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Study of low temperature embrittlement properties of fluoroelastomers

Views: 0     Author: Site Editor     Publish Time: 2023-08-22      Origin: Site

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The outstanding disadvantage of fluoroelastomer (FKM) is poor low temperature resistance. The low-temperature retraction temperature (TR10) of binary FKM is generally -18 to -16 ℃, and the glass transition temperature (Tg) is -20 ℃; the low-temperature resistance of ternary FKM is poorer than that of binary FKM. Special low-temperature type FKM has good low-temperature resistance, but the price is very high.


FKM in the ASTM D 2000-2012 "automotive rubber products standard classification system" of the M2, M5, M6 grade products required to pass the low-temperature embrittlement F15 (-25 ℃) test, M4 grade products required to pass the low-temperature embrittlement F17 (-40 ℃) test. In recent years, more and more FKM products are required to meet the requirements of both high temperature resistance (250-275 ℃) and low temperature embrittlement F15 or F17. The best way is to use low-temperature FKM, such as brittleness temperature of -45 ~ -40 ℃ Viton GLT type FKM, but this FKM high-temperature performance is poor and the price is difficult to make the market acceptable. By improving the performance of binary FKM compounds can simultaneously meet the requirements of low and high temperature resistance, and the cost is also reasonable, has become a hot spot of research.


This paper analyzes the characterization of FKM low-temperature performance of Tg, TR10 and brittleness temperature (Tbri) of the relationship between postgraduate students, fillers, mixing process, etc. on the binary and ternary FKM adhesive low-temperature brittleness performance for the preparation of low-temperature-resistant FKM adhesive to provide reference!


1. Characterization of FKM low-temperature embrittlement properties


Rubber has reversible deformation, can produce large deformation under the action of a small external force, and can be restored to its original state after removing the external force, so it is widely used. However, as the temperature decreases, the elasticity of rubber gradually deteriorates, and when it reaches Tg, the rubber loses its elasticity and fails. Because of the variety of rubber products, in the process of use may be subjected to impact, tension, shear, torsion, extrusion, abrasion, etc., its low-temperature embrittlement performance should be based on its working condition to choose the appropriate test method. Commonly used low-temperature embrittlement performance test methods include Tg test, impact brittleness temperature test, low-temperature retraction test, low-temperature torsional rigidity test (Gimen test), resistance to stretching and cold coefficient test, low-temperature hardness test, low-temperature compression permanent deformation and stress relaxation test, etc.


The low temperature resistance of FKM can be characterized by Tg, Tbri, TR10 and Torsion Temperature at low temperature (TGem), etc. These parameters have different meanings but have certain relationship with each other.

(1) Tg is the temperature at which the rubber changes from the highly elastic state to the glassy state and the glassy state to the highly elastic state, which usually characterizes the microscopic movement of the rubber molecular chains.

(2) Tbri is the temperature at which no damage occurs to the rubber under the specified conditions of impact force deformation, which usually reflects the strength of the rubber for use at low temperatures and its ability to withstand damage.

(3) TR10 is used to evaluate the viscoelasticity and crystallization effect of rubber at low temperatures, and usually reflects the lowest temperature at which a rubber material can maintain its elastic recovery.

(4) TGem uses a torsional steel wire with a known torsional constant as a reference material to twist the specimen at a large angle, while as the temperature decreases, the modulus of the rubber increases, the rigidity increases, and the angle of torsion decreases to the point where it barely twists at Tg. The torsion angle of the rubber according to the temperature change can evaluate its low temperature performance, usually reflecting the lowest temperature at which the rubber maintains its elasticity.


2 FKM Tg, TR10 and the relationship between Tbri


Commonly used FKM low-temperature embrittlement performance parameters are Tg, TR10 and Tbri, the supplier usually adopts the parameter Tg and TR10, ASTM adopts the parameter Tbri, there are certain differences and relationships between these three.


2. 1 The relationship between Tg and TR10


TR10 of each grade of FKM is close to Tg (the difference is not more than 3 ℃), indicating that both Tg and TR10 can reflect the low-temperature movement and glass state temperature of rubber molecular chain.


2.2 Relationship between FKM fluorine content and low-temperature embrittlement properties


In common binary and ternary FKM, TR10 increases with the increase of fluorine content; when a fourth monomer, PMVE, is introduced, its content has a great influence on TR10.

Its content has a great influence on TR10. Although the increase of fluorine content can improve the upper limit of FKM use temperature, but at the same time, due to the C-F bond replaces the C-H bond, reducing the softness of the molecular chain and the low-temperature performance of the rubber.


2.3 Influence of fillers on the low-temperature embrittlement properties of FKM compounds


Carbon black N774 adhesive has the lowest Tbri and the best low-temperature resistance; zinc oxide adhesive has the highest Tbri and the worst low-temperature resistance; the low-temperature embrittlement performance of the five types of adhesives does not differ much. After analyzing, after adding different fillers, the gap and structure between FKM molecular chains are different, and the corresponding low-temperature embrittlement performance is different.

After analyzing, after adding different fillers, the gaps and structures between FKM molecular chains are different, and the corresponding low-temperature embrittlement properties are different, and the filler with a small amount of rubber has a larger gel content and better low-temperature resistance.


2.4 The effect of mixing process on the low-temperature embrittlement properties of FKM compounds


The mixing process also has an effect on the low-temperature embrittlement properties of FKM compounds. Plasticizing before mixing can obtain rubber molecule flexibility better

The low temperature resistance of the compound can be improved by plasticizing before mixing. Thin pass treatment after the compound is parked can improve the dispersion properties of fillers and compatibilizers, and improve the low-temperature resistance. Generally speaking, the longer the molding, the lower the Mooney viscosity of the rubber. Increasing the number of molding times will reduce the Tg of the rubber, but the phenomenon is not significant, and there is no significant difference between the Tg and Tbri of FKM rubber with different number of molding times.


3 Conclusion

(1) The Tg of FKM is close to that of TR10, which can reflect the low temperature movement of FKM molecular chain and glass state temperature, and the Tbri is lower than that of Tg and TR10.

is lower than Tg and TR10.

(2) The low temperature embrittlement property of peroxide sulfurized high fluorine FKM is better, and the low temperature embrittlement property of bisphenol binary sulfurized FKM is worse.

(3) FKM rubber filled with carbon black N774 has better low-temperature resistance; rubber with a small amount of filler has a higher gel content and better low-temperature resistance.

(4) The number of molding times has little effect on the low-temperature embrittlement property of FKM compounds.


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