A 1K RTV silicone cures with atmospheric moisture. That reaction always releases something – and this by-product decides half the application. Not the Shore hardness, not the elongation at break, not the price. The by-product.
Once you have internalised that, it takes five minutes to understand why the cheap sanitary silicone ruins the mirror.
The three systems at a glance
| Acetoxy | Oxime | Alkoxy | |
|---|---|---|---|
| By-product | acetic acid | ketoxime | alcohol |
| Reaction | acidic | neutral | neutral |
| Odour | distinct, vinegar | characteristic, weaker | low |
| Skin formation | fast | medium | slower |
| Metals | corrosive | mostly fine, copper/brass critical | good |
| Alkaline substrates | unsuitable | limited | good |
| Cost | low | medium | higher |
Acetoxy: fast, cheap, aggressive
The acetoxy-curing system is the classic – and for good reason: it is inexpensive, skins over quickly and adheres excellently to glass and glazed ceramic, often without any primer. That is why it sits in the typical sanitary and glazing silicone.
The catch has a name: acetic acid. It does not just smell, it attacks.
- Metals – copper, brass, zinc and galvanised steel corrode. What at first merely looks unsightly turns into debonding at the adhesion face.
- Alkaline substrates – concrete, mortar, marble and natural stone. The acid reacts with the substrate; the typical consequences are discolouration and edge-zone damage. On marble the damage is irreversible.
- Mirrors – the backing coating is attacked. A mirror bonded with acetoxy silicone blinds from the edge inwards.
Acetoxy is therefore not a bad system. It is a system with a clear territory: glass, glazed ceramic, enamel. There it is hard to beat.
Oxime: the broad all-rounder
Oxime-curing systems cure neutrally and cover considerably more substrates – many plastics, painted surfaces, most metals. They are the reason “construction silicone” for mixed substrates exists at all.
Two limitations belong on the table nonetheless:
Copper and brass remain critical. On these metals, depending on the formulation, discolouration up to visible corrosion can occur. If you have non-ferrous metal in the application, test it – always, on the original, not on the data sheet.
The regulatory frame is moving. The ketoximes in use are under more toxicological discussion than they once were, and the classifications have shifted over the years. If you are designing a formulation to last ten years, the current status of the specific crosslinker is a question that comes before the formulation – not after it.
Alkoxy: the compatible one
Alkoxy-curing systems split off alcohol. Neutral, low-odour, non-corrosive – which makes them the choice for everything sensitive: natural stone, mirrors, electronics, non-ferrous metal.
You pay for that in three places:
- Speed – skin formation takes longer, and so does cure-through.
- Catalysis – the system is more demanding to dial in and reacts more sensitively to raw material variation.
- Adhesion – what works by itself on glass with an acetoxy system needs an adhesion promoter here, depending on the substrate.
Alkoxy is rarely the cheapest answer. But often the only one that holds for good.
The sequence that saves time
In practice the question almost always tips the same way. Not: which silicone shall we take? But:
- Which substrates actually occur? Not the ones from the specification – the ones from the site.
- What does the most sensitive of them tolerate? The weakest substrate determines the system, not the most common one.
- Only then: formulation, characteristics, price.
Turn the sequence around and start with the price, and you develop twice.
And in testing?
The by-product is part of what decides whether a sealant reaches the substrate compatibility required for an application to DIN EN 15651 at all. A system that attacks the substrate loses adhesion – and without adhesion, any movement capability is theory.
That is why the substrate question belongs at the start of development. Not in the complaint.