Loss of tensile strength in cellulose tissue on the surface of spruce (Picea abies) caused by natural photodegradation and delignification

Abstract

Photodegradation of wood leads to discolouration and delignification, inducing damage to the cellular structure. This study is an investigation of the damage and loss of tensile strength of the cellulose fibres in spruce (Picea abies). Microtensile tests in the longitudinal direction on 250-μm-thick microtomed samples were performed on undegraded and degraded spruce specimens. The tested length of the samples for the first series was 10 mm (t-span) to evaluate the entire wood composite degradation process and the interaction between cellulose and lignin. For the second series of samples, the tested length was 0 mm (z-span) to evaluate the fibre strength only. The samples were exposed outdoors on a 45° rack facing south for zero, 12, 16, and 70 weeks. The depth of degradation was studied by testing three 250-μm-thick layers, starting from the surface. With this approach, the real degradation behaviour can be evaluated because the cutting is performed after the exposure, in contrast to classical studies in which the microcuts are exposed directly. The touchless measurement of roughness did support the evaluation of the surface and helps to explain the degradation and erosion of the material.The results revealed a significant loss of tensile strength for the t- and z-spans of the degraded surface (0–250 μm) after 16 weeks of exposure and highlighted the degradation of the cellulose fibres. With the applied method, no significant loss of strength below 250 μm could be measured. However, quantitative analyses via scanning electron microscopy (SEM) showed typical degradation patterns, such as pit destruction and cracking between the cells below 250 μm.For the degraded and nondegraded samples, the tensile strength of the z-span was always twice that for the t-span. In addition, the time-dependent loss of strength always demonstrated the same tendency for both testing methods. This observation indicates a similar degree of degradation of lignin and cellulose in terms of strength loss.