Wood pulp

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Wood grinder in the “Alte Dombach” industrial museum in Bergisch Gladbach

Wood pulp (English mechanical pulp ) is a general term for fiber materials that are obtained from wood by mechanical defibration and are used for the production of certain types of paper , cardboard and cardboard . The production takes place either through purely mechanical wood pulping or with thermal and / or chemical pretreatment. Economically important types of wood pulp are wood pulp , pressure grinding and thermomechanical pulp (also called TMP , abbreviation for thermomechanical pulp ).

Unlike cellulose , wood pulp contains large amounts of lignin . The high proportion of lignin leads to yellowing of so - called wood - containing paper , which contains large amounts of wood pulp. Wood pulp is therefore used to manufacture briefly used papers such as newsprint and papers for magazines ( LWC paper and SC paper ). Cellulose is used for aging-resistant papers.

history

In the past, paper was mainly made from rags . Because of the limited availability of this raw material, alternatives were sought from around the year 1700 (see paper # Industrialization ). In the middle of the 19th century, Friedrich Gottlob Keller developed the method for producing pulp, which is still common today, which opened up a new raw material base for paper production. In 1846, Keller sold his patent for wood pulp production to the Swabian manufacturer Heinrich Voelter (1817–1887) from Heidenheim, who together with the mechanical engineer Johann Matthäus Voith (1803–1874) further developed the Keller wood pulp process to industrial maturity. In the 19th century, processes for the production of the higher quality, but more expensive to produce pulp were developed.

production

Wood grinder scheme

Wood pulp is obtained from the raw material wood , which mainly consists of lignocellulose . Lignocellulose consists of cellulose molecules that are assembled into fibers. A matrix made of lignin penetrates the cellulose, creating a pressure and tear-resistant bond. During the production of wood pulp, the wood is defibrated using various processes. Due to the almost complete use of the raw material, high yields are possible. However, the fibers are partially damaged or shortened by the mechanical action, which leads to a reduction in paper strength. In the production of cellulose, on the other hand, the lignin content is removed using chemical methods, so that the higher-quality cellulose, which consists almost entirely of cellulose, is obtained with a lower yield and greater effort.

Most of the time, round wood or felling wood is used in the production of wood , with softwood being preferred because of its long fiber content. Different processes lead to the different types of wood pulp.

Grinding process

Cut products are made by cutting wood with the help of whetstones .

Wood pulp

The production of ground wood (white ground) is done mechanically. Debarked wooden sticks ( meter logs or logs) of coniferous wood , mostly spruce , are pressed onto rotating whetstones with the addition of warm water and then ground (frayed). Further grinding can then take place to adjust the fibers. The white cut produced in the cold cut is quite light in color, but only of poor strength because the fibers are torn. If the wood is steamed before sanding (mainly resin-rich pine wood ), the result is brown sanding with better strength properties than white sanding .

Pressure cut

A more modern production variant is pressure grinding. Compared to conventional wood pulp, which is produced at a temperature below 100 ° C, the encapsulation of the grinder allows a higher pressure and thus a higher temperature. This softens the lignin matrix. As a result, the starting wood can be frayed more easily and a greater length of cellulose fibers can be achieved.

Refiner process


Active pairs of a refiner: ➀ edge against edge
➁ face against face
➂ knife cell against knife cell

In the refining process, the first step is the shredding into wood chips and, after further process steps, the defibration in the refiner.

Thermomechanical pulp (TMP)

When producing TMP, wood chips are heated for 3 to 5 minutes with steam at around 110–130 ° C. At this temperature the lignin softens and the fiber composite begins to dissolve. The wood is then frayed between the edges of a refiner. This pulp grinding usually takes place in two passes in order to gently extract the individual fibers : In the first refining stage, the softened wood chips are pre-shredded at overpressure, in a second refiner the coarse pulp is further defibrated at atmospheric pressure. Due to the thermal pretreatment, the properties partly approach those of the more expensive pulp, e.g. B. higher tear strength of the paper made from it.

Chemithermomechanical pulp (CTMP)

A variant of the TMP is the chemo-thermal wood pulp production (CTMP), in which the wood chips are also chemically pretreated. The chemical pretreatment makes the product darker than TMP. CTMP can also be bleached and is then referred to as BCTMP (bleached chemi-thermo-mechanical pulp) .

use

Depending on the desired paper properties - opacity (light scattering ability), volume (stiffness), brightness, surface properties, binding and strength properties - and according to economic aspects, different wood pulps are used. Often there is also a combination with other fibrous materials, e.g. B. with recycled waste paper ( waste paper ) or long-fiber pulp made from softwood to achieve the desired paper quality.

advantages

  • Wood pulp is much more advantageous than wood pulp for cardboard production, as the lignin in the pulp composite provides additional rigidity. Lignin-free paper made from cellulose is also unsuitable for newsprint. The production of wood pulp is also simpler and therefore more cost-effective than the production of wood pulp, as the complex cooking process for lignin removal is no longer necessary.
  • The yield based on the wood used is around 90% for wood pulp (for wood pulp and TMP wood pulp 90–96%, for CTMP wood pulp just under 90%), since almost all of the wood's ingredients, including lignin , are in the wood pulp are present unchanged. In cellulose production, the yield is significantly lower (around 50%) due to the removal of the lignin.

disadvantage

  • The main disadvantage of wood pulp in many applications is its lignin content. The lignin turns yellow when exposed to light and oxygen . Paper with a high wood pulp content will quickly discolor (newsprint).
  • The visible paper quality with wood pulp is poorer than with cellulose, since there is a significantly stronger flake formation, which affects the formation.
  • The grinding process requires a lot of energy to drive the grinding wheels and chopping systems. The energy requirement depends on the intensity of the grinding and is between 1500 and 3500 kWh / t. This high energy requirement and the availability of inexpensive waste paper have led to the use of almost 100% waste paper in Central Europe for the production of newsprint. Within Europe, TMP is only processed into newsprint in Scandinavia.
  • In particular, paper made from wood pulp has low strength values ​​because the fibers are severely damaged during manufacture. (Refiner wood pulp has better strength properties; CTMP is already comparable to wood pulp in this regard.) The strength can be increased by mixing with wood pulp.

Chemical proof

Wood pulp can be detected by the red coloring of the lignin it contains with hydrochloric acid phloroglucinol solution and thus differentiated from cellulose. Wurster's blue and red (after Casimir Wurster ) and aniline sulfate were also often used .

See also

literature

Web links

Individual evidence

  1. ↑ a b Refiner-Holzstoff papierundtechnik.de, July 30, 2009.
  2. ↑ Jürgen Blechschmidt (Ed.): Taschenbuch der Papiertechnik , Fachbuchverlag Leipzig in Carl Hanser Verlag, 2nd, updated edition 2013, p. 106.
  3. ↑ a b Holzstoff Lexikon der Papierindustrie, austropapier.at
  4. ↑ Fasstoff … papierundtechnik.de, July 31, 2008.