Materials & Resins # 1998 Gothenburg
Some years ago the polyolefin research and development was quite predictable and had to some extent the characteristics of a mature business. Today, it is one of the most interesting - and dynamic areas in the whole plastics industry. The reason for the excitement is mainly based on the development of new possibilities to better design the polymer molecules and as a consequence better utilise its inherent physical properties.
The major areas of improvement come from (I) the possibilities to tailor-make the molecular weight distribution (MWD) through a bimodal process and (2) a better way to make more defined polyolefins through the use of a new generation of catalysts - metallocenes.
If two reactors are combined in series. it is possible to obtain a polymer resin that exhibits a bimodal MWD. In this way, the resulting polymeric material combines good processability and high mechanical strength. Through the introduction of the Borstar process, which uses a loop reactor in combination with a gas phase reactor, it has also been posSible to produce completely new materials like bimodal LLD PE.
Another significant breakthrough is the introduction of the single site or metallocene catalysts. The products resulting from such polymerisations exhibit enhanced physical properties than comparable chromium or Ziegler Natta produced material.
The "molecular engineer" has by this catalyst family obtained a new tool to better design the polyolefin molecules. The polymerisation results in the most probable MWD. which means that heterogeneity index MwIMn equals 2. This should be compared with much broader MWD's for polyolefins prepared with other catalysts. The incorporation of copolymers is also appearing in a much more orderly fashion, which again gives improved physical properties. In the case of PP, very high levels of isotacticity and syndiotacticity are achievable.