Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use


Composition: Papermaker's alum is aluminum sulfate. Concentrations and addition amounts are usually based on either (a) the equivalent amount of Al2O3 or (b) the equivalent amount of the hydrate, Al2O3.14H2O. Alum is most commonly delivered as a liquid concentration, having a solids level of 8.3% as Al2O3 or about 50% as hydrate. Alum solutions are acidic. For instance, a 1% solution has a pH of around 3. Ionic species present in alum solutions are highly dependent on the degree of reaction with hydroxyl ions. It is tempting to say that "the composition is pH-dependent." However, in the vicinity of pH=4.3 the composition of alum solutions changes a great deal with very little change in pH. At pH=3 and lower the main species is a hydrated form of Al3+. At a pH near to 4.3 there may be oligomers such as Al13O40H487+ or related species that contain sulfate. In a broad range of pH between about 6 and 9.5 the main species will be Al(OH)3 (and possibly related species that contain sulfate). The soluble aluminate ion predominates at pH values greater that 9.5.

Functions: Drainage enhancement, rosin sizing, part of certain retention aid programs, dye fixation, cationic source, acidic buffer. See also rosin soap size, rosin emulsion size.

Strategies for Use: A general discussion of alum use strategies has to start with the subject of pH. At pH values of about 4.5 and lower alum can be described as a "forgiving" additive. In other words, the user has a lot of flexibility with respect to where and how much alum to use for rosin sizing and drainage. Because of the affinity of the sulfate ion for alum ions, an overdose of alum is not expected to cause the net colloidal charge of a papermaking furnish to be strongly cationic. Usually alum is added after rosin soap size to "set" the size onto the fiber surfaces. When the water hardness is high some papermakers reverse the order in order to minimize formation of calcium abietate and related rosin salts that are tacky and not efficient at making the paper hydrophobic. The same "reverse" order of addition also is used often when papermakers use rosin emulsion size products. Since the emulsion size is used at a somewhat higher pH range of 4.5 to 5.5 (or higher in the case of pseudo-neutral and neutral sizing) it becomes more critical that the time between the addition points for alum and rosin be minimized (but there has to be good mixing in between the additions). Alum still can play a very effective role in alkaline papermaking, even though the equilibrium species are uncharged. Apparently the alum is able to rapidly complex with and neutralize carboxylate species in neutral or alkaline furnish before it has reached its equilibrium ionic composition. Careful dilution with clean water and good agitation are required to avoid deposit formation when alum is used above about pH 5.5.

Cautions: Alum is acidic. See details in MSDS.

Aluminum ion species vs. pH   Diagram showing the estimated distribution of species of aluminum versus pH under assumed under conditions of equilibrium.

PLEASE NOTE: Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification.


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This page is maintained by Martin hubbe, Associate Professor of Wood and Paper
Science, NC State University, .