Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: This class of papermaking additives includes a wide range of molecular mass, degree of branching (often linear copolymers), and relative proportion of charged monomeric groups. By definition one of the monomers is plain acrylamide, i.e. -CH=C[-CONH2]- in which the group in brackets is attached to the second carbon. For production of an anionic copolymer, the co-monomer is often acrylic acid, -CH=C[-COOH]-. Cationic co-monomers typically have quaternary ammonium groups, and they are attached to the nitrogen of the amide, -CH=C[-CONH-RN+(CH3)3]-, where R can take a variety of forms. Polymerization is achieved by "zipping" the double bonds together - so the polymer backbone consists of single bonds (unsaturated). PAM copolymers used as retention aids have molecular masses in the range 2 to 15 million g/mole, and many of them are delivered as water-in-oil emulsions. PAM copolymers used as dry-strength resins are usually solution copolymers (no oil present) with molecular masses up to 1 million g/mole.
Functions: Retention aids (if molecular mass > 2 million g/mole); dry-strength resins (if molecular mass < 1 million g/mole); pitch-control agent (if highly cationic with molecular mass < 1 million g/mole); micro-polymer drainage aids (if highly anionic and very highly branched with a corona structure).
Strategies for Use: In their role as retention aids, acrylamide copolymers are added very late to the process, typically either right before or after the pressure screens. The pressure screens always degrade the molecular mass, hurting efficiency of retaining fine particles during paper formation. However, when the screens are used to deflocculate the fibers after the furnish has been treated with retention aid, then the resulting paper is more uniform. In their role as dry-strength agents the most critical aspect of acrylamide copolymer use is to balance the colloidal charge. Typically an oppositely charged material has to be added to the furnish either before or after a negatively charged strength agent. For control of pitch it is often recommended to treat the offending stream of thick-stock with just enough polymer so that filtrate from that stream has a low turbidity. Micro-polymers (see microparticles) usually are added last to the process, after a very-high-mass retention aid of opposite charge. The most critical factor is that the furnish have the same net colloidal charge as the retention aid at that point (usually by adding a charge scavenger).
Cautions: These materials can be very slippery when spilled. The emulsions of very-high-mass retention aids are particularly hazardous in this regard. Spills should be first wiped up with dry absorbent before rinsing.
|Synthesis of an acrylamide copolymer.|
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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