Liquid Silicone Rubber for Textile Belt RH-G2

Liquid Silicone Rubber for Textile Belt RH-G2 is a two-component, platinum-catalysed addition-cure silicone elastomer supplied at a 1:1 mix ratio by weight and formulated for knife-over-roll coating, rotary screen deposition, and transfer coating onto woven and nonwoven belts used in printing, paper transport, vacuum conveying, food-contact conveying, and timing belt top-coating. The uncured mixture has a viscosity of 42,000 mPa·s at 25 °C according to ISO 3219:1994, a thixotropic index of 2.2, and a working pot life greater than 48 h at 23 °C. Specific gravity is 1.12 per ASTM D792-20. After curing for 5 min at 150 °C, the elastomer reaches 40 Shore A hardness per ASTM D2240-15, 7.5 MPa tensile strength and 600 % elongation at break per ISO 37:2017, and 28 N/mm tear strength per ASTM D624-00(2012) Die B. Linear shrinkage measured on 100 mm × 100 mm × 2 mm plaques is 0.4 %. The grade is formulated to maintain low compression set and flexural stability under continuous belt cycling; it is not intended for solvent-borne silicone replacement or for applications requiring hardness above 60 Shore A.

Why Does Addition-Cure LSR Generate Lower Volatile By-Products than Peroxide-Cured Solid Silicone on High-Speed Belt Lines?

The curing mechanism of RH-G2 is hydrosilylation, in which vinyl-functional siloxane polymers react with silane crosslinkers in the presence of a platinum complex. This addition reaction releases no peroxide decomposition residues, no acetone or alcohol split products, and no acid by-products. In contrast, peroxide-cured high-consistency rubber may liberate low-molecular-weight organic acids and diketones during post-cure, which can deposit on oven walls or transfer to the belt surface. The absence of those by-products with RH-G2 reduces the need for multi-stage oven post-cure and permits direct lamination to heat-sensitive textile backings. Cure development is monitored by moving die rheometry per ISO 6502-1:2018, with a scorch time tS2 above 60 s at 120 °C and a t90 below 240 s at 150 °C. Cured sections exhibit a compression set of 15 % after 22 h at 175 °C per ISO 815-1:2020, which is typically lower than equivalent peroxide-cured solid belts tested under the same conditions. A further distinction is that RH-G2 does not require two-roll milling or preforming; the liquid state permits direct metering, filtration, and coating without the high-shear heat history that can alter peroxide bloom in solid rubber compounds.

For continuous coating, the A and B components are metered through a 1:1 positive-displacement gear pump into a static mixer with 24 elements, followed by vacuum deaeration at −0.09 MPa for 15 min to reduce air entrainment. The mixed liquid is then applied to polyester, polyamide, or aramid fabric by knife-over-roll or comma coater at line speeds between 5 m/min and 30 m/min. Target wet film thickness ranges from 0.1 mm to 1.5 mm; thinner coatings require a higher shear to prevent slump on vertical web sections. Because the material is thixotropic, viscosity under a shear rate of 10 s⁻¹ is lower than the low-shear value by a factor of approximately 2.2, allowing leveling without excessive strike-through on open-weave fabrics. Oven residence is set to reach a surface temperature of 150 °C for at least 5 min; thicker sections require step-cure profiles beginning at 120 °C to avoid surface skinning and internal microvoids. Air velocity across the web should be maintained between 1.5 m/s and 3.0 m/s in the first oven zone, with extraction sufficient to remove low-level siloxane cyclics without causing surface wrinkle or edge curl.

Adhesion Development on Polyester, Aramid, and Glass-Fibre Belt Substrates

Adhesion of RH-G2 to synthetic textile belts depends on substrate preparation and primer selection. On corona-treated polyester fabric with surface energy above 40 mN/m, a silane-based primer containing methacryl and vinyl functionality is applied at 2–5 g/m² dry coat weight and dried at 100 °C for 60 s. Peel adhesion measured according to ASTM D903-98(2017) on 25 mm wide specimens is typically 18–30 N/25 mm for primed polyester and 25–40 N/25 mm for aramid with a two-component primer. Without primer, peel values fall below 8 N/25 mm on untreated polyester, indicating that adhesion is governed by chemical coupling rather than mechanical interlock alone. For glass-fibre belts, the fabric should be heat-cleaned and sized with an epoxy-silane pre-treatment to resist hydrolytic attack at the fibre-matrix interface. RH-G2 does not require solvent-based tie coats for all substrates, but closed-weave nylon fabrics with high internal lubricant content may require a preliminary scouring with dilute alkaline detergent followed by drying to 0.1 % moisture content. Adhesion failure analysis on production belts has shown that interfacial voids are typically caused by trapped volatiles from the fabric finish rather than insufficient primer coverage, so fabric pre-drying and finish consistency are critical variables.

Rheological measurements during pumping indicate that the mixed material follows pseudoplastic behaviour from 0.1 s⁻¹ to 100 s⁻¹, with a low-shear apparent viscosity of 92,000 mPa·s and a high-shear apparent viscosity of 38,000 mPa·s at 25 °C. The thixotropic recovery time after blade shear is 45 s, measured by a rotational rheometer with a 35 mm cone-plate geometry at 25 °C. This recovery window defines the maximum open time before the coated web enters the oven: at line speeds below 5 m/min, the coating has sufficient time to level, but at speeds above 30 m/min, air bubbles can become frozen into the surface if deaeration is incomplete. Pump selection should favour low-pulsation gear pumps with hardened rotors; progressive cavity pumps are acceptable only if the stator is rated for filled silicone because abrasive fillers can accelerate elastomer stator wear. The mixed material should pass through a 100 μm filter screen before the coating head to remove agglomerates, but filters below 50 μm can create excessive backpressure and risk shear heating.

The table compares representative cured property profiles for RH-G2, a peroxide-cured high-consistency rubber, and a condensation-cure RTV silicone when evaluated under textile belt coating conditions.

When cured for food-contact conveyor belts, RH-G2 is formulated to comply with the compositional requirements of FDA 21 CFR 177.2600 for rubber articles intended for repeated use, with extraction testing performed according to the specified aqueous and fatty-food simulants. For the European market, the cured elastomer is evaluated under Regulation (EC) No 1935/2004 and the appropriate organ-specific migration limits, with German BfR Recommendation XV applied for elastomers in food-contact belts. For industrial and electronic belt applications, the cured material meets the restriction limits of RoHS Directive 2011/65/EU and the SVHC obligations of REACH Regulation (EC) No 1907/2006. Compliance documentation should be confirmed against the specific grade lot and the final laminate construction, because textile backings, primers, and process oils can alter overall migration behaviour. The product is not formulated for implantation or long-term mucosal contact, and no ISO 10993 claims are made unless separately validated.

When Sulfur-Containing Dyes or Heat-Set Lubricants Remain on the Belt Surface

Platinum-catalysed addition cure is sensitive to compounds that coordinate to the platinum complex. Residual sulfur-based dyes, heat-set lubricants, amine-based antistats, and certain organotin stabilisers can inhibit cure at the silicone-textile interface, producing a tacky interlayer. If the belt fabric contains sulfur-dyed polyamide or is lubricated with stearate-based processing aids, a pre-treatment with a compatible solvent scour is required. In production trials, intermittent tacky surface defects were traced to glove contamination from sulfur-vulcanised rubber hoses used in the coating line. The cure inhibition typically appears as a gradient of low hardness from the fabric interface upward, measurable as a reduction of 12–20 Shore A points in the first 50 μm above the substrate when compared with the bulk elastomer. This is not a general property failure; the inhibition layer is localised and can be avoided by replacing sulfur-cured rubber hoses with PTFE-lined or polyethylene hoses, by segregating latex gloves used upstream, and by monitoring the fabric surface pH. If inhibition is suspected, a cure check on a fabric swatch with a thin 0.5 mm coating should be performed before full-width production. The material should not be exposed to organotin-catalysed condensation silicones, amine-containing primers, or sulfur-vulcanised rubber contact surfaces in the uncured state.

In transfer moulding of fabric-backed timing belts, RH-G2 can be injected into closed moulds at 120–140 °C with a moulding pressure of 3–8 MPa. The low viscosity permits filling of tooth profiles with minimal knit lines, but venting must be sufficient to allow air escape because the addition-cure system does not produce volatile by-products that would otherwise sweep trapped air from the cavity. Compared with solid high-consistency rubber, RH-G2 does not require a separate preform cutting step, and compared with condensation-cure RTV, RH-G2 reaches demoulding hardness faster under heat and does not depend on ambient moisture for crosslinking. However, the mould surface must be clean and free of amine-based release agents; a fluoropolymer or non-reactive silicone release agent is preferred. Demould time at 130 °C is typically 90–120 s for a 3 mm section, depending on mould temperature uniformity. Mould temperature variation greater than ±5 °C across the cavity can produce local under-cure and a tacky surface on the side of the belt in contact with the cooler mould half.

Storage stability is specified at 5–30 °C in unopened containers for 12 months from the date of manufacture. The A and B components must be kept sealed because exposure to ambient humidity can increase viscosity of the B component and shift the mix ratio at the mixer inlet. Pre-drying of the textile web is required when relative humidity exceeds 60 %; moisture flash-off at the curing oven can generate microvoids in coatings above 0.8 mm wet thickness. Continuous operation with less than 0.2 MPa backpressure at the static mixer may result in off-ratio mixing, while backpressure above 1.5 MPa can cause gear-pump cavitation and air entrainment. The recommended mixing system is a flow-through static mixer with a shortened after-mixer length so that the mixed material reaches the coating head within 5 min to prevent premature gelation at high ambient temperatures. Avoid combination with amine-based additives, sulfur-containing pigments, or organotin compounds because these materials can interfere with platinum cure and produce localised tacky regions.

On a 1,800 mm wide knife-over-roll coating line, the grade has been run with a fixed gap of 0.3 mm and line speed of 12 m/min. In this configuration, the observed wet coating mass was 310 g/m² before cure and 270 g/m² after oven drying and crosslinking. The difference corresponds to a total volatile loss of 0.4 % and is dominated by low-level siloxane cyclics rather than water or organic solvents. When the fabric tension exceeded 80 N/cm, edge strike-through increased on open-weave polyester scrim, producing intermittent coating weight variation of ±8 %. Lowering the blade angle and maintaining fabric tension between 40 N/cm and 60 N/cm reduced variation to ±3 %. Because the material has a low plasticizer content and forms a covalently crosslinked network, migration into the textile layer under continuous load is minimal; however, published multi-year dynamic fatigue data for RH-G2 on high-load aramid vacuum belts in abrasive paper transport remains limited, and validation on the specific belt construction is required.