Doug Sharpe President of Elasto Proxy
In my last blog entry, I recommended grabbing a cup of coffee before diving into this week’s discussion about the coefficient of thermal expansion. Yes, the caffeine will help if you prefer explanations over calculations. But our topic this week is less about math and more about the physical properties of elastomers. Let me explain.
To choose the right compound for your sealing application, you need to know how that material will perform at specific temperatures. Physical properties such as modulus of elasticity are important, too, but let’s stick to temperature while you’ve got last week’s blog entry in mind and a hot cup of coffee in hand.
Elastomers and Changes in Temperature
All elastomers have a coefficient of thermal expansion. Simply put, this value describes how the material changes in length, area, or volume with changes in temperature. In the case of rubber door and window seals, linear expansion is important because it helps to predict how a change in temperature will literally lengthen or shorten the seal.
Let’s consider two examples, both involving a rubber door seal and a metal door frame. At high temperatures, the rubber seal expands more than the metal frame. At low temperatures, the seal contracts more than the surrounding metal material. So what happens if you choose the wrong rubber? The door may not shut if it’s hot, or may admit wind and weather if it’s cold.
Now think back to last week’s blog entry, in which we learned about tractor trailers that make northbound runs from Miami to Montreal. For drivers and vehicles alike, the temperature changes can be extreme – especially during winter. If a rubber door seal is made of a compound that can’t handle these changes, the seal may fail and jeopardize the load.
Temperature Range and Temperature Change
Seal performance isn’t just about temperature range then. To select the right compound, you must also consider temperature change – how the rubber reacts when the temperature rises and falls.
Take a look at the chart below. The data required some conversions – and some may quibble with the math – but our takeaway here is simpler than the calculations. As you can see by looking at the right-hand column, all rubber is not the same when it comes to temperature changes!
|Urethane||100° - 150°||180|
|Neoprene||130 - 150|
|Teflon||230||50 - 80|
Table 1: Some Common Elastomers and Their Coefficients of Thermal Expansion
For more information, including the coefficient of thermal expansion calculation itself, please visit the National Physical Laboratory. Another good on-line technical resource is Rubber as an Engineering Material: Guidelines for Users.
Feeling Stressed Out?
Don’t spill your coffee, but the relationship between elongation and temperature isn’t always so straightforward. For starters, elastomer elongation increases over a specific temperature range and then decreases at still higher temperatures.
Then there’s something called the Joule effect, which occurs only when an elastomer is under tensile stress. The easiest way to explain this is to imagine a rubber band suspending your coffee cup. If you warm the elongated rubber band with an infrared lamp (your desk lamp, perhaps), the rubber band doesn’t expand. In fact, it retracts to support the load.
Choose a Partner – Not Just a Provider
Experimenting with rubber bands and coffee cups makes for a fun science project (and perhaps a coffee-stained desk), but our job at Elasto Proxy is to help you choose the right sealing solution for your specific application. By analyzing all of your application requirements and listening to all of your needs, we can offer answers to your sealing questions – and not just explanations of coefficients and calculations.