The role of lignin in wood working processes using elevated temperatures: an abbreviated literature survey
- PDF / 995,239 Bytes
- 16 Pages / 595.276 x 790.866 pts Page_size
- 12 Downloads / 154 Views
REVIEW ARTICLE
The role of lignin in wood working processes using elevated temperatures: an abbreviated literature survey Zoltán Börcsök1 · Zoltán Pásztory1 Received: 30 March 2020 / Accepted: 17 November 2020 © The Author(s) 2020
Abstract The lignin, cellulose and hemicelluloses in wood are polymers that behave similarly to the artificial polymers and are bonded together in wood. Lignin differs from the other two substances by its highly branched, amorphous, three-dimensional structure. Under appropriate conditions, the moist lignin incorporated in the wood softens at about 100 °C and allows the molecules of it to deform in the cell walls. There are many advantages and disadvantages to this phenomenon. If we know this process accurately and the industrial areas where it matters, we may be able to improve these industrial processes. This article provides a brief theoretical summary of lignin softening and the woodworking processes where it plays a role: wood welding, pellet manufacturing, manufacturing binderless boards, solid wood bending, veneer manufacturing, and solid wood surface densification.
1 Introduction: thermoplastic behavior of lignin Wood is actually a mixture of polymers, composed of partially crystalline cellulose microfibrils and large amorphous hemicellulose and lignin molecules. In lignin, phenyl propane units create chains, which are crosslinked in an amorphous, three-dimensional structure, linked to the cellulose fibrils via hemicelluloses (Sakakibara 1991; Zandersons et al. 2004; Rowell et al. 2005). Lignin can be classified based on the chemical structure of its monomer units. Three major groups can be distinguished: grass lignin, softwood lignin and hardwood lignin. Depending on the configuration of guaiacyl (G), syringyl (S) and p-hydroxyphenylpropane (H) units, grass lignin is classified as a GSH lignin, softwood lignin is classified as G lignin and GS in case of hardwoods (Sarkanen 1975; Stelte et al. 2011b; Sakakibara 1991). The amount of lignin varies between species, and also between individual tissues such as bark, earlywood, latewood, normal wood and compressed wood, branch wood, wood from the roots; also by cell types (parenchyma or fibers), and cell wall layers, for example middle lamella, primary and secondary wall layer, and cell corners. Lignin and its chemical * Zoltán Pásztory pasztory.zoltan@uni‑sopron.hu 1
University of Sopron, 4 Bajcsy‑Zsilinszky E Str., Sopron 9400, Hungary
composition can be examined by various techniques, such as ultraviolet microscopy (Lange 1954; Scott et al. 1969; Adler 1977; Fergus and Goring 1970a, b) densitometric analysis (Scott et al. 1969), interference microscopy (Donaldson 1985), SEM-EDXA (Westermark et al. 1988), transmissionelectron-microscopy (Fromm et al. 2003), and by confocal Raman microscopy (Gierlinger and Schwanninger 2006). The greatest concentrations of lignin were found in the compound middle lamella and in the cell corners; lower concentrations were found in the secondary cell walls (Fergus et al. 1969; Scott et al. 1969
Data Loading...
