Jasna S. Stevanic

Orientation of the wood polymers in the cell wall of spruce wood fibres.

Abstract The mechanical and physical properties of wood fibres depend to a large extent on the orientation of the polymers, mainly the cellulose microfibrils, within the supramolecular structure of the cell wall. Under moist conditions, the arrangement within the polymer matrix may play a dominant role for mechanical properties in general and, especially, in the transverse direction. In this context, it is of special interest to determine the orientation of glucomannan and xylan, being the essential components of softwood hemicelluloses, and of lignin in wood fibres. Fourier transform infrared (FTIR) microscopy was used to examine the orientation of the main wood polymers in transversal and longitudinal direction of spruce fibres. We investigated fibres made from a thermomechanical pulp, in which the outer fibre wall layers were removed by mechanical action, and chemically delignified fibres. The polarised FTIR measurements indicated that glucomannan and xylan appear to have a parallel orientation with regard to the orientation of cellulose and, in all probability, an almost parallel orientation with regard to the fibre axis. Lignin was found to be less oriented in the fibre wall, although its arrangement is not fully isotropic. In the longitudinal direction of the fibres, there were no significant changes in the molecular orientation of the studied polymers.

FT-IR imaging microscopy to localise and characterise simultaneous and selective white-rot decay within spruce wood cells

Abstract Spruce wood that had been degraded by white-rot fungi (Trametes versicolor or Ceriporiopsis subvermispora) and suffered mass losses up to 17% was investigated by transmission Fourier transform infrared (FT-IR) imaging microscopy. A significant marker during incipient simultaneous white-rot (T. versicolor) was the cleavage of glycosidic bonds of polysaccharides that preceded their metabolisation. Simultaneous white-rot processes were also characterised by a relative decrease of the overall lignin content and a relative accumulation of wood polysaccharides. No early marker was found for selective white-rot (C. subvermispora) that removes mainly lignin by an oxidative process. This feature was detected only in wood samples exhibiting mass losses higher than 12%. Furthermore, it was shown, that simultaneous and selective white-rot processes were unevenly distributed within the wood samples but quite evenly distributed within single tracheids.

Analysis of thermally treated wood samples using dynamic FT-IR-spectroscopy

Thermal treatment of wood has been performed from thevery beginning – e.g., Stamm (1956), Kollman andSchneider (1963), Rusche (1973) – with the aim toenhance the durability of wood for outdoor constructionsand indoor humid areas. Extensive studies by VTT (Viita-niemi et al. 1995) in this area have led to the developmentof Thermowood which is obtained by heat treatment attemperatures between 1608C and 2308C in an oxygen-free atmosphere for several hours. As an effect of thethermal treatment, Thermowood obtains a characteris-tic darker colour and develops a higher resistance tohumidity as well as reduced swelling and shrinkagebehaviour. The new technical properties follow fromchanges of chemical nature in the wood-structure, suchas a reduction in hemicellulose content, which reducesthe water absorption capability.The following improvements have been obtained forthermally treated wood: higher shape stability (swellingand shrinking could be reduced to 50% of the untreatedmaterial); reduction of equilibrium moisture content (themoisture content of Thermowood is up to 50% lowerthan for normal wood); reduction of internal stresses; andhigher durability against fungi.Also, negative changes by the thermal treatment areobserved: the density is reduced up to 15% which caus-es reduced fire resistance; the strength properties are upto 20–60% reduced; breaking by splintering is more like-ly; the colour does not resist UV radiation so that theThermowood must be treated by UV blockers similar tonon-treated wood; Thermowood becomes grey byweathering similar to untreated wood.The main goal of this research was to further increasethe understanding regarding the molecular changesoccurring due to thermal treatment in relation to the vis-coelastic behaviour of the wood. FT-IR spectroscopy iswell established for characterising the chemical natureof substances. With the combination with dynamicmechanical analysis, it has been shown that informationregarding the molecular structures taking the load inpolymer materials may be analysed (Noda 1990). Dynam-ic FT-IR is also applicable to studies of cellulose andwood fibre materials (Salme´n et al. 2005). For this reason,dynamic FT-IR spectroscopy was used for characterisingthe response of wood components to thermal treatment.The samples investigated were taken from commerciallyavailable beech Thermowood .

Contribution of lignin to the strength properties in wood fibres studied by dynamic FTIR spectroscopy and dynamic mechanical analysis (DMA)

Abstract A deeper insight into the molecular interactions in the highly intermixed structure of the wood cell wall, from the point of view of both basic and applied science, is necessary. In particular, the role of the different matrix materials within the cell wall needs to be better understood, especially concerning how lignin contributes to the mechanical properties. In the present paper, the mechanical properties of spruce wood have been studied on a molecular scale by means of dynamic Fourier transform infrared (FTIR) spectroscopy. To this purpose, native spruce wood was subjected to chemical changes by impregnation and a mild pre-cooking with white liquor with a composition usual for kraft pulping. For comparison, lignin-rich primary cell wall material was also isolated by means of thermomechanical pulp (TMP) refining. Dynamic FTIR spectroscopy revealed that lignin took part in the stress transfer in all investigated samples. This finding is in contrast to literature data. A strong indirect coupling between lignin and cellulose was seen in the primary cell wall (P) material. In case of native wood, the lignin signal was much weaker and also indicated an indirect coupling to cellulose. In the case of pre-cooked wood samples (submitted to mild pulping), the interactions were modified so that the molecular straining of lignin was stronger and more directly related to that of cellulose. In other words, in these samples, lignin played a more active role in the stress transfer as compared to native wood. These findings were supported by a narrower lignin-softening region as measured by dynamic mechanical analysis (DMA). The interpretation is plausible in terms of the superior stiffness seen for high-yield pulps of a similar yield as the studied pre-cooked wood samples.

Effect of the size of the charged group on the properties of alkoxylated NFCs

The impact of the size of the charged group on the properties of alkoxylated NFC was studied by two chloroalkyl acid reagents. It was found that the employment of the larger 2-chloropropionic acid reagent leads to improved properties, e.g. higher fraction of nano-sized materials, and significantly better redispersion as compared to when the smaller monochloroacetic acid was employed. The differences in the impacts of the different reagents were hypothesized to be due to a more efficient disruption of the cohesion between the nanofibrils when a larger charged group was employed.

Miniaturized determination of ash content in kraft lignin samples using oxidative thermogravimetric analysis

A study has been made of several aspects of determination of ash content in kraft lignin samples using thermogravimetric analysis (TGA). Three different methods were used; with the main differences ...