Lightweight 100% polypropylene material. 1x1 soft rib knit fabric. Cold weather comfort. Wicks moisture away from your body to keep you dry and warm. Can be worn alone or as a base layer for extra warmth. Odor and mildew resistant. Shrink resistant. Polyproplene clothing has the lowest thermal conductivity (the rate at which heat flows through a material) of any traditional material. Black single layer polypropylene top. Black - performance thermal underwear top shirt (polypropylene).