Abstract
The paper presents changes in color and selected physico-chemical properties of alder (Alnus glutinosa) wood during the process of thermal treatment of the wood with a saturated steam-air mixture or saturated water steam in the temperature range t = 95–125 °C for τ = 3 to 12 h. During the process of thermal treatment of alder wood, the original light white-gray color changes depending on the temperature and time of modification to soft reddish-brown to dark brown color shades. Color changes of alder wood expressed in the form of the total color difference are in the range of values ∆E* = 2.7–31.7. Measurements of the density of thermally treated alder wood in the dry state indicate that due to the thermal treatment of alder wood, the density decreases by ρ ≤ 4.6% compared to the average density of native alder wood. Due to the hydrolysis of hemicelluloses, in the process of thermal treatment of wet alder wood, its acidity changes in the range of values: pH = 4.9 to 3.1. Analyzes of ATR-FTIR spectroscopy indicate changes in alder wood extractants and hemicellulose degradation. A decrease in unconjugated and an increase in conjugated carbonyls was observed at all temperatures of thermal modification of alder wood. Measurements indicate changes in the lignin of alder wood and the fact that as the temperature increases, the formation of new carbonyls increases, which is reflected in the change of the chromophoric system.
Highlights
A decrease in unconjugated and an increase in conjugated carbonyls was observed at all temperatures of thermal modification of alder wood
Color changes of darkening in the technological process of wood steaming were used to remove unwanted color differences between light white and dark cores, or to remove unwanted color spots caused by steaming, browning or molding, in recent times research has focused on targeted color changes wood of individual domestic woody plants in more or less pronounced color shades, with the purpose of imitating exotic woods [12,13,14,15,16,17,18]
FTIR analysis help explain the color changes of alder wood occurring during the thermal treatment process
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Targeted physical-mechanical changes are achieved by thermal treatment of wood used in the production of veneers, plywood, bentwood furniture, or pressed wood [1,2,3]. In addition to targeted physico-mechanical changes, thermal treatment of wood with saturated steam creates conditions for chemical reactions, such as water-soluble extraction processes, hydrolysis of alder wood hemicelluloses, depolymerization of polysaccharides and chemical changes in lignin causing modification of the beech wood chromophore system [4,5,6,7,8,9,10,11]. FTIR analysis help explain the color changes of alder wood occurring during the thermal treatment process
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