Heterophasic Propylene Research Articles

Solution and solid-state NMR characterization of heterophasic propylene–ethylene copolymers (HEPC) with increasing ethylene content

Solid-state and solution 13C NMR were used as complementary techniques to investigate changes in the molecular dynamics and chemical composition of different semi-crystalline fractions obtained using temperature rising elution fractionation (TREF) from heterophasic ethylene/propylene copolymers (HEPCs). The HEPCs were selected during a transition from homopolymer to copolymer and represent a range of total ethylene contents between 2 and 12mol%. It was found that these techniques yielded complimentary information, further breaking down the complexity of these polymers and providing observations that have not yet been obtained for these polymers by other experimental techniques. Ethylene dependent changes in the methylene region of solid-state spectra obtained for the bulk HEPCs was observed. Solid-state analysis of the semi crystalline fractions of these polymers provided more insight on this observation as well as on the distribution of these components between fractions with different crystallinities. From the solution NMR results, the distribution of different ethylene sequences between the fractions was evaluated and a statistical model proposed for this evolution. Furthermore the development of longer ethylene sequences observed by solution NMR was found to correspond with an increase of ethylene within rigid areas in the higher temperature semi-crystalline fractions. Thus a clearer understanding was obtained of how the different semi-crystalline copolymers, responsible for phase interactions and ultimately final product properties, develop with increasing ethylene incorporation.

Measuring Diffusion Coefficients of Nitroxide Radicals in Heterophasic Propylene−Ethylene Copolymers by Electron Spin Resonance Imaging

Heterophasic propylene-ethylene copolymers (HPECs), also known as impact-modified polypropylene copolymers (IPC), are an important class of materials because of their attractive mechanical properties and low cost. Their chemical composition and phase heterogeneity have important effects on the physical and mechanical properties of these systems. Commercially used HPECs usually contain stabilizers such as hindered amines, which can be lost by diffusion We present the measurement of the diffusion coefficient of nitroxide radicals by electron spin resonance imaging (ESRI); the radicals were obtained by oxidation of the commercial Tinuvin 770 hindered amine stabilizer (HAS) with m-chloroperoxybenzoic acid and consist of a mixture of mono- and biradicals. The diffusion rates were measured in two polymers, HPECI and HPEC2, which differed in the ethylene (E) to propylene (P) ratio, 25:75 and 10:90 wt %, respectively. One dimensional (1D) ESRI allowed the visualization of the radical distribution (profiles) within the sample as a function of storing time at 393 K. Simulation of this distribution led to the determination of the macroscopic diffusion coefficients, D, of the paramagnetic tracer in HPEC1 and HPEC2. The results indicate that the main factor determining the diffusion rates in these polymers is the amount of the crystalline phase, mostly composed of isotactic polypropylene (iPP). The D value was the same within experimental error in both systems, 1.42 x 10 -8 cm 2 s -1 . Comparison with literature data of the temperature variation of D for the same diffusant in polyethylene (PE) and polypropylene (PP) suggest that in both HPEC and HPEC2 the diffusion takes place in the amorphous phase restrained by the proximity of the crystalline domains. The results also indicate that ≈20% of the diffusant is lost by evaporation (blooming). Results of spectral-spatial 2D ESR1 show that the ESR spectra of the nitroxides do not change significantly along the sample length during diffusion, suggesting that the loss of stabilizer is due to diffusion and not to chemical reactions.

Stabilization and Early Degradation of UV-Irradiated Heterophasic Propylene−Ethylene Copolymers Based on ESR, ESR Imaging, UV−Vis, and DSC: Effect of Ethylene Content and UV Wavelength

Heterophasic propylene−ethylene copolymers (HPEC) containing 1 wt % bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were aged by exposure of ≈3 mm thick plaques to UVA and UVB sources in weathering chambers at 318 K. Two types of copolymers, HPEC1 and HPEC2, were examined, which contained 25 and 10 wt % ethylene (E), respectively, as ethylene/propylene rubber (EPR). Spatial and temporal effects of the aging process were studied by electron spin resonance (ESR) and ESR imaging (ESRI) of HAS-derived nitroxide radicals (HAS-NO), by comparison of thermal transitions in original and aged samples using DSC, and by measuring the UV−vis absorption of HPEC films as a function of their thickness. ESR spectra indicated the presence of nitroxide radicals in two amorphous sites, fast (F) and slow (S). The spatial distribution of HAS-NO was obtained by 1D ESRI. Nondestructive (“virtual”) slicing of the 2D spectral−spatial ESR images resulted in a series of ESR spectra ...

Thermal Aging of Heterophasic Propylene−Ethylene Copolymers: Spatial and Temporal Aspects of Degradation Based on ESR, ESR Imaging, and FTIR

Heterophasic propylene−ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were thermally aged at 393 and 433 K. Two types of HPEC were examined, containing 25 and 10% ethylene (E), respectively, as ethylene/propylene rubber (EPR). Spatial and temporal effects of the aging process were studied by electron spin resonance (ESR) and ESR imaging (ESRI) of HAS-derived nitroxide radicals and FTIR of films prepared by compression molding. The spatial distribution of the HAS-derived nitroxide radicals was obtained by 1D ESRI. These experiments enabled the visualization of an outer region of thickness ≈100 μm that contained a lower amount of nitroxides and believed to arise from the loss of the stabilizer by diffusion (“blooming”) and in chemical reactions during aging. Nondestructive (“virtual”) slicing of the 2D spectral spatial ESR images resulted in a series of ESR spectra that indicated the presence of nitroxide radicals in two amorphous sites, fast and slow; the corresponding relative intensity varied with sample depth. 1D and 2D ESRI allowed the detection of faster degradation of EPR in the amorphous phase represented by the fast spectral component. Both ESRI and FTIR experiments suggested a faster degradation rate in HPEC containing 25% E compared to 10% E; moreover, a larger Tinuvin 770 content in the polymers led to less efficient stabilization. FTIR spectra indicated increased ordering of polypropylene segments in HPEC during aging at 433 K.

Thermal Aging of Heterophasic Propylene−Ethylene Copolymers: Morphological Aspects Based on ESR, FTIR, and DSC

Electron spin resonance (ESR) spectra of nitroxide radicals formed in thermally treated heterophasic propylene−ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were studied in the temperature range 100−433 K; the nitroxides are derived from the HAS and are termed HAS−NO. The results were compared with ESR spectra of the same radicals obtained first by oxidation of Tinuvin 770 and then doped in HPEC and related homopolymers polyethylene (PE) and polypropylene (PP); these nitroxides are termed spin probes. ESR spectra indicated that HAS−NO and the spin probes in HPEC and homopolymers reside in a range of amorphous sites differing in their dynamical properties. Evidence for the various sites was obtained from the ESR line widths, the temperature variation of the extreme separation in the ESR spectra, and the effect of stress on the ratio of the fast and slow components in the spectra of HAS−NO in thermally treated HPEC. The relative population of sites was explained by assuming that the crystalline domains exert a restraining effect on chains located in vicinal amorphous domains; in PE the restraining effect was more pronounced in the polymer with the higher crystallinity (HDPE vs LDPE). Additional support for this assumption was provided by Fourier transform infrared (FTIR) spectra of HPEC and related polymers and by differential scanning calorimetry (DSC). The dynamically restrained chains evidenced by the spin probes are thought to be located in a rigid amorphous phase, which was described in the literature. In HPEC the fast spectral component represents unrestrained ethylene−propylene rubber (EPR) chains, and the slow spectral component represents amorphous PP and EPR chains in the rigid amorphous phase. This study has demonstrated the exceptional sensitivity of ESR spectra from nitroxide radicals to polymer morphology and degree of crystallinity.

Combining Ziegler‐Natta and mettalocene catalysis: New heterophasic propylene copolymers from the novel multicatalyst reactor granule technology

A new process that combines the excellent morphology control of Ti-based catalysts with the unique features of polyolefins from mettalocenes has been devised. First, propylene is polymerized with the Ti-based catalysts, having spherical form; then, activated mettalocene catalysts are introduced into the porous PP spheres, through an in situ impregnation and drying process, once the Ti has been deactivated. Then, the olefins polymerization, catalyzed by the mettalocene, is carried out in gas phase. This step, as well as the morphology of the as-polymerized polymer particles, has been investigated in detail. New heterophasic copolymers (PP/EPR) having free-flowing spherical form have been achieved, and the morphology of the copolymer spheres pointed out that the rubber is well dispersed inside the polymer granule. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1831–1837, 1997

Combining Ziegler-Natta and mettalocene catalysis: New heterophasic propylene copolymers from the novel multicatalyst reactor granule technology

A new process that combines the excellent morphology control of Ti-based catalysts with the unique features of polyolefins from mettalocenes has been devised. First, propylene is polymerized with the Ti-based catalysts, having spherical form; then, activated mettalocene catalysts are introduced into the porous PP spheres, through an in situ impregnation and drying process, once the Ti has been deactivated. Then, the olefins polymerization, catalyzed by the mettalocene, is carried out in gas phase. This step, as well as the morphology of the as-polymerized polymer particles, has been investigated in detail. New heterophasic copolymers (PP/EPR) having free-flowing spherical form have been achieved, and the morphology of the copolymer spheres pointed out that the rubber is well dispersed inside the polymer granule. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1831–1837, 1997