If it is Swedish newsprint – and the chances are it is – it will mostly be spruce fibres you can see. In the paper they are blond and slim, around one to three millimetres long. They’re also very beautiful, if you get the chance to view them under a microscope. But not all of it is spruce fibres; some will probably be
recovered fibre of mixed origin.
In the briefest of terms, newsprint is manufactured as follows: the spruce fibre is taken from the forest as pulpwood that is driven to the pulp factory. Here the wood raw material is chopped into small pieces, called chips, which are then ground down in refiners to separate the fibres. The paper pulp is then
pumped on to the paper mill, where the paper machine basically reforms the fibres
into a light but strong mesh that creates what we know as paper.
One person who can give a much more in-depth description of this process is Emilia Liiri, development manager at Braviken Paper Mill. Her department works on developing the whole process from wood to finished reels of paper. The challenge is to understand exactly what is happening at each stage in this chain. This knowledge is vital in ensuring that the fibres have the exact properties needed for the paper to meet customer requirements now and in the future.
“It’s only logical to focus so much on the fibre. We want to get it right from the start. The stage when the wood chips are ground down to separate the fibres is really when we form our material,” she explains.
When the process and product developers examine their fibres, it is often at a microscopic level. The original spruce wood fibre in Holmen XLNT paper is 3.5 mm long and 0.035 mm wide on average. The fibres are tubular, with a complex wall structure of several layers that differ in design and chemical composition.
A key part of the mechanical pulp process – and perhaps the most important – is to peel off some of these layers effectively but not too brutally. The aim is to get down to the ideal layer for stiff and strong fibres that have good optical properties. There is also a need for the appropriate amount of fine material to ensure the intended paper quality. The challenge is to achieve all this while being energy efficient.
When the experts discuss this stage in the process, it almost sounds like brain
surgery. They talk about ‘where to make the cut’ so that the correct place in the
fibre wall is targeted and ‘fibrillated’ just the right amount. This can be the key to
giving the Holmen XLNT paper the right strength, optical properties and desired bulk.
The comparison with surgery may not be far off the mark, since the focus is on
processing fibre walls measuring hundredths of a millimetre. The catch is that all this has to be done using large, heavy steel refiners running at 1 500 revolutions per minute. To the uninitiated, it might seem unlikely that such precision work can be carried out in these machines.
But Emilia explains: It is not a matter of working on the individual fibre (which is lucky since one gram of Holmen XLNT paper contains a million fibres).
The key is to develop the TMP process so that the fibres are separated and the
pulp has on average been processed in exactly the desired way.
“It really is possible and we are good at it. But then, we’ve been making TMP here at Braviken since 1977,” says Emilia with a smile. “At the same time, the Holmen Paper Development Centre has spent more than ten years researching and developing the TMP process and building up considerable expertise in the area.”
This accrued knowledge means that the properties of the paper can now be varied
within ever wider parameters. Holmen’s experts test different methods of fibre
separation, for example by adjusting the design of the grinding section in the refiners
or by changing the process conditions. Developing the natural strength and stiffness is one of the top priorities in fibre processing. Good tensile strength and elasticity are important in ensuring that the paper can survive the rigours of the printing presses.
Although the development work has come a long way, Emilia Liiri feels that there is still significant potential, not least when it comes to energy efficiencies.
In recent years, almost SEK 500 million has been invested in a modern line at Braviken for more energy efficient production of TMP pulp. The result has been a 20 per cent cut in electricity consumption since 2008 and the aim is to achieve further major savings in
“It’s difficult, but certainly not impossible. All the detailed knowledge we have about the fibres will be invaluable,” says Emilia Liiri.
Text and images: Strateg Marknadsföring Anders Thorén
Image Newsletter illustration (spruce) SLU, Innventia, MoRe Research