Extrudate swell of high density polyethylene. Part III: Extrusion blow molding die geometry effects

Two high density polyethylene resins—801 and 802— are examined with regard to their isothermal, time-dependent, and nonisothermal swelling properties when emerging from two annular and three diverging dies. The short time swelling characteristics of samples 801 and 802 are very important for these dies, resulting in a lower diameter swell for the latter, independent of the die geometry or flow rate. Output variations have much less impact on the swelling behavior than small changes in the geometry of the die mandrel. Accordingly, shear stress and shear rate parameters alone cannot be used to explain the swelling properties of a HDPE resin in the different die geometries. Straight annular dies induce higher diameter swelling than diverging dies.     

Extrudate swell of high density polyethylene. Part I: Aspects of molecular structure and rheological characterization methods

Two high density polyethylene (HDPE) resins–samples 801 and 802–both nominally the same material, as they are taken from successive batches of the same commercial grade, are characterized for their molecular structure and rheological properties. Gel permeation chromatography (GPC) and low angle laser light scattering (LALLS) results must be interpreted in combination with rheological data to show the presence of somewhat more high molecular weight material in 802 that in 801. Small amplitude oscillatory shear, steady shear, and capillary shear measurements performed in different laboratories show consistently higher shear viscosity values at low shear rates for sample 802. Extensional viscosity measurements show similar results. The interpretation of rheological data in terms of molecular structure could be complicated by the possible presence of long chain branching (LCB). The zero shear viscosity and discrete relaxation spectrum is estimated for both samples. The small rheological difference between 801 and 802 forms the basic information for understanding their time dependent extrudate swell behavior, as will be described in Part II.     

Effect of die temperature on the flow of polymer melts. Part II: Extrudate swell

The effect of die wall temperature on the extrudate swell of polymer melts flowing through dies with single and dual circular channels was studied. Extrudate swell was measured at constant flow rates using an Instron capillary rheometer with a modified die section. It was found that under isothermal conditions, extrudate swell plotted against the average wall shear stress gave rise to a temperature independent correlation for polystyrene. Under non-isothermal conditions, such a correlation did not exist, which might be due to the change of wall shear stress in the axial direction. The extrudate swell in the non-isothermal cases can be better correlated with the wall shear stress at die exit. For the two-hole die, changes of die wall temperature varied both the flow rate ratio and the extru date swell ratio. The latter is, however, much less sensitive to the die wall temperature than the former.     

Nonlinear mechanical response of high density polyethylene. Part II: Uniaxial constitutive modeling

Based on the experimental data presented in Part I, two uniaxial constitutive models are constructed. The first, a nonlinear viscoelastic (NVE) model, is formulated using the mechanical analogy consisting of one independent spring and six Kelvin elements in series. Creep data are used to determine the model parameters. The second model, a viscoplastic (VP) formulation, is developed using the viscoplastic theory proposed by Bodner to characterize the uniaxial viscoplastic behavior of metals. Inelastic strain rate is introduced into the state variable in addition to inelastic work to depict the strong rate dependent behavior of HDPE. Experimental data from constant strain rate tests are employed to construct the material functions of the model. Limitations in the application of each model are discussed in conjunction with possibilities for future work.     

Chlorinated high density polyethylene. II. Solid state structure

Chlorination of high density; polyethylene results in polymers which consist of unmodified methylene units and chlorinated methylene co-units. The effect of the concentration and distribution of chlorinated units on the solid state structure has been examined by thermal, wide angle X-ray diffraction and dynamic mechanical analysis. As the substitution becomes more random, the crystallinity, crystallite size, and crystalline perfection decrease for a given chlorine content. The chlorinated units are shown to be capable of co-crystallizing, and the concentration of chlorine in the crystalline phase increases as the distribution is made more random. Concurrently, the chlorine concentration of the amorphous phase decreases. Segregation of chlorine into the amorphous regions is most efficient when the substitution is blocky.     

Drawing of high density polyethylene tube: Effect of different cooling conditions

A method of producing biaxially oriented high density polyethylene tube is described where some of the deformation parameters may be controlled by a post mandrel cooling bath application. Experimental results show that dimensional stability of the drawn products primarily depend on uniform temperature distribution around the tube circumference. This research gives a clear understanding of the post mandrel deformation process and shows that natural cooling conditions may lead to inhomogeneous products having properties significantly different from water cooled products.     

Nonlinear mechanical response of high density polyethylene. Part I: Experimental investigation and model evaluation

The nonlinear behavior of high density polyethylene (HDPE) is investigated for samples cut from thick-walled HDPE pipe. Extensive experimental work has been performed to characterize the non-linear time-dependent response of the material tested under uniaxial compression. Tests were conducted under conditions of constant strain rate, creep, stress relaxation, constant loading rate, abrupt change of strain rate, creep-recovery, cyclic strain rate, and various combinations of these loading conditions. Creep and stress relaxation response after strain reversal and the effect of the transient response on the following stress-strain behavior is examined. Permanent strains for the test specimens and their dependence on loading histories are investigated. Specimens cut at various orientations from the pipe are used to quantify the small amounts of local anisotropy in the pipe specimen. The experimental work has been used to develop both nonlinear viscoelastic (NVE) and viscoplastic (VP) constitutive models in a companion paper. Both the test results and the corresponding model predictions are reported in this paper. It is found that the VP model reproduces the nonlinear viscoelastic-viscoplastic behavior of HDPE very well provided that the current strain is not below the maximum strain imposed (there is no strain reversal). The NVE model predicts the material behavior reasonably well for some loading conditions, but inadequately for others.     

Extrudate swell from slit and capillary dies: An experimental and theoretical study

A comparative experimental study of extrudate swell from long slit and capillary dies is reported for rheologically characterized polystyrene and polypropylene melts. Generally extrudate swell from a slit is greater than that from a capillary die. At low die wall shear rates it goes to a value of about 1.2 as opposed to about 1.1 found for capillary dies. The onset and character of extrudate distortion have been studied. The experimental results are compared with theories of swell based on unconstrained recovery from Poiseuille flow in these geometries. A detailed analysis of such theories of extrudate swell based on the original work of Tanner has been carried out. The analysis is placed in a more general form which should be valid for a range of die cross-sections.     

Rheology of high solid coatings. II. Analysis of combined sagging and leveling

The rheology of combined sagging and leveling of high solid coatings is analyzed in terms of non-Newtonian power-law model. The results indicate that, in order to have good leveling, good sag control, and good sprayability at the same time, high solid coatings should have pseudoplastic rheology with power constant of about 0.5 and viscosity at sec^-1 of about 50 poises. This theoretical prediction is confirmed experimentally.     

Photoinitiated crosslinking of low density polyethylene: II. Morphology and properties

The crystalline morphology and physical properties of photocrosslinked polyethylene in a low density polyethylene (LDPE)-benzophenone (BP)-triallyl isocyanurate (TAIC) system have been studied. The results of X-ray diffraction and DSC thermal analysis indicate that the effect of crosslinking on the degree of crystallinity of the material is quite limited, but the presence of crosslinks can affect the regularity of the crystal regions. This is proposed to be responsible for the slight variation in properties of the crosslinked material at room temperature, such as the decrease of modulus and the changes of yield behavior. The main improvements in properties of the photocrosslinked polyethylene are found at high temperatures, especially above the melting point. The strength is increased, and the heat distortion temperature is enhanced significantly, as previously reported in the literature.     

High density polyethylene foams. II. Elastic modulus

High density polyethylene (HDPE) foams (450–950 kg/m³) were prepared by compression molding and their tensile moduli were measured in order to study the normalized elastic modulus as a function of the normalized density of closed‐cell foams. The tensile data were then used to compare several models of cellular materials and polymer composites to determine which would fit our results. Of all models used, the simple empirical equation of Moore (square power‐law) and the differential scheme predict the data very well in the range of voids volume fraction under study (0–55%). © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2120–2129, 2003     

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     

Chlorinated high density polyethylene. I. Chain characterization

High density polyethylene has been chlorinated by three different methods: in suspension and in solutions of two different solvents. Carbon-13 NMR and infrared analysis show that chlorination in chlorobenzene solution leads to statistically random distribution while chlorination in suspension gives highly blocky substitution. An intermediate distribution was obtained by chlorination in tetrachloroethane solution.     

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.     

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.     

Measurement and simulation of low‐density polyethylene extrudate swell through a circular die

BACKGROUND: Extrudate swell is a common phenomenon in polymer processing. The investigation of its mechanism is of both scientific and industrial interest. RESULTS: The rheological parameters of a material described by the viscoelastic PTT (Phan‐Thien–Tanner) constitutive model are obtained by fitting the distributions of material functions detected with a strain‐controlled rheometer. The swelling ratios of low‐density polyethylene (LDPE) under different volume flow rates are indirectly obtained using a photographic technique. A mathematical model of extrudate swell is established and its finite element model is derived. A penalty method is employed to solve the extrudate swell problem with a decoupled algorithm. Computation stability is improved by using the discrete elastic‐viscous split stress algorithm incorporating the inconsistent streamline‐upwind scheme. CONCLUSION: The swell phenomenon of LDPE through a circular die is investigated using both experimental measurement and numerical simulation. The swelling ratios obtained from the simulation are compared with those measured: they agree well with each other. The essential flow characteristics of polymer melts are predicted and the mechanism of the swell phenomenon is further discussed. Copyright © 2009 Society of Chemical Industry     

High density polyethylene/ultra high molecular weight polyethylene blend. II. Effect of hydroxyapatite on processing,thermal, and mechanical properties

Hydroxyapatite (HA) is part of bone mineral composition. Several attempts have been made to incorporate HA into high density polyethylene (HDPE) to produce bone replacement biomaterials since neat HDPE is not suitable as bone replacement. The blending of HDPE with ultra high molecular weight polyethylene (UHMWPE) up to 50% by weight was performed with the aim of improving the toughness of composites. Reinforcement of blend with HA of up to 50% by weight was carried out. Methods of characterizing the composites included density, differential scanning calorimetry, thermal gravimetric analysis, ash content, and morphological examination using scanning electron microscope. For the mechanical properties of the composites, tensile, flexural, and impact tests were carried out. Incorporation of HA into HDPE has resulted in the brittleness of the composites. Blending of HDPE with UHMWPE in the presence of HA was found to improve the mechanical properties and promote a ductile failure of the resulting composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3931–3942, 2006     

Degradation of polyethylene during extrusion. II. Degradation of low‐density polyethylene,linear low‐density polyethylene,and high‐density polyethylene in film extrusion

The degradation of different polyethylenes—low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and high‐density polyethylene (HDPE)—with and without antioxidants and at different oxygen concentrations in the polymer granulates, have been studied in extrusion coating processing. The degradation was followed by online rheometry, size exclusion chromatography, surface oxidation index measurements, and gas chromatography–mass spectrometry. The degradations start in the extruder where primary radicals are formed, which are subject to the auto‐oxidation when oxygen is present. In the extruder, crosslinking or chain scissions reactions are dominating at low and high melt temperatures, respectively, for LDPE, and chain scission is overall dominating for the more linear LLDPE and HDPE resins. Additives such as antioxidants react with primary radicals formed in the melt. Degradation taking place in the film between the die orifice, and the quenching point is mainly related to the exposure time to air oxygen. Melt temperatures above 280°C give a dominating surface oxidation, which increases with the exposure time to air between die orifice and quenching too. A number of degradation products were identified—for example, aldehydes and organic acids—which were present in homologous series. The total amount of aldehydes and acids for each number of chain carbon atoms were appeared in the order of C5>C4>C6>C7?C2 for LDPE, C5>C6>C4>C7?C2 for LLDPE, and C5>C6>C7>C4?C2 for HDPE. The total amounts of oxidized compounds presented in the films were related to the processing conditions. Polymer melts exposed to oxygen at the highest temperatures and longest times showed the presence dialdehydes, in addition to the aldehydes and acids. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1525–1537, 2004     

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.