Unraveling the structural characteristics of lignin in hydrothermal pretreated fibers and manufactured binderless boards from Eucalyptus grandis

Abstract

Background: Eucalyptus grandis is one of the most abundant biomass from plantation in many parts of the world. The binderless board were manufactured from hydrothermal pretreated fibers of Eucalyptus wood and characterized for the chemical analyses and mechanical strengths in order to assess the mechanism of self-bonding. To make clear the self-bonding mechanism of these binderless boards, the structural characteristics of cellulolytic enzyme lignin (CEL) isolated from Eucalyptus wood, its hydrothermal pretreated fibers, and binderless boards were thoroughly investigated by chemical and spectroscopic methods. Results: The result revealed that hydrothermal pretreatment and hot pressing process could change cellulose crystalline structures by disrupting inter/intra hydrogen bonding of cellulose chains. During the hydrothermal pretreatment of Eucalyptus wood, acid-catalyzed cleavage of β-O-4′ linkages and ester bonds were the major mechanisms of lignin cleavage. This degradation pathway led to a more condensed lignin which has a high average molecular weight and more phenolic hydroxyl groups than the control. The hot pressing process resulted in the binderless boards with reduced lignin contents and decreased the glass transition temperature, thus making the lignin more accessible to the fiber surface. CEL isolated from the binderless boards showed an increased syringyl to guaiacyl propane (S/G) ratio but a lower molecular weight than those of the untreated Eucalyptus wood and the hydrothermal pretreated fibers.Conclusions: Based on the finding of this study, it is suggested that the combination of hydrothermal pretreatment and hot pressing process is a good way for conditioning hardwood sawdust for the production of binderless boards. The thermal softening of lignin, rich in phenolic hydroxyl groups, and increased condensed lignin structure contributed to the self-bonding formation of lignocellulosic materials.

Linear viscoelasticity of hot-pressed hybrid poplar relates to densification and to the in situ molecular parameters of cellulose

“…While this crystallinity increase remained small, the increase was consistent with treatment severity. The increase in crystallinity was attributed to the reduction in the amorphous region by crystallization of amorphous cellulose induced by hot pressing, in particular, under the plasticizing action of water (Bhuyian et al 2000;Xiao et al 2014). For the TCI method, band area was used after peak fitting, and this resulted in a better fit than using band height.…”

“…High-pressure steam pretreatment is also known to cause partial hydrolysis of hemicelluloses resulting Fig. 1 a FTIR spectra for control and hot-pressed poplar (9 % MC) samples and b expanded spectral region of hot-pressed poplar at 250°C and 9 % MC showing curve fitting of bands in increased wood compressibility (Hsu et al 1988;Xiao et al 2014). In the absence of moisture, the cell wall is more rigid and brittle and is easily crushed during hot pressing.…”

“…However, clearly evidenced here are quantifiable parallel trends between a molecular parameter of wood polymeric constituent and a macroscopic property of bulk wood at various degrees of heat compression. In addition to the changes occurring on wood carbohydrates, changes on lignin may play a role in alteration of wood hygroscopicity and viscoelasticity (Osman 2010;Xiao et al 2014). Tjeerdsma et al (1998) hypothesized that heat enhances the demethoxylation of guaiacyl and syringyl units in lignin, which increases the amount of reactive phenolic lignin sites available for reaction.…”

“…In addition, conformational changes in polysaccharides due to water desorption (Hakkou et al 2005b) and crystallization of amorphous regions were proposed to increase the cellulose crystallinity (Akgúl et al 2007). Hydrolysis of amorphous cellulose has also been reported and results in higher crystallinity index of cellulose in hydrothermally treated wood (Silva et al 2013;Xiao et al 2014). As a result, heat-treated wood exhibits improved dimensional stability and decreased hygroscopicity; for example, heat-treated wood conditioned at 96 % relative humidity experiences a moisture content decrease from 30 % to roughly 10 % but only a minor reduction of strength.…”

Chemisorption and sustained release of cefotaxime between a layered double hydroxide and polyvinyl alcohol nanofibers for enhanced efficacy against second degree burn wound infection

“…In spite of its low binding energy, the great importance of the hydrogen bond stems from its effects on the physical and chemical properties of the material, and is related to the degree of crystallinity, the regularity of the crystal system, and the amount of adsorbed water molecules bound. 55,56 In general, a decrease in the intensity or a disappearance of a band involved in the H-bond interactions means that this interaction is an 'intermolecular' one, and vice versa for 'intramolecular' interactions. The formation of intramolecular hydrogen bonding did not show any shi in absorption upon dilution, while the intermolecular did.…”