Gui-Fang Shan

Mechanical Properties and Morphology of LDPE/PP Blends

Two types of polypropylene (PP) with different molecular structure, namely, homogeneous PP (PPH) and PP block‐copolymer (PPC), were blended with a long chain, branched, low density polyethylene (LDPE) in a twin screw extruder and then injection moulded into test specimens; the mechanical properties and morphology of the blends are reported. The tensile strength, elastic modulus, flexural strength, and flexural modulus of the blends increased monotonically with increasing PP content, although exhibiting a slightly negative deviation from the rules of mixtures due to the relatively poor compatibility of the components, which caused the blends to separate into individual phases. Comparatively, these mechanical properties of the LDPE/PPH blend were much higher than that of the LDPE/PPC blend, which was attributable mainly to the fact that the mechanical properties of neat PPH are stronger than that of neat PPC. With respect to the impact strength of the blends, a maximum value appeared in LDPE/PPH blends when PPH content was about 20% and also in LDPE/PPC blends when PPC content was about 40%.

Investigation on Tensile Deformation Behavior of Semi-Crystalline Polymers

The strain rate, temperature, and microstructure-dependent, tensile-yielding behavior of three semi-crystalline polymers, namely high-density polyethylene (HDPE), polyamide 6 (PA6) and low-density polyethylene (LDPE), was investigated. It is found that, depending on the strain rate and temperature, the three polymers exhibit markedly different tensile deformation behavior, especially the shape of the stress-strain curves. LDPE exhibits a uniform extension and shows no obvious geometrically unstable effect, such as necking, during the overall tensile process. HDPE and PA6, on the other hand, show clear necking and cold-drawing phenomena during the uniaxial tensile process. When considering the effect of strain temperature on necking, significant differences between HDPE and PA6 emerge. For both, the heterogeneous necking disappears and homogeneous deformation occurs with increasing temperature. For HDPE, the homogeneous deformation takes place in the vicinity of the melting temperature, while for PA6, it takes place close to the glass transition temperature instead. The conventional yield point, corresponding to the force maximum in stress-strain curves, becomes less defined as the testing temperature is increased. It is applicable, to some extent, to combine the Brereton analysis and Considère construction to predict such a point quantitatively. However, this combination can only be suitable for homogeneously deformed material. In addition, it is found that the special, double yielding behavior will take place under certain deformation conditions for all three semi-crystalline polymers. With respect to judging the appearance of the double yielding of polymers, it seems that it can be estimated qualitatively by plotting the compression residual strain-applied strain curves of the samples.

The β Phase of Isotactic Polypropylene in TPVs Based on PP/EPDM

β‐Nucleating agent was added into maleic anhydride (MAH) compatiblized isotactic polypropylene (iPP)/ethylene‐propylene‐diene monomer (EPDM) blend systems, dynamically vulcanized by dicumyl peroxide (DCP). The β phase of iPP formed in TPVs was investigated by means of WAXD, and the effect of DCP content, MAH content and system viscosity on the β phase of iPP was also studied. With increasing DCP content, the MFR values of blend systems increased a little and then decreased, indicating that the DCP content showed a remarkable effect on the system viscosity. Also, with increasing MFR, the β phase content increased first and then decreased, while with increasing MAH content, the β phase content decreased sharply.