Chemistry in the Comics Part 3. The Acidity of Paper Hemy A. Carler Camrose Lutheran College. Camrose, AB, Canada T4V 2R3 Despite the current uncertainty in the U.S. economy, the latest edition of the Official Ouerstreet Comic Book Price Guide indicates a strong market for collectors' comics (I). In fact, a recent advertisement in The Comic Buyers Guide (No. 742) offered the sale of 25 valuable comic books for a total mice of $142.400 . . (U.S.). Manv of these comics were deacidified, certaindescri'bed as having been Ivan item of interest for nrosnective comic book investors. * Like other collectibles'and'rare items, the condition of a ootentiallv valuable comic hook is all-imuortant. This condition must also be maintained hy proper storage and permanent deacidification. One of the underlying reasons for the inherent instability of comic books is the presence of acids found in pulp paper in general. Thus, over a period of time, a comic hook with white pages will gradually turn yellow (or rapidly, if continuously exposed to sunlight) and may eventually turn brown or even brittle. Needless to say, no collector wishes to see his or her collection (valuable or not) turn slowlv into dust. ~ h danger k of acids in paper is not unique to collectors of comic hooks hut is part of a more general problem also faced by our nation's libraries (2-4). 1; has been estimated that approximately one-third of the 19 million books and pamphlets in the Lihrary of Congress are too brittle for circulation (5,6). It has also been estimated that approximately 90 million hooks (about 30% of the total) found in American research libraries cannot he used because of brittleness due to acid attack (7). Arecent large-scale study on bookdeterioration in the Yale University Lihrary has provided some statisties to suonort these estimates (8).It was found that 37.1%0f the ho&sampled overall at Y& had brittle paper, while 82.6% of the books overall had acidic uaoer with a DH of below 5.4. Book deterioration is indeed a serious problem, and, as a result, many university libraries are now engaged in preservation studies and in the establishment of guidelines for the selection of books for conservation treatment (9-12). This paper will focus on the nature of acidity in pulp paper as found in comic hooks (and library collections). Some of the various factors that contribute to the deterioration of paper will also he considered from a chemical perspective. The Acld-Catalyzed Hydrolysis ot Cellulose The acid-catalyzed hydrolysis of cellulose in paper fibers has been established as the main cause of paper deterioration in librarv books (13, 14). The same statement can be made for com-ic books made of pulp paper. The structure of cellulose has previously been described in this Journal as a long, linear polymer consisting of anhydroglucose units linked through 14-beta-glycosidic bonds (15, 16). Recent Raman and solid state 'W NMR spectroscopic studies suggest that the adjacent anhydroglucose units are nonequivalent and that the basic repeating unit of the cellulose structure is the dimeric anhydrocellobioseunit (17.18) as seen in Figure 1. In addition, the cellulose chains are cross-linked by hydrogen bonds, resulting in a rigid, fibrous, and water-insoluble polymer. The orientations of these chains vary in cellulose leading to crystalline regions with parallel chains strongly bound together by hydrogen
---
.111..
-tru
Figure 1. Three Blinked anhydroglucose u n b of the cellulose chain.
Figwe 2. Acld-catalyzed hydrolysis of Uwt cellulose Chain.
bonding and amorphous regions with nonoriented chains and considerable less intermolecular bonding. The number of beta-linked anhydroglucose unim found in one cellulose chain is known as the degree of oolymerization (DP). Cellulose as found in wood has a DP of appronim&eli 10,000. During the manufacture of paper, the DP of cellulose drops to about 1000 (13). The presence of acid in paper can catalyze the hydrolysis of cellulose (19) resulting in a cleavage of the heta-acetal linkages between the anhydroglucose subunits of cellulose as shown in Firmre 2. This nartial hvdrolvsis results in a sienifiW r k g e of about 400'1500, cant loweriig of the D P ~ B ~ I Oa-DP naoer loses streneth and further hvrolvsis . . will result in emb r h m e n t (13L'ks only the amorphous regions of cellulose that contain water are oenetratcd bv acid, the DP tends to level off around 200 (i3). Further attack by acids on the remaining cwstalline cellulose structure is not likely to continue unliss ihe paper were to be directly exposed toconcentrated acids. Under thisextreme condition, further hydrolysis of cellulose can produce the disaccharide, cellohiose, while total hydrolysis, which can be achieved by reaction with 40%aqueous hydrochloric acid, produces D-glucosein a 95%yield (20). At this point, the destruction of the 1,4-hetaglycosidic linkages is complete. The mechanism for the acid-catalyzed hydrolysis of cellulose has been widely studied (21-27). The first step of the mechanism involves rapid proton attack of the oxygen atom in the 1,4-beta-glycosidic linkage (Fig. 3a). In the second rate-determining step (Fig. 3b), a shift of the electron pair between the C(1) and acetal oxygen atoms results in a rupVolume 66
Number 11 November 1989
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of cellulose (34). Thus, acombination of acid impurities and adsorbed moisture can produce the undesired degradation of cellulose fibers in the paper of comic books. Unfortunately, there are a number of other contributing sources to the overall aciditv of nauer that develon during the storage of hooks. ~hese-inelideair pollutants-and the acidic degradation uroducts of the noncellulose comuonents of paper fibers such as hemicellulose and lignin. The major air pollutants that can permeate books while resting in storage are sulfur dioxide and nitrogen dioxide. There is considerable evidence to show that low concentrations of sulfur dioxide (2-9 ppm) can result in significant acid attack on paper (34). As a result of fossil fuel combustion, the lower end of this concentration range for sulfur dioxide is found in some cities. At the same time, nitrogen dioxide is oroduced as an air nollutant, mainlvfromautomobile engin&. During the sp&k plug ignition gasoline fuel, nitric oxide forms and is instantaneously oxidized to nitrogen dioxide
of
Figwe 3. Mechanism for the acidcatalyzed hydrolysis of cellulose. (a) W o n anack at the 1.4-beta-glycosidic linkage. (b) Scission ol the cellulose chain and formation of a carbonium cation. (c) Reaction of me carbonium cation with a water molecule. (d) me formation of two neutral cellulase chain residues and regeneration of the acidic proton.
ture of the cellulose chain. This heterolvsis . .nroduces an intermediate carbonium cation and a hydroxyl group at the C(4) atom of the neutral residue as shown in Fiwre 3c. The reaction between the carbonium cation and a water molecule (Fig. 3d) results in the formation of a neutral end group and the regeneration of the acid proton. This acid-catalyzed depolymerization of the cellulose chain, which can occur over and over again, is the major cause for the deterioration of paper and aging of comic books. Sources of Acldky In Paper Having descrihed the effect of acids on cellulose, it is necessary to explain why acids are found in paper in the first place. The major source of acidity in paper arises during its manufacture. A sizing is used to prevent the running of .. agent .. printing ink on paper, and, in to introduce to the hydrophilic cellulose fibers a dearee of resistance to wetting and penetration by aqueous Gquids. The waterproofing agent that is generally used is alum-precipitated rosin. The sizing process involves the introduction of the hydrophobic "rosin size" by precipitation with papermaker's alum, aluminum sulfate.. AldSOa)~.18H~0 - . .. .. . .(28.29). . . As is the case with other AI(II1) salts, an aqueous solution of aluminum sulfate is extensivelv hvdrolvzed to vield an acidic solution. lnitially, alumin- s~lfatedissoci~tes in water to form sulfate ions and hexaaquaaluminum (111) ions that can undergo acid hydrolysis as follows: ~
+
+ H,O + [AI(OH)(H,O),]~+H,O+ [AL(H,o)J~+
(1)
The acid ionization constant, K., for this reaction equals (30). However, in addition to the speciesfound in 1.12 X eq 1, a numher of complex ions are formed in aqueous aluminum(II1) solutions, including the Alz(OH)z(Hz0)84+dimer (31). The nature and concentrations of the various species depend on the pH (32). In any case, the precipitation of rosin size by aluminum sulfate is most effective in a pH range between 4.2 and 4.8 (33). The acidic environment required in the papermaking process results in sufficient acidity in the paper to cause hydrolysis of the 1,4-betaglycosidic bonds in cellulose. In addition, water is readily adsorhed by paper to the extent of about &lo% of the weight 884
Journal of Chemical Education
1/2N, + 1/20, -NO NO 1/20, -NO,
+
(2) (3)
Nitrogen dioxide could react with water present in paper to yield nitric acid: 3N0,
+ H,O
-
2HN0,
+ NO
(4)
The formation of nitric acid would. of course,. dezrade the cellulose chain. Instead, nitrogen dioxide reacts with cellulose mainly bv oxidizinn the hvdroxvl - group - - of the C(6) atom of the anhydioglucose knit td form an uronic acid grouping (34-36) as follows: Cellulose-CH,OH
+ ZNO,
-
Cellulose-COOH + 2NO + H20 (5)
While oxidation with nitrogen dioxide does not directly disturb the fiber structure of cellulose (19), the conversion of alcoholic groups to carboxylic acid groups will certainly increase the acidity of paper. The use of wood as a source of cellulose fiber and the chemical pulping and bleaching processes that are subsequently employed in papermaking led to an acidic product. Before the 1850's, paper was made from cotton and linen rags, which were relatively pure forms of cellulose (>go%). With the invention of the papermaking machine by the Fourdrinier brothers in 1804, cotton and linen materials could no longer meet the demands for fiber in paper manufacture (37).In 1866, Watt and Burgess began producing paper from wood pulp, a virtually unlimited and cheap source of cellulose fiber. However, cellulose obtained from wood pulp is less durable and of lower molecular weight. In addition, unlike cotton rags, which contain no lignin, wood must be delienified bv chemical *nulnine . -.orocesses. Not all lignin is removed, and the final paper product contains residual lienin as well as hemicelluloses and other extraneous components (38) that can slowly degrade, particularly in the presence of oxygen, to acidic products (13). The amount of residual lignin and bemicellulose contained in the resulting fibers is reflected in the pulp yield. A high pulp yield indicates a relatively high content of lignin and hemicellulose. The paper generally used in comic books is derived from a blend of groundwood pulp (7&80%) and an unhleached kraft pulp (20-25%), and, as such, contains a large amount of lignin (39). Other sources of acidity to paper can include residual chemicals from the pulping and bleaching processes, acid migration from book storage materials such as cardboard, and even from the printing ink media that slowly oxidize over time to yieldacidicmaterials. I t hasoften been noted by the author that the inside cover of an old comic book has browned where it has made contact with the actual panels
-
but not the margins of the "splash page". The problem of acidity in paper is indeed complex. In summary, the manufacture of paper from wood pulp using modern chemical techniques (such as alum sizing) results in a product that is acidic with an initial pH of between 4 and 5. Over a period of time, however, the pH will d r o even ~ further as more aciditv is eenerated through external pollutants or through the degradation of natural products present in the paper. In addition, a numher of other factors can enhance the acid-catalyzed hydrolysis and random scission of the 1,4-beta-glycosidiclinkages in cellulose. These factors will be briefly described helow. The Eff& of Temperature
An old mle of thumbstates that a temperature increeseof 10 OC will increase the rate of a chemical reaction by a factor of two or more. Thus, temperature can play an important role in determining the rate of acid-catalvzed hvdrolvsis . . of cellulose and acid-producing reactions in paper. The Arrhenius eauation in its various forms (40, 41) is commonly used to relate the temperature.(T) and rate constant (k)for a given chemical reaction. In its exponential form, the Arrhenius equation can be stated as follows: k = A~-E/RT
(6)
where A is the freauencv - - factor, E is the Arrhenius activation energy (42), and R is the gas constant. Most chemical reactions do not strictly obey the Arrhenius equation, although a plot of the natural logarithm of k versus the reciprocal of temperature is generally linear over a limited temperature range. The effect of temperature on the deterioration of paper has been studied extensively using methods of color reversion (yellowing),fiber strength, and the foldingendurance of naoer (43-46). (The folding endurance is a measure of the , numher of double folds thatlcan he obtained until the paper breaks under a snecified tension (47. 48).Acid-catalvzed hydrolysis of cellthose produces a loss i'n folding endurance.) Tem~eraturestudies are based on accelerated aging Drocedures that involve heating the test paper generkl; in the range of 6&120 OC under constant moisture conditions. Arrhenius plots (Ink m. 117') for the aging of paper are approximately linear so that onecan estimate rates of paper deterioration at lower temperatures by extrapolation. I n order to obtain data for "normal temperatures", one must ensure as much as oossihle that all factors (includine moisture) are held constant. An average value of 30.0 kcallmol (126 kJ1mol) has been estimated 6 r the activation energy f i r the deterioration of paper (43). It should be understood that this value is strictly an empirical numher that encompasses a number of chemical reactions that take olace during the aging of paper. Nevertheless, one can draw some co&lusions on the-effect of temperature on the rate of deterioration of paper. Using the logarithmic form of the Arrhenius equation for two rate constants at two temperatures,
..
oerature accelerated wine studies will vield somewhat difierent Arrhenius actiGatYon energies for different paper oroducts. However, it should be clear that the rate of deteribration of paper increases markedly with temperature. The Effect of Oxygen
There seems to be cousiderahle evidence to show that atmosphericoxygenalsocancontributegreatly to thedeterioration of paper (49,50). In addition, new methods involving ohotoeranhii film have been used to studv the autooxidation of paper through the detection of organic peroxides or hvdroeen ~eroxide(51). Free radical chain mechanisms have heen Gopbsed to ex&in the autooxidation of cellulose leadine to the formation of oreanic oeroxides and carhoxyl g ~ & ~ This s . autooxidation i f cellthose is also catalyzed by metallic cations such as Cu2+and Fe3+,which are naturally found as trace amounts in wood and which are also introduced as contaminants during.the pulping . . . and papermaking operations (52). Accelerated aging tests performed at 90 OC at 0%and 100% relative humidity indicate a direct linear relationship between the aging of paper (as evident by yellowing and loss of strength) and the concentration of oxygen (49). However, the contribution of atmospheric oxidation to the overall deterioration of paper decreases markedly as the temperature is lowered from that used during the accelerated aging test (e.g., 90 OC) to room temperature (50). As a result, the acid-catalyzed hydrolysis of cellulose remains the dominant factor in the deterioration of paper at room temperature. -
A
The EHed of LlgM -
It is well known that modem paper tends to discolor rapidly when it is continuously exposed to natural sunlight. Oh;viously, photoinduced chemical reactions must be-involved in causing this discoloration. According to the Grotthuss-Draper principle of 1818 (53), only the light that is absorbed by a substance is effective in nroducine a nhotochemical chance. Hiehlv cellulose .. .nurified . hoes not =b;orb light in the v i s h region hut does absorb weakly in the ultraviolet region at 260-270 nm and strongly below 200 nm (54,55). In addition, it has been observed that ultraviolet lieht emitted from a mercurv lamp at 253.7 nm will photodegrade cellulose with the foimatidn of carboxyl groups and a lowering in the degree of polymerization (54). A numher of degradation product8 occur including malondialdehyde, a variety of sugars, and volatile and nonvolatile acids. Hieher intensitv ultraviolet radiation will nroduce an even number i f degradation products, in&ding CO, COz, and Ha Once degradation of cellulose begins by direct exposure to ultraviolet light, new functional groups formed on the cellulose polymeric chain can exert an autocatalytic influence by behaving as new absorbing centers. The oresence of oxveen can lead to the formation of hvdroperoxides during ultraviolet photolysis of cellulose. These hydroperoxides are decomposed readily by light to form free radicals that can cause scission of the cellulose chain. Further oxidation can then occur with the formation of carbonyl and carhoxyl groups (55). Degradation of cellulose by light can also occur indirectly bv ohotosensitization. wherehv imourities absorb lieht . quanta and transfer their excitation energy to the cellulose chain. Modern oaDer contains various im~uritiesincludine oxidized cellulose; hemicellulose, lignin, metal ions, dye: and extraneous materials. Thus. Daoer can absorb both ultraviolet and visible light leadingti the degradation of the cellulose chain and discoloration. The cause of discoloration or yellowing in paper is attributable to the formation of aldehyde groups on the anhydroglucose units (55). Studies on the yellowing of cotton cellulose upon aging also indicate a relationship between yellowing and aldehyde group content (56,57).
--
..
one can calculate that the rate of deterioration of paper increases hy a factor of 5.5 for an increase of temperature from 20 OC to 30 "C.This value depends dramatically on the activation energy obtained from the Arrhenius plot. Thus, if we use the average activation energy of 25.3 kcallmol (106 kJ1mol) based on more than 200 Arrhenius plots done by the Barrow Laboratory for temperatures ranging from 38 OC to 125 "C (45). a factor of 4.2 is obtained. Considering the numher of variables that are involved in the various phases of paper manufacture, it seems reasonable that multitem-
.
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Number 11 November 1989
885
Lignin, a very complex polymer based on phenylpropane huildine units (38.58).contains a numher of chromoohoric groups ;hat can absorb in the near ultraviolet and ksihle suectral region. The ahsomtion of lieht can result in the ~hotochem~cal oxidation A d subsequent discoloration of lignin (55). At the same time, lignin in its excited state can behave as a sensitizer in inducing the degradation of cellulose. As aresult, comic hooks and other high-yield pulps that contain a suhstantial amount of lignintuin yeflow when continuously exposed to sunlight. The overall effect of light on paper products is to cause a degradation of the cellulose chain and a lowering in the deeree of ~olvmerization. At the same time. hoth lieht and . . atmospheric oxygen cause the formation of carhoxyl groups, which generates more aciditv in the oaoer. While cellulose in its state is relativel~unaffec% by natural sunlight, the presence of impurities such as lienin and metallic cations will-initiate photorhemical reactions resulting in the degradation of the cellulose chain. Thew photoinduced reactions are enhanced by acidity, a high moisture content, and the presence of oxygen. Concluslons The deterioration of oulu - oaper . . as found in comic hooks and library collections &I general is largely due to acid attack. While the nature of this attack, involving a scission of the 1,4-heta-glycosidiclinkages of the cellulosdchain, seems clear, the numher of ways in which acidity can he introduced into paper is varied and complex. Certainly, high-yield paper products, such as comic hooks, which contain large amounts of lienin and other imourities. are verv vulnerable to aging - via acid-catalyzed hydrolysis. ' Acid attack is not the onlv factor that affects the lifetimes of pulp paper products. MAY other factors such as sunlight, moisture content. atmosoheric oxygen, temperature, and the presence of m k t a ~ i ccations c&contrihuti to the degradation of cellulose fibers and the overall acidity of paper. All of these factors are interrelated. An increase in temperature, for example, will increase hoth the rates of acid-catalyzed hydrolysis and the atmospheric oxidation of cellulose. However, the effect of temperature on the rate of atmospheric oxidation is more dramatic. While the oresence of transition metal cations will catalyze the autooxidation of cellulose, a hieh relative humiditv will enhance acid-catalvzed hsdrolysi; At the same time," hoth light and atmosphkric oxidation can result in the formation of carhoxyl . groups - . on the cellulose chain and an increase in the acidity of paper. It has also heen reported recentlv that the effect of atmospheric oxygen diminished by a reduction in acidity on the &ing of (59). An understanding of the chemistry involved in the acidity and subsequent aging of paper is essential for finding the necessary means of extending the lifetimes of pulp paper products such as comic hooks and library collections. The recent use of nondestructive techniaues such as Fourier transform IR spectroscopy (FTIR) and electron spectroscoov for chemical analvsis for the detection of acidic . (ESCA) . functional groups in paper will certainly prove valuable for in future conservation research (60). An upcoming- -paper this miniseries will discuss a numher of recommended procedures for the storage and deacidification of comic hooks. Because this paper is intended mainly to serve as an introduction to the suhiect of acidity in paper, interested readers may wish to consdt the Literature Cited. ~
~
~~
~
~
Literature Clted I. Overstreet, R. M. O/fieiol Owrsfreof Comic Book R i c e Guide; The H o v e of Callectiblen:NcwVork, L989:pA-17. J Chrm.Edue. 1986.63.336. Ahalson, P. H. Science 1987,238,595. Young.P.H. CBRLNewa 1988,49.147. Malin,H. M. Chemistry 1979,52(2), 17. 6. J. Chem. Eng. News 1979,5710& 1). 37. 7. Smith, R. D. C& RL News 1987.41.2. 8. Wdker,G.;Gmonfield. J.;For. J.;Simonoff. J. S. ColleseondResoorchLihrari~s1985, 46,111. 9. Bond,R.; DeCarlo, M.:Henes. E.; Snyder,E. Colkge ondRasearch Libreria 1987.43,
2. 3. 4. 5.
,2?
10. 11. 12. 13. 14.
Simon. C. Sei. N e w 1982,122.2.3 Williams. L. B. College and Research Libmriea 1985,46,153. Weher, D. C. C & RL Now8 1981.43.238. Smith, R. D. PhD Thesis, Universityof Chiciyo, 1970. Cadton, A.M. Hondbaob of Pulp and Popsr Technology, 2nd ed.; Britt, K. W., Ed.; Van Nostrand Reinhold: New York. 1970:C h a ~ t e9-6. r
New York, 1943: Chsptcr 2.1. 20. Streit%eser. A.; Heathmek. C. H. Inlmduclion to Oiganie Chemistry; MaeMiUsn: New York, 1976:p 727. 21. Peters. R. H.; Still, R. H. Applied F i b n Science; Hsppw, F., Ed.; Academic: New York. 1979: Vol. 2. Chanter 10. 24. Whistler, R. L.; Richarb, G. N. J. Am. Chrm. Soc. 1958,80,4888. 25. Mitts, E.; Hixon, R. M. J. Am. Chem. Sm. 1944,66,483. 28. Blszei. A.: Kasik. M. Cellulose ond Ifr Deriuoliws: Kennedv. J. F.: Phillios. . . G. 0.;. w;dl&, D. J.: W~U-, P. A , Eds.; Wiley: N ~ W Y O 1986 ~ X , chapter8 27. Daniel. J. R. Bncvrlo~edPdloafPolymer SciencoandEReimerina, 2nd ed.: Kroaehwita.
.
.
.
K. 1988 TAPPI Paper Presaruafion Symposium Notes: lanta, GA. 1986; pp 146154. :Wilkinaon, G.; Gaus. P. L. Bmic lnorgonie Chemistry. 2nd ed.: Wilev: York. 1967: p316. ,F.A.;Wi1kinaon.G. Adwncodlnorponic Chomislry.5thrd.; Wiley: New York, 1986: pp 21b217. 32. Gildea, D. R.; Phipps, A. M.: Fevuson, J. H.; Kustm, K. lnorg. Cham. 1977,16,1257. 33. Sfuhrkc, R. A. Proseruotion of Poper ond Textiles o/ Historic and Adstic Value: Williams. J. C.. Ed.: American Chemical Society: WashinSon, DC, 1977: Vol. 1,
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