About LDPE. LDPE (a.k.a. Low Density Polyethylene), the first thermoplastic polyolefin used commercially, is one of the three main types of polyethylene. LDPE differs from high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) primarily due to its unique molecular structure, where large amounts of long-chain branching, impart different rheological behavior in both shear and extension. LDPE offers an unparalleled combination of low cost, ease of fabrication and balance of physical properties (toughness, flexibility and transparency). The first thermoplastic polyolefin used commercially, LDPE is still widely used in a wide range of applications.

LDPE is a polymer of ethylene manufactured on commercial scale exclusively via free-radical-initiated, high-pressure polymerization processes. The main technologies suitable for LDPE production use autoclave or tubular reactors. It may be produced as homopolymer (LDPE itself) or as copolymer, being the most common copolymers ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), ethylene ethyl acrylate (EEA), Ethylene Acrylic Acid (EAA), produced respectively from vinyl acetate, methyl acrylate, ethyl acrylate, and acrylic acid comonomers.

Although it offers unique properties for some applications, LDPE has been losing ground to other ethylene polymers, especially HDPE and LLDPE. It can also find other substitutes, such as polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), biodegradable polymers, and bio-based polymers.

LDPE Handling. The majority of LDPE is sold as small pellets. Most of the LDPE used in North America is shipped in 82-ton rail cars. Other common forms of shipment are 18-ton bulk trucks, 500-kg boxes, and 25-kg bags. In other parts of the world, the most common mode of shipment is in 25-kg bags.

LDPE Production. LDPE production consists of the polymerization of ethylene, in such a way that different LDPE manufacturing routes are related to different sources of such raw material - ethylene can be manufactured, for instance, via pyrolysis of hydrocarbons, recovery from refinery products, ethanol dehydration or methanol-to-olefins plants. Raw materials and the respective production processes employed in the manufacturing of LDPE are listed below.

LDPE Uses. Generically, it is possible to establish commercial grades based on LDPE applications: film and sheet, extrusion coating, injection molding, wire and cable, and blow molding. The film and sheet industry represents the largest application sector for low-density polyethylene, constituting approximately 50% of North American demand. Resins used for films typically have melt indices ranging from 0.2 to 6 and densities from 0.917 to 0.930. Typically, the extrusion coating process employs resins with melt indices between 4 and 10 and densities from 0.920 to 0.930, particularly those produced in autoclave reactors, with the essential combination of neck-in and drawdown. One of the resin specifications commonly used in extrusion coating is the general-purpose grade. The injection molding grade is obtained by injecting molten LDPE into a metal mold at high temperature and pressure. The resulting solid article, taking the mold's shape, is then cooled and extracted.

Although linear low-density polyethylene (LLDPE) and blends of LDPE and LLDPE have gained wide prominence in the film market because of their superior physical properties, LDPE remains preferred in specific film applications requiring characteristics like easy extrusion, clarity, and low taste and odor. This sector includes packaging applications like pallet wrapping, heavy-duty sacks, food packaging, and non-packaging uses such as refuse bags, storage bags, agricultural film, and industrial sheeting. LLDPE has significantly entered the injection molding market (e.g., lids, buckets, toys, and freezer containers) because of its higher melt index, which contributes to increased production speed and cost reduction. Nevertheless, LDPE is still chosen for applications where clarity is necessary, especially in products like caps and closures. On the other hand, the resulting structure of extrusion coating processes is used in various food packaging applications such as snack food packaging, juice boxes, and milk containers. LLDPE remains restricted in this market.