Strength Additives

It is often necessary to increase tensile and other strength properties in paper by chemical means rather than mechanically via the refining process. This is usually the case when the other effects of refining, such as the decreases in opacity and air permeability, are not desired. 

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. 

The retention of cationic polymers by cellulose is most readily explained by ionic interaction between the cationic group of the polymer and the acidic groups of cellulose, but there is also evidence that hydrogen bonding participates to a lesser extent in the adsorption process. 

Cationic starch is usually added to a blend of furnish components 

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Wet-laid nonwovens Wetox process Wet-strength additives... 

Urea—formaldehyde resins are also used as mol ding compounds and as wet strength additives for paper products. Melamine—formaldehyde resins find use in decorative laminates, thermoset surface coatings, and mol ding compounds such as dinnerware. 

In papemiaking, chemicals can be added either to the pulp slurry prior to sheet formation, ie, internal or wet-end addition, or to the resulting sheet after complete or partial drying, ie, surface or dry-end addition. The method chosen depends on retention and the desired effect. For example, strength additives usually are added internally if uniform strength throughout the sheet is wanted, but they are applied to the surface if the need is for increased surface strength. If an additive caimot be retained efftciendy from a dilute pulp slurry, then it is better to apply it to the surface of the sheet. 

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). 

Plasticizers. Plasticizers (36—38) are often added to a binder to reduce cross-link density and increase flexibiHty. Plasticizers improve toughness, springback, and flexibiHty, but degrade overall green strength. Additionally, plasticizers can increase the sensitivity of a binder system to moisture. 

High ultimate strength. Addition of phenolic resin is needed. 

As in CE, changing system variables (e.g., pH, ionic strength, additive concentration) is very easy in any of the continuous free flow electrophoresis systems reported here because all the interactions take place in free solution. Indeed, changing system variables may be easier in continuous free flow electrophoresis systems than in a CE system because there are essentially no wall effects. Of course, changing system variables in the continuous free flow electrophoresis apparatus may also be easier... 

Gel strength additives for the preparation of spacers (usually fragile gel additives). 

Diaikanol aminoalkyl phenols as admixtures enhance the strength [675]. The additives are useful in very small amounts and do not affect the initial properties of the fluid. The strength additive does not cause set acceleration or early set strength enhancement but provides enhanced compressive strength of the cement in later stages. Addition of small amounts of potassium ferricyanide and nitrile-trimethyl phosphonic acid promotes the formation of complex compounds and thus increases the strength of cement rock [1771]. 

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