Performance of Improved Poly vinyl alcohol as an Ageing Resistance Agent
by ALTAF H. BASTA
introduction
It is well documented that acid hydrolysis of cellulose and related carbohydrates is one of the key factors responsible for the degradation of paper during ageing1. Agricultural waste, and later wood, have been used in the manufacture of paper since the eighteenth century, and rosin, precipitated to the fibre by alum, became the favourite sizing agent2'3. Alum-rosin size can contribute to an increase in the acidity of paper. Paper produced since the introduction of alum is subject to yellowing, embrittlement, and as a result, loss in strength4. A number of deacidifica-tion techniques have been developed to reduce paper acidity and thus to lessen its deterioration. However, while most treated papers degrade less rapidly, some results from accelerated ageing experiments have shown that there can be increased degradation of paper the pH of which has been changed from acidic to alkaline using deacidification treatments5' 6.
Literature concerned about the permanence of paper has shown that rosin-sized paper has a lower thermal decomposition temperature than unsized and AKD-sized paper. Paper sized with hydrogenated rosin is considerably more stable than paper sized with natural rosin. Also, paper sized with starch and animal glue have shown relatively less degradation than those papers which are rosin sized7'8. Koura and Krause9 reported that impregnation of groundwood-containing paper with ethyl- or butyl-acrylate improved ageing resistance. Also, numerous researchers have studied the strengthening of weakened paper. On the whole, impregnation with cellulose ether and polyvinyl alcohol have been suggested10'11. Another method, which has been suggested, is graft polymerization12' 13 Recently, the consolidation property of Parylene for historical books14'15 has been studied.
In our earlier work in this field we demonstrated that Egyptian permanent paper is prepared from rosin-alum sized local pulps by incorporating the sodium silicate in sizing system to control the pH of the sizing system to pH 6.7-7.04'16, or by surface treatment the rosin-sized paper with chitosan-sodium silicate sys-
tems17. In this present work we evaluate the PVA-borax system used as a treating agent to produce durable rosin-sized papers, and as a strengthening polymer of weakened paper, i.e., a polymer for restoration purposes, in comparison with an application of standard PVA alone.
experimental
Paper samples
The samples for our research were made from a mixture of bleached pulps:
• 20% wood pulp,
• 45-50% rice straw,
• 20-25% bagasse,
• 5-10% repulped paper.
Other characteristics: grammage ca. 50 g/m2; 0.7% rosin; alum to adjust the pH to ca. 4.5-5.5. Producer: RAKTA Paper Mill, Alexandria, Egypt.
Treating agents
Polyvinyl alcohol: PVA (Mowiol grade 28-99), a product of Hoechst Co. (Germany). Specifications:
• Degree of saponification: 99.4 %,
• Viscosity of 4% aqueous solution: 28 cP,
• Ester no.: 8 mg KOH/g,
• Residual acetyl: 0.5,
• Ash: 0.5%
Borax: a product of Adwic Chemical Co., Egypt.
Treatment
The samples were dipped for two minutes at room temperature (ca. 22°C) into different concentrations of PVA (0.25-1% wt/vol H2O) and then into a 2% solution of borax. The treated samples were allowed to dry for 2 h in an electric drum.
Ageing
Untreated and treated paper samples were submitted to accelerate ageing at 100°C for 144 h. It is said that 72 h at 100°C would be equal to 25 years under ambient conditions18' 19. Durability was estimated according to the following relation20:
The higher the result, the better the durability.
Measurements
Breaking length, tear factor and yellowness of the untreated and the PVA-borax treated samples were measured before and after accelerated ageing according to standard procedures21'22. For breaking and tear factor tests, the specimens were cut with their edges parallel to the machine direction (MD) and in cross machine direction (CD) and then conditioned at 20°C and 65 °/o RH. For each test, at least five measurements were carried out.
Thermal analysis:
Thermogravimetric analyses (TG & DTG) of PVA and PVA-borax films were done using Perkin Elimer Thermo-gravimetric Analyzer TGA 7. Analysis was performed with a heating rate 10°C/minute and flow rate 50cc/minute, under non-isothermal conditions, in the presence of nitrogen. The reference material was α-alumina. This analysis was done to examine the thermal behaviour of borax incorporated PVA film.
IR-Spectra
The changes in the hydrogen bond strength and the C=O group, resulting from accelerated ageing of paper samples, were examined by IR-spectra technique. IR-spectra were recorded on an Insco FT/IR red spectrophotometer using KBr disc. The mean strength of hydrogen bonds (MSHB) and relative absorbance of car-bonyl group (Ec=0) were calculated as described elsewhere17'23.
results and discussion
Evaluation of PVA-borax system as an ageing resistance agent
A series of experiments were undertaken to examine the effect of the PVA-borax system as a sizing and ageing resistance agent for alum-rosin sized paper made from local short fibre pulps. For comparison samples using PVA alone were also tested.
Fig. 1 shows the variation of paper properties (strength & yellowness) as a function of the concentration of PVA, with or without the 2% borax treatment before ageing. It was clear that surface treatment of rosin-sized paper with PVA improved the strength properties of paper, in both the machine direction (MD) and cross direction (CD), reaching an optimum at the 0.5% concentration of PVA. Further increase in the PVA concentration (> 0.5%) caused a reduction in the strength properties of paper, compared with the 0.5% concentration of PVA. However, these values were still higher than those obtained from untreated paper. The PVA-borax system had a more profound effect on the samples than the PVA treatment alone. This is ascribed to the cross-linking reaction between PVA and borax with the formation of didiol-type bonds24 , in addition to hydrogen bonds forming during drying of the treated papers.
From Fig. 1 it is also clear that, the treatment using PVA alone produced less initial yellowing of the paper sheets than untreated (PVA = 0%) PVA-borax treated papers.
Fig. 2 shows that after accelerated ageing the strength properties of the untreated paper (PVA = 0%) were lower than those of the treated samples. As with the unaged samples, the presence of borax in the PVA treating solution led to greater improvement (Fig. 3), i.e. greater stabilized paper towards ageing than using PVA alone.
Fig. 2 shows that the treatment with PVA increased the yellowness of the paper, while the opposite was observed with samples using the PVA-borax system.
The advantage of using a PVA-borax system instead of PVA alone is probably related to the behaviour of borax to precipitate the PVA with the formation of a so called "skin" on the paper surface. The thermal behaviour of borax may also prevent the thermal degradation of PVA by enhancing the rate of dehydration instead of degradation and thermo-oxidation, where the decomposition temperature (Td) of PVA & PVA-borax films are 140 °C and 215°C, respectively (see Fig. 4). Therefore, on accelerated ageing the PVA-borax film insulated the components of the paper sample from the surrounding oxidizing atmosphere better than PVA alone, and consequently decreased the cleavage of the bonds between pulp and rosin, which would be responsible for the increase in acidity and loss in paper strength.
Fig. 1: Effect of PVA PVA-borax treatment on alum-rosin sized paper
Fig. 2: Effect of PVA and PVA-borax treatment on the ageing behaviour of rosin-alum sized paper.
Fig. 3: Durability according to the equation given on p. xy of treated and untreated samples.
The IR-measurements mean strength of hydrogen bond (MSHB) and relative absorbance of carbonyl group (Ec=o) in Fig. 5 emphasize the above view. It can be seen that, after accelerated ageing, PVA-borax treated papers had a relatively high MSHB and relatively low Ec=o compared to the PVA-treated samples. Also, at a relatively high PVA concentration the unexpected increase in Ec=o with the increase in MSHB, compared to unaged and untreated paper sample, suggested that the thermal ageing of treated paper yielded mainly carbonyl groups without chain scission, which was directly related to a loss of strength25.
Evaluation of the PVA-borax system as a strengthening agent
In addition to the tests described above, samples of the paper (cf. p. 130) were subjected to accelerated ageing for 6 days (equivalent, it is believed, to 50 years real ageing under ambient conditions) and then treated with 0.25-1.0 % PVA-2% borax system. Finally, these samples were submitted to the same measurements as the unaged samples. The results are given in Fig. 6
It can be seen that the treatment led to improved paper strength and that this effect was higher using the PVA-borax system. The relatively high PVA-borax system (up to 0.75%) was more successful in strengthening damaged rosin-sized paper. However, there was no appreciable difference in the values of yellowness (YI) whether using PVA-borax or PVA as a treating agent. These data may be ascribed to the behaviour of borax as a deacidification agent of aged paper in addition to the formation of a stronger PVA film on the paper surface during drying. This explanation is based on the assumption proposed by Pugina and Machulis24.
Fig. 4: Thermogravimetric (TG: .....) and its 1st derative (DTG:------) curves of PVA and
PVA-borax films.
Fig. 5: MSHB & Ec=o of treated paper after ageing for 6 days and untreated paper before and after ageing.
They reported that, on heating and at pH < 8 the PVA-borax film is transformed into a three-unit ester with the following structure:
From all the previous data it could be concluded that the PVA-borax system is able to retard the degradation of rosin-alum sized paper and to strengthen decayed papers for restoration purposes.
summaries
Performance of Improved Polyvinyl Alcohol as an Ageing Resistance Agent
Investigations have continued for improving the ageing resistance of local (Egyptian) alum-rosin sized paper sheets. In this work we intended to investigate the durability of polyvinyl alcohol (PVA) as a treatment polymer on locally produced paper and as a resizing agent to strengthen
Fig. 6: Effect of PVA & PVA-borax treatment on pre-aged alum-rosin sized paper.
decayed papers. In this respect, the treatment of such paper samples with a PVA-borax system was carried out before and after accelerated ageing for 6 days. The concentration of PVA was also optimized. The strength and optical properties, as well as thermogravimetric and IR-spectra analyses showed the success of using a PVA-borax system for the intended purpose in comparison with using individual PVA alone. Optimum improvement was achieved using 0.5-0.75 % PVA and 2% borax.
Le comportement de l'alcoolpolyvinylique en tant qu'agent stabilisateur contre le vieillissement
De plus amples investigations ont été faites pour améliorer la résistance au vieillissement des feuilles de papier local (égyptien) précipité à l'alun et encollé à la résine. L'intention était d'optimaliser l'utilisation du polyalcool de vinyle (PVA) comme agent polymère dont on se sert dans la fabrication du papier local ou comme agent de réencollage dans la restauration et le renforcement du papier endommagé. A cette fin des expériences ont été faites sur de tels échantillons en ajoutant du borax à la solution de PVA avant et après le vieillissement accéléré pendant 6 jours. Leur taux de concentration a été également étudié. Les mesures effectuées concernant la résistance, les propriétés optiques du papier, ainsi que les analyses IR et thermogravimétriques ont démontré que pour le but recherché l'utilisation du système PVA-borax était plus efficace que le PVA tout seul. Le meilleur résultant était atteint avec une concentration de 0,5—0,75% de PVA et 2% de borax.
Das Verhalten von Polyvinylalkohol ab Stabilisierungsmittel gegen die Alterung
Es wurden weiterführende Untersuchungen zur Verbesserung der Alterungsbeständigkeit von alaungefällt harzgeleimtem Papier heimischer (=ägyptischer) Herstellung durchgeführt. Die Intention ist es, den Einsatz von Polyvinylalcohol (PVA) zu befördern, sei es in der Herstellung oder als restauratorisches Nachleimmittel zur Festigung abgebauten Papiers. Hierzu wurden Versuche mit einem Zusatz von Borax zu der PVA-Lösung durchgeführt. Auch die Konzentration derselben wurde untersucht. Messungen zur Festigkeit und zur Vergilbung wie auch thermo-gravimetrische und IR-Analysen zeigten, daß für den angestrebten Zweck das PVA-Borax-Systems wirksamer ist als PVA allein. Die beste Wirkung wurde mit 0,5-0,75 % PVA und 2% Borax erzielt.
references
1. Zou,X., N. Gurnagul, T. Uesaka & J. Bouchard: Accelerated ageing of papers of pure cellulose-mechanism of cellulose degradation and paper embrittlement. Polym. Degrad. Stab.34 (1994): 393.
2. Grohmann, A.: From the world of Arabic Papyri. Royal Society of Historical Studies 1952.
3. Hunter, D.: Papermaking, the history and technique of ancient craft. NewYork 1967.
4. Norayer, G. Z. Xuejun: The effect of atmospheric pollutants on paper permanence. Tappi 77 (1994): 199.
5. El-Saied, H., A.H. Basta & M.M. Abdu: Permanence of paper. I: Problems and permanency of alum-rosin sized paper sheets from wood pulp. Restaurator 19 (1998): 155.
6. Kolar, J., & G. Novak: Effect of various deacidification techniques on the stability of cellulose pulp. Restaurator 17 (1996): 25.
7. Kolar, J., & G. Novak: Deacidification of paper.A progress report. Proc. Intl. Book Paper Conservation Conf., Ljubljana, Slovenia (1997): 305.
8. Jozef, D., & G. Henryk: Trwalepapiery drukowe idopisania. Przeglad Papier 51 (1995): 344.
9. Casey, J.P.: Pulp and paper chemistry and chemical technology 3. New York: Interscience Publ. 1981: 1914.
10. Koura, A. & T. Krause: Studies on conservation and restoration of paper by impregnation with acrylic resins. Papier 36 (1982): 117.
11. Strnadova, J. & M. Durovic: The cellulose ethers in paper conservation. Restaurator 15 (1994): 220.
12. Bicchieri, M., M. Bortolani & E. Veca: Characterization of low-molecular weight polyvinyl alcohol for restoration purposes. Restaurator 14 (1993): 11.
13. Burstall, M.L., C.E. Butler & C.C. Mollet: Improving the properties of paper by graft polymerization.]. Institute of Paper Conservation 10 (1986): 95.
14. Mirjam, F.: The reinforcement of papers by graft copolymerizption. Paris: ARSAG 1991: 228.
15. Humphrey, BJ.: The application of Parylene conformal coating technology to archival and artefact conservation. Studies in Conservation 29 (1984): 117.
16. Humphreys, B. J.: Paper strengthening with gas-phase parylene polymers. Practical considerations. Restaurator 11 (1990): 48:67.
17. El-Saied, H., A.H. Basta & M.M. Abdou: Permanence of paper. II: Correlation between paper permanence from straws and ageing variables. Restaurator 21 (2000): 158.
18. Basta, A.H.: The role ofchitosan in improving the ageing resistance of rosin sized paper. Restaurator 24 (2003): 106.
19. Rozmarin, Gh: Ageing of fibrous cellulose materials. Celluloza si Hirtie, 10 (1961): 428.
20. Kirova, Kn.L., LA. Stepanova, D.M. Flyate & I.S. Shul'man: Problemy Sokhraunosti Documental-nykh Materialov. Leningrad (Sankt Peterburg) 1977: 13.
21. Bobu, E. & E. Poppel: Effects of paperstocks parameters on paper ageing behaviour. Cellul. Chem. Technol.,22(1988): 123.
22. The Institute of Paper Chemistry, Appleton, Wisconsin, institute Method No. 411 (1952).
23. ASTM Method D1925-70.
24. Levdic, I.Y., M.D. Inshakov, E.P. Misyurova & V.N. Nikitin: Study of pulp structure by Infrared spectroscopy. Tr. Vses-Nauch. Issled. Inst. Tsellyul. Bum.Prom. 52 (1967): 109.
25. Finch, C.A.: Polyvinyl alcohol Developments., NewYork: Wiley 1992: 724.
26. Lee, S.B., R.L. Feller &J. Bogaard: Relation of cellulose chain scission to hot-alkali-soluble content during thermal and photochemical degradation of paper.]. Imaging Sci. 29 (1985): 61.
AltafH. Basta
Cellulose & Paper Dept.
National Research Centre
Dokki-12622, Cairo
Egypt
E-mail: