As continuing of the latest topic, this post will show you some more disadvantages of wood and the way to Minimize the Problems of Wood.
Most of the commonly employed strategies for protecting wood involve drying, coating and or impregnation.
Careful selection of wood
Some species have naturally decay resistant heartwood. Such species include sweet chestnut (Castanea sative Mill.), oak (Quercus spp.), juniper (Juniperus spp.). Sapwood is never naturally durable species has little or no decay resistance and must be treated if long-term durability is desired.
Coating provides protection to wood used both indoors and outdoors. Coating prevents rapid uptake and loss of moisture and reduces shrinking and swelling that can lead to surface cracking and other problems. But coating does not totally prevent changes in moisture content. Coating slows, but does not halts moisture level. Coating with solid color or pigmented stains protects wood against ultraviolet rays.
The addition of fungicides to coating provides some protection against development of decay and mold fungi.
Deteriorating paint film actually increases the decay hazard. Cracked paint allows moisture to come into contact with wood surface, and poses a barrier to rapid and complete redrying.
Generally wood will not be attacked by the common fungi at moisture content below the fiber saturation point (FSP). FSP for different wood lie between 20-35%, but 30% is accepted generally: Fungi can not attack wood used indoor and in heated rooms, since the equilibrium moisture content (EMC) is much more below than FSP. e.g. 6%
If wood is soaked in water, wood absorps water and is saturated with it. Finally there will be no more oxygen in wood. In this situation fungi can not grow in them. This is the main reason why woods are kept in water for a while. Besides underwater constructions, it is impossible to use woods completely wet; so when they are used out of water, they have to be completely dried out to EMC in order to protect them against fungi attack. In heated rooms, where the EMC lie between 5-10%, fungi can not survive on them.
One of the most effective ways to prevent degradation of wood is to thoroughly dry it and keep it dry. The last case is very important since even wood that has been klin dried will readily regain moisture if placed in a hamid enviroment.
Wood can be dried in air or in some type of dry klin. Air drying alone is not sufficient for wood items which are used in heated rooms. Therefore klin drying is necessary. Kiln drying has many advantages: One of them is the killing of staining or wood destroying fungi or insects that may be attack the wood and lower its grade.
Wood that will be used indoor need only be dried to provide for long term protection against rot.
Treating With Wood Preservatives
We can prevent decaying of wood by treating it with wood preservatives. But some of the wood preservatives may harm humans and other creatures. For this reason if wood is used outdoor in situations where it is often wet or in close proximitly to liquid water, then wood must be treated with wood preserving chemicals to achieve long term durability.
Wood preservatives are divided into two groups: Waterborne and oilborne chemicals.
About %75 of wood that is commercially treated today is treated with waterborne salts, and CCA is the compound used in treating for the greatest volume of wood.
Only creosote and pentachlorophenel are effective protecting wood in direct ground contact. These are also the only two oilborne preservatives that provide general protection against decay causing fungi, termites, marine borer and other insects.
Oil based or oilborne preservatives are generally used for treating of wood used outdoors in industrial applications; such as ties, piling and poles.
In a serious situation, wood is treated with waterborne preservatives for example chromated copper arsenate and, after thorough seasoning, is retreated with creosote.
Wood in service must be periodically retreated by brushing or a variety of other methods.
Retreatment of wood window frames, door frames and wood timber and beams is sometimes carried out by drilling holes in areas where decay has begun and filling these holes with a suitable treating compound. Treating compound in the form of solid rods are mostly preferred since it provides a slow release of active ingredients.
Retreatment of wood used in ground contact must be realized by application of pastes and wrapping with preservative impregnate bandages.
Abiotic Deterioration of wood
Another disadvantage of wood is that it easily catches fire. Wood consists of organic compounds which are composed mainly of carbon and hydrogen. They can combine with oxygen and burns. Because of these properties, wood is classified as a combustible material.
If the temperature of a inflammable gas is between 225°-260°C, it burns with a touch of flame. After the withdrawal of flame it will stop burning. If the temperature increases to 250°-270°C, it burns with a touch of flame and goes on to burn without a flame. If the temperature increases to 330°-520°C, wood begins to burn spontaneously. Chemical materials, especially extractives in woods structure cause the burning point to change. For example, a resinous piece of pinewood can catch fire in lower temperatures. In addition to this, specific gravity and surface mass (m2/kg) affect the duration of flame. Wood burns harder when the specific gravity and surface mass and moisture content increase, and vice versa.
Using thick wood as a structure element is another way of extension of burning point. Outer surface burns and turns into charcoal. Charcoal, which forms on the surface of wood as it burns is a very effective heat insulator. Therefore large timbers burn very slowly. In addition to this, wood is very good heat insulator too. The outer surface of the wood is 1000°C and the interior part is still 40°C when a piece of thick wood is burning. For this reason, buildings with thick structure elements such as beams and columns do not collapse easily on fire. On the other hand, in steel constructions, as heat increases, steel faces deformation, and their resistance decreases and collapses, where wood is used preventive measures must be taken for safety against fire. In this case wood is not a dangerous material.
It is impossible to make wood noncombustible like inorganic materials. In order to prevent potential dangers, wood can be processed in some fire retardants.
Fire retardants may be divided into two categories: Coating and chemicals-water soluble salts-that are impregnated into the wood structure.
Coatings are used to reduce the formation of volatile, frammable gases by promoting rapid decomposition of the wood surface to charcoal and water. They also protect wood surface against high temperature water soluble salts e.g. diammonium phosphate, ammonium tetraborate, sodium acetat, alkali silicates, borax are used against fire hazards in wood. Wood can be impregnated by these chemicals. This type of process can contribute to the increase of the burning point and retard spread and penetration of flame.
Fire retardants only reduce the flammability of wood and slow or eliminate progressive combustion. They do not prevent burning totaly in the presence of an external source of fire. In this case, wood does not go on burning once an external source of flames is removed.
Prof. Dr. Ramazan ÖZEN
President, Zonguldak Karaelmas University
HOT LINE: 0866 831 049 – Ms TU
PHONE: 0903 666 014 – Mr DUC