Cationic starches systems

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). 

Cationic starches will have an increasing importance in papermaking with the revived interest in alkaline sizing of paper. Systems using the alkaline sizing agents. 

Combinations of cationic starch and anionic microparticles are useful commercial systems. Shear-sensitive flocculation occurs, allowing microscale reflocculation in the formed paper sheet, which improves dewatering and retention.63,75,76 The microparticles can be colloidal silica, aluminum silicate, poly(silicic acid) or bentonite of specific size and surface area.77 79 Cationic, anionic or polymeric aluminum-containing compounds can be additional components. A three-part coacervate system uses a high molecular weight anionic polyacrylamide, cationic starch and silica.80 Cooking cationic starch in the presence of an anionic silica hydrosol was reported to improve drainage and retention.81... 

Controlled crosslinking of cationic starches improves performance in microparticle-containing papermaking systems.84-86 Superior performance over cationic potato starch was achieved with crosslinked cationic or amphoteric waxy maize, tapioca or potato starch in microparticle systems when the starch cooking was optimized to produce the proper colloidal dispersions.86... 

Cationic starch is mainly adsorbed by the small fiber fractions and filler particles, according to their high specific surface. At low cationic starch levels (0.5%), only the small fiber fraction is saturated and rendered cationic a mosaic system is established which results in small flocks and good retention, but poor drainage. Increasing the... 

Carr et al.119 investigated grafting via reactive extrusion of starch with cationic methacrylate, acrylamide and acrylonitrile monomers. Starch, monomer and CAN initiator were metered into a twin-screw extruder at starch contents of approximately 35% solids. The cationic methacrylate monomer showed poor reactivity during extrusion, with essentially no add-on. Acrylamide-starch systems (1 1 w/w) gave conversions of approximately 20% and add-ons of 16% to 18%. Acrylonitrile displayed the greatest reactivity during extrusion, with conversions of 74% and 63% for 1 1 and 1 2 w/w acrylonitrile/starch ratios, respectively. The corresponding add-ons were 27% and 42%. 

Retention systems have become increasingly more sophisticated as our understanding of wet-end chemistry has improved and paper machines have become more demanding. Often two, three or even four chemicals are necessary to attain the retention, drainage and formation balance on modem machines. This sometimes involves a coagulant to neutralise anionic trash, then silica sol or bentonite, anionic or cationic polyacrylamides (or even both) and cationic starch to flocculate the stock. 

The use of anionic starches in the wet-end declined rapidly following the introduction of cationic starches and their use almost ceased with the subsequent move to neutral sizing. There has been a revival of interest in anionic products for use in dual-starch systems [5]. 

Early cationic starches were tertiary aminoalkyl starches. Tertiary amines require a proton to remain cationic and were suitable for use in acid papermaking systems. At increasing pH levels, the tertiary amine eventually looses its charge. Quaternary amines remaining cationic throughout the papermaking pH range and now represent the majority of cationic starch types. 

Very early in the use of cationic starches, it was recognised that acidity from alnm in moderate to high levels can change the requirements for a cationic starch to those of an amphoteric starch. The highly cationic alnm at low pH competes for fibre and filler adsorption sites with the cationic starch [13]. It is postulated that interaction between the alum and the anionic phosphate affects starch retention. The anionic charge does not appear to provide an appreciable benefit in alkaline systems [14]. 

The sequential use of anionic and cationic wet-end starch was described as early as 1975. In an acid furnish, anionic starch would be added first, adsorbed onto the fibre by alum, and then a cationic starch was added. A reversed addition sequence has been proposed for alkaline papermaking systems [8]. There are numerous patents covering sequential additions of oppositely charged starches [17,18]. 

The introduction of cationic starch gave significant retention improvements over the use of alum. The use of cationic starch as a retention aid was eclipsed by the introduction of polyacrylamides and other polymers, being consigned mainly to the role of a strengthening agent. The advent of micro-particle retention systems revived the position of cationic starch as a key retention system element - particularly in silica nano-particle systems. (These are covered elsewhere in this book.)... 

Anionic colloids may complex with cationic starch, reducing the ability of the starch to bond to the fibre surface. Hydrocolloid complexes may also be formed, resulting in poor de-watering and pressing efficiencies. Disturbing anionic trash should be neutralised with a suitable anionic trash catcher before addition of cationic starch to the system. This topic is well covered in several woiks by Pieter Brower [22-27]. 

The development of micro-particle retention systems radically changes the chemistry. In these systems an overdose of cationic material is added to the thin stock, such that the system becomes cationically dispersed. An anionic micro-particle is later added to flocculate the stock. Under these circumstances a complete starch cook out may not be necessary - especially if the system is being over-charged with a polymer rather than starch. Cross-linked cationic starches that do not fully disperse may be used in such systems. The partially dispersed starch acts partly as a colloid and partly as a particle, having a larger active radius than a fully dispersed starch molecule. Benefits in starch retention are claimed. 

Dual system Cationic starch + anionic component... 

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