Polyethylene (PE) is the most widely produced plastic globally, a synthetic polymer created from ethylene. Its appearance and feel change dramatically based on how its molecular chains are structured. The amount of branching in the polymer chain determines the material’s density, which dictates its rigidity, clarity, and texture. This results in a spectrum ranging from thin, transparent films to thick, opaque slabs.
The Fundamental Physical Properties of Polyethylene
Polyethylene resin commonly appears as small, waxy pellets or granules that are naturally translucent white or milky in color. A foundational characteristic of all PE is its relatively low specific gravity, making it extremely lightweight. The material has a distinctively slick or waxy feel to the touch, a texture maintained even after molding into final forms.
Polyethylene exhibits chemical resistance against water, most strong mineral acids, bases, and many organic solvents, ensuring product durability. Although PE is a thermoplastic, its melting point is relatively low, typically falling between 100°C and 140°C depending on the specific type. This aids in processing but limits its use in high-heat applications.
High-Density Polyethylene (HDPE): Rigid Forms and Visual Traits
High-Density Polyethylene (HDPE) is characterized by its opacity and rigid form, achieved through a molecular structure with minimal branching. The linear chains pack tightly, creating high crystallinity that scatters light, resulting in an opaque product that is often milky white or colored with pigments. HDPE feels hard and strong, possessing a high strength-to-density ratio and tensile strength. Unlike lower-density relatives, it is not easily bent or stretched, making it ideal when structural integrity is required. Its surface often has a matte or slightly waxy finish.
HDPE is used for common rigid containers, including milk jugs, laundry detergent bottles, and large storage bins. Other examples include durable piping and industrial cutting boards. HDPE maintains its shape due to its resistance to impact, even when exposed to harsh chemicals.
Low-Density Polyethylene (LDPE and LLDPE): Flexible Films and Sheeting
Low-Density Polyethylene (LDPE) and Linear Low-Density Polyethylene (LLDPE) are the flexible side of the PE spectrum. The highly branched molecular structure of LDPE prevents tight packing, resulting in low crystallinity and a translucent appearance. Translucent means light passes through the material, but clear shapes or text cannot be distinctly seen on the other side.
LDPE is generally softer and more flexible than LLDPE, and standard LDPE appears relatively clear, as seen in many food bags. LLDPE has a more linear structure with short branches, giving it a slightly hazy appearance but offering superior strength and resistance to punctures. Both are highly pliable and used in thin film applications.
These low-density forms are defined by their use in flexible sheeting and thin films, contrasting sharply with the rigidity of HDPE. Common examples include plastic shopping bags, sandwich bags, and various types of plastic wrap. LLDPE is used for industrial stretch films where elasticity and toughness are prioritized. Squeeze bottles, such as those used for honey or mustard, demonstrate the soft nature of LDPE, allowing the container to be easily deformed to dispense contents.