Be Visible in The Dark
Retro-reflective clothing either contains reams of tiny, partly mirrored glass particles, so called Luneburg lenses, or micro trifold prisms located on a polymer film. These micro prisms reflect impinging light in the direction of the source of light.
Fleece contains of very fine polyester (PES) yarn processed into a knit fabric. The plush loops have been opened and roughened, giving space for air pockets on both sides of the material. This creates a fleecy material with a high heat insulation capacity, very low absorption of water (moisture-resistant) and low weight.
For simple rainwear, PES cloth is coated on one side with polyurethane (PU) or polyvinylchloride (PVC): The outer PES side provides strength, the inner PU one water resistance.
To add breathability, the inner layer of PU can be replaced with a polytetrafluoroethylene (PTFE) membrane. The pores of the membrane are approximately 1/20,000 the size of a water droplet, making the membrane impenetrable to liquid water while still allowing the water vapor molecules from sweat to pass through. This type of membrane is known under the trade name Gore-Tex®.
The breathability of such climatic membranes only works as long as the humidity on the outside of the membrane is much lower than on the inside; the chemical potential of the water vapour on the inside is larger than on the outside. Above 15 °C the difference between the chemical potentials becomes so small, that the breathability vanishes.
Other articles from this series
► Combating Old Man Winter – Physics helps to get rid of snow and ice in the winter ̶ but chemistry is more efficient