FEATURES
Treated with a process that ensures penetration of the preservative and gives a long service life.
Continued from page 38
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.
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.
40 APRIL / MAY 2019 //
The permit
conditions of the
certification bodies
ensure that there
are daily checks
that take place
to assess if the
correct methods
are being used
and the products
are coming out
at an acceptable
standard.
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°C 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 uptake of preservative. Oil type preservatives
such as creosote is used, and proper seasoning of the wood
is necessary before treatment.
WHAT DOES THE LAW SAY?
According to SAWPA, there are two main regulations that
apply to preservative treated timber in South Africa. The first
being VC 9092, the compulsory specification for the
preservative treatment of timber. This regulation is
administered and regulated by the National Regulator for
Compulsory Specifications (NRCS). It covers and regulates
the supply of preservative treated timber to the market, with
the chief aim to protect the consumer.
Secondly, preservative treatment has to meet the
requirement of the building standards, such as SANS 10400,
that primarily focuses on the application of the National
Building Regulations. Both VC 9092 and SANS 10400 refers
to compliance to SANS 10005. This standard addresses the
classification and types of timber preservatives that can be
used, the approved processes and the hazard classification
and end uses (H classes) for preservative treated timber. It
also specifies in which areas of South Africa the use of
preservative treated timber in structural applications, of
the two main species, such as softwoods and hardwoods,
is required.
De Klerk feels it is important to have a law for
timber treatment.
See more on page 42
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