Shear and elongational rheology of polyethylenes with different molecular characteristics. I. Shear rheology

Four metallocene polyethylenes (PE), one conventional low density polyethylene (LDPE), and one conventional linear low density polyethylene (LLDPE) were characterized in terms of their complex viscosity, storage and loss moduli, and phase angle at different temperatures. The effects of molecular weight, breadth of molecular weight distribution, and long‐chain branching (LCB) on the shear rheological properties of PEs are studied. For the sparsely long‐chain branched metallocene PEs, LCB increases the zero‐shear viscosity. The onsets of shear thinning are shifted to lower shear rates. There is also a plateau in the phase angle, δ, for these materials. Master curves for the complex viscosity and dynamic moduli were generated for all PE samples. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Comparison between a linear and a branched low-density polyethylene

Two low-density polyethylenes, a linear low-pressure (LLDPE) and a branched high-pressure (LDPE), have been compared. Their shear and extensional behavior and melt fracture phenomena have been investigated, and some mechanical and optical properties of their blown films have been measured. The rheological analysis showed major differences between the samples, both in shear viscosity and in elongational viscosity. The LLDPE exhibited two types of melt fracture, the first of which—a fine scale extrudate roughness—was not shown by the LDPE and appeared at a very low shear rate. The concomitance in LLDPE of a high shear viscosity and a low elongational viscosity and the presence of melt fracture at low shear rate resulted in its more difficult processing into film. The mechanical properties of the LLDPE film approached those of high-density polyethylene while the optical characteristics were in the range of LDPE. Such a coexistence of properties makes LLDPE an interesting material for film production.     

Effect of molecular structure in branched polyethylene on adhesion properties with polypropylene

Adhesion properties between branched polyethylene (PE) and isotactic polypropylene (PP) were studied by a peel test and scanning electron microscopy. In this study, two types of branched PEs were used; one is a linear low density polyethylene (LLDPE) and the other is a high pressure low density polyethylene (LDPE). The adhesive strength of the LLDPE/PP is much higher than that of LDPE/PP. Furthermore, the formation of PE influxes between PP spherulites has a small effect on the adhesion. The dynamic viscoelastic measurements for the binary blends composed of branched PE and PP were also carried out to estimate the interfacial tension by using a rheological emulsion model proposed by Palierne. The interfacial tension is 1.0 mN for LLDPE/PP and 2.1 mN for LDPE/PP, suggesting that the interfacial thickness of LLDPE/PP is about twice that of LDPE/PP. The adhesive strength between branched PE and PP will be determined by the interfacial thickness, which represents the entanglements between two polymers. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 457–463, 1998     

聚乙烯挤出拉伸流动行为的研究

介绍了１种测定表观拉伸流动性能的新方法,通过测量不同聚合物的拉伸流动阻力和表观拉伸应变速率,来比较它们的加工流动性能,并就低密度聚乙烯和线型低密度聚乙烯的分子结构和加工流动行为的影响进行了探讨。     

Elongational rheology of LLDPE / LDPE blends

The objective of this study is to investigate the effect of low density polyethylene (LDPE) content in linear low density polyethylene (LLDPE) on the crystallinity and strain hardening of LDPE / LLDPE blends. Three different linear low density polyethylenes (LL‐1, LL‐2 and LL‐3) and low density polyethylenes (LD‐1, LD‐2 and LD‐3) were investigated. Eight blends of LL‐1 with 10, 20, 30 and 70 wt % of LD‐1 and LD‐3, respectively, were prepared using a single screw extruder. The elongational behavior of the blends and their constituents were measured at 150°C using an RME rheometer. For the blends of LL‐1 with LD‐1, the low shear rate viscosity indicated a synergistic effect over the whole range of concentrations, whereas for the blends of LL‐1 with LD‐3, a different behavior was observed. For the elongational viscosity behavior, no significant differences were observed for the strain hardening of the 10–30% LDPE blends. Thermal analysis indicated that at concentrations up to 20%, LDPE does not significantly affect the melting and crystallization temperatures of LLDPE blends. In conclusion, the crystallinity and rheological results indicate that 10–20% LDPE is sufficient to provide improved strain hardening in LLDPE. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3070–3077, 2003     

The role of the interface in melt linear viscoelastic properties of LLDPE/LDPE blends: Effect of the molecular architecture of the matrix

The rheological properties of blends consisting of a long chain branched low‐density polyethylene (LDPE) and two linear low‐density polyethylenes (LLDPE) are studied in detail. The weight fractions of the LDPE used in the blends are 5 and 15%. The linear viscoelastic characterization is performed at different temperatures for all the blends to check thermorheological behavior and miscibility in the melt state. Blends containing metallocene LLDPE as the matrix display thermorheologically complex behavior and show evidences of immiscibility in the melt state. The linear viscoelastic response exhibits the typical additional relaxation ascribed to the form deformation mechanism of dispersed phase droplets (LDPE). The Palierne model satisfactorily describes the behavior of these blends in the whole frequency range explored. However, those blends with Ziegler‐Natta LLDPE as the matrix fulfill the time‐temperature superposition, but exhibit a broad linear viscoelastic response, further than the expected for an immiscible system with a sharp interface. The rheological analysis reveals that, in addition to the droplets form relaxation, another mechanism at lower frequencies exists. The broad linear response of the blends with the Ziegler‐Natta LLDPE can be explained by hypothesizing a strong interaction between the high molecular weight linear fraction of the LLDPE and the low molecular weight (almost linear) chains of the LDPE phase, forming a thick interface with its own viscoelastic properties. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Influence of branching characteristics on thermal and mechanical properties of Ziegler–Natta and metallocene hexene linear low‐density polyethylene blends with low‐density polyethylene

The effect of the branch content (BC) and composition distribution (CD) of linear low‐density polyethylene (LLDPE) on the thermal and mechanical properties of its blends with LDPE were studied. All blends and pure resins were conditioned in a Haake PolyDrive blender at 190°C and in the presence of adequate amounts of antioxidant. Two metallocene LLDPEs (m‐LLDPE) and one Ziegler–Natta (ZN) hexene LLDPE were melt blended with the same LDPE. The effect of the BC was investigated by blending two hexene m‐LLDPEs of similar weight‐average molecular weights and molecular weight distributions but different BCs with the same LDPE. The effect of the CD was studied by using a ZN and an m‐LLDPE with similar weight‐average molecular weights, BCs, and comonomer type. Low‐BC m‐LLDPE blends showed separate crystallization whereas cocrystallization was observed in the high‐BC m‐LLDPE‐rich blends. However, ZN‐LLDPE/LDPE blends showed separate crystallization together with a third population of cocrystals. The influence of the crystallization behavior was reflected in the mechanical properties. The BC influenced the modulus, ultimate tensile strength, and toughness. The addition of a small amount of LDPE to a low‐BC m‐LLDPE resulted in a major improvement in the toughness, whereas the results for the high‐BC pair followed the additivity rule. ZN‐LLDPE blends with LDPE blends were found to be more compatible and exhibited superior mechanical properties compared to m‐LLDPE counterparts with the same weight‐average molecular weight and BC. All mechanical properties of ZN‐LLDPE blends follow the linear rule of mixtures. However, the CD had a stronger influence on the mechanical properties in comparison to the BC. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2488–2498, 2005     

Elongational behavior of polyethylene melts—effect of deformation

Transient elongational viscosity of linear low density polyethylene (LLDPE) and two low density polyethylenes (LDPE1 and LDPE2) was measured at one temperature and different deformation rates in constant strain rate elongational rheometer. The elongational viscosity measurements revealed stronger strain hardening characteristics for LDPEs than that observed for LLDPE. The different response to stretching of these polymers is thought to relate to the presence of long chain branches in LDPEs, which affect the elongation viscosity profoundly. The onset of strain hardening for all long chain branched LDPEs as well as for linear LLDPE occurs at the same value of the critical strain, which is independent of temperature or deformation rate. An attempt has been made to explain this phenomenon in terms of the changes that occur in the macromolecular network upon stretching.     

Properties of mLLDPE/LDPE blends in film blowing

We evaluated the inline birefringence of two blend systems in film blowing. The first system consisted of a metallocene catalyzed linear low density polyethylene (mLLDPE‐1) and a low density polyethylene (LDPE‐1); the second one was made of a metallocene catalyzed polyethylene containing sparse long chain branches (mLLDPE‐2) and another low density polyethylene (LDPE‐2). Experimental data show that before the crystallization starts, the birefringence of the mLLDPE‐2/LDPE‐2 blends is a linear function of blend composition, suggesting miscibility of the mLLDPE‐2/LDPE‐2 blends. However, the birefringence of the mLLDPE‐1/LDPE‐1 blends shows positive deviations with respect to a linear function of blend composition. This is caused by the existence of form birefringence, suggesting immiscibility of the mLLDPE‐1/LDPE‐1 blends. The nonuniform biaxial elongational viscosity (NUBEV) at the reference temperature of 175°C for LDPE‐1 was evaluated for different operating conditions. The results show that NUBEV is approximately a unique function of the deformation rate, confirming the validity of the assumptions and technique used for the NUBEV calculation. The NUBEV and the nonuniform biaxial Trouton ratio (NUTR) of the mLLDPE‐2/LDPE‐2 blends was also evaluated using the same technique. The NUBEV of all mLLDPE‐2/LDPE‐2 blends shows a strain‐thinning behavior within the deformation rates investigated. Furthermore, the NUTR results show that LDPE‐2 deviates largely from the Newtonian fluid behavior, whereas mLLDPE‐2 is quite close to the Newtonian behavior. Nevertheless, the NUTR of the mLLDPE‐2/LDPE‐2 blends is almost a linear function of blend composition. POLYM. ENG. Sci., 45:343–353, 2005. © 2005 Society of Plastics Engineers     

Waste and Virgin LDPE/PET Blends Compatibilized with an Ethylene‐Butyl Acrylate‐Glycidyl Methacrylate (EBAGMA) Terpolymer, 1

Summary: This work is aimed at studying the morphology and the mechanical properties of blends of low density polyethylene (LDPE) and poly(ethylene terephthalate) (PET) (10, 20, and 30 wt.‐% of PET), obtained as both virgin polymers and urban plastic waste, and the effect of a terpolymer of ethylene‐butyl acrylate‐glycidyl methacrylate (EBAGMA) as a compatibilizer. LDPE and PET are blended in a single screw extruder twice; the first extrusion to homogenize the two components, and the second to improve the compatibilization degree when the EBAGMA terpolymer is applied. Scanning electron microscopy (SEM) analysis shows that the fractured surface of both the virgin polymer and the waste binary blends is characterized by a gross phase segregation morphology that leads to the formation of large PET aggregates (10–50 µm). Furthermore, a sharp decrease in the elongation at break and impact strength is observed, which denotes the brittleness of the binary blends. The addition of the EBAGMA terpolymer to the binary LDPE/PET blends reduces the size of the PET inclusions to 1–5 µm with a finer dispersion, as a result of an improvement of the interfacial adhesion strength between LDPE and PET. Consequently, increases of the tensile properties and impact strength are observed.

SEM micrographs of the fracture surface of a waste 70/30 LDPE/PET blend (R30) and of its blend with 15 pph of EBAGMA (R30C). Magnification × 1 000.     

Elongational flow properties of low-density polyethylene and linear low-density polyethylene from nonisothermal melt spinning experiments

Low-density polyethylene (LDPE) and also linear low-density polyethylene (LLDPE) resins can be characterized by the degree of strain hardening and down-gaging during elongation. A new method for the determination of the apparent elongational flow characteristics is presented. In a small scale apparatus, a molten monofilament is stretched under nonisothermal conditions similar to those found in tubular film extrusion. Measurement of resistance to elongational flow and apparent elongational strain rates permit the comparison of the process-ability of different resins under specified conditions. The effect of melt temperature and extension ratio are examined. The importance of the molecular structure of both LDPE and LLDPE resins on these properties is also outlined.     

Compatibilization of Conductive Polyethylene/Polyaniline Blends

Summary: The compatibilization of polyethylene/polyaniline (PE/PANI) blends and the preparation of plasticized PANI/camphorsulfonic acid (CSA) complexes suitable for melt blending were studied. Rheological properties of the components essentially affected the morphology of the blend and thereby the electrical conductivity. The hydrogen bonds between the PANI complex and the functionalized metallocene PE used as compatibilizer compensated the unfavorable viscosities of the components. Mechanical properties of PE/PANI blends were improved, and electrical conductivity of the blends remained the same or increased through addition of functionalized metallocene polyethylene. Plasticized PANI/CSA complex with good electrical conductivity was successfully prepared.

Compatibilization of PANI/CSA complex and OH‐functionalized polyethylene.     

Co‐crystallization in ternary polyethylene blends: tie crystal formation and mechanical properties improvement

Understanding the co‐crystallization behavior of ternary polyethylene (PE) blends is a challenging task. Herein, in addition to co‐crystallization behavior, the rheological and mechanical properties of melt compounded high density polyethylene (HDPE)/low density polyethylene (LDPE)/Zeigler ? Natta linear low density polyethylene (ZN‐LLDPE) blends have been studied in detail. The HDPE content of the blends was kept constant at 40 wt% and the LDPE/ZN‐LLDPE ratio was varied from 0.5 to 2. Rheological measurements confirmed the melt miscibility of the entire blends. Study of the crystalline structure of the blends using DSC, wide angle X‐ray scattering, small angle X‐ray scattering and field emission SEM techniques revealed the formation of two distinct co‐crystals in the blends. Fine LDPE/ZN‐LLDPE co‐crystals, named tie crystals, dispersed within the amorphous gallery between the coarse HDPE/ZN‐LLDPE co‐crystals were characterized for the first time in this study. It is shown that the tie crystals strengthen the amorphous gallery and play a major role in the mechanical performance of the blend.© 2016 Society of Chemical Industry     

Properties of films made from ternary blends of metallocene and conventional polyolefins

Thin films were blown from a composition of 75% linear low density polyethylene (LLDPE) and 25% LDPE. The LLDPE content was made up of different % of metallocene‐based and conventional octene‐based LLDPE. Tensile strength, dart impact strength, hot tack strength, heat seal strength, and the barrier properties of these films were measured. All the properties showed significant improvement when conventional LLDPE (cLLDPE) was replaced by metallocene‐based LLDPE (mLLDPE), even to the extent of only 25%. The blends of 50% mLLDPE and 50% LDPE showed attractive properties. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 53–57, 2001     

Isothermal crystallization and spherulite nucleation in blends of polypropylene with metallocene‐prepared polyethylene

The isothermal crystallization behavior and morphology of a polypropylene (PP)‐based copolymer, a metallocene‐prepared linear low‐density polyethylene (M‐LLDPE) and their three 10/90, 30/70 and 50/50 M‐LLDPE/PP blends have been investigated. The PP and M‐LLDPE contained 5 ethylene and 3.3 mol% hexene‐1 as a comonomer, respectively. Isothermal crystallization studies revealed a different temperature‐dependence on crystallization for M‐LLDPE, PP and their blends and the crystallization half‐life for the M‐LLDPE was higher than either PP or the blends at a certain temperature. The PP‐rich blends also showed a quite similar crystallization rate to that of PP. Investigations on the variation of spherulite growth rate of PP in the blends at different temperatures revealed no significant change and was quite independent of the amount of M‐LLDPE being employed. The morphology studies revealed that the nucleation densities of the PP spherulites decreased by introducing M‐LLDPE into PP and the M‐LLDPE remained as discrete droplets dispersed throughout the PP spherulites. The results obtained were consistent with no miscibility between the two components. Copyright © 2005 Society of Chemical Industry     

Dynamic mechanical properties of binary blends of different polyethylenes. II. Isothermal treatment

The relaxation processes and thermal properties of a series of blends of a highly linear high-density polyethylene (HDPE) with several branched high-density, linear low-density (LLDPE), and low-density polyethylenes (LDPE) have been measured as a function of crystallization temperature, T_c, and content of branched polyethylene (BPE). The influence of composition on the dynamic mechanical spectrum of the HDPE has been rationalized taking into account the dilution with increasing content of BPE of those crystals formed during the isothermal crystallization. The influence of the type of second constituent (HDPE, LLDPE or LDPE) on the relaxation process of the HDPE has been explained in terms of segregation material data.     

A Novel Intumescent Flame‐Retardant Polyethylene System

Summary: A novel carbonization agent was synthesized and characterized. The flame retardancy and thermal behavior of a new IFR system with and without metal chelates for LDPE were investigated by LOI, UL‐94 test, and TGA. Metal chelates can remarkably improve the flame retardant performance of intumescent systems according to the tested results of LOI values and UL‐94 ratings because they can act both as inhibitors of radicals and as promoters of carbonization. The TG curves show that the amounts of residue of IFR‐PE/metal chelate systems increase compared to those of PE and IFR‐PE at temperatures ranging from 400 to 650 °C. The IFR‐PE/metal chelate system can obviously reduce the amounts of decomposing products at higher temperatures and promotes the formation of carbonaceous charred layers. A LOI value of 29.8 and UL‐94 V‐0 rating could be achieved when the synthesized carbonization agent, APP, MP, and CoOSA were added into LDPE.

TG curves of PE, IFR‐PE, and IFR‐PE/metal chelate systems.     

Application of FT Rheology to Industrial Linear and Branched Polyethylene Blends

Numerous analytical techniques are used to quantify branching topologies in polyethylene. One of these methods, FT rheology, studies the higher harmonic contributions of the stress response to large amplitude oscillatory shear deformation. The sensitivity of FT rheology in the presence of sparse long‐chain branched chains blended with linear ones is addressed. FT rheology is sensitive even for small concentrations of a partly long‐chain branched component blended with a linear melt (1.5–5 wt.‐%). The non‐linear parameters present a strong dependence on the fraction of long‐chain branched species in a polydisperse melt, the miscibility of which is confirmed via rheological techniques.

     

Fabrication of Tunable Submicro‐ or Nano‐Structured Polyethylene Materials from Immiscible Blends with Cellulose Acetate Butyrate

Low density polyethylene (LDPE) was prepared into micro‐ or submicro‐spheres or nanofibers via melt blending or extrusion of cellulose acetate butyrate (CAB)/LDPE immiscible blends and subsequent removal of the CAB matrix. The sizes of the PE spheres or fibers can be successfully controlled by varying the composition ratio and modifying the interfacial properties of the blends. The surface structures of LDPE micro‐ or submicro‐spheres and nanofibers were analyzed using SEM and FTIR‐ATR spectroscopy. In addition, the crystalline structures of the LDPE nanofibers were characterized.

     

The effect of matrix composition on the properties of cellulosic fibers–polyethylene composites

The solution/precipitation method was used for the preparation of polyethylene (PE)/cellulosic fibers composites. Blends of modified linear low density PE [linear low density PE‐grafted maleic anhydride (LLDPE‐g‐MAH)] with low density PE (LDPE) were used as matrices for the aforementioned composites. Blends of LDPE with a copolymer of LDPE and acrylic acid (AA)/n‐butyl acrylate (n‐BA) [(AA/n‐BA)–LDPE] were also studied for the same purpose. The reinforcing effect of cellulosic fibers in terms of tensile strength is more enhanced when mixtures of the modified polar polymer with pure PE were used as matrices, as compared with that corresponding to matrices consisting of modified PE alone. Regarding the Izod impact strength, composites of LLDPE‐g‐MAH presented the best performance with an improvement of 135% in comparison with specimens consisting of LDPE matrix, whereas composites of (AA/n‐BA)‐LDPE matrix showed a modest improvement of their impact resistance. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008