Author Archives: Joy Wu

Measurement Modes in Thermogravimetric Analysis (TGA)

TGA analysis is a technique that measures sample weight change in-situ with temperature.  There are two frequently used modes in TGA.  One is constant heating rate and the other is isothermal temperature hold.

When an analyst is looking for an overall survey of how a sample behaves as temperature increases, such as temperature regions relating to volatiles or sublimation or degradation, a constant heating rate is applied in TGA testing.  On the other hand, the isothermal temperature hold is utilized for moisture or volatile content analysis.  A sample is heated to a temperature slightly higher than the boiling point of the volatile compound and held for some time until sample weight becomes stable.  This mode is similar to drying a sample in an oven.

Sometimes a more complex measurement mode is needed to improve resolution after a survey scan of constant heating rate.  The TGA method may apply variable heating rates and/or isothermal steps in order to distinguish each weight loss step more clearly.  Techniques to achieve better resolution include high resolution (“hi-res”) TGA, modulated TGA, or constant rate of weight change methodology.

Frequently Asked Questions about TMA CTE Measurement

The linear coefficient of thermal expansion (CLTE, or CTE or α) is often measured by a thermomechanical analyzer (TMA).  There are two standard test methods that we are often asked to follow: one is the ASTM E 831 test method and the other is the IPC-TM-650 2.4.24 test method.  To help clients prepare samples for testing and interpret test results, I compile a FAQ for your references in this blog.

Q.  Can you describe briefly how the test is done?
A.  In a TMA, the specimen is placed in the sample holder at ambient temperature. The sample height is measured by the probe with a minimal amount of force to maintain contact throughout the experiment. The furnace is closed around the sample stage to control temperature.  The initial temperature is usually 20 oC below the lowest temperature of interest. The specimen is then heated at a specified heating rate, for example, 5 oC/min, over the desired temperature range.  A thermograph is produced where the sample height dimensional change is tracked over the entire temperature range of interest.

Q.  What are the specimen dimension requirements for testing?
A.  Each specimen is a cube or a cylindrical shape. The base (i.e., x – y dimensions) should be 5 to 8 mm by 5 to 8 mm.  The measuring direction (may be called height, or length, or z-direction) should be 5 to 8 mm and flat and parallel.  The ASTM test method specifies length to be between 2 and 10 mm.  The IPC test method specifies thickness to be greater than 0.51mm.

Q.  Why a wide range in specimen height is allowed in test method?
A.  There are several reasons for this. One reason is dependent on the sample shape.  For example, the IPC method is specific for printed circuit boards which is flat and thin.  Another reason is the expected thermal expansion of the sample.  For samples with low thermal expansion, a longer length is preferable to improve measurement accuracy.  However, specimen longer than 10 mm is discouraged to minimize specimen temperature gradients.

Q.  Can I use a different heating rate than specified in the standard test method?
A.  Yes. A different heating rate is allowed as long as any deviation from the standard test method be reported in the test report.

Q.  How many specimens should be tested of the same material?
A.  It all depends… The ASTM test method recommends at least three specimens.  The IPC test method recommends two specimens.  Each test should use a fresh specimen.  Bottom line, do at least one!

Q.  What is the precision of the TMA CTE test?
A.  5 to 10% is typical. The precision depends on imprecisions of measurements (i.e., temperature and length measurements from the TMA equipment), sample preparations (i.e., surface parallelism), and sample type (i.e., specimen thickness and expected thermal expansion).

Q.  What is the dimensional unit for values of CTE, α
A.  The unit for α is length change over total length per temperature scale. The following expressions in SI units are frequently used:

ppm/ oC
10-6/ oC

Q.  How is CTE calculated
A.  The coefficient of linear thermal expansion (CLTE or CTE or a) is length change over temperature change over total initial length. In math expression,

α = (ΔL/ΔT) x (1 / Lo)

Determine CTE values from TMA thermograph; graph from ASTM E831 test method

CTE values are expressed either as “mean” value over a temperature range or as “instantaneous” value at a specific temperature.  The mean CTE value is the slope of direct line between two points on a TMA thermograph.  For example, in the TMA thermograph shown below, the mean CTE value between temperature of interest 0 to 100 oC is 24.26 mm/m.oC.  The instantaneous value is the right y-axis a value at a temperature.  This right y-axis (the blue line) is the derivative values of the left y-axis (the green line).  For example, the instantaneous CTE value at 150 oC is 26.3 mm/m.oC.

A typical TMA thermograph from TMA CTE testing

Thermal Analysis References

Occasionally I send references to my clients when questions were asked on complicated concepts or on general material behaviors under thermal analysis.  It is imperative to have scholarly references to explain these concepts in these occasions.  It is unfortunate, however, that the subject is not taught as a regular discipline at schools and that most of the research were done in the 70’s and 80’s and since then has been scattered due to its wide use but not in one concentrated area.

Given the readily available web search it has made literature research much easier now than ever.  You can search for a technical term you are interested in and many scholarly articles will appear from your web search.  Here I give a list of literature on thermal analysis, both books and websites that I refer to frequently as I find them very useful.

Useful books that summarize various techniques of thermal analysis:
Thermal characterization of polymeric materials / 2nd ed., edited by Edith A. Turi, 1981
Thermal analysis of polymers: fundamentals and applications / edited by Joseph D. Menczel and R. Bruce Prime, 2009

Useful application notes you can find from instrumentation companies:
TA Instruments website
Perkin Elmer website
Mettler Toledo website

And here are some more books that are useful for in-depth and theoretical concepts concerning thermal analysis:
Thermal Analysis / by Bernhard Wunderlich, 1990
Dynamic Mechanical Analysis of Polymeric Material / by T. Murayama, 1978
Thermal Methods / by James W. Dodd and Kenneth H. Tonge, 1987

Do you have a reference you use a lot with thermal analysis besides the ones I listed?  You can share your favorite books or sites here.

DMA Clamp Selections and Sample Geometries

A modern DMA instrument nowadays comes with several clamping fixtures to accommodate a wide variety of samples.  Modern DMA instrument also comes with sophisticated computer programming where you have a wider range of test parameter choices such as fixed frequency, frequency sweep, fixed force, fixed strain, stress relaxation, etc.  This is an evolution from early commercially available DMA instruments such as the Rheovibron, DuPont DMA 983, or Polymer Laboratories DMTA Mk II.  In my mind, it is analogous to how kitchen appliance hand-held blenders evolved: a modern hand-held blender allows for multiple rotating speed, and blade styles can be changed out for different culinary tasks.

The main advantage that comes with this evolution is versatility.  Now you have one instrument in the lab that you may use for various sample types and test modes instead of buying several machines.  The main disadvantage that comes with this evolution is, alas, also versatility.  Now you have a more complicated machine to learn, to use and to maintain. You may eventually find that some clamps or modes are never useful on this more expensive equipment you bought.

I describe DMA testing in simple terms: You hold a sample firmly, put the sample in an oven so you can heat it up, and observe how it moves when you push it this way and that way.  The said sample is usually a polymer, usually a solid, and usually cut to a rectangular shape so that the math can be applied.  The said force is usually small so that the math can be applied to calculate modulus values.

(1)  This said force is called “flexure” when you push the sample in the middle or at the end of it.  Sample is positioned either in a cantilever clamp (dual cantilever mode or single cantilever mode) or in a three-point bend clamp.

DMA flexure set up: three-point bend and cantilever

DMA flexure set up: three-point bend and cantilever

(2)  This said force is called “tension” when you pull on the sample along its length, with the sample positioned in a film clamp.

DMA tensile set up

DMA tensile set up

These two, flexure and tension, are the most commonly used set up in a DMA instrument.  The tension set up is also commonly used in a TMA instrument for film testing.

Two other clamp set up that are also included in a modern DMA instrument but are less used are compression clamp and shear clamp, which are designed for soft materials such as foam, gel, rubber, etc. and which would overlap with applications with a rheometer instrument.

DMA compression set up

DMA compression set up

DMA shear set up

DMA shear set up

In addition to flexure and tension set up as mentioned above, there is one more mode for a solid, stiff sample.  This said force is called “torsion” when you twist on the sample, with the sample held on both ends.  This set up requires a rheometer (note: not using a “DMA” instrument!) with a solid sample accessory fixture.  Historically this set up uses the Rheometric Scientific RDA instrument.

Solid torsion DMA

Solid torsion DMA

Measuring in-plane direction thermal expansion and shrinkage of a film or a sheet on a TMA

Thermal mechanical analyzer (TMA) is frequently used for coefficient of thermal expansion (CTE) measurement.  In a previous blog  CTE by TMA I mentioned that the expansion probe is used to measure linear CTE of a solid block of sample and the through-thickness expansion of a film sample.  To measure in-plane behaviors such as thermal expansion and shrinkage of a film sample, the tension probe with film sample assembly is used, see the photograph below.

TMA - film probe set up

TMA – film probe set up

Depending on how a film or a sheet is made it is likely to have directional differences in its machine direction (MD) and transverse direction (TD).  One common example that everyone can relate to is the shrinkage of a T-shirt after wash and dry.  I’ve had T-shirts that did not change size or shape after wash and dry, and I’ve had T-shirts that became smaller all around afterwards.  There was one T-shirt I had that became shorter but not narrower after wash and dry.  All these differences point to the various materials and processes that can be used to weave a fabric and to make a T-shirt which cause a difference in how the shirt behaves after wash and dry cycles.

This means the film or sheet sample should be tested in both directions and care should be taken to align directions during sample preparation.  One problem that can happen when sample is not aligned or when a sample is particularly heterogeneous is that the thermal stress differences in directions might cause the sample to twist during thermal testing.

Using DMA Analysis for Heat Forming Applications

Many plastics come in flat sheet and one way to bend the sheet to desired shapes is to perform what is called heat forming or thermoforming process.  In thermoforming, the plastic sheet is heated until it is softened to allow bending without breaking.  Once the shape is formed the plastic is cooled and it will retain its shape without springing back to flat sheet.

So how do you know what temperature to heat up the plastic in order to thermoforming?  One way to find out is to ask the plastic manufacturer.  The plastic manufacturer typically would provide a recommended working temperature for the said plastic for thermal forming processes.  Another way to find out the process temperature is to do a little bit of scientific investigation yourself.  The rule of thumb for working temperature range is above its glass transition temperature where it softens significantly but below its melting temperature (if it has one) where it may flow too freely.  Once you find out what kind of polymer your plastic sheet is, you can look up its Tg and Tm values and come up with a working temperature for the thermoforming process.

If you are dealing with specialty polymers and you do need hard data, you can perform a simple DMA analysis to help you determine the temperature range and to verify thermoforming process ability.  A DMA thermograph will display the material’s storage modulus values with respect to temperature, as illustrated in the graph below.

DMA thermograph showing storage modulus E’ of the polymer dropped during Tg transition into the rubbery plateau region

DMA thermograph showing storage modulus E’ of the polymer dropped during Tg transition into the rubbery plateau region

The DMA thermograph shows that the polymer material softens during the Tg transition.  Typically, the modulus drop is between one to three orders of magnitude.  For example, a polymer that has a modulus drop of 2-order of magnitude during its Tg transition means its modulus value is reduced from ~ 100 MPa in the glassy state to ~ 1 MPa in the rubbery state.

The rule of thumb for working temperature range I mentioned several paragraphs before means that you should process your polymer while it is in the rubbery state, and at its rubbery plateau.

There are polymers such as high crystallinity thermoplastic polymers or highly crosslinked thermoset polymers, however, where this rule of thumb does not work well.  The reason is because the modulus drop from glassy state to the rubbery plateau region is simply not significant enough to bend the plastic.  The modulus changes in thermographs with respect to polymer structures are well documented in scientific literature and textbooks.  The classic DMA thermograph (shown below as a cartoon drawing) you find in textbooks shows how Mc (molecular weight between crosslinks) of a thermoset polymer or crystallinity of a thermoplastic polymer affect the material modulus.  In a thermoset polymer, the higher the crosslinking density (i.e., low Mc) the less modulus drop after Tg transition.  In parallel, in a thermoplastic polymer, the higher the crystallinity the less modulus drop after Tg transition.

DMA thermograph to illustrate how storage modulus of a polymer changes with increasing crosslinking density or increasing crystallinity, around Tg transition

DMA thermograph to illustrate how storage modulus of a polymer changes with increasing crosslinking density or increasing crystallinity, around Tg transition

With little changes in modulus values even by taking up the temperature to rubbery plateau region, we can explain why those high crystallinity polymers or those high thermoset polymers are difficult to form shapes using thermoforming process.

In summary, thermoforming process works well for polymers at temperature range between its Tg and Tm except for polymers that are either highly crosslinked or crystallinity.  When data is needed to assess thermoforming ability, a simple DMA analysis can be performed to obtain the needed answer.

Measuring Glass Transition Temperature (Tg)

Glass transition (Tg) describes a material moving from rubbery to glassy state.  Unlike phase transitions, which are first-order phenomena, glass transition is a second-order phenomenon.  Glass transition is a kinetic phenomenon where it occurs over a range of temperature and its activation energy can be evaluated and it has relevancy to time-temperature-superposition.

Thermal analysis is particularly useful for glass transition temperature (Tg) evaluations of polymers.  For example, Tg can be studied using DSC, DMA and TMA analyses.  We show cartoon drawings below to illustrate typical thermographs depicting glass transition using thermal analysis techniques.  Depending on material type and sample geometry limitation, certain techniques are better suited than others.

Because glass transition is a kinetic phenomenon, Tg varies with heat rate in the thermal analysis testing.  This is one reason that heat rate should always be specified when reporting Tg values.  In general, Tg measured with a slower heat rate (such as at 1C/min) is lower than Tg measured with a higher heat rate (such as at 20 C/min).  The difference in measured Tg values due to heat rate differences may be as little as a few degrees to as much as 10 deg-C.

And because different material properties are being measured depending on the techniques, the measured glass transition temperature from one technique to another should not be expected to be “identical” or “equivalent” even with the same heat rate.  This is the reason that the type of thermal analysis should always be specified when reporting Tg values.  Although the measured Tg values should not be expected to be identical, the variation in Tg values obtained from various techniques are typically to be less than 10 or 20 deg-C.


Typical Tg transition in DSC

Typical Tg transition in DSC

Typical Tg transition in TMA

Typical Tg transition in TMA

Typical glass transitions in DMTA

Typical glass transitions in DMTA

Measuring Linear Thermal Expansion of Solid Materials by TMA

The thermal mechanical analyzer (TMA) with an expansion probe can be used to determine linear thermal expansion (aka CTE) of solid materials. The standard test method ASTM E 831 describes the instrumentation and how the testing is performed and analyzed.  A sample is cut to a small cube or cylinder to fit on the sample stage with the two faces flat and parallel in the direction of interest for CTE measurement, see picture below.  The sample length should be long enough to provide adequate resolution but not too long to cause thermal gradient within the sample.  A small load or force is placed on the sample from the contacting probe to ensure good contact but not inhibit sample expansion during heating.

To measure the through-thickness CTE of a film sample it is sometimes necessary to stack multiple sheets to increase overall sample length/thickness in order to improve resolution of the measurement.

TMA expansion probe with sample

TMA expansion probe with sample