Dry strength additives

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. 

Combinations of anionic and cationic resins are used. Some of the eady systems involved the use of a cationic wet-strength resin with an anionic dry-strength additive to provide both increased wet and dry strength (63). Combinations of anionic and cationic dry-strength additives also are used to provide strength effects which cannot be achieved by using either polymer alone (64). The ratio of the two polymers must be optimized to achieve maximum performance (see Acrylamide polymers). 

This reduces pulp tensile strength but improves tear strength not because of fiber degradation but because the hemiceUuloses bond chemically to give added tensile strength in the final paper product. To overcome this loss in hemiceUulose polymers, starch is added on the wet end of the paper machine as a dry strength additive. 

Recycled paper contains more fines, short fibers, and anionic trash. This will increase demand for process chemicals such as drainage aids and both wet- and dry-strength additives (43). 

Dry strength additives are usually water soluble, hydrophilic natural or synthetic polymers, the commercially most important of which are starch, natural vegetable gums and polyacrylamides. These polymers are often made in cationic form by the introduction of tertiary or quaternary amino groups into the polymer, and are therefore polyelectrolytes. They are thus also able to function to some extent as drainage and retention aids. 

Unmodified and anionically modified starches, soluble cellulose derivatives such as carboxymethylcellulose, polyvinyl alcohol, latex and other polymers are also used in some specialised applications. Starch, however, because of its cost, is by far the most common dry strength additive, about twenty times more being used than, for example, polyacrylamide. 

Starch is the most widely used dry strength additive and is normally made in a cationic form by introducing a reactive monomeric or polymeric tertiary amine or quaternary ammonium derivative into the molecule. The most commonly used reagent for tertiary amino starch is 2-chloroethyldiethylammonium chloride, and for quaternary starch is 2,3-epoxypropyltrimethylammonium chloride (Figure 7.10). 

Because dry strength additives are often polyelectrolytes, they are able to behave as retention aids. This leads to flocculation of the fibres and to mass distribution (formation) variations in the sheet which can reduce strength. The flocculation can be destroyed irreversibly by shear, which suggests that bridging flocculation is involved. 

Cationic starches - [STARCH] (Vol 22) - [STARCH] (Vol 22) -dry-strength additives [PAPERMAKING ADDITIVES] (Vol 18)... 

Starches - [FILLERS] (Vol 10) - [FOODS, NONCONVENTIONAL] (Vol 11) -dry-strength additives [PAPERMAKING ADDITIVES] (Vol 18) -as food additives [FOOD ADDITIVES] (Vol 11) -mpotash flotation [POTASSIUMCOMPOUNDS] (Vol 19) -for printing papers [PRINTING PROCESSES] (Vol 20)... 

---