By Said Ammar, P.Eng. 

At Hi-Tech Seals, our focus is on an elastomer’s sealing performance at low temperatures and identifying the lowest temperature at which a seal can function effectively.

Upon cooling, elastomeric compounds start to lose their elasticity which in turn means they start to lose their ability to maintain an effective seal. At these lower temperatures they start to harden and eventually develop glasslike brittleness which makes them more prone to shattering. This is due to the fact that as the temperature is decreased the thermal energy of the material is reduced; the polymer molecules essentially come to a standstill. It is important that an elastomer’s mechanical properties are not affected at lower temperatures. If mechanical properties remain intact then an elastomer will regain its original properties upon returning to normal temperatures.

At low temperatures, the chemical degradation of an elastomer is of little concern because chemical processes become extremely slow. At these temperatures, diffusion of gases and liquids into
the elastomer will also slow down.

Material Properties and Testing

There are four important low temperature measurements/tests which we should be aware of: Melting Temperature (Tm), Glass Transition Temperature (Tg), Low Temperature Brittleness Test and the Temperature Retraction Test (TR-10).

Melting Temperature

The first property we will look at is not really a low temperature property for our purposes; however, it is important we have a basic understanding of it. In simple terms, the Melting Temperature (Tm), or melting point, of a material is the temperature at which the solid and liquid phases of that material exist in equilibrium. When considered as the temperature of the change from liquid to solid, it is referred to as the freezing point or crystallization point. For a crystalline thermoplastic, it is the temperature at which the material changes from a solid to a liquid and is measured using ASTM D 3418 and ISO 1218. Simply speaking, if the temperature of a solid polymer or plastic reaches the material's Tm, the material will begin to melt.

Glass Transition Temperature

Another property that is commonly tested using ASTM D 3418 is the Glass Transition Temperature (Tg). This is more of a low temperature property for elastomers rather than plastics. The Tg is the temperature above which an amorphous polymer is soft and rubbery. Below the Tg an amorphous polymer is hard and brittle.

  • In an amorphous polymer the molecules are oriented randomly and are intertwined, much like cooked spaghetti, and the polymer has a glasslike, transparent appearance. Ex: PS, PVC, PC etc.
  • Most crystalline polymers are really semi-crystalline. They have an amorphous phase and a crystalline phase. The crystalline phase has a more ordered structure where the molecules are arranged in a very orderly fashion. Ex: PEEK, PTFE, PA, POM, PPS, LDPE, HDPE etc.
  • Tg is different than Tm. A material will reach its Tg before it reaches its Tm.
  • At temperatures between Tg and Tm the polymer has the ability to shift and/or deform, thus affecting the material's structural properties.
  • For plastics, they should be used below their Tg in order to ensure proper mechanical performance.
  • For rubber elastomers, they are best used above their Tg in order to ensure they are soft and flexible. Ex: Silicone typically has a Tg lower than room temperature and therefore has a fair amount of pliability at room temperature.
  • Fully amorphous polymers do not exhibit Tm but all polymers exhibit Tg.

Low Temperature Brittleness Test

The next property of concern is the Low Temperature Brittleness (LTB) Test and is commonly seen on elastomer specification sheets. This property is tested according to ASTM D 2137 (Brittleness Point of Flexible Polymers and Coated Fabrics). A specimen is cooled to a certain temperature and after 3 minutes its brittleness is checked by attempting to fracture the specimen. The Low Temperature Brittleness Test represents the lowest temperature at which a sample will not fracture or crack when struck once. It is not necessarily the lowest temperature at which a material may be used but is a useful value for specification purposes. In the ASTM classification system, it is represented by the suffix letter “F”. The common tests you will see on specification sheets are as follows:

  • F14 (-18°C) – NBR 90A
  • F15 (-25°C) – NBR 90A
  • F16 (-35°C) – NBR 70A, NBR 90A, HNBR 90A
  • F17 (-40°C) – NBR 70A, HNBR 70A, HNBR 80A, HNBR 90A, EPDM 70A, CR 70A
  • F18 (-50°C) – HNBR 70A, HNBR 80A
  • F19 (-55°C) – HNBR 80A, LTN, EPDM 70A, FVMQ 70A, VMQ 70A

Temperature Retraction Test

The final property we will look at is the Temperature Retraction Test (TR-10). This property is tested according to ASTM D 1329 (Retraction at Lower Temperatures). This test is not as common on specifications sheets as the Low Temperature Brittleness Test; however, it is a valuable test. It is commonly accepted that the TR-10 test is the most useful indicator for determining a compounds ability to seal at low temperatures. In this test a specimen is stretched about 50%, frozen and then gradually warmed at a constant rate. The temperature at which the material retracts 10% from the elongated position is considered the TR-10 value.

The TR-10 is considered a good indicator of the low temperature dynamic sealing limit since it is the lowest temperature at which the specimen exhibits rubber-like properties and resilience. The static low temperature limit (which we normally associate with LTB) is normally considered to be approximately 8 °C below the TR-10 value.

For example, the TR-10 value of the Viton A-401C polymer is -17 °C, so this would be a good indicator of its low temperature dynamic sealing limit. Therefore, -25 °C would be a good indicator of Viton A-401C’s low temperature static sealing limit. To summarize:

Low Temp Static Limit = LTB
Low Temp Dynamic Limit = TR-10

Hi-Tech Seals Temperature Ratings System

The temperature ranges we list on our Technical Data Sheets (TDS) is a general guideline.
Temperature max/min limits will depend on various application conditions. Temperature capabilities
may not meet the range listed or may exceed the range. Below is a general summary:

GENERAL: Low Temp = LTB / High Temp = ASTM Defined Test Temp
EXCEPTION: FEPM (Low Temp = LTB / High Temp = 232C)
EXCEPTION: FFKM (Low/High Temp = Manufacturer Supplied Value)

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