(708c) Heterogeneous Morphologies, Crystallization Behaviors, Rheological and Thermo-Mechanical Properties of Thermoplastic Polyolefins of Ipp and Obc Blends

High flow
impact thermoplastic polyolefin (TPOs) compounds are widely used in automotive
industry as exterior and interior parts. The injection molded TPOs lack
sufficient melt strength that limits their expanded applications in high
temperature and shear fabrication processing. Next generation high melt
strength impact thermoplastic elastomers (HMS-TPOs) possess higher shear
rheology response and extensional viscosity. They have extensive applications
in extrusion profiles, thermoforming and extrusion blow molding. However, inhomogeneous
multiphase morphologies of TPOs are realized and multiphase transition occurs
during processing,^[1-2] which
markedly influence stiffness, impact resistance and other properties.^[3-4]
The multiphase morphologies and transition depend on the relative viscosity, relative
thermal expansion, stress strass, interfacial tension, and mostly on intrinsic
instability.^[1-2]

Rigid
TPOs generally consist of a blend of polypropylene (iPP,
75 - 80 wt%),
a high performance impact modifier (20 - 25 wt%), and
a filler (10 wt%). Compared to statistical ethylene-octene copolymer (POE), the olefin block copolymers (OBC) exhibits
higher heat resistance, higher crystallization temperature, and better impact
resistance in TPOs.^[3] The multiphase morphology and related
properties of iPP/OBC-based HMS-TPOs have not been
studied. In this work, the influence of weight fraction and molecular structures
of OBC on the multiphase morphologies of compression molded HMS-TPO are
studied. The scanning electron microscope results demonstrate that HMS-TPOs,
composed of 70wt% HMS-iPP and 30wt% OBC with low melt
mess flow rate (MFR) and low density, have co-continuous morphology.
Interestingly, the HMS-TPOs containing 70wt% HMS-iPP
and 30 wt% OBC with moderate MFR and low-to-high
densities have heterogeneous phase morphologies of co-continuous skin and sea-island
core (average size of dispersed OBC ¡Ö 1
um) ; the gradient sea-island morphologies (average size of dispersed OBC = 714 ¨C
968 nm) are observed in the HMS-TPOs containing 85 wt%
HMS-iPP and 15 wt% OBC with
moderate MFR and high density, as well as the L-TPOs containing 70 wt% linear iPP and 30 wt% OBC with moderate MFR and low-to-high densities.

The
crystallization behaviors, shear rheological properties and thermo-mechanical
properties of these HMS-TPOs are also investigated, and compared with that of L-TPOs.
The X-ray powder diffraction profiles show that (040) planes of HMS-iPP in HMS-TPOs (70 wt%, 50 wt% iPP) are orientated to a
greater extent along the surface of the compression molded sheets than that of pure
HMS-iPP. The DSC analyses show that melting temperature
and crystallization temperature of HMS-TPO do not decrease with increasing weight
concentrations of OBC. In addition, the complex viscosity of TPOs do not
significantly decrease and the melt elasticity slightly increase as compared
with that of pure iPP, according to rotational
rheology testing (ARES). The strain-stress measurements indicate that the OBC
with low MFR and low densities are more suitable for achieving higher the
stiffness and toughness for the HMS-iPP. The HMS-TPOs
have yield strength of 17 - 20 MPa and elongation at break of 900% compared to that
of 30 MPa and 225% for pure HMS-iPP.

High
energy irradiation cause chain scission in iPP
matrix, and cross-linking in dispersed elastomer phase, and grafted chains
between iPP and elastomers, which result in in-situ compatibilization of iPP with
elastomers and higher mechanical properties.^[2,4]
Radiation-induced crosslinking can stabilize the optimum morphology during
injection molding.^[2] Therefore, the influence of high energy
irradiation on the morphological transition and mechanical properties of HMS-iPP will be investigated.

References.

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3.     G. Liu, X. Zhang, C. Liu, H. Chen, K. Walton, D. Wang. J. Appl. Polym. Sci.,
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4.     P. Svoboda, D. Svobodova, P. Slobodian, T. Ougizawa , T. Inoue. Poly. Test., 2009, 28, 215 - 222.

5.     M. R. Aghjeh, H. A. Khonakdar,
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