Fishing Craft Technology

Unit 4 - Boat building materials and construction of boat

4.4. Preservation of wood
In view of the limited availability of naturally durable species of timber like teak and Sal there is need to utilize less durable species of wood. This is commonly known as secondary timbers, which are used for boat construction. These timbers if properly selected, seasoned and treated with preservatives will be more durable and economical for boat construction. Treatment of timbers with preservatives is known as timber treatment.

Objectives

• To increase the life of timber structure
• To make the timber structure durable
• To protect the timber structures from the attack of destroying agencies such as fungi, insects etc.

Many timbers used in boat building yards are susceptible to fungal infection, attack by white ants, beetles and marine fouler and borers unless treated with a suitable preservative. Hence to increase the serviceability, the treatment of timbers is highly essential. While white ants like fungal infection attack interval timber structures, the marine borers attack from outside below the water lines.

1) Requirements of a good preservative
The requirements of good chemical preservatives should be

• High toxicity to fungi, termites beetles, marine borers and fouler.
• Good retention for considerable time.
• Non corrosive to metals
• Should not adversely effect the strength of wood
• Safe and easy to handle
• Free from fire hazards
• Good penetration
• Abundant supply
• Economical to use
• Should not taint fish and free from health hazards for human beings

2) Types of preservatives
There are three types of preservatives

• Oil type
• Organic solvent type
• Water borne type

i) Oil type:
In this group byproducts of oils such as coal tar creosotes, creosote petroleum solutions, creosote coal tar solution and lignite tar oils are included. The coal tar creosote is specially suited for exterior application. Coal tar is seldom used alone for preserving wood as with its use good penetration is usually difficult and it is less poisonous to wood – destroying fungi other than coal tar creosote. Coal tar creosote is normally used with a mixture of petroleum. The petroleum gives stability to creosote against evaporation and also protects the timber from splitting and cracking.

Advantages:

• Highly toxicity to word – destroying organisms
• Ease of application
• General availability and relatively low cost
• Non-corrosive

Disadvantages:

• Satisfactory painting is difficult
• Unpleasant odour
• Involves risk of fire
• Weight increases
• Taints fish

ii) Organic solvent type
These preservatives are used after dissolving them in suitable organic solvents. The choice of solvent depends on suitability of the preservative and the use to which the treated timber is to be put. The preservatives prepared by dissolving toxic chemicals in low cost solvent oils are also included in this group. The solvents may be heavy petroleum fuel oils, light fuel oils or any other cheap oil. The principal toxic chemicals used are pentachlorophenol, copper napthenate and tributyltinoxide. Oil solutions of toxic chemicals has an important advantage over byproduct oils in that their effectiveness is due to definite chemical compound, the concentration of which can be controlled as desired and determined accurately by chemical analysis.

Advantages:

• Treated timbers can be painted waxed or polished.

Disadvantages:

• As some of the solvents are highly inflammable, care should be taken while, handling the solution. In such cases the preservatives should preferably be applied cold expensive.

iii) Water borne type:
These preservatives include (a) Acid copper chromate (b) Chromated copper arsenate (c) Chromated zinc chloride. These preservatives are often employed when cleanliness and paintability of the treated wood are required. Water borne type preservatives are inorganic salts soluble in water.

Several formulations involving combinations of copper, chromium, arsenic etc., show a high resistance to leaching and good performance in service. These preservatives consist of a mixture of various salts with the addition of a fixative salt –usually sodium dichromate or potassium dichromate. The effect of chromium is to fix element, arsenic, copper zinc etc., is the wood so that the toxic salts are not leached by the action of water. It is however, necessary to allow timber to dry for 3-6 weeks for the fixation process to be complete.

Advantages:

• Easy to handle
• Available and cheap
• Treated timber can be painted and sheated
• Free from fire and health hazards
• Treated wood light and dry

Disadvantages:

• Unstable at high temperature hence application should be at low temperature
• Treated wood needs re drying
• No protection against weathering and mechanical wear
• Dissolves in course of time.

3) Selection of preservatives:
The type of preservative used depends on the working conditions. Water borne preservatives are best suited when cleanliness, freedom from odour, low increase in weight and relatively low cost are desired. Tributyl tin derivatives are found more suitable for fungi, insect attack as well as antifouling paints. Penta chloro phenyl is found to be having more penetration into wood than the copper napthanate though it may not be as effective as the latter. Organic solvent types are needed where painting has to be carryout and where cost is not a limiting factor. By products of oil are recommended where high toxicity is needed, painting is not required and where weight is not a factor.

4) Choice of treatment:
The choice of treatment is governed by the timber species, its sap wood content and the use to which it is to be put. The treatment is necessary for sap wood of all species of timber heart wood of non-durable species, heart wood of durable species if the timber members are to be placed on the ground or are required to give long life under severe conditions of service. In the case of marine structures all timbers should be treated certain timbers are resistant to treatment. Based on the degree of penetration timbers are classified as follows for the purpose of treatment.

• Easily treatable (100% penetration).
• 75-100% penetration (gurjan)
• Partially treatable 50-75% (siris, kindal)
• Difficult to treat – 25-50% (Dhaman)
• Not treatable -0-25% (teak and venteak)

5) Types of treatment
The following are the methods used for treatment for timbers.

• Surface application
• Soaking treatment
• Hot and cold process
• Boucheries process
• Pressure process
• Diffusion process

1) Surface application:
This is done either by brushing, spraying or dipping in the preservative, for a short period. For this treatment the timber is debarked thoroughly. For oil type preservative the moisture content in timber shall not be more than 14% with aqueous solutions, moisture content of 20-30% is permissible. At least two coats should be applied; the initial coat has to dry up before application of subsequent coats. This type is used mostly for treating material at site and for treatment of cut surface.
2) Soaking treatment:
In this treatment, the timber is debarked thoroughly and the treatment is carried out by submerging the timber in the preservative solution for a sufficiently long period until the required absorption of preservatives has been obtained.
3) Hot and cold process:
In this process, the timber is submerged in the preservative oil or solution which is then heated to 90oC and maintained at this temperature for a suitable period depending on the change. It is then allowed to cool until the required absorption of preservative is obtained. During the heating period, the air in the timber expands and is partially expelled during cooling the residual air in the timber contracts creating a partially vacuum which causes the preservative to be sucked into the timber.
4) Butcheries process:
This process is applied for the treatment of sap wood of green timbers soon after felling using any of the inorganic water soluble preservatives. The treatment is carried out by attaching to the butt end of a pole with back on, a rubber hose connected to a reservoir containing preservation solution and placed at a sufficiently high level. The pole is held in an inclined position, generally at an angle of 45oC to the horizontal with the butt end up. Due to hydrostatic pressure the preservatives displaces the sap in the timber which is then forced out at the narrow end. The treatment is stopped when the concentrating of preservative in the drip is nearby the same as that of the solution in the reservoir.
5) Pressure process:
This is a process of treating wood in a closed container of preservative. It is forced into the wood under pressure greater than one atmosphere pressure and is generally preceded or followed by vacuum. The pressure process may be done either with oil type or with water soluble type preservatives. In the former case a temperature of 80-90oC should be maintained during the pressure period.

There are two methods of pressure process. (1) Full cell process (2) Empty cell process

The full cell process:
It is used when maximum absorption of the preservative is desired. The timber is introduced into the cylinder. The door is closed, and then a vacuum at least 56 cm of mercury is created and maintained for half an hour. At the end of the vacuum period, the preservative is introduced into the cylinder, with the vacuum pump working. When the cylinder has been filled with the preservative the vacuum pump is stopped and the cylinder is subjected to 3.5–12.5 kg/cm2 pressure depending on the species and size of timber. After the desired absorption is obtained the preservative is withdrawn from the cylinder and finally a vacuum of 38-56 cm of mercury for about 15 minutes is once again applied to free the timber from dripping preservative.

Empty Cell Process:
It is a process for impregnating wood with preservatives or chemicals in which air imprisoned in wood under pressure expands when pressure is released to drive out part of the injected preservative or chemical. The distinguishing feature of empty cell process is that no vacuum is drawn before applying the preservative. The cell cavities are only partially filled.

Diffusion process:
This process is used in the case of timbers which are not easy to impregnate under pressure in dry conditions and also when there is danger of the timber getting deteriorated during seasoning. The depth of penetration and the amount of preservatives absorption depends on the concentration of preservative, period of diffusion species of timber and its moisture content. The preservatives are applied in different ways. Momentary dipping of timber in concentrated solution or prolonged immersion in dilute solution or application of paste over all the surfaces of timber and then these are close slacked under cover.