Metallocene-catalyzed Polyethylene Research Articles

Rheological criteria to characterize metallocene catalyzed polyethylenes

Dynamic viscoelastic and capillary extrusion rheometry measurements were carried out with a series of 13 metallocene catalyzed polyethylenes and copolymers of ethene and 1-hexene. The structural parameters were analyzed by size exclusion chromatography (SEC) and 13 C NMR, showing that the molecular weights range from M w = 80000 to 308000, the polydispersity index from 2 to 3.5 and the degree of short chain branching (SCB) from 0 to 13.8 SCB/1 000 C. In order to extract the maximum information from the experimental data, the following rheological methods were used: a) Viscosity and relaxation time dependence on molecular weight M w , b) Reduced dynamic viscosity η'/η 0 dependence on the product ω. η 0 . c) Frequency dependence of dynamic moduli, storage modulus (G') and loss modulus (G). d) log G' versus log G plots. e) Storage compliance J' dependence on storage modulus G'. f) Phase angle δ dependence on complex modulus G*. g) Relaxation spectra. h) Dependence of the exponent n of the power law model for the viscosity function η(γ) on of molecular weight, i) Analysis of the critical rate for sharkskin. These methods, except the last one, allow to separate the samples into three different groups, at least when low frequencies (below 10-' Hz) or times higher than 10 s are involved. The definition of these groups cannot be undertaken considering only the molecular parameters obtained by SEC and 13 C NMR. Analyzing our rheological results in comparison with long chain branched polyethylenes (LCB) and looking at the theoretical aspect of the dynamics of long branched chains, we assume that among our samples there are five linear (non-LCB, Group I) polyethylenes and two groups of slightly long chain branched polyethylenes, which differ in the number of branches.

The Blending and Coextrusion of Metallocene Catalysed Polyethylene in Blown Film Applications

As Metallocene Catalysed Polyethylenes (mPEs) become more commercially available they are finding ever increasing application in the blown film industry. However, it is becoming apparent that no two mPE resins behave in exactly the same manner. This paper examines the Theological properties of a range of mPEs with different densities and MFIs. These resins were then blended with a conventional low density polyethylene (LDPE) and extruded to form monolayer films. The mechanical properties of these films were compared to three layer coextruded films of A:B:A structure, where A is LDPE and B is mPE.

Hot tack of metallocene catalyzed polyethylene and low-density polyethylene blend

Hot tack of metallocene catalyzed polyethylene and low-density polyethylene blend

Structure correlated properties of metallocene catalysed polyethylenes by modulated temperature differential scanning calorimetry

Conventional and modulated temperature differential scanning calorimetry (MT-DSC) have been used to evaluate quantitatively the thermal properties of a set of advanced polyethylenes synthesised from metallocene catalysts. This study is sought to establish correlations between polymer chain molecular characteristics and crystallisation behaviour in a set of tailor-made polyethylenes. Melting is described by at least two different structure forming processes that behave independently of each other with Avrami analysis showing singular exponents and low rate constants from melting kinetic data. Avrami exponents from crystallisation kinetic data are broadly in line with typical values for polyethylene. Non-isothermal crystallisation kinetic studies revealed the presence of dual crystallisation peaks on linear cooling from the melt. The first crystallisation peak was found to shift moderately with cooling rate to lower temperatures of crystallisation whereas the second crystallisation peak remained invariant. Application of MT-DSC following discrete step-wise crystallisation has been successful at identifying initial crystallinities and the presence of reorganisational crystallisation during polymer melting. Melting is seen primarily in the reversing signal of the modulated heat flow, and reorganisational crystallisation during melting in the non-reversing heat flow signal.

Hot tack of metallocene catalyzed polyethylene and low-density polyethylene blend

Films made of metallocene catalyzed polyethylene (mPE), low-density polyethylene (LDPE), and their blend were prepared to investigate how LDPE influences the hot tack of film. Experimental results showed hot tack is independent of film thickness. The addition of 30 wt % of LDPE can increase the hot tack of mPE film. The thermograms of differential scanning calorimetry (DSC) suggest the respective partial melting and recrystallization of those smaller size crystals at the bond forming and joint fracture stages play very important roles. The large amount of partial melting and high flow may induce a higher degree of molecular diffusion. Higher residual crystallinity and recrystallization at the hot tack testing process may induce higher resistant to bond fracture. Those two positive influences show that the mPE/LDPE film has the higher hot tack. The evidence from optical (higher optical transmission and lower haze) as well as viscoelastic (higher storage modulus and lower melt viscosity) properties further support this hypothesis. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1769–1773, 1999

The Use of Metallocene Catalysed Polyethylene as a Blend Material in Blown Film Extrusion

AbstractMetallocene catalysed polyethylenes are the latest addition to the polyethylene family and are reported to have superior mechanical properties over conventional polyethylenes. This paper sought to investigate the effect of blending these materials with conventional low density polyethylene for use in thin films, suitable as packaging materials and produced by the blown film process. Two grades of Metallocene catalysed polyethylene were used in the study. Films containing increasing percentages of these resins were produced at various processing conditions and their mechanical properties considered. These films were then characterised using Differential Scanning Calorimetry and Dynamic Mechanical Thermal Analysis techniques. This work showed that increasing the percentage of metallocene catalysed polyethylene in a low‐density polyethylene film improved the mechanical properties of the resultant film.

Small-Amplitude Oscillatory Shear Flow Measurements as a Tool To Detect Very Low Amounts of Long Chain Branching in Polyethylenes

Dynamic viscoelastic results of 23 noncommercial metallocene-catalyzed polyethylenes and poly(ethylene/1-hexene) copolymers, in the range 130−190 °C, are presented. The effects of well-determined structural parameters such as molecular weight, polydispersity, and degree of short chain branching (SCB), are analyzed. The molecular weight varies between 60 000 and 325 000, the polydispersity between 1.8 and 7.3, and SCB between 0 and 48.5 branches/1000 C atoms. It is observed that a group of 11 polymers displays rheological specific features which can be summarized as follows: (a) higher dynamic viscosities at low frequencies than other polyethylenes and ethylene/1-hexene copolymers of similar molecular weight, polydispersity and SCB degree; (b) higher relaxation times than narrow molecular weight distribution polyethylenes of similar dynamic viscosities at low frequencies but similar relaxation times to those of broad molecular weight distribution; (c) higher values of elastic modulus, in comparison with polyethylenes of similar molecular weight, polydispersity, and SCB but of the same order of magnitude as those of broader molecular weight distribution; (d) higher activation energy of flow than linear polyethylenes of the same molecular weight, polydispersity, and SCB level. An analysis of the literature results leads us to suspect that the polymers which show a “dissident” behavior possess a certain very low degree of long chain branching (LCB). The analysis of the samples by SEC coupled with intrinsic viscosimetry reveals that some of these 11 polymers are long chain branched. However, this technique does not appear to be enough sensitive to detect very small amounts of LCB, and an alternative single rheological method, based on the effect of temperature on dynamic viscosity, is proposed to evaluate the possible presence of LCB.

Comparison of the Rheological Properties of Metallocene-Catalyzed and Conventional High-Density Polyethylenes Volume 29, Number 3, January 29, 1996, p 260

Comparison of the Rheological Properties of Metallocene-Catalyzed and Conventional High-Density Polyethylenes Volume 29, Number 3, January 29, 1996, p 260