I. The Word "Vulcanization" Is Actually a Historical Misnomer

In the silicone industry, you'll hear one word constantly: vulcanization. Whether the vulcanization is complete, whether post-curing is needed, how much vulcanizing agent to add... It all sounds like dealings with sulfur.

The word was indeed borrowed from the rubber industry. More than a century ago, people found that natural rubber was soft and sticky, melting in the summer sun and turning brittle in the winter cold—practically useless. Then an American named Goodyear heated rubber together with sulfur, and the rubber suddenly became strong, elastic, and no longer tacky—this process was called "vulcanization." Sulfur atoms, like little hooks, strung rubber's long chains together, turning loosely arranged molecules into a network. From then on, "vulcanization" became synonymous with "rubber going from soft to hard, from useless to useful."

Later, silicone rubber came along. There's no sulfur at all in silicone rubber—its crosslinking relies on an entirely different chemical mechanism—but the word "vulcanization" had become so ingrained in the industry that it simply stuck. So when an engineer today says "how well has this silicone vulcanized," they simply mean "how far along is its crosslinking cure"—it has nothing to do with sulfur anymore.

Only with this background in hand can we understand one thing: silicone "vulcanization" has several technological routes, and which route a material takes determines the applications it can serve.

II. Three Routes: How Silicone Goes from Liquid to Solid

If you imagine silicone molecules as long strands of noodles, then the essence of "curing" is creating connection points between these strands, weaving a pile of loose noodles into a sturdy net.

Different technological routes are simply different "ways of tying the knots."

The first route relies on moisture in the air. This is the familiar glass sealant and caulking. It comes out of the tube as a paste; after contacting the air, moisture slowly seeps in, triggering a reaction that hardens layer by layer from the surface inward. This type of silicone is called condensation-cure. Its advantage is convenience—ready to use straight from the tube, no mixing ratios, no heating required. Its drawbacks are equally obvious: the reaction releases small molecules (alcohols, acids, ketoximes, for example), producing odor; deep-section curing is slow, and thick applications easily end up "dry outside, wet inside." That tube of glass sealant at home that stays soft inside for ages when applied too thickly? Exactly this principle.

The second route relies on organic peroxides. This was the mainstream method for silicone rubber in the early days: mix the silicone with peroxide and heat it past one hundred degrees Celsius. The peroxide decomposes into free radicals, which act like little scissors, cutting active sites into the molecular chains so they can link to one another. This route is crude and cheap, and it's still widely used in heat-vulcanized (HTV) silicone rubber today. But its shortcomings: the reaction isn't clean, leaving decomposition residues behind; the products have odor and can't achieve high transparency; and thick parts can't be molded without pressure.

The third route is the protagonist of this series—addition-cure. It relies neither on moisture nor on radicals cutting wildly about, but on two types of "hooked" molecules pairing with precision, matched and clicked together one by one by a platinum catalyst. The whole process resembles precision mortise-and-tenon joinery: where to connect, and how much, is all engineered in advance.

Here's an analogy: the first two routes are like "gluing a pile of wooden sticks together haphazardly," while the third is "building a mortise-and-tenon structure according to blueprints." The latter is naturally more expensive and more demanding, but the result is more precise, cleaner, and more reliable.

III. Why This Route Simply Cannot Bypass Platinum

With the story told this far, the question becomes concrete: if mortise-and-tenon construction is so good, why must platinum do the "building"?

It comes down to the nature of the joinery. The two types of "hooks" in addition-cure silicone are carbon-carbon double bonds (called vinyl groups in the industry) and silicon-hydrogen bonds. Both groups are extreme homebodies—chemically stable, they can sit next to each other for a year or more without ever acknowledging each other's presence. This is a good thing: precisely because of it, properly mixed silicone can sit in a drum without solidifying, giving workers time to calmly cast molds, dispense, and coat.

But a blessing is also a curse. To make them "click" together exactly when needed, an outside force must intervene. And this pairing is exceptionally picky about who intervenes:

Push them with strong acids or alkalis? No—the silicone molecules themselves get destroyed first. Catalyze with ordinary metal salts? No—efficiency is too low, requiring long high-temperature baking, and the catalysis is incomplete. Initiate with free radicals? No—radicals are indiscriminate, cutting molecular chains and destroying the hooks, making an orderly network impossible.

Chemists thus discovered that the platinum-group metals—platinum in particular—are among the very few players capable of accomplishing this "gently and precisely." It gently pulls the silicon-hydrogen bond over, places it beside the vinyl group, lets the two click together, then withdraws—leaving no trace, producing no byproducts, damaging no other structures. Very few elements on the periodic table possess this ability.

A blunter analogy: you need to matchmake an extremely shy, extremely picky couple, and you need a matchmaker with patience, tact, and the grace to exit once the introduction is made. Platinum is that matchmaker.

IV. Platinum Brings More Than "The Ability to Cure"

If platinum's role were merely "making silicone cure," its value wouldn't be so irreplaceable. What truly elevates addition-cure systems to the high end is a cascade of "bonus benefits":

First, cleanliness. The reaction is an addition reaction: the two hooks click together with not one atom more or less, and no small molecules are released. This is why addition-cure silicone achieves low volatility, low odor, and non-corrosiveness—it can go into a baby's mouth, rest against blood vessels in the human body, sit beside a sealed chip—all on the strength of this cleanliness.

Second, controllability. Platinum's catalytic speed is exquisitely sensitive to temperature. At room temperature, with the inhibitor's help, it can "hold its position" for hours; once heated, the reaction launches rapidly and the silicone sets within minutes. This switch-like behavior is the very foundation of modern continuous injection molding and rapid mass production. You can't run a line producing a part every few tens of seconds with a tube of glass sealant—but you can with a platinum system.

Third, design freedom. Because crosslinking is engineered precisely through formulation, engineers can adjust the number, ratio, and distribution of the hooks to create textures ranging from jelly-soft to rubber-tough, while achieving high transparency, high strength, and high tear resistance. This "designability" is difficult for peroxide systems to match.

Fourth, deep sections cure fully too. Addition-cure doesn't depend on air and moisture penetrating inward; as long as heat reaches it, thick parts cure thoroughly inside, avoiding the "cooked outside, raw inside" sandwich problem—critical for thick-walled seals and large potting components.

V. The Price: No Free Lunch

Of course, there's no such thing as a free lunch. Choosing a platinum system means accepting its temperament:

Platinum is expensive—the most obvious point. Platinum is "delicate"—it is poisoned and deactivated upon contact with certain substances containing sulfur, nitrogen, phosphorus, or tin, the biggest headache on the shop floor. Platinum systems are sensitive to mixing ratios—even slight deviations in the A/B component ratio or mixing uniformity produce wildly different curing results. And storage conditions matter too: it fears high temperatures, contamination, and long shelf times.

Hence the industry's half-joking summary: with a platinum system, you trade "money and fastidiousness for cleanliness and control." Whether the price is worth paying depends on the product. For an ordinary sealing strip worth a few cents, a peroxide system may well be the better deal. But for a catheter headed into an operating room, or a chip that must stay sealed for ten years, there is no second choice.

VI. Summary

Returning to the question in the title: why can't addition-cure silicone do without platinum?

Because the core of the addition-cure route is a chemical reaction of "precision matchmaking"—and platinum is one of the few players who can preside over that matchmaking and emerge unscathed. It lets silicone wait quietly at room temperature, form rapidly under heat, and cure completely clean. Without platinum, addition-cure silicone could still exist—but it would no longer be the material that shines today across medical, food, electronics, and automotive applications.


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