Mechanical properties of thermally treated hemp fibers in inert atmosphere for potential composite reinforcement

Abstract

Hemp (Cannabis Sativ L.) is an important lignocellulosic raw material for the manufacture of cost-effective environmentally friendly composite materials. From an earlier study it was found that when hemp bast fibers were heated above the glass transition temperature of lignin, there was a migration of lignin to the surface of the fiber. The preliminary observations showed that heat treatment in inert environment seemed to provide enough fiber opening without affecting the associated tissues of the fibers. Here, hemp fibers were given heat treatment in an enclosed vessel in air as well as inert environment and their mechanical properties were compared to the raw hemp fiber. It was found that there were openings of fibers upon heating, both along the length as well as along the diameter or the width directions. For the same weight of the fiber, the total count of fibers increased during heat treatment, with increment up to 32% for inert environment and 39% for air environment; the increment was mainly due to opening up of fibers into lesser diameters than the original fibers. The strength properties were strongly influenced by the diameter of the fibers, with the lesser fibers contributing to greater tensile strength and modulus. The overall tensile strength and modulus of fibers treated in inert environment were found to have increased, probably due to production of fibers of lesser diameters, presumably with less number of natural defects. The overall strength of fiber treated in air environment, however, decreased even though there was opening up of fibers in this case as well. This was due to oxidation of various constituents of fiber which contributes strength.