Rheo-optical Raman study of microscopic deformation in high-density polyethylene under hot drawing

In situ observation of the microscopic structural changes in high-density polyethylene during hot drawing was performed by incorporating a temperature-controlled tensile machine into a Raman spectroscopy apparatus. It was found that the load sharing and molecular orientation during elongation drastically changed at 50°C. The microscopic stress of the crystalline chains decreased with increasing temperature and diminished around 50°C. Moreover, the orientation of the crystalline chains was greatly promoted above 50°C. These microscopic structural changes were caused by the thermal activation of the molecular motion within lamellar crystalline chains owing to the onset of relaxation of the crystalline phase.     

Thermal and shrinkage behaviour of stretched peroxide-crosslinked high-density polyethylene

The thermal shrinkage of stretched crosslinked high-density polyethylene (HDPE) was investigated with the aim to produce heat shrinkable materials. The heat shrinkable property was achieved by a process of heating-stretching-cooling by aid of tensile machine on crosslinked HDPE obtained by compounding with various amount of peroxide. Effect of stretching ratio and stretching temperature on thermal and shrinkage behaviour at varying peroxide contents was investigated. The results showed that crosslinking hindered the crystallization process by decreasing the melting and crystallization temperatures as well as the total degree of crystallinity. The stretching ratio had no significant effect on shrink temperature but rather on ultimate shrinkage. The stretching temperature had relatively significant influence on the shrink temperature. Crosslinked HDPE stretched at above melting point (140 °C) had higher shrink temperature as compared to those stretched at lower temperature (90 °C). These effects could be reasonably explained by Hoffman theory and changes in crystallites size and total amount of crystallinity.     

Conduction stability of high-density polyethylene/carbon black composites due to electric field action

Conduction stability of high-density polyethylene/carbon black (HDPE/CB) composites with a CB volume fraction slightly above the percolation threshold is studied in relation to electric field action at various ambient temperatures below the melting point of HDPE. It is found that resistance of the composites shows considerable changes after the electric field is switched off. Influence of irradiation crosslinking of HDPE on the conduction stability is also discussed.     

Capillary extrusion of polypropylene/high-density polyethylene immiscible blends as studied by rheo-particle image velocimetry

The capillary extrusion of polypropylene (PP) and high-density polyethylene (HDPE) immiscible blends was studied in this work by rheo-particle image velocimetry (Rheo-PIV). The PP/HDPE blends were prepared by single screw extrusion and extruded through a transparent capillary die at a temperature of 200 °C and concentrations of 80/20, 60/40, 40/60 and 20/80 wt%, respectively. PIV measurements described accurately the flow behavior of PP/HDPE blends and revealed continuous velocity profiles in the die, without macroscopic phase separation, for all the blends in the resolution range of the PIV technique. The flow behavior of all the blends was shear-thinning (power-law) type and their viscosities laid in between the values corresponding to the neat polymers and increased in an exponential way along with the concentration of the highest viscosity component in the blend (HDPE). Also, it was found that the extruded blends acquired a stratified morphology and that HDPE mitigates extrudate distortions in PP, meanwhile PP eliminates slip and flow instabilities in HDPE by migrating to the region of highest shear stresses in the die. Migration of PP to the capillary wall was corroborated by Raman spectroscopy measurements on the periphery of solid extrudates. Finally, via calculations of the density of the molten blends under flow using the velocity profiles in the die, we show that the homopolymers are compatible in the molten state and follow a simple inverse relation for their density, and an exponential one for their viscosity.     

Shape memory and mechanical properties of cross-linked polyethylene/clay nanocomposites

Shape memory polymer (SMP) such as cross-linked low-density polyethylene (XLDPE), can return from its temporary shape to the original (permanent) shape upon heating. SMP in comparison with shape memory alloy (SMA) and shape memory ceramic (SMC) has lower stiffness, so generates lower recovery force when it is being used as an actuator. Also, when SMP is reinforced with traditional micro-fillers, it often loses its shape memory effect due to the high weight fraction of filler (20-30%). To overcome these disadvantages, nanoclays can be used. The smart resultant nanocomposite, even in small clay loading level (0-10 wt.%), shows higher modulus, strength, and the other physical properties such as higher recovery force, required to act as an actuator.In this work, the effect of modified montmorillonite on mechanical and shape memory properties as well as the force generation of a shape memory cross-linked low density polyethylene were investigated.The results show that the modulus of elasticity, the recovery temperature, the recovery force and force recovery rate increase with increasing organoclay in nanocomposites, but final recovery strain decreases slightly.     

Synthesis of exfoliated polystyrene/montmorillonite nanocomposite by emulsion polymerization using a zwitterion as the clay modifier

Montmorillonite (MMT) was modified with zwitterion aminoundecanoic acid (AUA). First AUA was protonized to facilitate molecules to get into the galleries of the montmorillonite to accomplish ion exchange, and the carboxyl groups were then ionized in the alkaline aqueous media to enable exfoliation of the clay. It was demonstrated by rheological measurements and atomic force microscopic studies that exfoliation of the clay driven by the electrostatic repulsion took place in an alkaline medium. Polystyrene/montmorillonite (PS/MMT) nanocomposite was synthesized via emulsion polymerization in the presence of the modified MMT. The exfoliated microstructure of the composites was studied by the X-ray diffraction and transmission electron microscopy. The exfoliated PS/MMT nanocomposite showed a greatly improved modulus, a higher glass transition temperature and a better thermal stability compared to the neat polystyrene and the intercalated PS/MMT composites.     

Deformation mechanism of epoxy/clay nanocomposite

Polymer-layered silicate nanocomposites have been noticed recently due to their outstanding properties. The mechanical properties and deformation mechanism of epoxy/montmorillonite nanocomposites under compressive and flexural loadings were investigated. A reduction in compressive and flexural yield stress and also glass transition temperature with increasing the amount of organoclay was observed. This change in mechanical behavior of epoxy can be explained with observation of plastic deformation mechanism. The study of deformation mechanism revealed that presence of organoclay accelerates shear yielding in epoxy. Microscopic evaluation illustrated that nanoparticles in this system act as initiation sites for shear bands, and thus, enable the epoxy to undergo massive shear yielding.     

Structural evolution of melt-drawn transparent high-density polyethylene during heating and annealing: Synchrotron small-angle X-ray scattering study

The morphological development of melt-drawn transparent high-density polyethylene during heating was investigated employing in-situ synchrotron small-angle X-ray scattering (SAXS) technique. The results confirm that at lower temperatures only meridional scattering peaks aligned perpendicular to the extensional flow direction can be observed, indicating a highly oriented lamellar crystallite structure; whereas at higher temperatures an equatorial streak additional to the layer-like meridional scattering pattern develops, reflecting the presence of shish-kebab-like objects in the specimen under investigation. Upon heating, the average thickness of the kebab crystals remains essentially unaffected below 110 °C, and subsequently the selective melting of the less stable kebabs proceeds yielding thicker layered lamellar crystals. When the temperature is raised to 131 °C, the shish-like formation and the thermally stable kebab crystals melt simultaneously. In addition, the microstructure of the melt-drawn specimen subjected to annealing at elevated temperatures was probed at room temperature. As opposed to the SAXS patterns registered at high temperatures, the SAXS diagram measured after annealing shows no equatorial streak, suggesting that the cylindrical structures could be re-formed. This observation can be explained by assuming that the plate-like kebab crystals with their normal parallel to the stretching direction grow and impinge during cooling to room temperature due to secondary crystallization, which can be verified by in-situ SAXS experiments during annealing and subsequent cooling.     

Detailed mechanistic modeling of high-density polyethylene pyrolysis: Low molecular weight product evolution

A detailed, mechanistic model for high-density polyethylene pyrolysis was created based on the modeling framework developed in our previous work and was used to study the time evolution of low molecular weight products formed. Specifically, the role that unzipping, backbiting, and random scission reaction pathways play in the evolution of low molecular weight species was probed. The model tracked 151 species and included over 11,000 reactions. Rate parameters were adapted from our previous work, literature values, and regression against experimental data. The model results were found to be in excellent agreement with experimental data for the evolution of condensable low molecular weight products. The time evolution curves of specific low molecular weight products indicated that the random scission pathway was important for all species, while the backbiting pathway played a complementary role. Net rate analysis was used to further elucidate the competition between the pathways. Net rate analysis of end-chain radicals showed that the unzipping pathway was not competitive with the other pathways, as expected based on experimental yields of ethylene. The random scission pathway was found to be controlling, with the backbiting pathway playing a more minor role for product formation. By comparing the net rates for formation of specific mid-chain radicals via intramolecular hydrogen shift reactions, the contribution of the backbiting pathway was shown to vary, with radicals formed via the most facile x,x + 4-intramolecular hydrogen transfer reactions being favored.     

Structure development during isothermal crystallisation of high-density polyethylene: Synchrotron small-angle X-ray scattering study

Isothermal melt crystallisation in high-density polyethylene (HDPE) was studied using the time-resolved SAXS method with synchrotron radiation over a wide range of crystallisation temperatures. The SAXS profile was analysed by an interface distribution function, g₁(r), which is a superposition of three contributions associated with the size distributions of crystalline (L_C) and amorphous (L_A) layers and a distribution of long period (LP). The morphological parameters extracted from the g₁(r) functions show that the lamellar thickness increases with time, obeying a logarithmic time dependence. The time evolution of L_C observed for the sample crystallised at 122 °C leads to the conclusion that crystallisation proceeds according to the mechanism of thickening growth. For samples crystallised at lower temperatures (116 °C and 118 °C), the lamellar thickening mechanism has been observed. The rate of lamellar thickening in these cases is much lower than that at 122 °C. At 40 °C, thickening of the crystalline layer does not occur. The interface distribution functions were deconvoluted, and the relative standard deviation σ_C/L_C obtained in this way is an additional parameter that is varied during crystallisation and can be used for analysis of this process. Time-dependent changes in the σ_C/L_C at large supercooling (T_C=40 °C) indicates that L_C presents a broad distribution in which the relative standard deviation increases with time. At lower supercooling (T_C=122 °C), L_C shows a much sharper distribution. In this case, the relative standard deviation decreases with time.     

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.     

Thermal degradation and combustion behavior of the polyethylene/clay nanocomposite prepared by melt intercalation

Studies of thermal and fire-resistant properties of the polyethylene/organically modified montmorillonite (PE/MMT) nanocomposites prepared by means of melt intercalation are discussed. The sets of the data acquired with the aid of non-isothermal TG experiments have been treated by the model kinetic analysis. The extra acceleration of thermal-oxidative degradation of the nanocomposite which has been observed at the first stage of the overall process has been analyzed and is explained by the catalytic effect of the clay nanoparticles. The results of cone calorimetric tests lead to the conclusion that char formation plays a key role in the mechanism of flame retardation for nanocomposites.     

Determination of volatile organic compounds in recycled polyethylene terephthalate and high-density polyethylene by headspace solid phase microextraction gas chromatography mass spectrometry to evaluate the efficiency of recycling processes

A method for the determination of volatile organic compounds (VOCs) in recycled polyethylene terephthalate and high-density polyethylene using headspace sampling by solid-phase microextraction and gas chromatography coupled to mass spectrometry detection is presented. This method was used to evaluate the efficiency of cleaning processes for VOC removal from recycled PET. In addition, the method was also employed to evaluate the level of VOC contamination in multilayer packaging material containing recycled HDPE material. The optimisation of the extraction procedure for volatile compounds was performed and the best extraction conditions were found using a 75 μm carboxen-polydimethylsiloxane (CAR-PDMS) fibre for 20 min at 60 °C. The validation parameters for the established method were linear range, linearity, sensitivity, precision (repeatability), accuracy (recovery) and detection and quantification limits. The results indicated that the method could easily be used in quality control for the production of recycled PET and HDPE.     

Study on solvent permeation resistance properties of nylon6/clay nanocomposite

Nylon6/clay nanocomposite is prepared by mixing organized montmorillonite with nylon6 in HAAKE mixer. Solvent permeation resistance of the nanocomposite is measured to estimate the resistance to solvent permeation. The nanocomposite shows resistance to solvent permeation superior to that of pure nylon6. In addition, the clay content was found to significantly influence the solvent permeation resistance of nylon6, and the maximum improvement in barrier properties of nylon6/clay composite was found as the clay content reached an “optimum” value. By using proper composites and processing conditions, the permeation rate of toluene and ethanol in nylon6/clay nanocomposite is about 3 and 4 times slower than that in pure nylon6 at 50 °C. Our investigation indicated that the crystalline property of nylon6 has a strong impact on the sorption and diffusion of small molecules in the polymer. The improvement in solvent barrier properties of nylon6/clay nanocomposite is attributable to incorporation of an impermeable phase such as the layered silicate, improvement in crystallinity and decrease of crystalline dimension, which are evidenced by XRD, AFM, DSC and polarized optical microscopy (POM) studies.     

Preparation and structure of highly confined intercalated polystyrene/montmorillonite nanocomposite via a two-step method

A two-step approach with a combination of emulsion polymerization and melt intercalation with higher clay loading of 33 wt.% is disclosed to highly confine the polystyrene (PS) chains by montmorillonite. The product of the emulsion polymerization is an easily crushable fine powder. And the powder is readily processible by open mill to form a transparent sheet. In the melt intercalation process, further intercalation of polystyrene narrows the space among the tactoids and results a highly confined intercalated nanocomposite. The results of dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) showed that the cooperative motions of PS segments were substantially depressed, indicative of the highly intercalated structure formed in the nanocomposites. A structural model is proposed to explain the highly confined mesostruture of the PS/MMT nanocomposite.     

Effect of montmorillonite on structure and properties of nanocomposite with PA6/PS/elastomer matrix

Polyamide nanocomposites with fair balance of mechanical properties were recently obtained by addition of finely dispersed clay-compatibilized rubber or rigid PS phase. This work deals with combination of both components, which recently led also to enhanced mechanical behaviour in an analogous reactively compatibilized ternary system.Application of clay to PA6/PS/EPR matrix leads to a decrease in particle size analogously to corresponding binary blends, but the effect of clay on toughness is predominantly contradictory, i.e., a decrease with increasing clay content was found. Also the toughening effect of formed core-shell (elastomer/clay) particles is lower in comparison with binary PA6/EPR. At the same time, in contrast to the PA/PS system, the presence of core-shell particles formed by PS/C15 preblending leads to fair mechanical behaviour including enhanced toughness. This documents a complex affecting of the system behaviour by clay and the expected synergistic cooperation of numerous clay-induced changes in both component parameters and structure. The obtained results indicate that a proper combination of rigid and elastomeric inclusions can lead to nanocomposites with balanced and enhanced mechanical behaviour.     

Photochemical stabilization of linear low-density polyethylene/clay nanocomposites: Towards durable nanocomposites

This article reports a study of the chemical modifications of LLDPE/nanoblend nanocomposites exposed to UV light in conditions of artificially accelerated ageing and natural weathering. Analysis by infrared spectroscopy of the chemical modifications produced by photoageing shows that the presence of an organo-clay leads to the decrease of the oxidation induction time of the polymer (LLDPE), which results in lower durability of the nanocomposites. Protection against photooxidation was tested with different kinds of UV stabilizers and with a metal deactivator. It is shown that the metal deactivator is very efficient in stabilizing the nanocomposite since it totally cancels the prodegradant effect of the organo-clay. This confirms the role played by iron impurities in natural clays. The use of a metal deactivator offers a new insight into the stabilization strategy for nanocomposites.     

The thermal stability of highly filled high-density polyethylene quaternary composites: Interactive effects and improved measures

A range of formulations — consisting of high-density polyethylene, surface-coated calcium carbonate, masterbatched carbon black and a stabiliser package — were compared in terms of their thermal stability through the use of several techniques and measures. Notably, calcium carbonate loadings of up to 60 wt% were investigated. The formulations were designed to be I- and D-optimal, based on a quadratic Scheffé polynomial model. Novel and established measures for the thermal stability of the materials were compared, based on data generated using techniques for the determination of oxidative induction time and for studies by time-sweep oscillatory rheometry. For ease of comparison, all tests were performed in air. Fourier-transform infrared spectroscopy was performed on the materials subjected to time-sweep oscillatory rheometry, to ascertain the locality and modes of degradation. Variable effects were found, depending on the technique used. Broadly, strong primary effects were found: positive in the cases of the carbon black and stabiliser pack, negative in the case of calcium carbonate. The latter, however, was offset by the highly synergistic interaction of calcium carbonate with the stabiliser pack. Time-sweep oscillatory rheometry was shown to be a promising method, with it yielding high-quality, comparable results when interpreted using a novel measure.     

Cure behavior of epoxy resin/montmorillonite/imidazole nanocomposite by dynamic torsional vibration method

Flory’s gelation theory, the non-equilibrium thermodynamic fluctuation theory and the Avrami equation have been used to predict the cure behavior of epoxy resin/organo-montmorillonite (Org-MMT)/imidazole intercalated nanocomposites at various temperatures and Org-MMT loadings. The theoretical prediction is in good agreement with the experimental results obtained by a dynamic torsional vibration method. The results show that the addition of Org-MMT reduces the gelation time t_g and increases the rate of the curing reaction, the value of the kinetic constant k. The half-time t_1/2 of cure after the gel point decreases with increasing of cure temperature, and the value of n is around 3 at lower temperature (<90 °C) and decreases to ∼2 as the temperature increases. The addition of Org-MMT has no obvious effect on the apparent activation energy of the cure reaction. There is no special curing process required for the formation of an epoxy resin/Org-MMT/imidazole intercalated nanocomposite.