Review of Methods of Timber Preservation

Introduction

The biological beginning of the wood makes more susceptible for the foreign being onslaught and impairment due to them. Wood can be protected from the onslaught of decay Fungis, harmful insects or Marine bore bits by using selected chemicals as wood preservatives. The grade of protection obtained depends on the sort of preservative used an on accomplishing proper incursion and keeping of chemicals ( US Department of Agriculture, 1974 ) . The aim of wood saving is to present the preservative in to the wood so that a deep uninterrupted bed of treated wood contains sufficient preservative to forestall decay and insect onslaught ( FAO,1986 ) .

Timber saving methods

Non force per unit area intervention

Non force per unit area procedures are carried out without the usage of unreal force per unit area under the atmospheric force per unit area ( FAO, 1986 )

• Brush intervention

The simplest method of using a preservative is brushing and is usually used for continuing little single points ( FAO, 1986 ) . Brushing is convenient manner of using a wood preservative to little single points and it is desirable to use preservative to timber already in situ in a edifice ( Findlay,1985 ) .

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• Spraying

Spraying is convenient method of using preservatives to any big countries. ( Findlay, 1985 ) .

Spraying offers more broad and effectual covering of the lumber than brushing. The possibility of the preservative penetrating in to holes, clefts, splits is more in spraying ( FAO, 1986 ) . Brush application is absolutely satisfactory when it is necessary to use a superficial 13 coating of a high viscousness fluid, such as a pigment or varnish but even so the burden on the wood surface is merely approximately 25 % of that which can be achieved by a simple spray application ( Richardson, 1978 ) .

• Hot and cold unfastened armored combat vehicle method

In this procedure lumber is immersed in a bath or preservative which is heated for few hours and allowed to chill while the lumber is still submerged in the liquid. During the warming period the air in the cells expands and much of it is expelled as bubbles. During the warming period the air in the cells contracts making a vacuity and the preservative is drawn in to wood. Therefore practically whole of the soaking up takes topographic point during the cooling period ( FAO, 1986 )

• Sap supplanting method

Sap supplanting method can merely be applied to round lumbers in green status and uses the hydrostatic force per unit area due to gravitation to coerce the preservative from the butt terminal of the unit of ammunition lumber. In this method a cap is fitted to the butt terminal of a freshly sawn pole or unit of ammunition lumber and so one terminal of a flexible tubing is connected to the cap and the other terminal to a armored combat vehicle incorporating the preservative at a topographic point every bit high as possible. There are alterations to this procedure such as the application of a vacuity from the top terminal ( FAO, 1986 ) .

• Diffusion intervention for wood

Diffusion intervention with B compounds is the 1 of the simplest, cheapest and an effectual manner of protecting wood from biodegradation ( Dhamodaran and Gnanaharan, 1996 ) .

Preservative intervention of wood by diffusion involves the motion of molecules by random gesture from parts of high concentration to parts of lower concentration. This is typically slower method of intervention. This involve some consumption of preservative by majority flow and thenceforth preservative is farther distributed by diffusion. The rate of diffusion is straight relative to the concentration gradient as expressed for steady province diffusion in FickaˆYs jurisprudence of diffusion. Under ideal status this can be mathematically expressed as: The rate of diffusion rises with additions in temperature because there is greater available energy to get the better of the kinetic force keeping a molecule stationary. In add-on ion speed will increase.

Application of FickaˆYs jurisprudence indicates that after a period of diffusion the preservative concentration at a given deepness is relative to the concentration of the original applied solution. In practical applications duplicating the concentration of preservative constituent dosage non doubles the deepness of incursion in a given clip. Surveies indicate that rates of solute diffusion in wood follow the undermentioned ranking: longitudinal tangential, which is besides noted for majority flow ( Eaton and Hale, 1993 ) .

The longitudinal diffusion coefficients were 10 to 20 times larger than the radial diffusion coefficients, and the radial diffusion coefficients were 2 to 4 times larger than the digressive diffusion coefficients. The longitudinal diffusion rate increased quickly with wet content ( MC ) , while a slow addition in the radial diffusion rate was observed at MCs above 90 % . MC did non impact the rate of digressive diffusion within the scope of MCs experimented ( 70- 110 % ) . The consequence of temperature on B diffusion rates was more marked than the consequence of MC. The diffusion rate increased with temperature, although little way dependant differences were observed ( Caldaria, 2010 ) . The lumber merely after the submergence is closed block stacked and wholly covered with a polythene sheet so that drying is prevented every bit much as possible ( FAO, 1986 ) .

• Steam cold quench method

Steam cold quench method is presently non practising in Sri Lanka which is practising in Australia for intervention of rail manner slumberers. The methodological analysis of this intervention method is 4 hr steam of wood and so flooded with preservative solution and kept to the full immersed over dark or for about 5-15 hours depending on the thickness of lumber ( Findlay 1985 and Negi,1997 ) .

Pressure interventions

Using a positive external force per unit area to coerce the liquid in to the pores of wood ( Findlay,1985 ) . There are several types of high force per unit area processes some utilizing lone force per unit area and others utilizing vacuity and force per unit area ( FAO, 1986 ) .

• Bethell or Full cell procedure

This is the normal procedures used when handling with H2O born solutions ( FAO, 1986 ) . The chief stairss in the full cell procedure are

1. 1. 1. The charge of wood sealed in the treating cylinder, and a preliminary vacuity is applied for & A ; frac12 ; hr or more to take the air from the cylinder and every bit much as possible from the wood.
      2. The preservative, antecedently heated to slightly above the coveted treating temperature, is admitted to the cylinder without admittance of air.
      3. After the cylinder is filled force per unit area is applied until the needed preservative is obtained.
      4. When the force per unit area period is completed the preservative is withdrawn from the cylinder ( US Department of Agriculture,1974 ) .

• Empty cell procedure

There are two chief types of empty cell procedures which are called rueping and lowery procedures. In both procedures there is no initial vacuity applied, the preservative is forced in to the wood under force per unit area and later a vacuity is applied to take the surplus of the preservative. This procedure is usually used with tar oil preservatives. The chief stairss in the ruepning procedure are ;

1. 1. 1. Preliminary air force per unit area applied
      2. Fill cylinder/ keep air force per unit area
      3. Construct up force per unit area
      4. Maximal force per unit area held
      5. Release force per unit area
      6. Empty cylinder of preservative
      7. Concluding vacuity period
      8. Release vacuity

• And the chief stairss in the lowery procedure are

1. 1. 1. Fill cylinder with preservative at atmospheric force per unit area
      2. Construct up force per unit area
      3. Maximal force per unit area held
      4. Release force per unit area
      5. Empty cylinder of preservative
      6. Final vacuity preservative
      7. Release vacuity ( FAO, 1986 )

• Low force per unit area procedure
• The chief stairss of this procedure are,

1. 1. 1. Application of an initial vacuity
      2. Deluging with preservative
      3. Application of force per unit area
      4. Release of force per unit area
      5. Application of concluding vacuity

The above stairss are really similar to the stairss employed in the full cell procedure except the force per unit area applied is much less ( FAO, 1986 ) .

• Double vacuity procedure

A method has been called the dual vacuity procedure, involves exposing the to the full machined pieces at a wet content non transcending 25 % to an initial vacuity, followed by a short period at atmospheric or somewhat raised force per unit area completed by a concluding vacuity which is a kind of cleansing operation to take excess fluid from the cell pits ( Findlay, 1985 ) .

Timber preservatives

Wood preservatives are chemical substances which, when ab initio applied to wood, do it resistant to assail by disintegrating agents. The protective consequence is achieved by doing the wood preservative or repellant in to the beings that would assail it ( FAO,1986 ) .

An ideal wood preservative should hold following features

1. Lethal toxicity of the preservative should be high even in the low concentration against Fungis, insects and marine beings and low degree of deadly effects for mammals
2. The permanency of the wood preservative in the treated wood during assorted utilizations, such as opposition to
   1. • Leaching by H2O
      • Quick vaporization due to the heat
      • Chemical transmutation caused by oxidization, decrease and polymerisation
      • Chemical or enzymatic action doing a lowering of toxicity degree
3. High degree of amenableness to infuse in the full cross subdivision of the wood that is being treated in order to give a more permanent consequence
4. Preservative should be stable during the intervention
5. The inflammability of treated wood should non increase by the preservative
6. Make non do inauspicious effects on homo
7. The cost of the preservative should be low and readily available
8. Ease of transit over long distances in wood
9. The preservative must non do corrosion in the metals that are used in the equipment used for wood saving
10. The picture and varnishing of the treated wood should non forestall by the preservative
11. The strength and the other belongingss of the wood should non be adversely affected by the preservatives ( Negi,1997 )

None of the current used preservatives meet all these demands. Preservative effectivity is influenced by the protective value of the preservative chemical itself, the method of application and extent of incursion and keeping of the preservative in the treated wood ( US Department of Agriculture, 1974 ) .

Wood preservatives are normally classified in to four chief groups ;

• Oil type such as creosote
• Organic dissolver type
• Non-fixed H2O soluble type
• Fixed H2O soluble type

Oil type preservatives

There are figure of oil type preservatives such as creosote, coal pitch creosote. Creosote is a brown-black oily liquid that comes from the pitch produced during the distillment or the carbonisation of bituminous coal ( FAO, 1986 ) . Creosote is loosely defined as those fractions of distillations from coal pitch that furuncle between 200-400 & A ; ordm ; C. It is complicated mixture of a big figure of organic compounds. The comparative proportions of these depend on the composing of the original coal pitch and the method by which it was carbonized

Its advantages are ;

1. High toxicity to wood destructing beings
2. Relative unsolvability in H2O and low volatility, which impart to it a great grade of permanency under the most varied usage conditions
3. Ease of application
4. Ease with which its deepness of incursion can be determined
5. General handiness and comparative low cost
6. Long record of satisfactory usage ( US Department of Agriculture, 1974 )
7. Impregnation with creosote provides excess protection against wet content alterations, so that treated wood is stable and really immune to splitting.
8. It is non usually caustic to metals and has a high electrical opposition.

They have belongingss that are disadvantages for some intents.

1. The colour of the creosote and the fact that creosote-treated wood normally can non be painted satisfactorily do this preservative unsuitable for finish timber or other timber used where visual aspect and paintability are of import.
2. The olfactory property of the creosoted wood is unpleasant to some individuals. Besides creosote bluess are harmful to turning workss, and groceries that are sensitive to olfactory properties should non be stored where creosote olfactory properties are present.
3. Newly creosoted lumber can be ignited easy and will fire readily, bring forthing a heavy fume.

Organic dissolvers

Organic dissolver preservatives are really active chemicals that are extremely effectual as antifungals and insect powders. The chemicals are dissolved in organic dissolvers before application. The of import and normally used organic solvent type preservatives have been described below.

1. Benzene hexachloride ( BHC )
2. Dichloro-diphenyl-Trichloroethane ( DDT )
3. Synthetic pyrethroides
4. Metallic soaps
5. Precipitated soaps
6. Fused soaps

• Water soluble type

Waterborne preservatives leave the wood surface relatively clean, paintable and free from obnoxious olfactory property ( US Department of Agriculture, 1974 ) . These preservatives may farther be of two types:

• Leaching type

These are inorganic or organic salts that are soluble in H2O. They are suited merely for lumber which has to be used indoors as the preservatives may readily leach out when it contact with rain H2O. Important preservatives in this class are:

1. Zinc chloride
2. Boric acid and borax
3.  Sodium pentachlorophenate
4. Benzene hexachloride

• Fixed type

1. Copper-Chrome- Arsenic ( CCA )
2. Acid-Cupric-Chrome composing ( ACC )
3. Chromated Zinc Chloride ( CZC )
4. Copper Chrome boracic composing ( CCB ) ( Negi, 1997 ) 20

Boron

The H2O diffusing belongingss of B and its stableness at comparative high temperatures, it can be applied utilizing a broad assortment of intervention procedures than most other wood preservatives such as CCA or creosote ( Negi, 1997 ) . In malice of many advantages of borates boron intervention did non go established as a preferable method of protecting lumber because of the leachability of B ( Dhamodaran and Gnanaharan, 1996 ) but boron have favorable environmental features lower wellness impacts ( Calderia, 2010 ) .

Boron-based systems are campaigners for the future scope of preservative preparations with lower environmental and wellness impacts. Boron compounds have been used for over 40 old ages in Australia and Europe. The chief compound being used in the United States is disodium octaborate tetrahydrate [ Na2B8O13.4H2O ] ( B ) , which is soluble in H2O, is colourless and diffuses readily into green wood or can be force per unit area impregnated into dry wood. Borates have favourable environmental features, but their high susceptibleness to leaching is the chief obstruction to the widespread usage of B as a major constituent of wide spectrum wood preservatives ( Calderia, 2010 ) .