The carbon graphite rings used in mechanical seals. During the sintering process, there will be pyrolysis substances and pores discharged, and the porosity, calculated by volume, accounts for approximately 10% to 30%. This porous carbon graphite ring cannot be used as a mechanical seal ring. Therefore, it needs to undergo impregnation treatment to make it an impermeable material. Through impregnation, its strength and wear resistance can also be improved.
Phenolic resin is a general term for a class of resins obtained by condensation polymerization of phenolic compounds under the action of a catalyst.
1) Machining of the blank
The graphite rod material is processed into tube material, and the processing allowance of 2-3mm is reserved for the inner and outer diameters according to the finished product size. If the blank is a large block, it is advisable to cut it down using a thin abrasive wheel and then processed into tube material. The blank must not be contaminated with oil.
2) Cleaning
Use compressed air to clean the surface of the tube material of graphite dust, or wash it with tap water to prevent the impregnation surface pores from being blocked, which will affect the impregnation effect.
3) Low-temperature baking
Heat to 70-80℃, remove after 2 hours and let it cool naturally to remove the moisture in the gas pores of the graphite tube material.
4) Selection of impregnating agent
Phenolic resin has two types of water-soluble and alcohol-soluble. Generally, 7222 alcohol-soluble resin is selected. Alcohol-soluble resin is prone to volatilization, and its storage period is shorter than that of water-soluble resin. After multiple uses, its viscosity is high. When using again, add anhydrous ethanol to stir and dilute.
The impregnation process is as follows.
1) Place the graphite tube material into the wire basket frame, maintaining a distance of more than 2mm between the tubes. Use wire mesh to isolate each layer, then place the basket frame into the reactor, leaving a distance of 1/5 of the reactor body depth from the reactor flange. Close the reactor cover and evenly tighten the fastening bolts.
2) Vacuum. When the vacuum degree is around 0.1 MPa (750-760 mmHg column), maintain for 30 minutes.
3) Inhale the resin in the impregnation reactor, with the height being suitable to bury the workpiece by 100mm.
4) Introduce 0.6-0.8 MPa of compressed air and maintain for 4-5 hours.
5) Drain the resin, release the pressure, and remove the product. After the product is removed from the reactor for 2 hours, use an alkaline solution mixed with sawdust chips to wipe off the resin on the outer surface of the product.
6) Room temperature curing. Place the dried product in the curing reactor, apply pressure and heat for curing. The pressure should be 0.1 MPa higher than that during impregnation.
3) Curing process
Place the dried product in the curing reactor, apply pressure and heat for curing. The pressure should be 0.1 MPa higher than that during impregnation.
The curing of phenolic resin by immersion usually occurs at a temperature of 130℃. However, if the curing temperature is appropriately increased, it can enhance the curing effect of the phenolic resin and improve its corrosion resistance.
After the products have been cured, the透气ity can be tested using the following method: randomly select 3 pieces from each batch of products, remove the outer surface, place the two ends of the tube material between two rubber sealing gaskets on a special fixture, then introduce dry nitrogen gas into the inner cavity of the tube material. The test pressure is the maximum operating pressure of the product， and it is maintained for 15 minutes. During the pressurization process， a layer of soap solution is applied around the periphery of the test piece， and then check for the presence of bubbles.
The impregnation and curing treatment of the blank tube material usually requires repeating twice to meet the requirements. If the porosity is large (>15%)， or if the operating pressure is high and the shaft diameter is large， it is generally necessary to perform the impregnation and curing treatment three times. The first two times are to impregnate and cure the blank， and the last treatment is to impregnate and cure the finished product with a low-viscosity resin. Before each curing process， the surface of the sealing ring should be wiped clean with alcohol.
Furan resin is a type of thermosetting resin. It is a general term for a class of polymers made from furfural. It has a furan ring in its molecular structure, hence it is called furan resin.
1. Selection of Resin
Furfural resin is one type of furan resin. It is a viscous liquid formed by the one-step condensation of furfural and acetone. Its viscosity varies greatly depending on the production process. Therefore, an appropriate viscosity should be selected to simplify the impregnation process, improve the impregnation effect, and make full use of the impregnating agent. The viscosity is measured using the paint cup No. 4, and a resin with a measurement value of around 30 seconds is preferred as the impregnating agent.
2. Selection of Curing Agent
The furfural resin must be cured with a curing agent. Therefore, correctly selecting the curing agent is also very important for improving the impregnation effect and extending the usage time of the resin. Currently, the commonly used curing agents for furfural resin are ethyl sulfate, zinc chloride, and benzene sulfonate chloride. Among them, benzene sulfonate chloride is the most effective as a curing agent, and using it as a curing agent results in a longer usage period for the resin. However, it can release inhibitory gases, so proper ventilation in the room must be ensured during operation.
3. Heating and curing
To prevent resin bubble formation and overflow during the heating process, the temperature should be increased according to the heating rate specified in Table 9-20. The 60-90℃ range is the most critical during the entire curing process. If the temperature rises too quickly, the resin is prone to overflow.
1. Properties of Epoxy Resin
Epoxy resin is a type of high-molecular polymer containing epoxy groups. Among them, the resin obtained by the condensation of bisphenol A and epichlorohydrin is the best for impregnation. Representative brands include 610, 618, 634, etc. In its molecular structure, there are hydroxyl groups, ether groups, and extremely active epoxy groups. Therefore, graphite products impregnated with this resin have increased weight efficiency, high mechanical strength, and low porosity.
2. Dilution of Epoxy Resin
Epoxy resin has a high viscosity and must be diluted with a certain amount of diluent to reduce its viscosity before it can be used for impregnation of graphite. Diluents are divided into non-active and active types. The former includes acetone, xylene, etc.; the latter contains alkyl butyl ether. The commonly used 501# (with a relative molecular weight of 130 and a viscosity of ≤ 2x10-3 Pa.s at 25℃) is a new active diluent, which is basically non-toxic and has good performance.
3. Curing of Epoxy Resin Impregnated Graphite
Epoxy resin is a thermoplastic linear resin. It must be cured with a curing agent to crosslink the linear molecules into a network structure of macromolecules and become an insoluble high polymer. The curing agents include polyamines, acid anhydrides, etc. The 595 curing agent has the best effect. 595 curing agent is an amine borane-type compound with a viscosity of (3~5) x10-3 Pa.s.
4. Impregnation and Curing Process Flow
1) Prepare Impregnating Agent
The formula of the impregnating agent is: 75% epoxy resin of 6101, 5% 595 curing agent, and 25% 501 diluent. The epoxy resin is preheated to 40℃ to increase its fluidity, and then the diluent is added and thoroughly mixed.
2) Impregnation process conditions
Place the product in the reactor, evacuate to a pressure of 0.096 kPa (720 mmHg column) or higher, introduce the prepared impregnating agent, then apply pressure to 0.7 MPa, and maintain the pressure for 4 hours. Remove the product and clean the surface with acetone.
3) Curing treatment process conditions
Curing pressure: 0.7 MPa
1. Characteristics of Polytetrafluoroethylene
The internal cohesion of the molecules of polytetrafluoroethylene is low, which determines its excellent wear resistance and low friction coefficient. It also has excellent chemical stability, good corrosion resistance, and good thermal stability, capable of adapting to a temperature range of -180 to 250 degrees Celsius. Impregnating graphite with polytetrafluoroethylene dispersion liquid can be used as a corrosion-resistant mechanical seal friction pair material.
2. Impregnation Process
1) The impregnating agent is a polytetrafluoroethylene dispersion liquid with a concentration of 60%, a particle size of 0.2 to 0.25 microns, a pH of 10, and a viscosity of (4 to 8) x 10-3 Pa.s.
2) The graphite tube material requires a porosity of 20% to 30%.
3) Impregnation process: After rough processing and cleaning of the blank, it is dried at 120 degrees Celsius for 1 hour, then naturally cooled, and then placed in a reactor to be evacuated [vacuum degree > approximately 0.1 MPa (750 mmHg), maintained for 1 hour], then undergoes plasticization (375 to 380 degrees Celsius).
4. Plasticization
The purpose of plasticization is to make the dispersed liquid particles in the graphite pores fuse together in the molten state to form a film-like structure. The heating rate during plasticization must be strictly controlled. The heating rate is: room temperature to 100 degrees Celsius 1 hour, 100 to 160 degrees Celsius 1 hour, 160 to 220 degrees Celsius 1 hour, 220 to 270 degrees Celsius 1 hour, 270 to 320 degrees Celsius 1 hour, 320 to 370 degrees Celsius for 1 hour, and then 340 to 380 degrees Celsius for 1 hour for holding, followed by a temperature reduction to 160 degrees Celsius and then turning off the power, and then naturally cooling to room temperature.
5. Precautions
1) The impregnation effect of polytetrafluoroethylene is poor. Therefore, the impregnation and plasticization process should be repeated according to the above process, and repeated treatment is required to achieve the expected result. The number of treatments depends on the pressure resistance requirements of the product. Generally, it needs to be repeated 3 times.
2) The polytetrafluoroethylene dispersion liquid of the impregnating agent should be kept fresh and should not be reused. The impregnation temperature is preferably controlled at 20-40 degrees Celsius.
