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For. 485: Lignocellulosic Composite Materials

For. 485: Lignocellulosic Composite Materials. Lecture 1-5-2: Consolidation Behavior of Lignocellulosics in Thermal Processes, Part II. Consolidation Behavior: Wood as a Cellular, Viscoelastic Material. 2 levels of cellularity: Interparticle voids (space between furnish particles)

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For. 485: Lignocellulosic Composite Materials

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  1. For. 485:Lignocellulosic Composite Materials Lecture 1-5-2: Consolidation Behavior of Lignocellulosics in Thermal Processes, Part II

  2. Consolidation Behavior: Wood as a Cellular, Viscoelastic Material • 2 levels of cellularity: • Interparticle voids (space between furnish particles) • Intraparticle voids (cell lumens within the anatomical structure of wood) • Collapse of voids at both levels during consolidation leads to nonlinear compression behavior (between points C & D on next diagram)

  3. A: Linear-elastic compression; unrestrained particles slide past one another B: Particle contact from top-to-bottom of mat; particle bending begins C: Compression of particles; reduction of void space (intra- and inter-particle) D: Most cell lumens collapsed; compression of cell wall substance Compression behavior… FPL-GTR-149

  4. Transient (time- and location-dependent) Changes • Dynamic temperature and moisture content conditions within mat influence Tg of cell wall polymers • As Tg is exceeded, polymers soften, resulting in decreased modulus of the mat, resulting in densification as platen pressure is exerted • Densification: Density of compressed furnish exceeds that of raw material input

  5. Temp and MC Effects on SG

  6. Effects of Moisture and Temp on Tg

  7. Moisture Influence on Mat Counterpressure

  8. Temperature Influence on Mat Counterpressure

  9. Springback • As platen pressure is released at completion of press cycle (press opening): • Some (hopefully most) of the densification is not recovered; this is unrecoverable viscous strain or permanent deformation • Some of the densification resulting from consolidation is reversed; this is recoverable elastic strain or “springback”

  10. Thickness swell of waferboard • Diagram represents dissection of industrial waferboard into 1012 specimens, each 100 x 100 mm; after conditioning at 20 C and 65% RH, specimens were soaked in water for 24 hours at 20 C, and then the percent thickness swell was determined. • Thickness swell is primarily due to recovery of viscous strain (so-called permanent deformation), which, as we see here, is not necessarily “permanent” when excess water is introduced! Bolton et al., 1989

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