SUMMARY

      The purpose of this PhD thesis is to study and predict the properties and biodegradability of new biocomposites based of biodegradable polymers obtained from renawable resources. Two different starch-based thermoplastics have been used as polymeric matrices (Mater-Bi KE03B1 and Mater-Bi NF01U), and have been reinforced with different natural fibres (cotton, flax, kenaf, hemp, and jute) with the aim to prepare biodegradable materials with improved service life properties.
      The polymeric matrices, the natural fibres and the reinforced biocomposites were characterised, with the objective of assessing the influence of each natural fibre in the polymeric matrix properties. The methodology used involves the combined application of Spectroscopic Analysis (FTIR-ATR), Mechanical Analysis (Tensile Tests), Mophological Analysis (SEM) and Thermal Analysis (DMTA, TGA, DSC). Moreover, the properties of the reinforced biocomposites has been correlated with the chemical composition of the natural fibres. The degradability of the matrices and biocomposites has been studied through the performance of degradation in soil and water absorption tests. The hydrolysis process is considered as a first stage of the biodegradation  process. The degradation and absorption processes have been monotorised by means of changes in the thermal properties of the materials. In particular, the control parameters selected are the onset and the maximum rate of thermodegradation temperatures, and the activation energies of each thermal decomposition process. These parameters are used to control the thermal stability and designate the changes in the molecular environments that promote or hinder the thermodegradation process. These results are completed with morphological and spectroscopic studies.

      In general, these studies reveal some relevant results. The addition of the natural fibres on the polymeric matrices increases the stiffness of the matrix and its thermal stability, and reduces the break deformation. Moreover, the presence of the fibres increases the absorption capacity and assures the degradation of the polymeric matrix components. Each reinforced biocomposite shows specific properties depending on the matrix and natural fibre used, as well as their combination. The reinforced biocomposites of jute are the stiffest and toughest. On the other hand, although the presence of the rest of natural fibres (kenaf, hemp, cotton and flax) enhances the stiffness of the biocomposites, cotton and flax seem to promote a decrease in the thoughness. The thermal stability of the biocomposites increases with higher hemicellulose and lignin contents in the fibres, with kenaf and jute biocomposites displaying the greatest thermal stabilities. The addition of natural fibres to the polymeric matrices results in slower degradation of the starch component wherease the behaviour of the synthetic component depends on the hydrofility of the polymeric matrix. In the reinforced Mater-Bi KE biocomposites, the degradation of the natural fibres facilitates the microorganisms attack to the synthetic component from the begining of the test. Contrarialy, the synthetic component in the reinforced Mater-Bi NF present the same behaviour as the polymeric matrix. The biocomposites of flax shows the lowest capacity of degradation, while the presence of kenaf seems to promote the degradation of the synthetic component of the resulting biocomposites.

      This study confirms that the reinforcement of Mater-Bi NF and Mater-Bi KE with natural fibres broadens their applications, at the same time that the capacity of the degradation in soil is assured at the end of the service life. In the design of the reinforced biocomposites, the selection of one fibre will depend on the final required properties.