What is Polymer Processing?.....page 2



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:

Performance
Characteristic
Comments
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.

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.

Performance
Characteristic
Material
Contribution
Process
Contribution
High strength/weight ratio The composite incorporates high strength carbon fibers in a matrix of a low density (compared to metals) epoxy. 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.
Small radar signature. The materials have low radar reflection compared to metals, which can be further improved by the use of additives. The hand lay-up process is extremely flexible, allowing for the design and manufacture of shapes with low radar reflectance.
Temperature resistance. 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. Shaping of the part prior to crosslinking (curing) allows forming of a part which is eventually highly crosslinked.
Chemical resistance. Since the epoxy is crosslinked, it is very resistant to chemical pentration. It is difficult to swell and cannot be dissolved. Shaping of the part prior to crosslinking (curing) allows forming of a part which eventually is highly crosslinked.

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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