Mechanical properties of ultra-oriented polyethylene

Semicrystalline polymers generally exhibit moduli well below their theoretical limit due to chain folding and to lack of crystal alignment. Modulus increases attainable through standard drawing procedures are limited by sample fracture before large draw ratios are reached. Using an Instron capillary rheometer which allowed a draw ratio of > 300, transparent polyethylene strands of unusually high c-axis orientation have been produced by a combination of pressure and shear. The virtually perfect crystalline orientation and evidence for extended chains confirm that a significant improvement in modulus can be realized by this technique. The dynamic tensile storage modulus was measured by Vibron over the temperature range ?160°C to +120°C. Room-temperature moduli were 7 × 10¹¹ dyne/cm², higher than any reported values for drawn polyethylene. Values also remained above 10¹¹ dyne/cm² even at 120°C. The moduli and morphological data have been related by a model consisting of an extended-chain component in paralled with a conventional drawn morphology. Experimental and calculated moduli are compared and related to available theory.     

Local process-dependent structural and mechanical properties of extrusion blow molded high-density polyethylene hollow parts

Although applied for several decades, production of hollow plastic parts by extrusion blow molding (EBM) is still over-dimensioned. To overcome this issue, a thorough investigation of the process-structure-property relationship is required. In this study, the local process-structure-property relationship for high-density polyethylene EBM containers is analyzed with differential scanning calorimetry and dynamic mechanic analysis microindentation. Local process-dependent crystallinity and complex modulus data at various processing conditions are supplemented with wide-angle X-ray diffraction and transmission electron microscopy (TEM). The crystallinities and the complex moduli clearly show lower values close to the mold side than at the inner side and the middle of the cross-section, which reflects the temperature gradient during processing. Additionally, the orientation of the polymer chain (c-axis) reveals a low level of biaxiality with a slight tendency towards transverse direction. The biaxiality increases for low mold temperature and high draw ratio. Finally, biaxiality is confirmed with TEM, which reveals no preferred lamellar orientation.     

Characterization and properties of polyethylene blends I. Linear low-density polyethylene with high-density polyethylene

A blend system of linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with high-density (linear) polyethylene (HDPE) is investigated by differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), Raman longitudinal-acoustic-mode spectroscopy (LAM), and light scattering (LS). For slowly cooled or quenched samples, one single endotherm is evident in the DSC curve which depends on the composition. No separate peaks are observed in the WAXD, SAXS, Raman-LAM, and LS studies on the LLDPE/HDPE blends. This observation along with the fact that no peak broadening is observed suggests that these peaks are associated with the presence of a single component. In no case did we see double peaks or a broadened peak that might be associated with two closely spaced unresolved peaks. This suggests that segregation has not taken place at the structural levels of crystalline, lamellar, and spherulitic textures. A single-step drop in the scattered intensity (I_Hv) as a function of temperature is seen in the LS studies. It is therefore concluded that cocrystallization between the LLDPE and HDPE components occurs. The mechanical and optical α, β, and γ relaxations of these blends are explored by dynamic birefringence. The 50/50 blend displays the intermediate relaxation behavior between those of the components in all α, β, and γ regions. This observation is reminiscent of the characteristic of the typical miscible blends.     

Electret properties of blends of high-density polyethylene and polystyrene

Corona-electrets based on heterogeneous blends of high-density polyethylene and polystyrene were studied. The observed double-maximum composition dependence of the electret properties of polymer blends was explained in terms of the concept of their colloid structure and electrical conductivity.     

Modelling of the degradation of mechanical properties of high-density polyethylene based-packaging exposed to amyl acetate solution

In this paper, we studied the container-content interaction between a high-density polyethylene-based packaging (HDPE) and amyl acetate solution considered as diffusing agent. We were specifically interested in the impact of sorption and diffusion phenomena on the mechanical properties of HDPE bottles under static conditions at different stages of physical aging. Several uniaxial tests were realized on samples cut from bottles and structural tests were performed on bottles. The analysis of such various experimental results highlighted the Fickian nature of the diffusion phenomenon and the decrease of the compression strength of the bottles with the preconditioning time. We have also focused on the modelling of the mechanical behavior of HDPE taking into account the mass transfer induced by the diffusion phenomenon. Elastic-viscoplastic model was proposed and implemented in the finite element code ABAQUS. The parameters of this model were identified from tensile tests by solving an optimization problem. The identified models have been validated by numerical simulation of top-load compression of HDPE bottles. The obtained numerical results are in a good agreement with experimental measurements.     

The effect of electron irradiation on the structure and mechanical properties of highly drawn polyethylene fibers

Two very different high-modulus polyethylene fiber samples, a low molecular weight melt-spun and drawn fiber, and a high molecular weight gel-spun and drawn fiber, have been subjected to electron beam irradiation to various doses in vacuum and in the presence of acetylene. The gel content after irradiation in acetylene was found to be much greater than for an equivalent dose in vacuum. The gel content–dose relationship could not be described by either Charlesby–Pinner analysis or the Inokuti equation. This is attributed to the polydispersity and the complications introduced by the unique morphologies of highly drawn fibers. Following previous studies, the tensile creep behavior was interpreted in terms of a model comprising two thermally activated processes in parallel, a low stress process relating to the amorphous network, and a high stress process relating to the continuous crystal fraction. Analysis of the creep behavior of the melt-spun, low molecular weight fiber irradiated in vacuum revealed crosslinking in the amorphous regions and chain scission in the crystal. Chain scission was found to be much reduced when irradiating in acetylene, for which a mechanism has been proposed. The creep rates and activation volumes of the high molecular weight, gel-spun fiber were found to be significantly lower, probably due to the unique morphology. In this case the dominant effect of irradiation on the mechanical properties can be attributed to chain scission rather than crosslinking.     

Thermal properties and morphology of high-density polyethylene filled with coffee dregs

Composites of high-density polyethylene (HDPE) and coffee dregs (COFD) were elaborated using four different types (integral, extracted, major size, and minor size) of COFD. The aim was to study the effects of particle size and soluble extraction over the properties of the HDPE. Four blends were made at the proportion of 90–10 % polymer-filler. The materials were evaluated through optical and scanning electron microscopy, differential scanning calorimetry and thermogravimetry/derivative thermogravimetry. The results showed that the integral COFD has a performance similar to the minor size one, and superior to the extracted. The composites degraded in two steps. The first one was in a temperature lower than the neat HDPE, but higher than the processing temperature of the polymer. The melting temperature and the degree of crystallinity of the composites resulted similar to the neat HDPE ones. In general, extraction and particle size of the COFD have little influence on the behavior of the HDPE. The results show that COFD can be used as filler in polymeric composites.     

Recycled high-density polyethylene/gypsum composites: evaluation of the microscopic,thermal, flammability,and mechanical properties

Sustainable composites comprising scraps of high-density polyethylene (HDPE) and gypsum waste in proportions HDPE/gypsum 100/0, 50/50, 40/60, and 30/70 wt% were prepared. The morphology of the injected specimens was core-shell. Thermal, flammability, water absorption, and compression resistance were also evaluated. Progressively, the presence of gypsum increased the HDPE crystallinity and T_onset. Concerning the flammability, the composite 30/70 exhibited the burning rate three times lower than HDPE, indicating that the gypsum played a role as a flame retardant. The HDPE acted as waterproofing for gypsum. The compression resistance of the composites was similar to HDPE.     

Morphology and mechanical property of high-density polyethylene parts prepared by gas-assisted injection molding

The crystal morphology, melting behavior, and mechanical properties of high-density polyethylene (HDPE) samples obtained via gas-assisted injection molding (GAIM) under different gas pressures were investigated. Moreover, the non-isothermal crystallization kinetics of HDPE under different cooling rates was also studied. The obtained samples were characterized via differential scanning calorimetry, two-dimensional wide-angle X-ray scattering (2D-WAXS), tensile testing, dynamic mechanical analysis (DMA) and scanning electron microscopy techniques. It was found that the properties were intimately related to each other. Macroscopically, the flow-induced morphology of the various HDPE samples was characterized with a hierarchical crystalline structure, possessing oriented lamellar structure, shish?Ckebab structure, and common spherulites in the skin, sub-skin, and gas channel region, respectively. The 2D-WAXS results demonstrated that the degree of orientation of the high gas pressure sample was larger than that of the low pressure sample at the corresponding layer. The tensile testing results of GAIM parts showed that the mechanical properties of the GAIM parts were improved with an increase of the gas pressure. Furthermore, the DMA was utilized to obtain the dynamic mechanical properties of the GAIM samples, and the results indicated that significant improvement of the orientation was observed with an increase of the gas pressure.     

Computer simulation of the mechanical alpha relaxation in isothermally crystallized high-density polyethylene

The influence of thermal history on the dynamic mechanical behavior in the alpha relaxation region of high-density polyethylene (HDPE) is predicted using computer modeling. The model considers the mechanical alpha relaxation as a succession of cooperative single movements occurring within the defect region of the crystal lattice that finally produce a relative displacement between lamellae. A distribution function of the number of such single steps is postulated considering topological properties of the sample: branch content and molecular weight. To carry out the simulation, the model uses the spectrum of the sample subjected to fast cooling as a reference system. In addition, the crystallinity and the segregated material content for the particular thermal history of the object of study are also required. Loss modulus spectra are generated using balance of energy considerations, and a few examples are presented together with several experimental results. © 1993 John Wiley & Sons, Inc.     

The electrical properties of schungite-containing compositions based on polypropylene and high-density polyethylene

Variations in the dc and ac conductivities of schungite-containing compositions based on polypropylene-high-density polyethylene (PP-PE) blends were studied depending on the composition of the polymeric blend, the volume concentration of the filler, and the order of the introduction of the composition components during the preparation of compositions. It was shown that the conductivities of the compositions could depend on the order of the introduction of the components. The structure of initial and schungite-containing PP-PE blends of different compositions was studied by atomic-force microscopy. It was shown that the structure of the compositions depended on the composition of the initial PP-PE blends and the order of the introduction of the components into schungite-filled PP-PE compositions.     

Mechanical properties and crystallization of high-density polyethylene composites with mesostructured cellular silica foam

In this study, composites of high-density polyethylene (HDPE) with mesostructured cellular foam (MCF) silicas have been prepared by melt mixing and studied for the first time. Two different MCF silica analogues having different pore size were used, i.e., 12 nm (MCF-12) and 50 nm (MCF-50). The MCF content in the mesocomposites was 1, 2.5, 5, and 10 mass%. All HDPE/MCF-50 mesocomposites exhibited improved mechanical properties compared with neat HDPE, indicating that the mesocellular silica foam particles with the large mesopore size can act as efficient reinforcing agents. On the other hand, the MCF-12 silica with the smaller size mesopores induced inferior mechanical properties, mainly due to the poorer dispersion of the silica particles and the formation of large aggregates. The mesocellular silica foam particles also affected the thermal properties and the crystallization characteristics of HDPE. Crystallization of mesocomposites was faster than that of neat HDPE. Crystallization kinetics was analyzed with the Avrami equation for both isothermal and non-isothermal conditions. For isothermal crystallization, the Avrami exponent increased with increasing crystallization temperature from 2 to 3. In non-isothermal crystallization, the values of the Avrami exponent increased from 3 to 6.3 with decreasing cooling rate. Lower activation energy values of non-isothermal crystallization were calculated using the isoconversional method of Friedman, as well as using the Kissinger’s equation. Finally, the nucleation efficiency of the mesocellular silica foam particles was estimated from data associated with non-isothermal crystallization, according to the method of Dobreva.     

Effects of spinning conditions on the mechanical properties of ultrahigh‐molecular‐weight polyethylene fibers

We investigated the tensile strength and modulus of ultrahigh‐strength polyethylene (UHSPE) fibers obtained by using the special two‐step‐drawing process of as‐spun fiber (ASFs) which were prepared by the so‐called gel‐spinning method. We have found that the higher the ASF's spinning speed is, the higher the attainable tensile strength σ_f and modulus E are. For all the fibers drawn from ASFs with various spinning speed except for 120 m/min, we have found a master curve for the inverse of σ_f which is plotted as a function of T^1/4E^?1/2, where T is the linear density of the drawn fibers, in consistent with the Griffith theory: a thicker fiber obtained with a lower spinning speed exhibits lower strength, although all the AFSs possess the same value of E. This also suggests that a thicker fiber contains more defects which would lead to the Griffith‐type crack propagation breakage. Moreover, from morphological observation of ASFs under transmission electron microscopy, the ASF obtained at a relatively low spinning speed possesses a heterogeneous cross‐sectional morphology, whereas that obtained at relatively high spinning speed possesses a relatively homogenous morphology. We propose that this morphological evidence may account for the experimental findings of the behavior of the mechanical properties described above. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2639–2652, 2005     

Dynamic mechanical properties of crystalline,linear polyethylene

Two sets of dynamic mechanical property data and some stress relaxation data for semicrystalline, linear polyethylene are treated by data reduction methods previously described. These data can be represented by a master plot of reduced modulus versus reduced frequency and two sets of temperature-dependent shift factors. The first of these factors reflects the change of viscoelastic relaxation times with temperature. The second represents a separable change of modulus with temperature which applies over the entire time or frequency range of the experiments. This change is larger and in the opposite direction to that found applicable in the behavior of noncrystalline plastics and rubbers. The two sets of dynamic data show the same frequency–temperature dependence which can be represented by an activation energy of 22 kcal./mole. Small differences in the modulus–temperature dependence are attributed to differences in molecular weight or annealing conditions. The stress relaxation data superposes to a curve in good agreement with the dynamic data but with a factor of 20 difference in time scale. This difference is attributed to the finite strains used in the stress relaxation measurements. Such strains might be expected to increase free volume in simple extension deformations and so accelerate the relaxation.     

Relations between thermomechanical properties and chain folding of polyethylene terephthalate fibers

Summary The molecular mechanism of the nonlinear relationship between the transition temperatures (T _g resp.Tinm) and the outer tensile stress is discussed.On the basis of thermomechanical curves (deformation-temperature) taken at different external tensile force on drawn annealed PET fibers and films it has been shown that the transition temperatures depend nonlinearly on the applied tension stress. The maxima observed for bothT _g andT _m confirm the theoretical results ofCiferri andSmith andFrenkel assuming a crystallization of oriented polymer with chains in folded or helical conformation.A reasonable explanation is proposed for the increase ofT _m followed by decrease and again increase with progressively rising of the applied tension stress using the model ofBonart-Hosemann for structure of semicrystalline polymers: at low tension values an orientation of macromolecules in noncrystalline zones takes place followed by defolding of chains from crystallites and finally (at highest tension values) an extreme stretching with additional orientation proceeds. This mechanism is supported by infra-red measurements.The thermomechanical data plotted as external tension divided by the corresp. melting temperature versus deformation confirm the theoretical curve derived byFlory. The experimental curves demonstrate that (1) the crystallization under strain with negative elongation as well as (2) the regeneration of the amorphous phase and its additional stretching are physically realisable situations when the crystallization is accompanied by chain folding or building of helices. It is shown that the thermomechanical method could be used as a simple tool for investigating the chain folding problem.The data reported are an additional proof of the existence of regular folded chains in the crystalline PET too.

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Zusammenfassung Es wird der molekulare Mechanismus des nichtlinearen Zusammenhangs zwischen den Übergangstemperaturen (T_g bzw. T_m) und der äußeren Spannung diskutiert. Auf Grund der thermomechanischen Kurven (Deformation - Temperatur), aufgenommen bei verschiedenen äußeren Spannungen an verstreckten und getemperten PET Fasern, wird festgestellt, daß die Übergangstemperaturen (T_g und T_m) von der äußeren Spannung abhängig sind. Die beobachteten Maxima für die beiden Temperaturen T_g und T_m bestätigen die theoretischen Ergebnisse vonCiferri undSmith undFrenkel, die eine Kristallisation der orientierten Polymeren mit Kettenfaltung oder Spiral-Bildung annehmen. Von dem Bonart-Hosemann-Modell ausgehend wird eine Erklärung für die festgestellte Zunahme von T_m, nach welcher eine Abnahme und wieder neue Zunahme folgt, mit progressiv wachsender äußerer Spannung vorgeschlagen: bei niedrigen Spannungswerten findet eine Orientierung von Makromolekülen in nichtkristallinen Bereichen statt, nach welcher eine Entfaltung von Ketten in den Kristalliten eintritt und zuletzt (bei höchsten Spannungen) eine extreme Verstreckung mit zusätzlicher Orientierung stattfindet. Dieser Mechanismus wird von Infrarotmessungen gestützt. Die dargestellten thermomechanischen Daten in den Koordinaten äußere Spannung/Schmelztemperatur gegen Deformation bestätigen die vonFlory theoretisch berechnete Kurve. Die experimentellen Kurven demonstrieren, daß (1) die Kristallisation mit negativer Deformation sowie (2) die Regenerierung der amorphen Phase und ihre zusätzliche Verstreckung physikalisch realisierbare Situationen sind, wenn die Kristallisation mit Kettenfaltung oder Spiral-Bildung auftritt. Es ist damit gezeigt, daß thermomechanische Messungen ein erfolgreiches Mittel zur Untersuchung des Kettenfaltungsproblems darstellen. Die experimentellen Daten sind ein neuer Beweis für die Existenz von regulärer Kettenfaltung auch im kristallinen PET.

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With 11 figures     

Surface treatment of ultra‐high molecular weight polyethylene fibers using potassium permanganate and mechanical properties of its composites

Potassium permanganate was applied to improve the surface properties of the ultra‐high molecular weight polyethylene (UHMWPE) fibers. The results suggested that the surface oxygen atoms increased dramatically and the O/C ratio increased from 0.030 to 0.563 after treatment. The increased surface roughness and the O‐containing groups on the treated fiber surface decreased the contact angles with water and ethylene glycol. The crystallinity and the crystallite size of the treated fibers increased, and the DSC results indicated that chain scission and the formation of ―C═O chemical defects in the amorphous region were the main mechanisms of the deterioration of the treated UHMEPE fibers. The breaking strength and the elongation at break of the fibers decreased, but the modulus increased after treatment. The treated fibers exhibited better adhesion with epoxy matrix. An improvement of 27.6% from 101.4 to 129.4 MPa in ILSS confirmed the improvement in the interfacial adhesion strength of composites. The impact and bending strength of composites were both improved.     

Effects of different ultrahigh molecular weight polyethylene contents on the formation and evolution of hierarchical crystal structure of high-density polyethylene/ultrahigh molecular weight polyethylene blend fibers

In this paper, the blend fibers of ultrahigh molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) were prepared by solution blending and gel spinning process. The uniformity of the blend fibers has been confirmed by rheological data and thermodynamic unimodal curve. They were further characterized by single fiber strength test, scanning electron microscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, and so forth, to explore the structural evolution mechanism with the change of UHMWPE content. The results showed that when the molar content of UHMWPE was only 2.9 mol%, entanglement appeared in the structure of shish-kebab, and when the proportion reached 20 mol%, an interlocking structure could be observed. With the increase of UHMWPE content, kebab began to be networked, and when the content reached 33 mol%, kebab's orientation reached its peak. After that, the interlocking network structure gradually improved. When the content reached 50 mol%, the shish's orientation reached saturation, and the shish-kebab network became perfect. In addition, with the increase of UHMWPE content, stress-induced recrystallization occurred on the wafer, some kebab would be converted into shish crystals, and when the content exceeded 50 mol%, the microfibers began to merge, and the wafer became denser, but still had entanglements. Our work has proposed a quantitative explanation for the evolution of hierarchical crystal structure of HDPE/UHMWPE blend fibers.     

Influence of extrusion ratio on the thermal properties and morphology of ultradrawn high-density polyethylene

Solid-state extrusion of high-density polyethylene (HDPE) has received considerable attention. It has been shown that extrudate may have high values of optical clarity, tensile modulus (～70 GPa = 7 × 10¹¹ dyn/cm²), and c-axis orientation. The effects of extrusion conditions on the properties of the resultant fibers have, however, not yet been clarified. A systematic study has thus been made here to evaluate extrusion pressure, temperature, and extrusion (draw) ratio, and the molecular weight of extruded HDPE. The effects of extrusion ratio on the degree of crystallinity, melting behavior, crystal orientation, and dimensional change along the extrusion direction are reported.     

Dynamic birefringence studies of high-density polyethylene

A rheo-optical investigation has been carried out on a sample of high-density polyethylene (HDPE) in an attempt to examine the nature of the α-relaxation mechanism. Dynamic mechanical and bi-refringence behavior was measured over the frequency range of 0.008-4.3 Hz and temperature range ?40 to 100°C. The dynamic mechanical and birefringence data were reduced to a reference temperature of 50°C by a combination of horizontal and vertical superposition. The significance of the vertical shift factor has been discussed extensively in previous papers and is not dealt with here. An Arrhenius plot was made of the log of the horizontal shift factor versus reciprocal temperature for the mechanical and optical data. The mechanical data exhibited three distinct regions, the slopes of which led to activation energies of 70, 90, and 150 kJ mol^?1. The temperature at which these dispersions occurred suggested the observation of the β, α₁, and α₂ relaxation processes. The optical data contained two distinct regions from which activation energies of 55 and 95 kJ mol^?1 were obtained. The high-temperature α₂ process was not observed in the Arrhenius plot; however, a maximum in K′ and a change in sign of K″ probably reflects a contribution from the α₂ relaxation mechanism.