Editing Paperboard (section)

===Pulping===
{{Main|Pulp (paper)}}
Two principal methods for extracting fibres from their sources are:

* '''[[Chemical pulp]]ing''' uses chemical solutions to convert wood into pulp, yielding around 30% less than mechanical pulping; however, pulp made by the kraft process has superior strength
* '''[[Thermo mechanical pulp]]''' is a two-stage process which results in a very high yield of wood fibres at the expense of strength.

====Bleaching====
{{Main|Bleaching of wood pulp}}
Pulp used in the manufacture of paperboard can be [[bleaching of wood pulp|bleached]] to decrease colour and increase purity. Virgin fibre pulp is naturally brown in colour, because of the presence of [[lignin]]. Recycled paperboard may contain traces of [[ink]]s, [[Adhesive|bonding agents]] and other residue which colors it grey.
Although [[bleaching of wood pulp|bleaching]] is not necessary for all end-uses, it is vital for many graphical and packaging purposes. There are various methods of [[bleaching of wood pulp|bleaching]], which are used according to a number of factors for example, the degree of colour change required, chemicals chosen and method of treatment. There are three categories of [[bleaching of wood pulp|bleaching methods]]:

* Bleaching by '''delignification''' using [[chlorine gas]], which is a method that has been largely replaced by procedures which are gentler to the environment such as the use of oxygen as a replacement for the chlorine gas.	
* Bleaching by '''[[oxidation]]''' using chemicals such as [[chlorine dioxide]], [[hydrogen peroxide]] or [[sodium hypochlorite]].
* Bleaching by '''[[redox|reduction]]''' using chemicals such as [[sodium bisulphite]].<ref name="igg"/>