(555g) Continuous High-Temperature Polymerization of Mma at Pilot Scale

In the scope of this work, a continuous high-temperature polymerization process for methyl(methacrylate) (MMA) copolymers is developed at pilot scale. At a first instance, the kinetic properties of the reaction system for the given temperature range were determined. This includes thermal initiation by MMA-peroxides, depropagation of the living chains as well as a high temperature gel effect. All different aspects were included in a model for the description of the continuous process, allowing the prediction of conversion evolution and molecular weight distributions.

As many other unsaturated compounds, MMA tends to form peroxides with physically dissolved oxygen from air. The latter is usually present in large amounts during monomer storage since it is indispensable for its stabilization, which is commonly achieved by addition of hydroquinone derivatives. The MMA peroxides have a polymeric character and accumulate in the monomer during storage and heating cycles. At temperatures above 100°C, they decompose quickly into radicals and initiate polymerization. The formation and decomposition of MMA peroxides is determined and included in the model.

The ceiling temperature, i.e. the thermodynamic equilibrium between propagation and depropagation, for MMA is with approximately 220°C quite low, causing a significant limitation of monomer conversion at reaction temperatures above 150°C. This effect is included in the model using an empiric expression.

Finally, the existing gel effect models are all not suitable for the description of a high-temperature gel effect in a continuous polymerization being regulated by chain transfer agent, since most of them were derived for below glass transition temperature ranges or use initial initiator / chain transfer agent concentrations, which for example in a CSTR does not make sense. For these reasons, a new gel effect model is derived which fulfils the prerequisites for the modeling of the investigated process.

For model verification, a pilot plant with a maximum capacity of 10kg/h is used to obtain experimental data. The reactor is built from tubes equipped with Sulzer Chemtech static mixing elements and consists of two parts: a loop with high recycle ratio (CSTR mode) and a tubular part with plug flow conditions. For inline conversion measurement, the reactor contains two ultrasound probes calibrated for the reaction system. Additional conversion data is available by offline Headspace-GC. Molecular weight data is obtained by offline triple detection size exclusion chromatography. The final polymer is granulated for further analysis.

First runs show good agreement between model and experiment. Stepwise increase of the reaction temperature will reveal the change in product properties as well as reaction parameters such as necessary initiator and chain transfer agent concentration to achieve desired conversion and molecular weight distribution.