Starch dispersion

Cationic Starches. The two general categories of commercial cationic starches are tertiary and quaternary aminoalkyl ethers. Tertiary aminoalkyl ethers are prepared by treating an alkaline starch dispersion with a tertiary amine containing a P-halogenated alkyl, 3-chloto-2-hydtoxyptopyl radical, or a 2,3-epoxypropyl group. Under these reaction conditions, starch ethers are formed that contain tertiary amine free bases. Treatment with acid easily produces the cationic form. Amines used in this reaction include 2-dimethylaminoethyl chloride, 2-diethylaminoethyl chloride, and A/-(2,3-epoxypropyl) diethylamine. Commercial preparation of low DS derivatives employ reaction times of 6—12 h at 40—45°C for complete reaction. The final product is filtered, washed, and dried. 

Figure 15.7 Morphology of aqueous potato starch dispersion (10%, w/w) heated to 180°C and quenched at 150°C/min to 10°C viewed between crossed polarizers. (Reprinted from Ziegler et al., 2003, with permission from Elsevier).

Figure 15.8 SEM image of spherulite aggregates of jet cooked high amylose starch dispersion slowly cooled without stirring. (Reprinted from Fanta et al., 2008, with permission from Elsevier).

Figure 8.1 (a) Typical swelling pattern of a 50% starch dispersion (TMA dilatometry pattern) along with its corresponding... 

On cooling sufficiently concentrated starch dispersions, amylose and amylopectin molecules begin to reassociate, eventually forming crystallites of the B-type. This... 

Viscoelastic behavior during gelatinization of aqueous starch dispersions (10% w/w, wheat, com, potato and waxy barley) in the presence of emulsifiers was investigated... 

Heat-induced changes in rye starch dispersions have been studied using the Brabender ViscoAmylograph.11 The problem with this method is that the changes monitored are... 

Starch fibrids are produced by spraying alkaline starch dispersion into an agitated, concentrated solution of ammonium sulfate.41,42 Amylose fibers or films are likewise produced by extmsion of a hot paste into a coagulating bath. Starch fibrids (starch pulp) can be utilized as a bonding additive in paper, but its use is rare. 

The Brabender AmyloViscograph and the Rapid ViscoAnalyzer49 51 are tools to monitor the starch dispersion process as a function of temperature, time and shear intensity. A typical viscosity trace is depicted in Figure 18.3. An initial ascending... 

Multivalent ions in starch dispersions, particularly those of aluminum, calcium, sulfate and oxalate, will induce retrogradation due to complexation or competition for water of hydration. The ions can be introduced by hard process water or accumulate by leaching from paper during surface sizing or coating. The destabilizing effect of ions follows the Schulze-Hardy rule. 

Particles of papermaker s amylose (reformed amylose particles, RAPS) appear in two shapes, Types I and II. The onset of retrogradation is indicated when a clear and transparent starch dispersion becomes opaque (turbid). Type I amylose particles are small. They are shaped like small bow ties or wedges of pie. Starch solutions with Type... 

Starches often contain a substantial amount of salt (sodium chloride or sodium sulfate) as a residue from the modification process. Small quantities of salt in a hot starch dispersion will attack low-grade stainless steel and can cause severe corrosion of tanks, pipelines and coating application equipment. The salt content should not exceed 0.2% or 2000 ppm chloride. 

Efficiency of starch dispersion can be further improved when an excess quantity of steam is used97 or when the starch is held under pressure in a retention device (tank or coil) before discharge to the atmosphere. Both processes require a flash chamber (cyclone) to remove excess steam and steam relieved by the pressure drop. Excess steam refers to a multiple of the quantity required to reach target temperature and pressure. The quantity of saturated steam to reach a target temperature can be calculated from its heat content, the slurry temperature, the specific heats of starch and water, and the enthalpy of starch gelatinization. The action of excess steam in the dispersion zone between the jet and back-pressure valve causes a shear effect that enhances disintegration of the starch granules. In many applications, a three-fold excess of steam is used. 

A widely-used method to adjust the viscosity of a starch dispersion to a target level uses amylases. Alpha-amylase catalyzes hydrolysis of starch molecules by breaking a-D-(l,4)-glucosidic bonds at random, generating depolymerization products, a-(l,4 ... 

Excess starch dispersion is drained from the ends of the nip. The surface size is transferred to paper by capillary penetration, pressure penetration and by hydrodynamic force during nip passage. 

The quantity of starch transferred to paper by a size press depends on several factors concentration of dispersed starch in the surface size viscosity of the starch dispersion diameter of the size press rolls size press pond height cover hardness of the size press rolls size press nip loading pressure paper machine speeds wet-end sizing of the sheet and water content of the sheet. The concentration of starch in the surface size liquid can range from 2% to —15%, depending on product requirements. Frequently, pigments and other materials are added, which further increases total dispersed and suspended solids content. The viscosity ranges from water thin to several hundred cP (mPa-s). 

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