Internal stresses and activation volumes from the stress relaxation behavior of polyethylene at low deformations

Internal stress levels and values of the activation volume have been evaluated from the kinetics of stress relaxation in annealed samples of LD and HD polyethylene. The initial deformation of the samples was varied, the maximum values amounting to ca. 1%. The temperature of the experiments was 24°C for LDPE, and 24°, 50°, and 69°C for HDPE. The internal stress level was found to be approximately proportional to the initial deformation and independent of the temperature used. Such internal stresses appear to be introduced upon deformation, since permanent stresses had been removed by careful annealing. The activation volume (v) was found to satisfy the relation vσ^* ≈ 10kT, where σ^* is the effective stress, i.e., the difference between the applied and internal stress, k is Boltzmann's constant, and T is the absolute temperature. This is in good agreement with results reported elsewhere for a wide variety of materials. This relation applies primarily to the exponential flow portion of the relaxation curves, but by a simple transformation the power-law region can also be encompassed.     

Experimental study of elongational flow and failure of polymer melts

A new and simple instrument for measurement of elongational flow response of polymer melts in constant uniaxial extension rate experiments is described. Quantitative stress development data are presented for a series of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polystyrene (PS), polypropylene (PP), and poly(methyl methacrylate) (PMMA) melts. For small elongation rate E, linear viscoelastic behavior was observed; while for large E, LDPE and PS showed exponential stress growth, while HDPE and PP showed only linear stress growth. Stress relaxation experiments were carried out for several of the same melts in the instrument. Elongation to break and mechanisms of filament failure were studied. HDPE and PP have a tendency to neck and exhibit ductile failure, while at high E, LDPE and PS seem to show cohesive fracture. The elongational flow stress response data were compared to predictions of nonlinear viscoelastic fluid theory, specifically the Bogue-White formulation. The qualitative differences in responses of the melts studied were explained in terms of different dependences of the effective relaxation times on deformation rate and, more specifically, on values of the a parameter in the theory.     

Time‐resolved small‐angle X‐ray scattering study of void fraction evolution in high‐density polyethylene during stress unloading and strain recovery

By means of time‐resolved small‐angle X‐ray scattering, we developed an analysis methodology to assess the void volume fraction ?_v in high‐density polyethylene (HDPE) during tensile testing. The specimens were first drawn up to different imposed strains, and subsequently were subjected to stress unloading and strain recovery stages. During the loading stage, ?_v progressively increased with the strain level, starting from a well‐defined onset strain prior to the yield point. In particular, ?_v reached a maximum of 8.75 vol% for a strain of 12.5% in the case of a HDPE grade with a molecular weight of 105 000 g mol^?1. Stress unloading and strain recovery caused a decrease in ?_v attained at the end of the loading stage. For a HDPE grade with a molecular weight of 55 000 g mol^?1, ?_v was more important during the loading stage and the decrease in ?_v was less marked during the stress unloading stage when compared to the HDPE with molecular weight of 105 000 g mol^?1. The residual and reversible components of void volume fraction were revealed. © 2015 Society of Chemical Industry     

Melt spinning flow behaviour of high-density polyethylene blended with low-density polyethylene

ABSTRACT

The melt spinning flow behaviour of a high-density polyethylene (HDPE) blended with a low-density polyethylene (LDPE) was studied using a melt spinning technique in temperature ranging from 160 to 200°C and die extrusion velocity varying from 9 to 36?mm?s^?1. The results showed that the melt apparent extension viscosity of the blends was higher than those of the LDPE and HDPE; the melt apparent extension viscosity decreased with increasing temperature; while the melt apparent extension viscosity increased with increasing extension strain rate when the extension strain rate was lower than 0.2?s^?1, and then decreased; the melt apparent extension viscosity reached up to a maximum value when extension strain rate was about 0.2?s^?1; the relationship between the melt apparent extension viscosity and the LDPE weight fraction did not follow the mixing rule.     

Dynamic mechanical properties of binary blends of different polyethylenes. II. Isothermal treatment

The relaxation processes and thermal properties of a series of blends of a highly linear high-density polyethylene (HDPE) with several branched high-density, linear low-density (LLDPE), and low-density polyethylenes (LDPE) have been measured as a function of crystallization temperature, T_c, and content of branched polyethylene (BPE). The influence of composition on the dynamic mechanical spectrum of the HDPE has been rationalized taking into account the dilution with increasing content of BPE of those crystals formed during the isothermal crystallization. The influence of the type of second constituent (HDPE, LLDPE or LDPE) on the relaxation process of the HDPE has been explained in terms of segregation material data.     

Understanding of the tensile deformation in HDPE/LDPE blends based on their crystal structure and phase morphology

The mechanisms of tensile deformation in high density polyethylene/low density polyethylene (HDPE/LDPE) blends were studied by a video-controlled tensile set-up, combined with dynamic mechanical analysis and small angle X-ray scattering. When quenching from the melt to room temperature, HDPE forms well-organized spherulits with high crystallinity and rigid amorphous layers between lamellae, and LDPE forms irregular aggregates with low crystallinity and mobile amorphous layers between lamellae. A separate lamellar stack-like structure is formed in HDPE/LDPE blends during the quenching. The deformation is affected by both the crystal structure and the phase morphology. Because the semi-crystalline polymers are made up of two interpenetrating networks, one is built up by the entangled fluid part and the other by the crystallites, at low deformations the coupling and coarse slips of the crystalline blocks dominate the mechanical properties, which allows the system to maintain a homogeneous strain distribution in the sample. The assumption of a homogeneous strain distribution can now be further proved by the tensile deformation in HDPE/LDPE blends, which shows two-step processes, with HDPE crystallites being broken down first at imposed strain of 0.4 and then LDPE crystallites being broken later, at an imposed strain of 0.6.     

Silane grafting and moisture crosslinking of polyethylene: The effect of molecular structure

The silane grafting and moisture crosslinking of different grades of polyethylene have been investigated. Three types of polyethylene (HDPE, LLDPE, and LDPE) with different molecular structures and similar melt flow indices were selected. The initiator was dicumyl peroxide (DCP), and the silane was vinyltrimethoxysilane. The grafting reaction was carried out in an internal mixer. The extent of grafting and the degree of crosslinking were determined, and hot‐set tests were carried out to evaluate the crosslink structure of the different polyethylenes. The LLDPE had the highest degree of grafting, while the LDPE had the least. The rate of crosslinking for LDPE was higher than that of HDPE and LLDPE. The gel content of LDPE was higher than that of HDPE and LLDPE. Hot‐set elongation and the number‐average molecular weight between crosslinks (M_c) were lower for LLDPE and LDPE than for HDPE. Increasing the silane/DCP percentage led to peroxide crosslinking, thereby decreasing the M_c and hot‐set elongation. The number‐average molecular weight (M_n), molecular weight distribution, and number of chain branches were the most important parameters affecting the silane grafting and moisture crosslinking. J. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers     

Comparative ozonization of LDPE and HDPE and grafting of some monomers to elaborate new ion exchange membranes

A comparative study of the ozonization of low density polyethylene (LDPE) and high density polyethylene (HDPE) was carried out. A grafting study of acrylic acid (AA), N,N‐dimethylamino‐2‐ethylmethacrylate (MADAME) and vinyl phosphonic acid (VPA) on LDPE and HDPE was performed in mass and solution. The ozonized polyethylene and the grafting polymers were characterized by IR spectroscopy and elementary analysis. Ion exchange membranes were prepared from grafted copolymers and characterized by the exchange capacity and electrical resistance. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4423–4429, 2006     

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

The 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.     

Elongation properties of polyethylene melts

The extensional rheological properties of three grades of polyethylene melts, low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and high density polyethylene (HDPE) were measured using a melt spinning technique under the test conditions with temperature ranging from 150 to 210°C and extrusion rate varying from 11.25 to 22.50 mm s^?1. The results showed that the melt strength decreased with a rise of temperature while increased with an increase of extensional rate. With the rise of extensional strain rate and temperature, the melt extensional viscosity decreased. The extensional stress and viscosity reduced with increasing extrusion velocity when the temperature and extensional rate were constant. Moreover, the melt strength and extensional viscosity of the LDPE resin was the highest and the LLDPE was the lowest under the same experimental conditions. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Dichotomous behavior of polymer melts in isothermal melt spinning

Isothermal melt spinning experiments have been conducted using two polyethylene melts of low density (LDPE) and high density (HDPE) to produce steady state spinline profiles. The data revealed the threadline extensional viscosity exhibiting a contrasting picture : extension thickening behavior for LDPE and extension thinning one for HDPE. A White-Metzner model having a strain rate-dependent relaxation time was then found to be able to simulate this dichotomy in melt spinning fairly well: the fluids whose relaxation times have smaller strain rate-dependence can fit LDPE data with extension thickening extensional viscosity whereas the fluids whose relaxation times have larger strain rate-dependence can fit HDPE data with extension thinning extensional viscosity. This dichotomous nature of viscoelastic fluids is also believed to be able to explain other similar contrasting phenomena exhibited by polymer melts, such as vortex/no vortex in entry flows, cohesive/ductile fracture modes in extension, and more/less stable draw resonance than Newtonian fluids.     

Effect of strain rate on the fracture toughness of some ductile polymers using the essential work of fracture (EWF) approach

The plane strain fracture toughness of two ductile polymers, ultra high molecular weight polyethylene (UHMWPE) and acrylonitrile‐butadiene‐styrene (ABS), was measured by using the essential work of fracture approach. Truly plane strain fracture toughness (w_Ie) was measured for ABS at quasi‐static and impact rates of loading. For UHMWPE, the measured values were only “near” plane strain values (w_Ie*). It was confirmed both w_Ie* and w_Ie were independent of specimen type but dependent on strain rate. For UHMWPE, there was a negative strain rate effect, i.e., w_Ie* decreased with increasing loading rate. At low quasi‐static loading rate (v = 10 mm/min), w_Ie* was constant at 55 kJ/m². It then decreased to 15 KJ/m² when the loading rate was increased to 100 mm/min, and remained at that value even up to impact rate of loading (v = 3.7 m/s). For ABS, a mild positive strain rate effect was observed. w_Ie increased from 13 kJ/m² at v = 10 mm/min to 17 kJ/m² at v = 3.7 m/s.     

Effect of pro‐oxidants on biodegradation of polyethylene (LDPE) by indigenous fungal isolate,Aspergillus oryzae

Proxidant additives represent a promising solution to the problem of the environment contamination with polyethylene film litter. Pro‐oxidants accelerate photo‐ and thermo‐oxidation and consequent polymer chain cleavage rendering the product apparently more susceptible to biodegradation. In the present study, fungal strain, Aspergillus oryzae isolated from HDPE film (buried in soil for 3 months) utilized abiotically treated polyethylene (LDPE) as a sole carbon source and degraded it. Treatment with pro‐oxidant, manganese stearate followed by UV irradiation and incubation with A. oryzae resulted in maximum decrease in percentage of elongation and tensile strength by 62 and 51%, respectively, compared with other pro‐oxidant treated LDPE films which showed 45% (titanium stearate), 40% (iron stearate), and 39% (cobalt stearate) decrease in tensile strength. Fourier transform infrared (FTIR) analysis of proxidant treated LDPE films revealed generation of more number of carbonyl and carboxylic groups (1630–1840 cm⁻¹ and 1220–1340 cm⁻¹) compared with UV treated film. When these films were incubated with A. oryzae for 3 months complete degradation of carbonyl and carboxylic groups was achieved. Scanning electron microscopy of untreated and treated LDPE films also revealed that polymer has undergone degradation after abiotic and biotic treatments. This concludes proxidant treatment before UV irradiation accelerated photo‐oxidation of LDPE, caused functional groups to be generated in the polyethylene film and this resulted in biodegradation due to the consumption of carbonyl and carboxylic groups by A. oryzae which was evident by reduction in carbonyl peaks. Among the pro‐oxidants, manganese stearate treatment caused maximum degradation of polyethylene. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of sparse long‐chain branching on the step‐strain behavior of a series of well‐defined polyethylenes

The effect of sparse long chain branching, LCB, on the shear step‐strain relaxation modulus is analyzed using a series of eight high‐density polyethylene (HDPE) resins. Strains of 1 to 1250% are imposed on materials with LCB content ranging from zero to 3.33 LCB per 10,000 carbon atoms. All materials are observed to obey time–strain separation beyond some characteristic time, τ_k. The presence of LCB is observed to increase the value of τ_k relative to the linear resin. The behavior of the relaxation modulus at times shorter than τ_k is investigated by an analysis of the enhancement seen in the linear relaxation modulus, G⁰(t), as a function of strain and LCB content. This enhancement is seen to (1) increase with increasing strain in all resins, (2) be significantly larger in the sparsely branched HDPE resins relative to the linear HDPE resin, and (3) increase in magnitude with increasing LCB content. The shape and smoothness of the damping function is also investigated. The finite rise time to impose the desired strain is compared to the Rouse relaxation time of linear HDPE resins studied. Sparse LCB is found to increase the magnitude of the relaxation modulus at short times relative to the linear resin. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Compressive mechanical properties of sawdust/high density polyethylene composites under various strain rate loadings

This article is concerned with the static and dynamic mechanical properties of high‐density polyethylene (HDPE) reinforced with sawdust (SD) at a strain rate of up to 10³ s^?1. In this study, the static and dynamic properties of HDPE/SD composites with different filler loadings of 5, 10, 15, 20, and 30 wt% SD were deliberated at different levels of strain rates (0.001, 0.01, 0.1, 650, 900, and 1100 s^?1) using a conventional universal testing machine and the split Hopkinson pressure bar apparatus. The results showed that the stress–strain curves, yield behavior, stiffness, and strength properties of the HDPE/SD composites were strongly affected by both the strain rate and the filler loadings. Furthermore, the rate sensitivityof the HDPE/SD composites showed a great dependency on the applied strain rate, increasing as the strain rate increased. However, the thermal activation values showed a contrary trend. Meanwhile, for the postdamage analysis, the results showed that the applied strain rates influenced the deformation behavior of the tested HDPE/SD composites. Moreover, for the fractographic analysis at dynamic loading, the composites showed that all the specimens underwent a severe catastrophic deformation. J. VINYL ADDIT. TECHNOL., 24:162–173, 2018. © 2016 Society of Plastics Engineers     

Behavior of high density polyethylene and its nanocomposites under static and dynamic compression loadings

In this study, the deformation behavior of high density polyethylene (HDPE) and its nanocomposites under uniaxial static and dynamic compression loadings were experimentally investigated. The nanofillers used were carbon nanofibers (CNF) with surface treatments and pristine graphite nanoplatelets (GNP) respectively. The dynamic tests were performed at the strain rates of 1 × 10³, 4 × 10³, and 7 × 10³/s using the split Hopkinson pressure bar and the static tests were done at the strain rate of 1 × 10⁻²/s. In addition, microstructual examinations were performed to gain insights into the observed macroscopic behavior. It was observed that all the materials showed appreciable strain‐rate sensitivity. CNF‐based nanocomposites exhibited higher strength compared to that of the HDPE matrix material. However, the strength enhancement by GNP was very limited. The lower strength in HDPE/GNP relative to that of HDPE/CNF is likely due to the defect formation around the poor interface between polyethylene matrix and GNP reinforcement. Furthermore, the GNP with lamellate structure is also likely to create two‐dimensional interfacial cracks between the two phases, and hence weaken the strength and stability of the composites. It was also observed that for HDPE/CNF composites, different surface treatments did not seem to show significant effects on material strength. POLYM. COMPOS., 2013 © 2013 Society of Plastics Engineers     

Flow stress of high density polyethylene and nylon 66 at high rates of strain

Measurement of the flow stress of high density polyethylene (HDPE) and nylon 66 at strain rates of 10³ s^?1 using a split Hopkinson pressure bar technique is discussed. The flow stress at a strain of 10% has been determined for both polymers at 20°C. The intrinsic errors involved in this technique are briefly reviewed. The results indicate that the flow stress of HDPE and nylon 66 were 50MPa and 150MPa, respectively, at strain rates of about 10³s^?1.     

HDPE liquefaction: Random chain scission model

Liquefaction of commodity polymers to oils and gases can be used to recover the energy value of these materials. This article reports liquefaction data for high-density polyethylene (HDPE), one of the major plastics in recycled material. Thermal degradation of HDPE to oil–gas mixtures required higher temperatures (450–490°C) than low-density polyethylene (LDPE) (430–460°C) because of fewer chain branching points for HDPE, which are more susceptible to chain scission reactions. The addition of hydrogen (0.1–1.5 MPa) had negligible effect on product distribution. HDPE thermal degradation is consistent with a random chain scission mechanism. Product distributions for degradation at 450°C were modeled assuming random chain scission with a rate constant k(x) dependent on the molecular weight x by a power law model dependence, k(x) = k_b x^b, where k_b is the pseudo-first-order rate constant, and b is the power index of dependence on molecular weight. Degradation rates dropped rapidly after initial breakup of the chains, and 2 sets of coefficients were needed to describe the molecular weight distributions as functions of reaction time. The error in model was about 10%. This model can be used to optimize the production of oils from thermal degradation of HDPE. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1239–1251, 1998     

Kinetic approach on stabilization of LDPE in the presence of carnosic acid and related compounds. I. Thermal investigation

Carnosic acid and similar compounds exhibit antioxidant behavior in a polyethylene matrix. Thermal resistance of LDPE was investigated at three temperatures (190, 200, and 210°C) by isothermal chemiluminescence. The main kinetic parameters: oxidation induction period (t_i), half oxidation time (t_1/2), maximum oxidation time (t_max), and propagation rate of oxidation (v_ox^max) were calculated. The inhibition of thermal degradation is proved by the values of these parameters relative to unstabilized polymer: the induction times of stabilized low density polyethylene are of one order of magnitude greater that raw polyethylene, and half oxidation periods are three to five times longer than initial LDPE. Thermal aging of protected low density polyethylene occurs at a much slower rate in comparison with unmodified LDPE. The depletion of stabilizers was also evaluated and the kinetic characteristics (the half‐life and the rate constant of consumption for each antioxidant) at three concentrations of all tested additives (0.125, 0.25, 0.50, and 0.75% w/w) were determined. The effectiveness of stabilization was depicted by two values of activation energies calculated from oxidation induction times and maximum oxidation periods. Some considerations on stabilizing mechanism are presented. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1571–1577, 2005     

Morphology and texture of high-density polyethylene–polystyrene blends deformed by plane-strain compression

The blends of high-density polyethylene (HDPE) with atactic polystyrene (PS) were deformed plastically by plane-strain compression in a channel die. The samples were deformed up to the true strain 1.8 (compression ratio 6) in three temperature regimes: below, near, and above the glass transition temperature of polystyrene component. The morphology and the texture of crystalline component in the deformed blend samples were investigated by means of scanning electron microscopy and wide angle X-ray diffraction (pole figures technique). It was found that the deformation process in the blend of immiscible HDPE and PS does not differ markedly from the deformation of the one-component system from the point of view of the deformation mechanisms involved. The crystalline textures of the blend samples are qualitatively the same as in the plain HDPE deformed under similar conditions. The active deformation mechanisms are the same in deformation of both the plain HDPE and HDPE/PS blend. The mechanism identified are crystallographic slips: (100)[001], (100)[010], and (010[001] supported by the interlamellar slip. The presence of PS in blends modifies to some extent the deformation process and resulting orientation of the crystalline component of HDPE by modification of the stress distribution within HDPE matrix around PS inclusions. This influence is much stronger at low deformation temperatures, when PS is in a glassy state, than at temperatures above T_g of PS. © 1996 John Wiley & Sons, Inc.