Wet-end additives

Wentworth process Weston cell Wet air oxidation Wet ball mill Wet-end additives Wet etching Wetfastness Wet grinding... 

Binders. Latices are used as fiber binders by the paper and textile industries. The two principal methods of appHcation are (/) wet-end addition, wherein the ionic latex is added to a fiber slurry and then coagulated in the slurry prior to sheet formation, and (2) saturation of the latex into a formed fiber web wherein the latex is coagulated by dehydration. Latices are also used as binders for particulate matter such as mbber scrap. 

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. 

Amino resins are used by the paper industry in large volume for a variety of apphcations. The resins are divided into two classes according to the mode of appHcation. Resins added to the fiber slurry before the sheet is formed are called wet-end additives and are used to improve wet and dry strength and stiffness. Resins appHed to the surface of formed paper or board, almost invariably together with other additives, are used to improve the water resistance of coatings, the sag resistance in ceiling tiles, and the scuff resistance in cartons and labels. 

The performance and quality of starch can be improved through chemical modification (see Chapter 17). Chemical modifications provide processed foods, such as frozen, instant, dehydrated, encapsulated and heat-and-serve products, the appropriate texture, quality and shelf life (see Chapter 21), and improved processing condition tolerance, such as improved heat, shear and acid stability. Modification also allows starches to be used in the paper industry (see Chapter 19) as wet-end additives, sizing agents, coating binders, and adhesives and as textile sizes. 

Cationic starches are widely used as wet-end additives in the pulp and paper industry to enhance starch and filler retention during papermaking. Use of cationic starches increases paper strength and decreases biological oxygen demand (BOD) of paper mill effluent. Presently only cationic corn and potato starches are used by Canadian paper mills. The degree of substitution (DS) of normal, waxy, high-amylose barley... 

Solarek D, Tessler MM, Jobe P, Peek L. Cationic Starch Aldehydes - Wet End Additives for Temporary Wet Strength and Improved Dry Strength. Paper Chemistry Symposium Stockholm, Sweden STFI and SPCI organizers 1988 Sept. 27-290... 

Wet-end additives - Are the wet-end chemical additives within normal consumption rate ranges ... 

There is no substitute for hard work to survey the process systems and learn the pulp and papermaking process to understand potential incompatibilities and synergies with other wet-end additives and the process conditions, followed by laboratory studies evaluating different defoamer chemistries. Defoamers should be evaluated for their immediate effect to knock down surface foam and achieve longevity in performance, as well as impact entrained air. 

Today, the most common types of synthetic dry strength additive are based on polyacrylamide although polyvinylamine strength additives are growing in popularity. This is because of the relatively low cost, and the ability to readily put an ionic charge onto these polymers. Other resins such as polyvinyl alcohol or lattices are used, but generally these are seen in surface applications to the paper, rather than as wet-end additives. Similarly styrene acrylic resins have shown superior performance when applied through surface application rather than as stock additives [8],... 

Apparent sheet density is increased to a much larger extent by wet-end addition than by impregnation addition. 

D.S. Greif and L.A. Gasper, Cationic starch as a wet-end additive, Dry Strength Additives, TAPPI Press, p. 95. 

B.M. Jensen and J.C. Winters (1986), Cationic potato starches proven effective as wet-end additives, Pulp Paper, April. 

T. Lindstrom, P. Kolseth and P. Naslund, The dry strengthening effect of cationic starch wet-end addition on filled papers, P589, FR3285 Papermaking Raw Materials... 

This low-density latex dispersion (Neoprene Latex 735A) is designed by DuPont as a wet-end additive to fibrous slurries. The fraction of the latex in the initial dispersion is 42.8% by weight (37.3% by volume). The pH value at 25°C is 11.5. The physical properties of the neoprene (slow crystallizing polychloroprene homopolymer) have been measured in die DuPont laboratories many years ago. 

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