- The objective of this research project was to increase the density, strength,
and stiffness of two low to medium-density diffuse-porous hardwood species. Wood
used in this study was processed using a method of wood densification known as
“Viscoelastic Thermal Compression” (VTC). The VTC process is a three phase
treatment which incorporates saturated steam treatment, mechanical compression,
and a heat treatment. The strategy of the VTC process is to increase the density,
strength, and stiffness of wood through non-destructive means. The two hardwood
species chosen for this study were 7-year old Eastern cottonwood (Populus
deltoides) and sweetgum (Liquidambar styraciflua) from the Pensacola, Florida area.
The primary justification for this research was to test the suitability of VTC
processed wood for future use in a high-strength structural composite. Analysis of
VTC processed Eastern cottonwood has shown increases in density, MOE, and MOR
of up to 178, 254, and 156 percent, respectively. VTC processed sweetgum showed
increases in density, MOE, and MOR of up to 168, 213, and 182 percent,
respectively. Both species exhibited increases in MOE and MOR which were
approximately proportional to increases in wood density. Data obtained from this
research project was used to develop equations to estimate the potential increase in
wood strength and stiffness that can be achieved with the VTC process.
Eastern cottonwood control specimens showed statistical differences in wood
density as a result of vertical location. The upper stem locations were of higher
density than the lower sections. Controls from the upper stem were found to contain
a greater percentage of branchwood and earlywood vessels. A significantly smaller
lumen diameter and presence of a gelatinous layer was seen in the latewood regions
of the upper stem controls. This resulted in thicker lumen cell walls in the
longitudinal fibers, which may have contributed to the wood density increase seen.
Sweetgum control specimens showed no statistical difference in wood density as a
result of stem location.
The findings from this research showed that significant gains in wood
strength and stiffness could be obtained with the VTC process, without causing cell
wall fractures. These results were seen in two low to medium-density, diffuse porous
hardwood species. Similar results could likely be seen in other species, both
hardwood and softwood, providing the same procedure is followed. VTC processed
wood shows potential for use as a high-strength laminate in a composite product.