František Kačík

Spruce wood is widely used in outdoor applications, but its susceptibility to degradation under exposure to sunlight and moisture is a major concern. This study investigates the impact of accelerated aging on spruce wood’s surface chemistry, microstructure, geometry, and discoloration. The study was performed in two outdoor aging modes: dry and wet. The accelerated aging effects were evident in the changes in spruce wood structure, as well as in the other studied properties. During aging, it developed significant discoloration. Under simulated rainless outdoor conditions (dry mode), spruce wood gradually became dark brown. Under conditions involving rain (wet mode), the discoloration was qualitatively different from the dry mode. FTIR spectroscopy showed that during the accelerated aging of wood, lignin was mainly degraded, especially in the early stages of the process. A linear correlation was found between the changes in lignin and the color changes in the wood. There was an increase in carbonyl groups in the dry mode, which contributed to the color change and was also influenced by changes in extractives. The wet mode caused the leaching out of carbonyl groups. The observed decrease in cellulose crystallinity, together with the degradation of hydrophobic lignin, may result in the increased hydrophilicity of photodegraded wood. For both modes, there were different changes in the wood micro- and macrostructure, reflected in the surface morphology. The roughness increased during the aging process in both modes. The slightest changes in the roughness parameters were identified in the grain direction in the dry mode; the most evident was that the roughness parameters increased perpendicular to the grain in the wet mode. The demonstrated mechanism backing up the aging-related changes to the spruce wood structure and the relations unveiled between these changes and the changes in the spruce wood surface properties can provide an issue point for seeking ways how to mitigate the negative effects of the environmental factors the wood is exposed to.

This work evaluates the effect of using selected inorganic chemicals as the main components of waterborne wood preservative systems on the degradation of the cellulose constituent in wood from model samples. The polymeric properties of cellulose and the homogeneity of the degradation process primarily reflect very well the degree of cellulose deterioration. Whatman papers, as pure cellulose model samples, were impregnated with 10 different 5 wt% solutions of inorganic salts and distilled water and consequently subjected to wet-thermal accelerated aging (T = 85 °C, RH = 65%, for 30 days). The samples were then derivatized to cellulose tricarbanilates (CTCs) through two different procedures (by precipitation in a methanol–water mixture/by evaporation of pyridine from the reaction mixture) and finally analyzed using size exclusion chromatography (SEC). Chemically treated and aged cellulose samples showed different changes in the degree of polymerization (DP) and polydispersity (PD) in terms of untreated non-aged standard caused by different ongoing degradation reactions, such as dehydration, hydrolysis, oxidation, and crosslinking. In general, the lowest degradation rate after treatment by chemicals and after accelerated aging was observed in samples treated by borates, NaCl, and ZnSO4·7H2O. The greatest depolymerization after treatment and after accelerated aging was caused by sulphates containing NH4+, Cu2+, and Fe3+ cations, with aging by NH4Cl and (NH4)2HPO4-treated samples also leading to significant depolymerization. The higher DP values are linked to the precipitated method of CTC preparation, though not for chlorides and phosphates. PD is also generally higher in precipitated and aged samples and is heavily influenced by the presence of low molecular weight products. This paper brings new insights regarding the complex evaluation of the polymeric properties of degraded cellulose by considering all important factors affecting the sample and the analysis itself through the use of statistics. From the statistical point of view, the influences of all factors (solution, aging, method) and their interactions (except aging*method) on DP are statistically significant. The influence of the sample processing method used for analysis of the desired results becomes important mainly in practice. This work recommends the evaporation method for more accurate description of more degraded cellulose.