The Multifunctional Role of Aluminum Sulfate (Alum) in Papermaking

Aluminum Sulfate, commonly referred to as alum with the chemical formula Al₂(SO₄)₃, is a versatile inorganic compound that has been integral to the papermaking industry for centuries. Historically, its use dates back to at least the 14th century in Europe, initially as aluminum potassium sulfate imported for dyeing and later as aluminum sulfate in the 19th century, earning the moniker "papermaker's alum" due to its affordability and efficacy in rosin sizing. In modern papermaking, alum serves multiple functions, including as a sizing agent, retention aid, pH adjuster, and flocculant in water treatment. It enhances paper quality by improving resistance to liquids, controlling ink absorption, and aiding in the retention of fillers and fibers. Alum is particularly valuable in producing cultural papers (e.g., writing and printing papers), packaging papers, and industrial papers, though it is typically omitted from specialty papers like blotting or filter paper that require high absorbency. Its applications span acidic papermaking systems (pH 4-6) and, to a lesser extent, alkaline ones, where it interacts with pulp components to form complexes that modify surface properties. While cost-effective and widely used, alum's acidity can pose challenges for long-term paper durability.

Role in Paper Coating: Improving Ink Resistance

Paper coating involves applying a layer of pigments, binders, and additives to the paper surface to enhance printability, gloss, and surface smoothness. Aluminum sulfate plays a pivotal role in this process, particularly in formulations involving glue or gelatin binders, where it acts as a hardening agent and stabilizer to improve ink resistance—the ability of the paper to prevent ink penetration, bleeding, or feathering during printing.

Mechanism in Coating

Alum is often used in the production of satin white, a high-gloss pigment formed by reacting aluminum sulfate with slaked lime (calcium hydroxide). This reaction produces calcium sulfate and aluminum hydroxide, which contribute to a smooth, bright coating layer suitable for high-quality printing papers. In glue-bound coated papers, alum is added to the coating mixture to harden the glue (typically hide or bone glue), increasing the setting and melting points of the binder. This hardening is achieved through cross-linking reactions where aluminum ions form complexes with the glue's protein molecules, reducing the coating's solubility and enhancing its resistance to moisture and inks. For instance, in historical gelatin-alum coatings, alum concentrations of 5-40% (based on dry gelatin weight) were used to stabilize the viscosity and prevent spoilage, while also imparting a uniform luster and reducing ink absorbency.

Improving Ink Resistance

Alum's primary contribution to ink resistance in coatings is through controlling ink absorption and adhesion. By forming a less porous surface, it delays the penetration of ink vehicles (e.g., oils or solvents), preventing bleed-through and ensuring sharper, more defined prints. Studies on alum-glue solutions, such as those used in traditional Xuan paper, show that alum treatment can decrease ink absorbency by up to 80% compared to glue-only coatings, as the aluminum ions create hydrophobic barriers on the fiber surfaces. In modern applications, this results in better ink holdout, where the paper surface retains ink on top rather than allowing it to soak in, leading to improved color vibrancy and reduced ink usage. Experimental printing tests on glue-bound coated papers treated with alum (e.g., at 1.5-3.5% of glue weight) demonstrated no picking, lifting, or dusting, with print quality comparable to casein-bound papers, including clear highlights, deep blacks, and smooth gradations. Alum also aids in offset and lithographic printing by enhancing water resistance, which indirectly supports ink stability during wet processes.

Practical Applications and Benefits in Coating

In high-grade coated papers, alum enables the use of satin white for superior gloss and whiteness, requiring higher adhesive proportions but yielding papers with excellent printability. It is particularly effective in watercolor papers, where surface applications of alum solutions ensure uniform moisture resistance and prevent pigment alterations. Benefits include enhanced tensile, burst, and tear strength, as well as resistance to water, ink, and other liquids, making coated papers more durable for packaging and printing. However, overdosing can lead to excessive acidity, potentially causing clay settling or reduced coating uniformity.

Other Areas of Application in Papermaking

Beyond coating, aluminum sulfate's versatility extends to several key areas in papermaking, where it acts through charge neutralization, precipitation, and complex formation.

Sizing (Internal and Surface)

Alum is a primary precipitant in rosin sizing, where it reacts with rosin soap or emulsion to form insoluble aluminum resinate, anchoring it to pulp fibers. This creates a hydrophobic layer that improves water, ink, and oil resistance, essential for 80% of papers to prevent fluid penetration and maintain strength. In historical gelatin sizing, alum hardened the size, reducing absorbency for writing papers. Dosages typically range from 1.5 parts alum per part rosin, with pH optimized at 4.5-5.5 for best retention.

Retention Aid

As a coagulant, alum neutralizes anionic charges on fibers and fillers, promoting flocculation and retention of fines, pigments, and additives during sheet formation. This reduces losses in white water and improves paper uniformity, especially in systems with high anionic trash.

pH Control and Adjustment

Alum's acidic nature (pH ~3 in 1% solution) buffers the pulp to acidic levels, facilitating reactions in rosin sizing and anionic agents like AKD or ASA. It forms various aluminum species (e.g., Al³⁺ at low pH, Al(OH)₃ at neutral) that interact with carboxyl groups.

Water Treatment in Papermaking

In treating papermaking white water (recycled process water), alum acts as a flocculant to remove suspended solids, fibers, and turbidity through coagulation and settling. This clarifies water for reuse, reduces color, and minimizes environmental discharge.

Pitch Control and Dye Fixation

Alum binds pitch (resinous deposits) to fibers, preventing equipment fouling, and fixes cationic dyes to reduce bleeding in colored papers.

Historical and Miscellaneous Uses

In bookbinding, alum toughened decorative paste papers. It also aided bleaching and deflocculation in early mills.

Benefits and Challenges

Alum offers economic benefits like improved paper strength, printability, and resource efficiency, but challenges include potential paper degradation from acidity (e.g., pH drop leading to brittleness and discoloration over time) and the need for careful dosing to avoid deposits or reduced brightness. Modern alternatives like polyaluminum chloride address some issues, but alum remains a cornerstone due to its proven efficacy.