HDPE stands for high-density polyethylene. The thin, rustling T-shirt bag handed out at the supermarket checkout is most likely made from this material. HDPE owes its place in bag production to its ability to do the carrying job even as a thin film.
All members of the polyethylene family are built from the same basic building block; the difference between them lies in how their molecular chains are arranged. It is this arrangement that sets HDPE apart from the others.
The structure that makes HDPE different
The molecular chains of HDPE have little branching; the almost straight chains can pack closely together. This tight arrangement increases the material’s density and crystallinity. The result is a harder, more rigid film with higher tensile strength at the same thickness.
The same structure also determines the film’s appearance. HDPE film usually looks translucent and matt, has a paper-like feel to the touch and makes a distinct rustle when crumpled. Its moisture permeability is low, which makes it useful in many packaging applications.
Where is it used in bag production?
The most familiar use of HDPE is the T-shirt bag. A thin HDPE film can carry the weight of a supermarket shop when the handle area is designed correctly. This means the same job is done with less material.
- T-shirt and carrier bags: Thin, stiff-handling bags designed for carrying.
- Bags on a roll: Thin roll bags used for fresh produce and counter goods.
- Garbage bags: Especially thin bags intended for light use.
- Sack and liner films: Applications where stiffness and a moisture barrier matter.
Beyond bags, HDPE is also widely used in injection- and blow-moulded products such as bottles, jerrycans, crates, caps and pipes. However, the HDPE grades chosen for film have different flow properties from those used in these products.
HDPE in film extrusion
On a blown film line, HDPE behaves differently from LDPE. A bubble shape is often used in which the melt is allowed to rise like a narrow neck for a while after leaving the die and then expand. This shape helps the film to be stretched evenly in both directions.
The cooling and solidification of HDPE on the film line are also carefully managed. The cooling rate affects the crystal structure of the film and therefore its stiffness, clarity and tearing behaviour.
Very thin films can be produced from HDPE; however, the thinner the film, the more important it becomes to keep the thickness distribution even. Because the suitable range for sealing temperature can be narrower, settings need to be made carefully at the cutting and sealing stage.
Strengths and limits
Like every raw material, HDPE has its own balance; knowing this balance makes it easier to match what is expected of the bag with what the material can offer.
HDPE’s strength is that it offers good tensile strength and stiffness even at low thickness. This allows the bag to stay light while still doing the carrying job. Its low moisture permeability and general resistance to chemicals also broaden its range of uses.
Its limits come from the same structure. HDPE film can let a tear that has started run easily, especially in one direction; it is not as resistant to puncture by sharp-cornered products as LLDPE. In applications where clarity and gloss are expected, films based mainly on LDPE or LLDPE may be more suitable.
For this reason, a small amount of LLDPE is sometimes added to HDPE in practice. The blend can give the film a little more flexibility and tear resistance while largely preserving HDPE’s stiffness. The right ratio is determined through trials according to the bag’s use and the line’s conditions.
Compared with other types of polyethylene
Placing HDPE side by side with LDPE and LLDPE makes the choice easier. At the same thickness, HDPE gives the stiffest and most rustling film; LDPE offers a softer, glossier film that seals more easily; LLDPE stands out for its puncture and tear resistance.
These differences also affect the thickness needed to do the same job. In a thin bag designed for carrying, HDPE may be sufficient with less material, whereas the same bag made from LDPE usually needs a thicker film. The comparison should always be made on the basis of the conditions of use; there is no single “better” type.
When is HDPE the right choice?
If the bag is to be thin, light and designed for carrying; if function matters more than clarity in its appearance; and if the contents have no sharp corners, HDPE is a strong candidate. Where a soft feel, gloss, high flexibility or heavy, sharp-cornered loads are involved, other types of polyethylene and blends should also be considered.
Before deciding, the following questions can serve as a guide:
- How much load will the bag carry, and does that load have sharp corners?
- How should the bag feel in the hand and how should it look?
- Will it be printed, and if so, how detailed will the print be?
The choice of raw material should always be considered together with the bag’s conditions of use. Special uses such as food contact should be assessed separately according to the application and the relevant legislation.