Stress relaxation of elastomer compounds

Stress relaxation of elastomer compounds

FEATURE Stress relaxation of elastomer compounds Buc Slay, Halliburton Energy Services, Carrollton, Texas, USA, and Winston Webber, Halliburton Energ...

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FEATURE

Stress relaxation of elastomer compounds Buc Slay, Halliburton Energy Services, Carrollton, Texas, USA, and Winston Webber, Halliburton Energy Services, Arbroath, UK This article discusses the importance of conducting evaluation and stress-relaxation testing of elastomers. It covers the measurement of stress relaxation and the seal-force retention properties of elastomers using compression set, compression set with reheat and compressive stress relaxation tests. In particular, it promotes the use of a compression set test with a 24-hour unconstrained reheat cycle to generate valuable stress relaxation and seal-force retention trends in elastomers. Selecting a seal system for high-performance oilfield service requires a thorough knowledge of a seal’s performance envelope and the conditions of the service environment in which the seal will be used. Oilfield applications offer unique challenges, as tool systems endure extreme temperature and pressure changes throughout their service lifetimes. Production companies are also concerned about seal reliability and expected lifetime as these features have a large impact on the total well cost. In this article we look at the importance of conducting evaluation and stressrelaxation testing of elastomers. Elastomer evaluation includes basic materialproperty laboratory testing and performance testing that simulates the field environments. Basic laboratory tests include compression set, hardness, tensile properties and tear resistance. Performance testing covers extrusion resistance, rapid gas decompression, fluid compatibility, low temperature ‘sealability’ and seal-force retention. Here, we focus on the measurement of stress relaxation and the seal-force retention properties of elastomers using compression set (CS), compression set with reheat (CS+24) and compressive stress relaxation (CSR) tests. This article promotes the use of a compression set test with a 24-hour unconstrained reheat cycle to generate valuable stress relaxation and sealforce retention trends in elastomers.

Purpose of seals In the petroleum industry, seal systems are used to manipulate the flow of fluids, and they often rely on a combination of rubber and plastics to create an effective seal. O-rings, T-rings, bonded seals and other similar, thin cross-sectional seals are selfenergising because they only work when they are deformed or squeezed so that a seal contact force is exerted onto the seal-gland surfaces.

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The decay of this sealing contact force results in a lower-pressure holding capability as these seals typically are not pressure energised. Seals such as V-rings and U-cups are more immune to stress relaxation, as they can be energised by differential pressure. Packer elements are large, thick, crosssectional seals that incorporate elastomers, thermoplastics and various metal back-up shoe configurations. Packer elements are energised with an axial force, when the packer is set. This axial force significantly deforms the element in the radial direction in order to create a seal between the packer mandrel and the casing. The axial and radial response forces provided by the elastomer will decay with time – also reducing the retained sealing force.

Elastomer qualification The goal of any compound qualification test programme should be to meet performance and reliability criteria required by the complete tool system. Qualification often involves characterisation of physical properties of rubber by using hardness, tensile and tear properties. These tests do not predict seal performance accurately, as they are short timescale methods that do not account for creep and relaxation. Therefore, caution should be taken when using standard bench-test data for engineering design and creation of performance envelopes.

Characterising performance To more accurately characterise performance, one should evaluate extrusion resistance, rapid gas decompression performance, low-temperature sealability and seal-force retention. Stress relaxation and creep testing of elastomers are important, because they more closely evaluate the performance of seals over a longer

timescale. Creep is best simulated by highpressure extrusion tests, with constant loading applied to the seal. Stress relaxation can be evaluated indirectly with low-temperature testing. Compression set and compressive-stress relaxation methods also can be used. This article promotes the use of a compression set test with a 24-hour unconstrained reheat cycle (CS+24) to generate valuable stressrelaxation trends in elastomers.

Compression set Elastomers are complex materials with properties that change with temperature, time and the environment. An elastomer is a viscoelastic material, which means that it exhibits both viscous and elastic behaviour. At room temperature, elastomers can be deformed repeatedly, and upon immediate removal of the deforming load will return with force to their approximate original shape. The level of this returning force will vary, depending on the contribution from the viscous and elastic characteristics of the elastomer compound. However, when deformed at high temperatures over time, the rubber material may not completely recover, but rather, will take on a permanent set. This is known as compression set (CS) as it measures the loss of shape memory. CS can also occur at low temperatures, but this phenomenon is usually reversible, as the material recovers when it is heated. This recoverable CS is viewed as temporary set. It must be noted that standard experimental CS values are a combination of permanent and temporary set. CS is the per cent of deflection by which a seal fails to recover to its original dimension after a fixed time under a specific squeeze and temperature. The test can be run according to ASTM D395 Method B.

Figure 1. Example of low compression set and high compression set.

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FEATURE was removed from the oven and fixture. The remaining CS+24 value represents the effects of permanent set. Therefore, the permanent set measurements attained after the 24-hour reheat may represent the sealability of the rubber more accurately during service at an elevated temperature. The 24-hour reheat cycle is especially important for low-resilience materials. TFE/P is an example of an elastomer compound that can have poor resilience and a high glass transition temperature (7J) that will contribute to artificially high CS values. Once removed from the test fixture the viscous TFE/P does not have enough time to fully recover before it cools. The 24-hour reheat enables any residual temporary set to recover, leaving the user with only the permanent set and a more accurate representation of the sealing behaviour of TFE/P. This method also removes any variation in CS values caused by inconsistent cooling rates, removal of the fixture from the oven and sample from the fixture.

Material

FFKM

FFKM

FFKM

Specimen

330 O-ring

331 O-ring

332 O-ring

Environment

Air oven

Air oven

Air oven

Temp. (°F)

400

400

400

Time

22 hours

70 hours

1 week

Comp. set %

23%

40%

66%

24-hour reheat CS %

15%

31%

54%

Environment

Air oven

Air oven

Air oven

Temp. (°F)

500

500

500

Time

22 hours

70 hours

1 week

Comp. set %

40%

83%

99%

24-hour reheat CS %

26%

61%

94%

Environment

Ucon 500

Silicone

Temp. (°F)

400

400

Time

70 hours

70 hours

Comp. set %

44%

40%

Compression set environment

24-hour reheat CS %

10%

9%

Compression set tests are most often run in air per standard requirements but do not represent down-hole conditions. To investigate the effects of high-temperature oxygen-free environments, tests were run in air, glycol fluid (Ucon 500) and silicone fluid. The test results are shown in Table 1. When considering only the 70-hour compression set data of an FFKM, the air environment has the most severe effect and creates the worst CS and permanent set (CS+24). This is caused by the oxidation of the rubber. The glycol and silicone exposures lead to very low permanent set (CS+24) in the O-rings. The fluid vessels were cooled to 65ºC before the fixtures were removed, further contributing to the ‘freezing’ of the sample into its compressed state (temporary set) and high CS value. Therefore, one can see how the CS+24 data provide a much more accurate representation of seal force than the initial CS. This effect is even greater in oxygen-free environments that are more similar to well environments.

Table 1. Compression set results.

Figure 2. Compression set test fixture.

CS is evidenced by an O-ring that becomes permanently flattened after service, as opposed to a ring that has a low CS and returns to its original shape after being deformed, as shown in Figure 1. A material with a low CS has good long-term, self-energising seal properties. A simple CS test fixture for O-rings is shown in Figure 2.

Figure 3. CSR test fixture and fluid-compatibility vessel.

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Permanent versus temporary set In a standard ASTM or ISO CS test, fixtures are removed from an air oven and the samples are quickly removed from the fixture. The resulting CS value is a combination of temporary and permanent set. To separate permanent set from the temporary set, a 24-hour reheat can be performed. Once a sample is removed from its fixture and allowed to cool for the initial CS measurements, the unconstrained samples are placed back into an oven at test temperature for 24 hours. This reheat enables the seal to recover the temporary set that may have been caused by the rapid cooling of the sample when it

Compressive stress relaxation Time is an important parameter that influences the behaviour of viscoelastic materials. The equilibrium load and recovery response is elastic, but because of the viscous nature of the material, these responses change with time. Since the behaviour is time dependent, it is necessary to determine how time affects the sealability.

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FEATURE

Figure 4. CSR testing of O-rings.

When a constant strain is applied to the elastomer, the resulting force exerted by the material relaxes with time. The compressive stress relaxation (CSR) test attempts to measure this loss of sealing force over time. The processes causing stress relaxation may be physical or chemical and can occur simultaneously. However, at normal or low temperatures and/or short durations, stress relaxation is dominated by physical processes; while at high temperatures and/or long durations, chemical processes are dominant.>@ To demonstrate these facts, fixtures were assembled, and initial seal forces were obtained at room temperature and at an elevated temperature (Figure 3). These seal forces serve as points of reference for later measurements. They were then conditioned in an air oven and CSR tested at a high temperature as well as at room temperature after one day, three days and seven days. One particular study looked at the CSR changes of HNBR, FKM, and TFE/P (AFLAS) after conditioning in an air oven at 148°C and 204°C, as shown in Figure 4. This test shows how the seal force – measured at the conditioning temperature – changes over time . ASTM D-6147 - ‘Standard Test Method for Vulcanized Rubber and Thermoplastic Elastomer Determination of Force Decay (Stress Relaxation) in Compression’ was referenced and slightly modified for this work. There are many references available for CSR testing.>±@

Figure 5. Compression set before and after reheat.

ture of 150ºC and an accelerated ageing temperature of 200ºC. During the test, CS and CSR were compared. The chart on the left in Figure 5 shows the CS data without the 24-hour reheat, while the one on the right shows CS data with the 24-hour reheat cycle. It can be seen that the results are dramatically different and the CS+24 data show that the TFE/P is far superior to HNBR over 365 days at 150ºC. Figure 6 shows the CSR data for tests at 22ºC (on the left) and then tested at the conditioning temperature of 150ºC or 200ºC. The 22ºC CSR results, after hot conditioning, match the standard CS data reasonably well. However, when the CSR tests are carried out at high temperatures they more closely match the CS+24 results up until about 100 days, but then begin

to deviate. The CS+24 data show that the 150ºC HNBR has no squeeze after 180 days, but the CSR shows remaining seal force after 365 days. Ultimately, 24-hour reheat CS correlates well with hot CSR testing, and both are much better predictors of O-ring performance over time than traditional compression set testing.

Conclusions Selecting the appropriate seals and seal configurations for high-risk completions in deep-water applications is a task that has been approached with considerable apprehension. To remove the guesswork from this daunting task when addressing harsh environments and extreme conditions of temperature and fluid compatibility, selection and qualification procedures have been developed.>@

Case study – compression set versus CSR A one-year fluid compatibility study was developed to look at HNBR and TFE/P compounds in a particular brine solution at a well tempera-

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Figure 6. CSR testing at ambient 22ºC, and hot 150ºC and 200ºC temperatures.

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PATENTS These selection and qualification procedures are expensive and time-consuming. To eliminate the testing of unsuitable elastomers, short-term laboratory tests are used to give a guide to the long-term performance of the compound. Test methods that are available to elastomer suppliers without the need for expensive equipment present a significant advantage. Traditional hardness, tensile and tear-strength data are good for QC work but do not predict seal-system performance. The CS test, with a 24-hour unconstrained reheat cycle, is suggested as a test that does come close to predicting performance. It also offers good repeatability as a quality test. This method is particularly interesting as it does correlate well with

PATENTS Plunger seal for a syringe Applicant: Terumo Kabushiki Kaisha, Japan This patent describes a plunger seal for a syringe, which has an internal thread for fixing it to the plunger shaft. The seal thread is designed as an interference fit on the plunger thread. Patent number: WO/2010/103919 Inventors: K. Tachikawa and J. Ogawa Publication date: 16 September 2010

the more complicated stress-relaxation test methods.

References 1. Smith, L.P., ‘The Language of Rubber: An Introduction to the Specification and Testing of Elastomers’, Du Pont de Nemours International SA, 1993. 2. Tuckner, P., SAE Technical Paper Series: The Effects of Configuration on Sealing Force Measurement and Compression Stress Relaxation Response (2003-01-0946), Dyneon Llc, Society of Automotive Engineers, 2003. 3. Tuckner, P., SAE Technical Paper Series: Compression Stress Relaxation Testing Comparisons, Methods and Correlations (2001-1-0742), Dyneon Llc, Society of Automotive Engineers, 2001.

molybdenum, 1.2–2.0 wt% of silicon, 16–18 wt% of chrome and 0.8–1.5 wt% of manganese, with the remainder being iron. The seal is produced by centrifugal casting. This process improves productivity and the wear-resistance characteristics of the seal. Patent number: WO/2010/104322 Inventor: Y.J. Ma Publication date: 16 September 2010 Technical Editor’s comment: The application illustrated by this patent appears to be a shaft bearing seal.

4. Tuckner, P., SAE Technical Paper Series: Compression Stress Relaxation Test Comparisons and Development (200001-0752, 2000), Dyneon Llc, Society of Automotive Engineers, 2000. 5. Slay, B. and Ferrell, K., Performance Qualification of Seal Systems for Deepwater Completions (OTC paper 19626), Offshore Technology Conference, Houston, Texas, USA, 5–8 May 2008. Contact: Buc Slay, Halliburton Energy Services, 2601 Belt Line Road, MS C5-104, Carrollton, TX 75006, USA. Tel: +1 972 418 3166, Fax: +1 972 418 3598, Email: [email protected]

Mechanical seal Applicant: Weir Minerals Australia Ltd, Australia A mechanical seal, designed especially for slurry pump applications, has been developed. The ‘biasing device’ is manufactured from elastomer and is also impervious to the sealed fluid. In other words, it is a form of elastomer bellows that also acts as a spring. A series of three patents discusses the features of this seal. The seal plate assembly is able to rotate relative to the station-

Seal material Applicant: Bridgestone Corp, Japan A material that has good sealing characteristics and handleability forms the subject of this patent. The material is formed by injection moulding a thermoplastic elastomer. This material has, on its contact surface, a sealing section and a non-sealing section. The surface roughness Ras of the seal section is between 0.2 μm and 1.4 μm, and the surface roughness Rah of the non-seal section is greater than 3.0 μm to 7.0 μm. Patent number: WO/2010/104108 Inventor: T. Sano. Publication date: 16 September 2010

Alloy cast-iron for producing a seal $pplicant: JCS Co Ltd, Korea This invention relates to an alloy cast-iron for producing a seal and to a method of making such a seal. The alloy cast-iron contains 3.8–4.2 wt% of carbon, 3.3–4.7 wt% of nickel, 2–5 wt% of 12

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This mechanical seal – designed for slurry pump applications – uses a rubber bellows as both a secondary seal and spring. It also can be realigned with the shaft (WO/2010/105294).

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