Chemistry with a clear advantage

Automotive glazing is a growing market for polycarbonate.

Through development work in the USA and Netherlands, Shell technologists have found a new route to one of the key intermediates needed to make polycarbonate that avoids the use of phosgene – a toxic chemical - and which is also expected to have significant advantages in terms of cost and CO2 footprint.

Polycarbonate (PC) is the largest volume engineering thermoplastic and has enjoyed strong demand growth, thanks to performance characteristics that include strength, light weight, thermal stability and excellent polymer clarity.

The main applications for PC are optical media, electronics and sheeting/film. Automotive glazing also has strong growth potential because PC offers important weight savings over glass, as well as more design options and easy handling within automotive production processes.

Despite the size and potential of the market, there are some significant issues related to the sustainability of current production processes.

Most PC production has been based on reacting phosgene with bisphenol A (BPA), but this chemistry is inefficient and has significant environmental drawbacks. Phosgene is highly toxic and so requires stringent exposure management and controls, while the process involved is also complex and energy intensive.

Increasing environmental restrictions have added impetus to the search for safer and more sustainable technology.

The development of different processes, where phosgene is replaced with diphenyl carbonate (DPC), has been the key to this. However, while these alternative processes eliminate phosgene from the initial polymer synthesis, many producers still rely on the use of phosgene for making the DPC, via reaction with phenol.

A new process developed by Shell offers a route to DPC which is phosgene-free, and 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.

“As well as the fact that there is no phosgene involved, the reaction conditions are relatively mild and so can take place in one continuous process, which translates into a significant economic advantage,” says Garo Vaporciyan, a Principal Scientist with Shell Global Solutions.

“Thanks to efficient catalysts we are also able to achieve high conversion rates in a single pass, with over 99% selectivity and impressive yields.”

This feature was added to the Innovations section in June 2010