Mechanical, thermal and morphological properties of montmorillonite filled linear low density polyethylene-toughened polylactic acid nanocomposites

Abstract

Linear low density polyethylene (LLDPE) toughened polylactic acid (PLA) nanocomposites containing organophilic modified montmorillonite (MMT) were prepared by melt extrusion using a counter-rotating twin-screw extruder followed by injection molding in order to examine the mechanical, morphological and thermal properties of the nanocomposites. The mechanical properties of PLA/LLDPE nanocomposites were studied through tensile, flexural and impact tests. Scanning electron microscopy (SEM) was used to investigate the phase morphology and LLDPE particle’s size in PLA/LLDPE blends and nanocomposites. X-ray diffraction (XRD) was employed to characterize the formation of nanocomposites while the thermal properties were determined using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The dynamic mechanical properties were examined via dynamic mechanical analysis (DMA) while moisture permeability properties of the PLA/LLDPE nanocomposites were assessed through water absorption and hygrothermal aging. Subsequently, for PLA/LLPDE blends, the loadings of LLPDE were varied from 5-15 wt% and PLA/LLDPE nanocomposites with 2 phr and 4 phr loadings of MMT were prepared only for the optimum formulation (10 wt% of LLDPE). The results showed that the blending of LLDPE significantly increased the toughness but at the expense of stiffness and strength. Conversely, the incorporation of the MMT increased the stiffness, while the toughness and strength decreased. The PLA/LLDPE nanocomposites containing 2 phr of MMT and 10 wt% of LLDPE had the best balance of stiffness, strength and toughness. The impact strength results also proved that PLA nanocomposites were successfully toughened with LLDPE. XRD established that MMT were well dispersed and preferentially embedded in the PLA phase. SEM revealed that blend ratio and the presence of MMT were found to influence the morphology (e.g. LLDPE particle size and distribution) of the system. Finer particles’ size and better distribution of LLDPE has been observed in higher MMT loadings in the system. The SEM micrographs also revealed that increasing content of LLDPE has increased the particle size of LLDPE in PLA. DMA analysis discovered that the storage modulus at 30ºC increased with the presence of MMT for PLA nanocomposites. The DSC results showed that the crystallization temperature (Tc) dropped gradually with increasing content of MMT for both PLA and PLA/LLDPE nanocomposites while the glass transition (Tg) and melting temperature (Tm) remained unchanged. TGA also exhibited an increase in T10% decomposition temperature for PLA and PLA/LLDPE nanocomposites. Water absorption curves obeyed the Fick’s law with rapid moisture absorption to maximum saturation level (Mm) and the value of Mm of PLA increased with addition of LLDPE and 2 phr of MMT. Hygrothermal aging revealed that the Mm increased significantly at elevated temperatures (60ºC and 90ºC) and addition of LLDPE and MMT improved the hygrothermal stability of PLA.