Physical Properties of Rubber, Part 1 of 2

Written by Dale T. McGrosky

In this article we will describe the physical properties of rubber that you will see on Physical Property Data Sheets for elastomers that are tested to ASTM D2000 specifications. I have described what the property is, why it is important, and how you test it. There are many more physical properties of rubber than what are described here, but, we will limit them to the most common physical properties you will see in the ASTM D2000 standard.

I have broken this article into 2 parts. First part will cover Hardness, Ultimate Tensile Strength, Elongation, Tensile Set, Young's Modulus and Yield Strength. The results of these properties, except Hardness and Tensile Set, are found on the stress-strain curve that is generated during a Ultimate Tensile Strength test. Part 2 will cover Tear Resistance, Compression Set, Ozone Resistance, Fluid Resistance, Low Temperature Resistance, Abrasion Resistance.

Forces

Figure 1 – Types of Forces

Lets take a quick look at some forces before we get started on explaining properties. These forces will be applied to the specimen during testing. Being familiar with these will also give you and understanding of the forces that may be applied to the rubber parts while in your application.

Lets take a rectangular block of rubber as a specimen. If you squeeze the small sides together this is compressive force. If you stretch the block, this is tension or tensile force. If you twist the block this is torsional force and if you apply an opposing force to the side on top and opposite side on bottom, this is shear force. Some of these forces will be applied to the specimen during testing.

Hardness

Hardness is the measure of how resistant solid material is when a force is applied. There are 3 main type of hardness measurements, scratch, indentation and rebound. We will only be talking about the indentation hardness for elastomers. Indentation hardness is the materials resistance to indentation by an indentor.

Rubber is made in different hardness' for several reasons. Some sealing surfaces may not be totally smooth. The little voids, pits and scratches allow a pathway for fluid or air to escape through. Softer materials tend to flow better into these voids and imperfections on the sealing surface creating a better seal. On the other hand, harder rubbers will not do this as well but they do resist extrusion cause by high pressures. Also, coefficient of friction is also affected by the hardness of the rubber. Softer rubber has a higher coefficient of friction and harder rubber has a lower coefficient of friction. Coefficient of friction plays a factor when the rubber seal is sealing a part that moves.

Measuring Hardness

The durometer gauge is used to test the hardness of elastomers. The 3 most common durometer gauges used to measure rubber are Type A, Type M and Type D. Type A is used to test soft rubber materials while Type D is used to test hard rubber and plastic materials. Type M, also for soft materials, was developed to test small specimens, typically O-rings, that do not meet the physical size requirements specified in ASTM D2240. Is is important to know that although each of the hardness scales are graduated from 1-100, these scales are not the same. 90 Shore A is not the same as 90 Shore D or 90 Shore M. A piece of rubber measuring 90 on a Shore A gauge will read around 42-43 on a Shore D gauge.

Figure 2 – Stress-Strain curve showing Ultimate Tensile Strength, Ultimate Elongation and Modulus @ 100% Elongation

Tensile Strength

Ultimate tensile strength, or just tensile strength, is the maximum force a material can withstand without fracturing when stretched. It is the opposite of compressive strength. Have you ever purchased a pair of shoes and they came joined together with a piece of string? Instead of getting a pair of scissors, did you opted to test your physical strength against the tensile strength of the string and try to break it by pulling on it? If the string has a low tensile strength you should be able to pull and break the string easily. You can apply more tensional force than the string can withstand. If it has a high tensile strength it will be much harder to break by pulling. Are you starting to understand what tensile strength is?

Tensile strength is an indication of how strong a compound is. Any time you have an application where you are pulling on the part, tensile strength is important to know. Whether your product is designed to break easily or not at all the tensile strength will let you know how the object will react to the tensional forces. A few rubber products that tensile strength are important would be bungee cords, rubber tie downs, drive belts. Some elastomeric compounds, like Silicone, have a low tensile strength making them unsuitable for a dynamic types of seal because they can fracture easily.

Measuring Tensile Strength

Tensile strength is measured with a tensometer. A tensometer is special machine that is designed to apply a tensional or compressive force to a specimen, in our case a die cut dumbbell shape, and measure how much force it takes to deform and fracture the specimen. The force is typically displayed on a stress-strain curve that shows how much force was required to stretch the specimen to deformation and ultimately break.

Elongation

Maximum elongation, with respect to tensile testing, is the measure of how much a specimen stretches before it breaks. Elongation is usually expressed as a percentage. I had an application where a very small O-ring with an inside diameter of .056 inches had to stretch over a rod with a diameter of .170 inches. A Nitrile O-ring worked fine since it's ultimate elongation was well over 400% and the O-ring was able to withstand the 200% stretch during installation. But when we tried to use a fluorocarbon compound several of the O-rings were breaking during installation. This fluorocarbon compound had an ultimate elongation of 150% and could not withstand being stretched to over 200% during the installation and the o-ring would break.

Measuring Elongation

Elongation is measured with a ruler or an extensometer. An extensometer is an electronic ruler that is attached to the tensometer and will measure the extension of the specimen while torsional force is being applied. Another way of measuring elongation is with a regular ruler. To measure the elongation with a ruler, make two bench marks 1 inch a part on the specimen. This is the Initial Gage Length (Lo) and then measure the distance between the marks just before the specimen breaks. This is the Final Gage Length ( Lx). Calculate the elongation with the following equation: elongation % = 100( Lx – Lo ) / Lo.

Tensile Set

While we are using bench marks, let quickly talk about Tensile Set. Tensile Set is the extension remaining after a specimen has been stretched and allowed to relax for a predefined period of time. Tensile Set is expressed as a percentage of the original length. Tensile set results are not found on the stress-strain curve. It's a measurement that can be performed after the tensile strength test. Do not mistake Tensile Set with Elasticity. Elasticity is the mechanical property of a material to return to its original shape where Tensile Set is the amount on extension remaining after being stretched.

A rubber band would have a low Tensile Set percentage. After stretched it relaxes close to, if not exactly to, its original length. Now take a piece of Teflon and stretch it. It does not return to its original length and it stays in its stretched state. This would have a high Tensile Set percentage.

One test we perform in our Q.C. inspection is to pull on the O-ring and see how fast and how close it returns to its original diameter. The O-ring should fairly quickly return close to its original diameter. Often times a seal has to be stretched during installation and the last thing you want to happen is the O-ring stay stretched and not fit which could cause problems during assembly.

Measuring Tensile Set

Remember the 2 bench marks 1 inch apart on the specimen in the elongation test? To determine Tensile Set after break, wait 10 minutes after the specimen breaks and then fit the two halves of the specimen back together so there is good contact along the full length of the break. Measure the distance between the bench marks. Use the same equation used in the elongation test except the Final Gage Length (Lx) is the final measured distance between the bench marks. Another way to test without breaking is to stretch the specimen to a specified elongation and hold for 10 minutes. Release the specimen as quickly as possible, making sure not to allow it to snap back, and let sit for 10 minutes. Measure the distance between the bench marks. Again, use the same equation used in the elongation test except the Final Gage Length (Lx) is the final measured distance between the bench marks.


Figure 3 - The steep slope would indicate this material is the tougher of the two material curves shown.	Figure 4 - The shallow gentle slope shown on this curve would indicate this material is not very tough.

Young's Modulus

Young's Modulus is also known as Tensile Modulus, Elastic Modulus and Modulus of Elasticity (“Measure” of Elasticity). It's the measure of the stiffness of the material. You will see this on a physical property data sheet written something like “Modulus @ 100% Elongation.”

When performing a Tensile Strength test a plot is made of the stress vs. strain or amount of force required to stretch (deform) the specimen given length. This plot is called a stress-strain curve. The peak of the curve is the Tensile Strength and the Young's Modulus is the slope of the stress-strain curve. If you have a steep curve the specimen resists deformation (it's tougher) and a if the slope is gentle the material will deform easily. At any given point on the stress-strain curve we can read the Tangent Modulus. “Modulus @ 100% Elongation” says we want to know the amount of force required to stretch (elongate) the specimen 100%. We can also ask for Modulus @ 200% or any given point on the stress-strain curve.

Knowing how easily a material deforms under strain can be important in some applications. An engineer was installing a rubber seal on a door. The rubber he used had high modulus. The door was hard to close because the rubber resisted being deformed. He then used a compound with low modulus that deformed easily allowing the door to close easily.

Measuring Young's Modulus

Young's Modulus is measured during a Tensile Strength test. As stated above, when performing a Tensile Strength test a stress-strain curve is plotted. The slope of this curve is the Young's Modulus and any point on that curve is a Tangent Modulus.

Figure 5 – Young's Modulus (Linear Elastic Region) and Yield Point (Strength)

Yield Point

Yield Point is the force at which the specimen starts to deform permanently. It is difficult to point to the exact Yield Point on the curve because the transition is gradual, so a 2% offset (0.2% for metals) from the Linear Elastic Region is used to indicate the Offset Yield Strength. Although Yield Strength is meant to show the exact point where the specimen becomes permanently deformed, a 2% offset is an acceptable sacrifice because of how much easier it makes it to determine yield strength.

Just prior to the Yield Point is the Linear Elastic Region. The slope of the line in this region is called Young's Modulus. This is the area which the specimen retains its elasticity. When the force is removed in this area the specimen will return to its original shape. After this area the specimen transitions from elastic to plastic behavior. This means that after the Yield Point, permanent deformation occurs in the specimen and it will no longer return to its original shape. Here is where I am going to throw a curve at you. In most elastomer stress-strain curves you will not see a definite yield point or plastic region. The elastomer specimen will remain in the linear elastic region throughout most of the curve as shown in the figures 3 and 4 except Urethane compounds shown in figure 5.

To summarize the above properties lets take a look at the stress-strain curve that is generated during the Tensile Strength test, see figures 2 and 5.

Ultimate Tensile Strength – The amount of tensional force required to fracture a specimen.

Ultimate Elongation – the amount a specimen deforms by stretching.

Young's Modulus – The slope of the stress-strain curve that is generated during a tensile strength test.

Tangent Modulus – Any point on the stress-strain curve.

Yield Point – The force at which a material will begin to deform permanently.

These Properties are not seen on the stress-strain curve
Hardness – The measure of how resistant solid material is when a force is applied. There are 3 main type of hardness measurements, scratch, indentation and rebound.

Tensile Set – A measurement showing the extension remaining after a specimen has been stretched and allowed to relax for a predefined period of time. Tensile Set is not show on the stress-strain curve. Is is a measurement that can be done after a tensile strength test.