Single- use plastic bags are made from the petrochemical called polyethylene, specifically high-density polyethylene (HDPE). Polyethylene is the most common of plastics due to their production of over 100 million tons annually, accounting for 34 percent of the total plastics. After the oil and gas are sent to a refinery, the petrochemicals are separated by their densities and the desired petrochemical is extracted. HDPE is known for its large strength-to-density ratio. The density of HDPE can range from 930 to 970 kg/m3. HDPE has little branching (little replacement of a substituent, for example, replacing a hydrogen atom by another covalently bonded chain of a polymer). It also has strong intermolecular forces (the forces which mediate the interaction between molecules). The difference in strength exceeds the difference in density, giving HDPE a high specific strength, making this one of the main reasons why they are often used for carriage.