Wood Preservative Treatments

All of the wood preservation processes in current use can be placed under one of the following
categories: non-pressure/ diffusion,, sap replacement, or pressure and vacuum impregnation. Cost is
often a major consideration. The simplest Processing method generally requires the least expensive
equipment, but at the some time, it is often the least effective for long term protection of the material.
However,, for some requirements it is not necessary to extend the service life of untreated material. So
the choice of wood preservative and application method depends upon many factors, each of which
should be considered from the technological and economic viewpoint.

I. Non-pressure Processes

Non-pressure processes include any method where no external pressure is applied to force the wood
preservative into the timber. These include brushing, spraying, dipping, steeping, cold socking, and hot-
and-cold both. Diffusion method can also be considered in this category.

(A) Brushing or spraying -The application of wood preservatives by brushing, painting, or spraying i
simplest treatment available. It requires a minimum investment in equipment and can be employed for
applying oil-borne and waterborne preservative chemicals, coat-tar cresote or other low viscosity oils.
This method permits treatment to be carried out at the construction site or on wood parts already in
service.

Even when used in treating well seasoned material, the effect of brush or spray treatments is superficial.
They cannot be recommended except as temporary expedients. The moderate penetration that results
is seldom more than a few millimeters in depth. If any physical damage should rupture the thin
protective shell, the piece is subject to attack through the open area. Water-borne chemicals are readily
leached out of such timbers if they are in the open, but oily materials should provide protection for
somewhat longer periods. Under optimum conditions the normal service life of the wood can be
extended one to three years, assuming that surface cracks and checks are thoroughly filled and that
generous quantities of preservatives are applied over the entire surface of the timber.
 (B) Dipping - Another non-pressure process for applying preservative material is dipping - This method
involves the immersion of wood in a treating solution for a period of a few seconds to a few minutes. It
provides little more effectiveness than brushing or spraying except that end penetration is frequently
better in easily treated species. Complete immersion probably provides greater uniformity of coverage
than brushing and gives more assurance that all checks are filled. Dip treatment is not effective if the
wood is at a moisture content above the fiber saturation point. Dipping has been found to be
particularly well suitable for the treatment of millwork such as window sash at the factory. Paintable
preservative in a non-swelling carrier (NSP), of which pentachlorophenol is a good example, is widely
used. Dipping times are approximately three minutes. Dip treatment extends service life two to four
years when the wood is not subjected to physical damage.

(C) Steeping and Cold Soaking - Steeping and cold soaking are merely prolonged immersions of wood in
preservative solutions. The term cold soaking is generally applied to the use of oil solutions while
steeping refers to the soaking of wood in water solutions of preservative.

Cold soaking has been shown to be a rather effective method of treating seasoned material for farm use
because of this simplicity. Fuel oil solutions of pentachlorophenol are commonly employed. The more
viscous oils are not as satisfactory unless heated to reduce viscosity. Soaking times are not critical and
may be extended for long periods although two to several days is usually sufficient. Actually, a large
proportion of the absorption takes place during the first day of treatment, but prolonged soaking does
increase depth of penetration and amount of retention. As would be expected from anatomical
considerations,, the sapwood of certain softwoods is easily treated by this method. Hardwoods having
tylosis free vessels show good end penetration, but generally poor transverse penetration.

In the steeping process it is possible to use green as well as seasoned timber because salt from the
treating solution can move into wet wood by diffusion. Seasoned timber absorbs both water and salt so
that lower concentrations of salt solutions can be used. When treating green material, stronger
solutions should be used to offset the dilution and to speed up the rate of diffusion.

(D) Hot-and-Cold Bath - The hot-and-cold bath process is undoubtedly the most effective of the so-called
non-pressure treatments. The effectiveness of the method can actually be attributed to the mild
vacuum which is produced by the process, through not by mechanical equipment. The poles, posts, or
other timbers are first heated in preservative solution, or in a dry kiln in some cases. This causes the air
in the cells of the outer layer of the wood to expand. The heated material is then transferred to the cold
preservative solution. The warm air in the wood cells contracts upon cooling and creates a partial
vacuum in the outer portions of the wood. As atmospheric pressure tends to satisfy this mild vacuum,
penetration of preservative into the wood is aided.

The mechanics of the operation are very flexible and can be adjusted to the conditions at hand. Heating
can be accomplished in a kiln, in a tank of preservative or in water, depending on the choice of treating
chemical. The material being treated need not be removed from the hot tank if the solution can be
pumped out and quickly replaced by cold preservative. Though possibly not quite as effective, the hot
liquid can simply be allowed to cool.

Though coal-tar creosote and other preservative oils are generally used in this treatment process,
water-soluble salts can also be applied very effectively by this method. Care must be exercised to limit
the temperature of the hot both to a level that is safe for the particular solution being employed while
the cold bath must be warm enough to ensure liquid flow. If too high temperatures are reached, the
oily preservatives are likely to evaporate, Water solutions are subject to this danger as well as to the
possibility of precipitating part of the salts out of solution. The recommended temperatures are 190'F
to 2350 for the hot both and 90OF to 150OF for the cold bath.

(E) Diffusion Method - In the diffusion method of treatment, green timber is gradually penetrated by a
water-soluble salt which is generally applied in concentrated form. The best known example is the
Osmose process in which the toxic chemicals in paste form are coated over the surface of green, peeled
timber. Over a period of weeks the preservative diffuses into the green wood, provided the timber is
stacked and carefully covered to prevent moisture loss. Variation of this method include the use of
preservative bandages which are wrapped around individual poles, either after an application of
chemicals, or with a layer of preservative lining the bandage itself. The purpose of the bandage is to
prevent the loss of moisture from the unseasoned timber and to keep a supply of chemicals in contact
with the wood.

As with other non-pressure treatment methods, there is not great degree of control over depth of
penetration except through duration of stacking. Duration of treatment can be adjusted from about 30
days for small material to 90 days for timbers requiring greater penetration.

Diffusion treatment is also employed in the protection of the groundline zone of standing poles. Holes
are bored into the poles near the groundline, preservative is introduced into them and then the holes
are plugged. The toxic chemical is believed to diffuse through this critical zone over a period of time.
2. Pressure Processes

The preservative treatment of wood by pressure methods is the preferred commercial approach
because of its greater efficiency and effectiveness. Its efficiency stems from the much closer control
over treating conditions than is possible with the non-pressure processes. Its effectiveness is due to the
more uniform, deeper penetration and greater absorption of preservative than can usually be attained
by other means. The fact that the timber is totally enclosed in a cylinder in which conditions can be
varied widely offers a great advantage.

The cylinder is the heart of a pressure treating plant. It is a steel tank, usually horizontal, designed to
withstand high working pressures. Door may be installed at either or both ends of the cylinder,
depending on its size, the nature of the material to be treated in it, and the loading system used. In the
treatment of poles, pilings, and other large timbers, the charge can be rolled into the cylinder on
standard or narrow gauge rail trams and rolled out of the cylinder to the yards. Hand loading or crane
systems, are used for smaller material and small treating cylinders.

Accessory equipment must be provided for heating and storing preservative, for transferring it in and
out of the treating cylinder and for measuring the amount of preservative consumed in treating a
charge. In addition, compressors and pumps are required for vacuum and pressure phases of the
treating schedule and gauges must be installed to indicate those conditions.

Pre-treatment with steam, vacuum or air pressure permits a greater range of control over the final
treatment with preservative. Each of the well-known treating methods using pressure is based on a
variation in treating schedule of one or more of the above factors. The desired method is selected
because of its characteristic preservative retention which in turn can be related directly to the cost of
treatment chemicals.

The terms "empty-cell process" and "full-cell process" are frequently applied to treatments by pressure
methods. Though these terms may not be strictly accurate, they can be applied in a relative way in
describing the effect of a particular treatment schedule. Cell lumens in the penetrated portions of the
wood treated by the full-cell process are supposedly full of preservative. In the empty-cell process the
lumen walls are left with only a coating of chemical. Diffusion from the lumen into the cell wall must
take place in both cases but it would be expected that greater concentrations of preservative would
ultimately be found in the walls of wood treated by the full-cell process.

(A) Full-Cell Process - The primary objective of a full-cell treatment is to attain maximum retention of
preservative in the treated portion of the lumber. The factor which distinguishes it from empty-cell
treatment is the preliminary vacuum which is designed to remove as much air from the cells as possible,
thereby removing the air cushion which resists preservative penetration. A further advantage is that
there is a minimizing of preservative release ("kick-back")' caused by the expansion of trapped air when
pressure is removed from the cylinder.

The Bethell process is a full-cell process that is employed in treating with oils. A preliminary vacuum is
applied to the charge and held for a period of time. Then, without releasing the vacuum, the cylinder is
filled with preservative and pressure is applied and maintained until the desired absorption is reached.
After the preservative has been drained from the cylinder, it is customary to apply a mild final vacuum to
reduce preservative dripping from the treated timber.

The high net retentions attainable with the full-cell process can result in rather high preservative costs.
Its use can be justified for marine applications where maximum retentions of creosote are necessary for
effective protection. In certain tropical situations the high costs are also justifiable. However, in many
other uses,, the service life obtained from the timber treated by the empty-cell process is adequate and
the treatment costs are lower.

(B) Empty-Cell Processes - The empty-cell process differs from the full-cell process in that some means
of recovering much of the preservative is used, leaving no liquid preservative in the cell lumens of the
treated portions of the wood. In the original Rueping process, this is accomplished by applying
compressed air to the timber before forcing the preservative into it. The preservative can be admitted
into the treating cylinder from an equalizing tank where the air in the cylinder can interchange with the
preservative. This procedure traps air in the cells and when the pressure is released after treatment, the
trapped air expands and forces the preservative out. A final vacuum serves to remove even more of the
solution. A diagram of the Rueping process is shown in Figure 11.12. on Page 282 in Textbook.

In the Lowry process there is no preliminary air pressure applied, but the schedule is otherwise the
same as for the Rueping process. This eliminates the need for an air compressor.