High-performance HT-ACMs for automotive molded and extruded applications.

Polyacrylate polymers are known as specialty elastomers, which are classified as high-temperature and oil resistant elastomers. The ASTM D2000 and SAE J200 designation for polyacrylic polymer is ACM (acrylic co-monomer). The majority of end uses for ACM polymers are in applications where high-temperature and oil resistance is required. Six years ago, Zeon introduced the first of a range of new ACM elastomers (HT-ACM) with a unique functional cure site (refs. 1-3). Since then, the range has been expanded to four grades, with additional grades under development. These elastomers offer significant improvements in long-term high-temperature resistance and their use and superiority in a number of demanding automotive applications, including valve cover gaskets, oil pan gaskets, rocker cover gaskets and various seals, has already been successfully demonstrated. These applications, where sealing force retention for >160,000 kilometers is a typical expectation, require the excellent long-term high-temperature performance, excellent compression set, compressive stress relaxation (CSR) resistance, and stable modulus that these elastomers provide. Development of the latest of these HT-ACM elastomers, HyTemp AR212HR, has focused on enhanced extrusion specific processing characteristics. This has expanded the use of HT-ACM to applications such as hoses, tubes and air ducts.

This article describes the continuous development of Zeon HT-ACM elastomers and associated materials and will demonstrate the latest grade's ability to meet the high-temperature performance requirements of applications like (high temperature) HT-TDI hose and air ducts used in charged air systems. Benefits include longer scorch times for extrusion processing combined with economical and flexible curing characteristics, which offer significant cost benefits compared to current high-temperature solutions.

Experimental

Procedure--mixing and curing

The ACM compound formulation was based on meeting the VW standard for TDI hoses TL526.34. All compound masterbatches were prepared in a 1,600 cc internal mixer with high rotor speeds. All ingredients, except for curatives, were charged using an upsidedown mixing technique. After incorporation of fillers (approximately four minutes) the masterbatch was discharged at a temperature of 170[degrees]C.

This masterbatch was then transferred to a cool 20 cm two-roll mill, sheeted to approximately three millimeters thickness, and allowed to rest for one hour. After the rest period, the compound masterbatch was then placed back onto the mill, allowed to form a smooth rolling bank, and curatives were added. Test articles prepared from this final compound were compression molded at 190[degrees]C for eight minutes and post cured in a forced air convection oven for four hours at 177[degrees]C.

Rheological properties

Mooney viscosity and Mooney scorch measurements were conducted according to ASTM D1646 at 100[degrees] and 120[degrees]C, respectively. Cure characteristics were determined using a rheometer according to ASTM D2084.

Vulcanized evaluation

Tensile strength, elongation and hardness values were obtained according to ASTM D412 and D2240. The tensile and elongation data were collected using a United model E-VI-60 six-station tensile test instrument, while the hardness data were collected using a Zwick hardness tester. Air oven aged physical properties were obtained according to ASTM D573 and were evaluated on the United tester. Compression set was determined using the PV 3307 and VDA 675 218 (VW compression set) methods.

Background

The HT-ACM elastomers were primarily designed to be fast curing for injection molding processes and have been used successfully in many molded applications. The cure system used for optimum heat resistance results in typical scorch times for these compounds in the range of 4 to 8 minutes at 120[degrees]C. Hose manufacturers require safer processing characteristics combined with high throughput for economical production. This led to the development of the latest HT-ACM elastomer and specialized curative systems, which can be used for both molding and extrusion applications. A new protection system has also been developed to further enhance high-temperature aging performance to meet demanding high-temperature specifications.

Compound development

The series of ACM compounds was designed to evaluate cure systems as well as antioxidants systems. These four ACM compounds will be compared against similar standard AEM compounds. All the compounds were compounded to equal hardness (60 durometer).

Compounds 1 through 4 were based on HyTemp AR212HR. Compound 1 was based on the new protection package along with the extrusion cure system, and the compound is designated as HT ACM EX. Compound 2 evaluated the standard antioxidant package using the extrusion cure package, which is referred to as HT ACM EXAO. Compound 3 was similar to the first compound, except the molding cure system was evaluated. This compound is referred to as HT ACM M. Compound 4 is similar to compound 3 except for the antioxidant. This compound contained the standard antioxidant package and will be referred to as HT ACM MAO.

The four ACM compounds could be compared against themselves for heat resistance. There are other acrylate elastomers in the market place, and these have found homes in extrusion applications. It was a natural comparison for the ACM compounds to be evaluated against AEM compounds used for similar applications. Compound 5 was based on Vamac G and will be designated AEM-G. Compound 6 was based on Vamac GLS and will be designated AEM-GLS. Compounds 7 and 8 were based on Vamac GXF. Compound 8 was a slightly modified version of compound 7. The compounds will be referred to as AEM-GXF and AEM GXF M respectively. The AEM recipes can be seen in table 1.

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The rheological properties of the compounds can be seen in table 2. The scorch times ([t.sub.s]5) for the HT-ACMs range from 7 to 16 minutes. The scorch times of the AEM compounds range from 6 to 14 minutes. This could be considered acceptable for both molding and extrusion operations. All the compounds had T'90 cure times less than 20 minutes at 180[degrees]C, which fits in with most extrusion capabilities.

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All the compounds were in the target hardness range of 55 to 65 durometer A. The tensile strengths of the AEMs were higher than those of the HT-ACMs when measured at room temperature (23[degrees]C). The tensile strength of the HT-ACM based compounds was slightly higher than the AEM compounds when the test was performed at an operating temperature of 150[degrees]C. The ultimate elongations of the AEM compounds were higher than those of the HT-ACM compounds when measured at room temperature; however, the elongation of the HT-ACMs was higher at elevated temperature. The results can be seen in figures 1 through 3.

Compression set properties were evaluated under different conditions. The test temperature was 175[degrees]C and the test specimens were plied discs. Time intervals evaluated were 24, 168 and 504 hours. A noticeable pattern appears in the HT-ACMs (figure 4). The compounds with the standard AO (EXAO and MAO) had lower compression set than that of the compounds with the extrusion AO package. The molded compounds had lower compression set than the extrusion compounds, as expected. The AEM GXF based compound had higher compression set than the ones based on AEM-G and GLS.

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VW-type compression set was tested 24 hours at 175[degrees]C. The compression set of the HT-ACM molded compounds was less than the extruded compounds, as expected. One could observe a similar trend in the values associated with the antioxidant package used, as noted above. The compression set values of all the extrusion HT-ACM compounds were considerably lower than the limits seen in various automotive specifications of 80% max. The compression set of the HT-ACMs was equal to or better than the AEM based compounds. The results can be viewed in figure 5.

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Air oven aging was performed at three different temperatures: 175[degrees]C (1,000 hours), 190[degrees]C (168 hours) and 200[degrees]C (168 hours). A comparison of aged tensile values can be seen in figure 6. The tensile strengths of the HT-ACM compounds after aging were compound dependent; this was noticed after the different aging times/temperatures. The compounds containing the new AO package (EX and M) had higher tensile strengths than the ones with the standard AO package. The difference in tensile values between the AEM and HT-ACMs noted in the originals has now been minimized after aging. This should translate into higher percent tensile loss for the AEM compounds. Or, said another way, the tensile strength of the HT-ACM compounds is similar to that of the AEM compounds after aging. Data can be seen in figure 6.

Higher elongation values were observed in the compounds with the standard AO package (EXAO and MAO) than with the new AO package in both cure systems. Basically, the standard AO package gives better (lower) change in percent elongation. Elongation values of the HT-ACM compounds were higher than the values of the AEM compounds after aging at all three conditions: 175[degrees]C (1,000 hours), 190[degrees]C (168 hours) and 200[degrees]C (168 hours). Higher elongation values after aging result in lower elongation loss and better flexibility for the HT-ACM based compounds. The percent elongation data can be seen in figure 7, while the change in properties at the different temperatures can be seen in figures 8 through 10.

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All of the compounds were aged in two European motor oils, Lubrisol OS 206304 and Shell Helix Ultra. The samples were aged 94 hours at 175[degrees]C and 168 hours at 190[degrees]C in the above-mentioned oils, respectively. These were unusual temperatures, the typical test temperature used in testing is 150[degrees]C. These test conditions are considered severe (oils cannot take the heat for extended periods of time). Under these severe test conditions the HT-ACM based compounds exhibited lower property change than did the AEM based compounds. The property changes can be viewed in figures 11 and 12.

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The compounds were exposed to a mixture of diesel oil and RME (rapeseed oil methyl ester [European biodiesel]). The first mixture was composed of a low sulfur diesel fuel from Texaco at 95% blended with 5% RME. The second diesel fuel mixture evaluated was Diesel Ref. Fuel: ISO 1817 Type F Fluid 95%/RME 5%. This is a very aggressive fuel mixture. The test time/temperature duration for the fuel mixtures immersion was 46 hours at 23[degrees]C. The effect of the fuel exposure on the compounds can be seen in figure 13. All four of the HT-ACM based compounds exhibited excellent resistance to the low sulfur/RME mixture. The HT-ACM based compounds exhibited lower property change than did all the AEM based compounds.

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The diesel fuel mixture, which contained ISO 1817 Type F fluid/RME, was more aggressive toward the rubber compounds. The compounds exhibited larger change in hardness, tensile strength, percent elongation and weight. Despite the more aggressive nature of this fluid, the HT-ACM based compounds exhibited lower property change than did all the AEM based compounds. The effect of the fuel exposure on the compounds can be seen in figure 14.

The physical and chemical properties of the HT-ACMs compounds have been discussed. An application which requires the heat resistance of the HT-ACMs is turbo diesel injection hose (TDI).

High-temperature hose applications

The automotive industry continues to develop new high-performance engines to meet both environmental and low maintenance requirements. Diesel engine technology, with increased injection pressures, generates more heat. The general trend continues to be higher engine bay temperatures, and automotive fluids and oils that are more aggressive towards elastomer components. HT-ACM elastomers continue to offer the improved long-term heat and fluid resistance that is required. Efforts by the automotive industry to maximize cost/ performance benefits in material selection are leading to the replacement of more expensive solutions, such as fluoroelastomers, silicones and fluorosilicones with new, more cost-effective alternatives. One such example includes HT-ACM in charged air systems, specifically Hot Side TDI charged air hoses. Zeon HT-ACM hose components also readily operate at lower temperatures in areas such as transmission (TOC) and engine oil cooler (EOC) hose applications.

[FIGURE 14 OMITTED]

Car producers recently have investigated HT-ACM for continuous service at 175 [degrees]C/185[degrees]C, with peaks to 190[degrees]C/200[degrees]C in the hot side HT TDI area of charged air systems. As noted, due to the temperatures involved, the incumbent materials have traditionally been FKM, VMQ and FVMQ. Until the recent introduction of HT-ACM elastomers, there had been no cost-effective alternative material available which could bridge the gap between the standard ACM and AEM hoses operating at 150[degrees]C/160[degrees]C and these higher temperatures. In some cases, metal shielding is required to protect the lower temperature standard hoses. The introduction of HT-ACM hoses can eliminate the need for such shielding, thus providing weight, space and cost savings, while also potentially reducing noise, vibration and harshness (NVH). Of course, at temperatures above 200[degrees]C the fluoroelastomers and silicones will continue to be required, and in some cases where extreme temperatures are encountered, metal air ducts are employed.

HT-A CM performance There are several key properties of a TDI hose. These include, but are not limited to, the following:

* Compound must be extrudable at

--hardness: 50 to 65 durometer A

* excellent heat resistance

--continuous operating conditions--175[degrees]C to 185[degrees]C,

--peak operating conditions--190[degrees]C to 200[degrees]C;

* good tear resistance;

* excellent oil resistance; and

* excellent diesel fuel resistance.

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Heat resistance

In classifying the performance of elastomeric compounds, test data are typically presented in terms of percent loss of a particular property. In both standard ACM and HT-ACM compounds, retention of elongation at break and tensile strength are considered as two of the most relevant properties in assessing heat resistance performance. The SAE J2236 document (ref. 4) (standard method for determining continuous upper temperature resistance of elastomers) defines the continuous use temperature of a material as being related to the retention of more than 50% the value of an original property at the given test temperature.

The heat resistance of the HT-ACM AR212HT was measured using the SAE J2236 as a guideline (ref. 4). Air oven aging was conducted at three different temperatures, 150[degrees]C, 175[degrees]C and 190[degrees]C. Tensile loss vs. time was plotted for the 190[degrees]C and 175[degrees]C agings. Figures 15 and 17 represent this data. Figures 16 and 18 represent the plot of % elongation loss plotted against time. In each of the two test temperatures, elongation was the first property to lose 50% of its original value. The percent elongation retention/loss is the property that determines the heat resistance by using this method.

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Figure 19 shows a plot of % elongation change at 150[degrees]C. This compound was tested to 2,000 hours, and the compound still retained ~90% of its original elongation. The curve was then extrapolated to 50% loss, as is shown in figure 19.

The time to 50% loss was calculated for each of the three temperatures. The log of time was plotted vs. log 1/T ([degrees]K absolute temperature), resulting in a line with an [R.sup.2] value of >0.99, indicating a good fit. The plot was changed to a more user-friendly plot, and can be seen in figure 20.

Tear resistance

The typical tear resistance test that is used is trouser tear against the grain using the DIN 53507A method. The test specimens were oven aged 168 hours at 175[degrees]C and 72 hours at 200[degrees]C. The results were plotted and can be viewed in figure 21. Most of the TDI specifications specify tear values above 2 kN/m. The compound based on the HT-ACM212HR met this requirement.

Oil and fuel resistance

Resistance to automotive engine oils and fuels is important in many automotive hose applications. HT-ACM compounds offer excellent resistance to a range of fully and semi-synthetic engine oils; diesel and diesel/bio-diesel fuel blends. It should be noted that ACM elastomers are not resistant to gasoline fuel. Figures 22 and 23 show the excellent performance of HT-ACM AR212HR hose compound in the reference oil, Cecilia 20 (C20) and engine oils, Castrol SLX and Shell Helix Plus. The performance offered by the HT-ACM elastomers enables their use in a whole range of hose applications where these and other similar lubricants may be encountered.

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[FIGURE 20 OMITTED]

The diesel fuel resistance can be seen in figure 24. The fuels that were used in this evaluation of the HT-ACM AR212HR were standard diesel and a mixture of Diesel/5%RME (95/5). The test time and temperature used were 48 hours at 23[degrees]C. Minimal change in physical properties was observed after the samples had been exposed to the different fuels/mixtures.

Functional performance

Specifications for HT-TDI hoses calling for the use of HT-ACM have already been or are in the process of being introduced by many European car manufacturers. Hoses developed and now in production with HyTemp AR212HR have satisfied the dynamic impulse performance requirements of many of these specifications at the typical expected operating temperatures of 170[degrees]C to 180[degrees]C, with peaks up to 190[degrees]C.

TDI hose specifications have been introduced or revised to accommodate HT-ACM at seven different auto manufacturers..

The SAE definition of heat resistance does not mean that the compound cannot function above this temperature. For example, the HT-ACM is rated at 175[degrees]C by this definition, however compounds based on this polymer have functioned quite well at 180[degrees]C with intermittent temperature spikes to 200[degrees]C. It should be noted that this is a guideline to a functional absolute temperature.

Summary and conclusions

A new HT-ACM elastomer, new curative and new protection system have been developed, providing both long-term high-temperature performance and excellent processing characteristics for extrusion operations. Applications for this new elastomer include extruded HT TDI hose and air ducts. The Volkswagen Group was the first to introduce the new class of HT-ACM TDI hoses in 2004, with the requirements specified in the VW TL 526.34 standard (ref. 5). Other car producers have followed with a similar approach. A fundamental change in philosophy was adopted by VW with respect to the HT hoses, being much softer (55 to 65 durometer A) compared to the well-established standard (70 durometer A) TDI hoses. These HT hoses are expected to perform continuously at 175 [degrees]C/185 [degrees]C and intermittently to temperatures as high as 190[degrees]C/200[degrees]C. They were introduced to fulfill the performance deficiencies of standard ACM and/or AEM TDI hoses (150[degrees]C to 160[degrees]C) and provide a more cost-effective alternative to the more expensive current high-temperature solutions, such as FKM, VMQ and FVMQ hoses. These softer hoses also offer benefits in reduced noise, improved vibration characteristics and enhanced flexibility during aging in service.

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Benefits of the new HT-ACM elastomer can be summarized as:

* HT-ACMs yield higher retention of property than AEM compounds.

* The new antioxidant package present in the HT-ACM gives better tensile strength after aging, and should only be used when trying to meet a specification that has minimum tensile values like VW for extruded products.

* Excellent long-term aging characteristics, minimal property changes.

* Temperature range -30[degrees]C to 175[degrees]C (SAE definition) with peaks to 190[degrees]C/200[degrees]C.

* Enhanced scorch behavior and excellent extrusion characteristics.

* Upgrade vs. AEM and standard ACM hoses operating at 150[degrees]C/160[degrees]C.

* Cost effective alternative to present high-cost TDI solutions.

* Wide range of hose hardness can be extruded: 50 to 80 durometer A.

* Excellent oil and diesel fuel resistance.

* Suitable for molded and extrusion applications.

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This article is based on a paper presented at a meeting of the Rubber Division, ACS (www.rubber.org).

References

(1.) I. Kubota, et al., "Improved heat and compression set resistance polyacrylate elastomers; Part A, polymer development," Paper 126, Rubber Division, ACS, Cincinnati, OH, 2000.

(2.) P. Manley, et al., "Improved heat and compression set resistance polyacrylate elastomers; Part B--applications in engine and automatic transmission fluids requiring low compressive stress relaxation," Paper 127, Rubber Division, ACS, Cincinnati, OH, 2000.

(3.) I. Kubota, et al., "Improved low-temperature properties polyacrylate elastomers," Paper 32, Rubber Division, ACS, Cleveland, OH, 2001.

(4.) SAE J2236--Standard method for determining continuous upper temperature resistance of elastomers.

(5.) Volkswagen AG, TL 526.34, September 2005--Charge Air Hoses Made of High-Temperature ACM Elastomer.

by Andy Anderson, Paul Manley, John Moore, Peter Abraham, Ivan Burczak, Jeff Dickerhoof Yoshiaki Aimura and Hirofumi Masuda, Zeon Chemicals (http://www.zeonchemicals.com/hytemp.aspx)

Table 1--AEM recipes

                   AEM-G     AEM-      AEM      AEM
                              GLS      GXF    GXF M
AEM G              100.00
AEM GLS                     100.00
AEM GXF                              100.00   100.00
N772                50.00    50.00    50.00    50.00
Stearic acid         1.50     1.50     1.50     1.50
AO 445               2.00     2.00     2.00     2.00
Process aid VAM      1.00     1.00     1.00     1.00
Process aid 18 D     0.50     0.50     0.50
TP759                5.00     5.00     5.00     5.00
Subtotal           160.00   160.00   160.00   159.50
Cure system
DOTG                    4        4        4        4
HDC                  1.50     1.50     1.50     1.50
Total              165.50   165.50   165.50   165.00

Table 2--rheological properties

                         HT ACM   HT-ACM   HT-ACM   HT-ACM   AEM
                            -EX    -EXAO       -M     -MAO    -G

Mooney scorch ML at 120[degrees]C
Minimum viscosity          30.0     32.0     29.0     30.0   16.0
T5, (min.)                 11.3      7.3     15.1     12.6    8.2
T10, (min.)                14.0      8.5     18.3     15.3   10.3
MDR-2000 rheometerat 180[degrees]C
ML, (lbf.-in.)              1.0      1.0      0.9      1.0    0.2
MH, (lbf.-in.)              4.8      4.9      6.4      6.5    8.5
Ts2, (min.)                 1.9      1.5      2.0      1.7    2.0
T90, (min.)                18.6     18.4     12.7     11.4    6.2

                          AEM    AEM     AEM
                         -GLS    GXF   GXF M

Mooney scorch ML at 120[degrees]C
Minimum viscosity        16.0   15.0    16.0
T5, (min.)                6.5   13.1    11.3
T10, (min.)               8.5   18.4    15.5
MDR-2000 rheometerat 180[degrees]C
ML, (lbf.-in.)            0.2    0.2     0.2
MH, (lbf.-in.)            7.7    7.4     8.0
Ts2, (min.)               1.0    2.0     1.7
T90, (min.)               5.0    9.6     8.7