Liquid silicone rubber — LSR in industry parlance — is the class of product with the highest process demands in the silicone family. Its manufacturing method is peculiar: the barrel must be cooled while the mould is heated. The compound is pumped, metered, mixed and injected at ambient temperature, cures and sets within tens of seconds of entering the mould, and is then demoulded and removed.

This process imposes an almost contradictory dual requirement on the catalyst — and it is a requirement only platinum can meet.

1. LSR's Form and Process

LSR is a two-component (A/B) system:

• Side A: vinyl-terminated polysiloxane base polymer + platinum catalyst + inhibitor
• Side B: hydrogen silicone fluid (crosslinker) + base polymer + filler

Its viscosity is low enough to be pumped; it is mixed 1:1 by precision metering equipment, homogenized through a static mixer and injected directly into the mould cavity. The whole process is closed, continuous and automated — which is why it achieves high cleanliness and high consistency.

Typical process window: material temperature in the twenty-something degrees Celsius, mould temperature 150–200 °C, cure time from tens of seconds to two minutes.

2. That Near-Contradictory Requirement

Set out the process conditions and the problem becomes clear:

In the barrel and piping at twenty-something degrees, A and B once mixed must not react for hours or even days — otherwise the lines and the nozzle plug, and the line stops.

In the mould cavity, on rising above 150 °C, it must cure completely within tens of seconds — otherwise takt time cannot be brought down, and the part deforms and stays tacky at demoulding.

In other words, the catalyst must lie almost entirely dormant at low temperature and start at full speed instantaneously at high temperature, and that switch must be precise, repeatable and adjustable.

This is not a question of "how high the activity is" but of how controllable the activity is.

3. How Platinum Achieves It

The platinum complex itself is active even at low temperature, so on its own it cannot be made to lie dormant. The real mechanism is the acetylenic alcohol inhibitor:

The inhibitor molecule (commonly ethynylcyclohexanol) forms a π complex with platinum stronger than that of the vinyl group, temporarily occupying the active centre. The binding is reversible — it holds firmly at ambient temperature, and on heating the inhibitor dissociates and desorbs, whereupon platinum immediately regains its activity.

Pot life thus becomes an adjustable parameter: more inhibitor gives longer working time and a correspondingly higher cure temperature; less gives the reverse. When an LSR supplier ships, what it is essentially delivering is a "temperature–time" curve, which the customer selects against its own moulds and takt time.

More critical still: the cure releases no small molecules at all. Hydrosilylation is the direct addition of Si–H across C=C to form a stable Si–C bond — no alcohol, no acid, no gas.

Inside a closed mould cavity this is a matter of life and death: any gaseous by-product becomes bubbles, short shots or surface defects, and there is no way for it to escape. Condensation cure releases small molecules and simply cannot be used in a closed mould.

4. Why the Other Two Routes Cannot Enter

Peroxide cure (radical crosslinking) will not work. It requires decomposition at high temperature to generate the radicals that initiate the reaction, so cold-feed injection is not feasible — heat the barrel to decomposition temperature and it crosslinks prematurely and plugs solid. Its by-products are organic acids and ketones, bringing odour, yellowing and degraded electrical performance, and the part must undergo a post-cure (baking for hours) to remove residues, at which point LSR's high-efficiency takt advantage is entirely gone.

Condensation cure will not work. The small-molecule by-products cause high shrinkage (which can exceed one percent), so dimensional accuracy cannot be held; deep-section cure depends on small molecules diffusing outward, so the interior of thick-walled parts never cures through; and the reaction rate is affected by ambient humidity, giving poor reproducibility.

Comparing them, the four properties LSR requires — cold-feed pumpability, fast high-temperature cure, no by-products, low shrinkage — are all met by the addition route, while each of the other two falls short on two or more. This is not a question of cost or habit; it is decided by chemical mechanism.

5. What These Properties Have Won

Precisely because shrinkage is as low as a few parts per thousand, there are no by-products and precision moulding is possible, LSR has captured several high-barrier markets:

• Baby products: pacifiers, teethers. Odourless, non-migrating, able to withstand repeated high-temperature sterilization.
• Medical devices: catheters, respiratory masks, seals, implant-related components. Medical-grade compliance almost exclusively recognizes addition cure.
• Optics and electronics: LED encapsulation, lenses, sensor overmoulding. Demanding high transparency, low stress and stable dielectric properties.
• Automotive: seals, connector housings, wiring-harness sealing, demanding thermal-cycling and oil resistance.
• Keypads and precision structural parts: high-speed moulding in multi-cavity tools, where dimensional consistency is a hard specification.

What these fields have in common: the cost of a single failure far exceeds the material cost. That is why they are willing to pay the premium for platinum.

6. The Price: LSR Is Famously Delicate

The platinum-catalyzed route gives LSR every one of its advantages and every one of its troubles — extreme sensitivity to impurities.

The most common field failure is failure to cure or a tacky surface, and the source of the poison is often unexpected:

• Sulfur- and phosphorus-bearing substances in gloves, greases and mould release agents
• Amine-bearing substances (certain cleaners, some filler surface treatments)
• Tin compounds — equipment previously used for condensation-cure or organotin-catalyzed work will leave residues that poison platinum directly
• Solder, and certain metal ions
• Amine hardeners from epoxy resins and unsaturated polyesters

Hence several iron rules in an LSR workshop: dedicated equipment and utensils, no sharing with condensation-cure systems, validated cleaning of moulds and piping, and incoming compatibility checks on raw materials. Inadequate cleaning when switching product lines accounts for a very high share of failure cases.

Another field problem is metering and mixing. Deviation from the 1:1 ratio or a failed static mixer produces locally over-hard and locally tacky material — the defect known as "marbled" compound. Metering pumps must be calibrated regularly and mixers replaced on the prescribed schedule; this is not maintenance that can be skipped.

Then there is seasonal drift: in winter the workshop is cold, so pot life lengthens and cure slows; in summer the reverse. The correct response is to adjust the process temperature profile, not to change the formulation or the platinum level.

7. The One-Sentence Summary

LSR must use platinum catalysis not because platinum's activity is so high, but because a platinum complex paired with a reversible inhibitor can simultaneously satisfy all four process requirements — dormant when cold, instant cure in a hot mould, no by-products, low shrinkage. No other curing mechanism can assemble that combination.

The price is platinum's cost and its sensitivity to impurities. But LSR's target markets happen to be exactly those high-end fields where "failure is not permitted," and on that arithmetic the trade works. This is also why LSR remains the segment of the silicone industry with the highest technical content and the highest added value.


Diethylenetetramethyldisiloxane platinum complex (platinum catalyst) MY 8116