FVMQ Fluorosilicone – The advantages of normal silicone supplemented with media resistance
What is fluorovinylmethylsilicone rubber?
FVMQ is a further development of classic silicone. Due to the structural change of the rubber with a fluorine group, fluorosilicone exhibits significantly better oil and fuel resistance while retaining the properties of standard silicone.
History of FVMQ
The foundation for the development of silicones was laid by Frederic Kipping at the beginning of the 20th century through his intensive research into organosilicon compounds, which led to the introduction of the term „silicone“ in 1904. However, the compounds produced at that time were dismissed as unusable. It wasn't until half a century later that the importance of silicone was recognised and subsequently developed further. Today, components made from a wide variety of silicone compounds are found in diverse applications. Base silicone has been further developed and optimised in various forms, leading to the invention of the fluorinated counterpart FVMQ by Dow Corning in the early 1950s and its distribution by them as the first silicone manufacturer.
Silicone Terminology
The designations VMQ, MVQ, FVMQ and PVMQ all refer to silicone rubber and differ only in terms of the chemical side groups attached.
- VMQ / MVQ: Methyl and vinyl side groups
- FVMQ: additional fluorinated side groups
- PVMQ: additional phenyl-containing side groups
The Q in all cases stands for the identical siloxane backbone. The preceding letters indicate which organic groups are attached. By varying the attached organic groups, different properties of the final elastomer can be achieved, but the fundamental material class does not change.
Chemical composition of FVMQ
The silicone-based elastomer stands out from other rubbers as it is not a purely organic material. Silicone polymers are composed of silicon (Si), oxygen (O), hydrogen (H) and carbon (C) atoms, formed by two different monomers, so-called copolymer, consisting of 2,2,4,4,6,6,8,8-octamethyl-1,3,5,7,2,4,6,8-tetraoxatetrasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl-1,3,5,7,2,4,6,8-tetraoxatetrasiloxane.
Figure 1: Chemical structure of FVMQ: Polymerisation of octamethylcyclotetrasiloxane (left), methylvinylcyclotetrasiloxane (centre left) and 3,3,3-trifluoropropyl methylsiloxane cyclic tetramer (centre right) produces the FVMQ terpolymer (right).
In contrast to VMQ, FVMQ possesses fluoroalkyl groups on the silicon in addition to methyl and vinyl groups. The incorporated highly polar, high-energy carbon-fluorine bond leads to lower reactivity of the polymer side chain and, due to its hydrophobic character, contributes to a repulsion from polar and aromatic hydrocarbons. This minimises swelling and volume changes upon contact with fuels.
2. Properties and characteristics of FVMQ
The properties of FVMQ can be divided into chemical, mechanical, and physical. The properties of classic silicone are retained, with improvements in certain resistances, for example, to fuels and oils.
Physical Properties of FVMQ
Depending on the formulation, fluorosilicone offers one of the widest temperature ranges among technical silicones, from -65°C to +200°C. This makes FVMQ suitable for applications with extreme thermal stress or significant temperature fluctuations. Furthermore, fluorosilicone rubber exhibits high electrical insulating properties over a broad temperature and frequency range, as well as a density in the typical silicone rubber range of approximately 1.2–1.4 g/cm³. However, fluorosilicone can also be made electrically conductive or dissipative by adding carbon black or conductive particles. Additionally, its vapour and gas permeability is comparable to that of standard silicone, making it excellent for FVMQ seals.
Despite good physical properties, FVMQ exhibits limited compression set under sustained pressure and at high temperatures. As a result, other special FKM types are significantly more suitable and resilient for applications in highly stressed sealing systems, ensuring functionality without performance degradation.
Specialised FVMQ materials with flame retardancy can be specifically formulated by adding special phosphorus, nitrogen or halogen-based additives and fillers. These materials meet the UL94-V0 or FMVSS 302 fire safety standards while retaining the core strengths of FVMQ, such as temperature and media resistance, but often increase viscosity.
Chemical properties of FVMQ
FVMQ exhibits significantly improved resistance to mineral oils, air and automotive oils, fuels, and many aromatic hydrocarbons such as benzene or toluene compared to conventional silicone rubber. In addition, the high resistance to ozone and UV radiation, as well as the high weather resistance, are maintained. FVMQ shows greater stability against strong acids and alkalis than VMQ, but generally lags behind fluorocarbons such as FKM. The attached fluorine atoms protect the polymer chain from attack by other reactants such as organic solvents, thereby increasing the stability and inertness of the material.
In contrast to alternative fluoroelastomers such as FKM, FVMQ does not achieve the same stability against very aggressive acids, alkalis or strongly oxidising media, which limits the use of FVMQ in these applications.
As FVMQ is based on the same siloxane chemistry as VMQ, good physiological compatibility is fundamentally given. However, unlike standard silicone, FVMQ is less frequently used in medical or food applications. For some special grades of FVMQ, like standard silicone, corresponding FDA conformities (21 CFR 177.2600) and extensive biocompatibility data (KTW-/W270, EN 549) are available. Therefore, for physiological applications, these specifically approved grades must be selected.
Mechanical properties of FVMQ
FVMQ is mechanically similar to conventional silicone rubber: the soft, elastic behaviour of FVMQ results from its moderate to low tensile strength and high elongation at break. Its abrasion and tear resistance is comparable to that of VMQ and thus significantly lower than that of many carbon-hydrogen elastomers such as NBR or HNBR. Furthermore, FVMQ types remain elastic at low temperatures and do not show brittle deficiencies, which makes fluorosilicone particularly interesting for sealing applications at appropriate temperatures and stresses.
The hardness of FVMQ is specified on the Shore A scale, as it is a soft rubber. Fluorosilicone rubber falls within the range of 40 to 80 Shore A, with typical standard formulations lying between 50 and 70 Shore A. This places fluorosilicone in the soft to medium-hard range when compared to many standard elastomers.
Dynamically heavily stressed applications with high contact friction are challenging for components made of FVMQ due to their relatively low mechanical strength and abrasion resistance, and alternative elastomers such as NBR, HNBR, or FKM should be considered.
3. Processing of FVMQ
Polymerisation
For the synthesis of FVMQ, VMQ is first produced from the cyclic monomers 2,2,4,4,6,6,8,8-octamethyl-1,3,5,7,2,4,6,8-tetraoxatetrasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl-1,3,5,7,2,4,6,8-tetraoxatetrasiloxane in an anionic ring-opening polymerisation. In a second step, the methyl groups are substituted by trifluoropropyl groups, with the vinyl groups being retained as cross-linking sites. This fluorination is usually carried out in solution with specific proportions of fluorine species, resulting in a copolymer with a defined fluorine content. Subsequently, additives and fillers are added to the fluoro-silicone rubber to achieve the desired technical properties.
Vulcanisation processes of FVMQ
The vulcanisation of FVMQ elastomers is usually carried out radically using peroxides as radical initiators, followed by the addition of crosslinking agents such as triallyl isocyanurate (TAIC). The peroxides decompose upon heating into radicals, which break the double bond present in the polymer and form new radicals within the polymer. TAIC then attacks here and, due to its trifunctionality, forms crosslinks between the different polymer chains. The use of TAIC as a vulcanising agent shortens the curing time of the material while simultaneously improving its strength and resistance to abrasion and corrosion.
Figure 2: Linear FVMQ polymer chains with fluorinated side groups are activated under peroxide action and subsequently cross-linked via TAIC to a three-dimensional siloxane-based elastomer network.
Technical processing of FVMQ
Fluorosilicone rubber is processed both for high-temperature silicone (HTV) and liquid silicone (LSR) using classic processing methods such as compression moulding (compression, transfer), injection moulding, and extrusion. Careful process control must be ensured to avoid scorch (premature vulcanisation) while simultaneously guaranteeing the crosslinking density for good mechanical, physical, and chemical properties. Through targeted, precise process control, the FVMQ material is suitable for O-rings, seals, hoses, and moulded parts subjected to high temperatures and media loads in various industrial sectors.
4. Material Comparison – FVMQ vs. Other Elastomers
Fluorosilicone rubber is characterised by excellent resistance to weathering and ozone and broad temperature resistance, and combines this with significantly improved resistance to fuels and oils. Compared to natural rubber (NR), styrene-butadiene rubber (SBR), ethylene-propylene rubber (EPDM), butyl rubber (IIR), chloroprene rubber (CR), nitrile rubber (NBR) and hydrogenated nitrile rubber (HNBR), FVMQ offers a wider temperature application range and clear superiority in media resistance. However, for mechanically demanding applications, particularly in terms of abrasion and tear resistance, FVMQ is less robust and lags behind alternative rubbers. FVMQ offers similar fuel resistance to FKM, but lags slightly behind in chemical and high temperature limits, but offers better temperature flexibility. Furthermore, FVMQ shows typical silicone weaknesses in tear behaviour, whereby FVMQ is significantly below NR, SBR, HNBR or FKM, so that FVMQ is primarily suitable for static or only moderately dynamically stressed seals. FVMQ performs significantly better than NR, SBR, IIR, NBR and HNBR in terms of compression set at maximum continuous temperature, all of which have higher values and therefore more permanent deformation. At room temperature, FVMQ also exhibits very good compression set and low permanent deformation, outperforming possible alternatives such as IIR, HNBR or FKM.
| International abbreviation | FVMQ | VMQ | SBR | EPDM | |
| Hardness range (in Shore) | 40A-80A | 20A-90A | 20A-70D | 20A-95A | |
| Mechanical properties at room temp. | Tear resistance | 1 | 1 | 3 | 3 |
| Elongation at break | 2 | 4 | 3 | 3 | |
| Rebound Elasticity |
1 | 3 | 3 | 3 | |
| Continue tearing resistance |
1 | 1 | 3 | 3 | |
| Abrasion resistance | 3 | 3 | 1 | 1 | |
| Compression set rest |
at max. continuous operating temperature | 0 | 0 | 1 | 0 |
| at room temperature | 0 | 0 | 0 | 0 | |
| Thermal behaviour | Cooling behaviour (Tg) up to °C | -65 | -50 | -45 | -50 |
| Max. Continuous operating temperature up to °C | 200 | 220 | 90 | 130 | |
| Resistance to | Petrol | 2 | 2 | 1 | 1 |
| Mineral oil (at 100 °C) | 2 | 2 | 1 | 1 | |
| Acids (aqueous inorganic acids at RT) | 2 | 2 | 2 | 3 | |
| Alkalis (aqueous inorganic alkalis at RT) | 2 | 2 | 2 | 3 | |
| Water (at 100 °C, distilled) | 2 | 2 | 2 | 3 | |
| Weather and ozone | 3 | 3 | 1 | 3 | |
Table 1: Comparison of FVMQ Properties with Other Rubber and Silicone Materials
FVMQ is particularly convincing when a very wide temperature range, excellent weather and ozone resistance, good compression set properties, and significantly better resistance to oils and fuels compared to standard and silicone elastomers are required. In mechanically demanding applications, FVMQ falls short compared to alternative materials such as NR, SBR, EPDM, IIR, CR, NBR, and HNBR due to its poor abrasion and tear resistance.
Cost comparison between FVMQ and other elastomers
Fluorosilicone are more expensive than standard VMQ silicone and many other classic standard elastomers such as EPDM or NBR due to their two-step manufacturing process, more expensive raw materials, and significantly lower production volume – however, they are generally priced below the cost of high-quality alternatives like FKM.
| Mate rial | FVMQ | EPDM | NBR | CR | TPE/ TPU | Silicone (LSR) | Silicone (HTV) | HNBR | FKM |
|---|---|---|---|---|---|---|---|---|---|
| Cost factor | x 3,6 | x 1,0 | x 1,0 | x 1,2 | x 1,3 | x 1,4 | x 1,8 | x 2,9 | x 3,7 |
Table 2: Cost comparison of FVMQ with other rubber and silicone materials
FVMQ proves to be an economically sensible material primarily when applications involve high temperatures, harsh weather conditions, and simultaneous contact with oil or fuel. By using FVMQ, the lifespans of components can be extended due to its increased fatigue resistance, thereby reducing overall costs. In less demanding application areas, VMQ, EPDM, or NBR are often a more cost-effective and sensible alternative.
