4 INTERNAL SIZING OF PAPER
4.2 Internal sizing sub-processes
The aim of internal sizing is to cover the paper surfaces with a strongly anchored monomolecular hydrophobic layer. Since most sizing agents are added in the form of emulsions/dispersions, this involves a number of steps before the individual sizing molecules can anchor at fibre and filler surfaces. The following principal steps can be identified as critical for the buildup of hydrophobicity during internal sizing:
• Retention of sizing agent at fibre and filler surfaces
• Spreading and redistribution of sizing agent over fibre and filler surfaces
• Anchoring of sizing agents with surface groups on fibres and fillers
Depending on the sizing system used, the importance of these different processes varies. Rosin soap sizes used together with alum as a retention aid in acidic paper-making processes have high melting points and do not extensively redistribute following the retention step. For these sizing agents, it is very important that the reten-tion process results in a uniform distribureten-tion of rosin particles at the fibre surfaces and that the particle size is small in order to achieve a high sizing efficiency.
So-called free rosin dispersions (with high fractions of free rosin acid) do, however, redistribute after the depo-sition step of the dispersed rosin particles/droplets. For these systems, the spreading and surface anchoring of individual size molecules during curing will also affect the outcome of the rosin sizing process. The same is true for synthetic sizing agents like AKD and ASA, which are often referred to as cellulose-reactive sizes.
Note that the extent to which these molecules actually bond covalently to cellulose under papermaking condi-tions is rather unclear. Nevertheless, they do redistribute on fibres and filler surfaces and through this process the hydrophobicity of the fibre surfaces is increased. The mode in which redistribution occurs is also a matter of some debate, as will be discussed later in this section.
All of the above-mentioned processes are indeed occur-ring after the size deposition/retention step. However, their respective importance in terms of the resulting siz-ing efficiency will vary dependsiz-ing on drysiz-ing and storage conditions, molecular structure and the melting point of the sizing agent.
We will now give some illustrative examples of the importance of different surface chemistry related mech-anisms for sizing, concentrating chiefly on AKD sizing, but also with a mention of some aspects of ASA and rosin sizing. In addition to sizing mechanisms, some useful techniques for measuring the surface chemical Figure 7.18. Chemical structures of: (a) abietic acid, which is
one of the more common constituents of rosin; (b) alkyl ketene dimer (AKD), with R = C14-C18 (saturated or unsaturated);
(c) alkenyl succinic anhydride (ASA) with R = C14-C18 (unsaturated)
properties of sized paper are also presented, together with a discussion of the implications for product perfor-mance. More extensive reviews on the subject of internal sizing can be found in refs (19, 20).
4.2.1 Retention of sizing agents
Cationic polyelectrolytes are generally used as reten-tion aids for synthetic sizing agents such as AKD and ASA. In addition to their role as retention aids, these polyelectrolytes may also serve as a part or all of the emulsifying/stabilization system. The driving force for size retention at neutral and basic pH values is the electrostatic attraction, resulting in adsorption of positively charged (due to adsorption of, e.g. cationic starch or synthetic cationic polyelectrolytes) AKD par-ticles at the negatively charged fibre surface. The charge character of the polyelectrolyte determines the elec-trophoretic mobility of the particles, as is demonstrated in Figure 7.19 (21). In this case, the cationic groups were tertiary amino groups for which deprotonation occurs at higher pH, which is clearly indicated by the decrease in their mobility. For particles stabilized by a polyelectrolyte quaternary amino group with constant charge, no change in mobility is expected over the same pH interval. It is worth noting, however, that other polymers and surfactants may be added to increase the stability of the size particles against homoflocculation.
These and ionizable groups on the size molecule (see
below) may also influence the pH dependence of the electrophoretic mobility of the size particles. The pulp type is another important factor for the retention effi-ciency. It is well known that thermomechanical pulp (TMP) and waste ground-wood is difficult to size. This may be related to less effective retention, but can alterna-tively be due to slow surface spreading of size molecules or weak anchoring. Besides attractive electrostatic inter-actions, retention of size particles may also be promoted by charge neutralization and bridging phenomena, in particular in situations where the polyelectrolyte cov-erage on size particles and fibres is limited.
The traditional retention systems used for rosin sizes under acidic papermaking conditions differ to some extent from the systems used for synthetic sizing agents.
Aluminium sulfate is generally used for retaining rosin soaps (and dispersed rosin emulsion droplets) at the fibre surfaces. Ferrous or ferric ions can also be effi-cient, but these are not used commercially. The role of the metal salts is effectively to impart a positive charge to the dispersed rosin particles, which then are retended on to the negatively charged fibre surfaces by attractive electrostatic interactions. Figure 7.20 shows the mobility of rosin particle-aluminium ion complexes at different pH values (22). Increasing the pH decreases the retention due to dissociation and deprotonation of the hydrated aluminium complex. Hydrated aluminium ion complexes are sufficiently polyvalent up to pH val-ues of about 6 to efficiently recharge the surfaces of dispersed rosin (p^aAl(H2O)6 3 + = 4.9). Retention is therefore usually relatively effective up to pH 6. Above this value, a noticeable decrease in retention is usually
PH
Figure 7.19. Electrophoretic mobility as a function of pH for AKD particles stabilized by cationic starch and for bleached kraft pulp. (Redrawn from ref. (21))
Electrophoretic mobility [((xm/s)/(V/cm)] Electrophoretic mobility [(nm/s)/(V/cm)]
PH
Figure 7.20. Electrophoretic mobility of rosin precipitate as a function of pH in the presence of A1(NC>3)3, Al2 (804)3 and H2SO4. (Redrawn from ref. (22))
Bleached kraft pulp AKD/starch particles
PH
Figure 7.21. Retention of rosin precipitate (Hab) on bleached kraft pulp as a function of pH in deionized water with different additions of aluminium sulfate. (Redrawn from ref. (23))
observed, as shown in Figure 7.21 (23). Note that it is not sufficient that the retention in itself is efficient.
A further requirement is that the rosin particles are well dispersed against homoflocculation and are thereby retained evenly on the fibre surfaces. Both the stability and the driving force for retention by heterocoagula-tion decreases with increasing salt concentraheterocoagula-tion due to the screening of electrostatic interactions. Increas-ing ionic strength results in more effective screenIncreas-ing of the double-layer forces. High concentrations of diva-lent anions are for the same reason more detrimental to the retention efficiency than monovalent anions. It is further observed that the stability of dilute rosin solu-tions results in less aggregation and hence in a more even surface coverage than retention from a more con-centrated solution of rosin-aluminium hydroxide com-plexes.
It has been difficult to use rosin as a sizing agent at neutral and basic pH values, due partly to the relatively low pKa value of the aluminium ion complex used for promoting retention. The substitution of aluminum sulfate by poly(aluminium chloride) (PAC) and organic polyelectrolytes has however extended the pH range in which rosin sizing is applicable. These are only a few of many different strategies for increasing the pH interval for rosin sizing which can be found in the literature (see, e.g. (19, 20)).
4.2.2 Redistribution of sizing agent on paper surfaces
Internal sizing agents are almost always added in the form of emulsions or dispersions. After retention at a surface, these will naturally only cover a limited area, given by the number density of particles/droplets at the surfaces and their respective areas. Potentially, however, the size content in these particles/droplets may spread at the surface and form a thin hydrophobic film, which then will cover a much greater surface area and thereby render the paper significantly more hydrophobic.
The common view has been that after retention, AKD spreads in a wetting fashion to ultimately form a thin monomolecular film. A simple calculation shows that this would lead to a large increase of exposed hydrophobic area. One adsorbed AKD droplet/particle with a radius of, say, 0.1 urn, covers an area of approximately 0.03 urn2. If the content of the same AKD droplet could spread into a monomolecular film with a thickness of about 10 A, the resulting hydrophobic area would be about 2.4 urn2, i.e. roughly 100 times larger.
It is easy to understand that spreading would greatly benefit the sizing efficiency. However, the mechanism of spreading of sizing agents is not yet well understood.
Potentially, there exist at least three likely routes for redistribution of sizing agent on the fibre surfaces:
• Wetting flow when the drop spreads to an equilibrium contact angle between 180 and 0° by surface tension forces.
• Surface diffusion of a monolayer from the foot of the macroscopic drop. The sizing agent in the macroscopic drop does not spread on the monolayer due to surface-ordering effects (an effect referred to as autophobicity).
• Gas-phase transfer of vaporized sizing agent followed by readsorption on paper surfaces.
The first mechanism, resulting in a monolayer coverage (i.e. complete wetting), has long been assumed to be the relevant one. However, recent studies have shown that AKD also does not form a zero contact angle when contacted with a relatively high-energy surface, such as silica or pure cellulose. Wetting does indeed occur, but results in non-zero contact angles. The reason for this behaviour is that a monomolecular film of surface-ordered sizing molecules spreads ahead of the macroscopic drop. The bulk liquid cannot spread on this film, due to surface ordering and preferential orientation of the hydrophobic groups towards air.
The effect is commonly referred to as autophobicity.
However, further spreading of sizing agents occurs by
HAb retention (%)
surface diffusion and spreading of the monolayer, as is clearly seen in Figure 7.22 (24). The apparent diffusion coefficient in the AKD monolayer is ~ 1 0 ~n m2/s. This value was found to increase in proportion to the ambient temperature, whereas it decreased with the increasing melting point of the sizing agent. The relatively slow rate of monolayer spreading explains why, in practice, the sizing efficiency builds up with time during the storage of paper, as shown in Figure 7.23 (24). Attention has also been focused on the redistribution of sizing agent through a different mechanism, involving desorption, vapour diffusion, and readsorption (25). At temperatures above 80°, higher than usual storage temperatures in the paper rolls, this mechanism seems to play a role.
However, if the tendency for readsorption is low, this can also cause de-sizing, which is seen for single paper sheets exposed to ambient air after long storage times in Figure 7.23 (24). Despite the importance of size redistribution through surface spreading, few fundamental studies have dealt with this issue and very little is indeed known for systems other than AKD.
In the case of ASA, it is clear that sizing develops much faster than with AKD. However, the reason for this has not yet been established. In the case of rosins, spreading is claimed for the dispersion type with high fractions of free rosin acids, as opposed to what occurs with the sodium soap particulate form.
Little mechanistic information about the redistribution process is available for rosin sizes. Most likely, similar mechanisms are involved as in the case of AKD. Their respective importance will depend on the temperature,
Vf/Vh
Figure 7.23. Influence of storage time and storage temperature on the hydrophobicity of bleached kraft pulp sized with 0.04 wt% AKD. The hydrophobicity is given in terms of advancing water contact angle recorded when the drop base expansion was first observed to stabilize. (From ref. (24) with permission)
melting points, vapour pressure, surface tension and other process conditions.
4.2.3 Reactions and side-reactions of sizing agents
The commonly claimed reaction mechanism for AKD is a direct covalent linkage with cellulose via /?-keto
Contact angle/degrees
Distance/mm
Figure 7.22. Laterally resolved ellipsometry profile showing an AKD precursor that spreads out from the foot of a macroscopic AKD drop (melting point <10°C), which has been put in contact with a silica surface. The inset shows schematically the spreading of an autophobic precursor from the foot of the AKD droplet. (From ref. (24) with permission)
Thickness/A
Figure 7.24. Proposed reactions of AKD with cellulose and hydrolysis in aqueous media
ester formation, as shown in Figure 7.24. The molecule can also be hydrolysed by water leading to a /3-keto acid that will spontaneously transform to a ketone. The reactivity with cellulose under papermaking conditions is quite low and it is not clear to which extent AKD actually reacts with cellulose and other surface groups on papermaking fibres. There are many claims about reaction promotors and so forth, but their roles are unclear and promotion may well be related to improved retention. Despite the fact that AKD sizing research has a long history, the issue is still not resolved and remains under debate by researchers active in the paper chemistry field. In the case of AKD, the hydrolysis with water to form keto acids and subsequently ketones is also shown to have important implications for the sizing result. In particular, ketones are claimed to make paper slippery and difficult to handle in some operations. If AKD actually forms covalent linkages with cellulose, the competing ketone formation will also decrease the reaction efficiency with cellulose. Even if this is not the case and AKD is only physisorbed at the surface, the ketone formation may result in less efficient binding between the size molecules and the fibre surface. The fact that the ketone has a higher melting point than AKD is also noteworthy. This may, for instance, reduce the spreading tendency of the ketone on the paper surface, in comparison to AKD.
Alkenyl succinic anhydride (ASA) may also, like AKD, either react with cellulose hydroxyl groups or with water, as shown in Figure 7.25. Note that the side reaction of ASA leads to the formation of a dicar-boxylic acid, which due to its amphiphilic nature can lower the surface tension of polar liquids, such as water, and thereby also decrease the sizing efficiency. The reactivity of ASA is claimed to be much higher than that of AKD and "full sizing" develops immediately in the paper machine, and suggests that covalent bonds
Figure 7.25. Proposed reactions of ASA with cellulose and hydrolysis in aqueous media
between sizing agents and fibre surfaces are more fre-quent in ASA-sized paper than for AKD-sized paper.
The high reactivity of ASA necessitates on-site emulsi-fication, using cationic starch as the stabilizing system.
The surfactant nature and higher solubility of the dicar-boxylic acid side product may affect the properties of the ASA emulsion. This may change emulsion stabil-ity criteria and also influence the surface charge of the emulsion droplets. This can further increase the acces-sibility of water and thereby also the rate of hydrolysis.
The dicarboxylic acid by-product may also increase the paper wettability due to its surfactant nature.
4.2.4 Analysis of size content in paper
The sizing efficiency is commonly determined indi-rectly through various wetting and absorption measure-ments made on sized paper products. However, it is often preferable to quantify the amount of sizing agents retained in the paper. Furthermore, it is desirable to identify whether or not these are bound at papermak-ing surfaces and if they are non-reacted, reacted with fibre surfaces, or hydrolysed. This is usually done indi-rectly by extracting the paper in different steps and then analysing the liquid phase by gas chromatography or other analytical techniques. This is still the most com-mon approach, but there are many questions unanswered regarding the specificity and efficiency of the extraction steps used in the different analyses employed. X-ray photoelectron spectroscopy (XPS) (discussed below in Section 8.1.1) is a straightforward technique with the
Alkenyl succinic anhydride (R = C14-C18 unsaturated)
£-Keto acid
HO—Cellulose
/0-Keto ester Ketone
Cellulose-HO
cellulose
capacity to give quantitative information on the content of a surface-spread sizing agent in a sheet. However, it cannot be used to determine if the sizing agent is in its natural, surface-reacted or hydrolysed state. For this purpose, time-of-flight secondary ion mass spec-trometry (ToF-SIMS) seems to be a good candidate, as described below in Section 8.1.2. This technique can also provide information about the in-plane distribution of sizing agents, but does not provide quantitative infor-mation such as that obtained by XPS.