Material Technical Guides
Silicone Rubber Technical Guide
Technical guide to silicone rubber: temperature range -60 to +200°C, FDA/USP Class VI biocompatibility, LSR vs HCR processing, and medical/food applications.
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- Category
- Material Technical Guides
- Tags
- silicone rubberVMQLSRHCRmedical gradefood gradehigh temperature
- Keywords
- silicone rubber technical guide / VMQ properties / LSR vs HCR / FDA silicone rubber / medical grade silicone / Nanjing Yuhang Rubber
Expertise Signal
- Technical review
- YuHang Rubber Technical Team
- Review Role
- Industrial Rubber Product Technical Review
- Known For
- Rubber FenderRubber TrackRubber SheetRubber HoseRubber ExtrusionCustom Rubber Parts
Industrial rubber product manufacturer covering rubber fenders, rubber tracks, rubber sheets, rubber hoses, extrusions, belts and custom molded rubber parts.
1. What Is Silicone Rubber?
Silicone rubber (VMQ per ASTM D1418, also designated MQ for methyl silicone and PVMQ for phenyl-methyl-vinyl silicone) is fundamentally different from carbon-backbone elastomers. Its polymer backbone consists of alternating silicon and oxygen atoms (Si–O–Si), with organic methyl (CH₃) groups attached to the silicon atoms.
The siloxane bond (Si–O) has a bond energy of approximately 444 kJ/mol, significantly higher than the carbon-carbon bond (~348 kJ/mol) in conventional rubbers. This is the structural basis for silicone's exceptional thermal stability across an extraordinarily wide temperature range.
Silicone rubber holds a unique position among elastomers: it is the only rubber that is simultaneously biocompatible (USP Class VI, ISO 10993), thermally stable at both extremes (-60 to +200°C), and electrically insulating — making it indispensable for medical, food, and electrical applications.
2. Types of Silicone Rubber
2.1 Classification by Cure Mechanism
| Type | Cure Chemistry | Processing Method | Key Characteristics |
|---|---|---|---|
| HCR (High Consistency Rubber) | Peroxide or addition-cure (Pt catalyst) | Compression molding, extrusion, calendering | Gum-like; requires milling; lower tooling cost |
| LSR (Liquid Silicone Rubber) | Addition-cure (Pt catalyst) | Injection molding (LIM) | Low viscosity; fully automated; high precision; faster cycles |
| RTV (Room Temperature Vulcanizing) | Condensation or addition-cure | Casting, potting, sealant application | Cures at ambient; one-part or two-part |
2.2 Classification by Composition
| Designation | Pendant Groups | Low-Temperature Limit | Key Feature |
|---|---|---|---|
| MQ | Methyl (-CH₃) | -55°C | Basic grade; limited low-temp |
| VMQ | Vinyl + Methyl | -55 to -60°C | Most common; vinyl enables efficient peroxide cure |
| PVMQ | Phenyl + Vinyl + Methyl | -70 to -100°C | Phenyl disrupts crystallization; best low-temp |
| FVMQ | Trifluoropropyl + Vinyl + Methyl | -55 to -60°C | Fluorosilicone; improved fuel/oil resistance |
3. Mechanical and Physical Properties
Silicone's mechanical properties are notably lower than most organic elastomers — this is its primary limitation for structural applications.
| Property | VMQ (General) | LSR | FVMQ | Test Method |
|---|---|---|---|---|
| Hardness Range (Shore A) | 20–80 | 20–70 | 30–80 | ASTM D2240 |
| Tensile Strength (MPa) | 4–11 | 5–12 | 5–10 | ASTM D412 |
| Elongation at Break | 100–900% | 200–800% | 100–500% | ASTM D412 |
| Tear Strength (kN/m) | 8–45 | 15–50 | 10–30 | ASTM D624 (Die B) |
| Compression Set (22h/175°C) | 15–50% | 10–30% | 20–40% | ASTM D395 |
| Specific Gravity | 1.10–1.25 | 1.10–1.20 | 1.35–1.45 | ASTM D297 |
| Rebound Resilience | 50–70% | 55–75% | 40–55% | ISO 4662 |
| Dielectric Strength (kV/mm) | 15–25 | 18–25 | 12–18 | ASTM D149 |
| Volume Resistivity (Ω·cm) | 10¹⁴–10¹⁶ | 10¹⁴–10¹⁶ | 10¹³–10¹⁴ | ASTM D257 |
Key limitation — Low mechanical strength (5–10 MPa): Silicone's tensile strength is approximately 1/3 to 1/2 that of NR, NBR, or CR. It also has poor tear propagation resistance unless specifically reinforced (high-tear grades with >40 kN/m tear strength are achievable with specialized silica reinforcement). Engineers must account for this when designing silicone components — thicker cross-sections and gentler radii are often required compared to a comparable NBR or EPDM part.
4. Thermal Properties
Silicone rubber possesses the widest continuous service temperature range of any commercially available elastomer.
| Temperature Property | Value |
|---|---|
| Continuous Service (Lower) | -55 to -60°C (standard VMQ); -100°C (PVMQ) |
| Continuous Service (Upper) | 200°C (standard VMQ); 225°C (heat-stabilized grades) |
| Intermittent Peak | 250–300°C (short duration, minutes) |
| Brittle Point (VMQ) | -73°C |
| TR10 (VMQ) | -55 to -65°C |
| Thermal Conductivity (W/m·K) | 0.2–0.3 |
The upper limit is governed by the thermal stability of the methyl side groups; the lower limit by the flexibility of the siloxane backbone, which maintains segmental motion even at extremely low temperatures due to the low rotational energy barrier around the Si–O bond.
5. Biocompatibility and Regulatory Compliance
Silicone is the gold-standard biocompatible elastomer:
| Standard | Scope | Application |
|---|---|---|
| USP Class VI | Biological reactivity (acute systemic, intracutaneous, implantation) | Pharmaceutical, medical devices |
| ISO 10993-5, -6, -10, -11 | Cytotoxicity, implantation, sensitization, systemic toxicity | Medical devices (global) |
| FDA 21 CFR 177.2600 | Rubber articles intended for repeated food contact | Food processing |
| FDA 21 CFR 177.1210 | Closures with sealing gaskets for food containers | Food packaging |
| 3-A Sanitary Standards | Dairy and food processing equipment | Food/dairy industry |
| NSF 51 and NSF 61 | Food equipment materials; drinking water system components | Food service, potable water |
| BfR Recommendation XV | Silicones for food contact (EU) | European food contact |
| EU 1935/2004 | Food contact materials (framework regulation) | EU food contact |
Post-cure (secondary cure) is mandatory for food-grade and medical-grade silicone to remove volatile low-molecular-weight siloxanes and peroxide decomposition byproducts. A typical post-cure cycle involves 4 hours at 200°C in a ventilated oven.
6. LSR vs HCR: Processing Comparison
| Parameter | HCR (Gum Silicone) | LSR (Liquid Silicone) |
|---|---|---|
| Raw Material Form | Gum-like solid | Low-viscosity liquid (2-component) |
| Mixing | Two-roll mill or internal mixer | Static mixer or metering pump |
| Molding Process | Compression or transfer molding | Liquid Injection Molding (LIM) |
| Cycle Time | 3–10 minutes | 30–90 seconds |
| Tooling Cost | Lower | Higher (cold-runner, precision molds) |
| Flash | More flash; manual deflashing | Minimal flash; fully automated |
| Part Consistency | Moderate (operator-dependent) | Excellent (automated, closed-loop) |
| Labor Content | Higher | Lower (automated) |
| Best For | Low-to-medium volumes; large thick parts; sheet/extrusions | High volumes (>50k pcs/yr); precision small parts; overmolding |
| Waste Rate | 3–8% | <2% |
LSR has largely displaced HCR for high-volume medical and automotive components due to its automation compatibility, faster cycle times, and superior consistency. HCR remains preferred for low-volume production, thick-section parts, extrusions, and calendered sheet.
7. Key Applications
7.1 Medical Devices and Pharmaceutical
Silicone's USP Class VI and ISO 10993 biocompatibility makes it the dominant elastomer for implantable and disposable medical devices: septa and stoppers for drug vials (pierceable, resealing), medical tubing for peristaltic pumps, catheters and drainage tubes, respiratory masks and ventilator bellows, syringe plunger tips, and wound care (silicone gel sheets for scar management).
For long-term implantables (>30 days), additional testing per ISO 10993-1 is required, including chronic toxicity and carcinogenicity evaluation.
7.2 Food Processing and Service
Silicone seals, gaskets, and tubing for food processing must comply with FDA 21 CFR 177.2600 or BfR XV: oven door gaskets (continuous 200–230°C), beverage dispensing tubing (peristaltic pump), coffee machine seals and gaskets, dairy processing (milking machine inflations, pipeline gaskets), and bakery pan coatings and mold liners.
7.3 Electrical Insulation
High dielectric strength and arc resistance make silicone the premier material for high-voltage electrical insulation: cable terminations and joints (up to 400 kV), surge arrester housings, insulator coatings (Room Temperature Vulcanizing silicone), wire and cable insulation for high-temperature (appliance wire, Class H), and busbar insulation in switchgear.
7.4 Automotive Under-Hood
The automotive shift to electric vehicles (EVs) has increased demand for silicone: EV battery pack seals and gaskets (thermal management), turbocharger and charge air cooler hoses (high-temperature air), coolant system seals and O-rings (extended-life coolants, >150°C), sensor potting and encapsulation, and headlamp and lighting gaskets.
8. Comparison with Other Elastomers
| Property | VMQ (Silicone) | EPDM | NBR | FKM |
|---|---|---|---|---|
| Continuous High Temp | 200°C | 150°C | 120°C | 200°C |
| Low-Temp Limit | -60°C | -50°C | -30°C | -20°C |
| Tensile Strength (MPa) | 5–10 | 7–20 | 10–25 | 8–18 |
| Oil Resistance | Poor | Poor | Excellent | Excellent |
| Weathering Resistance | Excellent | Excellent | Poor | Excellent |
| Flame Resistance | Good (with additives) | Poor | Poor | Fair |
| Gas Permeability | Very High | Low | Low | Low |
| Biocompatibility | Excellent | Good | Limited | Limited |
| Cost Index (NR=1.0) | 5–10 | 1.5–2.0 | 1.5–2.5 | 30–60 |
9. Limitations
- Low mechanical strength: 5–10 MPa tensile means silicone cannot replace structural rubber components.
- High gas permeability: 10–100× more permeable than butyl rubber; unsuitable for gas barrier applications.
- Poor oil/fuel resistance: Silicone swells severely (>100%) in hydrocarbons. FVMQ offers improvement.
- Abrasion resistance: Very poor; silicone is quickly abraded in dynamic contact applications.
- High cost: 5–10× the cost of commodity elastomers per kilogram.
- Cure inhibition: Platinum-cured silicones are sensitive to amines, sulfur compounds, and certain organics.
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Nanjing Yuhang Rubber Co., Ltd. manufactures premium silicone rubber (VMQ) sheets, gaskets, molded parts, extruded profiles, and tubing for medical, food processing, pharmaceutical, and industrial applications. Our silicone compounds are available in FDA 21 CFR 177.2600, USP Class VI, and ISO 10993 biocompatible grades. Custom formulation and post-cure processing ensures full traceability and regulatory compliance documentation.
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