Know Your Plastics Series: Low Density Polyethylene

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Low-Density Polyethylene Acronyms and Resin Identification Number.

As members of the polyolefin resin family, Low-Density Polyethylene and High-Density Polyethylene are the lightweight, versatile resins used in producing a majority of plastic products in the world. However, LDPE and HDPE differ at one central point – their molecular arrangement..


This difference lies in the structure of the long, chainlike molecules in polyethylene. When these molecules are linear, they form High-Density Polyethylene (HDPE); when branched, they are known as Low-Density Polyethylene (LDPE).

Molecular Structure of HDPE and LDPE

Low-Density Polyethylene is the result of placing gaseous ethylene with oxide initiators under very high pressures and temperatures. This process creates a structure consisting of both long and short branches that limits the compacting of the polyethylene molecules that should make it crystalline, stiff and hardy. LDPE is the very flexible material used in squeeze bottles, trash and grocery bags, wire and cable insulation and packaging film.


Background

While studying the effects of extremely high pressures on polyethylene during the polymerisation process, Imperial Chemical Industries Ltd, England, produced the first LDPE in 1933.


In fact, LDPE was the very first polyethylene produced by the English company, even before HDPE.(Read article on HDPE)


Commercial production began in 1939 after receiving the patent for their process two years before. The first application of LDPE was as insulation for radar cables during World War II. That method of production is still in use today.


Uses of LDPE

LDPE Film Applications

70% of Low-Density Polyethylene produces the film for:

  • Plastic bags (groceries, frozen food, bread, food crops, cereal box liners, bubble wrap and dry-cleaning)
  • Lightweight packaging materials – sacks, six-pack rings, waterproof carton lining, plastic wraps, snap-on lids, pallet stretch film and pallet shrink film

Other applications include:

    Left to right: LDPE packaging, Clear Plastic and ColoredPlastic

  • Trays
  • Milk and juice boxes
  • Sterile blister packs for packaging drugs
  • Wash bottles and dispensing bottles
  • Playground slides
  • Computer components, hardware covers and packaging
  • Corrosion-resistant layer for work surfaces

Properties of LDPE

  • It has more side branches that lower intermolecular forces and make it weak.
  • LDPE has a lower density because its molecules are less tightly packed and less crystalline.
  • A lower density of 0.91-0.94 g/cm3 makes it lightweight.
  • Lower tensile strength than HDPE
  • Higher resilience than HDPE, i.e. LDPE can store energy during deformation and release that energy as it returns to form.
  • It is semi-rigid
  • It is more elastic than HDPE.
  • Its amorphous nature gives it high transparency especially, in thin foil.
  • It comes in translucent and opaque forms.
  • Its impact strength increases relatively in cold conditions making it quite rugged.
  • It cannot withstand very high temperatures (90oC for short periods and 80oC continuously)
  • At room temperatures, LDPE is non-reactive except for powerful oxidising agents and solvents.
  • Good chemical resistance
  • Resistant to most alcohols, acids and alkalis
  • Low resistance to oxidising agents and select hydrocarbons
  • Weatherproof
  • Low water absorption
  • It falls under the resin identification code “4” used in recycling.

Benefits of LDPE

  • Lower density means less mass compared to its volume
  • It is highly pliable.
  • It melts at a lower temperature than HDPE, so it uses less energy
  • Low cost
  • Good electrical insulation
  • Waterproof
  • It is easy to process and mould by most methods
  • LDPE doesn’t break easily
  • It is an excellent heat-seal layer in a flexible package
  • Good product protection when used in packaging

Disadvantages of LDPE

  • Prone to cracks due to stress
  • Its lower density means that LDPE breaks down much easier over time than other plastics.
  • Low resistance to extreme temperatures (high and low)
  • Little or no resistance to Ultraviolent rays
  • Consistent exposure to solar radiation AKA sun-light produces significant amounts of greenhouse gases (methane and ethylene) than other plastics
  • High permeability to carbon dioxide and other gases

Recycling LDPE

Currently, only a section of rigid LDPE products (containers, bottles, caps and lids) are welcome recycling. The more readily recycled products are LDPE bags and wraps. These film-like materials are re-purposed into furniture, panelling, floor tiles, landscape timber, shipping envelopes, compost and trash bins and compost bin liners.


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