SURFACE STRUCTURE FORMATION OF CUPRAAMMONIUM RAYON FIBERS

Abstract

The formation of the characteristic surface of cupraammonium rayon fibers was investigated using the electron microscope. The rough surface of cupraammonium rayon, similar to that of pine trees, was often considered a result of the coagulation that takes place under flowing at large deformation rate in the coagulation zone of the spinning process. In this experiment, never-dried gel fibers were obtained at a stage just before the drying process of the production line. Water in the gel fibers was first removed by a centrifuge, and then the fibers were dried under various drying conditions. Freeze and critical-point drying methods were employed for sample preparation to reduce distortions of the structure of the dried fibers. Electron microscopic observation proved that the surface layer of the gel fibers consists of microfibrilar networks. The water content leveled off at about 400% when fibers being dried with the use of the centrifuge. The surface topology of the fibers up to 100% moisture content approximately preserved the fibrilar structure such as the never-dried gel fibers. Upon further drying, fibril to fibril contact caused the fibrilar networks collapse and a new surface developed. The surface structure of cupraammonium rayon fiber, therefore, largely depends upon the drying process. Air drying at room temperature formed a relatively smooth surface. The rough and granulated surface appeared with the rapid drying in an air oven at high temperatures. Shrinkages and/or wrinklings of the fibrils introduced during the drying process played an important role in forming the characteristic surface structure. The experimental results show that the drying process rather than the coagulation process, governs the formation of the surface structure of the cupraammonium rayon fibers.