Making Pulp from Wood
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.