A wave of excitement about this size range - involving phenomena just a bit larger than typical molecular dimensions - is stimulated by recent advances in science. Now, as never before, it is possible to make, measure and manipulate structures in this tiny range of size.

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 nanometer scale already have become well-developed in papermaking processes.

Nanoparticles

In terms of the amounts of materials consumed, papermakers' use of nanoparticles (NPs) 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 annual tons of paper and paperboard products.

A brief review of colloidal silica attributes can help to make sense of how these additives function. As illustrated in Figure 1, 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 nonporous 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/g but this still isn't nearly as high a number as the surface area of commercially available colloidal silica nanoparticles for retention and drainage enhancement.

As illustrated in Figure 2, 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. In addition to their high surface area and negative surface charge, colloidal silica products also can vary with respect to structure. In other words, 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.As shown in Figures 3 and 4, two of the ways in which NPs can interact with adsorbed polyelectrolytes in the wet end of a paper machine can be described as "semi-reversible bridging" and "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, 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 to complexation between two different organic polyelectrolytes; evidence suggests that the rigid, three-dimensional nature of commercial NP additives is helpful for achieving the bridging and deswelling effects illustrated in Figure 4.

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 5 contrasts the size and shape of montmorillonite 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 to 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 NP 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 the 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 1, the so-called gel-type NP products used in papermaking applications usually are limited to small chains and clusters of primary particles. Greater effectiveness of such partially gelled structures makes sense in view of the bridging mechanism that was illustrated in Figure 3. 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 further 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 to either repel each other or 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.As illustrated in Figures 6 and 7, chain formation is favoured by intermediate repulsive forces. An individual primary particle, approaching the side of an existing chain, experiences greater net repulsion, compared to 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 already can be appreciated based on the success of innovations introduced during the 1980s and 1990s. For instance, various patents describe how aluminum 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 aluminum sulfate (papermaker's alum) and polyaluminum chloride (PAC).

Whereas the ionic species resulting from addition of these water-soluble aluminum compounds to aqueous solution 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 aluminum 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 result is that 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 aluminum that is used in the preparation.

An emerging emphasis on flexibilityTwo of the most frequently mentioned benefits of NP 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 new attention to designing of microparticle programmes that are suitable for addition after the pressure screens, thus avoiding unnecessary breakdown of the added retention aid polymer.