Processing and properties of nanocomposites based on polylactic acid, chitin and cellulose

Sammanfattning: The production of bio-based and biodegradable nanocomposites has gained attention during recent years for environmental reasons; however, the large-scale production of these nanocomposites still poses challenges. The objective of this work has been to prepare bio-based and biodegradable nanocomposites via liquid-assisted extrusion and to gain a deeper understanding of the process and the relationship between the process, composition, structure and properties. Extrusion is a common industrial process and thus, the development of this technique for the preparation of bionanocomposites can promote the commercialization of these materials in future.In this work, nanocomposites based on polylactic acid (PLA), cellulose nanofibers (CNF), cellulose nanocrystals (CNC), and chitin nanocrystals (ChNC) with varying nanomaterial content were prepared via liquid-assisted extrusion using a plasticizer as a dispersing and processing aid. This process consists of dispersing the nanomaterial in a liquid composed of water, a plasticizer and/or a solvent, and then feeding this suspension directly into the extruder during the process. To be able to carry out this process successfully, parameters such as the amount of liquid, the liquid feeding rate or the water-to-solvent ratio, among others, should be taken in account.CNF and ChNC were produced from banana rachis waste and crustacean waste, respectively, whereas CNC were available as a commercial product. Glycerol triacetate (GTA) and triethyl citrate (TEC) were used as plasticizers, dispersing and processing aids. The effects of the liquids used during extrusion, the plasticizers and the nanomaterials in the PLA properties were studied. Furthermore, the effects of the cooling rate during the compression molding and the solid-state drawing on the properties of the PLA nanocomposites were investigated. Additionally, the effect of ChNC on the processing and properties of blown films was evaluated.The results presented in this work demonstrated that the use of water and a solvent during the liquid-assisted extrusion did not decrease the molecular weight of the PLA. It was also found that the feeding of nanomaterials in aqueous or aqueous/solvent suspension resulted in PLA micro-composite with lower mechanical properties than PLA. However, when a nanomaterial was fed together with a plasticizer, its dispersion and distribution into the PLA were progressively improved with increasing plasticizer content. The plasticized PLA nanocomposites showed improved properties compared to their respective counterpart without nanomaterials when the plasticizer content was ≥7.5 wt%. Furthermore, it was demonstrated that the properties of PLA can be tailored through the composition of the nanocomposite or during the processing. It was observed that the modification of PLA with only plasticizer in high amounts (20 wt%) resulted in enhanced elongation at break and toughness but it had negative effects on the thermal and mechanical properties; however, the incorporation of nanomaterials minimized these effects. The addition of a small amount of nanomaterial (1 wt%), either CNF, CNC or ChNC, to plasticized PLA resulted in enhanced mechanical properties. It was also demonstrated that the cooling rate during compression molding and the solid-state drawing significantly affected the crystallinity of the PLA nanocomposites and, thus, their final properties. The fast cooling rate during compression molding resulted in more flexible and transparent materials than when a slow cooling rate was used, and as a result, PLA films with different mechanical properties were obtained. The drawing of the PLA/CNF nanocomposite at a drawing temperature slightly above the Tg, a high draw speed and at the highest drawing ratio, resulted in the highest mechanical properties. It was also found that the increased toughness after adding CNF to the plasticized PLA or after drawing the PLA/CNF nanocomposite, was attributed to the occurrence of massive crazing effect as a result of the presence of CNF and its effect on the crystallinity and/or on the spherulite growth. Finally, 6 kg of plasticized PLA nanocomposite with 5 wt% of ChNC was prepared and used as a masterbatch to produce bio-nanocomposite blown films. The nanocomposite material showed easier processability during the film-blowing process when compared with the reference material without nanocrystals. In addition, the nanocomposite blown films exhibited higher tear and puncture strength, lower fungal activity and lower electrostatic attraction properties, which are favorable in packaging applications. In conclusion, this thesis shows that the liquid-assisted extrusion process is an excellent approach for producing PLA nanocomposites using cellulose and chitin nanomaterials. The results indicated that the addition of these nanomaterials, together with a plasticizer and further processing, can result in PLA nanocomposites with varied properties that can be used for packing applications. It was also shown that the processing technique presented can be a step forward for the large-scale production of bionanocomposites.