Monday, 19 August 2019

Reviewing Key Engineering Plastics – Polycarbonate [incl. Video]




Hello and welcome to this post on reviewing key engineering plastics. Today, we have a closer look at polycarbonate (PC).
Similarly to the last review on the aliphatic polyamides PA 6 and PA 6.6, we review the chemistry including the simplified petrochemical flowchart, discuss the properties and applications of polycarbonate and look at their global demand and producers.
Here you can find the youtube video of the review:

Before we start with the chemistry I would like to uncover the history of polycarbonate. In 1953, Mr. Fox from GE Plastics prepared the first sample of polycarbonate in his lab. At the same time, Mr. Schnell from Bayer AG discovered polycarbonate too. A patent fight between the two inventors started. This could luckily be settled, which allowed the start of the high volume production of polycarbonate.

General properties and chemistry
Polycarbonate is an amorphous polymer with a glass transition of 147°C and a melting temperature of 160°C. It is transparent and has a density of 1.2 g/cm3.

There are two processes to obtain polycarbonate:
1. Phase transfer process
2. Melt process route

The phase transfer process involves an interfacial polycondensation between phosgene (COCl2) and bisphenol A (BPA) in an organic solvent. It is a batch process suitable for specialty polycarbonate grades.
The melt process has been developed as an alternative to the utilization of phosgene due to environmental concerns. It consists of melt transesterification of BPA and diphenyl carbonate (DPC). DPC is obtained from dimethyl carbonate and phenol or directly over carbonylation of phenol. In a next step, DPC reacts with BPA to form polycarbonate precursors. With the precursors at hand, polycondensation takes place to obtain high molecular weight polycarbonate.
The melt process has been developed as an alternative to the utilization of phosgene due to environmental concerns. It consists of melt transesterification of BPA and diphenyl carbonate (DPC). DPC is obtained from dimethyl carbonate and phenol or directly over carbonylation of phenol. In a next step, DPC reacts with BPA to form polycarbonate precursors. With the precursors at hand, polycondensation takes place to obtain high molecular weight polycarbonate.




Simplified flow chart: Where does PC has its chemical roots?
As already mentioned, bisphenol A (BPA) is a major building block for PC. The simplified chemical flow chart helps us understand how BPA is produced. BPA production needs phenol which is obtained by the so called cumene process. Benzene and propylene are required in the cumene process too. In detail, benzene is alkylated with propylene which results in cumene. In a next step cumene is oxidized to obtain phenol and acetone. Acid catalyzed condensation of phenol and acetone leads to BPA. In a next step, BPA reacts with diphenyl carbonate to form polycarbonate.




Polycarbonate properties
Impact resistance, ductility, clarity, and dimensional stability are the major advantage properties of PC which makes it an excellent engineering plastic. Furthermore, PC has inherent flame resistance, good electrical properties and can be used at elevated temperatures up to 120-140°C. PC has a poor solvent resistance, limited hydrolytic stability and notch sensitivity. This needs be taken into account during your material selection.




PC global demand
In 2016, global PC demand was 4 million tons. The electrical and electronics markets make up for 27% of the total PC consumption. Next in line for PC consumption are the construction and the automotive markets, respectively. Geographically, one can state that Asia is the largest market for PC with 59% of its global consumption.

 
PC Price to performance
Polycarbonates form the base of amorphous engineering thermoplastics with a price of ca. 2.5 €/kg for base grades. High heat modified grades cost in the range of 3 to 3.6 €/kg.



PC end uses
PC covers a wide range of applications in different markets:
- Electronic components: good electrical insulator; heat-resistant and flame-retardant properties
- Construction materials: domelights, flat or curved glazing, and sound walls
- Data storage: Compact Discs, DVDs, and Blu-ray Discs.
- Transportation (Automotive, aircraft, railway, and security components): headlamp lenses, decorative bezels and optical reflectors
- Medical applications: complies with both ISO 10993-1 and USP Class VI standards
- Smartphones: cases, battery covers.


A new application field is battery cage housings and battery management systems components in electrical vehicles, including hybrid electrical cars.

A famous application was the use of Lexan® PC for the helmet visor of the Apollo moon mission 50 years ago.
I wrote a separate post on this topic which you can find here.


This was the review on polycarbonate, a versatile amorphous engineering thermoplastic used in many applications.


Thanks for reading & till next time!


Greetings,
Herwig Juster


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Literature
[1] https://pubs.acs.org/doi/10.1021/ie034004z
[2] https://nexant.com

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