- Email: [email protected]
A study of the factors that affect extrusion resistance of elastomers
FEATURE
A study of the factors that affect extrusion resistance of elastomers Peter Warren, Stephen Winterbottom and Andrew Douglas, James Walker & Co Ltd, Cockermouth, UK This article describes a simple method for evaluating extrusion resistance, and studies fluorocarbon and hydrogenated nitrile rubber compounds at different temperatures and extrusion gaps. It also considers any correlation between standard laboratory test data and failure conditions. Having benchmarked those elastomers which use ‘‘conventional’’ compounding technology, modifications are made to improve their performance using a number of techniques. Finally, an assessment of the maximum extrusion resistance that may be expected from an elastomer at elevated temperatures, while maintaining a balance of elastomeric properties, concludes this article. The extrusion of elastomeric seal material occurs through creep and/or by fracture. Materials with poor set characteristics tend to initially creep into the gaps while those with good set properties are more likely to suffer initial damage from the edge of the gap. The forces applied in application can be considerable. Hertz[1] states: ‘for viscoelastic materials such as rubber, there is a stress amplification factor that causes local concentrations well in excess of the mean applied stress’. Although back-up rings and other elements are often used to reduce extrusion to a minimum, it is preferable to use a material with good resistance to extrusion damage whether or not back-up rings are used. Extrusion damage occurs when housing clearances are too large or when a seal, which has either no or inadequate anti-extrusion elements, is forced into or through a clearance. This may be observed around the whole circumference of a seal or may be limited to a portion of a seal where housing offset has occurred. It manifests
Figure 1. A classic case of ‘lace’ extrusion.
July 2012
itself in various forms and is normally evident on the low-pressure side unless, for example, chemical swelling or thermal expansion has occurred. Classic extrusion into a small clearance occurs over medium-to-long periods and results in lace-like debris (Figure 1). Extrusion may also happen catastrophically over a localised portion of the seal, through sudden failure of portions of any anti-extrusion device, or because of housing dilation at high pressures, causing the clearance to increase. Nibbling damage is normally observed when pressure cycling occurs. When system pressure is applied the housing “lifts” or “dilates” causing the clearance to increase. A nub of rubber extrudes into this clearance and is subsequently “nibbled” off when the pressure is dropped and the clearance is reduced.
Figure 2. A cutaway view of the test rig showing the extraction device.
The “Rizla effect” is normally associated with a continuous application of pressure and most often occurs with O-rings (or other designs that may rotate in a housing). Here the seal is forced into a clearance and with time literally unwinds into that clearance (hence the “Rizla effect”).
Test equipment Although there are numerous test methods which have been used over the years, we decided to concentrate on testing O-rings in a jig designed to emulate the type of failure found in the field. While this is based on earlier work at James Walker, our test-rig design (Figure 2) for testing this extrusion resistance is similar in principle to that used by Halliburton for its evaluations.[2] This arrangement has narrow lands and large spill areas in order to exaggerate the process and do not allow extruded material to hinder further extrusion. As this work was primarily initiated to understand materials it was felt that this particular design would enable us to readily distinguish between compounds. For this reason the pressure values quoted will not be fully representative of application. The extrusion testing can be controlled manually or by a computer. This is a modified version of one of our existing test rigs – the only change being the test block that is used. For ease of use the test block employed a bobbin Figure 3 with two ports in the housing for the oil. This bobbin enables
Figure 3. The test bobbin containing a failed seal.
Sealing Technology
7
FEATURE O-rings (50-329) with an inside diameter of 50.17 mm and a section diameter of 5.33 mm to be used. Furthermore, a simple screw method was employed to enable easy fitment and extraction of the bobbin. The control system enables us to ramp up to a specific pressure, hold that pressure for a fixed time and then repeat at a selected pressure increment, or to continuously ramp at a set rate. The system also enables a fixed pressure to be applied with or without ramping. The pressure within the test block is monitored and can be plotted against time to produce charts that show pressure, and any drop in pressure, during testing. The extrusion gaps employed were 0.4 mm and 0.8 mm (0.2 mm and 0.4 mm radial clearances). Although these gaps may be considered large by industry standards, they were chosen to demonstrate the material characteristics rather than give application values. Two temperatures – 100°C and 150°C – were initially chosen, although later work will examine lower and higher temperatures.
Fluoroelastomers (FKMs) and hydrogenated nitrile rubbers (HNBRs) were evaluated – compared with each other and within each family.
Test method The rings were fitted to the bobbin and the outside of the seals was lightly lubricated with the test oil (Shell Heat Transfer Oil S2). The bobbin was loaded into the block, sealed and oil introduced at minimal pressure. After ensuring there was no air trapped within the system, the block (seated on a heated platen) was allowed to stabilise at the test temperature for an hour. For our material evaluations it was decided to gradually ramp the pressure at a rate of 40 bar per minute. This was a large screening exercise and it is likely that a hold time at incremental pressures will be used for future evaluations when time is less restrictive. The pressure was increased until a significant pressure drop occurred. At this point the rig
Figure 4. Chart of extrusion failure pressures at 100°C.
was cooled, dismantled and the seals examined. Although this drop may be the end of the testing, the charts may indicate when the failure begins – by showing a series of small pressure drops earlier. An initial test run was normally followed by two replicates to give three data points under each set of conditions.
Extrusion resistance of FKMs Bisphenol cures Initially we tested a bisphenol-cured 90 hard material which has a long history of use in application. This had median failure values of 588 bar (0.4 mm gap) and 369 bar (0.8 mm gap) at 100°C. At 150°C the results were 290 bar and 202 bar, respectively. This gave us our initial points of reference for further developments. Readers will notice the high level of consistency in failure pressure between test runs. A similar level of consistency prevailed throughout the remainder of the test programme. In order to establish the effect of filler, the level was increased dramatically in this compound and this was then tested in comparison. The cure rate was similar to that of the standard product and sample rings were moulded under the same conditions and given the same post bake. The material was made stiffer, by the addition of filler, and harder, with 50% modulus increase, from 4.30 MPa to 6.10 MPa. Tensile strength, however, was reduced and elongation at break dropped to less than 100%. As can be seen from the charts in Figure 4 and Figure 5 the addition of filler increased extrusion resistance, but this was especially evident at the lowest temperature and narrowest gap. The improvement was in the order of 64% for the 0.4-mm gap at 100°C, but only 33% at 0.8 mm. The physical properties are dramatically reduced when such a high level of filler is used. However, a moderate increase in filler level may increase stiffness and reduce creep without compromising processing or final properties. Improvement in extrusion resistance may be less evident with smaller increases, but this improvement still may be significant at perhaps 20%.
Peroxide cures
Figure 5. Chart of extrusion failure pressures at 150°C.
8
Sealing Technology
Our next task was to look at the comparative values for peroxide-cured polymers. These were more extrusion resistant than the bisphenolcured compound at both gaps and both temperatures. The results (Figure 6) for these materials were similar despite using different polymers and quite different formulations. However, the results
July 2012
FEATURE are significantly better than those obtained from the bisphenol-cured compound. To complete the picture, a batch of lowtemperature peroxide-cured FKM (Figure 7) was made using the terpolymer formulation and substituting the polymer. Using the same polymer with minor changes to cure and filler gave a slight improvement in extrusion resistance over the standard formulation. There are a variety of peroxide-cured polymers available and compounds were mixed using both high fluorine and high fluorine, low-temperature grades. These gave similar results to the previous polymers although no effort had been made to optimise the formulations for extrusion resistance. All the compounds included in the chart in Figure 8 are 90 IRHD nominal. Actual values range from 87 to 92. The extrusion resistance, however, does not directly relate to specific hardness in this evaluation. Tear strengths for bisphenol and peroxide grades both at 23°C and 100°C were similar. Elongation of 50% and 100% reveal that while modulus values for varying formulations within a polymer type do show a trend in line with extrusion resistance, for different polymers this does not apply. As mentioned by Halliburton,[2] Aflas FKM is a classic example of a material with high modulus, but poor extrusion resistance (Figure 9, on page 10). We confirmed its findings as part of our evaluations.
Figure 6. Chart of extrusion failure pressures at 150°C.
Extrusion resistance of HNBRs Although not as good as peroxide-cured FKMs at a lower temperature the resistance of HNBRs does not decay as much at a higher temperature (Figures 10 & 11, on page 10). Again the resistance seems to be polymer dependent. The modulus does not coincide with resistance. As with the FKMs, an increase in the filler level will increase extrusion resistance. As increasing the filler level further would not give satisfactory dispersion in the HNBR, the comparison used involved a reduction in black level. As expected, the 90 hard material was significantly better than the 75 hard material, but again this was most evident at the smallest gap and lowest temperature.
Figure 7. Chart of extrusion failure pressures at 150°C
Cross-link density It is well known that shear modulus increases as the distance between cross-links is reduced. This is obviously both polymer-related and cure-related.
July 2012
Figure 8. Chart of extrusion failure pressures at 150°C
Sealing Technology
9
FEATURE
Figure 9. Chart of extrusion failure pressures at 100°C plotted against modulus values at 23°C.
Figure 10. Chart of extrusion failure pressures at 100°C.
Figure 11. Chart of extrusion failure pressures at 150°C.
10
Sealing Technology
Increasing cross-link density has a significant effect on physical properties, but the optimal density for one property may be very different to the optimal density for another. A balancing act rather than compromise is required. The ultimate aim is to produce materials that will resist extrusion in application, but there is no single physical test which will show this ability. The evaluation has shown that it is dependent on the choice of polymer first and then the optimisation of both cure and fillers to “toughen” the material. But what is toughness? It has been defined in various ways such as “resistance to crack initiation plus resistance to crack propagation” and “the ability of a material to absorb energy and plastically deform without fracturing”. Gordon[10] states: “The worst sin in an engineering material is not lack of strength or lack of stiffness, as desirable as these properties are, but lack of toughness, that is to say, lack of resistance to the propagation of cracks”. When considering extrusion resistance, key factors for consideration are hydrostatic compression and shear, therefore conventional wisdom does not apply.[4,5] For the highest tear resistance it is accepted that cross-link density should be as low as illustrated in Figure 12. This, however, does not apply for tear resistance in compression. A material that has been optimised for toughness may be improved further by cure and postcure conditions. However, this improvement has limitations and can only make marginal improvements to a well compounded material. Selecting an HNBR compound that is usually post cured and testing it with and without post cure does show a reduction in extrusion resistance for the latter of about 17% at 100°C and 23% at 150°C. It should be noted, however, that this decrease is of the same order at both temperatures and both extrusion gaps in contrast to increasing filler levels. A formulation which is specifically compounded so that it does not need a post cure would not, however, improve significantly if it were to be given one. Just simply increasing cross-link density does not result in a significant improvement. While some properties increase others decrease. Taken too far, the increase in cross-link density will tend towards brittleness, especially at elevated temperatures, and in some cases results in a dramatic drop in resilience. It is known, however, that increasing crosslink density does reduce the creep of rubbers and it is likely that part of the extrusion resistance is enhanced.[7] This work also concluded that creep was not directly related to stiffness, hardness or resilience, but was most closely related to compression set.
July 2012
FEATURE
Figure 12. Graphical representation showing the influence of cross-link density on given properties (A.Y. Coran[6]).
From an extrusion resistance principle, our evaluations suggest that a poor set is likely to give poor extrusion resistance. However, it does not mean that an outstanding compression set result will lead to outstanding extrusion resistance.
Other reinforcements In order to understand the effects of using other reinforcements, an HNBR compound was mixed in three versions in the laboratory. These were a standard version, one with multi-wall carbon nano-tubes and another using an Aramid powder. The addition of these materials was at 10 phr (5%), and dispersion was satisfactory at this level. The use of these reinforcing materials increased the modulus, but also severely reduced elongation. We believe that using a higher level would have compromised use in application. In effect, there was no improvement in resistance to extrusion and a small decrease was observed. This may be related to a tendency towards brittle failure or orientation.
Summary UÊ ÝÌÀÕÃÊÀiÃÃÌ>ViÊÃÊÌÊi>ÃÞÊÌÊiÃÌ>ÌiÊ using conventional laboratory tests. However, it may be possible to discount certain materials based on such tests. Our test and others like it give a better understanding and do enable predictions with some confidence to be made. UÊ / iÊ«ÞiÀÊ>««i>ÀÃÊÌÊLiÊÌ iÊ>ÊVÃ`eration in obtaining good performance. The reason why particular polymers are better or worse than others has not been fully investigated, but it would appear that it is not related simply to molecular weight. UÊ VÀi>Ã}ÊviÀÊiÛiÃÊÜÊ«ÀÛiÊÀiÃÃÌ>ViÊ with narrow gaps and lower temperatures, but it is not sufficient as severity increases. UÊ / iÊVÀÃÃÊ`iÃÌÞÊ>}>ÊV>ÊLiÊÕÃi`ÊÌÊ>iÊ moderate improvements, but there are limita-
July 2012
tions because of other physical requirements. UÊ / iÊLiÃÌÊÀiÃÕÌÃÊ>ÀiÊLÌ>i`ÊLÞÊV>ÀivÕÊ optimisation of the individual ingredients to maximise their effects. We had hoped to give an indication of the limits of a well compounded material under constant pressure, though will need to quantify this using a rig which relates more closely to application. Applying full pressure without ramping tended to encourage rotation of samples. Such rotation was not noticeable during the ramped tests used for this study. We will modify our rig to avoid this problem before re-evaluating constant pressure effects. Using ‘‘exotic’’ reinforcements may give better results with smaller extrusion gaps, or differing techniques, although the polymer is still the main consideration and, therefore, always may be the limiting factor. The values obtained from this study are purely comparative, and operating limits for a given elastomer will vary according to the conditions prevailing in a specific application. However, simple test methods, such as those described, do enable materials to be selected that will offer improved performance when assessed under conditions replicating field operation.
Acknowledgements The authors would like to extend their thanks to Paul Bowman, Duncan Smith and James Wilkinson for the design of the test rig, and provision of extrusion test data.
References 1. Hertz Jr, D.L., Mechanics of Elastomers at High Temperatures, presented at the High Temperature Electronics and Instrumentation Seminar, Houston, Texas, USA, 3–4 December 1979.
2. Slay, B., (Halliburton), Seal System Testing for High Performance Completions, Oilfield Engineering with Polymers Conference 2008. 3. Gordon, J.E. ‘The New Science of Strong Materials – or Why You Don’t Fall Through the Floor”, Penguin Books, 1968. 4. Bridgman, P.W., ‘The Physics of High Pressure’, G. Bell and Sons Ltd, London, UK, 1949 and ‘Studies in Large Plastic Flaw and Fracture’, McGraw-Hill, New York, USA. 1952. 5. Lindsey, G.H., ‘Hydrostatic Tensile Fracture of Polyurethane Elastomer’, California Institute of Technology, USA. 1966. 6. Coran, A.Y., chapter entitled Vulcanisation, in ‘Science & Technology of Rubber’, Academic Press, New York, USA. 1978. 7. Creep Phenomena (chapter 6), ‘The Services Rubber Investigations: Users Memoranda’, 1954. Contact: Peter Warren, James Walker & Co Ltd, Cockermouth, Cumbria CA13 0NH, UK. Tel: +44 1900 898277, Email [email protected], Web: www.jameswalker.biz
(Peter Warren has been in the sealing industry for over 33 years. As well as being a Fellow of the Institute of Materials, Minerals and Mining he is also a chartered scientist. His current role is head of Materials Engineering at James Walker & Co Ltd where he leads a team of 12 highly qualified scientists, technologists and technicians.)
This feature – by Peter Warren, Stephen Winterbottom and Andrew Douglas – is based on a paper entitled ‘Extrusion resistance of elastomers: a study of factors that affect performance’, which was presented in Aberdeen, Scotland, UK, on 17–18 April at this year’s High Performance Elastomers and Polymers for Oil & Gas Applications conference that was organised by iSmithers, part of Smithers Rapra Technology Ltd. Contact: Helen Charlesworth, Conferences, Smithers Rapra Technology Ltd, Shawbury, Shropshire SY4 4NR, UK. Tel: +44 1939 250383, Fax: +44 1939 1118, Email: [email protected], Web: http://info.smithersrapra.com
Editor’s comment: Also see the interview with James Walker’s Peter Warren and the feature entitled ‘Achieving RGD resistance to meet current oilfield needs’ which appear in Sealing Technology March 2012, pages 8–9 and October 2011, pages 9–12, respectively.
Sealing Technology
11
A study of the factors that affect extrusion resistance of elastomers Peter Warren, Stephen Winterbottom and Andrew Douglas, James Walker & Co Ltd, Cockermouth, UK This article describes a simple method for evaluating extrusion resistance, and studies fluorocarbon and hydrogenated nitrile rubber compounds at different temperatures and extrusion gaps. It also considers any correlation between standard laboratory test data and failure conditions. Having benchmarked those elastomers which use ‘‘conventional’’ compounding technology, modifications are made to improve their performance using a number of techniques. Finally, an assessment of the maximum extrusion resistance that may be expected from an elastomer at elevated temperatures, while maintaining a balance of elastomeric properties, concludes this article. The extrusion of elastomeric seal material occurs through creep and/or by fracture. Materials with poor set characteristics tend to initially creep into the gaps while those with good set properties are more likely to suffer initial damage from the edge of the gap. The forces applied in application can be considerable. Hertz[1] states: ‘for viscoelastic materials such as rubber, there is a stress amplification factor that causes local concentrations well in excess of the mean applied stress’. Although back-up rings and other elements are often used to reduce extrusion to a minimum, it is preferable to use a material with good resistance to extrusion damage whether or not back-up rings are used. Extrusion damage occurs when housing clearances are too large or when a seal, which has either no or inadequate anti-extrusion elements, is forced into or through a clearance. This may be observed around the whole circumference of a seal or may be limited to a portion of a seal where housing offset has occurred. It manifests
Figure 1. A classic case of ‘lace’ extrusion.
July 2012
itself in various forms and is normally evident on the low-pressure side unless, for example, chemical swelling or thermal expansion has occurred. Classic extrusion into a small clearance occurs over medium-to-long periods and results in lace-like debris (Figure 1). Extrusion may also happen catastrophically over a localised portion of the seal, through sudden failure of portions of any anti-extrusion device, or because of housing dilation at high pressures, causing the clearance to increase. Nibbling damage is normally observed when pressure cycling occurs. When system pressure is applied the housing “lifts” or “dilates” causing the clearance to increase. A nub of rubber extrudes into this clearance and is subsequently “nibbled” off when the pressure is dropped and the clearance is reduced.
Figure 2. A cutaway view of the test rig showing the extraction device.
The “Rizla effect” is normally associated with a continuous application of pressure and most often occurs with O-rings (or other designs that may rotate in a housing). Here the seal is forced into a clearance and with time literally unwinds into that clearance (hence the “Rizla effect”).
Test equipment Although there are numerous test methods which have been used over the years, we decided to concentrate on testing O-rings in a jig designed to emulate the type of failure found in the field. While this is based on earlier work at James Walker, our test-rig design (Figure 2) for testing this extrusion resistance is similar in principle to that used by Halliburton for its evaluations.[2] This arrangement has narrow lands and large spill areas in order to exaggerate the process and do not allow extruded material to hinder further extrusion. As this work was primarily initiated to understand materials it was felt that this particular design would enable us to readily distinguish between compounds. For this reason the pressure values quoted will not be fully representative of application. The extrusion testing can be controlled manually or by a computer. This is a modified version of one of our existing test rigs – the only change being the test block that is used. For ease of use the test block employed a bobbin Figure 3 with two ports in the housing for the oil. This bobbin enables
Figure 3. The test bobbin containing a failed seal.
Sealing Technology
7
FEATURE O-rings (50-329) with an inside diameter of 50.17 mm and a section diameter of 5.33 mm to be used. Furthermore, a simple screw method was employed to enable easy fitment and extraction of the bobbin. The control system enables us to ramp up to a specific pressure, hold that pressure for a fixed time and then repeat at a selected pressure increment, or to continuously ramp at a set rate. The system also enables a fixed pressure to be applied with or without ramping. The pressure within the test block is monitored and can be plotted against time to produce charts that show pressure, and any drop in pressure, during testing. The extrusion gaps employed were 0.4 mm and 0.8 mm (0.2 mm and 0.4 mm radial clearances). Although these gaps may be considered large by industry standards, they were chosen to demonstrate the material characteristics rather than give application values. Two temperatures – 100°C and 150°C – were initially chosen, although later work will examine lower and higher temperatures.
Fluoroelastomers (FKMs) and hydrogenated nitrile rubbers (HNBRs) were evaluated – compared with each other and within each family.
Test method The rings were fitted to the bobbin and the outside of the seals was lightly lubricated with the test oil (Shell Heat Transfer Oil S2). The bobbin was loaded into the block, sealed and oil introduced at minimal pressure. After ensuring there was no air trapped within the system, the block (seated on a heated platen) was allowed to stabilise at the test temperature for an hour. For our material evaluations it was decided to gradually ramp the pressure at a rate of 40 bar per minute. This was a large screening exercise and it is likely that a hold time at incremental pressures will be used for future evaluations when time is less restrictive. The pressure was increased until a significant pressure drop occurred. At this point the rig
Figure 4. Chart of extrusion failure pressures at 100°C.
was cooled, dismantled and the seals examined. Although this drop may be the end of the testing, the charts may indicate when the failure begins – by showing a series of small pressure drops earlier. An initial test run was normally followed by two replicates to give three data points under each set of conditions.
Extrusion resistance of FKMs Bisphenol cures Initially we tested a bisphenol-cured 90 hard material which has a long history of use in application. This had median failure values of 588 bar (0.4 mm gap) and 369 bar (0.8 mm gap) at 100°C. At 150°C the results were 290 bar and 202 bar, respectively. This gave us our initial points of reference for further developments. Readers will notice the high level of consistency in failure pressure between test runs. A similar level of consistency prevailed throughout the remainder of the test programme. In order to establish the effect of filler, the level was increased dramatically in this compound and this was then tested in comparison. The cure rate was similar to that of the standard product and sample rings were moulded under the same conditions and given the same post bake. The material was made stiffer, by the addition of filler, and harder, with 50% modulus increase, from 4.30 MPa to 6.10 MPa. Tensile strength, however, was reduced and elongation at break dropped to less than 100%. As can be seen from the charts in Figure 4 and Figure 5 the addition of filler increased extrusion resistance, but this was especially evident at the lowest temperature and narrowest gap. The improvement was in the order of 64% for the 0.4-mm gap at 100°C, but only 33% at 0.8 mm. The physical properties are dramatically reduced when such a high level of filler is used. However, a moderate increase in filler level may increase stiffness and reduce creep without compromising processing or final properties. Improvement in extrusion resistance may be less evident with smaller increases, but this improvement still may be significant at perhaps 20%.
Peroxide cures
Figure 5. Chart of extrusion failure pressures at 150°C.
8
Sealing Technology
Our next task was to look at the comparative values for peroxide-cured polymers. These were more extrusion resistant than the bisphenolcured compound at both gaps and both temperatures. The results (Figure 6) for these materials were similar despite using different polymers and quite different formulations. However, the results
July 2012
FEATURE are significantly better than those obtained from the bisphenol-cured compound. To complete the picture, a batch of lowtemperature peroxide-cured FKM (Figure 7) was made using the terpolymer formulation and substituting the polymer. Using the same polymer with minor changes to cure and filler gave a slight improvement in extrusion resistance over the standard formulation. There are a variety of peroxide-cured polymers available and compounds were mixed using both high fluorine and high fluorine, low-temperature grades. These gave similar results to the previous polymers although no effort had been made to optimise the formulations for extrusion resistance. All the compounds included in the chart in Figure 8 are 90 IRHD nominal. Actual values range from 87 to 92. The extrusion resistance, however, does not directly relate to specific hardness in this evaluation. Tear strengths for bisphenol and peroxide grades both at 23°C and 100°C were similar. Elongation of 50% and 100% reveal that while modulus values for varying formulations within a polymer type do show a trend in line with extrusion resistance, for different polymers this does not apply. As mentioned by Halliburton,[2] Aflas FKM is a classic example of a material with high modulus, but poor extrusion resistance (Figure 9, on page 10). We confirmed its findings as part of our evaluations.
Figure 6. Chart of extrusion failure pressures at 150°C.
Extrusion resistance of HNBRs Although not as good as peroxide-cured FKMs at a lower temperature the resistance of HNBRs does not decay as much at a higher temperature (Figures 10 & 11, on page 10). Again the resistance seems to be polymer dependent. The modulus does not coincide with resistance. As with the FKMs, an increase in the filler level will increase extrusion resistance. As increasing the filler level further would not give satisfactory dispersion in the HNBR, the comparison used involved a reduction in black level. As expected, the 90 hard material was significantly better than the 75 hard material, but again this was most evident at the smallest gap and lowest temperature.
Figure 7. Chart of extrusion failure pressures at 150°C
Cross-link density It is well known that shear modulus increases as the distance between cross-links is reduced. This is obviously both polymer-related and cure-related.
July 2012
Figure 8. Chart of extrusion failure pressures at 150°C
Sealing Technology
9
FEATURE
Figure 9. Chart of extrusion failure pressures at 100°C plotted against modulus values at 23°C.
Figure 10. Chart of extrusion failure pressures at 100°C.
Figure 11. Chart of extrusion failure pressures at 150°C.
10
Sealing Technology
Increasing cross-link density has a significant effect on physical properties, but the optimal density for one property may be very different to the optimal density for another. A balancing act rather than compromise is required. The ultimate aim is to produce materials that will resist extrusion in application, but there is no single physical test which will show this ability. The evaluation has shown that it is dependent on the choice of polymer first and then the optimisation of both cure and fillers to “toughen” the material. But what is toughness? It has been defined in various ways such as “resistance to crack initiation plus resistance to crack propagation” and “the ability of a material to absorb energy and plastically deform without fracturing”. Gordon[10] states: “The worst sin in an engineering material is not lack of strength or lack of stiffness, as desirable as these properties are, but lack of toughness, that is to say, lack of resistance to the propagation of cracks”. When considering extrusion resistance, key factors for consideration are hydrostatic compression and shear, therefore conventional wisdom does not apply.[4,5] For the highest tear resistance it is accepted that cross-link density should be as low as illustrated in Figure 12. This, however, does not apply for tear resistance in compression. A material that has been optimised for toughness may be improved further by cure and postcure conditions. However, this improvement has limitations and can only make marginal improvements to a well compounded material. Selecting an HNBR compound that is usually post cured and testing it with and without post cure does show a reduction in extrusion resistance for the latter of about 17% at 100°C and 23% at 150°C. It should be noted, however, that this decrease is of the same order at both temperatures and both extrusion gaps in contrast to increasing filler levels. A formulation which is specifically compounded so that it does not need a post cure would not, however, improve significantly if it were to be given one. Just simply increasing cross-link density does not result in a significant improvement. While some properties increase others decrease. Taken too far, the increase in cross-link density will tend towards brittleness, especially at elevated temperatures, and in some cases results in a dramatic drop in resilience. It is known, however, that increasing crosslink density does reduce the creep of rubbers and it is likely that part of the extrusion resistance is enhanced.[7] This work also concluded that creep was not directly related to stiffness, hardness or resilience, but was most closely related to compression set.
July 2012
FEATURE
Figure 12. Graphical representation showing the influence of cross-link density on given properties (A.Y. Coran[6]).
From an extrusion resistance principle, our evaluations suggest that a poor set is likely to give poor extrusion resistance. However, it does not mean that an outstanding compression set result will lead to outstanding extrusion resistance.
Other reinforcements In order to understand the effects of using other reinforcements, an HNBR compound was mixed in three versions in the laboratory. These were a standard version, one with multi-wall carbon nano-tubes and another using an Aramid powder. The addition of these materials was at 10 phr (5%), and dispersion was satisfactory at this level. The use of these reinforcing materials increased the modulus, but also severely reduced elongation. We believe that using a higher level would have compromised use in application. In effect, there was no improvement in resistance to extrusion and a small decrease was observed. This may be related to a tendency towards brittle failure or orientation.
Summary UÊ ÝÌÀÕÃÊÀiÃÃÌ>ViÊÃÊÌÊi>ÃÞÊÌÊiÃÌ>ÌiÊ using conventional laboratory tests. However, it may be possible to discount certain materials based on such tests. Our test and others like it give a better understanding and do enable predictions with some confidence to be made. UÊ / iÊ«ÞiÀÊ>««i>ÀÃÊÌÊLiÊÌ iÊ>ÊVÃ`eration in obtaining good performance. The reason why particular polymers are better or worse than others has not been fully investigated, but it would appear that it is not related simply to molecular weight. UÊ VÀi>Ã}ÊviÀÊiÛiÃÊÜÊ«ÀÛiÊÀiÃÃÌ>ViÊ with narrow gaps and lower temperatures, but it is not sufficient as severity increases. UÊ / iÊVÀÃÃÊ`iÃÌÞÊ>}>ÊV>ÊLiÊÕÃi`ÊÌÊ>iÊ moderate improvements, but there are limita-
July 2012
tions because of other physical requirements. UÊ / iÊLiÃÌÊÀiÃÕÌÃÊ>ÀiÊLÌ>i`ÊLÞÊV>ÀivÕÊ optimisation of the individual ingredients to maximise their effects. We had hoped to give an indication of the limits of a well compounded material under constant pressure, though will need to quantify this using a rig which relates more closely to application. Applying full pressure without ramping tended to encourage rotation of samples. Such rotation was not noticeable during the ramped tests used for this study. We will modify our rig to avoid this problem before re-evaluating constant pressure effects. Using ‘‘exotic’’ reinforcements may give better results with smaller extrusion gaps, or differing techniques, although the polymer is still the main consideration and, therefore, always may be the limiting factor. The values obtained from this study are purely comparative, and operating limits for a given elastomer will vary according to the conditions prevailing in a specific application. However, simple test methods, such as those described, do enable materials to be selected that will offer improved performance when assessed under conditions replicating field operation.
Acknowledgements The authors would like to extend their thanks to Paul Bowman, Duncan Smith and James Wilkinson for the design of the test rig, and provision of extrusion test data.
References 1. Hertz Jr, D.L., Mechanics of Elastomers at High Temperatures, presented at the High Temperature Electronics and Instrumentation Seminar, Houston, Texas, USA, 3–4 December 1979.
2. Slay, B., (Halliburton), Seal System Testing for High Performance Completions, Oilfield Engineering with Polymers Conference 2008. 3. Gordon, J.E. ‘The New Science of Strong Materials – or Why You Don’t Fall Through the Floor”, Penguin Books, 1968. 4. Bridgman, P.W., ‘The Physics of High Pressure’, G. Bell and Sons Ltd, London, UK, 1949 and ‘Studies in Large Plastic Flaw and Fracture’, McGraw-Hill, New York, USA. 1952. 5. Lindsey, G.H., ‘Hydrostatic Tensile Fracture of Polyurethane Elastomer’, California Institute of Technology, USA. 1966. 6. Coran, A.Y., chapter entitled Vulcanisation, in ‘Science & Technology of Rubber’, Academic Press, New York, USA. 1978. 7. Creep Phenomena (chapter 6), ‘The Services Rubber Investigations: Users Memoranda’, 1954. Contact: Peter Warren, James Walker & Co Ltd, Cockermouth, Cumbria CA13 0NH, UK. Tel: +44 1900 898277, Email [email protected], Web: www.jameswalker.biz
(Peter Warren has been in the sealing industry for over 33 years. As well as being a Fellow of the Institute of Materials, Minerals and Mining he is also a chartered scientist. His current role is head of Materials Engineering at James Walker & Co Ltd where he leads a team of 12 highly qualified scientists, technologists and technicians.)
This feature – by Peter Warren, Stephen Winterbottom and Andrew Douglas – is based on a paper entitled ‘Extrusion resistance of elastomers: a study of factors that affect performance’, which was presented in Aberdeen, Scotland, UK, on 17–18 April at this year’s High Performance Elastomers and Polymers for Oil & Gas Applications conference that was organised by iSmithers, part of Smithers Rapra Technology Ltd. Contact: Helen Charlesworth, Conferences, Smithers Rapra Technology Ltd, Shawbury, Shropshire SY4 4NR, UK. Tel: +44 1939 250383, Fax: +44 1939 1118, Email: [email protected], Web: http://info.smithersrapra.com
Editor’s comment: Also see the interview with James Walker’s Peter Warren and the feature entitled ‘Achieving RGD resistance to meet current oilfield needs’ which appear in Sealing Technology March 2012, pages 8–9 and October 2011, pages 9–12, respectively.
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