Shrinkage of Three Tropical Hardwoods Below and Above the Fiber Saturation Point
Keywords:
Equilibrium moisture content, fiber saturation point, shrinkage, <i>Cedrelinga cateniformis</i>, <i>Aspidosperma macrocarpon</i>, <i>Ormosia coccinea</i>Abstract
Two experimental techniques were used to perform moisture sorption tests at 25°C on samples of three tropical hardwood species: tornillo (Cedrelinga cateniformis Ducke), pumaquiro (Aspidosperma macrocarpon Mart.), and huayruro (Ormosia coccinea Jackson) woods. The first technique used saturated salt solutions at a relative humidity from 0% to 90%, and the second one used the pressure membrane method at above 96% relative humidity. These sorption tests were combined with shrinkage measurements. The fiber saturation point (FSP), estimated by interpolation to zero volumetric shrinkage, was 28%, 22.5%, and 21.5% for tornillo, pumaquiro, and huayruro, respectively. Results confirmed that at equilibrium moisture content, radial, tangential, and volumetric shrinkage occur above the actual FSP. This behavior can be explained by the effect of hysteresis at saturation on wood properties. This hysteresis indicates that loss of bound water takes place in the presence of liquid or capillary water, which contradicts the traditional concept of FSP. The initial equilibrium moisture content at which bound water starts to leave cell walls varied largely among the species: 52%, 36%, and 77% for tornillo, pumaquiro, and huayruro, respectively. The liquid water remaining in wood could be principally located in the least permeable flow paths of these wood species.References
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