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RubberTech Articles
Design Guide Article Series #1 - Selecting the Right Elastomer
Article: # 2
Author: Ernest B. Ferro, President, Corry RubberElastomer Properties and ValuesThe Importance of the Right Elastomer
The best solution to your problem requires:
- Selecting the lowest cost elastomer alternative that meets your application needs. Some elastomers are much more suited for some applications than others. For instance, low temperature flexibility to –100C is only available in some types of Silicone elastomers.
- Formulating the elastomer correctly, because of the dramatic difference in the resulting product’s properties.
This is why it is essential that the designer or purchaser of the component consult an Elastomer Technologist as early as possible during the design stage.
Valuable Elastomeric Properties
Elastomers have a most unique property - their ability to bounce. When a given shape of elastomeric material is subjected to an external force, it deforms storing some percentage of the energy that was used to deform it. However, when that force is released, the elastomer will return to essentially its original shape releasing most of the energy that it stored during deformation. Just about every application in which elastomers are used relies on this unique property.
There are other properties of elastomers that make them suitable and in some cases unique as engineering materials. These properties depend on the elastomer selected and how the elastomer is compounded.
Depending on the elastomers selected and how they are compounded, elastomeric products can:
- Absorb vibration and noise.
- Resist exposure to oxidation, oils, fuels and chemicals and still retain their usefulness.
- Maintain tension and compression forces indefinitely making them useful in sealing applications.
- Be serviceable in a range of temperatures between –100C to +275C.
- Be formed into almost any shape or configuration to accommodate thermal changes, differences in tolerances between components, movement, shocks and vibration.
- Be used as barriers to separate two different media, water and air in an accumulator.
- Provide thermal barriers to heat transfer.8. Withstand weather and outdoor exposure indefinitely with minimal change in properties.
- Be used in medical environments to provide barriers between the body and electronic components as in sheaths for endoscopes.
Overview of Dynamic Properties
Elastomers that are rubbery are actually a two phase or two component system. In use, these two phases or components seem to work against each other.
The first phase is called the elastic phase or more commonly, the ‘spring’ phase. Before electronic scales were readily available, people commonly used spring scales to weigh things. When we place a load of magnitude Z on a spring scale, the spring stretches or compresses a fixed amount in proportion to the load put on the scale. Assuming friction in the scale is negligible, the scale will indicate a load of Z. Using a load that is 4 times that of Z will result in a stretch or compression that is 4 times that of the first load. Because the second load produces a stretch that is exactly 4 times that of the first load, we can say that the system responds linearly to the load. When the load is removed, the scale returns to the zero point. In this example, the energy put into the spring to cause a deflection is equal to the energy given up by the spring when the indicator moves back to zero. Because the pointer always returns to zero when the load is removed, we say that the scale is 100% efficient. That is, all of the energy that went into the scale to produce the deflection is given up to return the scale to the zero point when the mass is removed.
What happens if we apply the load to the scale very slowly? Suppose it takes us 5 minutes to apply the Z load. Will the deflection of the spring change if we apply the load slowly? Assuming there are no significant friction issues, the needle will deflect to the same point that it did in the example above when the mass was applied at a typical rate. Because of this, we can say that the springs are not dependent on the rate of application of the load.
We have just described what is known as Hooke’s Law. That is, the deflection produced in a spring is proportional to the load applied and independent of the rate of application. The constant of proportionality that relates the load and deflection is called the spring constant and is specific to the type of spring being used. Hooke’s Law is represented mathematically as F=kX where F is the force or load applied, k is the spring constant and X is the displacement. In elastomers, the response described by Hooke’s Law is called the ‘elastic response’ and is referred to as E’.
The other part of our two-phase elastomer system is called the viscous phase. The viscous phase of the elastomer system results in a viscous response when the system is subjected to a load.
Let’s consider a tall, cylindrical jar filled with thick molasses. On the bottom of the jar, let’s place a round disk that’s just slightly smaller in diameter than the inside diameter of the jar. Furthermore, let’s attach a rod to the center of this disk. The length of the rod is greater than the height of the jar so that the rod sticks out of the jar and can be grasped by our hand.
If we grasp the rod and try to pull the disk out of the molasses or push it back into the molasses, we find that the viscosity of the molasses makes it difficult to move the disk back and forth inside the molasses. If we move the disk slowly back and forth in the molasses, we find that it takes less force to move it than if we try to move the disk rapidly back and forth. In the process of moving the disk through the molasses, we put energy into the molasses by shearing it between the disk and the walls of the jar. Furthermore, if we move the disk half way to the top of the jar and let go of the rod, we find that the disk tends to stay in place as if it were suspended there. Eventually, gravity will pull it to the bottom of the jar.
When we moved the disk in the molasses, we found that it took more force to move it rapidly than it did to move it slowly. In the process of moving the disk, we put energy into the molasses and work was done. Since the force required was greater with rapid motion than slow motion, we say that the force is rate dependent. So, we say that the force is proportional to the rate of application. It is not always linearly proportional.
When we moved the disk to the center of the molasses and let go of the rod, the disk stayed in the center of the molasses. We put energy into the system to move the disk but it didn’t return to the bottom of the jar. All of the energy we put in was lost. All of the energy put into a viscous system is lost. It is generally turned into heat and we therefore say that a viscous system is 0% efficient. In elastomers, the viscous response is referred to as E’’.
If we join the two phases, the elastic phase and the viscous phase, we now have our two phase system or elastomer. When both phases are joined and we put energy into both of them, the response of each phase is exactly opposite. Both phases determine how the cured elastomeric product will respond to the energy that is put into it.
Measurement of Dynamic Properties
In the mid 1950’s, the engineers at what is now Damlier-Chrysler conceived and built a system to determine the dynamic properties of elastomeric components. In particular, they were interested in the determination of the dynamic properties of engine mounts. The system that evolved was to become known as a ‘resonant beam’ system. It was a small system with an electrodynamic shaker that excited the system with a 5 pound force. A beam was attached to a set of pivots on one end. The sample was placed roughly in the center of the beam between the beam and a rigid specimen holder. Attached to the end of the beam opposite the pivot was the electrodynamic shaker. Instrumentation consisted of a force transducer attached to the shaker and a displacement transducer attached to the beam above the specimen.
One of the problems with the resonant beam system was that the data obtained on the same samples varied depending on which system was used. In the early 1960’s, a study was undertaken and it was determined that the bearings on which the resonant beam rotated were a key contributor to data variation. It was felt that if the bearings could be improved or eliminated, the consistency of data would be improved.
The problems with data obtained with the resonant beam system led to the development by MTS Systems Corp. of the electrohydraulic test machine in approximately 1966. The electrohydraulic machine uses a hydraulic cylinder to apply force directly to the specimen. Load and displacement are determined through the use of a load cell and displacement transducer directly attached to the specimen or to the test fixture. The electrohydraulic test machine eliminates the pivot bearings completely. In October, 1971, based on the conclusion of a statistically designed experiment, it was reported that the electrohydraulic test machines provided agreement between laboratories. The report that there was agreement between laboratories allowed a standard test procedure to be formulated by SAE and ASTM in 1973.
Between 1973 and now, the basic electrohydraulic test machine has stayed the same but improvements in electronics and computer aided data acquisition have further improved repeatability and reproducibility of the tests. Today, Corry Rubber Corporation uses a state of the art MTS 830 elastomer test system for determination of both static and dynamic elastomer physical properties.
The system is used to excite a specimen or component using a force or displacement. The forces and displacements and the phase angle between them are determined. From this data, the spring rates, damping and tan delta (the phase angle between the applied force and the resultant displacement) are determined.
Elasticity - As we mentioned, elastomers are unique in that they can bounce. Related to their ability to bounce is their ability to stretch and return to their original length when the force doing the stretching is released. Extremely hard elastomers will only stretch a few percent. However, high quality, natural rubber compounds such as those used in rubber bands can stretch more than 10 times their original length and still return to their original length. This property allows elastomers to supply constant forces such as in seals and tie down straps.
Electrical Properties - Generally, one thinks of elastomers as good insulators. We see the people working on the electrical poles outside of our houses wearing rubber gloves to protect themselves from shock. However, if properly compounded, rubber can be made to be electrically conductive. This is done in gurney wheels for gurneys used in surgery. The wheels are made conductive to dissipate static electrical charges to prevent static discharge in the operating room. Static discharges can disrupt electrical instruments or cause fire if flammable substances are used during surgery.
Environmental Resistance - The need to for improved environmental resistance was the driving force behind the development of the first synthetic elastomers. Fr. Julius Nieuwland of the University of Notre Dame performed some very early work on basic reactions involving vinyl-acetylene. Fr. Nieuwland’s research was later used by Du Pont chemists to create the first synthetic rubber, neoprene (1904). Neoprene had better resistance to oils, fuels and weather than natural rubber.
With proper selection, elastomeric compounds can be made to be resistant to just about any environment from hot fuels and oils to concentrated acids and bases. They can be made resistant to temperatures as high as +275°C or as low as –100°C. Even though exposed to these environments for long periods of time, we can still expect them to exhibit the elastomeric properties that make them the unique materials that they are.
Hardness - Hardness is probably the most widely used and recognized property of elastomers. Most people can discern differences in hardness between two similar parts simply by their feel. This elastomeric quality is analogous to Rockwell Hardness in steel. Elastomer hardness is measured using an instrument called a durometer. The durometer is a device with a spring loaded indentor. The indentor is pushed against the elastomeric sample. Depth of penetration of the indentor into the sample indicates the hardness. The farther the indentor penetrates, the lower the durometer or the softer the sample. In general, hardness will range from approximately 15 to 100 on the A scale. Material with a 15 durometer will be very soft like sponge and material with a harness of 100 will be extremely hard just like a rigid plastic. Not all types of elastomers can be compounded to the extremes listed above. Many cannot be compounded below approximately 40 or above approximately 90.
Resilience - Resilience or the ability to bounce is what makes elastomers unique as a class of materials. Depending on the type of elastomer chosen and how it is compounded, the resilience of a material can range from -----
Weathering - Depending upon the type of elastomer chosen and the way it is compounded, elastomeric compounds can be made which will last indefinitely under all types of weather conditions.
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