HDPE is high-density polyethylene, a polymer made from petroleum. It's extremely versatile, durable and flexible. Its properties make it very useful for large irrigation projects where its flexibility can cope with sharper bending radii for example and the kind of fluctuations in pressure, movement and external factors like UV rays and heat or cold, more easily than PVC. Because the joints are heat-fused, they are extremely strong and are more likely to create a leak-free system.