closure is used, i.e., circulation of the process water is employed. This leads to
increasing concentrations of the mentioned substances in the circulating water [38].
With large amounts of DCS in the process water, aggregates may form. Deposits of
aggregates of substances on machine parts may cause severe disturbances to paper
machine runnability [39] and, in this respect, the addition of cationic polyelectrolyte can be detrimental to process efficiency. Although this is sometimes viewed as
a problem leading to a high consumption of cationic additives, the process can have
several advantages. The viscosity of the aqueous phase is lowered by the removal of
unwanted dissolved or colloidal material and this facilitates drainage and
dewatering [40, 41], which is an economic advantage because it reduces the energy
needed for drying the paper sheet. Also, the PEC structures formed can be deposited
onto the fibre surfaces, thus increasing the adhesive contact between the fibres.
In the papermaking process, lignin is generally considered to be a more difficult
substance than hemicelluloses such as xylan, which can even act as a strength agent.
The positive effect of released hemicelluloses on paper strength is well established
in the literature [38]. Lignin, on the other hand (retained by the addition of CPAM,
forming PECs in situ) acts only as a filler, i.e., a non-bonding spacer in the fibre
crossings [38]. Extensive investigations of the interaction between different common retention chemicals and wood components were carried out by Stro ¨m et al.
during the 1980s [42–46]. When a high-charge-density polycation (here, polyethyleneimine, PEI) was added to a pulp suspension, any lignosulfonates were consumed first and hemicelluloses such as xylan later [43, 47]. The effect of the
lignosulfonate complexes depended on the relative amounts of the two oppositely
charged polymers, forming complexes with different net charges. If the added
polycation was in deficiency, the negatively charged PECs formed merely acted
as a larger amount of fines material in the water phase. With increasing polycation
dosage, cationic PECs were eventually formed, which could be deposited on the
negative fibre surfaces and thereby remove them from the water phase.
The interactions between different polysaccharide additives (such as guar gum
or starch) and the components in a variety of pulp suspensions (including whole
pulp and washed pulp) have been described as a four-step process (Fig. 7): mixing
of polysaccharides into the pulp suspension; complexation with DCS; adsorption of
polysaccharides (free or complexed) onto fibres and fines material; and, finally,
association of fines to the adsorbed polysaccharides on the fibres [48]. According to
the authors, this agrees with the maximum fines retention, drainage and paper
strength observed in industrial applications,
In a study with a similar purpose, i.e., the better understanding of the mechanisms
by which cationic retention aids operate in the presence of DCS, the formation of
colloidal and coacervate complexes from CPAM and sulfonated Kraft lignin was
studied [49]. Using a CPAM with lower molecular weight, the formation of colloidal
complexes was promoted over coacervate formation. With CPAM of higher molecular weight, the re-conformation (into colloidal PECs) was too slow, and coacervate
complexes were formed.
Other reports concerning in-situ-prepared PEC focus on the formation of PECs
and their effect on drainage and dewatering or as an aid for washing pulp [40, 41].
Polyelectrolyte Complexes for Tailoring of Wood Fibre Surfaces
11
increasing concentrations of the mentioned substances in the circulating water [38].
With large amounts of DCS in the process water, aggregates may form. Deposits of
aggregates of substances on machine parts may cause severe disturbances to paper
machine runnability [39] and, in this respect, the addition of cationic polyelectrolyte can be detrimental to process efficiency. Although this is sometimes viewed as
a problem leading to a high consumption of cationic additives, the process can have
several advantages. The viscosity of the aqueous phase is lowered by the removal of
unwanted dissolved or colloidal material and this facilitates drainage and
dewatering [40, 41], which is an economic advantage because it reduces the energy
needed for drying the paper sheet. Also, the PEC structures formed can be deposited
onto the fibre surfaces, thus increasing the adhesive contact between the fibres.
In the papermaking process, lignin is generally considered to be a more difficult
substance than hemicelluloses such as xylan, which can even act as a strength agent.
The positive effect of released hemicelluloses on paper strength is well established
in the literature [38]. Lignin, on the other hand (retained by the addition of CPAM,
forming PECs in situ) acts only as a filler, i.e., a non-bonding spacer in the fibre
crossings [38]. Extensive investigations of the interaction between different common retention chemicals and wood components were carried out by Stro ¨m et al.
during the 1980s [42–46]. When a high-charge-density polycation (here, polyethyleneimine, PEI) was added to a pulp suspension, any lignosulfonates were consumed first and hemicelluloses such as xylan later [43, 47]. The effect of the
lignosulfonate complexes depended on the relative amounts of the two oppositely
charged polymers, forming complexes with different net charges. If the added
polycation was in deficiency, the negatively charged PECs formed merely acted
as a larger amount of fines material in the water phase. With increasing polycation
dosage, cationic PECs were eventually formed, which could be deposited on the
negative fibre surfaces and thereby remove them from the water phase.
The interactions between different polysaccharide additives (such as guar gum
or starch) and the components in a variety of pulp suspensions (including whole
pulp and washed pulp) have been described as a four-step process (Fig. 7): mixing
of polysaccharides into the pulp suspension; complexation with DCS; adsorption of
polysaccharides (free or complexed) onto fibres and fines material; and, finally,
association of fines to the adsorbed polysaccharides on the fibres [48]. According to
the authors, this agrees with the maximum fines retention, drainage and paper
strength observed in industrial applications,
In a study with a similar purpose, i.e., the better understanding of the mechanisms
by which cationic retention aids operate in the presence of DCS, the formation of
colloidal and coacervate complexes from CPAM and sulfonated Kraft lignin was
studied [49]. Using a CPAM with lower molecular weight, the formation of colloidal
complexes was promoted over coacervate formation. With CPAM of higher molecular weight, the re-conformation (into colloidal PECs) was too slow, and coacervate
complexes were formed.
Other reports concerning in-situ-prepared PEC focus on the formation of PECs
and their effect on drainage and dewatering or as an aid for washing pulp [40, 41].
Polyelectrolyte Complexes for Tailoring of Wood Fibre Surfaces
11