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
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.
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.