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)  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)

Compression behavior… 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 FPL-GTR-149

Transient (time- and location- dependent) Changes Dynamic temperature and moisture content conditions within mat influence T g 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

Temp and MC Effects on SG

Effects of Moisture and Temp on T g

Moisture Influence on Mat Counterpressure

Temperature Influence on Mat Counterpressure

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”

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