Abstract
AbstractBackgroundEucalyptus grandisis one of the most abundant biomass from plantation in many parts of the world. The binderless board were manufactured from hydrothermal pretreated fibers ofEucalyptuswood 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 fromEucalyptuswood, its hydrothermal pretreated fibers, and binderless boards were thoroughly investigated by chemical and spectroscopic methods.ResultsThe 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 ofEucalyptuswood, 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 untreatedEucalyptuswood and the hydrothermal pretreated fibers.ConclusionsBased 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.
Highlights
Eucalyptus grandis is one of the most abundant biomass from plantation in many parts of the world
The untreated Eucalyptus feedstock had a typical composition of hardwood with 43.4% cellulose, 22.3% hemicelluloses, and 31.7% lignin
The hot pressing process resulted in the binderless boards with reduced lignin contents and decreased the glass transition temperature, making the lignin more accessible to the fiber surface
Summary
Eucalyptus grandis is one of the most abundant biomass from plantation in many parts of the world. The softening point of lignin (spherical droplets) after streaming treatment is believed to be lower than that of the original material and this makes it possible for a plastic flow to occur in situ [10] These lignin droplets were found by hotwater extraction or dilute acid pretreatment of lignocellulosic biomass [9,11,12]. Steam pretreatment can cause partial hydrolysis of hemicellulose for both hardwoods and softwoods which markedly increases the compressibility of wood and in turn significantly reduces the build-up of internal stresses in composites during hot pressing [13,14]. The self-bonding mechanism during hydrothermal treatment and hot pressing has not been completely elucidated so far
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