Making Pulp From Wood
Woodpulp currently represents 44% of the fibre used to manufacture UK paper and board. 11% is home produced and used by the four UK integrated mills - that is a mill that carries out the entire papermaking process from tree to end product. None of these mills uses mature fully grown trees, but rather small dimension timber, (which is no use to other commercial users such as furniture makers and builders), saw mill waste and forest thinning.
75% of the woodpulp used in the UK is imported, mainly from Scandinavia and North America. Woodpulp is also imported from Brazil but not from tropical rain forests because, apart from environmental considerations, the fibres from tropical hardwoods are unsuitable for papermaking.
In the past the industry used softwoods such as spruce, pine, fir, larch and cedar almost exclusively, but hardwoods such as birch and aspen are gaining in popularity. Fast growing eucalyptus have been successfully cultivated in Northern Spain, Portugal and Brazil and provide the papermaker with very high quality pulp. Softwoods provide long strong cellulose fibres and are used to produce papers where strength is a requirement, for example, packaging papers. The shorter hardwood fibres provide bulk, smoothness and opacity and are used to produce fluting medium and printings and writings.
Trees vary enormously in the time they take to reach full size. Much depends on climate and the soil, but these figures give a rough comparison:
Tree Type Life Expectancy
| Willow |
25-30 years |
| Sitka Spruce |
50 years |
| Douglas Fir |
55 - 60 years |
| Norway Spruce |
70 - 75 years |
| Scots Pine |
70 - 80 years |
| Oak |
100 years (not used for papermaking) |
(Oak trees usually live 200 - 300 years, but some have been known to live for more than 1,000)
When a tree trunk or thick branch is cut across, a series of dark rings can be seen. These are annual rings which result for the seasonal growth. Each ring represents one year's growth. The grain in timber is caused by the annual rings which show when a log is cut. Teak, mahogany, oak, beech and elm have attractive grain and are used for furniture making (rather than papermaking).
Woodpulp comes from trees from managed forests where more trees are planted than harvested to ensure that demand for timber products will never outstrip supply. The forest industry has become more aware over recent years that it has a responsibility, not only to ensure the economic viability of its operations, but also to ensure that the requirements of local communities and of flora and fauna are catered for. Greater care is taken to ensure that logged areas blend in more readily with the landscape.
Certain pulp characteristics depend on the process used to reduce the wood to its component fibres. There are three main ones:
Mechanical Pulp
This is a very cost effective but energy intensive process because all the log apart from the bark is used. The log is put through a grinding or refining process to separate the fibres. Chemicals are not used, but there is a high energy use which can only partly be offset by using the bark as fuel. Mechanical pulp retains all the natural tree substances - sap, sulphur compounds, sugar and a lignin which binds the fibres together. Because lignin reacts with ultra violet light, papers made from mechanical pulp tend to 'yellow' when exposed to light.
Mechanical pulps are used to manufacture newsprint and some packaging boards.
Chemical Pulp
Most chemical woodpulp is made by the sulphate process. Chips from de-barked logs are dissolved in caustic soda and sulphur by heat and pressure leaving a strong brown pulp, coloured by the effect of the chemicals on the lignin and wood sap. The pulp is commonly known as kraft - the German word for strong. About 20% of chemical pulp is made by the sulphite process. Chemical pulping uses less energy than mechanical pulping. In modern mills, recovery boiler operations and the controlled burning of bark and other residues make the chemical pulp mill a net energy producer which can often supply power to the grid, or steam to local domestic heating plants.
Chemical pulp, however, produces a strong liquid effluent that needs to be treated. The term "woodfree" is often used in the trade to signify that the chemical pulping process has been used. it is not meant to be misleading and has been used for decades to mean "free from mechanical wood" (or "ground wood free").
Combination Pulps
Thermo-Mechanical-Pulp (TMP) and Chemi-Thermo-Mechanical-Pulp (CTMP) are a combination of the mechanical and chemical processes. De-barked logs are chipped in both, then heated to extreme temperatures to soften them before passing through grindstones for mechanical reduction to fibres. The difference is that chemicals are sprayed onto the chips in the CTMP process to reduce the undesirable effects of the retained natural wood substances. The main disadvantage of this form of pulping is the high energy demand.
The consumer often requires high quality papers which retain strength, do not discolour during storage or go yellow when exposed to light. One of the most effective ways of achieving all three is bleaching which has the added advantages of improving absorption capacity and getting rid of small residues of bark or wood, as well as giving a high level of purity, particularly important when the paper is being used for hygiene, medical or certain food packaging purposes.
For many years chlorine was used because of its efficiency. In recent years however it has been discovered that alternative bleaches such as chlorine dioxide or oxygen have a lower impact on the environment and they are now increasingly being used throughout the industry.
