Epichlorohydrin (ECH), CAS 106-89-8, is the bifunctional epoxide intermediate responsible for the commercial epichlorohydrin elastomer family and the majority of liquid epoxy resins used in bonding and protective film applications. In chlorohydrin rubber, ECH polymerizes through the epoxide ring to form a polyether backbone that retains a chloromethyl substituent on every repeat unit; this side group contributes polarity, fuel and oil resistance, solvent resistance, and flame retardancy. Commercial grades designated CO, ECO, and GECO under ISO 1629 differ by incorporation of ethylene oxide and allyl glycidyl ether, and these structural variations directly control the balance of low-temperature flexibility, scorch safety, adhesion, and permeation resistance. ECH-derived bisphenol-A diglycidyl ether resins, typically with epoxide equivalent weights of 185–192 g/eq and dynamic viscosities of 11–14 Pa·s at 25 °C, serve as the binder foundation for high-solids protective coatings and structural adhesives. This treatment of Epichlorohydrin in Chlorohydrin Rubber, Adhesives & Coatings Applications examines grade selection, cure chemistry, processing limits, and regulatory test boundaries.

Chlorohydrin Rubber Grades and the Function of the Chloromethyl Side Group

In the elastomer designation system of ISO 1629, CO is the homopolymer of epichlorohydrin, ECO is the copolymer of epichlorohydrin and ethylene oxide, and GECO is the terpolymer of epichlorohydrin, ethylene oxide, and allyl glycidyl ether. The chloromethyl side group in each repeat unit introduces a permanent dipole that raises cohesive energy density and reduces equilibrium solvent uptake. This effect is most pronounced in CO, where chlorine content is approximately 38 wt%, while ECO grades contain approximately 26 wt% chlorine and exhibit improved low-temperature flexibility due to the ethylene oxide sequences. The allyl glycidyl ether in GECO provides pendant allylic unsaturation for peroxide or sulfur cure, at the expense of increased compound odor and potentially higher compression set after aging. Typical Mooney viscosity ML(1+4) at 100 °C values for commercial chlorohydrin rubber grades fall within 40–80 MU; selection below 45 MU favours transfer molding and calendering, while values above 60 MU improve extrudate green strength in fuel hose profiles and roll covers.

Shear heating during two-roll milling of ECO/GECO compounds is a critical processing variable. Mill rolls are maintained at 60–70 °C, front-to-rear friction ratios are set at 1.2:1, and carbon black N550 is added after the polymer has formed a continuous band. Addition of paraffinic plasticizer at 10–15 phr reduces compound viscosity and limits heat buildup, but excessive plasticizer migration can elevate Fuel C volume swell by 5–10 percentage points. Cure systems for ECO compounds are commonly based on triazine derivatives at 1.0–1.5 phr, combined with magnesium oxide or calcium carbonate acid acceptors at 3–5 phr. Scorch safety measured as Mooney scorch t5 at 121 °C for a productive triazine-loaded ECO compound generally exceeds 20 min, yet the same compound reaches 90% cure torque within 8–10 min at 170 °C. Ethylene thiourea remains a historical benchmark because of its balance of cure rate and modulus, but its reproductive toxicity classification under CLP Regulation (EC) No 1272/2008 has driven production-scale replacement with triazine-accelerated formulations. In injection molding, barrel zones are held between 70 °C and 80 °C, the mould is set at 170–190 °C for cured parts, and clamps are sized for projected areas at 30–60 MPa cavity pressure. Vent depths should not exceed 0.02–0.03 mm to prevent flash with low-viscosity GECO compounds.

How Does Fuel Immersion Affect CO, ECO, and GECO Compounds Differently?

Volume swell of peroxide-cured GECO after 70 h immersion in ASTM D471 Reference Fuel C at 23 °C is typically in the 25–40% range, while the same formulation can show 15–25% swell in ASTM D471 Reference Fuel B and 5–10% swell in mineral oil. The ranking follows the free-volume contribution of ethylene oxide; CO has the lowest equilibrium solvent uptake and highest glass transition, while GECO shifts the service range downward and increases permeation. Hardness change after Fuel C aging is commonly measured as −8 to −20 Shore A for ECO compounds, depending on plasticizer type. Linear polar plasticizers such as butyl carbitol formal are preferred over dioctyl adipate where low extraction is required. Tensile retention after immersion generally falls to 50–70% of unaged values, with failure usually initiated at the polymer-filler interface rather than by backbone degradation.

Bisphenol-A diglycidyl ether resins produced from two moles of ECH per mole of bisphenol A are the base liquid epoxies for structural adhesives. The resin fraction used in ambient-cure adhesives typically has an epoxide equivalent weight of 185–192 g/eq, a hydrolyzable chlorine content below 500 ppm, and a viscosity at 25 °C of 11–14 Pa·s. Formulators combine this resin with amine-functional curing agents at stoichiometric ratios calculated from amine hydrogen equivalent weight; a stoichiometric error of ±5% can reduce lap shear values by 10–20% in ASTM D1002 tests on aluminum coupons. In single-part heat-cure adhesives, dicyandiamide at 5–7 phr with a urea accelerator at 1–3 phr gives latency of 30–60 days at 25 °C and cures in 20–30 min at 150 °C. Epichlorohydrin rubber itself is added to structural adhesive films at 5–15 phr to reduce internal stress and improve peel strength in ASTM D1876 T-peel testing. Because the chloromethyl groups create polar attraction to aluminum and steel, the elastomer modifies the brittle epoxy network without suppressing glass transition temperature below 80 °C.

When Epoxy Coatings Formulated with ECH Must Meet Migration Limits in Food-Contact Service

In food-contact can and coil coatings, the relevant regulatory framework for ECH-derived epoxy resins is not the polymer content but the concentration of residual bisphenol A diglycidyl ether and its hydrolysis products. Under Regulation (EC) No 1895/2005, the specific migration limit for the sum of BADGE, BADGE·H₂O, and BADGE·2H₂O is 9 mg/kg food or food simulant for most applications. Coated can ends and bodies are tested using EN 1186 migration protocols with 10-day testing at 60 °C in 3% acetic acid, 10% ethanol, or olive oil simulants. In U.S. food-contact use, 21 CFR 175.300 allows epoxy resins as components of resinous and polymeric coatings subject to end-test limitations and appropriate curing. Residual ECH itself is controlled below detectable limits because ECH is classified as a Carcinogen Category 1B under CLP Regulation (EC) No 1272/2008. Industrial high-solids epoxy primers formulated with ECH-based bisphenol-A resins commonly require induction time of 15–20 min after mixing before airless spraying at 60–80 bar tip pressure. Initial dry film thickness is held to 100–150 µm for salt-spray resistance under ISO 9227 beyond 1,000 h.

Low-temperature sealing performance of GECO compounds in automatic transmission systems is controlled by brittle point, TR-10 retraction, and compression set. Press-cured 2 mm sheets tested under ISO 2921 show TR-10 values near −38 °C for ECO and −42 °C for GECO with 30 phr carbon black N550 and 5 phr dioctyl adipate. Compression set after 70 h at 150 °C evaluated with ASTM D395 Method B remains 25–35% for triazine-cured ECO and 35–45% for GECO, limiting use in high-temperature static seal grooves where set exceeds 40%. Fuel hose covers based on CO/ECO blends are frequently extruded through pin-and-die heads with 20–24 mm diameter dies and 1.2–1.5 draw-down ratios. Melt fracture is controlled by maintaining compound viscosity below 55 MU and die land temperatures below 95 °C. Published data for low-temperature dynamic ozone exposure on non-carbon-black GECO formulations is limited.

Selecting Cure Systems for Chlorohydrin Rubber Roll Compounds

Crack resistance in rubber-covered rolls under high nip pressure depends on balancing crosslink density with network homogeneity. For ECO rolls, a triazine derivative at 1.2 phr with 5 phr magnesium oxide and 1 phr calcium stearate achieves a moving die rheometer torque inflection of 8–12 dN·m at 170 °C within 12 min, but the same cure system can produce visible surface bloom if the roll is left unsheeted after cooling. Where ozone and weathering dominate, ECO is blended with 10–20 phr of EPDM or chlorosulfonated polyethylene. The partition of curatives between phases can cause overcure in the ECH phase and undercure in the EPDM phase, observed as low Shore A hardness pockets. Roll compounds are compounded with lower filler surface area, often N330 replaced by N550 at 50–60 phr, and process oil at 5 phr to limit heat generation during grinding. Surface roughness after grinding is specified as Ra 0.8–1.6 µm, and grinding wheels with 40–60 grit silicon carbide are used dry or with water cooling. The operational boundary is a stock temperature below 110 °C during milling to avoid premature crosslinking. Exceeding this threshold for more than 10 min can elevate Mooney viscosity by 5–10 MU and cause gel particles in the final roll cover.

Moisture sensitivity in ECH-derived coatings and adhesives is governed by the polarity of the chloromethyl side group and residual hydrolyzable chloride. Liquid epoxy resins with hydrolyzable chloride above 500 ppm can generate hydrochloric acid during high-temperature cure, causing amine curative salt formation and interfacial adhesion loss on steel primed with zinc silicate. In formulated adhesives, paste viscosity response at 25 °C is typically 30–80 Pa·s for trowelable systems, while ASTM D2196 rotary viscometer measurements at 20 rpm are used to control thixotropy index. A thixotropy index below 2.0 leads to sag on vertical joints. Open time for ambient-cure amine systems is 30–60 min at 23 °C and 50% RH, but above 60% RH film-forming amines can blush or carbamate, reducing ASTM D1002 steel lap shear strength by 15–25%. Pre-drying of steel substrates is required when the dew point is within 3 °C of substrate temperature; otherwise moisture condenses in the bond line and produces interfacial voids. Where ECH-based elastomer sealants are used in glazing applications, adhesion to glass is promoted by silane primers containing γ-glycidoxypropyltrimethoxysilane at 1–2 wt%; omission of primer lowers ASTM C794 peel adhesion to below 0.5 N/mm. Strong-base amine additives should be avoided in chlorohydrin rubber compounds because the chlorinated polyether backbone can undergo dehydrochlorination at elevated processing temperatures.