The call sounds the same every month: “Your silicone isn’t curing.” Then the cartridge arrives at the lab, and in nine out of ten cases the material is perfectly fine. The fault lies elsewhere – in the joint, in the air, in the time allowed.
That is down to a property which sets 1K RTV silicone apart from almost every other material: it does not cure by itself. It needs water from the air.
How 1K RTV actually cures
A one-component RTV silicone is stable in the cartridge because there is no moisture in there. Only when it is extruded does it meet humidity. The water splits off the crosslinker, the polymer chains link up – and this is exactly where the thinking goes wrong:
Curing runs from the outside in. First a skin forms, then the front slowly works its way into the depth. As a rule of thumb, expect roughly 2 to 3 mm of cure depth per day under standard conditions – 23 °C and 50 % relative humidity.
Do the arithmetic. A 12 mm deep joint therefore needs four to six days to cure through. Anyone who probes it after 24 hours and finds it still soft inside has not found a material defect. They have found physics.
The five most common causes
1. The joint is too deep
By far the most frequent case. A joint that is deeper than it is wide will practically never cure through in the middle – the water simply has no route in. Hence the rule: use a backer rod. A round profile limits the depth and prevents three-sided adhesion at the same time.
The proven target geometry is a width-to-depth ratio of roughly 2:1. A 20 mm wide joint should therefore be about 10 mm deep – not 20.
2. Air that is too dry
Below roughly 30 % relative humidity, curing slows noticeably. In winter, in heated interiors or in air-conditioned production halls, that point is reached quickly. The material is not defective; it is simply waiting for a reagent nobody is supplying.
3. The closed cavity
Silicone in an enclosed volume – between two sealed components, inside a closed cavity – never receives enough moisture. It stays soft. Not for weeks: permanently. For such applications 1K RTV is fundamentally the wrong system; this is where a 2K system belongs, one that crosslinks from within itself.
4. Cold
Curing is a chemical reaction, and reactions slow down in the cold. Below 5 °C, cure-through can drop off so far that it effectively stalls. On top of that, cold air carries less water in absolute terms, even at high relative humidity.
5. Aged stock
This is the one case where the material genuinely is at fault – and the rarest. If moisture has worked its way in over time through a leaking cartridge, part of the crosslinker has already reacted. You recognise it by an extended skin-over time and a softer end state. A look at the batch and the storage conditions settles it quickly.
How to tell the causes apart
The test is simpler than most people think. From the same container, draw a thin layer of about 2 mm onto a plate and store it under standard conditions.
- The layer cures through cleanly: the material is fine. The fault lies in the geometry or in the conditions on site – causes 1 to 4.
- Even the thin layer stays soft or takes conspicuously long: now the batch is worth a look – cause 5.
This single test separates material faults from application faults within 24 hours. It costs next to nothing and saves the long discussion.
What this means for your formulation
If cure-through is critical in your application, that is a formulation task, not a complaints task. Cure speed can be adjusted within limits via the crosslinker system, the catalysis and the formulation – but not arbitrarily: as long as the system depends on atmospheric humidity, the depth limit remains.
So the honest answer is sometimes: do not change the formulation, change the system. A 2K system cures independently of ambient humidity across the entire cross-section – at the price of mixing equipment and pot life.
Precisely this trade-off – 1K or 2K, which crosslinker, which geometry – is where development time and complaint rates are decided.