Natural Fibers are derived from plants which are rich in lignin and cellulose. For the fibers to be used as a drop-in replacement for traditionally used glass fibers, they have to undergo several types of mechanical and/or chemical processes to be compatible with polymer matrix. The most common and usually the first step is delignification process in which the outer high molecular weight lignin is stripped using chemical process similar to the Kraft pulp process. After delignification, the remaining cellulosic content is exposed on the surface. Cellulose is a macropolymer consisting of linear chain of several D-Glucose molecules. It is a hydrophilic constituent of cell walls of green plants. It is the ability of these chains to hydrogen-bond together (intramolecular) and to water (intermolecular) that gives cellulose its unique fibrous properties and also mechanical strength and chemical stability.
Cellulose by itself is highly unreactive due to the presence of inter and intra molecular hydrogen bonds. Polymers such as polypropylene are hydrophobic in nature. TO make composite materials with good tensile strength and dispersion of natural fibers within the polymer, the hydrophilic cellulosic fibers are often subjected to chemical treatments so that they can be made compatible with the hydrophobic polymer matrix. The main mechanism of promoting Fiber-Polymer adhesion or compatibility is to be able to break the intra and inter molecular hydrogen bonds so that the –OH group of the fibers can be accessed to modify the surface.
The most common surface treatments are as follows:
- Silane treatments– Various silanes can be grafted on the fiber surface to promote compatibility with polymer matrix. Some examples are vinyl triethoxy silane, Aminopropyl tri ethoxy silane, Isocyanonato propyl tri ethoxy silane etc.
- Coupling Agents – Coupling agents such as Maleic Anhydride are commonly used to promote compatibility of the fibers with the polymers
- Acetylation– The fibers are acetylated using acetic anhydride. The acetylation makes the fibers rough thereby increasing their compatibility in the polymer and also reduce moisture absorption.