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For. 485: Lignocellulosic Composite Materials. Lecture 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).
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For. 485: Lignocellulosic Composite Materials Lecture 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 change Tg = “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.
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
