1 Selection of raw rubber
Plate heat exchanger rubber gasket at the same time by the gas side of the high temperature, water side of the role of high pressure, and also have contact with the air, the working conditions are more demanding. Therefore, the material should be very good heat compression resistance, and can withstand the medium of water and its steam. In such conditions, should choose propylene mass fraction of 40% ~ 50 % of EPDM, because this type of EPDM elasticity is the best.
EPDM with high mass fraction of third monomer has good elasticity and low compression set; however, due to the high degree of cross-linking, the tensile strength and elongation at break are lower, and the aging resistance is also poor. In addition, the aging resistance of EPDM with ENB as the third monomer is better than that of EPDM with HD.
2 Curing system
Generally, EPDM rubber can be vulcanized with two kinds of vulcanization systems: peroxide and sulfur yellow vulcanization. The cross-linking bond structure of peroxide vulcanization system is C-C bond, while the cross-linking bond structure of sulfur vulcanization system is C-S bond. The C-C bond is much more thermally stable than the C-S bond, so high temperature resistant EPDM rubber is vulcanized with peroxide. The most widely used peroxide is DCP. With the increase of DCP dosage, the vulcanized rubber is gradually transformed from the initial undersulfurization, low strength and large deformation to the increase of strength and decrease of deformation; then the degree of vulcanization is further increased, the strength begins to decline, and the deformation reaches a minimum; finally, after the excess of vulcanizing agent, part of the molecular chain begins to break the chain degradation, the strength continues to decline, and the deformation is gradually increased.
3.Filler
EPDM belongs to non-crystalline rubber, raw rubber strength is not high. But after adding reinforcing filler, the strength is greatly increased. Reinforcing fillers generally have little effect on heat resistance, but increasing the amount of filler still helps to improve the heat resistance of the rubber, but also reduce costs.
As the grade of carbon black increases, the compression set decreases, the elasticity increases, and the strength decreases. The strength of N990 carbon black is too low and difficult to control in production, so it is not used. By choosing N762 carbon black, a low compression set value can be obtained. In order to facilitate the dispersion of carbon black and get good processing performance, choose a small amount of plasticizer paraffin oil, which is very compatible with ethylene propylene rubber.
4. Crosslinking agent
There is no unsaturated bond on the main chain of EPDM, although peroxide vulcanization can be used, but the vulcanization speed is slow, crosslinking efficiency is low. In order to stabilize and accelerate the vulcanization process, it is necessary to use crosslinking agent. The treated TAC/GR is better for dispersion and weighing.
With the increase of the amount of TAC/GR, the tensile strength, elongation at break and compression permanent deformation of vulcanized rubber decreased, and the MH value of constant tensile stress increased. It can be seen that the crosslinking density of vulcanized rubber was improved, the mechanical properties decreased, but the elasticity became bigger. At the same time, the ML value decreased, indicating that the Mooney viscosity decreased, the fluidity of the rubber was enhanced, and it was easy to be processed; ts1 was basically unchanged, and the t 90 value was shortened, and the vulcanization speed was obviously improved. The aging performance was poorer at 1.5phr of TAC/GR. This may be due to the presence of cross-linking aid, activate the peroxide vulcanization process, shorten the vulcanization time, reduce the possibility of rubber molecular chain breakage and disproportionation at high temperature. When the amount is too much, it enhances the aging cross-linking of the rubber and reduces the heat resistance of EPDM. After comparing, the compression permanent deformation is better at 2phr, and the heat resistance is not sacrificed much, so the TAC/GR is used at 2phr.
5.Antioxidant
EPDM can be used for a long time at 150 ℃, but beyond 150 ℃, the molecules begin to age gradually, and at 180 ℃, the rubber molecular chain will slowly decompose. In order to further improve its performance at high temperatures, protective additives must be used. In the case of water-vapor media, the antioxidant RD can be used, which is resistant to high temperatures but does not easily affect the vulcanization. In addition, 0.5phr antioxidant D can be used to strengthen the ability to resist oxygen in the air and improve the flexural fatigue resistance. The ratio of RD/anti-butyl=1.8/0.5.
Conclusion
The development of high-temperature water vapor-resistant high elasticity EPDM rubber, the focus is to meet the mechanical properties under the premise of maximizing the aging resistance and reduce the compression permanent deformation value.
( 1) The raw EPDM rubber is selected with 40%~50% of propylene and medium mass fraction of ENB as the third monomer.
( 2) The vulcanization system adopts DCP/ TAC, which helps to improve the crosslinking efficiency, aging resistance and process performance.
( 3) Use high elasticity carbon black N762 as much as possible, which can help to improve the resistance to media permeability and aging performance, and can reduce the cost.
( 4) Adopt the protection system of antioxidant RD/antioxidant D, with high-temperature antioxidant RD as the main one, and flexural fatigue-resistant and ozone-resistant antioxidant D as the auxiliary one.
( 5) The production process adopts high-temperature secondary vulcanization to increase the degree of vulcanization.
(6) The performance of EPDM in superheated water and water vapor is obviously better than that in air (same temperature). But even so, its long-term use of temperature is still not more than 150 ℃; impact temperature of 165 ℃ is appropriate, the highest can not exceed 180 ℃.
