WOOD (Materials 2)

Wood

a natural polymer-matrix composite

• made of cellulose + hemicellulose and lignin

Cellulose 90% (Fibers/natural polymers)

long fibers that provide strength, along direction of wood grain (glucose-based polymer)

Hemicellulose & Lignin 10% (Matrix/transverse rays)

• acts like glue, holds cellulose fibers together, makes wood stiff/rigid

• hemicellulose - polymers made from different sugar molecules

• lignins - provides rigidity

  • 3D structures of phenyl propane units

  • more like thermoset

Anisotropic

wood’s strength depends on direction force is applied

• parallel to grain = longitudinal

  • stronger/stiffer when along grain

• perpendicular to grain = radial & tangential

  • weaker because fibers separate and crush more easily

Free Water (Moisture)

water inside hollow spaces of wood cells (wet)

• drippy wet sponge

Fiber Saturation Point (FSP) (Moisture)

point where hollow spaces are empty of free water, but cell walls are still full of water

• squeezed sponge but still wet

Below FSP (Moisture)

water leaves cell walls, wood shrinks/warps/bends

• wood becomes stronger/stiffer

• oven-dried sponge?

Natural Flaws

defects in wood (knots/cracks/uneven grain) that decrease strength

• mechanical properties are highly variable

Engineered Wood

composite made from wood + wood waste products along with a synthetic polymer matrix (glue)

• to overcome limitations

  • anisotropy

  • size limited

  • warping/dimensional stability

  • flaws

Engineered Lumber Products

• Consistent

  • reduce swelling/shrinkage problems

  • no warp

• Design capabilities

  • increased strength and stiffness

• Environmental

  • reduced availability of old-growth timber

  • more efficient to use

  • use of undervalued species

Particle Board

particles of wood as small as sawdust

• use waste

• glued together + fully isotropic

• not size limited

• flaws don’t matter

Plywood

sheet veneer

• large sizes

• stacked and glued, rotate the grain

• defects not important