1:1 Addition-Cure vs. 10:1 Condensation-Cure Silicone: Mix Ratio, Performance and Selection

Buyers comparing RTV-2 silicone quotations often ask why one supplier quotes a 1:1 system and another a 10:1 system at a similar price. The ratio is not an arbitrary house style. It tells you which cure chemistry is inside the drum, and that chemistry determines curing speed in thick sections, tolerance to substrate contamination, what by-products are released, how precisely the operator must measure, and whether the material can be used in a sealed electronic enclosure at all.

Understanding the two families is the fastest way to avoid a costly specification mistake.

RTV-2 (two-component room-temperature-vulcanising) silicone is supplied as part A (base polymer with filler and, in some systems, catalyst) and part B (crosslinker and catalyst package). The ratio refers to the mixing weight or volume ratio of A to B.

• 1:1 addition cure (platinum): a hydrosilylation reaction between vinyl groups on the polymer and Si–H crosslinker, catalysed by a platinum complex. No small molecules are released — the cure is addition, not condensation. Because both parts are similar in viscosity and specific gravity, 1:1 works by weight or by volume, which is ideal for cartridge and meter-mix dispensing.
• 10:1 condensation cure (tin or titanate): a moisture-triggered condensation reaction where the crosslinker reacts with hydroxyl-terminated polymer, releasing alcohol (alkoxy system) or acetic acid (acetoxy system) as a by-product. Part B is a small, catalyst-rich package, hence the high ratio.

Both are genuine RTV-2 silicones and both are used in electronics, but they behave very differently in production.

• Electronic potting and encapsulation. No by-product means no corrosive vapour attacking copper traces, no bubbles forming inside a sealed housing, and no stress from shrinkage. This is why nearly all serious PCBA potting and gel grades are platinum systems.
• Thick sections. A 30 mm deep potted module cures evenly in hours rather than over a week.
• Optical and sensor modules. No fogging of lenses, windows or LED surfaces.
• Fast, repeatable production. Heat cure at 80–120 °C shortens cycle time to minutes and produces consistent hardness.
• Low compression set and stable modulus over thousands of thermal cycles — the reason automotive and EV designs specify 1:1 platinum grades.

• Contamination. Sulphur-cured gloves, tin-catalysed RTV used nearby, amine-cured epoxy fixtures, and some no-clean flux residues will inhibit cure. Isolate the line and specify sulphur-free PPE.
• Mix ratio accuracy. A 10 % lean part B mix can leave the surface permanently tacky.
• Pot life is shorter once mixed; plan batch sizes accordingly.

• Bonding and sealing rather than deep potting. Gasketing, seam sealing, sensor bonding, and thin-section encapsulation where the material is exposed to air.
• Difficult substrates. Unprimed adhesion to glass, ceramics, anodised aluminium and many engineering plastics is generally better.
• Cost-sensitive, large-area, thin applications such as form-in-place gaskets and weather sealing.
• Field repair and manual application where 10:1 cartridges with a hand gun are convenient and low waste.

• By-product management. Never encapsulate a sealed enclosure with an acetic system; the acid vapour attacks copper and can pressurise the housing. Use alkoxy (alcohol) systems for electronics, and vent during cure.
• Section depth. Do not design a 20 mm deep pour with a condensation product and expect a 24-hour cure.
• Humidity dependence. Cure rate varies with plant climate; a dry winter line and a humid summer line will not behave the same.

1. Never share mixing equipment. Traces of tin-catalysed condensation product in a platinum system will inhibit cure. Use dedicated static mixers, pots and tools.
2. Test on the real substrate. Cure a small sample in the actual housing, with the actual components and flux residue, before committing a production batch.
3. Recalculate cost per litre of cured compound, not per kilogram. A 1:1 system at a higher price per kg may still be cheaper per finished part if it cures in an hour instead of two days, or if it eliminates primer and rework.
4. Check the ratio by weight versus volume. 1:1 grades are usually formulated to be 1:1 by both weight and volume, but confirm on the datasheet — many 10:1 grades are 10:1 by weight only and will be off-ratio on volumetric dispensing.
5. Verify shelf life and storage. Platinum part B can drift if stored warm; condensation part B is moisture-sensitive and must stay sealed.

Is 1:1 always better than 10:1?
For potting electronics in sealed enclosures, almost always yes. For gasketing, bonding and mould making, 10:1 is often the more economical and more practical choice.

Can I use a 10:1 product for PCBA potting if I vent the housing?
Only for thin sections and with an alkoxy (alcohol) system. Deep sections will not cure fully and acid-cure systems should never be used on copper.

Why did my 1:1 silicone stay sticky?
Cure inhibition, an off-ratio mix, or expired part B. Check gloves and nearby tin-catalysed products first — inhibition is far more common than a bad batch.

Does mix ratio affect hardness?
Yes, within limits. Excess part B in a condensation system can plasticise the cured rubber; in an addition system, off-ratio typically causes tackiness or a soft core rather than a harder product.

The ratio printed on the label is the clearest single indicator of what a silicone will do in your process. Choose 1:1 addition cure when you need deep, by-product-free, reproducible encapsulation with long-term thermal stability; choose 10:1 condensation cure when you need economical bonding, sealing and thin-section protection on difficult substrates. Then control contamination and mix accuracy accordingly.

We supply both chemistries — 1:1 platinum-catalysed potting compounds and gels for electronics, and 10:1 alkoxy systems for sealing and bonding — and can run a substrate cure test with your housing material before you commit to a specification.