See-through Kettles may not be a transparent material selection.
We like our see-through kettles and take them for granted but they have a punishing life cycle. Over 10 years the kettle could undergo a total of 10,000 cycles. It is filled with cold water, heated up in a few minutes to boiling point and rapidly cooled when emptied.
You might expect polycarbonate, with a heat distortion temperature of 140oC, to cope with the temperatures involved. However, you also have to think about the time factor and the rapid and repeated temperature swings. A designer might consult the data sheets and assume that polycarbonate, with a ‘continuous use temperature’ in excess of 100oC would be a safe material to use. On the other hand, a chemist would see polycarbonate as an ester, subject to attack by water, particularly at elevated temperatures. A physicist would have concerns about the cyclic expansion and contraction leading to fatigue. A moulder would be wondering about residual moulding strain leading to environmental stress cracking.
When the BSH Group were designing their latest kettle, they set a target of 10,000 test cycles and found that most polycarbonates had difficulty in meeting that requirement. They had to turn to Lexan 1443T, a copolymer of polycarbonate and polysiloxane, from Sabic Innovative Plastics. It survived 12,000 cycles and offered good chemical resistance, outstanding surface finish and the required balance of ductility and melt viscosity.
Finally, more information on this topic may be available on the excellent British Plastics Federation website.