Chemistry Everyday for Everyone
The Chemistry of Paper Preservation Part 4. Alkaline Paper Henry A. Carter Department of Chemistry, Augustana University College, Camrose, AB, Canada, T4V 2R3 The inherent instability of paper due to the presence of acids that catalyze the hydrolytic degradation of cellulose has been previously documented in this Journal (1–3). The major source of acidity in paper arises during the manufacturing process, which since 1850 has commonly employed alum-rosin sizing (4). While deacidification methods can be used by conservators to retard the deterioration of old papers (3), preventive measures are needed to enhance the long-term stability of new papers (5–6). One possible solution to alleviate the problem of acidity in paper is to produce alkaline paper. Indeed, the papermaking industry is now converting to the manufacture of alkaline paper although the reasons may be more economic and environmental than cultural (7). The following recent developments have contributed to the demand for alkaline papermaking (8): •
increase in cost of pulp fibers and the filler pigment TiO2
•
increased demand for higher brightness and opacity in paper
•
availability of finely divided CaCO3 that can be used as a filler in paper
•
the influence of Western European markets, which have used alkaline papermaking since the 1970s
•
need for new technology to address environmental concerns.
Some of the major changes in going from acid paper to alkaline paper involve the nature of the chemical additives used in sizing and the composition of the filler. This paper will focus on the chemistry involved in the sizing of both acid and alkaline papers and the types of fillers used. Later, some advantages and also some problems of alkaline papermaking will be presented. Alum-Rosin (Acid) Sizing Sizing is the process whereby paper is chemically treated to give the hydrophilic cellulose fibers a degree of resistance to wetting and penetration by aqueous liquids (9). This is necessary to prevent the running of printing ink on paper. The waterproofing agent used in acid paper is alumprecipitated rosin size. Rosin is an amorphous, glassy material consisting mainly (about 90%) of monocarboxylic resin acids with alkylated hydrophenanthrene structures having the empirical formula C 19H27–33COOH (9). The most important resin acid found in commercial rosin is abietic acid, C 19H 29COOH, shown in Figure 1 (10). These resin acids are soluble in organic solvents but virtually insoluble in water owing to their large hydrophobic Figure 1. Abietic acid, portion, which shields the carone of several resin acboxyl groups. As a result, the ids found in rosin. resin acids of rosin provide an ef-
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fective hydrophobic coating for paper. Before use, however, the resin acids in rosin are either partially or completely neutralized by aqueous alkali to produce “rosin size” (9, 10). Before addition to the pulp slurry (or to the formed sheet of paper), the rosin size is mixed with water to form a suspension. Papermaker’s alum, Al2(SO4 )3 ?18H 2O, is then added to the aqueous dispersion of rosin size particles (9–11). Aluminum sulfate dissolves in water to form sulfate ions and hexaaquaaluminum(III) [Al(H2O)63+ ], ions. While a number of complex ions are formed in aqueous aluminum(III) solutions (12), the most important reaction is the first step of the acid hydrolysis of the Al(H2 O)63+ ion: [Al(H2O)6]3+ + H2O
[Al(OH)(H2O)5]2+ + H3O+ (1)
The acid ionization constant, Ka, for the equilibrium in eq 1 equals 1.12 × 10{5, making the Al(H2O)6 3+ cation the dominant species in solution (13). Precipitation of completely neutralized rosin size with alum occurs as follows (9): [Al(H 2O)6]3+ + R{
[Al(OH)(H2O)5]2+ + HR
(2)
where R{ = resinate ion and HR = resin acid. In eq 2, the resinate ion abstracts a proton from the Al(H2O)6 3+ ion to form the resin acid and a hydroxide group bound to aluminum. The formation of the Al–OH bond weakens the bonding of the remaining water molecules to aluminum, with the result that two water molecules can be displaced by resinate ions to form hydrated aluminum diresinate monohydroxide: [Al(OH)(H2O)5]2+ + 2R{
AlR2(OH)(H2O)3 + 2H2O (3)
Both the resin acid formed in eq 2 and aluminium diresinate monohydroxide formed in eq 3 are insoluble in water and precipitation occurs. The precipitate containing an approximately 1:1 molar ratio of resin acids and aluminum diresinate monohydroxide is, in fact, the sizing agent found in acid paper. The actual composition of the precipitate will vary according to the degree of hydrolysis of aluminum diresinate monohydroxide, the presence of other ions in the pulp slurry such as Ca2+, HCO3 { and OH {, the initial ratios of the reactants, and the possible formation of polymeric products (9). Electrostatic interaction, physical adsorption, and van der Waals attractive forces all play a role in the adhesion of the “sticky” resin derivatives to the cellulose polymer (9). The size may also be fortified or stabilized by the addition of a number of “modifiers” such as polyfunctional acids or their anhydrides. During the drying process at 110–120 °C, the precipitate tends to spread and polymerize, creating a hydrophobic surface over the cellulose fibers. The degree of sizing is adversely affected by conditions of high acidity or high alkalinity (9). In fact, the adhesion of the size precipitate to the cellulose fibers in the slurry is most effective in an acidic environment with the pH range between 4.2 and 4.8 (14).
Journal of Chemical Education • Vol. 74 No. 5 May 1997
Chemistry Everyday for Everyone (a)
(b)
Figure 2. Sizing reactions of (a) alkyl ketene dimers and (b) alkyl succinic anhydrides. Cell–OH = cellulose; R, R9 = short alkyl chains.
(a)
lose (15). However, the hydrocarbon chains protect the 4-membered and 5-membered rings of AKD and ASA respectively and the hydrolysis of the sizing agents is kept to a minimum (9). Both AKD and ASA are introduced into the paper in the form of an aqueous emulsion to which cationic starch has been added as a stabilizer and retention aid (9, 15). For both sizing reactions, the sizing agent is chemically bound to the cellulose substrate with the hydrophobic hydrocarbon chains pointing outward, thereby producing a water-repellent surface. Since chemical bonds are formed between cellulose and the sizing agents, alum is not required and the sizing reactions can take place efficiently in a neutral-to-alkaline medium (pH = 6.5–8.5). Accelerated aging tests suggest that alkaline sized paper can last several hundred years, in contrast to alum-rosin sized paper, which may last only 25–50 years, depending on the conditions of storage (9). Fillers
For many years, kaolin and titanium dioxide have been used as fillers in acid papermaking (7, 16). Although expensive, TiO2 adds opacity and brightness to paper. Alkaline papermaking, however, allows for greater choices in mineral fillers such as calcium carbonate. Different forms of calcium carbonate can be used. In Western Figure 3. Hydrolysis reactions of (a) alkyl ketene dimers and (b) alkyl succinic anhyEurope, CaCO3 is readily available from drides. R, R9 = short alkyl chains. chalk deposits and this has contributed to the growth of alkaline papermaking there Alkaline Sizing (6). In the U.S.A., CaCO3 can be obtained from precipitation by carbonating calcium hydroxide (7, 15). This precipiAn alternative to alum-rosin sizing is the use of the tated form of CaCO3 (PCC) is much purer and finer than synthetic sizing agents, alkyl ketene dimers (AKD) and naturally occurring limestone and can be produced on site alkyl succinic anhydrides (ASA) (6, 9, 15). These sizing at the paper mill (17). A second form of CaCO3 available to agents operate in the neutral-to-alkaline pH range, and the papermaker is ultrafine ground limestone (UFGL) obchemically react with the hydroxyl groups of cellulose. tained from the wet grinding of CaCO3 (8). Thus, AKD and ASA are referred to as “chemically reactive Finely precipitated calcium carbonate imparts high sizing agents”. These sizing agents consist of a hydrophilic brightness and adds good print quality to the paper. As group, which chemically binds the sizing agent to the cellusuch, CaCO3 can be used to replace expensive TiO2. In adlose fibers, and hydrophobic groups that, when orientated dition, more filler can be used in alkaline papermaking, alaway from the paper fibers, impart water repellency. though the amount of CaCO3 must be controlled for optiThe sizing reaction of the alkyl ketene dimer with celmum paper strength and oil absorptivity. Calcium carbonlulose is depicted in Figure 2(a). AKD contains two hydroate is ideally suited for neutral–alkaline systems but not carbon chains, R and R9, which are attached to a 4-memfor acid systems, where chemical reaction occurs evolving bered lactone ring. As the alkyl ketene dimer reacts with carbon dioxide. the hydroxyl groups of cellulose, the lactone ring opens and a β-ketoester linkage forms between cellulose and the Advantages of Alkaline Paper ketene dimer at the site of the carbonyl group. At the same time, the hydrophobic groups, R and R9, are necessary to Some advantages of alkaline papermaking can be sumimpart sizing to the paper. marized as follows (18–27): The sizing reaction of the alkyl succinic dimer with cellulose is depicted in Figure 2(b). ASA also contains two hy1. Improved Quality of Paper: The use of finely predrocarbon chains, R and R9, which are attached to a 5-memcipitated CaCO3 offers a built-in alkaline rebered ring. Again, during reaction between ASA and the serve and improved brightness over kaolin/TiO2 hydroxyl groups of cellulose, the ring opens and a βused in acid paper. Accelerated aging tests indiketoester linkage is formed between the sizing agent and cate a greater retention of tear strength and cellulose. folding endurance for papers containing CaCO 3 Both AKD and ASA are hydrolyzed by water as shown filler over papers containing a clay filler (18). in Figure 3 to form products which do not react with cellu(b)
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Chemistry Everyday for Everyone 2. Reduced Energy Expenditure: Less energy is expended during the alkaline papermaking process as drying is easier and the efficiency of refining or beating fibers increases in going from an acidic to an alkaline environment (15). The increase in paper strength corresponding to the increase in refining efficiency allows for more flexibility in the composition of the final paper product. To cut costs, a greater use of mineral filler can be used to replace paper fibers increasing the brightness and opacity of the paper but maintaining an acceptable mechanical strength. 3. Reduced Use of Fresh Water: It is possible to recycle alkaline water at many stages during the papermaking process. As a result, less fresh water is consumed during alkaline papermaking. 4. Reduced Effluent Treatment: Alkaline waste waters can be treated directly without adjusting the pH (23). The use of CaCO3 to achieve natural brightness may allow for less chlorine bleaching, and the discharge of alkaline waste water containing fewer organic chlorine compounds is less damaging to the environment (18, 19, 21, 23). 5. Reduced Machine Corrosion: In comparison to alum–rosin (acid) sizing, alkaline sizing is less corrosive to the papermaking equipment (18, 23). The alkaline pH allows for the use of steel and iron in place of the more expensive stainless and specialty steels. In addition, scales and deposits from the buildup of corrosive salts are considerably reduced for alkaline papermaking, leaving the equipment cleaner. 6. Increased Ease of Recycling Paper: Alkaline papermaking facilitates the recycling of paper that contains calcium carbonate as a pigment in its coating. The recycling of CaCO3 -coated paper under acid conditions is difficult owing to the reaction of acid with calcium carbonate, which uses up alum, produces foam, and generally interferes with the alum–rosin sizing process (19). 7. Improved Efficiency of Sizing: Alkaline systems require only 2–5 pounds of synthetic size per ton of paper, in contrast to rosin-based systems, which normally require about 10–15 pounds of size per ton (23, 24). Control is easier with synthetic sizes, as more or less sizing can be achieved by adjusting the addition rate of the size.
the diacid salts formed from hydrolysis of ASA, as seen in Figure 3(b), tend to deposit on the wet press rolls (36). The slow reaction of AKD with water, as seen in Figure 3(a), is enhanced in an alkaline medium, and stearone, an inert waxy ketone, can form, leading to undesirable deposits (36, 37). The correct choice and amount of retention aid additive can minimize hydrolysis and increase sizing retention (32, 37). Other problems with AKD and ASA involve adsorption of the size by PCC filler, preventing the formation of βketoester bonds with cellulose (37, 39). The use of surface sizing agents, such as styrene maleic anhydride, styrene acrylic acid, and polyurethane polymers, in combination with the internal sizing agents AKA and ASA, can alleviate this problem as well as minimize problems associated with using only an internal size (38, 39). Some calcium carbonate filler of fine particle size as well as additives can also enter the papermaking system and contribute to deposit formation. The use of coagulants, such as cationic starches, and flocculants, such as acrylamide-based polymers to bridge the coagulated particles, can cause agglomeration of the fine particles and prevent dispersion into the system (31). Otherwise, CaCO 3 filler and other salt deposits in alkaline machine systems can be removed by acidic solutions (30). Small amounts of alum and other sources of cationic aluminum are frequently used as additives in alkaline papermaking to improve the sizing performance of synthetic sizes and overall retention of fillers, fibers, and sizes (40– 42). This can lead to the slow formation of Al(OH)3 and also Al2(PO4)3 deposits, if phosphate ions are present in the system. Possible solutions to the problem of aluminum compound deposits include the replacement of alum by organic cationic donors, the use of chelating agents such as EDTA to tie up metal cations, and the use of “threshold inhibitors” such as polyphosphates, which inhibit crystal growth and maintain ion pairs in solution beyond that predicted by the solubility product (31). Still other forms of deposits must be controlled—for example, wood pitch, which can appear as a sticky residue under alkaline conditions (30). Alkaline papers using only precipitated CaCO3 (PCC) as a filler tend to be overly bright, porous, and slippery and to require more sizing (35). Noncarbonate filler pigments can be added to improve the quality of alkaline papers. For example, addition of clays can reduce the brightness, porosity, and size demand of alkaline paper, while TiO 2 can improve its opacity. The addition of silica and metal silicates can increase the coefficient of friction of alkaline paper, as can use of styrene maleic anhydride as a surface sizing agent (35, 38). Conclusions
Problems in Alkaline Papermaking and Possible Solutions While alkaline papermaking has been gaining momentum in the pulp and paper industry, there are some problems in the alkaline process that need to be overcome (26– 39). One problem is that microbiological growth occurs in neutral–alkaline environments, resulting in corrosive deposits. This can be alleviated by cleaning and use of oxidizing biocides (30, 32). Problems also arise from use of AKD and ASA as sizing agents. Hydrolysis of unretained sizing agent produces sticky deposits on the papermaking equipment. ASA emulsions tend to be more reactive than AKD emulsions, and
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Since the 1970s, alkaline papermaking has been happening in Western Europe, where there are large supplies of CaCO3 in the form of chalk, but short supplies of fiber and clay. With the availability of ultrafine and precipitated calcium carbonate, the conversion to alkaline papermaking has been taking place in the U.S.A. and Canada over the last 5–10 years. Alkaline papermaking offers both economical and environmental advantages over the traditional acid papermaking. Some concerns of librarians and archivists are also being met, as the quality of alkaline paper is superior to acid paper in terms of permanence. While many paper mills have successfully switched to alkaline papermaking, the conversion does not occur without some problems. Adjustments to equipment, operating conditions, retention aids, and other chemicals must be an-
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Chemistry Everyday for Everyone ticipated and corrections made. It should be noted that alkaline paper is not to be confused with permanent paper of archival quality. Folding endurance and tear strength tests following accelerated aging do indicate that alkaline paper will outlast the same paper that has been acid sized (18). However, other requirements must be met for permanent papers. Suggestions for standards for permanent paper include a minimum mechanical strength as measured by tear resistance, a minimum alkaline reserve, a minimum resistance to oxidation, a maximum lignin content, and a minimum and maximum pH value for an aqueous extract (43). The choice of standards for papers of archival quality has been the subject of much heated debate, particularly in regard to lignin content and whether lignin is detrimental or beneficial to the stability of cellulose in paper (44–48). Literature Cited 1. 2. 3. 4.
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