Study On Modification Of Polypropylene And Its Rheological Behavior And Foamablity

Isotactic polypropylene (iPP) is one of the leading and fastest-growing polyolefins due to their attractive properties, such as high melting point, low density, excellent chemical resistance and high tensile strength. Most of commodity iPP is produced by Ziegler-Natta or metallocene catalysts, resulting in highly linear chains with a relatively narrow molecular weight distribution. As consequent of this, the iPPs possess low melt strength, limiting uses in processes involving substantial elongational flow, such as thermoforming, film blowing, extrusion coating, blow molding and foaming. It is necessary to improve the melt strength of iPP in order to further widen its applications.This thesis is composed of five parts:1. We report our investigation on the possible presence of physical cross-links in polypropylene containing long chain branches (LCBPP) or amine moiety (PP-g-NH2), The materials were prepared via reaction between maleic anhydride grafted polypropylene (PP-g-MAH) and ethylene diamine (EDA). Through shear and extensional rheometry, we investigated the materials behavior in both linear and nonlinear viscoelastic regimes. We found that in many aspects the shear response of the linear PP-g-NH2 resembled that of a networked system. Furthermore, the uniaxial extensional measurement revealed that such network might possess considerable strength. The observed behavior can be attributed to the presence of physical cross-links in the modified products, which are in the form of phase separated domains and hydrogen bonding. Such physical cross-links are also present in some of the LCBPPs and the observed behavior of these modified products reflects the combined effects of LCB and physical cross-linking.2. We coupled both the MAH grafting and long chain branching reaction to produce the LCBPPs in one step through reactive extrusion. Detailed structural and rheological characterizations were conducted subsequently. The investigation indicated that the concentration of DCP as free radical initiator plays the key role in control of the chain structure. The study presents a versatile methodology to tailor the degree of long chain branching.3. In this study, four kinds of PP ionmers were prepared by grafted special function group onto the backbone of the PP chain. The relative degree of ionic interaction in these products can be summarized in the following order:Zn Salt> AM-Zn salt> MAM-Zn salt> DMAM-Zn salt. The result indicated that the strength of the ionic interactions can be varied simply by varying the substituent.4. The modified PPs were foamed in a batch process by depressurization and injection molding. A significant improvement in foamability and foam stability of the PPs was observed for LCBPPs, comparing with PP. The physical cross-linking method can also used to improve the foamability of the modified PPs. Moreover, foaming of amine functionalized PP by supercritical carbon dioxide (scCO2) is further facilitated by the favorable interactions between amine group and CO2.5. A mathematical model was established to simulate the bubble growth process during the foaming of the PPs by scCO2, taking into account of a wide range of physical and rheological properties (solubility, diffusivity, surface tension, long chain branching, zero shear viscosity, relaxation time, strain hardening), as well as processing conditions. By employing the Considere construction, the possibility of growth instability and bubble rupture at later stage of bubble growth was predicted. The simulation revealed that the improvement of foamability of the PPs by introducing long chain branching was due to the well-defined viscoelastic characteristics of the melt. Rheological factors that impede bubble growth are beneficial in stabilizing the bubble growth. Stability during bubble growth further facilitated by moderate strain hardening characteristics and elastics properties of the PPs. The diffusivity, solubility characteristics also have profound impact on the bubble growth stability, while the influence of the surface tension is negligible.