Timber iQ February - March 2020 // Issue: 48 | Page 16

ASSOCIATIONS
Timber preserving processes
By the South African Wood Preservers
Association (SAWPA)
The effectiveness of all wood
preservatives depends on the
penetration achieved and the retention
of preservative in the penetrated zones.
C
orrect methods in the application of preservatives
must be used to cater for the wide variation in
timber absorption characteristics.
HOT AND COLD BATH
Hot and cold bath treatment in open tanks is an immersion
treatment and consists of immersing the timber in the bath,
raising the temperature to about 85° for a predetermined
period, then allowing the preservative to cool or transferring
the timber to an adjacent cold preservative bath until cool.
During this process a vacuum is formed in the cell cavities that
ensures the uptake of preservative. Oil type preservatives such
as creosote is used, and proper seasoning of the wood is
necessary before treatment.
VACUUM PRESSURE
Vacuum pressure methods of preservative impregnation are
widely used commercially and have proved to be a most
effective means of controlling preservative loading and depth
of penetration. The most suitable method is generally
determined by timber characteristics, preservative type
required and the specified product end use.
FULL CELL PROCESS OR BETHELL PROCESS
The timber is placed in a cylinder which is then sealed. A
vacuum is drawn on the timber for a predetermined period of
time and the cylinder flooded with preservative while
maintaining the vacuum.
When flooding is complete, the preservative pressure in the
cylinder is raised and held until the timber refuses to
absorb further preservative or until the required retention
has been obtained.
The pressure is then released, the preservative pumped back
to the holding tank and a final vacuum drawn to remove
excess preservative. The complete treatment cycle can vary
from 1½ to 5 hours, depending on timber species permeability.
Water soluble preservatives are applied at ambient
temperature while oil types such as creosote are introduced
into the cylinder at temperatures from 80-90°. A
modification of the process, developed in South Africa, is
the application of the initial vacuum once the cylinder is
flooded to approximately 90% with water soluble
preservative. A series of modified Bethell cycles is
employed by the industry to provide better control of the
treatment process, reduce post treatment drip and facilitate
faster post treatment drying where applicable. The process
is called the full cell process as the cell cavities are filled
with preservatives.
EMPTY CELL PROCESS
The two basic procedures are the Lowry Process and the
Rueping process and both require that the timber be placed in
an impregnation cylinder. The Lowry Process is used for some
reasonably permeable species or to limit the liquid absorption
and omits the initial vacuum of the Full Cell Process.
The cylinder is flooded with preservative, the pressure raised
and held until refusal is reached. After releasing the pressure,
the preservative is returned to the holding tanks and a final
vacuum is drawn on the timber to remove excess preservative.
The Rueping Process is used when it is desired to minimise
the retention of preservative liquid and air pressure is applied
to the timber prior to flooding with preservative.
A normal preservative pressure period follows and when the
final vacuum is applied, the expansion of the air in the timber
forces out excess preservative liquid. The process is referred
to as the empty cell process as the preservative is forced
mainly into the cell walls and not the cavities.
DOUBLE VACUUM PROCESS
This method is a simple variation of the Full Cell process and
is used for the controlled application of light organic solvent
preservatives to building and joinery timbers where end uses
do not include high hazard or ground contact applications.
A pressure-treatment plant.
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FEBRUARY / MARCH 2020 //
The timber is enclosed in a sealed container and a partial
vacuum drawn. The unit is then flooded with preservative and
the fluid pressure may or may not be increased to a
predetermined level depending on timber permeability. The
pressure if applied, is then released. A final vacuum is applied
to remove excess preservative. The timber is then removed,
and the organic carrier evaporates after a period of 48 hours,
leaving the timber dry.
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