Flocculation of cellulosic fibres following addition of cationic starch

The results presented in this paper indicate that flocculation of cellulosic fibres induced by cationic starch is influenced by the charge density of the starch, the amount of starch, the contact time between starch and fibres, and the concentration of inorganic ions. At low levels of starch addition the degree of flocculation is low and unaffected by the degree of substitution (DS) of the starch. When the amount of starch addition is increased, a more highly charged starch induces a higher degree of flocculation. This is probably due to the higher electrostatic interaction between the more highly charged starch and the fibres. At high levels of addition of highly charged starch, the flocculation decreases. Calculations indicate that about 50% of the available fibre charges are neutralised by the adsorbed starch charges at maximum flocculation, which is in accordance with early theories of flocculation caused by polymer bridging. The decrease in flocculation can hence be ascribed to an increasingly complete saturation of the adsorption sites on the fibres. For the starch with the lower DS, there was a more br less continuous increase in degree of flocculation over the polymer addition range studied. This is also ascribed to a continuously increasing interaction between starch covered surfaces as the addition and adsorption of the starch increases. No maximum was found for this latter starch and this is supported by calculations which show that a surface coverage, of available charges, of 50% was never exceeded in these experiments. A moderate increase in ionic strength, i.e. 10(-2) M NaCl, increased the flocculation at a low level of starch addition. This effect is interpreted as a sensitisation flocculation. When the concentration of inorganic ions was further increased to 10(-1) M, flocculation decreased and eventually no flocs were observed. This effect is independent of the DS of the starch and of the amount of added starch. The findings in this study suggest that this decrease in flocculation is due to a decreased interaction between the starch-covered surfaces and eventually, at higher concentration of inorganic electrolytes, to a reduction in starch adsorption.