Gamma Irradiation of Washi
Part 2*: Changes in degree of polymerization and crystallinity
of cellulose
by ABDEL-SALAM EL-ESSEILY & MASAMITSU INABA
introduction: previous research
In conservation, gamma irradiation has been used for several specific purposes. Its application is based on the high penetration power of gamma rays and their high energy. Gamma irradiation has been used for the destruction of various fungi on several types of paper. Gamma irradiation under nitrogen at 3 kGy has been reported as destroying all fungal spores tested within a 24 hour period at 50°C and 95% RH'. One early application of gamma irradiation in conservation aimed at developing a process for the strengthening and protection of paper. Graft co-polymerization seemed to offer this potential. The basic concept is to create radical sites on the cellulose macromolecule, attach monomer to them and provoke polymerization. The final result is a polymer chain, which grows from the cellulose and forms, so to say, a backbone2.
High energy irradiation causes excitation and ionization leading to the degradation of the molecules and the creation of free radicals. The accompanying structural changes are in many respects analogous to those observed when cellulose is oxidized, finally resulting in the loss of tensile strength, reduced degree of polymerization and an increase in the number of carboxyl and carbonyl groups1.
Keeney et. al.4 reported that, when irradiated in an electron accelerator, paper showed a decrease in burst strength, tear strength and tensile strength as the dose increased, regardless of the pulp used. They also concluded that the loss in strength appeared to be in line with the dosage, with an overall reduction of about 25%, going from 0 to 80 kGy, suggesting, as a rule-of-thumb, a reduction in strength of 0.3-0.4% for each kGy.
Kubát1 remarked that the changes were small as long as the irradiation dose did not exceed 1 kGy, but that they increased rapidly at higher irradiation levels.
He also mentioned that for a given dose the number of carbonyl groups formed was approximately 20 times greater than the number of carboxyl groups. The most probable sites for the formation of stable free radicals on glucose units are the carbon atoms in positions 1 and 5 or even 2 and 6. Kubat added that the viscosity changed inversely with the storage time because the irradiated cellulose continued to degrade in the presence of oxygen. It has been reported that up to one third of the after-effect was due to the production of alkali instable cellulose. Gamma irradiation does not affect moisture absorption to any great extent. Although a decrease in the degree of polymerization depends on the chain length, it does not necessarily match a decrease in strength.
Butterfield6 has studied the effects of gamma irradiation on four types of pulp: pure rag pulp, two kinds of mixed rag pulps and mechanical wood pulp. Treatments of the paper samples included gamma irradiation with 10 kGy, dry thermal ageing and gamma irradiation followed by thermal ageing. She found that gamma irradiation caused a decrease in the mechanical strength of the papers similar to that resulting from dry thermal ageing. A synergistic effect was found when both irradiation and thermal ageing were performed on the paper samples. The reduction in fold endurance and tear resistance was higher as a result of irradiation followed by thermal ageing than that resulting from either gamma irradiation or accelerated ageing.
Recently, Adamo et al.7,8,9 have undertaken a series of research aimed at finding a new system for the decontamination of books and documents stored in libraries and archives, that are damaged by biological organisms, such as insects and mildews. They also pointed out that tensile strength and degree of acidity were not affected by irradiation up to lO0 kGy before accelerated ageing.
In brief, irradiation resistance is thought to depend on the composition of the paper, with lignin providing the cellulose with some protection against gamma irradiation. In addition, the moisture of the paper plays an important role in the natural ageing process4
Ueno et. al.10 studied the effect of irradiation on paper and paperboard. They concluded that irradiation had a very slight effect on the brightness of the samples at a dosage below 100 kGy, namely a reduction of ca. 2% change, and thatthere was no effect below 25 kGy. Dry-tensile, burst and tear strength reduction were directly proportional to the dosage especially above 25 kGy. On the other hand, wet-tensile strength increased proportionally to the dosage. Fold endurance and degree of polymerization both decreased with increased dosage due to fiber deterioration.
For restoration of historic papers damaged by vermin, it is preferable that the quality of paper used for repair resembles that of the original. In our previous article11 we aimed at modifying paper by using gamma irradiation (Cobalt 60, 49-
390 kGy), thermal ageing (80°C 60%RH) and a combination of both for suitably preparing modern Japanese paper for repairing historic paper objects. Generally, all treated samples showed colour changes due to both thermal ageing and irradiation (Fig.la). However, irradiated samples followed by thermal ageing showed greater discolouration, proportional to their irradiation doses, than samples thermally treated and then irradiated. This difference can probably be attributed to reactions occurring during irradiation and resulting in products which, by further reactions occurring during accelerated ageing, cause discolouration.
The samples irradiated at 390 kGy had deteriorated too severely to be measured for folding endurance. Kozosamples, which were thermally aged, showed high stability against folding, in contrast to the irradiated samples (Fig.1b). The samples, which were first irradiated and then thermally aged, showed a remarkably rapid decrease in folding endurance.
For the continuation paper presented here we measured degree of polymerization and crystallinity of cellulose, in order to clear up the effect of gamma irradiation and thermal ageing on washi in molecular order.
experiment
• Sample: Kozo paper: Hon-mino-gami; grammage: 22-29 g/m2; density: 0.32-0.38 g/cm3 pH of 6.9.
• Accelerated ageing: 80°C 60% RH 1, 2, 3 and 4 weeks.
• Irradiation: Cobalt 60 source at 49, 98, 195 and 390 kGy over 48 hours.
Selected samples were aged both thermally and by gamma irradiation. Some were thermally aged for 2 and 4 weeks and then irradiated according to the programme of irradiation doses; others were exposed to 98 and 390 kGy then thermally aged.
Crystallinity was measured by X-ray diffractometry; average degree of polymerization was determined by the viscosity measurement of nitrocellulose derivatives as in a previous paper12.
results and discussion
Degree of polymerization
The DP of thermally aged samples decreased gradually, that of irradiated samples drastically. Thermal treatment decreased the DP proportionally to the ageing
time. Thermal ageing of the irradiated samples resulted in a faster DP decrease than that of the only thermally aged samples (Fig. 1d). DP decreased gradually with longer period of thermal ageing, while folding endurance suffered only little reduction. (Fig. Ib). This was in accordance with previous results13, where it was found that after four weeks of thermal ageing the degree of polymerization was still at ca. 700, above which the strength properties did not change remarkably.
Crystallinity
It was found that the crystallinity of Kozo paper increased as the time of thermal ageing and radiation dose increased (Fig. 1c). Part of the increase, especially in the latter case, probably took place because of several humidity cycle treatments
(30°C 32% RH to 84°/oRH) for dimensional stability test before measuring crys-tallinity. The crystallinity of the samples was increased by both gamma irradiation after thermal and thermal ageing after irradiation. However, the latter sequence increased crystallinity more than the former.
conclusion
During thermal ageing DP changed gradually with time, while it changed drastically with the increase in radiation dose. The strength of the samples changed noticeably only when the DP dropped below about an average of 700. Crystallinity of kozofiber increased with gamma irradiation and with thermal ageing, in the same manner as the DP decreased. These changes were most noticeable when irradiation followed thermal ageing. These results explain well the decrease of physical strength as reported in a previous paper11.
acknowledgement
We would like to acknowledge the support for our experiments of Fukutake Science and Culture Foundation, of the Centre for Atomic Energy (The University of Tokyo) and of The Japan Atomic Energy Research Institute (Takasaki Institute). Furthermore, we would like to acknowledge Asahi Shimbun for granting a fellowship to El-Esseily.
summaries
Gamma Irradiation of Washi (Japanese Paper)
To simulate the properties of old, i.e. naturally aged paper, so that it would be suitable for the restoration of historical paper objects, modern Japanese paper (Kozo) was subjected to accelerated ageing techniques: thermal ageing at 80°C and 60% RH over periods of 1, 2, 3 and 4 weeks, and irradiation with a cobalt 60 source with doses of 49, 98, 195 and 390 kGy over a total period of 48 hours.
During thermal ageing degree of polymerization changed gradually with time, while it changed drastically with increased irradiation dosage. The strength of the samples changed noticeably only when the DP dropped below ca. 700. The crystallinity of kozo fiber was increased by gamma irradiation and thermal ageing, and paralleled the decrease of DP. These changes were most noticeable when irradiation followed thermal ageing. They explain well the decrease of physical strength reported in our previous paper.
Irradiation de papier japon (Washi) aux rayons Gamma
Afin de reproduire les proprietes du papier ancien, c'est-a-dire du papier vieilli naturellement, pour qu'il soil apte a la restauration de documents historiques, on a soumis du papier japon mo-deme (Kozo) a differentes techniques de vieillissement accelere : vieillissement thermique a 80°C et 60% d'humidite relative sur des periodes de 1, 2, 3 et 4 semaines ainsi qu'a ['irradiation d'une source de cobalt 60 a des doses de 49, 98, 195 et 390 kGy sur une periode de 48 heures.
Au cours du vieillissement thermique le degre de polymerisation a varie graduellement avec le temps alors que pendant Firradiation il a change radicalement lorsque les doses augmentaient. La resistance mecanique des echantilons n'a change significativement que lorsque le DP tombait au-dessous d'environ 700. La cristallinite de la fibre Kozo a augmente au cours des deux traite-ments, irradiation gamma et vieillissement thermique, parallelement a la reduction du DP. Ces changements etaient plus evidents lorsque rirradiation etait realisee apres le vieillissement thermique. Us montrent bien la reduction de la resistance mecanique evoquee dans notre article precedent.
Gammabestrahlung von Washi (Japanpapier)
Um neues Papier altem anzugleichen, auf daB es geeignet werde zur Restaurierung historischer Papierobjekte, wurde modernes Japanpapier (Kozp) zwei verschiedenen Methoden der beschleu-nigten Aliening unterworfen: Hitzealterung 80°C 60% RF iiber 1, 2, 3 und 4 Wochen; Bestrah-lung.mit einer Kobalt-60-Lampe bei 49, 98, 195 und 390 kGy iiber insgesamt 48 Stunden.
Wahrend der Hitzealterung anderte sich der Polymerisationsgrad allmahlich mit zunehmen-der Dauer; wahrend der Bestrahlung anderte er sich drastisch mit zunehmender Dosis. Die rnechanische Festigkeit zeigte eine signifikante Anderung erst wenn der DP unter ca. 700 sank. Die Kristallinitat der Ko^o-Faser nahm bei beiden Behandlungen zu, parallel zum Riickgang des DP. Diese Anderunger waren bei der Behandlungsfolge Bestrahlung und Aliening deutlicher als bei der umgekehrten.
references
1. Flieder, F.: Disinfection of paper using gamma-rays, electron beams and microwaves. Biodeteriora-tion of Cultural Property. 3rd International Conference. 3 (Bangkok 1995): 283-401.
2. Butler, C.E., C.A. Millington & D.W.G. Clements: Graft polymerization; a means of strengthening paper and increasing life expectancy ofcellulosic archival material. Historic Textile and Paper Materials II. Conservation and Characterization. Washington: American Chemical Society 1989: 34-53.
3. Philips, C. O., PJ. Baugh,J.F. Makellar & C. Von Sonntag: Interaction of irradiation with cellulose in the solid state. Cellulose Chemistry and Technology. American Chemical Society Symposium Series 48 (1977): 313-333.
4. Keeney, N. H.,J. H. Reynoldsjr. & D. A. Trageser: Effects of irradiation of handsheets. Tappi 51 (1968): 47A-49A.
5. Kubat, J., S. Martin-L6f & A. De Ruvo: The effect of gamma radiation on some paper properties. Svensk Papperstidnig 23 (1968): 851-856.
* Author to whom correspodence should be addressed