Wood Properties and Defects Quiz

• Lecture 1 of the CM 252 course covers the structure and properties of wood.
• Structure is the architectural organization of wood, including the nature and arrangement of its physical and chemical components.
• There is a close relationship between structure, properties, and utilization of wood.

Lecture 1 Units

• Unit 1 contains information on wood classification, processing, and characteristics.
• Unit 2 contains information on drying / seasoning of wood.
• Unit 3 contains information on the physical properties of wood.

Lecture 1 Objectives

• Know the structure and properties of wood.
• Understand why wood needs to be dried before utilization.
• Identify and select wood for various applications.

Wood Classification

• Trees are broadly classified into softwoods and hardwoods
• Softwoods refer to trees with needle-like leaves and seed-containing cones, which remain green for most of the year, examples include pine and cedar.
• Hardwoods refer to trees with broad leaves that are deciduous in temperate regions with seeds enclosed in fruiting bodies, such as nuts, examples include spruce and mahogany.
• The terms softwood and hardwood have little relation to the hardness or softness of the wood produced.
• Pine is a softwood but stronger than Wawa, a hardwood.

Log Processing

• Lumber or timber is produced from logs cut from tree stems.
• Lumber is wood in solid form cut to various sizes and is one of the simplest of forest products.
• Lumber is the basis for many products like mouldings, frames, dowels, and furniture parts.
• A mill designed to produce lumber is relatively simple.
• Machines in a typical mill include:
• Debarker: removes bark.
• Headsaw: breaks down round logs.
• Carriage or conveyor: handles logs during sawing.
• Edger: trims boards for smooth, parallel edges.
• Trimmer: cuts boards to square and precise lengths.

Wood Characteristics

• Macroscopic characteristics are features visible with the naked eye or a 2-3x hand lens.
• Macroscopic appearance differs according to the plane in view.
• Types: -Cross Section / Transverse Surface: Surface seen when looking at the end of a log or the top of a tree stump, usually circular.
  • Three Parts: pith, wood and bark
  • Within Wood Zone: heartwood, sapwood, annual growth rings and rays
• Radial Surface: Surface created by cutting along a radius of a round log, features pith, growth rings, heartwood, sapwood and bark.
• Location: appears as longitudinal strips depending on its location
• Tangential Surface: Surface created by cutting a tangent to the growth rings
• Also: It is the surface seen when viewing the outside of a log with the bark removed
• Features include: ends of rays, and sapwood

Wood Cells

• Three distinct surfaces of wood can be identified including cross section, radial and tangential.
• Wood has considerably different properties depending upon its orientation.
• Wood looks different in cross section, radial and tangential views.
• Hardwood is not necessarily hard, dense, or strong.
• Wood cells are the structural elements of wood tissues.
• Softwoods are mainly composed of long and narrow tube-cells called tracheids, which are mostly vertical, but can be horizontal.
• Hardwood cells are called fibres, and they are long and narrow.
• Fibres are similar to tracheids but are much shorter, about 1mm in length.
• Hardwood cells with open ends are called Vessels.
• Vessels help in conducting sap in trees for wood formation and may be plugged with gums, resins, and tylosis.
• Tylosis is an outgrowth from an adjoining ray into the lumen of a vessel

Chemical Composition of Wood

• Wood substance is made up of cellulose, hemi-cellulose, and lignin and may contain small amounts of extraneous materials
• The principle organic components are carbon, hydrogen and oxygen
• Woody substance is mainly carbohydrates produced through photosynthesis, with differing proportions in softwoods and hardwoods.
• Cellulose is the major component, about 50% of wood, composed of linked glucose (carbohydrate) molecules joined by polymerization.
• Hemi Cellulose constitutes about 20% of wood substance in softwoods and 15-35% in hardwoods, is also a carbohydrate but with a smaller degree of polymerization.
• Lignin constitutes 23-33% of softwood wood substance and 15-25% in hardwoods, aromatic in nature. It has an unknown chemical structure and reactivity.
• Extraneous Materials are organic and inorganic can be found in wood including tannins and other polyphenolics, colouring matter, essential oils, fats resins, gums and starch.
• Contribute to wood properties like colour, odour, taste, decay resistance, density, hygroscopicity and flammability
• Structure of hardwoods is more variable than softwoods.

Juvenile Wood

• It is an inferior quality wood formed when a stem is very young and found in the center of tree stems.
• The properties differ from, and are inferior to, the wood that forms later in the life of the tree.
• Occurrence in Lumber: It was once rare with use of large diameter trees as stem center constituted a small log volume.
• Today logs as small as 200mm in diameter are being used and are composed almost entirely of juvenile wood.
• It shrinks and swells excessively along the grain as moisture content changes.
• Strength is lower, and in some cases much lower, than mature wood of the same tree

Reaction Wood

• Reaction wood occurs more commonly in young rather than very old trees.
• Those seeking to produce quality wood products find interest in reaction wood because many of its properties are inferior to normal mature wood.
• Like juvenile wood, reaction wood has inferior properties and is much less desirable than normal mature wood.
• Compression wood is common in temperate trees, it is formed when growing stems tip a few degrees from the vertical and produce abnormal cells in orientation at the lower side.
• Tension wood is common in tropical trees, which is formed on the upper side of the leaning stem.

Drying/Seasoning Wood (Unit 2)

• Most problems that arise with wood in use are related to moisture or improper drying.
• Drying wood prior to use is necessary because:
• Wet wood usually dries out
• Wet wood of many species is susceptible to attack by decay and stain fungi, timber below 20% m.c. is too dry to decay
• Water must be removed to provide void space for preservatives for fungal and insect protection.
• Water removal saves on transportation weight
• Facilitates machining
• Dried wood enables strong glue joints
• Improves load carrying ability

Moisture and Equilibrium

• Moisture content (MC) describes amount of water in a piece of wood. Wood Moisture Content is calculated on a dry basis as: MC% = (Green Weight - Dry Weight) / Dry Weight × 100
• Dry weight in the formula refers to oven dry weight (i.e. weight of wood which has been dried to remove all moisture)
• The dry basis moisture content is the weight of the water in wood expressed as a percentage of the weight of the dry wood substance: MC% = Water Weight / Dry Weight × 100
• Water is a chemical constituent of wood, not just a contaminant

Water in Wood

• Freshly sawn low density heartwood may contain nearly 2.5x as much water by weight as dry wood, with approximately 250% dry basis moisture content.
• Freshly Sawn wood may have 100% moisture meaning amount of water in the wood is equal in weight to the wood when dried
• Composed of hollow fibres (vessels) whose lumens are connected by pits.
• Hygroscopicity is the wood's property to attract moisture to from the air to be held as water and water vapour.
• Hygroscopicity is a very important property of wood since the moisture held in the wood affects all other properties.
• Equilibrium Moisture Content (EMC): When water molecules returning to wood surface are the same as it leaves.
• Drying: More molecule of water leave rather than return.
• Wetting: More molecules of water return rather than leave.
• EMC is determined by water/humidity content of the surrounding air.(See research brochure for EMC of various locations in Ghana).

Moisture Movement

• Wood swells when it takes in water and shrinks when it loses water.
• Wood does not react and contract like metal.
• Dimensional changes in wood only happen when moisture drops below the fibre saturation point (fsp)
• Anisotrophy: The reduction or increase in wood dimensions that differs depending on the tree growth direction, for same moisture change
• Factors affecting shrinkage and swelling:
• Density
• Structure
• Extractives
• The magnitude of shrinkage and swelling increases with density since high-density wood has thicker cell walls.
• Extractive content causes a reduction in shrinkage and swelling.
• Porous wood shrinks less since cell walls are thinner.
• There is linear relationship between magnitude of shrinkage/swelling and the amount of moisture change.

Wood Drying

• Air Drying; Simplest way to dry lumber.
  • Carefully stacked piles of timber are placed outside drying under roofs to improve drying rate -Shields the wood from direct moisture/sun and use “stickers” between layers to aid air movement.
  • Simple, but can be expensive with rainy or cold weather lengthening drying time from months to a year and may need to be stored for a while. Completely enclosed buildings with fans are sometimes used to improve circulation of air.
• Kiln Drying:
  • The timber can be Sawmills dry under controlled conditions using kilns, containing:
  • Insulated chamber
  • Heating system
  • Humidification system
  • Temperature and relative humidity monitoring devices
  • Air circulation system
  • Moisture discharge system Timber stacked in kilns dried from only a few days, but species like Denya may need several months

Concerns

• Drying time, inventory size decrease, and moisture lowers with control/expertise through kiln use increases expenses.
• Kiln dried wood can regain moisture if it is exposed
• Dying leads to shrinkage associated with change in dimension that may cause degradation to the wood.
• Wood drying defects include opening of floor joints and furniture, as well as discolouration, warping, honeycombing, collapse (severe distortion of cells),and loosened or raised grain -Drying too rapidly results in seasoning checks, honeycomb, and others; relating to gradient level during process .High MC relates to tensile stress at the wood surface and a compression stress in wood core
• If the tensile stresses the wood will fail in tension at the surface, creating surface checks.
• The savings through rapid drying, offsets with defect costs

Wood Drying Processes

• Wood may assume several shapes in drying due to differential radial and tangential shrinkage and swelling resulting in unequal dimension change

Wood Properties (Unit 3)

• Wood physical characteristics are useful for describing a piece of wood in macroscopic terms

Colour

• Comes in variety of nature, sapwood species can be almost white, Black Ebony (example)
• Colour difference can be within the same sample of within different species. Is hard to determine if the weather breaks down, by extractive woods like tannis.

Odour

• This arises from volatile extraneous materials from heartwood, more intense, but vanishes over time, (such as Essia's rotten cabbage smell disappears with steaming).Is very hard to describe like colour.

Texture

• The deposit left affects these feature, (fresh is better dry). More distinctive in heartwood, can tell apart in wood types. Refers to sizes/amounts with the light, for lens
• Diameter = coarse (fine)
• Texture can describe medium/even -Grain (fibre line) in diagonal + spiral with fibers etc.

Weight

• Helpful way to determine, (influenced by heartwood, late and early types and light).

Hardness

• How strong and resistant the wood is. measured by loading the wood that measures the diameter.

Wood Density

• Wood amount, express mass, meter number.

Wood Density Factors

• Wood quality, such as strengths, with is impacted -Moisture is good for wood if there is moisture content
• Texture has amount of materials and voids

Chemical Components

Cell parts differ due quality.

This quiz covers topics related to juvenile wood, wood defects, and the drying process. It tests knowledge on the characteristics of juvenile wood, the impact of moisture content, and common defects associated with drying such as warping and seasoning.