Thermal instrument research brings new materials insights.
Developed initially to quantify polymer processes--which is still their largest user base--thermal analysis systems now are used in environmental, catalysis, pharmaceutical, food processing, inorganic materials, ceramics, and petroleum applications.
Trends in the development of thermal analysis techniques focus on hyphenated techniques, computer-controlled processing, and software enhancements.
While thermal analysis techniques provide a variety of physical information on material changes occurring during a thermal excursion, they cannot identify the evolved gas species being generated by the sample.
To remedy this problem, researchers found that by using a thermal analysis instrument as the front end to an analytical instrument, they could identify these evolved gases for specific regions of a thermal cycle.
These combined, or hyphenated, techniques provide a more complete characterization of materials, and in particular organic materials.
"Alternatively, the thermal analyzer brings a lot of control to the sampling technique for analytical instruments," says John Dwan, the business manager for TA products, Perkin-Elmer, Norwalk, Conn.
The most popular hyphenated systems are combinations of TGA (thermogravimetric analyzers) with FTIR (Fourier transform infrared), MS (mass spectrometer), or GC (gas chromatography) instruments; DMA (dynamic mechanical analyzers) with DEA (dielectric analyzers); and combined TGA and DSC (differential scanning calorimeters) with FTIR, MS, or FTIR and MS in combination.
The market for hyphenated systems peaked in 1989 and is increasing again after a period of flat growth in the early 1990s, says Paul Newbatt, product manager for thermal analysis products for Rheometrics, Piscataway, N.J.
For the most part, the basic technology of thermal analysis systems and hyphenated systems has not changed in more than 10 years. In a TGA system, for example, a sample pan is suspended in a furnace core on a microbalance, and weight changes in the sample are recorded as the furnace temperature is changed.
Recent research has focused on developing the interfaces for hyphenated systems. For a TGA-FTIR or a TGA-MS system, a line is taken off the dead space above the TGA's sample pan and run to the input port on the analytical instrument. The line is heated to prevent condensation of the gas on the walls of the transfer line. In MS systems, the vacuum of operation serves to pump the evolved gas from the TGA to the MS. In FTIR systems, a pump is placed after the FTIR cell.
While the line heater is designed to prevent condensation, it needs to be precisely controlled to prevent cracking of the gases if it gets too hot.
In a typical resin-curing application, researchers can experiment with different cure cycles in the TGA portion of the system and then use the MS or FTIR portion to profile the thermal evolution of each component, such as stabilizers, plasticizers, and colorants.
Several thermal analyzer manufacturers collaborate with various analytical instrument manufacturers to supply hyphenated systems. TA Instruments, New Castle, Del., collaborates with VG Instruments (Fisons) in TGA-MS systems and with Nicolet and BioRad for TGA-FTIR systems. Four TA Instruments TGA systems, with up to 0.1-[[micro]gram] sensitivity, 1-g capacity, and ambient to 1000 [degrees] C, can be used as part of these systems.
Rheometrics supplies a simultaneous thermal analyzer (STA) with an FTIR or MS interface. The STA combines a TGA with a quantitative heat flux DSC. This system is especially useful for characterizing complex reactions, such as those for hot-melt adhesives, neat resins, composites, thermoplastics, and elastomers.
ATI Instruments, Madison, Wis., also supplies TGA-FTIR/MS systems with ATI/Mattson supplying the FTIR systems.
The simplicity of the heated tube interfaces on these systems can and has resulted in researcher-implemented retrofits of existing TGA systems to existing MS or FTIR instruments.
Another hyphenated technique is the simultaneous DMA (dynamic mechanical analysis) and DEA (dielectric analysis) system. Developed jointly by Perkin-Elmer and Micromet, Newton Centre, Mass., this system provides complete characterization of curing reactions of organic materials during a single run by measuring changes in polymer viscosity.
With the DMA, users can determine flow, absolute viscosity, gel point, and vitrification. With the DEA, they determine the cure rate, cure state, and end of cure.
While TGA-based techniques are a fast growing area, they are second to calorimetry, the most widely used thermal analysis technique.
The biggest news in calorimetry is the development of dynamic differential scanning calorimetry (DDSC). A DDSC technique developed by TA Instruments is based on exposing the sample to a linear heating method, whose computer-generated superimposed sinusoidal temperature oscillation results in a cyclic heating profile.
Separation of the resultant heat flow during this cyclic heating provides a "total" heat flow from the conventional DSC and separates it into its heat capacity and kinetic components.
DDSC also provides a measure of the sample's thermal conductivity, and a characterization of its melting and crystallization.
Perkin-Elmer's approach to DDSC was developed with help from Jurgen Schawe, a thermal analysis researcher at the Univ. of Ulm, Germany.
Unlike current empirically based products, this approach is based on a dynamic response theory that uses existing data to provide more precise control and faster and more reliable DDSC measurements.
Computerization of automatic loading systems has also increased the efficiency and reliability of thermal analysis systems. The TGA-601 from Leco, St. Joseph, Mich., TGA-601 with its [+ or -]0.0001-g sensitivity and 20-position carousel, opens up its use for new applications, such as that for measuring flour in the food industry. Older systems with only [+ or -]0.0005-g sensitivity developed for measuring coal and rubber could not do this.
The implementation of Microsoft Windows-based software controllers has done as much as anything to increase thermal analysis systems' acceptance, says Krishnan Rajeshwa, a researcher at the Univ. of Texas, Arlington.
"First systems had their own proprietary software, which now has been replaced with more compatible systems," he says.
For example, the Windows-based TA50WS from Shimadzu Scientific Instruments, Columbia, Md., provides simultaneous control of up to four thermal analysis modules--TGA, DSC, TMA, and DTA (differential thermal analysis).
You also can view each analysis simultaneously on your computer display. Instrumentation programming, data acquisition, analysis, data storage, file transformation, and reporting are all obtained via keyboard or mouse.
Shimadzu's software also has a feature that lets you preview thermograms while the system is still in the measuring mode.
"The use of thermal analysis systems is expanding from its base in R&D and analytical service labs to near-line QA labs," says Perkin-Elmer's Dwan. "Improvements in microprocessing and our understanding of the various techniques will continue to shorten measurement times and increase its usefulness."
Real-World DSC Tests On High-Pressure Cell
Materials that operate or are processed in extreme high-pressure environments, such as gunpowder, rocket propellant, engine oil, or polymers, must be tested in a similar environment.
The new DSC high-pressure cell from Perkin-Elmer, Norwalk, Conn., is designed to handle those tests. The cell itself is a conventional DSC cell, with added high-pressure fittings, cover, clamping arrangement, and analog pressure gauge. Replacing a conventional cell with this unit is no harder than exchanging wiring, harnesses, according to John Dwan, thermal product manager at Perkin-Elmer.
These extras give it an operating window that runs from 40 [degrees] C to 600 [degrees] C and from 40 psi to 600 psi. Previous systems only went to 400 psi.
With its added ability to increase the oxygen content in the cell, the device can also perform tests on the effectiveness of anti-oxidants on lubricants, such as motor oil.
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|Publication:||R & D|
|Date:||Oct 1, 1994|
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