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Poisoning of silicones (cure inhibition)
Poisoning of silicones (cure inhibition)
Poisoning of silicones, also called cure inhibition, occurs when platinum silicones do not cure properly because the platinum catalyst reacts with substances in the surroundings. On this page you will read in simple terms what poisoning means, which substances cause it and how you prevent or solve it.
Addition-curing silicones
Addition-curing silicones are also called platinum silicones. Poisoning of platinum silicones is also called inhibition or cure inhibition.
Addition-curing silicones cure through polymerisation. In this process the short silicone molecules form long chains and cross-links. This network ultimately gives a rubbery silicone. The denser the network, the harder and stiffer the silicone.
Platinum is needed as a catalyst for this process. Platinum is an expensive metal and is therefore always added as sparingly as possible. Platinum also reacts with other components. If that happens, too little platinum may remain to let the silicone cure well. This is amplified because there is already very little platinum in the silicone due to cost.
The result is then that the silicone stays sticky or even liquid, and that it does not have the tensile strength and firmness you would expect, because the network is not or insufficiently formed.
Which substances poison addition silicones?
Sulphur
Sulphides and combinations of these. These occur among other things in latex gloves. Therefore always check your gloves.
Nitrogen
Amines and amides, nitriles and cyanates, and combinations and variations of these. These occur among other things in epoxies, paints and adhesives. If an epoxy or paint is fully cured this is often harmless, but the chance that an epoxy or paint is not fully cured is quite high.
Tin
Tin salts and similar substances. These are widely used in condensation silicones. Condensation silicones come in a 2-component variant, but also in 1-component variants. A very well-known form of 1-component condensation silicone is silicone sealant for joints. We advise against using such sealant when making moulds or enclosures for platinum silicone casting.
Phosphorus
Phosphorus components such as phosphine and phosphite. Used among other things in the metal industry for corrosion protection. So not all metal is safe for silicones.
Arsenic, antimony, selenium, tellurium
These elements and their compounds can also inhibit the curing of platinum silicones.
Some solvents
- Alcohols such as ethanol and methanol
- Esters such as ethyl acetate and vinyl acetate
- Some compounds with unsaturated bonds
Primers and coatings
- Polyethylene with one of the additives mentioned above
- Primers with additives from the list above
- Sodium salt primers
- Polyvinyl acetate or acrylic latex paints
- Coatings with calcium carbonate
- Coatings with natural rubber, styrene or polyvinyl acetate
Condensation silicones
The above does not apply to condensation silicones. These are silicones where the catalyst is often a tin compound. This type of silicone almost always cures, unless there is too little moisture in the air or in the system, or if the moisture is drawn out of the silicone, as with very dry river clay.
Preventing and solving inhibition of addition silicones
Prevention
The easiest way to prevent inhibition is to use materials and tools that contain none of these substances. Pay attention to the adhesives and paints you use, the material of your master model and even your gloves. Smoking can also poison the silicone, as can sanding or grinding epoxy or polyurethane products where dust lingers in the air for a long time.
Cure well
If you cannot avoid using one of these materials, let it cure very well. Let the products cure for several days if necessary, preferably at an elevated temperature. For UV-curing epoxy we recommend curing it especially well with UV light and possibly post-treating it. There are machines to fully cure epoxy prints.
Sealing
You can also coat the product with, for example, polyvinyl alcohol (PVA). This seals the material so the silicone does not come into contact with it. It is important that the seal is fully closed.
Extra platinum
You can add extra platinum to the silicone, or choose a silicone with a higher platinum percentage.
Raising the temperature
You can cure the silicone with mould at an elevated temperature, if the model and the other materials can handle it. The idea is that the silicone cures so quickly that the platinum has less time to react with contaminants. Unfortunately this does not work in all cases.
Preventing and solving inhibition of condensation silicones
Sufficient moisture
Make sure there is enough water in the condensation silicone. You can add a teaspoon of water per 200 grams, or use a condensation silicone accelerator (essentially a water emulsion). You can also slightly moisten the model or the materials around it. For example, make sure river clay does not dry out completely. Also use enough silicone, because a thin layer dries out faster on a dry surface than a thick layer.
Sufficient B component
Condensation silicones almost always cure when B component has been added. Too small an amount of B component can greatly slow down curing and gives a softer and less strong result. Note: a large excess is also not good, because it makes the silicone too hard and shortens the lifespan of the mould.
Note: this article does not contain all possible problems and is partly based on information from third parties. We cannot give any guarantees based on this document.