Polypropylene Process by The Dow Chemical Co

To produce homopolymer, random copolymer and impact copolymer polypropylene using the Dow gas-phase UNIPOL PP process.

Polypropylene Process by The Dow Chemical Co

A wide range of polypropylene is made in a gas-phase, fluidized-bed reactor using proprietary catalysts. Melt index, isotactic level and molecular weight distribution are controlled by utilizing the proper catalyst, adjusting operating conditions and adding molecular-weight control agents. Random copolymers are produced by adding ethylene
or butene to the reactor. Ethylene addition to a second reactor in series is used to produce the rubber phase of impact copolymers.

The UNIPOL PP process’ simple yet capable design results in low investment and operating costs, low environmental impact, minimal potential fire and explosion hazards, and easy operation and maintenance. To produce homopolymers and random copolymers, gaseous propylene, comonomer and catalyst are fed to a reactor (1) containing a fluidizedbed of growing polymer particles and operating near 35 kg/cm2 and approximately 70°C. A conventional, single-stage, centrifugal compressor (2) circulates the reaction gas, which fluidizes the reaction bed, provides raw materials for the polymerization reaction and removes the heat of the reaction from the bed. Circulating gas is cooled in a conventional heat exchanger (3). Granular product flows intermittently into product discharge tanks (4); unreacted gas is separated from the product and returned to the reactor.

To make impact copolymers, the polypropylene resin formed in the first reactor (1) is transferred into the second reactor (5). Gaseous propylene and ethylene, with no additional catalyst, are fed into the second reactor to produce the polymeric rubber phase within the existing polypropylene particles. The second reactor operates in the same manner as the initial reactor, but at approximately half the pressure, with a centrifugal compressor (6) circulating gas through a heat exchanger (7) and back to the fluid-bed reactor. Polypropylene product is removed by product discharge tanks (8) and unreacted gas is returned to the reactor.

Hydrocarbons remaining in the product are removed by purging with nitrogen. Granular products are pelletized in systems available from multiple vendors (9). Dow has ongoing development programs with these suppliers to optimize their systems for UNIPOL PP resins, guaranteeing low energy input and high product quality. Controlled rheology, high melt-flow grades are produced in the pelleting system through the addition of selected peroxides.

Products: Homopolymers can be produced with melt flows from less than 0.1 to 3,000 dg/min and isotactic content in excess of 99%. Random copolymers can be produced with up to 12 wt% ethylene or up to 21 wt% butene over a wide melt flow range (< 0.1 to > 100 dg/min).

A full range of impact copolymers can be polymerized with excellent stiffness for even the most demanding applications. Products from narrow to broad molecular-weight distribution can be manufactured in grades with proven advantage for film injection, molding, blow molding, extrusion and textile applications.

Proprietary catalyst and donor systems: The Dow Chemical Co. manufactures and provides a family of polypropylene catalysts (SHAC catalyst) and a family of external electron donors (SHAC ADT) for use in the UNIPOL PP process. SHAC catalyst offers high catalyst activity (up to 35 kg/g-hr using conventional external donor and up to 60 kg/g-hr using
a SHAC ADT) and competitive polymer properties. SHAC ADT performs three functions:

1) Controlling polymer isoctaticity similar to conventional external donors, such as D donor or C donor
2) Further improving process continuity by preventing agglomeration of polymer particles
3) Enhancing polymer properties.

The combination of SHAC catalyst and SHAC ADT enables Dow’s catalyst systems to achieve low cost and easy production of polypropylene polymer with superior properties. The exceptional performance of the SHAC catalyst systems also finds applications in other PP processes.

Licensor: The Dow Chemical Co. Univation Technologies is the licensor of the UNIPOL PP process.

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