3 PAPER FORMATION 3.1 The paper machine and the
3.3 Control of interparticle interactions J Colloidal stability and surface forces
3.3.2 Polymer adsorption
Nearly all retention aid systems used in paper manufacturing contain a high-molecular-weight polymer as a key component. The adsorption of polymers at the surfaces of fibres, fines and fillers in the papermaking
Increasing salt concentration
Distance
Interaction energy/J
Figure 7.7. DLVO-type interaction (continuous line) obtained as the sum of the electrostatic repulsion (dashed line) and van der Waals attraction (dotted line). The interaction curve is calculated for a sphere with a 1 ^m radius, a surface potential of 20 mV, a Hamaker constant of 20 x 10~20 J, immersed in a 10 mM 1:1 salt solution
Van der Waals attraction Distance/nm DLVO potential Electrostatic repulsion
Interaction energy/J
Bulk polymer concentration/ppm
Figure 7.10. Adsorption isotherms for different narrow-molecular-weight poly (vinyl alcohol) at the polystyrene-water interface. The molecular weights used are 8000 (filled diamonds), 28 000 (open triangles) and 67 000 (open circles).
(Redrawn from ref. (13))
furnish is used in different ways to facilitate retention.
The purpose of polymer adsorption is to modify the interactions between the colloidal components so that the fines and fillers are effectively retended at fibre surfaces. Introducing polymers into a colloidal system can result in a colloidal stability increase or decrease depending on the structure and density of the polymers at the surfaces of the colloidal material. There are several fundamental reasons for a polymer to adsorb at a surface. The first reason is specific attractive interactions
between the polymeric groups and the surface, such as the attraction of opposite charges of a cationic polymer (polyelectrolyte) and a negatively charged surface. The other, which is more specific to polymers, is the entropy gained when surface-bound molecules are released into solution when replaced by surface-anchoring polymer groups. The entropy gained by the many solvated molecules is much larger than the rather small loss in degree of conformational freedom of the polymer.
Due to the fact that polymer adsorption is entropically favoured, only a small specific adsorption energy is needed for polymers to anchor at a surface and substitute small surface-bound molecules. By considering the entropic driving force for polymer adsorption, it is easy to understand that the necessary energy for substituting, for instance, a solvent molecule with a polymer group at the surface will decrease with an increasing size of the polymer molecule. High-molecular-weight polymers are more inclined to adsorb than their low-molecular-weight analogues. Indeed, high-molecular-weight polymers adsorb strongly even at very low concentrations, thus leading to high-affinity isotherms having constant adsorption at higher concentrations where the surface has become saturated, as shown in Figure 7.10 (13). Generally, the adsorbed amount increases with the molecular weight of the polymer, becoming independent of molecular weight for very large polymers (12). As can be understood from the earlier discussion, the saturation adsorption tends to increase with the molecular weight. The high-affinity character of polymer isotherms means that the polymers in a practical sense are irreversibly adsorbed, since the concentration difference between any rinsing solution and the sub-surface outside the adsorbed layer is very small. This will lead to very slow mass-transfer from the interfacial region to the bulk. Even fairly low-molecular-weight polymers can appear irreversibly adsorbed. This is often due to the polydispersity, i.e. the polymer contains a high-molecular-weight fraction that adsorbs preferentially due to the larger entropic driving force.
Before going into the details of polymers at inter-faces, a few points about solution behaviour are worth mentioning. Long flexible-chain polymers tend to adopt a random coil conformation in dilute solutions. An important measure of this structure is the radius of gyra-tion, which depends on the molecular weight and the solvency in the ambient medium. Solvency describes the relative strength of the polymer segment/segment and segment/solvent interactions. A high affinity of the polymer for the solvent, i.e. a good solvent, leads to an osmotically swollen coil with a radius of gyration tend-ing to its maximum. The coil collapses and adopts a Adsorbed amount/(mg/m2 )
Figure 7.9. Influence of the surface potential on the DLVO potential between two charged surfaces. The system is, apart from the varying surface potential/charge, the same as in Figure 7.7
Distance Decreasing potential/charge
Interaction energy/J
more close-packed unit when the polymer affinity for the solvent decreases. Finally, the polymer will precipitate.
The intermediate situation when the affinities are equal is referred to as the #-temperature. Decreasing the poly-mer solvency will also promote adsorption of polypoly-mers at surfaces. Close to conditions of hulk precipitation, this tendency will be high and may result in the formation of multiple layers of polymers at the surface. For poly-mers that carry a net charge, commonly referred to as polyelectrolytes, the intra-chain electrostatic repulsion between charged groups along the polymer backbone results in a more extended conformation than that of a random coil. A common measure of the chain rigidity is the persistence length Lp, which increases with increas-ing charge density of the macromolecule and decreasincreas-ing ionic strength. A high rigidity leads to deviation from the random-coil conformation. A highly charged poly-electrolyte will, at very low ionic strengths, behave more like a stiff rod. As salt is added to a polyelectrolyte solu-tion, the intra-chain repulsion becomes screened and the polyelectrolyte will behave more and more as a flexible non-ionic polymer. The effects described above can also be seen for adsorbed polymers, as is emphasized below.
We shall now mention some basic facts about the manner in which polymers organize at interfaces.
Adsorbed polymer segments are classified into trains, loops and tails, depending on whether the segment is anchored at the interface, forms a loop out from the sur-face, or extends into the solution with only one end attached at the surface, as illustrated in Figure 7.11.
The relative contribution of trains, loops, and tails will depend on the interaction strength between the polymer monomers and the surface, the solvency of the polymer chain, and also the polymer charge density in the case of polyelectrolytes (12). If the polymer is a copolymer built up of different monomers, then the structure will also depend on the distribution of different segments along the chain. The following trends apply in most
cases, providing all other factors affecting adsorption are constant:
• Increasing the molecular weight usually results in an increase of adsorption, thicker layers, and a larger relative fraction of loops at the expense of tails.
• Improving the solvency conditions results in a decrease of the overall adsorption, but frequently to an increase of the size of loops and tails and their extension into solution.
• Increasing the strength of the monomer surface inter-action leads, at least initially, to an increase of adsorp-tion and an increase of the loop and tail extensions into solution.
Since polymers used in papermaking are often oppo-sitely charged to the surface, we need also to consider some basic electrostatic effects:
• Increasing the fraction of charged groups results, for purely electrosorbing polymers, initially in a strong increase of adsorption followed by a slower decrease at higher charge densities. In the latter regime, the layer thickness tends to decrease strongly with increasing polymer charge density. The maximum is for low-ionic-strength solutions observed at polymer charge densities of only a few percent.
• Increasing the salt concentration shifts the position of the maximum to higher charge densities and the adsorption to lower values. However, the layer thick-ness given by loop and tail sizes increases with the salt concentration. In the limit of high ionic strengths, electrostatic interactions are strongly screened, thus resulting in desorption. This effect becomes even larger if the competing ions are multivalent or interact specifically with the surface.
The adsorption maximum observed on increasing the polymer charge density can be explained as follows.
Initially, the increased electrostatic attraction results in more polymers being bound to the surface. However, this effect competes with the fact that an increased charge density results in more train segments, and thus fewer and shorter loops and tails. Low-charged polyelec-trolytes tend as a general rule to adsorb in more extended conformations with loops and tails than highly charged polyelectrolytes, which adopt flatter conformations, due both to the larger stiffness of the polymer chain and the increased chain density of conceivable anchoring sites. It should in this context be noted that purely electrosorbing polymers tend to fully charge-compensate the surface in the low-salt limit (12).
In many cases, adsorption is driven both by electro-static and non-electroelectro-static interactions. In conjunction Figure 7.11. Schematic illustration of the structure of an
adsorbed polymer chain. Segments are distributed into trains, loops and tails
Loop
Train Tail
with adsorption close to the saturation limit, this often results in an overcompensation of the surface charge.
The outcome of varying different properties, such as the ionic strength, becomes less transparent when both elec-trostatic and non-elecelec-trostatic interactions are important.
Increasing the salt concentration may, for instance, also lead to an increased adsorption in the high-salt limit for such a system. The reasons for the increase may in such a case be twofold. Increasing the salt concentration decreases the electrostatic intra-chain repulsion and can thereby result in denser packing of the polyelectrolyte chain in the interfacial region. It may also reduce the sol-ubility of the polymer and thereby cause an increased adsorption. The two effects are strongly interrelated.
In the case of like-charged polymers and surfaces, polymers are depleted from the surface (negative adsorp-tion) unless non-electrostatic interactions aid adsorption (typically hydrophobic attraction in aqueous media). In such a case, adsorption increases with increasing salt concentration, due to the screening of repulsive electro-static interactions between the polymer and the surface.
The retention systems used in papermaking are often multi-component systems. Nanoparticles, for instance silica sols, are frequently added to a high molecular weight cationic flocculants in order to improve retention and flocculation properties. Nanoparticles compete with the colloidal fibre, filler and fines surfaces for the poly-electrolyte. In terms of adsorption properties, nanoparti-cles seem to have similar, but more pronounced, effects than salts. Relatively small additions result in a large swelling of the adsorbed cationic polyelectrolyte layers.
The nanoparticle can be viewed as a multi-ion compet-ing with the surface for polyelectrolyte charges. When present in the adsorbed layer, they may mutually repel each other and thereby further aid in swelling of the adsorbed polyelectrolyte layer. Many multi-component systems are used in papermaking, but an attempt to ratio-nalize their respective complex interfacial behaviour is beyond the scope of this present chapter.
Before discussing the effects that adsorbed polymers have on interactions between surfaces and colloidal bodies, a brief discussion about the kinetic aspects of polymer adsorption phenomena is given. This is an important aspect to consider since papermaking is a dynamic process. At least at low coverage, polymer adsorption tends to be mass-transfer limited, implying that the adsorption rate can be written as follows:
-T =-J-[Cb - Ct(T)] (7.8) at 8
where Dp is the diffusion coefficient for the polymer, 8 is the diffusion distance (or stagnant layer thick-ness), Cb the bulk polymer concentration, and C8(F)
the sub-surface concentration for a given surface excess.
This is equal to the inverse isotherm value C(F) and hence directly accessible from experimental adsorption isotherm data. Due to the high-affinity nature of most polymer isotherms, the surface concentration C8(F) can be set to zero for the major part of the adsorption pro-cess ("perfect sink" condition). Surprisingly good fits to experimental adsorption data for polymers and block copolymers have been obtained by using the simple local equilibrium approach of equation (7.8). The simple mass-transfer-limited model (based on the assumption of local equilibrium) has indeed been shown to pre-dict rather nicely the adsorption kinetics up to 75% of the plateau surface coverage and for Reynolds numbers ranging from 1 to 200 (the experimentally accessible regime), as discussed in ref. (12). By using this simple model, we can also, as was mentioned above, explain the slow desorption kinetics observed for polymer and poly-electrolyte systems. The parameter C8(F) also decays very rapidly during desorption under local equilibrium conditions, and extremely slow mass-transfer is pre-dicted. The prediction is, indeed, in agreement with experimentally observed desorption rates, which, how-ever, are often interpreted in terms of adsorption irre-versibility. Therefore, the concept of irreversibility is not necessary for explaining the very slow desorp-tion rates observed experimentally for adsorbed poly-mer systems. A further indication of the reversibil-ity of polymer adsorption processes is the fact that most polymers are easily displaced by associates of higher molecular weight (12). The driving force for the displacement is the concomitant entropy increase of the system. In the case of charged polymer adsorp-tion, it is more likely that adsorption is partially irre-versible. The strong electrostatic bonds with the sur-face may inhibit relaxation and trap the polymer in a metastable state. This issue is, however, still unre-solved.
There are naturally many situations when polymer adsorption is not mass-transfer limited, and mixed kinet-ics, taking into account adsorption barriers and slow sur-face relaxation phenomena, more appropriately describe the adsorption kinetics. However, in many practical cases it is quite sufficient to consider the implications of the simple mass-transfer-limited kinetics discussed above, keeping in mind that the technical retention sys-tems used are inherently polydisperse. The papermaking furnish is furthermore exposed to high shear, which must be carefully considered in the kinetic analysis. The colli-sion frequency of the colloidal components in the system is also high. This is important and means that poly-mers may be transferred between colliding surfaces as
well as becoming structurally distorted in the adsorbed state.
The fact that most solid components in a papermak-ing furnish are rough and porous is an issue that we so far have not discussed. Conceptually, it is logical that the adsorption at fibre surfaces resembles the corresponding process occurring at a smooth surface having the same surface properties. A distribution of trains, loops, and tails, will thus form on the surface, depending on the various properties discussed earlier. However, with time, polymer may start to penetrate the small pores present in fibre surfaces, and relax in crevices. This is schemati-cally illustrated in Figure 7.12. Such relaxation phenom-ena will naturally affect the impact that the polymers have on colloidal interactions in terms of flocculation