Melting of drawn polyethylene films

If drawn, crosslinked, high molecular weight polyethylene films are restrained during melting, two endothermic peaks may be noted on a scanning calorimeter. In x-ray diffraction experiments, the appearance of a single new peak accompanies the disappearance of the orthorhombic pattern. The new peak is thought to be due to hexagonal chain packing above the melting temperature. The results are interpreted in terms of a small amount of fibrillar crystals in the drawn films.     

Paper-like filled polyethylene films

Filled high-density-polyethylene (HDPE) films for packaging applications have been developed. These films are of low permeability, are water and grease resistant, and can be heat-sealed like HDPE films but have stiffness, deadfold, cuttability, and appearance similar to those of paper. The films are produced in thicknesses of 10 to 100 + μm by the standard film blowing technique. The effect of the filler type, resin parameters, and film blowing process variables on the resultant filled film properties is discussed. The commercialization of filled films and the market acceptance of the products is also briefly elaborated upon.     

Folded chain model of highly drawn polyethylene

Neither the fringed-micelle, fringed-fibril or extreme folded-chain models explain the morphological and mechanical properties of highly drawn polyethylene. The modified folded-chain model, assuming that a substantial fraction (between 5 and 30 percent) of the molecules do not fold back at the crystal surface but go through the “amorphous” surface layer and enter the next crystal, avoids the insufficiencies of the above mentioned models. The elastic modulus and tensile strength of drawn polyethylene, both increasing with draw ratio, are to a large extent the consequence of the molecules bridging the quasi-amorphous layers and interconnecting the folded-chain lamellae oriented more-or-less perpendicular to the machine direction. The folds create enough space for the accommodation of more-or-less extended tie molecules in the quasi-amorphous layers between the lamellae. Electron microscopy and calorimetry of samples as drawn, annealed and/or etched with fuming nitric acid support the model.     

Characterization of polyethylene terephthalate films drawn in hot water

Sorbed water molecules in PET (around 1% in mass) lead to classic plasticizing effects, basically evidenced by a decrease of the glass transition (T_g) and of the cold crystallisation (T_c) temperatures with increasing water content. During drawing of dry PET film and depending on the draw ratio, the initial amorphous phase is oriented at first, then a strain‐induced crystallisation appears. This work deals with the conjugate effects of drawing and water sorption in PET films drawn in hot water. Differential scanning calorimetry and birefringence measurements shown that drawing performed in hot water leads to modifications of T_g and T_c without modification of the degree of crystallinity. Moreover, the formation of water clusters is observed when the strain‐induced crystalline phase occurs.     

Microstructure of biaxially drawn ultrahigh molecular weight polyethylene

The microstructure of ultrahigh molecular weight polyethylene (UHMW-PE) sheets biaxially drawn in the molten state was investigated by means of wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and electron microscopy. The crystallographic c-axis tended to be oriented in the sheet plane by the biaxial drawing in the molten state. The microstructure of the biaxially drawn UHMW-PE was shown to depend upon molecular weight of UHMW-PE. The biaxially drawn sheet of higher molecular weight (M_v = 2,700,000) showed a fibrous structure, while the lower molecular weight sample (M_v 700,000) had a lamellar structure. The result of DSC measurements suggested that a small number of nucleating extended chain crystals was produced by biaxial melt drawing of the UHMW-PE sheet with higher molecular weight.     

Lamellar deformation and its variation in drawn isolated polyethylene spherulites

Observations of individual lamellae within spherulites of linear polyethylene, drawn under affine conditions between room temperature and ∼100 °C, show lamellae surviving to sample failure, thereby providing a strong memory of the initial morphology in the final product. Lamellae rotate and deform according to the angle their plane makes with the draw direction. Those parallel to the draw direction extend, to the full draw ratio, by shear in the basal plane, probably in {110} planes and at constant lamellar thickness. The same mechanism appears to occur for lamellae at higher angles with chain slip expected increasingly to operate. This latter mechanism is responsible for lamellar thinning, which becomes universal in elongated lamellae at higher draw ratios. Lamellae whose planes are transverse to the draw direction contract, with kinking—also by chain slip—to produce bands of sheared lamellae in spherulite centres. The temperature of drawing has little pronounced effect on the drawn morphology unlike draw rate whose influence is evident. Faster draw produces more severe local damage and less-well-organized co-operative kinking. The amelioration of these effects at a slower rate is attributed to molecular mobility and the influence of the surrounding molecular network.     

High-density polyethylene filled with modified chalk

High-density polyethylene was filled with chalk in various concentrations ranging from 10% to 60% by weight. Ethylene oxide oligomer M_w = 300 was used as a liquid modifier for chalk in the amount 0 ÷ 20 wt % of the filler. The mechanical properties of these composites showed that high density polyethylene filled with chalk have quite high ultimate elongation and impact strength while their elastic modulus and tensile strength are very near to those values for pure high density polyethylene. On the base of mechanical properties and microscopical observations, the crack and microcrack damping is attributed to the presence of ethylene oxide oligomer.     

Recycled carbon fiber filled polyethylene composites

Composites of recycled carbon fiber (CF) with up to 30 wt % loading with polyethylene (PE) were prepared via melt compounding. The morphology of the composites and the degree of dispersion of the CF in the PE matrix was examined using scanning electron microscopy, and revealed the CF to be highly dispersed at all loadings and strong interfacial adhesion to exist between the CF and PE. Raman and FTIR spectroscopy were used to characterize the surface chemistry and potential bonding sites of recycled CF. Both the Young's modulus and ultimate tensile stress increased with increasing CF loading, but the percentage stress at break was unchanged up to 5 wt % loading, then decreased with further successive addition of CF. The effect of CF on the elastic modulus of PE was examined using the Halpin‐Tsai and modified Cox models, the former giving a better fit with the values determined experimentally. The electrical conductivity of the PE matrix was enhanced by about 11 orders of magnitude on addition of recycled CF with a percolation threshold of 7 and 15 wt % for 500‐μm and 3‐mm thick samples. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The thermal stabilization of polyethylene sulfide

It has been shown that the resistance of certain polyethylene sulfides to thermal degradation can be markedly improved by the addition of small amounts of metal dithiocarbamates, thioureas, or diphenyl acetylene together with a suitable carbodiimide. The extent of degradation, as indicated by the concurrent changes in polymer melt viscosity in the temperature range 220°–250°C, was dramatically reduced, in particular, by a stabilizer comprised of 4% hexamethylenebis(tert-butyl)carbodiimide and 2% diphenylacetylene. At 250°C polyethylene sulfide (Dabco initiated) was stable in the melt viscometer for a period of 8–10 min and the resulting extrudate showed little sign of discoloration. The detailed mechanism of stabilizer action cannot be put forward at this stage because of uncertainty regarding the mode of polymer degradation which could occur by radical or ionic processes.     

Surface modification of drawn gel-cast ultra-high molecular weight polyethylene films

Gel-spun ultra-high molecular weight polyethylene (UHMWPE) fibers have superior properties but their use in composite material applications is limited by their poor adhesion to polymer matrices. Previous studies have shown that etching improves the adhesion of epoxy to the fibers, but leads to a reduction in mechanical properties. The purpose of this research was to use uniaxially drawn gel-cast UHMWPE films as a model system since both films and fibers have a highly oriented fibrillar structural hierarchy. Etching has detrimental effects on the mechanical properties and crystallinity of these very thin films. The small amount of carbonyl and carboxyl groups added to the surface through etching raises the film's surface tension and enhances wetting by epoxy. Even though the unmodified film cannot be bonded with epoxy, the interlaminar shear strength between epoxy and the etched films approaches the cohesive strength of the epoxy. A combination of interfacial and UHMWPE cohesive failures is observed. The increase in adhesion is attributed to the slight increase in surface oxygen.     

Scanning force microscopy of nanofibrillar structure of drawn polyethylene tapes

Summary Atomic force microscopy studies of drawn ultra high molecular weight polyethylene tapes were conducted under water, where the operating repulsive forces and the contact area between probe and sample are smaller than in ambient conditions measurements. In this way a higher image resolution allows to identify nanofibrils with widths of 15–25nm, which are formed during stretching. Numerous linear features with separation of 5–8 nm were resolved on the surfaces of nanofibrils in a tape with draw ratio 70. Periodical contrast variations along the stretching direction with a repeat distance of ca. 25 nm-long period-were found on drawn tapes only at stronger operation forces. This finding indicates that these features are related not to the surface topography but to differences in surface hardness. From the molecular scale images it is evident that the harder parts of nanofibrils consist of crystalline domains of extended polymer chains, while no ordered features were found between the elevated image patterns.     

Electrical properties of carbon black filled polyethylene

Resistivity and dielectric constant of polyethylene/carbon black compounds were measured from room temperature to 140°C. Within the polyethylene melting region a PTC/NTC (positive followed by negative temperature coefficients) phenomenon is observed, whose intensity depends on the type of carbon black, its concentration and other parameters. Reproducibility of the PTC phenomenon in polyethylene compounds containing a single type of carbon black is rather low. However, by using mixtures of carbon blacks differing appreciably in their particle size, remarkable reproducibility improvements can be achieved. Several other aspects are also discussed in the present paper covering current-voltage relationships in these materials, comparison of PTC curves with DSC thermograms upon heating and cooling, and dielectric constant-temperature relations. The carbon black concentration giving the optimum PTC intensity can be predicted approximately from room temperature data.     

Electrical properties of exfoliated-graphite filled polyethylene composites

An experimental study has been carried out to prepare and characterize high density polyethylene-based composites filled with exfoliated graphite particles of varied aspect ratios. It has been demonstrated that the dielectric constant of the composite correlates with the in situ volumetric fraction of the compressible exfoliated filler particles. The dielectric constant has been found to range from 2.23 to 125. The minimum electrical resistivity of the composites obtained was 0.1 Ω.cm at a graphite content of 40 percent by volume. The use of these composites is intended in electromagnetic radiation interference (EMI) shielding.     

Structure characterization of cold drawn high density polyethylene thin film

This work throws light to study the changes of optical birefringence for cold drawn high density polyethylene (HDPE) thin film at different stresses. A stress–strain device connected to a scattering optical system was used to investigate the dynamical behavior of opto‐mechanical properties at room temperature (27°C ± 1°C). Some structural parameters, optical and mechanical orientation factors, f(θ), f₂(θ), f₄(θ), f₆(θ), F_av, P₂(θ), P₄(θ), f_c, and f_m, were calculated. Also, the distribution segments at an angle (θ), the number of random links per chain (N₁), the number of chains per unit volume (N_c), and the average work per chain W′ were calculated. The average value of the maximum birefringence was evaluated. The obtained studies demonstrate changes to the molecular orientation functions and evaluated micro structural parameters as a result of the applied cold‐drawing process on (HDPE) thin film. Relationships between the calculated parameters and draw ratios were presented for illustration. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of annealing on transport properties of organic vapours in drawn polyethylene

Drawing of quenched polyethylene at 60°C transforms the spherulitic sample into a material with fibrous structure. As a consequence, the sorption and still more the diffusion of C₂Cl₄ vapours are drastically reduced. Annealing at 120 and 125°C increases the crystallinity and hence reduces the amount of the amorphous component. In spite of that, it enormously increases the transport properties. It restores the sorption per unit volume of amorphous component to a value which, corresponding to completely relaxed amorphous component, is even higher than that of the starting undrawn but quenched material. The diffusion constant after annealing is a little smaller than in the undeformed original sample and shows the same small dependence on concentration in striking contrast with the drawn material. One concludes that the restoration by annealing of the sorption and diffusion coefficients for drawn polyethylene to the values of the undrawn material is a consequence of the complete relaxation of polyethylene chains, in particular of tie molecules, in the amorphous component which completely overcompensates the effects of a reduced amorphous fraction and the increase in crystal perfection.     

填充式高效密封填料在聚乙烯装置的应用

YQSM-01填充式高效密封填料可显著降低电耗、备件损耗、设备损耗,适应性强,在聚乙烯装置应用这种填充式填料,改善了现场环境,减少了事故发生率,同时也降低了检修维护劳动强度,实现绿色环境操作.     

The structure and properties of drawn blends of polyethylene and polypropylene

A series of highly oriented tapes has been prepared from a blend consisting of equal proportions of polyethylene and polypropylene. The mechanical properties and the structure and morphology of the samples have been investigated using DSC, optical microscopy, and wide angle and small angle diffraction, including measurements of crystal strain on samples under stress. It has been confirmed that the blend is incompatible, and a structural model has been proposed which is consistent with the observation that the polyethylene and polypropylene components act essentially independently in their response to external macroscopic stress.     

Electrical properties of carbon black filled crosslinked polyethylene

The electrical resistivity of peroxide and radiation crosslinked polyethylene/carbon black compounds was studied as a function of black concentration and, temperature in heating/cooling cycles. Different carbon blacks in compounds with polyethylene responded differently, regarding electrical resistivity, to the effect of crosslinking. In one case (fine black) the resistivity and PTC (positive temperature coefficient) effect did not significantly change, whereas in another case (coarse black) the effect of crosslinking-was to significantly increase the resistivity and reduce the PTC effect. The main advantages of crosslinking were to give compounds having good electrical reproducibility and to practically eliminate the NTC (negative temperature coefficient) effect in compounds containing fine, coarse, or mixtures of carbon blacks. Crosslinked compounds containing mixtures of carbon blacks have shown good conductivity, electrical reproducibility, and switching properties.     

Low-density polyethylene filled with chalk and liquid modifier

Based on the concept of introducing a liquid layer between the polymer and filler, a composition containing low-density polyethylene, chalk, and oligomer of ethylene oxide was proposed. Compositions containing up to 50% chalk modified with ethylene oxide exhibit typical thermoplastic behavior, i.e., neck formation and plastic deformation. They also show high-impact strength and other good properties. On the basis of mechanical data, the main action of the ethylene oxide oligomer in the system is to inhibit crack generation and propagation.