Insulator Liquid Silicone Rubber RH-G601/40

Insulator Liquid Silicone Rubber RH-G601/40 is a two-component, platinum-catalyzed addition-cure silicone elastomer supplied in a 1:1 mix ratio. The “40” designation corresponds to a nominal hardness of 40 Shore A after full cure. The grade is intended for high-voltage outdoor insulation components, including hollow-core composite insulator housings, cable termination stress cones, bushing covers, and surge-arrester jackets requiring resistance to tracking, corona discharge, and wet-surface leakage current. Unlike general-purpose liquid silicone rubber, the formulation includes a modified alumina trihydrate and fumed-silica package that balances low-shear pumpability against filled-system electrical stability. The uncured base is free of organotin compounds, and the addition-cure network releases no acetic acid or oxime by-products during crosslinking.

Because the hydrosilylation reaction proceeds without condensation by-products, cured parts exhibit lower volatile-siloxane carryover after the specified post-cure than condensation-cure RTV grades. The filler system increases the mixed viscosity relative to unfilled electronic-grade LSR, but it also raises comparative tracking index values and reduces surface tracking under contaminated wetting conditions. RH-G601/40 is formulated for closed-loop liquid injection molding and vacuum-assisted casting rather than hand-mixed open-mold processing, which distinguishes it from low-shear RTV silicones used in maintenance and field repair.

Which Measured Values Define the Grade for Wet-Arc Insulator Applications?

Table 1 lists representative first-party batch-certificate values for RH-G601/40. The values are measured on 2 mm sheets compression-molded at 120 °C for 10 min and post-cured at 200 °C for 4 h, unless otherwise noted. Electrical measurements are performed after 24 h recovery at 23 °C ± 2 °C and 50% ± 5% relative humidity.

The compression set value is obtained after post-cure. Without post-cure, the value measured on 120 °C-cured sheets is typically 25%. The volume resistivity and dielectric strength values fall within the expected range for a heavily filled LSR; unfilled general-purpose LSR grades often show volume resistivity one decade higher, but their tracking resistance under IEC 60112:2020 is commonly below 300 V. The alumina hydrate filler shifts tracking performance upward but increases mixed viscosity and reduces low-frequency dielectric constant stability in very thin sections.

For closed-loop injection molding on 200 L drum feed systems, the A and B components are metered through a 30 mm static mixer at a ratio of 1:1 by volume. The mixed viscosity at low shear is approximately 30,000 mPa·s at 25 °C; at injection shear rates above 100 s⁻¹ the material exhibits pseudoplastic behavior and fills fully heated multi-cavity molds at cavity pressures between 0.2 MPa and 0.5 MPa. Shot-size control uses a screw or piston injector with a nozzle jacket held below 50 °C to prevent premature hydrosilylation in the barrel. Molds are heated to 120 °C to 180 °C, with a preferred surface set point of 140 °C for parts thicker than 5 mm. The lower set point avoids a thermal-modulus gradient that appears when alumina-trihydrate-filled LSR is cured above 180 °C and can produce surface scorch at vent locations. Production-floor measurements on 4-cavity surge-arrester housing tools indicate that mold-surface temperature deviation above ±5 °C across the cavities shifts local Type A durometer values by up to 3 points and can leave tacky under-cured sprues.

The practical pot life of mixed material is 72 h at 25 °C under sealed storage; no comparable ISO pot-life test exists, and the value is a production-floor observation. Lower temperatures extend handling time, but cooling below 5 °C increases apparent viscosity and can starve feed throats in unheated drum pumps. No post-mixing vacuum is required for thickness below 5 mm; for thicker sections, vacuum degassing is applied according to the humidity limits discussed below. LSR-specific check valves and static mixer elements should be used, and mixing equipment previously contaminated with sulfur-cured EPDM or amine-cured epoxy must be purged with xylene followed by isopropanol before production.

When Ambient Humidity Exceeds 60% During Drum-Off and Metering

Although addition-cure LSR does not cure by moisture, absorbed water on the alumina hydrate filler can produce steam-induced porosity when the part cross-section exceeds 5 mm and mold temperature exceeds 130 °C. The failure mode appears as discrete spherical voids concentrated around metal inserts and gate regions rather than uniform foaming. When the ambient dew point exceeds 10 °C or relative humidity remains above 60% for more than 4 h, individual shots are degassed at −0.09 MPa for 2 min and the cavity is evacuated to −0.08 MPa before injection. Mold release agents based on silicone oil or soap are acceptable; amine-based release agents and sulfur-containing gloves are not used because platinum catalyst inhibition produces tacky, under-cured surfaces. The A and B components should be stored in unopened drums at 5 °C to 30 °C for a shelf life of 6 months; freezing does not crosslink the material but can cause filler sedimentation requiring low-shear rehomogenization.

A systematic comparison against peroxide-cure high-temperature vulcanized solid silicone, condensation-cure room-temperature vulcanized silicone, and hydrophobic epoxy clarifies the differences in processing and electrical behavior. Table 2 summarizes the primary distinctions. The addition-cure LSR enters the mold at ambient temperature and crosslinks by hydrosilylation rather than free-radical decomposition or moisture condensation. That removes post-mold bloom associated with peroxide residues and allows automated closed-loop metering, but it also imposes stricter contamination control. Condensation-cure RTV grades release acetic acid or oxime by-products and are typically processed in open molds at low shear, making their electrical tracking performance more sensitive to incomplete room-temperature cure and to entrained bubbles. Hydrophobic epoxy and rigid EPDM exhibit higher modulus and lower hydrophobicity recovery after water immersion; under dynamic bending at low strain, cured LSR retains elongation above 300%, whereas rigid epoxy systems may crack at the same deformation.

The comparative tracking index values in Table 2 are lot-dependent and may vary with filler surface treatment; the RH-G601/40 value in Table 1 is the release limit, not a guaranteed field-service rating for contaminated high-voltage environments. Direct published multilaboratory data for this specific grade under combined salt fog and inclined-plane aging are limited, so formal type testing should be performed on production-representative specimens.

Tracking and Erosion Performance in Salt-Fog and Inclined-Plane Testing

Inclined-plane tracking evaluations for outdoor insulation are performed in accordance with IEC 60587:2007 using a contaminated liquid composed of ammonium chloride and nonionic wetting agent at a flow rate of 0.6 mL/min. The initial tracking voltage is commonly set at 4.5 kV for filled silicone elastomers. First-party type-test records for RH-G601/40 show erosion depth of less than 3 mm after 6 h at 4.5 kV when specimens are post-cured at 200 °C for 4 h; however, independent multilaboratory data for this exact grade are limited. For customer qualification, specimens should be molded on the same equipment and post-cured under the same thermal profile intended for production, because wet-arc performance is controlled by residual cyclosiloxane concentration, filler dispersion, and surface contamination. In production audits, parts showing localized erosion to the glass-reinforced epoxy core are normally associated with under-dispersion of the alumina hydrate filler or with mold contamination rather than with the base polymer.

Salt-fog chamber exposure under IEC 62217:2012, or the relevant ANSI/IEEE dynamic test, is generally used for housing material validation. The key parameter is not initial hydrophobicity but hydrophobicity recovery after corona and water immersion. Silicone elastomers are distinguished from epoxy and EPDM by the ability of low-molecular-weight siloxanes to migrate to the surface and restore a hydrophobic layer after wetting. For RH-G601/40, the post-cure cycle reduces free cyclosiloxane content; excessive post-cure can slow hydrophobicity recovery and reduce migration capacity, while insufficient post-cure leaves volatile siloxanes that can condense on conductor hardware and attract dust. The specified 200 °C/4 h profile represents the production trade-off between volatile removal and preservation of hydrophobicity recovery.

In hollow-core composite insulator manufacturing, metering of RH-G601/40 into a vacuum-assisted casting station or injection mold over a 60 mm pultruded glass-reinforced epoxy core is performed at cavity pressures between 0.2 MPa and 0.5 MPa. On a 1,500 kN clamp-force LSR injection machine, the material fills annular gaps of 5 mm to 20 mm without exceeding the core crush strength of the filament-wound substrate. Flash generation is controlled by parting-line design with 0.02 mm to 0.05 mm clearance; larger clearances permit air entrapment and surface porosity at the parting line. Bonding to the epoxy core generally requires a primer or an organofunctional silane adhesion promoter applied by automated brush or spray; unprimed adhesion is not reliable under thermal cycling from −40 °C to 80 °C because differential thermal expansion concentrates shear stress at the silicone-epoxy interface.

For cable termination and bushing cover molding, the grade is processed with the same closed-loop LSR equipment but with lower cavity pressures. The filled material resists wet-arc tracking better than unfilled electronic-grade LSR, while retaining the low-temperature flexibility required for cold-weather installation. Published data for severe desert or marine salt-spray service at this specific hardness is limited; application qualification should include the relevant IEC 60815 pollution class and IEEE C62.11 impulse tests on finished assemblies rather than on plaques alone.