| • Introduction
• Wood Destroying Organisms
• Preservation of Timber
• Preservative Treatment of Plywood
• Vacuum Pressure Process
• Glue Line Treatment
| Introduction | Reconstituted wood panel products are essentially articles of wood.Hence, they need to be protected against infestation by wood destroying organisms.
Most commonly used commercial timbers are
non-durable. With the increasing use of wood
in its natural and reconstituted forms like plywood, blockboard, particleboard and MDF, it is necessary to know thedifferentorganisms,which attack
and cause destruction to wood and wood-based
products in service.
| | | Top | Wood Destroying Organisms
Wood Destroying organisms can be broadly classified into
two groups:Micro organisms called “fungi” and
“wood boring insects”, consisiting of beetles and termites. | | Wood Staining
Fungi | | Fungi of this type thrive in the sapportion of wet Fungi wood and produce harmless discolouration. They do not attack the cellularsubstance of wood and hence no structural damage is caused to the affected wood. | | Wood Roting
Fungi | | Fungi of this type attack wet wood causing a Fungi state of decomposition called “rot” or “decay”. They decompose the wood tissues and destroy the fibrous structure | | Wood Boring
Beetles | |
There are several varieties of wood
boring beetles; but those which
attack articles of wood in India
belong to the family “Lyctus borers”
which cause the well known “Powder
Discharge”.These are very small insects.
They attack only the sapwood portion in
any article as they draw their
nourishment from the stored food in the sapwood. | | Termites | | Termites usually attack articles,
which are fixed to the building
elements like wall panelling, ceilings,
wall units etc.They usually settle down
around neglectedareas inside the buildings.
The infestationand destruction caused by
termites is usuallymassive.They attack
wood from below the surface leaving a
sound, deceptive outer skin. | | | | Preservation of Timber | | In order to make any article of wood durable it has to be protected against all the above organisms. In the past a large quantity of timber was used in the construction of buildings and various exposed structural applications like railway sleepers,transmission poles etc. On account of the shortage of naturally durable timbers like teak, it was necessary to evolve materials and methods to protect woodenstructures from wood destroying organisms. | | | Top | | Preservative Treatment of Plywood | | The materials and methods used for the preservative treatment of plywood
is essentially the same as those employedfor treating naturalwood.
But plywood also lends itself to different treatment methods at
various stages of manufacture as follows:
- By soaking, diffusion or impregnation
of the individual veneers before
gluing, with water borne chemicals.
- By adding preservative chemicals in
the glue used for bonding.
- By impregnating the consolidated
plywood with water borne
chemicals by vacuum pressure process
or other methods. | | Top | | Vacuum Pressure Process | | A pressure impregnation plant consists of a pressure cylinder provided with a seal door, a vacuum pump, a pump for pumping preservative solution into the pressure vessel and measuring instruments. The plywood to be treated is stacked vertically inside the cylinder.After closing the seal door tightly, the air inside is evacuated using a vacuum pump. The cylinder is then filled with the preservative solution and a steady pressure is applied for a certain period. Thereafter the preservative solution is drained off, and the plywood is dried.
| | | Top | | Glue Line Treatment | |
Plywood which is not water proof, cannot be subjected to vacuum pressure impregnation with waterborne chemicals.
In the case of commercial plywood and other
varieties of plywood which do not possess a
water-proof bonding system, glue line treatment is
generally advised as a precaution against borer
attack. On account of the ease of use and the
economy, a number of plywood manufacturers are
now using Chlorpyrifos as a preservative in the
glue line even for water-proof plywood.
Experiments carried out by IPIRTI has confirmed
the efficancy of glue line treatment with a chemical
called Chlopyrifos at a concentration of 1%, against
borer attack. The efficacy of the same treatment
against attack by termites or wood rotting fungi
was not investigated and reported by IPIRTI in
their report.
The method of treatment stipulated by IS 710:1976, which is the IS Specification for Marine Plywood is vacuum pressure impregnation with fixed type chemicals.
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