Apple Rubber provides a wide range of seals for the fluid power market. A common question is which rubber material is best suited for specific applications. Since engineering schools often offer limited coverage of rubber materials, engineers frequently need to conduct extensive research to identify their best options.
This paper examines a category of rubber known as thermoset rubber. Thermoset rubbers are typically cured with sulfur or peroxide. This curing process allows them to return close to their original shape when subjected to heat or stress. In contrast, thermoplastic rubbers do not undergo curing and retain their shape when exposed to heat or stress, commonly called compression set. Examples of thermoplastic rubbers, often called thermoplastic elastomers (TPE), include SantopreneTM and urethanes (excluding cast or millable types).
ASTM D2000
Fluid power applications depend on the compatibility of the fluid with the rubber and the temperature range in which the rubber can operate. ASTM D2000 classifies rubber based on its heat and oil resistance. The classification uses letters “A” to “K” for oil resistance, where “A” represents the highest oil swell and “K” indicates the lowest. The classification uses letters “A” to “H” for operating temperature, with “A” corresponding to 70°C and “H” to 250°C.
For example:
- AA: Natural Rubber
- BG: Nitrile Rubber
- HK: Fluorocarbon Rubber
Note: ASTM D1418 provides standard abbreviation codes for different types of rubber: NR for natural rubber, NBR for nitrile rubber, and FKM for fluorocarbon rubber.
Key Concepts
**Saturated vs. Unsaturated Rubber:**
The terms “saturated” and “unsaturated” refer to the number of single, double, or triple bonds present in the polymer structure of rubber. This distinction is significant because it affects the compound’s polarity, influencing its ability to repel fluids. Furthermore, double bonds can be a weak point in the polymer backbone, as they make the material susceptible to attack by free radicals, leading to rubber degradation.
Double bonds are particularly vulnerable because they require relatively little energy to break. An example is acrylonitrile-butadiene rubber (NBR), which typically operates from 100°C to 125°C. NBR can undergo hydrogenation, adding hydrogen to the double bonds in the polymer backbone, resulting in hydrogenated acrylonitrile-butadiene Rubber (HNBR). HNBR has an improved operating temperature range of 150°C to 175°C.
Polarity: The most basic chemistry statement is “likes dissolve likes.” Polarity concerns the chemical structure and electrical charges in that structure.
| UnPolar Media | Polar Rubber |
| Mineral Oil | NBR, CR |
| Hydrocarbons (benzene, aromatics) |
ACM, HNBR, FKM, ECO, Silicone, FluoroSilicone |
| Polar Media | Unpolar Rubber |
| Water / Steam | NR, IR, SBR, BR |
| Alcohols / Glycols | EPDM, IIR, CIIR |
| Acids / Alkalies | |
| Esters / Ketones |
For most applications, polymer is the primary concern for special applications; the entire compound needs to be considered, including the polymer, fillers, protectants, processing aids, and curatives. Each ingredient can either enhance or detract from the performance of the rubber compound in specific applications.
Material Choices:
NBR (Buna-N): Acrylonitrile-butadiene rubber
ASTM D1418: NBR
ASTM D2000: BF, BG (Standard), BK, CH
Advantages: Excellent oil, fuel, and petroleum-based hydraulic fluids, wide operating temperature range (-40°C to +135°C).
Disadvantages: Poor resistance to ozone, UV, and environmental aging. Not suitable for continuous operation above 125°C. It can show dry-out shrinkage in fuel applications.
Polymer Consideration: NBR polymers are formulated with a specific acrylonitrile (ACN) content. As the ACN content increases, the material exhibits less oil swelling but sacrifices low-temperature flexibility (-25°C). Conversely, a lower ACN content results in higher oil swelling but provides better performance at low temperatures (-60°C). Most standard NBR compounds contain approximately 33% ACN to achieve optimal operating conditions. In industry literature, NBR compounds with high ACN levels are often referred to as “low swell,” while those with low ACN levels are labeled as “low temp.”
FKM: Fluorocarbon rubber
ASTM D1418: FKM, Type 1, 2, 3, and 5
ASTM D2000: HK
Advantages: Excellent oil, fuel, and petroleum-based hydraulic fluids, operating temperature range (-25°C to +400°C). Weather and ozone resistance. Lower friction for dynamic sealing.
Disadvantages: Higher expense. Poor low-temperature and flex-fuel resistance for standard grades. Not suitable for amine-based oil additives. Considered a PFAS.
Polymer Consideration: The base polymer used determines properties. Type 1 – standard copolymers that are typically used. Type 2 – terpolymers, which are higher fluorine which gives better chemical resistance. Type 3 – a low-temperature polymer for static applications to -40°C. Type 5 – marketed as Viton ETP®, with very high chemical resistance.
Note: FFKM is the highest level of fluorine and the highest chemical resistance. It can be 10 to 20 times the price of standard polymers.
Urethane: Polyester / polyether urethane rubber
ASTM D1418: AU / EU
ASTM D2000: BG
Advantages: Excellent abrasion and tear resistance. Higher tensile strengths. Resistance to ozone, UV, and weathering. Good resistance to oil and fuel.
Disadvantages: Poor resistance to hot water, acids, alkalis, and oxygenated solvents. Higher compression sets than NBR.
Polymer Considerations: Polyester-based polymers offer superior fuel and oil properties but are prone to hydrolysis. In contrast, polyether-based polymers exhibit hydrolysis resistance, although they typically perform worse at lower temperatures. Three main types of urethanes: Millable urethane, used in conventional molding; cast urethane, which are liquid grades that are poured into molds; and injection urethanes, which are used in plastic injection molding processes.
Other polymers to consider are HNBR for high temperature, EPDM for water applications, and Silicone for extreme temperature and dampening applications.
**References
ARPM Training Academy, (2024, July) Seals Product Design and Manufacturing
www.applerubber.com
Sponsored content by Apple Rubber
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