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

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Unlike an extruder, the shafts and paddles of a kneader reactor do not come into contact with the barrel, instead being supported by bearings at both ends. The ratio of length to diameter is 10–25 times less than that of an extruder. Also unlike an extruder, the kneader is characterized by gentle conveying with low shear rates (100 s^–1).

The kneader processor overlaps in function with both the stirred tank and the extruder, yet is very different from both (see table on page 32). Understanding these differences helps us to see how kneaders can best meet the needs of the polymer industry.

Kneader as reactor

The table shows that the stirred-tank reactor and the kneader processor are competitors for the reaction step. Both are capable of processing large volumes of polymers with similar residence times (longer than ten minutes). The great advantage of the kneader processor, however, is its ability to improve environmental performance by eliminating solvents from the reaction step. This is especially important for biopolymers such as polylactic acid (PLA).

The kneader processor acts as a series of self-cleaning mixing chambers. The self-cleaning mechanism can be based on the use of either one or two shafts:

single-shaft with static counter-hooks; or

two intermeshing shafts.

The number of mixing chambers forms part of the process specification. Reduction in foaming and the control of free-radical based polymerizations are typical requirements to be addressed by careful specification of the number of mixing chambers.

The kneader reactor is an ideal technology for producing free-radical based copolymers in the concentrated phase. The bulk reaction is controlled by evaporative cooling. A typical example of this application is acrylates, for which there is a world-scale reference plant based on this technology. Another example is reactive block copolymerization, which uses controlled radical polymerization to tailor polymer properties.

Biopolymers can also be processed in a kneader reactor. PLA and polysuccinimide are examples of how solvents can be eliminated from the reaction step during biopolymer manufacture. Monomer conversions above 97 percent are possible.

List is currently investigating bulk polymerization of other products in a kneader reactor.

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