Bimodal

Occasionally, we come across the term ‘bimodal’ in the technical datasheets or marketing introductions of certain polyolefin resins. Bimodal represents a distinctive method in production technology, and materials with bimodal characteristics possess unique attributes that position them as advanced solutions for specific applications. In this article, let’s delve into bimodal materials and explore the processes behind their creation.

Bimodal HDPE

High-Density Polyethylene (HDPE) is the most widely used polyethylene with market value is USD 66.23 billion in 2022 and is poised to grow at a CAGR of 4.77% from 2023-2029.

HDPE has a lower branching degree than low-density polyethylene, resulting in a stronger intermolecular force and tensile strength. The short-chain branching, incorporated by comonomers such as 1-butene or 1-hexene at low concentrations, play a crucial role in controlling density (ranges from 0.93 g/cm3 to 0.97 g/cm3) and enhancing the flexibility and impact strength of HDPE resin. Because the strength difference surpasses the density difference, HDPE has a higher specific strength, helping to produce light weight product and packaging. In general, HDPE resin has low moisture absorption, chemical & corrosion resistance, excellent tensile strength, and strong impact resistance characteristics. Due to these properties, it is used in various applications and industries which increases the market growth.

HDPE can be divided into three major types:

• Monomodal HDPE produced with Ziegler catalysts.
• Monomodal, broad molecular distribution HDPE produced with chrome catalysts.
• Bimodal HDPE produced with Ziegler catalysts.

In contrast to monomodal HDPE (or unimodal HDPE), which involves having a unique mode per reactor to produce either low-molecular-weight (LMW) or high-molecular-weight (HMW) resin, bimodal HDPE combines both LMW and HMW in a single reactor. LMW chains improve processability and HMW fraction is required to get good mechanical properties [Figure 1]. By this combination, bimodal HDPE exhibits a balance of processability, mechanical properties, and high stress-crack resistance.

The question arises: why don’t producers simply create two unimodal HDPE resins in each reactor and then mix them at the downstream stage, or blend them at the extruder? These approaches are indeed viable options and have been implemented in various production modes, as well as in end-users’ processing. However, intermolecular analysis underscores the advance of bimodal HDPE, as measured by polydispersity (Mw/Mn), homogeneity, and short chain branching distribution, surpassing other types. Practical results also prove the superior mechanical properties of bimodal HDPE in specific applications. In this article, due to limited space, we intentionally avoid going deeper into molecular structure, as it may be considered too academic.

The pioneering work on bimodal HDPE resins dates back to the 1980s, with key contributions from Oxychem (Nissan), Dow (Asahi), and Hoechst Celanese (Hoechst). ExxonMobil later licensed bimodal slurry technology from Mitsui. Today, a significant portion of the demand growth for HDPE is attributed to bimodal HDPE products, which possess exceptional high performance:

Bimodal HDPE film: excellent bubble stability at high extrusion rates and excellent film toughness as evidenced by high dart impact strength. Bimodal HDPE blow molding: high ESCR at very high stiffness, which allows the thin-walling and light-weighting of bottles. Bottles can also be produced in different shapes, sizes, and attractive colors for brand recognition. Bimodal HDPE pipe: meet PE100 performance with respect to minimum required strength (MRS), slow crack growth resistance (SCGR), and rapid crack propagation (RCP) requirements. Increase extrusion rates and remarkable low-sag performance in large diameter pipes of up to 2 meters in diameter [Figure 2]

Bimodal HDPE Technologies

Bimodal HDPE could be produced by two methods: by multiple reactors operating in series or by single reactors with advanced binary catalyst.

The first method normally utilizes 2 reactors operating in series. In the first reactor, high amounts of hydrogen are fed with ethylene. This process leads to the formation of low-molar-mass polyethylene. The second reactor is loaded with much less hydrogen to form a high- or very high-molar-mass polyethylene. This process allows for further essential product modification, specifically the incorporation of comonomer in the long polymer chains within the second reactor. There is also a reversed mode wherein the high-molar-mass component is produced firstly, followed by a low-molar-mass component. These products build up a polymer alloy in solid state with crystalline and amorphous regions in between. The crystalline regions are mainly formed by low-molar-mass homo-polyethylene. The high-molar-mass copolymers form the amorphous regions and act as tie molecules that connect the crystal lamellae. These tie molecules that possess short-chain branching (SCB) effectively hinder the pullout of the polymer chains from the crystallites. In this manner, the strength and resistance to slow crack growth of the resin are greatly improved finally. [Figure 3]

The second method, known as the single-reactor bimodal process, was developed by Univation for its Unipol gas phase process. This technology relies on an advanced engineered catalyst (a mixed catalyst, indeed), providing the capability to produce bimodal molecular weight distribution HDPE grades in a single gas phase reactor. This approach is estimated to result in a 40% reduction in investment costs compared to alternative multiple reactor systems. [Figure 4]

Current commercialized bimodal HDPE processes

Bimodal Polypropylene

The same definition of bimodal can also be applied to bimodal polypropylene, although it is less well-known than bimodal HDPE. For many years, polypropylene process development has focused on bimodal technology to improve processability (in high-speed production line) while maintaining outstanding mechanical properties. Bimodality has been achieved by operating two reactors in series at different operating conditions (i.e., in the presence and absence of hydrogen), resulting in a polymer with two performance characteristics combined on an “intraparticle” level [Figure 5]. In some processes (e.g., Spherizone), bimodality involves one single reactor operating different conditions within each zone, resulting in the blending of multiple properties on a macromolecular level, continues to emerge as the new benchmark for polypropylene production.