Agro-industry feedstock and side stream materials for wood panel manufacturing

Abstract

Wood-based panels are indispensable in many areas, such as the construction industry and furniture production. The intensified demand for renewable materials, rising wood prices and increasing protection zones of forest areas make the wood panel industry consider alternative raw materials. The agricultural sector provides, at the same time, large amounts of sustainable and renewable lignocellulosic materials. By-products can arise along the entire agricultural production chain, i.e., during harvesting or further processing into food, but their potential has not yet been fully exploited. This thesis explored the potential of agro-industry feedstocks and side streams as raw materials for wood panel manufacturing. A literature review on the research of agricultural residues as a raw material in wood panels provided an overview of the investigated wood alternatives and their performance in final products. Most of the studies focused on the production of particleboard and its mechanical and physical properties. Often only up to 30% of wood could be replaced by alternative raw materials before the properties decreased remarkably.This thesis focused on an intensive material characterisation of barley husks (BH), oat husks (OH) and wheat bran (WB). Husks are the protective surrounding of their cereal grain and have an anatomical leaf structure. Wheat bran is a side stream of flour production and consists of the grain's outer layers. It was found that BH and OH have at 70% and 66% a slightly lower holocellulose content than wooden materials (poplar, spruce), while their hemicelluloses content exceeding that of cellulose. Additionally, WB had a very high lignin content of 43%. The chemical composition, especially the ash content (5% BH, 6% OH) and the high silicon occurrence on the husks’surfaces, reduced their wettability, as demonstrated by low contact angle measurements. Micromechanical tests showed that OH could resist a higher ultimate stress load than BH and WB, but the modulus of elasticity (MOE) was lower. The MOE was noticeably affected by the microfibril angle, which was three to four-times larger in the husks compared to wooden materials. Furthermore, the results of OH showed larger particle lengths and widths on average, approximately half as much extractive content and slightly higher thermal stability compared to BH. Therefore, OH was suggested as promising raw material and evaluated for particleboard manufacturing. In an experimental investigation, OH was explored as raw material in aspecial particleboard type, i.e., tubular particleboards. Although the boards showed higher insulation properties than wood particle-based ones, the mechanical properties were considerably affected by the reduced wettability, and the manufacturing method led to poor density distribution. In addition, the agricultural feedstock wheat starch, in combination with microfibrillated cellulose (MFC) and emulsifiable diphenylmethane diisocyanate (eMDI), was investigated as an adhesive system for fibreboard production. Wheat starch was modified to dialdehyde starch (DAS) and served as the backbone in an adhesive formulation of 99.5% bio-based content using 1% MFC and 4% eMDI based on DAS, which showed excellent mechanical and water resistance performance in fibreboards. Especially, internal bond and MOE values even exceeded those obtained in boards manufactured with commercial formaldehyde-based adhesive. The application process should be optimized in the future since the DAS was applied in powder form, and long press times were necessary because the adhesive system required a high-water content. The DAS-based adhesive was used to bond OH in particleboards, where as challenges in practical implementation were encountered. The severely shortened starch molecule reacted with the proteins of the OH, and from temperatures of 160°C, it led to accelerating degradation and reduced bonding capacity of the adhesive. Finally, this thesis provided a deeper knowledge of husked-based raw materials' properties in the context of panel manufacturing and showed that they are a possible but challenging alternative to wood. Further experimental investigations are necessary to improve the interfacial adhesion of OH and there spective adhesive system in order to produce panels with mechanical and physical properties that meet current requirements. The investigation of a DAS based adhesive opened a promising path for bio-based adhesives and the independence of formaldehyde systems. But subsequent studies must convert the used application method into a sprayable process for industrial integration