Thermal Effusivity: What is it and Why is it Important?

What is Effusivity? 

Touch. It’s one of our critical senses in perceiving the world – everything from the clothes we wear to the bedding we sleep in. The thermoreceptors in our skin monitor and relay information to our brains about everything we come in contact with. This helps us make decisions about what feels pleasant to the touch and keeps us safe: Using Thermal Effusivity to Predict Burn Hazard from the Result of Testing Using Human Skin Simulation. Human skin is very good at detecting differences in a material’s ability to transfer heat, such as the warmth of a fleece sweater compared to the coolness of leather seats in a vehicle. This material property is known as thermal effusivity – as a metric, it can be used to quantify a material’s ability to exchange thermal energy between skin and material.

Why is this important? Because of the positive correlations that have been established through human test panels and correlating touch perception to the materials thermal effusivity, groups are looking further to better understand a materials effusivity and be able to compare materials where an individual may not be able to notice a perceived difference. Moreover, instead of having to organize an entire panel, the effusivity measurement can help as a quick, objective measurement to determine whether the claims being made about the material are valid have established a positive correlation between our touch perception of the warmth or coolness of a textile, to its thermal effusivity. 

How do People Feel it?

Even if two different materials are at the same ambient temperature, the material with the higher thermal effusivity will “feel cooler” and the material with the lower effusivity will “feel warmer” to the touch. All material types have an effusivity, not just textiles. The following is an exaggerated example that can be used as though exercise to experience effusivity at home or in the office.

Example:

Look around the room you are currently in. Try to identify something made of wood and something else made of metal. Both objects are at the same ambient temperature, but if you touch the metal it will feel much cooler than the wooden object. This is because the metal has an effusivity that can be 2-3 orders of magnitude higher than the wood, meaning it will more effectively draw heat away from your hand and therefore result in a greater cooling sensation than wood. 

The same concept can be applied to why you put on a pair of slippers in the morning before heading to the bathroom. The fabric/padding from your slippers are much lower in effusivity then the ceramic tiles on the floor. This means less heat is pulled away from the base of your foot when wearing slippers compared to walking barefoot on the ceramic floor.

How is it Represented on Products?

Though effusivity is most often observed on textiles, effusivity is also used to help groups imitate the thermal properties of a natural material.  You can see effusivity when observing the tags or promotional material of a garment that list “cooling” or “warming” effects, or mentioning that a garment will “keep you warm/cool” under certain conditions. The instantaneous feedback when touching a garment is directly related to thermal effusivity.  This correlates directly to point-of-sale perceptions when a consumer is feeling products on a rack, or in a dressing room.

Is effusivity the Same as Q-max? 

No. Qmax approximates thermal effusivity via heat flux, rather than measuring it directly. Qmax is an older metric (dating back to the 1970s) that was at a time the best option for estimating thermal touch properties. One of the biggest problems with Qmax is it has been shown to suffer from poor reproducibility between different instruments and users. While there is no way to directly convert Qmax to effusivity (or vice-versa)  having a large enough dataset if obtained from a single user on a single system data has been shown to be able to be correlated.

Effusivity and Q-Max Correlation Dataset

Why do brands care about effusivity? 

The C-Therm Tx (Touch Experience) Platforms MTPS sensor – quantifies warm or cool-touch properties of a material   Where previously the only way of understanding a materials thermal touch properties was by using indirect or subjective data,  the MTPS sensor provides groups with the ability to directly measure the thermal effusivity of material and receive objective data that can support the material development and purchasing decisions being made. 

This means that any brand claims relating to effusivity can be quantitatively proven. This helps brands make strong marketing, packaging, and product decisions, by knowing the actual performance of their materials.   

How can consumer brands use Effusivity? 

Product Decisions 

Whether trying to achieve a warm or cool feeling material, a direct effusivity measurement allows product designers to make more educated decisions when trying to decide between a range of materials, especially when the properties between two materials cannot be distinguished by a test panel. And once the group has decided on the material(s) that are going to be used for specific product lines, the effusivity measurement can also be used as a quick QC screening tool to ensure that each batch of material is meeting the specifications that were originally cited.

Marketing Decisions 

With the Tx (Touch Experience) Platform, you’ll have the hard numbers to refer to. Whether the marketing campaign is developed first, or the development efforts support a new campaign, having the material data will support whether certain claims can be made.

Research and Development Decisions 

In the  R&D phase of development, scientists will be able to truly understand the thermal characteristics of the materials they are creating. With thermal effusivity as another uncertainty put to rest, scientists and researchers can spend more time on other projects that they are tasked with.  

How Researchers are Using Effusivity

C-Therms MTPS sensor has been around for ~20 years and was developed for the characterization of powders, foams, liquids, pastes, and solids.  And as a result of a push from groups in the textile industry the sensor was adapted for characterizing a wide range of textiles and other materials where touch properties are important. These include: 

• Apparel (knits, wovens, and non-wovens):- Understanding the thermal properties of new materials being developed, compare different materials to understand which is better suited for a specific application, and validate claims of material against other materials being considered.
• Bedding components: Support development of bedding components that are supposed to have  cooling/warming properties that might be better suited for different seasons/climates/geographies (think flannel sheets for winter and cool summer sheets
• Car interiors:  This can be used similar to apparel applications, but more often this is used to determine if an imitating material can appropriately replicate the thermal properties of the natural materials like leather seats or mock wood/metal accenting.
• Diapers & Personal hygiene products: As materials become saturated the effusivity of a material tends to increase significantly. Where there is an emphasis on reducing the chilling effect or discomfort when saturated – effusivity is used to compare different material constructions when they become saturated.  
• Home Finishes and Fixings: Effusivity is often used for replicating the properties of a natural or more expensive material. This can range from countertops, cupboards, replicated wood accenting, to metal coating fixtures,
• Electronics and accessories: Though we may not think of this as part of the design process, the tactile response of different materials used to make electronics or accessories that are paired with electronics is very important. This can range from watch straps, cases, coatings, and even buttons/keys.

Thermal Effusivity Measurement & Standardizations 

Thermal effusivity testing has been standardized by ASTM International (American Society for Testing and Materials), a third-party standardization organization. https://www.astm.org/. Included in the standard is strict process and experiment parameters that will ensure the most accurate measurement of the thermal effusivity of your material. 

ASTM D7984 lays out the exact experiment parameters to find the thermal effusivity of textile materials, regardless of point of origin.  

This ensures fair and accurate representation of thermal characteristics across materials. If you use ingredient materials, this also ensures that your distributor is selling you the same consistency of materials, order after order.  

Companies that have implemented Standardization across the supply chain have found that they’re able to reduce the amount of textile waste and increase transparency along the supply chain. This ensures that the end-user of the garment experiences the most comfort for their exact needs.  

Tx (Touch Experience) is currently the only instrument to comply with ASTM D7984 and recognized by the standard, guaranteeing an accurate thermal effusivity measurement across all types of textiles.  

Remove uncertainties from your material characteristics and open up the door to endless marketing, R&D, and product opportunities by leveraging true thermal touch experience through the measurement of thermal effusivity.  

C-Therm Tx Warm-Feel Cool-Touch Instrument

Learn more on our textiles page, full of case studies and resources.