eFunda: Comparisons between Polymers (Plastics) and Metals and among Polymers (Plastics)

Plastics vs. Metals

From an engineering point of view, plastics differ from metallic materials in the following ways:
-	Plastics tend to have pronounced non-linear time-dependent stress/strain behavior. Below the yield point, metals typically have elastic behavior which is nearly linear.
-	Instead of a single melting point, plastics typically have a Glass Transition Temperature (T_G). Metals typically have a clearly-defined melting point.
-	Plastics in general exhibit pronounced creep. Metals undergo creep to a certain extent, but for most engineering configurations their creep is insignificant.
-	Plastics tend to degrade or denature (due to heat) rather than corrode within a typical atmosphere. Of course, chemical degradation can occur when reactive chemicals are present. Plastics impregnated with organic fillers can be subject to bacterial infestation. Metals can corrode even in a benign atmosphere from reaction with oxygen and water, but are not affected by bacteria.

Plastics vs. Plastics

AMORPHOUS POLYMERS: e.g., PVC, Polystyrene, Acrylic, ABS, PPO, PC, Polyetherimide have
-	a wide melting range,
-	low shrinkage after molding,
-	better impact and lower chemical resistance than crystalline polymers,
-	moderate heat resistance,
-	dimensional stability,
-	and superior cosmetics of outer surfaces.

CRYSTALLINE POLYMERS: e.g., PPS, PBT (Valox), Polypropylene (PP), have
-	a sharply-defined melting point,
-	high shrinkage after molding,
-	low impact strength,
-	high heat resistance,
-	good fatigue endurance,
-	good lubricity; wear resistance,
-	good chemical resistance,
-	and the ability to flow in thin-walled sections.

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