Polypropylene is a synthetic material, and polypropylene tanks are a light-weight construction.
Wikipedia definition (similar term):
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and is partially crystalline and non-polar. Its properties are similar to polyethylene, but it is slightly harder and more heat resistant. It is a white, mechanically rugged material and has a high chemical resistance. Polypropylene is the second-most widely produced commodity plastic (after polyethylene) and it is often used in packaging and labeling. In 2019, the global market for polypropylene was worth $126.03 billion.
Source: Wikipedia - Polypropylene