As new and challenging applications are being developed for polymers and plastics, a parameter that helps define the temperature limit for use of that polymer is required. The continuous use temperature, also known as the continuous service temperature, is one parameter that is often quoted in that regard.
Are you also searching how to determine its value? Do you know the benefits it can offer to your development? Find out here…
Learn how to determine the continuous use temperature of a material along with the factors affecting the same. Also, explore the difference in its values for amorphous and semi-crystalline polymers.
Factors Affecting Continuous Use Temperature
The maximum continuous use temperature of a material is defined as the maximum acceptable temperature above which the mechanical properties or the electrical properties of a part made from the material are significantly degrading over the reasonable lifetime of the tested product.
In reality, the true maximum continuous use temperature depends on how the term 'continuous' is defined.
− The time that is involved and the loading levels that are used in the testing can affect the value.
− Also, any additives and reinforcements that are used in the formulation may potentially have an effect on the maximum continuous use temperature that is determined.
Continuous Use Temperature Defined by Relative Thermal Index of a Material
A common parameter that is often used for comparing different materials in terms of continuous use temperature is the Underwriter Laboratory (UL) Relative Thermal Index or RTI.
UL 746 is the test method that is usually used to determine the RTI values. The RTI is based on a loss of properties of the plastic versus time. In general, when the plastic is exposed to this 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 for 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.
Determination of RTI Value
− In the determination of an RTI value, sets of test specimens are placed in ovens at four different pre-set temperatures.
− At certain time intervals, specimens are removed from the ovens and tested for the specific mechanical or electrical property of interest.
− The obtained results are plotted on a property versus time graph 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 property is referred to as 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 is a straight line that can be extrapolated, if needed, to indicate the half-life of the property at other temperatures.
The results that are obtained in this testing procedure can also be compared to a material with a known aging performance.
Types of RTI
As briefly mentioned already, the RTI values that are determined are somewhat dependent on the property that is being examined. There are three general classes of properties that are associated with the RTI.
− The RTI Electrical that is associated with insulating properties.
− The RTI Mechanical Impact is related to the impact resistance, toughness, elongation and flexibility.
− Finally, the RTI Mechanical Strength is associated with the mechanical properties or the structural integrity of the plastics.
The three values for a particular polymer are often different from each other.
» Continue reading and explore the difference in continuous use temperatures for amorphous and semi-crystalline polymers