Chemical Composition of Wood

Chemical Composition of Wood PowerPoint PPT Presentation

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--Cellulose: Cellulose is a long linear molecule with over 7,000 glucoses linked end to end. Bundles of cellulose molecules fasiculate laterally (by hydrogen bonds to form microfibrils. Microfibrils are the framework of the cell wall and are responsible for the s

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Chemical Composition of Wood

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1. Chemical Composition of Wood Species Cellulose Hemicellulose Lignin Softwoods 43% 28% 29%** Hardwoods 45% 34%** 21% ** These differences are significant Chemical composition of crop residues similar to hardwoods

3. Protection of Wood and Wood products Biological Deterioration of wood -- Woods containing fungistatic extractives, such as western redcedar and redwood, are durable against fungal and insect attacks. Fungal Decay --Brown Rot: Brown-rot fungi decompose carbohydrates (cellulose and hemicelluloses) and use them as foods and leave lignin behind; wood may loss as much as 70% of its weight and all of its strength --White Rot: White-rot fungi may decompose carbohydrates and lignin simultaneously or sequentially (lignin first), and infested wood appears to be bleached. --Soft Rot: Soft-rot fungi attacks moist wood slowly resulting in a spongy wood surface

4. Brown Rot

5. White Rot

6. Soft Rot

7. Wood-Destroying Insects Termites: This group of insects is responsible for destruction of wood in use because they feed on wood; the major types are subterranean termites and drywood termites Beetles: This group is next in economic importance; insects in this group mainly attack stressed or recently felled trees; only few of them seriously attack wood in use, such as powder-post beetles. Bark beetles attack and kill stressed trees by girdling the inner bark; they bring staining fungi into infested woodand cause blue stain of the wood. Carpenter Ants: They do not feed on wood, only nest in wood. They make nests by carving out decayed or partially decay wood to build the colonies. Carpenter Bees: They also do not feed on wood; usually nest in dead branches.

8. Termites and Ants Recognizing termites and ants

9. Termites Subterranean Termites Can only stay alive in humid environment; nest in soil and build tunnels to reach wood in houses; presence of active tunnels around the foundation of buildings indicates their attack. They consume only the softer earlywood, leaving harder latewood behind. Drywood Termites Do not depend on liquid water, they get water from digesting wood; nest in dry wood; very difficult to detect their presence but sometimes may find fecal materials in corners; must find professional help once active colonies are found. They consume both earlywood and latewood, leaving only an empty shell.

10. Wood Damages by Beetles

11. Damages by Carpenter Ants and Bees

12. Management of Decay and Insects Measures against Decay Use dry and decay-free wood Use durable or preservative-treated wood for places of high hazard. Keep woodwork dry (< 20% MC) Good designs for dryness and good ventilation in foundation, basement and attic. Frequent inspection. Measures against Insects Use kiln-dried wood (heat kills them) Keep woodwork dry. Avoid direct contact with soil; use treated wood if contact is necessary. Clean rotted or insect-infested wood, scraps and stumps around buildings

13. Thermal Degradation of Wood Fire destroys more wood in use than fungi and insects combined, and building fires also cause loss of human lives. Exposure of wood to temperatures below 200 oC for some time causes permanent loss of wood strength as discussed. The following events occur when wood is exposed to high temperatures: --Pyloysis: Tthermal degradation in the absence of oxygen; wood substances are degraded into gases and oil, leaving a surface charcoal layer. The charcoal layer may act as a insulation to prevent further damages from external heat. --Combustion: It is burning of flammable gases evolved from pyrolysis on the wood surface. Ignition of wood depends on surface/volume ratio, degree of confinement and temperature (generally 200 oC, could be as low as 66 oC) --Growing: It is flameless burning of charcoal in two steps; in the first step charcoal is oxidized to carbon monoxide (CO), followed by further oxidation of CO to produce CO2 large amount of heat. --Smoking: Smoke is an aerosol of gases, small oil droplets, charcoal particles and water vapor. It is the most deadly part of a building fire. Wood & wood products often are not the culprit to start a building fire, but they are combustible and always contribute to spread the flame. Treat them with fire retardants reduces flame spread.

16. Performance of Wood Beam Under Fire

17. Preservative Oils Creosote --The first wood preservative for treating railroad ties --The main ingredient is coal tar (distill of raw coat) --Often mix with wood tar or oil tar to reduce cost and viscosity --May be fortified with penta and copper naphthanates --For treating railroad ties, utility poles and fence posts

18. Oil-borne Wood Preservatives Penta (pentachlorophenol): --soluble in organic solvents and different grades of oils --often prepared as 5% solution to treat wood -- Due to its toxicity interior use of penta is prohibited Copper naphthanates and Copper-8-quinolinolate --Low mammalian toxicity and leachability, but expensive --Suitable for treating wood for human contact --Also used as dip treatments to prevent molds Biocides (IPBC,tubeconazoel and propiconazole) --Low mammalian toxicity and leachability, but expensive --Dip treating fine woodworks for interior uses

19. Water-borne Preservatives CCA (chromated copper arsenate): --The most used water-borne preservative in the past (1930s to 2003) --Treating solution contains chromium compounds, copper sulfate and arsenate --Chemicals are stable in treated wood if well seasoned --CCA-treated lumber for frequent human contact is not allowed since 2004 ACQ (ammoniacal copper quat): --Aqueous solution containing 50% copper sulfate and 50% quaternary ammonium compounds in ammonium hydroxide; much more benign than CCA; --Replaced CCA since 2004 to treat lumber for decks an playground structures, etc. --Current problem is its metal corrosiveness. Borates: borax (Na2B4O7) and boric acid (H3BO3), --Benign to the environment but very effective against fungi and insect attacks --Because of water solubility they can be leached out when treated lumber is used outdoors.

20. Fire Retardants Treating wood with fire retardants can not protect wood from being destroyed by fire. Under elevated temperatures, effective fire retardants accelerate wood decomposition, increase charcoal formation and reduce production of flammable gases. Effective fire retardants are those contain at least one of the elements phosphor (P), nitrogen (N), boron (B) and chlorine (Cl), such as ammonium phosphates (NH4H2PO4 and (NH4) 2HPO4), borax (Na2B4O7) and zinc chloride (ZnCl2). To be effective, Wood must be treated to a high loading of fire retardants, more than 2 pounds/ft3. Most inorganic fire retardants can cause chemical degradation of wood when the treated wood is used in warm and humid conditions. It is more desirable to treat wood with combinations of chemicals so that water-insoluble organic compounds containing P, N, B or Cl are formed in wood. These water-insoluble organic fire retardants would not harm the wood under warm and humid conditions, but under very high temperatures will breakdown into components to perform the tasks of wood decomposition, charcoal formation and reduction of flammable gas evolution.

21. Preservative and Fire Retardant Treatments Pre-treatments --Poles, pilings and lumber must be dried (water removed) to accept treatments. --All machining done before treating: There is a limit how deep the treatments can penetrate into the wood but after treating the treatments form a protective shell. If machining is done after treating the protective envelope would be broken, also wastes the treatments and creates a problem of disposing the wastes. --Some species of are very difficult to treating, therefore the surfaces of large wood members such as poles, pilings and railroad ties are incised to facilitate penetration.

22. Preservative and Fire Retardant Treatments Full-Cell Process: When done the wood cells are filled with treatments; for maximum treatment (> 2 lbs/ft3); fire retardant treatments is done with this process.

23. Preservative and Fire Retardant Treatments Empty-Cell Process: When done the cell walls are coated with treatments; usually used for preservative treatments (~ 0.2 to 0.5 lbs/ft3).

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