Rubber sheets are commonly used as flange gaskets for pipes or manholes that require frequent disassembly, with a pressure not exceeding 1.568MPa. Among various sealing gaskets, rubber gaskets are the softest and have good conformability, providing effective sealing even with lower preload. However, due to the thicker thickness or lower hardness of rubber gaskets, they are prone to extrusion when subjected to internal pressure.
Rubber sheets are susceptible to swelling, weight gain, softening, and stickiness when used in organic solvents such as benzene, ketones, and ethers, leading to seal failure. Generally, if the swelling degree exceeds 30%, it is not suitable for further use.
Rubber gaskets are more suitable for low-pressure conditions (especially below 0.6MPa) and vacuum. Rubber materials have good density and low permeability. For example, fluororubber is most suitable for sealing gaskets in vacuum containers, with a maximum vacuum degree of up to 1.3×10-7Pa. When using rubber gaskets in the vacuum range of 10-1~10-7Pa, baking and degassing treatment are required.
Asbestos rubber sheets have a lower price and are easy to use. However, the main issue is that even though rubber and some fillers are added to the sealing gasket material, they still cannot completely fill all the interconnected micropores, resulting in slight permeation. Therefore, they should not be used in highly contaminated media, even at low pressure and temperature. When used in high-temperature oil media, the rubber and fillers in the gasket can carbonize, reducing strength and causing permeation at the interface and inside the rubber gasket, resulting in coking and smoking. Additionally, asbestos rubber sheets are prone to adhere to flange sealing surfaces at high temperatures, causing difficulties in gasket replacement.
The operating pressure of rubber gaskets in various media under heated conditions depends on the strength retention rate of the gasket material. Asbestos fiber materials contain crystalline water and adsorbed water. At 110°C, about 2/3 of the adsorbed water between the fibers is released, leading to a decrease in tensile strength by approximately 10%. At 368°C, all the adsorbed water is released, resulting in a decrease in tensile strength by about 20%. At temperatures above 500°C, crystalline water starts to be released, further reducing strength.
The media also has a significant impact on the strength of asbestos rubber sheets. For example, in aviation lubricating oil and aviation fuel, the transverse tensile strength of 400-grade oil-resistant asbestos rubber sheets can differ by 80%, which is due to the severe rubber swelling caused by aviation fuel compared to aviation lubricating oil. Taking these factors into consideration, the recommended safe operating range for domestically produced XB450 asbestos rubber sheets is a temperature of 250°C to 300°C and a pressure of 3 to 3.5MPa. The usage temperature of 400-grade oil-resistant asbestos rubber sheets should not exceed 350°C.
Asbestos rubber sheets contain chloride ions and sulfides, which can form a corrosive galvanic cell with metal flanges after absorbing water. This is especially true for oil-resistant asbestos rubber sheets, which have a sulfur content several times higher than ordinary asbestos rubber sheets, making them unsuitable for use in non-oil media. Rubber gaskets can swell in oil and solvent media, but within a certain range, it has little effect on sealing performance. For example, 400-grade oil-resistant asbestos rubber sheets are required to have an increase in weight due to oil absorption of no more than 15% after a 24-hour immersion test in ambient temperature aviation fuel.
