Table 1 demonstrates the large differences in the composition of dual systems.
The quantity of polymers is mostly given in weight percent (wt%) without consideration
of the charge content of the systems to be flocculated.
In contrast to microparticle-containing systems, which were widely used in the
paper industry, the “classical” dual retention aid systems (polymer–polymer) are said
to give high retention combined with a poor dewatering [19]. But, some researchers
were able to show that an optimized polymer–polymer system has many advantages,
such as good dewatering, superior retention, and shear-resistant flocs.
These effects are obtained by a combination of charge patch formation and
bridging. The behavior especially depends on the concentration of PC (according to
the anionic character of the suspension) and the molar ratio of anionic and cationic
charges.
We have studied the mechanism of such polymer–polymer interactions as well as
the interaction between polymers and fibers or particles [20]. After investigating
the reaction between the strong PC poly(diallyl-dimethyl-ammonium chloride)
(PDADMAC) and two cellulosic model suspensions with low or high anionic trash
content, the influence of added PA on flocculation and dewatering was studied [20, 21].
As shown in Fig. 2a, for both suspensions, the drainage volume increased with
increasing polymer (PC) concentration in the suspension up to a maximum. The charge
of the suspension is slightly negative at this point, as shown in Fig. 2b. However, at
higher polymer dosage the efficiency of flocculation decreases. Something like a
plateau can be found; however, at the polymer dosage where the system is neutral,
the volume V SR strongly decreases. The polymer amount that is necessary for optimum
flocculation depends on the charge of the suspension to be flocculated. For instance, the
optimum polymer concentration (cationic demand) significantly increases with increasing trash content, but the drainage behavior (maximum of the drainage volume) was
similar in both cases (Fig. 2a). The amount of PC necessary for optimum flocculation is
of great importance for the optimization of the whole dual system and, as a rule, it
should be determined first.
It was also found that the molar mass of the added PA has large influence on
flocculation and dewatering. The higher the molar mass, the better the efficiency in
flocculation. In contrast to this observation, the charge density of the PAs should not
be very high. Most excellent results were obtained with PAA having a low (less
than about 30%) anionic charge. The difference between a step-by-step addition of
polymers to cellulosic suspension and the addition of pre-mixed complexes [25]
will be discussed in Sect. 4.
2.2 Interaction with Humic Acid
The influence of charge and molar mass on the flocculation mechanism was also
confirmed by results obtained for water treatment, where the influence of humic acid
on the flocculation of clay was investigated [26]. The separation of clay particles was
influenced by the presence of humic compounds, which act like an additional PA in
the clay dispersion. Owing to their adsorption on clay and the higher anionic charge
34
G. Petzold and S. Schwarz
The quantity of polymers is mostly given in weight percent (wt%) without consideration
of the charge content of the systems to be flocculated.
In contrast to microparticle-containing systems, which were widely used in the
paper industry, the “classical” dual retention aid systems (polymer–polymer) are said
to give high retention combined with a poor dewatering [19]. But, some researchers
were able to show that an optimized polymer–polymer system has many advantages,
such as good dewatering, superior retention, and shear-resistant flocs.
These effects are obtained by a combination of charge patch formation and
bridging. The behavior especially depends on the concentration of PC (according to
the anionic character of the suspension) and the molar ratio of anionic and cationic
charges.
We have studied the mechanism of such polymer–polymer interactions as well as
the interaction between polymers and fibers or particles [20]. After investigating
the reaction between the strong PC poly(diallyl-dimethyl-ammonium chloride)
(PDADMAC) and two cellulosic model suspensions with low or high anionic trash
content, the influence of added PA on flocculation and dewatering was studied [20, 21].
As shown in Fig. 2a, for both suspensions, the drainage volume increased with
increasing polymer (PC) concentration in the suspension up to a maximum. The charge
of the suspension is slightly negative at this point, as shown in Fig. 2b. However, at
higher polymer dosage the efficiency of flocculation decreases. Something like a
plateau can be found; however, at the polymer dosage where the system is neutral,
the volume V SR strongly decreases. The polymer amount that is necessary for optimum
flocculation depends on the charge of the suspension to be flocculated. For instance, the
optimum polymer concentration (cationic demand) significantly increases with increasing trash content, but the drainage behavior (maximum of the drainage volume) was
similar in both cases (Fig. 2a). The amount of PC necessary for optimum flocculation is
of great importance for the optimization of the whole dual system and, as a rule, it
should be determined first.
It was also found that the molar mass of the added PA has large influence on
flocculation and dewatering. The higher the molar mass, the better the efficiency in
flocculation. In contrast to this observation, the charge density of the PAs should not
be very high. Most excellent results were obtained with PAA having a low (less
than about 30%) anionic charge. The difference between a step-by-step addition of
polymers to cellulosic suspension and the addition of pre-mixed complexes [25]
will be discussed in Sect. 4.
2.2 Interaction with Humic Acid
The influence of charge and molar mass on the flocculation mechanism was also
confirmed by results obtained for water treatment, where the influence of humic acid
on the flocculation of clay was investigated [26]. The separation of clay particles was
influenced by the presence of humic compounds, which act like an additional PA in
the clay dispersion. Owing to their adsorption on clay and the higher anionic charge
34
G. Petzold and S. Schwarz