Date: December 3, 2003
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).
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
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
• 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
• 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
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
• 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
• 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
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
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
• 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
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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