While getting smaller is a massive problem for the sector, one solution may be to set its sights smaller still. Paper mills are currently scrutinising their core products on a truly tiny scale, literally, by investigating the potential of nanocellulose.
Cellulose is one of the building blocks of plants. We’ve been exploiting it for hundreds of years in paper and board. In that respect it is one of the earliest and most abundant forms of engineered materials around.
Nanotechnology is driven by advances in science that have allowed us to understand and manipulate the properties of matter at an ever-smaller scale. To provide some idea of that scale consider that one nanometre is 10-9m or a marginally less mind-boggling but less scientifically standard notation of one millionth of a millimetre. To put that in the context consider a human hair is 100,000nm across, a virus is 50nm and a bacteria 500nm.
Real-world uses?
Applying nanotechnology to cellulose has revealed that it has a range of special properties, which, if the processes can scale from laboratory to factory, should enable a range of new applications.
It is these properties that make it attractive for a vast range of potential applications. Mechanically it has the tensile strength of aluminium, stiffness of Kevlar and strength/weight ratio eight times that of stainless steel. Its viscosity is unusual in that liquids that include nanocellulose are normally thick and stable but become runnier when stirred or agitated, a property called thixothropy. Nanocellulose foams and aerogels have an enormous surface area, with 1g of nanocellulose having the surface area of a football pitch. Lastly, they are attracting attention as barrier materials. Gas permeability is very low, but can be tuned.
“Wood pulp is the main source of nanocellulose,” says Rob English Professor of Material Chemistry at Edinburgh Napier University’s School of Engineering and the Built Environment.
His department is working with paper manufacturer Sappi Europe to commercialise a process to make one sort of nanocellulose called cellulose nanofibres (CNF), which are sometimes referred to as fibrils. The other major kind is cellulose nano-crystals (CNC). A third class is produced by bacteria.
“The major differences are the size – fibrils are larger than crystals – and fibrils are produced mechanically whereas crystals are produced chemically,” says English.
The processes impact the cost and complexity of making the different types of nanocellulose, which, English says, means fibres are more amenable to commercialise. They are also much less expensive to make.
“CNC are produced by the breakdown of wood by strong acids, leaving only the crystalline form of cellulose,” says Doug Bousfield at the department of Chemical and Biological Engineering, University of Maine, which is one of the leaders in the development and application of nanocellulose.
“This is a uniform rod shaped particle. CNF are produced by mechanical action from a refiner, disk grinder, or homogeniser. These fibres are much less uniform and have a wide size range.”
While both CNC and CNF are narrow in cross-section – in the region of several to tens of nanometres across – the big difference between the two is length. Both are long and thin but the crystals at hundreds of nanometres long are comparatively short compared to fibres, which can be thousands of nanometres long. For example Canadian firm CelluForce produces crystals that are 100-250nm long by 5-10nm in diameter.
“They can produce colour effects in the same way as fish scales, butterfly wings and beetle carapaces,” says the firm’s chief technology officer Richard Berry. “This is structural colour, rather than dye or pigments.”
That means the resulting colours can be very pure and stable; additionally, as in nature, effects such as iridescence are possible.
“The key question right now is ‘where is the best application?’” says Bousfield.
“A lot of applications are paper and packaging or adjacent to paper and packaging,” says English. “Others include composites – for example for car parts – viscosity modifiers/thickeners, concrete additives, paints, inks and coatings, and foods. Niche applications include biomedical, including bone scaffolding. Another is to produce transparent films for OLED displays and membranes for flexible electronics.”
Behind that wide range of applications are a couple more of nanocellulose’s special properties.
“Nanocellulose can hold other materials in solution that currently separate out,” says Berry. “Cellulose has a good toxicological profile, even at nanoscale. Microcrystalline cellulose is already used in pills.
According to the RISI report Nanocellulose: Technology Applications, and Markets, which was produced by Jack Miller, principal consultant at US paper industry research firm Market-Intell, the potential size of the global market for nanocellulose is 23m tonnes. Of that paper and board make up the biggest single application accounting for 20m tonnes with paints and coatings next at 800,000 tonnes.
To date, and for the foreseeable future the production of nanocellulose is a fraction of that. The same report shows that in 2013 total production was around 10,000 tonnes, of which the majority was CNF. It projects rapid growth of 1,000%, so that by 2020 the volumes will be 100,000 tonnes of CNF and 8,000 tonnes of CNC. For 2015 it projects a more modest but still huge growth to 400,000 tonnes of CNF and 50,000 tonnes of CNC.
As those figures show the potential growth in nanocellulose is huge, although the actual volumes are modest in the overall paper market.
“At this point, the applications for nanocellulose are not that great,” says Bousfield. “It is mostly to help paper companies remain viable by having another product to sell.”
The volumes used by other sectors may seem small but that is because it only takes tiny amounts of nanocellulose to be added to most products for them to have a huge impact.
“The first application for our product is a wood adhesive where we can replace a lot of one material with a small amount of nanocellulose,” says Berry.
Also, talking about the volumes is only half the story, the price for nanocellulose is significantly higher than paper.
Nanocellulose and papers
Nanocellulose’s initial and biggest impact within print will be as a component of paper. It is an area that is currently being researched in both industry and academia, and the results aren’t too far from reaching the market.
“Nanocellulose is a natural for modifying paper to be more ‘closed’ and stiff,” says Bousfield. “It may help produce paper with more filler and less kraft fibres, reducing the cost of producing paper.”
Bousfield’s team has experimented with adding CNF to both the base and coating. Their findings are that adding CNF into paper can improve the strength and improve printing, because it helps retain the ink pigment at the paper surface. A small amount of CNF in a coating reduced the latex demand and increased the stiffness of the paper and also improved the picking properties.
“We’ve completed successful paper machine trials with a coating additive based on our CNC,” says Berry. “That’s likely to become a strong application over the next year.”
In addition to improving the properties of paper for existing applications nanocellulose also promises to help develop new ones too.
“The barrier properties of nanocellulose make the resulting paper suitable for applications where paper isn’t suitable today, for example in packaging that needs water vapour and oxygen barriers,” says Math Jennekens, R&D director at Sappi Europe.
Sappi’s efforts with Napier University on nanocellulose are moving rapidly from the research lab to wrapped reams.
“We have developed a process to manufacture nanocellulose and are working on a pilot plant to understand the issues with scaling up production,” says Jennekens. “We aim to productise in 2018. Papers using CNF will be commercially available before 2020.”
Besides its use as a component of papers and boards nanocellulose is being explored for other print and packaging uses. Thanks to their special colour characteristics, inks, coatings and films are one low-hanging opportunity
“Potentially, one application [for our CNC] will be inks for security printing, but it’s a little bit too soon to see if it’s cost-competitive,” says CelluForce’s Berry.
A more long-term development, which could be for more significant, is the development of a replacement for plastics that are currently produced from oil.
“Nano crystals can produce totally transparent materials, it is our belief we can replace synthetics with nanocellulose,” says Jennekens.
The bio refinery
The current turmoil in the oil industry, geopolitical instability, erratic pricing, worries about diminishing oil reserves and controversy over fracking are all making alternative sources of hydrocarbons for fuel and chemical feedstocks look increasingly attractive.
For forest product firms the dwindling demand for paper is making them look to other uses for trees. This may lead to the development of new industries where rather than waiting millions of years for fossil fuels to form, naturally clever chemistry is used to extract the right raw ingredients from the forest.
“Nanocellulose may end up being produced in integrated bio-refineries along with hemi-cellulose and lignin,” says Rob English Professor of Material Chemistry at Edinburgh Napier University’s School of Engineering and the Built Environment. “The base feedstock will be the forest.”
“Nanocellulose and lignin are the two building blocks of a whole new bio-derived products industry,” says Math Jennekens R&D director Sappi Europe. “Not only are there applications for nanocellulose, lignin can be further processed, which enables the production of a range of aromatic hydrocarbons.”
