The plastic bags are normally manufactured with polyethelene (PE). There are many kinds of PE but the main kinds used for making plastic shopping bags are HDPE, LDPE and LLDPE.
In practice the thin bags are normally made with HDPE and thicker bags are made with LDPE or HDPE. LLDPE is mixed with other material.
Difference between HDPE, LDPE, LLDPE
- LDPE (Low Density Polyethylene) has the most excessive branching. This causes the low density to have a less compact molecular structure which is what makes it less dense. It has a density of 0.910-0.925 g/cm3. The LDPE bags are more flexible but with less tensile strength.
- HDPE (High Density Polyethylene) has minimal branching of its’ polymer chains. Because it is denser it is more rigid and less permeable then the LDPE. It has a density of 0.941-0.965 g/cm3. The HDPE bags has better tensile strength but is not as flexible as LDPE bags.
- LLDPE (Linear Low Density Polyethylene) has a significant number of short branches. Because it has shorter and more branches its chains are able slide against each other upon elongation without becoming entangled like LDPE which has long branching chains that would get caught on each other. This gives LLDPE higher tensile strength and higher impact and puncture resistance than the LDPE. It has a density of 0.91-0.94 g/cm3. It is somewhere between HDPE and LDPE.
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