Improved PP Impact Copolymers With High Molecular Weight Ethylene-Propylene Rubber

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Publication Date: 04-Dec-2009
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Abstract

This paper describes an improvement of the stiffness/impact balance of polypropylene impact copolymers using high molecular weight ethylene-propylene rubber. The polypropylene injection molding market is looking for improved processing speed whilst maintaining mechanical properties. Until now it was not possible to improve processability and keep the stiffness/impact balance the same. According to the prior art and the current industry practice, the molecular weight of the rubber needs to be in a certain ratio to the molecular weight of the matrix in order to obtain good product properties. Unexpectedly it was found that by significantly exceeding this ratio, improved properties are being obtained.

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English (United States)

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United States

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IMPROVED PP IMPACT COPOLYMERS WITH HIGH MOLECULAR WEIGHT ETHYLENE-PROPYLENE RUBBER

This paper describes an improvement of the stiffness/impact balance of polypropylene impact copolymers using high molecular weight ethylene-propylene rubber. The polypropylene injection molding market is looking for improved processing speed whilst maintaining mechanical properties. Until now it was not possible to improve processability and keep the stiffness/impact balance the same. According to the prior art and the current industry practice, the molecular weight of the rubber needs to be in a certain ratio to the molecular weight of the matrix in order to obtain good product properties. Unexpectedly it was found that by significantly exceeding this ratio, improved properties are being obtained.

The technology can be employed in 3 ways:
1. Improve polymer processability at constant stiffness/impact
2. Improve stiffness and Heat Distortion Temperature (HDT) at constant processability and impact
3. Improve impact at constant processability and stiffness

Market opportunities exist for all three fields.

This technology relates to conventional polypropylene impact copolymer technology like bulk or gas phase processes and makes use of conventional catalysts. The rubber molecular weight is controlled by carefully controlling the hydrogen concentration in the reactor where the rubber is made. Excess hydrogen in the reactor feed streams can be removed by stripping, hydrogenation, osmosis or other means. Conditions in the reactor like temperature, pressure and residence time are chosen such that the desired amount of rubber at high rubber molecular weight is obtained. The rubber molecular weight is measured as the viscosity of a rubber solution in decalin at 135°C and expressed as an Intrinsic Viscosity (IV) number (according to ISO1628-1). Conventional polypropylene impact copolymers have IV numbers ranging from 1.5 - 3.5dl/g. The IV numbers of the impact copolymers related to the current publication are > 3.5dl/g.

Other technologies used to improve stiffness/impact include the use of alternative rubbers like ethylene/octene or ethylene/butene. Such solutions however carry considerably more cost. The concept of high rubber molecular weight to improve stiffness/impact balance is unique in the industry. This technology allows improved properties using existing equipment and monomers and hence provides a much lower cost solution to the problem. The key step in this technology is the control of hydrogen concentration prior to making the rubber part of the polymer. This technology is useful over a broad range of MFR's (ISO1133, 230°C/21.6kg), ranging from 0.5-100 g/10min, more specifically ranging from 1-50g/10min. The optimum rubber IV is > 3.5dl/g more preferably > 5dl/g.

A detailed example is shown here. A miniplant batch reactor is being fed with monomer and catalyst and polypropylene homopolymer is produced with...

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