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Low Temperature Testing for Seals Using Rubber Compounds

Dilip Dhupia

Applications Engineer, India

When seals lose their flexibility and ability to respond to a reaction force at low temperature, the change is physical. Thus, the compound must be formulated for optimum performance of the final product, while ensuring the correct V/E Ratio is maintained. Seals are used in many critical applications including (but not limited to): Aerospace; Oil & Gas; Food & Dairy; Automobile; Pharmaceutical / medical; and Petrochemical.

The following test methods can be used to assess the suitability of a compound in low temperature applications. We break these out into more detail throughout the rest of this blog post.

  • Brittleness test: ASTM D2137; ISO 812
  • Low Temperature retraction test: ASTM D1329; ISO 2921
  • Low temperature stiffening test: ASTM D1053; ISO 1432
  • Low temperature compression set: ASTM D1229; ISO 815-2
  • Dynamic Mechanical Analysis: ASTM D5992; ISO 4664-1

Brittleness Test

ASTM D2137 and ISO 812

Figure 1: Brittleness test

This test uses a hammer of a specific mass, moving at a specified speed toward five cured rubber samples held by a grip. The samples and the grip are cooled to a specified low temperature. The hammer strikes the samples at the specified speed and continues moving past. The temperature at which fracture occurs in three of the five samples is said to be the brittle point of the rubber compound.

Low Temperature Retraction Test

ASTM D1329; ISO 2921

This test is conducted in the sequence as:

  1. Elongating the specimen
  2. Locking it in the elongated condition
  3. Freezing it to a state of reduced elasticity
  4. Releasing the frozen specimen and allowing it to retract freely while raising the temperature at a uniform rate
  5. Measuring the length of the specimen at regular temperature intervals while it is retracting, and (“ASTM D1329 Specimen Cutting Dies – CCSi”)
  6. Computing the percentage retraction at these temperatures from the data obtained.

The temperatures corresponding to 10% and 70% retraction are designated as TR10 and TR70, respectively. The difference between the temperature TR10 and TR70 increases as the tendency to crystallize increases. TR70 correlates with low-temperature compression set. TR10 has been found to correlate with brittle point in vulcanizates based on polymers of similar type. (“ASTM International – Sign In”) In general, the retraction rate is believed to correlate with low-temperature flexibility of both crystallizable and non-crystallizable rubbers.

Low Temperature Stiffening Test

ASTM D1053 and ISO 1432

Figure 2: Low Temperature Stiffening test

The test method is employed to determine the subnormal temperature stiffening of flexible polymers or fabrics coated with flexible polymers. Temperatures at which the low temperature modulus is a specified multiple or ratio of the modulus at room temperature are interpolated from the twist versus temperature curve. These specified ratios of low temperature modulus to room-temperature modulus are called relative moduli. (“Standard Test Methods for Rubber Property—Stiffening at Low …”) These temperatures at the relative moduli encompass the transition region between the glassy and rubbery states of the material under testing. (“Standard Test Methods for Rubber Property—Stiffening at Low …”)

Dynamic Mechanical Analysis

ASTM D5992 and ISO 4664-1

DMA stands for Dynamic Mechanical Analysis. “ASTM D5992 describes tests for determining glass transition temperature via DMRT and tan delta data.” (“LOW TEMPERATURE TESTING OF ELASTOMERS”) A frequency of oscillation is chosen, and the temperature is ramped up from a temperature well below Tg. (i.e., the test uses temperature sweeps at a given frequency). The point at which tan delta reaches a maximum is said to be the material’s glass transition temperature. Once again, time and temperature are directly coupled via a specified heating rate, and again time and temperature effects are indistinguishable. ASTM D5992 also describes a method of determining Tg using results for E’’, or loss modulus.

At low temperature physical changes dominate over chemical changes in rubber seal. At low temperature sufficient flexibility of seal is desired to overcome initial drag for the elastomer to adjust its shape to maintain the seal lip to provide sealing action. Whist cold and inflexible seam may result in damage to seal lip and in turn sealing action is lost. Premier RPA + SZ is an effective DMA in torsional shear mode to provide data on V/E ratio and low temperature flexibility of seal rubber compound. The instrument is designed to measure the mixed rubber compound processability, cure properties and cured dynamic properties in a one test definition. It reduces the cycle time by approx. 35%.

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