Tensile property and interfacial dewetting in the calcite filled HDPE, LDPE, and LLDPE composites

Mechanical properties and complex melt viscosity of unfilled and the calcite (calcium carbonate: CaCO₃) filled high density polyethylene (HDPE), low density polyethylene (LDPE), and linear low density polyethylene (LLDPE) composites using dumbbell bar and film specimens are studied. In addition, the formation of air holes between calcium carbonate and the resin matrix was investigated from the phase morphology and interfacial behavior between the above constituents upon stretching using scanning electron microscopy. The tensile stress and the complex melt viscosity of the calcite filled (50%) polyethylene composites were higher than that of unfilled ones, implying that the reinforcing effect of calcium carbonate. The crack was initiated up to first 50% elongation along the transverse direction and the formation of air holes was originated by dewetting occurring through machine direction in the interface between calcium carbonate surface and HDPE. The propagation mechanism of the air hole formation was proposed to firstly originate by dewetting up to 300% elongation, and enlarged not only by breaking of a superimposed fibril structure, but also by merging effect air holes between fibrous resin matrix. However, the crack propagation was not observed at the very beginning elongation for the calcite filled LDPE and LLDPE systems. Less fibril structure was observed in LLDPE, then LDPE composites. The observed shape and the average size of the air holes were different from system to system. This sort of different interfacial behavior and mechanical properties may arise from different configuration of polyethylene.     

Phase structural analyses of polyethylene extrusion coatings on high‐density papers. II. Influence of paper surface properties on the polyethylene morphology

Paper samples of three different qualities were extrusion coated with low‐density polyethylene (LDPE) and high‐density polyethylene (HDPE). The morphological phases of the polyethylene layers have been quantified by ¹³C solid‐state high‐resolution NMR. Shear forces in the process initiate the formation of the monoclinic crystallites. The surface tensions of the high‐density papers have influence on the degree of interaction between the two materials and how these shear forces work. The paper surface properties will thus have an influence on properties and the size of the monoclinic crystalline mass fraction of the polyethylene coating. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 226–234, 2004     

Experimental study of the rheological,mechanical, and dielectric properties of MgO/LDPE nanocomposites

The properties of the polymer nanocomposites (PNCs), consisting low density polyethylene (LDPE) and magnesium oxide nanoparticles (MgO‐NPs), were systematically discussed in this paper. The shear mixing time and MgO concentration were considered as the two factors affecting the dispersion state, which was found to be effective to change the crystallinity and the mechanical performance of MgO/LDPE PNCs. A reduction in the dynamic shear viscosity was observed when the concentration of MgO‐NPs at a relative low level, which was also dominant by the dispersion states of MgO‐NPs. Evident enhancement of the static yield stress was revealed only by introducing a minute amount of MgO‐NPs (0.25 wt %). Meanwhile, the elastic and loss modulus were also found to be dependent on the dispersion state of MgO‐NPs. A positive increase in dielectric permittivity was identified by uniaxial stretching the MgO/LDPE PNCs strips owing to the orientation enhancement of internal microstructure. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43038.     

Thermal,mechanical, and barrier properties of polyethylene/surlyn/organoclay nanocomposites blown films prepared by different mixing methods

(Low‐density polyethylene) (LDPE)/clay nanocomposites were prepared by melt blending in a twin‐screw extruder by using different mixing methods. Zinc‐neutralized carboxylate ionomer was used as a compatibilizer. Blown films of the nanocomposites were then prepared. The effect of mixing method on the clay dispersion and properties of the nanocomposites was evaluated by wide‐angle X‐ray diffraction analysis, mechanical properties, thermal properties, and barrier properties. The structure and properties of nanocomposites containing different amounts of nanoclay prepared by selected mixing techniques were also investigated. It was found that melt compounding of Surlyn/clay masterbatch with pure LDPE and Surlyn (two‐step‐a method) results in better dispersion and intercalation of the nanofillers than melt mixing of LDPE/Surlyn/clay masterbatch with pure LDPE and surlyn (two‐step‐b method) and direct mixing of LDPE with clay. The films containing ionomer have good barrier properties. A wide‐angle X‐ray diffraction pattern indicates that intercalation of polymer chains into the clay galleries decreases by increasing the clay content. Barrier properties and tensile modulus of the films were improved by increasing the clay content. In addition, tensile strength increased in the machine direction, but it decreased in the transverse direction by increasing the clay content. DSC results showed that increasing the clay content does not show significant change in the melting and crystallization temperatures. The results of thermogravimetric analysis showed that the thermal stability of the nanocomposites decreased by increasing the clay content more than 1 wt%. J. VINYL ADDIT. TECHNOL., 21:60–69, 2015. © 2014 Society of Plastics Engineers     

Method of analyzing and quantifying the performance of mixing sections

In this article, the performance of three different mixing elements on color dispersion in high‐density polyethylene and linear low‐density polyethylene polymer stream during extrusion is studied. Two similarly designed Maddock mixers and a Stratablend II mixer are used as the last part of a general purpose single screw. Moreover, an inline melt camera is used for the quantification of mixing quality by visualization of grayscale of the color dispersion and thus mixing. The Stratablend II mixer produces the lowest and most uniform standard deviation. Both the Maddock mixers showed the same trend but higher values of standard deviation. All results are then compared with a full 3D finite element method simulation. Simulations clearly indicate that the Stratablend II mixer has the best mixing abilities and that these are mainly given by its unique design with high average value of shear stress. The role of elongational stress does not appear to have a high influence on mixing for these mixers. The results also suggest that the key factor for achieving better mixing is the frequency by which a large fraction of the material passes through the high shear stress regions of the mixer. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Effect of Graphene and Carbon Nanotube on Low‐Density Polyethylene Nanocomposites

The nanocomposites of low‐density polyethylene contain graphene (LDPE/Gr) and low‐density polyethylene contains carbon‐nanotubes (LDPE/CNTs) with different Gr loadings (0.5, 1, and 3 wt%) were formulated with a melt‐mixing method. The distribution of Grs in LDPE was detected by scanning electron microscopy. In this study, morphological, electrical, thermal, tensile, and rheological properties of nanocomposites were comparatively investigated. The outcomes were reviewed and it was recognized that LDPE/Gr nanocomposites reveal superior properties than LDPE/CNTs nanocomposites at the same loadings. The superior performance of LDPE/Gr nanocomposites attributes to the large aspect ratio of Gr and its two dimensional flat surfaces which effect in increasing physical interlinking with LDPE chains and expanded the interface zone at filler–LDPE interface. It was also identified that the achieved results for LDPE/CNT nanocomposites, which has a compact surface area and linkage with LDPE, are less noticeable than similar Gr compounds due to higher interfacial interactions between Gr and LDPE. The thermomechanical results of LDPE/Gr nanocomposites have been studied and the influence of nanoscaled strengthening in the thermoplastic matrix has been investigated. The existence of Gr limits the flexibility of LDPE chains, increases the rigidity and the strength of the LDPE‐nanocomposites. This study compares how a flat or roll structure of carbon nano‐structure additive (Grs vs. CNTs) can change the various properties of LDPE nanocomposites. J. VINYL ADDIT. TECHNOL., 25:35–40, 2019. © 2018 Society of Plastics Engineers     

Study of polyethylene solution fractionation and resulting fractional crystallization behavior

Solution fractionation for four different polyethylenes including high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and very low‐density polyethylene (VLDPE) are conducted by stepwise controlling both the temperature and the amount of precipitant. The size exclusion chromatograph (SEC) measurements indicate that solution fractionation technique can successfully separate all the polyethylene samples in accordance with their molecular weight and molecular‐weight distributions. In addition, infrared spectroscopy analysis shows that the degree of short‐chain branching for each fraction of each polyethylene varies with the fraction's molecular weight. The effect of the molecular weight with different short‐chain branching on each fraction's crystallinity represents the characteristics of chain components for different polyethylenes. The crystallinities of HDPE, LLDPE, and LDPE decrease with the increase in their molecular weights; however, for VLDPE, its crystallinity increases with the increase in the molecular weight. The research revealed that the degree of short‐chain branching, together with the molecular weight, can greatly affect the crystallinity of polyethylene. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2542–2549, 2004     

Temporal distributions: The basis for the development of mixing indexes for scale‐up of polymer processing equipment

New and better mixing criteria are needed to assess dispersive and distributive mixing efficiency in polymer processing equipment. Such criteria can serve the purpose of process optimization and machine scale‐up. In this work, the history of flow strength and shear stresses experienced by a number of particles in a twin‐flight, single‐screw extruder serve as the basis to produce temporal distributions of these parameters. In turn, the temporal distributions can be used for developing new mixing indexes for process optimization and scale‐up. Using models for dispersion kinetics and experimental data, calculations of agglomerate size distribution and average agglomerate size can be used as a dispersive mixing criterion.     

Effect of chemical modification of oil fly ash and compatibilization on the rheological and morphological properties of low‐density polyethylene composites

In this study, the effect of oil fly ash (OFA), a by‐product of oil fuel power plants, on the rheological and morphological behavior of low‐density polyethylene (LDPE) is investigated. As received and acid‐functionalized OFA (COOH‐OFA) are used to examine the effect of surface modification of OFA on polymer–filler composites. LDPE/OFA composites were prepared by melt mixing with filler loading in the range 1–10 wt %. The results are compared with pure LDPE. The effect of polyethylene‐grafted‐maleic anhydride (PE‐g‐MA) as a compatibilizer was also studied. Both viscous and elastic properties of composites increased with OFA loading especially at low frequency. The surface modification of OFA has influenced the properties of OFA. As‐received OFA showed some agglomeration at high loading that resulted in two‐phase system as described by scanning electron microscopy (SEM) and Cole–Cole plot. Field emission‐SEM (FE‐SEM) images showed improvement in the dispersion of COOH‐LDPE/OFA composites. In addition, the surface modification reduced the size of agglomeration. In general, the COOH modification of OFA improved both the dispersion and rheological properties of OFA. With chemical modification, the concentration of the filler can be increased to 10% without compromising the properties of the composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Morphology evolution of immiscible LDPE/PVC blends in the presence of compatibilizer and phase dispersant

Phase dispersion and coalescence in low‐density polyethylene (LDPE)/polyvinyl chloride (PVC) (70/30) blends influenced by compatibilizer and phase dispersant was studied. It was found that the morphology evolution of blends is sensitive to not only processing conditions (shear strength and mixing time) but also the added compatibilizer or phase dispersant. In our conditions, the stable phase morphology of each blend is obtained after mixing 15–25 min. In addition, the dispersed PVC phase in blends is easy to aggregate when the mixing rotor speed changed from high to low for the binary blends. As a compatibilizer, chlorided polyethylene (CPE) or nitrile rubber (NBR) can stabilize the morphology and hinder the coalescence of the dispersed PVC phase when added to the blends. However, the phase dispersant butadiene rubber (BR) or styrene butadiene rubber (SBR) could not stabilize the phase structure, although it could accelerate phase dispersion. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 763–772, 2004     

Binary mixtures of polyethylene and oxidized wax: Dependency of thermal and mechanical properties upon mixing procedure

The influence of the preparation procedure on the thermal and mechanical properties of linear low‐density polyethylene (LLDPE)– and LDPE–oxidized wax blends was investigated. It was found that mechanically mixed blends show reduced thermal stability as well as ultimate mechanical properties (stress and strain at break) compared to that of extrusion mixed blends. However, the structure of the blend and consequently its thermal and mechanical behavior also depend on the initial morphology of polyethylene. DSC measurements show miscibility up to high wax contents in both blend types, but increasing the amount of wax in LDPE blends induces increasing crystallinity. As a result, the LDPE/wax blends show improved thermal stability of between 20 and 50°C at low wax concentrations. Although the elasticity modulus of the blends increases, increasing the amount of wax generally degrades the mechanical properties. The main reason for this is the reduced number of tie chains. Changes in the average concentration of tie chains with increasing wax content were calculated and a correlation was made with the ultimate properties of the blends. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2446–2456, 2003     

Phase dispersion–crosslinking synergism in binary blends of poly(vinyl chloride) with low‐density polyethylene: Entrapping phenomenon in PVC/LDPE/DCP blend

The co‐crosslinked products and the entrapping phenomenon that may exist in a poly(vinyl chloride)/low density polyethylene/dicumyl peroxide (PVC/LDPE/DCP) blend were investigated. The results of selective extraction show that unextracted PVC was due to not being co‐crosslinked with LDPE but being entrapped by the networks formed by the LDPE phase. SBR, as a solid‐phase dispersant, can promote the perfection of networks of the LDPE phase when it is added to the PVC/LDPE blends together with DCP, which leads to more PVC unextracted and improvement of the mechanical properties of PVC/LDPE blends. Meanwhile, the improvement of the tensile properties is dependent mainly on the properties of the LDPE networks. Finally, the mechanism of phase dispersion–crosslinking synergism is presented. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1296–1303, 2003     

Mixing in reactive extrusion of low-density polyethylene melts: Linear vs. branched

The melt flows of linear low-density polyethylene (LLDPE) and branched low-density polyethylene (LDPE) have been compared in a fully intermeshing co-rotating twin-screw extruder. The polyethylene melts were selected in order to investigate the effects of the melt rheology on the mixing. Their shear vicosity curves are quite similar, but the LDPE has a markedly higher apparent extensional viscosity over a wide range of stretch rates. The stagger of the paddles in the mixing zone of the extruder creates axial pressure-driven axial flow can have significant extensional strain components. Residence time distributions obtained in the melt zones of the extruder with tracer dye reveal that the LDPE has a narrower residence time distribution than the LLDPE over a wide range of operating conditions. The axial dispersion for the LDPE is significantly lower than the axial dispersion for the LLDPE. This is attributed to the greater extensional viscosity of the LDPE. During the reactive extrusion process, solid maleic anhydride and polyethylene were added at the feed port but the peroxide provides better control of the crosslinking reaction. Residence time distributions measured for the chemically more reactive LLDPE melt indicate reduced levels of axial mixing with reaction. The reduction in mixing is due to a crosslinking reaction that occurs in parallel to the grafting reaction. This change in mixing is smaller than the difference in mixing between LDPE and LLDPE.     

Comonomer effect on the mechanical and morphological behavior on the calcite‐filled PP,CoPP, and TerPP

Comonomer effect on the mechanical and morphological behavior of the calcite (stearic acid coated calcium carbonate)‐filled polypropylene (PP), poly(propylene‐random‐ethylene) copolymer (CoPP), and poly(propylene‐co‐ethylene‐co‐1‐butene) terpolymer (TerPP) composites were investigated by using dumbbell bar and film specimens. The tensile properties of the calcite‐filled PP, CoPP, and TerPP composites exhibited lower values than those of the pure polymers (calcite‐unfilled polymers), whereas the complex viscosity of the calcite‐filled polymers exhibited slightly higher values than that of the pure polymers. Mechanical properties studied by using various strain rates and draw ratios rationalized in terms of comonomer units and contents in various PP systems. Morphological behavior of the specimens stretched at various strain rates and draw ratios was investigated by using SEM microphotographs and the mechanism of the formation of air holes was proposed. The air hole initiated from crack propagation and followed by dewetting between the calcite surface and the polymer interface in the weakened region. The crack propagated along the transverse direction; then the air hole developed parallel to the machine direction with fibril structure of the resin in PP and CoPP systems. However, TerPP composite exhibited no cracks in the beginning of the elongation, but the air hole was initiated due to dewetting; then its enlargement was exhibited by broken fibril structure of the resin. In the final stage of stretching, the air hole was dominated by merging of the neighboring air holes. Thus, different comonomer units, which are the small content of ethylene and 1‐butene in CoPP and TepPP, are responsible for these systems behaving in a different manner on the mechanical and morphological properties. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2041–2053, 2002     

Positron annihilation lifetime studies of gas sorption and desorption in polyethylene and poly[1‐(trimethylsilyl)‐1‐propyne]

The variation of free‐volume parameters (lifetime, intensity, and distribution) after sorption and desorption of CO₂ and CH₄ gases in the glassy polymer poly[1‐(trimethylsilyl)‐1‐propyne] (PTMSP) and in the rubbery high‐density polyethylene (HDPE) and low‐density polyethylene (LDPE) were determined by the PAL technique. Size distributions deduced from PAL measurements reveal the presence of large free‐volume holes in PTMSP with an average size of 0.725 nm³ and intensity of 22% in addition to a free‐volume hole size of 0.197 nm³ with an intensity of 11%. In polyethylene free‐volume hole sizes of 0.107 and 0.153 nm³ with intensities of 21% and 25% could be deduced for HDPE and LDPE, respectively. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 970–974, 2001     

Rheological and mechanical properties in polyethylene blends

Melt rheology and mechanical properties in linear low density polyethylene (LLDPE)/low density polyethylene (LDPE), LLDPE/high density polyethylene (HDPE), and HDPE/LDPE blends were investigated. All three blends were miscible in the melt, but the LLDPE/LDPE and HDPE/LDPE blends exibiled two crystallization and melting temperatures, indicating that those blends phase separated upon cooling from the melt. The melt strength of the blends increased with increasing molecular weight of the LDPE that was used. The mechanical properties of the LLDPE/LDPE blend were higher than claculated from a simple rule of mixtures, whiele those of the LLDPE/HDPE blend conformed to the rule of mixtures, but the properties of HDPE/LDPE were less than the rule of mixtures prediction.     

Air‐hole properties of calcite‐filled polypropylene copolymer films

This work was designed to study the effects of inorganic calcite powder on structurally different copolymer [poly(propylene‐co‐ethylene)] and terpolymer [poly (propylene‐co‐ethylene‐co‐1‐butene)] matrices and the possibility of making a suitable porous composite film. The yield stress of the composites did not improve, but the modulus increased gradually with the filler loading. The theoretical and experimental modulus and yield stress of the composites provided evidence of filler and polymer adhesion behavior. The impact strength showed little enhancement up to a 20 wt % loading for the poly(propylene‐co‐ethylene‐co‐1‐butene) system. The number‐average, weight‐average, and z‐average air‐hole diameters were compared with respect to the draw ratio as well as the calcite loading. The morphology of a micromechanically deformed composite, studied with an image analyzer, revealed that the aspect ratio and area of the air holes increased linearly as a function of the draw ratio, but the change in the aspect ratio upon filler loading was not remarkable. A suitable loading of a filler up to 30 wt % was good for controlling the porosity in the composite films. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effects of carbon nanofibers on crystalline structures and properties of ultrahigh molecular weight polyethylene blend fabricated using twin‐screw extrusion

Melt mixing in an extruder with polymers is an effective approach for forming nanocomposites, allowing mass production applications. The intent of this study is to investigate carbon nanofiber composites with ultrahigh molecular weight polyethylene (UHMWPE) matrix using the twin‐screw extruder. To decrease the high viscosity of UHMWPE, a low density polyethylene (LDPE) was added into the UHMWPE. The effects of carbon nanofibers (CNFs) on the crystalline structures and properties of the nanocomposites were analyzed. The differential scanning calorimetry (DSC) and X‐ray diffraction (XRD) measurements showed the addition of CNFs decreases the degree of crystallinity, but does not impart significant effects on the crystalline structure of the UHMWPE/LDPE blend. Tensile test results showed that the nanocomposite with loading of 3 wt % CNFs had an increase of 38% in tensile strength and 15% in modulus. The thermal stability and thermal conductivity of UHMWPE/LDPE blends were also enhanced by the addition of CNFs. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Morphology and physical properties of composite foams based on low‐density polyethylene and ground tire rubber

Low‐density polyethylene (LDPE)/(ground tire rubber) (GTR) composite foams with an average cell size smaller than 100 µm were produced by compression molding. The effects of the mechanochemical pretreatment of the LDPE/GTR blends and their GTR content on the cell morphology and physical properties were investigated. The results indicated that the pretreatment of LDPE/GTR blends by using a pan‐mill type of mechanochemical reactor made in our laboratory improved the dispersion state and thereby enhanced the foamability of the blends. The average cell size of the composite foams decreased from 64.0 to 42.3 µm with an increase in GTR content from 0 to 50 phr (parts by weight per hundred parts of resin). Moreover, the addition of 20–40 phr of GTR into the LDPE foams increased their rebound resistance and decreased their compression set. J. VINYL ADDIT. TECHNOL., 19:105–112, 2013. © 2013 Society of Plastics Engineers     

Resistivity‐temperature behavior of carbon fiber filled semicrystalline composites

Carbon fiber (CF) filled low‐density polyethylene (LDPE) composites were prepared by the conventional melt‐mixing method. The distribution of CF in the composite was studied by wide‐angle X‐ray diffraction (WAXD) and scanning electron microscope (SEM) observations. A phenomenological model was proposed to illustrate the resistivity‐temperature behavior of CF‐filled semicrystalline composites. The effects of the content and aspect ratio of CF on the positive temperature coefficient (PTC) and the room temperature resistivity were elucidated. A balance between the PTC intensity and the room‐temperature resistivity can be achieved by using a mixture of CFs with low and high aspect ratios. The negative temperature coefficient (NTC) phenomenon can be effectively eliminated by crosslinking under γ‐ray radiation, and the crosslinked composite exhibits a higher PTC intensity and PTC transition temperature than the noncrosslinked counterpart. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1222–1228, 2004