マルチセル構造を有する天然繊維の引張負荷による回転現象: ケナフ繊維による確認実験と理論考察

摘要

昨今，環境負荷低減の観点から，亜麻，大麻，ジュート，ケナフなどの植物系天然繊維が複合材料の強化材として多々用いられるようになった.このような複合材料は，一方向単層板や射出成形品として幅広い実用化が期待されている.一般に，植物系天然繊維は単繊維ではなく，多くの素繊維細胞(以下，セルと略記する）が集まって構成された繊維束状構造体である.セル内にはセルロースミクロフィブリル（Cellulose microfibrils,以下CMFと略記する）と呼ばれるセルロースの集合体がセルの軸周にらせん状に配向する.CMFのらせん配向角度（以下，CMF角と略記する）は，天然繊維の強度•剛性を左右する重要なパラメータであり，実験的解析的にCMF角の役割が明らかにされている.このような生体由来内部構造と機械的性質の関係について，従来はセル1本から成る天然繊維(以下，シングルセル繊維と記す）に基づいて論じられてきたが,最近では上述の多数のセルから成る天然繊維(以下，マルチセル繊維と記す）を対象とした議論も展開されている.%The purpose of this study is to confirm a rotation phenomenon around the fiber axis of a kenaf fiber, a representative multi-cell type fiber, during tensile loading, and clarify the rotation mechanism through 3D finite element analysis. The degree of rotation was quantified by measuring rotation angle during the tensile test of a single fiber, by giving a free mechanical condition perpendicular to the tensile direction. Results showed that, although many fiber specimens did not rotate during tensile loading, the rotation was confirmed for approximately one fourth of all the specimens. Young's moduli of the rotation identified specimens are almost the same level as non-rotating fibers, while these fibers indicated a slight decrease in strength, as compared with non-rotating fibers. The coordinates transformation matrix simulating a spatial spiral structure of cellulose microfibrils (CMF) was constructed, and applied for 3D finite element analysis. The calculation results show that rotation occurs in one-cell and seven-cell finite element models, but the magnitude of rotation is much smaller in the seven-cell model. Furthermore, a cell twisted finite element model was carried out to simulate a small-scale disorder in the multi-cell orientation structure, which was observed on the kenaf fiber surface. Finally, it is estimated from the calculation results that rotation occurs only for the fiber in which the mechanical balance between CMF spiral structure and multi-cells' intertwist is not enough.