Tensile properties in the oriented blends of high-density polyethylene and isotactic polypropylene obtained by dynamic packing injection molding

In this article, tensile properties have been discussed in terms of phase morphology, crystallinity and molecular orientation in the HDPE/iPP blends, prepared via dynamic packing injection molding, with aid of scanning electron microscopy (SEM), differential scanning calorimetry (DSC) as well as two dimensional X-ray scattering (2D WAXS). For the un-oriented blends, the tensile properties (tensile strength and modulus) are mainly dominated by the phase morphology and interfacial adhesion related to the influenced crystallization between HDPE and iPP component. A maximum in tensile strength and modulus is found at iPP content in the range of 70-80 v/v%. As for the oriented blends, however, the presence of dispersed phase in the blends, independent of phase morphology and crystallinity, always makes tensile properties to be deteriorated through reducing molecular orientation of matrix. It is molecular orientation of matrix that determines the tensile properties of oriented blends. In the blends with HDPE as matrix, steep decreasing of tensile properties is related to the rapid reducing of molecular orientation of HDPE, whereas in the blends with iPP as a major component, slight decreasing of molecular orientation of iPP results in slight reducing of tensile properties. Other factors, such as interfacial properties and phase morphology, seem to be little contribution to the modulus and tensile strength.     

Three-dimensional phase morphologies in HDPE/EVA blends obtained via dynamic injection packing molding

The formation of phase morphology of injection molded HDPE/EVA blends, under the effect of shear stress, has been investigated in detail. The shear stress was induced by dynamic packing injection molding, by which a specimen is forced to move repeatedly in the model by two pistons that move reversibly with the same frequency during cooling. Two kinds of EVA with VA content 16 wt% (16EVA) and 33 wt% (33EVA) were used to investigate the effect of interfacial tension. The phase morphology was viewed both parallel and perpendicular to the shear flow direction, so one can get an overall three-dimensional phase morphology. Low shear stress provided by the pistons has a substantial effect on the phase morphology along the flow direction but is insignificant in the direction perpendicular to the flow direction. Generally, a much elongated and layer-like structure is formed along the flow direction, and spherical droplet-like morphology is formed perpendicular to the flow direction, and the degree of deformation of rubber particles also depends upon their size and elasticity as well as on the interfacial properties between matrix and dispersed phase. For static samples of HDPE/16EVA blends (without shearing), only droplet morphology is formed as 16EVA content increases from10 to 40 wt%. However, under the effect of shear stress (dynamic samples), both droplet and cylinder morphologies can be formed depending on the volume ratio. For static samples of HDPE/33EVA blends, not only droplet, but also cylinder and co-continuous morphology (perpendicular to flow direction) can be formed depending on the volume ratio. For dynamic samples of HDPE/33EVA blends, droplet, cylinder and co-continuous network (co-continuous in both parallel and perpendicular to flow direction) can be formed under the effect of shear stress. The formation of phase morphology is discussed based on interfacial interaction, viscosity ratio, shear stress, and phase inversion.     

Studies on blends of high‐density polyethylene and polypropylene produced by oscillating shear stress field

Tensile strength and morphology of blends of high‐density polyethylene (HDPE) and polypropylene (PP) obtained by oscillating packing injection molding were investigated via Universal Testing Machine, DSC, and SAXS. Tensile strength is greatly enhanced from 24.5 MPa to more than 90 MPa for pure HDPE and for blends with PP content less than 10 wt %. There exists a sharp decrease of tensile strength when PP content is more than 10 wt %. The shear‐induced morphologies with core in the center, oriented zone surrounding the core and skin layer are observed in the cross‐section areas of the samples. Interestingly, a sharp decrease of oriented zone is seen when PP content is more than 10 wt %, associated with the sharp decrease of tensile strength. DSC result shows double melting peaks with a high‐temperature melting peak that is not present in the endotherm obtained from the central core and obtained from the samples by static packing injection molding, which indicates the existence of shish‐kebab structure in the oriented zone. However, there is no difference of crystallinity between the samples by oscillating and by static packing injection molding. SAXS was used to analyze the complicated morphologies induced by shear stress, and results show that the crystal thickness could be greatly increased under shear stress. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 58–63, 2002     

Orientation effects on the deformation and fracture properties of high‐density polyethylene/ethylene vinyl acetate (HDPE/EVA) blends

In this paper, the tensile deformation and fracture toughness of high‐density polyethylene (HDPE)/ethylene vinyl acetate (EVA) blends, obtained by dynamic packing injection moulding, have been comprehensively investigated in different directions of rectangle samples, including longitudinal, latitudinal and oblique directions relative to the flow direction. Two kinds of EVA were used with VA content 16 wt% (16EVA) and 33 wt% (33EVA) to control the interfacial interactions. The results indicate that molecular orientation and interfacial interaction play very important roles to determine the tensile behaviour and fracture toughness. Biaxial‐reinforcement of tensile strength was seen for HDPE/16EVA blends but only uniaxial‐reinforcement was observed for HDPE/33EVA blends. The difference is caused by the different interfacial interactions as highlighted by the peel test, scanning electron microscopy (SEM) observation as well as theoretical evaluation. Very high impact strength, decreasing with increasing EVA content, was observed when the fracture propagation is perpendicular to the shear flow direction, while a low impact strength, increasing slightly increasing with EVA content, was seen when the fracture propagation is parallel to the shear flow. The fracture of oblique samples is always along the flow direction instead of along the impact direction or tensile direction. The tensile behaviour and fracture toughness are discussed on the basis of the formation of transcrystalline zones, orientation of EVA particles and matrix toughness of HDPE in different directions. Copyright © 2004 Society of Chemical Industry     

Super polyolefin blends achieved via dynamic packing injection molding: Tensile strength

The tensile strength of some polyolefin blends, HDPE/PP, HDPE/LDPE, HDPE/ LLDPE, and PP/LLDPE, achieved by dynamic packing injection molding have been investigated as a function of composition and melt temperature. Molecular architecture and phase behavior play an important role in chain orientation, hence the tensile strength. For HDPE, which has a linear structure, the highest enhancement of tensile strength is obtained. LDPE, which has a highly branched structure, the smallest enhancement is seen. PP and LLDPE lie in between. Super polyolefin blends with high tensile strength and high elongation have been obtained by this method. The shear‐induced morphologies with core in the center, oriented zone surrounding the core and skin layer were observed in the cross‐section areas of the samples. The tensile strength was found to be directly proportional to the area of the oriented zone. When the area of oriented zone is less than 35%, the tensile strength is not only the orientation dependency but the blending components dependency as well. When the area of oriented zone is more than 35%, however, our new finding is that the orientation will be the dominating parameter to determine the tensile strength of the blends, independent of the components, the composition, molecular architecture, phase behavior, and crystal morphology. The maximum tensile strength for all the polyolefin blends is extrapolated as to 230MPa, as the area of oriented zone reaches to 100%. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 236–243, 2002     

The morphology and mechanical properties of dynamic packing injection molded PP/PS blends

As a part of long-term project aimed at super polyolefin blends, in this work, we report the mechanical reinforcement and phase morphology of the immiscible blends of polypropylene (PP) and polystyrene (PS) achieved by dynamic packing injection molding (DPIM). The shear stress (achieved by DPIM) and interfacial interaction (obtained by using styrene-butadiene-styrene (SBS) as a compatibilizer) have a great effect on phase morphology thus mechanical properties. The shear-induced morphology with core in the center and oriented zone surrounding the core was observed in the cross-section areas of the samples. The phase inversion was also found to shift towards lower PS content under shear stress, at 70 wt% in the core and 30 wt% in the oriented zone, compared with 80 wt% for static samples (without shear). The tensile strength, tensile modules and impact strength were found largely increase by means of either shear stress or compatibilizer. The PS particle size is greatly reduced with adding of SBS, and the reduced particle size results in greater resistance to deformation, which causes the co-continuous structure at oriented zone change into droplet morphology. The morphology resulting from blending and processing was discussed based on effect of interfacial tension, shear rate, phase viscosity ratio and composition. The observed change of mechanical properties was explained based on the combined effect of phase morphology (droplet-matrix or co-continuous phase) and molecular orientation under shear stress.     

Mechanical behaviors of ethylene/styrene interpolymer compatibilized polystyrene/polyethylene blends

In this study, ethylene/styrene interpolymer was used as a compatibilizer for the blends of polystyrene (PS) and high‐density polyethylene (HDPE). The mechanical properties including tensile and impact properties and morphology of the blends were investigated by means of uniaxial tension, instrumented falling‐weight impact measurements, and scanning electron microscopy. Tensile tests showed that the yield strength of the PS/HDPE/ESI blends decreases considerably with increasing HDPE content. However, the elongation at break of the blends tended to increase significantly with increasing HDPE content. The excellent tensile ductility of the HDPE‐rich blends resulted from shield yielding of the matrix. Izod and Charpy impact measurements indicated that the impact strength of the blends increases slowly with HDPE content up to 40 wt %; thereafter, it increases sharply with increasing HDPE content. The impact energy of the HDPE‐rich blends exceeded that of pure HDPE, implying that the HDPE polymer can be further toughened by the incorporation of brittle PS minor phase in the presence of ESI compatibilizer. The correlation between the impact property and morphology of the blends is discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 4001–4007, 2007     

Influence of crosslinking on crystallization,rheological, and mechanical behaviors of high density polyethylene/ethylene‐vinyl acetate copolymer blends

High density polyethylene (HDPE)/ethylene‐vinyl acetate copolymer (EVA) blends with selective crosslinking the EVA phase were prepared and the crystallization, rheological, and mechanical behaviors were studied. Selective crosslinking of EVA component could greatly improve both tensile and impact strengths of the HDPE‐rich blends and influence melting enthalpy at different annealing temperature in successive self‐nucleation and annealing procedure. Dynamic mechanical analysis reveals that glass transition temperatures of both the HDPE and EVA components are lowered upon blending and are raised upon crosslinking. The uncrosslinked HDPE/EVA blends are unstable in the melt and show increment in storage modulus (G′) and decay in loss tangent (tanδ) with annealing time associated with phase coarsening. However, morphology of selectively crosslinked blends in the melt state is highly unstable, characterized by a fast migration of uncrosslinked HDPE component out of the crosslinked EVA phase to the surface resulting in a rapid decay in G′ and an increment in tanδ at the early stage of annealing. POLYM. ENG. SCI., 54:2848–2858, 2014. © 2014 Society of Plastics Engineers     

动态保压注射成型技术对PA6/EPDM-g-MA 共混物相形态与力学性能的影响

采用动态保压注塑成型技术来控制分散相相形态和橡胶粒子在基体中的取向排列.纯尼龙,动态样与静态样具有相同的冲击强度.在加入橡胶后,动态样与静态样的冲击强度变化趋势基本一致,在橡胶含量为20%t时,体系完成脆韧转变冲击强度达到最大,在橡胶含量为30%和40%时冲击强度又下降.但与静态样相比,当橡胶含量为10%t时,动态样与静态样的冲击强度一致,而当橡胶含量超过10%时,动态样的冲击强度较静态样高.结合平行于熔体流动方向的SEM照片,在橡胶含量为10%时,动态样中的橡胶粒子与静态样一样并未被拉长与取向,而在橡胶含量超过10%时,动态样剪切层中的橡胶粒子被拉长且沿熔体流动方向取向.实验表明,在改善共混物界面相容性的基础上,适当的低剪切应力场能进一步提高橡胶分散相对冲击强度的贡献.     

Influence of short glass fiber reinforcement on the morphology development and mechanical properties of PET/HDPE blends

Poly(ethylene‐terephthalate) (PET)—high density polyethylene (HDPE) blends with varying component ratios and with or without short glass fiber reinforcement (10 gv%) were prepared. The morphology of the blends was studied by scanning electron microscopy (SEM) and optical microscopy. The interfacial shear strength between PET and HDPE as well as between the polymers and the glass fiber was determined with microbond testing. The static mechanical properties were defined with tensile tests and Charpy impact tests, while the dynamic mechanical properties with DMTA. In blends that do not contain glass fibers, the co‐continuous structure formed in the vicinity of phase inversion increased both the static tensile modulus as well as the impact strength compared to the mechanical properties of polymers. Based on the observations simple morphological models were set up, with the help of which the change in the mechanical properties of blends of different composition can be explained. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Morphologies and mechanical properties of HDPE induced by small amount of high‐molecular‐weight polyolefin and shear stress produced by dynamic packing injection molding

To better understand the effect of a small amount of high‐molecular‐weight polyethylene (HMWPE) on the mechanical properties and crystal morphology under the shear stress field, the dynamic packing injection molding (DPIM) was used to prepare the oriented pure polyethylene and its blends with 4% HMWPE. The experiment substantiated that the further improvement of tensile strength along the flow direction (MD) of high‐density polyethylene (HDPE)/HMWPE samples was achieved, whereas the tensile strength along the transverse direction (TD) still substantially exceeded that of conventional molding. Tensile strength in both flow and TDs were highly enhanced, with improvements from 23 to 76 MPa in MD and from 23 to 31 MPa in TD, besides the toughness was highly improved. So, the samples of HDPE/HMWPE transformed from high strength and brittleness to high strength and toughness. The obtained samples were characterized via SEM and TEM. For HDPE/HMWPE, the lamellae of the one shish‐kebab in the oriented region may be stretched into other shish‐kebab structures, and one lamella enjoys two shish or even more. This unique crystal morphology could lead to no yielding and necking phenomena in the stress–strain curves of HDPE/HMWPE samples by DPIM. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Impact and tensile properties of SEBS copolymer compatibilized PS/HDPE blends

In this study, polystyrene–hydrogenated polybutadiene–polystyrene (SEBS) triblock copolymer was used as a compatibilizer for the blends of polystyrene (PS) and high-density polyethylene (HDPE). The morphology and static mechanical and impact properties of the blends were investigated by means of scanning electron microscopy, uniaxial tension, and instrumented falling-weight impact measurements. Tensile tests showed that the yield strength of the PS/HDPE/SEBS blends decreases considerably with increasing HDPE content. However, the elongation at break of the blends tended to increase significantly with increasing HDPE content. The excellent tensile ductility of the HDPE-rich blends resulted from shield yielding of the matrix. Charpy impact measurements indicated that the impact strength of the blends increases slowly with HDPE content up to 50 wt %; thereafter, it increases sharply with increasing HDPE content. The impact energy of the HDPE-rich blends exceeded that of pure HDPE, implying that the HDPE polymer can be further toughened by the incorporation of brittle PS minor phase in the presence of SEBS compatibilizer. The correlation between the impact property and morphology of the blends is discussed. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1099–1108, 1998     

i-PP/HDPE blends. III. Characterization and compatibilization at lower i-PP contents

The mechanical properties of high-density polyethylene (HDPE)-rich i-PP/HDPE blends were studied. Two grades of HDPE were investigated, one with a melt viscosity close to that of the polypropylene (PP) and the other having a much lower melt viscosity. Compatibilization of the 10/90 i-PP/HDPE blend with three copolymers (an ethylene/propylene/diene [EPDM] copolymer and two ethylene/vinylacetate [EVA] copolymers, differing in their VA content) was also investigated. Blends of PP with the low melt viscosity HDPE displayed poor mechanical properties. It was not possible to improve these properties sufficiently with EPDM or EVA. In the case where viscosity matching was achieved between PP and HDPE, addition of i-PP (up to 30%) to HDPE resulted in a large drop in the impact strength of the blends, compared to that of the neat HDPE. A large drop (>50%) was also observed in the ultimate tensile elongation. However, the flexural modulus, yield stress, and ultimate tensile strength all increased with the introduction of i-PP into HDPE. Modification of these blends with an EPDM resulted in the return of all properties to values very close to those of the neat HDPE. The ultimate tensile elongation of the EPDM-modified i-PP/HDPE blend even exceeded that of the virgin HDPE. It was also found that although EVAs can be used to compatibilize these blends these additives were not as effective as was the EPDM. © 1996 John Wiley & Sons, Inc.     

Morphology and mechanical properties of biaxially oriented films of polypropylene and HDPE blends

Biaxially oriented films of blends of high-density polyethylene (HDPE) with polypropylene (PP) homopolymer and PP copolymers prepared by twin-screw extrusion and lab-stretcher have been investigated by scanning electron microscopy (SEM), polarized microscopy, differential-scanning calorimeter, and universal testing machine. Three different kinds of PP copolymers were used: (i) ethylene–propylene (EP) random copolymer; (ii) ethylene–propylene (EP) block copolymer; (iii) ethylene–propylene–buttylene (EPB) terpolymer. In the SEM study of the morphology of films of HDPE with various PP blends, phase separation is observed between the PP phase and the HDPE phase for all blends and compositions. In all blends, HDPE serves to reduce the average spherulites size, probably acting as a nucleating agent for PP. The reduction of spherulite size appeared most significantly in the blend of EPB terpolymer and HDPE. A large increase of crystallization temperature was found in the blend of EPB terpolymer and HDPE compared with the unblended EPB terpolymer. For the blend of EPB terpolymer and HDPE, the improvement of tensile strength and modulus is observed with an increase of HDPE content, and this can be considered as a result of the role of HDPE in reducing average spherulite size. © 1994 John Wiley & Sons, Inc.     

Characterization of PA6/EPDM-g-MA/HDPE ternary blends: The role of core-shell structure

In this work, the relationship between properties and morphologies of PA6/EPDM-g-MA/HDPE ternary blends was studied. Two processing methods (one- and two-step methods) were applied to prepare the PA6/EPDM-g-MA/HDPE ternary blends. The dependence of the phase morphology on interfacial interaction and processing method was discussed here. It was found that core-shell morphology (core: HDPE, shell: EPDM-g-MA in PA6 matrix) appeared in PA6/EPDM-g-MA/HDPE ternary blends, and in comparison to the blend prepared by one-step method, the core-shell morphology with thicker EPDM-g-MA shell appeared in the blend prepared by two-step method. In this case, a super toughened PA6 ternary blends with the Izod impact strength of 72.51 kJ/m² which is 4–5 times higher than PA6/EPDM-g-MA binary blend and 9–10 times higher than pure PA6 could be achieved. Moreover, the rheological results indicated that the storage modulus of ternary blends was heavily dependent on the phase morphology. The core-shell structure with thicker EPDM-g-MA shell would weaken the contribution of interfacial energy to the storage modulus of ternary blends.     

Cocontinuous morphologies in polystyrene/ethylene–vinyl acetate blends: The influence of the processing temperature

The influence of the compression‐molding temperature on the range of cocontinuity in polystyrene (PS)/ethylene–vinyl acetate (EVA) copolymer blends was studied. The blends presented a broad range of cocontinuity when compression‐molded at 160°C, and they became narrower when compression‐molded at higher temperatures. A coarsening effect was observed in PS/EVA (60:40 vol %) blends upon compression molding at higher temperature with an increase in the phase size of the cocontinuous structure. Concerning PS/EVA (40:60 vol %) blends, an increase in the mixing and molding temperatures resulted in a change from a cocontinuous morphology to a droplet–matrix morphology. This effect was observed by selective extraction experiments and scanning electron microscopy. The changes in the morphology with the molding conditions affected the storage modulus. An increase in the storage modulus in blends compression‐molded at 160°C was observed as a result of dual‐phase continuity. An EVA copolymer with a higher vinyl acetate content (28 wt %) and a higher melt‐flow index resulted in blends with a broader range of cocontinuity. This effect was more pronounced in blends with lower amounts of PS, that is, when EVA formed the matrix. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 386–398, 2003     

Radiation effects on HDPE/EVA blends

In this article, we discuss the radiation effects of high‐density polyethylene (HDPE)/ethylene–vinyl acetate (EVA) copolymer blends. In comparison with the low‐density polyethylene/EVA blends, the EVA content in the HDPE/EVA blends had a lower enhancement effect on radiation crosslinking by γ‐ray irradiation in air. The phenomenon is discussed with the compatibility, morphology, and thermal properties of HDPE/EVA blends. The HDPE/EVA blends were partly compatible in the amorphous region, and radiation crosslinking of the HDPE/EVA blend was less significant, although increasing the amorphous region's content of the HDPE/EVA blends and the vinyl acetate content of EVA were beneficial to radiation crosslinking. The good compatibility was a prerequisite for the enhancement effect of EVA on the radiation crosslinking of the polyethylene/EVA copolymer. The radiation crosslinking and the degradation mechanism of HDPE/EVA blends were examined quantitatively by a novel method, the step analysis process of irradiated HDPE/EVA blends with a thermal gravimetric analysis technique. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 553–558, 2002     

Mechanical properties and morphology of PA6/EVA blends

The mechanical properties of blends of polyamide6 (PA6) and ethylene vinyl acetate (EVA) at a blending composition of 0–50 wt % EVA were studied. The notched Izod impact strength of PA6 increased with the incorporation of EVA, the increase being more than 100% compared to PA6 at 10% EVA. The tensile strength and the tensile modulus of the blends decreased steadily as the weight percent of EVA increased. Analysis of the tensile data using predictive theories indicated the extent of the interaction of the dispersed phase and the matrix up to 20 wt % EVA. SEM studies of the cryogenically fractured surfaces indicated increase in the dispersed phase domain size with EVA concentrations. On the other hand, impact fractured surfaces of PA6/EVA blends indicated debonding of EVA particles, leaving hemispherical bumps, indicating inadequate interfacial adhesion between PA6 and EVA. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1593–1606, 2002     

Mechanical properties of HDPE/(PEC/PS)/SEBS blends

Various amounts of a styrene-butadiene-based triblock copolymer (SEBS) was used to compatibilize immiscible blends of high density polyethylene (HDPE) and an amorphous glassy phase consisting of either pure polystyrene (PS) or a miscible blend of PS and a polyether copolymer (PEC). PEC is structurally similar to poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). Mechanical properties were determined for blends fabricated by injection and compression molding. The inherently brittle two-phase HDPE/(PEC/PS) blends show significant increases in ductility and impact strength resulting from addition of SEBS. These improvements coincide with a slight loss in modulus and yield strength. If the amount of HDPE and SEBS is held constant, impact strength and ductility increase with the amount of PEC in the glassy phase. These trends evidently result from the added ductility of glassy phases containing PEC and perhaps from better interfacial adhesion in blends after adding SEBS. The latter stems from the thermodynamic miscibility between PEC and PS endblocks of SEBS which provide an enthalpic driving force for compatibilization. Differences between the properties of compression and injection-molded blends can be attributed to the degree of crystallinity and orientation induced during molding.     

Tensile properties and morphology of the dynamically cured EPDM and PP/HDPE ternary blends

The tensile properties and morphology of the polyolefin ternary blends of ethylenepropylene–diene terpolymer (EPDM), polypropylene and high density polyethylene were studied. Blends were prepared in a laboratory internal mixer where EPDM was cured in the presence of PP and HDPE under shear with dicumyl peroxide (DCP). For comparison, blends were also prepared from EPDM which was dynamically cured alone and blended with PP and HDPE later (cure–blend). The effect of DCP concentration, intensity of the shear mixing, and rubber/plastics composition was studied. The tensile strength and modulus increased with increasing DCP concentration in the blends of EPDM-rich compositions but decreased with increasing DCP concentration in blends of PP-rich compositions. In the morphological analysis by scanning electron microscopy (SEM), the small amount of EPDM acted as a compatibilizer to HDPE and PP. It was also revealed that the dynamic curing process could reduce the domain size of the crosslinked EPDM phase. When the EPDM forms the matrix, the phase separation effect becomes dominant between the EPDM matrix and PP or HDPE domain due to the crosslinking in the matrix.