Addition-curing silicone does not harden: Identify inhibitors
If an addition-curing (platinum-catalyzed) silicone remains sticky or liquid, inhibition is almost always the culprit: certain substances poison the platinum catalyst. Knowing the typical inhibitors prevents this failure – and saves the mold or potting.
To the point
The most common cause is contact with a catalyst poison: sulfur, amines, tin, or certain plastics. In addition, incorrect mixing ratios, incomplete mixing, or excessively low temperatures curing. If in doubt, a small preliminary test on the actual substrate is advisable.
The typical inhibitors
Addition-curing silicones harden via a platinum catalyst. This catalyst is poisoned by several groups of substances, causing cross-linking to fail locally or completely:
- Sulfur and sulfur compounds – latex gloves, sulfur-crosslinked rubbers (often EPDM, NBR), some putties and modeling clays.
- Amines – amine hardeners of epoxies, some polyurethanes and adhesives.
- Tin and organotin – residues of condensation-crosslinking (tin-catalyzed) silicones, also via common tools.
- Plasticizers and stabilizers – from PVC and some adhesive tapes.
Cross-contamination via poorly cleaned tools, mixing cups or gloves is one of the most common – and most easily overlooked – causes.
Rule out other causes
| symptom | Probable cause |
|---|---|
| Sticky layer only at the contact surface | Inhibition by the subsurface |
| Completely unconnected | incorrect ratio or incomplete mixing |
| Very slow curing | Too low temperature |
Here's how to prevent it
Use nitrile-free, low-sulfur gloves (no latex gloves), keep tools strictly separate from tin-curing silicones, keep amines and PVC away, and mix according to the exact ratios (background: understanding adhesive curing). Perform a compatibility test on critical substrates. Basic information on types and processing can be found under "Silicone for Mold Making and "Silicone Rubber: The Complete Guide". Suitable addition-curing types include, for example, SILISIL RTV MF-Flex 20, MF-Dura 35 , or PRO-Cast 45.
Frequently Asked Questions
Why does my silicone remain sticky? Most likely inhibition by sulfur, amines, tin, or an unsuitable substrate – or an incorrect mixing ratio.
Can I pour more silicone over sticky silicone? No. The inhibited layer must be removed and the cause eliminated, otherwise it will inhibit again.
How do I test the compatibility? With a small preliminary test on the actual material; if it hardens cleanly, the substrate is unproblematic.
Consultation and samples
Describe the substrate and the nature of the defect – we will help you find the source of the inhibition and select the appropriate silicone. Contact us or write to info@silitech.ch.
