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Karstedt Platinum Catalyst (Platinum-Divinyltetramethyldisiloxane Complex): The Industry-Standard Hydrosilylation Catalyst for LSR and RTV-2
SEMITECH supplies Karstedt platinum complex at 2%, 3% and 5% Pt loadings in vinyl-functional siloxane solvent — the modern industry standard for hydrosilylation cure of LSR injection moulding, RTV-2 medical and optical encapsulation, and food-contact silicones. Far more selective than chloroplatinic acid (Speier), no chloride residue, no part discolouration. 1 kg / 500 g packing under nitrogen blanket.
Contents
| 2 / 3 / 5% | 5–20 ppm | 1 kg |
|---|---|---|
| Pt loadings stocked | Typical use level | MOQ |
Chemistry & Specifications
Pt(0) complex with 1,1,3,3-tetramethyl-1,3-divinyldisiloxane ligand — soluble in vinyl-PDMS, no chloride.
Karstedt’s catalyst is a zerovalent platinum complex — Pt(0) coordinated by 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (DVTMDS) ligands — first reported by Bruce Karstedt at General Electric in 1973 (US Patent 3,775,452). The active catalytic species is a Pt(0)-DVTMDS dimeric complex with two divinyl-disiloxane ligands per Pt₂ unit, soluble in non-polar siloxane media. SEMITECH supplies Karstedt at three platinum loadings — 2 wt%, 3 wt%, and 5 wt% Pt — diluted in vinyl-functional siloxane solvent (typically vinyl-terminated PDMS at 5,000–10,000 cP) to allow direct dosing into the A-side of two-part LSR or RTV-2 systems without requiring a separate solvent strip.
The complex is supplied as a clear pale-yellow to amber liquid with platinum content verified by ICP-MS to ±0.05% of nominal. Storage form: tinted glass bottles or stainless-steel pressure cans under dry nitrogen blanket; light-sensitive (slow Pt aggregation under UV) and moisture-resistant (the DVTMDS ligand suppresses hydrolysis). The vinyl-siloxane solvent is reactive — it incorporates into the hydrosilylation network as part of normal cure, leaving no extractable solvent in the cured part.
Cure Mechanism: Pt(0)-Catalysed Hydrosilylation of Vinyl-PDMS + Si-H
Vinyl-PDMS coordinates to Pt centre, oxidative addition of Si-H, β-vinyl insertion, reductive elimination — Chalk-Harrod cycle.
The Karstedt-catalysed hydrosilylation cycle (Chalk-Harrod mechanism, modified for Pt(0)) proceeds via: (1) coordination of vinyl-PDMS olefin to the Pt(0) centre displacing one DVTMDS ligand; (2) oxidative addition of the Si-H bond from the crosslinker, giving a Pt(II)-H-Si intermediate; (3) β-migratory insertion of the coordinated vinyl into the Pt-H bond, forming a β-silyl alkyl-Pt(II) species; (4) reductive elimination releasing the new C-Si bond and regenerating Pt(0). The net result is Markovnikov-selective addition of Si-H across the C=C of vinyl-PDMS, forming a Si-CH₂-CH₂-Si crosslink with no byproduct released.
- Loading 5–10 ppm Pt — fast-cure LSR injection moulding (10–30 sec at 150–200°C)
- Loading 10–20 ppm Pt — RTV-2 cast moulding (5–30 min at 80–120°C); medical and optical
- Loading 20–50 ppm Pt — partial-poisoning override; high-clarity optical encapsulant; aged inventory recovery
Karstedt is far more selective and active than the historical Speier (chloroplatinic acid) catalyst — the DVTMDS ligand stabilises Pt(0) against premature reduction to colloidal platinum and provides sufficient steric bulk to suppress side reactions (β-hydride elimination giving allyl-PDMS, Si-H homologation). It produces no chloride residue, allowing optically clear and food-contact compliant cured silicones. The catalyst is fully consumed in cure — there is no separate quench step.
Inhibition is the dominant operational issue. Karstedt is reversibly inhibited by alkynes (used as cure-rate moderators — ECH, MBT inhibitors at 100–500 ppm extend pot-life from seconds to weeks), and irreversibly poisoned by sulphur, tertiary amines, organotin compounds, phosphines, and certain heavy-metal salts (Pb, Hg, Bi). Cure-failure on rubber moulds, EPDM gaskets, sulphur-vulcanised silicone, or surfaces previously contacted by DBTDL is the textbook Pt-poisoning symptom.
Applications & Formulation Guidance
LSR injection moulding (largest), RTV-2 medical/optical, food-contact silicone, electronic encapsulation, skin adhesives.
Liquid silicone rubber (LSR) injection moulding is the largest application — automotive sealing components, baby-care nipples and pacifiers, medical-device elastomers, kitchenware. Karstedt at 5–10 ppm Pt loaded in the A-side (vinyl-PDMS + filler) reacts with B-side (Si-H crosslinker + alkynyl alcohol inhibitor) on injection at 150–200°C tool temperature, with cycle times of 10–30 seconds for thin-section parts. RTV-2 mould rubber for prototype tooling and decorative casting uses Karstedt at 10–20 ppm Pt with cure cycles of 30 minutes at 80°C or 4 hours at 23°C; chosen over condensation-cure tin systems when faster cure, lower shrinkage, and food-contact compliance are required.
Optical and electronic encapsulation uses Karstedt at 10–15 ppm Pt with high-clarity vinyl-PDMS to seal LED packages, photovoltaic cell encapsulants, and PCB conformal coatings — chloride-free chemistry is required for reliability under DC bias and humidity stress (chloride induces silver migration and corrosion). Skin-adhesive silicone (medical wound dressings, scar treatment sheets, transdermal patches) uses Karstedt at 15–25 ppm Pt with low-modulus vinyl-PDMS and partial Si-H loading to give a soft, tacky cured surface. Food-contact silicone (bakeware, baby bottle nipples, beverage tubing) uses Karstedt with FDA-compliant vinyl-PDMS and silica filler — the absence of chloride byproduct and organotin catalyst makes Karstedt-cured silicone the only chemistry meeting most national food-contact regulations.
Procurement, Storage and Quality Control
1 kg / 500 g tinted glass under N₂; ICP-MS Pt assay; light-sensitive; 12-month shelf life.
SEMITECH issues a CoA on every batch with: platinum content (ICP-MS, target ±0.05% of nominal 2/3/5%), appearance and colour (APHA), viscosity at 25°C, and active species verification by ²⁹Si-NMR if specified. Standard packing: 500 g (5% Pt grade) or 1 kg (2% / 3% Pt grades) in tinted amber glass bottles with PTFE-lined caps under dry nitrogen blanket. MOQ is 500 g for the 5% grade, 1 kg for 2% and 3%. Bulk orders over 25 kg ship in stainless-steel pressure cans on a quarterly schedule synchronised to platinum sourcing cycles. Lead time 2–3 weeks to Asia, 3–4 weeks to Europe and North America. Air freight available for laboratory and qualification quantities (50–100 g samples) within 5 working days.
Storage discipline is critical: Karstedt is light-sensitive (UV slowly reduces Pt(0) to colloidal Pt black, visible as fine grey precipitate that is no longer catalytically active) and inhibitor-sensitive (any contact with sulphur-containing materials, EPDM elastomer fittings, or amine-containing solvents will deactivate fractional active Pt content). Recommended: store in original tinted glass bottles, sealed under nitrogen, below 25°C and away from direct sunlight; consume opened bottles within 60 days; replace any rubber gaskets, hoses, or seals that contact the catalyst with PTFE or perfluoroelastomer (Kalrez) materials. Shelf life 12 months sealed. Pt as commodity: Pt content drives ~85% of the catalyst cost; SEMITECH passes through monthly LBMA fixings on bulk orders. CoA documents original Pt source for compliance with conflict-mineral and ESG sourcing audits.
Karstedt platinum is the modern industry-standard hydrosilylation catalyst — far more selective than Speier, no chloride residue, fast cure (10–30 sec for LSR injection moulding at 150–200°C), required for medical, food-contact, optical, and electronic silicones. Stocked at 2/3/5% Pt loadings. Cross-contamination with sulphur, amine, organotin = dead catalyst.
Karstedt Platinum Catalyst Specification Sheet
SEMITECH stocked grades; CoA per batch with ICP-MS Pt assay.
| Property | Karstedt 2% | Karstedt 3% | Karstedt 5% |
|---|---|---|---|
| Pt content (ICP-MS) | 2.00 ± 0.05% | 3.00 ± 0.05% | 5.00 ± 0.05% |
| Solvent | Vinyl-PDMS, 5,000 cP | Vinyl-PDMS, 5,000 cP | Vinyl-PDMS, 8,000 cP |
| Appearance | Pale yellow liquid | Yellow liquid | Amber liquid |
| APHA colour | ≤200 | ≤300 | ≤500 |
| Viscosity (25°C) | 5,000–6,000 cP | 5,000–6,000 cP | 8,000–10,000 cP |
| Density (20°C) | 0.97 g/cm³ | 0.97 g/cm³ | 0.98 g/cm³ |
| Active complex | Pt(0)-DVTMDS dimer | Pt(0)-DVTMDS dimer | Pt(0)-DVTMDS dimer |
| Typical use level | 5–20 ppm Pt | 5–20 ppm Pt | 5–20 ppm Pt |
| Packaging | 1 kg amber glass | 1 kg amber glass | 500 g amber glass |
| Storage | Tinted glass, N₂ blanket, <25°C | Tinted glass, N₂ blanket, <25°C | Tinted glass, N₂ blanket, <25°C |
| Shelf life | 12 months sealed | 12 months sealed | 12 months sealed |
FAQ
+Why does my LSR fail to cure on a sulphur-vulcanised rubber tool insert?
Sulphur is the most aggressive Pt poison — even ppm-level migration from a sulphur-vulcanised rubber surface (EPDM, NBR, natural rubber) into the silicone-tool interface is sufficient to deactivate the catalyst at the boundary, producing tacky uncured layers 0.1–1 mm thick. Solutions: (1) replace the rubber insert with a sulphur-free elastomer (peroxide-cured EPDM, perfluoroelastomer, silicone); (2) bake the rubber surface at 200°C for 4 hours to outgas residual sulphur; (3) coat the interface with a barrier primer (vinyl-functional silane); (4) raise Pt dosage from 5 ppm to 30–50 ppm to overcome partial poisoning at the boundary. Also poisoning: tertiary amines (DABCO, IPDI processing residues), organotin (DBTDL traces from shared equipment), phosphines.
+Which Pt loading should I order — 2%, 3% or 5%?
Order the 5% Pt grade for: production-scale LSR moulders dispensing < 50 g catalyst per tonne of formulation (lower shipping cost per Pt unit, smaller storage footprint); RTV-2 mould-makers running daily small-batch dosing where 1 kg lasts 6+ months. Order the 2% or 3% grade for: lab-scale formulation development where dosing accuracy at small volumes is critical (more dilute = easier metering); custom formulations where Pt-to-vinyl ratio must be controlled tightly (the more dilute grades reduce the impact of weighing error on final Pt ppm).
+How do I confirm a Karstedt batch is still active before using it on a production run?
Run a small-scale gel test: weigh 50 g vinyl-PDMS (10,000 cP), add 10 ppm Pt from the test batch (e.g. 2 mg of 5% Pt grade), mix, add 1.5 wt% Si-H crosslinker, mix, and observe gel time at 23°C. A fresh active batch gels in 15–30 minutes at room temperature; an aged or partially-deactivated batch shows extended gel time (45–90+ minutes) or fails to cure tack-free. Confirm Pt content quantitatively by sending a 1 g sample for ICP-MS — costs USD 50–80 per sample at commercial labs. SEMITECH offers free re-assay on any batch <12 months from manufacture date.