Five decades from its first introduction, LLDPE has undergone significant development in various applications, particularly in film production. Its linear structure with short branches of comonomer has enhanced its properties, focusing on excellent toughness while maintaining good clarity. From the original LLDPE, produced with the fabulous Ziegler Natta catalyst, to the present-day rise of metallocene LLDPE capable of being applied in high-end products, the evolution of LLDPE is still ongoing to meet market’s needs. With this article, our goal is to provide valuable information regarding the history of LLDPE, property comparison with other polyethylenes, the comonomers in molecular structure, and the catalysts used.
History of polyethylene and the birth of LLDPE
Table 1 Progress of ethylene polymerization and catalysts used
| Year | Progress of ethylene polymerization and catalysts used |
| 1930 | Carl Shipp Marvel, an American chemist working at E.I. du Pont de Nemours & Company (now DuPont Company), discovered a high-density material, but the company failed to recognize the potential of the product. |
| 1933 | Low-density polyethylene was first produced in England by Imperial Chemical Industries Ltd. (ICI) during studies of the effects of extremely high pressures on the polymerization of polyethylene. ICI was granted a patent on its process in 1937 and began commercial production in 1939. It was first used during World War II as an insulator for radar cables. |
| 1953 | Karl Ziegler polymerizes ethene into high MW-HDPE (high density polyethylene) with the discovery of the catalyst based on titanium tetrachloride, and diethyl-aluminum chloride as co-catalyst. |
| 1953 | Robert L. Banks and J. Paul Hogan, both at Phillips Petroleum, filed the first patents on the Phillips catalyst, or Chromium catalyst, which is a heterogeneous catalyst, consists of a chromium oxide supported on silica gel. |
| 1954 | Giulio Natta utilizes the catalyst suggested by Ziegler to produce PP. Ziegler and Natta are both awarded the Nobel Prize for Chemistry 1963 in recognition of their work on the Ziegler-Natta catalyst. Since that time, by using different catalysts and polymerization methods, scientists have produced polyethylene with various properties and structures. |
| 1970s | Ziegler–Natta-catalyzed linear low-density polyethylene (LLDPE) was first commercialized. In 1968, LLDPE resin was also introduced by the Phillips Petroleum Company. |
| 1990s | Metallocene-catalyzed LLDPE (mLLDPE) resins were commercialized. |
We have explored the invention of three main catalysts employed commercially to produce polyethylene: the Phillips catalyst (chromium oxide), Ziegler–Natta catalysts (based on titanium trichloride), and Metallocene-based catalysts. The Phillips supported chromium catalyst is mainly used to produce HDPE. Since our focus is on LLDPE, in the latter part of this article, let’s go deeper into LLDPE’s comonomers and its commonly used catalysts: Ziegler-Natta and Metallocene.
LLDPE and Comonomers
The basic property of each polymer is influenced by their molecular structure. Unlike low density polyethylene (LDPE) produced in the high-pressure process, LLDPE has no long-chain branching and has a narrower molecular weight distribution, meaning the molecular weights of all molecules are more similar and do not vary significantly. LLDPE’s structure features a linear backbone with short and uniform branches, preventing the linear polymer chains from packing too closely together [Figure 1]. This results in LLDPE has lower density compared to HDPE, which lacks branches in its structure, exhibits a dense, highly crystalline material with high strength, moderate stiffness and low clarity (In essence, a molecular structure with fewer branches tends to pack tightly together, contain less porous area through which light can transmit). LLDPE is stiffer, more heat resistant, and as tough or tougher than conventional LDPE. It also had greater ESCR.
Apart from attractive resin properties, momentum behind LLDPE came from resin suppliers who foresaw lower production costs from reactor pressures of only 100 to 300 psi instead of the usual 30,000 to 50,000 psi. LLDPE has penetrated almost all traditional markets for polyethylene; it is used for plastic bags and sheets (where it allows using lower thickness than comparable LDPE), plastic wrap, stretch wrap, pouches, toys, covers, lids, pipes, buckets and containers, covering of cables, geomembranes, and mainly flexible tubing. Meanwhile, LDPE did not disappear, and HDPE still plays an important role in the polyolefin film industry due to their distinctive features, which are necessary for processing improvement and specific film applications.
Additional information about the production technologies, fundamentals, and properties comparison among LDPE, LLDPE, and HDPE can be found in another article titled ‘Polyethylene Film Processing Guide’.
The “linear low” resins were actually copolymers of ethylene and other comonomers such as 1-butene, 1-hexene, and 1-octene (known as α-olefins).
- Octene: : This is the highest-performance co-monomer used, so it is by far the most expensive. The octene co-monomer has the longest branch chains.
- Hexene: This co-monomer provides a good middle ground between octene and butene, but is also categorized as a high-performance co-monomer [Figure 2].
- Butene: Due to its lower cost, butene is the most common co-monomer for commodity plastic applications. The butene co-monomer has the shortest branch chains.
Both type and concentration of comonomer play an important role in the properties of the LLDPE in terms of melting behavior, density, crystallinity, and mechanical properties. For instance, the introduction of short-chain branching derived from the comonomer decreases the crystallinity and melting temperature of the copolymer. In film application, mechanical properties such as toughness and tensile strength, increases with the increase of comonomer length, i.e., 1-butene (C4 LLDPE), 1-hexene (C6 LLDPE), and 1-octene (C8 LLDPE).
C6 LLDPE exhibits superior toughness and elongation (the longer C6 branches allow the LLDPE molecules to slip on each other more easily), making it increasingly popular in machinery stretch film and high-quality lamination film. However, C4 LLDPE continues to be predominantly used in commodity production due to its excellent processability and lower cost.
Metallocene-LLDPE
Since the commercialized introduction in the 1990s, metallocene LLDPE (mLLDPE) resins have continued to evolve and improve. mLLDPE is produced in a low-pressure polymerization process using a metallocene catalyst to copolymerize ethylene and a comonomer such as 1-hexene (most common) or 1-octene. Metallocene catalysts are effective because they provide greater control over the polymerization reaction.
The discovery of these catalysts led to a breakthrough in the control of the crystalline structure or “architecture” of polyethylene. mLLDPE resins are widely acknowledged for their exceptional dart impact and puncture resistance, superior organoleptic properties, brilliant clarity, and outstanding hot tack and heat seal benefits compared to conventional LLDPE resins. These properties were achieved by improved control over the locations of the short chain branches in the polyethylene chain.
[Figure 3] shows conventional LLDPE resins have short chain branches predominantly located in the lower molecular weight chains whereas first generation mLLDPE resins have branches distributed evenly across the whole molecular weight distribution. This is of particular importance to the mechanical properties as only long chains containing short chain branches are able to act as tie molecules, which hold together the tertiary semicrystalline structure of the polymer, providing toughness and creep resistance.
It turned out that the advantages came with trade-offs. Metallocene catalysts were expensive and mLLDPE resin, due to its narrower molecular weight distribution, were harder to process than traditional Ziegler-Natta catalyst-based LLDPE or low-density polyethylene (LDPE). Recently, the demand for mLLDPE has risen again, with advancements in the plastic industry and the introduction of new generations of metallocene catalysts, the price gap is closing, and the processability of the resin is improving.
Ziegler-Natta and Metallocene catalyst in LLDPE production
- Ziegler Natta catalyst
These catalysts were originated in the 1950s by the German chemist Karl Ziegler for the polymerization of ethylene at atmospheric pressure. Ziegler employed a catalyst consisting of a mixture of titanium tetrachloride (TiCl4) and an alkyl derivative of aluminum (AL(C2H5)3). Giulio Natta, an Italian chemist, extended the method to other olefins and developed further variations of the Ziegler catalyst based on his findings on the mechanism of the polymerization reaction. The Ziegler-Natta catalysts include many mixtures of halides of transition metals, especially titanium, chromium, vanadium, and zirconium, with organic derivatives of non-transition metals, particularly alkyl aluminum compounds.
In the 1970s, magnesium chloride (MgCl2) was discovered to greatly enhance the activity of the titanium-based catalysts. These catalysts were so active that the removal of unwanted amorphous polymer and removal of residual titanium was no longer removed from the product (so called deashing). They enabled the commercialization of linear low-density polyethylene (LLDPE) resins and allowed the development of non-crystalline copolymers.
The modern Ziegler–Natta catalysts are mixtures of solid and liquid compounds, often containing MgCl2/TiCl4/Al(C2H5)3. (For propene polymerization, different internal and external donors such as ethyl benzoate, silanes or ethers to increase the tacticity). As Ziegler–Natta catalysts are heterogeneous and complex systems with different active sites, the polymer structure can be influenced only to a limited degree.
- Metallocene catalyst
Chemists have been aware of metallocene structures since the 1950s. But their commercial feasibility didn’t become apparent until 1977, when Walter Kaminsky at the University of Hamburg demonstrated that metallocene, with the help of a methyl aluminoxane cocatalyst (MAO), can be useful in polymerizing olefins. The active site, sandwiched between the constrained geometries of the cyclopentadienyl (Cp) structures, had the potential to knit together olefin monomers with pinpoint accuracy. Metallocene catalysts show in contrast to Ziegler systems only one type of active site (single site catalysts), which produces polymers with a narrow molar mass distribution (Mw/Mn=2), and their structure can be easily changed.
In comparison to Ziegler-Natta systems, metallocene catalysts are soluble in hydrocarbons; show only one type of active site and their chemical structure can be easily changed. These qualities would allow to predict the properties of the resulting polyolefins accurately by knowing the structure of the metallocene used during their synthetic process and to control the resulting molecular weight and distribution, comonomer content and tacticity by selection of the appropriate reactor conditions.
The first generation of metallocene catalysts also produce highly uniform polyethylene molecules with very consistent molecular weight. [Figure 4] illustrates that the molecular weight distribution (MWD) of these mLLDPE resins is significantly narrower compared to conventional LLDPE. The narrow molecular weight distribution typically caused these first generation metallocene resins to be harder to process than regular LLDPE or LDPE. With fewer shorter molecules, these resins required higher extruder motor loads, generated higher head pressures, increased shear heating and had a higher propensity to experience melt fracture. In addition, as these resins have fewer longer molecules, they tended to exhibit lower melt strength and reduced bubble stability. The more challenging processability and lower output rates have impeded the uptake of these products in some applications.
The second generation of metallocene resins was designed to compete with LDPE more effectively by addressing the processability issues. To improve processability, these resins have a broader molecular weight distribution and contain low levels of long chain branches. While these modifications improve processability, they also reduce the toughness and sealing performance.
The recently introduced third generation metallocene catalyst addresses the technical challenges faced by earlier generations and provides excellent performance characteristics and easier processability. This improvement is achieved through tailoring the short chain branching distribution by inserting branching preferentially in the high molecular weight chains. This dramatically increases the number of tie chain molecules available for holding the structure together, providing toughness. At the same time, the MWD for the third generation mLLDPE is broader than for first generation mLLDPE and more like conventional LLDPE. The result is a resin family with optimized processability and exceptional toughness.
