CSM (Hypalon) Rubber Technical Guide: Weather-Resistant, Chemical-Resistant Industrial Coating & Lining Material

CSM Chlorosulfonated Polyethylene (Hypalon): The Weather-Chemical Dual-Resistance Elastomer

Published: 2026-01-28 | Reading time: 10 minutes

1. What Is CSM, and Why Does It Matter?

CSM -- Chlorosulfonated Polyethylene, historically known by the DuPont trademark Hypalon -- occupies a distinct niche among synthetic elastomers. It delivers three performance characteristics that rarely coexist in a single material: outstanding outdoor weathering (20+ years), robust resistance to concentrated acids and alkalis, and moderate oil resistance. No other commodity or mid-tier elastomer achieves this combination at a comparable cost.

The material starts as polyethylene (PE) and undergoes two sequential chemical modifications that convert a rigid semi-crystalline thermoplastic into a crosslinkable elastomer:

Step 1 -- Chlorination. PE dissolved in a chlorinated solvent reacts with chlorine gas (Cl2). Chlorine atoms replace hydrogen atoms along the polymer backbone at a controlled level, typically 25--43% by weight. This chlorination disrupts the regularity of the PE chain, suppressing crystallinity and transforming the material from a rigid plastic into an amorphous elastomer with a glass transition temperature (Tg) in the range of -20 to -30degC.

Step 2 -- Chlorosulfonation. The chlorinated intermediate reacts with a mixture of SO2 and Cl2, introducing sulfonyl chloride groups (-SO2Cl) pendent to the backbone. These groups, present at a sulfur content of 0.8--1.4 wt%, serve as the reactive crosslinking sites -- a fundamentally different cure chemistry from the sulfur vulcanization used in diene rubbers.

The resulting polymer can be represented approximately as:

[-(CH2-CH2)n-(CHCl-CH2)m-(CH-SO2Cl)-]x

The practical significance of these two structural features is profound:

• • Chlorine (25--43 wt%) provides inherent flame retardancy (LOI 28--32), polar chemical resistance, and moderate oil resistance. Higher chlorine content improves chemical resistance and flame retardancy but reduces low-temperature flexibility.
• • Sulfonyl chloride groups enable metal-oxide crosslinking that produces C-SO2-O-Me linkages. These bonds provide better thermal stability than polysulfidic crosslinks (C-Sx-C) but less than peroxide-generated C-C bonds -- a trade-off that defines CSM's position in the performance hierarchy.

Chlorine Content: The Primary Performance Lever

2. CSM Crosslinking Chemistry

Unlike diene rubbers (NR, SBR, NBR) that crosslink through sulfur-accelerator systems reacting with C=C double bonds, CSM crosslinks through its sulfonyl chloride groups. The standard cure system employs:

• • Metal oxides (MgO as the primary crosslinking agent, often with PbO as a co-agent for enhanced water resistance)
• • Organic accelerators (pentaerythritol, thiourea derivatives, or thiuram disulfides)
• • Optional acid acceptors (epoxidized soybean oil, calcium stearate) to neutralize HCl evolved during processing

The crosslinking reaction proceeds via hydrolysis and condensation of the -SO2Cl groups in the presence of MgO, forming sulfonate crosslinks:

2 -SO2Cl + 2 MgO + H2O  ->  -SO2-O-Mg-O-O2S- + 2 HCl

The HCl liberated during cure must be scavenged by the metal oxide; this is why MgO loading in CSM compounds is substantially higher (10--20 phr) than in conventional rubber formulations where ZnO at 3--5 phr is typical.

Peroxide cure is an alternative for applications demanding maximum thermal stability. Peroxide-generated C-C crosslinks (bond dissociation energy ~350 kJ/mol) outperform sulfonate crosslinks at sustained temperatures above 120degC. The trade-off is reduced scorch safety and generally lower elongation at break.

3. Performance Profile: Where CSM Excels

3.1 Weathering and Ozone Resistance

CSM's saturated polyethylene backbone lacks the allylic hydrogens adjacent to C=C double bonds that serve as initiation sites for ozone and oxidative attack in diene rubbers. The result is exceptional outdoor durability that matches or exceeds EPDM:

• • Expected outdoor service life: 20--25 years (properly formulated, without requiring antiozonant protection)
• • Ozone resistance: No cracking after 1000+ hours at 50 pphm ozone, 20% elongation
• • UV resistance: Excellent; the chlorine in the backbone absorbs UV and dissipates the energy without chain scission at rates comparable to EPDM
• • Colour stability: Unlike carbon-black-loaded EPDM (which must be black for UV protection), CSM can be formulated in light colours (white, grey, pastels) while retaining full weatherability -- a significant advantage for architectural applications and rubber dams where solar heat reflection reduces service temperature

3.2 Chemical Resistance

CSM's resistance to concentrated mineral acids and strong alkalis is its most commercially valuable characteristic. The polar chlorine atoms create a solubility parameter (~9.0--9.5 cal^(1/2)/cm^(3/2)) that resists penetration by aqueous acids and polar solvents, while the saturated backbone avoids the acid-catalyzed degradation that attacks unsaturated rubbers.

Immersion test data for CSM (35% Cl grade) at 23degC equilibrium exposure:

Critical temperature caveat: While CSM holds up well against aggressive chemicals at ambient temperature, its performance degrades significantly above 70degC. At 70degC, 70% H2SO4 produces +15% volume swell (vs. +3% at 23degC), and 70% HNO3 produces +18% swell. This is attributed to the thermal instability of the sulfonate crosslinks under hot, acidic conditions. For hot concentrated acid service above 80degC, FKM or PTFE-lined systems should be evaluated.

3.3 Flame Resistance

With 25--43% chlorine by weight, CSM is intrinsically flame-retardant. The chlorine atoms release HCl in the gas phase during combustion, acting as radical scavengers that quench the flame-propagation chain reaction. CSM achieves:

• • Limiting Oxygen Index (LOI): 28--32% (air is 21% O2, so CSM will self-extinguish in ambient air)
• • Flame rating: Self-extinguishing; passes many industry flame-resistance specifications without halogenated additives

This gives CSM a significant advantage over EPDM (LOI <20, requires heavy loading of flame retardants), and positions it alongside CR (LOI 27--46) as a preferred material for mining and tunnel cable jackets.

3.4 Oil Resistance

CSM's oil resistance sits in the middle ground: substantially better than EPDM or NR (which swell excessively in hydrocarbons) but notably inferior to NBR. Volume swell in ASTM IRM 903 oil at 70degC reaches 40--50%. This moderate resistance makes CSM suitable for applications with occasional oil contact (e.g., industrial flooring, chemical plant environments) but disqualifies it for immersed oil service.

4. Full-Dimensional Comparison: CSM vs. CR vs. EPDM vs. NBR vs. PVC

The following table provides a direct comparison of CSM against the materials it most often competes with or is specified alongside. Ratings use a five-star scale.

The decision logic: CSM wins when the application demands simultaneous weather resistance and chemical resistance. CR is preferable when superior flame retardancy (higher LOI) or better low-temperature flexibility is required. EPDM wins on cost and low-temperature performance for applications with only weathering requirements and no chemical exposure. NBR is the choice for oil-immersed service. Flexible PVC competes on chemical resistance at ambient temperature but cannot match CSM's mechanical properties or service temperature range.

5. Physical Properties -- Typical Engineering Values

6. Key Applications

6.1 Rubber Dams (Inflatable Weirs)

This is arguably CSM's most demanding and highest-volume specialty application. A rubber dam consists of a fabric-reinforced elastomeric bladder anchored across a river or spillway, inflated with air or water to control upstream water level. The outer cover compound is exposed to:

• • Continuous sunlight/UV (often in subtropical or tropical locations)
• • Immersion or splash-zone water contact encouraging biological fouling
• • Potentially polluted water carrying industrial acids, alkalis, or hydrocarbons
• • Mechanical abrasion from suspended sediment during flood events

CSM is the material of choice for the outer cover layer (typically 2--3 mm thick) because EPDM -- the only other elastomer with comparable 20+ year weatherability -- suffers from biological fouling (algae and mollusk attachment) under long-term immersion and lacks CSM's chemical resistance to polluted water. Typical dam bag construction:

Design life: 20--25 years, determined primarily by the CSM outer cover's weathering endurance. Hot-vulcanized seam joints must achieve seam strength >= 80% of the parent material strength -- this is the critical quality control parameter during fabrication.

6.2 Chemical Storage and Process Plant Linings

CSM sheet linings (2--5 mm thick, often applied as multi-ply systems) protect steel and concrete vessels in:

• • Acid storage tanks (sulfuric, hydrochloric, phosphoric at ambient to warm temperatures)
• • Electroplating and pickling baths
• • Chemical processing floors and sumps
• • Wastewater treatment containment

For extreme chemical environments (hot concentrated oxidizing acids, organic solvents), CSM is evaluated alongside bromobutyl, EPDM, FKM, and PTFE. CSM's sweet spot is ambient-to-warm (<=50degC) mineral acid and alkali service where the combination of chemical resistance, outdoor durability (for external tank surfaces), and moderate cost provides a compelling value proposition.

6.3 Wire and Cable Jacketing

CSM is widely specified for cable jackets in:

• • Mining cables: Flame resistance (self-extinguishing) plus abrasion resistance
• • Shipboard and offshore cables: Weathering + oil splatter resistance + flame retardancy
• • Railway and mass transit: Low smoke, low toxicity, flame retardancy (EN 45545-2 compliant formulations)
• • Industrial power cables: Chemical splash resistance in process plant environments

The extrudability and surface finish of cable-grade CSM (e.g., Toso-CSM CN-350) rival CR, while offering better color stability for identification jackets.

6.4 Architectural Applications

CSM's ability to be formulated in light colours while retaining full UV/weather resistance makes it unique. Applications include:

• • Coloured roofing membranes: White/grey membranes reflect solar radiation, reducing building cooling loads
• • Decorative rubber roofing tiles: Colour-stable exterior rubber tiles
• • Expansion joint covers: Weather-resistant joint seals for building facades

7. The Post-Hypalon Landscape: Supplier and Grade Selection

DuPont discontinued Hypalon production in 2010. The following table maps legacy Hypalon grades to currently available CSM products from the primary global suppliers:

Grade selection guidance by application:

Contingency if CSM is unavailable:

• • For mild chemical + weathering applications: ECO (epichlorohydrin rubber) may be an acceptable substitute
• • For extreme chemical environments above 80degC: evaluate FKM + PTFE composite lining systems
• • For weathering-only applications: EPDM is generally the more cost-effective choice

8. When to Specify CSM -- Decision Framework

CSM is the right choice when the application simultaneously requires two or more of:

1. Outdoor weathering >= 15 years with colour flexibility (not restricted to black)

1. Resistance to concentrated mineral acids or strong alkalis at ambient to warm temperatures

1. Inherent flame retardancy without additive flame retardants

1. Moderate oil resistance as a supplementary requirement

Avoid CSM when:

• • The primary requirement is oil/fuel resistance -- specify NBR or FKM
• • The service temperature consistently exceeds 120degC -- specify FKM or silicone
• • The application requires low-temperature flexibility below -25degC -- specify EPDM (to -50degC) or CR (to -35degC)
• • Cost is the dominant constraint and only weathering resistance is needed -- EPDM is half the cost

CSM occupies a narrow but commercially significant performance window that no other mid-tier elastomer fills: it is the only material that combines EPDM-grade weatherability with CR-grade chemical resistance and inherent flame retardancy, at a price point well below fluorocarbon elastomers.

Inquiry & Technical Support

Nanjing Yuhang Rubber provides CSM material selection guidance and compound development support. For technical consultation on CSM grade selection, rubber dam compounds, or chemical-resistant lining formulations, contact our engineering team: Products | Material Database | Contact