HDPE or LDPE: two polyethylenes, two uses
A rigid jerrycan and a bottle you squeeze between your fingers can be made of the same molecule, ethylene. What separates them is not the starting chemistry, but the shape of the chains it has built.
By Tecmom editorial team · 2 min read
A matter of branches
Polyethylene is a long chain of ethylene. Depending on how it is made, that chain stays nearly straight or becomes covered with branches.
- In HDPE, high-density polyethylene, the chains have few branches: they pack tightly into crystalline zones. The material is denser, stiffer, stronger.
- In LDPE, low-density polyethylene, many branches keep the chains from lining up. The material is less dense, more flexible, more translucent.
The density figures that separate them vary slightly between sources: around 0.94 grams per cubic centimetre.
Two codes, two uses
Each material has its identification code: 2 for HDPE, 4 for LDPE.
HDPE makes rigid bottles, cans, jerrycans and many caps: wherever a pack must stand up, resist knocks and slow moisture. LDPE makes films, bags, and bottles meant to be squeezed, such as some drop bottles: wherever flexibility is the function.
What density changes
More crystallinity generally means a stiffer wall and a better water vapour barrier, because crystalline zones slow the passage of molecules. Less crystallinity means a more flexible wall that recovers its shape after squeezing.
In both cases the oxygen barrier remains modest: that is the weak point of polyethylenes. See permeability.
Choosing
The choice is made on function: stand or squeeze, slow moisture, resist the product. Our advice page HDPE, PET, PP compares the most common bottle materials, and the Tecmom catalogue lists its HDPE bottles.
Two materials born of the same gas, yet everything sets them apart to the touch. That is often the way in plastics: it is not the molecule that decides, but how it is organised.
Glossary
- HDPE
- High-density polyethylene: few branches, rigid material, code 2.
- LDPE
- Low-density polyethylene: branched chains, flexible material, code 4.
- Crystallinity
- The share of a material in which the chains are arranged in an orderly way.
- Branch
- A side chain attached to a polymer's main chain.