Polypropylene

When Ziegler polyethylene was first revealed (1954), there was much speculation that the high pressure polyethylene plant would become a thing of the past. It did not turn out that way.

There are at least two reasons why the high pressure plants are in the market. First, the two polymers have important property differences, and second, the new processes, although they appeared at first glance to be cheaper, turned out to be more expensive. The principal reason for the higher cost is that the polymer had to be purified in expensive facilities to remove catalyst residues and low molecular weight polymers.

The first important change in this situation was brought about by Phillips Petroleum, when they introduced their "particle form" process. In the original Phillips process, polymer is formed at 140°C in solution; then the solution is centrifuged to remove the catalyst (chromium oxide on a silicaalumina support). The "particle form" process was made possible by the development of new, more active catalysts, enabling the reaction to be carried out at lower temperatures so that the polymer pre- cipitates as it is formed. The catalyst is used in such small amounts that it does not have to be removed from the product.

In 1967, reports appeared that Solvay had developed a new process based on superactive Ziegler catalyst of high productivity. The high productivity appears to result from depositing the active catalyst ingredients in a thin layer over the relatively large surface of a solid support, such as silica gel. As in the "particle form" process, catalyst residues remain in the product, and the plant simplifications considerably improve the manufacturing economics.

Recently, Union Carbide introduced a vapor-phase process for high density polyethylene, In this process the catalyst also remains in the product.

These second-generation high density polyethylene processes provide a significant step in the direction of eliminating the cost difference between low and high density material.

Now, a similar development is taking place in polypropylene manufacture. From trade journal and patent information, three companies appear to have reached commercial production scale with new, simplified processes: Mitsubishi Petrochemical, Solvay, and BASF.