Thermoplastics 201

Proper selection of a thermoplastic material for a given application requires assessment of the environment the material will be exposed to. This includes:

• Chemical exposure
• Temperature range
• Environmental exposure (UV, weather)
• Mechanical stresses (flexion, tension, impact)

The selected material has to be compatible with the manufacturing process (extrusion, molding, thermoforming) and any secondary processes involved (bonding, printing, cutting). Other considerations that factor into the decision include:

• Required dimensional accuracy and stability
• Cost
• Regulatory (FDA, NSF, USP)
• Product lifetime and disposal

Thermoplastic materials can also be categorized in terms of their morphology, which can either be amorphous or semi-crystalline.

The polymer chains in amorphous thermoplastics such as polystyrene, polycarbonate, and polysulfone have a randomly order structure. The thermal properties of these materials are generally characterized by a glass transition temperature (T_g). Below its Tg an amorphous thermoplastic is a rigid glass. Above its Tg an amorphous thermoplastic will transition to a leathery solid and then gradually soften as temperature is increased.

The polymer chains in semi-crystalline thermoplastics such as polypropylene, polyamides, and PEEK are able to organize to a degree into tightly-packed, ordered structures or crystals. These crystalline structures will exhibit a sharp melting point. Polymer chains and segments that aren’t able to crystallize will remain amorphous. The amorphous phase of the material will often exhibit a glass transition temperature. At its melting point, a semi-crystalline thermoplastic will become a low viscosity melt.

A thermoplastics morphology type has significant influence on its properties and, therefore, is a key consideration when selecting the proper thermoplastic for a given application. The following table gives examples and general characteristics of each morphology. While each individual thermoplastic is unique and every application is different, these general characteristics are useful in assessing which morphology class is best suited for a given application.

Morphology	Example thermoplastics	General Characteristics
Amorphous	PS, PC, PSU	Soften above Tg Transparent/translucent Low shrinkage
		Dimensional accuracy Dimensional stability Good toughness Melt strength for extrusion, thermoforming Good bondability
Semi-crystalline	PP, PA, PEEK	Sharp melting point Usually Opaque Higher shrinkage Chemical resistance Wear resistance Load bearing and fatigue resistance Good flow characteristics for injection molding Lower bondability

Thermoplastic materials can also be formulated with a variety of additives in order to meet application requirements. Examples of such additives include UV stabilizers, antioxidants, antimicrobials, antistats, reinforcements, colorants, and processing aids.

Considerations unique to medical applications such as medical tubing include regulatory restrictions on base resin and additive selection and compatibility with sterilization (autoclave, gamma, EtO). Medical materials often will have to pass specific biological testing such as cytotoxicity or hemotoxicity and have resistance and compatibility to biological and medical fluids.