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handbook of modern sensors

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18.1 Materials 539<br />

The following is a nonexhaustive list <strong>of</strong> thermoplastics:<br />

ABS (acrylonitrile–butadiene–styrene) is very tough, yet hard and rigid. It has fair<br />

chemical resistance, low water absorption, and good dimensional stability. Some<br />

grades may be electroplated.<br />

Acrylic has high optical clarity and excellent resistance to outdoor weathering. This<br />

is a hard, glossy material with good electrical properties. It is available in a variety<br />

<strong>of</strong> colors.<br />

Fluoroplastics comprise a large family <strong>of</strong> materials (PTFE, FEP, PFA, CTFE,<br />

ECTFE, ETFE, and PFDF) with excellent electrical properties and chemical<br />

resistance, low friction, and outstanding stability at high temperatures. However,<br />

their strength is moderate and the cost is high.<br />

Nylon (polyimide) has outstanding toughness and wear resistance with a low coefficient<br />

<strong>of</strong> friction. It has good electrical and chemical properties. However, it is<br />

hygroscopic and dimensional stability is worst than in most other plastics.<br />

Polycarbonate has the highest impact resistance. It is transparent with excellent<br />

outdoor stability and resistance to creep under load. It may have some problems<br />

with chemicals.<br />

Polyester has excellent dimensional stability but is not suitable for outdoor use or<br />

for service in hot water.<br />

Polyethylene is lightweight and inexpensive with excellent chemical stability and<br />

good electrical properties. It has moderate transparency in the broad spectral<br />

range from visible to far infrared; it has poor dimensional and thermal stability.<br />

Polypropylene has outstanding resistance to flex and stress cracking with excellent<br />

chemical and electrical properties with good thermal stability. It is lightweight<br />

and inexpensive. Optical transparency is good down to the far-infrared spectral<br />

range. However, absorption and scattering <strong>of</strong> photons in the mid-infrared range<br />

is higher than in polyethylene.<br />

Polyurethane is tough, extremely abrasion, and impact resistant. It can be made into<br />

films and foams. It has good chemical and electrical properties; however, UV<br />

exposure degrades its quality.<br />

Another type <strong>of</strong> plastic is called thermoset, in which polymerization (curing) is<br />

done in two stages: one by the material manufacturer and the other by the molder.<br />

An example is phenolic, which during the molding process is liquefied under pressure,<br />

producing a cross-linking reaction between molecular chains. After it has been<br />

molded, a thermoset plastic has virtually all <strong>of</strong> its molecules interconnected with<br />

strong physical bonds, which are not heat reversible. In effect, curing, a thermoset is<br />

like cooking an egg. Once it is cooked, it will remain hard. In general, thermoset plastics<br />

resist higher temperatures and provide greater dimensional stability. This is the<br />

reason why such thermoset plastics such as polyester (reinforced) is used to make boat<br />

hulls and circuit-breaker components, epoxy is used to make printed circuit boards,<br />

and melamine is used to make dinnerware. On the other hand, thermoplastics <strong>of</strong>fer<br />

higher impact strength, easier processing, and better adaptability to complex designs<br />

than do thermosets.

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