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Definition
There are many potential definitions of polymer processing.
One of my favorites is that given by Tadmor and Gogos in their
classic textbook: polymer processing concerns "operations
carried out on polymeric materials or systems to increase
their utility." [1] Personally, I prefer the use of the
word "value" in place of "utility." The
goal of polymer processing is to increase the value of the
polymer or formulation. I view polymer processing as a tool
to achieve the desired shape, properties, and performance
for a polymer article. It has broad applicability from packaging
to aerospace.
Performance
It is the performance of a material which generates value.
Thus, for each particular application, one should ask:
"What are the critical performance characteristics for
this application?"
"How can this performance be achieved by a combination
of the material and the process?"
Let's take a particular example, the body and wings of the
F-117A stealth fighter/bomber. What are the critical performance
characteristics for this application? A few of them are listed
in the table below:
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Performance Characteristic
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Comments
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| High
strength/weight ratio |
Strong
and light materials for aerospace. |
| Small
radar signature. |
Must
be a "stealthy" aircraft. |
| Temperature
resistance. |
Hot engines
and exhaust gases. |
| Chemical
resistance. |
Intermittent
contact with rain, jet fuel, etc. |
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How are these characteristics achieved? The body and wings
of these stealth aircraft are constructed primarily of an
epoxy/carbon fiber composite which is shaped and manufactured
using a process called hand lay-up. For example, the starting
material may be preforms in the shape of flat sheets which
consist of uniaxial carbon fibers impregnated with a low or
medium molecular weight epoxy. These sheets are stacked and
formed in the desired shape of the wings and/or body and then
cured to polymerize the epoxy. Both the materials used and
the processing operation are critical for achievement of the
desired performance.
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Performance
Characteristic
|
Material
Contribution
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Process
Contribution
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High strength/weight ratio
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The composite incorporates high
strength carbon fibers in a matrix of a low density
(compared to metals) epoxy.
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The hand lay-up process allows
for detailed control of fiber orientation at different
locations on the part. The fiber orientation can
be optimized in order to support the loads which
are expected to be generated during flight.
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Small radar signature.
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The materials have low radar reflection
compared to metals, which can be further improved
by the use of additives.
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The hand lay-up process is extremely
flexible, allowing for the design and manufacture
of shapes with low radar reflectance.
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Temperature resistance.
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Since the epoxy is crosslinked,
it has very high dimensional stability. In fact,
this material will thermally degrade before it
begins to soften and flow.
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Shaping of the part prior to crosslinking
(curing) allows forming of a part which is eventually
highly crosslinked.
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Chemical resistance.
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Since the epoxy is crosslinked,
it is very resistant to chemical pentration. It
is difficult to swell and cannot be dissolved.
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Shaping of the part prior to crosslinking
(curing) allows forming of a part which eventually
is highly crosslinked.
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There is one key characteristic which we have not yet discussed.
The MOST important property of ANY material is its PRICE.
The MOST important property of ANY process is its COST.
In this sense the stealth aircraft is a poor example. High
performance epoxy/carbon composites are very expensive. In
addition, the hand lay-up process is very expensive because
it requires a large amount of highly skilled labor. In this
case, high costs are justified, due to the concurrent high
costs of the engines and electronics on the aircraft. In addition,
consider the military and political costs of having a pilot
shot down behind enemy lines.
As an exercise, list the critical performance characteristics
of another polymer product, and consider how these are achieved
by a combination of the material and processing. Choose something
that you are interested in. How about
a standard two liter soft drink bottle?
a compact disk?
an optical fiber?
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Technical
Papers 
