Chemistry Reference
In-Depth Information
exclude other
routes to stabilization which may be practical under some
circumstances.
To illustrate this point consider the production of lacquers for PVC films and
sheeting. Such lacquers contain a PVC homopolymer or low-acetate vinyl
chloride-vinyl acetate copolymer, poly(methyl methacrylate), a plasticizer and
perhaps some stabilizers, dulling agents (such as silica), pigments, and so on.
Methyl ethyl ketone (MEK) is the solvent of choice because it gives the best bal-
ance of low toxicity, volatility, and cost. Any other solvent is effectively excluded
for a variety of reasons such as cost, inadequate volatility for coating machines
designed to dry MEK, unfamiliar odor, toxicity, and so on. Unfortunately, MEK
is really a poor solvent for this mixture. The solids concentrations required
for effective coatings result in a mucuslike consistency if the lacquer is
produced by conventional slow speed stirring and heating. The mixture is
very thixotropic and tends to form uneven coatings and streaks when applied by
the usual roller coating methods. For reasons listed above the addition of better
solvents is not an acceptable route to improvement of the quality of the coating
mixture.
A practical procedure is readily apparent, however, if one proceeds by steps 1
and 2 above. A good dispersion is first made by intensive mechanical shearing.
High energy mixers are available which can boil the solvent in a few minutes just
from the input of mechanical work. The solid ingredients are added slowly to the
initially cold solvent while it is being sheared in such an apparatus. This produces
a finely dispersed, hot mixture. It is not a true solution, however, and will revert
eventually to a mucuslike state. To retard this demixing process one can add a
small concentration of an inexpensive nonsolvent like toluene. This makes the liq-
uid environment less hospitable for the solvated polymer coils which shrink and
are thus less likely to overlap and segregate. The final mixture is still not
stable indefinitely, but it can be easily redispersed by whipping with an air mixer
at the coating machine.
Although the scientific principles behind this simple example of practical tech-
nology are easily understood, it illustrates the benefits that can be realized by con-
sidering the blending process as a dispersion operation that may be followed, if
necessary, by an operation to retard the rate at which the ingredients of the blend
demix. In special cases, of course, the latter operation may be rendered unneces-
sary by the selection of blend ingredients that are miscible in the first instance.
The basic requirements for achieving good dispersions of polymeric mixtures
have been reviewed elsewhere
[21
23]
and will not be repeated here in any
detail. Extruders and intensive mixers produce mainly laminar mixing in which
the interfacial area between components of the mixture is increased in proportion
to the total amount of shear strain which is imparted to the fluid substrate. Better
laminar mixing is realized if the viscosities of the components of the blend are
reasonably well matched. Such mixers operate by moving their inner metal sur-
faces relative to each other. Shear strain is imparted to the polymer mixture if it
adheres to the moving walls of the mixer. When the ingredients of the mixture
