Comparative Analysis of Thermal Conductivity Techniques for Performance Textiles

Authors:

David Horinek – Atlas Industries

Trenton Rae - Low Impact, LLC

Dale Hume - Thermtest Inc.

Date of Publication:  Jan 25, 2025

Abstract

Thermal conductivity is a critical parameter influencing the performance of textiles in cooling and insulation applications. This white paper presents a comparative analysis of two widely used measurement techniques: the Modified Transient Plane Source (MTPS) method following ASTM D7984 and the Heat Flow Meter (HFM) method following ASTM C518. By evaluating various performance textiles developed by Low Impact and Atlas Industries, we highlight the importance of selecting the appropriate testing method for accurate thermal characterization.

Table of Contents

1. Introduction

2. Problem Statement

3. Methodology

4. Analysis and Discussion

   • 4.1 MTPS (ASTM D7984) Method

   • 4.2 HFM (ASTM C518) Method

   • 4.3 Comparative Data Analysis

5. Solution and Recommendations

6. Conclusion

7. References

1. Introduction

Performance textiles play a vital role in various industries, from apparel to automotive, where thermal management is essential. Atlas Industries and Low Impact are leaders in textile innovation and have been at the forefront of developing materials that offer superior cooling and warmth (insulation) properties.  

Market leading temperature modulation textiles:

1.        KinetiCool®

2.        HeatLock®

Accurate measurement of thermal conductivity in textiles is essential for optimizing performance in applications like insulation, moisture management, and temperature regulation. In product development, it helps engineers and designers select materials that provide the desired level of warmth or cooling, ensuring that fabrics meet functional and comfort requirements. By analyzing thermal conductivity, manufacturers can refine fiber blends, coatings, and fabric structures to enhance heat retention or dissipation, ultimately improving product effectiveness in various environments.

Traditionally, the MTPS method following ASTM D7984 is used to measure thermal effusivity, representing the qualitative feeling of coolness or warmth upon touch. Mathematically, thermal effusivity is equal to the square root of the product of thermal conductivity, density, and specific heat capacity.  With special care, can be used to measure thermal conductivity of homogeneous materials.

However, when applied to heterogeneous materials like fabrics, ASTM D7984 can lead to inaccuracies in thermal conductivity measurements. This paper explores the importance of selecting the appropriate testing method and introduces the HFM method following ASTM C518 as a superior alternative for heterogeneous textiles.

2. Problem Statement

The use of MTPS (ASTM D7984) for measuring thermal conductivity in heterogeneous materials such as fabrics introduces errors due to the method's assumption of material homogeneity. This results in the overestimation of thermal conductivity, leading to inaccurate assessments of a textile's performance. There is a need for a more accurate method that accounts for material heterogeneity to ensure reliable thermal characterization.

Textiles are heterogeneous because they are composed of a combination of different materials, structures, and properties that are designed to meet specific functional and aesthetic purposes. This heterogeneity is a result of the complex interplay of fibers, yarns, and weaves that make up a fabric (Figure 1). 

FIGURE 1

3. Methodology

• Sample Selection: Atlas Industries and LIT provided 4-5 textile samples designed for performance applications.

• MTPS Testing: Thermal effusivity and thermal conductivity were measured using the MTPS method as per ASTM D7984.

• HFM Testing: Thermal resistance and thermal conductivity were measured using the HFM method following ASTM C518.

• Data Analysis: Results from both methods were compared to evaluate discrepancies and assess the accuracy of each technique.

4. Analysis and Discussion

4.1 MTPS (ASTM D7984) Method

The transient MTPS method measures thermal effusivity, which relates to the initial feeling of coolness or warmth upon touch. While suitable for homogeneous materials, its application to heterogeneous textiles is problematic:

• Assumption of Homogeneity: Fabrics are inherently heterogeneous, comprising fibers, air gaps, and varying weave patterns.

• Overestimation of Conductivity: The MTPS method does not account for these variations, leading to inflated thermal conductivity values.

• Impact on Performance Evaluation: Overestimated conductivity can misinform design decisions and product specifications.

4.2 HFM (ASTM C518) Method

The HFM method offers a more accurate assessment for heterogeneous materials:

• Measurement of Thermal Resistance: It directly measures thermal resistance, a critical factor in insulating properties.

• Steady-State Conditions: The method operates under steady-state, providing reliable data over time.

• Calculation of Thermal Conductivity: Thermal conductivity is derived from thermal resistance and measured thickness, accounting for material heterogeneity.

4.3 Comparative Data Analysis

• Data Discrepancies: Samples tested with MTPS showed higher thermal conductivity values compared to HFM results.

• Correlation with Material Structure: HFM results correlated more closely with the known structural properties of the textiles.

• Implications for Product Development: Accurate HFM measurements allow for better prediction of a textile's thermal performance in real-world applications.

Table 1. Comparative analysis of thermal conductivity measurements methods for heterogeneous textiles

Sample Name	Method	Thermal Conductivity (W/m·K)	% Difference Between Methods
Textured Polyester 270D	MTPS (ASTM D7984)	0.0755	45.5
HFM (ASTM C518)	0.0475
Polyester 300-68	MTPS (ASTM D7984)	0.0412	14.4
HFM (ASTM C518)	0.0476
Heatlock® Polyester 110-698	MTPS (ASTM D7984)	0.0506	14.2
HFM (ASTM C518)	0.0439
Kineticool® 24-25-2	MTPS (ASTM D7984)	0.0544	20.2
HFM (ASTM C518)	0.0444

5. Solution and Recommendations

• Adoption of HFM Testing: For heterogeneous textiles, the HFM method should be the standard for thermal conductivity measurement.

• Reevaluation of Past Data: Products previously evaluated using MTPS should be retested to ensure data accuracy.

• Collaboration with Testing Experts: Partnering with specialists like Thermtest with broad knowledge of transient and steady-state methods can enhance testing protocols and data interpretation.

• Education and Training: Industry professionals should be educated on the limitations of certain testing methods and the benefits of using appropriate techniques.

6. Conclusion

Accurate thermal characterization of performance textiles is essential for the development and quality assurance of cooling and insulating products. The MTPS method, while useful for homogeneous materials, is inadequate for heterogeneous textiles due to its propensity to overestimate thermal conductivity. The HFM method provides a superior alternative, offering accurate measurements that reflect the true thermal properties of the material. By adopting the HFM method, Atlas Industries and others in the industry can ensure their products meet the desired performance standards.

7. References

1. ASTM C518-21, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus, ASTM International, West Conshohocken, PA, 2017.

2. ASTM D7984-21, Standard Test Method for Measurement of Thermal Effusivity of Fabrics Using a Modified Transient Plane Source (MTPS) Instrument, ASTM International, West Conshohocken, PA, 2016.

3. Thermtest Inc., Advancements in Thermal Conductivity Measurement Techniques, Technical Report, 2023.

Need more information?

For detailed insights into performance textiles visit Low Impact’s website (www.lowimpactllc.com). If you have specific questions or need assistance in selecting the right materials for your applications, feel free to contact us.

For detailed insights into accurate thermal conductivity measurement and how it can enhance your textile products, contact Thermtest Inc. Their expertise and advanced testing methods can help you achieve optimal performance and innovation in your materials.