Measuring thermal expansion by thermomechanical analysis (TMA)
When a material is heated or cooled, its size changes proportional to the original size and the change in temperature. This thermal expansion (or shrinkage) of materials needs to be considered in numerous applications.
We highlight this importance using two common examples, shown in Figure 1, and briefly discuss how we can measure as well as influence this property
Figure 1. Examples of applications where control over thermal expansion is critical. Left: Antenna tower; right: reflector of a car headlight assembly, visible behind the transparent cover.
The first example comprises 5G antennae, which are increasingly important in our daily lives as they transmit data from our phones, cars, and many other devices. In any antenna, data transfer is most efficient when transmitter and receiver wavelengths are matched. Since 5G operates at high frequency (up to 54 GHz), these antennae can be quite small, as the length of an antenna is inversely proportional to the frequency. Such antenna modules are often densely constructed, in which heat from the integrated circuits can build up. Moreover, many of these antennae are exposed to the everyday weather and fluctuating temperatures. As such, the antenna can undergo thermal expansion, which can lead to reduced efficiency or even damage to the antenna due to material warpage. Therefore, the thermal expansion coefficient is a crucial parameter in material selection for this application.
The other example is a car headlight assembly, particularly focusing on the reflector part (behind the transparent cover), which has a main function to direct the light towards the road. These reflectors are molded plastic parts, coated with an aluminum reflective layer. In such an assembly the temperature can vary greatly, not just from ambient conditions but also from heat generated by the lightbulb itself (although with modern LEDs this is less of an issue). However, the plastic, often polycarbonate (PC), expands much more under increasing temperature than the metal coating. It is not hard to imagine how this can result in delamination of the coating from the plastic, causing the reflector to malfunction. This example also demonstrates the importance of considering the thermal expansion behavior of materials, especially when combining them for any application.
A material’s expansion behavior is captured in the coefficient of thermal expansion (CTE), which can be measured using thermomechanical analysis (TMA). TMA is a technique that accurately measures dimensional changes in a sample, as a function of temperature (or time). Expansion can be measured using quartz compression or tensile probes, but other quartz probes (3-point bending, penetration) can also be used to measure heat deflection temperatures or softening points. Our TMA sample holder and quartz compression probe can be seen in Figure 2 below.
Figure 2. Left: Aluminum sample in the TMA holder. The quartz compression probe accurately measures any length changes. The thermocouple on the right side monitors the temperature. Right: TMA measurement data, with CTE values for each material.
To demonstrate differences in thermal expansion for the materials used in a car headlight assembly, we have measured several materials using our PerkinElmer Diamond TMA: aluminum, PC, and glass. The measurement data is also shown in Figure 2.
We can calculate the CTE of the materials from the slope of the curves in Figure 2, as shown, which are in close agreement with values from literature. Additionally, TMA data can also be used to measure the glass transition temperature (Tg) of a polymer sample; for this PC sample Tg was found to be 148 °C.
We clearly see a large difference in CTE for PC and aluminum, demonstrating the issue for the reflector part of our car headlight assembly, and allowing us to think of a solution. A common method to decrease the CTE of polymers is to add fillers with a low CTE, like glass. Therefore, one possible solution would be to make a PC composite with glass fibers, which is cheap and additionally strengthens the polymer. For example, the CTE of a 30 % glass-filled PC was determined as 22 · 10-6 K-1, which is a close match to the CTE of aluminum and would therefore be a suitable composite material for the headlight reflector application.
However, when compounding such composite materials more aspects need to be considered, such as fiber dispersion during compounding or fiber orientation during processing of the parts. This can result in anisotropy in the material properties, which can be expressed as greatly differing CTEs in the machine or transverse direction. As such, careful analysis of CTE is crucial in many aspects of material and product development.
We are happy to support you with TMA measurements for any of your thermal expansion challenges, so please feel free to contact us for further information.
Improving polymer properties
PTG/e’s unique multipurpose drawing line is capable of stretching tapes and fibers from polyolefins to high-temperature polymers such as polyamides or ketone polymers. By drawing these materials in one direction, improved mechanical properties (stiffness and strength) can be obtained.
As an example, the higher stiffness of artificial grass is the reason why it stands upright. And higher tensile strength is the reason debris/construction bags and woven shoppers are so strong. In addition to these everyday applications, the composite industry is very interested in the combination of low-density (light-weight) materials and their improved mechanical properties upon drawing. You can find composites based on drawn thermoplastic materials in all kinds of extreme applications, such as aerospace engineering, ballistic protection, oil & gas transport and sports.
The stiffness (tensile modulus) of thermoplastic polymers, like polyolefins, polyamides or ketone polymers, can be controlled by orientating polymer chains. This orientation can be achieved during stretching of the material between the glass transition and the melting temperature of the polymer, which is also called solid-state drawing. See the image below.
How does a drawing line works?
The drawing line consists of a draw unit with 5 godet rolls, a draw unit with 3 godet rolls separated by 2 heating zones in an oven, another draw unit with 3 godet rolls, and finally a winder. The drawing speed can be adjusted per unit, or even per roll on the oven unit. The temperature of each induction-heated roll and oven plate can be controlled separately, up to 280 °C. Afterwards, the materials are collected on a metal or cardboard tube for further analysis either at PTG/e or at the customer.
Advantages PTG/e drawing line
Since we are able to draw small lengths of tapes and fibers (starting from 20 meters) on our drawing line, less material is needed for your tests. Also, your own production line can keep running, since you are performing your tests in the PTG/e laboratory.
If you like to learn more about our capabilities, please contact us.
Especially for polymers size matters
Important properties of polymeric materials like tensile strength and viscosity critically depend on the size, or rather, the chain length of the macromolecules that they consist of. In other words, these properties are defined by the molecular weight distribution and the average molecular weight of the polymer.
Measuring the molecular weight of a polymer therefore provides crucial information for understanding many aspects related to the behaviour of polymeric materials.
Thus, chemically identical polymers can show different tensile properties as a result of differing molecular weight. Such differences frequently result from polymer degradation and are especially relevant in the context of recycling. Also in polymer production, the molecular weight is a key parameter in quality control. While often only the melt flow index (MFI) is measured, knowledge of the actual molecular weight (distribution) provides a more detailed picture. Finally, in the development of new polymer materials, assessment of the molecular weight is a key factor for optimizing synthesis conditions.
The molecular weight of many conventional polymers can conveniently be assessed by a technique called Size Exclusion Chromatography (SEC). This technique separates dissolved polymer molecules according to their size by passing them through a column packed with porous particles. While the larger molecules cannot enter the pores and therefore elute from the column relatively rapidly, the smaller ones do enter the pores of the column material and thus experience a net retardation. The final result is a chromatogram, showing the amount of material eluting from the column versus the elution time, with the elution time being inversely related to the molecular weight.
One key issue with SEC is the fact that polymeric materials need to be dissolved in a suitable solvent. However, not all polymers are the same and their solubility heavily depends on their chemical nature and molecular weight. While many common polymers (e.g. perspex or polystyrene) are easily dissolved in tetrahydrofuran (THF), more polar materials like polyamides or some polyesters, require hexafluoroisopropanol (HFIP) for complete dissolution. Even water (H2O) may be the only suitable solvent for certain polymers. On the other hand, the industrially important class of polyolefins (e.g. polyethylene, polypropylene) can only be dissolved in a chlorinated solvent and the complete SEC analysis is performed at 160 °C!
It is clear that every type of material needs specific measurement conditions to assess its molecular weight. At PTG/e, we have many years of experience with polymers of widely varying nature. Our state-of-the-art SEC equipment, running on different solvents, enables us to cover molecular weight determinations of almost any polymeric material, including those that are notoriously ‘difficult’ to dissolve.
Please contact us if you would like to find out whether molecular weight determination by SEC can provide a breakthrough insight into your material of interest!
Surface structure analysis by profilometry
The analysis of surface structures is of great importance in many industries, such as chip/sensor manufacturing, inkjet printing or membrane production. In these industries surface analyses are used for instance as quality control, checking surface roughness or finding the root cause of defects.
As an example, one of our customers had approached us to help solve an issue with a curable resin product. The application of this product requires a very flat surface. At first, the resin was cast on a Teflon film, in order to ‘copy’ the flat surface of these films onto the resin product. By the naked eye, such a film indeed appears very flat, but surface profiling revealed that the film has depth differences of 1-1,5 micrometers (see Figure 1). A silicon wafer, a known flat substrate material, shows depth differences of just 30 nanometers (see Figure 2). Using this silicon wafer as substrate for the curable resin did result in the desired smoothness of the final product. Therefore, surface profiling enabled our customer to choose the right substrate for their product.
Figure 1: Surface profile of Teflon foil.
Figure 2: Surface profile of the silicon (Si) wafer with a line profile analysis, indicated by the pink raster. The line profile is represented in the graph.
Another example below shows a microchip, which can be found in everyday devices like laptops or smartphones. A detailed image of its complex surface profile can be used to inspect the chip for any damages or incorrect assembly. The surface images in Figure 3 were obtained by an optical surface profiling technique.
Figure 3: Surface profile in 2D and 3D of a microchip in common electronics.
Using this technique, surfaces can be analysed quickly and accurately. Surface profiling can be done optically (optical profilometry), in which case light is used to illuminate a surface. The reflected light is detected and translated into a 2D/3D profile image. However, profiling can also be performed physically (stylus profilometry), where a stylus is used to probe a surface. Both techniques are extremely sensitive, capable of measuring depth differences of less than 1 nanometer. The choice of which technique is preferred mostly depends on the sample surface. For a very soft surface, you want to choose optical profilometry, so the surface is not changed as a result of the measurement. If a surface is absorbing (almost) all light, stylus profilometry is preferred.
At PTG/e, we offer both optical and stylus profilometry, as each technique has its pros and cons (which can often be compensated by the other technique). As such, we will always decide together with our customers which technique is best suited for their samples.
Interested in optical and physical surface profiling? Please contact us, it’s our pleasure to discuss the possibilities.
TGA-IR-GC-MS Hyphenation setup
By combining the analysis techniques TGA, FT-IR and GC-MS via coupling of the devices one obtains a powerful analysis technique. With this TGA-IR-GC-MS ‘Hyphenation setup’ it is possible to identify complex and unknown materials.
PTG/e makes use of a TGA-IR-GC-MS ‘Hyphenation setup’ from PerkinElmer.
Some applications for this technique are:
- The identification of additives, like plasticizers, in plastics
- Determination of the primary components of a material
- Analysis of unknown contaminations in a material, like fragrances or solvents
To characterize a material, it is first placed in the oven of the TGA, where it is heated with a programmed heating rate. During heating, weight loss can occur due to thermal decomposition or solvent evaporation. The evolved gases from these events are transferred via a transfer line to a heated chamber of the FT-IR. There, the gas is exposed to an infrared beam. The functional groups in the measured gas each respond differently to infrared radiation, which helps to identify the molecules.
Following the FT-IR, the gas is transferred via another transfer line to be injected onto the column of the GC. Depending of the affinity of the injected gas with the column, the temperature of the column and the speed of the carrier gas, the injected gas has a certain retention time. Different components have a different retention time, which enables the GC to separate the individual components of the injected gas.
An mass spectrometer (MS) is located at the end of the column. In the MS, the separated components are ionized and brought into an electric field. This field will accelerate the ionized components to a certain speed, depending on their mass and charge. The MS uses this principle to create a spectrum, which, together with the retention time of the GC, is unique for each component. Combining the data from FT-IR and GC-MS, the individual components of the evolved gases from TGA can be characterized.
Aside from the fully hyphenated setup, it is also possible to use the individual analysis techniques, or a partially hyphenated setup, for example TGA – IR or TGA – GC-MS. The specific required setup will depend on what needs to be analyzed exactly.
Clarus 690 GC & SQ8T MS
Moisture content determination of PET
Determining moisture content in PET granulate. An explanation of which analysis technique is most suitable, but also why.
Polyethylene terephthalate (PET) is widely known from the PET bottle. It is a thermoplastic material with many other applications besides the PET bottle. Although it may sound contradictory, PET is sensitive to water, especially during processing at high temperatures and pressures. When the material contains water (> 0.02 %) and is subsequently heated, the water can cause the molecular chains to break down via hydrolysis. This can have a drastic effect on the quality of the material.
For many companies that process PET, it is therefore extremely important to dry PET as much as possible before processing. But how can one check whether this drying process has taken place correctly?
This can be done in two ways, following the weight loss of the material during drying or via Karl Fischer titration.
Monitoring the weight loss of the material during drying seems to be the most obvious method. However, in order to be able to indicate small amounts of water in ppm range (parts per million), one must have a very accurate balance, such as a thermogravimetric analysis (TGA). With this analysis technique, it must be taken into account that not only the decrease in weight of water is monitored, but the decrease of all volatile components (such as used solvents or gaseous degradation products). This can lead to incorrect assumptions about the volatile component present when a small weight decrease is registered.
The most suitable technique for determining the amount of water is by Karl Fischer titration. The PET granulate is heated in a glass vial, so that water is released and blown into a titration liquid by means of a dry nitrogen flow. The liquid only reacts with water and the amount of water can be accurately determined on a ppm scale. An additional advantage is that the granulate can be sampled at various locations or after various time intervals in hermetically sealed bottles in order to determine the course of the drying process.
PTG /e has a wide range of analysis techniques and can therefore choose the most suitable technique or choose to combine techniques.
Interested in the various techniques and what they can be used for?
Download our overview of analysis equipment and techniques.