HXNBR for oil well specialties and roll applications.

Elastomeric components are required to perform at elevated temperatures and severe service environments for extended periods of time. It is well known in industry that the choice of polymer and the choice of ingredients in compounding the polymer are paramount to the success of the article in meeting the application needs (ref. 1). To achieve further improvement in performance, elastomers such as HNBR, which have total or near total saturation of the double bonds, in the polymer backbone have been developed (ref. 2). These offer superior resistance to degradation in hot air environments. Peroxide cure systems are most preferred for highly thermo-oxidative environments (ref. 3).

The properties of a polymer can be modified by introducing one or more functionalized monomers into the backbone of the main polymer. The addition of a monomer with carboxylic acid groups to the acrylonitrile butadiene (NBR) polymer backbone significantly alters cured compound properties, as well as processing behavior of the modified polymer. One of the primary reasons for including carboxylic acid functionality is to provide a reactive site that is useful in forming a network of ionic bonds to itself and to substrates. These ionic bonds supplement the conventional sulfur crosslinks of sulfur/sulfur donor cures or carbon-carbon linkages of peroxide cure systems. The ionic network is formed through crosslinking reactions between the carboxylic acid group and metal ions. This network leads to significantly increased strength and durability that is manifested by improved tensile, higher tear strength, higher low strain modulus and significantly improved abrasion resistance.

Recent developments have allowed these two technologies to be combined, and a patent application (ref. 4) has been filed for the hydrogenation of carboxylated acrylonitrile butadiene polymers. The new hydrogenated carboxylated acrylonitrile butadiene polymer, HXNBR, has been shown to have an interesting combination of properties (ref. 5). Properly formulated, this material can be used to develop compounds to meet the harsh demands in the oil well drilling and the roll industries, where high abrasion environments exist. These useful properties arise from the controlled level of residual double bonds in the HXNBR, the concentration of reactive carboxylic acid sites available to form bonds and the dosage of metal oxide to control the ionic crosslink network density. The effect of changing the ratio of HXNBR/HNBR (carboxylic acid concentration) and the amount of zinc peroxide on the physical and aging characteristics has been thoroughly studied in an earlier work (ref. 6).

The first part of this article was undertaken to investigate the effect of varying carbon black type on compound physical properties and abrasion resistance. In the second part, the differences between HXNBR and XNBR in a typical papermaking roll formulation are examined.

Experimental

Materials

A hydrogenated acrylonitrile butadiene rubber, with an acrylonitrile (ACN) content of 36% and residual double bond (RDB) content of 2.0%, and one with an ACN content of 33% and RDB content of 3.5% were used. The metal oxide had a typical Zn[O.sub.2]/ZnO content of 50%. This product will be referred to as zinc peroxide.

Compounding and physical testing

Compounds were mixed in a 1.6 liter internal mixer at 55 rpm with drop temperatures less than 150[degrees]C. Standard laboratory mill mixing procedures were used to incorporate the curatives in a separate mixing step. The compounds are based on Therban polymers, and all compounding ingredients were used as received from the suppliers without further refining. All testing was carried out according to standard ASTM or DIN procedures.

Aging environments

Oxidative aging, at the specified temperatures, was carried out in a recirculating hot air oven, conforming to ASTM E145, and aged specimens were tested according to ASTM D573. Test specimens were also aged in standard reference fluids (IRM 903, ASTM Oil #1). and tested according to procedure ASTM D471.

Results and discussion

A comprehensive study was undertaken to optimize the enhanced performance properties of compounds based on hydrogenated carboxylated acrylonitrile butadiene terpolymer (HXNBR). Compounds of this type are useful in the oil well services industry (OWS) and in the roll cover industries, where properties such as abrasion resistance, compression set and high strength are routinely required.

The polymers selected for this study were the 33% ACN content/3.5% RDB polymer, which will be referred to as HXNBR; the 36% ACN content/2.0% RDB polymer, a partially saturated medium ACN regular grade which will be referred to as HNBR: and Krynac X 750, referred to as XNBR. The first part of this article describes the further optimization of physical properties and abrasion resistance via carbon black selection for OWS applications. The second part of the article compares the abrasion resistance and heat aging properties of HXNBR compounds with those of XNBR compounds for printing roll applications.

Carbon black study for OWS

The principle OWS compound formulation used for this work is shown in table 1. This formulation was optimized in a previous study (ref. 6), where it was shown that a ratio of 65 phr HXNBR and 35 phr HNBR with 4.5 phr zinc peroxide resulted in the best balance of compression set and abrasion resistance. This formulation was used as the basis for further lab studies. The main factor examined in this study was the type of carbon black (particle size and structure). The compound hardness was kept constant at 70 durometer A for all the compounds by changing the carbon black loading for each carbon black type.

A key objective of this study was to further enhance the excellent abrasion resistance of the HXNBR compound via carbon black selection, as shown in table 2. It is well known in the industry that improved abrasion resistance is obtained with smaller particle size blacks (high surface area) and to a lesser extent, carbon blacks with higher structure. Accordingly, the N110 and N300 carbon black series are expected to give improved abrasion resistance compared to N774 and N990. All the carbon blacks in table 2, except for N990 (thermal) and N472 (conductive) are designated as furnace blacks. The carbon black level required to obtain a 70 durometer A compound varied from as little as 40 phr for N472 to as high as 100 phr for N990.

The structure and surface area mapping of the different carbon blacks used in the study is shown in figure 1. There are many textbooks that explain in further detail the effect of carbon blacks on elastomer properties. The abrasion resistance results are discussed in a later section.

[FIGURE 1 OMITTED]

Physical properties

The room temperature unaged and aged hardness results for the carbon black study are shown in figure 2. The unaged average durometer A hardness for all the compounds in this study was 68 [+ or -]2 pts. The hardness was also measured after seven day hot air aging at 125[degrees]C, 135[degrees]C and 150[degrees]C. As expected, the compound hardness increased as the aging temperature increased. Typically, the hardness increased by 10-12 units alter seven days at 150[degrees]C with little or no difference observed between the carbon black types.

[FIGURE 2 OMITTED]

The effect of carbon black type on compound scorch (t5) is shown in figure 3. The scorch safety time measured at 125[degrees]C remained greater than 30 minutes for all compounds; however, the scorch safety at 135[degrees]C decreases with decreasing surface area and structure. For example, the three compounds containing N650, N774 and N990 had scorch safety values of less than 20 minutes, with the N990 compound scorch safety at only 12 minutes. All the remaining compounds had scorch times in excess of 20 minutes, thus ensuring a high degree of processing safety. Note that at equal carbon black loadings, the reverse trend should be expected.

[FIGURE 3 OMITTED]

The unaged physical properties (stress at 100% and 200% elongation, tensile strength and elongation) are shown in figure 4. For compounds of the same hardness, it can be observed that the larger particle size (lower surface area) blacks impart more stiffness to the polymer network, resulting in higher M 100 and M200 values. Except for the N990 thermal black, tensile strength values are comparable for all compounds, thus independent of furnace carbon black type. The thermal black shows a reduction in tensile strength at break. Since the N990 black is used in larger quantities, the rubber content of the matrix is significantly lower than that of the other compounds, and the non-polymeric portion of the matrix has greater influence on the ultimate tensile.

[FIGURE 4 OMITTED]

Although the differences are not large, higher elongation to break values are favored, in general, by the higher surface area blacks. The most reinforcing black, N110, gave an elongation in excess of 300%. The high modulus values, especially at low strain, are beneficial for improved performance in explosive decompression environments that can be encountered in the oil well industry (ref. 7).

The peroxide curing, forming carbon-carbon linkages in the matrix, coupled with the formation of an ionic network via the carboxylic acid groups and the metal ions, clearly are the main contributors to physical properties such as modulus and tensile strength. The effect of carbon black type, as a typical reinforcing filler, is a minor contributor to final compound properties.

Abrasion resistance

The improved abrasion resistance is a key feature of HXNBR compounds. Many applications in the OWS and roll industry require superior abrasion resistance, along with heat resistance; thus the use of HXNBR will provide further benefits in these areas. The DIN and Taber abrasion resistance is shown in figure 5. In general, the smaller particle size carbon blacks generate the best abrasion resistance, as measured by volume loss. However, there is very little difference between the furnace blacks, e.g. N 110 to N774. This may be attributable to the presence of an ionic network, instead of the carbon black-rubber reinforcement matrix, as the main contributor to overall mechanical properties of the compound. As expected, N990 thermal carbon black results in lower abrasion resistance.

[FIGURE 5 OMITTED]

Tear resistance and resilience

The Die B tear resistance measured at 23[degrees]C, 125[degrees]C and 150[degrees]C is shown in figure 6. This test measures the compounds ability to resist tear propagation. It can be observed from the figure that the larger particle size blacks tend to resist and stop tear propagation better than the smaller ones. This trend is also observed at the higher temperatures.

[FIGURE 6 OMITTED]

The Zwick rebound (resilience) is shown in figure 7. Results were obtained at 0[degrees]C, 23[degrees]C and 100[degrees]C. In general, the elastic characteristics measured by this technique are similar from 0[degrees]C to 100[degrees]C for all eight compounds. Thus, a highly filled thermal black compound is portraying similar resilient behavior to a compound which is less loaded, containing a more reinforcing furnace black.

Paper making roll applications

The roll industry requires high hardness compounds with excellent mechanical strength and abrasion resistance. Currently, XNBR is used in many rubber" paper-making rolls. However, increased demands on production output have resulted in the need for heat-resistant rolls. HXNBR, with a much higher heat resistance rating than XNBR, is a suitable candidate for heat and abrasion-resistant rolls.

Table 3 summarizes the HXNBR and XNBR formulations used in this comparison study. The amount of peroxide in the XNBR recipe was decreased in order to compensate for the difference in double bond concentration between the two polymers.

Table 4 summarizes the unaged physical properties measured at 23[degrees]C for both formulations. Target specifications for a typical paper making roll compound are also included in the last column. The hardness requirement is easily met using either polymer, however, as it can be seen, HXNBR can readily satisfy both the tensile and elongation requirements of the specification.

Room temperature tear strength (Die B and Die C) of the HXNBR and XNBR formulations is presented in table 5. Both formulations meet the minimum 40 kN/m requirement of Die B tear, while the HXNBR formulation possesses a superior tear initiation resistance (Die C) compared to the XNBR formulation.

The heat aging properties under 25% compression for the roll compounds are shown in table 6. Both formulations meet the critical specification limit of 30% maximum set for this application. The compression set of HXNBR compounds can be lowered further by reducing the zinc peroxide level (ref. 6).

Figure 8 illustrates the stress-strain properties (hardness change, % tensile and elongation change) after aging both formulations for 70 and 504 hours at 100[degrees]C. After 70 hours of aging, it is evident that the HXNBR formulation has a better retention of physical properties, in particular the elongation. This difference becomes even more obvious after 504 hours of aging. XNBR loses about 50% of its elongation after this time, while HXNBR has lost less than 30% of its original elongation. The HXNBR compound has better heat aging properties compared to the XNBR compound.

[FIGURE 8 OMITTED]

Abrasion resistance testing results obtained from both formulations are given in table 7. The DIN abrasion test is commonly used as a guideline for wear characteristics in the roll industry. HXNBR has more than 36% better DIN abrasion resistance compared to XNBR. The results are mirrored in the Taber testing, which shows the superiority of HXNBR over XNBR compounds. The cutting abrasion resistance, which is represented by the Pico test, is also significantly better for HXNBR. Overall, HXNBR printing roll compounds show improved heat aging and abrasion resistance compared to XNBR compounds.

Conclusions

The enhanced performance properties of compounds based on HXNBR for use in the oil well services (OWS) and in the roll cover industries have been illustrated. For the same compound hardness, it was found that changing the carbon black type in an OWS formulation leads to minor performance changes in the final compound. The crosslinking provided by both the peroxide cure (carbon-carbon links) and the ionic network (carboxylate groups and zinc ions) is the major contributor to the enhanced performance properties. Slight differences are observed however, between the furnace and the thermal blacks in their effect on final compound properties, thus providing a degree of freedom in compounding to a specific 70 durometer A hardness OWS specification.

In the paper roll study, it was clearly shown how HXNBR outperforms XNBR for physical properties, hot air aging and abrasion resistance. Such property improvements meet and exceed paper roll specifications.

Table 1--70 dur. A
OWS compound

Ingredients            Phr

HXNBR                  65
HNBR                   35
ODA                   0.5
ODPA                  1.5
OPEFA                1.25
Carbon black     Variable
TOTM                   10
TAlC                  2.5
Zinc peroxide         4.5
Peroxide 1              9

Table 2--carbon black
type and level

Carbon    Level
black     (phr)
type

N472         40
N110         45
N339         45
N330         45
N326         55
N650         50
N774         65
N990        100

Table 3--paper making roll formulations

Ingredients           HXNBR     XNBR

HXNBR                 100        0
XNBR                    0      100
ODPA                    1.1      1.0
OPEFA                   2.0      2.0
ODA                     0.5      0.5
Carbon black, N550     70       70
Coagent 1               0       30
Coagent 2              25        0
Zinc peroxide           8        8
Peroxide 2              7        3.5

Table 4--unaged physical properties

Stress strain               HXNBR    XNBR     Target

Hardness duro. A2 (pts.)     88       89        90
Ultimate tensile (MPa)       22       16        20
Ultimate elongation (%)      85       10     70 to 90
Stress @ 25 (MPa)            10
Stress @ 50 (MPa)            16

Table 5--tear strength

Tear strength    HXNBR    XNBR     Target

Die B             44       49        40
Die C             34       22

Table 6--compression set at 100[degrees]C

Compression set    HXNBR    XNBR     Target
(100[degrees]C)

 70 h.              27       17        30
168 h.              41       21

Table 7--abrasion resistance

Abrasion resistance                       HXNBR    XNBR

DIN abrasion volume loss ([mm.sup.3])      257     404
Taber abrasion volume loss (ml/Kc)        .18      .48
Pico abrasion volume loss ([cm.sup.3])   .0009    .003

References

(1.) J.A. Kuczkowski and J.C. Gillick Rubber Chemistry and Technology, 57, 621 (1984).

(2.) D. Oppelt, H. Schuster: J. Thormer and R. Braden, British Patent 1,558,491 (1976); German Patent DE 2,539,132; CA 87:168798m.

(3.) R.C. Kellez Rubber Chemistry and Technology, 61, 238 (1988).

(4.) H. Bender and S.X. Guo (Bayer Inc.) CA 2304501, April 10, 2000.

(5.) S.X. Guo and W. von Hellens, Rubber World, Vol. 225, No. 5, 51 (2002).

(6.) E.C. Campomizzi, L.P. Ferrari and W. von Hellens, ACS paper no. 122, Cleveland, OH, October 11. 2002.

(7.) J.R. Moore, J.J. Ridland and J. Timar, "Optimization of HNBR for explosive decompression service," Rubbercon 92, Brighton, England. June 1992.