2. What did the first fully synthetic fibers, like nylon, make possible?
Nylon is a good place to start, because it was the first fiber made entirely in a laboratory rather than grown or spun from something natural. The Science History Institute traces it to the 1930s, when the chemist Wallace Carothers and his team at the DuPont company developed it. It went into production in 1939 [4][5]. When World War II began, nylon was used for war purposes such as parachute canopies [4].
Synthetic fibers are made by people, so makers can design them for properties such as strength and smoothness and produce them in large quantities. That freedom helps explain why synthetic fibers now outnumber natural ones. It does not mean they win at everything, as the next problem shows. (The exact year Carothers first made nylon is told slightly differently in different histories, so this article only says "the 1930s.")
3. How can a jacket block rain but still let sweat escape?
Here is the puzzle. A fabric that blocks liquid water can also trap the water vapor your body gives off when you sweat, unless it is designed to let that vapor out. Then you get damp from the inside. Gore-Tex, the best-known solution, uses a thin membrane full of tiny holes. Its maker says the membrane has nine billion pores per square inch, which works out to roughly 1.4 billion per square centimeter (9 billion divided by 6.45). In short, the pores are small enough to keep out liquid water but large enough for water vapor to pass through. In numbers: each pore is about 20,000 times smaller than a drop of water, so rain cannot get through, but about 700 times larger than a molecule of water vapor, so sweat that has evaporated can [2].
Think of it this way: a raindrop is too big to fit through a pore, while a vapor molecule slips through easily. The maker adds that the outer layer gets a water-repellent coating so that rain beads up and rolls away, and that wet clothing pulls heat from your body about three times faster than dry clothing [2].
One honest limit: every one of these numbers comes from the company that sells the product. I could not find an independent measurement, so they are best read as "the maker says."
4. What can wool do that a smooth synthetic fiber cannot?
Wool is only about 0.9% of the world's fiber, but it handles damp in a different way. Under a microscope, a wool fiber is covered with overlapping scales, unlike the smooth surface of a typical synthetic fiber. The Science Learning Hub, a New Zealand science education site, explains that these cells have a waxy coating, so wool repels liquid water while still taking in water vapor. Wool can absorb up to 30% of its own weight in water without feeling wet [3].
Wool fibers are also wavy. That waviness, called crimp, makes the fibers bulky and traps a lot of air between them, which gives good insulation [3]. So wool tackles the damp problem from the opposite direction to a membrane: instead of a clever hole size, it uses the fiber's own structure.
Rarely made does not mean poorly made. How much of something is produced tells you about cost and convenience, not about what it is good at.
5. What jobs do leather and down do in a jacket?
Leather starts as animal skin. Raw skin rots quickly, so it is tanned, a treatment that helps make it a more stable material, more resistant to decay [6]. Tanning is what makes skin usable as leather; how long a leather jacket lasts also depends on how it is made and cared for.
Down jackets are stuffed with the soft undercoat feathers of waterfowl. I could not open a reliable source on exactly why down is so warm, so this article leaves that question open rather than guess. It is a good one to look into yourself, along with what happens to a down jacket when it gets soaked.
Put together, the pattern is clear. Synthetic fibers are the most widely made base. A membrane solves rain-versus-sweat. Wool deals with damp by its structure. Leather starts as tanned skin. No single material wins against rain, sweat and cold all at once, which is why jackets use different materials for rain, sweat and cold.
6. How can you read the fiber label on your own jacket?
Pick a jacket at home and find the care label inside. Write down each fiber and its percentage. Add up the synthetic ones (polyester, nylon, acrylic). Is your jacket mostly synthetic? Does it have a lining made of something different? Then compare it with your rain-and-cold feeling: which problem do you think the jacket was designed for?
Be careful comparing your number with the world figure of 59%. That figure includes fibers used in carpets, sheets and curtains, so it cannot tell you whether your jacket is typical. Label rules differ by country, and some jackets, like leather ones, may not list a fiber at all. If you want to dig further, the Science Learning Hub page on wool [3] and the Science History Institute's nylon exhibition [4] are good places to start.