MITSUHIRO SHIBATA

ABSTRACT

In recent years, renewable resources-derived polymers (bio-based polymers) and composites (biocomposites) are attracting a great deal of attention because of the advantages of these polymers such as conservation of limited petroleum resources, possible biodegradability, the control of carbon dioxide emissions that lead to global warming. This chapter deals with the preparation, thermal and mechanical proper­ties of the bio-based network polymers prepared by bio-based epoxy resins and bio-based polyphenols, and their biocomposites with lignocellulosic fibers. As bio­based epoxy resins, glycerol polyglycidyl ether (GPE), polyglycerol polyglycidyl ether (PGPE), sorbitol polyglycidyl ether (SPE) and epoxidized soybean oil (ESO) were used. As bio-based polyphenols, tannic acid (TA) which is a hydrolysable tan­nin and quercetin (QC) which is a flavonoid were used. Also, the polyphenols (TPG) prepared by the reaction of tung oil (TO) and pyrogallol (PG) and guaiacyl pyrogal — lol[4]arene (PGVNC) prepared by the reaction of PG and vanillin (VN) were also used. As lignocellulosic fibers, wood flour (WF) made from Sanbu cedar crushed into powders through 3 mm screen mesh and microfibrillated cellulose fiber (MFC) were used. The thermal and mechanical properties of the bio-based polymer net­works and their biocomposites were investigated in detail by means of dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA) and tensile test. The morphology of the fractured surface of the biocomposites was observed by field emission-scanning electron microscopy (FE-SEM). Consequently, the SPE cured with PGVNC showed the highest tan 5 peak temperature (148 °C). The PGPE/TA/ WF and GPE/TA/WF biocomposites with WF content 50-60 wt.% showed the high­est Young’s modulus (4-5 GPa). The SPE/TA/MFC biocomposites with MFC con­tent 10 wt.% showed the highest tensile strength (80 MPa).

4.1 INTRODUCTION

Biocomposites which are composed of matrix resins and natural fibers such as wood and plant fibers have recently gained much attention due to their low cost, environ­mental friendliness, and their potential to compete with man-made fiber-reinforced composites.13 Furthermore, the concept of using bio-based polymers as matrix res­ins for biocomposites is becoming increasingly important due to dwindling petro­leum resources.45 Such biocomposites composed of bio-based polymers and natural fibers are especially termed as green composites. For example, the green compos­ites of bio-based polymers such as poly(lactic acid),614 poly(hydroxyalkanoate),1518 and cellulose acetate1920 with lignocellulosic natural fibers such as flax, jute, hemp, kenaf, abaca, bamboo, and wood flour have been reported by several groups. How­ever, the main problem encountered in using their bio-based polymers is its rather poor interfacial adhesion between the polar lignocellulose and the more hydropho­bic characteristics of those polymeric matrices. The poor adhesiveness results in a poor strength, a relatively low stiffness and high moisture uptake. Another major shortcoming of this type of matrix is the relatively low fiber content, typically of less than 50-60 wt.%. One way to improve the poor adhesion is a modification of the interface of matrix and fiber.689121516 However, the surface treatment normally increases both processing steps and its cost. Bio-based epoxy resin cured with poly­phenol-based hardener should be a good candidate for a matrix resin of the green composite, because high loading of natural fiber is possible due to a low viscosity of the resin before curing, and superior interfacial adhesion is expected due to the hydrogen bonding interaction between lignocellulosic fiber and hydroxypropyl (or hydroxyethyl) moiety formed by the curing reaction of glycidyl (or epoxy) group with phenol. In this chapter, after the promising bio-based aliphatic epoxy resins and bio-based polyphenol hardeners are introduced, the thermal and mechanical properties of the biocomposites using their bio-based epoxy resins/hardeners and lignocellulosic fibers are reviewed based on our previous studies.2126