice of these conditions was based on the results obtained in the STEP CT 90-0100 project. It was noted that the accelerated ageing over 12 days at a temperature of 90°C and relative hu­midity of 50% brought about approximately the same deterioration of paper as ac­celerated ageing according to ISO 5630-1 (1991), i.e. 80°C, 60% RH, for 24 days (ref. 3 p. 38). The accelerated ageing experiments were made at the TNO Centre for Paper and Board Research, Delft, the Netherlands.

paper analyses

All paper analyses were done on samples from stacks of 13 paper sheets; thus eliminating the effects of uneven distribution of chemicals and paper composition.

• pH. The pH of cold water extract was determined according to ISO 6588 (1981) procedure with the modification that the pH of the paper extract was measured in a 0.1M sodium chloride solution^6.7.

• Alkaline reserve. The alkaline reserve was measured according to ISO 10716 (1994). This procedure was modified by determining the endpoint of the titra-tion potentiometrically from the solution prepared for pH measurements'.

• Degree of polymerisation. We measured number average degree of polymeri­sation (DPn) and weight average degree of polymerisation (DPw). Details on the procedure are given in the annexe.

• Tensile strength. The tensile strength was measured using Alwetron TH 1 tester according to the SCAN-P 38:80 standard. The geometric mean of the tensile strength in the machine and cross direction of the paper was used.

• Zero-span tensile strength of paper, wet and dry. The Pulmac Zero Span Tester and short-span analysis were used to evaluate the dry and wet zero-span ten­sile strength⁸. The measuring procedure was carried out according to the ISO 15361:2000(E) standard. The short-span measurements were done only in the machine direction of the paper for both samples. Fibre bonding in sheets of paper was evaluated as dry/wet span ratio for span value 0.6⁹.

• ISO-brightness of paper. The ISO-brightness of the samples was measured by Elreoho-2000 reflectometer according to the SCAN-P 1:75 standard.

• Scanning electron microscopy. In order to evaluate the distribution and size of calcium carbonate crystals in the treated papers micrographs of the samples were taken using a DSM 962 scanning electron microscope (Germany, LEO). The samples were cut into 0.5 cm² squares, which were coated with platinum by vacuum evaporation.

The tensile strength, zero-span strength and ISO-brightness measurements were performed at the Laboratory of Paper Technology, Department of Forest Products Technology, Helsinki University of Technology, Espoo (Finland). For scanning electron microscopy, the Electron Microscopy Unit, Institute of Bio­technology, University of Helsinki (Finland) was used.

RESULTS AND DISCUSSION

As we have mentioned above the conditions of the calcium hydroxide/methyl cellulose conservation treatment for the manual deacidification of paper were

studied and developed in the frame of the "Deacidification Project" at the Hel­sinki University Library (ref. 2 p. 44). However, for the neutral paper no pre-treatment was used. The main aim of the immersion and rinsing stages was to remove water-soluble degradation products and contaminants as well as to im­prove the penetration of the conservation solution into the paper. The neutral pa­per was relatively new and had been stored under archive conditions so that no contamination was observed (ref. 3, p.59). It is also important to note that the penetration of the conservation solution into the neutral paper was very fast be­cause this paper contained no sizing.

The recommended level of alkaline reserve in an archival paper is 400 mmol/ kg^4.5. The purpose of the conservation treatment of old paper is to build up an alkaline reserve which decreases the destructive effects of aluminium resinate sizing and acidic degradation products formed in the paper during storage. The SPM treatment resulted in a much higher alkaline reserve in both papers than those treated by the LPM (cf. Table 1 and Fig. 1}. One possible explanation for this difference is that the dissolution of calcium hydroxide particles in the aque­ous methyl cellulose solution was hampered by the formation of loose aggregates between calcium hydroxide and methyl cellulose because methyl cellulose is known as a protective colloid¹⁰. The difference between the two treatments is seen clearly from micrographs of the treated papers. A comparison of figures 2A and 3A as well as 4A and 5A revealed that the precipitated calcium hydroxide was more voluminous in the SPM treatment. Furthermore, the calcium carbonate crystals formed after the SPM treatment were larger than the crystals formed after the LPM treatment, see Figs. 2, 3, 4, 5.

pH measurements of the cold extracts of the papers were carried out to evaluate the effect of the conservation treatment on the original acidity levels. Generally, the pH of the cold extract of paper depends on several factors, such as the original acidity of pulp, fillers and sizing, as well as on the age of the paper and the storage conditions. It should be noted that the pH of cold extracts and the acidity of the paper as the total number of acidic groups in the paper, correlate with each other only for papers prepared from the same pulp type⁷. In addition, the pH of the cold extract does not reveal the true pH in the fibre in the dry state¹¹. The pH of cold extract of the untreated neutral paper (5.54) indicated the original acidity of the sulphite pulp from which it was made and the presence of the degradation products formed during storage under the conditions described above. In the case of the acidic paper, the pH of the cold extract of the untreated paper (4.54) in-

Table 1: Measured data and relations

The main reason for paper deterioration is cellulose degradation¹¹. This is mani­fested in a significant and irreversible decrease in the molecular mass of cellulose. For the method to measure molar mass distribution (MWD) in this study see the annexe. For both the neutral and the acidic papers MWD did not change after die conservation treatments; they were 2.33 and 2.18, respectively. Therefore we used only the number average degree of polymerisation values (DPn) to evaluate possible cellulose degradation processes.

The evaluation of the DPn changes after a conservation treatment is very im­portant, because, if reducing groups are present in the cellulose molecules at a high pH, diey can initiate the "peeling-of" reaction of cellulose, which results in the decrease of the DPn. Besides, transition metal ions such as Fe²⁺ or Cu⁺, which occur in inks (including printing inks) and paper fillers, can promote the oxidative degradation of cellulose in alkaline media^12,13,14.

For the untreated neutral paper DP_n was 1475. It is difficult to draw any definite conclusions from this number because there are no reports of DPn measurements for the same type of pulp (bleached sulphite) at the same elution temperature (30°C). On the other hand, the DP of bleached bisulphite pulp calculated by-others¹⁵ from the intrinsic viscosity in cupriethylenediamine solution was in the same range as our result, i.e.1340.

Fig. 4: Electron micrographs of acidic paper treated by SPM, before (A) and after (B) ageing magnification 5OOOx.

Fig. 5: Electron micrographs of acidic paper treated by LPM, before (A) and after (B) ageing; magnification 5OOOx.

Fig. 6: Fibre bonding of papers treated with solutions of Ca(OH)₂ and methyl cellulose by solid phase method (SPM) and liquid phase method (LPM).

The DP_n of the papers used for this study was not changed by the calcium hydroxide/methyl cellulose conservation treatment, whether applied by SPM or by LPM. This conservation treatment does not cause the degradation of the cellu­lose.

Tensile strength of the papers increased after both SPM and LPM treatments. This is probably due to the strengthening effect of the methyl cellulose. The wet zero-span tensile strength of the paper is a criterion for fibre strength. The tensile strength of the neutral paper did not change significantly after either the SPM or the LPM treatments. We estimated the fibre bonding of the papers as dry-to-wet zero-span tensile strength ratio for 0.6 span value⁹. The fibre bonding of the neu­tral paper increased after both the SPM and LPM treatments (Fig. 6). It is clear that methyl cellulose increased the interfibre bonding in the neutral paper and did not influence the fibre strength.

In the case of the acidic paper the situation was complicated by the presence of the aluminium resinate sizing in the paper. Wetting of the paper strip, as it oc-

Paper Conservation Using Aqueous Solutions of Calcium Hydroxide/Methyl Cellulose

curs at the wet zero-span measurements for an elimination of hydrophilic inter-fibre bonding, cannot eliminate the contribution of the hydrophobic aluminium resinate sizing on the strength of the acidic paper. Thus, the wet zero-span value is not the measure of the fibre strength only, but includes the influence of the res­inate matrix as well¹⁷. Therefore, we concluded that the wet zero-span tensile strength measurement did not give a complete picture of the influence of the methyl cellulose on the fibre strength of the acidic paper.

The LPM treatment increased the tensile strength and fibre bonding of both neutral and acidic papers to a greater degree than the SPM treatment. The strength­ening effect of the LPM treatment was more pronounced than that of the SPM treatment. We drew the conclusion that the distribution of the methyl cellulose in the paper was less homogeneous with the SPM than with the LPM. This may be due to differences in the formation of the calcium hydroxide/methyl cellulose ag­gregates during the preparation of the solution.

Brightness

Paper brightness is considered a very important factor for aesthetics and readability. We found that the calcium hydroxide/methyl cellulose conservation treatment did not affect the brightness of the neutral paper, irrespective of the preparation method. In the case of the acidic paper the ISO-brightness increased probably due to the pre-washing with water, but the calcium hydroxide /methyl cellulose treatment did not affect the brightness.

In order to study the influence the preparation method of the conservation solu­tion has on paper permanence, both the LPM and SPM treated papers were sub­jected to accelerated ageing. The pH (cold aqueous extract) decreased slightly, obviously caused by the formation of the usual cellulose degradation products: formic, laevulinic, oxalic, saccharinic acid, acetaldehyde^{17, 18}.

For the untreated acidic paper, the decrease in pH after ageing was quite small: see Table 1 and Fig. 1, that is quite typical for old acidic papers (ref. 2, p. 89 and ref. 3, p. 187). The drop in pH of the SPM treated neutral paper was less than of the LPM treated neutral paper. The acidic paper showed the reverse. The alka­line reserve of the treated papers decreased in the ageing process, but the loss for the neutral paper was greater than for the acidic paper: ca. 10 units for die first and ca. 5 units for the last. From electron micrographs of the papers treated by

the SPM and the LPM we saw that, after ageing, the calcium carbonate crystals became smaller in both the neutral and acidic papers (Figs. 4 and 5). It is impor­tant to note that the decrease in the pH and the alkaline reserve of treated papers was indicative of similar processes in the paper, which resulted in the formation of acidic substances. As discussed above the pH of cold extract of paper did not reflect the true pH of the paper fibres; the true pH of the fibres in the treated pa­pers, having a moisture content was 5-6%, could be much higher than the numbers given in Table 1. It was shown by Begin et al.⁷ that even at as high an alkaline reserve as 2% calcium carbonate or 400mmol/kg, a slow degradation process is possible. The results represented by Haas et al.¹⁹ denoted the alkaline degradation of hydrocellulose at such low a temperature as 65°C. The alkaline degradation of hydrocellulose below 170°C causes the formation of 3-deoxy-2-C-(hydroximethyl)-pentonic acids and the presence of calcium ions improves their yield²⁰. The water-insoluble products of hydrolytic degradation formed during storage and their re­actions in alkaline media also influence the pH and the consumption of the alkaline reserve. In addition, the alkaline degradation of methyl cellulose may be re­sponsible for the decrease in pH and alkaline reserve.

A small decrease in the DPn after the accelerated ageing can be seen for the treated samples of both neutral and acidic papers, but the changes are not signifi­cant: less than 8% maximum. Taken into account these small changes and the above mentioned decreases in the pH and alkaline reserve we came to the con­clusion that the pH, alkaline reserve and DPn decreased could be due to the re­actions of the hydrolytic degradation products present in the papers before the conservation treatment, as well as the oxidation (ref. 21, p. 256) and degradation of cellulose and methyl cellulose in alkaline media. It should also be noted that the alkaline reserve added to the acidic paper was good enough to protect the pa­per cellulose against acid-catalysed hydrolytic degradation during accelerated ageing, irrespective of the method used to prepare the conservation solution.

The tensile strength of the untreated neutral paper and the neutral paper treated with the LPM decreased only very little during accelerated ageing; for the SPM treated neutral paper the loss was more than double. In the case of the acidic pa­per a loss of 11% for the treated and more than double (25%) for untreated paper was observed. The wet zero-span tensile strength of the neutral paper suffered a slight decrease in the case of the LPM treatment and virtually none in the case of the SPM. The wet zero-span tensile strength of the acidic paper did not change irrespective of the treatment. For the untreated papers it decreased during ac­celerated ageing by 5.7% for the neutral and by 40.1% for the acidic paper. The fibre bonding decreased for all treated and untreated papers in the ageing process (Fig. 6). As the fibre strength of the acidic paper treated by SPM or LPM and aged did not change but the tensile strength decreased, the changes in SPM or

LPM treated acidic paper after ageing can be explained rather by the degradation of methyl cellulose than the degradation of cellulose.

For both papers the decrease of the ISO-brightness caused by accelerated age­ing was greater for the untreated than for the treated samples; for the latter both treatments resulted in approximately the same loss. These changes can be ex­plained by the conversion of the end-wise degradation product (4-deoxy-D-glycero-2,3-hexodiulose) into a yellow compound (-diketone) (ref. 21, p.237).

conclusions

• Our study showed that the SPM treatment was superior to the LPM treatment with respect to the alkaline reserve. The alkaline reserve decreased during ac­celerated ageing both in the neutral and in the acidic paper, but it remained at a level sufficient for paper protection against the acid-catalysed hydrolysis, es­pecially in the acidic paper.

• The strengthening effect of the LPM treatment was greater for both the neutral paper and the acidic paper than the effect of the SPM treatment.

• In the case of the neutral paper, a higher alkaline reserve was deposited by the SPM treatment. This provided good protection against cellulose degradation during ageing. In the acidic paper, the accelerated ageing procedure used in this study was not appropriate to estimate the difference between the LPM and SPM treatments.

• Both treatments protected the papers against deterioration compared to the untreated paper.

acknowledgments

The authors wish to thank:

• Professor Hannu Paulapuuro of Laboratory of Paper Technology, Department of Forest Products Technology, Helsinki University of Technology for making possible the evaluation of mechanical and optical properties of the papers,

• Jyrki Juhanoja and Mervi Lindman for the electron micrographs,

• TEKES, National Technology Agency of Finland for financial support of this work.

annexe: DP measurement

Number average DP (DPn) and weight average DP (DP_w) were calculated from the molecular weight distribution (MWD) of the cellulose. MWD of cellulose was determined by gel permeation chromatography (GPC). The GPC was performed using a mixture of N,N-dimetylacetamide (DMAc) and 0.8% lithium chloride as eluent, three columns connected in series (Waters Styragel HR6, HR4, HR2) and a differential refractometer (Waters 410) thermostatted at 30°C. The injection vol­ume was 20 pi and contained 1 mg/ml of sample filtered on a 45 pi filter. The flow rate of eluent was 1 ml/min. Pullulan Shodex P82 standards (Shadeko) with narrow molecular weight distribution were used for calibration curve. The sam­ples were prepared according to the procedure described by E. Sjoholm et al.²¹. We limited the determination of the accuracy of the DP measurements to only two GPC runs. The difference between them for the untreated neutral paper was 31 monomer units.

summaries

Paper conservation using aqueous solutions of calcium hydroxide/methyl cellulose. 1. Prepara­tion of the solution

The results of the paper conservation treatment using Ca(OH)₂/methyl cellulose solutions pre­pared by two different methods were estimated by measuring chemical, mechanical and optical properties of treated and untreated papers. The permanence of papers treated was tested by ac­celerated ageing. The results obtained showed that treatment with the solution prepared by mix­ing solid Ca(OH) ₂ with methyl cellulose solution (solid phase method, SPM) resulted in a higher alkaline reserve than the treatment with a solution prepared by mixing a solution of Ca(OH) ₂ with methyl cellulose solution (liquid phase method, LPM). The alkaline reserve deposited by SPM gave better protection during accelerating ageing.

Les résultats du traitement de conservation des papiers traités et non traites au moyen de solu­tions aqueuses de Ca(OH) ₂/méthylcellulose préparées de facons différentes ont été soumis a différents tests de mesures des propriétés chimiques, mécaniques et optiques. Afin de déterminer la permanence du papier a long terme les échantillons de papier ont été soumis a un traitement de vieillissement accéléré. Les résultats révelent que la solution préparée a partir des deux substan­ces sous leur forme solide Ca(OH) ₂ et la cellulose de méthyle (solid phase method, SFM) aboutit a une reserve alcaline supérieure a celle de la solution preparee a partir du melange des deux substances liquides isolees (liquid phase method LFM). La réserve alcaline supérieure deposee par SFM assure une meilleure protection lors du traitement de vieillissement accéléré.

Anhand von chemischen, mechanischen und optischen Meβparametem wurde die Wirkung einer konservatorischen Behandlung von Papier mil wäβrigen Losungen von Ca(OH)2 und Methyl-cellulose untersucht, die auf verschiedene Weise zubereitet waren. Zur Pruning der langfristigen Bestandigkeit wurden die Proben einer beschleunigten Aliening unterworfen. Die Ergebnisse zeigen, dafi die aus der festen Form der beiden Substanzen hergestellte Losung (SPM) zu einer hoheren alkalischen Reserve führt als eine Mischung aus den getrennt gelosten Substanzen (LPM). Diese höhere alkalische Reserve hat eine bessere Resistenz gegen die Vorgänge bei be-schleunigter Aliening zur Folge.

references

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