Papermaking/Fibres for paper

Different types of fibres can be used for producing paper. The most well-known is the wood fibre. In the beginning of papermaking mostly fibres from cotton and silk were used. A notorious fibre that at one time was used for producing a specific papergrade was asbestos. Until the late sixties of the 20th century this cancer causing fibre was used to produce asbestos paper. This lead to elevated cancer death rates specially within the group of papermakers, who produced this type of paper.

Different types of plants can and are used for producing paper, like bamboo from China. Phragmites can also be used as raw material for paper, still in the nineties of the 20th century producing paper from phragmites was investigated in Holland.

Wood pulp edit

Pulp from wood is the most important raw material for paper. Another, less correct name, which is used for pulp is cellulose.

Wood pulp as raw material is formed after the following treatments:

  • Collection of wood as raw material, usually harvesting from production forests, wood from thinning and wood rests from sawmills.
  • Sawing, debarking and woodchipping. Bark exist only of small amounts of useful fibres and therefore is used mostly for producing energy and steam. Woodchipping is the conversion of timber to woodchips.
  • Wood defibring
  • Bleaching
  • Drying and formation of pulp bales.

This last part of the process is only necessary when the production of paper is not integrated with the production of wood pulp. The last step is necessary when the paper mill is located on a different location than the raw material source.

Hardwood and softwood can both be used to for paper. Usually wood pulp from eucalypt trees is treated as a separate group although it is in fact hardwood, because it has special dimensions (relatively short, but thick), which gives it unique properties.

In some countries paper industry exist without the existence of pulp mills in this country, like Holland. All wood pulp for the production of fine paper is imported.

Typical hardwood used in the paper industry comes from:

  • aspen/populus
  • birch
  • beech
  • eucalypt

Typical softwood:

  • spruce
  • pine
  • fir
  • larch

Fibres from tropical hardwood trees are not as suitable for production of paper. This is contrary to what people think.

Pulp seen with a microscope
Pulp seen with a microscope
The structure of lignin
The structure of lignin
Paper fibres seen through a microscope
Chemical structure of lignin

Defibring of wood pulp edit

For this chapter some knowledge about chemistry is required

There are two main types for defibring: mechanical or chemical. There exist however also a intermediate. Mechanically defibred pulp is called "wood containing", whereas chemically defibred pulp is called "woodfree". Wood containing is misleading, because it only says if lignin is present in substantial amounts.
Wood consists besides of fibres also of ligin. Lignin functions as binder between the individual fibres in the tree and needs to be removed or treated so it can be turned into pulp.

Pulp, as is, is usually only used for chemically defibred fibres, but the mechanical defibring is also mentioned here.

Mechanical pulping process edit

The first method we consider is stone groundwood.

The stem of a tree is pushed onto a turning ground stone. This process leads to a lot of heat formation, which is why the wood needs cooling. The high temperature attenuates the lignin (which is the binding component between the fibres), which is why the individual fibre is freed. Part of the fibres is not only freed but is also shortened.

right|250px|Houtslijper met drukverhoging
Houtslijper met drukverhoging

The second defibring method is pressure groundwood.
Similar to stone groundwood, the difference being the elevated pressure under which the process takes place. Due to the higher pressure, the temperature tends to be higher as well, which causes a more thorough weakening of the lignin structure.

The third defibring method is thermo groundwood.
This process is performed with atmospheric pressure. The temperature is elevated and kept at a high temperature. Warm wood is used instead of cold wood.

The fourth defibring method is RMP, refiner mechanical pulp.
First wood is chipped and then lead into a refiner. A refiner consists of plates on which knives are mounted. The plates turn with high speed and is pressured against a stationary plate, so that the fibre during contact with plates or knives is pulled out of the wood. Hier wordt gebruik gemaakt van gechipt hout. Het hout wordt in een refiner geleid. Een refiner bestaat uit een of meer platen met messen erop. De platen draaien en drukken hierbij tegen een stilstaande plaat, waarbij de vezel bij contact met de platen en messen uit het hout wordt getrokken.

The fifth defibring method is CRMP, chemical refiner mechanical pulp.
Actually this is a method which is between mechanical and chemical defibring. Before the chips are feed into the refiner they are treated with sufite of bisulfite. This causes the fibre to be stronger compared to other mechanical methods. The efficiency of the process is alas lower so that a lower proportion of the wood is turned into pulp.

The sixth defibring method is TMP, thermo mechanical pulp.
The chips are preheated before it is feed into the refiner.

The seventh defibring method is is CTMP, chemo thermo mechanical pulp.
The chips get besides pretreatment with heat also a pretreatment with chemicals like sulfite. This pulp is regarded as the best wood containing mechanical pulp.

This last method is also called "semi-chemical" and is the transistion from mechanical to chemical pulp.

Chemical pulping process edit



"Paper from polemists yellows fast."
Simon Carmiggelt - Dutch writer and columnist (1913-1987)

Resources edit

Consistency Measurement - Pulp consistency measurement and control for paper industry.