A study on poly(lactic acid) and natural rubber blends compatibilized by organoclays

Abstract

In this thesis, the effect of organoclay as a compatibilizer in immiscible poly(lactic acid) (PLA) and natural rubber (NR) blends was studied. Structural development of PLA90/NR10 blend was examined depending on the clay content (0.5~10wt%). The drop size of the dispersed NR phase initially decreased and then remained constant as the clay content increased. Below the critical concentration (3wt%), the clay was selectively localized mostly at the interface between the matrix phase (PLA) and the dispersed phase (NR), whereas the clay particles were additionally observed in the matrix phase above 3wt%. XRD analysis revealed that the gallery structure of the clay was highly dependent on the concentration and selective location of the clays. The development of the structure and the physical properties of the blend were also investigated according to the clay contents by measuring the linear viscoelasticity and surface tension. The degree of clay exfoliation was analyzed quantitatively in terms of the modulus ratio of PLA/NR/clay and PLA/clay. The elongational behavior and mechanical properties of the blends were strongly dependent on the location of the clays. The clay worked as an efficient compatibilizer below the critical content, while it worked as a reinforcing filler above the threshold, making the matrix more rigid. Next, the compatibilizing effect of organoclay in immiscible poly(lactic acid) (PLA) and natural rubber (NR) blends was investigated through small amplitude oscillatory shear (SAOS) and large amplitude oscillatory shear (LAOS) tests by varying clay concentrations, mixing conditions and types of clay. A quantified nonlinear parameter (I3/1) from FT-rheology and complex moduli (

G*

) were used to analyze the structural development of the PLA/NR blends in response to added clays. The nonlinear-linear viscoelastic ratio (NLR) was also introduced to describe morphological changes in the blends. Observation of the TEM images revealed that the drop size of natural rubber decreased as the clay content increased from 0.5 to 3 wt%. However, the NR size did not change above 3wt% of the clay. The NLR value increased up to 3wt%, then reached a plateau. The drop size reduction due to increased mixing rates was reflected in the NLR value. In addition, the compatibilization efficiency of various types of clay was in accordance with the NLR value. A reciprocal relationship was observed between the NLR and inverse of the drop size, with respect to the clay concentration, mixing condition, or types of clay. The NLR value could be used to estimate the compatibilizing effect of organoclay in immiscible polymer blends (PLA/NR), even without direct morphological observation.