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.
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