Wood pulp

Wood pulp is the most common material used to make paper. Wood pulp generally comes from softwood trees such as spruce, pine, fir, larch and cedar.

Wood pulp is made in several stages:

1. First the bark must be removed from the wood. This can be done with or without water (wet stripping). The bark is generally recovered to reuse as fuel in the pulp and paper making process.
2. The cellulose fibres that keep the wood together must then be separated. This can be done in a number of ways:
   • The wood may be crushed with grinders (huge grindstones) and then soaked in water to produce mechanical pulp. Mechanical pulps are used for products that require less strength, such as newsprint and paperboards.
   • The wood may be crushed with grinders using steam at high pressures and temperatures to produce thermomechanical pulp (TMP). TMP differs in quality from mechanical pulp.
   • In additional to the grinders, chemicals may be used to break up the cellulose fibres. Pulp produced this way is known as chemithermomechanical pulp (CTMP).
   • Chemical pulp is produced by combining wood chips and chemicals in huge vats known as digesters. The effect of the heat and the chemicals dissolves the lignin, without breaking the wood fibres. Chemical pulp is used for materials that need to be stronger. Chemical pulps include kraft pulp (or sulphate pulp).
   • Pulp may also be made out of waste paper and paperboard. Recycled pulp is most often used to make paperboard or sanitary paper.
3. The pulp produced so far may need to be bleached. The chemicals used to bleach pulp have been a source of concern to the pulp and paper making industry, and recently the pulp industry has been using alternative chemicals to chlorine.
4. The pulp mixture is now sent to the paper machine, where it is shaped and dried.

Using wood to make paper is a fairly recent innovation. In the 1900s, linen and flax fibres were the primary material source, but a shortage led to experimentation with other materials. Around 1850, a German named Friedrich Gottlob Keller crushed wood with a wet grindstone to obtain wood pulp. Further experimentation by American chemist C.B. Tilghman and Swedish inventor C.F. Dahl enabled the manufacture of pulp by using chemicals to break down the fibres.