Why LSR survives -50 to 230°C, why it can be made as soft as Shore A 0, and why it is trusted inside the human body — every one of these answers lives in the molecule. This article starts at the root.
The short version: LSR is an elastomer built on a siloxane backbone, crosslinked by a two-component platinum-cure system, and processed by direct injection molding. It survives -50 to 230°C, it can be formulated as soft as Shore A 0, and it is accepted for food and medical contact. Those three capabilities come from three different places: the backbone chemistry, the crosslink density, and the cure chemistry. After reading this, you will know where each one comes from — and which lines to read first when a datasheet lands on your desk.
PP, PA and PC all share a carbon–carbon skeleton. Silicone's skeleton is a siloxane (Si–O–Si) chain — a semi-inorganic polymer. This is not a detail; it is the divide that explains nearly every "anomalous" property of silicone.
The Si–O bond carries roughly 452 kJ/mol versus about 346 kJ/mol for C–C. A stronger bond needs more thermal energy to break. That is the chemical root of why addition-cure silicone works continuously at 230°C and briefly beyond 250°C, while most engineering plastics start to degrade around 150°C.
Carbon chains bond at about 109.5°. The Si–O–Si angle is roughly 143°, and the Si–O bond rotates with very little resistance. The result is one of the most flexible polymer backbones known: segments keep moving far below zero, glass transition sits near -120°C, and silicone stays elastic where ordinary rubber turns stiff and brittle around -40°C.
In PDMS the side groups are methyls: symmetric, non-polar, wrapping the backbone in a low-energy skin. Macroscopically: surface energy near 20 mN/m (water is 72), poor wettability, low friction, and no reactivity with living tissue. The "silky, non-stick, mouth-safe" intuition about silicone all comes from this methyl shield.
Keep three pairs of numbers in mind: bond energy sets the heat ceiling; bond angle and rotational freedom set softness and cold performance; side-group polarity sets surface behavior. Every process discussion that follows stands on these three points.
"Liquid" is not a marketing word. It describes a specific packaging: two components, addition cure. A and B are each stable at room temperature; after mixing there is a workable window; inside a hot mold, crosslinking completes in seconds.
Under platinum catalysis, Si–H adds across the vinyl group and forms a new Si–C bond, stitching linear chains into a 3D network. The critical fact: no by-products. Peroxide systems release volatiles and leave residues; addition cure produces nothing that needs to escape. Two direct consequences:
An inhibitor holds the platinum catalyst in check at room temperature. Above roughly 120°C it loses its grip and cure completes within tens of seconds. LSR solidification is therefore temperature-triggered — which explains two shop-floor facts:
A TDS lists a dozen parameters. Six usually decide your project. Read them by asking "what does my part fear":
| Parameter | Typical range | What it decides for your part |
|---|---|---|
| Mixed viscosity | ~300–1,000 Pa·s | Whether material can fill thin walls and deep cores; shear thinning sets the injection-speed ceiling |
| Hardness | Shore A 0–90 (typically 10–80) | Feel, sealing pressure, demolding ease; overmolding layer hardness also drives interfacial stress |
| Tear strength | ~10–45 kN/m | Ability of thin sections to survive demolding; the lifeline of tear tabs and pull features |
| Elongation at break | ~200–700% | Elastic reserve under repeated bending and pulling |
| Compression set | 22h/175°C ~10–30% | The core seal metric: after years compressed, does it still rebound? |
| Shrinkage | ~2.5–3.0% | Mold scale-up factor; also the first suspect in any dimensional dispute |
One practical warning: TDS numbers describe standard test slabs, not your part. Thin walls, thick sections and overmolded structures can all deviate from the datasheet — final judgment belongs to part-level validation, which is the subject of our Quality & Validation series.
All three share the siloxane skeleton. What differs is how they crosslink, in what form, and how they are processed:
| LSR (liquid) | HTV (high-consistency) | RTV (room-temperature) | |
|---|---|---|---|
| Form | Two-component paste, pumpable | Solid gum, must be milled into sheets | One/two-component paste or fluid |
| Cure system | Platinum addition | Peroxide or addition | Condensation (alcohol/oxime) or addition |
| Processing | Injection molding, automated | Compression molding, extrusion | Coating, potting, in-situ forming |
| Typical cycle | Tens of seconds per shot | Minutes to tens of minutes | Hours to cure |
| Best for | Precision, thin-wall, multi-cavity, overmolding, volume | Large parts, thick sections, keypads, extrusions, low-mid volume | Sealing, bonding, potting, repair |
One sentence to remember: RTV forms in place, HTV forms under a press, LSR forms by injection. If your part needs dimensional consistency, fast cycles and overmolding, you have landed in the LSR box — which is why we have stayed in exactly this box for over a decade.
Not a supplier preference — three verifiable reasons stacked on top of each other:
When we evaluate a new material requirement, the first round never mentions grades. We ask five questions — and the order itself carries information:
With five answers on the table, the candidate list usually narrows from hundreds to two or three. The rest belongs to coupon-level validation.
This is chapter one of our Materials series. Next: platinum cure in engineering terms — how to read a cure curve, how to recognize under- and over-cured parts, and how to set cure time without guesswork. If you want to discuss a specific material now, send us the service environment and geometry.
Send the service environment, geometry and validation requirements — we will answer in your context.