On the Degradation and Stabilization of Poly(Methyl Methacrylate) in a Continuous Process By Philip Nising, Thomas Zeilmann, and Thierry Meyer* Thermally unstable polymers such as poly(methyl methacrylate) are degraded considerably during industrial processing. This degradationanditsreductiontoaminimumhavebeeninvestigatedinbothlabandcontinuouspilot-scaleexperiments.Athree- step degradation mechanism, starting at 180C, was proved by Thermogravimetrical Analysis (TGA) and a kinetic approach to describe it was derived. The knowledge of this degradation behavior was then applied to a pilot-scale process with a production rate of 10 kg/h and the process yield loss during the devolatilization step was investigated. Using heat stabilizers, the overall processyieldcouldbeimprovedby10%,whereastheresidualorganicvolatilesconcentration(VOC)wasdrasticallyreducedto values below 1000 ppm. In order to preserve the molecular weight of the final product these stabilizers were added into the process, separately, at the end of the polymerization reaction but before the devolatilization step. 1 Introduction With the increasing demand for lower molecular weight poly(methyl methacrylate) (PMMA) for extrusion and injec- tion molding, continuous polymerization processes become more and more important for its production. Solution polymerization processes are required in order to ensure a pumpable mass flow in the plant. At the end of the process, solvents and residual monomer must be removed from the polymermeltinadevolatilizationstep.Thisisusuallyrealized by heating up the solution followed by a strong pressure drop (flash), thus vaporizing the volatile fraction. Until now, since PMMA has a rather poor thermal stability, this last process steplimitstheoverallyieldsignificantlybydegradingpolymer chains back, mainly, into monomer. For sufficient devolatilization, the minimum preheating temperature, depending on the reactor setup, was determined bypilot-scaleexperimentstobe280C.Atthistemperature,a yield loss of 10 % is observed that can no longer be neglected in continuous production. This yield loss is temperature dependent,with4%at240Candgoingupto12%at300C. It is therefore necessary to first investigate the degradation behavior of unstabilized PMMA at temperatures similar to the one in the production process as well as to find strategies for stabilizing the polymer without changing its product properties. 2 Non-oxidative Degradation of PMMA Poly(methyl methacrylate) degrades back into monomer above180Cinthreedifferentstepsbyanunzippingreaction. The monomer yield for PMMA can be up to 90 % [1]. A typical degradation curve measured by TGA is presented in Fig. 1, where the different degradation steps are clearly visible. 0 10 20 30 40 50 60 70 80 90 100 0 50 100 150 200 250 300 350 400 450 temperature [ºC] relative mass loss H - H unsaturated end-groups random chain scission I II III Figure 1. TGA curve (0.5 C/min) for non-commercial PMMA from a batch experiment, radically polymerized at 130C with 30 mmol/L DBPO. As already shown by [2] and verified by our own measurements, these three steps are due to three different types of bonds within the polymer chains, formed by different reactions during radical polymerization, Fig. 2: l Regular polymer backbone bonds (r-c), zone III l Unsaturated end groups (d-b), zone II l Head-to-Head-bonds (h-h), zone I The polymer chain propagation consists of the addition of one monomer unit to the growing macroradical, in general by a ªhead-to-tailº mechanism. This means that, stereochemi- cally,themonomerisalwaysboundtothepolymerchainbyits least substituted carbon. Thus, the relatively big -COOMe group is always ahead of the growing chain. This ªhead-to- tailº propagation leads to a uniform backbone chain with C-C bonds that are thermally stable up to 300C. Above this, they break up statistically, known as random chain scission. The unsaturated end groups and the head-to-head bonds are formed mainly by termination reactions. Apart from termina- tion by other radical species, two active polymer chains can also terminate themselves in two different ways: either by disproportionation, leading to a double bond in one chains Chem. Eng. Technol. 26 (2003) 5, Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0930-7516/03/0505-0599 $ 17.50+.50/0 599 ± [*] Dipl.-Ing.P.Nising, Dr.T.Zeilmann, Dr.T.Meyer (author to whom correspondence should be addressed, e-mail: Thierry.Meyer@epfl.ch), Swiss Federal Institute of Technology, Institute of Chemical and Biological Process Science, Polymer Reaction Engineering Unit, SB- ISP-UPRE, CH-1015 Lausanne, Switzerland. 0930-7516/03/0505-0599 $ 17.50+.50/0 Full Paper