Comparison of the Rheological Properties of Metallocene-Catalyzed and Conventional High-Density Polyethylenes

Abstract

Dynamic measurements in a plate−plate system and steady state flow experiments in a capillary die are presented for conventional high-density polyethylenes (HDPEs) and a new type of polyolefin. The latter, the so-called metallocene-catalyzed HDPEs, are characterized by their low polydispersity and the total absence of branching. The metallocene-catalyzed materials show a different rheological behavior than commercial polyethylenes, which can be summarized as follows: (a) Higher viscosities than conventional HDPEs of the same molecular weight. The dependence of the viscosity on the molecular weight follows a power law equation with an exponent of 4.2 for metallocene catalyzed and 3.6 for conventionals. (b) For high molecular weight materials, the storage modulus overcomes the loss modulus (G‘ > G‘‘) at 190 °C in all frequency ranges. However, for conventional HDPEs, G‘‘ > G‘ at the same temperature and frequency range. (c) At long relaxation times, the values of H(τ) spectra of metallocene-catalyzed samples are significantly higher than those which correspond to a conventional sample of practically the same molecular weight. (d) Metallocene-catalyzed HDPEs are difficult to process, as sharkskin and slip-stick effects take place at very low shear rates. The onset of sharskin takes place at σc1 = 0.18 MPa, and the slip-stick regime occurs at σc2 = 0.25 MPa, independently of temperature. The values of the plateau modulus, G N° = 1.6 × 106 Pa, and the corresponding molecular weight between entanglements, M e = 1830, found for the metallocene-catalyzed materials, are very similar to those found for conventional polyethylenes. However, the activation energies of flow of the new polymers (7−9 kcal/mol) are slightly higher than those of conventional HDPEs.

A Comprehensive Review on Controlled Synthesis of Long‐Chain Branched Polyolefins: Part 3, Characterization of Long‐Chain Branched Polymers

“…Santamaría and co-workers [116] found that metallocene-catalyzed long-chain branched PEs have considerably higher G′(ω) and G″(ω) values than conventional linear PEs with similar MW. Ruymbeke et al [117] also reported that the experimental dynamic moduli are sensitive to very low level long-chain branches in PEs.…”

Correlations between the Shape of Viscosity Functions and the Molecular Structure of Long‐Chain Branched Polyethylenes

“…[6][7][8][9] For lower molar masses h 0 =h lin 0 < 1 might be observed. [10] Alternatively, it has been proposed that the dependence of the phase angle, d, on the magnitude of the complex modulus, jG Ã j, may be used for the detection of long-chain branches.…”

Polietileno: principais tipos, propriedades e aplicações

“…Devido a esse comportamento, os metalocenos dão origem a homopolímeros de etileno contendo ramificações longas, geradas a partir da incorporação de cadeias polietilênicas já terminadas. A presença dessas ramificações tem sido considerada a razão da boa processabilidade dos copolímeros de etileno-1-octeno produzido pela Dow Chemical [4,[35][36][37][38][39] .…”