Effects of halloysite nanotubes on the mechanical, crystallization, and weathering behaviour of polylactide

Kaygusuz, İlker
The purpose of the first part of this thesis was to investigate the usability of industrially friendly twin screw extruder melt compounding method and injection molding melt shaping method to produce polylactide (PLA) / halloysite nanotube (HNT) composites for engineering applications. One of the main problems of PLA to be used in engineering applications is its inherent brittleness. Therefore, another goal was to reveal the effects of HNTs on the significant thermal and mechanical engineering properties of PLA nanocomposites, including fracture toughness values. Various procedures applied revealed that these industrial methods could be used successively in the production of PLA/HNT nanocomposites. Mechanical tests and thermal analyses indicated that engineering performance of these nanocomposites could be improved for the use in many applications. For instance, by using only 3 wt% HNT, the increase are 25% in flexural modulus, 54% in fracture toughness, 39% in storage modulus, and 100% in crystallinity amount.vi The purpose of the second part of this thesis was to investigate influences of HNTs on the (i) isothermal and (ii) non-isothermal crystallization kinetics of PLA by DSC analyses, and (iii) crystallinity of injection molded and then annealed specimens by DSC and XRD analyses. Due to basically very effective heterogeneous nucleation effect, addition of HNTs resulted in significant increases in the crystallinity of PLA under all three cases. Crystallization time parameters and Avrami rate constants indicated that crystallization rate increased under isothermal crystallization while it decreased under non-isothermal crystallization due to the delayed conformational mobility of PLA chains by the physical barrier actions of HNTs. Avrami exponent also revealed that two-dimensional growth mechanism of crystallites transformed into three-dimensional growth during non-isothermal crystallization while there was no change during isothermal crystallization. Crystallinity determinations of the injection molded and then annealed specimens indicated that, the highest crystallinity degree of PLA, i.e. 47%, could be reached by the addition of only 1 wt% HNT. The purpose of the third part of this thesis was to explore consequences of accelerated weathering on the behavior of PLA/HNT nanocomposites. Behaviors of the specimens were compared before weathering and after exposure to UV irradiation and humidity steps according to Cycle-C of ISO 4892-3 standards for a total duration of 300 hours. IR studies revealed that photolysis and partly hydrolysis were the main degradation mechanisms leading to random main chain scission of the PLA macromolecular structure, which was reflected with drastic reductions in the mechanical properties of flexural strength and fracture toughness. On the other hand, no reductions were observed in the values of thermal degradation temperatures, storage modulus and flexural modulus of the specimens, which was due to the compensating effect of higher amounts of crystallinity formed during UV irradiation cycles at 70°C.
Citation Formats
İ. Kaygusuz, “Effects of halloysite nanotubes on the mechanical, crystallization, and weathering behaviour of polylactide,” M.S. - Master of Science, Middle East Technical University, 2014.