Down-to-earth role for imidized polymers

Richard W. Campbell
Product Development Manager
Quadrant Engineering Plastic Products
Reading, Pa. www.quandrantepp.com

Designers of the B-2 Stealth Bomber or Lunar Lander probably didn't hesitate specifying imidized polymers for their designs. But though these materials are tough and ultralight, some designerssee them as too exotic for mainstream uses. This perception, plus a need for high processing temperatures, together have kept this class of polymers out of wide deployment.

Now, however, the situation has changed. Techniques for processing imidized polymers today routinely include compression molding, extrusion, and injection molding. Flexible processing lets designers replace metals or ceramics in applications such as O-ring backups, seal rings, valve seats, thrust washers, and bearings. Imidized polymers are also specified for thermal insulators, high-performance bearings, electrical connectors, and ablative structures.

The family of imidized polymers include polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), and polyetherimide (PEI). Chemically, all contain at least one imide linkage (carbonyl — amine — carbonyl)/repeat unit, which is formed via a heterocyclic ring closure reaction. The cyclic rings help boost thermal and mechanical strength.

The molecular architecture of these polymers is often redesigned to ease processing or optimize physical or mechanical properties. To increase flexibility of PEI, for example, chemists add an ether linkage which contains oxygen. Alternatively, the addition of a nitrogen-carrying amide boosts PAI flexibility. In both cases, increasing molecular flexibility makes it easier to process the polymers by conventional means without degrading the polymer's high heat, chemical, or flame resistance.

Classifying PIs
Traditionally, the first group of imidized polymers, PIs, were thermoset polymers that couldn't be melt processed because of their highly branched or crosslinked molecular structure. Crosslinking boosts the polymer's heat and chemical resistance. Designers have long taken advantage of these properties employing PIs as the matrix in glassfiber or carbon-filled structural composites.

"Pure" PIs are often considered " pseudothermoplastics" because their processing resembles that of thermosets. But, incorporating ethers and other flexible links into the molecular backbone makes possible processing by more conventional thermoplastic techniques. It also opens the door to new shapes and design possibilities.

Thermoset PIs have continuous use temperature ratings (CUTR) to 680°F and can withstand brief exposures to 900°F. The combination of high stiffness and light weight makes thermoset PI composites ideal metal replacements in aerospace and other demanding applications.

Processing of thermoset PIs often takes place by shaping the partially imidized prepregs, then curing the shapes in high-temperature ovens, highpressure autoclaves, or both. Curing completes imidization making the thermoset PI or composites tough yet machinable. Examples of thermoset PI composites and adhesives include Kinel and Kerimid made by the Polymer Specialty Div. of Vantico Inc., Brewster, N.Y. (formerly Ciba Specialty Chemicals).

Non-Melting (NM) PIs, while not necessarily crosslinked, have stiff polymer backbones. This makes melt processing difficult to impossible because decomposition occurs before melting. Despite this, they represent at least 35% of the dollar volume for all PIs. They are typically highly aromatic in structure, which accounts for their highglass-transition (T_g) and continuous-use temperatures. Several NM-PIs have the unusual ability to be direct-formed (sintered) into near net shape parts.

As with most polymers, additives such as graphite, PTFE, or molybdenum disulfide, improve wear resistance, and other properties of NM-PI resins.

NM-PIs find use in electrical/electronic applications such as films for flexible printed circuit boards and insulation, or in molded components such as sockets and bobbins. NM-PIs are also used in bearings, valve seats, and seals thanks to high heat and chemical resistance. The high purity of NM-PIs makes them ideal candidates for semiconductor waferprocessing and handling applications.

Research labs have synthesized hundreds of NM-PIs, although only a handful have ever been commercialized. Most notable are Vespel shapes and Kapton films from DuPont, Wilmington, Del. These NMPIs are based on the condensation polymerization of pyromelletic dianhydride and oxydianiline. More recently, Quadrant Engineering Plastic Products, Reading, Pa., (formerly DSM EPP) introduced a NM-PI called Duratron XP.

Each member of this imide family is inherently flame resistant carrying an oxygen index (OI) on the order of 53. Under normal circumstances, materials with OI ratings of less than 25 can support combustion. For example, Vespel SP-1 has a rating of UL94 5VA at a0.75-mm thickness.

In addition to the "pure" PI, there are numerous other "hybrid" imidized polymers which contain imide linkages in their molecular backbone structure making them melt processible. These polymers typically have thermal properties below those of pure PIs.

True thermoplastic PIs only recently have been introduced by Mitsui Chemical America Inc., Purchase, N.Y., under the tradename Aurum. The thermal properties are somewhat lower than other PIs. Aurum has a Tg of 482°F and its heat-deflection temperature (HDT) is 464°F at 264 psi, for molded or extruded unfilled products. Unlike other PIs, which don't crystallize, annealing Aurum slowly crystallizes its microstructure.

A crystalline melting point of 730°F arises from first annealing the molded or extruded parts at 428°F for several hours depending on the thickness, followed by a 536°F anneal. This procedure also increases the PI's HDT to 610°F. However, thermal and mechanical properties greatly depend on the degree of crystallinity so careful processing is a must. Improved property consistency must be balanced against polymer toughness and thermal properties. The material has an OI of 47 which lets it meet UL94 5VA at > mm.

Thermoplastic PIs can also blend with other engineering thermoplastics to permit processing by more conventional methods. However, this modification makes them lose some or all of the thermal properties present in pure thermoplastic PIs.

The Other Imidized Polymer Contenders
PEIs, such as Ultem introduced by GE Plastics in the early 1980s, show up in a myriad of applications. PEIs have alternating aromatic imide links that boost high-temperature performance as well as flexible ether links which help ease processing. The HDT of the polymer at 264 psi is 392°F and can increase 20°F with the addition of glass or carbon fibers. Its continuous use temperature is 338°F. In addition to use in injection-molded parts and extruded films, PEIs compression mold or extrude into a wide variety of stock shapes.

With an OI rating of 47, PEI exceeds FAR 25.853 requirements for smoke and heat evolution. This lets designers put PEI components in aircraft interiors. Because they cost relatively little, PEIs show up in many high-volume automotive and lighting applications. PEIs also get deployed in electrical applications, because of their inherently low flammability — with no additives. In addition, the polymer's low dissipation factor and dielectric constant are stable over a wide temperature range and frequency. PEI has a UL94 5VA (>1.9 mm) rating.

PEI is the only material in the imide family, which has FDA-compliant grades and stands up to autoclave sterilization. These properties facilitate use in medical components such as sterilizing trays, instrument handles, and other operating-room equipment, as well as in food-processing components. The natural PEI polymer is amorphous and transparent, although a dark amber in color.

PAIs contain a flexible, amide linkage which eases processing. Invented by Amoco (now Solvay Advanced Polymers LLC, Alpharetta, Ga.), Torlon PAI thrives in harsh thermal, chemical, and stressed environments. Applications include high-temperature electrical connectors and switches, valve seats, chemical seals, bushings, and racecar engine components. Semiconductor parts such as chip nests and sockets also come from PAIs.

Graphite, carbon fibers, and/or PTFE additives make PAI a candidate for high-wear applications such as bearings, thrust washers, large labyrinth seals, and rotary compressor vanes. Piston rings, valves and seats, and wear pads are other examples. An OI rating of 45 makes PAI a solid UL-94 V-0-compliant material.

PAI parts are formed by injection molding, extrusion, and compression molding. Parts injection molded and extruded from PAI should be carefully cured to maximize chemical and wear resistance. Curing also improves strength and toughness.

Extruded stock shapes cure during manufacturing but if the end user removes more than 0.040 in. from the surface of the shape then postcuring is recommended for optimal chemical and wear resistance. Postcuring is not necessarily recommended for compression-molded parts because the resin cures before molding.

Like most amorphous polymers, PAIs have mechanical properties that stay relatively constant up to the material's T_g of 527°F. Here, the polymer softens and loses nearly all its mechanical attributes.

PAIs have good chemical resistance against hydrocarbons, sour gases, oils, and oxygenated organics — ethers, esters, alcohols, and nitros. But they have limited resistance to strong bases, amines, and certain high-temperature acidic environments. The darker color of the cured surface of unfilled PAI is due to minor oxidation.

The PBI (polybenzimidazole) molecular-backbone is a heterocylcic polymer. This inherently high-strength, ladderlike structure is responsible for the material's unusually good performance. But its molecular structure poses challenges during processing. It was not until 1987 that a practical process was invented by Hoechst Celanese, Charlotte, N.C., and Alpha Precision Plastics, Houston, to mold large shapes from PBI.

Parts from Hoechst's Celazole PBI, for example, offer a combination of mechanical and thermal properties that are said to surpass all other commercially available high-performance plastics. PBIs have a T_g of 750°F, an 800°F deflection temperature under load, and compressive strength equivalent to TP or TS materials. They have good wear resistance, even without additives, but can be further enhanced with internal lubricants.

PBIs won't burn in air (OI 58) and can withstand a 1,400°F flame for a few minutes with only slight distortion or charring. The major application for PBI is fabric for firefighter clothing. However, several hundred hours in 600°F air will make PBIs slowly oxidize and lose weight. Nevertheless, some high-temperature applications have been successful. PBI oxidation rates depend on surface area, pressure, temperature, and oxygen level in a particular environment. Bulkier items such as valve seats perform continuously to 700°F, whereas smaller thin-walled parts may be limited to 550°F. On the other end of the temperature spectrum, PBI doesn't embrittle at -100°F and PBI fibers remain ductile even in liquid nitrogen.

Designers employ PBI in oil-well/geothermal ( downhole) and chemical-process components such as O-ring backups, seal rings, valve seats, thrust washers, and bearings. Automotive and aerospace applications for PBI include thermal insulators, high-performance bearings, electrical connectors, and ablative structures. PBI's high thermal stability and mechanical toughness lets it replace ceramic gas-diffusing components in plasma-cutting torches. The material also serves in ball valves where temperatures range from -450 to 700°F as well as in highly corrosive environments. Despite its hardness, PBI is resilient enough to provide the compression elasticity required for valve seats.

Although other imidized polymers may not need diamond-tipped tools for machining, it's imperative that they are used for PBI. In addition, careful procedures and a learning curve are generally needed to machine PBI. Once the techniques are mastered, however, machined PBI parts have a hard, glossy black and smooth surface.

Common Imidized Polymer Traits
Because all imidized polymers are hygroscopic, several can exhibit a phenomenon known as thermal shock. This occurs when PAI or PBI parts that have absorbed moisture are heated suddenly above about 440°F. Trapped moisture will turn to steam and exert a high internal pressure against the somewhat impervious skin. The result is severe surface blistering and distortion of the part. The effect can be avoided by drying the part in a stepwise progression to 500°F over two days and holding this temperature for 24 hr. This removes the entrapped water. The part should then be cooled slowly to avoid introducing stress.

Imidized polymers are said to offer the best combination of thermal and mechanical properties of any commercially available plastic. All are UL-94 V-O, or better, without any fire-retarding additives. They resist hydrocarbons and most organic solvents except amines. They are amorphous or noncrystalline, except for annealed TPI, so there are no abrupt changes in the mechanical properties below the T_g.

The downside of imidized polymers is that they all are relatively hygroscopic and will expand as they absorb water. The dimensional effect is more pronounced in smaller parts than in larger ones. In addition, prolonged exposure to boiling water or steam causes irreversible degradation of properties, although to a lesser extent for PEI. The chemical resistance against strong alkalis or bases and amines is also limited.