Wood Notes

Wood (Chapter 7)

Intro & History

Wood is a structural tissue in stems and roots of trees and woody plants.
It's a natural composite of:
- Cellulose fibers (strong in tension).
- Lignin matrix (resists compression).
Sometimes defined as secondary xylem in tree stems or more broadly as similar tissue elsewhere (roots, shrubs).
Function:
- Support for woody plant growth.
- Conveys water/nutrients between leaves, growing tissues, and roots.
Wood can refer to similar plant materials, engineered wood, woodchips, or fiber.
Used for millennia as fuel, construction material, tools, weapons, furniture, and paper.
More recently, feedstock for purified cellulose and its derivatives (cellophane, cellulose acetate).
Earliest known wood plants: New Brunswick, Canada, approximately 395-400 million years ago (2011 discovery).
Wood can be dated using:
- Carbon dating.
- Dendrochronology (in some species) to determine creation date.
Used for fuel, construction, tools, weapons, furniture, packaging, artworks, and paper for thousands of years.
Known constructions date back ten thousand years (e.g., European Neolithic long house).
Recent use enhanced by steel and bronze in construction.
Tree-ring widths and isotopic abundances give climate clues at the time of cutting.

Types of Wood

Wood types vary significantly in color, density, and hardness.
Three main categories:
- Softwood.
- Hardwood.
- Engineered Wood.

Softwood

Comes from conifer trees (lose leaves in the fall).
Examples: Pine, Cedar, Redwood, Spruce, Douglas Fir.
Douglas Fir properties: Excellent strength-to-weight ratio, highest strength rating.
Uses: Ceilings, furniture, doors, and windows.
Advantages:
- Softer surface but still strong.
- Lightweight.
- Easier to work with than hardwoods.
- Easier to transport.
- Affordable alternative (pine is denser than some hardwoods).
- Quicker development pace, lower costs.
- Flexible.

Hardwood

Wood from dicot trees.
Found in broad-leaved temperate and tropical forests.
Mostly deciduous in temperate/boreal latitudes, mostly evergreen in tropics/subtropics.
Hardwood (angiosperm trees) contrasts with softwood (gymnosperm trees).
Temperate region hardwoods lose leaves in autumn and are dormant in winter.
Tropical region hardwoods may shed leaves due to seasonal or sporadic droughts.
Deciduous species (e.g., oak) typically show annual growth rings (may be absent in tropical hardwoods).
More complex structure than softwoods, often slower growing.
Key feature separating hardwoods from softwoods: presence of pores or vessels.
Vessels show variation in:
- Size.
- Shape of perforation plates (simple, scalariform, reticulate, foraminate).
- Structure of cell wall (spiral thickenings).
Hardness:
- Generally harder than softwoods, but exceptions exist.
- Enormous variation in actual wood hardness in both groups.
- Some hardwoods (e.g., balsa) are softer than most softwoods.
- Yew is an example of a hard softwood.
Uses:
- Flooring (most common).
- Cladding, paneling, cabinetry, fencing, boats, and outdoor decks.
Hardwoods tend to be more difficult to work with.
Perform best in high-traffic usage (flooring, furniture).
Lignin composition difference:
- Softwood lignin is mainly derived from coniferyl alcohol.
- Hardwood lignin is primarily derived from sinapyl alcohol.

Engineered Wood

Made of layers with 100% natural wood on top and bottom and a stable core in the middle.
A class of building products/materials.
Made by binding pieces of real wood, scrap wood, or shredded wood fibers.
Designed to be stronger and more durable.
Used in home construction, commercial buildings, and industrial products.

Uses of Wood

Heating: good energy source in fireplaces and campfires.
Construction materials:
- Building houses (especially in North America).
- Generates less waste and speeds up construction.
Boats: wood floats in water and prevents water entry.
Flooring: highly durable and resistant, holds warmth better than tiles.
Furniture: chairs, tables, and beds.
Tool handles and cutlery: chopsticks, toothpicks, pencils, wooden spoons.
Artistic medium:
- Art frames, carvings, and sculptures.
- Woodcut printmaking, engraving, panel painting.
Sports equipment: cricket, table tennis, hockey, baseball, skateboarding, skiing, snowboarding, and surfboarding.
Musical instruments:
- Piano, guitar, and drum.
- Importance: Soundboards transmit and radiate string vibration, adding color to tone.
Wooden toys: preferred by parents for safety (less toxic than plastic).
Paper: essential for modern life.

Physical Properties

Wood Growth Rings

Occur where there are clear seasons, leading to a discrete pattern on the end of a log.
If seasons are annual, growth rings are annual rings.
Where there are no seasonal differences, growth rings may be indistinct or absent.
Two parts of a growth ring:
- Inner portion (nearest the center of the tree):
  - Wider elements.
  - Lighter in color.
  - Formed early in the season (rapid growth).
  - Early wood or spring wood.
- Outer portion:
  - Late wood or summer wood.
  - Produced in summer.
White pines have little contrast in the ring, resulting in uniform texture and easy workability.
Hard pines have very dense and deep-colored late wood, presenting a decided contrast to the soft, straw-colored early wood.
Ring-porous woods: each season's growth is well-defined (large pores of spring abut denser tissue of fall).
Diffuse-porous woods: demarcation between rings is not always clear, sometimes almost invisible.

Knots

Imperfection in timber that reduces strength but may be exploited for artistic effect.
In a longitudinally sawn plank, a knot appears as a roughly circular, darker piece of wood around which the grain flows.
Knot is a portion of a side branch (or dormant bud) included in the wood of the stem or larger branch.
The included portion is irregularly conical.
Fibre direction within a knot is up to $90$ degrees different from stem fibres, producing local cross grain.
Lower limbs often die but persist, and subsequent growth layers are not intimately joined with the dead limb but grow around it.
Dead branches produce unattached knots, likely to drop out after sawing.
Knots are classified by form, size, soundness, and firmness in grading lumber/structural timber.
Firmness is affected by the length of time the branch was dead while the stem grew.
Knots materially affect cracking (checking), warping, working ease, and cleavability.
Weakening effect is more serious under forces perpendicular to the grain and/or tension than under load along the grain and/or compression.
The extent to which knots affect beam strength depends upon their position, size, number, and condition.
A knot on the upper side is compressed, while one on the lower side is subjected to tension.
Season check in the knot offers little resistance to tensile stress.
Small knots along the neutral plane of a beam may increase strength by preventing longitudinal shearing.
Knots are least injurious when they extend through the board at right angles to its broadest surface.
Knots near beam ends do not weaken it.
Sound knots in the central portion (one-fourth the height of the beam from either edge) are not serious defects.
Knots do not necessarily influence stiffness; stiffness and elastic strength are more dependent upon sound wood than localized defects.
Breaking strength is very susceptible to defects.
Sound knots do not weaken wood under compression parallel to the grain.
Wood with knots is sometimes preferred in decorative applications for visual interest.

Heartwood and Sapwood

Heartwood: Dead wood, more resistant to decay because of chemical substances (genetically programmed).
Appears as a colored circle (usually) following annual rings in shape.
Usually much darker than living wood and forms with age.
Many woody plants do not form heartwood, and decay can discolor wood similarly, leading to confusion.
It is uncertain whether heartwood is truly dead, as it can still chemically react to decay organisms, but only once.
Sapwood: Wood that is not heartwood; living wood in the growing tree.
All wood in a tree is first formed as sapwood.
Principal functions:
- Conduct water from roots to leaves.
- Store and give back food prepared in leaves according to the season.
More leaves and vigorous growth require a larger volume of sapwood.
Rapid growth in the open results in thicker sapwood than in trees growing in dense forests.
Some species require more growth in diameter before heartwood forms (e.g., second-growth hickory or open-grown pines).
Heartwood term derives from its position, not vital importance (a tree can thrive with a completely decayed heart).
Some species begin heartwood formation early (thin sapwood), while others change slowly.
Thin sapwood is characteristic of chestnut, black locust, mulberry, Osage-orange, and sassafras.
Thick sapwood is the rule in maple, ash, hickory, hackberry, beech, and pine.
Others never form heartwood.
No definite relation between annual rings and sapwood amount - sapwood cross-sectional area is roughly proportional to the tree crown size.
Narrow rings require more of them than wide rings; sapwood becomes thinner or increases in volume as the tree gets larger.
Sapwood is thicker in the upper trunk portion than near the base (upper sections are younger and have less diameter).
Young trees are covered with limbs to the ground, but as they grow, some or all limbs die and fall off.
Subsequent wood growth may conceal stubs, which remain as knots.
No matter how smooth an old log's outside is, it's knotty near the middle.
Sapwood of an old tree, especially a forest-grown tree, will be freer from knots than the inner heartwood.
Knots are defects that weaken timber, so sapwood may be stronger than heartwood from the same tree due to its position.
A tree growing in the open under constant conditions makes rapid growth in youth and gradually declines.
Growth rings are wide for many years but later become narrower as each ring is laid down on the outside of previously formed wood.
Unless a tree materially increases wood production yearly, rings must become thinner as the trunk gets wider.
As a tree reaches maturity, its crown opens, annual wood production lessens, and growth ring width reduces.
Forest-grown trees depend on the competition for light and nourishment, so rapid and slow growth periods may alternate.
Some trees (e.g., southern oaks) maintain the same ring width for hundreds of years.
Overall, growth ring width decreases as a tree gets larger in diameter.
Different pieces of wood cut from a large tree may differ considerably, especially if the tree is big and mature.
Wood laid on late in some trees' life is softer, lighter, weaker, and more even-textured than wood produced earlier.
In other trees, the reverse applies; this may or may not correspond to heartwood and sapwood.
In a large log, sapwood may be inferior in hardness, strength, and toughness to equally sound hardwood from the same log (reverse may be true in a smaller tree).

Chemistry of Wood

Wood is a three-dimensional biopolymer composite of interconnected:
- Cellulose.
- Hemicellulose.
- Lignin.
- Minor amounts of extractives and inorganics.
Wood cells consist of sugar-based polymers (carbohydrates, $65-75%$ ) and lignin ( $18-35%$ ).
Chemical composition (approximate):
- Carbon: $50%$ .
- Oxygen: $42%$ .
- Hydrogen: $6%$ .
- Nitrogen: $1%$ .
- Other elements (Ca, K, Na, Mg, Fe, Mn): $1%$ .
Also contains sulfur, chlorine, silicon, phosphorus, and other elements in small quantities.
Dry wood elemental composition: ~ $50%$ carbon, ~ $6%$ hydrogen, ~ $44%$ oxygen, and trace amounts of inorganics.
Softwoods vs. Hardwoods:
- Softwoods: higher cellulose ( $40-45%$ ) and lignin ( $26-34%$ ), lower pentosan ( $7-14%$ ).
- Hardwoods: cellulose ( $38-49%$ ), lignin ( $23-30%$ ), and pentosan ( $19-26%$ ).
Major carbohydrate portion comprised of cellulose and hemicellulose polymers with minor amounts of other sugar polymers (starch, pectin).
Holocellulose (cellulose + hemicelluloses) accounts for ~ $65-70%$ of wood dry weight.
These polymers are rich in hydroxyl groups for moisture sorption through hydrogen bonding.
Cellulose: most abundant organic chemical on Earth.
- A glucan polymer of d-glucopyranose units.
- Building block is cellobiose (two-sugar unit).
- Degree of polymerization refers to the number of glucose units in a cellulose molecule.
- Cellulose molecules are randomly oriented and form intra- and intermolecular hydrogen bonds.
- Crystalline regions are formed as packing density increases (highly crystalline wood-derived cellulose may contain up to $65%$ crystalline regions).
- Remaining portion is amorphous cellulose (lower packing density).
- Cellulose types: crystalline, nanocrystalline, accessible, and non-accessible.
- Accessible and non-accessible refer to water availability, microorganisms, etc.
- Crystalline cellulose surfaces are accessible; the rest is non-accessible.
- Most noncrystalline cellulose is accessible, but part is covered with hemicelluloses and lignin, becoming non-accessible.
Lignin: Amorphous polymer with an irregular chemical structure (different structural elements are not systematically linked).
- Roughly classified into softwood lignin, hardwood lignin, and grass lignin.
- Besides native lignin, various forms are available (e.g., milled wood lignin, dioxane lignin, enzymatically liberated lignin, Kraft lignin, alkali lignin).
- Native lignin behaves as an insoluble and three-dimensional network; isolated lignins exhibit maximum solubility in solvents (dioxane, acetone, methyl cello solves, tetrahydrofuran, dimethylformamide, dimethylsulfoxide).
- Lignin defined as a polyphenolic material arising primarily from enzymatic dehydrogenative polymerization of three phenylpropanoid units (p-hydroxy cinnamyl alcohols).
- Proportions of lignin precursors vary with botanical origin.
- Softwood lignins are mainly derived from trans-coniferyl alcohol ( $90%$ ) with the remainder consisting mainly of trans-p-coumaroyl alcohol.
- Hardwood lignins are mainly composed of trans-coniferyl alcohol and trans-sinapyl alcohol in varying ratios (about $50%$ for cache alcohol).
Hemicelluloses: Structure can be understood by considering the conformation of monomer units.
- Intimately associated with cellulose, contributing to the tree's structural component.
- Present in very large amounts when the tree is under stress (compression wood has higher d-galactose content and lignin).
- Contain a backbone consisting of one repeating sugar unit with branch points.
- Consist of more than one type of sugar unit; sometimes referred to by the sugars they contain (e.g., galactoglucan, arabino glucuronoxylan, arabinogalactan, glucuronoxylan, glucomannan).
- Also contain acetyl- and methyl-substituted groups.
- Soluble in alkali and easily hydrolyzed by acids.
- A gradient elution at varying alkali concentrations used for crude fractionation from wood; can then be precipitated from alkaline solution by acidification using acetic acid.
Extractives (natural products): Chemicals in wood that can be extracted using several solvents.
- Sometimes classified by the solvent used (e.g., water-soluble, toluene ethanol, or ether-soluble extractives).
- Hundreds identified; their role in the tree is well understood in some cases, not clear in others.
- Used for waterproofing wooden boats, in torches, and as a binder for centuries.
- Applications in medicine, cosmetics, and as a preservative.
- Some are precursors to other chemicals, some are in response to wounds, and some act as a defense mechanism.
- Cell wall chemicals mainly consisting of fats, fatty acids, fatty alcohols, phenols, terpenes, steroids, resin acids, rosin, waxes, and many other minor organic compounds.
- Exist as monomers, dimers, and polymers.
- Softwoods generally have higher extractives content than hardwoods.
- Most extractives in both are located in the heartwood, responsible for color, smell, and durability.
- Qualitative difference in extractive content from species to species is the basis of chemotaxonomy (taxonomy based on chemical constituents).
Inorganic content: Usually referred to as ash content, which is an approximate measure of mineral salts and other inorganic matter in the fiber after combustion at $575 ± 25°C$ .
- The inorganic content can be quite high in woods containing large amounts of silica; in most cases, it is less than $0.5%$ .
- This small amount contains a wide variety of elements (Ca, Mg, and K make up $80%$ of the ash).
- These elements existed as oxalates, carbonates, and sulfates in the wood or bound to carboxyl groups in pectic materials.
- Other elements present: Na, Si, B, Mn, Fe, Mo, Cu, Zn, Ag, Al, Ba, Co, Cr, Ni, Pb, Rb, Sr, Ti, Au, Ga, In, La, Li, Sn, V, and Zr.
- Some are essential for wood growth; inorganic ions are absorbed through the roots and transported throughout the tree.
- The distribution of 12 inorganic elements varies widely within and between species.

Production of Wood Process

Wood Manufacturing Process:

Head Rig: Primary saw cuts the tree into sawn pieces.
Edging: Removing irregular edges and defects from sawn pieces.
Trimming: Squaring off the ends of lumber into uniform pieces based on market dimensions.
Rough Lumber Sorting: Pieces are segregated based on dimension and final product production.
Stickering: Lumber destined for dry production is stacked with spacers (stickers) that allow air to circulate (green product skips this).
Drying: Lumber is kiln-dried to facilitate natural moisture content evaporation.
Planning: Smoothing the surface of each lumber piece and making its width and thickness uniform.
Grading: Assessing the characteristics of each lumber piece in order to assign quality.

The success of wood manufacturing rests on the ability to retain continually.
Loses or gains moisture until its amount is in balance with the surrounding environment.
The amount of moisture at that point is called the equilibrium moisture content (EMC)and depends mainly on the relative humidity and temperature of the surrounding air.
Kiln drying usually requires controlling the EMC conditions of the wood by monitoring and controlling the relative humidity (RH)and temperature in the kilns.
A primary job is to stabilize and maintain optimum drying conditions for the wood during the kiln drying process.
Moisture content technology is a vital component of modern lumber drying manufacturing; technicians monitor moisture management systems.
Numerous electronic manufacturers produce a range of lumber moisture content measurement systems for large lumber-drying manufacturing operations.
While maintaining and controlling the relative humidity and temperature levels in the kilns, lumber producers continuously measure and monitor the changes in the moisture content to eliminate the over-drying or under-drying of the wood.
These moisture content measurements can apply to a whole stack of wood in the kiln or to an individual spot-checked wood piece.
From the moment the log enters into production to its shipment, lumber manufacturers depend upon integrated moisture content measurement and management systems to achieve quality production on every piece of lumber manufactured by the mill.
The lumber drying process is the origin requiring mills to depend on proper wood moisture management procedures.
However, many consumers and builders mistakenly believe that wood moisture content concerns end when the.
Wood is shipped out of the mill. The moisture content of wood must be constantly measured and controlled as wood will continually lose and gain moisture until it is in balance with the surrounding atmospheric conditions.

Positive Impact on Environment

Wood is a sustainable and versatile material with a limitless range of uses (construction, tools, paper, fuel, etc.).
Making buildings from wood provides several health benefits.
Carbon absorbed by trees is locked up when used as timber.
Living in an environment with more wooden features and furniture can support good health.
Hardwood floors improve indoor air quality and prevent dust, mould and allergens from accumulating compared to carpets (US Environmental Protection Agency).
Having natural materials such as wood visible within our living environment can reduce stress and aid relaxation, attention and creativity.
Health benefits of sauna (reduced risk of heart disease and improvements to sleep, skin health, and blood flow as well as an overall sense of mental well-being).
The use of juniper (a softwood from a coniferous tree) in the sauna is also said to provide protective health properties.
Human health and well-being are intimately linked to the state of the environment.
Rejuvenation of forests is necessary.
The capacity of a tree to absorb carbon declines with age as growth slows down, and the decay of organic material increases, therefore, it begins to release $CO_2$ .
Carbon storage does not continue in the ecosystem; young forests have a higher capacity to absorb carbon than mature forests.
When trees are harvested and used to make wood products, the carbon remains stored in the wood for the life of the product.
About $50$ percent of the dry weight of wood is carbon; harvesting the tree does not return carbon to the atmosphere; rather, wood products play a key role in removing carbon from the atmosphere.
The use of harvested trees in production means an increase in healthy and environmentally friendly products in our lives.
More usage of wood means the decrease of using other raw materials that are non- environmentally, non-renewable, and require large amounts of fossil fuels to produce.
Wood is the best material to combat climate change.
In addition to removing carbon. From the atmosphere, it is also important.