Linear low density polyethylenes and their blends: Part 2. Shear flow of LLDPE's

The steady state and dynamic shear behavior of eleven commercial linear low density polyethylenes (LLDPE) and one low density polyethylene (LDPE) resin were measured in capillary and parallel plate geometries at T = 150 to 230°C. The extrudate swell and the Bagley correction were determined. A large pressure effect on capillary flow of narrow molecular weight distribution LLDPE was observed and a new corrective procedure was proposed. After the correction the steady state viscosity was found to be equal to the dynamic (not complex) viscosity: η(\documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document}) = η'(ω = \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document}). A newly proposed four parameter relation between η and the deformation rate was found to provide a simple means for computation of the zero shear viscosity, ηo, and the primary relaxation time. Both these parameters showed a high degree of correlation. The expected relation: ηo ∝? M_w^3.4 was observed for low molecular weight samples with low polydispersity. The LLDPE activation energy of flow, E_σ=29.9 ± 1.8 kJ/mole, was determined.     

Linear low density polyethylenes and their blends: Part 5 extensional flow of LLDPE blends

The uniaxial extensional flow at 150°C of two series of blends: I. LLDPE/LLDPE and II. LLDPE/LDPE was examined in full range of concentrations as well as that of accessible in the rheometer strains and strain rates. It was concluded that Series-I blends containing different LLD-type polymers are miscible. Their properties can be predicted on the basis of molecular weight and molecular weight distribution. By contrast, excepting low concentration limits, blends of Series-II are immiscible. Both series show strain hardening, due to higher values of the maximum strain at break. Series-II seems to be superior (under the test conditions). The stress growth function in shear, computed from the frequency relaxation spectrum, provided a good prediction of the linear viscoelastic component of the stress growth function in uniaxial extension.     

Linear low density polyethylenes and their blends: Part 1. Molecular characterization

Ten commercial linear low-density polyethylenes (LLDPE) were characterized by solution viscosity, size exclusion chromatography, SEC, and ¹³C nuclear magnetic resonance. The resins were copolymers of ethylene with butene, hexene, or octene. They were prepared in gas phase (with narrow or very broad molecular weight distribution), or in solution. The macromolecules were found to be linear. For all but the very broad molecular weight distribution resins the average comonomer sequence length was found to be 1; in the other case diad formation was observed. The weight average molecular weights calculated from SEC, and intrinsic viscosities agreed quite well. Mechanical degradation of LLDPE was observed during the solution viscosity measurements.     

Linear low density polyethylenes and their blends: Part 4 shear flow of LLDPE blends with LLDPE and LDPE

The steady state and dynamic shear behavior of linear low density polyethylenes (LLDPE) blended with low density polyethylene (LDPE) and with another LLDPE resin were measured in capillary and parallel plate geometries at T = 150, 190, and 230°C. The extrudate swell and the Bagley correction were determined. It was observed that the pressure correction plays a significant role in capillary flow of LLDPE/LDPE blends–an indication of immiscibility. Several other rheological functions also suggested a phase separation for the system. Nevertheless, the blend behaved as a “compatible” mixture of emulsion type. By contrast, blends of two LLDPE resins show expected miscibility. However, even in this case additivity was not always observed. A new simple method of calculating the relaxation spectrum was developed. The method is analytical and its accuracy depends on adequacy of the semiempirical relation (proposed previously) to describe dynamic viscosity dependence on the test frequency. For all samples the spectrum allowed computation of storage modulus in good agreement with experimental findings.     

Elongational behavior of low density/linear low density polyethylenes

Rheological data have been collected in isothermal elongational flow for three different types of blends, made from one low density polyethylene and three linear low density samples. In addition to the transient curves, elongation at break data are also reported. The influence of the composition and of the molecular weight of the linear low density polyethylene is discussed.     

Flow properties of low density/linear low density polyethylenes

Rheological data have been collected both in shear and non-isothermal elongational flow on three different types of blends, made from one low density polyethylene sample and three linear low density polyethylene samples. In addition to the flow curves, data are presented on the extrudate-swell phenomenon, on the instability arising in capillary flow and on the tensile behavior in the molten state.     

Crystalline-state extrusion of low density polyethylenes

Three low density polyethylenes, one long branched (A) and two linear (B and C), have been solid-state-extruded at several constant temperatures from ambient to 80°C and to draw ratios ? 8. The initial densities and melt indices of A, B, and C are 0.920, 0.920, and 0.935 g/cm³, and 1.9, 0.8, and 1.2, respectively. Melt-crystallized cylindrical billets were extruded through conical dies in an Instron Capillary Rheometer. The linear polymers were found to draw by extrusion more readily than the branched; all three strain-harden. Density, birefringence, tensile, and thermal properties have been evaluated as functions of extrusion temperature and draw ratio. Despite a measured loss via die swell, substantial orientation takes place during solid-state extrusion as evidenced by increases in transparency, birefringence, and tensile modulus (up to 4.5 times that of the original isotropic polymer). Depending on the polymer and the draw temperature, density does go through a minimum or shows a monotonic increase with draw by extrusion. A minimum in modulus is also observed at low draw and at all draw temperatures for all three polymers. The highest tensile moduli achieved are 0.73, 0.46, and 1.5 GPa for A, B, and C, respectively, at their highest draw ratio. The melting point for polymer B decreases with extrusion draw ratio, whereas it remains constant after a small initial drop, for the two others. For all three low density polyethylenes, birefringence increases rapidly with extrusion draw and then levels off at high draw. The birefringence limit is similar for A and B, i.e., 0.046 ± 0.004, but higher for C, i.e., 0.068 ± 0.009. This work extends beyond others in that it studies the effect of short as well as long branches in solid-state extrusion by comparing the linear and long branched LDPE polymers and LDPE with prior evaluations of HDPE.     

Post-yield behaviour of low density polyethylenes: 2. Plastic fracture

Diamond-shaped cavities have been observed to grow in a stable fashion through oriented low density polyethylenes, linear and high pressure, and account for the ultimate fracture characteristic of these polymers. Their growth has been examined by the application of fine grids on the specimen surface, and (adjacent) elements of material at the diamond tip deform in simple shear in turn, as the maximum shear is attained. Simple shear tests on drawn material confirm that the angle between the linear faces of the diamond cavities is determined by the onset of strain softening followed by strain hardening. The characteristics of these determine the extent of yielding in each element.     

Structural and rheological investigation of low density polyethylenes

Some low density polyethylenes (LDPE) with different melt flow index (MFI) or produced by different producers have been examined in detail by solvent gradient fractionation, ¹³C NMR analysis, FTIR spectroscopy and melt rheological measurements. It was found that the distribution curves of the samples resemble Wesslau's logarithmic-normal model. From branching analyses it can be concluded that the branching content in the analyzed LDPEs is independent from the molecular weight. Relations between viscosity curve parameters and molecular structure have been investigated. It has been found that the dependence of the first normal stress difference on the shear stress is influenced by polydispersity as well as by the character of samples branching.     

Crystallinity in chemically crosslinked low density polyethylenes: 3. Morphology of the XLPE-2 system

Studies of the morphology of low density branched polyethylene crosslinked using 2% dicumyl peroxide have been carried out using transmission electron microscopy of replicas of surfaces etched with permanganic reagent. Under optimal etching conditions detailed information regarding the morphologies of bulk crystallized samples of low crystallinity can be obtained. The morphology of crosslinked polyethylene is compared with that of the original branched polyethylene as a function of crystallization temperature. Branched polyethylene crystallizes in the form of banded spherulites from 88°C to 103°C with a temperature dependent band periodicity. Introduction of a small amount of crosslinking produces profound effects with banded sheaf or bundle-like morphologies resulting. The absence of lamellar banding in the sheaves of the gel fraction of the crosslinked polyethylene is attributed to pre-existent tie-molecules which are an inherent feature of network junctions in a crosslinked system. In the case of a typical unextracted crosslinked polyethylene, the morphology is found to vary with crystallization temperature. D.s.c. studies reveal that the thermal behaviour of XLPE-2 is not too different from that of OPE. The cause of multiple melting behaviour in this class of material is discussed and it is suggested that in addition to other processes of reorganization, hindrance to solid state thickening may be important. A method of monitoring different stages of lamellar melting for semicrystalline polymers was developed involving annealing at elevated temperatures in the presence of chlorosulphonic acid.     

Reversible crosslinked low density polyethylenes: structure and thermal properties

In the present study, low density polyethylene (LDPE) has been crosslinked at 170 °C with three different systems by a) using peroxide, b) peroxide and accelerator and c), peroxide, accelerator and sulfur. The effect of chemical crosslinking on LDPE structure has been investigated using torque measurements, Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXS), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Therefore, effects of each crosslinking system on the structural and thermal properties of the material in terms of crystallinity, thermal transitions and stability have been discussed. The reversible crosslinking of LDPE allow the recyclability of polyolefins, increasing the thermal properties.     

Processing properties of high- and low-density polyethylenes and their blends

Blends of two characterized linear polyethylenes with a branched polyethylene have been prepared by melt extrusion. It has been found that the linear polyethylenes can be shear modified in a reversible manner similar to branched polymers and that this shear modification and its reversal by re-heating does not change the molecular weight distribution, thereby indicating that the shear modification is a physical rather than chemical change in structure. Because both the high- and low-density polyethylene components of the blends are capable of undergoing reversible shear modification, it is possible to produce blends with either greater or less melt elasticity than the individual components by adjusting the conditions of blending. This demonstrates that the correlation of the properties of blends with the properties of their components should not be attempted without consideration of the effect of the blending process on the properties of the individual components.     

Dielectric relaxations of high and low density irradiated polyethylenes

A systematic study of dielectric spectrum on a series of high and low density commercial polyethylenes has been performed. Every polyethylene was characterized by determination of molecular weight distribution, the number of functional groups: ketone, aldehyde, vinyl, vinylidene, trans-vinylene, the fraction crystalline, and the degree of branching. We have observed that both γ and α dielectric relaxations zones are formed by two overlap relaxations which have been labeled γ_II, γ_I, and α′, and α respectively, in order of increasing temperatures. The intensity of the dielectric spectrum of all polyethylene depends over and above other parameters on the number of the carbonyl groups formed as a result of irradiation. This number is different for every polyethylene even when they receive the same dose of irradiation (20 Mrad). However, the participation of carbonyl groups in the γ_I and α′ dielectric relaxations decreases with the total crystalline content of each polyethylene. The α dielectric relaxation position in the temperature axis is governed by the most probable crystallite thickness.     

Sizes of long branches in low density polyethylenes

¹³C-NMR spectroscopy and size exclusion chromatography have been used to determine the mean length of long branches in a number of high pressure process low density polyethylenes (LDPEs). ¹³C-NMR analyses count all branches longer than C5 as “long.” The polyethylenes studied all had 2–3 long branches per 1000 carbons. The mean branch length was of the order of 200–300 carbons in length. The size of long branches increases with increasing M?_n of the parent polyethylene, but the size of long branches relative to the overall macromolecular size decreases with increasing M?_n. The mean molecular weight of the long branches is some 5–20% of M?_n of the particular polymer and decreases as M?_n increases. Both autoclave and tubular reactor products were studied.     

Crystallinity in chemically crosslinked low density polyethylenes: 1. Structural and fusion studies

The level of extractables, crystallinity, unit cell parameters and other structural information have been obtained for low density polyethylene crosslinked using weight fractions of dicumyl peroxide from 0.9 to 6.8%. Since the presence of the extractable fraction influences the overall structure as well as the crystallization and melting behaviour studies have been carried out of the gel and sol fractions in addition to the as-crosslinked polymers. Evidence suggests that when crystallized above 100°C the sol fraction of the as-crosslinked polymer either self-crystallizes or selectively crystallizes with the most linear section of the gel network.     

Rheological properties of linear low density polyethylene/polypropylene blends. Part 2: Solid state behavior

This second paper of a series continues the examination of the tensile properties of two series of linear low density polyethylene/polypropylene, (LLDPE/PP) blends. The blends were prepared using a twin-screw extruder and cover the whole concentration range, An Instron Universal Tensile Tester was used to measure the tensile properties of the blends between 10 and 70°C, and the temperature and composition dependences of the modulus were examined. A comparison is established between the solid state and melt properties to underline the behavior in the PP rich region. Results of dynamic mechanical experiments and differential scanning calorimetry on the same materials are also given, and the mechanical behavior is discussed in terms of the variation of the system's crystallinity.     

Characterization of ultra low density polyethylenes (PE-ULD)

Ultra low density polyethylene (PE-ULD), the newest and most recently commercialized member of polyethylene family, has been characterized in terms of structure, thermal and rheological properties, and molecular parameters. Since the prime use of PE-ULD is polyolefin modifier, emphasis is put on the melt rheology of the samples to provide data base for blending with other resins.     

Dynamic-mechanical relaxations in high and low density polyethylenes. Effects of irradiation and annealing

A study of the dynamic-mechanical relaxation spectrum in a series of commercial high and low density polyethylenes (Dow Chemical), irradiated as well as unirradiated, and subjected to different annealing process, has been performed. The effect of 20-Mrad dose of irradiation on the chemical structure has been analyzed and an increase in the number of aldehyde, ketone, and transvinylene groups and a decrease in the number of vinyl and vinylidene groups has been observed. The dynamic-mechanical spectrum of irradiated and unirradiated high and low density polyethylenes contains the γ_II-, γ_I-, β-, α_I-, α_II-, and α_III- relaxations, in order of increasing temperature. It has been observed that γ-irradiation followed by annealing modified the intensity and the position of relaxations in these polyethylenes.     

Bulk and surface compression viscosities of high and low density polyethylenes

Bulk compression flow of high density polyethylene (HDPE) and low density polyethylene (LDPE) have been measured at temperatures of 423 to 463K, pressure up to 150 MPa, and bulk compression rates of about 1.0 to 200.00 × 10^?5 s^?1. Bulk and surface compression modulus of elasticity (L and K_s), longitudinal bulk compression viscosity (η_L), and surface compression viscosity (η) are described as a function of compression rates (k_v and k_s), compression deformations (k_v percent and K_s percent), and temperature (T). Bulk and surface compression flow activation energies are of the order of 40 to 100 KJ/mol and 3.84 KJ/mol, respectively.     

Effect of polymerization catalyst technology on the melt processing stability of polyethylenes,Part 3: Additives blends performance

This article considers the interaction between additives that occur during the stabilization process. The simultaneous effects of the additives and associated interactions on melt processing stability and processing discoloration were of particular interest. Melt stability is an important factor to consider because physical changes in the processed polymer can occur during the compounding and fabrication steps. Furthermore, discoloration is one of the most important problems affecting commercial polymers. Most discoloration manifests itself as yellowing, especially in the case of polyolefins. Although yellowing can often be associated with degradation processes caused by various agents, such as light or heat, this is not always the case; yellowing can also be due to the interaction of additives in the stabilizer packages. Blends of primary antioxidants (AOs), secondary AOs, and hindered amine light stabilizers have been studied with the intention of further improving stabilization performance together with cost reduction of the stabilized polymer. Although synergism between AOs and a stabilizer is fairly common, antagonism was also observed in terms of melt flow protection and in color stability in some of the AOs tested. The effects of a range of thermal and light stabilizers on the melt stability (investigated via multiple pass extrusion) and color stability of three different polyethylenes (PEs) were examined. The PEs varied in terms of the catalyst system used to synthesize the polymers and included a high‐density polyethylene (HDPE) produced by using a chromium‐based Phillips catalyst and two linear low‐density polyethylenes (LLDPEs) produced via chromium‐based metallocene and titanium‐based Ziegler‐Natta catalysts. The apparent lack of influence of polymerization catalyst system on the mode of stabilizer interaction should lead to the reassessment of stabilizer formulation strategies in relation to PE type/catalyst system and associated commercial/economic considerations. J. VINYL ADDIT. TECHNOL., 22:117–127, 2016. © 2014 Society of Plastics Engineers