The Chemistry of Paper Preservation - American Chemical Society

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The Chemistry of Paper Preservation Part 2. The Yellowing of Paper and Conservation Bleaching Henry A. Carter Department of Chemistry, Augustana University College, Camrose, AB, Canada, T4V 2R3 Yellowing is often viewed as one of the early signs of the aging and deterioration of paper. Depending on the nature of the paper and conditions of storage, the yellow may eventually turn to brown and the paper may become brittle as it reaches the later stages of aging. This discoloration on aging can be attributed to the presence of chromophores found in some of the products formed from the degradation of one or more components of paper (1, 2). While there does not seem to be a correlation between discoloration and loss of mechanical strength (3–7), the yellow color in paper is considered an undesirable characteristic. With the development of new and improved mechanical pulps (8), there has been renewed interest from the pulp and paper industry in the photochemical reactions that accompany the yellowing of paper, particularly in lignin-rich papers such as newsprint. Research has centered around identifying the chromophores that are formed, the reaction mechanisms that cause the lightinduced yellowing of paper, and the possible use of chemical reagents to inhibit these mechanisms (9–15). Paper conservators are also interested in the nature of the discoloration of paper, not only from the aesthetic view that the appearance of the paper can be enhanced, but also from the conservation view that the paper might be stabilized. An understanding of the types of chromophores responsible for yellowing can assist the conservator in making intelligent choices regarding the use of bleaching methods in the treatment of old papers and artifacts. Many bleaching methods are now available and, in fact, conservation bleaching has become an important area of conservation research, second only to mass deacidification (16). This paper will discuss the causes of the yellowing of paper and then focus on the methods used in conservation bleaching. The Yellowing of Paper Paper is composed of cellulose, hemicellulose, lignin, and extraneous material from wood, plus dyes, additives, sizing agents, glue, starch, fillers, and pigments. As a consequence of oxidative reactions of these components, colored degradation products containing carbonyl (C =O) groups and carbon–carbon double bonds (C=C) can result (2, 16, 17). Oxidation is particularly affected by light, heat, moisture, the presence of metal cations such as Cu2+ and Fe3+, and pollutant gases such as SO2 and NO2 (1). The extent of discoloration as paper ages will depend on the percentage composition of the constituents in paper. High-yield pulps that contain large amounts of lignin and hemicellulose tend to yellow at a faster rate. The discoloration of paper is initiated by oxidation or photooxidation, a process whereby oxidation occurs following the absorption of light by the chromophores of the components of paper (16). Oxidative or photooxidative reactions can produce new functional groups that

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behave as new absorbing centers that account for the yellowing of paper (1). The nature of the chromophores in the photooxidized components of paper are briefly discussed in the following sections.

Cellulose Chromophores The structure of cellulose has been previously described in this Journal as a long, linear polymer based on repeating dimeric anhydrocellobiose units (1, 18, 19). During oxidation or photooxidation, carbonyl groups and carboxylic groups are formed from hydroxyl groups on the anhydroglucose units (16, 17, 20, 21). The formation of both aldehyde and ketone groups on carbon atoms C-2 and C-3 in the anhydroglucose units accounts for the yellowing of cellulose during aging (16, 17). Hemicellulose Chromophores Hemicellulose consists of a mixture of polysaccharides with short side chains and a much lower degree of polymerization than cellulose (22). Oxidation seems to occur more readily for hemicellulose than for cellulose, with the result that paper containing a higher percentage of hemicellulose is more susceptible to yellowing (16). The chromophores of oxidized hemicellulose have been identified as aldehyde groups on carbon atoms C-2 and C-3 in the anhydroglucose units (16, 17). Lignin Chromophores The structure of lignin has previously been described in this Journal (18, 23). Lignin is a very complex aromatic polymer based on substituted phenylpropane units linked primarily through ether bonds and to a lesser extent by C–C bonds (24). It contains many chromophores with aromatic conjugated bond systems and carbonyl groups, and as such, its interaction with light in the presence of oxygen is the main cause of photoyellowing of paper made from mechanical pulps. The contributions of oxidized cellulose and hemicellulose to the yellowing of lignin-containing paper appear to be minimal (25). In fact, there is evidence to suggest that lignin may protect cellulose from photodegradation by absorbing UV radiation and also by acting as an antioxidant by quenching free radicals formed in cellulose (6, 26). The photooxidation of lignin produces aromatic ketones, quinones, aldehydes (such as coniferaldehyde), and acids as photodegradation products (9). While the mechanism of the light-induced yellowing of lignin-containing papers has been widely studied for many years, new information, using lignin model compounds such as αguaiacoxylacetoveratrone, has become available for the formation of chromophores and the degradation of lignin during photooxidation (6, 9–11, 27–29). The mechanism for the photoyellowing of lignin-containing paper is shown in Figure 1 and can be briefly summarized as follows (6, 9): 1. The primary chromophores of lignin absorb nearultraviolet light (300–400 nm).

Journal of Chemical Education • Vol. 73 No. 11 November 1996


Yellow Chromophores

[O2]

LIG

OMe O Secondary Chromophores

Phenoxy Radicals

H2COH C Primary Chromophores

O

LIG

Light

Radicals

HOC OMe

near UV

Added Scavengers

Lignin Protection

Conservation Bleaching

OMe O

between phenoxides and metallic cations such as Al3+, Fe3+, Fe2+, Cu2+, and Mn 2+), extractives containing nonlignin polyphenol structures, resin acids containing conjugated systems of double bonds, chromophores produced by the action of microorganisms (particularly fungi) on cellulose, and chromophore systems associated with local discoloration in paper known as foxing (16). It should be noted that other components of paper such as dyes, pigments, metal ions, and additives can behave as photosensitizers by absorbing light quanta and transferring their excitation energy to oxygen. The activated oxygen and H2O2 (formed from activated O2 and moisture) can photooxidize the cellulose chain to form chromophores (33, 34).

LIG

Ketyl Radicals

H2COH CH2 C

O

O

LIG

Other Secondary Chromophores OMe Ketones

Figure 1. The mechanism for chromophore production and the degradation of lignin during the photoyellowing of lignin-containing paper. LIG = rest of lignin molecule. [From Leary, G. J. J. Pulp Paper Sci. 1994, 20(6), J154–J160. Reproduced with permission.] 2. The effect of light is to rupture some of the ether linkages of lignin to produce free radicals. 3. The free radicals react with lignin to produce phenoxy and ketyl radicals. 4. The ketyl radicals break down to form more phenoxy radicals and also ketones, which now function as secondary chromophores; the phenoxy radicals are oxidized by atmospheric oxygen to form yellow quinones that also function as secondary chromophores. 5. The secondary chromophores can absorb light, and the photodegradation of lignin and the yellowing of paper continue.

Current research is centered around the inhibition of the yellowing of paper in high-yield pulps (thereby making lignin-rich pulps more marketable) by introducing reagents that can interrupt any of the steps in the above mechanism (8, 9, 12, 13, 15, 30–32). For example, adding antioxidants such as ascorbic acid and thiols inhibits the yellowing mechanism by scavenging free radicals. Both ascorbic acid and thiols such as thioglycerol, HSCH2CHOHCH2OH, are easily oxidized and are believed to scavenge radicals involved in the photoyellowing process by acting as hydrogen donors (9). In general, methods of inhibiting the yellowing mechanism are based on the following: modifying the primary chromophores of lignin, using bleaching agents to eliminate the secondary chromophores once they are formed, chemically modifying lignin structures to prevent the formation of free radical intermediates, scavenging the free radicals, and, finally, eliminating the yellow color of the products (such as the o-quinones) with bleaching agents.

Other Chromophores in Paper These involve organometallic complexes (formed

Conservation bleaching involves chemically treating paper such as that found in rare books, historic documents, maps, and other archival materials, in order to remove unwanted discoloration or stains. As chemical bleaching can be potentially damaging to the artifact in question, bleaching should be used as a last resort to remove or modify the chromophores that are responsible for the discoloration. In addition, there may be the problem of colored inks that might be adversely affected by the bleaching. In any case, chemical bleaching should not be attempted until the artifact has been washed with water.

The Washing of Paper It may be possible to remove some of the discoloration by simply washing the paper with deionized water, made alkaline with Ca(OH)2 solution to maintain the Ca2+ ion concentration in the paper (35–38). Studies using gel permeation chromatography indicate that the removal of Mg 2+ or Ca 2+ during paper washing with deionized water alone results in increased degradation of the cellulose artifact (39, 40). Alcohol may also be added (after testing its effect on inks, dyes, and pigments) to increase the wettability of the paper (36). The nature of the resulting extract of colored, acidic, watersoluble degradation products will depend on the type of paper being treated. In particular, the washing of old lignin-rich papers produces colored solutions (37). Since only acidic material that is soluble can be removed by washing, it is necessary to follow the washing procedure by deacidification with aqueous Ca(OH)2 or Mg(HCO3)2 (36, 41). The effectiveness of the uptake of Ca2+ or Mg2+ ions into various papers that have been washed with solutions of either Ca(HCO3)2, Ca(OH)2, or Mg(HCO 3)2 has been studied by scanning electron microscopy and atomic absorption spectroscopy (42). The results indicate that the uptake of Ca2+ or Mg2+ ions becomes greater with increases in the amount of lignin in the paper, the pH of the solution, the concentration of the solution, and the degree of oxidation of the cellulosic fibers. Introduction to Chemical Bleaching If washing does not sufficiently remove the discoloration from a paper artifact, then chemical bleaching may be considered. Methods available for conservation bleaching are essentially based on industrial bleaching processes where the chemistry of the bleach is wellknown (43 - 50). From the point of view of the conservator, conservation bleaching will involve modifying the structure of the chromophore through either oxidation or reduction, removing the chromophore entirely or

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eliminating the auxochromic effect of the substituents to the chromophore (16). The success of a particular bleaching process is often evaluated by its bleaching efficiency, a quantity obtained by measuring the change in reflectance or brightness of the paper at a specific wavelength, often 457 nm (51). The extent of the return of discoloration following bleaching, known as brightness or color reversion, is also important and can be correlated to oxidative damage to the cellulose fibers (51). The potential damage to cellulose fibers through chemical bleaching must always be considered. Generally, oxidizing bleaches lead to some oxidation and degradation of cellulose fibers. Experiments involving cellulose cotton linters and a number of oxidizing bleaches indicate stabilized hydrogen peroxide to be the least degradative oxidative bleach while hypochlorite solutions (pH = 7 or less) seem to cause the most fiber damage (52). Reducing bleaches are suspected to have fewer negative chemical effects on cellulose (51). Information on the working procedures and experimental details of conservation bleaching is available (16, 51–56). This paper will concentrate on the chemistry involved in bleaching. Oxidizing Bleaches A number of oxidizing bleaches have been considered in the past for conservation work. These include chlorine dioxide, calcium and sodium hypochlorite, hydrogen peroxide, chloramine-T, potassium permanganate, sodium perborate, ozone, sodium chlorite, and light (natural or artificial) (53–56). Only those oxidizing bleaches that are in common use today will be discussed— namely, hydrogen peroxide, alkaline hypochlorite, chlorine dioxide, and light. Hydrogen Peroxide. A great deal of success in conservation bleaching has been achieved using stabilized H2O2 in the concentration range of 0.5–3.0% and pH range of 8–10 (51). The actual peroxide concentration and pH will depend on the nature of the paper artifact. Recent kinetic studies of aqueous alkaline hydrogen peroxide solutions have ruled out singlet oxygen and free radicals such as HO?, HOO?, and O2?{ as being involved in the bleaching mechanism (57). Instead, the active bleaching agent is the perhydroxyl anion, OOH {, obtained by the ionization of H2O2: H2O2 + H2O

H3O+ + OOH {

(1)

The acid ionization constant of hydrogen peroxide is very low (Ka = 2 × 10{12), with the result that solutions of H2O2 must be made alkaline to raise the concentration of OOH {. Otherwise, no noticeable bleaching action takes place (43). At the same time, the pH must not rise above 11, as decomposition of OOH { then begins to occur: 2 OOH{ → O2 + 2 OH {

H2COH

O

CH2

LIG

C

(2)

As peroxide decomposition is catalyzed by transition metal cations (particularly Fe3+, Mn2+, and Cu 2+) that may be found in paper, magnesium sulfate and sodium silicate are commonly added to the bleaching solution to produce colloidal Mg(OH)2 and MgSiO3, which can adsorb these metal cations (43, 51). The silicate ions also facilitate the penetration of the peroxide into the cellulose fibers. If the peroxide solution is still not fully stabilized, a chelating agent such as EDTA may be necessary to remove all traces of metal cations. Finally, a buffer system must be added to obtain the desired pH.

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Degradation of cellulose, hemicellulose and lignin appears to be very slight under conditions of alkaline peroxide bleaching (43, 51). Color reversion is slow, even for lignin-rich papers where hydrogen peroxide appears to be superior to chlorine-based bleaches which undergo chlorination reactions with lignin. Hydrogen peroxide is one of the few oxidizing bleaches that can be used on ligneous paper (51). Recent studies on the bleaching of mechanical and ultrahigh yield pulps with alkaline H2O2 indicate that important chromophores of lignin (shown in Fig. 2) are bleached. Specifically, the highly conjugated coniferaldehyde and quinone chromophores, which absorb at wavelengths in the region 360–460 nm, are oxidized to chromophores that absorb light at wavelengths below 300 nm (12). In addition, aromatic carbonyl groups are oxidized by a large excess of alkaline H2O2. Previous studies on ligneous paper have suggested reaction schemes that also indicate the oxidation of α-carbonyl groups and conjugated systems by H2O2 to produce carboxyl groups (16). Since there is the possibility that secondary chromophores arising during peroxide bleaching will lead to color reversion, subsequent bleaching with a reducing agent is recommended (16, 58). It should be noted that bleaching efficiency increases with pH and the concentration of the OOH { anion. In considering the strength and pH of the H2O2 solution, a compromise must be made between eliminating unwanted chromophores and avoiding any chemical degradation to the artifact. Burgess recommends a pH of 9.0 and a concentration of 1.0–2.0% H2O 2 as a general rule for most paper artifacts (51). Alkaline Hypochlorite. Hypochlorite bleach solutions made from either NaOCl or Ca(OCl)2 are used mainly as a last resort for heavily stained artifacts. While hypochlorite bleaching can be very effective in removing difficult stains including mold stains and foxing, degradation of cellulose during hypochlorite bleaching has been reported (51–56). The reaction between cellulose and hypochlorite involves oxidation of the hydroxyl groups on the anhydroglucose units to produce aldehyde, ketone, or carboxyl groups and scission of the 1,4-β-glycosidic linkages of the cellulose chain (16, 53). To keep damage to the cellulose fibers at a minimum, the hypochlorite concentration should not be higher that 0.5% and the bleaching time should not extend beyond 5 minutes (51). In addition, hypochlorite bleaching is not recommended for ligneous papers owing to the formation of yellow chlorolignin derivatives.

O

O O

OCH3 OLIG A

B

OCH3 OLIG C

Figure 2. Chromophores of lignin believed to be main contributors to the yellowing of ligneous paper. A = orthoquinone; B = coniferaldehyde; C = aromatic ketone. LIG = rest of lignin molecule.



The hypochlorite solution should be made alkaline to a pH > 9.0 to suppress the hydrolysis of OCl{ and prevent the formation of unstable HOCl: OCl{ + H 2O

HOCl + OH {

(3)

In acidic solutions, HOCl forms and decomposes as follows (43): 3 HOCl → HClO3 + 2 HCl

(4)

HOCl + HCl → H 2O + Cl 2

(5)

In the presence of trace amounts of metal cations, HOCl can decompose to liberate oxygen: 2 HOCl → 2 HCl + O2

(6)

Clearly, the active ingredients in hypochlorite bleaches vary with pH (16, 50): at pH < 2, Cl2 is the main component in solution; at pH 4–6, HOCl is the dominant species; at pH > 9, OCl{ is the only component present. For alkaline solutions the oxidation of cellulose produces carboxyl groups, which are immediately neutralized. For neutral or acidic solutions, carbonyl groups are produced; these cause subsequent color reversion (51, 53, 59). To minimize color reversion, hypochlorite bleaching should be followed by bleaching with a reducing agent such as NaBH4 (58). Chlorine Dioxide. Chlorine dioxide has been used both in the gas phase and in aqueous solution (0.1–2.0%) as a bleaching agent. It can often remove severe stains (16, 51, 53–56). As chlorine dioxide has been known to explode without warning, particularly in the gas phase (60), it should be handled with extreme caution by the conservator. A relatively safe method for the in situ preparation of ClO2 (51, 53, 61) involves the reaction between sodium chlorite and formaldehyde: H2CO + H+ + ClO 2{ → HOCl + HCOOH HCOOH + HOCl + 2 ClO2{ → 2 ClO2 + Cl { + H2O + HCOO{

(7) (8)

To keep the pH below 7, deacidification must not be performed before bleaching. As the reaction proceeds as according to eq 7, the pH of the solution drops as formic acid is produced. The increase in acidity results in the formation of ClO2 according to eq 8. Chlorine dioxide behaves as an oxidizing agent in acidic solution, and the complete reduction of ClO2 is shown in eq 9: ClO2 + 4 H+ + 5 e{ → Cl{ + 2 H2O

(9)

The individual steps of this overall reduction reaction produce HClO 2, HOCl, and Cl2, all of which behave as oxidizing agents (62). In alkaline solution, ClO2 disproportionates according to eq 10—another reason why paper should not be deacidified before ClO2 bleaching: 2 ClO2 + 2 OH { → ClO3{ + ClO2{ + H 2O

(10)

Chlorine dioxide reacts readily with lignin in numerous ways. One such reaction results in chlorination and the formation of colored ortho- and para-quinones (16, 49). As the chlorinated compounds cannot be washed out without using strong alkali, ligneous papers should not be treated with ClO2 (51, 56). Degradation of the cellulose chain can also occur, but can be minimized by keeping the concentration of ClO2 below 2% (51).

Light Light bleaching, using either natural sunlight or artificial light, is performed while the paper artifact is immersed in an alkaline solution of Ca(OH) 2 or Mg(HCO 3) 2 (53, 55, 56). Damaging ultraviolet light is removed by the use of filters such as Mylar (63), Plexiglas, or Lexan, leaving the effective range of wavelengths between 400 and 550 nm. The solution provides dissolved oxygen necessary for photooxidation of chromophores and can also remove solubilized oxidized products following light bleaching. The recommended time for bleaching is 2–4 hours for natural sunlight and 8– 10 hours for artificial light. Light bleaching is a relatively simple method that can be useful in removing many types of stains. It should, however, be avoided for ligneous paper, which yellows more than it bleaches (56). Reducing Bleaches Only two reducing bleaches are commonly used by conservators today (51, 56). These are sodium dithionite (hydrosulfite or hyposulfite, Na2S 2O4) and borohydride (BH4{) salts. Sodium Dithionite. Both sodium and zinc dithionite have found use in the bleaching of mechanical pulps; whereas only the sodium salt is used in conservation bleaching (16, 43, 44, 46, 51, 56). The dithionite ion, S 2O42{, behaves as a strong reducing agent in alkaline solution (16, 43): S 2O42{ + 4 OH { → 2 SO32{ + 2 H2O + 2 e{

(11)

Although the reducing power of the dithionite ion drops off as the pH is lowered, both the industrial bleaching of wood pulps and conservation bleaching of artifacts are most effective in the pH range 6.0–7.5 (44, 56). Degradation of cellulose during dithionite bleaching does not seem to occur (64). The bleaching action of S2O42{ on wood pulp consists mainly in the reduction of quinones to phenols, thereby increasing the brightness of the pulp (44). Unfortunately, the reduced chromophores reoxidize over time, causing the pulp to fade. Conservators, in working with ligneous paper, have experienced the brightness reversion properties of dithionites. As a result, conservation work involving the dithionite ion is usually limited to the removal of ferric oxide stains (51, 56). Recently, conservators have used neutral citrate solutions of Na2S 2O4 to remove iron corrosion products from artifacts raised from the wreck of the Titanic (65). Borohydrides (Tetrahydridoborates). Sodium borohydride, NaBH4, has been employed in the industrial bleaching of mechanical pulps (43, 44) and in conservation bleaching (16, 51, 53–56, 66–68). The BH4{ ion is a strong reducing agent in alkaline solution (43): BH 4{ + 10 OH { → BO 33{ + 8 e{ + 7 H2O

(12)

Sodium borohydride aqueous solutions can be used in the concentration range of 0.01–2.0% and they self-buffer to a pH of about 9 (51). The slow decomposition of the BH4{ in solution can be a problem (66): BH { + 4 H O → B(OH) { + 4 H (13) 4

2

4

2

Hydrogen gas evolved from the reaction in eq 13 can penetrate the cellulose fibers and physically damage the paper artifact being treated. Therefore, it is important that hydrogen evolution be allowed to subside before im-


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LIG C

LIG O

H

C

OH

NaBH4

OCH3

OCH3 OLIG

OLIG

Figure 3. The reduction of aromatic carbonyl groups in lignin by NaBH4 to hydroxyl groups. LIG = rest of lignin molecule.

mersion of the paper artifact. Alternatively, the tetramethylammonium and tetraethylammonium salts, (CH3)4NBH4 and (C2H 5)4NBH4, can be used in conservation work where lower rates of decomposition have been observed (51). In addition, the BH4{ salts may be dissolved in either CH3OH or the less toxic C2H5OH. The decomposition of the BH4{ ion in alcohols occurs at a much slower rate: BH4{ + 4 ROH → B(OR)4{ + 4 H2 (R = CH3, C2H5) (14) It is important to deacidify the paper artifact before bleaching to remove acid, which accelerates the rate of decomposition of the BH4{ ion (66): BH4{ + H + + 3 H2O → B(OH)3 + 4 H2

Acknowledgment The author extends his thanks to Gordon Leary (Paprican) and Catherine Findlay (J. Pulp Paper Sci.) for the use of Figure 1 in this paper.

(15)

Finally, the solvents must be free of metal cations that can catalyze the decomposition of the borohydride ion (66). The bleaching action of borohydrides on cellulose involves the reduction of carbonyl groups to hydroxyl groups (16, 67). This results in increased brightness to the paper. The removal of carbonyl groups by the BH4{ ion also stabilizes the cellulose chain, as carbonyl groups provide a point of attack by alkali leading to chain scission (67). It is important, however, that the paper artifact be washed thoroughly after borohydride bleaching to remove sodium borate deposits, which could later lead to alkaline degradation of the paper (68). The effect of borohydride treatment on lignin is to reduce quinone and aromatic ketones and produce increased brightness in ligneous paper (15, 16, 43). One such reduction of an α-carbonyl group in lignin is shown in Figure 3. Overall, NaBH4 is an effective bleaching agent for groundwood (45) and in the conservation bleaching of both ligneous and nonligneous paper (68). It is particularly useful for eliminating the adhesive residues and tape stains often found in old books (67). However, borohydride bleaching must be approached with caution when inks, dyes, or colored pigments are present. The effect of the solution’s high pH can dissolve or dislodge colorants from old papers. Pigments also catalyze the decomposition of both NaBH4 and H2O2 solutions, leading to extensive gas evolution and possible damage to the artifact (69). Conclusions The main chromophores responsible for the yellowing of paper are known. Ligneous paper, in particular, yellows relatively easy in the presence of light owing to a recently established photooxidative mechanism involv-

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ing free radicals. Current research by institutes connected with the pulp and paper industries centers around the scavenging of free radicals or interrupting any of the steps that occur during the photoyellowing of lignin-containing paper. The chemistry of the yellowing of paper is of interest to the conservation scientist faced with the task of selecting a suitable bleaching method. The effectiveness of the bleaching agent on the chromophore systems present in the paper artifact, the possibility of both physical and chemical degradation to the paper as a result of bleaching, and the problem of color reversion are all concerns of the conservator. While the general viewpoint has been expressed that conservation bleaching should be used only as a last resort, removal of chromophores—particularly the reduction of carbonyl groups to hydroxyl groups—can have some stabilizing effects on paper. While the choice of a bleaching agent will depend on the nature of the paper artifact, hydrogen peroxide and borohydride salts are generally recommended, particularly for ligneous paper. For interested readers, a recipe for whitening old newspaper clippings appears elsewhere in this Journal (70).

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