Zhejiang Runhe Organic Silicon New Materials Co., Ltd. Vinyl Silicone Oil for Liquid Silicone Rubber Formulation

Zhejiang Runhe Organic Silicon New Materials Co., Ltd. vinyl silicone oil for liquid silicone rubber formulation is a linear polydimethylsiloxane bearing vinyl dimethylsiloxy terminal units and, depending on grade, distributed vinyl methylsiloxane repeating units. In two-part platinum-catalyzed addition-cure LSR, the fluid serves simultaneously as a network-forming prepolymer and as a viscosity control agent because its molecular weight and vinyl molar content determine both crosslink density and the mixed-component rheology before thermal activation. The hydrosilylation reaction between vinyl groups and Si-H-functional polymethylhydrogensiloxane crosslinkers consumes vinyl functionality in the presence of Karstedt-type platinum catalysts at concentrations between 1 ppm and 20 ppm Pt relative to total compound mass, with typical industrial loadings falling between 5 ppm and 10 ppm. Technical literature from organosilicon suppliers identifies vinyl silicone oil grades with kinematic viscosity at 25°C ranging from 200 mPa·s to 100,000 mPa·s, vinyl contents from 0.05 mmol/g to 1.20 mmol/g, and volatile fractions below 0.50% after 3 h at 150°C. For LSR compounds designed for injection molding, the most frequently selected grades have viscosities between 500 mPa·s and 20,000 mPa·s and vinyl contents between 0.10 mmol/g and 0.50 mmol/g, because higher molecular weight fluids increase the plateau viscosity of the uncured mix while lower vinyl contents yield insufficient network density after curing. The fluid’s low silanol content, commonly specified below 200 ppm OH, prevents condensation side reactions that would otherwise compete with platinum-catalyzed addition and generate water or hydrogen by-products capable of creating voids in thick sections. In addition-cure LSR, the vinyl silicone oil is typically combined with a fumed silica filler pre-treated with hexamethyldisilazane or vinyl alkoxysilane, a platinum catalyst masterbatch, and a separate Side A containing the Si-H crosslinker; mixing occurs at temperatures maintained below 35°C to preserve latent inhibition and minimize premature gelation. The viscosity response of the unfilled fluid at 25°C follows a shear-thinning index that is less pronounced than that of filled LSR but remains non-Newtonian above 100 s⁻¹ due to polymer chain entanglement when the average chain length exceeds approximately 500 dimethylsiloxane units. Because the terminal vinyl content defines the maximum network chain extension and the pendant vinyl content controls the local crosslink density near filler surfaces, formulators adjust the ratio of terminal to pendant vinyl groups by blending vinyl fluids with different molecular architectures rather than relying on a single grade.

If Terminal Vinyl Concentration Falls Below 0.08 mmol/g, Tear Propagation Resistance Declines Rapidly

At low terminal vinyl concentration below 0.08 mmol/g, the addition-cure network lacks sufficient elastically active chains to distribute stress away from crack tips, causing tear strength measured according to ASTM D624 Die B to fall by as much as 40% relative to formulations containing 0.20 mmol/g vinyl, according to comparative data published in silicone rubber compounding literature. The same effect appears in tensile testing under ASTM D412, where tensile strength may drop from 8–11 MPa at 0.20–0.30 mmol/g vinyl to 4–6 MPa at 0.05–0.10 mmol/g vinyl when filler loading and cure conditions remain constant. Elongation at break often rises as crosslink density falls, but the increase is not a processing advantage because the network becomes mechanically weak and exhibits higher creep under static load. Compression set tested according to ISO 815-1:2014 method A at 22 h/175°C also degrades, frequently exceeding 35% for vinyl contents below 0.10 mmol/g, whereas formulations between 0.25 mmol/g and 0.40 mmol/g commonly produce compression set values of 10–18%. The critical threshold is not a single point but a function of the reinforcing silica structure: fumed silica with high surface area above 180 m²/g can partially compensate for low vinyl content through filler networking, but cannot restore the elastic recovery lost from insufficient covalent crosslinks. Conversely, vinyl contents above 0.50 mmol/g reduce elongation rapidly and may create hard, glassy networks with Shore A hardness above 60 and tear strength that plateaus or declines because the network cannot deform sufficiently to blunt crack tips. Table 1 presents a representative property progression assembled from published LSR formulation studies; absolute values shift with filler type, filler loading, and mixing history.

In production-scale LSR injection molding, the choice of vinyl silicone oil viscosity creates a direct conflict between dosing accuracy and mold filling pressure. A vinyl fluid at 20,000 mPa·s improves cured elastomer toughness by maintaining longer network chains, but elevates the apparent viscosity of the mixed A/B stream to 400–800 Pa·s at 10 s⁻¹ and 25°C; this requires static mixers with 24 to 32 elements and metering pumps with hardened rotors to maintain a dosing ratio tolerance of ±1%. Experience on LSR machines with 20:1 L/D water-cooled barrels and electric screw drives shows that mixed compound viscosity above 1,000 Pa·s at 10 s⁻¹ leads to short shots in thin-wall multi-cavity tools and increases injection pressure beyond 120 bar. Conversely, vinyl fluids with viscosity below 500 mPa·s reduce crosslinker demand and promote wetting of fumed silica aggregates during planetary mixing, but may increase the volatile weight loss during post-cure and create mold fouling when volatile cyclosiloxanes condense on vent surfaces. The processing window for a 1:1 LSR system using vinyl silicone oil as the main polymer is often narrower than ±5°C for barrel temperature control because the platinum catalyst becomes increasingly active above 35°C; accumulated residence time in static mixers exceeding 30 min at 30°C can produce localized gel particles that later appear as surface defects in cured parts. Vacuum degassing at -0.095 MPa after A/B mixing reduces dissolved air and low-molecular-weight siloxane vapor pressure, but degassing must be limited to 2–4 min because prolonged vacuum can strip inhibitor and shorten pot life. Equipment setup therefore commonly includes a water-cooled metering block, a static mixer mounted immediately before the shut-off nozzle, and a closed material supply system with 25 kg or 200 kg drum feeders under 0.5–1.0 bar dry nitrogen. These conditions are derived from production-scale LSR molding practice; published data for this specific vinyl silicone oil grade across all machine sizes is limited, and validation on the target injection unit remains necessary.

What Limits Cure Rate in Thin-Wall Molding at 110°C?

At mold temperatures between 110°C and 120°C, the cure rate of addition-cure LSR containing vinyl silicone oil is controlled by the catalytic cycle of platinum hydrosilylation, not by diffusion of the Si-H crosslinker. Karstedt-type platinum catalysts producing Pt(0) colloidal species require a minimum thermal activation threshold near 70°C; below this, the inhibitor, typically 1-ethynylcyclohexanol or tetravinyltetramethylcyclotetrasiloxane, binds reversibly to the platinum center and prevents the Chalk-Harrod cycle from initiating. At 110°C, the inhibitor desorbs within 5–15 s, after which the reaction consumes terminal vinyl groups at a rate that depends primarily on the molar ratio SiH:vinyl and the local platinum concentration. Formulations with SiH:vinyl ratios between 1.4:1 and 1.8:1 typically achieve T90 cure times of 15–40 s at 110°C and 40–80 s at 90°C, as measured by moving die rheometer ISO 6502. The presence of excess Si-H beyond 2.0:1 can reduce network completeness by generating hydrogen gas via side reactions with trace silanol groups, causing bubbles in sections thicker than 5 mm; SiH deficiency below 1.2:1 leaves unreacted vinyl groups that plasticize the network and raise compression set after 22 h/175°C. Thin-wall molding at wall thicknesses below 0.5 mm can use higher mold temperatures up to 180°C because heat transfer is rapid, but thick sections must not exceed 150°C to avoid thermal degradation and split lines from cure-rate mismatch. The cure rate also depends on the inhibitor concentration; typical levels of 0.01–0.05 wt% relative to Part A extend pot life at 25°C from 24 h to 72 h, but excess inhibitor increases the minimum mold temperature required to achieve demolding strength. Vinyl silicone oil with high internal vinyl content accelerates network gelation once the inhibitor is consumed, whereas terminal vinyl-rich fluids tend to build modulus more slowly and generate lower exothermic peaks. The exotherm during LSR cure is generally below 25°C in thin sections but can exceed 40°C in parts thicker than 20 mm when high vinyl content and platinum levels above 10 ppm are combined.

Fumed silica reinforcement in LSR based on vinyl silicone oil depends on the formation of hydrogen-bonded filler networks that are densified by polymer-filler interactions during planetary mixing and compounded storage. Vinyl fluids with viscosity between 2,000 mPa·s and 10,000 mPa·s penetrate silica aggregates more effectively than 100,000 mPa·s fluids, reducing the critical strain for Payne effect onset and producing lower uncured storage modulus at low shear amplitudes. However, low-viscosity fluids reduce the extensional viscosity of the uncured compound, which can cause wall slip in kneaders and limit the dispersion stability of hydrophobic silica after aging. The use of vinyl alkoxysilane-treated silica with surface vinyl concentrations between 0.3 mmol/g and 0.7 mmol/g introduces covalent filler-rubber coupling during platinum cure, raising tensile strength from less than 7 MPa to 8–11 MPa and tear strength from 15 kN/m to 25–35 kN/m. Tear propagation resistance measured according to ASTM D624 Die B improves when the vinyl silicone oil contains a fraction of pendant vinyl groups, because pendant groups create a bimodal network with short chains that resist crack opening at the silica interface and long terminal chains that permit large-scale deformation. The fatigue failure mode in LSR components produced from vinyl silicone oil is often cohesive in the polymer phase at hardness below 40 Shore A, transitioning to interfacial failure at the silica surface above 60 Shore A; the change occurs because high crosslink density restricts chain mobility and concentrates stress at filler aggregates. Milling or high-shear mixing in a planetary dispersion mixer with a vacuum cover and 40 rpm planetary blade speed for 60–90 min produces a uniform compound when the total filler loading is between 20 phr and 40 phr, but fillers above 40 phr require staged oil addition to prevent dry agglomerate formation. Production-scale compounding experience indicates that batch-to-batch variation in vinyl oil moisture content above 150 ppm H₂O creates bubble defects and must be controlled by drying the silica or by applying vacuum during compounding.

Injection Speed, Gate Freeze, and Air Entrapment Thresholds in Multi-Cavity Molds

Multi-cavity injection molding of LSR formulated with vinyl silicone oil requires gate dimensions and injection speed settings that account for the high thermal expansion of uncured compound and its tendency to flash. The mixed material at 25°C exhibits an apparent viscosity between 200 Pa·s and 600 Pa·s at a shear rate of 100 s⁻¹, but at injection velocities of 60–120 mm/s through cold-runner gates of 0.5–1.0 mm diameter, shear rates exceed 10,000 s⁻¹ and viscosity drops to 20–80 Pa·s. This shear thinning enables filling of thin sections down to 0.3 mm without excessive injection pressure, but also allows material to penetrate parting lines when clamp force is below 0.5 t/cm² of projected area. Molders typically set the LSR injection machine to a shot size 1.1–1.2 times the cavity volume, hold pressure 20–50 bar for 2–5 s, and vent depth 0.01 mm to prevent air entrapment at the end of fill. Air trapped at knit lines can lower tear resistance by 30–50% when measured on ASTM D624 specimens because the knit line behaves as a pre-existing crack. To eliminate air from multi-cavity tools with complex geometries, the mold is connected to a vacuum pump drawing -0.08 MPa to -0.09 MPa for 3–8 s before injection; this is more critical with low-viscosity vinyl silicone oil grades because lower viscosity does not sweep air out of blind pockets as effectively as high-viscosity compounds. Gate freeze is not a curing phenomenon in LSR as in thermoplastic molding; instead, the runner material remains flowable until the mold temperature initiates cure, so cold-runner systems maintain the entire runner at 20–25°C while the cavity is heated to 120–170°C. The thermal boundary between the cold runner and the hot cavity must be within ±3°C of the setpoint to prevent premature gelling in the gate tip; this is achieved with sequential valve gates and thermal separation rings. Injection pressure mapping on 16-cavity medical stopper molds shows a typical fill pressure of 80–150 bar when using vinyl silicone oil with viscosity 10,000 mPa·s, but pressure rises above 200 bar if the gate diameter is reduced below 0.4 mm or if the oil viscosity is above 60,000 mPa·s. These thresholds guide mold design but published data for this specific configuration is limited for tooling with more than 32 cavities.

Post-Cure Volatile Targets Are Set by Extraction Limits in ISO 10993-18 and USP <88>.

Addition-cure LSR formulations based on vinyl silicone oil contain residual octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) unless a post-cure cycle is used. The initial volatile content measured by headspace gas chromatography after 4 h at 200°C should fall below 0.5 wt% for general industrial parts, but medical and food-contact articles require lower extraction limits. The test method ISO 10993-18 provides chemical characterization for medical device materials, while USP <88> Class VI specifies systemic injection, intracutaneous reactivity, and implantation tests for elastomers in indirect patient contact. Residual platinum catalyst at levels above 10 ppm may require process validation because Pt is not entirely removed by post-cure. Post-curing in forced-air ovens at 200°C for 4 h removes most cyclosiloxanes and volatile silanols, but lower temperatures require longer times: at 150°C, removal of D4 below 100 ppm may require 16 h in thin sections. The post-cure cycle also completes the reaction of residual vinyl and Si-H groups, reducing compression set and eliminating surface tack. Parts intended for repeated food contact under FDA 21 CFR 177.2600 must meet total extractive limits in distilled water and n-heptane after specified time-temperature extraction; the extraction values are influenced by the molecular weight distribution of the vinyl silicone oil and by the residual cyclic siloxane fraction. Table 2 summarizes relevant compliance standards and test conditions for LSR based on vinyl silicone oil.

Compression set testing according to ISO 815-1:2014 method A at 175°C for 22 h reveals incomplete network formation when unreacted vinyl groups remain at the chain ends or when Si-H reserves are too low. Formulations with vinyl silicone oil content below 0.10 mmol/g regularly exhibit compression set above 30%, while network densities corresponding to 0.25–0.40 mmol/g reduce compression set to 10–18%. However, pushing vinyl content above 0.50 mmol/g without increasing the Si-H crosslinker proportion creates a stoichiometric imbalance that leaves polar vinyl moieties and lowers thermal stability at 200°C. Heat aging for 168 h at 200°C in a forced-air oven causes retention of tensile strength to fall from 85% at 0.20 mmol/g vinyl to below 55% at 0.60 mmol/g vinyl if the SiH:vinyl ratio is not adjusted above 1.6:1. The mechanism involves oxidative degradation at tertiary carbon centers formed by hydrosilylation when excess Si-H adds to the α-carbon of vinyl groups; therefore, stoichiometric control rather than maximum vinyl content is the dominant factor for long-term compression set. Low-VOC processing of vinyl silicone oil also requires control of platinum catalyst residues because platinum can catalyze thermal depolymerization during prolonged aging above 250°C. The compression set target for automotive seals and medical stoppers is often below 25% after 22 h/175°C, while gaskets used in continuous service at 150°C may require values below 15% to maintain sealing force. Experience on production-scale LSR molding lines shows that post-cure time below 2 h at 200°C leaves residual compression set of 28–35%, whereas extending post-cure to 4 h reduces the value to 12–18% for the same vinyl oil grade and platinum loading.

Metering Low-Viscosity Vinyl Silicone Oil Through 20:1 L/D Static Mixers

Low-viscosity vinyl silicone oil grades below 2,000 mPa·s are easier to meter than high-viscosity grades but introduce a different set of production limitations in continuous LSR mixing. In a 20:1 L/D static mixer with 24 helical elements, the pressure drop follows Darcy-type shear-thinning behavior; at a flow rate of 10 kg/h, a 1,000 mPa·s fluid generates approximately 3–6 bar pressure drop, while a 20,000 mPa·s fluid can require 30–60 bar at the same throughput and mixer diameter. This pressure difference affects the accuracy of the feeding loop: gear pumps with 0.6 cm³/rev to 3.0 cm³/rev displacement maintain ±0.5% repeatability for Newtonian fluids below 5,000 mPa·s, but shear heating in the pump and static mixer raises the fluid temperature by 2–5°C per 100 bar pressure drop. Since the platinum catalyst is usually premixed with the vinyl silicone oil in Part A, any shear heating above 35°C shortens pot life and can initiate early gelation in dead zones of the mixer. Dead zones are minimized by using static mixers with removable elements and no internal weld seams; production experience identifies that mixers with 32 elements exhibit lower wall residence time than 12-element mixers despite higher pressure drop because recirculation zones are broken by repeated splitting and recombination. The addition of a low-viscosity vinyl silicone oil at 5–10 wt% to a high-molecular-weight LSR base reduces the compounded viscosity by up to 40% while only slightly lowering Shore A hardness; this is often used to accommodate narrow gates or long flow paths in connector seals and gaskets. However, diluent grades with vinyl content below 0.20 mmol/g must be compensated with additional Si-H crosslinker to maintain the same network density. If the Si-H crosslinker is not adjusted, the resulting soft network can exhibit compression set above 30% and reduced tear strength below 15 kN/m. When metering two separate streams of high and low viscosity vinyl silicone oil, a simultaneous static mixing unit with mass-flow metering at ±0.5% repeatability is required; otherwise, the standard deviation of Shore A hardness across a 100-shot production run can exceed ±2 points.

LSR components produced from vinyl silicone oil are processed into diaphragms, needle-free valve membranes, peristaltic tubing, and optical lenses where clarity, low extractables, and compression set stability are simultaneous requirements. The optical clarity of cured LSR is affected by the refractive index match between the vinyl silicone oil and treated silica; oils with refractive index near 1.405 at 25°C and low silanol contents produce haze values below 5% at 2 mm thickness when measured according to ASTM D1003. For translucent or transparent LSR, vinyl silicone oil grades with narrow molecular weight distribution are preferred because broad dispersity introduces differential light scattering at the filler-polymer interface. In automotive electrical connector seals, the cured compound must retain hardness after long-term exposure to engine fluids; immersion in IRM 903 oil at 150°C for 168 h according to ISO 1817 typically increases hardness by 2–5 Shore A and reduces elongation by 10–20%, depending on the crosslink density and filler content. In medical applications, the absence of peroxide decomposition products is a key advantage of vinyl silicone oil-based addition-cure LSR over peroxide-cured elastomers; the relevant biocompatibility end-points are tested according to ISO 10993-5 and ISO 10993-10. The platinum content in cured parts must be controlled because residual Pt above 20 ppm can exceed pharmaceutical compatibility guidelines in some jurisdictions. Production of LSR for medical wearables with skin contact for more than 30 days often requires ISO 10993-23 irritation testing in addition to standard ISO 10993-10; the vinyl silicone oil supplier must provide consistent batch-to-batch vinyl content and volatile profiles because changes in D4 and D5 content alter the toxicological risk assessment. Published data for this specific application with Zhejiang Runhe vinyl silicone oil is limited, so compounding houses perform extraction tests on each incoming lot using gas chromatography with 0.5 ppm detection limits for D4, D5, and D6.