Because the range of performance requirements is so broad, Glenair manufactures and stocks an equally broad range of wire enclosures: from plastic convoluted tubing to various forms of metal-core conduit. Understanding the performance properties of the materials which go into these products is the key to providing outstanding service for our conduit customers. In this issue of QwikConnect we present a complete overview of one of the most important materials used in the manufacture of protective wire enclosures: plastic.

From Monomers to Polymers

Glenair's plastic convoluted tubing is available in the widest range of materials and constructions in the industry - the right weight, duty, and protection level for every application. The Series 74 (helical) convoluted tubing, for example, is available in PTFE, Tefzel (regular and irradiated), PFA, FEP, Kynar and PEEK. The tubing is fabricated in annular, helical, close convolutions, thin wall, dual wall and reinforced versions and can be supplied in random lengths (Glenair Standard Packaging) as well as specific lengths or spools.

The various plastic materials used in the manufacture of the tubing are referred to as polymers. Polymers are carbon based compounds built up from a series of smaller units, or monomers. Which monomers are used, how they are joined together (rings, strings, etc.) and the overall molecular weight of the compound are the three variables which determine the mechanical and physical properties of the resultant material. Polyether-etherketone (PEEK), for example, consists of ether, ketone and phenyl monomers. The ability to form PEEK into specific shapes using heat is a property provided by the ether groups. PEEK's high temperature tolerance and excellent mechanical strength are due to the structure of the ketone and phenyl groups.

Every one of the of the plastic polymers used in the fabrication of convoluted tubing has unique properties due to its distinctive molecular make-up. The following summaries provide an overview of material properties for all of the commercial polymers used in Glenair's line of plastic convoluted tubing:

Kynar- Kynar is the trade name for polyvinylidene, an opaque black, thermally stable material used in the standard manufacture of annular plastic tubing. It is highly resistant to UV light and gamma radiation. Kynar® is extremely resistant to oxidizing agents and halogens and is almost completely resistant to allphatic, aromatics, alcohols, acids and chlorinated solvents. Kynar is mechanically strong, abrasion resistant and features high dielectric strength. Kynar is also thermally stable, resistant to low temperatures, self-extinguishing and nontoxic.

PTFE- Polytetrafluoroethylene, one of the more expensive polymer tubings, provides one of the highest temperature ratings available in an exotic plastic (260°C). PTFE is inert to virtually all chemicals and insoluble in all solvents below 300°C. PTFE is considered the most slippery material in existence and has very low friction and adhesiveness coefficients. PTFE's moisture retention is less than 0.01% and it has an infinite shelf life. The material also has very good electrical (high resistance) and dielectrical (very low dielectric number and loss factor) properties.

ETFE- Also known by the trade name Tefzel, ethylene tetraflouroethylene provides the highest tensile strength and lubricity available. It is produced in opaque black and transparent clear. ETFE resin is a meltcut thermoplastic which combines mechanical toughness with outstanding chemical, dielectric and thermal properties. Its other main features include ease of processing, a lower density than Teflon® and improved radiation resistance. ETFE is suitable for continuous exposure at a maximum service temperature of 155°C.

FEP and PFA- Perfluoralkoxy (PFA), and fluorinated ethylene propylene (FEP) are two of the lower cost materials to provide relatively high thermal stability. These Teflon® copolymers provide continuous service temperatures up to 260°C for PFA and 205°C for FEP. Both have excellent dielectric properties and are unaffected by virtually all solvents and chemicals. As with PTFE, they also have excellent low temperature toughness and good resistance to adhesion.

PEEK- Polyetheretherketone is halogen free, extremely light weight and crush resistant. PEEK is semi-crystalline, with both crystalline and amorphous regions. The material is very tough under an extreme range of conditions. Melting occurs at about 340° C, continuous use is rated at 250° C, with short-term excursions to 300° C, or 572° F. PEEK far outperforms other plastics in this regard, and has a V-O flammability rating down to 0.057 inches without the use of additives. The mechanical and high temperature properties of this material make processing and manufacturing of PEEK tubing more costly than other polymers. PEEK combines strength, toughness, chemical resistance, purity, lubricity/wear resistance, and USP Class VI biocompatibility. This material maintains its properties under sustained pressure, elevated temperature, water, steam, and most chemical environments.

Gamma Processed ETFE- Ethylene tetrafluoroethylene provides a durable, highly flexible protective enclosure for wiring systems used in harsh application environments. The material provides exceptional flexibility in cable routing and electrical system architecture and has a higher operating temperature range than non-processed ETFE of -55° C to 200° C. The tubing also withstands temperature excursions to 300° C and is both crush resistant and cut resistant at elevated temperatures. The irradiated material provides outstanding resistance to hydraulic fluids, fuels, lubricating oils, solvents, and decontaminating agents.

Not So Fast

As you can see from the summaries and also from Table 1, each material has certain advantages over the rest, but there is also a great deal of overlap in terms of the most basic physical properties. In fact, some of the performance measures are so close that final selection is usually based on other factors such as ease of manufacture, price, color and so on. Two of DuPont's most popular materials, Telflon® PFA and Teflon® PTFE, for example, are so close in most major performance areas that selection has little to do with differences in performance. Both materials show outstanding chemical properties for resistance to corrosive agents, nonsolubility, and nonflammability. Both are virtually identical in terms of their electrical properties for dielectric and volume resistivity. Even in critical service temperature ratings, both materials have identical properties. In fact, it is only in three areas, cold flow, folding endurance and cost that these virtually identical fluoropolymer resins differ significantly. Because PFA is melt extruded, it exhibits better cold flow values than PTFE, which translates to better long-term sealing and reduced loosening of fittings. PTFE, on the other hand, exhibits better folding endurance than PFA, but at 500,000 bends before failure, PFA is well within the operational tolerances of most applications. In terms of cost, PFA convoluted tubing is typically the better performer, as the melt extruded product is less costly to fabricate and can be produced in any length, effectively eliminating the scrap associated with PTFE.

It is this caliber of understanding for the subtle differences between materials that Glenair has to offer our conduit customers. Nowhere else in the world does anyone offer the range of materials and constructions as Glenair, and nowhere else does anyone duplicate our expertise in materials selection.

Glossary of Performance Terms

Brittle Temperature: The temperature below which a material becomes brittle as measured by cold impact tests.

Biodegradation: The degree to which a material is resistant to enzymatic and microbiological attack or provides a substrate for fungal or bacterial growth.

Chemical Resistance: The degree to which a material resists contamination and degradation from organic and inorganic chemicals.

Coefficient of Expansion: The extent to which a material will expand or contract when heated or cooled.

Cold Flow: The degree to which a material changes or distorts over time when subjected to tensile or compression stresses.

Combustion: Rapid oxidation, such as occurs in the burning of any material.

Dielectric Strength: The peak voltage a material can withstand before failure. Usually expressed as volts per mil.

Dimensional Stability: The degree to which materials are unaffected dimensionally by, for example, long-term immersion in water.

Ductility: The measure of a material's ability to deform plastically without fracturing.

Elongation: The incremental changes in length of a material stressed under tension.

Environmental Protection: The general measure of a conduit system's ability to prevent moisture, dirt, air, or dust from polluting, contaminating or damaging enclosed wires and terminations.

Extrusion: A manufacturing process employing heat, forming dies, and a screw to form thermoplastic materials into predetermined shapes.

Flammability: The inherent measure of a material's resistance to burning.

Folding Endurance: The number of times a material can be bent or flexed before experiencing failure.

Hardness: The measure of a material's strength, rigidity, and resistance to abrasion or penetration.

Heat Aging: The changes in the performance of a material over time due to environmental factors such as heat and light.

Humidity: The degree to which materials are inherently hydrophobic and resistant to hydrolysis.

Irradiation: A method of altering the molecular structure and physical properties of a material through exposure to high-energy emissions. Also referred to as crosslinking.

Mechanical Properties: The physical and operational properties of a material as they pertain to its toughness and durability when subjected to various types of stress.

Permeability: The extent to which a material acts as an effective barrier to water permeation through absorption.

Radiation Resistance: The measure of a material's resistance to the destructive affects of high doses of gamma radiation.

Service Temperature: The maximum temperature at which a material can operate for extended periods with acceptable changes in its basic properties; also referred to as rated temperature.

Shelf Life: The length of time that a stored material remains usable.

Solvent Resistance: The ability of a material to retain physical and electrical properties after being immersed in specific solvents.

Thermal Conductivity: The degree to which a material acts as a conductor or heat sink.

Thermal Properties: The measure of a material's resistance to extremes in temperature, particularly its ability to retain its strength, dimensional stability and other physical properties.

Tensile Strength: The resistance of a material to pulling forces, and the point at which a material tears or fails under tension.

Toxic Gas Emission: The measure of poisonous gas, usually halogen, emitted by a material at high temperatures or upon combustion.

Ultraviolet Resistance: The measure of a material's resistance to damage caused by long-term exposure to sunlight.

Volume Resistivity: The resistance of a material to the flow of electrical current.