Retention, paper

General Considerations. With liquids and solutions the most serious losses are due to (a) transference from spherical flasks and difficulties of drainage, (b) retention by filter-papers, (c) absorption by large corks. As containers for small quantities of liquids it is therefore often convenient to use pear-shaped flasks A and conical test-tubes or centrifuge-tubes B (Fig. 29). (In this and subsequent figures, approximate dimensions are given to indicate a convenient size.)... 

The protonated form of poly(vinyl amine) (PVAm—HCl) has two advantages over many cationic polymers high cationic charge densities are possible and the pendent primary amines have high reactivity. It has been appHed in water treatment, paper making, and textiles (qv). The protonated forms modified with low molecular weight aldehydes are usehil as fines and filler retention agents and are in use with recycled fibers. As with all new products, unexpected appHcations, such as in clear antiperspirants, have been found. It is usehil in many metal complexation appHcations (49). 

Starch is a polysaccharide found in many plant species. Com and potatoes are two common sources of industrial starch. The composition of starch varies somewhat in terms of the amount of branching of the polymer chains (11). Its principal use as a flocculant is in the Bayer process for extracting aluminum from bauxite ore. The digestion of bauxite in sodium hydroxide solution produces a suspension of finely divided iron minerals and siUcates, called red mud, in a highly alkaline Hquor. Starch is used to settle the red mud so that relatively pure alumina can be produced from the clarified Hquor. It has been largely replaced by acryHc acid and acrylamide-based (11,12) polymers, although a number of plants stiH add some starch in addition to synthetic polymers to reduce the level of residual suspended soHds in the Hquor. Starch [9005-25-8] can be modified with various reagents to produce semisynthetic polymers. The principal one of these is cationic starch, which is used as a retention aid in paper production as a component of a dual system (13,14) or a microparticle system (15). 

Vacuum filters are usually simulated with a Buchner funnel test or filter leaf test (54). The measured parameters are cake weight, cake moisture, and filtration rate. Retention aids are usually evaluated using the Britt jar test, also called the Dynamic Drainage Jar, which simulates the shear conditions found on the paper machine and predicts performance (55). 

The retention of fillers in the sheet during the forming process is important. Both hydrodynamic mechanisms and colloidal or coflocculation phenomena are significant in determining filler retention (7). Polymeric retention aids are used to bridge between filler particles and fibers. Talc is sometimes used with mechanical pulp furnishes in order to reduce the deposition of pitch-like materials onto paper machinery. 

Papermaking additives can be categorized either as process additives or as functional additives. Process additives are materials that improve the operation of the paper machine, such as retention and drainage aids, biocides, dispersants, and defoamers they are primarily added at the wet end of the paper machine. Functional additives are materials that enhance or alter specific properties of the paper product, such as fillers (qv), sizing agents, dyes, optical brighteners, and wet- and dry-strength additives they may be added internally or to the surface of the sheet. 

Retention and drainage additives are vital to the use of recycled fibers. Papermakers consider recycled fibers to behave like virgin fines, while recycled fines behave like filler. Drainage on the paper machine can be impeded and first-pass retention reduced by the use of recycled fiber (9). Additionally, the negative impact of contaminants found in recycled fibers can be minimized by the appropriate use of dispersants and other pitch-control additives. 

Other. A large variety of additives are used in paper-coatiag colors primarily to modify the physical properties of the colors (102). At high soHds concentrations in water, mineral pigment particles tend to associate and form viscous pastes. Dispersants (qv) are used to prevent this and to provide low viscosity slurries. Common dispersants include polyphosphates and sodium polyacrylate [9003-04-7]. Various water-soluble polymers are added to coatiag colors and act as water-retention agents and as rheology modifiers. 

In the paper industry, PEO is widely used as a retention aid and pitch control agent in the newsprint industry (118—135). Typically, a phenol formaldehyde-type resin is added to the substrate before the addition of PEO. The chemical that is added before PEO has been referred to as an enhancer. Recent pubHcations on designing enhancers that work with PEO have resulted in expanding the use of PEO in flocculation of several substrates (128,129). 

The amino group is readily dia2oti2ed in aqueous solution, and this reaction forms a basis for the assay of sulfas. Aldehydes also react to form anils, and the yellow product formed with 4-(dimethylamino)hen2a1dehyde can be used for detection in thiu-layer and paper chromatography. Chromatographic retention values have been deterrnined in a number of thiu layer systems, and have been used as an expression of the lipophilic character of sulfonamides (23). These values have corresponded well with Hansch lipophilic parameters determined in an isobutyl alcohol—water system. 

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