LDPE
A branched polyethylene used in flexible packaging film.
- Also known as
- low density polyethylene, low-density poly(ethene), polythene
In everyday products
- Flexible packaging bag — Bag film
- Travel squeeze bottle — Squeezable bottle body
- Protective foam wrap — Thin polyethylene foam sheet
Structure and behaviour
[–CH₂–CH₂–]ₙ
The repeat unit shows the carbon backbone, not the short and long branches produced by the radical process. Branching interrupts regular packing, leaving more disordered material than in typical HDPE.
- Lower crystallinity than typical HDPE contributes to flexibility in bags and squeezable bottle walls.
- Long-chain branching affects how the molten polymer stretches, helping suitable grades sustain a blown-film bubble.
- When two film surfaces soften together and cool in contact, a heat seal forms without making fresh polymer from ethene.
- Gas-filled cells make LDPE foam lighter than solid LDPE; foam density and polymer density describe different things.
- Thin film bends easily partly because of its geometry. A thick section of the same resin will feel much stiffer.
Preparation routes
Ethene route
Free-radical addition polymerisation. Ethene molecules join into long chains. The high-pressure radical process also creates branches, giving the material its characteristic low-density structure. Branch formation during radical growth changes packing without adding a different main-chain repeat unit. This connects reaction conditions to film flexibility.
A radical initiator, such as an organic peroxide, starts chain growth. It is not a second monomer. Film blowing, heat sealing and foam expansion happen after the chemical chains have formed.
- Poly(ethene): structure, manufacture and usesChemical Industry Education Centre, University of York
Material limits
LDPE, LLDPE and HDPE are different material choices; many flexible packs are blends or multilayers. A flexible wall is not evidence of good oxygen-barrier performance. Film collection often differs from rigid-container collection: use the local scheme for clean wrapping, and keep compostable plastics out of a conventional PE recycling stream.
Sources
- Poly(ethene): structure, manufacture and usesChemical Industry Education Centre, University of York
- DOW LDPE 219M: film and bag applicationsDow
- Polymer structure, crystallinity and physical propertiesOpenStax, Rice University
- Plastic bags and wrapping: collection guidanceWRAP / Recycle Now