I've had many people ask me what the difference is between polypropylene
fabric and polyester
fabric used to make thermal underwear. In this post, I'll point out ony the factors that I think are the most relevant.
First of all, polypropylene and polyester are synthetic materials. They are both polymers, which is essentially plastic. As a result their colors won't fade or bleed when washed because the colors are built into the material. Polypropylene however, is more hydrophobic than polyester meaning that it does not absorb as much water. Since the water cannot be absorbed into the fabric, the water(or sweat) has a tendancy to spread evenly throughout the garment which in turn helps the water to evaporate much quicker than a fabric that absorbs and retains the water. As a quick example, if you cup your hand and put some water in it and continue to hold the water in your hand with your hand still in a cup shape, the water will take a very long time to dry. But, if you uncup your hand and use your other hand to spread the water evenly all over both of your hands, the water will dry in less than 1 tenth of the time.
So, having explained this, polypropylene will dry much faster than polyester. Polypropylene has a much lower melting point than polyester so you will want to avoid washing polypro in hot water or drying it. For this reason, polyester materials are much easier to care for than polypropylene. Polyester is also more UV resistant than polypro. If you wear polypropylene as an outer layer exposed to the sun, eventually the polypropylene fabric will break down and the color will fade. Polypropylene has a lower heat transfer rate which means that thermal underwear made from polypro will retain more heat than polyester.
So really, whether one fabric is better than the other really depends on what it will be used for. If you just need a fabric to wear next to your skin that will dry very quickly and thus keep you skin dry, then polypropylene is probably what you want. If you want just a good all around fabric that also dries quickly, but is very easy to launder and care for, then polyester is probably your best bet.