Silicone Elastomers RH-SEB-6066

RH-SEB-6066 is supplied as a two-part high-consistency silicone elastomer system for platinum-catalyzed hydrosilylation cure. The base component is millable on standard two-roll mills with a friction ratio of 1.2:1 and roll temperature between 25 °C and 40 °C; catalyst is incorporated at 0.8–1.5 phr on a cool mill before extrusion, calendering, or molding. Unlike peroxide-cured HCR grades, the cure reaction does not generate acetophenone or chlorinated aromatic byproducts, and closed-mold vulcanization can proceed at 150–165 °C within cycle times of 5–10 min for sections up to 5 mm, depending on mold temperature uniformity. The resulting network is a dimethylvinylsiloxy-modified polysiloxane crosslinked by short-chain hydride-functional oligomers. This chemistry is applied in food-contact and medical-grade elastomer formulations evaluated under 21 CFR 177.2600, USP <87>, and ISO 10993-5; however, compliance for RH-SEB-6066 must be confirmed from the supplier’s regulatory certificate because formulation batch state modifies the extraction profile.

What Limits the Processing Window for RH-SEB-6066 in Hot-Air Vulcanization?

The addition-cure kinetics are controlled by a low-temperature inhibitor that evaporates or decomposes above 120 °C. Below 30 °C, catalyzed compound remains malleable for 24–72 h; at 40 °C, the same compound can scorch within 2–4 h. This sensitivity creates a narrow temperature corridor for continuous hot-air vulcanization. A 90 mm extruder with an L/D of 20:1 and barrel zones held at 25–35 °C is typical for profiles; die temperature is usually 60–80 °C, and the downstream oven is ramped from 160 °C to 250 °C over 3–6 min. Screw speeds above 40 min⁻¹ can generate sufficient viscous heating to exceed the inhibitor threshold and produce hardened gel particles in the extrudate. Because platinum-catalyzed systems are also poisoned by amines, sulfur, organotin compounds, and sulfur-bearing carbon black, the production area should segregate RH-SEB-6066 from nitrile, neoprene, and sulfur-vulcanized EPDM scraps. Published data for this specific configuration is limited; therefore, a moving-die rheometer curve at 177 °C should be generated for each lot to establish t10 and t90 and to set maximum allowed stock residence time at the mill.

Comparative Byproduct Profile Against Peroxide- and Condensation-Cure Silicone Elastomers

Calendered sheet from RH-SEB-6066 is press-cured in chrome-plated or PTFE-coated molds at 150–165 °C under 80–120 bar (8–12 MPa) clamp pressure. A vacuum chamber capable of 30–70 mbar absolute during the first 30 s of the cure cycle is used to prevent air entrapment in gaskets and diaphragms. Demolded parts are post-cured at 200 °C for 4 h in a forced-air oven with an air change rate of at least 20 chamber volumes per hour. Post-cure reduces low-molecular-weight cyclic siloxanes and stabilizes the compression set value measured by ASTM D395-18 Method B or ISO 815-1:2019. Cross-sections above 20 mm should be step-cured because the low thermal conductivity of silicone can produce a thermal lag of 15–25 °C between surface and core. A step profile of 120 °C for 20 min, 150 °C for 20 min, and 200 °C for 4 h is commonly used for solid profiles; published data for this specific configuration is limited. The post-cure oven must not contain sulfur-vulcanized rubber residues, amine-containing release agents, or PVC trim scrap, because these substances can deactivate platinum at the part surface and leave a tacky skin.

Post-Cure Volatile Management Is Governed by Siloxane Ring Equilibration, Not Peroxide Residue Stripping

In peroxide-cured compounds, the post-cure step functions to volatilize acidic decomposition products and terminate residual radical activity. For RH-SEB-6066, the post-cure objective is different: silanol and hydride end-groups continue to condense or equilibrate, and cyclic dimethylsiloxane oligomers D4, D5, and D6 migrate to the surface. Headspace gas chromatography per DIN EN 14350-1 or the supplier’s internal method is used to quantify residual cyclics after post-cure. For food-contact sealing applications, EU Regulation 10/2011 and 21 CFR 177.2600 impose extraction limits; post-curing at 200 °C for 4 h typically reduces total cyclics to below 0.5 % by weight for addition-cure HCR, but batch-to-batch variation is observed and lot-specific extraction data should be requested. In medical or pharmaceutical applications, ISO 10993-12 extraction testing and USP <87> cytotoxicity are common; the material supplier must demonstrate the final formulation’s compliance rather than relying on generic class data. The absence of acidic peroxide residues allows RH-SEB-6066 to be used in closed electronic connector seals without the acetic acid corrosion risk of acetoxy RTV systems, provided the inhibitor package and catalyst level are controlled.

Transfer molding of RH-SEB-6066 requires a cold-runner design with coolant held at 20–25 °C to prevent crosslinking in the runner during multi-cavity cycles. If a hot runner is used, the runner temperature must not exceed 60 °C for hold times above 5 min. Injection pressures between 60–100 MPa are used for high-consistency silicone; clamp force should be calculated at 3–5 kN/cm² of projected area. Because addition-cure HCR has a lower viscosity under shear than peroxide-cured compounds at the same filler loading, the compound may flash more readily; mold parting-line fit should be held to 10–20 µm clearance, and vacuum drainage channels should be cleaned after every 500 shots to prevent silicone residue buildup. In cleanroom processing, the base is often pre-filtered through 200-mesh screens, but post-cure extraction is still required for low-volatile applications.

Storage of RH-SEB-6066 in sealed polyethylene bags inside cardboard drums below 30 °C extends usable life to 6–12 months from the date of manufacture. The catalyst component should be stored separately at 2–8 °C if the supplier’s instruction indicates. Condensed moisture above 60 % relative humidity can hydrolyze hydride groups and reduce the Si-H content required for stoichiometric cure. For this reason, the base should be conditioned for 12 h in a desiccated room at 20–25 °C and under 30 % relative humidity before catalyst addition. If the base is exposed to high-humidity air during milling, cured hardness can drop by 3–5 Shore A points and tensile strength by 10–15 % because the hydride-to-vinyl stoichiometry shifts. These property cliff effects are observed in platinum-catalyzed HCR processing and are not specific lot guarantees.

When the Application Requires Shore A 40–60 Without Post-Cure Complexity

Hardness in RH-SEB-6066 is adjusted by crosslinker ratio, not by plasticizer addition. At a nominal catalyst/crosslinker addition of 1.0 phr, the cured network typically falls within Shore A 40–60. Lower crosslinker levels reduce hardness and increase elongation; higher levels increase tensile modulus but can reduce tear resistance if crosslink density exceeds the optimum for energy dissipation. For a thermally conductive or flame-retardant variant, filler loading is the dominant variable, and the viscosity increase can shift the Mooney viscosity from 30 MU to above 80 MU. Processors should not extrapolate from general-purpose silicone molding parameters without obtaining the specific compound’s rheometer curve.

Swelling in hydrocarbons is higher than for nitrile or FKM elastomers; silicone elastomers are not intended for continuous immersion in gasoline, ketones, or strong acids. RH-SEB-6066 should be limited to air, water, steam, and selected polar fluids. In ASTM D471 reference fuel B or IRM 903 oil immersion, the volume swell of a 50 Shore A silicone HCR generally exceeds 60 % at 70 °C, so the compound is not a candidate for oil-contact sealing. For applications requiring hydrocarbon fluid resistance, fluorosilicone grades should be evaluated instead. Continuous use temperature for silicone elastomers is typically 200–230 °C for 10,000 h, with intermittent excursion to 250–280 °C in air. Above 300 °C, depolymerization accelerates and the network can revert to low-molecular-weight siloxanes. RH-SEB-6066 should not be used in closed anaerobic systems above 250 °C without aging tests because platinum-cured networks can soften via siloxane bond exchange; published data for this specific configuration is limited.

Unlike low-viscosity liquid silicone rubber, RH-SEB-6066 is handled as a high-consistency gum. LSR requires closed-loop pumping and cooled static mixers; RH-SEB-6066 runs through conventional two-roll mills, extruders, calenders, and compression presses. The trade-off is that HCR catalyst addition is a manual weigh-and-mill step, which introduces operator-dependent variance in catalyst dispersion. LSR can be injection molded with cycle times of 30–60 s for small parts; HCR compression molding cycles are longer but permit curing of high-hardness compounds and thicker preforms without the freeze-locked mixer residence-time constraints of LSR. This distinction is often the decisive factor when a processor already operates solid silicone conversion equipment and requires a low-volatile, platinum-cured compound without investing in LSR metering systems.