By Shawn Lee

Challenging applications in military and aerospace electronics, healthcare, and down-hole oil and gas applications have created the need for extreme-performance resins that combine very high heat and chemical resistance with good thermoplastic melt processability. These market re-quirements prompted the development of Extem resin, a new amorphous, melt-processable thermoplastic polyimide (TPI). Introduced by GE Plastics this past November, Extem resin is an offshoot of GE's 20-year experience in making Ultem polyetherimide resin.

GE Plastics expects the new TPI to help narrow the performance-vs.-processability gap facing current high-heat thermoplastic and thermoset materials. Extem resin boasts the highest glass transition (Tg) of any amorphous thermoplastic, at 311 C. Along with this extreme heat resistance, it offers high chemical resistance, high strength and stiffness, robust dimensional stability, and high inherent flame resistance. At the same time, it provides the manufacturing productivity of melt processing by in-jection molding, extrusion, blow molding, or thermoforming without re-quiring any post-cure or crystallization steps to yield its maximum performance.

Extem resins are the culmination of 14 years of R&D that included development of new mon-omers and a new manufacturing process. That process has been proven first at a pilot plant in the U.S., then at a just-launched semi-works plant in this country that can produce hundreds of tons annually.

Target applications
Exteme resin's combination of processability and outstanding performance can help catapult the use of TPIs from exotic, low-volume applications to broader mainstream use. More than 30 customers are sampling and developing applications utilizing Extem resin.

Extem resin's extreme heat, as well as improved chemical resistance to hydrocarbons and chlorinated solvents, will position it for components in the down-hole oil and gas industry. Producers of aerospace and military connectors also can benefit from its high as-molded performance without additional post-cure time or the long cycle times of non-melting and thermoset imidized resins. Extem resin's potential lead-free solder capability along with its balance of strength, stiffness, and dimensional stability makes it an excellent candidate for electrical, electronic, and semi-conductor applications.

In automotive, Extem resin could offer opportunities for metal replacement. And in healthcare, Extem resins offer an interesting blend of properties that have shown significant benefits in hemo-compatible membranes.

How it compares
Demand for high-performance polymers (HPP) is experiencing about 9% annual growth. The HPP market includes three major categories of polymers amorphous thermoplastics, semi-crystalline thermoplastics, and non-melting and imidized materials. Performance in the HPP world typically centers on chemical resistance and continuous-use temperature. Additionally, there are a variety of secondary considerations such as wear resistance and flammability.

Amorphous thermoplastics such as PEI and polyethersulfone (PES) are typically characterized by relatively high Tg?17 C and 230 C, respectively. This leads to retention of strength, stiffness, and dimensional stability across a wide range of elevated temperatures. However, they are generally limited in their range of chemical resistance, particularly to highly alkaline materials, and are restricted to continuous-use temperatures below 200 C.

Semi-crystalline polymers such as PEEK and PPS offer high melting temperatures?34 C and 285 C, respectively plus broad chemical resistance and high retention of properties under thermal aging. They are, however, susceptible to dimensional change and require glass fillers to maintain strength and stiffness above their relatively low Tg5 C for PPS and 150 C for PEEK.

Current imidized materials such as polyamide-imide (PAI) and thermoset polyimide (PI) offer a combination of amorphous and semi-crystalline attributes for extreme performance but fall short in productivity because of the need for long thermoset cure cycles, post-mold curing or crystallization, or milling parts from extruded stock shapes. For example, PAI can be injection molded as a thermoplastic but must be post-cured for up to 15 days in order to achieve its maximum physical properties. In its uncured state, PAI is a brittle, low-strength resin that must be handled very carefully in order to minimize part breakage. During the cure cycle, the imidization process is completed, which raises the strength, wear, and thermal characteristics. Once PAI parts are cured, the parts cannot be recycled or reused, as the material is now a thermoset.

Components produced from thermoset polyimide can be molded with long cure cycles using compression molding or vacuum-bag/autoclave techniques. Or parts can be cut or milled from cured shapes such as film and slab stock. This limits the geometry of the final parts. There is, in addition, one other melt-processable TPI on the market, but it has a lower Tg and requires post-mold crystallization to achieve its maximum properties.

Two resin families
The Extem resin platform will initially offer two families of materials, Extem UH and Extem XH resins. The Extem UH family will offer the highest heat resistance, with a Tg up to 311 C, plus broad chemical resistance. The Extem XH family will offer a Tg up to 267 C along with higher melt flow. Additional attributes of Extem resins include exceptional dimensional stability; high strength, stiffness, and creep resistance at elevated temperatures; transparency; outstanding flame, smoke, and smoke-toxicity performance without additives, such as a high limiting oxygen index (LOI) of 45% to 47%.

GE Plastics is investigating at least 75 potential products based on Extem resin, including various filled/reinforced compounds and blends with Ultem or other resins. Extem is also expected to join GE掇 LNP portfolio of compounds designed for wear resistance, electrostatic dissipation (ESD), and other properties.

Ultem resins have been used in applications with continuous-use temperatures as high as 200 C and UL Relative Thermal Index (RTI) ratings up to 180 C. However, Extem UH resins are expected to have continuous-use temperatures as high as 250 C and potentially achieve UL RTI ratings at or above 200 C, a level that no other melt-pro-cessable, amorphous thermoplastic can claim. Extem UH resins demonstrate outstanding thermal-oxidative stability at extreme temperatures.

In addition to very high heat resistance, Extem UH resin also possesses superior chemical resistance. For example, GE's Ultem resin, though amorphous, has good resistance to chemicals such as organics and acids. Ultem CRS5001 resin currently offers the best chemical resistance in that family. But Extem UH resins radically outperform Ultem CRS5001 resins or example, in resistance hydrocarbons such as toluene and aggressive solvents like methyl ethyl ketone (MEK) and chlorinated solvents such as methylene chloride. These new materials demonstrate chemical resistance comparable to cured PAI or PI.

Easier-flowing Extem XH
The Extem XH family of resins is characterized by its extreme heat capability balanced with greater processability. While maintaining high creep resistance and strength at elevated temperatures, it can be injection molded to fill thin-wall, complex parts.

Extem XH resin will offer a high-strength material for short-term heat resistance above 200 C. This offers the potential for use in lead-free soldering of electronics.

At room temperature, many high-performance amorphous and semi-crystalline thermoplastics exhibit very high tensile strength (over 100 MPa). Extem XH resin, while not the strongest material at ambient temperature, outperforms many others at temperatures up to 240 C by retaining more of its tensile strength and creep resistance. Figure 5 demonstrates the tensile strength of Extem XH resin vs. other high-performance resins at ambient temperature and 170 C.

About the Author:
Shawn Lee is the product manager for High Performance Polymers in the Americas at GE Plastics in Mount Vernon, Ind. He has over 11 years of experience with GE and 3M in a variety of roles in technology, product development, and marketing. Prior to his current position, Lee was a regional application development specialist for GE Ultem and Siltem resins.