Karstedt Platinum Catalyst

Overview

Karstedt's catalyst — a zerovalent platinum complex with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (DVTMDS) ligands, first reported by Bruce Karstedt at General Electric in 1973 — is the modern industry-standard catalyst for hydrosilylation cure of addition-cure silicone. The active species is a Pt(0)-DVTMDS dimeric complex, soluble in non-polar siloxane media, supplied at 2%, 3% or 5% Pt by weight diluted in vinyl-functional siloxane solvent (typically vinyl-PDMS at 5,000–10,000 cP) for direct dosing into the A-side of two-part LSR or RTV-2 systems.

The Karstedt-catalysed hydrosilylation cycle proceeds via the Chalk-Harrod mechanism: vinyl-PDMS coordinates to Pt(0), oxidative addition of Si–H from the crosslinker gives a Pt(II)–H–Si intermediate, β-migratory insertion of vinyl into the Pt–H bond, then reductive elimination forms a new Si–CH₂–CH₂–Si crosslink with no byproduct released. 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, Si–H homologation). It produces no chloride residue, allowing optically clear and food-contact compliant cured silicones.

The dominant operational issue is poisoning. Karstedt is reversibly inhibited by alkynes (used as cure-rate moderators — 1-ethynyl-1-cyclohexanol and similar 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. Production lines for addition-cure silicone must be fully segregated from any tin- or condensation-cure silicone production.

LSR (liquid silicone rubber) injection moulding is the largest single application — automotive sealing components, baby-care nipples and pacifiers, medical-device elastomers, kitchenware. Karstedt at 5–10 ppm Pt loaded in the A-side 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. Storage form: tinted glass bottles or stainless-steel pressure cans under dry nitrogen blanket; light-sensitive (slow Pt aggregation under UV) and inhibitor-sensitive (any contact with sulphur-containing materials, EPDM elastomer fittings, or amine-containing solvents will deactivate fractional active Pt content).

Technical Specifications

Applications

Liquid Silicone Rubber (LSR) Injection Moulding

The largest application — automotive sealing, baby-care nipples and pacifiers, medical-device elastomers, kitchenware. Karstedt at 5–10 ppm Pt in A-side cures in 10–30 seconds at 150–200°C tool temperature.

RTV-2 Mould Rubber

Prototype tooling, decorative casting, food-contact moulds use 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

LED packages, photovoltaic cell encapsulants, PCB conformal coatings use Karstedt at 10–15 ppm Pt with high-clarity vinyl-PDMS — chloride-free chemistry is required for reliability under DC bias and humidity stress (chloride induces silver migration and corrosion).

Skin-Adhesive Medical

Wound dressings, scar treatment sheets, transdermal patches use 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 — the absence of chloride byproduct and organotin catalyst makes Karstedt-cured silicone the only chemistry meeting most national food-contact regulations.

Selection Guide vs Other Catalysts

Choose Karstedt when: medical, food-contact, optical, or electronic application requires no chloride residue and no organotin contamination. Choose Speier (chloroplatinic acid) when: cost-per-Pt-mole drives the decision and slight cure colouration is acceptable; silane synthesis where chloride byproduct is anyway present. Choose tin (DBTDL/DOTL/T-9) when: condensation-cure RTV is the target chemistry and Pt-poisoning concerns rule out addition cure.

Equivalent Grades

• Umicore Catalyst PC072 — primary commercial Karstedt grade
• Heraeus Karstedt — alternative European supply
• Johnson Matthey Karstedt — alternative supplier
• Generic Karstedt 2/3/5% — Chinese commodity grade, ICP-MS verified