What is wood made of?
Wood has three main parts. Cellulose forms long, strong fibres, about 40 to 50% of dry wood. Hemicellulose is a shorter sugar-like part. Lignin (about 20 to 30%) is the natural glue that holds fibres together and makes wood stiff. If we break wood into chips, fibres or thin layers, and glue them again, we can make new materials that are bigger, flatter and more even than a plank cut from one tree.
Manufacturing wood-based materials
Wood-based boards use small pieces and waste well.
- Plywood: thin sheets (veneers) glued with the grain of each layer crossing the one below. It has an odd number of layers (3, 5, 7), so it is strong in both directions and stays flat.
- Particle board: chips and sawdust mixed with glue (resin), heated and pressed. Cheap, used for furniture and shelves, but it dislikes water.
- Fibreboard (MDF): wood is broken into fine fibres, mixed with glue and pressed. It has a smooth face and can be painted well.
- Strand board (OSB): long chips in layers, used for walls and floors.
- Glulam and CLT: planks glued into big beams and panels for buildings.
Steps: prepare pieces, dry, mix with glue, form a mat, hot-press, cool, trim and sand.
Wood pulp and washi paper
Pulp is wood broken down into loose fibres. In mechanical pulping machines grind the wood. In chemical pulping chips are cooked with chemicals that dissolve the lignin, giving stronger, whiter fibres. The pulp is mixed with water, spread on a moving screen, pressed and dried into paper. Used paper can be re-pulped and recycled many times.
Washi is traditional Japanese paper. It is made from the long fibres of plant bark (such as kozo, mitsumata and gampi), not from chipped trees. The fibres are cooked, beaten, mixed with water and a slippery plant gum, and scooped onto a screen by hand. The long fibres cross like a net, so washi is thin yet strong and lasts hundreds of years. It is used for writing, art, books, lamp shades and repairs of old documents.
Using woody biomass
Biomass is material from living things used as an energy source. Woody biomass includes chips from thinning, sawdust, bark, branches and old wood that cannot be reused. It can be:
- burned in a boiler for heat (schools, greenhouses, factories),
- used to make steam for electricity,
- pressed into pellets, which are dry, even and easy to carry and burn,
- turned into charcoal or liquid fuels.
It is renewable only if new trees grow to take back the carbon dioxide. It is best to use leftovers and waste, and to burn wood only after reuse and recycling.
Try it: one pile, four products
Tear a used paper into small pieces, soak it in water for a day, blend it, and spread the pulp on a mesh to dry. You have made recycled paper. In the 3D, tap each button and write which step is added: glue and press (board), cooking (paper), a net of long fibres (washi), or burning (energy).
Key formulas and definitions
- Dry wood: cellulose ≈ 40 to 50%, lignin ≈ 20 to 30%
- Plywood: odd number of veneer layers, grain crossed at 90°
- Pulp: separate fibres (mechanical or chemical), then sheet and dry
- Mass = density × volume; Area density (paper) = mass ÷ area
- Energy from fuel = mass × energy per kg (wood pellets ≈ 17 MJ/kg)
Worked examples
1. A plywood sheet has 5 veneers, each 3 mm thick. How thick is the plywood?
5 × 3 mm = 15 mm. (5 is an odd number of layers, as plywood needs.)
2. An A4 sheet is 0.21 m × 0.297 m and the paper is 80 g/m². Find the mass of one sheet and of a 500-sheet ream.
Area = 0.21 × 0.297 ≈ 0.0624 m². Mass = 0.0624 × 80 ≈ 5 g. A ream: 500 × 5 g = 2,500 g = 2.5 kg.
3. How much energy does 10 kg of wood pellets give if each kg gives about 17 MJ?
10 × 17 = 170 MJ (about 47 kWh, since 1 kWh = 3.6 MJ).
4. A sheet of particle board is 2.44 m × 1.22 m × 0.018 m with density 700 kg/m³. Find its mass.
Volume = 2.44 × 1.22 × 0.018 ≈ 0.0536 m³. Mass = 0.0536 × 700 ≈ 37.5 kg.
5. Which product keeps the long fibres: paper from chipped softwood or washi? Which is stronger for its thinness?
Washi keeps long plant-bark fibres that cross like a net, so it is strong even when very thin.
Common mistakes
- Thinking plywood has an even number of layers. It normally has an odd number so the grain balances.
- Saying lignin is the fibre. Cellulose is the fibre; lignin is the glue.
- Treating washi like ordinary wood-pulp paper. It uses long bark fibres and is mostly hand-made.
- Believing biomass burning is always carbon-free. It is renewable only if trees regrow and waste wood is used first.