(215f) Process Developments in Continuous Silicone-Polyether Copolymers

Authors: 
Kundan, A., Momentive Performance Materials Inc.
Bahr, S., Momentive Performance Materials Inc.
Silicone-polyether copolymer (SPC) is a term used to describe a generic chemical entity obtained by the combination of a methyl siloxy moiety-containing chemical entity with a minimum of one other chemical entity. The other chemical entity could be a polyether, an alkyl olefinically unsaturated group that may or may not contain another chemical functionality or a combination of such entities. The wide range of applications for SPC includes the lowering of surface tension for agricultural adjuvants, acting as stabilizers for polyurethane foams, additives for coating applications, antifoams and emulsifiers. These copolymers are produced via hydrosilation/hydrosilylation which is the reaction of a hydrosilatable olefin, such as allyl terminated polyalkyleneoxides, with hydrosiloxanes, such as poly(dimethyl)(methylhydrogen) siloxanes in the presence of an appropriate noble metal catalyst. Due to its wide range of applications, the total volume of SPC manufactured worldwide has been estimated to be in the excess of 100 million pounds per year. Due to a steady increase in worldwide demand over the last 15 years, efficient manufacture of such copolymers is desired for two primary reasons: 1) lower cost and 2) less waste. Continuous systems are much smaller than batch reactor systems, are less costly, contain less product, are easier to clean, generate less waste and exhibit higher efficiencies. As a result, a paradigm shift has occurred in process innovation and development over the last decade or so, with an increasing emphasis on the development of continuous or semi-continuous manufacturing processes over traditional batch processes. A review of these continuous processes will be discussed.

There are two primary types of continuous reactors available for SPC formation: continuous stirred tank reactors (CSTR) and plug flow reactors (PFR). In a CSTR, the probability of total uniform consumption of all the reactants is low. A SPC fluid containing unreacted hydrogen siloxane or having non-uniform consumption of reactants is unacceptable for making certain polyurethane foams. In a PFR, phase-separation of the immiscible reactants occurs after initial mixing, slowing down the rate of the reaction. Hence, neither the CSTR nor the PFR is sufficient alone to manufacture all classes of SPC continuously. However, a combination of both provides surprisingly effective results by driving the reaction to completion with acceptably uniform consumption and without undergoing phase separation. US Patent No. 5,986,022 discloses a process for the continuous manufacture of SPC that has at least one CSTR in the series and is continuously fed into at least one PFR from which product is withdrawn. The olefinic compound and the hydrogen siloxane in the end stream of the CSTR series are reacted to an extent that it is homogenous and does not phase separately going into the PFR. This process is known as a continuous copolymer unit (CCU). Another process invention is the multi-stage mixer (MSM) disclosed in US Patent No. 6,897,280 B2. This patent discloses continuous production of SPC in a tubular reactor equipped with stirring blades in a multi-stage environment and plug-flow maintaining device. Here, the reactants and the catalyst are continuously fed to the entry of the multi-stage blade-mixed plug flow reactor. The product is continuously withdrawn from the exit of the reactor and the stream and is comprised of SPC that is substantially free of unreacted hydrogen siloxane, provided that the residence time and reaction temperatures are sufficient for reaction completion. US Patent No. 6,410,772 B2 discloses an approach similar to MSM technology but describes separate tubular reactors for static plug flow and mixed blade flow. Additional patents relevant to the continuous manufacture of SPC over the last 15 years will also be discussed.

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