Rubber Seals: The Key Choice for Valve Sealing

In the industrial field, valves are indispensable equipment, and sealing performance is one of the key indicators for evaluating valve quality. As an important component of valve sealing, rubber seals play a crucial role in the sealing performance and service life of valves. This article, from a design perspective combined with practical experience, explores the factors to consider when selecting rubber seal materials, including operating temperature, media, hardness, compression set, and wear resistance, providing a reference for the mechanical sealing industry.

The primary function of a valve is to control the flow of fluid, and reliable sealing performance is the key to ensuring this function is properly executed. Poor sealing performance can not only cause fluid leakage but may also lead to safety accidents and environmental pollution. Many factors affect valve sealing, mainly including the design of the sealing surface, material selection, the pressure difference before and after the valve, and the composition of the medium. Among them, the choice of sealing material is particularly critical, especially rubber sealing materials.

As mentioned above, sealing performance is an important indicator for evaluating valve quality, and as the core component of valve sealing, the quality of rubber seals directly affects the sealing effect and service life of valves. Therefore, when designing and manufacturing valves, it is necessary to have a thorough understanding of the characteristics of rubber sealing materials and to make a reasonable choice according to the specific application scenario of the valve.

The performance of rubber materials changes with temperature. Rubber can maintain elasticity and flexibility only within its suitable working temperature range, thus achieving sealing. If the temperature is too low, the rubber will harden, lose elasticity, or even become brittle; if the temperature is too high, the rubber is prone to oxidation, thermal aging, or chemical reactions with the sealed fluid, leading to a decline in physical, chemical, and mechanical properties, ultimately losing sealing function. Different types of rubber materials have different temperature ranges. For example, Nitrile Butadiene Rubber (NBR) has an operating temperature range of -40 to 120 °C, while Hydrogenated Nitrile Butadiene Rubber (HNBR) has a range of -40 to 150 °C. Due to the diversity of rubber formulations, the performance of products from different companies may vary. Therefore, when selecting rubber sealing materials, specific product data should be referenced.

Rubber seals must be able to withstand the medium they directly contact. The medium not only includes the substance being sealed but may also include lubricating oil, cleaning agents, or solvents used during system cleaning. Valves are widely applied, and the types of media in contact with rubber seals are diverse, such as acids, alkalis, alcohols, oils, organic solvents, water, steam, oxygen, and so on. If the medium is simple, selecting a seal is relatively easy. For example, for ethanol (alcohol) or its aqueous solution, EPDM, silicone (SIL), NBR, and CNBR can all be used as alternative materials. However, if the medium contains multiple chemicals, it is necessary to consider whether the rubber seal can resist these chemicals. For instance, if the system contains both ethanol and isopropanol, EPDM or silicone rubber may be a better choice.

If the resistance of the seal material to a specific liquid medium is uncertain, an immersion test can be performed. At a specified temperature, the rubber part is immersed in the medium for several days or longer. If the rubber does not resist the medium, usually after 7–14 days at high temperature, the rubber may show changes in volume, weight, hardness, shape, or even decompose.

Rubber hardness is usually expressed in Shore A, commonly ranging from 40 to 90 Shore A. Softer rubber is more flexible but has lower wear resistance; harder rubber generally has a higher tensile modulus and better compression set performance. However, these rules are not absolute, as the actual hardness of the seal can sometimes be difficult to measure accurately, and properties such as wear resistance, modulus, and compression set are closely related to material type and formulation.

Compression set is an important indicator for evaluating rubber products. It reflects not only the ability of the rubber to recover its original shape after pressure is applied but also the quality of the rubber product (whether it is fully vulcanized). During testing, a rubber sample of known height is compressed to a specified extent, maintained at a certain temperature for a period, then released, and the final height of the sample in a free state is measured. If a rubber seal repeatedly undergoes “pressure-load and release” cycles in use, those with poor compression set may quickly lose sealing capability. In practice, compression set may be partially offset by the swelling of the rubber in the medium, so other factors such as solvent resistance, valve pressure, and wear resistance must be considered comprehensively.

Wear resistance is an important indicator for dynamic seals. However, results from standard tests sometimes do not match real-world conditions. The best approach is to combine material experience with functional testing of the rubber part. For example, polyurethane has the best wear resistance, while silicone has poorer wear resistance. However, special additives in rubber formulations can significantly improve wear resistance. For instance, self-lubricating EPDM has a lower surface friction coefficient than general grades and performs better than simply applying lubricating oil, since lubricating oil is consumed quickly during use, whereas special additives in self-lubricating products maintain a smooth surface for a longer time.

After understanding the key factors in selecting rubber sealing materials, we will introduce some common rubber seal materials and their performance characteristics in detail. These materials are widely used in various industrial applications, and each has unique advantages and applicable ranges.

NBR seals are suitable for petroleum-based hydraulic oils, glycol-based hydraulic oils, diester lubricants, gasoline, water, silicone grease, silicone oil, and other media. It is currently the most widely used and cost-effective rubber seal. However, it is not suitable for polar solvents such as ketones, ozone, nitro hydrocarbons, MEK, and chloroform. General operating temperature: -40 to 120 °C.

HNBR seals have excellent corrosion resistance, tear resistance, and compression set resistance, good ozone, sunlight, and weather resistance, and wear resistance superior to NBR. Suitable for washing machines, automotive engine systems, but not recommended for alcohols, esters, or aromatic solutions. General operating temperature: -40 to 150 °C.

Silicone rubber seals have excellent heat, cold, ozone, and atmospheric aging resistance, good insulation, but lower tensile strength than general rubber and lack oil resistance. Suitable for household appliances such as water heaters, irons, microwaves, and products in contact with the human body such as kettles and water dispensers. Not recommended for most concentrated solvents, oils, strong acids, or sodium hydroxide. General operating temperature: -55 to 250 °C.

Fluorocarbon rubber seals have superior high-temperature resistance compared to silicone, excellent weathering, ozone, and chemical resistance, but poor cold resistance. Resistant to most oils and solvents, especially acids, aliphatic hydrocarbons, aromatic hydrocarbons, and animal and vegetable oils. Suitable for diesel engines, fuel systems, and chemical plant sealing needs. Not recommended for ketones, low-molecular esters, and nitrate-containing mixtures. General operating temperature: -20 to 250 °C.

Fluorosilicone rubber seals combine the advantages of fluorocarbon and silicone rubbers, with excellent oil, solvent, fuel, and high- and low-temperature resistance. Resistant to oxygen-containing compounds, aromatic solvents, and chlorinated solvents. General operating temperature: -50 to 200 °C.

EPDM seals have good weather, ozone, water, and chemical resistance. Suitable for alcohols, ketones, and sealing in high-temperature steam environments. General operating temperature: -55 to 150 °C.

CR seals have excellent sunlight and weather resistance, resistant to refrigerants like dichlorodifluoromethane and ammonia, dilute acids, and silicone-based lubricants. Swells in mineral oils with low aniline points and may crystallize or harden at low temperatures. Suitable for atmospheric, sunlight, ozone exposure, and chemically resistant sealing. Not recommended for strong acids, nitro hydrocarbons, esters, chloroform, or ketones. General operating temperature: -55 to 120 °C.

Butyl rubber seals have excellent gas tightness, heat, sunlight, and ozone resistance, and good insulation. Resistant to polar solvents such as alcohols, ketones, and esters, as well as exposure to animal and vegetable oils or oxidizers. Suitable for chemical-resistant or vacuum equipment. Not recommended for petroleum solvents, kerosene, or aromatics. General operating temperature: -50 to 110 °C.

ACM seals have excellent oil resistance and high-temperature and weather resistance, but slightly lower mechanical strength, compression set, and water resistance. Generally used in automotive transmission and power steering systems. Not suitable for hot water, brake fluids, or phosphate esters. General operating temperature: -25 to 170 °C.

Natural rubber seals have good wear resistance, elasticity, tensile strength, and elongation, but age easily in air, become sticky with heat, and swell or dissolve in mineral oil or gasoline. Resistant to alkalis but not strong acids. Suitable for automotive brake fluids, ethanol, and other liquids with hydroxyl ions. General operating temperature: -20 to 100 °C.

Polyurethane rubber has excellent mechanical properties, wear resistance, and high-pressure resistance, superior to other rubbers. It also has good aging, ozone, and oil resistance, but is prone to hydrolysis at high temperatures. Generally used for high-pressure and high-wear sealing applications. General operating temperature: -45 to 90 °C.

Rubber seals play a crucial role in valve sealing. Selecting suitable rubber seal materials requires comprehensive consideration of operating temperature, media, hardness, compression set, and wear resistance. Different rubber materials have distinct performance characteristics and are suitable for different applications. In practice, material selection and testing should also consider specific working conditions and experience to ensure valve sealing performance and service life.