HDPE is a commonly used petroleum thermoplastic and the most used of the three polyethylenes for a wide range of applications. If you look at this plastic under a microscope, you would see that it has a linear structure with few branches lending to its optimal strength density ratio. As a result of its molecular makeup, this polymer shines brightest in applications where moisture resistance and cost-effectiveness are needed. HDPE was created in the 1930s and used in high-frequency radar cables during World War II. It was introduced to the market commercially soon after. While its higher density versions yield a more rigid result, HDPE can vary in flexibility. Low-density grades of the thermoplastic are less stiff and the high-density grades have equally high crystallinity.