The soft rubber edge of your phone case, a baby's pacifier, an IV tube, the transparent compound over an electronic component, the sealing sleeve on an automotive wiring harness — all of these are made of silicone rubber, and all of them are formed by the same chemical process: hydrosilylation.

That reaction does not proceed without platinum. Which is why an industry of modest size consumes a substantial quantity of platinum every year.

1. Three Routes to Curing Silicone Rubber

Turning a silicone material from a liquid or plastic state into an elastomer essentially means linking linear polysiloxane molecules to one another to form a three-dimensional network. There are three ways to make those links:

Condensation cure. Silanol groups react with alkoxy groups, releasing a small molecule (alcohol, acetic acid, oxime, etc.) in the process. It cures at room temperature, needs simple equipment and is low in cost. But it has three hard weaknesses: the released small molecules cause high shrinkage (which can exceed one percent) and poor dimensional accuracy; there is odour and corrosivity (the acetoxy type is not kind to metals); and deep-section cure is difficult — the interior of thick parts never cures through.

Peroxide cure (high-temperature vulcanization). Crosslinking is initiated by free radicals and requires high temperature; the products have good mechanical properties and low cost. But the by-products are the organic acids and ketones left after decomposition, bringing odour, yellowing and degraded electrical performance; and radical reactions are poorly selective and unkind to other components in the formulation, so the finished part must undergo a post-cure.

Addition cure. The silicon–hydrogen bond (Si–H) adds directly across the vinyl group (C=C), releasing no small molecule whatsoever. That single fact determines all of its advantages.

2. Why Addition Cure Is the Standard for High-End Products

"no small-molecule by-products," unpacked, yields a chain of benefits:

Extremely low shrinkage. Shrinkage for addition-cure silicone rubber is normally in the range of a few parts per thousand — one to two orders of magnitude below condensation cure. That is why precision moulding, optical-grade encapsulation and microfluidic chips, where dimensional demands are severe, can only be done this way.

Deep-section and fast cure. With no small molecules needing to diffuse out, thick sections cure through just as well, and under heat the cure completes in minutes — suited to automated injection moulding and continuous production.

No residual by-products. No acid, no alcohol, no ketone, so the product is odourless, does not yellow, does not corrode metals or electronic components, and has stable dielectric properties.

Precisely controllable pot life. With inhibitors, the system can be made to "stand for hours at room temperature without reacting, then cure within minutes on heating." That parameter is critical to line takt time, and neither of the other two routes can deliver it.

High formulation tolerance and designable properties. Crosslink density is set precisely by the ratio of Si–H to vinyl groups, so hardness, modulus and tear strength can all be tuned to target.

There is only one price: it is expensive, and it is delicate. Platinum is a cost item, and it is extremely sensitive to impurities such as sulfur, phosphorus, nitrogen, tin and amines — once it is poisoned, the result is either no cure or an incomplete cure. This is also where troubleshooting concentrates in addition-cure silicone rubber production.

3. The Reaction Itself: One Addition Step, One Turn of the Platinum

Chemically it is very clean:

≡Si–H + CH₂=CH–R →(Pt)→ ≡Si–CH₂–CH₂–R

The Si–H on the hydrogen silicone fluid (the crosslinker) adds across the double bond of the vinyl-terminated polysiloxane (the base polymer), forming a stable Si–C bond that ties the two molecular chains together. With multiple vinyl groups on one molecule and multiple Si–H groups on the crosslinker, multi-point linking builds the network.

The catalytic cycle runs in three steps: platinum first undergoes oxidative addition to the Si–H bond, taking silicon and hydrogen onto itself; the olefin inserts into the Pt–H or Pt–Si bond; then reductive elimination releases the product, and platinum returns to its starting state for the next round. Platinum is consumed and regenerated within the process, which is why a ppm-level charge can drive an entire kettle of material to react.

This is also what sets the platinum requirement for addition-cure systems: typically a few ppm to a few tens of ppm, which works out to a very small platinum consumption per tonne of product.

4. The Standard Two-Component Formulation

Industrial addition-cure systems are almost invariably two-component (A/B):

Component A: vinyl-terminated base polymer + platinum catalyst + inhibitor
Component B: hydrogen silicone fluid (crosslinker) + base polymer

The catalyst must sit in Component A, stored separately from Component B. The reason is straightforward: if platinum and Si–H are kept together for long, the reaction proceeds slowly, the compound thickens progressively and pot life runs out of control; and the Si–H may also undergo side reactions with other components during storage.

In use the two are mixed at ratio (commonly 1:1 or 10:1); the reaction begins immediately on mixing, so adequate working time is needed, and the mix must be stirred thoroughly — poor mixing produces local cure alongside local tackiness, the defect known as "marbled" compound.

5. The Industries It Underpins

Addition-cure silicone rubber reaches further than most people realize:

Liquid silicone rubber (LSR) injection moulding: baby products (pacifiers, teethers), medical devices (catheters, masks, seals), keypads and optical components. Medical and food-contact grades almost all run on addition cure, because it gives no by-products, no odour and low migration.

Electronic encapsulation and potting compounds: power modules, sensors, LEDs, automotive electronic control units. These demand low stress, high dielectric strength and thermal-cycling resistance — addition cure is the only route that satisfies them.

Silicone gels: electronic damping, medical dressings, prosthetic filler. These demand extreme softness, transparency and self-healing, with very low crosslink density, which places very high demands on the uniformity of platinum dispersion.

High-temperature vulcanization silicone rubber (HCR): seals, insulators, appliance components. Peroxide cure still dominates this field on cost grounds, but applications with requirements on odour, electrical performance or yellowing have already moved to addition cure.

Release agents, coatings, textile auxiliaries: waterborne systems use water-soluble platinum; solvent-based systems use oil-soluble platinum.

6. Why the Industry Cannot Do Without It

In one sentence: the move upmarket in silicones is essentially the spread of the addition-cure route.

Condensation cure and peroxide cure carry the commodity and low-end silicone market, but anything involving precision dimensions, medical compliance, electronic reliability or optical transparency can only run on addition cure. And the only practical catalyst for addition cure is platinum (rhodium and ruthenium systems are used in a few cases, but platinum is overwhelmingly dominant).

This creates a structural lock-in: the direction of technological upgrading in the silicone industry directly determines the long-term demand for platinum. Conversely, platinum price volatility transmits straight through to the cost structure of the entire silicone industry — which is why the industry has kept pushing platinum-reduction technology and kept improving platinum recovery systems.

For the user, the practical meaning of understanding this lock-in is: in an addition-cure system, platinum is not an auxiliary that can be economized away, but the key component that decides whether the product can be made at all. The risk of saving cost on it far outweighs the amount saved.

7. The Three Things That Go Wrong Most Often on Site

Poisoning causing failure to cure. Sulfur, phosphorus, nitrogen, amines, tin, lead — the source may be gloves, grease, mould release agent, peroxide curing agent left over from the previous kettle, or even solder. The symptom is a tacky surface or failure to cure throughout. The diagnostic direction is to run small samples and eliminate suspected contact sources one by one — not to double the platinum dose.

Uneven mixing. Deviation in the two-component ratio or insufficient agitation produces locally over-hard and locally tacky material. Metering pumps must be calibrated regularly, and static mixers replaced on schedule.

Inhibitor and temperature mismatch. In winter the workshop is cold, so pot life lengthens and cure slows; in summer the reverse. The line should fine-tune the temperature profile seasonally rather than change the formulation.


Diethylenetetramethyldisiloxane platinum complex (platinum catalyst) MY 8113