Continuous Service Temperature of Plastics

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.

Continuous Use Temperature is measured in °C.

Check out more on Continuous Service Temperature:

» Factors on Which Max. Continuous Use Temperature Depends
» How to Measure Continuous Service Temperature?
» Continuous Use Temperature of Common Polymers
» Effect of Incorporating Fillers

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.

How to Measure Continuous Service Temperature of Plastics?

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.

Continuous Use Temperature of Common Polymers

Amorphous Polymers

The table below lists the continuous use temperatures of some common amorphous polymers along with their Tg values:

Polymer Name	Minimum Value (°C)	Maximum Value (°C)	Tg (°C)
PC	100	140	147
PEI	170	170	217
PMMA	70	90	105
PESU	175	185	230
PSU	150	180	190

The primary reason why both a minimum and maximum value is quoted in this table is because, the various properties that are examined will yield slightly different values.

For these amorphous polymers, the glass transition temperature, or Tg, is the primary factor that controls the continuous use temperature for each of the various polymers. Thus, the values that are quoted in table above are lower than the reported Tg values by between 10 and 40°C.

This behavior can be understood in terms of the fact that there is a significant change in all of the properties of the polymer at the Tg. These properties include the mechanical properties as well as the electrical properties of the polymer.

Semi-Crystalline Polymers

The table below contains the continuous use temperatures of some common semi-crystalline polymers.

Polymer Name	Min Value (°C)	Max Value (°C)	Tg (°C)	Tm (°C)
PEEK	154	260	143	343
PET	80	140	69	255
PPS	200	220	126	279
PBT	80	140	40	223
PA6	80	120	50	220

The table lists the measured values of both the Tg and Tm for these same polymers. These values have been typically determined by thermal analysis techniques, such as DSC.

This data infers that unlike the amorphous polymers, the continuous use temperatures for all of these semi-crystalline polymers are above the Tg value and below the Tm value. In fact, all of the reported values are between the Tg and Tm parameters for all of these polymers. This observation is true of all of the different chemical structures for the various polymers.

Amorphous Vs Crystalline Polymers

Some insight into this behavior can be gained by an examination of the plot that is displayed in the figure below. The plot shows a typical graph for the modulus vs. temperature behavior of a typical semi-crystalline polymer compared to the same behavior for a completely amorphous polymer.

Modulus v/s Temperature for Amorphous & Semi-Crystalline Polymers

While there is some loss in the modulus at the Tg for the semi-crystalline polymer, the majority of the loss occurs near the Tm.
This is in contrast with the behavior for the amorphous polymer where a large decrease in the modulus is observed at the Tg.

While the present graph and argument explicitly deals with the modulus vs. temperature behavior of plastics, similar arguments that the largest change in behavior in properties for a semi-crystalline polymer occurs at the polymer Tm can be made.

This is true for both the mechanical properties as well as the electrical properties of plastics.

Effect of Incorporating Fillers on Continuous Use Temperature

The other point that should be discussed is the effect of the incorporation of fillers into polymers on the continuous use temperature. That effect can be investigated through an examination of the data that is displayed in the table below. The data from the table shows the continuous use temperature of polyether imide or PEI with several different fillers:

Polymer Name	Minimum Value (°C)	Maximum Value (°C)
PEI	170	170
PEI + 30 % Glass Fiber	170	170
PEI Mineral Filled	170	170

In this case, there is no effect of the fillers on the continuous use temperature values for this particular polymer.

While this is a limited set of data, it also suggests that the primary factor that determines the continuous use temperatures of plastics is the chemical structure of the polymer. Also, any reinforcing additive that is included in a formulation has a minimal effect. This is an area in which more work should be done to determine the general nature of the observed effect.

Checkout Values of Several Other Polymers:

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