Apple Rubber has been molding for over 50 years, and we have developed many compounds over that time. Because our parts serve the sealing industry, we select compounds based on properties that allow for robust sealing. Chemical compatibility is critical for applications exposed to fluids, such as fuels, oils, acids, and bases. Physical properties include excellent compression set, heat resistance, and high tensile strength. For environmental exposure, compounds require ozone and UV resistance; for specialty applications like medical, compounds are tested to FDA USP Class VI and ISO 10993. Here are the primary materials Apple Rubber molds to meet customer requirements. Most of our materials are compounded for compression, transfer, and injection molding.
Acrylonitrile-Butadiene (NBR, Buna-N)
This is the workhorse for the automotive industry. NBR resists oil and fuels and offers a wide operating temperature range. NBR polymer is polymerized with varying levels of acrylonitrile, which changes volume swell and low-temperature performance. Modified polymers like Hydrogenated (HNBR) offer higher-temperature performance and better ozone resistance. Carboxylated (XNBR) gives better abrasion resistance.
- Fuel and oil resistance. High ACN: lower swell; low ACN: high swell.
- Standard operating range: -40°C to +135°C (-40ºF to +275ºF). High ACN: -25°C (+13ºF) operating range; Low ACN: +60°C (+140ºF) operating range.
- Poor ozone resistance.
- Possible crazing of certain plastics.
- Typically the largest O-ring inventory selection due to the automotive industry.
- Not recommended for medical applications or outdoor exposure.
Silicone Rubber (SL, VMQ)
Silicone offers one of the widest operating ranges for low- to high-temperature applications. The polymer structure is very stable for outdoor applications and has high ozone resistance. The medical industry uses it because of its inert properties, biocompatibility, and low extraction. Other silicone families include fluorinated (FS) and Phynel (PVMQ). FS rubber gives better fuel and oil resistance. PVMQ provides extremely low-temperature resistance down to -100ºC (-148ºF).
- Some fuel and oil resistance. Will pass the splash test application.
- Standard operating range: -60°C to +150°C (-76ºF to +302ºF). Most compounds are post-cured for 4 hours at 200°C (+392ºF), so they can handle temperatures for short periods of time.
- Low outgassing when post-cured. Can meet ASTM E595.
- Select grades are tested to FDA Medical and ISO 10993 Medical standards.
- Very good damping properties.
Fluorocarbon Rubber (FKM)
Widely used for chemical and high-temperature resistance. Very low compression set at high temperature. Because of its formulation, very low extraction when exposed to most chemicals. Also, very low outgassing since post-cured. FKM families with higher fluorine content offer even better chemical resistance. Functional groups improve low-temperature properties. Typically more expensive than many sealing polymers.
- Very good fuel and oil resistance. Also, can handle acidic applications.
- Standard operating range: -25°C to +200°C (+13ºF to +392ºF). Low-temp FKM down to -40°C (-40ºF). New polymers down to -60°C (-76ºF).
- Can be compounded for medical applications.
- Low outgassing when post-cured. Can meet ASTM E595.
- Perfluoroelastomer (FFKM) compounds meet extreme high-temperature (+300°C/+572ºF) and high-chemical resistance requirements, but at a very high cost.
Ethylene-Propylene (EP, EPDM)
EPDM is a modified EPR rubber adding a third comonomer diene for additional cure sites. EPDM rubber is more commonly used. EPDM has gained wide acceptance in the seal industry for excellent ozone and chemical resistance to acids, alcohols, and basic materials. EPDM polymers have varying ethylene-to-propylene levels. Higher ethylene improves physical properties but reduces low-temperature performance. Higher propylene improves low-temperature performance and reduces compression set.
- No fuel or oil resistance.
- Standard operating range: sulfur-cured -60°C to +135°C (-76ºF to +275ºF). Peroxide-cured compounds can handle +150°C (+302ºF).
- Excellent ozone resistance and very low water permeability or swelling.
- Good chemical resistance to solvents.
All of these four polymers can be used in FDA and EU food-contact applications when compounded correctly.
Many other polymers are used in specific applications. Apple Rubber molds these four the most. Other polymers that we mold are Chloroprene (CR, Neoprene) for outdoor applications with some oil contact. Butyl rubber in the sealing industry is used where reduced gas permeability is important. We mold urethane in high-abrasion applications, such as hydraulics. HNBR is recommended for the same application and provides a better compression set. Epichlorohydrin (ECO) is a good replacement for CR. ECO was mainly used for hose applications. New polymers have an improved compression set, which gives better sealing performance. Polyacrylate is used in automotive transmission seals.
In summary, industry-specific performance requirements and regulatory standards dictate the selection and formulation of rubber compounds for seals. While some materials overlap, each industry tailors its approach to optimize safety, reliability, and functional longevity. Advances in polymer chemistry and compounding techniques continue to expand the capabilities of rubber seals, supporting the evolving needs of medical, automotive, and aerospace applications.
If you have a rubber material in mind for your next sealing needs, reach out to us and we’ll get the quote you need fast.