Temperature and moisture influence on compression-recovery behavior of wood

The primary limitation of non-veneer wood composites for applications in moist environments is dimensional instability. Thickness instabilities from moisture absorption primarily result from damaged cell structures that recover upon absorption of moisture. Previous research has shown that manipulating the pressing parameters involved in the manufacture of non-veneer wood composites (i.e., temperature and moisture) can lead to a more dimensionally stable product. However, the precise phenomena controlling these changes are not fully defined. To understand development of pressing-induced damage, the large strain, compression-recovery behavior of wood and polyurethane (PUR) foam (i.e., as a model system) was studied at a variety of compression temperatures spanning the glassy to rubber transition. The behavior is then related to polymer phase transitions to discern the role of viscoelastic behavior in damage evolution. The elastic modulus (E) and yield stress (σ_y) were used to characterize the elastic region of compression, whereas fractional recovery (R) and dissipated energy (ΔE) represented the inelastic component. The PUR foam displayed a distinct glassy plateau region dominated by E, σ _y and ΔE as well as low R. Wood with 22 and 12% MC behaved similarly to the elastomeric PUR foam; however, limits on environmental control prevented testing in the rubbery regime for the 12% MC samples. The E and σ_y also decreased with increasing compression temperature for oven-dried yellow-poplar. However, in contrast to yellow-poplar with either 12 or 22% MC, an increase in ΔE was accompanied by a decrease in R with increasing compression temperature of the oven-dried yellow-poplar. An apparent change in mechanism occurs when compressing wood at high temperatures without moisture present. This change was attributed to kinetic effects such as thermal degradation or crosslinking reactions.