High-density Polyethylene Samples Research Articles

Radiation crosslinking of polyethylene with electron beam at different temperatures

AbstractThe effect of temperature over the range −196 to 150°C on the crosslinking of polyethylenes irradiated by electron beam has been investigated on the basis of gel content determination and Fourier transform infra‐red (FTIR) spectroscopy. The crosslinking efficiency increases significantly with increasing irradiation does and at elevated irradiation temperature. The crosslinking rates of high density polyethylene (HDPE) and low density polyethylene (LDPE) samples above the melting point (TM) are much higher than those below Tm. The FTIR data give positive evidence: (i) that trans‐vinylene double bonds in cross linked HDPE and LDPE samples increase with increasing irradiation dose temperature (ii) that vinyl double bonds in HDPE decrease rapidly with increasing irradiation dose and temperature, and (iii) vinylidene groups in LDPE decrease slowly with increasing temperature at the lower dose and are almost independent of the irradiation temperature at above room temperature and the higher dose of more than 100 kGy. Gas bubbles are observed in LDPE samples irradiated at 100 and 150°C with high dose (200 to 250 kGy). The size of the bubbles increases gradually at high temperatures.

The morphology, chain structure and fracture behaviour of high-density polyethylene

High-density polyethylene (HDPE) is being used more and more in critical long-term applications. For this reason it is important to have a strong understanding of those parameters which control the fracture behaviour of HDPE. In Part I of this work, fracture results were presented for eleven HDPE samples tested using a tensile testing machine. Such short-term tests do not accurately reflect the in-service loads on HDPE components, which tend to be low and static. It is, therefore, important to perform fracture tests under long-term static loads. The results of such testing are presented in this paper. The resistance to static fatigue was found to be most strongly dependent on molecular weight. Short branch concentration and short branch length were also found to exert an influence on the resistance to static fatigue. This result is similar to the findings presented in Part I of this work. However, there is some evidence that molecular weight influences fracture behaviour to a greater extent in the long-term tests. Notwithstanding, the similarity between the short-term and long-term results is important. It means that an early indication of the long-term performance of HDPE resins can be obtained from rapid comparative tests conducted using a tensile testing machine.

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.

Dose effect in gamma-irradiated polyethylene studied by positron annihilation lifetime technique

Commercial high-density polyethylene (HDPE) samples were irradiated by λ-rays with doses ranging from 10 to 200 kGy. Positron annihilation lifetime measurements were performed to study the effect of λ-radiation on the structure of polyethylene. The lifetime π3, associated with the pick-off process, in which the positron in o-Ps annihilates an electron from the surrounding material, and its intensity I3 are measured as a function of λ-dose. It can be concluded that the effect of λ-radiation on polyethylene results in intensive formation of new bonds or crosslinking at doses ranging from 10 to 100 kGy. Saturation of I3 for the irradiated HDPE at doses ranging from 100 to 200 kGy may indicate the achievement of a structural stability.

The reactive modification of polyethylene. II: Mathematical modeling

AbstractIn Part I of this work, experimental data showed that the effect of low concentrations of free radical initiator injected into polyethylene during extrusion depended upon the degree of unsaturation and branching in the feed as well as the molecular weight. This paper shows attempts to quantitatively explain these reactive extrusion results through development of two kinetic models based upon the rate equations for reactions considered dominant. The first model developed incorporated unsaturation via consideration of simultaneous crosslinking‐endlinking reactions. It contains two variable parameters: the overall initiator efficiency and a ratio of two rate constants reflecting the reactivity of the unsaturated bonds. The model was able to fit the changes in molecular weight distribution of both the low density and linear low density polyethylene but not the high density polyethylene samples. In addition to fitting the molecular weight distribution, this model also provided reasonable values of initiator efficiencies for crosslinking, endlinking, and chain extension reactions, as well as the number of terminal vinyls of the products. The second model is a special case of the first: it neglects the presence of unsaturation in the feed. This second model is actually the usual “crosslinking” model widely known from a derivation based upon statistical arguments. It was not able to fit most molecular weight distributions obtained. However, the model was shown to be useful for accounting for observed molecular weight distribution for a high density polyethylene sample of low initial unsaturation. Also, it was able to explain the amount of gel formed as a function of initiator concentration.

The reactive modification of polyethylene. I: The effect of low initiator concentrations on molecular properties

AbstractCommercial samples of high density, linear low density, and low density polyethylene were modified by injection of low concentrations of free radical initiator during extrusion. Molecular properties monitored included molecular weight distribution, degree of unsaturation, and branching. When the polyethylene teed to this reactive extrusion process had similar values of Mw, but varying polydispersity, degree of branching and degree of unsaturation, the magnitude of the change in molecular weight distribution was found to be in the following order: HDPE 1 > LDPE2 > LLDPE. In general, terminal vinyls enhanced molecular weight increase, and branching promoted degradation. However, for a second high density polyethylene sample with Mw = 154,000 (rather than the previous sample's Mw of 85,600), the change in molecular weight distribution was small and located at the lower molecular weight end. This work provided data for the kinetic model development detailed in Part II.

Characterization of high-molecular-mass polyethylenes by gel permeation chromatography—low-angle laser-light scattering

Gel permeation chromatography—low-angle laser-light scattering (GPC-LALLS) has been applied to the analysis of high-molecular-mass polyolefins. It was found that changing the particle size of the column packing from 10 to approximately 50 μm resulted in an increase in the molecular mass of NBS 1476 of from 100 000 to 195 000. It was also found that changing the flow-rate from 1.0 to 0.1 ml/min resulted in an increase in the molecular mass of the same sample of from 195 000 to 280 000 for the large particle size gel columns. Increases were also seen with commercial low-density samples when changing these same conditions, but the increases were not as large in magnitude. A change in the frit size on the columns did not seem to have an effect on the measured molecular masses of the NBS 1476 samples. Another finding in this study was that slurry high-density polyethylene samples which previously had been very difficult to characterize using GPC-LALLS gave much higher molecular masses which were much more reproducible when characterized using the large particle size columns with low flow-rates.

Dose effect in gamma irradiated polyethylene studied by the positron annihilation lifetime technique

Commercial high density polyethylene samples have been irradiated by gamma rays with doses ranging from 10 to 200 KGy. Positron annihilation lifetime measurements have been performed to study the effect of gamma radiation on the structure of polyethylene. The lifetime, τ 3, associated with the pick-off process, in which the positron in o-Ps annihilates with an electron from the surrounding material, and its intensity, I 3, are measured as a function of gamma dose. It can be concluded that the effect of gamma irradiation on polyethylene results in intensive formation of new bonds or cross-linking from 10 to around 100 KGy. Saturation of I 3 from 100 to 200 KGy may indicate the achievement of structural stability.

Correlations between molecular weight, morphology and micromechanical deformation processes of polyethylenes

The influence of molecular weight on morphology and micromechanical deformation processes of polyethylene has been studied by conventional transmission and high-voltage electron microscopy. Fractions with very narrow molecular weight distributions and commercial samples of high-density polyethylene in the range of molecular weight 104 to 1.6·106 have been studied. With increasing molecular weight there is a change in the morphology from sheaf-like structures and banded spherulites, to small bundles of parallel lamellae or randomly distributed lamellae. For molecular weights greater than 105, the thickness of the lamellae increases more slowly than the thickness of the interlamellar, amorphous layers.

Effect of thermal oxidation on the spherulitic morphology of high‐density polyethylene

AbstractThe supermolecular structure of high‐density polyethylene (HDPE) at various stages of oxidation was studied using polarized optical microscopy and small‐angle x‐ray scattering (SAXS). Samples of HDPE crystallized isothermally at 123°C show a pronounced change in their spherulitic structure with progressive thermal oxidation. Polarized optical micrographs and SAXS data indicate that the average lamellar thickness decreases concomitantly with thermal treatment. Solid‐state oxidative scission occurs preferentially at the chain folds where the polymer molecules are strained. This process increases the level of crystallinity of the polymer due to the more efficient packing of crystallites formed by the shorter cleaved chains. The morphological changes are related to the polymer melt flow index, molecular weight distribution, crystallinity and peak melting temperature. A model is proposed to account for the changes in the spherulitic morphology.

Crazing on stretching of samples of high density polyethylene of different molecular mass in liquid media

The effect of MM on the strain of high density polyethylene (HDPE) in liquid media is studied. It is established that the effectiveness of the action of the liquid media, as evaluated from the value of the decrease in yield point of the polymer, is inversely related to the MM of the HDPE. This effect is determined by the decrease in the liability of the crystalline polymer to undergo crazing in liquid media with increase in MM.

Effect of radiochemical ageing on the tensile properties of high density polyethylene

Samples of high density polyethylene of 4 mm thickness were γ-irradiated in air at 0.69, 0.41 and 0.19 Gy sec −1 and in vacuum at 0.69 Gy · sec −1 dose rate. Spectrophotometric and sol-gel measurements showed that, for a given dose, the conversions of oxidative degradation in the skin zone and crosslinking in the core zone were nearly independent of the dose rate which controls essentially the depth of the oxidized zone. Tensile measurements showed that the mechanical properties are practically unaffected in the elastic and upper yield regions. The ultimate (true) stress can be considered practically constant. In constrast, the rate of ultimate elongation change is a decreasing function of the dose rate, i.e. an increasing function of the depth of the oxidized zone. The observed variations can be qualitatively explained by fracture mechanics considerations.

The effect of the gel content of crosslinked polyethylene on its physical properties

The changes in physical properties of crosslinked polyethylene were studied as a function of the gel content. The density, Shore hardness, wear resistance, softening point and linear expansion coefficient were measured for high and low density polyethylene samples, crosslinked with various amounts of dicumyl peroxide. The crosslinking was carried out in a Brabender Plasticorder at 160°C for 30 min. so that the crosslinked material was produced under mixing in a one-step process. The test specimens were prepared using a hydraulic press with heated plates working at various temperatures and pressures. The changes of properties are attributed mainly to structure variations arising from the network formation i.e. the destruction of crystallinity.

Low-temperature oxidation of oriented polyolefins

Oxidation of isotropic and oriented samples of high-density and low-density polyethylene (HDPE, LDPE) and of polypropylene (PP) was studied at 80° and p o 2 = 80 kPa. Orientation drawing prolongs the induction period of oxidation with HDPE, poly-4-methylpentene and PP, but shortens that of LPDE. The observed effects of orientation on the kinetics of polyolefin oxidation are explained by localization of the oxidation process in defect zones of intrafibrillar amorphous domains. Such localization is a consequence of changes in structural characteristics of the polyolefins caused by orientation drawing. In LDPE the oxidation process is accelerated due to the enhanced contribution of tertiary CH groups or side branches to the oxidation process.

The role of thermal expansions in the heat-induced relaxation of uniaxially-strained HDPE

Heat-induced changes in the linear dimensions of oriented high density polyethylene samples were studied. It has been shown that, strained under certain conditions, polyethylene (PE) samples may undergo thermal expansion prior to shrinkage in the initial stages of heating. Subsequent heating results in polymer shrinkage, which goes down to the equilibrium linear dimensions corresponding to those of an isotropic non-strained sample at a given temperature. The stage of shrinkage is followed by an expansion, the sample linear dimensions being governed by the coefficient of linear thermal expansion of the initial isotropic PE.

On Dispersed Crystallization in Polymer Melts as Proceeding from a Quenched Boundary

Abstract The present paper is concerned with a crystallization process which occurs in the interior of a polymer melt, when its boundary (mold wall etc.) is suddenly quenched. As polymer melts are prone to supercooling, dispersed nucleation can take place at a distance from the boundary, if the local temperature has become low enough. This results in (delayed) formation of a crystallization zone moving into the bulk of the material. An approximate methematical treatment of this phenomenon enables an easy (semi-quantitative) interpretation of experimental results which were obtained with the aid of a technique of visual inspection on a high density polyethylene (HDPE) sample.

Slurry abrasion of polymers under simulated submarine conditions

A submarine power cable to link the Hawaiian islands is being considered. It will be longer and go deeper than any previously deployed power cable and presents a significant technology extension. It is anticipated that failure by abrasion due to rubbing against submarine rocks could be a distinct hazard. The present study was initiated to gain relevant data on the behavior of some candidate commercial polymers when abraded by typical submarine rock particles. Samples of high density polyethylene, polypropylene, poly(vinyl chloride), poly(methyl methacrylate), Delrin and polytetrafluoroethylene were abraded by slurries of crushed submarine rocks (pillow lava and coral). Comparison tests were made with slurries of quartz abrasives and with a pin-on-disk apparatus using SiC sandpaper. It was found that the various polymers tested showed the same general ranking independent of the abrasive used, even though the rates of wear varied by several orders of magnitude between the different tests and even though there was a distinct change in wear mechanism from almost pure surface fatigue to almost pure cutting from one end of the spectrum to the other. The results do not directly fit any of the existing models for abrasive wear and the need for a refined model is discussed.

Measurement of the degree of crystallinity of polyethylene wear debris by means of Raman spectroscopy

The Raman spectroscopic method for measuring the degree of crystallinity of polyethylene, developed by Strobl and Hagedorn 1 and by Glotin and Mandelkern, 2 has been applied to the wear debris arising from a continuous dry friction experiment which consisted of friction of high density polyethylene (HDPE) on glass. The analytical method was first tested on two reference HDPE samples having a known degree of crystallinity. The degree of crystallinity of the wear debris is higher than that of the initial non-rubbed material (HDPE, M w = 4·10 6). This result is consistent with the conclusions of a previous tribological and rheological study.

Rate of spin exchange between oxygen and nitroxide radicals in polyolefins measured by continuous saturation EPR spectra

Rate constants of the spin exchange between nitroxide radicals and oxygen in oriented and isotropic samples of polypropylene, high density and low-density polyethylene were measured over the temperature range of 290–373 K by the method of continuous saturation of the EPR spectrum of the spin probe, and used to calculate oxygen permeability. The results agree with literature data for isotropic samples, but are smaller by 1·5 to 2·5 times upon orientation.

Effect of molecular characteristics of polyethylene on film strength in orientation elongation

High density polyethylene (HDPE) samples specially selected with M w = (66–290) × 10 3 and polydispersion (2–50) were used to examine the dependence of film strength on elongation. The rate of strength increase with elongation depends on W w for polydispersed samples. Optimum values of M w and polydispersion were found to ensure a film strength of 3 GPa.