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8Sealing Technology August 2005

IntroductionThe basic mechanical principles on which seal-ing systems are based can vary wildly betweendifferent applications and operating environ-ments. Hence the resultant demands on thephysical and chemical properties of a materialwill also vary. Toroidal elastomer seals, O-rings, dominate sealing applications because oftheir high sealing efficiency, ease of installationand robust design.

Dynamics or pressure within a system maycompel the use of more complex sealing pro-files or composite seal assemblies, consistingof multiple materials that, in combination,can offer the required properties. However, bymodifying and thus enhancing material per-formance, we can broaden the scope for appli-cations where elastomeric O-rings can beused, and also optimize the properties of com-posite seal assemblies. Figure 1 is a matrixsummarizing the requirements that differentindustries place on seals.

Simple analysis reveals that the requirementsfor chemical resistance and operating

temperature in any particular industry deter-mine those families of materials that can beused as part of sealing systems. However, this ismerely the first stage in specifying a seal mater-ial. Many differing grades of material, with sig-nificantly varying properties, may belong to asingle material designation. Synthetic elastomermaterials used in sealing systems generally con-sist of an organic polymer and inorganic rein-forcing filler systems. Although the polymerback-bone may be similar, determining manyof the physical properties, as demonstrated inFigure 2, there may be significant differencesto the cross-linking and filler systems. Fromthese differences in cross-linking and filler sys-tems come many of the differences in physicalproperties and hence sealing efficiency.

Patterns within the matrix in Figure 1 revealcorrelations between physical properties andsuitability to particular applications. For example, those materials that have low tensilestrength are only used for static applications,whereas dynamic applications utilize higherstrength materials. More detailed analysisreveals that high-pressure applications typically

use materials of high strength and relativelyhigh modulus, compared to low-pressureapplications where low-modulus materials willbe used.

Through the correct specification of fillersystems, it is possible to optimize the physicalproperties of a particular grade of materialwhen compared to others within the same des-ignation. The reinforcement effect of a filler iscomplex and dependent on its structure, parti-cle size and the chemical make-up of the parti-cles themselves. Carbon black, for instance, hasa very irregular surface, which makes the rein-forcement particularly effective. However, somesynthetic silicas are perfectly spherical, offeringvery little in terms of reinforcement. In order toachieve specific physical properties from amaterial the correct combination of reinforcingand non-reinforcing fillers must be selected. Byperforming a simple study we can identify theeffects of varying the volume fraction of fillerswithin the polymer.

Modulus versus hardnessThe modulus of a material is related to thehardness. As the modulus increases then so doesthe hardness (Figure 3). It is widely knownwithin the industry that O-rings of higherhardness are more capable of withstandingextrusion to higher pressures. Figure 4 showsan elastomer being extruded from the groove.The shear stress in the elastomer at the point ofextrusion can be simplified to be:

Applications of nanotechnology within high-performance sealingmaterialsBy Mick Holland – Precision Polymer Engineering Ltd, Blackburn, UK

The fillers used in an elastomer compound can directly affect both the physicalproperties and chemical resistance. This article reviews the types of fillers that maybe used, how they affect the material performance, and shows why the use of certainnanoparticle fillers can improve material properties. An example of a successfulapplication to perfluoroelastomers for aggressive chemical applications is discussed.

It can be seen from this that liquid lubricatedseals can still provide a reliable and effective sealwhere required. Flowserve has verified applica-tion limits for liquid lubricated compressor sealsat 300 bar and 100 m/s.

ConclusionsDry gas sealing of rotary compressors is wellestablished, and a number of options are

available from the seal industry. The Flowserve T-groove design offers a seal that provides equalperformance regardless of the direction of rota-tion. It is also possible to obtain a compactdesign and a high pressure drop per stage, bothof which offer the potential for hardware savingsand extending dry gas seals to new areas of appli-cation.

Compact liquid lubricated seals can also besupplied where they are a better solution, such as

with contaminated gas. Impressive reliability canbe achieved.

Contact:Compressor Seal Group Germany, FlowserveCorporation Flow Solutions, Flaspoete 101,D-44388 Dortmund, Germany. Tel: +49 231 69640,Fax: +49 231 6964 256, Email:MGruenewald@flowserve.com,Web: www.flowserve.com

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9Sealing Technology August 2005

Sheer stress (τ) =

Pressure differential (δP) × Area of extrusion gapArea of elastomer resisting extrusion

However, the width of the elastomer resistingextrusion is difficult to quantify and will varydepending upon the modulus of the elastomer.Therefore prediction of elastomer extrusionwithin a sealing system is generally based onwell publicised empirical measurements, orcomplex nonlinear finite-element analysis(FEA).

At the point of extrusion, on a molecularlevel, the elastomer will fail in tension, withthe elastomer exceeding its maximum elonga-tion at break. Materials with a higher modulusreduce the strain for a given stress; hence for agiven pressure, acting over a given area, theywill strain less and not reach their elongation atbreak. Harder materials tend to have a highermodulus and hence it is generalised that hardermaterials can withstand higher pressure differ-entials. Comparison of two materials of similarhardness and differing modulus show the high-er modulus material to withstand extrusion toa greater degree.

By increasing hardness we may reduce theability of an elastomer to conform to a roughsurface, or may overload the polymer withfiller, causing unacceptable compression set.Therefore for high pressure applications onemay choose to select fillers that will increasethe modulus to a greater degree than they willincrease hardness.

Modulus versus ultimatetensile strengthThe simple nature of a tensile dumbbell testmakes the tensile strength of a material anoften quoted property on material datasheets.The general use of this property is to measure

the consistency of product. Although seals bytheir very nature are typically used in compres-sion, their elastic properties can result in thedevelopment of tensile stresses within the bodyof the seal when subjected to compression orshear stress.

Typical dynamic applications are sealingagainst a reciprocating or rotating shaft orbore. The compression of the material, com-bined with shear frictional forces, can result in tensile stresses that exceed the ultimate ten-sile strength (UTS) of the material, causing atensile failure.

Referring to Figures 1 and 2, we see that mate-rial families with a high tensile strength are usedin dynamic applications. However, for highmodulus materials, the graph in Figure 5 wouldsuggest that for a given mix of fillers, increasingthe filler content will increase the modulus andhence reduce the ultimate tensile strength. Thismay initially suggest that dynamic applications

should use low-modulus grades of a particularmaterial; however, higher-modulus materialsoffer lower friction than low-modulus material,reducing shear stresses. Hence it is necessary tofind a careful compromise between low frictionand tensile strength, and the selection of a basepolymer with good physical properties becomescritical in maximizing the capabilities of the finalmaterial.

The addition of a small percentage of non-reinforcing lubricating fillers, such as polarizedgraphite, can reduce friction without signifi-cant detriment to the modulus and the tensilestrength.

Modulus versus elongation at breakFillers within a polymer may have two effects:they may act as stress raisers, reducing the

Figure 2. The relationship between typical physical properties and chemical resistance for the usualseal elastomers.

Figure 1. Summary of seal material usage in different industries: ✔ Typically used, X Rarely used, ✰ Used dynamically, ∆P Extremes of pressure.

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10Sealing Technology August 2005

energy at break; or they may arrest crack prop-agation, to increase the energy required forbreakage. Fillers can be classed as reinforcingor non-reinforcing, depending on whetherthey arrest crack propagation to a greaterextent than they raise stresses, or vice versa. Fora given mix of fillers one can construe that anincrease in filler level and hence modulus willreduce the elongation at break (EOB), asshown in Figure 6, even though the failureenergy, calculated from the area under thestress strain curve, may increase.

Together with physical effects fillers introducechemical effects. Similar examples can be usedto illustrate the chemical effects of each filler

type. Even though solid, the filler will have anassociated acidity or alkalinity (pH) which inrubber chemistry is very important; even slightchanges in pH can greatly affect the curingprocess. In carbon black, depending on theexact grade, the particles can be slightly acidic,which can lead to a slight retardation of thecure. For some silicas, however, the pH can be asgreat as 4.0, which is very acidic. If these arenotcompounded properly the cure can be com-pletely ineffective.

Further complications are also found withfiller selection, where the chemical compatibili-ty can be greatly affected. As discussed above,each of the fillers has an associated pH. This isessentially a function of the molecular groupson the surface of the particle and the mediainto which it is immersed. Using silica as anexample, with a fairly acidic pH, in alkalinemedia, the silica will try to absorb the alkalinein an attempt to reach neutrality, causing significant swelling of the seal.

A simple summary of the requirements ofphysical properties that may be optimizedthrough the use of fillers is detailed in Figure 7.

Selection of filler systems must be made toachieve the required physical properties with-out adversely affecting the chemistry of the

polymer cure or the final chemical resistance ofthe elastomer.

Nanofiller systemsThe introduction of nanoscale filler systemsgreatly increases the surface area to volumeratio of the filler system, and as such can significantly increase the extent to which a single, or group, of physical properties can bemodified. As with traditional fillers, differentnanofillers can be used to achieve differentphysical properties, and the selection of fillersremains just as critical. The total area under astress/strain curve represents the energyrequired to cause failure.

The increase in surface area to volume ratioincreases the energy required to cause failure,i.e. the area under the curve will increase, whencompared with a filler of similar nature withlarger dimensions. Although the area under astress strain curve will increase, this may occurin a number of ways. In the stress/strain curvein Figure 8, four possibilities are demonstrated.The slope of the line will define the Young’smodulus of the material:

• 1 is the initial stress strain curve.• 2 has an increase in both UTS and modulus

with similar EOB.• 3 has an increase in UTS and EOB.• 4 has an increase in EOB with a decrease in

modulus.

Careful selection of nanofillers will allow theproperties of the material to be enhanced to agreater extent than when using conventionalfiller systems. For example, curve 3 in Figure 8shows a material with an unchanged modulus,but both UTS and EOB have been increased.This could be achieved by using a similar filler ofsignificantly smaller scale.

Research by Gatos et al. performed at the Institute for Composite Materials, TUKaiserslautern in Germany[1] has shown that,by using appropriate modified layered silicates(organoclays), of nanoscale dimensions, onecan significantly increase both the UTS and

Figure 3. General relationship between modulus and hardness for conventional elastomers.

Figure 4. Extrusion of an O-ring from a groove.

Figure 5. Stress at 50% extension versus ultimate tensile strength of typical conventional elastomers.

Figure 6. Relationship between elongation and break with stress at 50% extension for traditional elastomers.

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11Sealing Technology August 2005

EOB of an elastomer, such as shown in curve 3of Figure 8. Selecting a layered silicate of high aspect ratio, a further modulus improve-ment is observed. There are obvious benefitsto this increase in the energy required for failure, ultimate tensile strength and elonga-tion, for example for use in high-pressuredynamic applications.

Low-pressure, chemically aggressive applica-tions require different physical properties thatare less obvious in nature. Low-pressure applications, such as vacuum applications, relyon the initial compression of the seal, rather thanthe differential pressure, to generate the normalsealing forces. These normal forces are far lowerthan those generated in high-pressure applica-tions, meaning that high-modulus, hard materi-als are unable to conform to the mating surfaces.Hence low-modulus materials are used for lowcontact force applications.

Lower modulus equates to lower stresseswithin the elastomer for a given strain.Reducing the stress levels in an elastomer mate-rial will reduce the likelihood of chemical

attack. Stress-induced chemical attack is amacroscopic material failure resulting frommechanically propagated cracks, initiated bychemical degradation of the material.

To fully understand this mechanism, it isnecessary to understand the nature of thematerial. An elastomer is made up of longchains of molecules onto which are bondedother atoms, or groups of atoms. The strengthof the bond is dependent on the covalentnature of the atoms, i.e. how capable they areof sharing electrons. Locations within thecross-linking molecules, or unsaturations onthe backbone, typically contain the weakestcovalent bonds. It is these locations that arefirst to be chemically attacked. By applying astress to an elastomer we deform the molecularchains, which both reduces the energy neededto break covalent bonds and introduces sterictwisting that exposes weaker bonds; this mech-anism is known as stress-induced chemicalattack. The chemical attack initiates the forma-tion of microscopic fissures. Once formed acrack will continue to propagate, growing

slowly until it reaches a critical length, atwhich point the fracture energy causes rapidpropagation of the crack across the whole section of the seal, causing the seal to break ina brittle-like manner.

When operating in chemically aggressive,low-pressure environments, we must thereforelook to introduce filler systems that enhancetensile strength and elongation at break, butoffer the low modulus needed to achieve good compliance and prevent acceleratedchemical attack.

A perfluoroelastomer specifically developedusing nanotechnology to optimize its charac-teristics for low-pressure, chemically aggressiveenvironments – such as those seen in bio-ana-lytical equipment and semiconductor processchambers – is Perlast G67P. This utilizes aunique spherical perfluoropolymer filler sys-tem of 30–35 nm diameter that enhances bothtensile strength and elongation at break, yetmaintains a low modulus. The perfluoropoly-mer fillers are highly inert, preventing chemi-cal attack of the fillers. By offering a low mod-ulus of 3.5 MPa, the material offers highsealing efficiency at low pressures and mini-mizes stresses within the polymer. By minimiz-ing stresses it can offer higher resistance tostress-induced chemical attack than conven-tional perfluoroelastomers, and has beenshown to maintain excellent chemical resis-tance under actual process conditions.

SummaryThe extent to which physical properties can beenhanced using nanofiller systems is greatlyincreased when compared to traditional fillers.This is a result of the increase in the surface areato volume ratio of the fillers increasing the fail-ure energy.

It follows that the fundamental decisions fac-ing the seal design engineer using nano ratherthan conventional filler systems remain: whichphysical properties require modification, andhow will this be achieved?

No one nanofiller system technology willbecome a panacea, as the basic mechanical prin-ciples on which sealing systems are based canvary wildly between different applications andoperating environments.

Reference1. K.G. Gatos et al.: Effects of platelets’ aspectratio on the properties of HNBR/organoclaynanocomposites. Conference on High Perform-ance and Speciality Elastomers 2005, Geneva,Switzerland, 20–21 April 2005 (RapraTechnology, UK).

Contact:Mick Holland, Perlast Ltd, Greenbank Road, BlackburnBB1 3EA, UK. Tel: +44 1254 295400, Fax: +44 1254680182, Email: mick.holland@prepol.com,Web: www.perlast.com

Figure 7. Summary of the properties that may be altered by the use of fillers to match the application.

Figure 8. Selectednanofillers may be usedto improve themechanical propertiesof the traditional material (1) in a number of ways to suit the application:(1) Initial stress/straincurve, (2) Increase UTS and modulus,(3) Increase UTS andEOB, (4) Increase EOBand decrease modulus.