zytel htn whitepaper r8!06!2008
TRANSCRIPT
1
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.
2
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.
3
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).
4
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
5
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.
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.
7
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,
8
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: [email protected]
9
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.
www.plastics.dupont.com