Vinyl Silicone Oil RH-Vi306B

Vinyl Silicone Oil RH-Vi306B is a linear polydimethylsiloxane fluid with vinyl functional groups positioned at the terminal silicon atoms. The product is supplied as a clear, colourless liquid and functions as a reactive diluent, chain extender, or base fluid in platinum-catalysed addition-cure silicone systems. Because the reactive sites are restricted to chain termini, the cured network tends toward a more uniform molecular weight between crosslinks than that obtained from fluids containing both terminal and pendant vinyl groups. Published lot-specific data for the RH-Vi306B commercial designation should be obtained from the manufacturer’s certificate of analysis; comparable vinyl-terminated polydimethylsiloxane fluids in the 100–1,000 mm²/s kinematic viscosity range have been described in supplier technical literature and in polymer science studies of hydrosilylation cure.

Representative specification ranges include kinematic viscosity at 25 °C determined by ASTM D445 or ISO 3104, vinyl content by quantitative 1H NMR or iodometric titration, volatile matter after 2 h at 150 °C according to ISO 3251, refractive index at 25 °C according to ASTM D1218, and density at 25 °C according to ASTM D4052. Users should verify the certificate of analysis for each lot because minor variations in vinyl content alter the required Si-H crosslinker ratio.

When Terminal Vinyl Placement Replaces Mixed-Vinyl Architectures

Replacing a mixed-vinyl architecture with terminal vinyl placement alters hydrosilylation cure stoichiometry and network topology. In a terminal vinyl fluid, each linear chain contains approximately two reactive vinyl termini, assuming no chain-transfer side reactions during polymerisation. This limits the maximum number of Si-H addition points per chain to two, which produces a more uniform molecular weight between crosslinks. Pendant vinyl fluids, by contrast, integrate vinyl groups along the dimethylsiloxane backbone at concentrations between 0.10 mmol/g and 0.80 mmol/g, depending on the copolymer composition. The resulting network contains both chain-end and mid-chain crosslinks, increasing crosslink density but also increasing the probability of sterically restricted hydrosilylation sites. In formulations where low compression set is required, terminal vinyl fluids are preferred because the network can relax more uniformly under cyclic deformation. The difference is measurable by dynamic mechanical analysis, with terminal vinyl networks exhibiting a lower plateau modulus relative to mixed-vinyl networks at equal total vinyl concentration.

In high-speed liquid silicone rubber compounding, RH-Vi306B is added to the base polymer before filler incorporation. On a co-rotating twin-screw extruder with L/D ratio of 44:1 and barrel temperatures maintained between 20 °C and 40 °C, the fluid wets precipitated or fumed silica within 15–45 s of residence time. A side-fed silica stream is preferred over a main-port addition because it reduces the peak shear rate observed at the screw elements. The resulting masterbatch shows a viscosity increase from the neat fluid value to several hundred thousand mPa·s under low shear, but the material remains pumpable at the injection unit of a liquid silicone rubber machine when the clamp force is at least 80 metric tons and the nozzle diameter is 8–12 mm. Injection pressure typically ranges from 30 MPa to 90 MPa depending on mould cavity geometry.

What Limits Shelf Life in Humid Storage Conditions

Shelf life of vinyl silicone oil is primarily limited by moisture ingress and subsequent hydrolysis of siloxane bonds. The product is supplied in sealed steel or lined fibre drums and should be stored at 5–35 °C in a dry area with relative humidity below 60%. When drums are opened repeatedly, atmospheric water can condense on the liquid surface and initiate slow chain re-equilibration, producing free silanol species. These silanols are detectable by FTIR as a broadening of the Si-O-Si band around 1000–1100 cm⁻¹ and by an increase in titratable acidity. In addition-cure formulations, free silanols can compete with vinyl groups for hydride-functional crosslinker, leading to hydrogen evolution and reduced final crosslink density. Pre-drying under vacuum at 80–100 °C for 1–2 h is therefore recommended when the oil has been exposed to ambient air for more than 8 h or when the storage area humidity exceeds 55%.

Representative specification ranges for RH-Vi306B and a typical pendant vinyl silicone fluid are listed below.

Unlike methyl silicone oil, which is non-reactive and often used as a heat-transfer fluid or lubricant, RH-Vi306B contains reactive vinyl termini. When both fluids are present in a formulation, the methyl silicone oil acts as a plasticiser and dilutes the crosslink density, shifting the glass transition temperature to lower values but reducing tensile strength. Hydrogen silicone oil, containing Si-H groups, can undergo dehydrogenative coupling with hydroxyl impurities in the presence of platinum or amine catalysts. In a formulation where RH-Vi306B is blended with a hydrogen silicone crosslinker, the intended hydrosilylation addition consumes one Si-H group per vinyl group. Excess Si-H groups can undergo hydrolysis or condensation side reactions if exposed to ambient moisture, generating hydrogen gas and causing bubbles in the cured part. Consequently, the molar ratio of Si-H to vinyl is usually held between 1.2:1 and 2.0:1 to ensure complete cure without excessive gas evolution.

Viscoelastic Behaviour and Shear Thinning Across High-Pressure Mixing Heads

Neat vinyl silicone oils exhibit near-Newtonian behaviour at shear rates up to 100 s⁻¹, with a shear-thinning onset typically observed above 1,000 s⁻¹ in narrow-gap rotational rheometry. When the fluid is blended with fumed silica at loading levels between 15 wt% and 25 wt%, the resulting compound develops a yield stress that depends on silica surface treatment and dispersion quality. In a high-pressure mixing head operating at 250–500 bar, the apparent viscosity at the injection nozzle can drop by an order of magnitude relative to low-shear viscosity. This shear-thinning behaviour permits filling of thin-walled mould sections but can also cause die swell and surface defects if the injection velocity profile is not matched to the curing rate. The cure profile of a standard addition-cure system containing RH-Vi306B and a methylhydrogen crosslinker with Si-H content of 0.30–0.60 wt% is typically measured by moving die rheometry according to ASTM D5289; scorch time at 120 °C is in the range of 30–90 s, and the time to 90% cure ranges from 2 min to 5 min. These times depend heavily on the platinum catalyst loading, which is usually expressed as parts per million of platinum metal relative to total silicone mass.

During vacuum degassing of the formulated compound, the vinyl fluid contributes to bubble coalescence and removal because of its low surface tension relative to high-viscosity gum bases. Batch degassing in a planetary mixer at −0.08 MPa to −0.095 MPa for 10–20 min reduces entrapped air content to below 0.5 vol%. If the compound is subsequently transferred through a filter mesh with absolute rating 50 µm, pressure drop across the mesh remains below 0.1 MPa for a batch temperature of 25 °C. Premature crosslinking in the transfer line is avoided by maintaining the cooling water inlet temperature below 15 °C and by limiting residence time above 40 °C to less than 2 min.

Inhibitor partitioning between the vinyl fluid and the platinum catalyst phase controls the induction period. Addition-cure silicone formulations often contain 2-methyl-3-butyn-2-ol or tetramethyltetravinylcyclotetrasiloxane as the inhibitor. In terminal vinyl fluids, the inhibitor partitions primarily into the continuous siloxane phase, whereas in pendant vinyl fluids the inhibitor may associate with higher-density crosslink regions after premature partial cure. This partitioning difference means that terminal vinyl fluids require lower inhibitor loadings to achieve the same pot life at room temperature. A typical pot life extension at 25 °C from 1 h to 8 h is achieved with 0.01–0.05 wt% inhibitor in a terminal vinyl system, compared with 0.05–0.15 wt% in a mixed-vinyl system. Batch release records should include lot number, date of manufacture, viscosity, vinyl content, volatile matter, and appearance. If a batch is to be used in optical or electronic applications, additional filtration through 1 µm absolute membranes is recommended. The resulting level of particulates above 5 µm should be below 100 particles/mL when tested by light obscuration according to USP <788>.

Material safety data for vinyl silicone oil grades list the product as non-flammable under ambient conditions, with a flash point above 150 °C when measured by ASTM D92. Process operators should use nitrile gloves and safety glasses when handling drums, because silicone oils are slippery and can create fall hazards. Spills should be contained with inert absorbent and disposed of according to local environmental rules. For medical or food-contact applications, end users must verify that the cured elastomer meets migration and extraction requirements under FDA 21 CFR §177.2600 or ISO 10993-1 as applicable. The vinyl silicone oil alone is not a finished device component; extractables and leachables data are formulation-dependent.