Chemistry Reference
In-Depth Information
5.5
Practical Aspects of Polymer Blending
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Polymer blends have become a very important subject for scientific investigation
in recent years because of their growing commercial acceptance. Copolymerization
and blending are alternative routes for modifications of properties of polymers.
Blending is the less expensive method. It does not always provide a satisfactory
alternative to copolymerization, of course, but polymer blends have been success-
fully used in an increasing number of applications in recent years. Such successes
encourage more attempts to apply this technique to a wider range of problems in
polymer-related industries.
5.5.1
Objectives in Making Blends
It is usually, but not always, desired to make a blend whose properties will not
change significantly during its normal usage period. Unstable blends are some-
times required, however. An example is the use of slip agents (surface lubricants)
in polyolefin films. The additives must be sufficiently compatible with the host
resin not to exude from the polymer melt onto the extruder barrel walls during
film extrusion. If the slip agent migrated to this boundary the resin would turn
with the screw and would not be extrudable. The slip agent must exude from the
solid polymer, however, since its lubrication function is exercised only on the sur-
face of the final film. Amides of long chain fatty acids have the right balance of
controlled immiscibility for such applications in polyolefin plastics. Lubricants
and antistatic agents are other examples of components of blends that are not
designed to be stable.
This section will concentrate on stable blends since these are of greater gen-
eral interest. It should be noted that stability in this context does not necessarily
imply miscibility or even that the mixture attains a state of thermodynamic equi-
librium during its useful lifetime. More generally, all that is required is that the
components of the mixture adhere to each other well enough to maintain an ade-
quate mechanical integrity for the particular application and that this capacity be
maintained for the expected reasonable lifetime of the particular article.
5.5.2
Blending Operations
The manufacture of useful, stable blends involves two major steps: (1) The com-
ponents are mixed to a degree of dispersion that is appropriate for the particular
purpose for which the blend is intended; (2) additional procedures are followed, if
necessary, to ensure that the dispersion produced in step 1 will not demix during
its use period.
Note that it is useful to consider step 2 as a problem involving retardation of a
kinetic process (demixing). The viewpoint that focuses on blending as a problem
in thermodynamic stability is included here as a special case but should not
