Nanotechnology has been used to improve retention and drainage systems for wet-end chemistry for some time, though
scientists are only just beginning to understand how it works and what it means for the future of papermaking. Nanoparticle technology for wet-end chemistry is basically the science of silica monomers grown into clusters.

The first generation, still used today, combined an anionic nanoparticle (colloidal silica sol) and cationic starch.

The technology has progressed over the years. In 1992, a new generation was introduced in which structured silica sols were developed so they could be used in combination with synthetic cationic polyacrylamide (C-PAM). This development was a marked improvement over the older technologies because the nanoparticles bonded much better with the C-PAM. The silica spheres created strong covalent siloxane bonds that would not be broken by shearing on the paper machine. Despite this improved characteristic, it tends to be effective only on cleaner fine paper and board grades. It is not as suitable for paper grades having high conductivity at the wet-end.

As we seek to take advantage of recent scientific advances for wet-end applications, it is important to bear in mind that papermaking technology is already a leader in the field of nanotechnology. Phenomena occurring on a nanometre scale already have become well-developed in papermaking processes.

Figure 1 gives a representative list of papermaking additives that typically have dimensions within the size range of about 1–100nm: colloidal silica, micelles of rosin soap size, the main ionic species of polyaluminium chloride, and the radius of gyration of some common polyelectrolytes added to the wet end, such as starch.

New experimental methods fuelling nanotechnology

Atomic force microscopy (AFM) and related methods are prime examples of scientific developments that have been fuelling interest in nanotechnology.

It is worth beginning with AFM, since these methods have particular promise in terms of future developments in submicroscopic stylus, cantilever and piezo-electric crystals that expand in response to an applied voltage in order to detect surface features over incredibly small dimensions.

On a superficial level, AFM-related methods can be compared with conventional imaging techniques, such as transmission electron microscopy (TEM). It is worth noting that TEM was already well-established by the 1930s, achieving resolutions down to approximately 5nm. More recently, TEM methods have been used to observe individual strands of retention aid molecules. However, there are several key ways in which AFM methods can surpass electron microscopic approaches, especially with respect to potential applications in wet-end chemistry:

• Measurements and manipulations can be carried out in a wet state, not just in vacuum. In fact, AFM methods can even be used in a humid atmosphere, depending on the goals of the study.
• It is not necessary to coat the sample with gold or other extraneous material in order to use AFM imaging and other AFM-related methods. This means that there is less chance that results will contain artifacts of preparation methods.
• The method is able to resolve distances quantitatively in three dimensions.
  

One of the benefits of recent progress in AFM and other nano-sensitive analytical methods has been a new understanding and appreciation of the kinds of surfaces important to the papermaking industry. For instance, Furuta and Gray (1998) used AFM to characterise microfibrils at the surfaces of refined kraft fibres. In the wet state it was found that these nano-sized structures tended to stick out from the surfaces of wet fibres to an extent that is sensitive to chemical factors.

Likewise, Neuman (1993) showed that even in the case of cellulose spin-coated from trifluoroacetic acid onto a smooth substrate, it was possible to sense the effects of cellulose macromolecular chains extending approximately 60–80nm from the surface into the solution phase.

Horn (2001) described the use of AFM to image single polyelectrolyte molecules of interest to papermakers, on solid surfaces. In addition, by attaching submicroscopic spheres to the tips of AFM cantilevers it has been possible to evaluate the subtle forces exerted by polyelectrolytes adsorbed on surfaces. Such methods have been further modified to measure the frictional forces between cellulosic surfaces at a nanoscale.

As a consequence of these advances, it has been necessary for paper scientists to think again about some issues that many people considered as having been resolved many years ago. For example, it is inherently challenging to try to predict the attractive and repulsive forces between surfaces covered by water-loving polymers, as in the case of cellulosic fibre surfaces. Because of recent gains in our knowledge of fibre surfaces, the simplified geometrical models of contact regions, as assumed in many earlier studies of forces between fibres and other solids in the wet state, can no longer be considered as being credible in many cases of potential interest to papermakers.

The emerging picture of sponge-like papermaking fibres, coated by ‘tails’ of water-loving hemicellulose and cellulose macromolecules, has the potential to lead to better systems for retention and dewatering. At one time, it was widely believed that the decay of zeta potential data, following treatment of a papermaking slurry with a highly cationic polymer, was due mainly to migration of the polymers inside submicroscopic pores in the cell walls of the fibres. It might be supposed that such cationic polymers would cause contraction of the pore diameters, helping to expel water.

However, such a concept cannot explain the increased effectiveness of polyethyleneimine (PEI) dewatering chemicals as a function of increasing molecular mass. A more recent study (Wang and Hubbe, 2002) showed that a decay in dewatering enhancement, with increased time after addition of a highly cationic polymer, was not well-correlated to changes in zeta potential.

Both sets of results are more consistent with concepts of polyelectrolyte complexation between the dewatering aid polymer molecules and the water-loving molecular chains extending from fibre surfaces. Such complexation is expected to cause the fibrils at fibre surfaces to become matted down so that they hold less water and don’t provide as much resistance to flow of water around the fibres.

Before leaving the subject of analytical tools, it is worth mentioning some other methods in nanotechnology that have the potential to accelerate developments in papermaking chemistry. The quartz crystal microbalance (QCM) is able to detect minute quantities of adsorbed materials in the wet state, even down to the monomolecular layer quantities. Because adsorption can be studied as a function of time onto
various prepared surfaces, including well-characterised cellulose surfaces, QCM has the potential to allow papermaking technologists to compare quantitatively the abilities of different candidate wet-end additives to interact with such surfaces. Even better, the method gives the user information about whether the adsorbed material is firmly attached in solid-like fashion, or whether it acts like an elastic or
viscous material in the adsorbed state.

Other methods capable of sensing effects of the outermost layers of molecules at a fibre surface include dynamic contact angle methods, charge and zeta potential tests, and traditional vacuum-type surface analytical methods, such as x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). In addition, ellipsometry and reflectometry can be used to measure the thickness of polymer monolayers on smooth surfaces.

Nanoparticles as dewatering and retention adjuncts In terms of the amounts of materials consumed, the use of nanoparticles by papermakers is arguably the world’s most important present application of nanotechnology. Worldwide, papermakers employ colloidal silica and related
products each year to promote dewatering and fine-particle retention on hundreds of paper machines during production of over ten million metric tonnes annually of paper and paperboard products.

A brief review of colloidal silica attributes can help make sense of how these additives function. The sizes of the primary particles in commercially available colloidal silica additives generally lie within the range of 1–5nm. Since these primary particles are non-porous and approximately spherical, the cited dimensions imply surface areas in the range of about 500–3,000m2/g.

To put those numbers into perspective, the surface area per unit mass of these nanoparticles is about 1,000 times higher than that of most other solid materials that constitute dry paper. Highly swollen wood-pulp fibres in their wet state can have surface areas as high as about 200m2/gram, but this still is not nearly as high a number as the surface area of commercially available colloidal silica nanoparticles for retention and drainage enhancement.

Figure 2 illustrates the approximate size and structure of two principal types of colloidal silica nanoparticle products that are used as part of treatment programmes to enhance fine-particle retention and dewatering during the formation of paper.

The surface of silica can be described as acidic, meaning that protons dissociate from silanol groups, leaving behind a negative charge. Both dissociation and the density of surface charge increase with increasing pH. As well as their high surface area and negative surface charge, the structure of colloidal silica products can also vary. The primary particles may exist as discrete entities, which then may become joined as clusters or chains during their preparation.

If one merely adds colloidal silica in dispersed form to a slurry of untreated fibres, nothing happens. The fibres are neither flocculated nor dispersed and there is no change in the rate of release of water when the slurry is placed on a screen. The negatively charged particles have little interaction with the negative surfaces of untreated papermaking fibres. Rather, the presence of a high-mass polyelectrolyte, usually of cationic charge, is required before one observes significant benefits in terms of dewatering rates or fine-particle retention.

One way that nanoparticles can interact with adsorbed polyelectrolytes in the wet-end of a paper machine can be described as semireversible bridging” and another as “contraction-deswelling”. Based on a mechanism first demonstrated by measurements of sediment densities, the reversible attachments between the fibres, resulting from the sequential treatment with polymeric flocculant and nanoparticles (NP), are expected to yield a more bulky paper structure with larger inter-fibre pores. Because the primary particles of colloidal silica can be much smaller than the extended conformations of the polymers, it is useful to envision the interactions between them as instances of polyelectrolyte complexation. There is one key distinction, however, compared with complexation, between two different organic polyelectrolytes; evidence suggests that the rigid, three-dimensional nature of commercial NP additives is helpful for achieving the required bridging and deswelling effects.

A second type of mineral additive used in combination with cationic polymers for retention and drainage enhancement is alkali montmorillonite, a so-called ‘swelling clay’ that is commonly known as bentonite or smectite. Figure 3 contrasts the size and shape of the larger, but very thin (see edge view), plates of montmorillonite microparticles that are also used as papermaking additives with that of colloidal silica particles, as already discussed.

Though montmorillonite particles are too big in two dimensions to fit the usual definition of nanoparticles, the platelets of clay are capable of being separated into layers as thin as about 10nm. To prepare these microparticles for papermaking applications, the mineral is treated with base, replacing the calcium ions in the mined material with sodium. Most applications of bentonite for retention and dewatering have employed sequential addition of the mineral before or after high-mass cationic acrylamide. Compared with microparticle systems employing colloidal silica particles, bentonite programmes have the reputation of providing cost-effective retention benefits over a wide range of pH and other wet-end conditions.

What next for nanoparticles in the wet end?

Some recent developments in nanoparticle technology involve the structure, surface area and charge of the inorganic solids. The word “structure” implies an optimisation in the degree and pattern by which primary particles are joined together. The word “gel” is often used in scientific literature to describe NP solids compositions in which the primary particles are extensively linked together in a three-dimensional pattern. By contrast, as was illustrated in Figure 2, the so-called gel-type NP products used in papermaking applications are usually limited to small chains and clusters of primary particles. The idea is that an NP composite particle having a chain-like structure is more likely to be able to interact simultaneously with polyelectrolyte chains extending from two different surfaces.

Though methods of producing NPs for papermaking differ in detail, there are well-known principles that can be exploited in an attempt to improve NP performance for future applications in papermaking. One of the keys will involve careful control of colloidal stability, i.e. the tendency of particles either to repel each other or to stick together following chance collisions. Different results can be achieved depending on the balance of attractive and repulsive surface forces present during the initial formation of the primary particles. Strong inter-particle repulsive forces cause the NPs to grow as discrete individuals, resulting in what colloid scientists call “sol” products.

By contrast, NP formation in the near-absence of repulsive forces allows the primary particles to collide and fuse together. Once two such particles successfully collide, they tend to be held together by Van der Waals forces and thereafter the continued precipitation of material from solution fuses the primary particles together. Further reduction of the effects of negative charges at the solids’ surfaces, either by reducing the pH or by adding salt ions, favours the formation of highly structured solids, which can be used as silica gel desiccants and absorbents. Between these two extremes there is room for innovations leading to well-controlled chain or cluster formation.

Chain formation is favoured by intermediate repulsive forces. An individual primary particle, approaching the side of an existing chain, experiences greater net repulsion than when it approaches the end of the same chain. Random diffusion will tend to favour the particle colliding with the end of a chain, where there is a lesser barrier of free energy opposing the collision. Multiple repetitions of this mechanism will tend to produce chain-like composite particles.

Some future opportunities to fine-tune the charged nature of NP products can already be appreciated based on the success of innovations introduced during the 1980s and 1990s. For instance, various patents describe how aluminium can be incorporated into colloidal silica NP products. Surprisingly, the effect of such addition on charge characteristics can be just the opposite of what would be expected, based on the well-known colloidal effects of aluminium sulphate (papermaker’s alum) and polyaluminium chloride. Whereas the ionic species resulting from addition of these water-soluble aluminium compounds to aqueous solutions ordinarily contribute to a positive charge of surfaces onto which they adsorb, just the opposite can occur within certain ranges of co-precipitation with sodium silicate. The reason is that tetrahedrally co-ordinated aluminium ions, having trivalent character, can occupy spaces in the semi-crystalline solid that otherwise would have been occupied by tetravalent silicon atoms having the same symmetry. The resulting colloidal particles maintain their negative charge to much lower pH values, allowing them to out-perform ordinary colloidal silica as a dewatering aid under acidic papermaking conditions.

More recent developments have been reported for the incorporation of boron into nanoparticle products and it seems likely that a similar mechanism may be at work. In addition, work with other synthetic silica microparticles (SMM) makes available a continuous range of particle surface charge characteristics, including positively charged particles, depending on the ratio of silicon to aluminium that is used in preparation.

An emerging emphasis on flexibility 

Two of the most frequently mentioned benefits of nanoparticle addition programmes during paper manufacture are increased fine-particle retention efficiency and faster dewatering. But papermakers’ needs vary widely with respect to these two kinds of effects. Dewatering effects are often a dominant concern for papermakers producing relatively heavyweight paperboard grades, especially if the furnish contains low-freeness mechanical fibres. Retention is of greater relative concern for those papermakers who are producing lightweight paper products on high-speed paper machines with modern forming sections.

To some extent, papermakers have been able to meet their unique needs by running trials with different kinds of NPs, different polymers, different relative dosage rates and different addition points. For example, a switch to a more highly structured colloidal silica product or to highly platy bentonite microparticles has been found to enhance retention effects, relative to dewatering benefits. However, there has been a need for systems that allow the user to make quick, reliable adjustments in the balance between retention and dewatering effects.
Recently, it has been proposed to use a mixture of different types of microparticles, varying the ratio between them, to optimise retention, dewatering and formation uniformity. Also, there is attention being paid to designing microparticle programmes suitable for addition after the pressure screens, thus avoiding unnecessary breakdown of the added retention aid polymer. It can be expected that more systems offering similar flexibility will become available in the future.

Nanocomposites for superior strength

A concept closely related to the nanoparticle-based dewatering and retention systems just considered can also be used, in principle, for development of superior bonding strength in composite-type products, including paper. Though the concept of nanocomposites has mainly been applied to extruded materials, such as plastics, it is not hard to imagine its applications in the wet-end of a paper machine to enhance the effects of dry-strength additives. It is worth noting that the earliest patents describing addition of cationic starch and colloidal silica to slurries of fibres and mineral refer to these additives as a “binder” system.

Markets and forecasts

It appears that nanoparticle retention and drainage systems are becoming more widely accepted and are even becoming more specialised. According to one industry estimate, there are
over 1,000 nanoparticle-driven paper machines in use today. In North America (1,100) and Europe (1,900) there are approximately 3,000 paper machines. As silicate nanoparticle retention and drainage systems are so attractive, it is expected that most machines will use them in the future.

Benefits:

• Improved formation;
• Improved retention;
• Improved drainage;
• Improved dry strength;
• Better reactivity.
• Increased production;
• Reduced steam consumption;
• Products with higher brightness.