High-performance 150[degrees]C capable TPVs-long-term aging behavior and processing.

Thermoset elastomers have traditionally been the material of choice for demanding underhood automotive parts requiring heat and/or oil resistance coupled with flexibility, noise and vibration dampening, or sealing ability. Included in these applications are dynamic shaft seals, inboard constant-velocity joint boots (CVJ), high-temperature air ducts, as well as connectors and tubes, all of which require rubber-like properties in addition to heat and fluid resistance.

Of the thermoset elastomers, silicone elastomers (VMQ) (ref. 1) and ethylene-propylene elastomers (EPDM) (ref. 2) find application where heat resistance above 135[degrees]C is required. However, both exhibit excessive loss of physicals and oil migration after extended exposure to mineral and synthetic fluids commonly used underhood. To combat this, polyacrylates (ACM) and ethylene-acrylates (AEM) have been largely adopted, due to their favorable cost-performance balance, for oil sealing applications where heat resistance up to 150[degrees]C is required (ref. 3). Hydrogenated nitrile (HNBR) (ref. 4) and fluoropolymers (FKM) (ref. 5) are typically reserved for applications requiring extreme toughness or severe permeation resistance.

Thermoplastic vulcanizates

Thermoplastic vulcanizates (TPVs), technology in which the rubber phase is dynamically vulcanized and dispersed in a thermoplastic matrix, have been gaining ground in automotive use. Olefinic-based TPVs (e.g., polypropylene plastic phase) are widely used in window/weather seals, interior components and only the least demanding underhood applications (ref. 6). TPVs combine the positive attributes of thermoset elastomers (softness, flexibility, sealing) with the processing advantages of thermoplastics.

TPVs have traditionally been excluded from underhood applications requiring fluid resistance and/or heat resistance above 125[degrees]C. EPDM/polypropylene TPVs are known to exhibit poor oil resistance, resulting in excessive swell and dramatic reduction of physical properties. And, above 125[degrees]C, all of the olefinic TPVs demonstrate a rapid drop in mechanical strength as they approach the melt point of the plastic phase. None of the olefinic-TPVs are suitable for powertrain and underbody applications where 150[degrees]C heat and hot fluids are commonplace.

New developments

Two new classes of TPVs have been introduced recently (refs. 7 and 8), promising dramatically improved heat and oil resistance compared to current generation TPVs. Both classes are reported as capable of withstanding long-term exposure to high temperature (150[degrees]C) air and oil.

The first of these classes is based on silicone rubber dispersed in an olefin plastic matrix (Si-TPV). While it would be expected that silicone should provide some level of oil resistance, as well as good heat resistance, it will be shown that this class suffers near complete loss of mechanical properties and swells excessively upon hot oil exposure. The poor oil resistance of Si-TPV mirrors the similar poor resistance of thermoset silicone.

The second class of materials has proven to satisfy Society of Automotive Engineers (SAE J2236) requirements for 150[degrees]C, 3,000 hour continuous use in hot air, as well as mineral and synthetic oils. This class of TPVs is based on elastomeric phases selected from ACM, dynamically vulcanized in plastic matrices of polyamide or polyester. The initial grades to be commercialized from this class are based on ACM/polyamide (ACM/PA).

Objective and criteria

The objective of this article is to compare the long-term heat and oil resistance of a representative ACM/PA TPV over a range of temperatures, fluids and exposure times to current generation TPVs, Si-TPV and thermoset elastomers. Copolyester (COPE) will be included in the comparison, as COPEs find application in CVJ boots.

The important criteria for confirming the heat and oil resistance are:

* Elongation and tensile strength retention in excess of 50% after 3,024 hours, 150[degrees]C in oil;

* no significant (more than 15 points, durometer A) change in hardness that would negatively impact component dampening characteristics and flexibility;

* volume change upon 512 hours, 150[degrees]C oil aging of less than 5%; and

* low-temperature flexibility to -40[degrees]C.

To assess economical processing, a criterion of injection molding cycle times for a reference test slab of less than 30 seconds was selected.

Experimental

Seven materials were selected for evaluation:

* A TPV based on polyacrylate (ACM) elastomer and Ny Nylon 6, designated ACM/PA;

* a low durometer copolyester resin, designated COPE;

* a TPV based on EPDM and polypropylene, designated EPDM/PP;

* a TPV based on NBR and polypropylene, designated NBR/PP;

* a melt processable rubber, designated melt proc.,

* a TPV based on VMQ and an olefinic plastic phase, designated Si-TPV; and

* a black-filled thermoset ACM compound, designated TS ACM.

Original properties

The original physical properties of the seven materials are summarized in table 1. All of the polymers were selected to have initial hardness between 70 and 80 durometer A, the exception being Si-TPV, which had an original hardness of 68 durometer A. This hardness range is comparable to automotive requirements for the targeted applications, seals, boots, air ducts and hose covers.

As shown in table 1, ACM/PA has a higher initial tensile strength than the polypropylene-containing TPVs (EPDM/PP and NBR/PP), and is similar to Si-TPV. COPE and Melt proc. each have higher initial tensile strengths than ACM/PA. While ACM/PA has a lower unaged (initial) elongation at break, it will be shown to have better retention of elongation upon aging than the other materials.

On balance, the initial physical properties of ACM/PA are equivalent to TS ACM, with the exception of specific gravity, which is lower for ACM/PA (1.15).

Physical properties as a function of temperature

The majority of laboratory material tests, including ASTM D471 and D573, assess the aging behavior of materials after set exposure times to various environments (temperatures, fluids). A limitation of these tests is that materials are conditioned to standard temperatures and pressures (STP) prior to measuring physical properties after aging.

It is necessary to determine the behavior of materials at temperature. That is, measure initial physical properties as a function of temperature. As shown in figure 1, ACM/PA shows negligible change in initial tensile strength from 100[degrees] to 150[degrees]C; it is quite stable over this range. In comparison, COPE, which has a high tensile strength at room temperature, begins to melt and lose tensile strength as it approaches 150[degrees]C. Olefinic-based TPVs show similar loss of mechanical properties as the temperature is increased above 100[degrees]C. A similar pattern is shown for elongation at break.

[FIGURE 1 OMITTED]

Comparison versus conventional TPVs and COPE

Retention of physical properties following prolonged exposure to hot air and immersion into mineral oil was measured on the test polymers following ASTM D471 and D573.

Air and fluid aging was conducted at 150[degrees]C, reflective of temperatures encountered underhood. ASTM #1 and IRM #903 reference oils were used for short-term aging, while ASTM Service Fluid 105 (SF105) was selected for extended aging. SF105 is an aggressive, additive-containing reference fluid designed to simulate modern semi-synthetic engine oils. It is designed to accentuate differences between materials, and is less susceptible to degradation upon long-term high heat exposure.

Figures 2 and 3 show the change in tensile strength and elongation after 336 hours, 150[degrees]C aging. The ACM/PA TPV can be seen to exhibit excellent retention of both in air and oil. In comparison, COPE, NBR/PP and melt proc. all were degraded beyond the ability to test the samples by this short-term, hot air exposure. When exposed to hot oil (IRM #903), ACM/PA was the only material to survive.

[FIGURES 2-3 OMITTED]

Further, figure 4 illustrates that even in a non-aggressive fluid (ASTM #1) designed to test for extracatables, EPDM/PP undergoes volume swell in excess of 50%, whereas ACM/PA undergoes minimal volume change.

[FIGURE 4 OMITTED]

It can be concluded that conventional TPVs, including EPDM/PP and NBR/PP, as well as melt proc. and COPE, are not suitable for moderate and longer-term exposure to 150[degrees]C environments.

Comparison versus silicone-based TPVs

As noted above, in addition to ACM/PA, a new class of TPVs based on silicone elastomer in a polyolefin matrix has been commercialized. A comparison of the heat and oil resistance of ACM/PA versus Si-TPV was made following hot air (150[degrees]C) and oil (IRM #903 and SF105) exposure.

As shown in figures 5 and 6, ACM/PA and Si-TPV have similar retention of physical properties upon hot air exposure. Both show approximately 50% loss of initial tensile after 1,008 hours at 150[degrees]C. In hot oil, however, ACM/PA shows superior retention of both tensile strength and elongation at break. Si-TPV shows greater than 95% loss of tensile and elongation after 168 hours at 150[degrees]C.

[FIGURES 5-6 OMITTED]

Further, figures 7 and 8 show Si-TPV to soften by more than 30 points (durometer A) and swell in excess of 12% after 504 hours exposure to 150[degrees]C SF105 oil. It is speculated that this behavior can be attributed, at least in part, to the combination of silicone and amorphous polyolefin, both of which are known to have poor oil resistance. Compared against Si-TPV, ACM/PA TPV has superior retention of properties following hot oil exposure.

[FIGURES 7-8 OMITTED]

Long-term 150[degrees]C aging of ACM/PA

To assess its suitability for long-term continuous use in today's underhood automotive environments, the 150[degrees]C immersions with ACM/PA TPV were continued for 3,024 hours.

Figures 9 and 10 show the ACM/PA TPV to have less than 25% loss of tensile strength and 50% loss of elongation after 3,024 hours aging in 150[degrees]C SF105. In air, approximately 50% of the initial tensile and elongation are retained after 3,024 hours. Following an initial drop at 504 hours, elongation retention is stable through 3,024 hours. A similar pattern is shown for tensile strength.

[FIGURES 9-10 OMITTED]

Hardness change upon long-term aging is shown in figure 11. ACM/PA shows less than 10 points hardness increase after 3,024 hours and, essentially, no increase after the initial 504 hours of aging. ACM/PA TPV can be classified as 150[degrees]C, 3,024 hours continuous use temperature material in air and SF105, according to SAE J2236.

[FIGURE 11 OMITTED]

Low-temperature properties of ACM/PA TPV

Many specifications for underhood automotive components require low-temperature properties to -40[degrees]C, in addition to 150[degrees]C upper heat resistance requirements. ACM/PA TPV was evaluated via Gehman and glass transition temperature (Tg) for its low temperature behavior. The Tg was measured at -42[degrees]C; whereas the Gehman results indicated a T10 value of -40[degrees]C and a T100 value of-52[degrees]C (figure 12). ACM/PA can be expected to function at -40[degrees]C.

[FIGURE 12 OMITTED]

Processability

TPVs afford three key processing advantages versus thermoset elastomers:

* Cycle times on the order of a third or less;

* pre- and post-consumer recyclable, including in-process scrap; and

* overmoldable to rigid thermoplastics.

The cycle time of ACM/PA TPV was compared to that of TS ACM; molding standard 10 cm x 10 cm and 15 cm x 15 cm x 2 mm test plaques. Via injection molding, ACM/PA TPV was found to have a cycle time of less than 30 seconds, compared with 90 seconds for TS ACM. Additionally, ACM/PA TPV does not require an off-line post-cure, whereas TS ACM was post-cured for 1.5 hours to achieve desired physical properties.

The ability to recycle ACM/PA was confirmed via examining initial tensile strength of virgin material compared with that containing 25% and 100% regrind. As shown in figure 13, incorporation of regrind has essentially no impact on initial physical properties. ACM/PA is judged as fully recyclable.

[FIGURE 13 OMITTED]

Overmoldability to thermoplastics

The ability to overmold TPVs to rigid thermoplastics, without the use of an adhesive system, is an area of increasing interest in the automotive market. Via overmolding, two-shot injection molding or 3-D blow molding, it is possible to combine the flexibility of a rubber-like material with the structural integrity of rigid thermoplastics.

The adhesion of ACM/PA to a number of engineering thermoplastics was evaluated (figure 14). ACM/PA shows good ability to adhere to polyamides, whether they be Nylon 6, Nylon 6,6, glass-filled or mineral-filled. Adhesion was also found to at least one Delrin grade of POM. as well as Amodel PPA. In all cases, it was necessary to pre-heat the rigid plastic prior to overmolding with ACM/PA, a practice common in the plastics industry if pre-molded plastic inserts are used.

[FIGURE 14 OMITTED]

Adhesion to metal was possible utilizing a standard rubber solvent-based primer/adhesive system. However, these adhesives must be heat activated and, therefore, the adhesive coated metal must be warmed prior to molding. The full results of the overmold adhesion results of ACM/PA are shown in figure 14.

Conclusions

Several conclusions can be drawn from the data presented:

* The ACM/PA TPV has superior hot oil resistance (150[degrees]C) compared to COPE and TPVs with polypropylene plastic matrices.

* ACM/PA TPV has superior oil resistance compared with Si-TPV. Si-TPV is almost completely destroyed in hot oil.

* ACM/PA TPV is capable of meeting the requirement for less than 50% loss of tensile strength and elongation after long-term, high-heat aging (3,024 hours, 150[degrees]C).

* ACM/PA can be readily molded and recycled. It shows good overmold adhesion without adhesives, to polyamide and some other engineering plastics.

Table 1--original physical properties of
evaluated polymers

                      ACM      CO      EPDM      NBR     Melt      Si-
                      PA       PE       PP       PP      proc.     TPV

Tensile, MPa           9.0     14.4      6.1      5.6      9.9     11.1
Elongation, %        294      644      410      158      400      580
Hardness, duro. A     78       79       82       73       76       68
Specific gravity       1.1      1.1      1.0      1.0      1.3      1.2

                      TS
                      ACM

Tensile, MPa           9.6
Elongation, %        234
Hardness, duro. A     71
Specific gravity       1.3

References

(1.) J. Hamilton II, " An overview of silicone rubber," Rubber World, p. 31, June, 2003.

(2.) A. Paeglis, "A simple model for predicting heat aging of EPDM rubber," paper 35, 162nd meeting of Rubber Division/ACS, October, 2002.

(3.) A. Anderson, R. Bruner and P. Manley, "Advances in heat resistant ACM compounding technology .for under-the-hood applications," paper 34, 162nd meeting of Rubber Division/ACS, October, 2002.

(4.) E. Files, "Novel compounding technology for improved temperature properties for HNBR," paper 32, 162nd meeting of Rubber Division/ACS, October, 2002.

(5.) R. Stevens, "Long term heat aging of various fluoroelastomers," paper 33, 162nd meeting of Rubber Division/ACS, October, 2002.

(6.) R. Eller, "An automotive world without elastomers," IISRP annual meeting, Naples, May 14, 2002.

(7.) J. Liao, "Silicone TPV offers high performance solutions," Rubber World, p. 40, February; 2003.

(8.) B. Cail and R.D. DeMarco, "New heat and oil resistant thermoplastic vulcanizates for demanding under hood applications," SAE paper 3M-173, February 2003.