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Encyclopedia of Physical Science and Technology EN012c-593 July 26, 2001 15:56

Polymer Processing 625

mixing. This diffusion is driven by the chemical potential conditions such as temperature, speed of rotating parts,
difference due to concentration variation, and it is a very flowing velocity, and residence time are the important fac-
slow process, because its timescale is proportional to the tors which determine the relative strengths of the mixing
value of the diffusion coefficient. Thus, this mechanism mechanisms. As a consequence, this relative strength af-
becomes important in gases and low molecular weight, fects the efficiency of mixing and the quality of the prod-
miscible liquid systems, although there are timescale dif- uct. In almost all cases, both good distribution and good
ferences in those two cases. dispersion are required. In some cases, only distributive
The major difference between mixing in general and in mixing can be tolerated if the next step offers dispersive
polymer processing stems from the fact that the viscosity characteristics and, respectively, dispersive mixing is used
2
ofpolymermeltsisusuallyhigherthan10 Pasec,andthus when a finely dispersed mixture is required and when the
mixing takes place in the laminar regime only (Re < 2000; next step does not offer any dispersion characteristics.
to achieve such a number the polymer would have to flow Some of the nomenclature mentioned above gets a spe-
down a 1-m-wide channel at a velocity of 20 cm/sec). This cific connotation when referred to polymer processing,
has a severe consequence: the lack of eddy and molecu- and thus we will give here some specificdefinitions. Com-
lar diffusion, which greatly enhance the rate of mixing pounding refers to the process of softening, melting, and
and reduce the scale of homogenization. Thus, all mixing compaction of the polymer matrix and dispersion of the
theories and practices should be adjusted to the laminar additive into that matrix. Blending refers to all processes in
regimetofindapplicabilityinthepolymerprocessingarea. which two or more components are intermingled without
This remark applies also to solid–solid mixing in polymer significant change of their physical state. Finally, knead-
processing, but it does not find application to the addition ing refers to mixing achieved by compression and folding
of low-molecular-weight substances into polymers, like of layers over one another; milling refers to a combination
dyes, where molecular diffusion plays a role. of smearing, wiping, and possibly grinding, and mulling
Two basic types of mixing can be identified as exten- refers to wiping and rolling actions.
sive and intensive mixing. Distributive, convective, repet- Dispersive mixing is the term used to describe mixing
itive, simple mixing, and blending are the main names associated with some fundamental change of the physical
that extensive mixing is also associated with, whereas characteristics of one or more of the components of the
compounding, dispersive, and dispersing mixing are the mixture. Generally, dispersive mixing is divided into two
corresponding names associated with intensive mixing. parts: The first part is the incorporation of the additives
Extensive mixing refers to processes that reduce the in terms of agglomerated particles or the second polymer
nonuniformity of the distribution (viewed on a scale larger component into the polymer matrix, and the second part is
than the size of the distributed components) of the minor the dispersion (or deagglomeration) of the second phase to
into the major component without disturbing the initial yield the final product. The microstructures of the blends
scale of the minor component. It can be achieved through are determined by rheological, hydrodynamic, and ther-
two mechanisms: rearrangement and deformation in lami- modynamic parameters. The rheological parameters are
nar flow. Also, deformation achieved in shear, elongation, viscosity, elasticity, and yield stress of all components.
and squeezing flows plays a major role in distributing the The hydrodynamic parameters determine the flow fields.
minor component. The thermodynamic parameters are related to solubility,
The term intensive mixing refers to processes that break adhesion, and diffusion of all components.
down the liquid dispersed phase or the initial particle ag- This type of dispersion has been applied in the polymer
glomerates, and they decrease the ultimate particle of the processing industry for at least 50 years. It is concerned
dispersion. A typical example is the dispersion of agglom- with the incorporation and deagglomeration of additives
erates of colloidal carbon black particles in PE. In this case in the polymer matrix with the ultimate goal being the
the initial ultimate particle is the agglomerate, and the final reduction of the price or the improvement of the proper-
is the particle itself. Another example is the dispersion of ties of the final product. Of course, if the additive exists
a polymer into another polymer where the minor compo- in the form of isolated noninteracting particles then the
nent should be dispersed into small droplets or elongated task of mixing is only to distribute these particles uni-
fibers (both of them have a length scale of about 10 µm). formly throughout the final product. However, when the
The analysis of dispersive mixing follows the lines of the additive exists in the form of clusters of particles (interact-
analysis of the distributive mixing with the complication ing or noninteracting), then dispersive mixing ensures that
that the breakup forces should now be included. the agglomerates break into isolated particles, which then
The geometry of the mixing equipment; physical pa- should be distributed by extensive mixing mechanisms.
rameters such as viscosity, density, interfacial tension, The size of the particles as well as their ability to in-
elasticity, and attractive forces for solids; and operating teract with each other characterize the type of cluster as