Oscillating flow behavior of high-density polyethylene melts

The oscillating flow behavior of a variety of high-density polyethylene and copolymer samples was studied in a constant displacement rate rheometer. At any plunger velocity, the period of the oscillations decreases linearly with melt depth, suggesting a resonance phenomenon. As plunger velocity is increased, the load waveform changes in a regular manner that indicates a progressive increases in the proportion of each cycle spent on the right-hand branch of the flow curve. Little difference was found in the shear stress at which oscillating flow began for samples differing in molecular weight, molecular weight distribution, and manufacturing process. However, the shear rate at which oscillating flow begins depends, strongly on both molecular weight and distribution. Oscillating flow is shifted to higher shear rates by broadening distribution, reducing molecular weight, increasing temperature, or decreasing the L/D ratio of the capillary.     

Crystallization behavior of high-density polyethylene/linear low-density polyethylene blend

Binary blend of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), prepared by melt mixing in an extruder, in the entire range of blending ratio, is studied for crystallization behavior by differential scanning calorimetry (DSC) and X-ray diffraction measurements. Cocrystallization was evident in the entire range of blend composition, from the single-peak character in both DSC crystallization exotherms and meltingendotherms and the X-ray diffraction peaks. A detailed analysis of DSC crystallization exotherms revealed a systematic effect of the addition of LLDPE on nucleation rate and the subsequently developed crystalline morphology, which could be distinguished in the three regions of blending ratio, viz, the “HDPE-rich blend,” “LLDPE-rich blend,” and the “middle range from 30–70% LLDPE content.” Variations in crystallinity, crystallite size, and d spacing are discussed in terms of differences in molecular structure of the components.     

Confined parison inflation behavior of a high-density polyethylene

An analysis of the confined parison inflation step associated with the extrusion blowmolding of a high-density polyethylene is presented. Based upon simple geometrical considerations and material conservation principles, relationships describing the wall thickness variation have been obtained for various mold configurations. The experimentally measured part thickness distribution is found to be in good qualitative agreement and reasonable quantitative correspondence with the theoretical predictions. Furthermore, the expressions for the thickness variation have been utilized in order to estimate the confined inflation time for the case of a power-law constitutive equation. In addition, a brief discussion of some practical mold design considerations is given based upon the theoretical analysis.     

Viscometric behavior of high-density polyethylene solutions at high temperatures

The steady shear viscosities of high density polyethylene disolved in highly branched isoparaffin solvents have been measured with a high-pressure autoclave viscometer at temperatures ranging from 150 to 250°C and over a range of shear rates from 0.02 to 170 sec^?1. Laboratory measurements on solutions ranging from 15 to 67 wt percent polymer were used to develop and calibrate a mathematical model for solution viscosity intended for a range of from 10 to 100 wt percent polymer. The foundation for the model consists fo two equations: the modified Martin equation is used to describe the effect of concentration, and the Sabia equation is used to describe pseudoplastic behavior. The model correlates viscosities that can range over nine orders of magnitude with sufficient accuracy for most process design work, averaging less than 10 percent error. Both the data and the model indicate that at constant stress the activation energies (E_A) for the effect of temperature on viscosity are higher for solutions in the midrange of composition than for either of the pure components. This peak in E_A is related to the density difference between polymer and solvent.     

A closer look into the behavior of oxygen plasma-treated high-density polyethylene

X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure investigation have been used to characterize the surfaces of two oxygen plasma-treated high-density polyethylene (HDPE) samples having different crystalline fractions. Both the observations indicate that a higher degree of crystallinity restricts the mobility of the polar functional groups on the HDPE surface. An increased crystalline order lowers the amount of oxidation and the aging of polar functional groups on the substrate surface. The results are supported by contact angle measurements and field emission scanning electron microscopic observations.     

Stabilization of high-density polyethylene

Several types of antioxidants are evaluated in high-density polyethylene for color and physical property stabilization during processing and thermal aging. A wide variety of evaluation tests are used and discussed. Heat- and light-induced oxidation mechanisms are reviewed. Antioxidants such as octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate and tetrakis[methylene 3-(3′,5′-di-tert-butyl-4′-hydroxy phenyl) propionate] methane were found to provide very high retention of physical properties, excellent initial color and color retention. Combinations of antioxidant- and ultraviolet light absorbers were evaluated for stabilizing high density polyethylene exposed to artificial light and outdoor weathering. The combination of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate and 2-(2′-5′-di-tert-butyl-2′-hydroxy-phenyl)-5-chlorobenzotriazole was found to be a very efficient stabilizer system. Polymer containing this stabilizer combination had excellent color stability and minimum crosslinking and embrittlement after Arizona weathering.     

A constitutive model for the nonlinear viscoelastic viscoplastic behavior of glassy polymers

Two features of the glassy state of an amorphous polymer, which play a key role in determining its mechanical properties, are the distributed nature of the microstructural state and the thermally activated (temporal) evolution of this state. In this work, we have sought to capture these features in a mechanistically motivated constitutive model by considering a distribution in the activation energy barrier to deformation in a thermally activated model of the deformation process. We thus model what is traditionally termed the nonlinear viscoelastic behavior as an elastic-inelastic transition, where the energetically distributed nature of inelastic events and their evolution with straining is taken into account. The thermoreversible nature of inelastic deformation is modeled by invoking the notion of strain energy stored by localized inelastic shear transformations. The model results are compared to experimental data for constant true strain rate uniaxial compression tests (nonmonotonic) at different rates and temperatures; its predictive capabilities are further tested by comparison with compressive creep tests at different stress levels and temperatures.     

Necking of high-density polyethylene

Necking draw of high-density polyethylene is studied at draw rates of 2.5 × 10^?2 to 25 mm/min and at temperatures of ?40° to 80°C. Effect of temperature and draw rate on necking stress is interpreted in terms of viscoelastic flow of amorphous phase accompanying orientation of crystallites. It is proved that reducibility of draw rate and temperature holds and that the reduction factor obeys approximately the Williams-Landel-Ferry equation. Necking stress at an extremely low draw rate, critical necking stress, is discussed in terms of the phase equilibrium under stress between two states before and after microfracture of crystallites. The theory, with some approximations, leads to the equation by Iida in which the critical necking stress is expressed by fusion parameters. The thermodynamic behavior of isothermal necking is discussed and a phenomenologic criterion for necking is presented.     

Design of experiments for thermo-mechanical behavior of polypropylene/high-density polyethylene/nanokaolinite clay composites

This paper deals with the preparation of nanocomposites using polypropylene (PP)/high-density polyethylene (PE) blend and low-cost nanokaolinite clay by melt compounding in a Thermo Haake Rheocord mixer. The optimization of processing parameters and nanoclay content is done using Box–Behnken design of response surface methodology. Mechanical properties are modeled in terms of processing parameters and nanoclay content and results are verified using statistical analysis. Most reports suggest that kaolinite clay is difficult to disperse in polymer matrix compared to costly montmorillonite clay. This difficulty is overcome by surface modification of nanokaolinite clay by an organic group and the effect of modification is studied using melt flow index, thermal stability and dynamic mechanical behavior. Morphological characterization is done by scanning electron microscopy and X-ray diffraction. Study shows that cheap and abundantly occurring nanokaolinite clay is an efficient reinforcing agent for PP/PE blend. Design of experiments can be effectively used to model such a system, which is influenced by a number of variables. It is also observed that surface modification of the nanoclay with an organic group leads to remarkable improvement in the thermal and mechanical properties of the blend.     

Melt rheology of high-density polyethylene

The melt rheology of high density polyethylene was investigated. Linear viscoelasticity, capillary flow properties, and molecular weight parameter were measured with a plate relaxometer, capillary rheometer, and gel permeation chromatography, respectively. Intimate correlations among the slope of relaxation modulus curve, non-Newtonian flow behavior, Barus effect, and molecular weight parameter, M_z(M_z+1)/M_w, respectively, were found.     

Suspension bromination of high-density polyethylene

High-density polyethylene powder has been brominated in suspension. The product was characterized by various physicochemical methods to determine the nature and effect of the substitution. Analysis of the infrared spectra indicates an initial addition reaction to terminal vinyl double bonds, followed by a substitution reaction on the polymer chain. The kinetic and thermal data show that the pseudo-first-order reaction occurs in the amorphous regions only. The brominated sites function as chain defects to decrease the crystallinity of the melt-recrystallized polymer, prevent annealing, and cause intensity changes in the mechanical α relaxation. Limits are observed in these effects, however, which confirms the expected “blocky” rather than random nature of the substitution.     

An investigation of chemical crosslinking effect on properties of high-density polyethylene

High-density polyethylene (HDPE) was chemically crosslinked with various amounts of di-tert butyl cumyl peroxide (BCUP). Crosslink density determined by rubber elasticity theory using hot set test showed an increase with increasing BCUP. Glass transition temperature (T_g), thermal stability, crystallization, melting behavior and tensile properties were studied. The results showed a new finding about decrease in T_g as a consequence of the ‘chemical crosslinking’ of HDPE. This was explained by observed reduction in crystallinity and expected increase in free volume as a result of restriction in chain packing. However, chemical crosslinking had no significant effect on the thermal stability. The stress at break, Young's modulus yield strength and elongation at break generally decreased with increase in BCUP. By increasing the temperature for slightly crosslinked HDPE, the elongation at break was increased but by increasing the crosslinking level an opposite effect was observed. Crosslinked HDPE showed an decrease in creep strain and an increase in creep modulus with increasing BCUP.     

Physical properties of extended-chain high-density polyethylene

Extended-chain high-density polyethylene prepared through crystallization at high pressure is substantially stiffer and somewhat stronger than normal folded-chain HDPE. With weight-average molecular weight in the range normal for molding or extrusion resins, the extended-chain material is inductile and brittle; but with molecular weight near 2,000,000, the resin can be rigid and tough. This rigid, tough material can be converted to articles through some of the solid-state processes developed for metals. The volume–temperature behavior of HDPE at 5000 atmospheres appears to reflect a polymorphic transition between orthorhombic and triclinic phases.     

Energy criterion for modelling creep rupture of high-density polyethylene

A technique involving the use of a three-element mechanical model with a critical stored energy criterion modelled accurately the creep rupture time of two types of high-density polyethylene (HDPE) specimens. The upper stress limit where the specimen ruptured immediately on application of load and the lower stress limit where the specimen sustained the load indefinitely were also features of the model. These two limits were found to depend on the resilience, elastic modulus and anelastic modulus of the HDPE.     

Morphology of cold-drawn high-density polyethylene fibres

Drawn fibres of high-density polyethylene, and their crystalline residues obtained by nitric acid digestion of the amorphous phase, exhibit single- or double-peaked melting endotherms depending on the degree of crystallinity of the fibre, the draw ratio and, in the case of some of the crystalline residues, whether or not they have been melted once or twice. The melting behaviour of the drawn fibres and their crystalline residues has been accounted for in terms of the rupture of a fraction of the original lamellar structure and the growth of a new crystalline structure.     

Crystallization kinetics of high-density polyethylene/linear low-density polyethylene blend

This article presents crystallization kinetics studies on a cocrystallizing polymer highdensity polyethylene (HDPE)/linear lowd-ensity polyethylene (LLDPE) blend. The nonisothermal crystallization exotherms obtained by differential scanning calorimetry (DSC) were analyzed to investigate the effect of cocrystallization on kinetics parameters, namely the Avrami exponent and activation energy. The regular change of Avrami exponent with blend composition from a value of about 3 corresponding to HDPE to a value of 2 corresponding to LLDPE is observed. A sheaf-like crystalline growth with variation of nucleation depending on blend composition is concluded from these results of DSC exotherm analysis in conjunction with the small-angle light scattering observations. The observed variation of activation energy of crystallization with blend composition suggests the role of interaction of side chains and comonomer units present in the LLDPE. © 1994 John Wiley & Sons, Inc.     

The effect of bromination and inherent vinyl unsaturation on the drawing behavior of high-density polyethylene

Various high-density polyethylenes were brominated in order to eliminate the inherent vinyl Unsaturation in the polymer. They were then drawn uniaxially at three different temperatures, and the resulting fibrous materials were studied under a scanning electron microscope. It was found that the brominated samples yielded transparent fibrous materials that had more regular fibrillar structure and fewer surface overgrowths, cracks, and voids compared with the unbrominated polymers drawn under identical conditions. Bromination also appeared to improve the mechanical properties of the drawn and the undrawn polymer. High-density polyethylene that had no inherent vinyl Unsaturation also yielded transparent fibrous material with fibrillar structure similar to that of brominated polymers.     

An anomaly in the necking behavior of polyethylene

Tensile creep measurements at constant load on nonoriented polyethylene have shown a marked transition at a certain stress level from a neck formation followed by instantaneous fracture to the formation of a neck which resists fracture for a considerable time. The transition, which shifts towards shorter time and higher nominal stress with increasing molecular weight, has been studied for 16 polyethylenes of different molecular weights, degrees of branching and crystalline structures. The marked. transition has only been observed for high density polyethylene of high molecular weight. Deformation measurements show a more distinct necking for the high density than for the medium density polyethylenes. This is consistent with current molecular deformation theories. A hypothesis for the transition is proposed based on the distinctness of the neck process in the high density polyethylene and the large difference in strength between the spherulitic structure and the fibrillar structure. The dependence of the transition on molecular weight is expected since the number of tic chains incrcrtses with increasing molecular weight.     

Cavitation during tensile deformation of high-density polyethylene

Cavitation process of high-density polyethylene during tensile deformation was studied. It has been shown that the crystallinity and perfection of HDPE crystals govern whether plastic deformation of the polymer is associated with cavitation or deformation occurs without cavitation. The strength of crystals may be controlled by crystallization conditions during preparation of a polymer. If the crystals are thin and the degree of crystallinity is low then the plastic deformation of crystals occurs before reaching the level of stress that initiates cavitation. On the other hand, if the crystals are thick, more perfect, and crystallinity is high, then the cavitation in an amorphous phase is initiated first, and later followed by the deformation of crystals. The cavitation process is usually initiated at the stress level close to the yield point. However, the level of stress necessary for cavitation may be decreased substantially by the orientation of crystalline lamellae, as it was observed in the skin layers of injection-molded material. Voids formed in the skin layer do not influence the yielding process. Typically, cavitation was initiated in volume at the stress of 29-30 MPa, but in the skin of injected samples voids were observed even at the macroscopic stresses of 2 MPa only. The development of voids with deformation was studied both for skin and volume of injection-molded HDPE sample. The shape of voids is strictly connected with deformation of crystalline phase around them.