Compounding for improved physical and heat aging properties in HXNBR.

Hydrogenated carboxylated acrylonitrile-butadiene rubber (HXNBR) conveys many improved properties compared to carboxylated nitrile rubber (XNBR), such as mechanical properties (modulus and hot tear strength), abrasion resistance, adhesion to fabrics and hot air aging resistance. The combination of these property improvements gives rise to a robust material that can be used in many diverse applications, including paper-making and printing rolls, as well as oil well specialties such as packer and drill bit seals, blow out preventers and pipe wipers. Hydrogenation of XNBR to a level of 3.5% residual double bond content provides a versatile elastomer that can be crosslinked using sulfur, peroxide and/or metal ions (ionic). The 34% acrylonitrile level provides oil resistance. In this article, improvements of the physical properties and hot air aging resistance of HXNBR by compounding techniques will be explored. Examples of property improvement will be taken from both the roller and oil well industries.

Hydrogenated carboxylated acrylonitrile rubber is made by selectively hydrogenating the carbon-carbon double bonds in carboxylated nitrile without chemically altering the carboxylic acid functional group. This group is distributed in a random fashion along the chain backbone. The currently available grade (Therban XT VP KA 8889) contains 34% ACN and 5% carboxylic acid groups, and has been hydrogenated to the level where only 3.5% residual double bonds remain (ref. 1).

It has been demonstrated that hydrogenation of carboxylated acrylonitrile-butadiene rubber (XNBR) conveys the following major property improvements over XNBR (ref. 1):

* Higher tensile strength and elongation for a given hardness;

* better tensile strength and elongation retention up to 170[degrees]C;

* superior die B and C tear strengths in the 23[degrees]C to 170[degrees]C range;

* better cutting abrasion resistance (Pico test);

* higher adhesive strength to nylon at 125[degrees]C; and

* improved hot air aging resistance.

HXNBR is particularly useful in blends with HNBR using zinc diaerylate (ZDA) as part of the crosslinking system. These blends result in a system characterized by higher modulus and tensile, better adhesion, improved low temperature properties and enhanced Pico abrasion resistance.

Von Hellens, et al., (ref. 2) have shown that although HNBR/HXNBR/ZDA blends form high modulus vulcanizates, good elongation properties can still be maintained. In their DOE (designed experiment), it was shown that the modulus is greatly enhanced in the presence of both HXNBR and ZDA in the blend mixture. Rubber product applications where such physical properties are useful include hard rolls, high tooth stiffness timing belts (for diesel engines or narrower belts) and high cut/heat/abrasion resistant conveyor belts.

Paper mill rubber roll compounds require improved heat and abrasion resistance characteristics in order to accommodate higher line speeds, reduce down time and to increase production output (ref. 3). Excellent retention of physical properties upon exposure to NaOH and sulfurous acid during pulp manufacture is also necessary. HXNBR was compared directly to a XNBR in a typical paper making roll formulation (ref. 4). For a 90 durometer A compound, the HXNBR based formulation portrayed improved tensile and elongation values, superior die C tear resistance, better elongation retention after aging at 100[degrees]C, and enhanced abrasion resistance measured by the DIN, Taber and Pico methods.

Rubber articles commonly used in the oil well specialty industry such as packer and drill bit seals, blow out preventers and pipe wipers, are required to perform in severe environments. Besides heat resistance requirements up to 150[degrees]C, such parts require water and oil based drilling mud resistance, wear resistance and excellent compression set (ref. 5). Moreover, the parts must perform in the presence of corrosive gases such as hydrogen sulfide and under conditions where they routinely undergo explosive decompression (rel. 6). Hydrogenated HNBR has been proven to be the elastomer of choice for such harsh environments.

Campomizzi, et al., (ref. 7) investigated the blend ratio of HNBR to HXNBR (0 to 100 phr), as well as zinc peroxide level (0 to 9 phr) in a DOE in order to enhance mechanical/ abrasion/compression set properties of a black, peroxide cured oil seal compound. The study essentially examined the effect of increasing the amount of ionic crosslinking between the zinc ions liberated by the zinc peroxide and the pendant carboxylic acid groups of the HXNBR elastomer. It was found that room temperature physical properties such as tensile strength and modulus were significantly improved upon increasing ionic crosslink concentration. Enhanced strength performance was also observed for temperatures up to 150[degrees]C. At these temperatures, hot tear strength was improved by as much as 50%. HXNBR/zinc peroxide rich compounds provided the best Pico abrasion resistance. However, it was found that HXNBR blends with HNBR (65/35) with low levels of zinc peroxide (3 phr) are required in order to optimize compression set. DIN abrasion resistance also improved upon lowering the amount of zinc peroxide in the designed experiment. Heat and oil resistance did not vary significantly within the design limits. In the second ensuing study, the effect of carbon black type (surface area and structure) for a 70 durometer A hardness oil seal compound possessing an excellent balance of compression set and abrasion resistance was investigated (65/35 HXNBR/HNBR blend ratio, 4.5 phr zinc peroxide, 9 phr 40% active peroxide) (ref. 4). For same hardness compounds, the higher surface area blacks provided moderately stifler compounds (Ml00 and M200) with a slight improvement in elongation to break values. Thermal black provided less tensile strength, as expected. N110 to N774 type blacks all provided improvement to DIN or Taber type abrasion resistance, with little difference among them. This is believed to be due to the ionic network along with carbon-carbon peroxide crosslinking contributing more to overall mechanical properties as compared to the carbon black-elastomer physical interactions. Resistance to tear (die B) was improved in using the lower surface area carbon blacks and this, for temperatures up to 150[degrees]C.

In order to enhance curing characteristics, as well as the overall compound performance, polymerizable multifunctional monomers known as co-agents are routinely added in peroxide curing formulations (ref. 8). Homopolymerization of the co-agent at vulcanization temperatures can cause the formation of an interpenetrating network within the rubber chains. The co-agents can also chemically graft onto the rubber chain backbone. This reaction, however, will only occur if the radical-containing chain is able to initiate the polymerization reaction. A successful graft, which finds termination on another radical containing rubber chain, leads to the formation of a crosslink. Co-agent addition will generally increase hardness and modulus of the vulcanizate, decrease elongation and compression set, while reducing both Mooney viscosity and scorch time.

The present study will continue to explore property enhancement in HXNBR/HNBR blends by screening a number of commercially available co-agents in an optimized oil seal compound. Among properties explored, particular emphasis will be on improving the hot air aging resistance, the compression set and the abrasion characteristics of the vulcanizate.

Experimental

The optimized HXNBR/HNBR compound from the earlier study (ref. 4) is given in table 1. Details of the mixing procedure and test methods used have been elaborated (ref. 4). The preferred peroxide co-agent that has been used in the previous work is triallyl isocyanurate (TALC) at a level of 1.5 phr. This formulation will act as the control for the co-agent variation study.

Table 2 lists the eight different peroxide co-agents and their levels screened in this investigation. Type I co-agents are those which increase overall compound properties while increasing cure rate and possible scorch issues (acrylates. methacrylates and maleimides). On the other hand, Type II co-agents enhance compound properties without substantially changing cure rate or increasing scorch. These include the polybutadiene type and allylated species such as triallyl isocyanurate. The co-agent levels were chosen to show typical low to high levels of each of the co-agents during which noticeable property changes (crosslinking density and scorch time) could be ascertained. Peroxide co-agents DA, TA, DMA and TMA all contain a non-nitroso scorch retarder in order to prevent premature vulcanization during processing (ref. 9).

Bender, et al., have shown that mPDM use in a peroxide cured HNBR gives rise to a clear deviation from first order kinetics during the crosslinking reaction, resulting in minimal scorch safety and high crosslinking levels (ref. 10).

Results and discussion

In addition to their role in the crosslinking mechanism, co-agents can also modify the processing characteristics of a compound. Compound mooney viscosity is a gross indicator of processability. Figure 1 shows the effect of the various co-agents on the compound Mooney viscosity relative to the base compound containing TAIC. In general, the Type I co-agents impart equal or better viscosity reduction. Increasing the level of co-agent increases the plasticizing effect. The high levels of DA, DMA and TMA are particularly useful in reducing the compound viscosity. However, the reverse trend is observed in the case of PB. This material is a low molecular weight polymer and is a very large molecule relative to the other co-agents and, in both cases where it is used, the compound viscosity is higher than the base case.

Figure 2 illustrates the effect of peroxide co-agent selection on the compound Mooney scorch safety measured at 135[degrees]C. As expected, the longest scorch safety is provided by the Type II co-agents TAIC and PB with scorch times ranging from 13 to 17 minutes. An increase in Type II co-agent concentration provides a slight beneficial increase in scorch safety. On the other hand, all Type I co-agents display less scorch safety with the shortest (<2 minutes) given by TMPTMA. The other methacrylate based co-agents provide a better scorch safety, even with the higher concentrations. [T.sub.05] values of about 15 minutes or longer at 135[degrees]C are considered excellent for optimum scorch safety protection in the case of injection molding.

The compound recipe was formulated in order to attain a 70 durometer A hardness material. In figure 3, the hardness values are given as a function of co-agent type. The 70 durometer A requirement is widely met by the 16 formulations; however, it can be noticed that higher levels of PB and TMPTMA do push the hardness values into the mid 70 range. PB co-agent addition normally causes a higher hardness compound.

The co-agent effect on 100% modulus values is displayed in figure 4. In all cases, an increase in co-agent level from low to high, gives rise to an enhancement in the 100% modulus values. Some co-agents, like TMPTMA and TA appear to be more effective in increasing the modulus. The highest modulus compounds had the PB co-agent at 10 phr. The lowest moduli compounds were the DA and DMA based compounds.

Tensile strength values for all 16 compounds range from about 20 up to 25 MPa (figure 5). The highest tensile values were obtained with low levels of TAIC and PB and with the high level of TA. Four phr of mPDM appears to be detrimental to tensile strength, as over 3 MPa of strength is lost compared to the control compound.

The effect of co-agent type and concentration on elongation at break is illustrated in figure 6. It is evident from this graph that higher co-agent concentration leads to a corresponding loss in elongation. It is expected that the higher levels of co-agent enhance the physical properties by providing a higher crosslink density, which would naturally lead to lower elongations. Compounds displaying significant increases in elongation contained DA, TA, DMA, TMA or TMPTMA as peroxide co-agent. In fact, an elongation of 300% can be achieved with 2.5 phr of DMA, which is a 50% improvement compared to the control compound. The mPDM co-agent gave the lowest elongation levels.

Compression set resistance was measured at 135[degrees]C after 70 hours (figure 7) and after 168 hours (figure 8). The best resistance to compression set is displayed by the TAIC compound at a level of 4 phr, an improvement of about 4% in set value over the control (with 1.5 phr TALC). The co-agent mPDM also provides good compression set resistance. The PB, as well as the acrylate and methacrylate based co-agents, provide generally poorer resistance to set.

Figure 9 illustrates the effect of co-agent type on the die B tear strength measured at room temperature. The control compound possesses moderate tear strength of 49 kN/m. The highest tear strengths were attained using PB, with values approaching 70 kN/m. DMA also appears to be quite effective, as tear strengths in the 60s can be easily reached. In most of the cases, an increase of co-agent concentration gives rise to a decrease in tear strength. Retention of tear strength at elevated temperatures is given in figure 10. At 125[degrees]C, is it readily seen that the best hot tear strengths are obtained with low levels of PB, or by using DMA independent of its loading. In all cases, the hot tear strength resistance decreases with increasing co-agent concentration.

The abrasion resistance of the 16 compounds, measured by the DIN method, is provided in figure 11. The control compound possesses a DIN value of 90. DIN values below 100 are indicative of very good abrasion resistant compounds. In every case, an increase in co-agent concentration brings about a loss in abrasion resistance. A substantial increase in volume loss of between 10 to 15 cubic millimeters is observed. Use of mPDM and DMA at low levels provides a slight improvement in abrasion resistance, with values in the low 80s.

Figure 12 shows the abrasion resistance of all the compounds measured by the Pico abrader, which provides a cutting abrasion resistance. Compared to the DIN method of measuring abrasion, the Pico abrasion index varies considerably between compounds. The compounds containing PB obviously display the best abrasion resistance. Compounds containing high levels of mPDM, TA, TMA and TMPTMA also show an improvement in abrasion resistance over the control. Less abrasion resistance is seen for higher levels of TAIC, as well as when using the DA based co-agent.

Figures 13 through 15 illustrate the effect of co-agent type and concentration on physical properties after hot air aging for one week at 135[degrees]C. The control compound shows an increase in hardness of 12 pts. during the aging period. The best resistance to hardening during hot air aging is provided by the PB co-agent as well the TMA co-agent at higher levels. With respect to tensile change during aging, most of the compounds give a l0 to 15% increase in tensile. Only the TA co-agent based compounds provide minimal change in tensile strength. Low levels ofmPDM and high levels of DA also bring about only a small positive increase in tensile strength. Elongation loss is negative for all compounds, ranging from -29% for the control up to over -50% for the TA co-agent. Both TAIC and PB provide the best elongation retention. Both the acrylate and methacrylate based co-agents show elongation retentions between -40 to -50%.

If increased compound flow is desired during processing, the compound Mooney viscosity data suggest using the nonpolymeric based co-agents at the higher levels. If scorch safety is a primary concern during injection molding, for example, then the data suggest that the use of a Type II co-agent such as TAIC or PB is highly preferred. With respect to end use properties, the co-agent choice and level will depend directly on the actual demands of the compound. Co-agent addition during a peroxide cure generally provides improved heat aging, a higher modulus, hardness, tensile strength and tear strength as well as enhanced abrasion and compression set resistance (ref. 8). Improved rubber to metal adhesion is another advantage seen in using certain peroxide co-agents.

The use of PB as peroxide co-agent is strongly indicated if one requires high hardness/modulus/tensile coupled with excellent room and high temperature tear strength. Good hot air aging resistance, as well as Pico abrasion, are also observed; however, the compression set tends to suffer to a certain extent. TAIC provides the best hot air aging and compression set resistance; however, increasing its concentration to 4 phr reduces the amount of Pico and DIN abrasion resistance. The inherent high reactivity of mPDM, exemplified by its low scorch safety, leads to compounds displaying lower tensile and elongation, but with good compression set and abrasion resistance. TMPTMA is quite effective in increasing modulus and hardness with ample elongation. Pico abrasion is improved with its use. If a lower modulus, high elongation/tear strength compound is important to final compound characteristics, DMA appears to be the most effective co-agent; however, the amount of compression set will increase.

Conclusions

A series of eight different peroxide co-agents compounded at low and high levels has been screened in an optimized HXNBR/HNBR drill bit seal formulation. It is obvious from this study that final co-agent selection and concentration will depend on the desired final properties. All co-agents enhanced the overall properties of the final compound, with noticeable differences between them. Non-polymeric peroxide co-agents provided the most improvement for compound flow behavior. Both PB and TAIC are indicated for injection molding applications where scorch safety is a concern. PB provides excellent overall property enhancement to the seal compound, but with less resistance to compression set. TAIC is the preferred co-agent where the compression set requirements are critical. The remaining acrylate/methacrylate and mPDM co-agents provide certain advantages and disadvantages which may be useful depending on the combination of properties required by the end specification.

References

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

(2.) von Hellens, W. and Guo, S.X, Rubber Division, ACS, Pittsburgh, PA, October 11, 2002.

(3.) Brown, EA., Fuchs, E. and Harrington, S.P, Rubber Division, Orlando, FL, October 26-29, 1993.

(4.) Ferrari, L.P., Pazur, R.J. and Campomizzi, E.C., Rubber Division, ACS, Cleveland, OH, October 16, 2003.

(5.) Thoermer, J., Mirza J. and Shoen, N., Elastomerics, 28, 1986.

(6.) Moore, J.R., Ridland, J.J. and Timar, J., Rubbercon 92, Brighton, U.K., June 15-19, 1992.

(7.) Campomizzi, E.C., Ferrari, L.P. and von Hellens, W., Rubber Division, ACS, Pittsburgh, PA, October 11, 2002.

(8.) Class, J.B. and Dluzneski, P.R. "Peroxide vulcanization," in Basic Elastomer Technology, 1st edition, ed.: Baranwal, K.C. and Stephens, H.L., Rubber Division, ACS, Akron, OH, 2001.

(9.) Costin, R., Nagel, W. and McElwee, C.B., Rubber Division, ACS, Cincinnati, OH, October 2000.

(10.) Bender, H. and Campomizzi, E.C., Rubber Division, ACS, Nashville, TN, May 1998.

Table 1--HXNBR/
HNBR optmized
formulation

Ingredient            PHR

HNBR                   35
HXNBR                  65
Processing aids      1.75
Antioxidant          1.50
Carbon black           55
TOTM                   10
Zinc peroxide           4
  (50% active)
Co-agent            Vari-
Peroxide             able
  (40% active)       9.00
Total (phr)

Table 2--peroxide co-agents evaluated in this investigation

Co-agent            Chemical                         Low     High
                    description                    level    level

TAIC (DIAK #7)      Triallyl isocyanurate            1.5        4
PB (Ricon 453-D)    1,2 -vinyl polybutadiene         5         10
mPDM (HVA #2)       N,N'-m-phenylenedimaleimide      1.5        4
DA (Saret 522)      Difunctional acrylate            2.5        8
TA (Saret 519)      Trifunctional acrylate           2.5        8
DMA (Saret 516)     Difunctional methacrylate        2.5        8
TMA (Saret 517)     Trifunctional methacrylate       2.5        8
TMPTMA (SR350)      Trimethyol propane               2.5        8
                      trimethacrylate