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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. Semi-

crystalline 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.

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Figure 1: Schematic of structure in the solid state for

amorphous and semi-crystalline polymers.

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 semi-

crystalline 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.

POLYAMIDE PRODUCT FAMILY

A polyamide is a polymer having an amide linkage

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 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.

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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).

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.

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 temperatures2 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).

Table I. Selected Properties of Typical Polyamides3

Polymer Tg (C) Tm (C)

DTUL @

1.8MPa

(C)

% H2O,

24 hrs

2mm Gra

de

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

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-as-

molded conditions. Water absorption data taken on 2 mm thick test

specimens. Tg estimated by DMA (Dynamic Mechanical Analysis).

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Figure 5: Chemical structures of three common PPAs: PA6T/66, PA6TDT, and PA6T/6I/66.

Shown in Figure 6 is the flex modulus4 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

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significance of that was reported for coolant systems in

reference 7.

Figure 6: Flex modulus (ISO 178) for various polyamides

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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.

Figure 7: Accelerated Flexural Creep by Dynamic

Mechanical Analysis (DMA) at 150C, 28 MPa5.

PERFORMANCE DATA IN AUTOMOTIVE

COOLANTS

5 Samples are annealed, tested under dry as molded condition

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

break6 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.

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.

6

Figure 8: Stress at break for 30-35% GR polyamides

exposed to 50% Zerex® LLC at 130°C.

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 10: Stress at break comparison in Zerex® and Dex-

cool® 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 11: Tensile modulus (ISO 527) in Dexcool® @

130°C for varying glass load of PA6T/DT and

PA66.

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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 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 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 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,

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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.

8. Keating, M.Y., L. Malone and W. Saunders,

"Annealing Effect on Semi-Crystalline Materials in

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

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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

Zytel® is a registered trademark of the DuPont Company or its affiliates. Go to Zytel® HTN home page. 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.

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.

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