Engineering Answers / Material & Selection
Two silicones with the same backbone and different chemistry. One leaves nothing behind and costs more; the other needs a bake-out and can still smell. The part decides, not the price per kilo.
Platinum (addition) cure crosslinks by hydrosilylation and produces no by-products, so it is the default for skin, food, baby and medical contact — but it is inhibited by sulfur, tin, amines and some plasticisers, and it costs more. Peroxide (free-radical) cure is cheaper and far more tolerant of a dirty mould, but it leaves acidic and aromatic decomposition products that must be driven off in a 200 °C post-cure. Skip that bake and the part smells, tastes and fogs windscreens.
Platinum cure is an addition reaction. A vinyl group on one polymer chain and a silicon-hydride group on another are joined by a platinum catalyst, and the only thing the reaction produces is the crosslink itself. Nothing has to evaporate afterwards, which is why the cured part has no taste, no odour and very low extractables. It also cures fast and in deep sections, because it is not diffusion-limited.
Peroxide cure is a free-radical reaction. Heat decomposes an organic peroxide, the radicals abstract hydrogen from the polymer and chains link through those sites. The peroxide does not disappear — it breaks down into fragments such as acetophenone, cumyl alcohol and methane, and those fragments stay in the rubber until they are driven out. That is the entire reason a post-cure exists.
The practical consequence is simple. Platinum cure is bought for what it does not contain. Peroxide cure is bought for what it costs and what it tolerates.
| What matters | Platinum (addition) cure | Peroxide (free-radical) cure |
|---|---|---|
| Reaction by-products | None | Ketones, alcohols, methane — driven off in post-cure |
| Odour and taste | Essentially none | Noticeable until fully post-cured |
| Extractables | Very low | Higher, and depends on post-cure completeness |
| Colour freedom | Any colour, including pale and translucent | Yellowing tendency; pale colours are difficult |
| Cure speed in thick sections | Fast, uniform | Slower; can be uneven through a thick wall |
| Sensitivity to contamination | High — sulfur, tin, amines inhibit the catalyst | Low; tolerates a dirtier mould and shop |
| Cost | Higher compound cost | Lower compound cost |
| Typical use | Medical, baby, food, skin contact, optical | Industrial gaskets, bumpers, sleeves |
The colour line in that table is the one buyers underestimate. A translucent or pastel part is difficult on peroxide cure because the decomposition fragments discolour the compound, and no amount of pigment reliably covers it. If the design calls for a clean, near-white or water-clear part, the cure system was decided the moment the colour was chosen — and the colour tolerance itself is harder to hold on silicone than on an opaque plastic.
Platinum cure has one weakness and it causes more scrap than any other material issue we see: the catalyst is easy to poison.
The symptom is distinctive: a tacky surface, a soft or uncured core in a thick section, or a part that cures normally on one face and stays sticky on the other. Operators often respond by adding catalyst or raising temperature, which does not work — a poisoned catalyst is poisoned regardless of dose.
We run both systems, which means the discipline is not optional: dedicated nitrile gloves, colour-coded tools, sealed compound storage, and a mould that is cleaned and verified before a platinum job goes in after a peroxide job. When a customer reports a "bad batch" of LSR, the first question is what the part was handled with, not which lot number it came from.
Peroxide cure is not obsolete, and a supplier who refuses to quote it is not saving you money — they are narrowing your options.
What has genuinely changed is the top end of the market. Automotive interior parts, which used to accept a slightly fogging peroxide compound, now mostly specify addition cure because interior fogging is a warranty issue. Medical and baby work has effectively standardised on platinum.
They will not be, and a quote that shows no difference is worth questioning. Peroxide compounds are cheaper per kilo and cheaper to process, but they add a post-cure step, an oven, and a sensory risk that has to be managed. Platinum compounds cost more and demand cleaner handling, but they remove an entire category of customer complaint.
For a part that touches a person, that trade is almost never worth taking.
Yes, if the compound is on the relevant food-contact listing and the part is properly post-cured. The regulatory question and the sensory question are separate. A peroxide-cured part can be compliant and still fail a taste test, because the volatiles that cause off-odour are not what the food-contact test measures. For anything with a mouth or nose near it, we run platinum cure unless the customer has data showing the peroxide route passes their own sensory panel.
That is cure inhibition, and it is almost always contamination rather than a bad batch. Sulfur from vulcanised rubber gloves or a latex-containing release agent, tin from a PVC plasticiser, amines from an epoxy or an amine-cured coating, and some adhesive tape residues all poison the platinum catalyst. The symptom is a sticky surface, a soft core in thick sections or a part that cures on one face and not the other. The fix is handling discipline — nitrile gloves, dedicated tools, sealed storage — not more catalyst.
Yes, in cost-driven industrial parts where odour, colour and extractables do not matter: general gaskets, bumpers, sleeves, some under-hood parts. It also tolerates compound types and mould conditions that platinum cure will not, and it is more forgiving of a shop that runs mixed materials on shared equipment. What has changed is that automotive interior and medical work, which used to accept it, now mostly specifies addition cure.