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 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

Temp and MC Effects on SG

Effects of Moisture and Temp on Tg

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