Maximum Continuous Service Temperature

The maximum continuous use temperature of plastics is the maximum acceptable temperature above which the mechanical properties or electrical properties of a part made from the material are significantly degrading over the reasonable lifetime of the tested product. These properties include tensile strength, impact strength, and dielectric strength linked to insulation.

 

The maximum continuous temperature is measured in:
 

• Degree Celsius (°C)
• Degree Fahrenheit (°F)
• Degree Kelvin (K)

In reality, the true maximum continuous use temperature depends on:
 

• The time involved in testing.
• The loading levels can affect the value.
• Additives and reinforcements used in the formulation.

Relative Thermal Index (RTI) is a parameter to compare the continuous use temperature of materials. The RTI is based on a loss of properties of the plastic versus time. In general, when the plastic is exposed to maximum continuous use temperature - good, long-term performance is observed. On the other hand, it does not consider short-term thermal spikes. 

RTI gives an indication of the aging temperature that a material can endure for 100,000 hours and still retain at least half of the initial property being measured. However, it does need to be noted that different properties of materials decay at dissimilar rates. This is the primary reason why often RTI values are associated with a particular property and the related continuous use temperatures are given as a range of values rather than as a single value.

 

The RTI values depend on the property that is being examined. There are three general classes of properties that are associated with the RTI.
 

1. RTI Electrical for insulating properties.
2. RTI Mechanical Impact for impact resistance, elongation, toughness, and flexibility.
3. RTI Mechanical Strength for mechanical properties or the structural integrity of the plastics.
    

The three values for a particular polymer are often different from each other. 

 

UL 746 is the test method to determine the RTI values.
 

• Place the sets of test specimens in ovens at four different pre-set temperatures.
• At certain time intervals, remove the specimens from the ovens. Determine the specific mechanical or electrical properties of interest.
• Plot the obtained results on a property versus a time graph. This is done until the property that is being tested declines to 50 percent or less of its initial value.

In this analysis, the 50 percent value of the half-life of that particular property. The half-life values are then, plotted against the reciprocal of the absolute aging temperature. This plot results in a straight line that can be extrapolated, if needed, to indicate the half-life of the property at other temperatures. 

We can also compare the results obtained in this testing procedure with a material of known aging performance.