For. 485: Lignocellulosic Composite MaterialsLecture 1-3: Thermoplastic Softening of Lignocellulose

Composite

“Combinations of materials in which the constituents retain their identities in the composite on a macro scale” (Dietz). Adhesive-bonded wood/lignocelluose

materials fall under this definition. Although useful, this definition is perhaps

unnecessarily limited by the word “macro” in that contemporary trends seem to favor the development of “nanocomposites.”

Lignocellulose cell wall

A multicomponent polymer system Structural polymers:

Cellulose, hemicelluloses, lignin

Cell wall constituents exhibit properties that are described by polymer theory

Polymer behavior with temperature changeTg = “Glass transition temperature”

This is the temperature at which a polymer undergoes a change in the slope of the specific volume versus temperature curve, or more simply, the temperature at which the material changes from a glassy (brittle) state to a rubbery state.

This generally corresponds to an abrupt decrease in stiffness. If this is the case, Tg should appear in a plot of modulus vs. temperature.

Glass Transition Temperature, Tg

Tg of Dry Wood Polymers

Data points indicate experimental variation in Tg measurements

Note that cellulose has highest Tg, followed by hemicelluloses and lignin.

Moisture Effect

Moisture has a “diluent” effect on cell wall polymers This means

that moisture tends to lower the Tg

Diluent Effect