Measuring the Thermal Conductivity of Insulation Materials
Thermal insulation materials are designed to reduce or prevent the transmission of heat in the regions they are applied. Such materials are widely seen and used in applications such as packaging, building, automotive, spacecraft, clothing and many others.
Inherently, a critical performance attribute of an insulation material is its thermal conductivity. Materials possessing a low thermal conductivity have a high thermal resistance.
Heat flow meters are traditionally employed to measure the thermal resistance of insulation. They provide a reliable means of characterizing such performance based on international standards including ASTM C518 and ISO 8301. Thermal resistance is the reciprocal of thermal conductivity. For a detailed outline of how to easily convert thermal conductivity and thermal resistance, see here.
For accelerated characterization, C-Therm’s TCi Thermal Conductivity Analyzer is offered with a range of transient methods, including the Modified Transient Plane Source (MTPS) technique. The MTPS method provides a fast, easy, and accurate way to measure the thermal conductivity of insulation samples with no sample preparation or contact agents required. It offers a complimentary means of testing insulation performance in a fraction of the time, making it suitable for rapid R&D testing and QC inspection. Additionally, due to the one-sided test method of the patented sensor, the technique empowers testing of samples under a wider range of conditions, e.g. temperature, humidity, pressure, glove box etc.
Case Highlight #1:
Thermal Conductivity Measurement of Expanded Polystyrene Foam Samples: Comparison with traditional Guarded Hot Plate Technique and Quality Control Application
Expanded Polystyrene Board (EPS Foam) is a widely used plastic for applications in packaging and building insulation. It has a very low thermal conductivity property, and therefore is an ideal material for providing thermal insulation.
The Chart I below shows the TCi’s results on a certified reference sample material of EPS provided by the National Institute of Standards and Technology (NIST) measured via the guarded hot plate (GHP) technique. GHP is a highly accurate and reliable method for measuring thermal conductivity – but it takes hours to run a sample and the wait-time for results - coupled with the onerous sample size requirements – can add up to a significant burden for some users.
As graphically presented in Chart I, the average test results generated with the C-Therm TCi Thermal Conductivity Analyzer on the NIST reference sample is 0.0329 W/mK with an RSD of 0.19%. This represents a difference of 2.13% from the stated NIST value measured with GHP of 0.0337 W/mK. All testing was done at approximately 24°C.
In testing within 2.13% of the stated reference value, the test results fall within the 2.4% uncertainty noted on the NIST certificate for the reference standard material. These test results exemplify the high accuracy clients typically achieve in characterizing a wide range of sample materials with the C-Therm TCi Thermal Conductivity Analyzer.
The following testing results highlight application of the TCi in quality control testing for insulation materials. A sample of EPS was sourced from the local Home Depot store in Fredericton, New Brunswick. The TrueFoam™ sample was tested in 10 different locations in assessing both the overall quality of the insulation material, and also the sample homogeneity. In Chart II these results are plotted – note that this time the X-axis plots different locations of measurement in contrast to Chart I above that plots multiple measurements on the same location. Test results demonstrate the product provides excellent insulation quality with an average thermal conductivity of 0.033 W/mK and is highly homogeneous in its performance across multiple locations with a relative standard deviation 0f 0.6%. As the product has a specification of better than 0.0363 W/mK, C-Therm found the insulation to surpass the stated performance specification for the product. All testing was completed within 10 minutes (each measurement took less than 3 seconds with a 60-second interval period between measurements).
In manufacturing, the C-Therm TCi Thermal Conductivity Analyzer offers added insight in being able to accurately and quickly measure the thermal conductivity of products so production can understand if they are meeting specifications consistently. In this example, the test results tested better than the stated thermal conductivity and the sample material is of excellent consistency.
Case Highlight #2:
Thermal Conductivity of High Temperature Insulation Material
High temperature thermal conductivity measurements are important for exploration and performance evaluation of materials existing in high temperature environments. Insulating materials, such as furnace insulation and high fluid transport plumbing are designed and used to help heat from exiting a system into its surroundings in such environment.
Accurately measuring thermal conductivity at high temperatures using transient methods has traditionally been very hard to achieve primarily due to limitations in the sensor materials. Traditional sensors use glass or plastic dielectric coatings and silicone-based sealants to protect the sensor chip, but can become soft at high temperature, damaging the sensor and/or inaccurately measuring the materials thermal conductivity. Some traditional transient methods use a mica-based insulation material, which is highly fragile and often limits the sensor to a single use. C-Therm’s TCi Thermal Conductivity Analyzer‘ shigh temperature module for insulation materials is the leading approach to handling thermal conductivity measurements up to 500°C, as it utilizes a special sensor chip with an alumina dielectric and ceramic sealant to ensure no softening and proper operation at elevated temperatures. Moreover – the unique, robust single-sided design and alumina sensor chip protect the sensor against mechanical damage in standard use and are not vulnerable to delamination or breakage in routine handling, resulting in a sensor that can be reused indefinitely.
Figure 1: High-density ceramic insulation board used in this study
The TCi’s high-temperature sensor capabilities were demonstrated by measuring the thermal conductivity of a high-density ceramic board (Figure 1) between 300 and 500°C. The ceramic board had previously been characterized by ASTM Standard C201, a steady-state method designed for the characterization of thermal refractories, which is similar in principle to a guarded heat-flow meter apparatus. In ASTM Standard C201, where a sample is placed within a heater chamber, and a copper calorimeter is used to measure the heat-flow while a set temperature difference across the hot and cold sides of the sample is maintained. As a steady-state method, the collection of data requires the use of a large sample machined to precise specifications, and collection of data for analysis can take hours or days. The TCi Thermal Conductivity Analyzer has several key advantages compared with steady-state techniques: The TCi performs a measurement in one to three seconds, as opposed to thirty minutes or more for steady-state techniques, allowing collection of more data in the same amount of time. Using smaller samples with greater flexibility than typical steady-state techniques, the TCi also takes less time to come to temperature than a typical steady-state technique and does not require precision machining, allowing faster, easier collection of data for high-temperature insulation applications.
Figure 2: Thermal conductivity of a high-density ceramic insulation board between 300 and 500°C with 3% error lines from the expected values
As shown in Figure 2, C-Therm’s TCi Thermal Conductivity Analyzer can measure accurate thermal conductivity of materials at elevated temperatures. The measurements were in great agreement with the expected value of the ceramic insulation board between 300 to 500°C (better than 3% accuracy). This sort of fast, accurate high-temperature thermal conductivity measurement is crucial in material selection and research for high temperature applications.
Case Highlight #3:
Rapid Assessment of Thermal Conductivity Performance of Aerogels
Aerogels are a relatively new class of ultralight, porous materials, typically derived from a gel. In an aerogel, the liquid component of the gel has been replaced by a gas (typically air). Owing to their very light nature, most aerogel samples have a translucent, blueish appearance. Porosity of aerogels is generally in excess of 98% (meaning that, per unit volume, >98% of an aerogel’s volume is pore volume). Aerogels can be made of a variety of chemical compounds.
Image Source: NASA/JPL-Caltech
Aerogels are known for their extremely low thermal conductivity, which is often lower than that of air. In this respect, the thermal conductivity of an aerogel material is typically identified as a critical performance specification. This low thermal conductivity makes aerogel materials exciting in the field of insulation research, where engineers are continually looking to improve energy efficiency without adding excessive weight.
Three commercially-available aerogel samples were recently provided by a client seeking performance data on them in comparison to the specification sheet. The samples were analyzed with the C-Therm TCi Thermal Conductivity Analyzer using the Modified Transient Plane Source (MTPS) technique. The results are displayed below:
It can be seen that the thermal conductivity performance measured is in good agreement with the specified thermal conductivity of these commercially-available aerogel samples. Agreement with the specified value in all three cases was better than 4%.
Case Highlight #4:
Thermal Conductivity Characterization of Candidate Materials for Bio-Based Insulation Application
Building insulation materials form the thermal envelope of a building and reduce heat transfer. They are part of the complex structural elements of a wall or a roof. Consequently, insulation materials are indispensable parts in the design and construction of buildings.
Bamboo is a sustainable material and its products are considered as building materials for the same uses as timber: floors, ceiling, wall, and building envelopes in both exterior and interior design elements. Moreover, bamboo materials have great advantages including low cost and attractive aesthetic appearance; they are therefore an ideal alternative to traditional materials for sustainable building.
Researchers at Université Savoie Mont Blanc, Vietnam National University and Ton Duc Thang University manufactured novel, environment friendly insulation fiberboards from bamboo fibers and protein-based bone glue using thermo-pressing on a heated hydraulic press. They investigated the relationship between thermal conductivity and density, the amount of proteins, humidity levels, and moisture content variation in their study.
The thermal conductivity of the specimens was determined with a TCi Thermal Conductivity Analyzer using method of Modified Transient Plane Source (MTPS) at 25 °C, and it was installed inside the RH-Box (Fig. 1).
Fig. 1. The photographs of specimens and thermal conductivity measurement inside RH-Box at 25 °C.
The specimens tested (50 ± 1 mm) were pre-conditioned at 57% RH and 25 °C until reaching a constant mass. First, the thermal conductivity of these specimens was assessed at 57%RH, followed by 75%RH; the thermal conductivity was regularly measured until reaching a constant mass. After reaching a constant mass at 75%RH, these specimens were evaluated at 33%RH; the thermal conductivity was also regularly measured until reaching a constant mass. This measurement was repeated in three cycles for each relative humidity level (33% and 75%). The thermal conductivity variation could be measured according to the density, relative humidity and moisture content variation of the materials.
Fig. 2. Thermal conductivity functions with amount of glue (a) and density (b) of bamboo fiberboards at 57% RH and 25 °C
Fig. 3. Evolution of thermal conductivity during three cycles between 75% and 33% RH at 25 °C inside RH-Box.
Fig. 4. Evolution of thermal conductivity with moisture content variations for the fiberboards at 25 °C (RH: 57%→ 75%).
In conclusion, the thermal conductivity of bamboo fiberboards at different glue ratios is fairly low, varying between 0.0582 and 0.0812 (W.m−1K−1) at 25 °C and 57%RH. Thermal conductivity is a function of relative humidity and moisture content.