Plastic bags are a ubiquitous form of packaging, used globally for everything from grocery shopping to waste containment. Understanding the fundamental properties of these items, which are overwhelmingly designed for single-use purposes, reveals why they are so prevalent in daily life and why they pose significant environmental challenges. These properties are directly linked to their base material, a synthetic polymer optimized for low cost and high utility.
Polymer Composition
Plastic bags are primarily manufactured from polyethylene (PE), a versatile thermoplastic polymer derived from petroleum. The process of polymerization links ethylene molecules into long chains of hydrocarbons, and the structure of these chains determines the final properties of the plastic film. Polyethylene is classified into different types based on its density and the amount of branching in its molecular structure.
The two main types used for bags are High-Density Polyethylene (HDPE) and Low-Density Polyethylene (LDPE). HDPE has a linear structure with minimal branching, allowing molecules to pack tightly, resulting in a rigid, strong material with a density of 0.93–0.97 g/cm³. Conversely, LDPE features a highly branched molecular structure, which creates more space between chains and yields a softer, more flexible material (0.910–0.940 g/cm³). HDPE is commonly used for thin grocery bags, while LDPE is used for thicker, stretchier bags like garbage or dry-cleaning bags.
Essential Physical Attributes
The design of plastic bags leverages a combination of physical properties to make them highly functional containers. They exhibit considerable flexibility, allowing them to be easily folded, stored, and conform to the shape of their contents without tearing. This malleability is especially characteristic of LDPE films, which are soft and pliable.
Despite being thin, plastic films possess high tensile strength, which is the maximum pulling stress they can withstand before breaking. This strength-to-weight ratio allows the bags to hold substantial loads relative to their own mass. The thickness, or gauge, of the film is a primary factor, with common grocery bags being extremely thin, sometimes around \(0.5\) mil (about 12.5 microns).
The low density of the polyethylene material contributes to the bag’s lightweight nature, making it economical to transport and handle. While HDPE is stronger and more durable for a given thickness, LDPE offers greater puncture and tear resistance due to its stretchiness and ability to deform under stress. These attributes ensure the bag remains functional for transport, resisting damage from the items placed inside.
Barrier Function and Chemical Stability
Polyethylene bags function as effective barriers, protecting their contents from external factors. Their structure gives them low moisture permeability, meaning they are highly resistant to water transfer. This impermeability helps keep contents dry and is useful for food safety and preventing leaks during transport.
Plastic bags also exhibit strong chemical stability. Polyethylene is largely chemically inert, meaning it does not readily react with most common household substances, including many acids, bases, and alcohols. This stability makes them a reliable choice for containing a wide variety of materials, preventing corrosion or degradation of the bag itself.
Environmental Persistence
The same molecular properties that make plastic bags durable also contribute to their long-term environmental presence. Polyethylene films are not inherently biodegradable, meaning they resist microbial breakdown by naturally occurring bacteria and fungi. Their chemical structure is too stable for natural processes to efficiently dismantle the long polymer chains.
Even plastic bags marketed as “biodegradable” often fail to fully decompose in natural environments like soil or sea over several years. The material’s durability allows conventional plastic bags to remain intact and carry a full load of shopping even after three years of exposure in marine or soil environments. Although polyethylene is technically recyclable, the extremely thin gauge and low weight of many single-use bags complicate the mechanical recycling process, leading to a significant portion ending up in landfills or as environmental litter.