The most important use of alum is its ability to impart paper with resistance ("sizing") to penetration by water or aqueous liquids. The sizing of paper stops writing inks from blurring and feathering and helps paper bags retain their strength by resisting moisture absorption. Rosin is the most commonly used sizing agent to impart a moisture-resisting property to paper. The role of alum in paper sizing is to precipitate rosin size on pulp fibers prior to paper formation. Most of alum's applications depend on the strong positive charge its polyvalent species have in solution. The aluminum-rosinate reaction is the key to paper sizing. When used with a soluble soap rosin sizing, alum fulfills three critical functions:

• It provides the acid condition and aluminum ions that precipitate the size.
• It gives the precipitate a positive charge that attracts it to the negatively charged fiber surface.
• During drying, the aluminum anchors the hydrophobic portion of the size onto the outer surface of the aluminum-rosinate precipitate.
  It improves the performance of synthetic sizes by providing cationic charges to the largely anionic pulp furnish and assists size retention accordingly.

Paper Sizing Theories

There are many theories regarding the sizing mechanism, however, a simplified explanation is outlined below:

In low pH aqueous solutions, alum dissociates to sulfate ions and hexahydrated, trivalent aluminum ions. Over the pH range 5 – 9, the hydrated aluminum ions exist as a mixture of species having net charges of +3, +2, and +1, with other polyvalent species having been suggested as possible. Soluble anionic aluminates (-1) will form at pHs > 10.

When rosin soap size and alum are dispersed in a pulp slurry, the aluminum ions react with the rosin size emulsion particles forming a very insoluble aluminum diresinate and free resin acid. At the same time, the positive charge of the aluminum ion imparts a positive surface charge to the size precipitate enabling it to be absorbed on the negatively charged fiber surface.

When acid-dispersed rosin size is used, the aluminum ion from alum imparts a positive charge to the fiber surface. The surface then absorbs the negatively charged acid-dispersed rosin particles.

The fibers are then formed into a paper web, are dewatered and heat dried. During drying, the aluminum ion anchors the polar (hydrophilic) carboxyl groups in the size precipitate. Thus, the non-polar (hydrophobic) hydrocarbon portion of the resin acids reorients on the outer surface, thereby imparting more lasting water repellency to the surface.

Improved Drainage & Retention

Alum also performs an important role as a retention aid for fines (fiber fragments) and fillers. At stock pH near and below 4.5, trivalent aluminum ions are present and act as an electrostatic bonding agent. As the negative charge of the system is reduced, particles can approach each other more closely. As pH approaches 6.0, cationic floc is formed, which behaves differently from the aluminum ion at the lower pH level. Aluminum hydroxy compounds absorb onto the negatively charged fiber surface to form positively charged patches. Negatively charged fiber fines and mineral fillers are attracted to and held by these patches with emphasis on electrostatic bonding and less on entrapment. With this mechanism, the retention is increased, allowing alum to precipitate and retain dyes, latex and dry strength additives as well as fines, fillers and microparticulates. With this more orderly retention, the sheet is "opened up", resulting in increased water drainage which permits faster machine operating speeds.

Other Alum Benefits

Alum can offer several other benefits when used in the paper industry, including:

• reducing deposits on equipment, particularly when added to highly resinous, unbleached stock to precipitate the resins naturally found in the fiber
• removing turbidity and color in high-quality water, by acting as a coagulant, particularly in mills producing bleached pulp & paper grades
• removing suspended solids, reducing BOD and color for effluent treatment in pulp & paper mills, by effectively acting as a flocculant and coagulant
• setting acid (anionic) dyes on paper as a mordant
• assisting in setting many wet strength chemicals
• improving paper machine save-alls operations, where fibers, solids and water are separated and reused
• adjusting wet end pH
• controlling foam
• reducing press picking
• wet strength resin curing and broke recovery