The List Kneader Processor Is a Unique Solution for Working in the Concentrated Phase

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Kneader as devolatilizer

When it comes to devolatilization, on the other hand, kneader processors provide serious competition for extruders.

The main issue is the short residence times (two minutes or less) that characterize extruders. To devolatilize the polymer mixture effectively, the extruder needs to renew the polymer surface three times within this short time span. Such intensive surface renewal places stringent requirements on the configuration of the extruder screws.

To ensure good energy transfer and mixing in the short residence times available, the extruder needs to generate the correct pressure profile along its length. This in turn requires very specific configuration of the mixing elements. The result is that for each polymer grade, the extruder needs a particular shaft geometry to create the correct pressure profile. EPDM rubber grades span Mooney viscosities of 20 to 100. Processing all these grades may require several different extruder configurations, and even then the turndown available from any one machine is generally poor.

Another issue with extruders is that contact between the screws and the barrel can contaminate the polymer with metallic particles; the kneader avoids this problem.

In early 2000 the kneader processor started to emerge as a serious contender for polymer devolatilization. The benefits:

• no metal-to-metal contact, so no metallic contamination;

• one unique shaft geometry covers Mooney viscosities of 20–100;

• low shear eliminates the risk of product damage, especially for highly unsaturated polymers;

• large turndown ratios.

The kneader processor is now well established for the devolatilization of metallocene-based elastomers. Following a final flash stage, the kneader re-heats the polymer and devolatilizes it further.

This technology now has been further developed for temperature-sensitive polymers such as BR, SBR, SBS, SIS, styrene-based elastomers, butyl rubber, halobutyl rubbers, and polyisoprene.

Water injection cools the melt, decreases the vapor pressure of volatiles in the gas phase, and strips the volatiles out of the melt.

The polymer solution delivered from a stirred-tank reactor is first evaporated in a kneader known as the main evaporator. This kneader uses mechanical energy at low shear rates to remove most of the solvent. The polymer is then transferred to a second machine, the kneader devolatilizer, to remove the remaining solvent and monomers. If a kneader reactor is used to carry out the polymerization, the main evaporator is not necessary; instead, polymer from the kneader reactor passes straight to the kneader devolatilizer to remove the remaining small amount of unconverted monomers.

* The author is an employee of LIST AG, Arisdorf/Switzerland.

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