Microhardness of extruded high density polyethylene fibers

High-density polyethylene of high tensile modulus has been produced by solid state extrusion using an Instron capillary rheometer. Microhardness measurements on these ultraoriented fibers have been made to assess their perfection from values of the tensile elastic modulus and shear strength. The microhardness tests were measured using a Vickers square diamond. The microhardness increased with the common temperature for crystallization and extrusion, likely due to improvement in the lateral packing of microfibrils. The variation of microhardness with draw ratio is also illustrated.     

振动挤出HDPE管材的结构与力学性能

使用自制的电磁动态塑化挤出机和螺旋芯棒式机头挤出高密度聚乙烯(HDPE)管材.采用爆破压力测试,拉伸性能测试,差示扫描量热法分析等研究振动频率和振幅对HDPE管材结构与力学性能的影响.振动挤出的HDPE管材周向强度显著提高,实现了管材的双向自增强.与稳态相比,振动挤出的HDPE管材结晶度提高,熔点升高,结晶完善;爆破压力最大提高了34.2%,轴向拉伸屈服应力最大提高了5.3%.     

Thermal conductivity of extruded polyethylene

The thermal conductivity of extruded polyethylene has been measured between 220 and 360K by a direct comparative method. For the material with the highest extrusion ratio the conductivity parallel to the extrusion direction, κ_∥, is 72 mW/cm K at room temperature and more or less constant over the complete temperature range. Between the same temperatures the conductivity perpendicular to the extrusion direction decreases with increasing T by about 50% with the anisotropy in the most highly extruded sample almost 30 at room temperature. The results on κ_∥ cannot be explained by the Takayanagi model used in our earlier papers and further modifications to the model are required.     

Structure and properties of extruded polyethylene film

A series of low-density polythylene extruded films was examined quantitatively by the birefringence, infrared dichroism, and x-ray pole figure techniques. The birefringence ranges from mildly positive to mildly negative with increasing severity of quenching conditions. The x-ray data show that the birefringence is largely due to the contribution of oriented crystallites, the amorphous orientation being quite low. The crystal orientation functions suggest equal degrees of a and c axis orientation parallel to the machine direction at low quenching rates, and increasing a axis orientation as the quenching rate increases, coupled with a shift in the c axis from parallel to perpendicular orientation. These results are confirmed by infrared dichroism data. The relative degree of a and c axis orientation ultimately reached is intermediate between that predicted by Keller's type I and type II models, but approximates the orientation previously observed in laboratory films prepared by oriented crystallization at 100% elongation. The crystalline orientation may be explained by the modified row orientation structure of Keller and Machin. However, the data can also be reconcilled with that of spherulitic entities observed in samples crystallized at 20-50% stretch. It is suggested that these spherulites may possess a combination type II and screw dislocation morphology in the equatorial and polar regions, respectively. Such a structure differs from the row structure in that the latter implies that the amount of polar material is negligible compared to the equatorial material. It is recognized, however, that orientation data cannot unambiguously decide between these alternatives.     

Compatibilizer system for extruded polyethylene foam and film

This paper discusses the processing/product benefits of adding additives to extruded polyethylene (PE) foam and film. Low-density polyethylene foam (over 30 times expansion) requires adequate cooling to stabilize the melt/gas system for optimal foaming efficiency. The presence of a small portion of high melting polyethylenes (i.e. LLDPE, HDPE) causes flow instability after cooling, resulting in surface defects in finished products. We found that the addition of metal oxide and fatty acid ester improved processing latitude and foam product quality. This formula was also tested in film processes at various ratios of LD/LLD/HD with promising results. It appears that this formula can allow us to accommodate post consumer resin (PCR) and to take advantage of PE price variations.     

Swell and distortions of high-density polyethylene extruded through capillary dies

The effect of varying the die entrance angle and the die length on extrudate swell and on the onset of extrudate distortion in capillary extrusion has been studied. Using theory from the literature, we have analyzed the contribution to the total pressure drop from the elongational and shear deformation in the entrance region, and from the capillary pressure drop in the land region of the die. From the contribution of the elongational deformation, we obtained an estimate for the elongational viscosity of the polymer. The same analysis was used to study the influence of the die geometry on the stick-slip instability. It is found that the elongational component at the inlet region mainly influences the extrudate distortions. The onset of the stick-slip instability occurs within 10% at a wall stress τ_w of 0.3MPa, where τ_w is calculated from expressions assuming fully developed flow. The variation around this average value is systematic with changes in die geometry, and the observed variations are probably due to the non-homogeneous pressure field in the die. We also propose a model for predicting extrudate swell. Input to the model are material parameters obtainable from oscillatoric measurements of the loss and storage modulus and residence times calculated from the geometry of the die. The swell model includes a fitting parameter that sets the overall scale of the swell.     

Effect of a separator in a sheet die on the anisotropy of tear strength of extruded medium density polyethylene

Sheets of medium density polyethylene (MDPE) were extruded through a slit die containing an internal separator. Thus, the melt stream was momentarily split before emerging from the die. A line of separation was evident in the extruded sheets. It is attributed to incomplete welding or healing. Measurements of tear energy G_c revealed that the extruded sheets were anisotropic and that the weld line was extremely weak after extrusion start-up, only about 1/5 of the strength elsewhere. As extrusion continued, the strength of the weld line increased to reach that of the bulk material after about 10 min. This is attributed to an increasing temperature of the melt in the die region, aiding interdiffusion. A sample containing 30% by weight of short glass fibers showed less initial weld-line weakness but the weld line remained weak in this case, even after long extrusion times.     

Morphology of extruded high-density polyethylene pipes studied by atomic force microscopy

Atomic force microscopy (AFM) was used to study the structure of extruded polyethylene (PE) pipe. During extrusion, the outer surface of the pipe was cooled with water. Two cross sections, parallel and transverse to the extrusion direction, were examined in order to spatially follow the structural development during extrusion. The morphology revealed was spherulitic, and the spherulites had a mostly banded appearance when viewed under the AFM. We were not able to distinguish an oriented skin layer at the surface of the pipe, either by AFM or polarizing microscopy. The changes in the pipe's structure resulting from the cooling conditions were found to be rather gradual, and no clearly defined zones were observed. A slight orientation towards the extrusion direction was detected only in the area of the pipe crystallized under the lowest degree of undercooling. Measured spherulitic size, band period, and lamellae thickness showed a gradual increase in their values from the cooled to the noncooled surface of the pipe. Transmission electron microscopy (TEM) was used to verify the band period and lamellae thickness measurements done by AFM. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 515–523, 1997     

Push–pull extrusion: A new approach for the solid-state deformation illustrated with high density polyethylene

The crystalline state deformation of high density polyethylene has been examined at an extrusion draw ratio of 30 over a range of temperatures and pressures. The experiments involve combined pushing (extrusion) and pulling through a conical die. The pressure dependence of the extrusion rate through conical dies is given by a logarithmic relation and the temperature dependence by an activation energy of ～95 kcal/mole. An equation established for the total applied force linearly relates the pulling and extrusion pressure components and represents a force balance at the die entrance and exit. Steady-state extrusion, with or without pulling, was feasible in a pressure range beyond which fractures occurred owing to strain rate and shear or tensile failure. Under some circumstances the extrusion rate was increased by ten times. The mechanical properties and mode of deformation were not affected by pull load and fibers with a tensile modulus of 55 GPa were produced at T < 110°C.     

Optical properties of extruded polyethylene thin films related to anisotropy and inhomogeneous microstructure

In this paper a method has been developed to extend the use of the well-known expression of Clausius-Mosotti for inhomogeneous materials, considering their anisotropy through a series expansion based on the structural factor of nonsphericity. This is related to experimental results available from the ordinary and extraordinary refractive indices of a set of samples of low-density polyethylene films, where such phenomena are observed. Here results are obtained which are valid for the microscopic optical response, as it is shown by the linear fitting of these general expressions, with excellent correlation coefficients. Furthermore, if we apply the Bergman-Milton limits, we can theoretically confirm the existence of two neutral lines experimentally obtained.     

Some solid-state properties of blends of polyethylene terephthalate and polyamide-6,6

Polymer blends incorporating polyamide-6,6 (PA) and polyethylene terephthalate (PET) and having the following PA wt,% concentrations were prepared: 0, 5, 10, 25, 30, 35, and 100. Samples were obtained by molding in a reciprocating-screw injection-molding machine. The samples were annealed to minimize frozen-in stresses without increasing the crystallinity level in the material. Melting and heat-of-fnsion data, obtained with the differential scanning calorimeter, suggest an overall increased crystallinity in the blends, as indicated by a significant excess heat of fusion. Whereas the neat polymers exhibit ductile failure under both tensile and impact testing conditions, the blends show brittle behavior. Finally, the abrasion resistance of the blends is inferior to that observed for PET but higher than the resistance of PA.     

氯化聚乙烯专用HDPE树脂剖析

采用凝胶渗透色谱仪、差示扫描量热仪、扫描电子显微镜等考察了氯化聚乙烯(CPE)专用高密度聚乙烯(HDPE)的结构、性能和颗粒形态.结果表明: CPE专用HDPE具有中等相对分子质量、单峰相对分子质量分布、低共聚单体含量、高结晶度、高密度和低蜡含量等特点;进口料的粒径控制较好,粗颗粒含量更少;国产料具有更大的比表面积、孔容和更精细的形态结构.     

Shear modification of low density polyethylene

A single screw extruder was used to impose different shear histories on a low density polyethylene with broad molecular weight distribution and high molecular weight tail that had very little long chain branching. This polymer exhibits relatively high melt elasticity and the viscoelastic properties of its melt are strongly affected by preshearing. Such changes are accomplished without significant changes in molecular weight distribution or chemical structure. Measured viscous and elastic properties of the melt are different from piston-driven and screw extruder capillary rheometers. Shear modification effects in single screw extruders are enhanced by decreasing melt temperature, increased screw rotation speeds, and higher screw compression ratios. Melt elasticity can be cycled between high and relatively low values, for the particular polymer, by annealing or shearing the polymer melt.     

Melt elasticity of low density polyethylene

The stress vs strain and strain recovery characteristics of a series of low density polyethylenes of various molecular size and molecular size distributions have been studied in the melt state. The results show that the high molecular size portion of the molecular size distribution dominates the stress vs strain behavior. The high molecular size component causes a large increase in the stress overshoot and steady state stress. The high molecular size component also has a strong influence on the magnitude of recoverable strain, The strain recovery characteristics are dependent on the amount of strain applied. For large strains (above the yield point) the strain recovers slowly to a large extent (Type I). For applied strains below the yield point the strain recovery is rapid and finished in a short period of time (Type II).     

Vibrations of crystallographic defects associated with a single chain in polyethylene

The vibrational behaviour of crystallographic defects associated with a single chain were investigated for a dispiration, disclination, and dislocation in polyethylene. An approximate longitudinal modulus for the defects was determined by using conformational calculations to estimate the energy changes associated with changes in the length of a defect. This modulus, combined with the mass per unit length of the defect, was used to estimate the lowest longitudinal frequency of the defect, which was found to be around 100 cm^?1 for all the defects considered. Normal mode vibrational calculations for oligomers containing defects showed that the predicted lowest longitudinal modes could be identified by examination of the displacements associated with modes occurring in the estimated frequency range. It was shown that the defects could be considered as localized oscillators embedded in the crystal and coupled to the vibrational modes of the crystal. The presence of defects provides special mechanisms for coupling light waves and lattice vibrations in the crystal which may affect the Raman spectrum.     

Impact yielding of high density polyethylene

We have used a projectile impact method to estimate the flow stress of high density polyethylene at a strain rate of 3 × 10³ sec⁻¹. The technique was developed initially by Taylor and applied successfully by Whiffin and others to ductile metals. The data from this experiment have been compared with data obtained in more conventional compression and drop hammer tests at lower strain rates at 20° and 100°C. The flow stress of high density polyethylene deduced from the impact test at 20°C is significantly higher than that anticipated from a simple extrapolation of the low strain rate data at 20°C. The data at 100°C are however in good agreement. The technique has also been used to estimate the flow stress of high density polyethylene as a function of temperature over the range −20° to +105°C. These data indicate that the discrepancy in the data for 20°C arises from a real discontinuity in the response of the polymer rather than from an inadequacy in the theoretical analysis of the impact experiment as applied to polymeric solids. We conclude that the impact method described is a useful technique for estimating the flow stress of polymers. It is however limited to a relatively narrow range of strain rates.     

Melt strength of linear low‐density polyethylene/low‐density polyethylene blends

Linear low‐density polyethylenes and low‐density polyethylenes of various compositions were melt‐blended with a batch mixer. The blends were characterized by their melt strengths and other rheological properties. A simple method for measuring melt strength is presented. The melt strength of a blend may vary according to the additive rule or deviate from the additive rule by showing a synergistic or antagonistic effect. This article reports our investigation of the parameters controlling variations of the melt strength of a blend. The reciprocal of the melt strength of a blend correlates well with the reciprocal of the zero‐shear viscosity and the reciprocal of the relaxation time of the melt. An empirical equation relating the maximum increment (or decrement) of the melt strength to the melt indices of the blend components is proposed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1408–1418, 2002     

Absorption of dicumyl peroxide by extruded polyethylene: Difference between surface and bulk morphology

Under some conditions, the absorption of dicumyl peroxide (DICUP) at 70°C by extruded low density polyethylene (LDPE) displayed features which are characteristic of two-stage sorption. For example, the initial slope of the DICUP absorption curve (plot of M_t/M_∝ against \documentclass {article}\pagestyle{empty}\begin{document}$\sqrt t$\end{document} ) reflecting diffusion in the surface region was 2.5 times higher than the slope of the second stage (reflecting diffusion in the bulk) for 4-mm-diameter LDPE rods extruded at 130°C and 180 rpm (die pressure = 6000 psig). Only a single stage curve was evident in the same rod with its surface removed or in smaller diameter extrudate. Increasing the screw speed from 10 to 45 rpm at nearly constant extrusion pressure (1800–2380 psig) resulted in an approximately 50% decrease in the initial (first stage) slope with a negligible effect on the second stage slope. Increasing the die pressure (4900–6000 psig), by decreasing the extrusion temperature, at constant screw speed (125 rpm) resulted in an almost threefold decrease in the second stage slope without apparent effect on the first stage slope. Photomicrographs demonstrated the presence of distinct surface and bulk morphologies with evidence of a transcrystalline surface layer oriented prependicular to the surface at low screw speeds and larger spherulites in the bulk of the high pressure extrudate. Such morphological features and the observed dependence of the two stage sorption curves on extrusion conditions are consistent with the surface and bulk morphology of the extruded LDPE rod being dependent on the screw speed and die pressure, respectively. These uptake curves were determined by modifying the classical sorption technique to separate the measurement of the equilibrium uptake from the continuous recording of the slow changes in mass during absorption. This technique may be useful in the characterization of the migration process in other plastic/penetrant systems where diffusion is too slow to be measured by conventional means.     

Blends of modified polycarbonate and high density polyethylene

In this work, blends of polycarbonate and a high density polyethylene were investigated through their morphology, mechanical properties, and the effect of compatibilizers: a copolymer styrene-butadiene-styrene and an ionomer. Blending was performed in the melt state at 220°C, and the concentration of the compatibilizers was varied from 1% to 5% by weight. In the case of the copolymer modified blend, the results showed no change in the mechanical properties compared to the neat blend, whereas the morphology showed that the copolymer might interact with only one phase. For the ionomer, the addition of 1% increased the Young's modulus and the tensile strength of the blend. For the morphology, a large change in the size of the dispersed phase (polyethylene) is observed. This was attributed to the compatibility of the ethylene group of the ionomer with the minor phase, and the reaction between the carbonate group of polycarbonate (PC) and the acid group of the ionomer. An investigation on the binary blends of PC and the ionomer showed the occurrence of a chemical reaction that might be of a transesterification type. Differential scanning calorimetry and Fourier transform infrared techniques were used to characterize these blends.