The classic approach to increasing the strength is mechanical beating of the pulp.
This strengthens the resulting paper by straightening and flexibilizing the fibres,
leading to stronger fibre–fibre joints and a more uniform stress distribution in the
sheet with beaten fibres [7]. The side effect, the production of fine material, leads to
more difficult dewatering, which results in a wetter sheet and thereby higher production costs. Beating also generally leads to the formation of denser sheets, which in
turn may lead to lower bending stiffness or lower light scattering, and greater
shrinkage upon drying, which are undesirable effects for some paper grades.
The problem of densification has to some extent been overcome by (partial) replacement of the beating with chemical treatment with polyelectrolytes.
Recently, it has been shown that it can be of both technical and economic interest
to pre-form polyelectrolyte complexes (PECs) before letting the polyelectrolytes
interact with the fibres [8, 9]. This procedure offers an industrially interesting
method for exploiting the advantages of the separate polyelectrolytes and for
creating a new, nanostructured unit (which in itself has a further advantage in
enhancing paper properties). It is also very interesting from an industrial point of
view because PECs with new properties can be formed from polyelectrolytes that
already have the necessary application permission from the authorities. It is usually
very expensive to introduce totally new chemicals.
In more general terms, the addition of PECs can be seen as a surface treatment that,
in combination with all the polymers available today with exciting functionalities,
such as antibacterial, conductive, UV- or thermoresponsive properties, offers very
interesting possibilities for adding new functionality to a large variety of fibre-based
materials.
2 Adsorption of PECs to Cellulose Fibres
In order to correctly describe the adsorption of PECs at the solid–liquid interface, it is
important to decide how to define the interaction between a PEC and a solid surface,
i.e., should it be considered an adsorption of a polyelectrolyte or an adsorption of
nanosized colloids with a net charge? This naturally depends on a number of factors,
but it is vital to establish what type of complex is being studied since a soluble
complex will differ from a coacervate, which in turn will differ from a colloidal
complex [10, 11]. In the present discussion, only colloidal complexes will be
discussed and, as shown in Fig. 1, this type of complex can be represented by a
nanosized colloid with a neutral core and a charged corona. The size of these colloids
depends on their composition and on how they are prepared but, according to
Dautzenberg [11], they are roughly in the size range between 10 and 100 nm. The
charge of the complexes naturally depends on the preparation conditions and on the
properties of the constituting polyelectrolytes. The corona naturally also has a cloud
of counter-ions to balance the non-neutralized charges in the external layer(s) of the
complex.
The adsorption of polyelectrolytes and charged colloids at the solid–liquid interface has attracted considerable interest over the years [12–14]. Since the early 1980s,
Polyelectrolyte Complexes for Tailoring of Wood Fibre Surfaces
3
This strengthens the resulting paper by straightening and flexibilizing the fibres,
leading to stronger fibre–fibre joints and a more uniform stress distribution in the
sheet with beaten fibres [7]. The side effect, the production of fine material, leads to
more difficult dewatering, which results in a wetter sheet and thereby higher production costs. Beating also generally leads to the formation of denser sheets, which in
turn may lead to lower bending stiffness or lower light scattering, and greater
shrinkage upon drying, which are undesirable effects for some paper grades.
The problem of densification has to some extent been overcome by (partial) replacement of the beating with chemical treatment with polyelectrolytes.
Recently, it has been shown that it can be of both technical and economic interest
to pre-form polyelectrolyte complexes (PECs) before letting the polyelectrolytes
interact with the fibres [8, 9]. This procedure offers an industrially interesting
method for exploiting the advantages of the separate polyelectrolytes and for
creating a new, nanostructured unit (which in itself has a further advantage in
enhancing paper properties). It is also very interesting from an industrial point of
view because PECs with new properties can be formed from polyelectrolytes that
already have the necessary application permission from the authorities. It is usually
very expensive to introduce totally new chemicals.
In more general terms, the addition of PECs can be seen as a surface treatment that,
in combination with all the polymers available today with exciting functionalities,
such as antibacterial, conductive, UV- or thermoresponsive properties, offers very
interesting possibilities for adding new functionality to a large variety of fibre-based
materials.
2 Adsorption of PECs to Cellulose Fibres
In order to correctly describe the adsorption of PECs at the solid–liquid interface, it is
important to decide how to define the interaction between a PEC and a solid surface,
i.e., should it be considered an adsorption of a polyelectrolyte or an adsorption of
nanosized colloids with a net charge? This naturally depends on a number of factors,
but it is vital to establish what type of complex is being studied since a soluble
complex will differ from a coacervate, which in turn will differ from a colloidal
complex [10, 11]. In the present discussion, only colloidal complexes will be
discussed and, as shown in Fig. 1, this type of complex can be represented by a
nanosized colloid with a neutral core and a charged corona. The size of these colloids
depends on their composition and on how they are prepared but, according to
Dautzenberg [11], they are roughly in the size range between 10 and 100 nm. The
charge of the complexes naturally depends on the preparation conditions and on the
properties of the constituting polyelectrolytes. The corona naturally also has a cloud
of counter-ions to balance the non-neutralized charges in the external layer(s) of the
complex.
The adsorption of polyelectrolytes and charged colloids at the solid–liquid interface has attracted considerable interest over the years [12–14]. Since the early 1980s,
Polyelectrolyte Complexes for Tailoring of Wood Fibre Surfaces
3