Liquid silicone rubber for Screen Printing RH-G8

On circular-knit cotton–polyamide hosiery intended for hospital, yoga and ballet applications, RH-G8 is prepared at the print head as a two-component, platinum-catalyzed addition-cure liquid silicone rubber with a 10:1 A:B mix ratio, then adjusted in the formulation room to maintain a Brookfield viscosity at 25°C in the 50 000–80 000 mPa·s range. Typical formulation additions on production runs are 5–8 parts by weight of a phthalate-free high-opacity pigment paste per 100 parts mixed base, 1.5–3.0 parts of a low-viscosity silicone diluent when line temperature drops below 22°C, and 1.0–2.0 parts of a silane-based fibre-adhesion promoter for nylon-rich knit constructions. Addition of diluent beyond 5.0 parts is known to reduce wet edge definition and produce dot area gain exceeding 12% on 40-denier nylon–spandex substrates, particularly at machine speeds above 15 m/min. Compliance documentation for this segment requires OEKO-TEX Standard 100 Annex 4 Class I, REACH Annex XVII entries 23 and 51, and for children’s hosiery 16 CFR 1303 with surface-coating lead below 90 ppm. The substrate is first de-sized and dried to residual moisture <5% because water release during flash curing creates pinhole defects at the silicone–fibre interface. Screen printing uses flatbed or rotary equipment with mesh counts of 24–40 threads/cm, open areas of 35–45%, and squeegee hardness of 65–70 Shore A set at an angle of 70–80° and a pressure of 2–4 bar. The wet film thickness is held between 0.3–0.6 mm for grip dot deposits, followed by forced hot-air curing at 130–150°C for 2–4 min. Finished product types include non-slip hospital socks, yoga grip socks, ballet sole dots, infant crib socks, and geriatric fall-prevention hosiery with retained crocking fastness after 30 repeated ISO 6330 washes.

What Determines Gel-Fraction Control on Silicone Heat-Transfer Label Systems Printed on PET Release Liners?

The controlling parameter for heat-transfer silicone label systems is the gel fraction after the flash-cure stage, because under-cured film migrates into the adhesive layer during storage while over-cured film loses interlayer adhesion during hot-peel application. RH-G8 is formulated in this process at 100 parts base with 5–12 parts high-opacity pigment and 2–5 parts matting powder to reduce gloss below 10 GU at 60°, keeping mixed viscosity between 40 000–60 000 mPa·s so that fine text remains printable through 60–90 threads/cm mesh with 28–38% open area. The PET release liner is printed at a wet deposit thickness of 80–120 µm, flash-gelled in an infrared tunnel at 90–110°C for 20–40 s, then fully cured in forced air at 150°C for 3 min. If gel fraction exceeds 65%, the subsequent laminating step with thermoplastic polyurethane heat-seal adhesive shows interfacial fracture at the silicone–TPU boundary; if gel fraction remains below 45%, the printed film blocks during roll storage. Compliance standards applied to this class are OEKO-TEX Standard 100 Class I, CPSIA Section 101 with lead below 90 ppm in surface coatings, and REACH Annex XVII. Final transfer onto garments uses a heat press at 150–170°C, 8–12 s dwell, and 2.5–4.0 bar pressure, producing tagless t-shirt labels, underwear size bands, sportswear logos, and medical uniform identification patches.

When Silicone Ink Is Printed onto Pre-Vulcanized Silicone Rubber Sheet, Adhesion Is Governed by Surface Energy, Not Solvent Etch

Adhesion to pre-vulcanized silicone rubber sheet does not respond to solvent etching because the substrate is already crosslinked and has low surface energy; instead, the surface is prepared by plasma or corona discharge until dye test ink indicates a surface energy of at least 38 mN/m, after which no organic solvent wiping is permitted. RH-G8 is compounded for this use at 100 parts base with 10–15 parts inorganic pigment, 2–5 parts high-temperature fumed silica for slump resistance, and 1–3 parts alkoxy silane adhesion promoter, with addition of amine-based additives prohibited because amine contamination interferes with platinum cure and leaves a tacky top surface. Compliance for skin-contact and food-contact silicone articles references US FDA 21 CFR 177.2600, RoHS Directive 2011/65/EU, and REACH, while printing facilities typically document batch traceability against ISO 9001 incoming raw material control. Screen printing uses 80–120 threads/cm mesh to deposit a thin film of 15–25 µm wet thickness, and two or three overprints are applied to achieve colour opacity on dark silicone rubber. Curing in forced air at 150–180°C for 5–8 min is followed by a 200°C post-cure for 4 h to reduce unreacted siloxane fractions, after which crosshatch adhesion to ISO 2409 is expected to reach classification 0. Terminal products include printed silicone rubber keypads, mobile phone case decorative markings, medical pump buttons, and silicone wristband identifiers.

Platinum-Cure Kinetics at the Lower End of the Tunnel Oven Profile

At the lower end of the curing window, the platinum-catalyzed addition-cure reaction of RH-G8 becomes highly sensitive to dwell time and airflow distribution, and knitted medical textiles with retained moisture above 3% may exhibit cure inhibition at the deposit centre. For medical compression stockings and orthopaedic brace straps, the formulation is limited to 100 parts base, 2–5 parts high-purity fumed silica, 2–4 parts biocompatible pigment, and 0.3–1.0 part silver-glass antimicrobial additive where clinical surface hygiene is required; no plasticizer or peroxide modifier is introduced because leachable residues are outside the intended skin-contact profile. Direct skin-contact applications must be supported by ISO 10993-5:2009 cytotoxicity testing and ISO 10993-10:2010 irritation and delayed-type hypersensitivity testing, with an additional US FDA 21 CFR 177.2600 extractives review for repeated-use rubber articles. Production occurs in an ISO Class 8 cleanroom, using pre-washed and vacuum-dried knit fabric, mesh counts of 43–60 threads/cm, and a controlled wet deposit thickness of 0.2–0.4 mm. The tunnel oven profile is set to 125–135°C for 4–6 min, followed by an offline post-cure at 150°C for 2 h; published production data indicate that dropping oven temperature below 120°C causes tensile retention values to fall below 70% of full-cure modulus and produces a tacky surface detectable by a polyethylene film contact test. Finished medical products include compression stocking grip dots, orthotic anti-slip straps, compression therapy wrap closures, and cervical collar positioning pads.

For high-abrasion panels on backpacks, luggage, waders and tactical outerwear, RH-G8 is printed as a thick elastomeric reinforcement that must retain flexibility below -40°C while resisting gouging under ISO 12947-2 abrasion conditions. The working formulation combines 100 parts base with 8–15 parts high-tensile silica masterbatch, 2–4 parts pigment, and 0.5–2.0 parts structurant to increase low-shear viscosity for heavy film build. Fabric panels made from Cordura, hypalon or neoprene are pre-dried at 90°C for 20 min or vacuum-dried to residual moisture <0.5% because thick deposits trap water vapour and form internal porosity during cure. The printing operation uses mesh counts of 20–35 threads/cm, open areas of 40–55%, and multiple passes to achieve a wet film thickness of 0.5–0.8 mm; platen adhesive is required to stabilize high-density woven substrates during squeegee return. Curing in a hot-air tunnel at 140–150°C for 5–8 min is followed by a room-temperature stabilization period of 24 h before adhesion tape testing. Compliance is documented under REACH and RoHS, with flammability reference to 16 CFR 1610 for wearing apparel where applicable. Terminal products include backpack bottom panels, luggage scuff guards, outdoor glove palm prints, dog harness grip patches, and wader knee reinforcements.

Food-Contact Conveyor Belt Grip Ridges and the EU 1935/2004 Migration Compliance Matrix

Conveyor belt topcoats require a gradient of print deposit thickness across the fibre base, because ridge height affects product release, belt tracking, and cleanability in food drying tunnels. Published data for this specific RH-G8 configuration on woven glass fibre is limited, but the curing profile must remain within the established 160–180°C range for 8–12 min, followed by a 200°C post-cure of 4 h to reduce volatile siloxane content before food-contact release testing. The screen-print formulation uses 100 parts base with 5–10 parts heat-stable inorganic pigment and 2–6 parts fumed silica for slump resistance, applied through 24–40 threads/cm mesh at a wet deposit thickness of 0.4–0.7 mm. Substrate preparation for silicone-coated glass fibre cloth includes an in-line corona treatment and an initial low-temperature evaporation pass at 90°C for 10 min before full cure. Compliance for food processing components requires US FDA 21 CFR 177.2600, EU 1935/2004 overall migration documentation, EU 10/2011 where plastic multi-layer structures are involved, and LFGB recommendation XV for silicone elastomers in German market applications. Terminal product types include food processing conveyor belts with raised grip ridges, non-stick baking mats with printed location marks, drying oven belt edge seals, and silicone-coated glass fibre release sheets with printed batch identifiers.