Polymer Properties
Polymers are characterized in many ways - by chemical or physical structure, by strength or thermal performance, by optical or electrical properties, etc.
Most textbooks will give qualitative and some quantitative data on polymer properties. Properties can vary widely however, between manufacturers, for different performance grades, due to the presence of additives and reinforcements, or other reasons. For more precise data, contact a representative from a polymer producer, compounder, or distributor for a spec sheet on a particular material and grade. Often grades are offered to suit the needs of specific types of applications.
Properties of interest typically include:
Physical Properties
- Specific Gravity
- Mold Shrinkage (in flow, cross-flow, and thickness directions)
Mechanical Properties
- Strength (Tensile and Flexural)
- Modulus (Tensile and Flexural)
- Elongation
- Hardness
- Impact Resistance
Thermal Properties
- Heat Deflection Temperature
- VICAT Softening Temperature
- Glass Transition Temp
- Heat Capacity
- Thermal Conductivity
- Thermal Expansion Coefficient
Processing Characteristics
- Melt Flow Index
- Melt Strength
- Melting Point, No-flow Temp
- Shear Rate/Viscosity Relation
- Compressibility (Pressure/Volume/Temperature Relation)
Optical Properties
- Light Transmission
- Haze
- Refractive Index
Electrical Properties
- Surface and Volume Resistivity
- Dielectric Constant
- Dielectric Strength
- Dissipation Factor
- Breakdown Voltage
Environmental Properties
- Chemical Resistance
- UV Resistance
- Flame Resistance (UL Rating)
- Oxygen Index
- Water Absorption
Morphology
- Crystallinity
- Orientation
- Composition (Neat, Blended, Filled)
(http://www.lexmark.com/ptc/book6.html has a brief overview of properties for a number of commonly used polymers)
Classification of Polymers
There are many ways in which polymer properties or behavior are classified to make general descriptions and understanding easier. Some common classifications are:
Thermoplastic vs. Thermoset Polymers
"Thermoplastics" are materials which can be heated and formed, then re-heated and re-formed repeatedly. The shape of the polymer molecules is generally linear, or slightly branched, allowing them to flow under pressure when heated above the effective melting point.
"Thermoset" materials undergo a chemical as well as a phase change when they are heated. Their molecules form a three-dimensional cross-linked network. Once they are heated and formed they can not be reprocessed - the three-dimensional molecules can not be made to flow under pressure when heated.
Amorphous vs Crystalline Polymers
Polymers with nearly linear structure, which have simple backbones, tend to be flexible and fold up to form very tightly packed and ordered "crystalline" areas. Levels of crystallinity can vary from zero to near 100%. Time and temperature during processing influence the degree of crystallinity. Crystalline polymers include: polyethylene, polypropylene, acetals, nylons, and most thermoplastic polyesters. Crystalline polymers have higher shrinkage, are generally opaque or translucent, with good to excellent chemical resistance, low surface friction, and good to excellent wear resistance.
Polymers with bulkier molecular chains or large branches or functional groups tend to be stiffer and will not fold up tight enough to form crystals. These materials are referred to as "amorphous" polymers. Common amorphous polymers include polystyrene, polycarbonate, acrylic, ABS, SAN, and polysulfone. Amorphous polymers have low shrinkage, good transparency, gradual softening when heated (no distinct melting point), average to poor chemical resistance, high surface friction, and average to low wear resistance.
Addition vs. Condensation Polymers
Polymers such as nylons, acetals, and polyesters are made by condensation or step-reaction polymerization, where small molecules (monomers) of two different chemicals combine to form chains of alternating chemical groups. The length of molecules is determined by the number of active chain ends available to react with more monomer or the active ends of other molecules.
Polymers such as polyethylene, polystyrene, acrylic, and polyvinyl chloride are made by addition or chain-reaction polymerization where only one monomer species is used. The reaction is begun by an initiator which activates monomer molecules by the breaking a double bond between atoms and creating two bonding sites. These sites quickly react with sites on other monomer or polymer molecules. The process continues until the initiator is used up and the reaction stops. The length of molecules is determined by the number of monomer molecules which can attach to a chain before the initiator is consumed and all molecules with initated bonding sites have reacted.
Commodity, Engineering, and Performance Polymers
Commodity polymers have relatively low physical properties. They are used for inexpensive or disposable consumer or industrial products or packaging. They have limited stress and low temperature resistance, but are well suited to high volume production. Polyethylene, polystyrene, and polypropylene are good examples. In recent years, material suppliers have achieved improved strength and thermal properties from some commodity materials, displacing low-end applications for engineering polymers.
Engineering polymers have higher strength and thermal resistance. Their price may range from two to ten times as much as a commodity polymer. They are used in enclosures, structural frames and and load bearing members, and applications requiring wear resistance, long life expectency, flame resistance, and the ability to endure cyclic stress loading. Good examples are polyesters, polycarbonates, ABS, and acetal.
Performance polymers are at the highest end of the spectrum, with very high strength and thermal resistance. They tend to be very expensive, priced two to five times above most engineering polymers. They are used in high temperature, high stress applications, in harsh environments, and in generally low to medium volume production. Examples include PEEK, polyetherimides, and LCP's.