Characterization of fibre surface treatments in natural fibre composites by infrared and Raman spectroscopy

Abstract

The incorporation of natural fibers as filler and/or reinforcing materials in polymeric matrices is an increasingly growing area of work in industrial and academic fields. One of the major problems found in the research and development of these materials has been to reach good compatibility between the two main phases, fibers and matrices. It is perfectly established in the literature that the mechanical performance of the composites depends not only on the properties of the main components but also on the nature and strength of the interface, that is responsible of the load transfer from the matrix to the fibers. In order to solve this problem, different treatments of the natural fibers have been proposed to modify the bonding and interactions at the interface. Aiming at this final goal, different fiber treatments have been investigated using spectroscopic techniques, among others, applied to the surface of the fibers, before its incorporation to the composite and after compounding in other fewer cases. Fourier Transform Infrared (FTIR) and Raman spectroscopies have proved to be essential tools for the characterization of structure, environment, and dynamic of different types of materials, including fibers and particles, as it is the present case. FTIR is probably the most frequently used method for the analysis of chemical structure and identification of functional groups in polymers, natural fibers and their composites. However, the use of Raman spectroscopy has been gaining momentum during the last years, mainly as a complementary technique for FTIR. Chemical and physical changes in cellulosic fibers produced by different applied treatments can be followed by these techniques. Different FTIR methods, such as transmission, diffuse reflectance and attenuated reflectance IR have been applied to identify characteristic groups on the surface of treated natural fibers and particles. Modifications of the fibers are generally obtained by leakage of different components from the fibers (as in alkaline treatments), esterifications, reactions with isocyanates, irradiation treatments, etc. The extent of the reactions can affect just the surface of the fibers or most of their volume. The use of FTIR and Raman spectroscopic techniques applied to the recognition of changes in composition of the fibers due to leakage of original components and / or incorporation of different species attached or adsorbed on the fiber surfaces, as well as changes of morphology (changes in crystallinity) are some of the results that will be discussed in this chapter.