High Performance Polyamides Fulfill Demanding Requirements for Automotive Thermal Management Components

High Performance Polyamides Fulfill Demanding Requirements for Automotive Thermal Management Components David Glasscock Walter Atolino Gary Kozielski Marv Martens DuPont Engineering Polymers Because they maintain excellent strength and toughness during exposure to hot, aggressive automotive fluids and to hot air whether humid or dry, high performance polyamides (HPPA) can make durable, functional components for automotive thermal management and other demanding applications. This paper reviews the basic chemistry of polyamides and demonstrates how the HPPA family differs from standard nylon. It focuses on semi-aromatic HPPA polymers known as polyphthalamides (PPA). INTRODUCTION The use of engineering thermoplastics in automotive components has grown significantly over the last 25 years with many new applications in powertrain, electrical components, chassis, trim components and other vehicle areas. Typical modern vehicles have more than 100 kg of plastic components (Ref. 1). Some of the main forces driving demand growth include weight reduction, production gains (easier assembling, integration of parts and systems) and more design flexibility. Under-the-hood applications have shown particularly high growth. Typical examples include air intake manifolds, rocker covers, radiator end tanks, fuel rails, electrical connectors and others. Polyamides have had great success in those areas due to their excellent balance of oil resistance, thermal stability, mechanical strength, toughness and other desirable properties. In recent years, temperatures in the engine compartment have been rising because of reduced space and more powerful engines. The temperature resistance of plastics parts has consequently become even more critical. Weight reduction also continues being an issue to help reduce fuel consumption. These factors can be expected to lead to increased use of polymers with higher temperature performance such as PPAs. The resistance of PPA s to antifreeze is another factor in their favor. In an investigation of the effect of antifreeze solutions on polyamides in 1995, Garrett and Owens (Ref. 7) concluded that the performance of semi-aromatic PPA is superior to that of aliphatic polyamides such as nylon 6 or nylon 66. We have extended their study by measuring the performance of different types of PPAs and their resistance to today s more aggressive long-life coolants in 5000 hour tests consistent with today s extended warranty intervals. BACKGROUND ON POLYMER CHEMISTRY Because people who need to design and use plastics have varying familiarity with plastics, we will briefly familiarize the reader with basics. For those wishing to gain more knowledge, references 14 and 15 are excellent guides. Polymers consist of repeating units of monomers (individual molecules) that combine to form a long chain. The polymers may consist of a single type of molecule (known as a homopolymer) or may be combinations of more than one molecule (known as a copolymer). A major class of polymers known as thermoplastics may be remelted, as opposed to thermosets, which form irreversible crosslinks between polymer chains. Within the thermoplastics category, there are amorphous and crystalline polymers. Amorphous polymers have random orientation of their polymer chains, whereas crystalline polymers form highly ordered crystal structures within an amorphous matrix (Figure 1). The term semi-crystalline polymers is used for polymers containing both crystalline and amorphous regions. As a general rule, amorphous polymers have advantages of transparency and toughness. Semicrystalline polymers have advantages in chemical resistance and temperature performance. These are general statements however, and the designer must consult product-specific literature and test data for specific properties. 1

POLYAMIDE PRODUCT FAMILY A polyamide is a polymer having an amide linkage Figure 1: Schematic of structure in the solid state for amorphous and semi-crystalline polymers. in the polymer backbone (Ref. 16). Aliphatic or semi-aromatic polyamides that are melt-processible are also referred to as nylon (Ref. 9). This definition encompasses a wide variety of products, most notably nylon 66 or PA66 and nylon 6 or PA6, which represent the vast majority of nylon produced in the world today. PA66 is produced by polymerizing hexamethylenediamine (HMD) and adipic acid (AA) polymerization. The "66" designation refers to the six carbon atoms in HMD and AA, respectively (Figure 3). Nylon 6 is a polymer of caprolactam, which contains both components of an amide linkage. These nylons are considered aliphatic because there are no aromatic ring structures along the backbone of the polymer chain. A less common polyamide, PA46, is a polymer of diaminobutane and adipic acid. It has a much higher melt point than PA6 or PA66. Figure 2: Classification of amorphous and semi-crystalline polymers by performance. For definitions of material acronyms, see end of document. Figure 2 shows various amorphous and crystalline plastics segmented by performance. Generally, the higher in the triangle, the higher the use temperature. The polymers discussed in this paper include aliphatic polyamides such as nylon 6 or 66, which are in the middle temperature range of semi-crystalline thermoplastics, and PPA, which is in the upper temperature range of the semicrystalline thermoplastics. Polymers are often used in combination with other ingredients to make a useful product. This combination of polymer and additives is often referred to as a plastic, or a composite. Typical ingredients used to produce composites are fiberglass, mineral, heat stabilizers, flame retardants and other processing aids. Most of the products we discuss in this paper are composites with 30 to 35% by weight of fiberglass reinforcement, or GR for short (Glass Reinforced). Fiberglass reinforcement provides strength and stiffness particularly as the temperature is increased beyond the polymer s glass transition temperature (Tg), where the amorphous region becomes mobile. Figure 3: Typical polyamides: PA6, PA66 and PA46. The addition of an aromatic ring [ ] structure to a polyamide provides many advantages to the polymer. These advantages include a higher Tg, higher melting point, and reduced absorption of moisture and solvents. These property advantages are manifested as improvements in dimensional stability, improved solvent (and hydrolysis) resistance, and better high temperature mechanical property retention. A more detailed discussion can be found in reference 19. The aromatic content for almost all commercially important semi-aromatic polyamides is provided in the form of terephthalic acid (TPA) or isophthalic acid (IPA) as shown in Figure 4. 2

Table I shows properties typical of glass-reinforced composites of the polyamides we have discussed. In general, the PPAs have higher glass transition temperatures, higher melt points and higher deflection temperatures 2 than the aliphatic PA66 and PA46. Also, the PPAs pick up less moisture so moisture exposure has a smaller effect on properties. Note however, there are differences in key properties within the PPA family, these translate into different performance (see references 5 and 11). Figure 4: Terephthalic acid (TPA) & isophthalic acid (IPA). ASTM D5336 defines a polyphthalamide (PPA) as a polyamide in which the residues of terephthalic acid or isophthalic acid or a combination of the two comprise at least 55 molar percent of the dicarboxylic acid portion of the repeating structural units in the polymer chain (ASTM, 2003). Referring back to Figure 3, this means that a portion of the acid segment is replaced with an aromatic component, terephthalic acid (TPA) and/or isophthalic acid (IPA). Table I. Selected Properties of Typical Polyamides 3 Polymer Tg (C) Tm (C) DTUL @ 1.8MPa (C) % H2O, 24 hrs 2mm PA6T/DT (PPA) 140 300 264 0.5% A PA6T/6I/66 (PPA) 125 312 278 0.5% B PA6T/66 (PPA) 90 310 285 0.5% C PA 46 80 295 290 1.5% D PA 66 65 263 252 1.2% E Test Method DMA ISO 11357-1/-3 ISO 75f ISO 62 Grade Figure 5 shows three common polyamides meeting the definition of a PPA as described in ASTM D5336. The first structure shown is 6T/66. The "6T/66" designation is as follows: HMD "6" + TPA "T" build the 6T molecule and "66" comes from the HMD + AA (PA66 described earlier). These two molecules form the copolymer 6T/66. The "x" and "y" designate that there is not necessarily a 1-to-1 ratio of 6T to 66. In fact by definition of a PPA, at least 55% of the adipic acid in the polymer chain has been replaced by TPA. Therefore, in the chemical formula, x 0.55, y = (1 x) will meet the definition of PPA. Another PPA structure, 6T/DT, is also shown in Figure 5. In this case, 100% of the AA has been replaced by a TPA component. However, the amine segment has some fraction of HMD replaced by 2-methyl pentamethylene diamine (MPMD), designated as "D" 1. The purpose of the MPMD is to modify the crystallinity just enough to allow it to be melt processed, creating a practical injection molding resin. For PA6T/DT, x 0 and y = (1 x). That is, any ratio of the copolymer units of "6T" and "DT" will meet the PPA definition, but the ratio is typically determined by optimizing the polymer properties. Also shown in Figure 5 is 6T/6I/66, a "terpolymer" of "6T", "6I" and "66" where "I" is isophthalic acid along with 66 serves to modify the crystallinity to allow for injection molding. To meet the definition of a PPA this polymer must have (x + y) 0.55, z = (1 x y). 1 Strictly speaking, per ASTM D6779-03, we should use PA6T/MPMDT. We abbreviate in this paper as PA6T/DT. 2 Deflection temperature under load (DTUL, defined by ISO 75f) represents the temperature at which a test specimen reaches a standard deflection with a given load (1.8MPa is used in this paper). 3 Grades representing the different product families are as follows: A = Zytel HTN51G35HSL, B = 33% GR PA6T/6I/66, C = Zytel HTN52G35HSL, D = Zytel 33-35% GR PA66, E = 30% GR PA46. Tg and flexural modulus vary with moisture content; values represent dry-asmolded conditions. Water absorption data taken on 2 mm thick test specimens. Tg estimated by DMA (Dynamic Mechanical Analysis). 3

Figure 5: Chemical structures of three common PPAs: PA6T/66, PA6TDT, and PA6T/6I/66. Shown in Figure 6 is the flex modulus 4 of various polyamides as a function of temperature. These are all typical 30-35% glass-reinforced commercially available grades. The drop in flex modulus corresponds to the glass transition temperature (Tg), when the amorphous region of the semi-crystalline polymer matrix becomes mobile. It is 4 Flex (flexural) modulus, defined by ISO 178, is an approximation to Young's modulus of a test specimen under a flexural (bending) load. the glass fibers that reinforce a structure between the crystalline regions and maintain significant properties above the Tg. PA46 has interesting properties due to its relatively high flex modulus at the very highest of temperatures. This is due to its higher level of crystallinity. However, referring back to Table I, PA46 picks up a significant amount of moisture relative to PPA, and this reduces its performance in many real-life conditions where humidity or aqueous chemicals are present. The 4

significance of that was reported for coolant systems in reference 7. Figure 6: Flex modulus (ISO 178) for various polyamides 3 with 30-35% glass reinforcement. Creep, also known as deformation under constant load, is one of the most important mechanical properties to characterize long-term performance of a plastic under load (Ref. 3). Materials with low creep retain their original dimensions longer than materials with high creep. Shown in Figure 7 is the creep inferred from accelerated testing via dynamic mechanical analysis (DMA) (Ref. 8). Measurements were taken on a specimen under flexural load of 28 MPa at 150C. Results indicate that the percent total strain of PPA6T/DT is about 50-75% that of PA6T/66, PA6T/6I/66 and PA66 under the same conditions. These results are consistent with the flexural modulus values at 150C shown in Figure 6. The chemistry of automotive coolants is quite complex, typically with an ethylene glycol / water mixture as a base. Corrosion inhibitors are added to the ethylene glycol. Conventional antifreezes have used inorganic corrosion inhibitors such as silicates but these tend to degrade quickly over time. Today, most of the current corrosion inhibitor technology is based on organic acid technology (OAT) or hybrid organic acid technology (HOAT). The organic acids used today have better stability, allowing for much longer time between changing of engine coolant, hence the term "long-life coolants" or "extended life coolants" (Ref. 17 and 18). Three materials, PA6T/DT, PA6T/6I/66 and PA66, were tested for property retention as a function of time up to 5000 hours. Results shown are based on 50/50 coolant with water. All three plastics are modified formulations designed for improved hydrolysis resistance compared to the standard formulations in Table I, with glass reinforcement levels of 30-35% unless otherwise specified. The coolants chosen were both long-life formulations: Valvoline Zerex G05, a hybrid organic acid technology (HOAT) coolant herein referred to as "Zerex G05" and Prestone Extended Life 5/150, a Dex-Cool approved formulation based on organic acid technology (OAT), herein referred to as "Dex-Cool ". Both coolants were tested as a 50/50 mix with water. The test protocol was ISO 527, measuring stress at break 6 and tensile modulus 7 on 4mm thick test specimens after immersion in the solution at 130 C. Test measurements were performed at 23 C. Results are shown in Figures 8 and 9. Clearly, the PA66 shows the largest drop in property retention, losing most of its properties within 1000 hours of testing. While all of the materials experienced a drop in properties over time, Figure 8 indicates that the PA6T/DT retains the highest stress-at-break values, particularly with respect to PA66. To put the results into context, PA6T/DT has the same or better property values at 5000 hours compared to PA66 at 1000 hours, allowing the use of thermoplastics in extended life coolant applications. Referring to Figure 9, the tensile modulus data highlights the PA6T/DT having higher retention vs. PA6T/6I/66 or PA66. Figure 7: Accelerated Flexural Creep by Dynamic Mechanical Analysis (DMA) at 150C, 28 MPa 5. PERFORMANCE DATA IN AUTOMOTIVE COOLANTS 5 Samples are annealed, tested under dry as molded condition 6 Stress at break, defined as tensile stress at break by ISO 527, is defined as the tensile stress at which the test specimen ruptures. 7 Tensile modulus, defined by ISO 527, is Young's modulus as measured on a test specimen in tension. 5

Figure 8: Stress at break for 30-35% GR polyamides exposed to 50% Zerex LLC at 130 C. Figure 10: Stress at break comparison in Zerex and Dexcool LLC for 35% GR PPA (PA6T/DT). Higher glass levels will help maintain an additional buffer of performance. After 5000 hrs in Dex-cool, a PA6T/DT with 45% GR maintained almost 20% higher tensile modulus compared to 35% GR, as indicated in Figure 11. Figure 9: Tensile modulus for 30-35% GR polyamides exposed to 50% Zerex LLC at 130 C. Figure 10 shows PA6T/DT @ 130 C in both Zerex G05 and Dex-cool long-life coolants. The performance was comparable, but the tests indicated that Dex-cool was a somewhat more aggressive coolant. Figure 11: Tensile modulus (ISO 527) in Dexcool @ 130 C for varying glass load of PA6T/DT and PA66. 6

APPLICATION OF PPA IN AUTOMOTIVE The following examples are commercial applications demonstrating the use of PPA in automotive thermal management applications. In all cases the materials have a PA6T/DT polymer base, though the filler level may vary by particular application. Figure 12 shows a water pump impeller. After rigorous evaluation and testing, a leading manufacturer of automotive water pumps in South America has adopted glass-reinforced PPA for impellers for a number of its aftermarket models. The parts were formerly made of cast iron, aluminum or glass-reinforced PA66. The manufacturer's technicians tested impellers molded from Zytel HTN for more than 1000 hours using standard automotive coolant at temperature conditions matching actual use. Service temperatures typically range from 110 to 115 C with peaks of 130 C. PPS was also tested, as it is known to have good chemical resistance. In evaluating PPS however, it was concluded that breakage would be a problem during handling. Figure 13: Water outlet valve. Figure 14 shows a novel example of PPA used at the heart of the engine recently commercialized by Aisan Industry for Toyota. In this application, the PPA is exposed on both sides to hot long-life coolant. Per Aisan Industry, a "water jacket spacer" improves the fuel economy by modifying the flow profile of coolant around the cylinder walls. This results in a more even cylinder temperature profile, more uniform viscosity of the oil and hence a reduction in friction. This results in an improvement in fuel economy by approximately 1% according to Toyota and Aisan Industry. Figure 12: Automotive water pump impeller. Engine water outlets and thermostat housings have been key application areas for PPA. These applications have been demonstrated in commercial success at a range of OEMs. Shown in Figure 13 is a water outlet valve as an example. Often these applications are replacing aluminum, providing weight reduction and reduced cost due to less secondary machining. Figure 14: Toyota water jacket spacer. CONCLUDING REMARKS Polyphthalamides have a fundamental advantage over other polyamide products in thermal management applications due to their aromatic nature. This translates into expanded opportunity for substitution of metal deeper into the powertrain, providing benefits in weight reduction, 7

feature flexibility and potential for cost reduction. Furthermore, polyphthalamides represent a class of polymers, differing in the polymer backbone and consequently showing differences in performance. We've demonstrated these performance differences through data, and demonstrated the commercial viability of PPA in thermal management applications through successful case studies. ACKNOWLEDGEMENT The authors wish to thank a number of people who helped with this work. Dino Tres, Clive Robertson, Craig Andrews and Hajime Ohke-san provided useful feedback. Linda Basso provided expertise on plastics testing in automotive coolants, and Kim Lantz organized much of the test data gathering shown here. Mimi Keating provided valuable, timely insight for thermal analysis. We also appreciate Aisan Industry allowing us to present a novel use of PPA in automotive coolant systems. REFERENCES 1. American Plastics Council (APC), Automotive Learning Center, http://www.plastics-car.com, 2004. 2. ASTM International, "Standard Specification for Polyphthalamide (PPA) Injection Molding Materials," D5336-03, 5 pages, 2003. 3. Birley, A., B. Haworth and J. Batchelor "Physics of Plastics: Processing, Properties and Material Engineering", Hanser Publishers, New York, 528 pages, 1991. 4. Eaton, E., W. Boon and C. Smith, "A Chemical Base for Engine Coolant / Antifreeze with Improved Thermal Stability Properties," SAE Technical Paper Series #2001-01-1182, 7 pages, 2001. 5. Ferrito, S., "An Analytical Approach Toward Monitoring Degradation in Engineering Thermoplastic Materials Used for Electrical Applications," Annu. Rep. Conf. Elec. Insul. Dielec., pages 833-837, 1996. 6. Gallini, J. "Polyamides, Aromatic", Encyclopedia of Polymer Science and Technology, John Wiley & Sons, 2005. 7. Garrett, D. and G. Owens, "Polyphthalamide Resins for Use as Automotive Engine Coolant Components," SAE Technical Paper Series #950192, 4 pages, 1995. Creep Behavior," Journal of Thermal Analysis and Calorimetry, vol. 69, pages 37-52, 2002. 9. Kohan, M., "Nylon Plastics Handbook," Hanser Publishers, New York, 631 pages, 1995. 10. Kohan, M., S. Mestemacher, R. Pagilagan and K. Redmond, "Polyamides," Ullmann s Encyclopedia of Industrial Chemistry, John Wiley & Sons, 2003. 11. Lapain, A. and E. Luibrand, "Compatibility of External Life Coolant Systems with Plastic Components," SAE Technical Paper Series #970075, 8 pages, 1997. 12. Mark, J. "Polymer Data Handbook", Oxford University Press, New York & Oxford, 928 pages, 1999. 13. Palmer, R., "Polyamides, Plastics", Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, 1996. 14. Ullmann s Encyclopedia of Industrial Chemistry Seventh Edition (John Wiley & Sons, Federal Republic of Germany, 2004) [www.wiley-vch.de/home/ullmanns 15. University of Southern Mississippi, Department of Polymer Science: "The Macrogalleria: A Cyberwonderland of Polymer Fun", http://www.pslc.ws/macrog/index.htm, 2002. 16. Weber, J., "Polyamides, General", Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, 1996. 17. Weir, T. and P. Van de Ven, "Review of Organic Acids as Inhibitors in Engine Coolants," SAE Technical Paper Series #960641, 11 pages, 1996. 18. Wilson, T., "A Comparison of Various Polymers in Select Organic Acid Technology (OAT) Coolants," SAE Technical Paper Series #2000-01-1095, 17 pages, 2000. 19. Zimmerman J., Polyamides, Encyclopedia of Polymer Science & Engineering, Wiley-Interscience, New York, Volume 11, pp. 340-349, 1988. FOR MORE INFORMATION Please contact your local DuPont Engineering Polymers representative. In North America, call 1-800-441-0575 or 1-302-999-4592; e-mail: web-inquiries.ddf@usa.dupont.com 8. Keating, M.Y., L. Malone and W. Saunders, "Annealing Effect on Semi-Crystalline Materials in 8

KEY WORDS Polyphthalamide, polyamides, coolant, long-life coolants, thermoplastics, thermostat housings, performance, physical properties, chemical resistance DEFINITIONS, ACRONYMS, ABBREVIATIONS ABS: Acrylonitrile-Butadiene-Styrene DMA: Dynamic Mechanical Analysis GR: Glass-reinforced HDPE: High-density Polyethylene HDT: Heat deflection Temperature HPPA: High Performance Polyamide IPA: Isophthalic acid LCP: Liquid Crystal Polymer LDPE: Low-density Polyethylene LLC: Long-Life Coolant MPPO: Modified Polyphenylene Oxide PA: Polyamide PBT: Polybutylene Terephthalate PC: Polycarbonate PCT: Polycyclohexylenedimethylene terephthalate PEEK: Polyetheretherketone PEI: Polyether Imide PES: Polyether Sulfone PET: Polyethylene Terephthalate PI: Polyimide POM: Polyoxymethylene PP: Polypropylene PPA: Polyphthalamide PPS: Polyphenylene Sulfide PS: Polystyrene PSU: Polysulfone PVC: Polyvinylchoride SAN: Styrene Acrylonitrile SMA: Styrene Maleic Anhydride PMMA: Polymethyl Methacrylate Tg: Glass Transition Temperature Tm: Melt Temperature TPA: Terephthalic acid DISCLAIMER Because we cannot anticipate or control the many different conditions under which this information and/or products may be used, neither DuPont nor the authors guarantees the applicability or the accuracy of this information or the suitability of its products in any given situation. Users of DuPont products should make their own tests to determine the suitability of each such product for their particular purposes. The data listed herein falls within the normal range of product properties but they should not be used to establish specification limits or used alone as the basis of design. Disclosure of this information is not a license to operate or a recommendation to infringe a patent of DuPont or others. Zytel is a registered trademark of the DuPont Company or its affiliates. Zerex is a registered trademark of Ashland Inc. Dex-Cool is a registered trademark of General Motors Corporation Prestone is a registered trademark of Honeywell International, Inc., or its subsidiaries or affiliates. 9