POSSIBILITIES FOR IMPROVEMENT OF MOISTURE AND STRENGTH PROPERTIES OF DECAYED SPRUCE WOOD WITH NATURAL RESINS

The paper presents effects of the selected natural conservation substances - resins (colophony, dam mar, mastic, sandarac and shellac) on some moisture and mechanical properties of intentionally decayed Norway spruce (Picea abies L. Karst.) wood, i.e. the kinetics of water sorption (S), the kinetics of volume swelling (beta), the compression strength (sigma), and the modulus of rupture in bending (MOR). Spruce samples 25x15x50 mm, 20x20x30 mm and 8.5x8.5x120 mm (RxTxL) were firstly attacked by the brown-rot fungus Coniophora puteana during 0, 2, 4, 6, 8, 10, or 12 weeks into various degrees of rot (Delta m - average weight losses varied between 0.48% and 28.11%, and density decreases ranged between 0.73% and 21.48%), and then impregnated with 20% solutions of natural resins with their retentions (R) from 66 to 118 kg.m(-3). The mastic proved to be the most effective for the stability of decayed spruce against water. For example, in the 168 hour of the water sorption test, the drop of S-168 was about 27.5% (shellac) up to 36.9% (mastic) or 37.6% (dammar), and the drop of beta(168) was about 14.0% (sandarac) up to 32.1% (mastic). The natural resins also partly improved the strength (sigma and MOR) properties of the decayed and sound spruce. At the sound spruce, the compression strength was increased approx. by about 6.3% (shellac) to 15.4% (sandarac), and the MOR by about 3.8% (sandarac) to 10.4% (colophony). A similar strengthening effect of the natural resins was determined for the decayed spruce. However, the natural conservation substances were not able to change the exponential decrease of the strength characteristics of decayed spruce with growth of the weight losses (sigma, or MOR = a + b.exp/-c.Delta m/).