Versatile, Diverse, Instrumentation, and Methods
Thermal analysis, or calorimetry, correlates temperature-dependent events to physical characteristics of a sample, such as mass, structure, strength, brittleness, elongation, decomposition, evolved gases, oxidation, reduction, or physicochemical structure. Any property that changes with temperature lends itself to some variant of thermal analysis.
Every industry concerned with the relationship between energy and how their products behave in the real world uses thermal analyzers. Thermal measurements provide food companies with values for caloric (energy) content, materials manufacturers with phase transition temperatures, and academic researchers with insights into phases of matter.
Thermal measurements are relevant in every phase of a product’s life cycle, from development to manufacturing, quality control, and release. “Everything you do to a material, everything you add to it, how you formulate it, where you store it, and how you beat up on it, all affect the final deliverable property,” notes Michael Zemo, a market manager at Mettler- Toledo (Columbus, OH). And all these are related to how the material or finished product handles heat.
Thermal analysis is not only about high temperatures. Some instruments have a cooling function that enables monitoring of low-temperature events such as glass transition in polymers.
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) are two specialties of Particle Technology Labs (Downer’s Grove, IL), a contract analytical services lab.
Particle Technology uses DSC to measure glass transitions, melting temperatures, and crystallization. “It can also determine the enthalpy of endothermic and exothermic events,” comments Dave Jovanovic, a fine particle analyst at the company.
TGA, a complementary technique, measures heat associated with gain or loss of mass resulting from evaporation, oxidation/reduction, or decomposition. Analysts often use TGA as a pre-test for DSC, to determine the sample’s heating limits before DSC analysis.
Pharmaceutical and polymer industries are increasingly interested in both TGA and DSC. Drug companies use these techniques to test for drug stability and crystalline state (or lack thereof). For example, some medicines work better in a particular crystalline polymorph, and others are more effective in an amorphous state. Polymer companies are interested in measuring numerous properties associated with heat, such as mechanical dimensions and stability, chemical stability, and physical states (e.g., glass transition) related to mechanical performance.
“Calorimetric profiling [thermal analysis] is a very powerful technique,” says Mr. Zemo, and not for just large changes like solid-to-liquid or liquid-to-gas transitions. “If there’s a physical or chemical change, you can pick it up.”
Yet thermal analysis works best when investigators know what they’re looking for, or at least know the identity of the sample. “You see a lot of weird things happening in TGA when you heat a sample to one thousand degrees celsius,” Mr. Jovanovic tells Lab Manager Magazine. Dehydration is an easy one because it occurs at 100° C, but often you can only speculate as to what is occurring.”
To reduce that uncertainty, some thermal analyzers incorporate a spectrophotometer in the mix. These techniques, known collectively as thermo-optical analysis, include thermospectrometry, thermorefractometry, thermoluminescence, and thermomicroscopy.
All work on the principle that a sample’s interaction with light changes with temperature. Numerous discrete and continuous thermally relevant events can be measured this way, including crystallization, melting, corrosion, phase transitions, drying, and polymorphism.
Two other limitations are related: accuracy and hysteresis. According to Mr. Jovanovic, typical DSCs can err in measuring even familiar events like water boiling or freezing by +/- 0.5° C. Micro-DSC is somewhat more accurate because it heats samples much more slowly. Again, it helps to know what to look for, particularly when heat capacity or process-related hysteresis is known to occur.
From his perspective as a thermal instrument manufacturer, Don Miller, president of Instrument Specialists (Twin Lakes, WI), sees the market as “quite a bit off ” from its heyday 15 years ago. “Thermal analysis is a mature market with slow growth if any,” he says, “and a much smaller dollar share than other types of instrumentation.”
To differentiate itself, the company provides upgrades for older systems, regardless of the manufacturer, as well as maintenance and repair. These services are all the more interesting since vintage thermal analyzers tend to be proprietary in terms of both hardware and software. “Today, nobody cares if they buy somebody else’s HPLC pump or detector, but thermal analysis wasn’t like that. If you bought an instrument, you were limited to the original manufacturer’s parts and software.”
Instrument Specialists produces and sells a very broad range of analyzers, including common TGA and DSC models, but it also offers an STA (simultaneous thermal analyzer) that combines TGA and DSC in a single instrument, a high-pressure DSC, and an instrument that allows users to swap out TGA and DSC cells. Some systems have the capability of adding mass spectrometry or Fourier- transform or infrared capabilities.
“But DSC is by far the number-one seller, with TGA coming in second,” Mr. Miller says. The two methods, moreover, are complementary, measuring fine or subtle properties and gross properties, respectively.
Thermal analysis may be a mature science, but that has not stopped manufacturers from tweaking it. In response to customer requests for faster heating and cooling, Mettler- Toledo introduced “flash” DSC in 2010. Mr. Zemo describes the flash variety as “not a replacement for, but complementary to, conventional DSC.” In the more common variety, materials are temperature-scanned slowly at around 0.2° per second, which gives them time to react to the heat input. With flash, temperatures rise by hundreds or thousands of degrees per second. “The material can’t adapt,” says Mr. Zemo. “You get a freeze-frame of how it really reacts to heat.”
Flash DSC provides insights into the world of rapid thermal processing, for example, molded plastic parts that cool from processing to room temperature in a second. It helps answer questions about why product batches fail.