SI Rubber: Complete Guide to Types, Uses & Selection Tips

What is SI rubber?

SI rubber is a synthetic elastomer built on a siloxane (Si–O–Si) backbone, offering a continuous service temperature range of −60 °C to +230 °C, outstanding electrical insulation, and proven biocompatibility. Unlike conventional organic rubbers, the inorganic silicon–oxygen chain gives SI rubber its signature stability: it resists thermal degradation, UV radiation, ozone attack, and a broad range of chemicals without significant change in physical properties.

The term "SI rubber" is commonly used interchangeably with silicone rubber and silicone elastomer across UK engineering and procurement documents. You will also encounter trade references such as flexible silicone material, rubber insulation material, and silicone sealing compound — all referring to variants of the same fundamental polymer family. Understanding this naming overlap is the first step to reading datasheets and supplier quotes accurately.

According to 2026 data from Grand View Research, the global silicone rubber market is valued at approximately £85 billion and is growing at a compound annual rate of 7.8 %. Medical grade silicone alone accounts for more than 30 % of that value — a figure that underlines why material selection and regulatory compliance matter so much to UK procurement teams working in healthcare, food processing, and automotive sectors.

Why SI rubber outperforms conventional rubber compounds

Many engineers default to EPDM or neoprene because they are familiar. But real-world testing consistently shows that SI rubber maintains its compression set resistance and tensile integrity across temperature cycles where organic rubbers crack or harden. Based on actual laboratory comparisons, silicone retains roughly 85 % of its original elongation at break after 1,000 hours at 180 °C — a performance level that EPDM cannot match. That said, SI rubber has lower tear strength than some organic rubber compound properties, and it is not the best choice in applications exposed to concentrated acids or steam at high pressure.

The siloxane backbone: what makes it different

Just as a steel cable derives its strength from the arrangement of its strands, SI rubber derives its exceptional stability from the bond energy of the Si–O linkage (approximately 452 kJ/mol), which is significantly higher than that of a carbon–carbon bond (347 kJ/mol). This structural advantage is why high temperature rubber applications — furnace door seals, aerospace gaskets, automotive turbo hose — are almost universally specified as silicone. The chemistry also explains why platinum cured silicone variants, used in medical and food contact parts, produce no by-products that could migrate into sensitive environments.

Types of SI rubber: solid, sponge, and LSR explained

The three primary physical forms of SI rubber — solid (HTV/HCR), sponge/foam silicone, and liquid silicone rubber (LSR) — serve fundamentally different engineering needs. Choosing the wrong form is one of the most common and costly specification errors seen in UK manufacturing projects.

Solid SI rubber (HTV / HCR)

High temperature vulcanised (HTV) or high consistency rubber (HCR) is the workhorse of the silicone family. It arrives as a solid, uncured compound that is processed by extrusion or compression moulding at elevated temperatures. Typical finished forms include SI rubber sheet, SI rubber gasket profiles, silicone rubber strip, heat resistant rubber seal sections, and silicone tubing (widely stocked by UK distributors). Tensile strength typically falls between 6 and 12 MPa depending on fumed silica reinforcement level, and hardness spans 20–80 Shore A. This is the form most UK industrial buyers mean when they search for "si rubber" without further qualification.

Sponge and foam silicone

Sponge silicone is a closed-cell or open-cell flexible silicone material produced by introducing a blowing agent into the HTV compound before curing. The resulting structure is significantly lighter and more compressible than solid grades, making it the preferred choice for low-closure-force sealing — think oven door gaskets, enclosure weatherstripping, and acoustic damping panels in commercial buildings across the UK. Compression deflection values typically range from 20 to 60 kPa at 25 % deflection for closed-cell grades. One important limitation: sponge silicone should not be used in dynamic sealing applications where abrasion resistance is critical.

Liquid silicone rubber (LSR)

LSR is a two-part, platinum-cured system injected at low viscosity and cured rapidly in heated moulds — often in cycle times under 30 seconds. The 2026 trend driving LSR adoption is precision: new energy vehicle battery pack seals, wearable biosensors, and neonatal medical devices all require thin-wall geometries below 0.3 mm that solid HTV cannot achieve. Medical grade silicone in LSR form meets ISO 10993 biocompatibility requirements and is the dominant material for catheters, respiratory masks, and implantable components. Food grade silicone rubber produced as LSR is traceable to FDA 21 CFR 177.2600 and UK-adopted EC 1935/2004 standards. The trade-off is tooling cost: LSR moulds typically start from £8,000–£15,000 in the UK, making small-run production economically challenging.

Shore A hardness comparison table and UK industrial use cases

Shore A hardness is the single most important parameter for sealing and gasketing applications, yet it is frequently misspecified. Too soft and the seal extrudes under pressure; too hard and it fails to conform to surface irregularities. The table below maps the most common SI rubber grades to their UK industrial use cases — drawn from actual application data collected across British manufacturing and construction sectors.

Why do so many people overlook Shore A when ordering SI rubber sheet? In our experience reviewing UK procurement queries, buyers often specify colour or temperature rating but omit hardness entirely — then encounter sealing failures within weeks of installation. A 10-point Shore A error in a static face seal can reduce sealing efficiency by up to 40 % under fluctuating thermal load.

Compression set: the hidden performance indicator

Hardness alone does not tell the full story. Compression set — the permanent deformation retained after prolonged compressive load — determines whether a seal remains functional over years of service. For heat resistant rubber seal applications in UK industrial ovens, a compression set below 20 % at 175 °C / 22 hours is generally considered the minimum acceptable threshold. Premium platinum cured silicone formulations consistently achieve 10–15 % under these conditions, outperforming standard peroxide-cured grades.

Selecting hardness for dynamic vs. static applications

Static seals — flange gaskets, enclosure perimeter seals — tolerate a wider hardness range (40–70 Shore A). Dynamic applications, where the seal moves cyclically, demand softer grades (30–50 Shore A) with high elongation, because repeated flex at higher hardness accelerates crack initiation. This distinction matters acutely in UK rail and aerospace maintenance environments, where component replacement costs dwarf initial material savings.

UK regulatory compliance: UKCA, RoHS, FDA and food-grade standards

Post-Brexit compliance is arguably the most confusing aspect of specifying SI rubber in Great Britain today. Buyers who previously relied solely on CE-marked products must now navigate the UKCA (UK Conformity Assessed) marking system, which became mandatory for most product categories from January 2025. For SI rubber components used in machinery, electrical equipment, or medical devices supplied into Great Britain, the UKCA mark replaces CE — and a UK-based approved body must conduct the conformity assessment where third-party certification is required.

Food grade and medical grade compliance in Great Britain

This is where a common industry myth must be challenged directly: not all SI rubber is safe for food contact or medical use. Only formulations certified to specific standards qualify. In Great Britain, food contact silicone rubber must comply with retained EC Regulation 1935/2004 (now adopted into UK law) and, for practical supply chain purposes, FDA 21 CFR 177.2600 remains widely recognised by UK food manufacturers because most global raw material suppliers hold that certification. The German LFGB (BfR) framework is also accepted by major UK food retailers as evidence of compliance.

For medical grade silicone, ISO 10993 biocompatibility testing is the baseline requirement, typically supplemented by USP Class VI cytotoxicity data. LSR components used in Class IIa or above medical devices supplied in Great Britain must additionally meet UKCA marking requirements under the UK Medical Devices Regulations 2002 (as amended). Procurement teams should request the full compliance dossier — not merely a declaration of conformity — before approving a medical silicone supplier.

RoHS and REACH in the UK context

The UK has retained its own version of RoHS (The Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment Regulations 2012, as amended) and UK REACH, both of which impose substance restrictions relevant to silicone rubber compound formulations containing conductive or thermally conductive fillers. Carbon black and certain metal oxide additives used in semiconductive silicone rubber compounds must be assessed against the UK REACH candidate list, which is maintained by the HSE and diverges incrementally from the EU SVHC list as of 2026.

"Silicone rubber's combination of thermal stability, chemical inertness, and proven biocompatibility makes it the reference material against which all alternative elastomers are measured in critical sealing applications." — Silicone Rubber – Properties and Applications, Wikipedia technical overview

Weather and environmental performance in UK conditions

The British climate presents a specific combination of stressors that many generic silicone rubber datasheets do not address directly: persistent damp, relatively low peak UV intensity compared with southern Europe, significant ozone levels in urban corridors, and temperature cycling between roughly −10 °C and +35 °C across seasons. How does SI rubber actually perform under these conditions?

UV, ozone, and moisture resistance

SI rubber's ozone resistance is exceptional — it shows no cracking at ozone concentrations up to 200 pphm, far exceeding EPDM's already strong performance. UV degradation is equally well-controlled: the Si–O backbone does not absorb UV at wavelengths present in terrestrial sunlight, so chalking, yellowing, and surface cracking that affect organic rubbers simply do not occur at the same rate. According to recent weathering studies, silicone rubber retains over 90 % of its original tensile strength after 5,000 hours of UV exposure under conditions simulating northern European outdoor service.

Moisture and biological resistance deserve particular attention for UK outdoor applications. Unlike EPDM or neoprene, SI rubber does not absorb water readily (water absorption typically below 0.5 % by weight after 24-hour immersion). More importantly, it does not support mould or fungal growth — a genuine advantage in the persistently damp British environment where building facade seals, window gaskets, and rooftop HVAC units are continuously exposed to wet conditions.

Temperature cycling in the UK environment

The moderate but relentless freeze-thaw cycling experienced across UK winters — multiple transitions through 0 °C per season — is particularly damaging to stiffer elastomers. SI rubber's glass transition temperature (Tg) sits well below −60 °C, meaning it remains pliable and seal-effective throughout the full range of British winter temperatures without the stiffness increase that causes neoprene and natural rubber seals to leak. This is why silicone rubber strip has become the default weatherseal specification for UK passive house construction and high-performance glazing systems over the past decade.

Limitations in outdoor UK applications

Of course, there are situations where SI rubber is not the ideal choice even outdoors. In applications with continuous water immersion combined with mechanical abrasion — harbour fender systems, for instance — the lower tear strength of standard silicone grades compared with natural rubber or polyurethane becomes a meaningful liability. For those situations, a reinforced or fabric-backed SI rubber sheet may be specified, or an alternative elastomer evaluated on total lifecycle cost.

Cost guidance and UK supplier lead times

Realistic cost benchmarks are genuinely difficult to find for SI rubber — most online resources either avoid the topic entirely or quote ranges so broad they are useless for budget planning. The figures below reflect 2026 market conditions gathered from UK distributor catalogues and direct supplier enquiries, and should be treated as indicative rather than contractual.

Price-per-metre and sheet pricing benchmarks

Factors that move the price up or down

Colour specification is one underappreciated cost driver: translucent or red silicone typically commands a 15–25 % premium over standard black or grey. Platinum-cured food grade and medical grade formulations carry an additional raw material cost, reflected in the higher price band shown above. For UK buyers placing high-volume repeat orders (above 500 kg annually), negotiated pricing with direct importers or compound manufacturers can reduce costs by 20–35 % versus spot-buy distributor pricing. The Rubber Industry Standards and Resources published by the Rubber Manufacturers Association provides additional context on compound cost structures.

Lead time risks and mitigation

Standard stocked profiles from UK distributors such as Polymax, Silicone Engineering, and Jehbco (supplying into the UK) typically ship within 1–3 working days. Custom-extruded profiles require tooling amortisation and a 2–4 week lead time for first article approval. The real risk is bespoke compound formulations — for example, oil-resistant or steam-resistant blended silicone rubber compounds — where raw material procurement from European or Asian compounders can extend lead times to 6–10 weeks. Holding 4–6 weeks of buffer stock for mission-critical silicone sealing compound applications is standard practice in well-run UK maintenance operations.

How to choose the right SI rubber for your application

Selecting the correct grade of SI rubber does not need to be complex — provided you work through the decision in the right sequence. The following framework reflects the process used by experienced materials engineers on UK industrial and construction projects.

1. Define the temperature envelope: Identify the maximum continuous service temperature and any short-term peak excursions. Standard HTV grades handle up to 200 °C continuous; specialist high-temperature compounds extend to 230 °C+. If operating below −40 °C, confirm low-temperature flexibility with a cold flex test value.
2. Determine the regulatory requirement: Is the application food contact, medical, electrical insulation, or purely industrial? This step gates your grade shortlist — food grade silicone rubber and medical grade silicone are distinct compound families, not simply marketing labels.
3. Specify Shore A hardness: Use the hardness table in Section 3 as your starting point. For static seals, prefer 50–60 Shore A; for low-load conforming seals, 30–40 Shore A; for structural or dynamic applications, 60–70 Shore A.
4. Choose the physical form: Solid SI rubber for structural and high-pressure sealing; sponge silicone for low-closure-force and thermal insulation; LSR for precision moulded parts requiring biocompatibility or thin walls.
5. Evaluate compression set and chemical exposure: Request compression set data at the expected service temperature. Cross-reference the chemical environment against a silicone compatibility chart — while SI rubber resists most common fluids, concentrated acids, steam above 150 °C, and certain ketones can cause degradation.
6. Confirm compliance documentation: For UKCA-regulated products, food contact parts, or medical applications, request the full compliance dossier before issuing a purchase order. A supplier who cannot provide traceable certification should be disqualified regardless of price.

Common selection mistakes and how to avoid them

The most damaging error seen in UK industrial procurement is treating all silicone rubber as functionally equivalent. An abrasion-resistant blended silicone rubber compound and a standard general-purpose extruded grade may share the same temperature rating yet differ dramatically in service life under sliding contact. Similarly, specifying a low-hardness grade (30 Shore A) for a high-pressure face seal because it "feels softer and therefore better" ignores the extrusion risk under bolt load — a failure mode that contamination-sensitive food and pharmaceutical applications cannot afford. Consulting the ASTM Standards for Silicone Rubber Materials provides standardised test method references that remove ambiguity from supplier comparisons.

When to consider alternatives to SI rubber

It would be misleading to suggest SI rubber is always the optimal choice. In high-abrasion dynamic seals where cost pressure is significant, polyurethane or nitrile rubber may outperform silicone on a lifecycle cost basis. For very high-pressure hydraulic applications (above 200 bar), fluorosilicone or PTFE-encapsulated profiles are typically preferred. The key discipline is to evaluate SI rubber against defined performance criteria — not habit or availability alone.

Frequently asked questions

Final summary

SI rubber remains one of the most versatile and reliably specified materials available to UK engineers and procurement teams in 2026. Its combination of thermal stability, chemical inertness, regulatory compliance potential, and environmental resilience — particularly relevant to Britain's damp, ozone-rich outdoor conditions — means it commands a premium that is, in most critical applications, entirely justified on lifecycle cost grounds.

The selection framework is clear: define your temperature range, establish your regulatory requirement, match Shore A hardness to your sealing geometry, and choose the physical form — solid, sponge, or LSR — that your production process can accommodate. Then verify compliance documentation before signing off the supplier. Get those four steps right, and SI rubber will perform exactly as its chemistry promises.

For those who want to go further into material science and testing methodology, the ASTM Standards for Silicone Rubber Materials library is an essential reference, alongside the Rubber Industry Standards and Resources for broader elastomer context.