Deformation Of High Density Polyethylene Research Articles

Plastic Deformation of High Density Polyethylene with Extended-Chain Crystal Morphology.

Samples of polyethylene with extended-chain crystal morphology, obtained by crystallization under high pressure, were subjected to uniaxial compression to various strains. Accompanying structural changes were analyzed using scanning electron microscopy. At the true strain of e = 0.2−0.3 the microbuckling instability was observed in longitudinally loaded lamellae, resulting in the formation of angular kinks. This induced a rapid reorientation of the lamellae, facilitating their further deformation by crystallographic slip. Microbuckling instability was found to occur earlier than in samples with folded-chain crystal morphology (e = 0.3−0.4) due to a smaller ratio of the amorphous to crystalline layer thickness. SEM observations demonstrated that the microbuckling instability begins with small undulation in long lamellae. Sharp angular lamellar kinks develop from the initial undulation through intense plastic deformation by crystallographic slip along the chain direction. The same slip system was found to operate throughout the kink, including the tip region as well as both limbs. In contrast to thin folded-chain lamellae that often undergo fragmentation during deformation, the thick extended-chain lamellae deform stably by chain slip and retain their continuity up to high strains, e > 1.6. This stability of deformation is related to the large thickness of extended-chain lamellae.

Simulation of heat transfer in a landfill with layered new and old municipal solid waste

Due to rapid degradation of the newly filled municipal solid waste (MSW), the local temperature of the waste layer increases greatly. The mechanical parameters related to waste degradation and the deformation of high-density polyethylene (HDPE) pipes in the waste body will be affected by the elevated temperature. To predict the temperature distribution in the anaerobic landfill, a one-dimensional heat transfer model is established in this study. This model considers the stratification of the saturated and unsaturated zones, and the layering of new and old waste. Furthermore, a single peak model for heat production is applied as the source term of heat production. The stratification of the unsaturated and saturated zones is considered by distinguishing the difference in heat conductivity and specific heat capacity. The layering of the new and old waste layers is considered by distinguishing the difference in the length of time that waste has been degraded to produce heat. Based on the numerical calculation method, the temperature distribution in a landfill with layered new and old MSW is well simulated. The position where the maximum temperature occurs and the variation in the temperature at the edge of new and old waste are elucidated. The sensitivity analysis shows that the influence of the density on the temperature distribution is more significant. Besides, the stratification of saturated–unsaturated waste should also be considered in landfills.

Temperature monitoring during a water-injection test using a vertical well in a newly filled MSW layer of a landfill.

High temperature may adversely affect municipal solid waste (MSW) biodegradation and lead to an increase in the deformation of high-density polyethylene (HDPE) pipes used for the collection of leachate and landfill gas in landfills. The test in this study was to change the waste temperature around the vertical injection well by water injection using a vertical well. The test was conducted intermittently with two different flowrates in a newly filled MSW layer of a landfill. The temperature, gas pressure and leachate level in the test area were simultaneously monitored during this study. The results showed that the waste temperature around the vertical injection well was effectively changed by water injection, which did not result in a significant rise in the leachate level. During water injection, the waste temperature influence distance in the horizontal direction increased with depth from the leachate level to the bottom of the injection well. The bottom temperature of the injection well decreased to near the water-injection temperature. The range of influence of the waste temperature caused by intermittent water injections slightly increased in this test. After water injection was stopped, the waste temperature near the vertical injection well increased quickly initially, and then the increments became more gradual with time. When the leachate level recovered stably, there was still a temperature gradient around the injection well within the range of influence. The temperature and gas pressure in the waste above the leachate level and far away from the injection well were slightly influenced by water injection.

Deformation of high density polyethylene by dynamic equal-channel-angular pressing.

The effect of high strain rate and large shear deformation on the orientation of crystallites in high density polyethylene (HDPE) was investigated with dynamic equal-channel-angular pressing (D-ECAP). The HDPE samples were processed by two loading routes, route A and route C. Grid lines were used to obtain macroscopic strain distributions, which were substantiated by finite element modeling. Owing to the strain rate effect, the number of D-ECAP processing passes has a minor effect on the shear strain accumulations compared to ECAP. D-ECAP leads to a decrease in the thickness of the crystalline stem, and the crystallinity. After route-A or route-C D-ECAP processing, a new monoclinic phase emerges, and two types of crystallographic c-axis orientations appear: the crystallographic c-axis is approximately parallel to the flow direction (FD), or is tilted at approximately 55° clockwise away from FD. However, only one type of crystallographic c-axis orientation is detected after 2 passes of route-C D-ECAP. It is viable to utilize D-ECAP to control the structure and orientation of crystalline polymers, as a complement to ECAP and other processing techniques.

Large strain, high rate semi‐solid deformation of high density polyethylene at elevated temperatures

High density polyethylene (HDPE) is predominately processed from the molten state. However, there is the potential to enhance the mechanical properties by forming below the melt temperature, in the semi‐solid phase. To further investigate this enhancement, HDPE sheets were deformed under constant width, simultaneous equal‐biaxial (EB), and sequential biaxial deformation. The samples were deformed at strain‐rates, from 4 to 16 s−1, up to nominal strains of 3.0 at temperatures below 130°C. The strain‐rate and high‐strain level applied were chosen so that the data was comparable to the thermoforming process. It was discovered that in order for the HDPE sheet to deform at the high‐rate and high strains the processing temperature must be between 126°C and 130°C. This resulted in a processing window that was particularly small, 5°C, and that within this window a temperature change of 1°C had a significant impact on the stress–strain response. The samples were analyzed post deformation, where the elastic modulus was found to increase by a factor of 2.08 in the machine direction, when deformed via EB at 129°C. The results showed that HDPE can be deformed to strain levels comparable to the thermoforming process. POLYM. ENG. SCI., 58:1516–1522, 2018. © 2017 Society of Plastics Engineers

Analysis of plastic deformation of semi-crystalline polymers during ECAE process using 135° die

In this paper, analysis of plastic deformation of high density polyethylene (HDPE) and polypropylene (PP) during an equal channel angular extrusion (ECAE) process is investigated. The effects of ram speed, number of passes, processing route and temperature are tested experimentally using a 135◦ die. The results show that the pressing force decreases with an increase in the number of passes and reaches a saturation state rapidly for routes A and C compared to routes BA and BC. Furthermore, it is found that the reduced curvature of the extruded samples is obtained by route C, however, the maximum warping is obtained by route A. A slight influence of temperature on the reduction of the warping is observed on the extruded samples. In order to predict the plastic strain inside the extruded samples, an elastic viscoplastic model is identified using compressive tests at different strain rates and coupled with the finite element method (FEM). A good correlation is found between the numerical modeling and experimental findings. FEM results show that the PP samples display a higher level of plastic strain compared to HDPE samples. However, almost the same degree of strain heterogeneity is obtained for both polymers. Finally, in order to reduce the warping and improve the strain homogeneity, a controlled back-pressure with small corner angle is expected to be an adequate solution.

Morphology and properties alterations in cavitating and non-cavitating high density polyethylene

Abstract Cavitation phenomenon has been marginalised in the past, although it alters morphology and properties of semicrystalline polymers during their plastic deformation. The formation of cavitational pores are due to the loss of cohesion by the amorphous phase. The research in this paper has been conducted on cavitating and non-cavitating high density polyethylenes with identical microstructure of the crystalline component, however, with modified amorphous phase. The influence of cavitation on selected properties and mechanisms activated during tensile deformation of high density polyethylene has been systematically studied including the alterations of yield stress and yield strain, the strain onset of activation of crystallographic slips and martensitic transformation, the intensity of the lamellae fragmentation, the degree of molecular orientation and the amount of heat generated during deformation.

Influence of reduced graphene oxide on the rheological response and chain orientation on shear deformation of high density polyethylene

The rheological response of high density polyethylene/reduced graphene oxide nanoplatelets (HDPE/rGON) composites, and the influence of rGON on chain orientation and crystallization behavior after shear flow are investigated. Melt rheology reveals the presence of strong interaction between polymer chains and the filler. Above 4.0 wt % of the filler concentration, the terminal region of frequency sweep shows changes in the linear viscoelastic properties of the composites. In particular, at these high concentrations the cross-over frequency at which the transition from predominantly elastic to viscous behavior occurs significantly shifts to lower values, indicating the formation of a solid-like percolated network. A drop in G′ at high frequency (100 rad/s) is observed in the presence of the filler, and the storage modulus shows minima at filler concentration between 2.0 and 4.0 wt %. The influence of chain–filler interaction on chain orientation, and subsequent crystallization behavior after application of shear is followed by time resolved WAXD/SAXS. The orientation of the crystalline domains was quantified by the Herman's orientation factor that supports the presence of strong chain–filler interaction. The Deborah number of reptation and retraction suggests that during the applied non-linear shear, polymer chains in the composites experience mild stretch that is not significant enough to induce crystallization at the high temperature (136 °C). However, restriction imposed by the filler on the chain mobility is pronounced enough to preserve oriented state that causes anisotropy in crystallization on cooling. The enhanced orientation with increasing filler content is conclusively attributed to the strong chain–filler interaction.

Structure evolution during uniaxial tensile deformation of high density polyethylene before and after irradiation by 1 MeV electrons

AbstractThe structural evolution of high density polyethylene (HDPE) during uniaxial tensile deformation, before and after irradiation by 1 MeV electrons, is in situ studied by synchrotron small angle X‐ray scattering (SAXS) and wide angle X‐ray diffraction (WAXD). Both the pristine and the irradiated HDPE exhibit three regions of deformation behavior. It is shown that the deformation in the first region is in accord with the change in long period of the lamellar structure. In the following two regions, both the strain‐induced melting and strain‐induced crystallization could occur. The tensile stress decreases with strain in the second region due to the dominant melting effect. In the third region, the synergistic effect of the melting and crystallization results in stress leveling off first, and then the tensile stress increases again because the crystallization effect becomes dominant at higher strains. For the irradiated HDPE, the irradiation‐induced crosslinking network slows down the deformation process. Compared to the pristine one, all the tensile stress is rather higher at a given strain for the irradiated HDPE. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40269.

Atomic force microscopic study of the structure of high‐density polyethylene deformed in liquid medium by crazing mechanism

A procedure has been developed for the direct atomic force microscopic (AFM) examination of the native structure of high-density polyethylene (HDPE) deformed in an adsorption-active liquid medium (AALM) by the crazing mechanism. The AFM investigation has been carried out in the presence of a liquid medium under conditions preventing deformed films from shrinkage. Deformation of HDPE in AALM has been shown to proceed through the delocalized crazing mechanism and result in the development of a fibrillar-porous structure. The structural parameters of the crazed polymer have been determined. The obtained AFM images demonstrate a nanosized nonuniformity of the deformation and enable one to observe the structural rearrangements that take place in the deformed polymer after removal of the liquid medium and stress relaxation. A structural similarity has been revealed between HDPE deformed in the AALM and hard elastic polymers.

Free volume change of elongated polyethylene films studied using a positron probe microanalyzer

Free volume change of low density polyethylene (LDPE) and high density polyethylene (HDPE) films upon mechanical deformation was microscopically investigated by positron probe microanalysis (PPMA). The ortho-positronium (o-Ps) lifetimes were gradually shortened by uniaxial deformation, indicative of shrinkage of the free volume. The o-Ps intensity for HDPE increased by deformation, whereas that for LDPE varied little. It suggests that destruction of crystallites plays an important role in the deformation of HDPE. PPMA is demonstrated to be a promising, powerful probe investigating free volume changes, at different local points subjected to different degrees of deformation, in elongated polymers.

The determining role of scratch indenter radius on surface deformation of high density polyethylene and calcium carbonate-reinforced composite

We have used microscopy techniques to examine mechanically induced surface damage during microscratching of high density polyethylene and calcium carbonate-reinforced composite. The radius of stylus has a dominant effect on surface deformation, which can be predicted from the characteristics of surface morphology and the scratch deformation parameters. However, the resistance to scratch deformation is not linearly related to radius but depends on the geometry of the stylus. The surface deformation behavior is discussed in terms of physical (crystallinity, spherulite size) and mechanical properties (elastic modulus, elastic recovery, toughness, and scratch hardness).

Microscopic aspects of surface deformation and fracture of high density polyethylene

Microstructural evolution during plastic deformation of high density polyethylene was studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques as a function of strain and strain rate. At low strain rates of tensile test, wedging was the dominant mode of deformation and produced fibrillated fracture. At higher strain rates, wedging was reduced and crazing was the dominant micromechanism of deformation. The final fracture was a combination of crazing/tearing and fibrillation. The micromechanisms of deformation and fracture are sensitive to strain rate. The domains of micromechanisms of deformation are depicted in terms of strain rate–strain diagrams providing a perspective of the domains of the deformation processes occurring in different strain rate–strain regimes. Atomic force microscopy was applied to study the mechanical response in wedge and craze modes of deformation, and quantify the surface relief. Atomic force microscopy indicated that wedge mode of deformation involved drawing and grouping or close packing of lamellae, while crazing was characterized by stretching, merging and splitting of lamellae, and formation of microvoids between the split lamellae. The surface relief associated with wedge was relatively less in comparison to craze deformed region.

粘塑性構成式による高密度ポリエチレンの単軸繰り返し変形のシミュレーション(OS12d ポリマの変形と破壊に関するモデリングとシミュレーション)

粘塑性構成式による高密度ポリエチレンの単軸繰り返し変形のシミュレーション(OS12d ポリマの変形と破壊に関するモデリングとシミュレーション)

Microstructural aspects of tensile deformation of high density polyethylene

The paper describes tensile flow behaviour and related microstructural aspects of injection moulded polyethylene. The flow stress increased with increase in strain rate indicating that the tensile flow behaviour is sensitive to strain rate. Also, the yield stress exhibited a linear relationship with strain rate and followed the thermal activation concept with the activation volume consistent with the thickness of the lamellar crystallite. The behaviour of mechanically induced surface damage on polyethylene was studied by SEM. At low strain rates, the deformation process was characterised by features that looked like deformation bands. With increase in strain rate and strain, the deformation bands developed into a distinct array of crazes and grew inwards, followed by tearing. Also, with increasing strain rate the crazes multiplied and secondary crazes were generated that were at an angle to the tensile axis. The examination of the morphology of the fracture surface of polyethylene at various strain rates provided an insight into the mode of fracture process. At low strain rates polyethylene exhibited a ductile type of fracture with extreme fibrillation and, at intermediate strain rates, crazing - tearing was the predominant mode of fracture at the edges, while fibrillar failure occurred in the mid-thickness of the fractured surface. But, at higher strain rates the percentage of fibrillation was relatively small in comparison to lower strain rates. The different modes of deformation processes can be represented in the form of mechanical deformation - strain diagrams, which provide a broad perspective of the deformation processes operating in the different regimes.

The adiabatic fracture of thermoplastic fibres

The development of methods for measuring true stresses and strains in thermoplastics and of models for representing the results, makes it possible to predict polymer performance in a number of ways. Recently this method was used to study the stability of the tensile deformation of high-density polyethylene under adiabatic conditions. It was proposed that at high strain rates, thermomechanical softening would render the plastic deformation process unstable, promoting localised deformation and fracture. In this paper, the isothermal extension process measured at different temperatures is assumed to be stopped and then restarted after different draw ratios have been attained, as in the drawing of a fibre. In this way the effect of draw ratio on fibre tensile properties can then be predicted. It is shown that, with fast deformation under adiabatic conditions, the softening effect due to the increase in temperature exceeds the opposing influence of strain hardening so that the nominal stress is predicted to fall continuously with increased strain. This leads to a ductile fracture process, which, in a fibre, can generate mushroom shaped blobs of polymer at the broken ends. This effect has previously been reported by Hearle and co-workers. The applicability of the model to different type of fibre is considered.

Texture evolution in large strain simple shear deformation of high density polyethylene

The deformational behaviour of linear high density polyethylene (HDPE) in simple shear to large strains and the mechanisms involved in this deformation were investigated. The specimens were sheared at a constant rate at room temperature. The structure and morphology of the samples of HDPE deformed to various permanent shear strains up to 6.3 were studied using wide-angle and small-angle X-ray scattering and transmission electron microscopy. The results show that the simple shear deformation of HDPE is controlled by crystallographic slip mechanisms, mostly the (100)[001] chain slip supported by the (100)[010] transverse slip and (010)[001] chain slip. The crystallographic deformation mechanisms were found to be active in the whole range of the strain history. At strains up to 3.0 the deformation by these mechanisms is supported by shear in the amorphous interlamellar layers (interlamellar sliding) which also alters the orientation of the crystalline phase. This sliding appears to be partially reversible on unloading. At moderate strains chain slip becomes unstable, which causes the localization of deformation within numerous fine shear bands. Further deformation concentrated in bands leads to the destruction of the crystalline lamellae and the transformation of the sample morphology into a microfibrillar one at a shear strain near 4.3. Further deformation, mostly by chain slip, in the crystals arranged after their fragmentation into microfibrils results in a sharp, nearly axial texture with the chain axis c tilted a few degrees away from the direction of shear.

The influence of interfacial friction on the deformation of high density polyethylene in a split hopkinson pressure bar

The results of an experimental study of the influence of interface friction on the yield behaviour of a high density polyethylene when it is compressively deformed to a large strain (25%) at high rates of strain ([if5 × 10 3 s −1) in a split Hopkinson pressure bar (SHPB) are described in this paper. The potential sources of error and uncertainty intrinsic in this method are described and evaluated. It is shown that for discs of polymer with diameterto-thickness ratios of about 10, which are required for these experiments, interface friction can produce a very significant increase in the apparent flow stress. Three model lubricants have been used, and it is found that even a modest friction coefficient of 0.05 produces an enhancement in the flow stress of about 25%. A simple rigid-plastic analysis based on the von Mises yield criterion is found to serve as a useful empirical correction. Results from previous studies of polymer deformation in the SHPB configuration have been evaluated using this analysis, and these results confirm the generality and importance of interface frictional constraint in these experiments. The most suitable lubricant for high speed deformation studies is found to be a petroleum gel (10 μm thick) which shear softens and produces a nearly ideal lubricating layer.

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

AbstractThe 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.

Lamellar separation during the deformation of high-density polyethylene

Upon tensile straining at low deformation rates (e=2×10−5 sec−1), spherulitic linear polyethylene behaves reversibly for extensions up to 40%. In stress relaxation experiments on unloaded specimens the stress increases with time. Samples kept under constant strain (e = 40%) over four months show macroscopic cracking. Microstructural investigation was performed using low- and wide-angle X-ray diffraction and transmission electron microscopy. These investigations reveal a very inhomogeneous deformation within those lamellar stacks for which the crystalline lamellae lie normal to the tensile axis. The deformation in that case is similar to what has been observed for “elastic hard fibres”. A two-mechanism model to explain the macroscopic observation on the basis of the microscopic observations is developed.