Platinum-cured silicone is addition-cure silicone rubber crosslinked by a platinum catalyst, through a reaction called hydrosilylation, rather than by heat-generated peroxide radicals or moisture-driven condensation. Commercial formulations run this catalyst at 1 to 100 parts per million of platinum by weight, and unlike almost every other industrial use of the metal, that platinum is never recovered once the silicone cures. The sections below cover the chemistry, the two named catalysts that carry it, typical loading, why the process makes silicone one of the few dissipative platinum uses, and where it fits against platinum's wider industrial demand.
What Is Platinum-Cured Silicone?#
Platinum-cured silicone is silicone rubber whose crosslinking reaction is catalyzed by platinum, adding a silicon-hydrogen bond across a carbon-carbon double bond to build the cured network. This addition-cure chemistry places platinum-cured silicone among the uses of platinum group metals in industrial catalysis, alongside catalytic converters, petroleum reforming and glass manufacturing, though its parts-per-million loading and its dissipative loss set it apart from those recovered-metal uses. Manufacturers formulate it as a two-part system: one part carries the platinum catalyst, the other carries the crosslinker, and the two mix and cure only once combined.
Addition Cure vs Condensation and Peroxide Cure#
Addition-cure (platinum) silicone produces no volatile byproduct as it crosslinks, unlike condensation-cure silicone, which releases a leaving group such as acetic acid or an alcohol as it sets, and unlike peroxide-cure silicone, which relies on heat-generated free radicals instead of a metal catalyst. Condensation-cure silicones, the familiar one-part sealants that cure at room temperature against atmospheric moisture, shrink slightly as their byproduct evaporates and are unsuitable for thick, enclosed sections where that byproduct cannot escape. Peroxide-cure silicones need an external heat source, typically 100 to 180°C, to decompose the peroxide into free radicals that crosslink the polymer chains, and they leave decomposition byproducts of their own that often require a post-cure bake to drive off. Platinum-catalyzed addition cure avoids both problems: it produces no shrinkage-causing byproduct, cures at temperatures from room temperature up to about 150°C depending on the inhibitor package, and cures reliably in thick sections and closed molds where a byproduct-releasing or purely thermal system would fail. That combination is why liquid silicone rubber (LSR), medical-grade silicone, release coatings and cast-and-cure encapsulants are formulated almost exclusively as platinum addition-cure systems.
What Is a Platinum Catalyst? How Platinum Cures Silicone#
A platinum catalyst in this context is the platinum complex that accelerates hydrosilylation without being consumed by the overall reaction, dispersed through the silicone at parts-per-million level. Hydrosilylation itself is the addition of a silicon-hydrogen (Si-H) bond across a carbon-carbon double bond (C=C), written as Si-H + CH2=CH-R to Si-CH2-CH2-R; the reaction builds the crosslinks that turn a liquid or paste silicone formulation into a solid elastomer. Platinum's role is purely catalytic: each platinum center cycles through the mechanism below many times over, so a formulation needs only a trace amount of metal to crosslink the entire polymer network. That repeated cycling, called catalytic turnover, is why a hydrosilylation formulation can crosslink a large volume of silicone from a platinum loading measured in single-digit or low double-digit parts per million rather than in weight percent, the concentration scale typical of a stoichiometric reagent.
The Chalk-Harrod Hydrosilylation Mechanism#
The Chalk-Harrod mechanism explains platinum's role in three steps: oxidative addition of the Si-H bond to platinum, migratory insertion of the coordinated alkene into the platinum-hydrogen bond, then reductive elimination forming the new silicon-carbon bond. A modified Chalk-Harrod route, in which the alkene inserts into the platinum-silicon bond instead of the platinum-hydrogen bond, accounts for the vinylsilane byproducts sometimes observed in hydrosilylation, according to the peer-reviewed hydrosilylation literature summarized in Polymers (MDPI, volume 12, article 2174, 2020) and further detailed in Inorganica Chimica Acta (1999). The cycle runs as follows:
- Oxidative addition: the Si-H bond adds across the platinum center, and platinum's oxidation state rises as it inserts into that bond.
- Migratory insertion: the coordinated alkene inserts into the platinum-hydrogen bond, forming a platinum-alkyl intermediate.
- Reductive elimination: the alkyl group and the silicon fragment combine to form the new silicon-carbon bond, releasing the crosslinked product and regenerating the active platinum catalyst for another cycle.
A modified route to the same outcome inserts the alkene into the platinum-silicon bond instead of the platinum-hydrogen bond at the second step, and that alternative pathway is what produces the small amount of vinylsilane byproduct sometimes seen alongside the main hydrosilylation product.
Speier's Catalyst and Karstedt's Catalyst#
Speier's catalyst (hexachloroplatinic acid, H2PtCl6, in isopropanol) was the first practical hydrosilylation catalyst, introduced at Dow Corning in the 1950s. The Karstedt catalyst (a platinum(0)-divinyltetramethyldisiloxane complex, CAS 11057-89-9 for the discrete complex) followed in 1973, developed by Bruce Karstedt at General Electric under US Patent 3,775,452, which was filed on 28 April 1971 and granted on 27 November 1973. Karstedt's catalyst is now the industry standard for silicone hydrosilylation, and it carries a specific, named chemical identity distinct from a generic platinum solution, the same standard this site uses for every named PGM compound it catalogs.
| Property | Speier's catalyst | Karstedt's catalyst |
|---|---|---|
| Composition | Hexachloroplatinic acid (H2PtCl6) in isopropanol | Platinum(0)-divinyltetramethyldisiloxane complex |
| Introduced | 1950s, at Dow Corning | 1973 (patent grant), at General Electric |
| Solubility in silicone matrix | Limited | High |
| Relative activity | Lower; historically used at higher ppm | Higher; effective at lower ppm |
| Colloidal platinum haze | Can occur on aging or overheating | Does not occur |
Table 1: the two named platinum hydrosilylation catalysts compared on composition, origin and behavior in the silicone matrix.
The Speier catalyst is chemically the same substance as chloroplatinic acid, both carrying CAS number 16941-12-1; the refining and general chemistry of that compound is covered on its own compound page, while this page covers only its role as a silicone hydrosilylation catalyst. Speier's catalyst historically ran at roughly 10 to 50 parts per million platinum by mass in industrial silicone hydrosilylation, an order of magnitude higher than Karstedt's catalyst now needs for the same cure, per the peer-reviewed hydrosilylation literature (Polymers, MDPI, 2020) and Platinum Metals Review; that loading gap is a large part of why Karstedt's catalyst became the default choice once it reached commercial scale.
Why Karstedt's Catalyst Replaced Speier's#
Karstedt's catalyst replaced Speier's as the industry standard because it is soluble in the silicone matrix, far more active, and does not produce the colloidal platinum haze that Speier's catalyst can. Speier's catalyst, being chloride-based, needs a higher platinum loading to reach the same cure speed and can leave a visible colloidal platinum discoloration in the cured part if the catalyst degrades during processing. Karstedt's catalyst dissolves fully in the silicone base, resists that degradation, and cures reliably at a lower platinum loading, which is why it displaced Speier's catalyst across most commercial addition-cure formulations. Reported on a mass basis, platinum makes up about 41.1% of the discrete Karstedt complex or 51.1% of the simplified single-platinum repeat unit that some suppliers use to describe the same chemistry; the commercial catalyst itself is sold as a dilute platinum solution, so neither figure applies directly to a bottle of the product as shipped, and any platinum content quoted for it should always state which of the three bases it uses.
How Much Platinum Is in Cured Silicone?#
Commercial hydrosilylation loadings range from 1 to 100 parts per million of platinum by weight of the total composition, with roughly 10 ppm typical at ambient cure and Johnson Matthey citing 10 to 15 ppm as effective for hydrogen-removal duty. The exact figure depends heavily on the application, the desired cure speed and the inhibitor system used to control pot life before cure.
| Loading basis | Platinum loading | Source |
|---|---|---|
| Ambient-cure typical | ~10 ppm | Industry-standard hydrosilylation practice |
| Commercial application range | 1 to 100 ppm (5 to 200 ppm depending on application) | BASF Catalysts Metals |
| Johnson Matthey hydrogen-removal duty | 10 to 15 ppm | Johnson Matthey |
| Published GE formulation | 90 ppm platinum as Karstedt's catalyst, with 900 to 1,800 ppm inhibitor | General Electric formulation, cited in the hydrosilylation literature |
| Patent-literature ambient-cure range | 0.5 to 10 ppm platinum | USPTO patent literature (single-source) |
| Patent-literature loading floor | As little as 0.001 part by weight platinum per million parts of the total composition (below 1 part per billion) | USPTO patent literature (single-source) |
Table 2: platinum loading in commercial hydrosilylation formulations, by basis and source. Figures state ppm platinum only; this page does not estimate a tonnage or ounce figure for platinum consumed by the silicone industry, since the addition-cure share of total silicone volume needed for that calculation is not published.
The lowest patent-literature figures sit well below the 10 ppm ambient-cure typical, illustrating how far a formulator can thrift platinum loading before cure speed and shelf stability start to suffer; most commercial formulations stay well above that floor for a working margin against inhibitor consumption and mold-surface poisoning, covered next.
Why Platinum-Cured Silicone Costs More: A Dissipative Platinum Use#
Platinum-cured silicone costs more than peroxide-cured silicone partly because its platinum is never recovered: the metal stays dispersed in the cured elastomer at ppm level, economically unrecoverable, making silicone one of the few genuinely dissipative platinum uses. That sets it apart from platinum in glass manufacturing, refining or nitric acid production, where 30 to 95% of the metal returns once the equipment reaches end of life. Once the catalyst has done its job of crosslinking the polymer network, it remains locked inside the finished part at a concentration far too low to justify recovery, so every ounce of platinum a silicone formulator buys is effectively consumed rather than lent. That dissipative economics sits behind part of the price premium platinum-cured silicone carries over peroxide-cured or condensation-cured grades, on top of the cure-quality advantages covered above. It also means silicone hydrosilylation adds one-way demand to the platinum market that recycling cannot later offset, in contrast with catalytic converters and refining catalysts, which return most of their platinum through scrap and spent-catalyst recovery once they reach end of life.
Why Silicone Sometimes Fails to Cure: Catalyst Poisoning#
Silicone sometimes fails to cure because amines, phosphines, sulfur compounds and tin compounds inhibit the platinum hydrosilylation catalyst, the practical reason a mold or substrate can stop a platinum-cure silicone from setting against it, a poisoning behavior documented in the peer-reviewed hydrosilylation literature (Polymers, MDPI, volume 9, article 534, 2017). These poisons work by binding strongly to the platinum center, blocking the coordination sites the Chalk-Harrod cycle needs and shutting down catalytic turnover. Common poisoning culprits include:
- Amine-cured epoxies and adhesives, whose residual amine groups coordinate platinum and stop cure at the contact surface.
- Organotin compounds, used as catalysts in condensation-cure silicones and some other rubbers, which cross-contaminate platinum-cure systems on shared tooling.
- Sulfur-vulcanized rubbers, whose sulfur compounds poison platinum on contact, a well-known reason silicone will not cure against certain natural or synthetic rubber substrates.
- Phosphine-containing materials, chemically similar in their platinum-binding behavior to the phosphine ligands used in other PGM catalysts.
No quantified inhibition threshold is published for any of these poison classes, so formulators generally address the risk with a barrier coating or a cure test on the actual substrate rather than by calculating a tolerance level.
Silicone's Place in Platinum Industrial Demand#
The global silicone market was estimated at 3.03 million tonnes in 2025 (Towards Chem and Materials) and 3.32 million tonnes in 2026 (Mordor Intelligence), of which only the addition-cure share carries platinum; condensation-cure and peroxide-cure silicones use no platinum at all. Silicone hydrosilylation sits within the wider category of industrial platinum demand alongside chemical, glass and petroleum uses, though it is measured in ppm loading rather than in the troy-ounce or kilogram terms used for platinum price today and other bullion-facing platinum demand. The World Platinum Investment Council reported that silicone platinum demand "remained broadly stable" in Q1 2026, with a modest increase forecast for the full year, a steadier trend than the swings seen in some of platinum's other industrial and investment demand categories. That stability reflects silicone's status as a mature, established platinum end use rather than a growth or decline story: demand tracks silicone production volume in addition-cure grades, not the investment flows or automotive-emissions rules that move other industrial and jewelry categories.
Platinum-Cured Silicone in Context#
Silicone manufacturers source platinum hydrosilylation catalyst, most commonly as Karstedt's catalyst, from precious-metal chemical suppliers rather than as raw metal. That sourcing route sits within the site's fuller catalog of platinum uses across catalysis, jewelry and investment demand, and it is distinct from either a scrap-recovery transaction or a bullion purchase, since the catalyst is bought, formulated and consumed rather than held or resold as metal.
Sourcing Platinum Catalyst for Silicone Manufacturing#
Buyers formulating addition-cure silicone can request a quote on platinum hydrosilylation catalyst through DailyPlatinum's dealer-neutral industrial RFQ, rather than a named vendor recommendation. This page does not name or rank molding-compound brands, since that retail-product comparison sits outside DailyPlatinum's scope as a chemistry and market source; the RFQ instead routes an industrial buyer's specification to precious-metal chemical suppliers. A useful request typically specifies:
- Target application, such as liquid silicone rubber, medical-grade silicone or a release coating, since loading and inhibitor needs differ by end use.
- Target platinum loading in ppm, informed by the ranges in Table 2 above.
- Inhibitor requirements, which set the working pot life before cure begins.
- Catalyst form, a dilute Karstedt's-catalyst solution versus a Speier's-catalyst-based system for chloride-tolerant applications.
Platinum-Cured Silicone FAQ#
What is platinum silicone?#
Platinum silicone is silicone rubber crosslinked by a platinum catalyst through hydrosilylation, rather than by peroxide or moisture cure.
Why is platinum silicone so expensive?#
Platinum-cured silicone costs more largely because its platinum catalyst is never recovered, unlike most other platinum industrial uses.
What is a platinum catalyst?#
A platinum catalyst is a platinum complex that speeds up a reaction, here hydrosilylation, without itself being permanently consumed.
What are platinum's symbol, atomic number and CAS number?#
Platinum has the symbol Pt, atomic number 78 and CAS number 7440-06-4; Karstedt's catalyst itself carries CAS number 11057-89-9. Full details on the element are on the platinum (Pt), element 78 page.