Local variability in chemical and physical properties of spruce wood fibers

Abstract: Wood is well known to be a heterogeneous native materialwith large variations in its composite properties, includingvariations whose origins have not yet been completelyidentified. Traditionally, the macroscopic variations in woodhave received the most attention; for example, the variation indensity from the pith to the bark in trees. On the fiber level,morphological variations, such as width and cell wallthickness, have been studied rather extensively, whereas thevariability in the properties of the cell wall has receivedless attention. The aim of this work was to demonstrate some ofthe variability in the chemical and physical properties offibers in annual rings and their effect on the elasticproperties in the transverse direction of the cell wall.Double radial cell walls were shown to express a largevariability in properties within the annual ring. FTIRmicroscopic measurements, combined with multivariate analysis,indicated an increase in the hemicellulose content and adecrease in the lignin content when going from earlywood tolatewood in the annual ring. Some of these variations were dueto an increased thickness and relative contribution of the cellwall layers, whereas some might have been due to a differencein their polymer concentrationin the secondary walls. Fibrilangle measurements, using polarization confocal microscopy andorientation of soft rot cavities, showed a large variability ofthe fibril angles, with a decreasing trend from thefirst-formed earlywood fibers to the middle of the annual ring.Latewood fibers had a lower fibril angle. The variability infibril angles of annual rings containing compression wood wasmore scattered and the fibril angles were generally higher bothin early- and latewood. The transverse elastic modulus wasfound to vary substantially from fiber to fiber, with earlywoodfibers tending to have a higher modulus than transition woodfibers. For example, the experimental transverse elasticmodulus of earlywood fibers was 1.5-3.0 GPa, whereas themodulus of transition wood fibers was 0.7-1.5 GPa. These valueswere lower than those values obtained from mathematical modelsbased upon the elastic properties of cellulose, lignin, andhemicellulose. It was shown, however, that it is possible toreduce the discrepancy between experimental and modeled data byusing lower and more orthotropic elastic constants ofhemicellulose and lignin in the modeling. In contrast to thelongitudinal elastic modulus of wood fibers, the variation inthe transverse elastic modulus between earlywood and transitionwood fibers was found to depend more on the relative thicknessand fibril angle of the S1-layer than on the fibril angle in the S2-layer.The main conclusion drawn from these observations was thatthe heterogeneity of the wood material is extensive, evenwithin an annual ring.Keywords: Cell walls, Chemical composition, Elasticconstants, Elastic strength, Fiber properties, Fibril angles,Mathematical models, Microscopy techniques,Picea abies, Stiffness, Tensile properties, Woodpolymers, Wood