Unlike thermoplastics, elastomers are typically used over a wide range of temperatures and significantly above their glass transition temperature (Tg). The advantages of elastomers over thermoplastics are their ability to recover almost completely from the tensile state (high elasticity), as well as their generalized elasticity, low hardness and low modulus properties. When elastomers are used below room temperature, they show an increase in hardness, an increase in modulus, and a decrease in elasticity. When elastomers are used below room temperature, there is a tendency for hardness to increase, modulus to increase, elasticity to decrease (low tensile) and compression set to increase. Depending on the problem with the elastomer, two phenomena can occur at the same time - glass hardening and partial crystallization - CR, EPDM, NR are some examples of elastomers that exhibit crystallization.
1. Overview of low temperature testing
Brittleness, compression permanent deformation, retraction, hardening and cryogenic hardening have been used for many years in order to characterize polymer properties at low temperatures. Compressive stress relaxation is relatively new and focuses on determining the sealing force of a material over a period of time under various environmental conditions.
2. Brittleness Temperature
ASTM D 2137 defines the brittleness temperature as the lowest temperature at which vulcanized rubber will not show fracture or rupture under specified impact conditions. Five rubber specimens of pre-determined shape are prepared, placed in a chamber or liquid medium, subjected to a set temperature for 3±0.5min, and then given an impact velocity of 2.0±0.2m/s. The specimens are removed and subjected to an impact or rupture test. The specimen is removed and tested for impact or fracture, all without damage. The test was repeated up to the brittleness temperature - the lowest temperature at which no fracture was found was very close to 1°C.
3. Low Temperature Compression Set and Low Temperature Hardening
The test procedure for low-temperature compression set is very close to that for standard compression set, except that the temperature is controlled by some energy method, such as dry ice, liquid nitrogen, or mechanical methods, and the value is within ± 1°C of the preset temperature. After recovery from the fixture, the specimen is also placed at the preset low temperature and molded to a diameter of 29 mm and a thickness of 12.5 mm. Low-temperature compression set is an indirect method for sealing applications of the compound in question. Compressive stress relaxation is the direct method and will be discussed later. Low temperature hardening is also usually determined using a vulcanized compression set specimen (29mm x 12.5mm), but re-tested at a low temperature control, which is the same as that for compression set, and then again at the same temperature as their set temperature. Hardening and low-temperature compression set are directly affected by cooling, but also by the tendency of the polymer to crystallize, with the rate of crystallization dependent on temperature, e.g., CR crystallizes fastest around -10°C, and then decreases at lower temperatures, mainly due to the immobility of the polymer chain segments (the molecular chains freeze before rearrangement).
4. Gehman Low Temperature Hardening
ASTM D 1053 describes the low-temperature hardening method as follows: a series of elastic polymer specimens are fixedly attached to a wire with a known torsional constant, and the other end of the wire is attached to a torsion head capable of permitting the wire to be twisted. The specimens are immersed in a heat transfer medium at a specific temperature below normal, at which time the torsion head is twisted by 180°, and then the specimens are twisted by an amount (less than 180°) that is dependent on the inverse of the specimen's flexibility and stiffness. Then use the amount of goniometer to determine the amount of specimen twist, the angle of twist and the hardness of the rubber material. The temperature of the system is gradually increased at this point, and a plot of the angle of twist against temperature is obtained. The temperatures at which the modulus reaches T2, T10, and T100 are usually recorded as equal to the modulus value at room temperature.
5. Low Temperature Retraction (TR Test)
The TR test is utilized to evaluate the ability of a specimen in the tensile state when compressive permanent deformation and compressive stress relaxation determined by compressive stress are used to determine low temperature effects. As covered earlier, many polymers like NR and PVC will crystallize at low temperatures, but stretching can also crystallize, leading to additional factors when looking at low temperature properties. For evaluation applications such as exhaust suspension, TR under tension is very appropriate and frequently used. In this test, the specimen is elongated (often by 50% or 100%) and frozen in the elongated state. The specimen is released, at which time the temperature is raised at a determined rate to measure the recovery of the specimen, the length of the shrinkage is measured and the elongation is recorded. The temperatures at which the specimen shrinks by 10%, 30%, 50%, and 70% are usually noted as TR10, TR30, TR50, and TR70. TR10 relates to the brittleness temperature; TR70 relates to the permanent deformation of the specimen in low-temperature compression; and the difference between TR10 and TR70 is used to measure crystallization of the specimen (the greater the difference, the greater the tendency to crystallize).
6 . Low Temperature Compressive Stress Relaxation (CSR)
The CSR test can be used to make predictions about the performance and life of sealing materials. When an elastomeric compound is given a constant deformation, a combined force is created, and the ability of the material to maintain this force within a certain environmental range measures its ability to seal. Both physical and chemical mechanisms contribute to stress relaxation, based on time and temperature, one factor will dominate, physical relaxation is observed at low temperatures, immediately after a given stress, which leads to chain rearrangement and changes in the rubber-filler and filler-filler surfaces, and the relaxation of the stress removal system is reversible. At higher temperatures, the chemical composition determines the rate of relaxation, when the physical processes are already small and the chemical relaxation is irreversible, leading to chain breakage and cross-linking reactions. Temperature cycling or sudden increases in temperature can have an effect on stress relaxation in elastomers. During the CSR test, the test specimen is placed
During CSR testing, stress relaxation is increased when the test specimen is subjected to elevated temperatures. If stress relaxation occurs early in the test, the amount of additional relaxation increases first and has a maximum value during the first cycle. In a tensile large test piece to produce gasket samples (19mm outer diameter, inner diameter of 15mm), with an elastic fixture will be compressed to the specimen to their room temperature thickness of 25%, and at 25 ℃ into the environmental test chamber, the temperature at 25 ℃ to maintain 24h, and then down to -20 ℃, maintained for 24h, followed by the next temperature between -20 ~ 110 ℃ cycle of 24h, the entire test time at test temperature, the test temperature, continuous force determination. The force measurement is performed continuously throughout the test time at the test temperature.
7. Effect of Ethylene Content
7.1 Ethylene content has the greatest impact on the low temperature performance of EPDM polymers. Polymers with Ethylene content ranging from 48% to 72% were evaluated under high quality sealing formulations. All aim to reduce the variation in mooney viscosity by introducing ENB in these different polymers.
EPDM rubber is amorphous if the ethylene/propylene ratio is equal and the distribution of the two monomers in the polymer chain is random. EPDM with 48% and 54% ethylene content does not crystallize at or above room temperature. When the ethylene content reaches 65%, the ethylene sequences begin to increase in number and length and can form crystals, which are observed in the crystallization peaks on the DSC curves around 40°C. The larger the DSC peaks, the larger the crystals that form.
7.2 In addition to the effect of ethylene content on low temperature properties discussed later, crystallite size affects the ease of mixing and processing of compounds containing crystals. The larger the crystallite size, the more heat and shear work is required at the mixing stage to fully blend the polymer with the other components. The raw rubber strength of EPDM compounds increases with increasing ethylene content. In sealing formulations where the effect of ethylene content was measured, an increase in ethylene content from 50% to 68% resulted in at least a four-fold increase in the strength of the rubber. The room-temperature hardness also increases with increasing ethylene content. The Shore A hardness of the amorphous polymer adhesive is 63°, whereas the Shore A hardness of the polymer with the highest ethylene content is 79°. This is due to the increase in the ethylene sequence, the increase in crystallization in the adhesive, and the corresponding increase in thermoplastic polymers.
7.3 When the hardness is measured at low temperatures, in contrast to the polymers with high ethylene content, the amorphous polymers show less change in hardness, whereas the change in hardness of the higher ethylene content does not show a linear pattern and the hardness remains high at room temperature, so that the polymers containing the higher ethylene content continue to have the highest hardness at low temperatures.
7.4 Compression set is largely dependent on the test temperature. If tested at 175°C, there is no difference in compression set between any of the polymers (set is influenced by the design of the compound and the choice of vulcanization system). After melting of the ethylene crystals, the polymer exhibits an amorphous form, and in order to examine the effect of the ethylene content, tests were done at 23°C. Polymers with a higher ethylene content clearly have higher permanent deformation (more than twice as much), and the effect of the ethylene content is even larger when tested at -20°C and -40°C. Polymers with more than 60% ethylene content have high permanent deformation (>80%); at -40°C, only the fully amorphous polymers have low permanent deformation (17%).
7.5 Effect of Ethylene Content on Low Temperature Hardening from Gehman Tests. Given a temperature, the higher the corner, the lower the increase in stiffness (or increase in modulus). At low temperatures, the stiffness modulus increases significantly with increasing ethylene content. For amorphous polymers, the T2 is -47°C, while the highest ethylene content polymer has a T2 of only -16°C.
7.6TR Measuring shrinkage recovery of specimens after extension freezing, the ethylene content has a significant effect on the test method, which is again similar to the Gehman test.
This is similar to the Gehman test. The shrinkage (%) of the various polymers varies as a function of temperature, with the amorphous polymers having the highest shrinkage recovery at low temperatures; however, as predicted, the recovery deteriorates as the ethylene content increases at a given temperature.
recovery deteriorates. The value of TR10 varies from -53°C for amorphous polymers to -28°C for polymers with high ethylene content.
7.7 Compressive stress relaxation (CSR) cycle
Cycle. Compress the compounds, allow them to relax at 25°C for 24 h, and then place them in a cycle of temperatures ranging from -20°C to 110°C intermittently for 24 h. When compressed for the first time, after the equilibration period, the crystalline polymer E has a higher loss of stress than the amorphous polymer, and when lowered to -20°C the sealing force of the two polymers decreases, while the amorphous polymer A has a high retention of stress (higher F/F0). Heating the compound to 110°C restored its sealing force, and when brought back down to -20°C, the remaining sealing force of the crystalline polymer was less than 20% of its value, which is generally considered too low for most applications, with the amorphous polymer retaining more than 50% of its sealing force, and the amorphous polymer again having a higher recovery than the crystalline polymer. The next cycle yielded similar conclusions. It is clear that amorphous polymers are superior for sealing applications where high and low temperature performance is required.
8. Effect of Diolefin Content
To provide the unsaturated point required for vulcanization, non-conjugated diolefins like ENB, HX and DCPD are added to ethylene propylene polymers. One double bond reacts in the polymer matrix, while the second acts as a complement to the polymerized molecular chain and provides the vulcanization point for sulfur yellow vulcanization. The effect of ENB was evaluated in windshield (rain) bar profiles. Polymers containing 2%, 6% and 8% ENB were compared.The addition of ENB had a significant effect on the vulcanization characteristics and crosslink density. Modulus increased while elongation decreased significantly. The hardness increased and the compression set improved during temperature rise. As the ENB content increases, the charring time becomes shorter.
ENB is an amorphous material, and when added to the polymer backbone, it disrupts the crystallization of the ethylene portion of the polymer, so that polymers with the same ethylene content can be obtained, and the higher content of ENB improves the low-temperature properties. At room temperature, the higher ENB content slightly improves the compression set due to the improved crosslink density. However, at low temperatures, the compression set of the polymers with higher ENB content is significantly better than that of the polymers with 2% ENB content. The effect of ENB content on brittleness temperature, temperature retraction, and Gehman's test did not show any significant difference in brittleness temperature between polymers in general, and for the Gehman's test and the TR test, each polymer showed an improvement in low-temperature properties with increasing ENB content.
9. Effect of mooney Viscosity on Low Temperature Properties
It is well known that mooney viscosity (molecular mass) has a significant effect on the processing behavior of elastomers. In extrusion and molding applications In extrusion and molding applications, it is important to select a compound with a suitable Mooney viscosity value. Using the same formulation that was used to investigate the effect of the third monomer, ENB, on low-temperature properties to examine Mooney viscosity, polymers with Mooney viscosities of 30, 60, and 80 were compared, and the Mooney viscosity of the compounds increased as the Mooney viscosity of the polymers used increased. Tensile strength, modulus, and raw rubber strength increased with increasing Mooney viscosity. The effect of Mooney viscosity on the low temperature properties of EPDM was not significant. However, the compression permanent deformation at room temperature, -20°C and -40°C increases with increasing molecular mass. However, the compression set at room temperature, -20°C and -40°C did not change significantly with increasing molecular mass, whereas the compression set at elevated temperatures (175°C) showed some changes for the higher mooney viscosities of the EPDM adhesives.
10. Conclusion
The ethylene and diolefin content has a significant effect on the performance of EPDM elastomers in low temperature applications, with polymers with low ethylene content performing well and polymers with high diolefin content improving due to disrupted crystallization of the ethylene portion of the polymer. Low ethylene content polymers should be used when low temperature performance is a limitation.
