Thermal Analysis Application No. UC 421Application published in METTLER TOLEDO Thermal Analysis UserCom 42

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Curve interpretation Part 5: TMA curvesDr. Melanie Nijman

The interpretation of TMA curves is often difficult because different physical effects pro- duce the same or similar effects on the measurement curves. In such cases, we must either vary the measurement parameters used for the TMA measurement (measurement mode, force program, temperature program) or obtain more information by using other thermal analysis methods. In this article, we discuss a number of practical approaches.

IntroductionDepending on the method and mode used for the actual measurement, it is not always possible to uniquely assign effects on a TMA curve to particular physical processes. For example, a step-like change to shorter length in a meas-urement performed in compression can indicate a glass transition, melting or a solid-solid transition. In such cases, ad-ditional measurements are needed to in-terpret the measured TMA curve. Some possibilities are:• to vary the force program using differ-

ent static forces (positive or negative) or oscillating forces (DLTMA);

• to vary the temperature program using heating-cooling-heating cycles or by changing the heating rate;

• to use a different measurement mode (e.g. expansion instead of compression);

• to use a different thermal analysis tech-nique such as DSC, TGA (-EGA) or DMA.

In this article, we will show how these possibilities can lead to a better under-standing of the processes occurring in a material.

Influence of the force program In a TMA measurement, the force pro-gram used has a decisive influence on the shape of the measurement curve. For example, if an amorphous material is measured using a small force, the glass transition appears as an increase in the slope of the TMA curve (the TMA curve becomes steeper).

If the same sample is measured using a large force, the glass transition is ob-served as a step in the TMA curve (the thickness of the sample decreases). The TMA provides forces ranging from –0.1 N to +1 N. In addition force programs can be used in which the applied force is not constant but varies as a sine wave or square wave function.

Example 1: Size of the applied force Figure 1 shows the shrinkage and ex-pansion behavior of a polyester fiber. The measurement of shrinkage usually requires the use of low forces. The black curve was recorded using a constant force of 0.01 N. We see that the sample shrinks (the sample length decreases).

The red curve was measured using a force of 0.02 N. Here we see that the fiber stretches after the glass transition. The shrinkage force of the material (the force at which the sample neither shrinks nor stretches) therefore lies between 0.01 N and 0.02 N for the sample measured here.

Shrinkage is typical for semicrystalline stretched fibers: as a result of the produc-tion process, the crystals are all oriented in the same direction. This orientation is lost after the glass transition and the fibers shrink in the direction of tension

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n(and become thicker). Comparison of materials in this way allows conclusions to be drawn about differences in produc-tion and about the materials themselves.

Example 2: DLTMA In Dynamic Load TMA or DLTMA, the force acting on the sample varies. We can use either a sine wave or a square wave force program. In the square wave force program, the applied force changes peri-odically from a smaller to a larger value. The values of the two forces and the pe-riod (the default value is 12 s) have to be specified in the method.

In the sine wave force program, the ap-plied force varies as a sine function be-tween two forces that also have to be speci-fied in the method. In this case, the period can also be freely chosen. Figure 2 shows the result of a DLTMA measurement of a thin 0.5-mm polyethylene terephthalate (PET) disk. For comparison, the curve in the upper diagram shows the TMA meas-urement of the same material performed in compression using a constant force.

The TMA curve shows three clear steps in which the thickness of the sample de-creases as the measurement probe pen-etrates more and more into the sample.

The measurement does not however al-low us to unambiguously assign the three effects - they could be due to a glass tran-sition, melting, crystallization, shrink-age, decomposition, or other effects.

The DLTMA experiment provides us with the information we need to interpret the TMA measurement curve [1]. Up until about 70 °C, the change in force has no effect and the material is hard. At about 70 °C, the sample softens and the dis-placement amplitude increases. At the same time, the material expands and there is a sudden change in the coeffi-cient of thermal expansion (CTE).

This behavior is typical for a glass transi-tion. The amplitude then decreases, the material becomes harder and shrinks. The behavior is characteristic for cold crystallization. After this transition, the displacement amplitude is almost zero and the material is now hard again but crystalline and no longer amorphous. Now that we have assigned the first two steps, the final step can only be due to the melting of crystallites formed.

Example 3: DLTMA with positive and negative forcesDLTMA measurements are usually per-formed using positive forces. Certain ap-plications however need alternating posi-tive and negative forces.

An example of this is shown in Figure 3, which displays measurement curves of an initially liquid adhesive. The adhesive was contained in a 40-µL aluminum cru-

Figure 1. Semicrystalline polyester fiber measured in the tension mode using forces of 0.01 N and 0.02 N.

Figure 2. Thin PET disk measured using a constant force (0.5 N, above) and an alternating force (DLTMA, 0.01 N/ 0.19 N, below).

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ncible to prevent it flowing. The measure-ment was performed using a flat meas-urement probe (surface area 1 mm2) at a constant temperature of 10 °C.

The force program is shown in the upper right corner of the diagram. The DLTMA curve (black) initially shows very large deflections. As long as the sample is liquid, the negative force lifts the probe completely out of the liquid.

After a certain time, with increasing crosslinking, the viscosity of the adhe-sive increases and the adhesive becomes increasingly sticky. As a result of this, the negative force is no longer sufficient to lift the probe out of the adhesive mass and the displacement amplitude becomes smaller until it is almost zero after about 40 minutes.

The time up to the point when the dis-placement amplitude first begins to de-crease is known as the gel time. This is usually evaluated as the onset of the step in the envelope difference curve of the upper and lower envelopes.

The gel time is the time up until the point when an adhesive or a resin system becomes highly viscous. Terms used in practical usage are the pot life and work-ing life. All three terms are related but defined in slightly different ways.

Essentially they indicate the period in which an adhesive or thermosetting system can still be used for its intended purpose before it gels or begins to cure and can no longer produce an acceptable result.

Example 4: Influence of frequency in DLTMA measurements When DLTMA measurements are per-formed using a sine function force pro-gram, the frequency or period of the

sine-shaped force excitation is an im-portant measurement parameter which can be used to distinguish frequency-dependent from frequency-independent effects. For example, melting always occurs at the same temperature and is independent of the frequency. The glass transition temperature, however, depends on the frequency and shifts to higher temperatures at higher frequencies. The frequency dependence of an effect can

best be seen by comparing the modu-lus curves. The modulus curve can be calculated from the measured DLTMA curve using the sample geometry and the applied force program [2].

This is illustrated in Figure 4. The up-per part of the diagram displays DLTMA curves of a printed circuit board. The curves were recorded using a sine wave force program with periods of 100 s or

Figure 3. Curing process of an adhesive measured by DLTMA.

Figure 4. DLTMA curves of a printed circuit board showing the frequency dependence of the glass transition temperature.

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n0.01 Hz (black curve) and 10 s or 0.1 Hz (red curve).

The amplitude of the force was 0.48 N. The lower part of the diagram displays the corresponding modulus curves. Com-parison of the two curves shows that the glass transition at the higher fre-quency is shifted by about 5 K to higher temperature.

A simple rule of thumb for the frequency dependence of the glass transition is that the glass transition shifts ±5 K per dec-ade. A similar rule of thumb also applies for DSC measurements: if the heating (or cooling rate) is changed by an order of magnitude, the glass transition also shifts by about ±5 K.

Temperature programs using heating–cooling–heating In thermal analysis, the so-called ther-mal history of a sample plays an impor-tant role. The thermal history of a sam-ple includes the processing conditions in production and the conditions a material was exposed to during storage or use. The thermal history can only be observed in the first heating run.

It is also the reason why the measure-ment curves of the first and second heat-ing runs are often different. In many cases, it is therefore advisable to meas-ure both the first and the second heating runs. This applies to TMA, and to DSC and DMA measurements.

The example in Figure 5 shows the first and second heating runs of a PET fiber measured by DLTMA [3]. In the first heating run (black curve), the length of the fiber remains constant up to about 80 °C and the displacement amplitude is small. The fiber then begins to shrink above 80 °C. At the same time, the fiber softens and the displacement amplitude increases. This behavior is characteristic for the glass transition of a stretched fiber.

After the first heating run, all the inter-nal stresses frozen into the fiber during the production process have been elimi-nated through relaxation. As a result, the sample no longer shrinks in the second heating run.

The use of other techniques Sometimes, the properties of a mate-rial cannot be clearly characterized by TMA even after varying the force or tem-perature program. In such cases, other thermal analysis techniques can provide valuable information and thereby con-

tribute to a better understanding of the material properties. In the following sec-tions we will discuss different examples.

TMA and DSC, and SDTA The curves colored black in the upper part of Figure 6 show the TMA curve and part of the DLTMA curve of a hot melt ad-hesive [5]. The TMA curve indicates tran-sitions at –37 °C and 0 °C.

These are interpreted as a glass transi-tion and the gel point. The gel point can be much more easily seen in the DLTMA

Figure 5. First and second heating runs of a PET fiber measured by DLTMA.

Figure 6. TMA, DLTMA and DSC measurements of a hot melt adhesive.

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ncurve, which is shown in the inset dia-gram below the TMA curve in the tem-perature range –20 °C to +20 °C. At 0 °C there is a very clear transition to a more mobile phase. At higher temperatures, two further effects are visible that cause the material to shrink or soften.

These latter effects can be identified with the aid of the DSC curve shown below in red as melting – the curve exhibits an endothermic melting peak in the relevant temperature range. The two steps of the TMA measurement indicate that this be-havior is due to a two-step melting process or to a material blend consisting of two components with similar melting points.

The DSC curve confirms the glass transi-tion at about –37 °C; the gelation point is mechanical property that cannot be detected by DSC.

Use of the SDTA signal for additional informationBesides sample length, the METTLER TOLEDO TMA also measures the sample temperature. The instrument calculates an SDTA signal from the reference tem-perature and the sample temperature. This provides calorimetric information.

The example in Figure 7 shows the first (black curve) and second (blue curve) heating runs of an epoxy resin that was measured by DLTMA. The SDTA signal recorded during the measurement is dis-played in the lower part of the diagram (red curve).

Based on the two DLTMA curves, we think that the epoxy resin cures during the first heating run at about 195 °C. At this tem-perature, the modulation becomes small-er as a result of the crosslinking reaction.

In the second heating run, the glass transition is shifted to a higher tempera-ture. The supposed curing is confirmed by the SDTA curve (exothermic peak at 195 °C). The SDTA signal simultane-ously measured with the TMA curve can therefore be used to confirm or dismiss assumptions about certain effects such as curing, melting, or crystallization.

Tip: Use of the first derivative for the evaluation of TMA curves The top curve shown in the diagram in Figure 8 is the TMA curve of a multilayer film measured in the penetration mode. The film consists of different layers of polyethylene and polyamide [4].

Figure 7. Curing of an epoxy resin measured by DLTMA and SDTA.

Figure 8. Determination of the thickness of layers of a multilayer film by TMA.

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nFor quality control purposes, it is impor-tant to know the thickness of the layers. The materials used for the film are all semicrystalline. When each layer melts, the TMA probe penetrates further into the material. The steps produced correspond approximately to the thickness of the in-dividual layers. The different steps can be clearly seen on the TMA curve.

They are not all completely separated from each other. This makes it difficult to determine the step heights. As in TGA measurements, the use of the first de-rivative of the TMA curve (middle curve) helps solve the problem. The first deriva-tive curve shows the steps as peaks and the area under a peak corresponds to the height of the corresponding TMA step.

For comparison, the bottom curve Fig-ure 8 shows the DSC heat flow curve. The melting peaks of the different materi-als in the film correspond to the effects observed in the TMA curve or the first derivative curve and allow the different materials in the film to be identified.

The weak effect at about 50 °C in the DSC curve is due to the glass transition of PA 11. It can also be seen in the TMA curve (see the inset diagram in the top right corner of Figure 8). The thickness of the individual layers cannot however be determined from the DSC curve.

TMA and TGACopper wire is used in electronics, for ex-ample in transformers and electrical mo-tors. The copper wire is usually insulated with a thin lacquer coating. In opera-tion, the coiled copper wire can become quite warm. To prevent short circuits, the lacquer coating must be stable at higher temperatures. In the following example, the stability of the coating of a copper wire is investigated by TMA and TGA.

The TMA measurement was performed with an approximately 3-mm long piece of copper wire using the ball-point probe. The applied force was 0.02 N. The TMA curve recorded (black curve) shows two clear steps. Without further information, it is not immediately clear what causes the two steps.

A TGA measurement clarifies the situa-tion. The TGA curve (blue curve) shows a loss of mass from about 260 °C on-ward which is due to decomposition of the lacquer coating. The step from about 260 °C onward on the TMA curve also corresponds to the decomposition of the coating.

The first step on the TMA curve at about 180 °C is not accompanied by a mass loss and must therefore be due to the glass transition of the coating. Above 180 °C, the coating becomes soft. Clearly, in practical use, the temperature of the cop-per wire must not exceed 180 °C.

The total step height on the TMA curve (about 8 µm) indicates that the thickness of the lacquer coating is about 4 µm.

TMA and EGAIn many cases, not only the temperature range in which a material can be used is of interest but also the nature of the gases released when it begins to decom-pose. To obtain this type of information, a TMA instrument can also be coupled to a mass spectrometer (MS) or a Fourier transform infrared spectrometer (FTIR). This enables you to determine the tem-perature at which possibly harmful gases are liberated.

Figure 10 shows the results of the TMA-MS analysis of a printed circuit board [6]. The TMA curve is displayed in the upper part of the figure. The curve ex-hibits a glass transition at about 93 °C and indicates that delamination begins at about 320 °C followed by decomposi-tion from 360 °C onward.

Brominated flame retardants (BFRs) such as tetrabromobisphenol A (TBBPA or TBBA) are often used in printed cir-cuit boards. Typical decomposition products of TBBPA are bromine and me-thyl bromide. These two molecules can be identified using mass spectrometric

Figure 9. Coating of a copper wire measured by TMA, TGA and DSC.

Mettler-Toledo AG, AnalyticalPostfach, CH-8603 SchwerzenbachPhone +41 44 806 73 87Fax +41 44 806 72 60Contact: urs.joerimann@mt.com

© 02/2016 Mettler-Toledo AG, 30304667Marketing MatChar / MarCom Analytical

For more information

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analysis by detecting the m/z 70 and m/z 94 ions.

The lower part of Figure 10 displays the MS ion curves for these two masses. It can be seen that elimination of TBBPA begins at the glass transition, that is, at a much lower temperature than the actual de-lamination or decomposition of the board.

ConclusionsDepending on the method and mode used for the actual measurement, it is not al-ways possible to definitely assign effects on a TMA curve to particular physical processes. In such cases, measurements performed under other conditions can lead to a better understanding of the pro-cesses that occur in the material.

The possibilities include measurements using different force programs (static force, DLTMA, DLTMA with different frequencies), measurements using a dif-ferent measurement mode (for example expansion instead of penetration) or measurements using a different tempera-ture program (heating-cooling-heating, variation of the heating rate). In many cases, the use of other thermal analysis techniques (DSC, TGA-EGA or DMA) can also provide important information to help you interpret TMA curves.

References [1] R. Riesen and J. Schawe, METTLER

TOLEDO Collected Applications Hand-book: Thermoplastics, 101–102.

[2] G. Widmann, J. Schawe, R. Riesen, Interpreting DMA curves, Part 1, User-Com 15, 1–6.

[3] Expansion and shrinkage of fibers, UserCom 11, 20–24.

[4] A. Hammer, Analysis of thin multilay-er polymer films by DSC, TMA, and microscopy, UserCom 30, 15–17.

[5] A. Hammer, Investigation of a hot melt adhesive by TMA, UserCom33, 21–22 .

[6] C. Darribère, Investigation of dela-mination and foaming by TMA-MS, UserCom 15, 21–22.

Publishing Note:This application has been published in the METTLER TOLEDO Thermal Analysis UserCom No. 42.See www.mt.com/ta-usercoms

Figure 10. Delamination and decomposition of a printed circuit board measured by TMA and EGA.