Seal Selection and Thermal Expansion

Doug Sharpe President of Elasto ProxyIn 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. Tochoose the right compoundfor 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 TemperatureAll elastomers have acoefficient of thermal expansion。简单地说,这个值描述斜纹布l changes in length, area, or volume with changes in temperature. In the case of rubber door and window seals,linear expansionis 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 arubber door sealand 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 ChangeSeal performance isn’t just about temperature range then. To select the right compound, you must also consider temperaturechange– 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!

Thermal Expansion
Material	Thermal Stability	X10-6mm/°C
EPDM	150° C	160
NBR	120° C	230
SBR	Ambient	220
Silicone	Ambient	2.5
Urethane	100° – 150°	180
Neoprene		130 – 150
Teflon	230	50 – 80

Table 1: Some Common Elastomers and Their Coefficients of Thermal ExpansionFor more information, including the coefficient of thermal expansion calculation itself, please visit theNational Physical Laboratory。Another good on-line technical resource isRubber 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,elastomerelongation 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 ProviderExperimenting 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 yourapplication requirementsand listening to all of your needs, we can offer answers to your sealing questions – and not just explanations of coefficients and calculations. For over 25 years, Elasto Proxy has provided sealing solutions to partners in a variety of industries.How can we help you?Please comment below, orcontact usat our website today.