Predicting the pore-filling ratio in lumen-impregnated wood

Abstract

Lumen impregnation, unlike most other wood modification methods, is typically assessed by the pore-filling ratio (PFR) (i.e. the fraction of luminal porosity filled) rather than by weight percentage gain (WPG). During lumen impregnation, the impregnants act on the voids in the wood rather than on the solid mass (e.g. cell walls), but the PFR cannot be measured as conveniently as the WPG during processing. Here, it is demonstrated how the PFR can be calculated directly from the WPG if the bulk density of the untreated wood is known. The relationship between the WPG and bulk density was examined experimentally by applying a pressured impregnation on knot-free specimens from Sitka spruce with a liquid mixture of methacrylate monomers. Based on the validated model, it was possible to further study the effect of different process-related parameters, such as hydraulic pressure, on lumen impregnation. Skeletal density is another key parameter in this model, which directly reflects the amount of inaccessible pores and closed lumens, and can be independently determined by helium pycnometry. The permeability can be qualitatively evaluated by PFR as well as skeletal density. For instance, poor permeability of knotty wood, due to the large extractives content around knots, was 123Wood Sci Technol (2017) 51:1277-1290 DOI 10.1007 reflected by a lower skeletal density and inefficient lumen impregnation (low PFR). Although this model was examined on a laboratory scale, it provides guidance on the precise effect of different parameters on lumen impregnation, thereby improving the fundamental understanding of and enabling better control over the modification of wood by impregnation.

The transport of liquids in softwood: timber as a model porous medium

“…At the end of each experiment the total fluid volume that had intruded within the specimen (deduced from measurements of the mass of chloroform) was found to be approximately 92% of the total pore volume. This is consistent with the findings of 25 who (based on measurements of timber specimens taken from the same batch of Sitka spruce as the specimens reported herein) determined that approximately 92% of the total pore space was ‘accessible’, in their case either accessible to liquid monomers or helium pycnometry, i.e. some 8% of the pore space was inaccessible even to some of the smallest gas molecules.…”

“…We define the accessible porosity of the wood as the proportion of void space that is accessible to fluid flow (e.g. determined by helium pycnometry 25 ). Guided by our understanding of the microstructure of softwood timbers 26 , we consider the pore-space to consist of relatively long thin tubes (the liquid transmitting tracheid lumena), the cross-sections of which do not vary significantly along their length.…”

“…Reassuringly, this indicates that, informed by the experiments of Wu et al . 25 , each of our experiments were run for a sufficiently long period of time that we achieved a pore filling ratio of 100% — see 25 for a full discussion of ‘accessible pore space’ and ‘pore filling ratio’.…”

An experimental study on change of gas permeability depending on pore structures in three species (hinoki, Douglas fir, and hemlock) of softwood

“…To measure the true density of the wood substance, internal voids must be ignored. Wood samples from the sapwood and the heartwood were pulverized into fine wood powder [34,37,38].…”

“…Bulk density of the cylindrical specimens was measured by the dimension method. True density of cylindrical specimens and sawdust was measured by gas pycnometer (model: PYC-100A-1, Porous Material Inc., USA) [5,37,38]. Before measuring the specimen, the gas pycnometer was calibrated to a standard stainless-steel cylindrical sample from the manufacturer, the volume and mass of which were 15.4346 cm 3 and 41.1767 g, respectively.…”