What LDPE Is
Low-density polyethylene (LDPE) is a thermoplastic made by the free-radical polymerisation of ethylene under high pressure. The high-pressure process produces a polymer with a long-chain branched structure, and that branching is the reason for its characteristic softness, clarity and toughness. LDPE has a density of about 0.910 to 0.925 g/cm3, which places it below high-density polyethylene and linear low-density polyethylene, and a melting range of roughly 105 to 120 °C. It is flexible, chemically inert, an excellent electrical insulator and a good barrier to water vapour, although it is a relatively poor barrier to oxygen and to many organic vapours.
Typical Properties That Shape the Applications
| Property | Typical value |
|---|---|
| Chemical identity | Polyethylene, CAS 9002-88-4 |
| Density | 0.910 – 0.925 g/cm3 |
| Melting range | 105 – 120 °C |
| Crystallinity | Roughly 40 – 55 % |
| Melt flow rate at 190 °C, 2.16 kg | 0.2 – 50 g/10 min depending on grade |
| Tensile strength at break | Typically 8 – 20 MPa |
| Recycling resin code | 4 |
Density and melt flow rate are the two numbers that select a grade. Low melt flow rate grades with a narrow molecular weight distribution are used for blown film and heavy-duty sacks, where melt strength matters, while higher melt flow rate grades flow more easily and suit thin-wall containers, coatings and injection-moulded parts.
Examples One: Bags and Film
The largest single use of LDPE is flexible film, and the most familiar products are bags: grocery carriers, refuse sacks, bread bags, frozen food bags, produce liners and dry-cleaning covers. LDPE is chosen because it seals at low temperature, tolerates the flexing and puncturing loads of packaged food, and stays soft in a domestic freezer. The same resin, usually in a lower melt flow rate grade, becomes agricultural film, shrink wrap, stretch film for palletising, bubble wrap and protective liners. Film grades are commonly specified by density, melt flow rate and film properties such as tensile strength and impact resistance.
Examples Two: Containers, Lids and Closures
LDPE appears in beverage cups and vending cups, ice cream lids, trays, squeeze bottles for sauces and detergents, and flexible lids for tubs and pails. Here the resin competes on softness, impact resistance at low temperature and good weldability rather than on rigidity, which is why liquid containers that must be squeezed rely on LDPE while stiffer HDPE or PET covers applications needing dimensional stability.
Examples Three: Packaging, Pipes and Industrial Goods
Flexible packaging laminates and coating layers, where LDPE acts as the sealant web in multi-material structures.
Gas and water pipes and irrigation tube, including the inner layers of some composite pipe constructions.
Corrosion-resistant work surfaces, cable insulation and jacketing, and agricultural or construction sheeting.
Heavy-duty industrial sacks, liners for bulk bags and stretch hoods for pallet unitising.
Examples Four: Medical, Consumer and Construction Products
Medical and assistive products use LDPE for orthotic and prosthetic components, drug delivery packaging and squeeze-type dropper bottles, taking advantage of the resin's chemical inertness and its ability to be sterilised. Consumer goods include toys, housewares and reusable carrier bags. In construction, low-density polyethylene is found in floor tiles, landscape timber and outdoor lumber composites, furniture components, compost bins and trash cans, and waterproofing or damp-proof membranes, where flexibility and resistance to water and soil chemicals are the deciding factors.
Choosing and Processing the Right LDPE Grade
Four parameters cover most selection decisions: density, which sets stiffness and barrier; melt flow rate, which sets flow and melt strength; additive package, which controls slip, antiblock and ultraviolet stability; and compliance status for food contact or medical use. LDPE is processed by blown and cast film extrusion, extrusion coating, blow moulding and injection moulding, all within a fairly wide processing window because the branched chains flow easily. Because it is coded as resin 4, LDPE is widely accepted in mechanical recycling streams and in energy recovery, and buyers increasingly look for grades containing a defined recycled content where the application allows it.
FAQ
Q: What are the examples of LDPE material?
Common examples are grocery and refuse bags, bread and frozen food bags, beverage cups, ice cream lids, squeeze bottles, shrink and stretch film, cable insulation, pipes and prosthetic components.
Q: What is the difference between LDPE and HDPE?
LDPE has a branched chain structure with a density of about 0.910 to 0.925 g/cm3, making it softer and clearer, while HDPE is linear, denser and stiffer with higher tensile strength.
Q: What is LDPE's melting point?
Low-density polyethylene softens and melts in the range of roughly 105 to 120 degrees Celsius, depending on the grade and its density.
Q: Is LDPE recyclable?
Yes. It carries recycling resin code 4 and is accepted in many film and flexible packaging recycling streams, including back-of-store collection for industrial film.
Q: Why is LDPE used for plastic bags?
Because it seals at low temperature, tolerate puncture and flexing loads even when cold, and provides good water vapour resistance at a low material cost.
Q: What does melt flow rate tell you about an LDPE grade?
Melt flow rate indicates how easily the resin flows; lower values give greater melt strength for film and blow moulding, while higher values suit thin-wall moulding and coating.
