Too Hot to Handle: Standards for Thermal Measurements
Too Hot to Handle: Standards for Thermal Measurements
Depending on how it’s formulated, steel will melt at a temperature between 2,500 and 2,800 F (1,370–1,538 C). Aluminum, on the other hand, will start to soften at just over 1,200 F (649 C). These numbers might sound hypothetical and abstract, but that changes when you’re driving over a steel bridge or sitting in an airplane. Everything around us responds to heat differently, so an understanding of how these thermal properties affect the products and materials we use every day is a critical part of designing safe, reliable systems.
That’s why the standards developed by the committee on thermal measurements (E37) – currently celebrating its 50th anniversary this year – are so important and used across so many different industries.
“We talk about the measurement of thermal properties as a function of temperature, from liquid air temperatures all the way up to white hot,” explains Roger Blaine, Ph.D., a retired member of the committee and former ASTM International Board of Directors member. He is also a regulatory affairs manager at TA Instruments, a worldwide supplier of thermal analysis and rheology equipment. “So, if you’re involved in the metals industry and you know that metals soften at certain high temperatures, you’re going to want to know exactly where that happens. You’ll also want to know how much energy you’ll have to use to heat them up to get to a melting point.”
FOR YOU: Testing Your Metal
E37 standards are also widely used in the polymer field, Blaine explains, since the thermal properties of plastics and polymers can actually change based on the use of new thermal treatments. As a result, the measurement of these properties continues to be an active part of the design and manufacture of composite and thermoplastic materials.
“As long as these kinds of materials are used in commerce, thermal analysis will be important for understanding and working with them,” he adds. “With metals, for example, once you measure how much energy it takes to get to the melting point, that goes into a handbook and is no longer of interest for a lot of study. But with plastics, because there's always this process going on of mixing different polymeric materials in such a way as to mold the characteristics to a particular application, temperature as a property is a tool for making those measurements and designing those products.”
As it celebrates its 50th anniversary in 2023, the committee is tasked with developing standard test methods, practices, terminology, and definitions as they relate to thermal measurements. As a generalist committee, the work of E37 isn’t focused on any specific products or industries but rather how thermophysical properties are measured – and the test methods used to make those measurements. It also helps provide reference values for thermal properties and to create calibration methods so that different testing apparatus can be correlated with each other.
Among the committee’s most impactful standards are:
1) Standard test method for assignment of the glass transition temperature by thermomechanical analysis (E1545)
The glass transition temperature is the temperature at which a material softens but doesn't fully melt, and it is typically measured using the thermomechanical analysis, dynamic mechanical analysis method (or the tension method). This standard in particular describes a test method for assigning the glass transition temperature using thermomechanical measurements to provide a quick way to detect changes in hardness or expansion associated with the glass transition. Being able to accurately measure that temperature line is particularly useful, according to Blaine, because more happens to materials at that point than just melting. “There are a number of other property changes that happen as well,” he says. “The concept of the glass transition temperature is that temperature where they go from being mostly solid to mostly liquid, and when that happens there are a number of physical properties that change including heat capacity, length and more.”
2) Standard test method for melting and crystallization temperatures by thermal analysis (E794)
Beyond the glass transition temperature, most manufacturers want to be able to determine the actual melting temperature of a given material, which is what standards like this one address. It’s the next step in measuring properties based around the glass transition temperature. “Now, why is that important?” says Blaine. “In the field of composites, if you’re working on an airplane wing and you're going to create a panel using what's called a thermal set material, which means it's a plastic material that is liquid at low temperature, you need to know how far you can go to do that. You can form it into a shape, and then you heat it up and the material is cured as it crosslinks and becomes hard and brittle.” Being able to measure that transition from a hard material to a soft material and back again is an indication of how well cured the material is and therefore how strong it is.
3) Standard test methods for kinetic parameters by differential scanning calorimetry using isothermal methods (E2070)
Finally, measuring the energy associated with each step in this process allows a producer to determine where on the thermal continuum they can ensure safe operation. If it takes less energy than expected to get a material to the glass transition, for example, that might indicate other problems with the makeup of the underlying material that could threaten safety. This is one standard among many that measures the energy used in the process.
Looking ahead, Blaine is excited about new thermal measurement analysis techniques such as thermal effusivity, which is becoming widely used in the space program to characterize extraterrestrial properties. For instance, when instruments are sent into orbit around Mars, they can use thermal effusivity as a way to map the planet’s surface, using heat to determine whether the surface is granular, whether it's hard rock, whether it's dust, how deep the surface goes, and so on.
And this has applications here at home, too.
“Thermal effusivity deals with the warmth or coolness we feel when we touch a material,” Blaine says. “So, for example, clothing apparel. We want the clothing that we wear to feel cool in the summertime and warm in the wintertime, so fabric cooling manufacturers want to measure the property. It’s the same in the automobile industry. We want the dashboards in our automobiles to have the touch and feel of wood paneling, and that property is related to heat. That’s why these new measurements will likely be at the forefront of what the committee is going to be looking at for the next five years or so.”
Tim Sprinkle is a freelance writer based in Colorado Springs, CO. He has written for Yahoo, The Street, and other websites.