Shell nears phosgene-free feedstock link-up with polycarbonate major

Amsterdam - Shell is “very, very close” to commercialising a technology for the production of diphenyl carbonate (DPC) as a feedstock for the production of polycarbonate, according to Ben van Beurden, executive vice president, Shell Chemicals.

Over recent years, the Dutch-based company has been in talks with major manufacturers of the engineering polymer, for whom DPC offers a potential replacement to phosgene - a highly toxic feedstock precursor.

Production of polycarbonate is, today, predominantly based on reacting phosgene with bisphenol A (BPA) to produce the polymer - so-called interfacial or solution polycarbonate.

Due to its toxicity, though, phosgene requires stringent exposure management and controls. The phosgene-based process, said Shell, is also complex and energy intensive, involving the use of carbon monoxide, caustic soda and a chlorinated solvent, which has its own health and safety issues.

During the reaction process the chlorine is converted to sodium chloride, which then has to be removed from the finished polymer in an additional washing stage in order to achieve the material’s optical clarity. Disposal of the waste salt presents another issue.

These issues have led polycarbonate manufacturers to develop phosgene-free routes to DPC. However, according to Shell, these are still highly capital- and energy-intensive.

Polycarbonate made by reacting DPC with the BPA is called melt-PC, because the process does not require a solvent and is performed neat - in the melt. Although the melt-PC process eliminates phosgene, chlorine and solvents from the polymer production, most producers still use phosgene in the production of DPC, via reaction with phenol.

Shell’s process is based on a multi-stage reaction involving carbon dioxide, phenol and either propylene oxide (PO) or ethylene oxide (EO). It produces DPC, as well as a glycol co-product, using a high-yield catalyst, derived from the company’s expertise in glycol production chemistry.

The feedstocks involved in the process, as well as the glycol co-products, are already produced by Shell. The conversion of epoxide to glycol at the heart of the process is similar to reactions in its MPG (mono propylene glycol) and MEG (mono ethylene glycol) manufacturing operations, the company points out.

As a C6 technology, DPC offers particular business attractions for Shell, which is the world’s largest producers of styrene - the main outlet for benzene in its production system, according to van Beurden.

“We have developed technology that will go without phosgene and offers a lower operating- and capital-cost structure,” van Beurden said a recent press conference at the Shell Technology Centre in Amsterdam.

“We are getting to the point where we don’t want any more styrene [which] can go through difficult periods in the economic cyclic. So we are looking for alternatives,” he added. “Polycarbonate is quite a strongly growing sector, and we need to participate in it to claim our space in the supply chain.”