30 research outputs found
Polymer rheology for melt processing : complex flows
No full textThe rheological behavior of polymer melts plays an important role during processing since it not only determines the extra stresses and pressure gradients and can reduce flow instabilities that yield process limits, but it also couples material, process, and production properties. Examples of (final) product properties determined by the process are frozen-in orientation, the structure of semicrystalline polymers, and long-term dimensional stability. For quantitative predictions of these qualities, the availability of reliable constitutive models is a prerequisite. Here, the performance of a new generation of constitutive differential equations (Verbeeten et al. 2000 W.M.H. Verbeeten, G.W.M. Peters and F.P.T. Baaijens, Differential constitutive equations for polymer melts Proc. XIII Int Congress on Rheology, British Society of Rheology, Glasgow, UK (2000).Verbeeten et al. 2000) based on the Pompom model (McLeish and Larson 1998) is presented
Thermodynamic admissibility of the extended Pom-Pom model for branched polymers
No full textThe thermodynamic consistency of the eXtended Pom-Pom (XPP) model for branched polymers of Verbeeten et al. [W.M.H. Verbeeten, G.W.M. Peters, F.P.T. Baaijens, Differential constitutive equations for polymer melts: the extended pom-pom model, J. Rheol. 45 (4) (2001) 823–843; W.M.H. Verbeeten, G.W.M. Peters, F.P.T. Baaijens, Differential constitutive equations for polymer melts: the extended pom-pom model (vol 45, pg 823–843, 2001), J. Rheol. 45 (6) (2001) 1489] as well as its modified version [J. van Meerveld, Note on the thermodynamic consistency of the integral pom-pom model, J. Non-Newtonian Fluid Mech. 108 (1–3) (2002) 291–299] is investigated from the perspective of non-equilibrium thermodynamics, namely the General Equation for Non-Equilibrium Reversible–Irreversible Coupling (GENERIC) framework. The thermodynamic admissibility of the XPP model is shown for both its original and modified form. According to the GENERIC formalism, the parameter a introduced by Verbeeten et al. to predict non-zero second normal stress in shear flows must fulfill the condition 0 = a = 1
Fibre fragmentation in multi-fibre micro and hybrid composites:the influence of fibre surface treatment
No full textAnisotropy parameter restrictions for the eXtended Pom-Pom model
No full textA significant step forward in modelling polymer melt rheology has been the introduction of the Pom-Pom constitutive model of McLeish & Larson [J. Rheol., 42(1):81¡§C110, 1998]. Various modifications of the Pom-Pom model have been published over the years in order to overcome several inconveniences of the original model. Amongst those modified models, the eXtended Pom-Pom (XPP) model of Verbeeten et. al. [J. Rheol., 45(4):823-843, 2001] has received quite some attention. However, the XPP model has been criticized for the generation of multiple and unphysical solutions. This paper deals with two issues. First, in the XPP model, anisotropy is implemented in a Giesekus-like manner which is known to result in unphysical solutions for non-linear parameter values ¦Á ¡Ý 0.5. Hence, we put forward the conjecture that a similar limitation holds for the XPP model. In the present paper, the limits for the anisotropy parameter are elaborated on and result to be most restraining at high deformation rates where the backbone tube is oriented and the backbone tube stretch approaches the number of arms q. By restricting the anisotropy parameter to a maximum critical value the XPP model produces only one solution, which is the correct physical rheology. In the second part we show that, contrary to the results published by Inkson and Phillips [J. Non-Newton Fluid, 145(2-3):92-101, 2007], for the special case where the anisotropy parameter equals zero, only one physically relevant solution exists in unaxial extensional. In addition to this physically relevant solution, also solutions exist in the physically unattainable part of the conformation space. However, the existence of these physically unattainable solutions is not a unique feature of the XPP model but rather general for non-linear differential type rheological equations
Analysis of viscoelastic polymer melt flow
No full textIn recent years significant progress has been made in the experimental and numerical analysis of viscoelastic polymer melt flow. From the numerical point of view a number of well established mixed finite element formulations have been developed that are sufficiently stable, robust and efficient to analyze a range of two and three dimensional flow problems.
Typical examples are the so-called DEVSS/DG and DEVSS/SUPG methods. Experimentally, an increasing variety of flow geometries has become available, satisfying requirements such as near twodimensionality of the flow (i.e. a depth-to-height aspect ratio of at least 8 in the whole flow domain).
The measurement resolution of fringe patterns has improved significantly by the application of lasers
such that accurate measurements can be made at critical locations in the geometry. Furthermore,
the use of Mueller calculus allows the prediction of the observable fringe patterns in three dimensional flow geometries.
Crucial to the quantitative prediction of velocity and stress fields in inhomogeneous flows is the availability of reliable constitutive models. Within the class of differential models, the multi-mode versions of the Giesekus and PTT model have been the most widely applied. Recently, a new generation of models [3] has been developed based on the concepts of the Pompom model. These Pompom based models are a major step forwards in describing the rheology of (polyethylene) melts.
The objective of this contribution is to investigate the performance of an enhanced, single equation
version of the Pompom model in predicting the inhomogeneous flow of an LDPE melt in the
cross-slot geometry. This particular version quantitatively predicts available rheometrical shear and
elongational data of a set of polymer melts, see Verbeeten et al
Yield stress distribution in injection-mouldedglassy polymers
No full textA methodology for structural analysis simulations is presented that incorporates the distribution of mechanical propertiesalong the geometrical dimensions of injection-moulded amorphous polymer products. It is based on a previously developedmodelling approach, where the thermomechanical history experienced during processing was used to determine the yield stressat the end of an injection-moulding cycle. Comparison between experimental data and simulation results showed an excellentquantitative agreement, both for short-term tensile tests as well as long-term creep experiments over a range of strain rates,applied stresses, and testing temperatures. Changes in mould temperature and component wall thickness, which directly affectthe cooling profiles and, hence, the mechanical properties, were well captured by the methodology presented. Furthermore, itturns out that the distribution of the yield stress along a tensile bar is one of the triggers for the onset of the (strong) localizationgenerally observed in experiments.Spanish Government (Ministry of Sci-ence and Innovation, Ministry of Economy and Competitiveness)through grant numbers RYC-2010-07171 and DPI2011-25470This is the peer reviewed version of the following article: Verbeeten, W. M., Kanters, M. J., Engels, T. A. and Govaert, L. E. (2015), Yield stress distribution in
injection-moulded glassy polymers. Polym. Int., 64(11): 1527–1536, which has been published in final form at http://dx.doi.org/10.1002/pi.4898. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archivin
The Impact of Business Unit Strategy, Structure and Technical Innovativeness on Change in Management Accounting and Control Systems at the Business Unit Level: An Empirical Analysis
No full textThis study tests the hypothesis that business unit strategy and business unit structure affect change in a business unit's Management Accounting & Control System (MACS). Business units are parts of a larger 'whole' organization, such as departments, teams, strategic groups or divisions. Change in MACS has been assessed by asking managers to estimate the number of changes that has taken place in their business unit's MACS over a two-year time period. Using data from a survey amongst 61 business unit managers in the Netherlands, the study suggests that the costing & transfer pricing and reward system are relatively 'resistant to change'. The results also indicate that the administrative capacity of a business unit is the main driver of change in MACS. Finally, business unit strategy and business unit structure affect change in specific components of MACS at the business unit level, apparently depending on whether the change in MACS facilitates or influences managerial decisions. [ABSTRACT FROM AUTHOR
