287680 Polypropylene (PP)-Elastomer Nanocomposites: Effect of PP and Elastomer MFI On Toughness and Thermal Expansion Behavior
Extruded PP/PP-g-MA/MMT/elastomer nanocomposites were prepared at fixed 30 wt% elastomer and 0-7 wt% MMT. The ratio of maleic anhydride grafted polypropylene, PP-g-MA and organoclay was fixed at 1.0. Elastomer particle morphology in the presence of MMT was evaluated for three different molecular weight grades of polypropylene (PP) (H= high, M= medium and L= low) and five different ethylene-co-octene elastomers (EOR) with different melt flow index (MFI). The MMT particles are located exclusively in the PP phase in the PP/PP-g-MA/MMT/EOR nanocomposites as seen from TEM images. PP/PP-g-MA/MMT/elastomer nanocomposites with toughness in range from 450 J/m to 800 J/m (super toughness) are obtained based on the molecular weight of the PP, elastomer type and MMT content. Elastomers having a melt flow index (MFI) in the range of 0.5-1.0 showed significant improvement in the impact strength of L-PP based nanocomposites while most H-PP/EOR blends gave ‘super-tough’ materials without MMT and maintain this toughness in the presence of MMT. For the similar elastomer particle sizes in nanocomposites, the impact strength varies as H-PP> M-PP> L-PP. The ductile-brittle (D-B) transition temperature decreases with increased molecular weight of the PP and the addition of MMT. Elastomers having a melt flow index (MFI) in the range of 0.5-1.0 showed a significant decrease in the D-B transition temperature for both L-PP and H-PP nanocomposites.
The effect of PP-g-MA/organoclay ratios varying from 0 to 1.5 was also studied on the toughness of L-PP/PP-g-MA/MMT/EOR nanocomposites prepared from elastomer having MFI of 0.5. The elastomer particle size reduced significantly and toughness is observed at lower MMT content as the PP-g-MA/organoclay ratio is increased. The PP-g-MA facilitates dispersion of MMT in the PP matrix which acts as a barrier to coalescence of elastomer particles leading to much smaller particle sizes at lower MMT content. The thermal expansion coefficient is significantly reduced in the presence of MMT and as the PP-g-MA/orgnoclay ratio is increased. The tensile modulus and yield stress improved with the MMT content and PP-g-MA/organoclay ratio. Overall, extruder-made TPO nanocomposite provides a well balance of stiffness and toughness and offers the advantage of using different molecular weight grades of PP and elastomer, and MMT to formulate TPOs in terms of particle size control and properties based on end use applications.
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