Mechanical properties and structure of high‐density polyethylene samples prepared by injection molding with low‐frequency vibration

To better understand the formation of different crystal structures and improve the mechanical properties of high‐density polyethylene samples, melt vibration technology, which generally includes shear vibration and hydrostatic pressure vibration, was used to prepare injection samples. Through melt vibration, the crystal structure changed from typical spherulites of the traditional injection sample to obviously orientated lamellae of vibration samples. Sizes and orientation degrees of lamellae were different according to different vibration conditions. Crystallinity degrees of vibration samples increased notably. Therefore, the tensile strength of vibration samples increased with increasing vibration frequency and vibration pressure, whereas elongation of vibration samples decreased during the first stage and then continued to increase as the vibration frequency increased. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 818–823, 2005     

Fine structure and physical properties of polyethylene/poly(ethylene terephthalate) bicomponent fibers in high‐speed spinning. I. Polyethylene sheath/poly(ethylene terephthalate) core fibers

The high‐speed melt spinning of sheath/core type bicomponent fibers was performed and the change of fiber structure with increasing take‐up velocity was investigated. Two kinds of polyethylene, high density and linear low density (HDPE, LLDPE) with melt flow rates (MFR) of 11 and 50, [HDPE(11), LLDPE(50)], and poly(ethylene terephthalate) (PET) were selected and two sets of sheath/core combinations [HDPE(11)/PET and LLDPE(50)/PET bicomponent fibers] were studied. The fiber structure formation and physical property effects on the take‐up velocities were investigated with birefringence, wide‐angle X‐ray diffraction, thermal analysis, tensile tests, and so forth. In the fiber structure formation of PE/PET, the PET component was developed but the PE components were suppressed in high‐speed spinning. The different kinds of PE had little affect on the fine structure formation of bicomponent fibers. The difference in the mechanical properties of the bicomponent fiber with the MFR was very small. The instability of the interface was shown above a take‐up velocity of 4 km/min, where the orientation‐induced crystallization of PET started. LLDPE(50)/PET has a larger difference in intrinsic viscosity and a higher stability of the interface compared to the HDPE(11)/PET bicomponent fibers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2254–2266, 2000     

Successive solution fractionation mechanisms for high‐density polyethylene

BACKGROUND: Preparative fractionation techniques are currently used in order to obtain large amounts of polyethylene fractions. Preparative successive solution fractionation (SSF) and temperature rising elution fractionation (TREF) are compared as regards obtaining, at a multi‐gram scale, low‐dispersity fractions of high‐density polyethylene (HDPE). The operative separation mechanisms during a SSF of a broad HDPE, which are not yet totally elucidated, are also studied in this work. RESULTS: SSF and TREF approaches lead to the separation of HDPE macromolecules according to their molar masses. If very homogeneous fractions (dispersities from 1.1 to 1.3) are isolated in TREF at the lowest elution temperature, the collected mass is too low. At higher elution temperatures, the fractions have too broad a molar mass distribution (dispersities from 2.7 to 3.7). With the SSF procedure, dispersities are not as low as for the first TREF fractions. But, the relative weight fraction is better distributed between the different extraction temperatures. The molar mass distribution exhibits a dispersity of around 1.9. CONCLUSION: The SSF method is the most suitable way to obtain large gram amounts of low‐dispersity (ca 2) HDPE fractions over a wide molar mass range. Complementary gram‐scale rheological characterization is thus possible enabling a better comprehension of the SSF mechanism. Liquid–liquid demixing is the main mechanism in SSF, but its relative importance depends on polymer characteristics and solvent quality. Copyright © 2008 Society of Chemical Industry     

Transcrystallinity effects in high‐density polyethylene. I. Experimental observations in differential scanning calorimetry analysis

The crystallization of a high‐density polyethylene was analyzed with differential scanning calorimetry (DSC) measurements. An intense transcrystallinity was observed at the contact between the polymer and the DSC pans. The modification of the crystallization kinetics induced by this phenomenon was studied as a function of cooling rate and sample thickness. We point out that most of the theoretical predictions of our previous model could be checked. The crystallization temperature was a function of the sample thickness and could be also correlated with the thickness of the transcrystalline zones. The shapes of the DSC traces were complex and correlated with the amount of trancrystallization. The usual interpretations of such DSC curves were not accurate. We conclude that specific experimental procedures must be proposed to understand and correctly use such measurements. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 725–733, 2002     

Studies on the kinetics of orientation and crystallization of nylon‐66 during high‐speed melt spinning

A new way of applying on‐line experimental data and basic theory to study the mechanism of orientation of a high‐speed melt spinning process is described. The relationship of birefringence and stress for Nylon‐66 was developed to understand the phenomena in the spinning line. The value of birefringence along the spinning line was calculated by various models to predict the orientation change. By comparison of the model prediction and on‐line experimental birefringence, a suitable mechanical model to simulate the change of the profiles along the spinning line was chosen, and the structural development mechanism is discussed. The results show that the orientation mechanism of high‐speed melt spinning of Nylon‐66 is determined by deformation and deformation rate along the spinning line. For Nylon‐66, molecular and crystal orientations develop independently and are controlled by the rotation of crystals and chain segments in the deformation field. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3157–3163, 2001     

Effect of silane grafting on the microstructure of high‐density polyethylene/organically modified montmorillonite nanocomposites

Organically modified montmorillonite (org‐MMT) and high‐density polyethylene (HDPE) grafted with silane groups (HDPE‐g‐silane) were melt compounded to give HDPE‐g‐silane‐blend‐org‐MMT nanocomposites. X‐ray diffractometry was performed to investigate the intercalation effect. Transmission electron microscopy was applied to observe the dispersion of org‐MMT layers in HDPE matrices. The results indicate that an intercalated structure can be easily obtained in HDPE‐g‐silane‐blend‐org‐MMT nanocomposites. Furthermore, positron annihilation lifetime spectroscopy was used to characterize the microstructure of the composites. It is found that the ortho‐positron (o‐Ps) intensity for HDPE‐g‐silane is decreased by approximately 10% with a narrower lifetime distribution than that for HDPE. With increasing org‐MMT concentration, the o‐Ps intensity I₃ increases for HDPE‐g‐silane‐blend‐org‐MMT nanocomposites; however, for HDPE‐blend‐org‐MMT composites I₃ decreases. It is found that HDPE composites with good dispersion can be obtained following appropriate modification of the HDPE. And silane grafting has an effect on the free volume of the HDPE nanocomposites. Copyright © 2007 Society of Chemical Industry     

Transcrystallinity effects in high‐density polyethylene. II. Determination of kinetics parameters

Transcrystallinity may occur during differential scanning calorimetry analysis at the surfaces of the samples. In such a case, measurements may be unsuitable. We propose simple methods for the determination of intrinsic crystallization data that are accurate for the polymer and for the determination of the nucleating ability of the surfaces. These methods are based on the experimental analysis of the crystallization of samples with different and calibrated thicknesses during experiments at different constant cooling rates. Analysis of thin samples allowed the characterization of transcrystallinity, whereas analysis of at least three samples of different thicknesses allowed us to determine the true crystallization kinetics of the bulk material. These two techniques were independent of each other and were successfully applied to the case of a high‐density polyethylene. The determinations were verified with simple analytical models. A further extension could be the study of the nucleating ability of different substrates. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 734–742, 2002     

Blend of high‐density polyethylene and a linear low‐density polyethylene with compositional‐invariant mechanical properties

This article presents the tensile properties and morphological characteristics of binary blends of the high‐density polyethylene (HDPE) and a linear low‐density polyethylene (LLDPE). Two constituents were melt blended in a single‐screw extruder. Injection‐molded specimens were evaluated for their mechanical properties by employing a Universal tensile tester and the morphological characteristics evaluated by using a differential scanning calorimeter and X‐ray diffractometer. It is interesting to observe that the mechanical properties remained invariant in the 10–90% LLDPE content. More specifically, the yield and breaking stresses of these blends are around 80% of the corresponding values of HDPE. The yield elongation and elongation‐at‐break are around 65% to corresponding values of HDPE and the modulus is 50% away. Furthermore, the melting endotherms and the crystallization exotherms of these blends are singlet in nature. They cluster around the corresponding thermal traces of HDPE. This singlet characteristic in thermal traces entails cocrystallization between these two constituting components. The clustering of thermal traces of blends near HDPE meant HDPE‐type of crystallites were formed. Being nearly similar crystallites of blends to that of HDPE indicates nearness in mechanical properties are observed. The X‐ray diffraction data also corroborate these observations. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2604–2608, 2002     

Influence of solvent quality on successive solution fractionation (SSF) efficiency of high‐density polyethylene

BACKGROUND: Preparative successive solution fractionation (SSF) is a powerful technique for obtaining narrow‐dispersity fractions on a multi‐gram scale of high‐density polyethylene (HDPE). In a previous paper, the operative separation mechanisms during SSF of a broad HDPE in cyclohexanone were studied. Two mechanisms, and not only one as expected from the literature, contribute to the separation of HDPE molecules according to their molar mass (MM). The very low MM chains are separated by a solid–liquid (S–L) mechanism, while the longer chains are isolated by a liquid–liquid (L–L) phase separation. In the present paper, the influence of a poorer solvent, diphenyl ether, is reported. RESULTS: It is shown that the relative importance of the S–L mechanism with respect to the L–L one is altered by the use of this solvent. The L–L temperature range is increased in diphenyl ether while the S–L transition temperature remains unchanged. Consequently, the SSF efficiency is improved. Large amounts (on a gram scale) of narrow‐dispersity fractions are isolated, mainly by the L–L mechanism. Polydispersities are about 1.5 (compared to 2.0 for cyclohexanone) and a broader MM range of closer molar mass distribution fractions is available. CONCLUSION: This work demonstrates that the use of diphenyl ether, a poorer solvent than cyclohexanone (always used as SSF solvent for polyethylene in the literature), leads to an improvement of SSF efficiency for an essentially linear HDPE. The differences of behaviour during the separation with cyclohexanone or diphenyl ether are explained by the establishment of a phase diagram. Copyright © 2009 Society of Chemical Industry     

Compatibilization of high‐impact polystyrene/high‐density polyethylene blends by styrene/ethylene–butylene/styrene block copolymer

Compatibilization of polymer blends of high‐impact polystyrene (HIPS) and high‐density polyethylene (HDPE) blend by styrene/ethylene–butylene/styrene (SEBS) was elucidated. Polymer blends containing many ratios of HIPS and HDPE with various concentrations of SEBS were prepared. The Izod impact strength and elongation at break of the blends increased with increases in SEBS content. They increased markedly when the HDPE content was higher than 50 wt %. Tensile strength of blends increased when the SEBS concentration was not higher than 5 pphr. Whenever the SEBS loading was higher than 5 pphr, the tensile strength decreased and a greater decrease was found in blends in which the HDPE concentration was more than 50 wt %. The log additivity rule model was applied to these blends, which showed that the blends containing the HIPS‐rich phase gave higher compatibility at the higher shear rates. Surprisingly, the blends containing the HDPE‐rich phase yielded greater compatibility at the lower shear rates. Morphology observations of the blends indicated better compatibility of the blends with increasing SEBS concentration. The relaxation time (T₂) values from the pulsed NMR measurements revealed that both polymer blends became more compatible when the SEBS concentration was increased. When integrating all the investigations of compatibility compared with the mechanical properties, it is possible to conclude that SEBS promotes a certain level of compatibilization for several ratios of HIPS/HDPE blends. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 742–755, 2004     

Effect of vibration extrusion on the structure and properties of high‐density polyethylene pipes

BACKGROUND: The axial strength of a plastic pipe is much higher than its circumferential strength due to the macromolecular orientation during extrusion. In this work, a custom‐made electromagnetic dynamic plasticating extruder was adopted to extrude high‐density polyethylene (HDPE) pipes. A vibration force field was introduced into the whole plasticating and extrusion process by axial vibration of the screw. The aim of superimposing a vibration force field was to change the crystalline structure of HDPE and improve the molecular orientation in the circumferential direction to obtain high‐circumferential‐strength pipes. RESULTS: Through vibration extrusion, the circumferential strength of HDPE pipes increased significantly, and biaxial self‐reinforcement pipes could be obtained. The maximum increase of bursting pressure and tensile yield strength was 34.2 and 5.3%, respectively. According to differential scanning calorimetry and wide‐angle X‐ray diffraction measurements, the HDPE pipes prepared by vibration extrusion had higher crystallinity, higher melting temperature, larger crystal sizes and more perfect crystals. CONCLUSION: Vibration extrusion can effectively enhance the mechanical properties of HDPE pipes, especially the circumferential strength. The improvement of mechanical properties of HDPE pipes obtained by vibration extrusion can be attributed to the higher degree of crystallinity and the improvement of the molecular orientation and of the crystalline morphology. Copyright © 2008 Society of Chemical Industry     

Polyethylene composite fibers. I. Composite fibers of high‐density polyethylene

Polymer matrix composites are generally studied in the form of bulk solids, and very few works have examined composite fibers. The research described here extended such bulk studies to fibers. The question is whether or not what has been reported for bulk polymers will be the same in fibers. In this article are reported studies of high‐density polyethylene (HDPE), whereas those of linear low‐density polyethylene are reported in part II of this article series. Two types of filler were used, that is, organically modified montmorillonite (OMMT), in which the nanosized filler particles had a high aspect ratio, and microsized calcium carbonate (CaCO₃), with an aspect ratio nearer to unity. Composite fibers of both as‐spun and highly drawn forms were prepared, and their structures, morphology, and mechanical properties were studied. It was found that the microsized particles gave HDPE composite fibers with mechanical properties that were the same as those of the neat polymer. In the case of clay composite fibers, the clay interfered with the yield process, and the usual yield point could not be observed. The particle shape did not affect the mechanical properties. The fibers showed different deformation morphologies at low draw ratios. The CaCO₃ composite fibers showed cavities, which were indicative of low interaction between the polymer and the filler. The OMMT composite fibers showed platelets aligned along the fibers and good polymer–filler interaction. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Radiation‐induced grafting of glycidyl methacrylate onto high‐density polyethylene (HDPE) and radiation lamination of HDPE

Radiation‐induced grafting of glycidyl meth‐acrylate (GMA) onto high‐density polyethylene (HDPE) and the radiation lamination of HDPE by bulk grafting of GMA were reported. The effects of irradiation dose, monomer concentration, and atmosphere on grafting were investigated. The extent of grafting initially increased with irradiation dose and then remained almost constant. The extent of grafting was higher in 2M GMA than in 1M GMA at the same irradiation dose. The extent of grafting in nitrogen was higher than that in air. The grafted samples were characterized with FTIR spectrometry and thermogravimetric (TG) analysis. A carbonyl group was found on grafted HDPE samples, and the carbonyl index increased with the extent of grafting. TG analyses proved the existence of grafted materials on HDPE and the grafted GMA thermally decomposes at a temperature lower than that of HDPE. Strong adhesion could be obtained with radiation lamination of HDPE by bulk grafting of GMA. Benzophenone facilitates the grafting in a proper concentration range. The adhesion mechanism of the laminated samples was the entanglement of the grafted chains. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 772–779, 2005     

Effect of initial crystallinity on the response of high‐density polyethylene to high‐energy radiation

Samples of each of two high‐density polyethylenes with various initial degrees of crystallinity, but otherwise identical, were exposed under a vacuum to moderate doses of gamma irradiation. The results indicate that, for otherwise initially identical polymer samples, the dose required to reach the gel point increases with increase of the initial degree of crystallinity. Above the critical dose for gelation, the gel content decreases with higher degrees of crystallinity at equal radiation doses. The mechanical behavior of the polymers changed progressively from ductile to brittle as the crystallinity was increased. The extensibility of originally ductile samples decreases with increasing radiation dose. The irradiation of samples having intermediate behavior produces a change to ductile behavior. Mechanical behavior is not modified substantially when brittle samples are irradiated. The initial modulus is little altered by irradiation, while the yield stress shows a slight increase with irradiation. The mechanical properties, such as draw ratio at break and ultimate tensile stress, decrease with dose in ductile samples. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1375–1384, 1999     

Thermal and morphological properties of high‐density polyethylene/ethylene–vinyl acetate copolymer composites with polyhedral oligomeric silsesquioxane nanostructure

The demand for improved properties of common polymers keeps increasing, and several new approaches have been investigated. In the study reported here, composites with a polymer matrix comprising a blend of high‐density polyethylene with ethylene–vinyl acetate copolymer (EVA), and with polyhedral oligomeric silsesquioxane (POSS) as a nanostructure, were processed and characterized in terms of their thermal and morphological properties. For the preparation of the composites, the concentrations of the blend components (0, 50 and 100 wt%) and of the POSS (0, 1 and 5 wt%) were varied. X‐ray diffraction results indicated that the presence of EVA in the composites led to the appearance of crystalline domains at lower POSS concentrations. Transmission and scanning electron microscopy showed that samples with 1 wt% of POSS had a homogeneous distribution in the polymer matrix with average dimensions of ca 150 nm. However, the formation of aggregates occurred in samples with 5 wt% of POSS. Differential scanning calorimetry and thermogravimetic analyses indicated that the POSS did not affect the melt and degradation temperatures of the polymer matrix. POSS underwent aggregation at higher concentrations during the composite processing, indicating a solubility limit of around 1 wt%. The presence of EVA in the composite favors POSS aggregation due to an increase in the polarity of the polymer matrix. Copyright © 2009 Society of Chemical Industry     

Necking behavior in high‐speed melt spinning of poly(ethylene terephthalate)

The necking behavior in the high‐speed melt‐spinning process of poly(ethylene terephthalate) (PET) was analyzed using a mathematical simulation under a nonisothermal condition. A constitutive model into which the strain‐rate dependence of viscosity and the strain‐hardening effect are incorporated was used. Based on the simulated results, the cause of a local reduction of apparent viscosity was found to be due mainly to high strain rate. Also the onset of crystallization, if it occurred, was found to happen near the end of the neck. In addition, with no crystallization involved, the necking can still occur. The deformation process in high‐speed spinning of PET was found to consist of two regions along the spin line: a Newtonian flow region and a rubberlike deformation region. The necking behavior is discussed here in terms of strain‐rate sensitivity and strain‐hardening parameter. As a result, a criterion for the onset of stable necking has been obtained. The necking behavior does not seem to be essentially different from that in cold drawing. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 446–456, 2000     

Self‐reinforcement of high‐density polyethylene/low‐density polyethylene prepared by oscillating packing injection molding under low pressure

This article reports the toughness improvement of high‐density polyethylene (HDPE) by low‐density polyethylene (LDPE) in oscillating packing injection molding, whereas tensile strength and modulus are greatly enhanced by oscillating packing at the same time. Compared with self‐reinforced pure HDPE, the tensile strength of HDPE/LDPE (80/20 wt %) keeps at the same level, and toughness increases. Multilayer structure on the fracture surface of self‐reinforced HDPE/LDPE specimens can be observed by scanning electron microscope. The central layer of the fracture surface breaks in a ductile manner, whereas the break of shear layer is somewhat brittle. The strength and modulus increase is due to the high orientation of macromolecules along the flow direction, refined crystallization, and shish‐kebab crystals. Differential scanning calorimetry and wide‐angle X‐ray diffraction find cocrystallization occurs between HDPE and LDPE. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 799–804, 1999     

Structure development and interfacial interactions in high‐density polyethylene/hydroxyapatite (HDPE/HA) composites molded with preferred orientation

Composites of high‐density polyethylene (HDPE) filled with sintered and nonsintered hydroxyapatite (HA) powders, designated as HAs and HAns, respectively, were compounded by twin screw extrusion. Compounds with neoalkoxy titanate or zirconate coupling agents were also produced to improve interfacial interaction and filler dispersion in the composites. The composites were molded into tensile test bars using (i) conventional injection molding and (ii) shear‐controlled orientation in injection molding (SCORIM). This latter molding technique was used to deliberately induce a strong anisotropic character to the composites. The mechanical characterization included tensile testing and microhardness measurements. The morphology of the moldings was studied by both polarized light microscopy and scanning electron microscopy, and the structure developed was assessed by wide‐angle X‐ray diffraction. The reinforcing effect of HA particles was found to depend on the molding technique employed. The higher mechanical performance of SCORIM processed composites results from the much higher orientation of the matrix and, to a lesser extent, from the superior degree of filler dispersion compared with conventional moldings. The strong anisotropy of the SCORIM moldings is associated with a clear laminated morphology developed during shear application stage. The titanate and the zirconate coupling agents caused significant variations in the tensile test behavior, but their influence was strongly dependent on the molding technique employed. The application of shear associated with the use of coupling agents promotes the disruption of the HA agglomerates and improves mechanical performance. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2873–2886, 2002     

Studies on high‐speed melt spinning of noncircular cross‐ section fibers. I. Structural analysis of as‐spun fibers

Flat fibers and hollow fibers were prepared through the high‐speed melt spinning of poly(ethylene terephthalate) (PET), and the structures of these fibers were compared with those of circular fibers. The cross‐sectional shape of each fiber changed to a dull shape in comparison with that of the respective spinning nozzle. The change in the cross‐sectional shape was slightly suppressed with an increase in the take‐up velocity. There was a significant development of structural variation in the cross section of flat fibers in that the molecular orientation and crystallization were enhanced at the edge. Despite the difference in the cross‐sectional shape, the structural development of flat, hollow, and circular fibers with increasing take‐up velocity showed almost similar behavior. Considering that the tensile stress at the solidification point of the spin line is known to govern the structure development of high‐speed spun PET fibers, it was speculated that the effects of the enhancement of cooling and air friction on the tensile stress at the solidification point cancel each other. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1575–1581, 2001     

Effect of low‐temperature annealing on dimensional stability of poly(ethylene 2,6‐naphthalene dicarboxylate) fibers melt‐spun at high speed

To investigate structural factors, necessary to obtain a valuable industrial fiber possessing excellent thermomechanical properties, poly(ethylene 2,6‐naphthalene dicarboxylate) (PEN) fibers were produced by high‐speed melt‐spinning to a take‐up speed of 8 km/min, followed by low‐temperature annealing between the glass‐transition temperature (T_g) and exothermic cold crystallization temperature (T_{c cold}), where little transition of crystalline phase, as well as little thermal degradation, takes place. Their thermomechanical behavior, as well as structural variations, were investigated through differential scanning calorimetry, Rheovibron, thermomechanical analysis (TMA), and tensile testing. Two types of the α‐ and α′‐dispersions were observed at near T_g and at a temperature 50–60°C higher than T_g, respectively. The dispersions were affected by rearranged structures, which are generated by developing an inhomogeneous taut structure with rigidity of aromatic segment and aliphatic segment. The α‐dispersion seemed to reflect an inhomogeneous taut structure by the less nearly arranged segments. Consequently, at intermediate take‐up speeds between 2 and 6 km/min the inhomogeneous taut structure may be partially formed, but the homogeneously ordered structure may be enlarged as the take‐up speed and annealing temperature increased. Thermal shrinkage increased above the α‐dispersion temperature, which suggested that the onset point of dimensional change in PEN fibers was attributed to α‐dispersion. In the case of annealed fibers, the start of length change coincided with the respective annealing temperatures, which indicated that dimensional stability could be gained from restraining the inhomogeneous taut structure in the amorphous region without the transition of crystalline phase by annealing between T_g and T_{c cold}. Therefore, to obtain dimensional stability in PEN fibers, it is supposed that the inhomogeneous taut structure exhibited by the α‐dispersion should be controlled. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 212–218, 2005