LLDPE stands for linear low-density polyethylene. Although its name resembles LDPE, it behaves differently: despite its low density, it is markedly resistant to puncture and tearing. This is why it is used in bag production as the raw material that “adds strength” to the film.
Pallet stretch films, bags that carry heavy loads and strong garbage bags are among the best-known uses of LLDPE. For bag manufacturers, LLDPE is less a material used on its own than a component that sets the balance in blends.
Linear chain, short branches
The main chain of LLDPE is linear, like that of HDPE; however, there are short side branches at regular intervals along the chain. These branches come from a second building block, called a comonomer, which is added to the main monomer during polymerisation. The short branches disrupt the tight packing of the chains and lower the density, while the linear structure preserves strength.
This structure gives LLDPE the best of both sides: flexibility close to that of LDPE and mechanical strength that matches HDPE and in some respects even exceeds it. The type of comonomer used and the production technology further affect the film’s strength and clarity.
How is it made?
LLDPE is produced by polymerising ethylene together with a comonomer such as butene, hexene or octene, with the help of a catalyst and at relatively low pressure. The type and amount of comonomer determine the length and frequency of the side branches. In general, longer side branches give the film higher strength.
Catalyst technology also affects the result. LLDPE grades produced with newer catalyst systems can offer a different balance of strength and clarity because their chain structure is more regular. On the market these grades go by separate names; when choosing, it is necessary to look at the properties in the technical data sheet rather than the name.
Where is it used in bag production?
LLDPE comes to the fore where the bag is put under strain. Heavy, sharp-cornered or irregularly shaped loads; bags that hang for long periods or are dragged along; films that stretch to wrap the product: these are the material’s natural uses.
- Garbage bags: Bags where puncture and tear resistance are the priority.
- Gusseted and courier bags: Bags that carry heavy and sharp-cornered products.
- Stretch and wrapping films: Films that stretch to wrap the load tightly.
- Blends: A component added to HDPE and LDPE films to give them strength.
LLDPE in film extrusion
The structure that gives LLDPE its strength makes it somewhat harder to process. Its melt flows more stiffly and puts more load on the extruder; because it does not recover as well as LDPE when stretched, the bubble can be less stable on a blown film line. For this reason, rather than pure LLDPE, it is usually used blended with some LDPE.
On multi-layer lines, LLDPE is frequently placed in the inner layer or on the sealing surface. Because it gives a sound seam over a wide temperature range during sealing, it is also a suitable option for the sealing layer.
Strengths and limits
LLDPE’s strengths are high puncture and impact strength, good tear resistance, the ability to elongate for a long time before breaking and a sound seam when sealed. These properties can also make it possible to do the same job with a thinner film.
When used as the sealing layer, LLDPE also keeps its flexibility around the seam. In bags carrying heavy loads, this can help the seam respond to load by stretching rather than breaking suddenly.
Its limits include difficulty in processing, lower gloss than LDPE in some grades and the film’s tendency to stretch and resist the blade during cutting. Where the stiff feel of HDPE is wanted, LLDPE alone does not meet the expectation.
When is LLDPE the right choice?
Another advantage of LLDPE is that its strength offers the possibility of doing the same job with a thinner film. Reducing the thickness also reduces the amount of material used per bag; however, this decision must always be verified with trial runs and use tests. Down-gauging can change not only strength but also the feel of the bag in the hand and the look of the print.
If resistance to puncture, tearing or sudden loads is the bag’s priority, LLDPE or an LLDPE-rich blend is a strong candidate. Where appearance and processability matter it can be used with LDPE, and where stiffness is wanted, with HDPE.
In practice, LLDPE’s contribution becomes visible under the strains a bag experiences in use: in a bag carrying a sharp-cornered box, rubbing against the ground or being packed tightly, the difference in puncture resistance is felt immediately.
The blend ratio is not set by a recipe alone; the bag’s conditions of use, the film thickness and the capabilities of the line are assessed together. For special uses such as food contact, suitability is verified separately by looking at the conditions of use and the legislation.