Addition-curing silicone does not harden: identifying inhibitors
If an addition-curing (platinum-catalyzed) silicone remains tacky or liquid, inhibition is almost always the cause: certain substances poison the platinum catalyst. Those who know the typical inhibitors can avoid failure and save the mold or the potting.
In a nutshell
The most common cause is contact with a catalyst poison: sulfur, amines, tin, or certain plastics. Additionally, incorrect mixing ratio, incomplete mixing, or too low a temperature prevent curing. When in doubt, a small preliminary test on the actual substrate helps.
The typical inhibitors
Addition-curing silicones cure via a platinum catalyst. This catalyst is poisoned by several groups of substances, causing the crosslinking to fail locally or completely:
- Sulfur and sulfur compounds: Latex gloves, sulfur-cured rubbers (often EPDM, NBR), some sealants and modeling compounds.
- Amines: Amine hardeners from epoxies, some polyurethanes and adhesives.
- Tin and organotin: residues from condensation-curing (tin-catalyzed) silicones, also via shared 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.
Exclude other causes
| Symptom | Probable cause |
|---|---|
| Sticky layer only on the contact surface | Inhibition by the substrate |
| Completely uncured | incorrect ratio or incomplete mixing |
| Very slow curing | temperature too low |
How to prevent it
Use nitrile-free, low-sulfur gloves (no latex gloves), strictly separate tools from tin-curing silicones, keep amines and PVC away, and mix exactly according to ratio (background: Understanding adhesive curing). For critical substrates, run a compatibility pre-test. Fundamentals on types and processing are available under Silicone for mold making and Silicone rubber: the complete guide. Suitable addition-curing types are, for example, SILISIL RTV MF-Flex 20, MF-Dura 35 , or PRO-Cast 45.
Frequently asked questions
Why does my silicone remain tacky? Very likely inhibition due to sulfur, amines, tin, or an unsuitable substrate, or an incorrect mixing ratio.
Can I pour over sticky silicone? No. The inhibited layer must be removed and the cause eliminated, otherwise it will inhibit again.
How do I test compatibility? With a small preliminary test on the actual material; if it cures cleanly, the substrate is not critical.
Consultation and samples
Describe the substrate and the failure pattern to us, and we will help identify the source of inhibition and select the appropriate silicone. Contact us or write to info@silitech.ch.
