Relative Temperature Index is the Best Measure of Heat Resistance

In specifying materials for certain applications, we might need to know what kind of heat resistance that the material has. This is a vague concept at best. What do we mean by heat resistance? Can the heat resistance of a material be altered with additives or fillers?

Let’s delve into a few of these issues and I will show you where to find the best indication of heat resistance of plastic materials as well as give you some guidelines that you can use for picking materials.

I have seen many people over the years use heat deflection temperature as an indication of heat resistance and assume that this was the maximum temperature that the material could be exposed to. HDT is often listed on data sheets so it is readily available. Heat deflection temperature measures one specific property, the temperature at which a plaque deflects under a given load. It is typically measured at two loads, 66 psi (455 Mpa) and 264 psi (1820 Mpa). The problem is that heat deflection testing measures short term property changes. It does not indicate how the properties of the material will change over long term exposure to elevated temperatures.

Some people also assume that as long as a material is kept below its melting point that it should be ok. The problem with this is that many materials will suffer significant loss of properties long before they get to their melting point. Also, most amorphous materials like PC and ABS do not have sharp melting points.

What we need to know is: what is the maximum temperature that a material can be exposed to continuously over its life without significant loss of properties and appearance? Neither HDT nor melt point will tell us this. Thankfully, there is a test that will. Well, kind of.

The test method that I speak of is an Underwriters Laboratory test method. The designation is UL746B. They call it Relative Temperature Index or RTI.

According to Underwriters Laboratory:

Relative temperature index (RTI) is the maximum service temperature for a material where a class of critical property will not be unacceptably compromised through chemical thermal degradation. This spans over the reasonable life of an electrical product relative to a reference material having a confirmed, acceptable corresponding performance-defined RTI.

And:

The end-of-life of a material at each test temperature in this program is assumed to be the time when the value of the critical property had decreased to 50 percent of its original (as received) value. If a material has been investigated under the thermal-aging program, the relative temperature index (RTI), in degrees C, shown is based on a comparison with a material which has acceptable service experience and correlates numerically with the temperatures above in which the material is likely to degrade prematurely.

Basically, plaques are molded and tested for tensile strength, Izod Impact and surface resistivity (electrical). Plaques are then aged at different temperatures and retested at various points to determine end of life at various temperatures.

RTI is the best measurement that we have of the maximum temperature that a material can be exposed to over the long term without significant loss of properties. However, there are a few things to keep in mind when using the RTI value:

1. The plaques being tested for RTI are exposed to the temperature continuously for up to 2000 hours. It is unlikely that the part that you are specifying a material for is continuously exposed. It is more likely intermittently exposed to elevated temperatures. For instance, a car is unlikely to be running continuously for 2000 hours. Parts under the hood will be exposed to elevated temperature for a few hours at a time and then allowed to cool. This will extend the life.
2. The UL746B test method does not measure appearance or color retention. If you are worried about the color changing or the appearance changing, RTI might be a decent starting point but you might want to have some heat aging testing performed before settling on a material. For instance, natural nylon will start to discolor above 80 C although the RTI is higher than that.
3. The plaques in the RTI testing are exposed to air at elevated temperature. Keep in mind that metal conducts heat much faster than air. The heat resistance may decrease if the part is in contact with metal that is at an elevated temperature.
4. Keep in mind that the RTI test method considers failure mode to be a 50% loss of properties. Your application may not be able to handle a 50% loss of properties without catastrophic failure. In this case, you will want to make sure that you have a safety margin of up to double the RTI. If your part will be exposed to 80C, you might want a material with an RTI of 160C.

Where to find the RTI for a material

Underwriters Laboratory has tested many materials for RTI and they report these numbers on what they call the UL Yellow Card for the material. You can find these on the UL Prospector website. Here is an example of one:

The RTI values are listed under the thermal section of the card, usually towards the bottom. You will also notice that there are typically a number of different RTIs listed for a material. They divide it into RTI Elec (electrical properties), RTI Imp (impact strength), and RTI Str (tensile strength). The property loss for most materials is not consistent for a given temperature. It might lose tensile strength at a lower temperature than it loses impact strength for instance. You might have to decide what the most important property is. If all three are critical, then use the lowest RTI figure.

You will also notice that many materials have different thicknesses listed under each category such as .71mm, 1.5mm etc. This represents the thickness of the plaques tested. Usually thicker sections can take more heat. Look at the RTI that corresponds to the thinnest section of your part.

To make things a little easier, here is a list of approximate RTIs for a bunch of generic materials. These are only guidelines; you should really then look up the actual RTI testing for the material and application that you are considering.

Material                                                               Approx. Relative Thermal Index (°C)

ABS                                                                        60

Acetal                                                                   105

Nylon 6                                                                 130

Nylon 6/6                                                            130

Polycarbonate                                                   125

Polyester (PBT)                                                 130

Polyester (PET)                                                 140

Polyetherimide                                                 170

Polyethersulphone                                         180

Polyethylene                                                     60

Polyphenylene Oxide (PPO/PPE)                105

PEEK                                                                    220

Polyphenylene Sulfide                                  220

Polypropylene                                                  115

Polystyrene                                                        50

Polysulphone                                                    150

TPO/TPE                                                              115

TPU                                                                        50

PVC                                                                        80

SAN                                                                       50

The approximate RTIs listed above are for unfilled versions of these materials. The addition of fillers and reinforcements like glass fiber will tend to increase the RTI. The addition of heat stabilizer packages to materials will increase the RTI as well. For instance, here are some RTI values for a few grades of Nylon:

Material                                                                                                         RTI Imp

Zytel 101L (unfilled Nylon 6/6)                                                               75 C

Zytel 70G33L (33% GF Nylon 6/6)                                                           120 C

Zytel 70G33HS1L (33% GF Nylon 6/6 with heat stabilizer)              125 C

Notice how much the addition of glass fiber increases the RTI. Also notice that the addition of heat stabilizer adds even more RTI.

When specifying materials for applications that will be exposed to elevated temperatures, it is important to consider the long term effects on the properties as opposed to the short term effects. Heat deflection temperature, vicat softening temperature and melting point all measure the short term effects. Relative Temperature Index has its limitations but is the best measurement of long term heat resistance that we have. If RTI does not provide enough information you might consider having additional heat aging testing performed. Many laboratories can heat age plaques or molded parts for you so that you could measure additional property changes.