The first commercial production of polypropylene was in the 1950s following the discovery of Ziegler-Natta catalysts. It was produced initially in a slurry process where propylene is reacted in the presence of a catalyst in solvent to produce a mixture of crystalline and amorphous polymer. The disadvantage of this route was that the catalyst had to be treated with alcohol to deactivate and extract it, while unwanted atactic polymer had to be extracted and removed.
Bulk technologies were then developed in which the solvent was replaced by liquid propylene. In addition, considerable efforts were made in improving the activity of the catalysts. One particular development was the commercialisation of the Spheripol process by Himont (now licensed by Basell) in 1982. This process combines bulk phase polymerisation in tubular loop reactors with gas phase polymerisation and has become the most dominant licensed process. More recently, Borealis has taken a similar route with the adaptation of its Borstar bimodal PE process to make PP.
The higher activity catalysts also enabled the introduction of gas phase technology. Although the gas phase route was first commercialised in 1967 by BASF (this process called Novolen is now owned by ABB Lummus Global and Equistar), it did not come into wide scale use until Union Carbide, and later others, offered gas phase technology for licensing in 1983. Advantages of the gas phase route include operating with lower purity propylene and low production costs.
Much development work is presently directed at introducing metallocene catalysts into the PP process to improve the properties of the resins with commercial quantities now becoming available from a number of producers. However, the commercialisation of such resins has been slow and an agreement between Basell and ExxonMobil Chemical to accelerate developments in metallocene PP technology should increase the rate these products are introduced.
A recent advancement in reactor technology has been the development of a fluid bed, multi-zone circulating reactor (MZCR) by Basell. Called Spherizone, the process has been retrofitted into an existing Spheripol PP plant at Brindisi, Italy. The Spherizone circulating reactor has two interconnected zones. In one, the riser, there is fast fluidisation while in the other, the downer, features a slower packed bed mode. The two zones can generate different materials and extend the range of PP properties.
Polypropylene
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Polypropylene
Uses and Outlook
Polypropylene's (PP) intrinsic properties of high stiffness, good tensile strength and inertness toward acids, alkalis and solvents, plus its cost advantage over other thermoplastic polymers, has secured its position in a wide range of consumer and industrial products, manufactured by several high-volume forming methods. In addition, its low density and cost advantage over other thermoplastic polymers make it suitable for applications that are weight and cost conscious.
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