12 Juli 2016

Fibres of the future 2: cellulosic fibres from wood

Second in a series of articles on fibres of the future

Wood contains cellulosic fibres, embedded in lignin and hemicellulose. Pure cellulose can be processed to a quality from which yarns can be spun. The most well-known of these products is viscose. The viscose process however involves the toxic and highly inflammable solvent carbon disulphide and is now abandoned in most countries. On the other hand, wood is an abundant and cheap resource, and new cellulosic fibre processes are in rapid development; within ten years, these products might change the fibre market completely.

This is the second in a series of articles on fibres of the future. The articles were published on 31 May and 19 June.

Eucalyptus

Eucalyptus (here: a beautiful Australian tree) is the feedstock for Lenzing’s Tencel cellulosic fibres.

Much R&D going on

Why produce biobased polyester and polyamide fibres, says Annita Westenbroek, if nature already provides us with excellent cellulosic fibres? In order to arrive at the new biobased polymers while starting from cellulose, we would have to perform many reaction steps: first break down the feedstock to simple monomers, and then restructure these bottom-up in order to arrive at the desired long molecular chains. Annita is the director of the Dutch Biorefinery Cluster and has her roots in the paper and pulp industry. Nature has lain down perfect fibres, she says, and if we develop sustainable ways to transform them into soft yarns, that would give them a head start on artificial biobased yarns.

The first commercial process to improve on viscose is called the lyocell process. Here, carbon disulphide has been substituted by NMMA (N-methylmorfoline-N-oxide). Moreover, the process has been changed and the solvent is recycled. The lyocell fibre is much like viscose, but it is stronger when whet, and does not wrinkle as much. For a decade already, lyocell has been produced and sold by Lenzing from Austria under the name Tencel. In addition to Lenzing, according to Annita Westenbroek, almost all major pulp companies now intensely research wood biorefinery: Stora Enso, Borregaard, Metsä, UPM, Mondi, Sappi; all of them have extensive knowledge of wood chemistry and develop many new applications starting from cellulose. The mere fact that we hear very little about this research, according to Annita, testifies to its promising nature: very competitive product developments. And all of them hope to be the first to bring it on the market.

Banknotes

Hemp could very well substitute cotton in banknotes.

Cellulosic fibres from new technologies

The real breakthrough in wood biorefinery is that we can now dissolve cellulose in innovative liquids that are cheaper and more environmentally friendly, e.g. ionic liquids (ILs) and protic ionic liquids (PILs). Once dissolved, it can be spun into textile fibres or chemically reacted to produce derivatised celluloses and cellulose-based plastics. In the framework of a BBI joint initiative, pulp companies cooperate to develop a cellulose dissolving process based on deep eutectic solvents (DES). The advantage of DES would be that we might even use biotechnology to process the contents of the solution. Other researchers have succeeded in decomposing cellulose from wood into fibrils, and binding them together again; different ways of reuniting the fibrils will determine the properties of the new product: soft as cotton, or hard as steel. So far, the main problem for fibres developed from these processes is that they are too expensive. This also holds true for Lenzing’s Tencel. Price is also the main obstacle to a larger market share of qualitatively very good fibres like linen, sisal and hemp. But given the amount of effort that goes into cellulosic fibres, they may well become cheaper fast. And given the fact that any other second generation bioplastic requires cellulose to be broken down first, there is a reasonable chance that the cellulosic polymers will prevail.

Still, there are many applications of cellulosic fibres, and for some price may not be decisive. One of these is bank notes. A Dutch paper mill producing paper for bank notes developed a sustainable process for hemp fibres to substitute cotton. Incurring lower costs, with a lower water footprint, grown without pesticides or insecticides, and without any fair trade issues because home grown. And with a lower ecological footprint. But so far, the product is not even being considered, ‘as we have always used cotton in our bank notes’.

Sustainability matters

The other major factor (beside price) that will determine the fate of new cellulosic fibres is the sustainability of the production process. Remember that cotton production has major environmental effects. Although cotton is not an energy intensive product, in LCAs it does not score well because of some very bad scores in important categories: land use, water use, ecotoxicity (both on surface waters and the soil) and eutrophication. Most effects result from cultivation of cotton. Pesticides use is responsible for the high ecotoxicities and fertiliser use is the main cause of eutrophication. In LCAs therefore, cotton tends to finish below its competitors, even below fibres from petrochemical origin. Even though the traditional viscose process tends to score even worse. In comparison, the Lenzing lyocell process has a better environmental score, its main weakness being its high energy use. And that’s why the pulp industry invests heavily in the development of innovative sustainable processes for cellulosic textile fibre production from wood.

In short, we expect many new cellulosic fibres to come to the market in the next decade. They will be competitors both to traditional biobased fibres like cotton, and to the fossil-based fibres that have gradually overtaken the former.

Source: Bio Based Press, 2016-06-19.
Author: Diederik van der Hoeven

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