Platinum catalysts are the "heart" of liquid silicone curing, but they are not standardized, one-size-fits-all products. Depending on the application—some require fast curing, others long storage life without premature curing, some demand yellowing resistance and high-temperature stability, while others need to work in aqueous environments—the industry has developed multiple types of platinum catalysts with distinct functionalities.

This article systematically reviews the six major categories of platinum catalysts currently available on the market to help clarify your selection strategy.

1. Karstedt Catalysts — The Industrial Workhorse

Karstedt catalysts are the most widely used variant in the platinum catalyst family. Chemically, they are complexes of platinum(0) with divinyltetramethyldisiloxane (TMDVDS), CAS No. 68478-92-2, molecular weight 381.48, appearing as colorless transparent or pale yellow liquids.1

Core Advantages:

• High catalytic activity: Rapidly initiates hydrosilylation reactions from room temperature up to 50°C.
• Light color: Produces highly transparent, non-yellowing products.
• Good compatibility with silicone systems: Easily dissolves in silicone oils, toluene, and other organic solvents.
• Good storage stability: Shelf life of 6–12 months when sealed and stored away from light.

Typical Specifications:
Karstedt catalysts typically come in platinum concentrations of 1,000–20,000 ppm, with 3,000 ppm and 5,000 ppm being the most common industrial grades. The typical addition level is 0.05%–0.5% of the total compound weight.1

Key Usage Note: Karstedt catalysts must be used together with an inhibitor (such as ethynylcyclohexanol, ECH); otherwise, their high room-temperature activity will cause the mixed compound to gel too quickly for processing.89

Typical Applications: General addition-cure liquid silicone rubber (LSR), electronic potting compounds, silicone gels, addition-cure HCRs, thermal conductive gels, and more.12

2. Speier Catalysts — The Classic Pioneer

Speier catalysts are solutions of chloroplatinic acid (H₂PtCl₆) in isopropanol or ethanol, representing the earliest platinum catalysts adopted for industrial use.612

Characteristics:

• Simple preparation and low cost.
• Lower catalytic activity (approximately 1/10 to 1/5 that of Karstedt catalysts).
• Contains chloride ions, posing a corrosion risk to certain substrates.
• Yellowish color, unsuitable for transparent products.

Typical Applications: Cost-sensitive applications with modest activity requirements, such as mid-to-low-grade silicone rubber products and silicone-to-metal bonding systems. Once the industry mainstay before Karstedt catalysts became widespread, Speier catalysts are now being gradually phased out.6

3. Ashby Catalysts — Premium Choice for High Transparency

Ashby catalysts are complexes of platinum(0) with vinyl cyclics (tetramethyltetravinylcyclotetrasiloxane).4

Core Advantages:

• High reaction activity.
• Excellent resistance to yellowing and oxidation.
• Better poison resistance than conventional Karstedt catalysts.
• Outstanding storage stability.

Typical Applications: High-end products with demanding appearance requirements, such as optical-grade silicones, high-transparency LED encapsulants, and precision mold silicones. Priced slightly higher than Karstedt catalysts, they are well worth the investment in critical applications.14

4. Poison-Resistant Catalysts — Survivors in "Toxic" Environments

The biggest weakness of platinum catalysts is their susceptibility to "poisoning"—compounds containing sulfur, nitrogen, or phosphorus, as well as certain heavy metal ions, can deactivate the platinum active sites, resulting in incomplete or failed curing.610

To address this, the industry has developed poison-resistant platinum catalysts. By engineering the platinum ligand structure (e.g., introducing N-heterocyclic carbene, NHC, ligands, or anchoring platinum onto metal-organic framework, MOF, carriers), the platinum active centers are effectively "shielded," enabling them to withstand attacks from common poisons.7

Typical Product Features:

• Maintain catalytic activity even in environments containing trace amines, thiols, organotin compounds, and other poisons.
• Better platinum dispersion (atom-level dispersion), reducing agglomeration.
• Some grades leverage steric hindrance effects of ligands to block poisons from reaching the platinum active center.

Typical Applications: Factories transitioning from peroxide-cure systems to platinum-cure systems, where equipment and pipelines may retain residual impurities; electronic potting applications where the compound may come into contact with flux residues.10

5. Latent (One-Component) Catalysts — Simplifying Processing

Traditional platinum-cure systems require separate A/B component storage, and the mixed compound must be used within a short time frame. This poses challenges for automated production and long-term storage. Latent (one-component) catalysts solve this pain point.

The principle of latent catalysts is to keep the platinum active centers in a "dormant" state at room temperature through special ligand encapsulation or microencapsulation techniques. Only when heated above a certain temperature does the encapsulation layer melt or the ligand detach, "awakening" the catalyst to begin working.28

Based on the duration of latency, they are divided into two types:

Short-acting latent catalysts (e.g., PC-17): Remain uncured for 30 minutes to 2 hours at room temperature, and fully cure within 5 minutes above 50°C. Suitable for scenarios requiring completion of operations within a few hours.

Long-acting latent catalysts (e.g., PC-52/PC-53): Remain uncured for 3–6 months at 20–25°C room temperature, stay stable for several hours at 90–100°C, and rapidly and fully cure above 140–180°C. They can directly replace peroxide-cure systems, enabling the simple process of one-component storage with heat-activated curing.8

Typical Applications: One-component addition-cure silicones requiring long-term storage, platinum vulcanizing agents for HCRs, and scenarios where peroxide systems need to be replaced.8

6. Water-Borne Catalysts — The New Eco-Friendly Direction

Traditional platinum catalysts are oil-soluble and cannot be used directly in aqueous systems. With increasingly stringent environmental regulations, demand for water-borne silicone rubbers and silicone oil emulsions is growing rapidly, giving rise to water-borne platinum catalysts.

Typical Features:

• Water-free in their native form but fully water-soluble.
• Remain stable for months in aqueous dilutions.
• Release catalytic activity when heated above 90°C.

Typical Applications: Water-borne silicone rubber emulsions, water-borne silicone oils, water-borne coating materials, paper release agents, and more.

Selection Decision Tree

Faced with so many types, how do you choose? Here is a simplified decision framework:

Step 1: Consider the application scenario.

• General liquid silicone, potting compounds, LSR injection molding → Karstedt type
• Optical-grade, high-transparency products → Ashby type or high-refractive-index type
• Food/medical-grade products → Karstedt or Ashby type compliant with FDA/LFGB standards
• Aqueous systems → Water-borne catalyst

Step 2: Consider processing conditions.

• Two-component, mix-and-use → Karstedt type + inhibitor
• One-component requiring long-term storage → Long-acting latent catalyst
• Short operation window needed → Short-acting latent catalyst

Step 3: Consider environmental compatibility.

• Clean production environment, no poisoning sources → Standard Karstedt type
• Equipment previously used for peroxide systems, possible residual impurities → Poison-resistant type
• System contains trace nitrogen- or sulfur-containing additives → Poison-resistant type

Step 4: Consider platinum content.
Higher platinum content is not always better. For products of常规 thickness with moderate curing requirements, low concentrations (1,500–3,000 ppm) are sufficient; high concentrations (5,000–20,000 ppm) are only needed for thin-wall rapid injection molding or deep-section curing scenarios. Excessive addition not only increases costs but may also trigger side reactions.3

Summary

Platinum catalysts are not "off-the-shelf components" but rather "tailored solutions." Selection is fundamentally about finding the optimal balance among activity, selectivity, poison resistance, processing window, regulatory compliance, and cost. Karstedt catalysts are the universal starting point, but if your production environment, product requirements, or processing conditions are特殊, be sure to consider the corresponding functional catalyst rather than simply "adding more catalyst" to solve the problem.


Diethylenetetramethyldisiloxane platinum complex (platinum catalyst) MY 8113