511 research outputs found
Extrusion compounding of polyethylene with blowing agent
Cellular plastics are very attractive for the production of lightweight, structural and/or large
dimension parts, e.g., boats, floaters, decks, etc. For their production, polymers and chemical
blowing agents are either mixed or compounded prior to processing by injection or rotational
moulding. It is essential to ensure good dispersion of the blowing agent in the polymeric matrix,
and prevent its activation from occurring during compounding, i.e., an optimal processing
window must be used. The aim of this work is to produce medium density polyethylene with
Azodicarbonamide (MDPE/ADCA) masterbatches in pellet form for further processing by
rotational moulding. For that purpose, a set of experimental procedures was conducted to
evaluate the correct processing window without premature expansion during extrusion. Upon
melt compounding of the masterbatch in pellets of different sizes, foamed parts were produced
and characterized in terms of visual aspect, expansion ability and morphology.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No. 734205 – H2020-MSCA-RISE-201
Film blowing of PHB-based systems for home compostable food packaging
One of the routes to minimize the environmental impact of plastics waste is the use of bio-sourced and biodegradable alternatives, particularly for packaging applications. Although Polyhydroxyalkanoates (PHA) are attractive candidates for food packaging, they have poor processability, particularly for extrusion film blowing. Thus, one relatively successful alternative has been blending PHA with a biodegradable polymer. This work proposes film blowing of a co-extruded Poly (hydroxybutyrate) (PHB) layer with a poly butylene adipate-co-terephtalate (PBAT) layer to enhance bubble stability, mechanical and barrier properties. Co-extrusion is detailed, together with the different strategies followed to improve adhesion between film layers and the PHB content in the films. Films with thicknesses below 50 micron and elongation at break beyond 500 % were consistently produced.This project has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No. 774088. LH also acknowledges
funding from the Portuguese Foundation for Science and Technology Investigator
Programme through grant IF/00606/2014
Numerical and experimental study of agglomerate dispersion in polymer extrusion
A model for agglomerate dispersion in screw extruders was developed and superimposed on the flow
patterns as simulated using the FIDAP software. A particle tracking algorithm with an adaptive time step was used to
follow the agglomerates trajectory. Along this flow path, the breakup probability was estimated using a Monte Carlo
method and in conjunction with the local fragmentation number. Particle size distributions and Shannon entropy were
computed along the screw channel. The results show good qualitative agreement between model predictions and
experimental data
The Radial Distribution of Magnetic Helicity in the Solar Convective Zone: Observations and Dynamo Theory
We continue our attempt to connect observational data on current helicity in
solar active regions with solar dynamo models. In addition to our previous
results about temporal and latitudinal distributions of current helicity
(Kleeorin et al. 2003), we argue that some information concerning the radial
profile of the current helicity averaged over time and latitude can be
extracted from the available observations. The main feature of this
distribution can be presented as follows. Both shallow and deep active regions
demonstrate a clear dominance of one sign of current helicity in a given
hemisphere during the whole cycle. Broadly speaking, current helicity has
opposite polarities in the Northern and Southern hemispheres, although there
are some active regions that violate this polarity rule. The relative number of
active regions violating the polarity rule is significantly higher for deeper
active regions. A separation of active regions into `shallow', `middle' and
`deep' is made by comparing their rotation rate and the helioseismic rotation
law. We use a version of Parker's dynamo model in two spatial dimensions, that
employs a nonlinearity based on magnetic helicity conservation arguments. The
predictions of this model about the radial distribution of solar current
helicity appear to be in remarkable agreement with the available observational
data; in particular the relative volume occupied by the current helicity of
"wrong" sign grows significantly with the depth.Comment: 12 pages, 8 Postscript figures, uses mn2e.cl
Global mixing indices for single screw extrusion
The present work presents numerical simulations of dispersive and distributive mixing of a two
phase system being processed in a single screw extruder under various conditions. Models were developed to
quantify the degree of mixing of liquid-liquid and solids-liquid systems, for a given set of material properties,
operating conditions and extruder geometry
Magnetic field generation in fully convective rotating spheres
Magnetohydrodynamic simulations of fully convective, rotating spheres with
volume heating near the center and cooling at the surface are presented. The
dynamo-generated magnetic field saturates at equipartition field strength near
the surface. In the interior, the field is dominated by small-scale structures,
but outside the sphere by the global scale. Azimuthal averages of the field
reveal a large-scale field of smaller amplitude also inside the star. The
internal angular velocity shows some tendency to be constant along cylinders
and is ``anti-solar'' (fastest at the poles and slowest at the equator).Comment: 12 pages, 11 figures, 2 tables, to appear in the 10 Feb issue of Ap
A Huygens principle for diffusion and anomalous diffusion in spatially extended systems
Magnetic Helicity Evolution During the Solar Activity Cycle: Observations and Dynamo Theory
We study a simple model for the solar dynamo in the framework of the Parker
migratory dynamo, with a nonlinear dynamo saturation mechanism based on
magnetic helicity conservation arguments. We find a parameter range in which
the model demonstrates a cyclic behaviour with properties similar to that of
Parker dynamo with the simplest form of algebraic alpha-quenching. We compare
the nonlinear current helicity evolution in this model with data for the
current helicity evolution obtained during 10 years of observations at the
Huairou Solar Station of China. On one hand, our simulated data demonstrate
behaviour comparable with the observed phenomenology, provided that a suitable
set of governing dynamo parameters is chosen. On the other hand, the
observational data are shown to be rich enough to reject some other sets of
governing parameters. We conclude that, in spite of the very preliminary state
of the observations and the crude nature of the model, the idea of using
observational data to constrain our ideas concerning magnetic field generation
in the framework of the solar dynamo appears promising.Comment: 10 pages, 3 Postscript figures, uses aa.cl
Solar Grand Minima and random fluctuations in dynamo parameters
We consider to what extent the long-term dynamics of cyclic solar activity in
the form of Grand Minima can be associated with random fluctuations of the
parameters governing the solar dynamo. We consider fluctuations of the
alpha-coefficient in the conventional Parker migratory dynamo, and also in
slightly more sophisticated dynamo models, and demonstrate that they can mimic
the gross features of the phenomenon of the occurrence of Grand Minima over a
suitable parameter range. The temporal distribution of these Grand Minima
appears chaotic, with a more or less exponential waiting time distribution,
typical of Poisson processes. In contrast however, the available reconstruction
of Grand Minima statistics based on cosmogenic isotope data demonstrates
substantial deviations from this exponential law. We were unable to reproduce
the non-Poissonic tail of the waiting time distribution either in the framework
of a simple alpha-quenched Parker model, or in its straightforward
generalization, nor in simple models with feedback on the differential
rotation. We suggest that the disagreement may only be apparent and is
plausibly related to the limited observational data, and that the observations
and results of numerical modeling can be consistent and represent physically
similar dynamo regimes.Comment: Solar Physics, in prin
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