Conductive Polymers

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.

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

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 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.

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.

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.

Abstract was taken from original journal pre-proof: Thermoforming process of thermoplastic-based continuous CFRP’s offer a major advantage in reducing cycle times for large-scale productions, but it can also have a significant impact on the structural performance of the parts by inducing undesirable effects. This necessitates the development of an optimal manufacturing process that minimizes the introduction of undesirable factors in the structure and thereby achieves the targeted mechanical performance. This can be done by first establishing a relationship between the manufacturing process and mechanical performance and successively optimizing it to achieve the desired targets. The current study focuses on the former part, where a manufacturing-to-response (MTR) pathway is established for a continuous fiber-reinforced thermoplastic composite hat structure. The MTR pathway incorporates the thermoforming process-induced effects while determining the mechanical performance and principally comprises of material characterization, finite element simulations, and experimental validation. The composite material system selected for this study is AS4/Nylon-6 (PA6) with a woven layup. At first, the thermoforming simulations are performed above the melt temperature of PA6 using an anisotropic hyperelastic material model, and the process-induced effects such as thickness variation, fiber orientations, and residual stresses are captured from the analysis. Residual stresses developed in the formed structure during quench cooling from the elevated temperature are predicted by the implementation of classical laminate theory (CLT). These results are then mapped onto a duplicate part meshed suitably for mechanical performance analysis. A quasi-static 3-point bend test and a dynamic impact test are carried out and the results are compared with experimental tests. Experimental results from thermoforming, bending and dynamic impact trials show good agreement with the simulation results for the hat structure under consideration. Further, the static and dynamic performance is evaluated for the thermoformed structure and the effects of the thermoforming process are compared numerically, for the cases with and without the inclusion of process effects. [1]

The thermal conductivity of composite samples is measured as per ASTM D 7984 using a C- Therm TCi TH91-13-00703 instrument. [1]