Blends of HXNBR and low viscosity HNBR for automotive applications.

Selective hydrogenation of the double bonds in acrylonitrile-butadiene rubber (NBR) produces a specialty elastomer known as hydrogenated acrylonitrile-butadiene rubber (HNBR) which may be formulated to produce a material conveying an excellent balance of properties (mechanical and dynamic, as well as improved resistance to hot air, oils, chemicals and abrasion) (ref. 1). Compared to other high performance specialty elastomers, HNBR exhibits the highest sealing force retention in automotive oil (150[degrees]C) as measured by compressive stress relaxation techniques (ref. 2). Given the exceptional performance qualities of this elastomer, its compounds can be used in the automobile industry (timing belts, hosing, seals and gaskets), in the oil industry (stator pumps, packing seals and blowout preventers), as well in other industries for applications such as cable coverings or laminated rollers.

The number of specialty grades of HNBR currently available on the market coincides with the development of emerging technologies designed to develop new grades that fill specific customer demands. At Lanxess, the regular HNBR grades developed in the 1980s were soon followed by the rapid expansion of specialty grades based on particular end use requirements, including: low temperature, heat resistant, acrylate reinforcement, carboxylate technology and finally, low viscosity technology. The final two grade families are proper only to Lanxess. The HXNBR currently available is known in the marketplace as Therban XT VP KA 8889 and contains 34% acrylonitrile (ACN), 5% carboxylic acid groups and 3.5% residual double bond (RDB) content. Hydrogenation of carboxylated nitrile (XNBR) to produce HXNBR brings about the following vulcanizate improvements (ref. 3):

* Mechanical properties (tensile strength and elongation) from 23[degrees]C to elevated temperatures (170[degrees]C):

* tear resistance from 23[degrees]C to 170[degrees]C;

* Pico abrasion resistance:

* adhesive strength to substrates like nylon at 125[degrees]C: and

* hot air oxidation resistance.

Due to this set of outstanding properties, HXNBR can be formulated alone or blended with HNBR. For example, the combination of HXNBR with HNBR and zinc diacrylate (ZDA) gives rise to vulcanizates with high moduli and good elongation properties which can be used in applications such as hard rolls, high tooth stiffness timing belts, as well as heat and abrasion resistant conveyor belts (ref. 4). Wrana et al. (ref. 5) have demonstrated the benefits of the HXNBR/ZDA system for attaining superior mechanical and dynamical-mechanical properties over a wide temperature and deformation range. This was attributed to the very stable ionic interaction of ZDA with the carboxylate group of HXNBR, which together provided exceptional reinforcing properties. In other work, HXNBR compounds have been formulated to 90 A hardness in order to meet the strict requirements of paper mill roll applications where improved heat. tear and abrasion resistance, along with enhanced mechanical properties, are required (ref. 6). HXNBR can be used for the extreme heat/oil/water conditions normally encountered in oil well drilling applications (packers, drill bit seals, blow out preventers and pipe wipers).

Substantial efforts have been undertaken to examine the use of HXNBR for drill bit seal applications (refs. 6-9). In the initial study, the effect of ionic crosslinking was investigated by varying the blend ratio of HNBR to HXNBR, as well as zinc peroxide level. Higher levels of ionic crosslinking were found to be advantageous for increasing stiffness, tensile and tear strength, hot tensile and tear, as well as Pico abrasion resistance; however, they were detrimental to the hot air compression set and DIN abrasion resistance. Consequently, a judicious mix of ionic to covalent crosslinking was chosen in order to optimize all properties of concern to the seal compound. In the second ensuing study, the effect of carbon black type (surface area and structure) was investigated for 70 A hardness compounds. Of the eight carbon blacks evaluated, it was determined that an N650 type black presented the best overall display of properties, with the added benefit of enhanced overall stiffness. In the third part of this series of work, the effect of varying the type and concentration of peroxide co-agent used in the peroxide based formulation was examined. All co-agents tested provided advantages and disadvantages, depending on the property measured. Both triallyl isocyanurate (TAIC) and 1,2-polybutadiene resin (PB) based formulations possess an excellent combination of properties for use in a drill bit seal type application. In the final study, the effects of a post cure (30 to 180 minutes in a hot air oven set at 150[degrees]C) on the optimized drill bit seal compound were examined. The results strongly suggest that a mild post cure of 30 up to 60 minutes at 150[degrees]C is beneficial for optimal seal properties (better compression set, physical properties and abrasion resistance). These improved properties were explained by' a beneficial optimization that takes place in the amount of ionic crosslinking in such dual peroxide/zinc peroxide systems. Although the formulation work was completed for a drill bit seal application, the optimized formulation can be considered for use in other applications where excellent mechanical properties coupled with abrasion resistance and good compression set are necessary.

The most recent and exciting breakthrough in HNBR technology has been the development of low viscosity HNBR-AT, previously impossible to manufacture due to limitations of the viscosity of the NBR feedstock (ref. 10). Commercially known as Therban AT VP KA 8966, it possesses a 34 wt. % ACN content, a viscosity of 39 MU (100[degrees]C) and < 0.9% residual double bonds (RDB). It has been clearly demonstrated to possess the following advantages over regular viscosity HNBR grades (refs. 10 and 11):

* Low molecular weight (low viscosity) provides improved processability, faster injection mold tilling leading to shorter cycle times, improved extrusion rates with compounds possessing smoother surfaces and sharper edges, and quicker mixing due to a faster black incorporation time.

* Narrower molecular weight distribution helps to maintain excellent physical strength properties, despite the drop in molecular weight.

* Elimination or reduction of plasticizer use is possible in the vulcanizate in order to improve heat aging resistance.

* Compound costs can be lowered by use of less plasticizer and slightly more filler (for same hardness compounds).

In addition, no mold fouling occurs when properly compounded. HNBR-AT is particularly suited for rubber compounds used as seals, gaskets, diaphragms, hoses, boots, o-rings, timing belts and cable applications.

Campomizzi et al. (ref. 12) recently carried out an elaborate designed experiment involving the screening of five factors, including: silicate filler, plasticizer level, antidegradant system, metal oxide level and HNBR polymer viscosity. HNBRAT based compounds with good scorch safety, fast cure times and low compound viscosity could be designed for injection molding type conditions. The data also suggested that combinations of HNBR-AT and HNBR-HT (heat resistant) technologies could produce compounds with improved heat and fluid aging characteristics. This study also demonstrated that HNBR compounds, normally excluded from consideration due to their high viscosities, are now feasible due to the existence of low viscosity HNBR-AT. This paves the way for a new generation of compoundable HNBRs with valuable properties for use by end users. The benefits of blending HNBRAT into regular normal viscosity grades of HNBR of varying RDB and ACN contents have been investigated (ref. 13). In addition to processability, heat and fluid aging resistance were noticeably improved in the blends. The study strongly indicated that any current functioning HNBR compound could benefit from blending in HNBR-AT.

One limiting factor in the use of HXNBR has been its high Mooney viscosity (77 MU at 100[degrees]C), which can cause concern for injection molding or extrusion type applications. The hot air aging resistance is an improvement over carboxylated nitrile, but not necessarily satisfactory for the temperatures encountered in an automobile. This current investigation will look into the effect of blending HNBR-AT into a standard peroxide/zinc peroxide curable HXNBR compound in order to help to improve its general processability profile. The effect of blending on physical properties, low temperature behavior, as well as heat and fluid resistance, will also be examined. Certain potential automotive applications with such blends will also be discussed.

Experimental

The peroxide cured HXNBR recipe used for this investigation is tabulated in table 1. The control formulation contains 100% HXNBR. HNBR-AT (34% ACN, < 0.9% RDB, 39 MU) is then blended into HXNBR at three different levels: 25/75, 50/50 and 75/25. In the first four compounds, the level of zinc peroxide was kept constant at 7 phr. Since the blends (HNBRAT rich in particular) will contain less total carboxylic acid content, it was decided to reduce the level of zinc peroxide for another four compounds to a level of 3 phr, therefore providing a total of eight compounds (two HXNBR controls and six blends of HXNBR/HNBR-AT).

A laboratory BR-82 internal mixer (1.6 L capacity) was used for first stage mixing. The rotor speed was set at 55 rpm and cooling carried out at 30[degrees]C. All ingredients except the curatives were added to the mixer using a conventional mix addition and sequence. The peroxide was added and incorporated during the second stage on a cool 10" by 20" two roll mill.

The processability and final compound properties of the HNBR blends were measured in accordance with the ensuing list of ASTM/DIN procedures:

* Mooney viscosity and scorch--ASTM D1646-81;

* moving die rheometer (MDR)--ASTM D5289;

* rubber process analyzer (RPA)--ASTM D6204-97;

* tear resistance--ASTM D624;

* temperature retraction--ASTM D1329;

* hardness--ASTM D2240;

* stress strain--ASTM D412 A;

* fluid resistance--ASTM D471;

* compression set--ASTM D395 B;

* hot air aging resistance--ASTM D573;

* DIN abrasion resistance--DIN 53 516;

* Pico abrasion resistance--ASTM D2228; and

* ozone resistance--ASTM D1149.

Results and discussion

Processability

Perhaps the easiest and most common method to gage the ease of processability of rubber is by measuring the Mooney viscosity. The raw elastomer Mooney viscosities (MLI+4@ 100[degrees]C) were measured on HNBR-AT and HXNBR as 40.0 and 74.4 MU, respectively. These values are near the midpoints of the viscosity ranges for these materials. Compound Mooney viscosity and scorch results are given in figures 1 and 2, respectively. The trends from these graphs are quite revealing. The systematic and progressive blending of HNBR-AT into HXNBR brings about a substantial lowering of the compound viscosity. Compound viscosity values decrease by up to 20 MU in the cases where the blend ratio is 25/75. Compound Mooney viscosities in the range of 50 to 60 are desirable for injection moldable products. It is also observed that the higher zinc peroxide level compounds (7 phr) possess a lower compound viscosity compared to the 3 phr ones. This may be attributed to the zinc peroxide dispersion providing a lubricating effect to the compound, lowering the compound viscosity at high temperatures. Mooney scorch is often an issue with carboxylated elastomers. The use of a slow release dispersion of zinc peroxide alleviates this issue in 100% HXNBR, however, as can be seen from the data in figure 2, HNBR-AT addition will help to prolong the safety period before vulcanization. Longer scorch safety is advantageous in injection molding in the case of long channels and intricate die designs that require good flow for complete mold filling.

[FIGURES 1-2 OMITTED]

The RPA 2000 was used to assess the processability characteristics of all eight compounds. Through use of dynamic mechanical measurements, the viscoelastic behavior of a rubber compound can be probed by changing the frequency, strain, time and temperature of the measurement (ref. 14). Results have been reported in terms of the storage modulus (G') and tan delta, which is the ratio of the loss modulus (G") to the storage modulus. Given the combination of strain and frequencies used, the RPA frequency sweep essentially looks at shear rates ranging from 0,01 [s.sup.-1] to 14.6 [s.sup.-1]. The frequency sweep results in terms of G' and tan delta for 7 and 3 phr zinc peroxide levels are illustrated in figures 3a and b respectively. As expected, the higher molecular weight compound (HXNBR alone) possesses the highest storage modulus as function of frequency, due primarily to a higher number of chain entanglements. Blending in HNBR-AT progressively lowers the storage modulus. Tan delta values at the high frequency end increase with the progressive addition of HNBR-AT. These results strongly suggest that better processability is expected for the HNBR-AT rich blends.

[FIGURES 3a-3b OMITTED]

Strain sweeps were also undertaken on all eight compounds using a frequency of six cycles per minute (cpm) and a test temperature of 100[degrees]C (figures 4a and b). In both of these figures, similar trends can be noticed. The highest storage moduli values and lowest tan deltas as a function of strain magnitude are observed with the HXNBR alone or HXNBR rich blend formulations. In other words, the progressive addition of HNBR-AT in the blend helps to reduce the overall elastic behavior in the unvulcanized state, providing a compound possessing improved processing characteristics.

[FIGURES 4a-4b OMITTED]

A capillary rheometer (Monsanto processability tester) possessing a barrel L/D of 30 and a die diameter of 0.0754 cm was employed to explore the injection moldability of the compounds in the higher shear rate zone. In figures 5a and 5b, the barrel pressure is plotted as a function of shear rate. In both figures, it is observed that increasing the concentration of HNBR-AT in the blend will help in decreasing barrel pressure; meaning that for the same barrel pressure, an HNBR-AT blend will flow quicker through the capillary compared to the HXNBR compound alone. Compounds containing a higher level of zinc peroxide (figure 5a) present a slighter lower barrel pressure than those at the 3 phr level; again, most likely due to the lubricating effect of the zinc peroxide dispersion. Barrel pressure became unreliable in the 3,000 [s.sup.-1] range of the rheometer as the 1,000 bar range limit of the apparatus was attained. Nevertheless, the trends observed in the lower shear rate range of the rheometer are favorable towards the use of HNBR-AT rich blends for lower barrel pressures and/or quicker flow behavior.

[FIGURES 5a-5b OMITTED]

Finally, the cure characteristics of the eight compounds were followed on a moving die rheometer (MDR) operating at a frequency of 1.7 Hz, an oscillating strain of 1[degrees] and at a temperature of 180[degrees]C. The corresponding cure curves are displayed in figures 6a and b respectively. These results clearly indicate that HNBR-AT rich blends possess lower minimum torques, which translates into enhanced flowability before the onset of cure. High levels of both zinc peroxide and HXNBR caused the appearance of a slight marching modulus (figure 6a), while at low levels of zinc peroxide, no marching modulus phenomenon was detected. These results also suggest a slightly higher state of cure for the compounds containing less zinc peroxide.

[FIGURES 6a-6b OMITTED]

Physical properties

A-2 type hardness was measured at 23[degrees], 100[degrees] and 150[degrees]C (figure 7). The highest hardnesses were obtained on the HXNBR compounds, as AT addition progressively lowered hardness values. Higher levels of zinc peroxide provided slightly higher hardness values. The best hardness retention at elevated temperature testing was observed with the AT rich compounds and for lower zinc peroxide concentrations.

[FIGURE 7 OMITTED]

Stress-strain data were collected on all eight compounds and reported in terms of stress at 100% elongation (figure 8), elongation (figure 9) and tensile strength (figure 10). The stress data in figure 8 mirror the trends observed in the hardness values. HNBR-AT addition to HXNBR causes a decrease in stiffness values. Higher stiffness values are possible with 7 phr of zinc peroxide addition. The high zinc peroxide addition will maximize the number of ionic crosslinks possible in the compounds. As seen elsewhere (ref. 9), the ionic crosslinking effect is not fully seen in the cure data due to its labile nature at very high temperatures (180[degrees]C), as it actually suggested the opposite stiffness effect in this work. The AT rich compounds display the least change in stiffness upon elevated temperature testing. Elongation increases with HNBR-AT addition and for the lower level of zinc peroxide (figure 9). Here it is observed that the best elongation at higher temperatures is seen with the HXNBR rich blends containing 7 phi of zinc peroxide. Extraordinary high tensile strengths of well over 25 MPa are possible using HXNBR (figure 10). Blending in HNBR-AT brings about a slight decrease in tensile strength, but only by about 1-3 MPa. A drop in tensile strength is noted during the elevated temperature testing: however, values of 8-13 MPa are possible at 150[degrees]C. Excellent mechanical properly retention at elevated temperatures is crucial for proper functioning of parts that provide good sealing behavior.

[FIGURES 8-10 OMITTED]

Tear resistance, using die B or die C cut specimens, was measured at room temperature (figure 11). The excellent tear strength of HXNBR is observed. Blending in HNBR-AT has only a moderate effect in decreasing the tear strength, as excellent values are retained up to the 25/75 blend ratio.

[FIGURE 11 OMITTED]

As shown by the temperature retraction data in figure 12, the low temperature resistance slightly improves upon HNBR-AT addition. As both materials are 34% ACN, the diluting of the amount of carboxylate functionality in the blends would give rise to a slight improvement in low temperature flexibility.

[FIGURE 12 OMITTED]

Ozone static testing (20% extension) was carried out on the eight compounds at 40[degrees]C and using an ozone pressure of 50 mPa. No signs of cracking were observed in any of the compounds alter 168 hours of testing (0/0 rating according to ASTM standard).

Hot air heat resistance

The hot air heat resistance of the eight compounds was tested by way of compression set at 135[degrees]C (figure 13) and 150[degrees]C (figure 14) up to 504 hours. The trends are readily apparent. Progressive HNBR-AT addition causes a lowering of compression set. Lower levels of zinc peroxide were advantageous in reducing set. The best compression set numbers were obtained using 75 phr of HNBR-AT and 3 phr of zinc peroxide. Lower ionic crosslink levels give rise to a better compression set resistance. It has been demonstrated that a mild post cure on such systems of 30 to 60 minutes at 150[degrees]C in hot air can optimize the overall crosslinking network and provide better compression set (ref. 9).

[FIGURES 13-14 OMITTED]

The hardness and stress strain data changes upon exposing die C dumbbell samples to hot air at 135[degrees]C for 504 hours are depicted in figure 15. In all cases, hardening and stiffening takes place with corresponding loss of elongation, a result normally depicted by HNBRs (ref. 15). It is clear, however, that adding more HNBR-AT can lessen elongation loss. Increased zinc peroxide level provided a moderate reduction in elongation loss as well.

[FIGURE 15 OMITTED]

Abrasive properties

Figures 16 and 17 demonstrate the abrasive characteristics by way of the DIN and Pico methods of abrasion measurement. Excellent DIN numbers between 50 and 60 (in terms of volume loss) were measured on all samples. Better DIN abrasion was seen with lower zinc peroxide levels, while blending in HNBR-AT had a minimal effect on this property. On the other hand, the Pico abrasion resistance was hugely affected by HNBR-AT concentration in the blend. Extraordinary high values of abrasion resistance can be observed for the HXNBR compounds. Blending in HNBR-AT lowers this sort of abrasion resistance, but still provides good abrasive properties to the compound.

[FIGURES 16-17 OMITTED]

Fluid immersion testing

A variety of test fluids and greases were chosen in order to probe possible automotive applications where a combination of HXNBR/HNBR-AT could be used. SF105 oil (figure 18), lithium grease (figure 19), polyurea grease (figure 20) and power steering fluid (figure 21) were all chosen for screening purposes. All immersion aging testing was carried out on specimens aged for 168 hours at 135[degrees]C. Property change in terms of hardness, tensile, elongation and volume swell is reported in each of these figures. SF105 oil resistance is favored by higher HNBR-AT concentrations, as seen by better elongation retention values. Lithium and polyurea based constant velocity joint grease resistance is improved in the HNBR-AT rich blends. Higher zinc peroxide concentrations display better resistance to polyurea grease. Finally, the power steering fluid resistance improves with HNBR-AT addition, with higher zinc peroxide levels being advantageous for elongation retention characteristics.

[FIGURES 18-21 OMITTED]

Conclusions

A study in which low viscosity HNBR-AT is progressively blended into HXNBR has been completed. Zinc peroxide levels were set at high and low levels in order to compensate for changing carboxylic acid levels in the blends. Compound Mooney viscosity, RPA and capillary rheometer data clearly showed improvement in processability upon blending HNBR-AT into HXNBR. Scorch safety was also seen to improve. Extremely high tensile strength compounds were observed using blends of HXNBR and HNBR-AT, with excellent retention of physical properties (stiffness, elongation) at elevated temperatures. HNBR-AT addition can moderately improve the low temperature resistance. Compounds were found to be ozone resistant independent of HNBR-AT concentration. The HNBR-AT rich blends with lower zinc peroxide levels provided the best resistance to hot air under compression and tensile testing. Excellent abrasion resistant compounds are possible using a blend combination of HXNBR and HNBR-AT. HNBR-AT rich blends (75 phr HNBR-AT/25 phr HXNBR) provided the most resistance to SF105 oil, lithium and polyurea greases, and to power steering fluid. Automotive engine seals, wheel seals, o-ring and CVJ boot type applications are prime candidates for a blend of HNBR-AT and HXNBR.

References

(1.) H. Buding, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co., Chapter 3: Synthetic Rubber, Sec. 6.3, 2002.

(2.) R.J. Pazur, J. Bielby and U. Dinges, Rubber Division, ACS, paper no. 107, Cleveland, OH, October 14-17, 2003.

(3.) S.X. Guo and W. von Hellens, Rubber World, 225 (5), 51-55, February 2002.

(4.) W. von Hellens and S.X. Guo, Rubber Division, ACS, paper no. 120, Pittsburgh, PA, October 11, 2002.

(5.) C. Wrana and D. Achten, Rubber Division, ACS, paper no. 48, San Antonio, TX, May 18, 2005.

(6.) L.P. Ferrari, R.J. Pazur and E.C. Campomizzi, Rubber Division, ACS, paper no. 104, Cleveland, OH, October 16, 2003.

(7.) E.C. Campomizzi, L.P. Ferrari and W. von Hellens, Rubber Division, ACS, paper no. 122, Pittsburgh, PA, October 11, 2002.

(8.) R.J. Pazur, L.P. Ferrari and E.C. Campomizzi, Rubber Division, ACS, paper no. 48, Columbus, OH, October 16, 2004.

(9.) R.J. Pazur, L.P. Ferrari and E.C. Campomizzi, Rubber Division, ACS, paper no. 70, San Antonio, TX, May 18, 2005.

(10.) S.X. Guo and F. Guerin, Rubber Division, ACS, paper no. 62, Columbus, OH, October 5-8, 2004.

(11.) R. Parg, C. Wrana and D. Achten, Kautsch. Gummi Kunstst. 58 (3) 2005, 109.

(12.) E.C. Campomizzi, L.P. Ferrari and R.J. Pazur, Rubber Division, ACS, paper no. 34, Pittsburgh, PA, November 1-3, 2005; Rubber World, 235 (3), December 2006.

(13.) R.J. Pazur, Automotive Elastomers Conference Papers, Dearborn, MI, June 14, 2005.

(14.) J. Sezna, Rubber & Plastic News, November 30, 1998.

(15.) H. Bender and E. Campomizzi, Kautsch. Gummi Kunstst. 5, 14 (2001).

Table - standard HXNBR formulation
used, with the blend ratios of HNBRs and
amounts of zinc peroxide given

Ingredient                              Phr
HXNBR                 100   75   50   25   100   75   50   25
HNBR-AT                 0   25   50   75     0   25   50   75
Carbon black N660                        50
ODA                                     0.5
ODPA                                    1.5
TOTM                                      5
zinc peroxide (50%)            7                     3
TAIC                                    1.5
Peroxide (40%)                          7.5
Total phr                    173                   169

by Richard J. Pazur and E.C. Campomizzi, Lanxess

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