Dynamic Rheological Behavior of HDPE/UHMWPE Blends

The rheological behaviors of high-density polyethylene (HDPE)/ultra-high molecular weight polyethylene (UHMWPE) blends prepared by melt blending and solution blending were studied. The results showed that the rheological parameters (G′, G ^″, and η*) of both types of blends increased gradually with increasing fraction of UHMWPE, while the tanδ decreased correspondingly. Comparing blends with the same UHMWPE content, all G′, G ^″, and η* values of solution blends were higher, and the tanδ of the solution blends were remarkably lower than those of the melt blends. Combined with the scanning electron microscopy (SEM) observations, it was proved that, because of its very high melt viscosity, the UHMWPE chain is difficult to diffuse and be distributed well in the HDPE matrix by melt blending, resulting in a two-phase-like morphology. On the other hand, the blends prepared by the solution blending showed a homogeneous distribution of UHMWPE in the HDPE matrix. In addition, the state of aggregation of the UHMWPE in the HDPE matrix can be distinguished well by time–temperature superposition (TTS) curves; i.e., the two-phase-like morphology in the melt blends can be detected by the failure of TTS in the high-frequency range, which cannot be reflected by Cole–Cole plots and Han curves.     

Morphology and mechanical properties of PP/HMWPE blends

Mechanical properties and morphology of blends of polypropylene (PP) with high molecular weight polyethylene (HMWPE) prepared by coprecipitation from xylene solution are investigated. Compared to blends of PP with commercial high-density polyethylene (HDPE), the mechanical properties of the blends of PP/HMWPE are much superior to those of PP/HDPE blends. Not only is the tensile strength stronger, but also the elongation at break is much higher than that of the PP/HDPE blends of the same composition. These differences increase with increasing HMWPE and HDPE content. Scanning electron microscopy of the fracture surface resulting from the tensile tests shows that the compatibility in PP/ HMWPE blends is much better than that in PP/HDPE blends. This is most likely attributable to the enhanced chain entanglement of HMWPE with the PP in the amorphous phase due to the lower crystallinity, owing to the high molecular weight of the HMWPE, and a much more flexible chain. The thermal behavior and spherulite morphology of both blends are also investigated.     

Thermorheology and Crystallization Behaviors of Polyethylenes: Effect of Molecular Attributes

Correlations between polyethylenes of different compositions and branching architectures and the temperature dependence of their viscoelastic behavior as well as the dependence of the nonisothermal crystallization behaviors on the cooling rate were described. To analyze the thermorheological behavior of the various classical polyethylenes, a method proposed by van Gurp and Palmen was utilized and the classical high-pressure low-density polyethylene (LDPE) was found to be thermorheologically complex, while for high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), thermorheological simplicity was observed. The Avrami and Mo methods were applied to describe the nonisothermal crystallization kinetics of the different PEs for various cooling rate. The values of the kinetic parameter F(T), kinetic crystallization rate constant (Z_c), and half-time of crystallization (t_1/2) indicated that long-chain branching (LCB) had the role of being a heterogeneous nucleating agent and accelerated the crystallization of polyethylene. Moreover, an HDPE sample of both high molecular weight (M_w) and molecular weight distribution (MWD) had a different crystallization rate dependence from the other samples at various corresponding cooling rates. The crystallization activation energy for nonisothermal crystallization of different PEs was determined using the Kissinger method and showed that the presence of LCB as well as high M_w can increase the crystallization activation energy of polyethylene.     

Mechanical,Thermal, and Surface Properties of Ultrahigh Molecular Weight Polyethylene/Polypropylene Blends

Various compositions of ultrahigh molecular weight polyethylene/polypropylene (UHMWPE/PP) blends were prepared in decalin, with the rheological, mechanical, thermal, and surface properties of the blends being determined using the solution cast film. Viscosity and mechanical properties of the blends decreased below the additivity value with increasing PP content implying that PP molecules disturb the entanglement of UHMWPE. Contact angle of the blend films with a water drop increased with increasing content of PP. The atomic force microscope (AFM) images showed that the surface of cast UHMWPE was very smooth whereas that of cast PP was very uneven. For blends, the surface became rough and uneven with increasing content of PP. The melting temperature of PP (T _mP) decreased in the blends with increasing UHMWPE content while that of UHMWPE (T _mU) remained almost constant in blends.     

Effect of High-Density Polyethylene Nanocomposite Compatibilizer Type on the Interfacial Adhesion and Mechanical Properties of Polyethylene Nano-Homocomposites

In this paper, the effect of nanocomposite compatibilizer type on the interfacial adhesion and mechanical properties of new class of polyethylene (PE) homocomposites, comprising PE/clay nanocomposites as matrix and ultra high molecular weight polyethylene (UHMWPE) fibers as reinforcement, was investigated. These were manufactured by a combination of powder impregnation and film stacking methods, introduced in previous research. Three types of high-density polyethylene (HDPE) Nanocomposites were prepared based on the various compatibilizers used: (i) nanocomposites containing HDPE-grafted maleic anhydride (HDPE-g-MA) as compatibilizer of clay and HDPE matrix, (ii) linear low-density polyethylene-grafted maleic anhydride (LLDPE-g-MA) used as compatibilizer, and (iii) nanocomposites without any compatibilizer. The effects of the presence and compatibilizer type on the quality of clay dispersion, and also the interface features of HDPE-nanocomposite and UHMWPE fibers were investigated and compared with each other. The results demonstrated that the kind of compatibilizer was an important factor determining the dispersion state of clay platelets, and influenced the UHMWPE fiber–PE matrix interface adhesion and the mechanical properties of the PE nano-homocomposites.     

An In situ Experimental Study on Solidification Kinetics of Polyolefins: Effect of Cooling Conditions

The solidification kinetics of polyolefins (PO) under three cooling conditions were investigated using an in situ measurement of the temperature decay within the PO resins. The phase-change temperature range of high-density polyethylene (HDPE) was located between 110 and 120°C, and those of low-density polyethylene (LDPE) and polypropylene (PP) were 90–110°C and 100–120°C, respectively. The cooling rate of the liquid-state stage is larger than that of the crystallization stage, primarily owing to the release of the latent heat of crystallization as well as the reduced temperature difference between the sample and cooling medium; they jointly slow down the cooling rate to an extent. The time with respect to phase transformation and its lasting period have close relations to the materials' molecular characteristics (e.g., Mw, MWD, LCB, etc.). Three empirical equations were proposed, and found to be applicable for the cooling analysis of the PO molten materials at relatively low cooling rates prior to crystallization.     

Improvement of Mechanical Properties and Ultraviolet Resistance of Polyethylene Pipe Materials Using High Density Polyethylene Matrix Grafted Carbon Black

In recent years, high grade high density polyethylene (HDPE) pipe materials are being more and more widely used for water and gas supply. Carbon black (CB) is usually used as an anti-UV-light reagent for pipe materials. However, homogeneous dispersion of CB in the HDPE matrix and modification of the interface has always been a great challenge. In this work, HDPE matrix grafted CB (HDPE-g-CB) was successfully prepared through HDPE radicals formation by a thermo-mechanical method and subsequent radical capture by the CB surface. The weight percentage of grafted HDPE approached 10 wt% and the modification sharply reduced the surface free energy of the CB. The SEM (scanning electron micrographs) and TEM (transmission electron microscopy) results showed that HDPE-g-CB was uniformly dispersed in the HDPE pipe materials and the domain size of the dispersed phase was remarkably decreased from that in HDPE/CB. Therefore, compared with the HDPE/CB, the mechanical properties and ultraviolet (UV) resistance of HDPE/HDPE-g-CB were significantly improved, positively influencing the expected life span of pipelines.     

Experimental study of transcrystallinity in UHMWPE/LLDPE composites

This study deals with the effect of a transcrystalline LLDPE (linear low-density polyethylene) layer grown on Spectra 1000 UHMWPE (ultrahigh molecular weight polyethylene) fibres. Chemical similarity between the fibre and the surrounding melt does not promote transcrystallinity as no transcrystalline microstructure appears from the surface of as-received Spectra 1000 UHMWPE fibres. However, oxygen plasma treatment of the UHMWPE fibres yields a degree of surface roughness that appears to promote easy nucleation and growth of LLDPE transcrystallinity. The kinetics of transcrystalline growth were investigated quantitatively. The growth rate increased by a factor of about 12 for a 10°C increase in supercooling, and at 105°C the maximum observed thickness of the transcrystalline layer was about one fibre diameter. The induction time was found to decrease as the crystallization isotherm increased. We discuss the possibility of using surface energy parameters to define a better criterion for the nucleation of transcrystallinity from the UHMWPE fibre substrate. Preliminary data were generated for the interfacial mechanical shear strength by means of the microbond test. It is conjectured that the combined effects of a thermal treatment and the presence/absence of a transcrystalline layer might produce significant changes in the interfacial shear strength, as illustrated here by a 43% increase observed with specimens subjected to different thermal treatments.     

Simulation of Gas-Assisted Injection Molding of High-Density Polyethylene: The Role of Rheological Properties and Physical Fields on the Crystalline Morphology

The shear and temperature fields in the filling process during gas penetration of gas-assisted injection molding (GAIM) of two kinds of high-density polyethylene (HDPE) with different molecular weights (M_w) were simulated numerically via Moldflow 6.1. The simulated results indicated that HDPE with higher M_w showed a relatively low shear rate in both the outer and inner zone of GAIM parts as compared to that with lower M_w, while the shear rate of the inner zone of the parts of the two materials were lower than that of the outer zone, which has a lower temperature and shorter time for crystallization. Considering the remarkable difference between their viscosities, a model of the shear thinning and recovering effects on crystallization during the GAIM process is proposed. Through the simulation, along with the analysis of their viscosities and rheological properties, the influence of molecular weight and shear and temperature fields on two special kinds of crystalline morphology (i.e., shish-kebabs and banded spherulites) during GAIM were interpreted.     

Effect of compatibilizers on the crystallization kinetics of cellulose-filled high density polyethylene

High density polyethylene (HDPE) is a ubiquitous material with versatile properties. It is produced and used in greater volume than any other thermoplastic. HDPE is often filled with a variety of materials for various applications. Glass fiber and wood flour are two common fillers for HDPE. This study investigated microcrystalline cellulose (MCC) as a filler in HDPE. The use of compatibilizers, or coupling agents, was investigated as a means of improving the dispersion of the cellulose filler in the HDPE matrix and the mechanical properties of the resulting composites. One compatibilizer was shown to improve the strength of the resulting composite. The stiffness was unaffected, as expected. Thermal properties were measured by means of differential scanning calorimetry. Analysis of the crystallization kinetics indicated that the Avrami coefficient was altered by the filler and was also modified by the presence of the compatibilizer. The presence of cellulose and/or compatibilizer increased the matrix degree of crystallinity. The two compatibilizers studied did not behave similarly and may have different mechanisms of compatibilization.     

Study on the Structure and Properties of UHMWPE/Epoxy Resin Composite Fiber

The preparation, crystallization behavior, and fiber structure and properties of ultrahigh molecular weight polyethylene (UHMWPE) epoxy resin composite fiber were studied by means of differential scanning calorimeter (DSC), X‐ray diffraction (XRD), Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and tensile testing. The morphology showed a different behavior from pure polyethylene (PE) fiber. The fiber mechanical properties, creep behavior, and thermal properties of UHMWPE fiber can be improved by adding epoxy resin. It's believed that the epoxy can serve as a physical cross‐linking agent to limit the motion or migration of PE molecules and consequently improve the fiber creep property. However, when the content of epoxy resin is higher than 5 wt%, all of the behavior and properties deteriorate.     

Co-crystallization of Blends of High-density Polyethylene with Linear Low-density Polyethylene: An Investigation with Successive Self-nucleation and Annealing (SSA) Technique

Two kinds of polyethylenes, high-density polyethylene (HDPE) with few chain branches and short-chain branched linear low-density polyethylene (LLDPE) with a relatively larger average molecular weight, were melt blended together in various mass ratios based on consideration of their practical applications. After identifying the good compatibility of the blends, their crystallization behaviors were studied by the successive self-nucleation and annealing (SSA) technique. The SSA analysis showed that not merely the number of melting fractions in the SSA curves changed with the blend composition, but also the content of the first two melting fractions at high temperature of SSA curves showed a positive deviation and a negative deviation with the blend composition, respectively. These phenomena, as well as the interesting appearance of a stepped increase of the lamellar thickness of each fraction with the highest temperature in each sample, indicated that co-crystallization occurred between HDPE and LLDPE. The results from wide-angle X-ray diffraction (WAXD) supported the conjecture obtained by the SSA analysis.     

Mechanochemical degradation kinetics of high-density polyethylene melt and its mechanism in the presence of ultrasonic irradiation

In this paper, the effect of ultrasonic intensity on the degradation of high-density polyethylene (HDPE) melt, degradation mechanism, ultrasonic degradation kinetics of HDPE melt as well as the development of molecular weight distribution of HDPE melt during ultrasonic degradation were studied. In the initial stage, the ultrasonic degradation of HDPE melt shows a random scission process, and the molecular weight distribution broadens. After that, the ultrasonic degradation of HDPE melt shows a nonrandom scission process, and the molecular weight distribution of HDPE melt narrows with ultrasonic irradiation time. The average molecular weight of HDPE decreases with the increase of ultrasonic intensity and increases and trends forward that of undegraded HDPE with the increase of distance from ultrasonic probe tip, indicating that attenuation of ultrasonic intensity in HDPE melt is very quick. Ultrasonic degradation kinetics of HDPE melt obeys the equation: Mt=M(infinity) + Ae(-kt). The theoretic calculation by this equation accords well with the experimental results. The plausible ultrasonic degradation mechanism of polymer melt based on molecular relaxation was also proposed in this paper.     

Morphology,phase structure,and properties of polyolefin/hydrogenated oligocyclopentadiene blends

The paper discusses the influence of an amorphous oligomer (namely hydrogenated oligocyclopentadiene — HOCP) on the morphology and the phase structure of its blends with several polyolefins as a function of composition and crystallization conditions. In particular the following polyolefins were studied: high-density polyethylene (HDPE), isotactic polypropylene (IPP), poly(l-butene) (PB-1), and poly(4-methyl pentene-1) (P4MP1). The blends under investigation are complex polymer systems. In fact, in dependence on temperature, blend composition, and cooling rate, they assume different morphologies and consequently show different thermal and mechanical behaviors. In the solid state the blends form a generally three-phase system: a crystalline phase of polyolefin and two amorphous phases, one rich in the amorphous polyolefin and the other in HOCP. The crystallization process and the properties are determined by the morphology and the phase structure, as well as by the physical state of the HOCP-rich phase.     

Correlation of Oscillation Cycles and Crystallization in HDPE Blends with Small Amounts of HMWPE Prepared by Dynamic-Packing Injection Molding

The effect of oscillation cycles on crystal morphology was investigated for high-density polyethylene (HDPE) in blends with 4 wt% high molecular weight polyethylene (HMWPE) (labeled B4) in samples prepared through dynamic-packing injection molding (DPIM). With the aid of differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), a weblike shish-kebab morphology that markedly increased stiffness and toughness was found at a specific oscillation cycle. The DSC and SEM results showed that crystal morphology was altered with changes in the oscillation cycle. The SEM and TEM results showed that a much better weblike shish-kebab structure, in which most of the lamellae connect different columns compared with conventional shish-kebabs, was formed in the B4 samples when the oscillation cycle was 10s. These results show that a proper oscillation cycle favors the improvement of crystal structures in HDPE blends induced by a small amount of HMWPE.     

Unexpected shish-kebab structure in a sheared polyethylene melt

Scanning electron micrographs of a solvent-extracted sheared polyethylene (PE) blend revealed, for the first time, an unexpected shish-kebab structure with multiple shish. The blend contained 2 wt % of crystallizing ultrahigh molecular weight polyethylene (UHMWPE) and 98 wt % of noncrystallizing PE matrix. The formation of multiple shish was attributed to the coil-stretch transition occurring in sections of UHMWPE chains. Synchrotron x-ray data provided clear evidence of the hypothesis that multiple shish originate from stretched chain sections and kebabs originate from coiled chain sections, following a diffusion-controlled crystallization process.     

Powder compaction,sintering, and rolling of ultra high molecular weight polyethylene and its composites

The applicability of powder compaction and sintering techniques to the processing of ultra high molecular weight polyethylene (UHMWPE) powder is demonstrated. With proper processing procedure and type of UHMWPE powder, the mechanical properties obtained are nearly equivalent to those obtained by conventional melt processes. The properties were optimized by selection of a sintering temperature just above the melting point and by close control of particle size and distribution. The processability of UHMWPE is dependent on the morphology of the powder. Only those powders with a fibrous morphology provided good mechanical properties after sintering. The mechanical properties of powder compacts can be improved by several techniques. Liquid sintering with added normal molecular weight polyethylene, with close control of particle size and distribution and amount of the second component, yielded improved properties. Composites of UHMWPE, with short glass and graphite fiber reinforcement, processed by powder compaction and sintering resulted in increased modulus. The properties of these composites depended upon the amount of fibers, fiber length, fiber-matrix bonding, and fiber orientation. Rolling the powder-processed UHMWPE oriented the structure and improved the mechanical properties, although it decreased the mechanical properties of the glass and graphite fiber composites because of debonding between fiber and matrix. The properties of carbon black—UHMWPE mixtures were improved by rolling because of a more uniform distribution of carbon black.     

Surface Modification of Ultrahigh-molecular-weight Polyethylene Fibers with Coupling Agent during Extraction Process

Ultrahigh molecular weight polyethylene (UHMWPE) fibers were treated with a coupling agent following the extraction of gel fibers, resulting in modified fibers after subsequent ultra-drawing. The structure and morphology of the modified UHMWPE fibers were characterized and their surface wetting, interfacial adhesion, and mechanical properties were investigated. It was found that the coupling agent was absorbed into the UHMWPE fiber and trapped on the fiber surface. Compared with unmodified UHMWPE fibers, the modified fibers had smaller contact angle, higher crystallinity, and smaller crystal size. The interfacial adhesion and mechanical properties of UHMWPE fibers were significantly improved with increasing coupling agent concentration and gradually reached a plateau value. After treatment with 1.5 wt% solution of a silane coupling agent (γ -aminopropyl triethoxysilane, SCA-KH-550), the interfacial shear strength of the UHMWPE-fiber/epoxy composites was increased by 108% and the tensile strength and modulus of modified UHMWPE fibers were increased by 11% and 37% respectively.     

Thermal and Mechanical Properties of Ultra High Molecular Weight Polyethylene Fiber Reinforced High-Density Polyethylene Homocomposites: Effect of Processing Condition and Nanoclay Addition

A new method to prepare single-polymer high-density (HDPE)-ultra high molecular weight polyethylene (UHMWPE) fiber (PE-PE homocomposites) composed and also PE-PE homocomposites containing HDPE organo montmorillonite clay (OMMT) nanocomposites as a matrix (PE nanohomocomposites) was used. Owing to the major importance of fiber impregnation by the matrix and its effect on the adhesion of matrix/fiber and, consequently, the mechanical properties of the composite, a combination of powder impregnation and film stacking methods, utilizing compression molding, were used for manufacturing the PE-PE homocomposites and PE nanohomocomposites. In addition, PE nanohomocomposites with the matrix containing different amounts of nanoclay were prepared to investigate the effect of the clay on the interfacial and mechanical properties of the PE-PE nanohomocomposites. Several different processing conditions were examined to determine the best conditions for manufacturing of the PE-PE homocomposite and PE nanohomocomposites and it was concluded that 40 bar and 10 min of compression molding resulted in the highest overall mechanical properties. The PE-PE homocomposites and PE-PE nanohomocomposites showed identical trends for the relationship between the effects of processing conditions and mechanical properties. Mechanical results demonstrated that clay platelets could increase the interfacial strength by improving physical entanglements between fiber and matrix through better cocrystallization.     

Tribological characteristics of UHMWPE composite and relationship with its compressive behavior

The triboogical characteristics and the mechanics compress behaviors of pure and composite ultra high molecular weight polyethylene (UHMWPE) were investigated using tribological apparatus and universal materials testing apparatus respectively.Results show that there are direct relationships between the sliding friction, wear characteristics, and compression behaviors of UHMWPE composite. The composite of UHMWPE with added copper particles had great improvement in tribological characteristics and mechanics behaviors. Based on the experimental results, a microstructure model of UHMWPE-copper composite is preliminarily proposed.