We’ve heard plenty about how nanotechnology is being used to improve performance of paper and packaging through its use in additives and coatings, but what about nanotechnology’s potential in the actual production process, even helping to grow the perfect trees for paper? It would be hard to think of a material that has contributed more to our civilisation than paper. The rise of Internet communication has not diminished paper’s role in the world; on the contrary, the Internet seems to have increased it.

Papermaking has a long history dating back almost two millennia and its production has been perfected over many centuries. A modern paper mill is an impressive sight, consuming vast quantities of trees, and possibly producing a thousand miles of paper every day. At first sight, nanotechnology appears to be the very opposite of this ancient and large-scale technology – something very modern and very small. Yet nanotechnology is creating revolutions in many fields working at much larger scales. A good example is electronics, where the relentless drive to reduce the size of components has brought integrated circuits down to nanometre dimensions. Another growth area is nanoparticles, used in an astonishing range of products, although many of the innovations simply continue trends that began with larger particles. Papermaking has in effect been practising nanotechnology for centuries. The basic structural elements of paper, i.e. cellulose fibres, are true nanostructures, as are many of the other added components such as fillers (nanoparticles) and sizing.

Nanotechnology offers papermaking great potential to enhance existing products and to enable new roles. Some critics raise objections to the idea that nanotechnology and papermaking are closely linked, on the grounds that papermaking is essentially a statistical process and therefore opposed to the ‘classical’ concept of nanotechnology, according to which the position of every atom is specified and controlled. Be that as it may, there is relentless pressure in the industry to increase the rate of production, and businesses will adopt any technological developments that increase efficiency and quality while decreasing costs.

Paper is a thin sheet material made from vegetable fibres felted together. Its earliest and most important use is as a substrate for writing produced by hand or by some mechanical process such as typing or printing. It is therefore distinct from other substrates such as papyrus, rice paper, parchment and vellum. According to D Hunter, paper is defined as a thin sheet made from fibres that have been macerated until each individual filament is a separate unit; the fibres are then intermixed with water and, using a sieve-like screen, they are lifted from the water in the form of a thin stratum; the water drains through the small openings of the screen, leaving a sheet of matted fibre on its surface. Most paper is made from cellulose, the main structural material in the plant kingdom. In principle, any form of cellulose may be used as raw material for papermaking, but the overwhelming majority comes from trees. Unusual plants may be used for special papers.

Papermaking dates from the beginning of the second century AD; it is reputed to have been invented in 105 AD by Ts’ai Lun in China. Via Samarkand, Baghdad, Damascus, Egypt, the Maghreb and Muslim Spain, papermaking arrived in Italy by the end of the 13th century. It appeared in England in about 1490. During this time it was made by hand in single sheets from rags. A flat sieve was dipped into water containing rag fibres then lifted out. The fibres were shaken together and the resulting mat was hung up to dry.

At the time of the French Revolution, workers in the French government paper mill in Nantes demanded higher pay and shorter hours. The manager, Robert, invented a papermaking machine to satisfy the government without hurting the business. The machine was later perfected by two Englishmen, the Fourdrinier brothers. It poured the fibres out in a stream of water onto a long wire screen looped over rollers. As the screen moved slowly over the rollers, the water drained off and delivered an endless sheet of wet paper. Its use spread rapidly and it was extremely successful, but its very success created an acute shortage of rags. It was therefore essential to find a new and cheaper source of fibre, which turned out to be wood.

The structure of wood

The inner part of a tree trunk is called the xylem. Its main functions are to support the branches and to provide channels for water and nutrients to flow from the roots to the leaves. Around the outermost layer of the xylem is a ring of cells called the cambium, where the new cells are created during the growing season. Encircling the xylem is the phloem, a narrow band of cells where sugars made in the leaves are transported to the cambium and the roots. Encircling the phloem is the bark, a layer of dead cells that give the other cells protection.

In the spring, when the tree is growing rapidly, the cells produced by the cambium are large and have thin walls; in the summer, growth slows down and the cells have a small diameter and thick walls. The two types of wood are known as springwood and summerwood, and their fibres have profoundly different properties for papermaking. The thick-walled summerwood fibres do not collapse on drying; papers made from summerwood are bulky and porous. The thin-walled springwood fibres collapse into flat ribbons on drying; paper made from springwood tends to be dense and strong.

The relative proportions of the two types profoundly affect the quality of the pulp, but this effect is often masked by varying proportions of mature and juvenile wood. Trees grow from the inside outwards. The conical growing tip of a tree is formed entirely out of juvenile wood; beneath the base of the cone there is a cylinder of juvenile wood extending down to the base of the tree. The juvenile fibres are shorter and have thinner walls than the outer ones. Both length and wall thickness increase with maturity. Trees taking a century or more to reach pulpwood size, as in northern Sweden, are likely to contain only a small proportion of juvenile wood. On the other hand, trees grown in warm, humid climates may be felled for pulping after a mere decade, and therefore contain mainly juvenile wood. Sawmill waste is a major raw material for pulp mills.

Regardless of its age, when a log is converted into timber, the waste is normally from the outer regions of the tree. Therefore the chips sent from the sawmill to the pulp mill are normally composed almost exclusively of mature wood, i.e. the raw material has longer, thicker fibres than pulp made from whole logs. The two great classes of wood are the hardwoods and the softwoods. Softwoods are gymnosperms and hardwoods are angiosperms; angiosperms include most of the familiar flowering plants. Gymnosperms appeared first and are more primitive than the angiosperms. In gymnosperms, a single type of cell, the tracheids, carries out both functions of support and water conduction. Summerwood tracheids are principally supporting fibres and springwood tracheids are principally water-conducting fibres. In the later and more advanced hardwoods, different cell types evolved to fulfil the different functions. Vessels are responsible for water conduction; they are short thin-walled cells and tend to be destroyed in the pulping process. Fibres perform the supporting function; they have small diameters, thick walls and are pointed at their ends. They are the most important cells for hardwood papermaking. Parenchyma is a third cell type; it stores nutrient reserves, mainly sugars, during winter. Parenchyma is present in hardwoods and softwoods but tends to be destroyed easily during pulping.

Cellulose

Cellulose is a long unbranched chain of glucose monomers linked head to tail. The chains are packed side by side to form microfibrils. Microfibrils can exist in two crystalline forms, and typically exist together in a mixture. The fibrils typically have a diameter of 3nm in plant cellulose. Cellulose is heavily hydroxylated and can therefore participate extensively in hydrogen bonding. Its pyranose form is in thermal equilibrium with the alcohol-aldehyde form, but the equilibrium considerably favours the pyranose form. Ultraviolet light causes dehydrogenation and radical formation.

Pulp

Trees are harvested from the forest and their bark and phloem are removed. The purpose of pulp manufacture is to disintegrate the cellular structure of the wood into individual cellulose fibres. The simplest way of doing this is to break down the wood using a purely mechanical process – bark-free sticks of wood are broken down in grinders. This method gives the greatest yield, but the quality is not particularly high. It is used extensively to make newsprint. The product is called groundwood or mechanical pulp.

Except in the case of cotton, which is a seed hair, cellulose vegetable fibres are cemented together with non-cellulosic material to form the woody mass, some 50 per cent of which is actually cellulose. A variety of chemical methods have been developed to yield purer material. In particular, the wood from the xylem is primarily constructed from cellulose fibres bonded together with lignin. Lignin is a complicated polyphenolic compound. The crude mechanical grinding process does not eliminate the lignin, but merely produces bundles of fibres still joined by lignin. In chemical processing, everything apart from the cellulose is dissolved away, leaving a pure white material. The wood is firstly chopped into small chips. The chips are then placed into digesters in which they are cooked together with chemicals. After washing, and possibly bleaching, the fibres emerge as pure cellulose wood pulp, generally called chemical pulp.

There are several chemical methods for digesting the lignin. The goal is to oxidise the lignin selectively (whereupon it breaks up and becomes soluble) without attacking the cellulose. Few inexpensive oxidants have this property. The main processes are sulphite pulp used with spruce and other softwoods,

in which the woodchips are treated with calcium bisulphite under acidic conditions; soda pulp used with hardwoods and esparto grass, where the oxidant is caustic soda; and kraft or sulphate pulp, where pine is treated with caustic soda and sodium sulphate, which is converted to sulphide. Many hardwoods are pulped using the sulphate process. Semi-alkaline pulp (SAP) is cooked at slightly alkaline pH (around pH 8) and has a higher pH than acid-cooked pulp. The industry has huge experience in choosing the best method for a given type of wood, and the properties of the final papers depend heavily on the choice of chemical process. For example, sulphate pulps tend to produce bulkier, more opaque papers than sulphite pulps, and the papers have less tendency to shrink.

Additional processing steps can be included to give the pulp special properties. For example, the wood can be prehydrolysed by digesting under slightly acidic conditions, then given a normal sulphate cook, and finally subjected to drastic mercerisation (usually with cold caustic soda) to yield pulp with a high cellulose content. A process midway between purely mechanical and purely chemical is particularly efficient for pulping shortfibre hardwoods. It is called the semichemical neutral sulphite method, in which the wood is only partially digested and its disintegration is completed by mechanical means. The pulp may pass directly to the paper machine, otherwise it flows onto a moving, shaking wire screen through which the water drains away to give a continuous ribbon that is dried and rolled up.

Paper production

When the paper mill is supplied with pulp sheets, they are broken up in a breaker, which is fitted with a bladed rotating roll, and mixed with water to give 5 per cent fibre and 95 per cent water. The resulting ‘stuff’ or ‘stock’ has about the consistency of porridge. When the pulp needs to have a good colour, a bleach is run into the breaker; it could be calcium hypochlorite liquor, chlorine or chlorine dioxide. Alternatives to the chlorine treatments are active oxygens, such as ozone or hydrogen peroxide.

Breaking and beating

The next operation is beating, in which the pulp is abraded between moving surfaces adjusted to work under pressure and very close to each other (Figure 4). This subjects the pulp to very high compressive and shear stresses that cause the

fibre surface and the fibre body to partly disintegrate, yielding a large number of fibrillae joined at their bases to the original fibre. This produces an enormous increase in the material’s surface area. Depending on the exact conditions in the beater, the fibres may also be cut to shorter lengths. Although beating is ostensibly a mechanical process, changes in the surface chemistry may take place, such as an increase of hydrophobicity, known as self-sizing; variation in beating alone can cause the same stock to produce greaseproof or blotting paper. The pH is very important in helping or hindering the beating process, so good pH control is essential for reproducible runs.

Colouring materials such as dyes or pigments can be added during beating. Size, traditionally based on rosin, is mixed in if required. It reduces water absorbency, increasing writeability. Rosin is chiefly abietic acid (abieta-7,13-dien-18-oic acid) and is usually made into a soap by cooking with soda ash; superior papers may be tub-sized with gelatine in a separate after-process. Alum is added to fix the dyes and precipitate the rosin onto the fibres. Mineral matter is added to give a good surface for printing and for economy – as much as 30 per cent is found in some papers. If size is not added at this stage, surface sizing can be carried out in the paper machine. Synthetic sizing agents include alkyl ketene dimer and alkenyl succinic anhydride, which are used to size alkaline paper. In recent years there has been a great increase in the variety and sophistication of additives. Many proprietary chemicals are now used to improve paper properties. Examples are additives for improving printability; surface strength improvers; cross-linkers; antistatics; flame-retardant treatments; antislip coating materials; microbiocides; materials for providing superior barrier properties for air, water and water vapour; fluorescent dyes; and miscellaneous chemicals for repelling or attracting various substances. Sizing too has come a long way from the simple rosin of 50 years ago.

Dyestuffs are also more sophisticated and include fluorescent whitening agents to enhance the whiteness and brightness of the paper. In addition, many chemicals are added for improving the wet-end process for better additive retention, pH control, water drainage, foaming control, fibre recovery and runnability in the paper machine. They are sometimes called auxiliary chemicals.

The beaten pulp mixture is diluted until there is about one part of fibre to 200 parts of water. Any grit is allowed to settle out and the stock is passed through screens to remove small lumps. It then flows through a slit-shaped orifice onto a travelling endless band of wire gauze (sometimes called the Fourdrinier wire), with about 60–80 meshes per inch, and supported on small rollers. Most of the water runs through the gauze (drainage) and the fibres remain as a mat of wet paper. Drainage is assisted in the later stages by applying a vacuum to the underside of the wire.

Then the wet paper is lifted away (couched) from the wire, carried on felts through pressing rolls to remove most of the remaining water, over steam-heated drying cylinders (air may also be blown along the direction of the web to assist drying), calendered (intense calendering can result in the paper becoming transparent, called glassine paper), and finally wound on a reel at the end of the machine. Processes such as creping or coating may be carried out on- or off-machine. Additional calendering, or supercalendering, is an off-machine process. Key production parameters are drainage and retention. Friction, temperature and pressure are important in calendering. Pressure, or nip, between pairs of rollers is also important.

Nanotechnology in production

Nanotechnology is used in additives and coatings but this is not considered here. We are looking at the actual processing itself, starting with the growth of timber through to recycling waste paper and disposing of waste in a profitable fashion.

Bionanotechnology

Nanotechnology can help to improve the supply of raw materials to the paper mill by intervention at the level of cellular processes – bionanotechnology. Genetic engineering could be favourably applied to tree planting, not least because trees can be readily cloned. Nevertheless, much basic research still needs to be performed on cellulose biosynthesis, and improving forest product yields is still very much at the research level. One of the difficulties is the slowness of tree growth, so experiments take a long time to reach fruition. Genetic engineering is having more effect on improving yields and increasing the pest resistance of cellulose-producing plants such as cotton.

Decades of extensive investigation into microbial enzymes, especially enzymes isolated from extremophiles, have yielded several important chemicals. Several are now used in the paper industry, such as xylanases for hydrolysing wood and cellulose. Generally this facilitates the chemical pulping and bleaching processes, reducing chemical consumption and enhancing the quality of the final product. Enzymes are particularly valuable for performing processes such as eliminating the fines present in some nonwood pulps such as sugar cane bagasse, bamboo, straw and grass. Contamination with fines is a wasteful sink for processing chemicals as well as decreasing paper strength and generating unwanted dust during paper conversion.

Interest in basidiomycetes producing cellulases, hemicellulases and lignolytic enzymes has markedly increased over the past ten years, mainly due to the potential use of these fungi in a variety of biotechnological applications, which include: biotransformation of plant raw materials into feeds and fuels; production of enzymes, antibiotics, polysaccharides and other physiologically active compounds; as well as biopulping, biobleaching of paper pulp, and bioremediation of soils and industrial waters polluted with toxins, especially dyes and other compounds containing aromatic rings. The majority of previous studies have focused on the lignin-degrading enzymes of Phanerochaete chrysosporium and Trametes versicolor as model organisms. Recently, however, interest in studying the lignin-modifying enzymes of a wider range of white rot fungi has increased not only from the standpoint of comparative biology but also with the expectation of finding better lignin-degrading systems for use in various biotechnological applications, especially in the pulp industry.

De-inking

Enzymes may also be useful for de-inking recycled fibres. Current de-inking technology disperses recovered paper in water and separates it from non-fibre impurities. Air is blown into the fibre suspension, the ink adheres to the bubbles of air and rises with them to the surface (flotation), which removes the ink. The fibre may be concentrated on a screen and made to undergo a second flotation stage before going to the paper machine.

Exotic catalysts for use with oxygen gas dissolved in water have been developed to replace the sulphate process, the sulphite process etc. A recently reported example is the polyoxometallate cluster ion [AlVW11O40]6-, which in the first step

of the process extracts an electron from lignin. This requires heating at 130°C for three hours. Other polyoxometallate cluster ions sequester the protons produced by the lignin oxidation. In the second step, the polyoxometallate cluster ion mixture

is reoxidised by molecular oxygen and returned to its original composition. Meanwhile the lignin fragments are further oxidised to carbon dioxide and water. The delignified pulp must then be separated from the catalysts and washed free of any remaining lignin fragments.

Unfortunately, the polyoxometallate cluster ions are not very efficient and they are massive molecules, hence a ratio of polyoxometallate cluster ions to pulp of 150–200 to 1 seems to be required. Note also the requirement for a high temperature. Given the exotic, hence expensive, nature of the polyoxometallate cluster ions, as well as their potential toxicity, they are unlikely to be used in practice. Nevertheless, it is a small step towards greener, more environmentally-friendly papermaking.

Cellulase and xylanase producers are thermophilic microscopic fungi that can grow on media containing microcrystalline cellulose as the sole carbon source. Some of them grow at 55–60°C. The cellulases with the best heat stability are produced by Allescheria terrestris and Chaetomium thermophilum. A. terrestris cellulase has especially good heat stability. After incubation at 70°C for two hours, the residual endoglucanase activity is 66 per cent and the residual cellobiase activity is 80 per cent. Chaetomium thermophilum endoglucanase is characterised by high activity and thermoresistance. This strain produces ®-glucosidase in trace amounts.

Other cellulase producers are the facultative thermophiles Sporotrichum pulverulentum, Aspergillus terreus, A. versicolor and A. wentii. The thermophilic fungi A. versicolor and A. wentii intensively form ®-glucosidase characterised by superior heat resistance under deep cultivation. The optimum working temperatures for the A. versicolor and A. wentii cellulase preparations are 57–60°C and 55–60°C, respectively.

These facultative thermophilic microscopic fungi convert a wide spectrum of lignocellulosic wastes into single-cell protein. The amount of raw protein in the biomass varies 25–40 per cent. Amino acid analyses showed that the protein contains most of the replaceable amino acids and all the irreplaceable amino acids.

Poor mixing is the bane of many processes in the chemical industry. Wherever a reaction can follow two or more alternative paths, product yields can depend enormously on the mixing regime in a typical paddle-stirred reaction vessel. Only 10 per cent of the volume in the vessel may be well-mixed. Dynamic photography of solutions with added tracers have revealed that a large fraction of the vessel volume is relatively stagnant. It is important to consider processes at the micrometre scale and possibly even the submicrometre scale.

This applies particularly to the moment when papermaking fillers and auxiliary chemicals are added to the stock. Stock may be treated as a complex fluid and its accurate description is still a very active research area in fluid dynamics. Solutions or suspensions of additives need to be rather diluted so they do not create unexpected problems by producing high local concentrations. When two completely miscible solutions are brought together, mixing assisted by stirring, called macromixing, does not directly produce complete mixing on a molecular level. It is known from hydrodynamics that so-called eddies are formed during the macromixing of liquid objects. The characteristic eddy length R is in the range 10–100µm.

Their degradation to a microhomogeneous solution, or micromixing, is determined by the rate of molecular diffusion. If this process is slower than any true chemical reaction, diffusion will mask the rate of that chemical reaction.

So there are several ways in which nanotechnology can improve the process of paper production. Some of these are already taking off, such as the use of enzymes, whilst others, in particular the potential to literally clone the right trees for paper pulp, are at the early R&D stage and will have other obstacles to overcome. Theses are not simply technical challenges, they will raise questions and no doubt generate new areas of debate around GM and cloning issues.

This article contains information from the technology study Nanotechnology in Paper Production, published by Pira.

To order a copy please contact:

Denise Davidson
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Takeaways

• At first glance nanotechnology appears to have very little potential in an established large-scale production process as papermaking.
• But nanotechnology, where it can increase production rates and efficiencies, is of great interest to the industry.
• Paper consists of a thin sheet of cellulose, the main structural material in plants, matted together once water has been drained away.
• Although papermaking dates back to China, second century AD, it was two English brothers, the Fourdriniers, who built the first machine for large-scale paper production, after the French Revolution.
• Trees are made from many different types of cells that support the tree, transport water and minerals, and protect them.
• Springwood and summerwood produce very different types of paper: dense and strong, and bulky and porous, respectively.
• Adding various proportions of mature and juvenile wood can affect the quality of wood pulp .
• Though nanotechnology can improve supply of raw materials by intervening at the cellular process, cellulose biosynthesis still requires lots of basic research.
• Other areas that nanotechnology can benefit are de-inking and mixing processes.
• There are several applications for nanotechnology in paper production, some already being commercialised whilst others could prove controversial and require lots more research.