Wood pulp

What is Wood Pulp?

Trees are the main raw material for the European pulp, paper and board industry but far from destroying the forests, the pulp industry is a custodian of this valuable resource. Pulp production can be based either on well managed natural forests or planted forests. In Europe sustainably-managed natural forests are the main raw material source for the pulp industry, with the exception of the Iberian peninsula, where planted forests are used, similar to pulp production in the Southern Hemisphere. The fact that the pulp industry is based on a renewable, recyclable raw material makes it perfectly positioned to play an important role in the bio-economy of the future.

Indeed, pulp mills are becoming more than just a source of wood pulp nowadays. The new generation of pulp mill is a platform for extracting environmentally-friendly and sustainable raw materials from wood for applications which have long been dominated by synthetic raw materials and chemicals. More often than not, these modern mills generate more green electricity than they use. They supply the surplus back to the national grid, and produce an ever-increasing range of sustainable by-products, from pellets and bio-chemicals to tall oil and biofuels

In traditional pulp making, the wood logs are broken down to extract the valuable cellulose fibres, preserving as much of the fibre as possible without damaging it. Separation is either mechanical, where the logs are disintegrated into fibres by physical shear force, or chemical, where the work is done with the help of heat, pressure and chemicals.

Once separated, the fibres will usually be bleached, washed and screened depending on the end use they are destined for. Unbleached pulp may be used directly to make different types of brown packaging papers and boards, and bleached pulp for making tissue or printing and writing papers. Pulp may be consumed directly as undried pulp in an integrated pulp/paper/board mill or it can be dried and pressed into bales to be used as a raw material by customers around the world.

It may seem surprising that a commodity like wood fibre needs to undergo such a complex and carefully-designed process to become pulp. But there is no simple way to turn a piece of wood into a tissue that feels soft to touch but strong enough to blow your nose on. The innovation and quality we see in paper products today are the result of years of research, investment and expertise. The work begins right back at the heart of the business, in the forest: The combination of the tree’s genetic history, macro environmental factors, soil type, even the amount of rain a tree receives; all these can impact the properties of the fibres before the wood has even been collected and the refining process has started.

No two trees are identical, so efficient well-designed mills and a high level of expertise are essential to achieve the highest, most consistent level of pulp quality. The natural differences between wood species mean that pulp producers can carefully combine fibres with different strengths and weaknesses to get the optimal properties they, and their customers, are looking for.

For example, the slow-growing pines and spruces of Northern Europe produce long fibres which can impart the additional strength essential for packaging, high-grade printing or tissue paper applications. Shorter fibres from rapidly-growing plantation trees in warmer climates are known for their good surface properties, printability and softness. Most of the time the final product is a combination of different types of fibres tailored to meet specific quality and property parameters.


Chemical pulp

Over 95% of the chemical pulp in the world today is made using the kraft process which uses caustic soda and sodium sulphate to dissolve the woodchips. In the kraft cooking process, wood chips exit the digester with an ISO brightness level varying between 35 to 50 % depending on the species and process. After cooking the fibres can be used in unbleached kraft pulp, or further bleached to over 90% ISO if required. Chemical pulping is about removing lignin and extractives from the cellulose components to achieve a bright, stable pulp which is easy to handle when it reaches the paper mill.

Today all bleached pulp is either ECF or TCF (elementally or totally chlorine-free, respectively). Both these processes bleach the fibres without producing harmful dioxins and furans.

A small percentage of chemical pulp is produced using either an acidic or neutral sulphite process. Yield and strength properties are lower, since this method tends to dissolve part of the cellulose, which probably explains the demise of the sulphite process. On the other hand, the sulphite process enables more efficient bleaching and higher brightness.


Mechanical pulp

Mechanical pulping usually has the highest yield from the pulpwood, of 90% or above, because it uses all the log except for the bark. However in the chemical pulp process, the separated lignin and other residues are processed into sustainable energy. This means that mechanical pulping is now a high-energy way to break down wood to fibre. The investment costs for mechanical pulp mills are relatively low in comparison with other types of pulp mills, however, and very often newsprint or board mills will have an integrated mechanical pulp plant.

Mechanical pulps have lower strength characteristics than chemical pulps due to their more rigid fibre morphology but they offer higher bulk. This makes mechanical pulp well suited to bulk grades of paper, such as newsprint and different packaging boards, due to its beneficial bulk-cost ratio. Old newspapers and books go yellowish with age because these papers have a high lignin content, together with the presence of oxygen and UV-radiation.




Wood pulp was first produced in the mid-19th century mechanically by grinding logs against a water-lubricated, rotating stone-faced drum. The heat generated by grinding softens the lignin and the mechanical forces separate the fibres to form ‘ground wood pulp'(GW).

This process is still used today, especially for newsprint. Around 60% is now produced using a new generation of mechanical refiners which subject the wood to intensive shearing forces between a rotating steel disc and a fixed plate instead of conventional grinding. The woodchips can also be pre-softened by heat (thermomechanical pulp – TMP) or by heat and a mild chemical treatment with sodium sulphite (chemi-thermomechanical pulp -CTMP). They can be further bleached (BCTMP) to support a wider range of products from tissue to paper/board grades in which higher brightness and bulk are required.