Tensile properties and morphology of blends of polyethylene and polypropylene

The tensile behavior of blends of linear polyethylene (PE) and isotactic polypropylene (PP) was examined in relation to their morphology. Yield stress increases monotonically with increasing PP content, while true ultimate strength is much lower in all blends than in the pure polymers as a result of early fracture. The blends fail at low elongation because of their two-phase structure, consisting of interpenetrating networks or of islands of PE in a PP matrix, as shown by scanning electron microscopy of fracture surfaces and transmission electron microscopy of thin films. While spherulites in PP are very large (～100 μm in diameter), addition of 10% or more of PE drastically reduces their average size. This, together with the profusion of intercrystalline links introduced by PE, may be associated with maximization of tensile modulus in blends containing ～80% PP. Introduction of special nucleating agents to PP reduces average spherulite size and is accompanied by slight improvements in modulus. Thin films of blends strained in the electron microscope neck and fibrillate in their PE regions, but fracture cleanly with little fibrillation in areas of PP.     

Linear low-density polyethylene addition to polypropylene/elastomer blends: Phase structure and impact properties

Morphology features and effects of particle size and composition of the disperse phase on the impact properties have been studied for the blends of isotactic polypropylene (PP)/ethylene-propylene-diene terpolymer and (EPDM)/linear low-density polyethylene (LLDPE). The blend components were mixed in a twin-screw extruder, press molded, and analyzed by scanning electron microscopy, SEM (fractured and toluene etched samples), and by transmission electron microscopy, TEM (RuO₄ stained samples). TEM was most effective for the identification of component distribution and particle size measurement. An increasing degree of LLDPE and EPDM interpenetration was observed with the PE content. Not one case of a neat component separation was detected. LLDPE addition improves the EPDM dispersability, affecting mainly the larger particles. The impact properties at room temperature were especially dependent on the rubber content, whereas at low temperature the particle diameter appears to be the controlling parameter. The affect of LLDPE on blend toughness is more evident in the latter case.     

The reactive extrusion of polyethylene/polypropylene blends

The objective of this work was to investigate the compatibilization of a blend of linear low density polyethylene with polypropylene by Injection of a free radical initiator during extrusion. The reactive extrusion process utilized a single screw extruder equipped with two static mixers. The initiator was injected into the extruder feedport and temperature programming used to cause most reaction to occur within the static mixers. Although elongation at yield was increased by 37 percent, impact strength and yield strength decreased by 17 and 54 percent, respectively. Scanning electron microscopy showed that the maximum size of the dispersed phase decreased from a maximum size of four microns to less than two microns upon addition of initiator. Size exclusion chromatography (SEC), temperature rising elution chromatography (TREF), and differential scanning calorimetry showed that the polypropylene in the blend was degrading while the polyethylene was increasing in molecular size. The combination of SEC and TREF was particularly useful in elucidating this result. No copolymer was discerned by any of the methods used.     

Adhesion of polyethylene blends to polypropylene

The effect of chain microstructure on adhesion of ethylene copolymers to polypropylene (PP) was studied using coextruded microlayers. Adhesion was measured by delamination toughness G using the T-peel test, and interfacial morphology was examined by atomic force microscopy. Good adhesion to PP was achieved with homogeneous metallocene catalyzed copolymers (mPE) with density 0.90 g cm⁻³ or less. Good adhesion was attributed to entanglement bridges. In contrast, a heterogeneous Ziegler-Natta catalyzed copolymer (ZNPE) of density 0.925 g cm⁻³ exhibited poor adhesion to PP due to an amorphous interfacial layer of low molecular weight, highly branched fractions that prevented effective interaction of ZNPE bulk chains with PP. Blending mPE with ZNPE eliminated the amorphous interfacial layer and resulted in epitaxial crystallization of ZNPE bulk chains with some increase in G. Increasing the mPE content of the blend past the amount required to completely resolve the amorphous interfacial layer of ZNPE resulted in a steady, almost linear, increase in G. Phase separation of mPE and ZNPE during crystallization produced an interface with regions of epitaxially crystallized ZNPE bulk chains and other regions of entangled mPE chains. Entanglement bridges imparted much better adhesion than did epitaxially crystallized lamellae.     

Ultralow density polyethylene blends with polypropylene

Two types of ultralow density polyethylene (ULDPE) of different melt viscosities were blended with a polypropylene (PP) in a twin screw extruder. Morphology, thermal, rheological, and mechanical properties of the blends were determined. Morphological observation from SEM showed a clean phase separation of PP/ULDPE blends. However, depending on the viscosity ratio, a significant difference in the extent of phase separation, as well as in the phase inversion composition, was demonstrated. The melting temperature of PP and ULDPE were respectively increased and decreased in the blend. Crystallization rate and the, crystallinity of PP and ULDPE were first increased and then decreased as the other component was increased. Yield at low frequencies was observed with 30 wt% ULDPE in PP. In ULDPE-rich compositions, complex viscosities of the blends gave negative deviation from the additive rule of mixing. Mechanical properties such as flexural modulus, elongation at break and Vicat softening point were closely relatable to the morphology. The impact strength of PP is significantly improved by ULDPE addition.     

Structure and properties of drawn tapes of high-density polyethylene/ethylene copolymer blends. I

A series of undrawn and drawn tapes has been prepared from HDPE, as well as, blends consisting of 90% HDPE and 10% ethylene copolymers. The influence of both the molecular irregularity of ethylene copolymers and resultant crystallization behavior on structure and mechanical properties of these blends has been investigated using differential scanning calorimetry, wide-and small-angle X-ray diffraction, mechanical response at small and large strains, and dynamic mechanical thermal analysis. The tensile drawing study of un-drawn tapes shows enhanced strain hardening and a consistent reduction in natural, as well as, maximum achievable draw ratio with an increase in molecular irregularity of ethylene copolymers. It has been confirmed that blends are partially miscible in the amorphous, as well as, in the crystalline phase through cocrystallization. The lateral crystallite thicknesses, crystallinity, and amorphous phase orientation of blends consistently decreases with an increase in molecular irregularity of ethylene copolymers because of a large-scale change in crystallization and drawing behavior of HDPE component in the blends. There is a distinct possibility that the molecular network exerts an important influence on physical and mechanical properties of undrawn and drawn tapes. © 1996 John Wiley & Sons, Inc.     

Alternating multilayer structure of polyethylene/polypropylene blends obtained through injection molding

The formation of multilayer structures in the high‐speed thin wall injection‐molded samples of high‐density polyethylene/isotactic polypropylene blends is reported. Based on the morphology development in injection runner and mold, a possible formation mechanism of multilayer structure was proposed in this study. Injection molding could be used as a simple and an effective method for the fabrication of multifunctional multilayer structure. This work is interesting and important for scientific research as well as several potential applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Phase separation in polypropylene and metallocene polyethylene blends

Blends of a metallocene linear low density polyethylene (m‐LLDPE) and polypropylene random copolymer (PP) have been prepared using a twin screw extruder and characterized by thermal analysis, mechanical properties, and wide angle X‐ray scattering to determine their degree of compatibility. The blends were either directly quenched in water from the melt‐ or slow‐cooled to room temperature. In both cases, the two components formed separate phases and crystallized independently. The slow‐cooled specimens had higher yield stress, tensile modulus, and lower elongation at break consistent with higher degree of crystallinity. The elongation to break also varied with composition reaching a minimum at 50% consistent with the incompatible nature of the blends. Crystallization kinetics and melting studies confirm that the two components formed separate phases and crystallized independently. POLYM. ENG. SCI., 46:889–895, 2006. © 2006 Society of Plastics Engineers     

Miscibility and crystallization of metallocene polyethylene blends with polypropylene

The crystallization and morphology of very‐low‐density polyethylene (VLDPE) and ultra‐low‐density polyethylene (ULDPE) blends with isotactic polypropylene (PP) were studied by differential scanning calorimetry (DSC) and hot‐stage optical microscopy (HSOM) with polarized light. In particular, the isothermal crystallization of PP in molten PE was investigated. A polypropylene homopolymer was melt‐blended with six types of VLDPEs and ULDPEs, with variations in branch content and length and in molecular weight. All the blends contained 20% PP by mass. It was found that the crystallization temperatures of PP and PE changed in the blends, and the crystallization of PP was affected by branch length and content and by the molecular weight of the PE, indicating a certain degree of miscibility between PP and PE. The isothermal crystallization rate of PP decreased in the blends; in particular, the crystallization rate of PP was slower in the ULDPE with lower MFI, suggesting that crystallization of PP was hindered by PE and that its rate was regulated by the viscosity of ULDPE. HSOM images showed that a portion of the PP crystallized from molten PE, although phase separation was obvious, providing additional information on the miscible behavior between PP and VLDPEs (or ULDPEs). Furthermore, the miscible level between the PP and the ULDPEs was higher than that between the PP and the VLDPEs because the degree of change in the crystallization behavior of the PP and PE was greater in the PP–ULDPE blends. This was possibly a result of the higher branch content in the ULDPE. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1179–1189, 2003     

Reactive compatibilization of polypropylene/polyethylene terephthalate blends

The reactive compatibilization of polypropylene/polyethylene terephthalate (PP/PET) blends by addition of glycidyl methacrylate grafted PP (PP-g-GMA) was studied. Two PP-g-GMA copolymers, containing either 0.2 or 1.2 wt% of GMA, were used as interface modifiers. These were incorporated into PP blends (with either 70 or 90 wt% PET), replacing 1/5 of PP in the system. The use of these modifiers changed the blends' tensile mechanical behavior from fragile to ductile. Blend tensile strength was improved by 10% and elongation at break showed 10 to 20-fold increases while stiffness remained constant. Scanning electron micrographs showed the PP average domain size in injection molded specimens to decrease to the micron/sub-micron size upon addition of the GMA modified resins, while the unmodified blends exhibited heterogeneous morphology comprising large lamellae 10–20 μm wide. The low-GMA graft content PP seemed slightly more efficient than the high GMA content PP in emulsifiying PP/PET blends. The GMA grafting level on PP had very limited effects on the blends' mechanical behavior in the range of GMA graft density provided by the two modified resins investigated.     

Dynamic mechanical behavior and structure of ternary blends of polypropylene/EPDM rubber/polyethylene

The dynamic mechanical behavior of ternary blends of isotactic polypropylene (80–0 percent)/EPDM rubber (20 percent)/high-density polyethylene (0–80 percent) was investigated in the temperature range from −196 to 100°C by means of a free-oscillating torsional pendulum. The structure of the blends was examined by a scanning electron microscope on etched surfaces cut by a fractured glass edge in liquid nitrogen. Dynamic mechanical response spectra and microphotographs of the systems studied show that the minor thermoplastic forms the core of EPDM rubber inclusions. At 20 percent rubber in the blends, the inclusions can accommodate from 20 to 30 percent polyethylene or polypropylene. Addition of either thermoplastic not exceeding this limit has almost the same effect on the stiffness, damping, and yield stress of the blends as the addition of the same amount of rubber. Ternary blends with equal or slightly different polypropylene and polyethylene fractions have the structure of interpenetrating phases in which EPDM rubber forms the interface layer.     

氯化聚乙烯橡胶改性聚丙烯的非等温结晶行为研究

用示差扫描量热仪(DSC)对氯化聚乙烯橡胶(CM)改性聚丙烯(PP)的非等温结晶行为进行了研究。结果表明,随着降温速度上升,样品结晶温度下降,结晶速度上升。在以15k/min与20k/min速度降温时,发现两者的结晶行为相一致。采用TCHC交联剂的1#样品的结晶温度与速度都大于采用DCP交联剂的2#样品,前者的硫化效率低,对结晶影响小。本文又通过求解非等温结晶动力学参数与结晶活化能,进一步解释了1#与2#样品的结晶行为。     

Tensile properties of polyethylene blends

Binary and ternary blends were prepared from low, medium, and high density polyethylene. The tensile properties of these materials indicated that the blends formed either compatible or semi-compatible mixtures. One of the ternary blends exhibited a slight synergism in properties which could be partially attributed to an enhancement in crystallinity. Such blends may have practical utility by yielding materials having a combination of strength, stiffness, and toughness.     

Studies on blends of polyethylene terephthalate with bisphenol-a-polycarbonate and polypropylene

Moldability and mechanical properties of polyethylene terephthalate (PET) under normal molding conditions were found to improve significantly when it was blended with bisphenol-A-polycarbonate (PC) and polypropylene (PP) to form ternary polymer blend systems. DSC results of these blends revealed that the PET and PC components formed a miscible blend while PP being incompatible with them, formed a separate phase. PP was also found to form a sleeve around the PET-PC miscible phase and, thereby, showed a skin-core type of morphology. Variations of mechanical properties with varying amounts of PP was measured keeping the ratio of PET and PC constant. Tensile and flexural properties of the blends decrease with the amount of PP. Notched impact strength increases up to a certain level of PP and then decreases, while the unnotched values decrease gradually. The effect of annealing on the mechanical properties of these blends have been discussed on the basis of the increased crystallinity of some of the components.     

Phase structure of impact-modified polypropylene blends

A morphological study of polypropylene/ethylene-propylene-diene terpolymer (PP/EPDM) and polypropylene (PP)/polyolefin thermoplastic rubber blends was conducted. Dispersion of the impact modifier in the blend was investigated by transmission and scanning electron microscopy (TEM and SEM). It was established that number-average particle size ($\text{D}\text{?}{}_{\mathrm{n}}$) of the EPDM impact modifier increased with its melt viscosity. The differences in melt viscosities of the blended components were characterized by the phase viscosity ratio (μ). The course of $\text{D}\text{?}{}_{\mathrm{n}}$ vs. log μ function was qualitatively in agreement with the Rayleigh—Taylor—Tomotika theory. Accordingly, high degree of dispersion of the impact modifier can be achieved if melt viscosities of the blended components are very closely matched, i.e. if $\text{μ ? 1}$. It was concluded from SEM results that, below an impact modifier content of 20%, the modifier formed the dispersed phase in the continuous PP matrix. In blends containing 50% of impact modifier, the latter may also form continuous phase depending on its type and μ value beside the still continuous PP phase (co-continuous network structure).     

Rheological and mechanical properties of ternary blends of linear-low-density polyethylene/polypropylene/ethylene-propylene block polymers

The effect of addition of an ethylene-propylene block polymer on rheological and mechanical properties of a linear-low-density polyethylene/polypropylene blend was examined. The samples were prepared by melt blending in a twin-screw extruder followed by injection molding. The single-, two- and three-component systems were treated the same way. The mechanical behavior of the blends was evaluated by means of tensile, and flexural, tests at 23 and ?40°C. The capillary, elongational, and dynamic-flow measurements were performed at 190°C.     

Electrical behavior and structure of polypropylene/ultrahigh molecular weight polyethylene/carbon black immiscible blends

This study investigates the electrical behavior, which is the positive temperature coefficient/negative temperature coefficient (PTC/NTC), and structure of polypropylene (PP)/ultrahigh molecular weight polyethylene (UHMWPE)/carbon black (CB) and PP/γ irradiated UHMWPE (XL‐UHMWPE)/CB blends. As‐received UHMWPE or XL‐UHMWPE particles are chosen as the dispersed phase because of their unusual structural and rheological properties (extremely high viscosity), which practically prevent CB particles penetration. Because of their stronger affinity to PE, CB particles initially form conductive networks in the UHMWPE phase, followed by distribution in the PP matrix, thus interconnecting the CB‐covered UHMWPE particles. This unusual CB distribution results in a reduced electrical percolation threshold and also a double‐PTC effect. The blends are also investigated as filaments for the effect of shear rate and processing temperature on their electrical properties using a capillary rheometer. Because of the different morphologies of the as‐received and XL‐UHMWPE particles in the filaments, the UHMWPE containing blends exhibit unpredictable resistivities with increasing shear rates, while their XL‐UHMWPE containing counterparts depict more stable trends. The different electrical properties of the produced filaments are also related to differences in the rheological behavior of PP/UHMWPE/CB and PP/XL‐UHMWPE/CB blends. Although the flow mechanism of the former blend is attributed to polymer viscous flow, the latter is attributed to particle slippage effects. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 104–115, 2001     

Effect of recycled polypropylene on polypropylene/high‐density polyethylene blends

The effect of recycled PP on incompatible blends of virgin polypropylene (PP) and high‐density polyethylene (HDPE) was studied. Recycled PP from urban solid waste was extracted with methyl ethyl ketone and the compatibilizing action of the product before and after extraction was examined. The characterization of the recycled PP was performed by FTIR, NMR, and DSC analyses. Mechanical properties of the blends were evaluated. The results showed partial compatibility of the blend components, reflected in the improvement of the tensile strength and elongation. Best results were achieved by the addition of extracted recycled PP on the 50/50 PP/HDPE blends. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1305–1311, 2001