Wisdom For Polyethylene vol.32
32. What Is mLLDPE (Metallocene LLDPE)?
2025-12-04
Common Issues in Film Processing
Film processors often encounter the following challenges:
- “Bags tear easily.”
- “Sharp-edged contents quickly cut the film.”
- “Thickness was increased to improve strength, but the film became stiff and difficult to handle.”
A high-performance material attracting attention as a solution to these issues is Metallocene Linear Low-Density Polyethylene Film (mLLDPE).
Here, we explain its characteristics, differences from conventional LLDPE, and applications in technical detail.
What is Metallocene Film?
In recent years, the term “metallocene” has become more common.
While it is becoming a standard material for film processors, broad recognition in the market is still limited.
While it is becoming a standard material for film processors, broad recognition in the market is still limited.
Basics of Polyethylene
Most commonly used transparent poly bags (often called “plastic bags” in general, but more accurately polyethylene bags) are made from Linear Low-Density Polyethylene (LLDPE).
LLDPE is produced via polymerization of ethylene, typically using chromium-based catalysts or Ziegler-Natta catalysts.
Metallocene Catalysts
Metallocene films are produced using metallocene catalysts instead of conventional catalysts, which is why they are called metallocene films.
Metallocene catalysts, also known as single-site catalysts, have the following features:
- Uniform molecular chain length
- Uniform incorporation of comonomers into polyethylene chains
- Narrow molecular weight and composition distribution
Characteristics of Metallocene Film
The narrow molecular weight and composition distribution improves the following properties:
- Increased melt viscosity
- Decreased melt tension
- Higher tensile strength
- Higher elongation
- Improved puncture resistance
As a result, metallocene film exhibits superior tear and puncture resistance compared to conventional polyethylene films, while maintaining flexibility even with increased strength.
It is therefore expected to find applications across various industries where both strength and suppleness are required.
It is therefore expected to find applications across various industries where both strength and suppleness are required.
In addition, the following physical property values are a comparative table of various physical properties between metallocene film and conventional LLDPE film.
At a glance, it can be seen that the values of the metallocene film surpass those of the conventional product.
At a glance, it can be seen that the values of the metallocene film surpass those of the conventional product.
Comparative Physical Property Table
Item | Direction | Specimen | LLDPE 0.08mm | Metallocene 0.08mm | JIS Standard (LLDPE) |
Tensile Strength (MPa) | Machine Direction (MD) | 1 | 23.4 | 37.0 | 16.7 or more |
2 | 25.0 | 35.4 | |||
3 | 24.2 | 32.3 | |||
4 | 22.5 | 32.5 | |||
5 | 23.0 | 32.3 | |||
Ave. | 23.6 | 33.9 | |||
Transverse Direction (TD) | 1 | 28.5 | 42.5 | ||
2 | 26.0 | 33.2 | |||
3 | 25.3 | 34.5 | |||
4 | 26.3 | 35.2 | |||
5 | 25.0 | 34.6 | |||
Ave. | 26.2 | 36.0 |
Item | Direction | Specimen | LLDPE 0.08mm | Metallocene 0.08mm | JIS Standard (LLDPE) |
Elongation (%) | Machine Direction (MD) | 1 | 331.3 | 522.8 | 250 or more |
2 | 368.8 | 491.1 | |||
3 | 351.0 | 454.0 | |||
4 | 313.0 | 454.3 | |||
5 | 324.6 | 447.3 | |||
Ave. | 337.7 | 473.9 | |||
Transverse Direction (TD) | 1 | 459.1 | 721.3 | ||
2 | 421.8 | 567.6 | |||
3 | 409.4 | 618.9 | |||
4 | 422.9 | 635.8 | |||
5 | 398.3 | 637.9 | |||
Ave. | 422.3 | 636.3 |
*These data are measured values and not guaranteed values.
Since it may be difficult to imagine based on numerical values alone, please refer to the photographs below.
These are photographs of metallocene film being pressed against a bottle approximately 18 cm in height.
It can be seen that the film stretches without tearing at all.
The conventional LLDPE film tore partway through.
L-LDPE Film
When pressed against the bottle, partway through…
The circled area in red has torn.
The film has torn, and the bottle inside can be seen.
Metallocene Film
When pressed against the bottle, it stretched down to near the bottom without tearing.
It is not torn.
Even when enlarged, it is still not torn.
As described above, while metallocene film has versatile excellent properties, there are also several issues.
As explained earlier, “the melt viscosity increases and the melt tension decreases,” and the problem caused by this characteristic is a “decline in processability.”
As explained earlier, “the melt viscosity increases and the melt tension decreases,” and the problem caused by this characteristic is a “decline in processability.”
Polyethylene bags are basically manufactured by the inflation method.
The inflation method is a manufacturing process in which molten pellets (granular resin) are extruded together with air from a circular die, inflated into a balloon shape, and then wound up.
The inflation method is a manufacturing process in which molten pellets (granular resin) are extruded together with air from a circular die, inflated into a balloon shape, and then wound up.
In this process, when the molten resin is blown up with air, if the melt viscosity is too high, melt fracture (surface irregularity) is likely to occur.
When a high-melt-viscosity resin is extruded at high speed, it is forced to flow “under a condition of high shear stress while increasing the shear rate,” causing unstable flow behavior.
As a result, the film surface becomes wavy, not only causing non-uniform thickness, but in some cases making the product unusable.
In addition, due to the low melt tension, there is also the problem of the film sagging during bubble formation.
When a high-melt-viscosity resin is extruded at high speed, it is forced to flow “under a condition of high shear stress while increasing the shear rate,” causing unstable flow behavior.
As a result, the film surface becomes wavy, not only causing non-uniform thickness, but in some cases making the product unusable.
In addition, due to the low melt tension, there is also the problem of the film sagging during bubble formation.
Furthermore, in the bag-making process, the appropriate temperature range during heat sealing becomes extremely narrow, and if thickness variation is present, “seal defects” occur frequently, leading to an increase in defective products and higher costs.
In the worst case, there is also a risk of bag breakage occurring during customer use.
In the worst case, there is also a risk of bag breakage occurring during customer use.
So, how can processability be improved while making use of the excellent properties of metallocene?
There are broadly two possible approaches.
There are broadly two possible approaches.
① Use of raw materials with a moderately broadened molecular weight distribution
In other words, this means using “raw materials that take processability into consideration.”
This can be achieved by adopting specific raw materials provided by material manufacturers; however, “dependence on the material manufacturer’s know-how” and “compatibility with the inflation machine” become issues.
With this method, the selection of appropriate raw materials becomes a key piece of know-how.
This can be achieved by adopting specific raw materials provided by material manufacturers; however, “dependence on the material manufacturer’s know-how” and “compatibility with the inflation machine” become issues.
With this method, the selection of appropriate raw materials becomes a key piece of know-how.
② Blending with conventional LLDPE
By mixing conventional LLDPE, which has excellent processability, the weakness of metallocene in terms of processability is reinforced.
With this method, the blending method and ratio become important.
・Increasing the metallocene ratio improves the physical properties of the film, but reduces processability.
・Increasing the conventional LLDPE ratio improves processability, but reduces the physical properties of the film.
With this method, the blending method and ratio become important.
・Increasing the metallocene ratio improves the physical properties of the film, but reduces processability.
・Increasing the conventional LLDPE ratio improves processability, but reduces the physical properties of the film.
This balance is like a set of scales, and by determining the optimal blending ratio while maintaining processability, it becomes possible to create products that excel in both film performance and processability.
Summary
Whichever method is adopted, in the end, the accumulation of know-how is the key to success.
In recent years, as reduction of environmental burden has been required, down-gauging (thinning) can be achieved by utilizing the excellent physical properties of metallocene film, thereby making it possible to reduce environmental impact.
In addition, by improving the physical properties of the film without changing the thickness, it is also possible to contribute to solving issues at manufacturing sites.
Conventional LLDPE attracted attention as a “revolutionary film,” but today it has become a common general-purpose material.
＊All rights reserved.
Next Article
33. Poly Bag Scratches and Pinholes