Polypropylene is a synthetic fibre and can be found in both carpet pile and backing. Polypropylene carpets are always solution dyed, this a process where the colour pigment is hot mixed with the polymer.
Because polypropylene carpets are dyed this way its hard for colour in drinks and food to find a way to bond and is impossible to colour stain.
Polypropylene carpets resist all chemicals used in cleaning. due to it low melt point it does not resist heat very well and will melt this is most common when sofas are dragged across and the heat from the friction will mark the carpet.
This fibre is oil loving and oil based stains can be an issue. The sign is yellowing