of the paper made from the fibres. The increased joint strength between the fibres is
due to both an increased molecular contact area between the fibres and an increased
molecular adhesion. The increased paper strength is also a result of an increased
number of fibre/fibre contacts/unit volume of the paper network.
Contents
1 Setting the Scene: Polyelectrolytes in Papermaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Adsorption of PECs to Cellulose Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 PEC for Controlling Adhesive Properties of Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 7
3.1 PECs Produced In Situ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 In Situ PEC Formation with Wood Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 Pre-formed PECs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4 What Controls the Adhesion? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5 Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1 Setting the Scene: Polyelectrolytes in Papermaking
Briefly, paper is made from a very dilute aqueous suspension of anionically charged
cellulosic fibres which, after water removal, form a fibrous network, i.e., a paper
sheet. In many cases, filler particles such as ground or precipitated calcium carbonate
or clay are added to the fibre suspension to enhance the optical performance or
printability of the paper. Also, a variety of other components are added to improve
specific properties of the paper sheet (e.g., wet and dry strength agents) or to facilitate
the paper production process (e.g., retention aids to minimize the loss of fines and
filler material to the process water, dewatering aids or defoaming chemicals).
The majority of these chemical additives are polymers.
The strength of a paper sheet is determined by the strength of the individual
constituent fibres, the strength of the joints bonding the fibres together, the number of
such joints per volume and the sheet formation (a measure of how evenly the fibres are
distributed in the sheet). The fact that the strength of the sheet is generally significantly
lower than the strength of a fibre leads to the conclusion that the joints between fibres
are of utmost importance [1, 2]. The strength of a fibre–fibre joint is due to mechanical
interlocking, van der Waals interactions, ionic bonds, hydrogen bonds, polymer interdiffusion between fibre surfaces and hydrophobic interactions [3]. For most of these
factors, the actual molecular contact area in the fibre–fibre joint is crucial [4].
Thus, it is of interest to modify the fibre–fibre joint in order to improve the various
parameters of the paper sheet. One example is the addition of a polyelectrolyte in the
papermaking process to increase the strength of the fibre–fibre joints and thus increase
the paper strength. A large variety of polyelectrolytes, natural and synthetic, are used
to modify fibre surfaces in papermaking, some of which (e.g., starch) have been used
for more than 50 years [3]. Instead of single polymers, alternating layers of cationic
and anionic polyelectrolytes can be adsorbed, forming a polyelectrolyte multilayer
(PEM) [5] on the fibre surface [6]. This procedure leads to a significant increase in
paper strength.
2
C. Ankerfors and L. Wa ˚gberg
due to both an increased molecular contact area between the fibres and an increased
molecular adhesion. The increased paper strength is also a result of an increased
number of fibre/fibre contacts/unit volume of the paper network.
Contents
1 Setting the Scene: Polyelectrolytes in Papermaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Adsorption of PECs to Cellulose Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 PEC for Controlling Adhesive Properties of Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 7
3.1 PECs Produced In Situ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 In Situ PEC Formation with Wood Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 Pre-formed PECs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4 What Controls the Adhesion? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5 Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1 Setting the Scene: Polyelectrolytes in Papermaking
Briefly, paper is made from a very dilute aqueous suspension of anionically charged
cellulosic fibres which, after water removal, form a fibrous network, i.e., a paper
sheet. In many cases, filler particles such as ground or precipitated calcium carbonate
or clay are added to the fibre suspension to enhance the optical performance or
printability of the paper. Also, a variety of other components are added to improve
specific properties of the paper sheet (e.g., wet and dry strength agents) or to facilitate
the paper production process (e.g., retention aids to minimize the loss of fines and
filler material to the process water, dewatering aids or defoaming chemicals).
The majority of these chemical additives are polymers.
The strength of a paper sheet is determined by the strength of the individual
constituent fibres, the strength of the joints bonding the fibres together, the number of
such joints per volume and the sheet formation (a measure of how evenly the fibres are
distributed in the sheet). The fact that the strength of the sheet is generally significantly
lower than the strength of a fibre leads to the conclusion that the joints between fibres
are of utmost importance [1, 2]. The strength of a fibre–fibre joint is due to mechanical
interlocking, van der Waals interactions, ionic bonds, hydrogen bonds, polymer interdiffusion between fibre surfaces and hydrophobic interactions [3]. For most of these
factors, the actual molecular contact area in the fibre–fibre joint is crucial [4].
Thus, it is of interest to modify the fibre–fibre joint in order to improve the various
parameters of the paper sheet. One example is the addition of a polyelectrolyte in the
papermaking process to increase the strength of the fibre–fibre joints and thus increase
the paper strength. A large variety of polyelectrolytes, natural and synthetic, are used
to modify fibre surfaces in papermaking, some of which (e.g., starch) have been used
for more than 50 years [3]. Instead of single polymers, alternating layers of cationic
and anionic polyelectrolytes can be adsorbed, forming a polyelectrolyte multilayer
(PEM) [5] on the fibre surface [6]. This procedure leads to a significant increase in
paper strength.
2
C. Ankerfors and L. Wa ˚gberg