What is Polypropylene?

Legal Definition
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging and labeling, textiles (e.g., ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes. An addition polymer made from the monomer propylene, it is rugged and unusually resistant to many chemical solvents, bases and acids.

Polypropylene has a relatively slippery "low energy surface" that means that many common glues will not form adequate joints. Joining of polypropylene is often done using welding processes.

In 2013, the global market for polypropylene was about 55 million tonnes. Polypropylene is the world's second-most widely produced synthetic plastic, after polyethylene.
-- Wikipedia
Legal Definition
A plastic that has a very smooth surface, and can be cracked easily, but is resistant to scatches. It is commonly used for the manufacture of battery cases, straws and other items like margarine tubs, etc.