SELECTED PROPERTIES OF WOOD PRETREATED BY ALKALI AND OXIDATION AGENT Solár R., Kurjatko S., Dibdiaková J., Hudec J., Reinprecht L. Faculty of Wood Sciences and Technology. Technical University of Zvolen, T.G. Masaryka 24, Zvolen, Slovakia.

Introduction Production of chemical pulps is an expensive matter connected with high consumption of energies, chemicals and water. The efficacy of pulping production might be improved by: - introduction of new technologies with flow through digesters (savings of the time and energies). - pre-treatment of chips by biotic (white-rot fungi) or abiotic agents (savings of pulping and bleaching chemicals, and energies, as well). Fungal pre-treatment based on the enzymatic release in the structure of partly delignified wood has not been used from practical reasons in the practice.

Introduction  Similar effect on the structure, chemical composition and physical properties of wood may have alkaline/oxidation sequences of the chemical pre-treatment.  Advantage of chemical pre-treatments dwells in their more uniform effect and shorter time of pre-treatment.

Aim Estimation of alterations in the selected physical properties of hornbeam, poplar and spruce wood due to pretreatment by chemical agents. To estimate the influence of changed physical properties of the pre-treated wood in relation to kraft pulping and properties of resulting pulps.

Material, selection of specimens Test specimens of the examined wood species with dimensions of 2.5x2.5x1 cm were prepared from three sections taken from middle parts of the tree trunks. Shorter dimension of the specimens was parallel, and the longer ones were perpendicular to grain. From the specimens comparable sets, each comprising 15 specimens, were selected. In the case of spruce and poplar wood the sets contained proportional number of specimens of mature (sap- poplar) and juvenile (heart-poplar) wood.

Pre-treatments Chemical pre-treatments comprised: 48 - h alkaline step (2.5 % NaOH) only, or combined by: a/ 72 - h oxidation with 7.5 % H 2 O 2 (with, or without dicyandiamide activator) at the temperature of 22 o C, b/ 24 - h oxidation with 7.5 % H 2 O 2 with dicyandiamide activator at 60 o C c/ 24 -h oxidation with 4 % per-acetic acid carried out at the ambient temperature (22 o C).

Measurements and analyses Weight loss. Facial swelling after 48 h dipping in H 2 O /untreated wood/ or wood after pre-treatment. Coefficients of axial permeability in wet state. Uptake of water by the specimens. Diffusion coefficient for the pseudostationary process. Lignin in kraft pulp (ASTM 13 m). Brightness of unbleached pulps.

Pulping Kraft pulping: sulfidity of liquor was 21.5 %; active alkali 16.0 %, time to reach the pulping temperature of 170 o C - 60 min, pulping at the temperature – 60 min; hydromodul 1:4.2. Note: pulping in small scale autoclaves without stirring, dimensions of chips cut from the specimens were 3x3x10mm (longer dimension parallel to grain), their moisture content before pulping %.

Results and discussion All pre-treatments resulted in weight loss of the selected wood species and changes in their physical properties, as well (Figs. 1-5).

Results - weight loss, swelling

All chemical pre-treatments of hornbeam wood led to a moderate and acceptable weight loss. Despite the weight loss, a series of the pre-treated specimens of hornbeam wood exhibited for 18 to 25 % higher facial swelling compared to sound material. This observation indirectly indicates better accessibility of the pre-treated wood for water.

Results – weight loss, swelling

In the case of poplar wood pre-treated under similar conditions the resulting weight loss, with exception of pre-treatment at 60 o C with activated H 2 O 2, was milder. Similarly, slightly milder was also increase in the facial swelling of the pre-treated poplar wood (for 14 to 19 %).

Results – axial permeability

Alkaline pre-treatments of the tested wood species resulted in marked reduction of their axial permeability. Alkaline treatment followed by hydrogen peroxide, or per- acetic acid step influenced the reduced permeability of hornbeam wood negligibly; in the case of poplar wood oxidation steps reduced a drop in the permeability for approximately 50 %. In the case of spruce wood, both alkaline and alkaline pre- treatment combined with peroxide step diminished its axial permeability for approximately 40 %.

Results – uptake of water

All chemical pre-treatments markedly increased the accessibility of wood for water. The increased uptake of water was almost the same for all tested wood species (cca %) submitted to identical alkali/oxidation pre- treatments. Increase in the uptake of water by the pre-treated wood results from its relaxed ultra-structure expressed outwardly in extreme increase of facial swelling of this material.

Results – uptake of water, diffusion Higher concentration of water in the cell walls of the pre- treated wood may promote their easier impregnation due to faster motion of the ions and water-soluble molecules. This statement concerns both diffusion of chemicals into wood and diffusion of delignification products out from wood into the pulping media. The above statements turned out to be true, as it follows from Table 1 presenting the axial diffusion coefficients of hornbeam wood and properties of kraft pulps (Figs ).

Results – diffusion Table 1: Diffusion coeficient (D) of the pseudo-stationary process for sound and pre-treated hornbeam wood (n = 6) Sample/pre-treatmentD (m 2.s -1 )v (%) Sound wood /1/ 0.81 E % NaOH (48 h, 20 o C) /1/ 1.50 E Sound wood /2/ 1.23 E % NaOH/H 2 O 2 (48 h,20 o C;72h, 20 o C) E

Results – unbleached kraft pulps

Conclussions The obtained data allowed to derive the following conclusions:  From the viewpoint of transport of dissolved chemicals in the pre-treated hornbeam, poplar or spruce wood, an interesting finding is reduction in its axial permeability.  Reduced permeability of the pre-treated wood is a result of its extreme swelling reducing the vessels lumina cros- sections. This process is accompanied by increased water uptake of such a material.

Conclussions An excessive amount of water in the released, swollen and less dense ultra-structure of the pre-treated wood affects positively the diffusion of water soluble molecules through the cell walls (chemicals, degradation products of wood). Enhanced diffusion through the pre-treated wood possibly over-compensates its reduced permeability and improves the rate of transport processes in wood.

Conclussions Improved conditions for transport of chemicals through the pre-treated wood and of delignification products from wood into puling media well represented the improved properties of kraft pulps from by alkali, or alkali and peroxide pre-treated chips.

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