Page 135 - CW E-Magazine (12-11-2024)

Point of View

India awaits its fi rst polycarbonate plant, but its no

longer a speciality polymer

Polycarbonate (PC) is a high quality, engineering plastic with a unique combination of properties including strength, lightness, durability,
high transparency, and heat resistance. It is easily processed and found in several products from essential medical devices, electronics,
computer housings, cars, buildings,and consumer goods. It was once extensively used for optical data storage,but that has all but vanished.

PC is an indispensable material to the medical industry; its exceptional transparency, comparable to glass, enable healthcare
professionals to easily observe procedures and monitor patient conditions.Moreover, the polymer can withstand temperatures
up to 135°C without deformation, meeting rigorous sterilization needs.

Manufacturing technology
PC was till recently produced by several variations of ‘interfacial’ technology, in which the alkali salt of Bisphenol-A (BPA) in aqueous
solution is phosgenated in the presence of an inert solvent. While this is a mature technology that delivers high-quality product, it needs
highly hazardous phosgene.

The leading producers of PC have considerable expertise in handling this toxic gas in a safe manner, but pressures to deliver less
hazardous routes led to the development of trans-esterification (or melt) processes that instead use diphenyl carbonate (DPC). Several
leading PC producers including SABIC Innovative Plastics, Covestro, Asahi and Mitsubishi Chemical/Mitsubishi Gas Chemical have all
independently developed non-phosgene PC technologies and are using these processes (though not exclusively) for making PC. While
there are some engineering design differences among the various technologies, the significant distinguishing features are the means
employed to make DPC and its precursors. Some routes require ethylene oxide, which poses its own set of safety challenges.

A few PC producers that have developed melt technology have also built ‘hybrid’ plants on the site of their interfacial plants, making
DPC from phosgene and phenol. The reasons for doing so have to do with the ready availability of phosgene and the on-site use of a
by-product (HCl).

The DPC process despite having the need to synthesize two carbonate intermediates still has great financial advantage over the
interfacial one. While the cash costs – raw material, energy, labour – and related factors are similar for the two,capital costs and hence
finance costs (depreciation plus interest on debt to build the plant) are typically higher for the phosgene-based plant due the need for
expensive equipment to protect workers and ensure safety for the area surrounding the plant. Presently, 70-75% of total PC capacity is
based on the interfacial process.

Application trends
The electronics/electricals sector is the leading and fastest growing market for PC currently, driven by growing consumer demand for
communication and entertainment devices, including mobile phones, tablets, personal computers, laptops, flat-screen TVs, etc. Demand
is also being driven by use as sheet/film in construction. For example, PC roof panels are ideal when there is a need for resistance to
weather and flame retardance. These roof panels are 250 times more resistant to shock than glass and 20 times more than polymethyl
methacrylate.
In the automotive industry, PC has made strong inroads into uses such as headlight lenses and as alternative to glass in selected
automotive windows (i.e., small rear side windows). But use in windscreens and panoramic roofs is a potentially huge sector,though
limited as of now due to concerns over scratching, yellowing, and heat intolerance. But recent advancements in design, nanotechnology,
and materials science are allowing car makers to create glazing profiles with never-before-seen capabilities and enabling designers
create distinctive looks.

For several end-uses, PC is blended or alloyed with other resins to tailor price and performance in a process called compounding,
which also uses several additives (e.g., flame retardants, stabilizers, reinforcing agents). Alloys with polybutylene terephthalate (PBT) and
polyethylene terephthalate (PET) take advantage of the good chemical resistance of polyesters. PC/PBT alloy was particularly successful
at one time in the automotive industry for front and rear bumpers,but this application has been taken over by thermoplastic elastomers.

Chemical Weekly November 5, 2024 135

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