Process Economics Program Report 19E
Bimodal Linear Low-Density Polyethylene
Published: June 1996
Poor processability of first-generation linear low-density polyethylene (LLDPE) has prevented full LLDPE penetration of high-pressure low-density polyethylene (LDPE) markets. In recent years, however, significant catalyst and process developments have improved the processability of PEs by making resins with broad bimodal molecular weight distributions.
In 1994, Union Carbide developed a new LLDPE catalyst and has built a 660 million lb/yr (300,000 t/y) two-reactor cascade gas-phase plant to make bimodal LLDPE and high-density PE (HDPE). In 1995, Borealis introduced its new supercritical slurry PE technology, Borstar®, which combines a loop reactor and a gas-phase reactor operating in series with a specialized proprietary catalyst to make bimodal high molecular weight HDPE and LLDPE. In 1996, Borealis employed the Borstar® technology in a new 265 million lb/yr (120,000 t/yr) plant in Porvoo, Finland. Other polyolefins producers such as Quantum (now Equistar) and Mobil have also developed mixed metallocene/Ziegler-Natta catalyst systems that are claimed to produce LLDPE resins in a single reactor system with molecular weight distribution similar to those produced in a dual-reactor system. Bimodal LLDPE is targeted to compete with high-pressure LDPE and with bimodal HDPE in thin film applications.
This report reviews the process technologies for manufacturing bimodal LLDPE products and their production economics. We evaluate UNIPOL II® technology, Borstar® technology, and a mixed catalyst technology. In addition to the process evaluations, we present summaries of patents relating to the processes we evaluate, and of the current status of the bimodal LLDPE industry.
Our estimates show that for the same capacity, a facility using Borstar® technology will require higher capital investment than a plant using either UNIPOL II®-type technology or a bimetallic catalyst system in one reactor. Bimodal LLDPE resins made in the UNIPOL II®- type and Borstar® facilities have similar net production costs, whereas resins made in plants using a metallocene/Ziegler-Natta catalyst system in a gas-phase reactor have the highest net production cost.