Chemistry Reference
In-Depth Information
materials for this application
[29]
. Cocrystallization is probably also a factor in
this case.
5.5.3.3
Prevention of Segregation
Once a satisfactory initial dispersion has been produced various operations can be
conducted to reduce or eliminate the rate of demixing. These are considered sepa-
rately below.
Cross-Linking. A thermoset system is produced when a polymer is cross-
linked under static conditions, as in a compression mold. This is the basis of the
production of vulcanized articles or cross-linked polyethylene pipe and wire insu-
lation. If the same polymer is lightly cross-linked while it is being sheared in the
molten state, however, it will remain thermoplastic. If it is more heavily cross-
linked during this process, the final product may contain significant quantities of
gel particles, but the whole mass will still be tractable.
This technique provides a method for incorporating fillers or reinforcing
agents into some polymers which ordinarily do not tolerate such additions. A high
loading of carbon black cannot normally be put into polyethylene, for example,
without serious deterioration of the mechanical properties of that polymer.
Various hydrocarbon elastomers will accept high carbon black contents. Such
black-loaded rubbers do not normally form stable mixtures with polyethylene, but
strong, permanent blends can be made by carrying out simultaneous blending and
cross-linking operations in an internal mixer. If conductive carbon black is mixed
carefully with a peroxide or other free radical source, rubber, and the polyolefin,
this technique can yield semiconductive compositions in hydrocarbon matrices.
When the peroxide decomposes it produces radicals that can abstract atoms from
the main chains of polymers. When the resulting macroradicals combine, the par-
ent polymers are linked by primary valence bonds.
The potential exists for chemical bonding of the two polymeric species in
such operations but it is not certain that this is always what happens. It is possible
in some instances that the stability of the mixture derives mainly from the entan-
glement of one polymer in a loose, cross-linked network of the other.
The “dynamic cross-linking” process is used to produce thermoplastic elasto-
mers from mixtures of crystallizable polyolefins and various rubbers. Variations
of basically the same method are employed to produce novel, stable polymer
alloys by performing chemical reactions during extrusion of such mixtures. In
that case, the current industrial term is reactive extrusion. Such processes are
used, for example, to improve processability of LLDPE's into tubular film (by
introducing long chain branches during extrusion with low levels of peroxides) or
to modify the molecular weight distribution of polypropylenes (again by extrusion
with radical-generating peroxides).
Interpenetrating Networks. Interpenetrating networks (IPNs) and related mate-
rials are formed by swelling a cross-linked polymer with a monomer and poly-
merizing and cross-linking the latter to produce interlocked networks. In
semi-interpenetrating systems, only the first polymer is cross-linked. Most of
