(The retention is the retention of the solids on the paper machine screen)
In the manufacture of paper, the constituents that will later constitute the paper are placed on a sieve as a dilute aqueous slurry and filtered. This filtration process is improved and partly accelerated by retention agents; at high doses, dehydration slows down due to mucilage. Retention aids are either aluminum sulfate or cationically charged polymers with a high molecular weight . The latter are often copolymers of acrylamide and quaternary ammonium compounds . Cationic wet strength agents can also act as retention aids due to their charge.
In addition to cellulose , which is the main component of paper, many papers (e.g. printing and writing papers) contain a large amount of pigments and fillers . A large part of these pigments and fillers would end up in the wastewater without retention agents , since they have little affinity for fibers. In addition to ecological damage, there is also economic damage, since most pigments and filler minerals are quite expensive. Retention agents reload the fillers and dyes in such a way that a higher affinity for the fibers is created and the loss of these materials with the white water is reduced during dewatering . At the same time, retention aids in the neutral pH range promote glue adhesion and the formation of clumps in the cellulose fibers. As a result, larger quantities of fine and residual fibers are retained, which, in particular in the recycling of waste paper (high proportion of fiber that has been shortened several times), represent a significant pollution factor for the wastewater (NOX value).
The retention indicates the relative proportion of the fibers, fillers and fines retained on the sieve compared to a complete retention of these substances in the wastewater. A high retention corresponds to a high percentage of additives remaining in the paper.