WOOD PRESERVATION

wood preservation mean all measures that are taken to ensure a long life of wood.

PEST THAT DAMAGE WOOD

Under proper use conditions, wood can give centuries of good service; but under unfavourable conditions wood ma readily be damaged and destroyed by fungi, insects and marine borers.

WOOD-INHABITING FUNGI
Fungi cause wood decay mold and most sap wood stains. These fungi feed on living or dead wood. The fungi that develop on or in wood can be divided into two major groups depending on damage they caused.
Wood-destroying fungi (decay fungi)
Wood-staining fungi (sap staining fungi, mold fungi).

Wood destroying fungi.

Wood-staining fungi

BASIC REQUIREMENTS OF FUNGI THAT GROW ON WOOD
i) Favourable temperature: usually ranging between 50 degrees and 90 degrees F. the optimum is about 70 degrees to 85 degrees F. blood is safe from decay at temperatures below 35 degrees F and above 100 degrees F.
ii) Adequate moisture: fungi will not attack dry wood (wood with a moisture content of 19 percent or less). Decay fungi require wood moisture content (M.C) of about 30 percent.
iii) Adequate oxygen: fungi cannot live in water saturated wood.
iv) Food source: cellulose component of the wood (wood itself).

wood destroying fungi

Both the sapwood and hardwood of most tree species are susceptible to decay. Decay fungi may grow in the interior of the wood or appear on wood surface as fan-shaped-patches of fine, threat like, cottony growths or as root-like shapes. The colour of these growths may range from white through light brown, bright yellow and dark brown. The spore-producing bodies may be mush rooms, shelf-like brackets or structures with a flattened, crust-like appearing. Fine threadlike fungal strands grow throughout the wood and digest parts of it food. In time, the strength of the wood is destroyed.
Decay will stop when the temperature of the wood is either too low or too high or when the moisture content is drier than the fungi’s requirement.
Wood decay fungi can be grouped into three major categories:
i) Brown rot
ii) White rot
iii) Soft rot

i) Brown rot: fungi that cause brown rot are able to break down the cellulose component of wood for food, leaving a brown residue of lignin. The final stage of wood decay by the brown rots can be identified by.
Dark brown colour of the wood
The ease with which the drying-wood substance can be crushed to powder.

ii) White rot: white rot fungi, which break down both lignin and cellulose, have a breading effect which may make the damaged of wood appear whiter than normal.

iii) Soft rot: soft rot fungi usually attack green (water-saturated) wood (high moisture content) cursing a gradual softening from the surface inward that resembles brown rot.

wood staining fungi

Sap staining fungi: these fungi penetrate and discolour sap-wood, particular of the soft wood species. Typical sap stain unlike staining by mod fungi cannot be removed by brushing or planning.

i) Mold Fungi:these fungi first noticeable as green, yellow, brown or black fuzzy or powdery surface growths on softwoods. Freshly out or seasoned stock piled during warm, humid weather, may be noticeable discoloured in 5 to 6 days or less. As with sap stains molds do not reduce wood strength however, they can increase the capacity of wood to absorb moisture, thereby opening the door to attack by decay fungi.

chemical stains

Chemical stains may resemble blue or brown stains, but are not caused by fungi. These stains result from chemical changes or seasoning. The most important chemical stains are brown stains that can downgrade lumber for some uses.

insects
Several kinds of insects attack living trees, logs, lumber and finished wood products food and/or shelter. These pests include various termites, ants and beetles.

1. Termites: Termites use wood for food and shelter, and are the most destructive of all wood insects.

Termites feeding on a wood.

Termites are divided into three major groups:
i) Subterranean or ground-inhabiting termites.
ii) Dry wood termites
iii) Damp wood termites.

i) Subterranean termites: At certain seasons of the year winged males and females are produced by the termite colony. They swarm, mate, lose their wings and attempt to begging a new colony in the soil. Termites build tunnels through earth and around obstructions of get a source of food (either sound or decaying wood). They also require a constant source of moisture usually obtained from the soil.

ii. Dry wood termites: dry wood termites are found naturally only in Hawaii, Puerto Rico and in a narrow strip of land extending from southern California and Texas to Florida and along the Atlantic cost to Virginia.
Although Dry wood are naturally found in these places, they can be transported to other parts of the country by wood infested with termite.
iii) damp wood termites: compared to the drywood termites, they usually do not cause much damage to buildings and man-made structures. they feed on wood that is not dried out.

1. Ants
   Ants cannot use wood for food but they are often confused with termites because the two look somewhat similar. However, there are several distinct differences in their physical appearance. Ants have allowed antennae: termites do not. Ants have narrow waists, whereas termite bodies are broad. Ant wings have five veins and the hind wings are smaller than the front wings. Both pairs of termite wings are similar in shape and size, ant have very small veins.
   Carpenter ants may be black or red. They usually live in stumps, trees, or logs, but often damage poles or structural timbers set in the ground. Elevated potions of buildings, such as window sills and porch columns, are susceptible to damage.
   
   Carpenter ants.

2. Bettles
   Powder post or Linctus Beetles: powder post beetles attack both freshly cut and seasoned hard wood and softwoods. Adults lay eggs in the wood pores. The larvae burrow through the wood making tunnels from 1/16 to 1/12 inch in diameter parked with a fine powder.

A Beetle damaging a wood.

Wood Handling and Types of Wood Preservatives
If wood is to be used where it will be subject to pest attack, it must be protected. This protection can be achieved by:
Control of moisture content.
Use of a wood that is naturally resistant to the pests chemical treatment.
In addition, mechanical barriers (such as metal termite shields and caps on pilings, poles and posts) are sometimes used, but are usually ineffective.

i) Moisture Control
The moisture content of living trees and the wood products obtained from them may range from about 30 percent to more than 20 percent. Much of this moisture must be removed for most uses. ‘Green’ lumber usually is dried.
to prevent stain and decay,
to reduce damage by insects,
to reduce uncontrolled dimensional change (shrinkage),
to reduce weight and increase strength, and
to prepare the wood for treatment with chemical preservatives.

ii) Chemical Control
The proper application of chemical preservatives can protect wood from decay and stain fungi, insects and marine borers, thus prolonging the service life of wood for many years.

Type of Preservatives
Wood Preservatives fall into two broad categories:

i) Oil-borne preservatives
ii) Water-borne preservatives

Many different chemical are in each of these classes, and each has differing effectiveness in various exposure conditions. The three exposure categories for preservative are:

• Ground contact (high decay hazard that needs a (heavy-duty preservatives).
• Above ground contact (low decay hazard that does not usually require pressure treatment).
• Marine exposure (high decay hazard that needs a heavy-duty preservative or possible dual treatment).

i) Oil-borne Preservatives
These chemicals are generally insoluble in water. They are usually dissolved in petroleum or other organic solvents in order to penetrate wood. Research developments have recently made available oil-borne preservatives formulated as water in oil emulsions or dispersions in water.

Advantages

• Toxic to fungi, insects and mold.
• Can be dissolved in oils having a wide range in viscosity, vapor pressure and colour.
• Low solubility.
• Can be glued depending on the diluents or carrier, and
• Ease of handling and use.

Disadvantage

• Can leave an oily, unpaintable surface, depending on the carrier.
• For some applications, provides somewhat less physical protection to wood than creosote.
• Should not be used in homes or other living areas because of toxic fumes, and
• Strong odor is toxic and irritating to plants, animals and humans.
• Dark colour.
• Oily, unpaintable surface.
• Tendency to bleed or exude from the wood surface.

ii) Waterborne Preservatives
This class of preservatives includes various metallic salts and other compounds. The principal compounds used are combinations of copper, chromium, arsenic and fluoride.
Water-borne preservatives have gained increasingly wider usage for lumber, plywood, fence posts, pilings and timbers.

Advantages

• Treatment presents no hazard from fire or explosion.
• The wood surface is left clean, paintable and free of objectionable odors.
• Safe for interior use and treatment of playground equipment, and
• Leach resistant.

Disadvantage

• Unless re-dried after treatment, the wood is subject to warping and checking.
• Does not protect the wood from excessive weathering.

Water-borne preservatives are often used when cleanliness and paintability of the treated wood are required.

Safety Precautions when Spraying Preservatives

i) Avoid inhaling vapors. If it cannot be avoided, you must wear a high efficiency half-mask canister or cartridge respirator.
ii) You must always wear a respirator when applying the preservatives by a spray method, or entering, cleaning or repairing vats, tanks, and other related equipment contaminated with the wood treatment solution.
ii) Respirators must fit properly, be well-maintained and approved for poly-nuclear aromatics organic vapors and acid gases. Do not over apply. Always check for leaks in the system
iv) After every use, the goggles and face piece should be washed with detergent and water, rinsed, dried and stored in a clean, dry place.
v) Change cartridges and canister if you have trouble breathing, or if you smell the preservatives. Disposal regulations for used cartridges and canisters are the same as for the preservatives.

First – Aid in Case of Accidents

i) The label on each preservative must include directions for first aid in case of an accident.
ii) Have the phone number of your local hospital, your doctor, or a poison control center on hand. Chemicals can be absorbed through the skin, inhaled, or swallowed.

Some Basic First-Aid Instruments to Follow in Case of an Accident:

i) For chemicals in your eyes – flush eyes with running water immediately for at least 15 minutes. Keep the water pressure low and see a doctor as possible.
ii) Be careful not to spill the chemicals on your skin or clothing. If you do, remove contaminated clothing immediately and wash the affected areas with mild soap and water. iii) Soap, clean water, or a waterless hand cleaner should be readily available. If skin becomes inflamed, see a doctor.
iv) If you swallow a chemical, get medical attention at once. Drink large quantities of water and induce vomiting. Do not make an unconscious person vomit.
v) Anyone overcome by fumes must be moved to fresh air. If the victim is not breathing, apply artificial respiration.
vi) Call for emergency medical assistance immediately.

Other Safety Precautions
i) Cut wood to size before treating it with creosote and penta.
ii) Wear a dust mask to avoid inhaling sawdust when sawing and machining treated wood.
iii) Wear goggles to protect your eyes from flying particles.
v) Whenever possible, work outdoors to avoid indoor accumulation of sawdust.
vi) Wash exposed body areas thoroughly after working the wood and before eating or drinking, or before using tobacco products.

Methods of Applying Wood Preservatives

The two major types of treatment are pressure and non-pressure methods.

i) Pressure Processes: We might expect wood to treat easily because of its porous structure, but wood id surprisingly resistant to deep penetration by preservatives. The basic principle of pressure processes involves the placement of wood materials in an airtight, steel cylinder or retort and immersing it in a preservative under pressure to force the preservative into the wood. Impregnation of preservatives by pressure is the most common method used in the commercial treatment of wood.

• It has several advantages:
• It gives a deeper and more uniform penetration.
• It allows better control over retention.
• Wood can be reconditioned in the chamber.
• It is quicker and more reliable than non-pressure methods, and
• It can comply with the code regulations and engineered specifications.
  There are two basic variations of the pressure treatment method: the full-cell process and the empty-cell process. With either method, it is important to closely follow established standards on:
  Preparation of the wood product to be treated.
  Amount and duration of vacuum and of pressure.
  Solution temperature (when critical),
  treating time, type of preservative, and
  Concentration of the preservative.

ii) Non-Pressure Processes: Non-pressure methods may be satisfactory where deep penetration; high levels of retention and precise treatment are not required. The effectiveness of non-pressure methods depends on the kind of wood, its moisture content, method and duration of treatment and the preservative used.
There are many methods applying preservatives to wood without the use of pressure. Some of the more widely-used methods are described:
Brushing, Spraying and Pouring Treatments: With these methods, creosotes, oil-borne preservatives, or water-borne salts are applied to the surfaces of the wood product o be treated. The wood should be thoroughly air-dried before treatment and if oil-borne preservatives are used; the wood should be warm enough to avoid congealing the oil.

• Dipping: Treatment by dipping consists of immersing wood in a preservative solution for several seconds or several minutes. As with brushing-type treatments, the wood should first be well dried before treatment. Although, dipping is better than brushing for penetration of preservatives into the checks and cracks of wood surfaces and may add 2 to 4 years of protection over untreated wood, dipping is unsatisfactory for uses subject to abrasion. Probably the principal use of dipping is for window frames employing an immersion of 3 minutes.

• Cold Soaking: Cold soaking is commonly used for treating round, fence, posts and sawn timbers using pentaenlorophenol or other viscous, oil-borne preservatives. The process involves the soaking of dried wood for 2 – 7 days in a vat containing the unheated liquid oil preservatives. Cold soaking has been popular for farm use because of its simplicity and low cost.

• Steeping: The steeping process employs a water- borne salt preservative solution to either dry or green wood. It consists of submerging the wood in a tank full of the solution at atmospheric temperature for several days or weeks (heating the solution would speed up penetration). Absorption is rapid the first 3 days then continues at a decreasing rate almost indefinitely, when treating flat-sawn, wood products space should be provided between and around each piece of wood to permit complete exposure to the preservative material.

• Hot and Cold Bath (Thermal Process): The hot and cold bath or thermal process, also called the boiling and cooling or open-tank treating method, is suitable with oil-based and water-borne preservatives. When used properly, the method provides a reasonably effective substitute for pressure impregnation. The process is quite simple involving the use of one or two tanks. With two tanks, the wood product first is immersed into a hot solution usually of the preservative, itself or even boiling water, followed by its immersion into a tank of cold solution.

• Double Diffusion: Treatment by double diffusion is a two dispersing of a preservative liquid into a piece of wood. An example of the process would be to first soak a green wood product, such as a post, in a solution of copper sulfate.

• Vacuum Process: In the vacuum process, wood products are enclosed in an airtight container from which air is removed with a vacuum pump. The container then is filled with the preservative without additional pressure and without the air-entering. The partial removal of air from the wood, by the vacuum, followed by addition of the preservative creates a slight pressure that derives the preservative into the wood.