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Conductive Polymers Conductive Polymers

Measuring the Thermal Conductivity of Polymers

Conductive Polymers

Measuring the thermal conductivity of polymers is necessary in understanding the material’s performance for a wide range of applications where heat dissipation is considered a critical quality attribute.  In electronics, the thermal conductivity directly supports the thermal management within the package and is critical to the performance, lifetime, and reliability of the electronic device.  Thermally conductive polymer compounds can replace metal, ceramics and conventional plastics for such heat-sensitive applications. The key is to have an accurate understanding of the effective thermal conductivity of the polymer composite – this requires testing as part of the candidate material’s characterization. 

Polymers are more and more used in various heat-sink applications such as electronics, biomedical devices, battery housings and automotive parts. The usage of thermally conductive plastics which are injection moldable leads to maximum design freedom for constructing parts. Furthermore, thermally conductive compounds show a lower density and can be much lighter than equivalent components.  Incorporation of thermal conductive fillers such as boron nitride (BN), carbon nanotubes (CNTs), aluminum nitride (AlN), copper (Cu), silver (Ag) and graphene nanoplatelets (GNPs) is the most common approach to improve thermal conductivity.

However, in formulating and compounding the polymer composites there are often challenges with the distribution of the filler additives and other factors that impact the effective thermal conductivity.  At the molecular level the thermal transport mechanisms in polymers in terms of polymer morphology, chain structure and inter-chain coupling impact thermal conductivity. 

Ultimately it is necessary to test and quantify the thermal conductivity performance. 

What is the best test method for testing the thermal conductivity of polymers?  The optimal method depends on the sample and conditions under which the material is desired to be tested.  C-Therm stresses the importance of testing samples under representative test conditions for the application.  Pressure, temperature and other environmental conditions (e.g. humidity) need to be representative of the intended use as they can significantly impact the effective thermal conductivity. 

With its modular offering, C-Therm’s Trident Thermal Conductivity instrument provides users with the necessary options to test polymers that range in sample formats.  As a complete solution – it is the only commercial test equipment that offers the versatility within the platform to test bulk isotropic, oriented/anisotropic, thin films, and polymer melt samples. 

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The modified transient plane source (MTPS) method available on Trident is recommended for bulk samples and offers a significant advantage in operating as a single-sided test which enables first and foremost the easiest way to measure thermal conductivity with excellent reproducibility between labs supported by the heavily automated test procedure which negates the opportunity for user bias.  The simplicity of the method greatly reduces training requirements such that anyone can measure thermal conductivity.  Furthermore, MTPS also enables the ability to thermally map the effective thermal conductivity of composite materials in investigating settling effects or distribution issues with the additives.  MTPS operates in compliance with ASTM D7984.

The FLEX transient plane source (TPS) hot disc method available on Trident is recommended for more advanced users looking to take advantage of the specialized utilities for testing polymers.  The Anisotropy utility provides the ability to profile the effective thermal conductivity in both the through-plane and in-plane orientation of the polymer composites.  The Thin Films utility provides a solution for testing the thermal conductivity of polymer thin films. C-Therm FLEX TPS method operates in accordance with ISO 22007-2.

 

Lastly, C-Therm’s transient line source (Needle) method offers an optimal solution for the sticky situations involved in testing polymer melts where high temperature and high pressure are often needed to represent processes in plastic injection moulding.  C-Therm TLS Needle operates in compliance with ASTM 5334 and D5930.

  • Measuring the thermal conductivity of a polymer composite with FLEX TPS sensor

    Measuring the thermal conductivity of a polymer composite with FLEX TPS sensor

  • Polymer Discs being measured with Flex TPS ISO 22007-2

    Polymer Discs being measured with Flex TPS ISO 22007-2

  • Thermally Conductive Thin Film

    Thermally Conductive Thin Film

  • Loading Sample on the MTPS Sensor

    Loading Sample on the MTPS Sensor

  • Thermally Conductive Potting Compound

    Thermally Conductive Potting Compound

  • Masterbatch polymers

    Masterbatch polymers

  • Covestro

    The C-Therm TCi Thermal Conductivity Analyzer has provided our group a fast, accurate capability to test the thermal conductivity of our polymers with C-Therm’spatented high-precision MTPS sensor. The instrument has become very popular within our group for its quick easy reliable measurement and the support from C-Therm has exceeded our expectations. We recently upgraded the unit with the new robust TLS module for work on polymer melts.”

    Jose Fonseca,
    Expert Thermodynamics, Covestro (Sector: Polymers)

    Covestro More Testimonials
  • Haydale Composites Solutions Ltd.

    The C-Therm TCi has been a key piece of testing equipment at Haydale, providing fast and accurate thermal conductivity measurements for our product development of nanocomposites. Having this capability has allowed a better understanding of the dispersion of nanomaterials in polymer matrices through thermal mapping sample surfaces. The support and customer service from C-Therm has been excellent over the years, we look forward to dealing with them again in the near future.”

    Stuart Sykes

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Case Highlights

Measuring the Thermal Conductivity of Anisotropic or Oriented Samples

Anisotropic materials are those that have varying thermal properties in different directions or orientations (i.e. in- vs through-plane). This can stem from the use of directional additives or simply be an inherent material property.  While the degree of anisotropy can vary, any material that is not isotropic in nature (i.e. same in- vs through-plane) may require specialized testing considerations to obtain valid results. For thermal conductivity measurements of anisotropic materials, C-Therm recommends the use of either the Modified Transient Plane Source (MTPS) or FLEX Transient Plane Source (TPS) methods.

The MTPS offers the simplest and fastest option for testing anisotropic materials. Thanks to the method’s Guard Ring Technology™. MTPS measurements are always directionally dependent and measure in the direction normal to the sensor surface. As such, the direction of measurement is dependent on the orientation of the sample on the sensor surface.

Figure 1. Example of material directionality in the x,y and z-direction.

Based on the above, assuming the xy surface is in contact with the sensor than the direction of the measurement would be along the z-direction. Assuming sufficient size, the sample can be rotated, and measurements can be performed along with the different directions completely independently of one another. In scenarios where the sample size is limited multiple samples can be clamped together to accommodate the size requirements (see below).

Figure 2. Bars of polymer resin with a carbon-based additive demonstrating the “clamp/stack” method for in- and through-plane measurements.

In this example the through-plane measurement was easily performed on a single sample. However, due to the limitation of sample thickness for the in-plane measurement multiple samples were stacked together. Result obtained from this measurement can be seen in the table below.

 

Through-plane

In-plane

Thermal Conductivity (W/mK)

1.32

2.59

Table 1. Measurement results using the MTPS method on an anisotropic sample.

C-Therm’s FLEX TPS (ISO 22007-2 Hot Disc) method also offers the ability to measure the thermal conductivity of anisotropic materials, however with some notable differences compared to the MTPS. The FLEX TPS method requires the manual adjustment of testing parameters. It provides the user greater control over the measurement – but requires more training accordingly and is a more complex operation.  It is highly customizable and provides users with the most flexibility in test setup and design. Because the FLEX TPS employs a double-sided thermal conductivity sensor with no controlled “directionality” (no guard ring) to the measurement there are some important requirements that need to be met.

Figure 3. Different sizes of Flex TPS sensor

 

First, this class of material will require the use of the specialized Anisotropic Utility, which operates differently from the traditional mode of operation with the FLEX TPS Bulk method.   While still conforming to ISO 22007-2, this special utility incorporates a user-inputted value for the material’s volumetric heat capacity. This inputted sample property is mandatory to extrapolate the thermal properties from the in- and through-plane direction.  This method is only recommended for materials that are orthotropic in nature (i.e. x = y ≠ z). If this condition does not exist, it is recommended to revert to the use of the MTPS method noted above. Lastly, the FLEX TPS thermal conductivity sensors come in various sizes ranging from 6 to 30 mm and the size of the sensor chosen dictates sample size requirements. As a general rule, the sample diameter must be at least 2.5x the sensor diameter and sample thickness at least equaling the diameter of the sensor. It is also important to ensure both samples used for the test are identical with similar surface and internal properties. Example data collected on anisotropic materials can be seen in the figure below.

 

Figure 4. Left) TPS sensor and sample setup and Right) Measurement results using the TPS Anisotropic utility.

The MTPS and FLEX TPS methods are powerful, complimentary tools for testing anisotropic materials. Being transient-based methods, they have a significant benefit over traditional steady-state options and are much more viable compared to other transient options such as laser flash which is generally not recommended for anisotropic samples (see ASTM E1461/E2585 for further details).

For more information, please contact us at info@ctherm.com or call +1 506 457 1515.

Measuring Thin Film Thermal Conductivity using Flex TPS Sensor

Thin Films are used for a variety of applications in protective optical and electric coatings, thin-film photovoltaic cells and thin film batteries (Figure 1). While thin film materials have existed for decades, thermal conductivity measurement methodologies have traditionally been focused on exploring bulk samples, and the capability to characterize these specialty materials has generally lagged. In recent years, the knowledge gap has shrunk, prompted by new and exciting markets in nano and microscale fabrications where thermal management is significantly important. A novel tool for such characterization is the transient plane source (TPS) adaptation for testing thin films as outlined in ISO 22007-2.

Thin Films have many applications ranging from flexible electronics, optics, and photovoltaics each with thermal management problems to solve. In this example photo, electronic components are embedded in a thermally insulative and electrically insulative material called Kapton. (Copyright: Wikipedia Commons, 2020).

Using a C-Therm Flex TPS thermal conductivity sensor and Trident’s Thin Films Utility, the measurement involves testing initially with just the selected backing material. The backing material is selected based on the type of sample being tested – according to the ISO guidance document it should be at least 10 times greater thermal conductivity than the test material.i This is followed by testing of 1 layer of the thin film and then 2 layers of the thin film. The successive addition of the thin film is used to generate a linear regression of temperature rise vs film thickness, which is then used to determine thermal conductivity (k). A general rule of thumb for the number of tested layers is to have a total layered thickness between 250 – 750 um for best results. The TPS method requires only a few layers of film to achieve a valid result, making it appealing for such characterization. The procedure does require an accurate measurement of the film thickness to achieve a high-quality result. 

TPS Thin Films Utility Basics The TPS method employs a double-sided sensor which is comprised of a spiral of electrically conductive nickel, sandwiched within an insulating polyimide material. By applying a voltage to the sensor, a temperature gradient is generated at the sample sensor interface and the temperature rise is measured and recorded by a Wheatstone bridge circuit with a data logging system.

C-Therm’s hot disc sensors for measuring thermal conductivity conform to ISO 22007-2.

C-Therm FLEX Transient Plane Source Thermal Conductivity Sensors

This is the general function of a bulk measurement for TPS. By measuring the sample’s temperature rise vs film layer from zero (just the backing material) to at least two total layers (Figure 3), thermal resistivity, thus thermal conductivity can be obtained. As noted above, for this method to work, the films must have a thermal conductivity much less than the backing material (more than one order of magnitude lower). The reason for this is because of the backing material and sample are too similar with respect to their thermal properties, the film itself will become too thermally transparent for the TPS sensor to measure. For example. If stainless steel was used as the backing material (typical k of 15W/mK), the sample being analyzed would have to be less than 1.5 W/mK. An example of a linear regression of temperature rise vs film thickness is shown in Figure 3. 

Figure 1 – Schematic for measuring thin film TPS measurements. A single thick film measurement is conducted, followed by a double thick film measurement and a triple thick film measurement. The resulting relationship between change in temperature and film thickness can give the thermal conductivity by relating thermal resistivity to thermal conductivity.

Download the complete case study here.

Investigation of Expandable Polymeric Microspheres for Packaging Applications


This case highlight investigates the feasibility of incorporating expandable polymeric microspheres into polyolefin films for food packaging application. There is also a focus on the ability of the microsphere-loaded film to reduce the weight of the packaging materials and to improve their thermal insulation, mechanical, and barrier properties.

The graph below features the thermal conductivity data acquired using the Trident. Both their thermal conductivity and thermal effusivity of the multilayer HDPE microsphere films decreased with increasing microsphere loading levels. The addition of 1% microsphere loading resulted in an 80% decrease in thermal conductivity. Overall, with the addition of up to 5% microsphere loading it was found that the polyolefin films would be lighter for ration packaging, would reduce cost through the use of less resin to produce the same thickness of film and could improve the thermal insulation for the pouches.

Measuring the Thermal Conductivity of a Polymer Melt by Transient Line Source (TLS) Technique

Thermal conductivity provides vital information for reliable process simulation of extrusion and injection molding processes.     

Injection molding is the most commonly used manufacturing process for the fabrication of plastic parts. The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part.

Figure 1- Plastic Injection Molding

The thermal conductivity of molten plastics is an important material property from the point of view of plastics processing since it affects temperature distribution and cooling behavior of the melt.  Accurate thermal conductivity characterization of the polymer feedstock supports increased productivity and better quality of finished product.  It is vital for reliable process simulation of extrusion and injection molding processes.

 
Figure 2 – C-Therm’s Trident Thermal Conductivity Analyzer in Transient Line Source (TLS) configuration. The TLS sensors provide a robust, efficient, and accurate capability to measure the thermal conductivity of polymer melts according to ASTM D5930.  

Historically, the setup parameters for such operations were discovered iteratively through trial-and-error and based on the experience of the operator’s “feel” for the equipment. In modern process development, it is expected to predict the polymer’s behavior during unit operations with the aid of rheological modelling. Polymer manufacturing processes can be optimized in a rational way using such a model – but a model is only as good as the data it’s built on. The thermal conductivity of the polymer feedstock from its initial state (often powdered or pelletized), through the melt transition, and then again as it cools to the melt, is a key thermophysical parameter for such processes, as it dictates important process parameters like heating rate and cooling time needed to avoid undesirable flaws such as blistering, burn marks, warping or sink marks.

Industry has standardized on measuring the thermal conductivity of thermoplastics via the Transient Line Source method C-Therm offers as part of its Trident Thermal Conductivity Analyzer modular instrument. C-Therm’s TLS sensor operates in accordance with industry standard ASTM D5930.  Using a TLS sensor and a sample vessel, as seen above, a powdered polymer may be melted in a bath or dry thermal chamber, then its thermal conductivity measured through the melt transition, and again as it re-solidifies.

Figure 3- Thermal Conductivity Test Results of Polyamide 12

A sample of powdered polyamide 12 (above), a thermoplastic material commonly used in injection molding, was measured for its thermal conductivity at 125 °C, 150 °C, and 200 °C using the C-Therm Trident Thermal Conductivity Analyzer with a Transient Line Source (TLS) sensor.

C-Therm’s TLS system provides researchers and manufacturing engineers in the polymer sector with a reliable, easy-to-use solution for measuring polymer melts.  The TLS option on the C-Therm Trident can also be bundled together with the broader capabilities of the MTPS sensor offering ever greater versatility in testing all types of materials including solids, liquids, powders and pastes with a thermal conductivity range of 0 to 500 W/mK. 

For more information on the C-Therm Trident Thermal Conductivity Analyzer, click here.

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