Articles posted by Jehbco Silicones

Jehbco manufactures custom engineered silicone extrusions to suit your application’s requirements.  Our engineers take every effort to ensure that our silicone’s properties match your application perfectly.

An important property of silicone is the coefficient of friction (CoF), which describes how hard it is to slide the silicone along a surface.  The CoF affects many areas: examples include the forces in moving seal systems (for example, o-ring seals on pistons); the ability of a seal to remain in place without adhesive; and the compression of a gasket between two hard surfaces.

In a previous article, we discussed how the coefficient of friction is measured.  In this article, we’ll look at what factors influence the CoF and how it can be tailored to your application.

The coefficient of friction is a property of the entire system, not only the silicone.  In general, the coefficient of friction will be higher (that is, sliding will be more difficult) when silicone is in contact with a rough surface than when silicone is in contact with a smooth surface.  The material itself will also affect the coefficient of friction – silicone will slide less against a smooth piece of aluminium than against a smooth piece of Teflon.

Figure 1: Measuring coefficient of friction.

One important system property that can affect the coefficient of friction is temperature.  As the temperature decreases, the silicone becomes slightly less soft.  This causes the coefficient of friction to decrease very slightly.  In addition, lower temperatures may produce condensation.  A thin film of water on the surface of the silicone will act as a lubricant and further lower the coefficient of friction, causing the silicone to slide more easily.

Silicone is naturally quite tacky, with a coefficient of friction of approximately 1.0 in many cases.  On many of our products, processes are applied to reduce the coefficient of friction.  These processes include applying a small amount of talc to the silicone surface and post curing the silicone in an oven for several hours.  In general, harder silicones with a higher durometer have a lower coefficient of friction, and our platinum silicone material has a lower coefficient of friction.

Jehbco’s engineers are able to tailor the properties of our silicone to produce a coefficient of friction that meets your requirements, and we have in-house testing capabilities that allow us to measure the coefficient of friction.  For any help with your application please review the Jehbco website www.Jehbco.com.au, and contact us with any questions.

Jehbco’s pure silicone extrusions are used in a wide range of applications, from aircraft seals to sea water piping gaskets.  These applications present a wide range of operating conditions that our silicone must work under, and Jehbco works with our customers to ensure the best silicone product is chosen for your application.

In order to determine the best silicone for your application, we must be able to measure the performance of our silicone.  A range of material properties are tested to describe how our silicone will perform under different conditions.  Several of these properties are measured using a stress/strain curve.

The stress/strain curve

The stress/strain curve is created by stretching a piece of silicone and measuring both the force required to stretch the silicone (tensile force) and how much it stretches (elongation).  The silicone is stretched until it breaks.  Since the silicone samples being stretched can be different sizes, the tensile force and elongation measurements are corrected.  Tensile force is divided by the cross sectional area of the silicone to get stress and elongation is divided by the original length of the silicone to get strain.  Stress is plotted on the y-axis and strain is plotted on the x-axis to get a stress/strain curve.   An example stress/strain curve is shown  below.

From the stress/strain curve, we can determine several important mechanical properties that tell us how the silicone behaves.  The modulus of the silicone tells us how stiff the silicone is, or how hard it is to stretch.  This is measured by calculating the slope of the first part of the stress/strain curve.  Stiff silicones will have a very steep curve, while softer silicones will have a shallower curve.

The tensile strength of the silicone is the amount of stress, or normalised force, needed to break the silicone.  This is easily read off the stress/strain curve – it is the stress at the very end of the curve, where the silicone breaks.

To ensure our product works as required, it is important that properties such as modulus and tensile strength are right for the application.  Jehbco has in-house facilities for generating stress/strain curves, to help us tailor the right product for your application.

For any help with your application please review the Jehbco website www.Jehbco.com.au, and contact us with any questions.

Seals are used to prevent the ingress of fluid or particles in a wide range of mechanical applications.  Applications vary from traffic light enclosures to hydraulic mining equipment, fluids range from seawater to DOT brake fluid and pressures range from a couple of kilopascals to hundreds of megapascals. While some applications have been standardised, many seals require individual design.  Jehbco manufactures custom silicone extrusions for a range of non-standard sealing applications.

Seal performance is influenced by a number of factors.  These include:

• The nature of the fluid being sealed, whether it be liquid or gas;
• The condition of the sealing surfaces and presence of any grooves or roughness;
• Materials;
• Geometry;
• Force exerted on the sealing surfaces.

This last factor, sealing force, is useful not only in estimating performance, but is essential for proper design of the sealing system.  For example, in a gasket system (Figure 1), sealing force is exerted on the gasket by a series of bolts.  To design the bolting system, the designer must know the force required to compress the gasket and form a seal.

Figure 1: A gasketted flanged pipe joint.

Sealing force is often determined through guesswork and expensive in-field testing.  Jehbco are breaking this mould by developing custom tooling to measure force on our sealing products.  With these tools, Jehbco will be able to provide our customers with accurate estimates of sealing force required under specific conditions – for example, the force required to compress an o-ring by 20%, or the force exerted by a seal on an expansion joint when the joint is 10 mm wide.  Jehbco aims to use these tools to enhance your design processes so we can more quickly produce a silicone extrusion that meets your sealing requirements.

For more information on silicone seals and assistance, please contact us at Jehbco.

Jehbco’s silicone extrusions are used in a wide range of applications, from aircraft seals to medical grade tubing.  To make sure you have the right silicone for your application, the application requirements have to be closely matched to the silicone properties.  There are many material properties that may affect our silicone’s performance in your application.  One of the most important properties is hardness or “durometer”.

The hardness of a material refers to how easily the material resists deformation under compression.  Put more simply, if something is pressed against the surface of a material, hardness tells us how easily the material is deformed or marked.   Depending on the material, there are several standard tests that can be used to determine hardness.  Metals are often tested using the Vickers or Rockwell procedures – these press a small tool into the surface of the metal under a standard load and measure the size of the resulting mark.

For elastomers such as silicone, hardness is measured using the Shore Durometer test.  This test measures the depth that a small cone can be pressed into the surface of the silicone.  The depth that the cone sinks into the silicone is converted to a value on the Shore Durometer scale.  The cone sinks deeper into soft silicones and these have a low value on the scale.  If the silicone is hard, the cone doesn’t sink very far and the value on the Shore scale is high.

Silicone hardness and the Durometer Scale with Jehbco Silicones

There are several variations of the Shore Durometer test for different types of elastomers.  The most common are Shore Durometer A for soft elastomers and Shore Durometer D for hard elastomers.  Each test gives a hardness value from 0 to 100.  The Shore A test is generally used to measure the hardness of Jehbco silicones – most of our silicones fall between 25 and 80 Duro Shore A.  Jehbco has equipment to test Shore hardness to the ASTM D2240 standard.

To get an idea of what the different hardness scales mean, 25 Duro silicone can be easily compressed with your fingers – think of the rubber that rubber bands are made of.  80 Duro silicone is much harder to compress, more like the rubber in a shoe sole.

To ensure that the silicone you choose for your application performs as required, it must have the right durometer.  For applications such as vacuum sheeting, a low durometer might be just right, while gaskets may require a medium to high durometer.  Jehbco can tailor a silicone material to the durometer required for your application.

For any further questions about how durometer will affect your application please contact our sales team.

Friction is the force opposing movement when two materials make contact.  Knowledge of friction is essential for many of our customers’ applications:

• it determines forces in moving systems such as O’Rings seals,
• it describes the ability of a seal to remain in place without adhesive
• it affects the compression of a gasket when pressed between two metal surfaces.

Figure 1: Friction force on a block

Figure illustrates the frictional force on a block resting on an incline.  Gravity pushes the block down the incline, and friction opposes this force.  Friction is a phenomenon that is still not understood completely, and frictional force cannot be calculated using any theoretical equation – it must be measured directly for the system of interest.  A surprising and useful empirical result is that frictional force depends only on the force pushing materials together and not on their area of contact.  This is because friction is due to atomic contact between surfaces. The area of atomic contact is very small and depends above all upon the force between surfaces, not their areas.

The coefficient of friction is the ratio of friction force to normal force.  Most material combinations show two types of friction – a strong frictional force when the materials are not sliding (static friction), and a weaker force when they are sliding (kinetic friction).  Different methods are used to measure each type of friction.

To measure the static coefficient of friction, the two materials can be tilted (as in Figure 1) until one of the materials begins to slide.  The static coefficient of friction is the tangent of the angle of tilt when the materials start sliding.

Measuring the kinetic coefficient of friction is more involved, and requires careful measurement of the force required to keep two materials sliding at a constant speed.  Many test rigs have been designed for this.  The most important factor is that the test conditions match application conditions as closely as possible to ensure results are relevant to the application.

Jehbco have developed in-house testing procedures and facilities to measure the coefficient of friction in customer applications.  If friction is important in your application, we can tailor a test to your application conditions and give detailed information on the performance of our products.

For any help with your application please review the Jehbco website www.Jehbco.com.au, and contact us with any questions.