The tale of The Emperor's New Clothes
is as much about marketing hype as it is about keeping up pretenses in a social setting. The hype over "smart clothing" is just that, but a new report
(open access!) from Stanford researchers about a radiative-transparent fabric appears to offer some real potential in comfortable clothing.
As humans, we feel either warm, cold or just-right depending on the heat balance that our bodies keep with the environment. Conduction, convection and radiation all play a part, with radiation dominating at over 50% according to the researchers [*]. The radiation is in the mid-IR with a peak at 9.5 μm. Unfortunately, most of the fabrics that we wear absorb rather strongly in that region, preventing that heat loss. As a result, we feel hot and need to kick down the A/C a few degrees to compensate. Polyethylene (PE) does not absorb in that region however, and so the researchers used it as the basis for a fabric that is transparent in the mid-IR.
The researchers found that a nano-porous polyethylene (normally used in the construction of lithium ion batteries) was opaque in the visible range due to the pores scattering visible light, but still transparent in the mid-IR. The PE fabric by itself was rather weak, so they ended up creating a laminate with a coarse cotton mesh to provide a stronger fabric. Tensile testing showed the laminate to be rather brittle (only about 15% elongation before break, vs. 70% for cotton), but that can be easily addressed in the future with other constructions.
Two comments: First, the use of polyethylene as an IR-transparent clothing choice is not a new idea. It was proposed (and modeled as effective) over a year ago
(and earlier research may well exist too). Second, the biggest challenge I foresee is coloring the fabric. Pigments are an option for monotonic colors (no, black is not an option since it will absorb the mid-IR), but for people that want flashy prints, it could be a real challenge.
Nonetheless, it will be interesting to see what happens to this concept. PE is plenty cheap, and so a fabric based on this could be quite affordable and all that much more valuable to people in hot areas where cooling cost can be budget busters. And the use of it in athletic wear, especially for endurance events, could prove significant as well.
[*] The researchers cite articles from 1937 and 1939 (!) for this datum. (References 18 and 19). Maybe someone should look into updating it, as I wonder how valid it truly is.