Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: Negatively charged copolymers of acrylamide are widely used as retention aids and dry-strength resins. However, different molecular mass ranges are used for these two roles. Anionic acrylamide copolymer retention aids typically have molecular masses in the range of 5 to 20 million grams per mole. The strength agents typically have molecular masses in the hundreds of thousands. Another difference is in the form delivered to the mill. Though anionic retention aids can be delivered as solid beads or in solution, it is more popular to get them as water-in-oil emulsions. Before such emulsion products can be used they have to be "inverted" with a dilution of about 100-to-1 with a lot of agitation. Further time (at least half an hour) is needed for the individual molecules to uncoil themselves and achieve their full potential as retention aids. The monomers used in their preparation are acrylamide and acrylic acid. The acrylic acid is usually present in its corresponding sodium salt form in the final formulation of retention aid or strength aid. The strength agents are usually shipped as solutions having solids levels in the range of 10 to 50%. A wide range of charge density is available in each case.
Function: Retention aids (molecular mass 5-20 million grams per mole); dry-strength agents (mass in the 100,000's g/mole). Some dispersants (lower mass, usually) have similar compositions.
Strategies for Use: First let's consider the anionic acrylamide retention aids. These are often used for alkaline papermaking, though they also can be used under acidic conditions. The most basic rule is that the furnish must have a suitable "cationic source" to serve as anchoring points for the anionic retention aid. Such materials as polydimethlammonium-epichlorohydrin (polyamine), polydiallyldimethylammonium chloride (poly-DADMAC), and cationic wet-strength resins are especially effective in this role as "promoters" of the retention aid. Alum also can be used as a promoter, especially if the pH is below 6 or if the alum is added at a point where it is in contact with the furnish for a few minutes or less before the retention aid addition. The most popular addition point for a retention aid is right after the pressure screen(s), since this gives the highest retention efficiency. The papermaker has an incentive to use as little retention aid as practical - only enough to keep the paper machine from getting too dirty, to avoid basis weight fluctuations, and to avoid runnability problems. Higher levels are likely to floc the fibers enough to hurt the uniform appearance of the paper, i.e. its "formation." Another alternative is to add the retention aid before the screen(s), possibly achieving a better balance between formation uniformity and retention.
Dry-strength resin performance tends to be highly dependent on the colloidal charge state of the system. Whereas anionic retention aids merely need cationic "anchoring points," the strength resins appear to need a stoichiometric match. Under acidic papermaking conditions alum can be used to achieve optimum resin retention, drainage, and dry-strength contribution. Under alkaline papermaking conditions the same can be achieved by a highly charged polymer, usually containing quaternary amines.
Cautions: Anionic retention aid polymers are probably the slipperiest materials to be found in a paper mill. Spills of retention aid emulsions should be collected with dry absorbent. Spraying with water will merely spread the problem around.
|Synthesis of anionic copolymers of acrylamide. Note: Similar products can be made by hydrolysis of polyacrylamide.|
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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