Q.
a) In timber construction the preservation of timber has to be carried out to increase the life of timber. Discuss the different types of preservatives use for timber. (12 marks)
b) Explain in detail using diagrams the construction of suspended timber ground floor. (13 marks)
(25 marks, 2018 Q2)
A.
a) Types of preservatives use for timber.
One of the disadvantages of wood is dimentional instability by the changes in relative humidity of air. To prevent the different changes in different dimentions and improve some properties wood is treated with.
a) water-repellent formulations
b) phenolic resins
c) polyetlhylene glycol
d) monomers polymerising in the wood (wood-polymer composites)
e) acetic anhydride with catalyzer for acetylation of the material.
See earlier posting here by the Republic of Turkey Ministry of Culture and Tourism at
http://www.kultur.gov.tr/EN,35286/preservation-of-wood-material-by-chemical-techniques.html
WOOD PRESERVATIVES :
WOOD PRESERVATIVES wood preservatives is a solution or mixture or compound used for preservative treatment of wood. This preservative treatment involves the placement of preservative chemicals which are antagonistic to wood destroying agencies like termites fungal decay, and etc...
Types of preservatives :
Types of preservatives - oil type, organic solvent type, water soluble type, leaching fixed
Specific Types of Preservatives :
Specific Types of Preservatives Modified creosote based preservatives. Light organic solvent based preservatives e.g. Protim, Vacsol, Impresol, Cuprinol. Copper chrome arsenic preservatives (fixed turbine preservatives) e.g. Tanalith, Celcure, Sarmix. Preservative compounds of boron or fluoride e.g. Polybor, Sodium Fluoride.
Oil types - creosote :
Oil types - creosote coal tar, wood tar, water gas-tar. widely used creosote-coal tar creosote. a product of distillation of coal-tar consists of a variety of products – tar acids, the tar bases and neutral hydrocarbons. used in admixture with petroleum oils in the ratio of 50: 50 by volume weight. High durability, protect timber against splitting
creosote :
creosote It is highly toxic, oily in a water proofing medium. odour and colour are objectionable. Its defects such as unpaintability of timber, area hindrance. Solignum and creosant are two proprietary preservatives containing high boiling creosote as a base. Coal tar is used widely than wood tar
Copper naphthenate :
Copper naphthenate Oil-borne preservative provides added durability and waterproofing. Semi-transparent color reveals natural wood grain. Steam cleaning after treatment removes surface oil residue for clean handling. Non-corrosive to hardware. All common wood can be used for treatment.
Water soluble type :
Water soluble type highly toxic to fungi low permanency on account of high solubility. Easily washed on exposure to rain corrodes iron bolts or fastenings, which have to be galvanized Two types Leaching type Fixed type
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Leaching type In organic or organic salts,soluable in water. It may be readily leach out when in contact with rainwater. Boric acid & borax Toxic to all types of organism used in wood panel products like plywood & for it’s penetration properties Sodium pentachloro phenate High degree of effectiveness against sap stain
Zinc chloride :
Zinc chloride cheapest of water soluble group of preservatives. highly toxic to fungi, but not permanent. leached out of wood if exposed to heavy rain. To prevent leaching, it is mixed with glue or sodium-dichromate is added to fix it. Fire retardant
Mercuric chloride :
Mercuric chloride extremely toxic to wood destroying fungi, extremely poisonous to human beings and animals, its use is not extensive. It also corrodes metals.
Zinc meta arsenate :
Zinc meta arsenate insoluble in water but soluble in dilute acids. impregnated into wood with the aid of volatile acids. after the acid has volatilized out, the preservative salts are deposited in the wood, more or less permanently.
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Fixed type Insoluble salts in the presence of wood & get permanently fixed. Treated with these can be used even under exposed condition. Take time to dry about 2-3 weeks kept Applied even room temperature. Treat with these type is very clean, can be painted, polished or waxed.
Ascu (CCA) :
Ascu (CCA) The ingredients of this proprietary preservative are: Arsenic penta-oxide 1 part by wt. Copper-sulphate CuSO4 5H2O 3 -do- Potassium or sodium- dichromate 4 -do- Combination of arsenic penta-oxide and copper-sulphate forms a good toxic preservative against wood destroying agencies like fungi, insects and borers.
Ascu :
Ascu Function of potassium or sodium dichromate, is to fix the other two chemicals in the wood so as to make them unleachable. The degree of fixation attained with this preservative is very good; it is one of the best preservative. Additional advantage of being used cold which eliminates the necessity of heating apparatus in pressure treatments.
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Acid- Cupric- Chrome Composition chemical % of weight. Copper-sulphate 47.5 to 50 Potassium or sodium- dichromate 42.5 to 45 Chromic acetate 4.75 to 5 Solid content not >95%, ph 2.7 to 0.2 Cromated Zinc Chloride (CZC) Zinc chloride -- 81.5 Potassium or sodium- dichromate –18.5
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Copper Chrome Boric Composition Copper-sulphate- 3 parts Potassium or sodium- dichromate- 4 parts Boric acid -- 1.5 parts
Celcure :
Celcure The constituents of this preservative are: Copper-sulphate CuSO4 5H2O 5.5 parts by wt. Sodium-dichromate Na2CrO7 5.5 -do- Acetic acid 0.2 -do-
Organic solvent type :
Organic solvent type consist of copper and zinc salts of organic acids as Naphthenic acid, stearic acid, abietic acid and chlorinated phenols and benzenes like pentachlorophenol and hexa chloro-benzene and chlorinated naphtenates. Used after dissolving in suitable organic solvent of low boiling point (vapourisation) They can painted and varnished. Very active chemicals, highly effective as fungicides, & insecticides. High degree of penetrability:
Penta chlorophenol - Fully chromated derivative of phenol - has lasting protective function & broad spectrum of efficacy Benzene hexa chloride ( BHC) - Fully chromated derivative of benzene - used to product from borers & termites. Dichloro diphenyl trichloroethane (DDT) - used for insecticidal & fungicidal.
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Synthetic Pyrithrides used for its insecticidal properties. most efficient for termites E.g. cypermethrin & deltamethrin. Metallic Soaps 2 types precipitated soaps copper napthenate, copper quinolinolate. fused soaps
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Copper & Zinc Preservatives Fine powers Copper & Zinc Abietate(resinate) Cashew Nut Shell Liquor (CNSL) Have 2 compounds Anacordium acid (90%)- derivative of salicyli acid Cardol(10%)- derivative of resorcinol
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Bhilaman Nut Shell Liquor (BNSL) Black ,viscous, highly vesicant dihydroxyphenol containing preservatives. Mainly used for High resistance to acid, alkalies, & heat. Good flexibility & adhesion Readily soluble in cheap & easily available organic solvents.
Creosote / Pigment Emulsified Creosote (PEC) :
Creosote / Pigment Emulsified Creosote (PEC) heavy duty preservatives used to protect such commodities as poles, sleepers, marine piles and railway sleepers against decay, insect and borer attack. complex preservatives made up of more than 300 components usually applied by vacuum-pressure impregnation . oil-borne, migrate slowly in the treated timber and may leave an oily surface finish. PEC is less likely to do this than normal creosote.
Benefits of creosote and PEC :
Benefits of creosote and PEC effective against decay, insect, and marine borers strongly water repellent giving good weather resistance very long lasting
Boron :
Boron to protect timber, such as house frames, against wood-boring insects. effective against the powder post beetle that attacks some starch rich hardwoods, the furniture beetle that can occur mainly in softwoods.
Water borne and oil type :
Water borne and oil type cleanliness, paintability, and free from odour. Waterborne treated should be dried after treatment. But mainly seasoned to prevent leaching and get impregnated. For example, while only five days are required for chemical fixations to occur in CCA-treated wood at 70° F, it takes two months for fixation when the temperature is only 35°. It is labeled as KDAT (kiln, dried after treatment). Arsenic and chromium preservatives are avoided due to new additions.
Types and their importance :
Types and their importance Modified creosote and other oil-type preservatives- heavy duty exterior work Fixed water-borne preservatives- domestic buildings, posts, poles, landscaping fencing, cooling towers, decking, cladding, etc. Light organic solvent preservatives- factory assembled joinery, e.g. window frames, elevated decking, pergolas and fascias. protection for garden furniture.
Advantages of Treated Timber :
Advantages of Treated Timber long life under hazardous conditions. cost efficient. It is versatile A variety of finishes provide additional attractiveness. provides flexibility for design and can economically overcome difficult site situations.
References :
References S.S.Negi - Wood science & technology Tribhawan Metha - A Handbook of Forest Utilization. www.wheeler-con.com www.decoratingdirect.co.uk www.pscap.net www.osmose.com/wood/usa/preservatives www.epa.gov/oppad001/reregistration/cca www.freepatentsonline.com
Ref:
Kanagaraj. Kinds of Wood Preservatives, available at:
http://www.authorstream.com/Presentation/kanagaraj007-166212-kinds-wood-preservatives-education-ppt-powerpoint/
Ref:
Earlier post.