Rheology and the breadmaking process

The aim of this paper is to investigate how the molecular structure of linear polypropylenes can be modified by electron beam irradiation. For this purpose a linear precursor was irradiated with different doses. The samples were analyzed by size-exclusion chromatography coupled with a light scattering detector. With increasing doses, a reduction of molar mass and an increasing number of long-chain branches were found. Moreover, conclusions with respect to the topography of the long-chain branches obtained were drawn from the molar mass dependence of the zero shear viscosity of the irradiated samples, which deviates significantly from that of linear polypropylenes. The experimental results can be interpreted in a way that at low doses very few but long branches occur. At higher doses more and shorter branches per molecule exist. The elongational experiments clearly exhibit a change of the strain-hardening behavior with irradiation dose, which is in agreement with the structural changes concluded from the s...

Long-Chain Branched Polypropylenes by Electron Beam Irradiation and Their Rheological Properties

Extensional viscosity of several polypropylene polymers and their blends was measured and the foam processing of these blends using carbon dioxide blowing agent was studied. Foaming was carried out on a co-rotating twin-screw extrusion line, with a gear pump to build pressure. A linear isotactic polypropylene and two branched polypropylenes were considered. The uniaxial extensional viscosity was quantified and the foam characterized based on bulk density, cell size, and cell concentration. The linear polymer exhibits no strain hardening, while both branched polymers show pronounced strain hardening. Blends of low concentrations of branched polymer in the linear polypropylene show significant strain hardening down to 10-wt% branched polypropylene. Strain hardening is expected to prevent cell coalescence and lead to higher cell concentrations. The branched polymers were found to have a lower cell concentration than the linear polymer. Yet blends of linear and branched polypropylenes attained a cell concentration higher than either of the neat polymers. This suggests that even small amounts of branched polypropylene blended in linear polypropylene can improve the foaming process. Polym. Eng. Sci. 44:2090–2100, 2004. © 2004 Society of Plastics Engineers.

Strain hardening in polypropylenes and its role in extrusion foaming

This book represents the first in a series entitled Polymeric Foams: Technology and Applications. Generally, thermoplastic and thermoset foams have been treated as two separate practices in industry. Polymeric Foams: Mechanisms and Materials provides the basics of foaming in general, providing a convenient and strong foundation to those working in both thermoplastic and thermoset foams. The book addresses scientific principles behind polymeric foaming and presents foaming chemistry and physics, resin and blowing agents, and foaming mechanisms in separate chapters, thus providing an overall and fundamental understanding of foaming for polymeric foam products and processes.

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Polymeric Foams : Mechanisms and Materials

Past and present developments in polymer bead foams and bead foaming technology

By generalizing the Doi-Edwards model to the Molecular Stress Function theory of Wagner and Schaeffer, the extensional viscosities of polyolefin melts in uniaxial, equibiaxial and planar constant strain-rate experiments starting from the isotropic state can be described quantitatively. While the strain hardening of four linear polymer melts (two high-density polyethylenes, a polystyrene and a polypropylene) can be accounted for by a tube diameter that decreases affinely with the average stretch, the two long-chain-branched polymer melts considered (a low-density polyethylene and a long-chain branched polypropylene) show enhanced strain hardening in extensional flows due to the presence of long-chain branches. This can be quantified by a molecular stress function, the square of which is quadratic in the average stretch and which follows from the junction fluctuation theory of Flory. The ultimate magnitude of the strain-hardening effect is governed by a maximum value of the molecular stress, which is specific to the polymer melt considered and which is the only free non-linear parameter of the theory.

The strain-hardening behaviour of linear and long-chain-branched polyolefin melts in extensional flows

The rheological characterisation of a high-density polyethylene is performed by means of measurements of the storage and loss moduli, the shear viscosity and the transient uniaxial elongational viscosity, the latter being obtained with the Meissner extensional rheometer. The rheological behaviour of the polymeric material is described by means of a multi-mode Phan Thien-Tanner fluid model, the parameters of which are successively fitted on the basis of the linear and non-linear properties. By using a semi-analytical technique and the finite element method, numerical investigations are performed for the shape recovery of the sample, and the predictions are compared with their experimental counterparts. Surface tension effects are also explored. We discuss the agreement between the experiments and the simulation results.

Rheological characterisation of a high-density polyethylene with a multi-mode differential viscoelastic model and numerical simulation of transient elongational recovery experiments

F. Langouche

Papers

The strain-hardening behaviour of linear and long-chain-branched polyolefin melts in extensional flows

Rheological characterisation of a high-density polyethylene with a multi-mode differential viscoelastic model and numerical simulation of transient elongational recovery experiments