Wet end applications

Commercially available cationic starches for wet end application are quaternary and tertiary products. These products have been available since about the mid 1950 s and no new basic chemistry has been developed since that time. The development in the late 1940 s and early 1950 s of starch ethers and esters made in the original granule form led to a torrent of starch derivatives for industrial use. Very few of these became commercial. This is possibly because the functions that were required by the industrial and food markets were far... 

Fig. 3.6 Starch modifications for wet-end application (PVAm = polyvinylamine), (QUAB = quaternary ammonium compound).

Chem. Descrip. Polyether derlv, of an aliphatic alcohol Uses Defoamer, liq, stock deaerator for wet-end applic. for all fibrous materials, paper and board, and esp, for slow stocks defoamerfor prod, of groundwood and... 

Paper Applications. In beater additions, the latex is mixed with the beaten paper pulp either by addition at the beater or to the stock chest at the wet end of the paper machine. In either case, the pH of the pulp is reduced to 4.0—4.5, usually by the addition of a solution of alum to the pulp—latex mixture which has been thoroughly agitated. The latex, which for this application must be based on an anionic emulsifier, coagulates as the pH drops. The latex solids separate in intimate association with the pulp fibers. The pulp is then screened and the paper web formed in the conventional way. A latex for this purpose must possess the proper balance between mechanical and chemical stability. 

Anionic starches are obtained by reaction with phosphoric acid and alkali metal phosphates or by derivatization with carboxymethyl groups.30,31 This modification is primarily used to introduce amphoteric properties into cationic com starch for application on the wet end of the paper machine. Anionic starches with carboxymethyl substitution are used as thickeners in coating colors or as binders in coatings for specialty paper grades. Oxidized starches are inherently anionic but without thickening action. Potato starch already carries sufficient natural anionic charge to provide amphoteric properties after cationization.32... 

However, as this book is concerned with the practical application of wet-end chemicals, these should be covered in other articles, but it is sufficient to say that good wet-end size application is required to allow these surface applied chemicals... 

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],... 

Cationic starch is the major wet-end starch. The phenomena involved in starch adsorption and the selection of cationic starch need careful consideration, as do the proper preparation and application of starch. The criteria for the use of native, anionic and amphoteric starches are also considered. 

For food applications it is desirable to only partially cook starch so that some texture remains. In food terms, a fuUy dispersed starch is overcooked . For papermaking, only fully dispersed starches will release their full bonding power. Partial cooking of ionic starches will release only part of the starch charge, with resulting variability and difficulties in wet-end charge control. Partially cooked starches may be referred to as undercooked in papermaking applications. 

Acknowledgements Particular reference is made to the following authors J.J.W. Swinkels for information on native starches and fundamental structure and chemistry, H.G.M. van de Steeg for her dissertation on starch adsorption, P.H. Brower for the development of starch application in the wet-end. Professor T. Lindstorm for fundamental wet-end chemistry. 

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