The Cochineal: An Online Student Journal and Repository of Conservation, Preservation, and Cultural Studies

Author: Jim Thurn
Date: December 3, 2003

History, Chemistry, and Long-Term Effects of Alum-Rosin Size in Paper

Sizing of paper is a common practice to reduce absorption of liquid, including ink, into the paper surface. Different methods of sizing paper have been practiced throughout the history of papermaking. Alum-rosin size was an important method of sizing paper since the early 1800's, although its addition to paper has decreased sharply since the mid- to late 1980's.

Due to the importance of alum-rosin size in producing paper with resistance to liquids, the paper industry devoted much effort to study the chemical interaction of alum, rosin, and paper fibers. Although alum-rosin size was very effective in reducing absorption of liquids, a negative impact was the reduction of pH in paper, leading to chemical breakdown of cellulose fibers. This report presents an overview of the history, chemistry, and long-term effects of alum-rosin size in relation to papermaking.
Paper may be sized internally and/or externally. The most common historical internal size for paper consisted of rosin. Pulp fibers, fillers (e.g., clay), and size particles are usually negatively charged. Since particles with the same charge repel one another, the papermaker must intercede to promote the attachment of size and fillers to the pulp fibers. Alum was found to greatly facilitate the attachment of negatively-charged molecules (Smook 1982). Thus, alum acts as a mordant to bind together the different components of paper. Optimum performance of alum-rosin size during paper manufacture occurs at a pH of 4 to 5.5 (Arnson 1982). An unfortunate consequence of such a low pH level is acidic paper, which promotes acid hydrolysis and scission of cellulose molecules.

Alum was also often used without rosin for the control of pH (i.e., to reduce pH), and to increase the retention of materials other than rosin (e.g., fines, fillers, and pigments) in the paper. Rosin is reportedly of little significance in the physical deterioration of paper, but the acidity associated with alum is a major cause of paper deterioration. In fact, the pH of paper is the most significant factor influencing strength loss over time (Casey 1981).

History of Alum-Rosin Size
The following presents a brief overview of the history of alum-rosin size. To provide an historical context for comparison purposes, the use of alum alone, gelatin alone, and alum in combination with gelatin, is described. The information is presented chronologically.

Historically, gelatin was very frequently applied as an external, surface size on finished sheets of paper. The gelatin was applied to finished paper either alone or in combination with alum. Alum was added to gelatin size to control the viscosity of gelatin at different temperatures and concentrations, to prevent the growth of molds and bacteria, and to reduce the absorption of inks into the gelatin and paper substrate (Barrow 1974).

The earliest use of gelatin size in paper probably occurred in Europe in 1337 (Hunter 1947). The earliest use of alum in combination with gelatin is believed to have occurred during the 1500's. Evidence of the first use of alum alone occurs in a papermaking handbook dated 1634 and, by 1660, alum usage was common at paper mills (Barrow 1974). Barrow (1974) studied the paper contained in 250 books produced throughout the 1700's, and detected alum in 213 (85%) of the books. Importantly, of the 37 books containing no alum, 31 were produced between 1700 and 1750. Only six of the books containing no alum were produced between 1750 and 1799. Thus, the results show a trend of increasing alum usage during the 1700's.

Alum-rosin size was invented by Moritz Friedrich Illig in Germany in 1807, which eventually replaced alum-gelatin size due to its lower cost (Barrow 1974; Green 1992). Paper mills were commonly adding alum-rosin size to the papermaking stock by the 1840's (Kusterer and Sproull 1973). In a study of books published throughout the 1800's, alum was detected in 91% of the book papers, and rosin was detected in approximately 70% of the papers tested from the mid- to late 1800's (Barrow 1974).

Studies conducted on the condition of paper in 500 books published between 1900 and 1949, showed alum was present in 98.7% of the papers, and rosin was present in 74% of the papers. These results suggest that alum alone was used in 24.7% of the papers to improve the operating performance of the paper machine (i.e., control pH and improve retention of fines, fillers, and pigments). Interestingly, even though the books published between 1900 and 1949 were the newest books evaluated in the studies, the paper contained in the newer books showed the lowest fold resistance of any of the papers studied, which included papers produced as early as 1507. The tear resistance of the newer papers was also low (Church 1959; Barrow 1974).

However, one should not assume the presence of alum-rosin size is soley responsible for the reduced strength of the newer papers. For example, the study revealed 27% of the tested papers produced between 1900 and 1949 contained groundwood (i.e., shortened fibers), but only 20% of the tested papers produced in the mid- to late 1800's contained groundwood (Barrow 1974). Thus, other variables likely contributed to a decrease in strength characteristics of the newer papers.

The first commercially-produced alkaline papers were manufactured by paper companies such as Hercules, Inc. and S.D. Warren during the 1950's (Smith 1990). The first commercially-produced, permanent, durable paper was manufactured in 1960 through the use of a synthetic, non-acidic size, and the addition of calcium carbonate as a mild, alkaline buffer (Barrow 1974). Synthetic polymer mordants commonly replaced alum at paper mills in the mid- to late 1980's, allowing mills to operate under neutral or alkaline conditions (Au and Thorn 1995). The first alkaline papermaking seminar sponsored by the Technical Association of the Pulp and Paper Industry was held in 1983, which provides evidence of the increased attention given by the paper industry to alkaline papermaking (Tappi Journal 1982).

The following factors contributed to the increasing trend of neutral and alkaline papermaking, and a concurrent decrease in acidic alum-rosin sizing (Smook 1982; Walkden 1990; Final Report to Congress 1995):
• Passage of 40 CFR 430 - Pulp, Paper, and Paperboard Point Source Category, 1993
• Passage of Public Law 101-423 - Joint Resolution to Establish a National Policy on Permanent Paper, 1990
• Availability of synthetic sizing chemicals suitable for alkaline processes
• Ability to use calcium carbonate in alkaline processes
• Lower cost of raw materials
• Lower energy consumption
• Reduced corrosion of papermaking machinery

Non-acidic, synthetic sizing chemicals allowed the paper industry to convert many mills from acidic processes to alkaline processes. This trend increased in 1993 after passage of the environmental regulation, 40 CFR 430- Pulp, Paper, and Paperboard Point Source Category, which governs effluent discharges from pulp and paper mills. The environmental regulation made conversion from acidic to alkaline processes economically favorable for paper mills, and was the most significant influence in the shift toward alkaline processes (Final Report to Congress 1995).

Passage of Public Law 101-423, Joint Resolution to Establish a National Policy on Permanent Paper, in 1990 also provided incentive for paper mills to convert to alkaline processes. The law states, "It is the policy of the United States that Federal records, books, and publications of enduring value be produced on acid free permanent papers". The law also encourages American publishers, as well as state and local governments, to use permanent papers for documents of enduring value (Final Report to Congress 1995).
A timeline showing major milestones of alum and rosin usage in paper is provided in Figure 1 (Barrow 1974; Au and Thorn 1995).

Figure 1. Major milestones of alum and rosin usage in papermaking

Chemistry of Alum-Rosin Size
Alum-rosin chemistry has been studied extensively by researchers in the pulp and paper industry. However, despite the vast amount of research conducted, the complexity of alum-rosin chemistry has resulted in several theories regarding how alum, rosin, cellulose, and other compounds interact during paper manufacture. One theory of alum-rosin chemistry, based on coordinate chemistry, is presented in the following paragraphs (Biermann 1996).

Both alum and rosin have been added to pulp in several different forms. The term "alum" refers to a group of double salts. Aluminum potassium sulfate [KAl(SO4)2] is a type of alum, which was often used in conjunction with rosin for sizing of paper. In contrast, "papermakers alum", aluminum sulfate [Al2(SO4)3], is technically not an alum since the compound is a single salt, but was also often used in alum-rosin sizing of paper (Casey 1981; Budavari 1989).

Aluminum is the active component in alum, and its properties are important to the sizing process. The aluminum ion has a high charge of +3, and a small ionic radius of 0.50 angstrom, which results in a high charge density. The high charge density is responsible for the diverse chemical reactions of Al+3 because the ion readily reacts with other species to form a lower energy state. Much of the complexity of alum-rosin chemistry (and the existence of differing theories) is due to the many possible reactions of Al+3 with other constituents in aqueous solutions. The occurrence of specific reactions, and the types of aluminum compounds formed, are dependent on many variables. One of the most important variables influencing alum-rosin chemistry is the pH of the solution. The reactions most favorable to alum-rosin sizing of paper occur in a pH range of 4.0-5.5 (Arnson 1982).

Rosin is an amber-colored, natural resin present in southern pine. The rosin is tapped from trees, extracted from stumps, or processed from tall oil (Smook 1982). Rosin consists of a group of closely-related diterpene acids. The molecular structure of the most common diterpene acid, abietic acid, is shown in Figure 2 on the following page (Roberts, 1996).

Figure 2. Molecular structure of abietic acid

Like alum, rosin has been added to papermaking stock in two different forms. One form of rosin is a free acid rosin dispersion, known as rosin acid emulsion. The second form of rosin is produced by saponification to create a soluble, alkali metal soap, known as rosin soap (Casey 1981). Research suggests that different sizing mechanisms occur depending on whether the rosin acid or rosin soap form is added to the papermaking stock (Roberts 1996).
Rosin is a twenty-carbon organic acid, and is considered an amphipathic material because the compound contains both hydrophilic and hydrophobic parts (Smook 1982). Figure 3 on the following page shows the aliphatic and aromatic forms of rosin, as well as the hydrophilic and hydrophobic portions of both forms (Gess 1989). The aliphatic form of rosin is abbreviated in Figure 3, as designated by the parentheses in "(CH2)", since the molecule actually contains twenty carbons.

Figure 3. Aliphatic (left) and aromatic (right) forms of rosin, showing the hydrophobic and hydrophilic portions of both forms.

The following conditions are required for proper sizing of paper (TAPPI 1970):
• Formation of size precipitate characterized by a low free-surface energy and, therefore, a high water repellency.
• Formation of a uniform coating of size precipitate over the fiber surfaces.
• Conversion of the liquid size on the fiber surface to a stable, low free-surface energy film (i.e., aluminum rosinate), which remains stable even if contact with fluids occurs.

Rosin is added to pulp and precipitated onto fibers by alum. To provide effective sizing, the hydrophobic parts must be oriented outward and away from the fibers, where they can perform their function of repelling water (Smook 1982). The proper orientation of rosin molecules for sizing of paper is shown in Figure 4 on the following page (Gess 1989).

Figure 4. Hydrophilic portions of rosin molecules are anchored to the cellulose surface. Hydrophobic portions of rosin molecules are oriented away from cellulose to repel water.

Normal covalent bonds occur when each atom donates one electron to the pair. However, coordinate covalent bonds are formed when one atom donates both electrons of the electron pair. The reactions of alum in aqueous solutions are often explained by coordinate chemistry. Coordination complexes occur when Lewis acids (compounds which accept electron pairs) react with Lewis bases (compounds which donate electron pairs). An example of a simple coordinate chemistry reaction is the reaction of hydronium ion with hydroxide ion to form water as shown below (Biermann 1996).

In the above reaction, the electron pair is donated by the hydroxide ion (base) to form a water molecule in which the electron pair is shared. A "ligand" is a species which donates an electron pair(s), so ligands are considered Lewis bases. When alum is added to an aqueous solution, the compound dissociates, liberating Al3+ cations. The aluminum cation has a coordination number of six, which results in the aluminum cation reacting with six electron pairs of ligands. If no other ligands are present, Al3+ reacts with six water molecules to form a hydrated complex with the formula [Al(H2O)6]3+. The octahedral-shaped aluminum complex is shown on the left side of the reaction in Figure 5 on the following page. The oxygen atom of the OH- ions forms bonds with two different coordinating cations, producing two hydroxo bridges as shown on the right side of the reaction in Figure 5 (Biermann 1996).

Figure 5. Hydrated aluminum complex (left) reacts with OH- to form hydroxo bridges (right).

During rosin sizing, alum also forms bonds with other ligands besides water, such as rosinate anions, carboxylate groups, hydroxyl groups, and sulfate anions to form a compound known as the aluminum-rosinate complex. The aluminum-rosinate complex is a double bridged oxo compound, which forms after the loss of hydrogen from the hydroxo bridges. One possible form of the aluminum-rosinate complex is shown in Figure 6 (Biermann 1996). The complex is a colloidal compound, so its solubility in water is limited. Reduced solubility occurs due to the increased size of the complex, hydrophobic linkages, and a reduction of charge due to coordination of the aluminum cation with anions (Biermann 1996).

Figure 6. Hypothetical aluminum-rosinate structure

Formation of oxo bridges are important to alum-rosin sizing since the bridges enlarge the aluminum-rosinate complex and decrease the solubility of the complex in water. The average degree of polymerization varies with pH. For example, at a pH of 5, as much as 90% of the aluminum is in the form of polymers, which provides effective sizing. However, at pH below 4, alum is ineffective because Al3+ ions do not complex with OH- ions, and, therefore, hydroxo bridges are not formed. As a result, the solubility of alum is increased and retention of rosin on fibers is reduced (Biermann 1996).
Coordination reactions occur in dilute pulp mixtures at the wet end of the paper machine, as well as in the press and dryer sections. In fact, coordination reactions in the press and dryer sections, with elevated temperatures and higher concentrations of species, are probably more important to the sizing process than reactions occurring in dilute solutions (Biermann 1996). Higher concentrations of constituents allow more opportunities for bonding, and higher temperatures may allow for a more even distribution of the size throughout the sheet due to a sintering effect (Gess 1989). Furthermore, the oxo bridges most likely form in the press and dryer sections of the paper machine (Biermann 1996).
Other theories, or modifications of existing theories, for alum-rosin chemistry also exist. For example, Gess (1989) proposed mechanisms of alum-rosin chemistry to explain differences observed in paper sized with rosin acid emulsion and paper sized with rosin soaps. The observed differences include:
• Size regression (i.e., gradual loss of size over time)
• Size migration in rolls of paper at elevated temperatures
• Size migration from sized paper to adjacent unsized paperLong-Term Effects of Alum-Rosin Size

The acidity associated with alum is known to have long term degradative effects on paper. In the presence of moisture, acidic paper undergoes acid hydrolysis causing scission of cellulose chains. Acid hydrolysis accounts for approximately 90% of chemical deterioration in paper. The mechanism for acid hydrolysis of cellulose is shown in Figures 7, 8, and 9 on the following page (Smook 1982; Smith 1990).

Figure 7. Cellulose in contact with acid (H+).

Figure 8. Hydrogen ion attaches to the oxygen atom between glucose units, removing an electron from the adjacent carbon.

Figure 9. Scission of cellulose chain.

As shown in Figure 9, a hydrogen ion is released each time scission occurs. This hydrogen ion is then able to catalyze the hydrolysis process again. As cellulose molecules become shorter and shorter due to hydrolysis, the paper becomes weaker and weaker. After one half to one percent of the cellulose molecules are broken, the paper is significantly weakened (Hollinger 1984).

Historically, alum-rosin sizing of paper was a very common method to impart resistance to liquid absorption. Although the method was very effective for its intended purpose, the method is deleterious to the long-term chemical stability of paper. Paper containing alum-rosin size is susceptible to hydrolysis due to the acidity caused by alum. Fortunately, most paper manufactured today is produced by neutral or alkaline processes and, therefore, is not as susceptible to hydrolysis reactions. However, important historical papers containing alum-rosin size require proper care to reduce the harmful effects of acidity.

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