Polyethylene and Polypropylene are very similar as far as physical properties.
However, Polypropylene can be produced optically clear where polyethylene can only be made translucent like a milk jug.
Polyethylene does have physical properties that allow it to stand up better in cold temperatures, particularly when using it as signs.
electrical insulator carbon black
Polypropylenes are light in weight. They have a high resistance to cracking, acids, organic solvents and electrolytes. They also have high melting point and good dielectric properties and are non-toxic.
Monomer of polyethylene is ethylene and monomer of polypropylene is propylene.
Polyethylene has a lower melting point compared to the higher melting point of polypropylene. (this may be a good test for you)
Polypropylene is not as sturdy as polyethylene.
Polypropylene is stiffer and resistant to chemicals and organic solvents compared to polyethylene.
Polypropylene is pure, non-stretching and generally more rigid than polyethylene.