High thermal conductivity in soft, fleece-like textiles

Question

Fleeces typically exhibit low thermal conductivity, and are often used in winter clothing for their insulating properties. I am seeking a material for a radiant heat application that is soft to the touch like fleece, but transmits heat well.

Measuring thermal conductivity in Watts per meter-Kelvin.

This study found 15 fleece samples to all have thermal conductivity below 0.004 W/mK. Textiles with higher thermal conductivity include leather (0.1 to 0.15 W/mK) and linen (0.188 W/mK).

I understand that the air trapped within fleece will impede heat flow. So a heat-transmitting fleece might have very dense 'hairs' to shrink air pockets. Alternatively, the hairs may be shorter.

Heated blankets often incorporate fleeces. Do these blankets use particular fabrics to reduce heat loss across the fleece? Or do they just put out enough heat to compensate for the loss through poor conductivity?

Answer 1

Fleece is a non-woven fabric that makes a good insulator when filled with air. Heating blankets use fleece and other insulating materials specifically because they ARE good insulators. The electric heating elements in the heat blanket are discrete wires and not smoothly distributed across the surface. Insulation allows the heat to travel on the inside of the blanket easier than exiting the blanket, resulting in move even heat distribution to the user.

Here is a thermal image of a heat blanket from the electric blanket wiki.

It is important to note that when considering electric heat production, the efficiency is not reduced by adding insulation. Electricity watts in equal heat watts out regardless of temperature. There is no "heat loss" in this case. This is not true for other types of heating such as a heat exchanger like a home steam radiator.

If you did need to increase the conductivity of fleece for a different application you have a couple options. It will still be in the conductivity range of an insulator, so expect very small improvements relative to other good thermal conductors like aluminum.

1. Compress the fleece thinner so there is less air volume. The acrylic will conduct better than the captured air.
2. Replace the air with a more conductive fluid like water, or oil, or thermal grease containing conductive metal powder.
3. Instead of using acrylic polymer for the fibers, fill or replace them with a more conductive material.
4. Design the material to have larger pockets that allows more internal convective currents.
5. Use black and darker colors as they have higher emissivity.