Silicone Rubber (VMQ)
Vinyl Methyl Silicone | Polysiloxane | Q-Group Elastomer
Silicone rubber is the only common elastomer with an inorganic silicon-oxygen backbone rather than a carbon-carbon chain. That structural difference is the source of its two defining properties: the widest continuous service temperature range of any commodity elastomer, and the inherent biocompatibility and chemical inertness that make it the default specification for food-contact, pharmaceutical, and medical device sealing.
Wayne Rubber manufactures custom silicone gaskets, seals, o-rings, diaphragms, and molded components from Wixom, Michigan. VMQ (Vinyl Methyl Silicone) is the standard designation; the silicone rubber family also includes PVMQ for extreme low-temperature service and FVMQ (fluorosilicone) for applications requiring both silicone’s temperature range and fuel or solvent resistance.
Why the Silicon-Oxygen Backbone Changes Everything
Every other common elastomer (nitrile, EPDM, neoprene, natural rubber) is built on a carbon-carbon polymer chain. Carbon-carbon bonds have a bond energy of approximately 347 kJ/mol and begin to break down thermally at temperatures that are typical operating limits for hydrocarbon rubbers. Silicon-oxygen bonds (Si-O) have a bond energy of approximately 452 kJ/mol and are substantially more thermally stable. This single structural difference is why silicone rubber maintains useful mechanical properties at temperatures that degrade or destroy every hydrocarbon elastomer.
The Si-O backbone also rotates more freely than C-C chains, giving the polymer an exceptionally low glass transition temperature (below -100°C for some silicone grades) and explaining why silicone retains its flexibility at extreme cold without the crystallization stiffening that affects neoprene or the glass transition rigidity that affects nitrile at low temperatures.
The same Si-O backbone is chemically inert to most biological systems. Silicone does not support microbial growth, does not leach plasticizers or antioxidants, has no taste or odor, and does not react with most aqueous solutions, dilute acids, or alkalis; this chemical inertness is the foundation of silicone’s role in food, pharmaceutical, and medical applications; this is a direct property of the polymer chemistry, not a formulation achievement.
The tradeoff is mechanical. The Si-O chain provides less intermolecular interaction and cohesive strength than a C-C chain. Silicone rubber has low tensile strength, low tear resistance, and poor abrasion resistance relative to nitrile, neoprene, and natural rubber at similar durometers. It is not a mechanical rubber; it is a sealing and environmental protection rubber, specified for its thermal and biological properties rather than its resistance to physical wear.
Silicone Sub-Types: VMQ, PVMQ, and FVMQ
The silicone elastomer family includes three sub-types relevant to sealing applications, distinguished by the side groups attached to the Si-O backbone. The side group chemistry controls low-temperature performance, chemical resistance, and cost. Standard VMQ covers most applications; PVMQ and FVMQ are specified when the application pushes beyond VMQ’s limits.
The standard silicone rubber grade. Methyl and vinyl side groups on the Si-O backbone. The vinyl groups provide crosslinking sites for peroxide or addition cure. Service temperature range of -65°F to +450°F (-54°C to +232°C), with short-term excursions to 500°F. The compound for general food, pharmaceutical, automotive, and high-temperature sealing. Available in FDA, NSF, and USP Class VI compliant formulations.
Adds phenyl side groups to the VMQ polymer. Phenyl groups disrupt polymer chain crystallization at very low temperatures, extending the low-temperature flexibility limit to approximately -175°F (-115°C) for specialty grades, significantly beyond standard VMQ’s -65°F limit. Used in aerospace applications, cryogenic sealing, and outdoor equipment in extreme cold climates where standard VMQ would lose flexibility. Higher cost than standard VMQ.
Replaces some methyl groups with trifluoropropyl groups, adding fluorine to the Si-O backbone. The fluorine dramatically improves resistance to petroleum oils and fuels, the primary weakness of standard VMQ. FVMQ provides silicone’s temperature range with a level of oil resistance approaching nitrile. Used where both extreme temperature and fuel or oil contact are simultaneously present. Temperature range slightly reduced relative to VMQ. See the FVMQ material page for full details.
HCR vs. LSR: Two Forms of Silicone Rubber
Silicone rubber is processed and supplied in two physically different forms: High Consistency Rubber (HCR) and Liquid Silicone Rubber (LSR), each suited to different manufacturing processes and part geometries. The final cured properties of HCR and LSR silicone are similar; the difference is in how the uncured material is handled and what manufacturing process it enables.
- Supplied as a solid gum or pre-mixed compound, similar in handling to conventional rubber
- Processed by compression molding, transfer molding, or extrusion, using the same equipment used for other elastomers
- Typically cured with a peroxide cure system; post-cure in an oven recommended to remove cure byproducts and improve compression set
- Addition (platinum) cure systems also available for HCR, required for FDA and USP Class VI compliance where peroxide extractables are a concern
- Appropriate for most custom molded parts: o-rings, gaskets, seals, diaphragms, and grommets at moderate volumes
- Wayne Rubber processes HCR silicone for the majority of custom compression-molded components
- Supplied as a two-component liquid system (Part A and Part B) that is mixed at the molding machine and injection-molded
- Cured exclusively with platinum-catalyzed addition cure system, inherently suitable for FDA and USP Class VI applications
- No post-cure required in most cases; the addition cure produces no volatile byproducts
- Enables complex geometries, thin walls, and tight dimensional tolerances that are difficult or impossible in compression-molded HCR
- High tooling investment and setup cost; economical at high volumes (typically 10,000+ parts per run)
- The preferred form for medical device components, baby products, and high-volume food-contact parts requiring consistent dimensions
Addition (Platinum) Cure vs. Peroxide Cure
The cure system determines the purity and compliance of the finished silicone part. For most industrial applications the choice is transparent; peroxide cure is the default for cost-effective HCR processing. For food, pharmaceutical, and medical applications, the cure system is a primary specification requirement.
- Crosslinks the silicone polymer via platinum-catalyzed hydrosilylation; a clean reaction that produces no volatile byproducts
- The finished part is inherently low in extractables, meeting FDA 21 CFR 177.2600, USP Class VI, and ISO 10993 biocompatibility requirements without post-cure extraction steps
- Required for all LSR processing; also available for HCR compounds in demanding applications
- Sensitive to contamination: sulfur-containing materials, organotin compounds, and certain amines can poison the platinum catalyst and prevent cure; parts must be molded in a clean environment free of these materials
- Higher compound cost than peroxide cure; justified for any application with regulatory compliance requirements
- Crosslinks the silicone polymer using organic peroxide initiators; the peroxide decomposes during vulcanization, leaving volatile organic byproducts in the part
- Post-cure (secondary oven cure) is required to remove these byproducts and achieve published mechanical properties and low compression set values
- Without post-cure, peroxide-cured silicone parts will have a noticeable odor, higher extractables, and significantly worse compression set, a common quality deficiency in low-cost silicone parts
- Lower compound and processing cost than addition cure; adequate for industrial sealing applications without regulatory compliance requirements
- Not suitable for direct food contact or implantable medical device applications due to residual peroxide byproducts even after post-cure
Silicone Chemical Resistance Summary
| Fluid / Environment | Resistance | Notes |
|---|---|---|
| High Temperature (dry heat) | Excellent | VMQ maintains useful mechanical properties and sealing performance at continuous temperatures to 450°F (232°C), with short-term excursions to 500°F. No other commodity elastomer approaches this continuous dry-heat limit. The correct specification for oven seals, exhaust-adjacent components, food processing equipment at elevated temperatures, and high-temperature autoclave sealing. |
| Low Temperature | Excellent | Standard VMQ retains flexibility to -65°F (-54°C) without crystallization stiffening (unlike neoprene) and without glass transition rigidity at the service temperature (unlike nitrile at its low-temperature limit). PVMQ extends this to approximately -175°F (-115°C). Silicone is the standard compound for cold-climate sealing, cryogenic-adjacent applications, and outdoor equipment in arctic service. |
| Ozone and UV | Excellent | The Si-O backbone has no double bonds susceptible to ozone attack. Silicone rubber does not crack or degrade in outdoor UV and ozone environments. Long outdoor service life comparable to EPDM. |
| Steam | Moderate: grade dependent | Silicone has better steam resistance than FKM (which degrades by dehydrofluorination in steam) but is not as steam-resistant as EPDM. Intermittent steam exposure in autoclave sterilization cycles is acceptable for properly formulated platinum-cured VMQ. Continuous steam service above 250°F is not recommended for standard VMQ; EPDM is the correct long-term steam compound. Specialty peroxide-cured VMQ grades formulated for steam resistance are available for specific applications. |
| Petroleum Oils and Fuels | Poor (VMQ) | Standard VMQ swells significantly in petroleum-based oils and fuels. This is silicone’s primary chemical limitation for sealing applications. FVMQ (fluorosilicone) is required when both silicone’s temperature range and oil/fuel contact are simultaneously present. Do not specify standard VMQ for oil-wetted sealing; nitrile, FVMQ, or FKM are the appropriate alternatives. |
| Dilute Acids and Alkalis | Good to Moderate | Good resistance to dilute aqueous acids and alkalis at room temperature; resistance decreases with increasing concentration and temperature. Not suitable for concentrated acids or strong alkali service. Verify specific fluid and concentration against the Chemical Resistance Guide. |
| Aqueous Solutions, Saline, Biological Fluids | Excellent | Silicone is highly inert to aqueous solutions, saline, body fluids, and most biological media. This inertness, combined with biocompatibility, makes VMQ the standard elastomer for medical device sealing, implant-adjacent components, and pharmaceutical process contact parts. |
| Alcohols | Good | Good resistance to most alcohols including ethanol, isopropanol, and methanol at normal concentrations. Relevant to pharmaceutical and food processing applications using alcohol-based sanitizing agents. |
| Ketones and Chlorinated Solvents | Poor | Silicone swells in ketones (acetone, MEK) and chlorinated solvents. Not suitable for applications involving these fluid types. FFKM is required for ketone service combined with high-temperature requirements. |
Food, Pharmaceutical, and Medical Compliance Tiers
Silicone rubber compliance for food and medical applications is governed by a hierarchy of standards, each with progressively more demanding test requirements. Specifying the correct tier for the application is essential; over-specifying adds cost and under-specifying creates regulatory liability. The cure system must be appropriate for the compliance tier required.
| Standard | Scope | Application |
|---|---|---|
| FDA 21 CFR 177.2600 | Lists permitted rubber ingredients for repeated food-contact use. Compliance means the compound is formulated from FDA-listed ingredients; it does not certify the finished part or test for extractables. | General food processing equipment, food-contact gaskets and seals, beverage equipment. The minimum compliance tier for most food industry applications. Platinum or peroxide cure acceptable if post-cure is complete. |
| NSF 51 / NSF 61 | NSF 51 covers food equipment materials (direct contact); NSF 61 covers drinking water system components. Both require testing of actual finished parts for extractables that might contaminate food or water, which is more rigorous than FDA ingredient listing alone. | Commercial food service equipment, potable water system seals, beverage dispensing equipment, water treatment components. Required when the equipment itself carries NSF certification or when the end customer specifies NSF-certified components. |
| USP Class VI | United States Pharmacopeia biological reactivity testing on finished parts, including systemic injection, intracutaneous, and implantation tests in animals. Tests the finished material’s biological response, not just its ingredient list. | Pharmaceutical process equipment, drug-contact seals, single-use bioprocessing components, medical device sealing. Required for any component with potential drug or biological fluid contact in pharmaceutical manufacturing. |
| ISO 10993 | International standard for biological evaluation of medical devices. More comprehensive than USP Class VI; covers cytotoxicity, sensitization, hemocompatibility, and other endpoints depending on device contact type and duration. | Implantable medical devices, long-term body contact components, class II and III medical device sealing. Required for medical device regulatory submissions in the EU (MDR) and US (FDA 510(k)/PMA) where the elastomeric component has patient contact. |
Silicone Rubber Applications
Silicone Rubber Products from Wayne Rubber
Wayne Rubber manufactures custom VMQ silicone components across the following product categories. Each page covers grade options, dimensional standards, cure system options, and compliance documentation available for that product form.
Request Custom Silicone Parts from Wayne Rubber
Provide your part drawing, operating temperature range, process fluid, and compliance requirement (FDA, NSF, USP Class VI, ISO 10993). Wayne Rubber will recommend the correct VMQ grade and cure system and provide a quote.