Wood Preservation Lecture 8
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Transcript of Wood Preservation Lecture 8
Th e University of the West Indies
Faculty of Engineering
M.Sc./Diploma in Construction Engineering and Management
SCIENCE OF MATERIALS TECHNOLOGY
Lecture PRESERVATIVE TREATMENT OF TIMBER (BS 1282)
Lecturer; Dr Mwasha Abrahams
Objectives of the lecture:
Health hazards during the preservation of timber
Classifications of timber preservatives
Application processes.
Termite control
References include:
BS 5268 CP for structural use of timber: part 5:1977 deal with Preservative
treatment for constructors’ timber
Specifications of the British Wood Preserving Association (BWPA)
BRE Digest 201 describe Wood preservatives application methods
A BRE report is: Methods of applying wood preservatives by D. F. Purslow ,
HMSO
http://en.wikipedia.org/wiki/Timber_treatment cite October 2006-10-20
1.0 Introduction
Wood can last for centuries if four elements needed by wood-destroying organisms are
eliminated –
1. Moisture (content above 20%),
2. Favorable temperature (50-90 degrees F),
3. Air or food source (wood fiber).
1
Chemical wood preservatives protect wood from decay and insect attack by eliminating
the wood fibres as a food source. There are over 52 varieties of borers and termites
present in Trinidad & Tobago.
1.1 History
Treatment of wood has been practiced for almost as long as the use of wood itself. Some
accounts reach back to the beginning of recorded history. For example the Bible in
Genesis, 6:13-14 “And God said unto Noah… make thee an ark of gopher wood; rooms
shalt thou make in the ark, and shalt pitch it within and without with pitch.” There are
also records of wood preservation reaching back to ancient Greece during Alexander the
Great’s rule, where bridge wood was soaked in olive oil. The Romans also protected their
wood by brushing their ship hulls with tar. During the Industrial Revolution wood
preservation became a corner stone of the wood processing industry.
1.2 Health and Safety during timber preservation
Unfortunately, many of the timber treatments have proven to be extremely hazardous
both to the workers and the environment; as a result many treatment centers have been
forced to close and undertake massive environmental restoration and ground remediation.
With many of the chemicals having a long period through which they are highly
dangerous, and probably never be totally eliminated.
Care should be taken on using preservatives e.g. Goggles should be worn on spraying
chemicals. Disposal of containers or pesticide wastes must have proper regulation and
procedures established by local authorities. Pesticide wastes are toxic.
2.0 Chemical Preservatives
Timber or lumber that is treated with a preservative generally have it applied through
vacuum and\or pressure treatment. The preservatives used to pressure-treat lumber are
classified as pesticides. Treating lumber provides long-term resistance to organisms that
cause deterioration. If it is applied correctly, it extends the productive life of lumber by
five to ten times. If left untreated, wood that is exposed to moisture or soil for sustained
periods of time will become weakened by various types of fungi, bacteria or insects.
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Chemical preservatives can be classified into three broad categories:
Water-Bourne salts, (BS 3452, 3453, 4072)
Oil-Bourne preservatives, (BS 144. BS 3051) and
Light Organic Solvent Preservatives (LOSPs) (BS 5707).
2.1 Water-Borne Preservatives
Water is the most common solvent carrier in preservative formulations due to its
availability and low cost. Water-Bourne systems do however have the drawback that they
swell timber, leading to increased twisting and splitting
Chromatic Copper Arsenate (CCA)
Alkaline copper quaternary
Other copper compounds
Borate preservatives
Sodium Silicate- Based
Bifenthrin Spray
2.1.1 Chromated Copper Arsenate (CCA)
Common preservative developed in the 1930. During CCA treatment, copper is the
primary fungicide, arsenic is a secondary fungicide and an insecticide, and chromium is a
fixative which also provides Ultraviolet (UV) light resistance. Recognized for the
greenish tint it imparts to lumber, CCA is a preservative that was extremely common for
many decades, however it contained arsenic. The chemicals may leach from the wood
into surrounding soil, resulting in concentrations higher than naturally occurring
background levels. Australian Pesticides and Veternary Medicines Authority (APVMA
2006) and (USEPA 2004) restricting the use of CCA in treated lumber in residential and
commercial construction, with the exception of shakes and shingles, permanent wood
foundations, and certain commercial applications. This was in an effort to reduce the use
of arsenic and increase environmental safety.
2.1.2 Alkaline Copper Quaternary(ACQ)
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Alkaline Copper Quaternary (ACQ) is a preservative made up of copper, a fungicide, and
NH4+ compound, an insecticide which also augments the fungicidal treatment. Since it
contains high levels of copper, ACQ-treated lumber is five times more corrosive to
common steel, according to American Wood Preservers association (AWPA) test results.
It is necessary to use double-galvanized or stainless steel fasteners in ACQ lumber. Use
of fasteners meeting or exceeding requirements for ASTM A 153 Class D meet the added
requirements for fastener durability. The U.S. began mandating the use of ACQ in end-
consumer lumber in 2004.
2.1.3 Other copper compounds
These include Copper Azole (CA), Copper Chromate (CCh), Copper Citrate (CC), Acid
Copper Chromate(ACC) and ammonia Copper Zinc Arsenate (ACZA). AZCA is
generally used for marine applications.
2.1.4 Borate preservatives
Borate treated wood is non-toxic to humans, and contains no coppers or other heavy
metals. Borate taken into the body is excreted, rather than building up as heavy metals do.
Unlike most other preservatives, borate compounds do not become fixed in the wood and
can be washed out.
B(OH)3 + H2O ⇌ B(OH)4− + H+
Therefore they cannot be used where they will be exposed to standing water. Recent
interest in low toxicity lumber for residential use, along with new regulations restricting
wood preservation agents, has resulted in a resurgence of the use in borate treated wood
for floor beams and internal structural members.
2.1.5 Sodium Silicate-based preservatives
Sodium Silicate is produced by fusing sodium with sand or heating both ingredients
under pressure. It has been in use since the 1800s. It can be a deterrent against insect
attack and possesses minor flame-resistant properties; however, it is easily washed out of
wood by moisture, forming a flake-like layer on top of the wood. Other uses include
fixing pigments in paintings and cloth printing, and for preserving eggs.
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2.1.6 Bifenthrin spray preservatives
In Australia, a water-based bifenthrin preservative has been developed to improve the
insect resistance of timber. As this preservative is applied by spray, it only penetrates the
outer 2mm of the timber cross-section. Concerns have been raised as to whether this thin-
envelope system will provide protection against insects in the longer term, particularly
when exposed to sunlight for extended periods.
2.2 Oil-borne Preservatives
Coal –Tar Creosote (BS 144: 1973; BS 3051:1972)
Linseed oil
2.2.1 Coal-tar Creosote(BS 144; BS 3051)
Creosote is a tar-based preservative that has been commonly used for telephone poles and
railroad ties. Creosote is one of the oldest wood preservatives, and was originally derived
from a wood distillate. It often collects inside chimneys causing a fire hazard. Creosote is
regulated as a pesticide and is not usually sold to the general public. It is still used for
railway sleepers and utility poles.
2.2.2 Linseed Oil
In recent years in Australia and New Zealand, Linseed has been used as a solvent to
'envelope treat' timber. This involves just treating the outer 5mm of the cross-section of a
timber member with preservative, leaving the core-untreated. While not as effective as
CCA or LOSP methods, envelope treatments are significantly cheaper as they use far less
preservative. Major preservative manufacturers add a blue dye to envelope treatments.
There is an on-going promotional campaign in Australia for this type of treatment.
2.3 Light Organic Solvent Preservatives (LOSP) (BS 5707)
This class of timber treatments use white spirit as the solvent carrier to deliver
preservative compounds into timber. Commonly used in Australia and New Zealand,
modern formulations use Permethrin as an insecticide and Propaconazole &
5
Tebuconazole as fungicides. While still using a chemical preservative, this formulation is
generally considered 'safer' than many other systems because it contains no heavy-metals.
This also means that LOSP treated timber is generally no more corrosive to metal fixings
than untreated timber. Commercial formulations typically come with a 25-year guarantee
against termite, borer and rot attack.
Unlike water-based 'pressure treated' wood methods, LOSP is applied under vacuum. The
combination of a low-viscosity carrier and vacuum means that the preservative is able to
enter the wood more easily than CCA preservation methods. In practice this means that
LOSP treated wood is more dimensionally stable and less prone to splitting and twisting
than alternative methods.
2.3.1 Micro-emulsions
Due to the introduction of strict Volatile Organic Compound (VOC) laws, in the
European Union LOSPs have lost favors due to the high cost and long process times
associated with vapour-recovery systems. As an alternative, LOSPs have been emulsified
into water-based solvents. While this does significantly reduce VOC emissions, micro-
emulsion swells timber during treatment thus removing many of the advantages of LOSP
formulations.
2.3.2 Organic Solvent types
They consist of the following preservative in organic solvents:
These include
Chloranated naphthalenes and Chloranated Hydrocarbons
Copper naphthenate (green in color),
Pentachlorophenol
3.0 Other Preservatives
3.1 Naturally rot-resistant woods
This includes hardwood (heartwood) including Afromosia, African Mahogany, European
Oak, Sapele, Teak, Utile and Softwood Western such as Red Cedar, Huon Pine, Merbau,
Ironbark, many Cypresses and Coast Redwood. These species are resistant to decay in
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their natural state, due to high levels of organic chemicals called "extractives", mainly
polyphenols. Extractives are chemicals that are deposited in the heartwood of certain tree
species as they convert sapwood to heartwood. However, many of these species tend to
be prohibitively expensive for general construction applications.
3.2 Tung oil
Tung oil has been known about for hundreds of years in China, where it was used as a
preservative for wooden ships. The oil penetrates the wood then hardens to form an
impermeable hydrophobic layer up to 5 mm into the wood. As a preservative it is
effective for exterior work above and below ground, but the thin layer makes it less
useful in practice. It is not available as a pressure treatment. Some manufacturers
recommend tung oil as a stabiliser for CCA.
3.3 Heat treatments
There is ongoing research as to whether heat treatments can be used to make timber more
durable. By heating timber to a certain temperature, it may be possible to make the wood-
fibre less appetizing to insects.
4.0 Application Processes
4.1 Introduction and History
Probably the first attempts made to protect wood from decay and insect attack consisted
of brushing or rubbing preservatives onto the surfaces of the treated wood. Through trial
and error the most effective preservatives and application processes where slowly
determined. In the Industrial Revolution demands for such things as telegraph poles and
railroad ties helped to fuel, an explosion of new techniques that emerged in the early 19th
century. The goal of modern day wood preservation is to ensure a deep uniform
penetration with reasonable cost without endangering the environment. The most
widespread application processes today are those using artificial pressure through which
many woods are being effectively treated, but several species (such as Spruce, Douglas
Fir, Larch, Hemlock and Fir) are very resistant to impregnation. With the use of incising,
the treatment of these woods has been somewhat successful but with a higher cost and is
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not always satisfactory results. One can divide the wood-preserving methods roughly into
either non pressure processes or pressure processes.
4.2 Non-pressure Processes
There are numerous non-pressure processes of treating wood which vary primarily in
their procedure. The most common of these treatments involve the application of the
preservative by means of brushing or spraying, dipping, soaking, steeping or by means of
hot and cold bath. There is also a variety of additional methods, involving charring,
applying preservatives in bored holes, diffusion processes and sap displacement.
4.2.1 Brush and Spray Treatments
Brushing preservatives is a long-practiced and often used in today’s carpentry workshops.
Through technology developments it is also possible to spray preservative over the
surface of the treated timber. Some of the liquid is drawn into the wood as the result of
capillary action, but this penetration is insignificant and not suitable for long time
weathering. By using the spray method, coal-tar creosote, oil-borne solutions and water-
borne salts (to some extent) can also be applied. A thorough brush or spray treatment
with coal-tar creosote can add 1 to 3 years to the lifespan of poles or posts. Two or more
coats provide better protection than one, but the successive coats should not be applied
until the prior coat has dried or soaked into the wood. The wood should be seasoned
before treatment.
4.3 Dipping
Dipping consists of simply immersing the wood in a bath of creosote or other
preservative for a few seconds or minutes. Similar penetrations to that of brushing and
spraying processes are achieved. It has the advantage of minimizing hand labor. It
requires more equipment and larger quantities of preservative and is not adequate to
treating small lots of timber. Usually the dipping process is useful in the treatment of
window sash and doors. Treatment with Copper salt preservatives is no longer allowed
with this method.
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4.4 Steeping
In this process the wood is submerged in a tank of water - preservative mix, and allowed
to soak for a longer period of time (several days to weeks). The depth and retention
achieved depends on factors such as species, wood moisture, preservative and soak
duration. The majority of the absorption takes place during the first two or three days, but
will continue at a slower pace for an indefinite period. As a result, the longer the wood
can be left in the solution, the better treatment it will receive. When treating seasoned
timber, both the water and the preservative salt soak into the wood making it necessary to
season the wood a second time. Posts and poles can be treated on direct endangered
areas, but should be treated at least 30 cm (1 ft) above the future ground level.
The depth obtained during regular steeping periods varies from 5 mm to 10 mm (1/8 to
1/3 in.) up to 30 mm (1 in.) by sap pine. Due to the low absorption, solution strength
should be somewhat stronger than that by pressure processes, around 5% by seasoned
timber and 10% by green timber (because the concentration slowly decreases as the
chemicals diffuse into the wood). The solution strength should be controlled continually
and if necessary be corrected with the salt additive. After being removed from the
treatment tank the chemical will continue to spread within the wood if it has sufficient
moisture content. The wood should be weighed down and piled so that the solution can
reach all surfaces (by sawed materials stickers should be placed between every board
layer). This process finds minimal use despite its former popularity in continental Europe
and Great Britain.
4.5 Kyanizing
Named after John Kyan, who patented this process in England in 1832, Kyanizing
consists of steeping wood, in a 0.67% mercuric chloride preservative solution.
4.5.1 Hot and Cold Bath
Patented by C. A. Seeley, this process achieves treatment by immersing seasoned wood
in successive baths of hot and cold preservatives. During the hot baths the air expands in
the timbers. When the timbers are changed to the cold bath (the preservative can also be
changed) a partial vacuum is created within the lumen of the cells causing the
9
preservative to be drawn into the wood. Some penetration occurs during the hot baths, but
most of it takes place during the cold baths. This cycle is repeated with a steep time
reduction compared to other steeping processes. Each bath may last 4 to 8 hours or in
some cases longer. The temperature of the preservative in the hot bath should be between
60 to 110 °C (140 to 225 °F) and 30 to 40 °C (85 to 105 °F) by the cold bathes
(depending on preservative and treespecies). The average penetration depths achieved
with this process ranges from 30 mm to 50 mm (1 to 12/3 in.). Both preservative oils and
water-soluble salts can be used with this treatment. Due to the longer treatment periods
this method finds little use in the commercial wood preservation industry today.
4.6 Osmosis Process
In this process, first developed in Germany, the preservative is applied to the surface of
green wood in the form of a cream or paste. The wood is then stacked in solid piles,
which are covered securely with waterproof tarp to prevent moisture loss. The treated
wood is left covered for 30 days (up to 90 days), as the water-soluble portions of the
preservative diffuse into the water of the green wood. The osmosis process is often used
in the United States and Canada for the treatment of fence posts, as well as the
subsequent treatment of ground-line areas for standing poles. But because of its intensive
time and labor consumption it is not used on a large scale basis.
4.7 Sap Displacement
Sap displacement takes place when one brings a preventative into or onto the sapwood of
a living tree which carries it within the sap stream for long distance. The idea of injecting
treatment in a tree to repel fungal, parasite (mistletoe) or insect attacks has also been
tested, with positive results in the domestic crop corn.
5.0 Pressure Processes
Pressure processes are those in which the treatment is carried at in closed cylinders with
applied pressure and/or vacuum. These processes have a number of advantages over the
non-pressure methods. In most cases, a deeper and more uniform penetration and a higher
absorption of preservative is achieved. Another advantage is that the treating conditions
10
can be controlled so that retention and penetration can be varied. These pressure
processes can be adapted to the large-scale production. The high initial costs for
equipment and the energy costs are the biggest disadvantages. The treatment is carried
out in cylinders. The timbers are loaded onto special tram cars, so called “buggies,” and
into the cylinder. These cylinders are then set under pressure often with the addition of
higher temperature. As final treatment a vacuum is frequently produced to obtain excess
preservatives. These cycles can be repeated to achieve better penetration.
5.1 Full-Cell Process
In the full-cell process, the intent is to keep as much of the liquid absorbed into the wood
during the pressure period as possible, thus leaving the maximum concentration of
preservatives in the treated area. Usually water solutions of preservative salts are
employed with this process but it is also possible to impregnate wood with oil. The
desired retention is achieved by changing the strength of the solution. William Burnett
patented this development in 1838 of Full-Cell Impregnation with water solutions. His
patent described the injection of tar and oils into wood by applying pressure in closed
cylinders. This process is still used today with some improvements.
5.2 Empty-Cell Processes
The empty-cell process should be used when a deep penetration with a limited final
retention of liquid is desired. In the empty-cell processes, excess preservative in the
timber is subsequently recovered, resulting in a coating of the cell instead of it being
filled with it. The empty-cell processes are mainly used to impregnate wood with oil.
There are two basic empty-cell processes and many other variations. Both the Rueping
and the Lowry process are usually limited to the treatment of timber with creosote or
other preservative oils, although they can also be used for injecting water solutions. The
main area of application is the impregnation of such products as railway ties, poles, posts,
lumber and many forms of construction timbers.
5.2.1 Rueping Process
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The Rueping process was patented by Max Rüping of Germany in 1902. The main
difference to other methods is the application of preliminary air pressure to the wood,
before the insertion of the preservative oil.
5.2.2 Lowry Process
This empty-cell process was patented in 1906 by C. B. Lowry. The process is similar to
that of Rueping but it does not require an initial air pressure. The preservative oil is
impregnated into the timber without any preliminary treatment.
5.3 Fluctuation Pressure Process
Contrary to the “static” Full-Cell and Empty-Cell processes, the Fluctuation Process is a
“dynamic” Process. By this process the pressure inside the impregnation cylinder changes
between pressure and vacuum within a few seconds. There have been erratic claims that
through this process it is possible to reverse the pit closure by Spruce. However the best
results that have been achieved with this process by Spruce do not exceed a penetration
deeper than 10 mm (1/3in.). Specialized equipment is necessary and therefore higher
investment costs incur.
5.4 Boucherie Process
Developed by Dr. Boucherie of France in 1838, this approach consisted of attaching a
bag or container of preservative solution to a standing or a freshly cut tree with bark,
branches, and leaves still attached, and so injecting the liquid into the sap stream.
Through transpiration of moisture from the leaves the preservative was drawn upward
through the sapwood of the tree trunk.
The modified Boucherie process consists of placing freshly cut, unpeeled timbers onto
declining skids, with the stump slightly elevated; then fastening watertight covering caps
or boring a number of holes into or onto the ends, and inserting a solution of copper
sulfate or other water- borne preservative into the caps or holes from an elevated
container. Preservative oils tend to not penetrate satisfactorily by this method. The
hydrostatic pressure of the liquid forces the preservative lengthwise into and through the
sapwood, thus pushing the sap out of the other end of the timber. After a few days, the
12
sapwood is completely impregnated; unfortunately little or no penetration takes place in
the heartwood. Only green wood can be treated in this manner. This process has found
considerable usage to impregnate poles and also larger trees in Europe and North
America, and has experienced a revival of usage to impregnate bamboo in countries such
as Costa Rica, Bangladesh, India and the state of Hawaii.
5.4.1 High pressure sap displacement system
Developed in the Philippines this method (abbreviated HPSD) consists of a cylinder
pressure cap made from 3 mm thick mild steel plate secured with 8 sets of bolts, a 2-HP
diesel engine and a pressure regulator with 1.4 - 14 kg/m2 capacity. The cap is placed
over the stump of a pole, tree or bamboo and the preservative is forced into the wood
with pressure from the motor.
6.0 Other Applications
6.1 Vacuum Processes
LOSP treatments often use a vacuum impregnation process. This is possible because of
the lower viscosity of the white-spirit carrier used.
6.2 Incising
This process consists of making shallow, slit-like holes in the surfaces of material to be
treated, so that deeper and more uniform penetration of preservative may be obtained. In
North America, where smaller timber dimensions are common, incision depths of 4 to 6
mm (1/6 to 1/4 in.) have become standard. In Europe, where larger dimensions are
widespread, incision depths of 10 to 12 mm (1/2 in.) are necessary. Incisions by LASER
are significantly smaller than those of spokes or needles. The costs for each process type
are approximately for spoke / conventional all-round incising €0.50 per m², by laser
incising €3.60 per m² and by needle incision €1.00 per m². (Figures originate from the
year 1998 and may vary to present day prices)
7.0 Inject/Plug inserts
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Preformed plugs can be effectively inserted in drilled holes. Almost similar method
developed by RENTOKIL Ltd whereby injected preservatives are injected into predrilled
holes in existing joinery (windows, door etc)
8.0 Termite Treatment
In Trinidad & Tobago termites can be divided into three distinct groups:
Subterranean,
Drywood and
Nasuties.
Subterranean termites are responsible for as much as ninety (90%) percent of the
structural damage in Trinidad & Tobago.
8.1 Types of termite treatment:
Pre-Construction Soil Treatment
Soil Impregnation and Post Construction Treatment
Pesticide company technicians must be familiar with the current control practices,
including trenching, rodding, subslab injection, and low-pressure spray applications.
These techniques must be correctly employed to prevent or control infestations by
subterranean termites species of Reticulitermes, Zootermopsis, Heterotermes and
Coptotermes. Choice of appropriate procedures should include consideration of such
viable factors as design of the structure, water table, soil type, soil compaction, grade
conditions, location and type of domestic water supplies and drainage systems.
8.2 Pre-construction soil treatment
Chemicals for soil treatment are used to establish a barrier against termite attack. The
chemical emulsion must be adequately dispersed in the soil to provide a barrier between
the wood in the structure and the termite colonies in the soil. Effective preconstruction
subterranean termite control requires the establishment of an unbroken vertical and/or
horizontal chemical barrier between the wood in the structure and the termite colonies in
the soil. It includes treating the soil below the proposed foundations at specific rates of
application while the building is under construction:
14
Putting the "approved" chemical as high in the fill as possible to increase the
coverage at termite entry points;
Flooding the area immediately under the slab with soil toxicant and using a rich
concrete mixture to fill grade stake holes;
Treating along the outside of the foundation (following label directions using
trench or roding techniques);
Covering the treated soil with at least 2 inches of untreated soil.
Pesticide companies should avoid contamination of public and private water supplies by
following precautions:
Use equipment or procedures, as outlined in many product methodologies.
Do not treat soil beneath structures that contain wells or cisterns. {There should
be local specifications for recommended distance of treatment areas from wells.}
Care must be taken to avoid runoff. Do not treat soil that is water-saturated.
8.3 Soil impregnation and post-construction treatment
Soil impregnation requires the drilling along both sides of the supporting walls of the
structure, and sometimes the floor itself, to set up a chemical barrier below the existing
foundation. Post construction applications may be made by sub-slab injection, Roding,
and/or trenching using low-pressure spray not exceeding 25 p.s.i. at the nozzle.
Where necessary, drill through the foundation walls from the outside and inject the
chemical just beneath the slab or along the inside of the foundation.
9.0 References
ALAN EVERETT (1994) Mitchell’s materials 5th Edition Pearson education
TAYLOR G. D. AND SMITH B. J. (1985) material in construction Longman Technician
Series, Construction and Civil engineering
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