Polyphosphazene elastomers in the oil field.

Polyphosphazene elastomers in the oil field

At Ethyl, we are developing new areas of applications for polyphosphazene elastomers. We feel that these elastomers are well suited for many applications in the oil field.

In this article we will:

* give some background on polyphosphazene elastomers;

* discuss their physical properties;

* look at their physical properties as a function of temperature;

* examine low temperature performance;

* review their resistance to fluids.

Polyphosphazene chemistry encompasses a broad family of compositions. The focus here will be on fluorinated polyphosphazene elastomers, referred to hereafter by the generic designation FZ (as designated by ASTM D1418). They are now available commercially from Ethyl as Eypel-F elastomers. Previously, these materials were commercially available from Firestone Tire and Rubber under the PNF trademark (ref. 1). Since this class of elastomers is now available commercially, we welcome this opportunity to review their characteristics and ways these materials can fit into applications in the oil field.

Overview

Phosphonitrilic elastomers, or polyphosphazenes, represent the first new class of semi-organic elastomers to be developed and successfully commercialized since silicone rubber (ref. 2). The application potential of this class of elastomer is only now beginning to be exploited, even though these polymers have been known for 20 years (ref. 3).

FZ elastomers differ from typical elastomers in that their backbone is phosphorus-nitrogen, whereas most other elastomers have a carbon-carbon backbone. The phosphorus-nitrogen backbone gives the FZ elastomer its inherent fatigue resistance and superior low-temperature performance (figures 1 and 2). This backbone also contributes to the excellent oxidative and thermal stability characteristics of FZ elastomers (figure 3).

Fluorinated polyphosphazene elastomers have a unique combination of properties that can meet needs in diverse and demanding applications. For example, FZ elastomer is used as the elastomer component in the air plenum seal of the M1 tank, where it provides fatigue resistance, low-temperature flexibility, broad operating temperature range and bondability.

These properties give the seal endurance for long life with trouble-free service. Also, fuel and hydraulic seals for high-performance aircraft are frequently made of FZ elastomer. Its wide operating temperature range and fuel/ hydraulic fluid resistance make FZ elastomer a prime choice for such applications.

People working on vibration-damping applications often find FZ elastomers to be the best vibration-absorbing material, which has hydrocarbon resistance that will also perform in the operating temperature range of -54 [degrees]C to 177 [degrees]C. Furthermore, evaluations in the automotive industry have created interest in this elastomer for use in specialty seals and diaphragms. Engineers have chosen FZ elastomer because of its high-temperature and low-temperature performance, bondability, flexibility and its resistance to fuels and oils. FZ elastomers have found their way into the oil field via valve-stem packings and seals for the wellhead of oxygen injection systems. Several other seals for wellhead applications are currently undergoing evaluation. Again, a broad operating temperature range and resistance to hydrocarbons are the key properties contributed by this elastomer.

Physical properties

Initial physical properties of a compound are of interest as one goes about selecting the right elastomer compound for a given application. EZ elastomer compounds are available in hardnesses of 35 to 90 Shore A, tensile strengths of 1,000 to 2,000 psi, elongations of 75 to 250% and compression sets of 15 to 55% after 70 hours at 150 [degrees]C (table 1).

Table : Table 1 - properties of FZ compounds

Physical property            Units        Value
Specific gravity                          1.70-1.85
Tensile strength             MPa (psi)    6.9-13.8 (1,000-2,000)
100% modulus                 MPa (psi)    2.8-13.8 (400-2,000)
Elongation                   %            75-250
Compression set              %            15-55

(70 hr @ 150 [degrees]C

Hardness, durometer          Shore A      35-90
TR-10                        [degrees]C   -56
Brittle point                [degrees]C   -68
Temperature range            [degrees]C   -65 to 175
Tear resistance, Die B       kN/M (psi)   to 26 (60 150)

Physical properties as a function of temperature

The physical properties of an elastomer are very dependent on the temperature at which they are measured. The magnitude of change in physical properties with temperature change is dependent on the base elastomer. In environments such as the oil field, where operating temperatures are wide ranging, an elastomer that gives consistent physical properties across such a temperature range is very desirable.

As can be seen in figure 4, the tensile strength of FZ elastomer changes less across a temperature range than that of the competitive elastomers, in this case fluorocarbon and highly saturated nitrile. The ultimate elongation of FZ elastomer is relatively constant regardless of the temperature at which it is measured. The ultimate elongation of the competitive elastomers varies greatly depending upon the temperature at which the measurement is taken (figure 5). Data from our laboratories indicate that at the high-temperature end the FZ elastomer exhibits higher elongations than the competitive elastomers. In an operating environment where a seal must function at something other than room temperature, performance of FZ elastomers suggested by these data should be very beneficial.

Low temperature performance of FZ elastomers

Low-temperature performance of elastomers is greatly influenced by the base polymer. This performance can be modified, sometimes significantly, by the addition of liquid plasticizers. However, plasticizers which improve the low-temperature performance are often volatile and subject to leaching. When plasticizers are leached from the compound, the low-temperature performance of the elastomer can be diminished greatly.

FZ elastomers are different from most other elastomers in that they do not require the addition of plasticizers to have excellent low-temperature performance. Therefore, their low-temperature performance will not deteriorate due to plasticizer extraction as functional parts age. This superior low-temperature performance is inherent in the polymer due to its unique phosphorus-nitrogen backbone. FZ elastomers have a glass transition temperature (Tg) of -65 [degrees]C, a TR-10 (low temperature retraction) value of -58 [degrees]C, and a low-temperature brittleness of -50 [degrees]C (ref. 3). Data comparing FZ low temperature properties with other elastomers are listed in tables 2 and 3.

Table : Table 2 - brittleness temperature

Elastomer type FZ FKM  HNBR

Brittleness

Temperature, [degrees]C -51 (-60) -27 (-16) -52 (-62)

Table : Table 3 - retraction at low temperatures

                      (TR test)
Elastomer type                     FZ         FKM        HNBR
TR-10, [degrees]C ([degrees]F)   -58 (-74)   -15 (+5)   -17 (+2)

Fluid resistance

Because FZ elastomers are fluorinated, they are resistant to a number of materials which may be encountered in the oil field. Hydrocarbons, water, amine corrosion inhibitors, diesel fuel, greases and many other materials may be encountered. Data showing the resistance of FZ elastomers to some of these materials are in tables 4-6.

Table : Table 4 - effects of oil immersion

                                              FZ
                                        Immersion data
Compound type                        811   751     752

Immersion,

166 hr @ 325 [degrees]F

Tensile strength, psi              2,020   1,880   2,150
  ASTM #3                          1,600   1,800   1,900
   5% NACE B/#3                    1,500   1,490   1,780
Elongation, %                        113     157     152
  ASTM #3                            104     168     153
  5% NACE B/#3                       111     145     147
Durometer hardness, Shore A pts.      77      71      80
  ASTM #3                             68      76      75
  5% NACE B/#3                        72      67      75

Volume swell, %

  ASTM #3                            1.8     2.5     1.9
  5% NACE B/#3                       5.3       8     6.8

Table : Table 5 - property changes on oil immersion

                                              FZ
                                        Immersion data
Compound type                        811       751     752

Immersion,

166 hr @ 325 [degrees]F

Tensile strength

  ASTM #3                          -20.8%    -4.3%   -11.6%
  5% NACE B/#3                     -25.7%   -25.7%   -17.2%

Elongation, %

  ASTM #3                           -8.0%     7.0%     0.7%
  5% NACE B/#3                      -1.8%    -7.6%    -2.2%

Durometer hardness, Shore A pts.

  ASTM #3                            -9       5       -5
  5% NACE B/#3                       -5      -4       -5

Volume swell, %

  ASTM #3                            1.8%     2.5%     1.9%
  5% NACE B/#3                       5.3%       8%     6.8%

Table : Table 6 - effects of aqueous immersion

FZ compound 751 - 1 week

Immersion       Temp.,   Volume   Hardness   Tensile
fluid             C      change   change     strength
                          %       points     change %
Water            23      3.5       -2         -5.3
                100      7.9        0         -7.5
10%              23      3.5       -2        -19.5
Sulfuric acid   100      2.8        1         0.75
10%              23      3.6       -1         -1.5
Ammonia         100      6.6       12        -59.4
10%              23      2.0       -2         -7.5
Sodium          100      7.8        2        -16.5

hydroxide

Summary

Elastomeric components which perform well in the oil field do so in a demanding environment. Today's oil exploration requires an elastomer which can perform across a wide temperature range. Low-temperature sealing is a must in the North Sea and the North Slope operations. Ideally, temperature change should have a minimal impact on the physical properties of the elastomeric component. In today's environment, elastomers are expected to withstand the aggressive fluids which will be encountered in petroleum exploration, production and distribution. This article has dealt with how FZ elastomers respond to these issues. We are hopeful that you have found this informative and look forward to helping you solve some of your most critical problems with FZ elastomers.

References

1. Books, J.T., "Elastomers and rubber technology," Singler, R.E.; Byrne, C.A., Ed.; Sagamore Army Materials Research Conference Proceedings, Lake Luzerne, New York, 1985; Volume 32, pp. 119-128. 2. Lohr, D.F. and Penton, H.R., "Handbook of elastomers new developments and technology," Bhowmick, A.K.; Stephens, H.L., Ed.; Marcel Dekker: New York, 1988. 3. Rose, S.H. (to Horizon Research) U.S. 3 ,315 ,688 (June 2, 1970).

PHOTO : Figure 1

PHOTO : Figure 2

PHOTO : Figure 3 - oil resistance and temperature range comparison for elastomers

PHOTO : Figure 4 - tensile strength vs. temperature

PHOTO : Figure 5 - ultimate elongation vs. temperature