PBT Parts Withstand Tropical Conditions
PBT is produced by means of a polycondensation reaction accompanied by release of water. Under certain so-called supercritical conditions such as those that can occur in the engine compartment of motor vehicles, this reaction can be reversed in response to hydrolysis. Breaking the chain molecules leads to degradation of material properties. To prevent this, PBT grades with special hydrolysis-inhibiting stabilizers are used in many fields of application.
Investigations have now shown that when PBT is used under naturally occurring climate conditions no such stabilization to prevent hydrolysis is necessary. The world’s harshest "natural" load case for testing the resistance of PBT to hydrolysis is the tropical climate. Temperatures of about 40 °C are encountered at a relative humidity of 85 %. It was determined in the investigations that unstabilized PBT exhibits severe degradation due to hydrolysis at elevated temperatures of 70°C, while essentially no degradation due to hydrolysis occurred at temperatures of 55°C.
Of the mechanical properties investigated, the Izod impact strength was the most sensitive to the test climates selected. For this reason, it was selected as the indicator variable. The long-term stability of PBT in natural climates was calculated with the aid of Arrhenius plots. The conclusions drawn are that parts made from unreinforced, flame retardant and non-hydrolysis-stabilized PBT can withstand a temperature of 40 °C at a relative humidity of 85% for 40,000 hours, i.e. about 4.5 years. These investigations confirmed for the first time that PBT can be used in all naturally occurring climates without stabilization to prevent hydrolysis. When the temperature and humidity fall into the supercritical range, however, e.g. in under-the-hood or offshore applications, the report recommends use of hydrolysis-stabilized PBT.