Standard condensation catalyst for PU- and epoxy-resistant silicone molds
Wacker Catalyst T 21 is the standard catalyst (B-component) for the condensation-curing ELASTOSIL® M silicone rubbers for mold making: Elastosil M 4511, M 4512, M 4514, and M 4541. This colorless, low-viscosity catalyst based on an organotin compound has a density of 0.96 g/cm³ and a dynamic viscosity of 15 mPa·s at 23 °C. According to Wacker, T 21 provides excellent chemical resistance to polyurethane resins, significantly extending the mold's lifespan. For Elastosil M 4514, Wacker also specifies T 21 for casting epoxy. The standard dosage is 5 weight percent of the A-component (100:5 ratio). At a 5% dosage with Elastosil M 4514, the pot life is approximately 60 to 90 minutes and the demolding time is 8 to 12 hours at 23 °C and 50% relative humidity.
Key Benefits
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Dosage 5% (100:5) – fixed, easy-to-measure weight ratio to the A-component for reliable curing.
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Excellent PU resistance – Wacker positions T 21 for a high number of casts in polyurethane casting resin.
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Also for epoxy – with Elastosil M 4514, the specified catalyst for casting epoxy resins.
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Density 0.96 g/cm³ and viscosity 15 mPa·s – very low-viscosity, mixes quickly into the base component.
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Mixable with Catalyst T 47 – at 95:5 (T 21 : T 47), the pot life drops to 20 to 40 minutes and the mold can be demolded after 4 to 6 hours; at 90:10, the pot life is approximately 20 minutes and the demolding time 2 to 4 hours.
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Colorless – the catalyst does not discolor the mold.
Typical Applications
- Curing Elastosil M 4511, M 4512, M 4514, and M 4541 condensation silicones.
- Mold making for casting polyurethane casting resin.
- Mold making for casting epoxy resins.
- Catalyst for our Silicone Condensation 20 Resin, which comes with T 21 as standard.
- Mixing with Catalyst T 47 for accelerated curing in time-critical production.
Processing Instructions
- Stir the condensation silicone (A-component) thoroughly before use to distribute any sedimentation.
- Weigh the catalyst at 5% of the A-component's weight, so 5 grams of T 21 per 100 grams of silicone. You can use 0.5% less or more: less extends curing (more time for de-aeration), more accelerates it but increases shrinkage. Never use too little catalyst, otherwise the silicone will not cure (properly).
- For faster curing, you can mix T 21 with Catalyst T 47, for example in a 95:5 ratio (per 100 grams of A: 4.75 grams of T 21 with 0.25 grams of T 47).
- Mix the catalyst carefully and evenly into the silicone, scraping the walls and bottom of the mixing container. T 21 is colorless, so you cannot tell by color when the catalyst is evenly distributed; therefore, mix thoroughly.
- Process the mixture within the pot life and never prepare more than you can process within that time.
- Wait until the mixture is fully cured before demolding.
Note: pot life (working time) and demolding time are highly dependent on the mixing ratio and humidity. With more catalyst or higher humidity, both pot life and demolding time will be shorter. Below 40% relative humidity, the silicone surface may remain tacky; in this case, increase the humidity in the room.
Casting epoxy: Wacker recommends post-curing condensation molds for epoxy for a few hours at approximately 150 °C before first use. Casting polyester? Then use Catalyst T 51, which Wacker developed specifically for polyester resins. More explanation about the right combination can be found at silicones for aggressive resins.
Wacker Chemie AG is a global market leader in the development and production of silicones for industrial and technical applications.
Shelf life: if stored in sealed packaging, dry and cool, the shelf life is at least the period stated on the label. Exceeding this does not automatically render the product unusable; check its properties before use.
This product is intended for the business market. For larger quantities or customized products, please contact us at info@siliconesandmore.com.
Professional Support
Are you undecided between T 21 and T 51, or unsure about mixing with T 47 for faster curing? Our specialists are happy to advise you on the best catalyst combination for your resin and application. Feel free to contact us.
