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.     

Ageing effects on the creep‐recovery behavior of blown extruded low density polyethylene film

Low density polyethylene films used for greenhouse covering were naturally weathered for 6 months in sub‐Saharan region (Algeria). The microstructural and morphological changes have been checked by infrared spectroscopy. The changes of the mechanical properties have been followed by tensile and creep‐recovery tests. The measurements were carried out in the two main directions of the film plane. Besides oxidation, chain scission and crosslinking are competing during all the ageing protocol, affecting thereby, the mechanical properties as well as the viscoelastic behavior revealed by creep‐recovery curves. The anisotropic character of the film is preserved during ageing. The improvement of the creep resistance via crosslinking in both directions, affects the different deformations. The short chain segments coming from chain scissions increase the crystallinity ratio (via a chemo‐crystallization process) lowering by consequence each of the minimum strain rate ( ), the instantaneous ( ), and delayed recoveries ( ) as well. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44209.     

Degradation of low density polyethylene during extrusion. I. Volatile compounds in smoke from extruded films

Many problems with odor and taste in food packaging can be traced to degradation of the packaging materials during processing. From this starting point, the degradation of polyethylene in a commercial extrusion coating process was studied by analyzing degradation products present in smoke sampled at the extruder die orifice. Two low‐density polyethylenes, A and B, with similar melt flow indexes and densities and obtained from different producers, were investigated. A third polymer, C, consisting of recycled material B, was also investigated. More than 40 aliphatic aldehydes and ketones, together with 14 different carboxylic acids, were identified in the smoke. The highest concentration was found for acetaldehyde, regardless of polymer and processing conditions. Increasing the extrusion temperatures in the range 280–325 °C increased the amounts of the oxidized products in the smoke. The extruded film thickness, 12 and 25 μm, influenced the concentrations of degradation products, with the thicker film giving higher amounts of product. The recycled polymer C generally gave lower concentrations of degradation products compared with the virgin polymer B. Differences in the product spectrum between the two virgin polymers may be related to differences in the manufacturing process. Many of the identified compounds have very characteristic taste and smell and are consequently of interest from an odor and taste point of view in food packaging applications. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1580–1586, 2002     

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.     

Filler reaggregation and network formation time scale in extruded high‐density polyethylene/multiwalled carbon nanotube composites

A high‐density polyethylene (HDPE) masterbatch containing 20.2 wt% multiwalled carbon nanotubes (MWNTs) was melt diluted with neat HDPE using two different methods: a twin screw microcompounder and a single‐screw extruder. The electrical properties of these composites were assessed using bulk electrical conductivity measurements, their mechanical properties were evaluated using tensile tests and dynamic mechanical analysis (DMA), and percent crystallinity was determined by wide angle x‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). A percolation threshold (p_c) of 4.5 wt% MWNTs was found in compression‐molded samples. Extruded samples were prepared with nanotube concentrations below and above the compression‐molded percolation threshold (2 and 7 wt% MWNTs) and passed through the extruder twice before entering a low‐shear melt annealing zone. Different melt annealing times were used and their effects on the electrical and mechanical properties of the resulting quench‐cooled composites were evaluated. Results showed that extruded composites were nonconductive, indicating that a conductive nanotube network did not form on the time scale of these experiments. Annealing time also did not affect significantly the mechanical properties of the resulting solid composites. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

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.     

Dynamic rheological properties of high‐density polyethylene/polystyrene blends extruded in the presence of ultrasonic oscillations

The linear rheological properties of high‐density polyethylene (HDPE), polystyrene (PS), and HDPE/PS (80/20) blends were used to characterize their structural development during extrusion in the presence of ultrasonic oscillations. The master curves of the storage shear modulus (G′) and loss shear modulus (G″) at 200°C for HDPE, PS, and HDPE/PS (80/20) blends were constructed with time–temperature superposition, and their zero shear viscosity was determined from Cole–Cole plots of the out‐of‐phase viscous component of the dynamic complex viscosity (η″) versus the dynamic shear viscosity. The experimental results showed that ultrasonic oscillations during extrusion reduced G′ and G″ as well as the zero shear viscosity of HDPE and PS because of their mechanochemical degradation in the presence of ultrasonic oscillations; this was confirmed by molecular weight measurements. Ultrasonic oscillations increased the slopes of log G′ versus log G″ for HDPE and PS in the low‐frequency terminal zone because of the increase in their molecular weight distributions. The slopes of log G′ versus log G″ for HDPE/PS (80/20) blends and an emulsion model were used to characterize the ultrasonic enhancement of the compatibility of the blends. The results showed that ultrasonic oscillations could reduce the interfacial tension and enhance the compatibility of the blends, and this was consistent with our previous work. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3153–3158, 2004     

Thermal and morphological evaluation of very low density polyethylene/high density polyethylene blends

Blends of very low density polyethylene (VLDPE) and high density polyethylene (HDPE) were prepared by melt extrusion. These blends exhibit a tendency to phase segregate when they are slow cooled from the melt. If they are cooled at increasingly faster rates, a finite population of co‐crystals can be isolated from the rest of the phase segregated material, indicating that this system is probably miscible in the melt but phase separates during cooling. Transmission electron microscopy observations are consistent with the blend melt miscibility since inter‐lamellar mixing was clearly appreciated in the samples examined. Other effects arising from interactions between the polymers were the nucleation of VLDPE rich phase by HDPE rich phase, and a melting point depression of HDPE rich phase caused by a dilution effect exerted by molten VLDPE rich phase. After a successive self‐nucleation and annealing thermal fractionation procedure is applied to the blends, phase separation dominates the behavior, although some small fraction of co‐crystals was still present.     

Degradation of low density polyethylene during extrusion. III. Volatile compounds in extruded films creating off‐flavor

This study was aimed at finding a correlation between the experienced off‐flavor in packaged foods and the presence of specific degradation products in PE packaging films. The possibility to trap degradation products by chemical reactions with scavengers, that is, zeolites and maleic anhydride grafted LLDPE, were investigated. This trapping would prevent the degradation products from migrating to the polymer film surface and further into food in contact with the film. This work concludes that off‐flavor in water packed in LDPE‐films depends on extrusion temperature and the content of oxidation products in the polymer film. At lower extrusion temperatures, reactive additives to the LDPE material could control the release of off‐flavor giving components. Adsorbents, such as zeolites, which are able to adsorb degradation products, are effective also at higher extrusion temperatures. The amount of oxidized degradation products in the films correlated well to the perceived off‐flavor in the packed water. The presence of aldehydes and ketones have a clear impact on the off‐flavor. The best correlation between off‐flavor and oxidized components were found for C₇? C₉ ketones, and aldehydes in the range of C₅ to C₈. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 847–858, 2005     

Morphology and property of polyethylene pipe extruded at the low mandrel rotation

During the rotation extrusion of polyethylene (PE) pipes, with the rotating mandrel, compressed air as a cooling medium was introduced through their interior to achieve the quick cooling of the inner wall. The experimental results showed that the hoop stress exerted by mandrel rotation could promote the molecular orientation in the hoop direction; moreover, the introduction of compressed air could quicken its inner wall's cooling rate so as to slow down the relaxation of the oriented molecule and to reserve the orientation structure. Therefore, the hoop orientation degree increased with the increasing inner wall's cooling rate. As a result, the performance of the PE pipe was greatly enhanced. The hoop tensile strength of the PE pipe produced by the novel extrusion method increased from original 24.1 MPa up to 35 MPa; the pipe's crack initiation time increased from 27 to 60 h and the crack growth rate slowed down. POLYM. ENG. SCI., 50:1743–1750, 2010. © 2010 Society of Plastics Engineers     

Degradation of low density polyethylene during extrusion. IV. Off‐flavor compounds in extruded films of stabilized LDPE

This study was aimed at finding a correlation between the experienced off‐flavor in packed foods and the presence of specific degradation products in LDPE packaging films. The possibility to trap degradation products by chemical reactions with scavengers, i.e., a zeolite additive or antioxidants, was investigated This would prevent degradation products from migrating to the polymer film surface and further into food in contact with the film. It was found that off‐flavor noted in water packed in LDPE films depended on extrusion temperature and exposure time for the melt to oxygen, that is, the parameters that influence the contents of oxidation products that are able to migrate from the polymer film. It was also found that adsorption of oxidative degradation products in a zeolite additive or protection of LDPE by using antioxidants could prevent off‐flavor in the packed product (water). However, the antioxidant should be selected with regard to extrusion temperature because thermal instability in the additive might jeopardize the intended effect. Multifunctional antioxidants seem to provide improved protection, the most effective one evaluated in this work being Irganox E201, i.e., vitamin E. Concentrations of oxidized degradation products are well correlated to the perceived off‐flavor in the packed water. The highest correlation between off‐flavor and oxidized components was found for ketones in the range of C₇ to C₉ and aldehydes in the range of C₆ to C₉. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 583–595, 2005     

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.     

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     

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.     

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.