Characterizing Melting and Softening Points Using the TMA 4000

The melting point is the basic property of any material, whether it is a ceramic, metal, or plastic. While a DSC can determine the temperature where the heat of crystalline melting takes place, or the heat capacity increases at the glass transition, it takes a mechanical analyzer to identify where the sample goes from being rigid to soft or flexible—i.e., where there is a dramatic decrease in modulus.

While a simple thermomechanical analysis (TMA) test is enough to quantify or qualify the changes that take place in a material during heating, dynamic mechanical analysis or DMA provides the broadest range of quantitative information regarding changes in modulus. In order to obtain this for different types of samples, a number of sample geometries are used by TMA, using unique probes for compression, extension, and flexure or bending. Upon heating a sample to its melting point, the force applied by the probe tends to deform the sample. Subsequently, the changes occurring in height of the sample are detected and recorded by TMA as a function of temperature. This method also forms the basis for specific melting point tests. Thanks to its fast test time and simplicity, TMA is currently used to perform most of these melting point tests.

TMA 4000 Analyzer

The TMA 4000 instrument can be used for conducting fast and consistent melt tests. A cross-sectional diagram of the TMA 4000 is shown in Figure 1.

Figure 1. Cross-sectional diagram of the TMA 4000

The TMA 4000 provides the following features:

• Cooled by a heat exchanger surface, the cold sink makes it possible to bolt a mechanical refrigerator or a water circulator with a single bolt. This provides users the flexibility to select low-cost coolers depending on their lowest temperature analysis requirements. Users can easily upgrade to a better cooler for lower temperature at a later time.
• The furnace heating the sample has a height of 40mm, which provides a wide uniform temperature zone. This allows for more precise temperature control, and provides more temperature information. The highly reliable furnace can go higher in temperature when compared to its use in the TMA 4000 analyzer.
• When compared to other materials, fused quartz is more resistant and exhibits the lowest thermal expansion. Fused quartz furnace tube, probes and sample platform are more rugged and heavier gauge in comparison to competitive TMAs.
• A linear variable differential transformer (LVDT) or position sensor has a wide 12mm linear detection zone and is highly sensitive. This provides sensitivity to small changes AND the ability to track large dimensional changes should they occur. The LVDT is temperature thermostatted to make its output independent of furnace temperature and laboratory temperature.
• The weight of the core rod and sample probe is supported by a fully submerged Archimedean float. This makes it possible to damp environmental vibration and helps protect the quartzware from freefall during the event of power outage or a loading process. This unique feature is not provided by other manufacturers as they would rather offer quasi-DMA functionality to their TMA.
• Wide-range and linear, the force transducer needs only to provide the user-selected up or down force as it does not have to support the weight of the probe and core rod.

Softening Under Compression

The simplest form of softening point is to mount the TMA with a sample, use an applied force to lower a flat tip probe on the sample, and then heat up the sample (Figure 2). Two different flat tip probes are available to choose the optimal applied stress.

Figure 2. Expansion probe on can liner sample

The identification of a liner’s softening point on an aluminum beverage can is shown in Figure 3.

Figure 3. Detection of Tg of a food/beverage can lining.

In order to prevent reactions between the aluminum and beverage, the resin lining needs to be used. Using the can fabricator, the softening point i.e., the glass transition can be adjusted to be lower or higher in temperature by utilizing a heat cycle to promote the crosslinking reaction.

This adjustment of Tg helps in optimizing throughput, while trying to meet processing, flexibility, and barrier maintenance needs. In spite of limited sample availability, TMA provides a means for making this measurement. Current research in this area is being applied to find replacements for the BPA-epoxy primers currently in use.

Figure 4. Softening point of a ceramic glaze—sample in a STA pan

Figure 4 shows the softening point for a ceramic glaze sample that had been run by simultaneous thermal analysis (STA). In this case the STA trace had shown evidence for two weak glass transitions, and running the sample in the TMA allowed confirmation of these events. This test can be carried out within a STA or DSC pan with the aid of a loose-fitting lid to reduce the cleanup process. The first Tg in the STA correlated in the TMA with an increase in the coefficient of expansion, the second is the melt.

In the series of tests discussed above, the onset melting temperature depends somewhat on the sample geometry and the amount of selected force. Using the probe and sample dimensions, it is possible to make this test less technique-dependent so that the stress being applied to the sample and the ensuing strain related to the probe displacement can be measured.

The Vicat softening temperature test like ASTM E2347 and the Heat Deflection Under Load test like ASTM E2092and ASTM D648 are two examples of such a test. The latter test is used for acquiring the heat deflection temperature (HDT) , also called the deflection temperature under load (DTUL) .

Heat Deflection Temperature

The flexure probe can be used when there is a need to detect the exact temperature where a sample material crosses a specific modulus threshold without viscous flow. Under such conditions, a dimension-defined sample is placed over two parallel knife edges, as shown in Figure 5. The probe is then lowered onto the sample, which is centered between the pair of knife edges, and this is followed by applying a specified force.

Figure 5. Diagram showing the geometry for the flexural analysis, or three point bending geometry used in the heat deflection test.

Both the knife spacing and sample geometry help in measuring the applied stress and the resultant strain. ASTM E2092 and ASTM D 648 are standard test methods that use this approach and replicate industry-specific tests for the HDT.

Figure 6. HDT/DTUL flexure test heat deflection at 66psi (0.455N) of polystyrene

An example of this test on a polystyrene sample is shown in Figure 6. For this type of analysis, the force applied to the sample is measured based on the dimensions of a sample for a stress of 0.445N or 66psi. The output data is the temperature at which the strain is 0.20%. The displacement corresponding to this standard strain is based on the sample geometry and knife edge spacing. This test can identify where a material may have become sufficiently flexible that a mechanical piece will bend under load.

Vicat Softening Temperature

The Vicat test is mainly used for detecting a temperature, where a highly localized stress leads to penetration into a component body. During the TMA simulation of this analysis, the force is focused on the sample’s small surface area using the penetration probe (Figure 7).

Figure 7. TMA geometry for simulated Vicat softening test

In the Vicat test, it is important to identify the exact temperature at which the material softens to a particular value of Young's Modulus. Using the highest applied force of 200g and the probe and sample geometry leads to a measured penetration of 0.32mm to acquire the Vicat A modulus. TMA-simulated Vicat softening temperature is the temperature relative to this penetration. TMA analysis of a porous and highly filled PVC material is shown in Figure 8.

Figure 8. Softening point by a Vicat-type technique

Conclusion

This article has shown how the simple and rugged TMA 4000 analyzer was evaluated over a wide range of application such as establishing the softening and melting points of ceramics, plastics, metals, and organics. It is sensitive enough to identify melting or weak transitions of thin layers, and has the required dynamic range to track the melting phenomena via a complete dimensional change. The TMA 4000 analyzer can be used in educational environments, thanks to its robust design. It can be used for standard testing processes employing industry-specific tests to determine melting properties without the use of rheological techniques.

This information has been sourced, reviewed and adapted from materials provided by PerkinElmer.

For more information on this source, please visit PerkinElmer.