Aggregated understanding of characteristics of wheat straw node and internode with their interfacial bonding mechanisms

Abstract

The demand for the efficient utilisation of straw biomass requires detailed analyses of its fundamental chemical structures, morphological complexity, individual cell wall components and the correlation of physicochemical to mechanical properties. The study involved two main areas: understanding the details of microstructure and characterisation/differentiation of properties of various profiled wheat straw. Comprehensive and systematic experimental programmes were therefore designed in order to thoroughly investigate the node and internode of wheat straw with quantitative appraisals and qualitative interpretations. This could contribute towards its valorisation in bio-refinery pathways. The sophisticated morphology of node and internode, inner and outer surface was investigated. It was found that the morphology across node area has a great variety when the longitudinal profile is investigated in the upwards direction to grain head. A 3D image of nodes illustrated the dense core with elliptical shaped rings organised in order to provide the echanical strength to the overall stem. The variation of cell wall composition across wheat straw node and internode showed that node yielded slightly higher Klason lignin, extractives and ash content than internode, which could be related to their morphology, precisely the higher ash and extractives content in the node are explained by thicker epidermis tissue. The physicochemical and mechanical properties of node and internode were differentiated and the effects of a combination of mild physical pre-treatment were monitored. The results indicated: i) the reduction of waxes from the outer surface, ii) significantly lower (P < 0.05) extractives and iii) the dissolution of silicon (Si weight %) on the outer surface of node and internode. The tensile strength of nodes and internodes after pre-treatments also resulted in a significant increase (P < 0.05). The accumulated characteristic data enabled the investigation of interfacial properties and bonding mechanisms of the inner and outer surface of wheat straw with thermosetting resins. Different surface functionalities and anatomical sections, altered the bonding performance, i.e. waxes and silica concentrated on the outer surface inhibited the quality of the interface. Nevertheless, the treatment improved interface (P < 0.05) between resins and the micro-porous surface of wheat straw by causing the microcellular structure of straw to expand and hence inspire the mechanical entanglement on a micro level upon resin solidification.