David Midgley
Welch Fluorocarbon Inc.
Dover, N.H.

The uninitiated may find the expanding volume of data for engineered plastics challenging to sort through and a bit confusing. The pressure put on plastic suppliers to constantly develop new materials only makes the material-selection process more daunting. Designers must wade through an expanding array of improved mechanical and physical properties. They must also gain a good understanding of which process — injection molding, extruding, thermoforming, laminating, casting, and blow molding — will be the optimum choice for transforming a new polymer into the latest complex part.

No discussion about high-performance engineering plastics can begin without a basic understanding of plastics and how they are produced. The term plastic covers a wide range of materials that contain different combinations of carbon, hydrogen, nitrogen, oxygen, and other organic and inorganic compounds.

Plastic production begins by heating (cracking) hydrocarbons in natural gas or crude oil and breaking the raw material down into individual components called monomers. Monomers are the basic building blocks of plastics. Through the process of polymerization, monomers combine to create chemical chains called polymers. Polymerization usually takes place under heat and/or pressure.

The term homopolymer refers to a polymer made from a single monomer. Copolymers come from two different monomers. In a copolymer, the monomer chains can be random, alternating, or grafted. A terpolymer is the combination of three different monomers. Combining monomers in various combinations is key to producing the wide range of plastic resins, all with differing properties.

Polymers are typically divided into two major groups. The first, thermo-plastics, can be melted and reprocessed over and over again. The second, thermosets, can't be remelted after processing. Though thermoplastics and thermosets are different, they are characterized by their mechanical, thermal, physical, and electrical properties. Certain resins perform better than others with respect to these properties, thus giving rise to another classification of plastics:

Commodity plastics rank low with respect to certain physical properties. They find use in disposable products that require low-cost and high-volume. Examples include poly-ethylene (PE), polypropylene (PP), and polystyrene (PS).

Engineering plastics are better performing polymers that typically carry a higher price tag and include acrylic (PMMA), polycarbonate (PC), and nylon (PA).

High-performance plastics are the highest-ranking polymers with respect to physical properties. These plastics operate over a wide thermal range and have excellent chemical, electrical, and moisture barrier properties. They include fluoropolymers that contain molecules of carbon and fluorine, which dramatically boost chemical and solvent resistance as well as no-stick properties. They also have extremely high working temperatures and electrical resistance. It's not uncommon to see high-performance polymers converted into films, sheets, tubing, rods, and bar stock shapes with a sticker price upwards of $75 to $100/lb.

STATE OF THE INDUSTRY
Today's polymer chemistries focus on creating superior materials by blending compatible polymers, fillers, and additives that change the physical properties of the base material. The generally accepted list of high-performance plastics includes the following base materials:

ECTFE (ethylene-ETFE (ethylene-tetrafluoroethylene) chlorotrifluoroethylene)
FEP
(fluorinated ethylene propylene)
LCP (liquid-crystal polymer)
PAI
(polyamide-imide)
PCTFE (polymonochloro-PEEK
(polyetherether ketone) trifluoroethylene)
PEI
(polyetherimide)
PFA
(perfluoroalkoxy)
PI (polyimide)
PPS
(polyphenylene sulfide)
PSU
(polysulfone)
PTFE (polytetrafluoroethylene)
PVDF
(polyvinylidene fluoride)
PVF (polyvinylfluoride)

The selection of a material for a specific end use begins with a complete understanding of the application and the material requirements. Material properties — mechanical, thermal, electrical, chemical — are typically where designers miss opportunities. There's often preconceived notions of what materials they "need" based on an incomplete knowledge of the breadth of materials available.

Another consideration that helps narrow the field of potential polymers is how the part will be fabricated: thermo-formed, injection molded, rotationally molded, machined, etc. Outside expertise can help. Material suppliers have specific processing data for their resins and may be able to share past product-design examples that can help shorten the learning curve.

The pace at which new polymers and coextrusions are developing makes certain designs obsolete in short order. Thankfully, there are film manufacturers such as the Ajedium Film Group LLC, Newark, Del., that can develop exotic films for specific applications and produce small film runs for faster prototyping. Material manufacturers are a good resource for determining the best material and manufacturing options. All high-performance manufacturers of film, for example, have a list of approved converters for their films. A quick call to the applications engineering department can help pinpoint the person able to answer technical questions and suggest possible alternatives.

HIGH-PERFORMANCE PLASTICS COMPARATIVE CHART
 
MECHANICAL
THERMAL
ELECTRICAL
CHEMICAL
ECTFE
Excellent (resists abrasions)
Better (maximum continuous use at 150°C)
Excellent
Excellent
ETFE
Excellent (high tensile and impact strength)
Better (–100 to 150°C)
Excellent
Good
FEP
Good (dimensional stability and low creep resistance)
Excellent(–190 to 205°C)
Better
Excellent
LCP
Excellent
Excellent
Good
Excellent
PAI
Excellent
Excellent (continuous use at 250°C)
Excellent
Better
PCTFE
Excellent (high creep resistance)
Better (–250 to 150°C)
Good
Excellent
PEEK
Excellent (especially at high temperatures)
Excellent (continuous use at 250°C)
Excellent
Better
PEI
Excellent (low shrink)
Better (continuous use at 180°C)
Better
Excellent
PI
Excellent (high tensile strength)
Excellent (especially at high temperatures)
Excellent
Better
PFA
Better (low creep and abrasion resistance)
Excellent (–150 to 260°C)
Better
Excellent
PTFE
Good (dimensional stability and low creep resistance)
Excellent (–180 to 260°C)
Excellent
Excellent
PPS
Excellent
Better (continuous use at 200°C)
Better
Excellent
PSU
Better (retains properties over a wide thermal range)
Better (continuous use at 200°C)
Good
Better
PVDF
Excellent (resists abrasions)
Good
Good
Good

MAKE CONTACT:
Ajedium Film Group LLC,
(302) 452-6609, www.ajedium.com
DuPont, (800) 441-7515, dupont.com
Honeywell, (800) 934-5679, honeywell-specialtyfilms.com
Saint Gobain Performance Plastics, (303) 562-9111, plastics.saint-gobain.com
The Society of the Plastics Industry,
socplas.org
Welch Fluorocarbon Inc.,
(603) 742-0164, welchfluorocarbon.com