CR (Neoprene) Rubber Technical Guide

1. What Is CR (Chloroprene / Neoprene) Rubber?

CR, more widely recognized by the DuPont trade name Neoprene, is a synthetic rubber produced by the emulsion polymerization of 2-chloro-2,1,3-butadiene (chloroprene monomer). First commercialized by DuPont in 1931, CR was one of the earliest oil-resistant synthetic rubbers and remains one of the most balanced all-purpose elastomers available today.

The chlorine atom in the polymer backbone gives CR its characteristic balance of properties: it resists both weathering/ozone and oils/fuels, a combination that neither natural rubber (excellent dynamic properties but poor oil resistance) nor NBR (excellent oil resistance but poor weathering) can deliver alone.

CR is classified under ASTM D2000 / SAE J200 designation BC, BE (Class B = service temp 100°C; oil resistance Class C/E). Under ISO 1629, it is designated "CR."

2. Mechanical and Physical Properties

CR offers a well-rounded property profile suitable for general industrial, marine, and construction applications.

Key insight: CR's tensile strength (10–25 MPa) sits between NR (15–30 MPa) and EPDM (7–20 MPa). Its tear resistance is notably superior to EPDM, making it the preferred choice for applications where both weather resistance and physical robustness are required.

3. Resistance Properties

3.1 Weathering and Ozone Resistance

CR exhibits excellent resistance to ozone, UV radiation, and general weathering. This is attributed to the chlorine atom in the polymer chain, which deactivates the double bond, making it less susceptible to ozone attack compared to the highly unsaturated backbones of NR, SBR, and NBR.

• • Ozone resistance: No cracks after 1000 h at 50 pphm ozone, 20% elongation (ASTM D1149)
• • Outdoor service life: 10–15 years without significant degradation (properly formulated)

3.2 Flame Resistance

CR is one of the few intrinsically flame-resistant elastomers. The chlorine content (~40% by weight) releases HCl when exposed to flame, which acts as a radical scavenger in the gas phase, quenching the flame propagation reaction.

• • UL94 rating: V-0 achievable with flame-retardant compounding
• • Oxygen Index (LOI): 27–46% (depending on formulation)
• • Chlorine content: 36–40%

This makes CR the rubber of choice for:

• • Mining conveyor belts (fire-resistant grades per MSHA/ISO 340)
• • Cable jacketing in tunnels and buildings
• • Mass transit interior components (EN 45545-2)

3.3 Oil and Chemical Resistance

CR offers moderate oil resistance, between that of NBR (excellent) and NR/EPDM (poor).

• • Volume swell in ASTM IRM 903 oil: 30–50% (70h/100°C)
• • Good resistance to refrigerants (R-12, R-22) and Freon
• • Good resistance to dilute acids and alkalis
• • Poor resistance to: aromatic solvents (benzene, toluene), ketones (MEK, acetone), chlorinated solvents

3.4 Other Resistances

4. Comparison: CR vs EPDM vs NBR

Understanding where CR sits relative to the other major elastomers is essential for material selection.

Decision logic:

• • Weathering + no oil exposure → choose EPDM (lower cost, better low-temp)
• • Oil exposure + no weathering → choose NBR (better oil resistance)
• • Both weathering AND oil + flame resistance needed → choose CR

5. Applications

5.1 Cable Jackets and Wire Insulation

CR is widely specified for cable jackets in environments requiring flame resistance and mechanical durability. Typical applications include mining trailing cables, shipboard cables, welding cables, and control cables in tunnels and underground railways. Specifications often reference IEC 60245, ICEA S-75-381, and UL 62 for flexible cord jackets.

5.2 Flame-Resistant Conveyor Belts

In underground coal mining and other hazardous environments, CR-based conveyor belt covers meet the flame-resistance requirements of ISO 340, MSHA 30 CFR Part 14, and EN 14973 Category M2. The combination of self-extinguishing behavior and good abrasion resistance makes CR the primary cover compound for these safety-critical applications.

5.3 Marine Dock Fenders and Oil-Spill Resistant Components

CR-based dock fenders and marine bumpers resist both saltwater weathering and incidental oil/fuel exposure on the water surface — conditions that would degrade NR-based fenders within a single season. CR cylindrical and D-type fenders are specified under PIANC WG33 guidelines and commonly meet the energy absorption and reaction force requirements of BS 6349 (Maritime Works).

5.4 Expansion Joints and Bridge Bearings

CR is the dominant elastomer for structural expansion joints in bridges and buildings. The combination of ozone resistance, moderate oil resistance (road de-icing salts, fuel spills), and good compression set resistance meets the requirements of AASHTO M251 and EN 1337-3 for structural bearings.

5.5 Industrial Hoses and Gaskets

CR is commonly used for the outer cover of industrial hoses (hydraulic, chemical transfer) where weather resistance and abrasion resistance are required. Gaskets for water treatment plants, HVAC systems, and general industrial service frequently use CR for its broad chemical compatibility and sealing performance.

6. Compounding and Formulation

6.1 Cure Systems

CR is most commonly vulcanized using metal oxide cure systems, typically zinc oxide (ZnO) and magnesium oxide (MgO). Unlike sulfur-cured diene rubbers, the metal oxide cure for CR involves:

• • ZnO functioning as a crosslinking agent
• • MgO acting as an acid acceptor (scavenging HCl released during cure and service)
• • Ethylene thiourea (ETU) as the primary accelerator (though regulatory pressure is driving alternatives)

Peroxide cures are generally not effective for CR due to the chlorine atom interfering with free-radical crosslinking.

6.2 Carbon Black Selection

6.3 Plasticizers and Processing Aids

• • Petroleum-based oils: moderate compatibility; low-aniline-point oils preferred
• • Ester plasticizers (DOS, DOA): improve low-temperature flexibility
• • Factice (vulcanized vegetable oil): improves processing and surface finish

7. Standards and Specifications

8. Limitations and Considerations

Despite its versatility, CR has several limitations:

1. Higher specific gravity (1.35–1.50) compared to NR (0.93) or EPDM (0.86). Parts weigh 40–50% more, increasing material cost per part.

1. Poor low-temperature performance: Brittle point at -40°C; most grades stiffen significantly below -25°C.

1. Crystallization: CR grades with high trans content can crystallize on storage, causing stiffening. Applying mild heat reverses this reversible stiffening.

1. Poor aromatic solvent resistance: Cannot be used in contact with fuels containing high aromatic content.

1. Limited continuous service temperature: 110°C is the practical limit; EPDM and ACM offer better heat resistance.

1. Storage stability concerns: CR compounds have more limited storage life than EPDM or NR; the uncured rubber can scorch in storage under hot ambient conditions.

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Nanjing Yuhang Rubber Co., Ltd. is a leading manufacturer of industrial rubber products in China, supplying high-quality CR (Neoprene) rubber sheets, gaskets, marine fenders, conveyor belts, and custom-molded components to customers in over 75 countries. Our CR compounds are formulated and tested in-house to meet ASTM D2000 BC/BE, ISO 340 flame-resistance, and PIANC marine fender standards.

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