Chemistry Everyday for Everyone
The Chemistry of Paper Preservation Part 3. The Strengthening of Paper Henry A. Carter Department of Chemistry, Augustana University College, Camrose, AB, Canada, T4V 2R3 The deterioration of paper as it ages is a serious problem for archival communities throughout the world. Research has helped to identify the chemical reactions that take place during the aging of paper, and the acidcatalyzed hydrolysis of cellulose in paper fibers continues to be viewed as the dominant factor in the deterioration of paper (1). While conservation bleaching (2) can improve the aesthetic appearance of paper artifacts, paper conservators have directed their attention to deacidification methods for stabilizing and removing damaging acids from paper (1). What happens, however, if the paper has already deteriorated to the extent that it has become brittle and is literally falling to pieces after years of acid attack or severe oxidation? While it is not thermodynamically possible to reverse the scission of the β-acetal linkages in the cellulose chains, paper strengthening processes have been developed to specifically treat fragile or brittle materials. In the past, physical methods such as lamination and polyester film encapsulation have been applied to bind damaged paper artifacts and to hold separated pieces intact (3). However, these methods are both timeconsuming and expensive, as each sheet of paper must be individually treated. Recently, two new processes have been developed to chemically treat papers and books on a large scale basis, namely, the parylene process and the graft copolymerization (British Library) method. The Parylene Process Parylene refers to the generic name given to a family of polymers derived from xylene called poly-paraxylylenes. Interesting uses for parylenes have been developed by Union Carbide Corporation, including electrical insulators, dry film lubricants, and protective barriers for electronic circuitry and medical implants such as pacemakers (4–6). These commercial applications are based on the ability of parylenes to deposit a uniform, transparent, and colorless film on desired objects. In 1982, investigations began to explore the potential of parylenes in strengthening old papers and historic artifacts (5). It was soon discovered that paper artifacts could be strengthened with parylenes with no apparent damage to the papers or inks. The parylene process is described in Figure 1. It takes place in three stages in a parylene vacuum deposition system. In the first stage, the powdered parylene dimer, di-para-xylylene, is placed in a vacuum and vaporized at 150–250 °C. Next, the dimeric parylene gas is passed through a pyrolysis chamber at 650–690 °C, where it splits into two reactive monomer units. During the third stage, the monomer vapor diffuses into the deposition chamber at room temperature, where the monomer molecules collide with the surface of the sample. Polymerization of the monomeric units to polypara-xylylenes occurs with the deposition of a thin, uniform film. As the diffusing monomer molecules are able
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to penetrate porous surfaces such as paper, the inner surfaces are also coated. The thickness of the film can be controlled by regulating the amount of exposure of monomer gas to the artifact. If the film is too thin, the artifact can be recoated with monomer to produce an apparently uninterrupted continuous film, as the growth sites on the sample remain active for a few hours (6). Finally, excess monomer can be trapped out at about {120 °C and later reused. Two types of parylene are commonly used in conservation work. Parylene-N, shown in the figure, is used for applications where greater diffusion of the monomer is required, as would be the case for books, in which each individual page must be penetrated and coated. Parylene-C, which contains one chlorine substituent per phenyl ring, is used when a stronger and more stable film is required. Parylenes in conservation work are not limited to the preservation of fragile papers and brittle books. The parylene process has found applications in the preservation of historic silk artifacts and other textile fragments (7), the conservation of wooden artifacts, the strengthening of deteriorated water- or fire-damaged objects, and the consolidation of fossils and plant, insect, bird, marine invertebrate, and animal specimens housed in museums (8, 9). Recently, parylenes have been used to preserve plant specimens from a 45-million-year-old fossil forest discovered in the Geodetic Hills region of Axel Heiberg Island in the Canadian Arctic (8, 9). Although the plant fossils (leaves, ferns, cones, needles, and tree trunks) were found intact, frozen, and preserved in the Arctic tundra, they turned out to be extremely delicate and would begin to disintegrate to ash on drying. Treatment with parylenes stabilized the fragile specimens with little noticeable change in appearance.
H2C
CH2 di-para-xylylene at 150 °C (dimer)
H2C
CH2
H2C
CH2
H2C
CH2
para-xylylene at 670 °C (monomer)
poly(para-xylylene) at 25 °C (polymer)
n
Figure 1. The three stages of the parylene process.
Journal of Chemical Education • Vol. 73 No. 12 December 1996
Chemistry Everday for Everyone
The Effect of Parylene on Paper and Bound Books During the parylene process described above, monomeric para-xylylene molecules penetrate the cellulose fibers. As polymerization occurs, the cellulose fibers become encased by a thin film of controllable thickness, usually 2–12 µm for conservation work. Where scission of the cellulose fibers has previously occurred, the parylene film acts to join the broken fibers. In addition, interfibrous bonding occurs where the parylene film contacts cellulose fibers that cross one another. No chemical reaction occurs between cellulose and parylene. The cellulose fibers are not chemically altered but simply held in place or reinforced by parylene mechanical links. After parylene treatment, the paper artifact is heavier and tends to feel smooth and slippery. The parylene film is hydrophobic, is less permeable to pollutant gases, and is resistant to chemical attack from the environment. Independent testing of papers coated with paryleneN has been carried out by the Canadian Conservation Institute (10). Specimens included ligneous and nonligneous single sheet papers as well as entire books. Some of the main results are summarized here. •
•
•
•
•
Parylene treatment results in an increase in strength of both ligneous and nonligneous papers, whether in single sheets or books. At the same time, the strength of the more porous ligneous paper is improved to an even greater degree. Aqueous washing and bleaching with 2% H2O 2 of the parylene-coated papers produces no significant change in color and strength. Deacidification of the parylene-coated papers with aqueous Mg(HCO 3)2 partially neutralizes the ligneous paper (which receives a larger deposit of parylene) and completely neutralizes the nonligneous paper, leaving a small alkaline reserve. The thickness of the parylene film (as analyzed by an IR baseline interference ripple) is uniform for single sheet papers. The film tends to be less evenly distributed for books (the middle pages and the areas near the spine receive thinner films). This can be an advantage, as regions of the book such as the page edges, which are more vulnerable to pollutant gases and sunlight, would also be more accessible to penetration by parylene and receive a thicker protective coating. While changes to the appearance of the paper artifact are often barely noticeable after parylene coating, the effect on colored materials is more pronounced and sometimes interference patterns appear if the parylene coating is unevenly distributed.
The Graft Copolymerization or British Library Process This method is based on the concept of strengthening and binding damaged cellulose fibers by grafting polymeric chains to the cellulose structure (11–20). The moisture content of the books is first reduced to permit greater strengthening. Gaseous, monomeric ethyl acrylate, H 2 C=CHCO2 C 2H 5, and methyl methacrylate, H2C=C(CH3)CO 2CH3, are then condensed onto the papers and allowed to diffuse through the cellulose fibers overnight to create a homogeneous distribution of monomers. Next day, copolymerization is initiated by irradiating the papers with low intensity gamma rays from a cobalt-60 source. Webs of long-chain polymers are deposited on and between the cellulose fibers, with the result
that the paper structure is strengthened. (At the same time, the depolymerization of cellulose as a result of gamma irradiation is measurable but small [14].) Finally, the papers are ventilated for several days to remove all residual monomers. The chemical reactions that take place involve the three classic steps of a free radical polymerization reaction: initiation, propagation, and termination (14). The initiation step involves the creation of radical sites throughout the cellulose chains from the effect of gamma radiation as shown in eq 1: Cell-H + hν → Cell? + H?
(1)
where Cell-H = cellulose. As can be seen, a hydrogen atom is abstracted from the cellulose structure. The cellulose radical site produced can then attach a monomer as according to eq 2 to produce yet another radical site, which is capable of continued growth (propagation): Cell? + CH2=C(R)(COOR9) → Cell–CH2–C(R)(COOR9)? (2)
where R = H and R9 = C2H5 for ethyl acrylate, and R = CH3 and R9 = CH 3 for methyl methacrylate. In eq 3, another monomer (either ethyl acrylate or methyl methacrylate) is added to the growing polymer chain: Cell–CH 2–C(R)(COOR9)? + CH2=C(R)(COOR9) → Cell–CH2–C(R)(COOR9)–CH 2–C(R)(COOR9)?
(3)
Monomers are added over and over again at each radical site until long-chain polymers, which we can represent by Cell–[CH2–C(R)(COOR9)] n–CH2–C(R)(COOR9)? , have grown at many sites throughout the cellulose structure. How the ethyl acrylate and methyl methacrylate monomers are incorporated into the chain will depend on the relative amounts of monomers used. A number of possibilities exist for termination, including the reaction in equation 4, where two radical sites of growing polymer chains couple together, thereby cross-linking broken cellulose fibers (14): Cell–[CH2–C(R)(COOR9)]n–CH2–C(R)(COOR9)? + Cell–[CH2–C(R)(COOR9)]m–CH2–C(R)(COOR9)? → Cell–[CH2–C(R)(COOR)9]n+1–[C(R)(COOR9)–CH2]m+1–Cell (4)
At the same time, there is the possibility that monomer radicals could be produced from ethyl acrylate and methyl methacrylate during gamma ray irradiation: CH2=C(R)(COOR9) + hν → ?CH 2–C(R)(COOR9)? (5) ?CH2–C(R)(COOR9)? + ?CH2–C(R)(COOR9)? → ?CH2–C(R)(COOR9)–CH2–C(R)(COOR9)?
(6)
These radicals would then join in a head-to-tail combination as seen in equation 6. The process could repeat itself over and over again, leading to a long-chain polymer of formula [–CH2–C(R)(COOR9)–]n This polymerization of the comonomers competes with the grafting process and, as such, is an undesirable side effect. Fortunately, the low dose of high-energy gamma radiation, about 1 MRad over 12 hours (18), is less likely to interact with the monomers than with the cellulose backbone, where free radicals are relatively easily
Vol. 73 No. 12 December 1996 • Journal of Chemical Education
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Chemistry Everyday for Everyone
formed (11, 14, 17). As a result, the extent of polymerization of the comonomers is kept to a minimum. The use of different monomers produces a polymeric chain with different properties from that produced from one monomer alone. Ethyl acrylate by itself produces a low polymer yield, but stabilizes and imparts flexibility to the polymeric chain (17). Methyl methacrylate produces a brittle polymer (Lucite™, Plexiglas™), but increases the yield of graft copolymerization. A volume ratio of 5:1 of ethyl acrylate to methyl methacrylate works well (13). The possibility of other monomers being added to the polymer chain has been considered. For example, a basic amino monomer such as 2-dimethylaminoethyl methacrylate, H2C=CHCO2CH 2CH 2N(CH3)2, could be added to neutralize acids, or a monomer containing more than one double bond, such as 1,6-hexanediol diacrylate, [H2C=CHCO2(CH 2)3–]2, could be included to provide additional strength through cross-linking (13). In fact, the University of Surrey has developed, for graft copolymerization, a “cocktail” of monomers containing ethyl acrylate, methyl methacrylate, 2-dimethylaminoethyl methacrylate, and 1,6-hexanediol diacrylate in a composition ratio of 5:1:0.2:0.01 (18, 19). Early results indicate that graft copolymerization can significantly strengthen both ligneous and nonligneous papers (13, 19). The amount of increase in strength depends on the nature and original strength of the paper. While strength increases of 20 to 30 times have been observed for some papers, very weak papers show only a small increase in strength after treatment (19). Increasing the amount of comonomers used does further strengthen the paper, but then the paper’s appearance is altered and it may become translucent. In addition, polymer deposition is poor for clay-coated papers and paper made from juniper wood or esparto grass. The experimental work done on sheets of paper has been successfully extended to books. The treatment of books can be controlled to give an even distribution of polymer over all regions of the pages contained within. While the paper increases in weight by 10–20% after treatment, there seems to be little change in appearance, and the process leaves the bindings and inks undamaged. Meanwhile, the graft copolymerization process, which has yet to be commercialized, is continuing to be developed at the pilot stage (18). Conclusions Both the parylene process and the graft copolymerization process offer conservators a method for strengthening paper artifacts that have become too brittle for handling. While both strengthening processes offer many advantages, it should be remembered that both processes are also irreversible, and that parylene coatings and graft polymers cannot simply be removed with solvents or by mechanical means. Recent research on the effect of alkali on the longterm stability of paper fibers containing lignin has caused paper conservators to become very cautious in using deacidification treatments on old papers (21). In particular, the permanence of lignin-containing papers can actually decrease when they are subjected to the
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high concentrations of alkali often used to deposit a buffer reserve after deacidification. There can be other problems associated with mass deacidification, such as damage to color photographs, color illustrations, plastics, manuscripts with soluble inks, bindings, dyes, and adhesives (19). Thus, paper strengthening techniques may provide a viable alternative for the preservation of paper artifacts. Another possibility may be the combined use of deacidification and strengthening agents. Some of the deacidification methods discussed in part 1 of this miniseries (1), such as the Viennese method (22, 23) and FMC method (24, 25), have already demonstrated combined strengthening and deacidification effects. As a recent example, the newly developed Batelle mass deacidification process (26) employs, as a deacidifying agent, the double alkoxide Ti(OC 2H 5) 4 ? Mg(OC2H 5) 2 dissolved in hexamethyl disiloxane, (CH 3) 3 SiOSi(CH 3) 3 . Paper strengthening involving bonding between titanium alkoxide and cellulose has been reported. Acknowledgments The author thanks Helen Burgess (Canadian Conservation Institute), Ken Harris (Library of Congress), Mirjam Foot (British Library), and Anthony Egan (Nordion International) for helpful discussions. Literature Cited 1. Carter, H. A. J. Chem. Educ. 1996, 73, 417. 2. Carter, H. A. J. Chem. Educ. 1996, 73, 1068. 3. Jones, N. M. M. 1988 TAPPI Paper Preservation Symposium Notes; TAPPI: Atlanta, 1988; pp 141–144. 4. Humphrey, B. J. J. Am. Inst. Conserv. 1986, 25, 15–29. 5. Humphrey, B. J. 1988 TAPPI Paper Preservation Symposium Notes; TAPPI: Atlanta, 1988; pp 157–162. 6. Humphrey, B. J. Restaurator 1990, 11, 48–68. 7. Hansen, E. F.; Ginell, W. S. Historic Textile and Paper Materials II: Conservation and Characterization; Zeronian, S. H.; Needles, H. L., Eds.; American Chemical Society: Washington, DC, 1989; Chapter 8. 8. Grattan, D. W. Can. Chem. News 1989, 41(9), 25–26. 9. Grattan, D. W. Can. Conserv. Inst. Newslett. 1993, No. 11, 3–4. 10. Burgess, H. D.; Grattan, D. W. Sauvegarde et Conservation des Photographies, Dessins, Imprimes et Manuscripts; Association pour la Recherche Scientifique sur les Arts Graphiques: Paris, 1991; pp 231–242. 11. Burstall, M. L.; Butler, C. E.; Mollett, C. C. Paper Conserv. 1986, 10, 95–100. 12. Clements, D. W. G. 1988 TAPPI Paper Preservation Symposium Notes; TAPPI: Atlanta, 1988; pp 155–156. 13. Butler, C. E.; Millington, C. A.; Clements, D. W. G. Historic Textile and Paper Materials II: Conservation and Characterization; Zeronian, S. H.; Needles, H. L., Eds.; American Chemical Society: Washington, DC, 1989; Chapter 3. 14. Hebeish, A.; Guthrie, J. T. The Chemistry and Technology of Cellulosic Copolymers; Springer-Verlag: New York, 1981; Chapters 1–3. 15. Arthur, J. C. Advances in Macromolecular Chemistry; Pasika, W. M., Ed.; Academic: New York, 1970; Vol. 2, pp 1–87. 16. Cunha, G. M. Library Technology Reports ; White, H. S., Ed.; American Library Association: Chicago, 1989; Vol. 25, pp 69–70. 17. Davis, N. P.; Garnett, J. L.; Long, M. A.; Major, G.; Nicol, K. J. Preservation of Paper and Textiles of Historic and Artistic Value; Williams, J. C., Ed.; American Chemical Society: Washington, DC, 1981; Vol. 2, Chapter 17. 18. Egan, A.; Mardian, J.; Foot, M.; King, E.; Millington, A.; Nevin, M.; Butler, C.; Barker, J.; Fletcher, D. 9th International Meeting on Radiation Processing; Istanbul, September 1994. 19. Foot, M. Liber Q. 1993, 3(2), 145–152. 20. Foot, M. Sauvegarde et Conservation des Photographies, Dessins, Imprimes et Manuscripts; Association pour la Recherche Scientifique sur les Arts Graphiques: Paris, 1991; pp 227–230. 21. Burgess, H. D.; Goltz, D. M. Archivaria 1994, 37, 182–202. 22. Banik, G.; Sobotka, W. K. 1988 TAPPI Paper Preservation Symposium Notes; TAPPI: Atlanta, 1988; pp 146–154. 23. Wächter, O. Restaurator 1987, 8, 111–123. 24. Wedinger, R. Chem. Br. 1992, 28, 898–900. 25. Wedinger, R. S. Restaurator 1991, 12, 1–17. 26. Wittekind, J. Restaurator 1994, 15, 189–207.
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