The plastic polycarbonate has become an indispensable part of our everyday lives – but until now, it has been manufactured using up a high level of resources and involving the highly toxic gas phosgene. A German industrial plant manufacturer has now commissioned its second polycarbonate production plant in China, which not only saves resources, but also dispenses with the use of that toxic gas. An important part of the new process is a conical dryer from BHS-Sonthofen.
Data storage devices, sporting goods, roofing, credit cards – polycarbonate can be found in practically every plastic item for everyday use. Since its discovery in the 1950s by German chemist Herrmann Schnell (1916-1999), and simultaneously in the USA by General Electric, the plastic’s success story has continued unabated. Given its low weight, temperature resistance and impact strength, as well as its colorless and transparent appearance, polycarbonate enjoys great popularity – around four million metric tons of it are now produced annually. Although polycarbonate is found virtually everywhere, the established manufacturing process is still costly and dangerous: in addition to bisphenol A, a condensate of acetone and phenol, polycarbonates consist of phosgene (COCl2, also known as carbon dioxide dichloride or carbonyl chloride), a toxic gas that was used as a chemical warfare agent during World War I.
Non-phosgene melting process
The conventional production of polycarbonate by polycondensation of phosgene requires a high input of water and energy: around ten tons of water for one ton of polycarbonate. In addition, the resulting wastewater must then be reprocessed at great expense in order to be reusable. “Production using phosgene is only possible under the highest safety precautions,” explains Thomas Boerboom, Marketing Manager at BHS-Sonthofen. “A process that does not involve this highly toxic gas is therefore not only more environmentally friendly, but also spares manufacturers some of the extensive and costly safety precautions.”
In the so-called non-phosgene melting process now in operation, the polycarbonate is obtained from the raw materials bisphenol A (BPA) and diphenyl carbonate (DPC). This production process, which is more environmentally friendly, consists of five steps: Once the raw material has been processed, the material is converted by the chemical reaction known as transesterification. This is followed by polycondensation. In this intermediate step, the material can be given individual properties, such as stability, transparency, as well as resistance to UV light or heat.
Vertical dryer with proven helical principle
The conical dryer from BHS-Sonthofen Process Technology, a subsidiary of BHS-Sonthofen, comes into play for the next coating process step: Thanks to an intelligent design in the lid of the dryer, a coating additive is added to the polycarbonate intermediate product, and distributed reliably and evenly in the container using the AVA helical principle. The single-supported, self-centering agitator in the form of a single conical spiral conveys the polycarbonate product upwards along the heated container wall, thus ensuring intensive contact of the product with the heat transfer surfaces. At the same time, the intermediate product inside the process chamber sinks to the bottom again. This permanent product circulation allows for uniform product properties, such as a homogeneous temperature and product moisture. “Our proven technology ensures precise temperature control and low residual moisture in the product,” explains Martin Specht, Sales Manager at BHS-Sonthofen.
The solvent, which acts as a carrier for the coating additive, is heated by the double jacket of the AVA conical dryer and dried by vacuum in the next step. The solvent steam follows the pressure gradient to the heat exchanger and is condensed there. It can then be returned to the process as a pure starting product, and reused as carrier material. The finished, coated polycarbonate material is now discharged completely residue-free in batches via the discharge valve in the bottom of the dryer. Finally, specific additives are added to the polymer melt in a downstream process before the high-quality polycarbonate chips can be produced for further processing in the final process step by means of a strand-guided pelletizing system.
New applications are constantly emerging for polycarbonate due to its special physical properties. The manufacturing process described here for the sought-after material is environmentally friendly with a highly efficient energy balance – and with lower investment costs compared to alternative processes.
