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Use the description below
to make sense of the drawing
· Wood pulp is diluted
in water (99.5% water) and held in a headbox.
· Liquid pulp flows onto a moving wire-mesh belt, water drains
· Pulp is then pressed with a wire-mesh on top to drain further
· The wet paper at this stage is called a web
· Pulp is then passed under a 'dandy roll' to smooth
out the fibres
· This is passed first through a series of steam-heated rollers,
that dry it, and then between heavy calendar rollers, which give
it a smooth finish.
· The final paper has a moisture content of 4-6%
How
is the wood pulp made?
Wood pulp come from trees (no
surprise there). Wood from trees can be processed in three different
ways.
Mechanical pulp
Coniferous logs are saturated with water and debarked
Ground down to about 1-2mm
Larger pieces are reground
Currently only suitable for low grade packaging
Bleached with peroxide or sodium hydroxide to give white appearance
This method offers High Yield (lots of paper per tree) , but
contains a lot of impurities (not good!).
Chemical pulp
Hardwood or softwood is debarked
Logs cut into 2cm long chips along the grain
These are pounded into fragments
The pulp is treated with acid or alkaline to break it down
Yield is low (not much paper per tree) but fibres are long and
therefore strong (very good).
(Semi-chemical pulp; Uses steam rather than
chemical, then bleached)
Waste pulp
Recycled paper and card used to make waste pulp
Only suitable for low grade paper
Durability, strength and colour are not as good as 'virgin' pulp
Waste pulp often mixed with virgin pulp to improve properties.
(E.g. 12% recycled paper)
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