There are two types of commonly used plastic bags, HDPE and LDPE.

HDPE or High Density Polyethylene is a plolyethylene thermoplastic made from petroleum.  Known for its large strength to density ration, HDPE is commonly used in the production of plastic bottle, corrosion-resistant piping, geomembranes, and plastic lumber.  HDPE is commonly recycled and has the number ‘2’ as its recycling symbol

LDPE or Low Density Polyethylene is a thermoplastic made from the monomer ethylnen.  It was the first grade of polyethylene made in 1933 using a high pressure process via free radical polymerization.  Its recycling symbol is ‘4’