Vinyl Terminated Silicone Oil RH-Vi302

Addition-Cure LSR Injection Moulding: Metering Rheology, Pt Inhibition, and Cavity Pressure Limitations

Injection moulding of addition-cure liquid silicone rubber begins with two components, typically designated Part A and Part B, that are transferred by servo-driven piston metering units from 200 L drums or 20 L pails into a static mixer. RH-Vi302 is incorporated as the α,ω-divinyl polydimethylsiloxane base fluid or as a controlled-viscosity reactive diluent on the Part A or Part B side, depending on whether the compounder requires a lower total mixed viscosity for thin-wall filling or a higher molecular weight between crosslinks for improved tear resistance. Viscosity of the vinyl-terminated fluid should be verified by ASTM D445 at 25 °C, and vinyl content should be confirmed by iodometric titration against the supplier certificate of analysis before batching, because batch-to-batch drift in vinyl content shifts the required hydride crosslinker addition and alters the final network density. The mixed LSR is injected into a closed mould at 120 °C to 180 °C, with cavity pressure typically managed between 5 MPa and 20 MPa depending on part thickness, gate geometry, and mould temperature uniformity. Clamp force is calculated from projected area multiplied by cavity pressure, with an additional safety factor of 10% to 20% for large tools and multi-cavity configurations. Cure proceeds by platinum-catalysed hydrosilylation between SiH and SiVi, with no condensation by-product; however, hydroxylated silica surfaces and atmospheric moisture consume SiH groups and release hydrogen gas, causing porosity in thick sections. Production-scale failure modes include mould fouling from low-molecular-weight cyclic siloxanes, platinum catalyst poisoning by sulfur-containing additives, organotin compounds, and amine-based release agents, and scorch in the static mixer when inhibitor levels are insufficient. Inhibitor loading with 1-ethynylcyclohexanol or an equivalent alkyne alcohol is typically adjusted from 0.02 phr to 0.10 phr to maintain pot life without delaying cure excessively. Tensile properties are measured by ISO 37, hardness by ASTM D2240, tear resistance by ASTM D624, and density by ASTM D792. Post-cure schedules at 200 °C for 4 h are used to reduce volatiles and stabilise the crosslinked network. The exact vinyl content and volatile fraction of RH-Vi302 are grade-specific and must be read from the supplier batch certificate; published data for this specific configuration is limited, and production trials should not substitute for batch-specific formulation work.

High-voltage cable accessory compounds are formulated with vinyl-terminated PDMS as the primary elastomer to provide stable dielectric loss and volume resistivity after thermal ageing in contact with copper and aluminium conductors. A typical compounding route combines the vinyl-functional fluid with hexamethyldisilazane-treated fumed silica, a poly(methylhydrosiloxane-co-dimethylsiloxane) crosslinker, and a platinum catalyst. The uncured compound is injection moulded or transfer moulded around EPDM and XLPE cable insulation at 130 °C to 170 °C, then post-cured at 200 °C for 2 h to 4 h to strip residual cyclics below the supplier-specified volatile threshold. Electrical acceptance is anchored to IEC 60243-1 for breakdown strength, ASTM D257 for volume resistivity, and ASTM D150 for permittivity and dissipation factor at 50 Hz or 60 Hz. Silicone compounds in this segment frequently specify volume resistivity not less than 1×10^14 Ω·cm at 25 °C and dielectric strength above 20 kV/mm on 2 mm plaques; however, final part geometry and electrode configuration rather than the material datasheet limit service stress. Batch records from production-scale moulding show that moisture levels above 200 ppm in fumed silica or filler produce bubbles and knit-line cracks at thick-section cable joints. Sulfur-cured rubber residues, organotin-stabilised materials, and amine-containing epoxy coatings must be excluded from the bonding interface because they release catalyst poisons and inhibit cure at the surface. Electrical treeing resistance and interfacial pressure are also influenced by filler dispersion; a three-roll mill with gap settings below 50 µm is typically required to break fumed silica agglomerates and achieve consistent breakdown strength across a production batch.

Why Does Stoichiometric Drift Occur in Alumina-Filled Electronic Potting Compounds?

Alumina and boron nitride fillers do not participate directly in hydrosilylation, but their surface silanol groups and adsorbed moisture consume SiH through hydrolysis, releasing hydrogen and reducing the effective SiH:SiVi ratio below the value calculated from the crosslinker batch sheet. The result in electronic potting compounds is a soft interface or tacky layer at the surface, lower hardness in the upper zone of the module, and increased coefficient of thermal expansion in the cured mass. To compensate, formulators often raise the crosslinker input from a nominal 1.2:1 molar SiH:SiVi ratio to 1.6:1 when filler loadings exceed 60 wt%; the precise adjustment must be determined by moving-die rheometer cure curves using ISO 6502 or an equivalent method. Potting compounds are mixed under vacuum at −0.095 MPa to −0.085 MPa to remove entrapped air, then dispensed into housings and cured at 80 °C to 120 °C for 30 min to 90 min. Thermal conductivity is measured by ASTM D5470, flammability by UL 94 V-0, and electrical insulation by IPC-CC-830B or end-user specification. High filler loadings above 70 wt% increase viscosity into the paste range, making vacuum degassing less effective and reducing pot life due to internal shear heating during planetary mixing. Production-scale failures include catalyst poisoning from organotin residues in enclosure gaskets, sulfur from polymer housings, and amine blush on printed circuit boards, all of which require surface cleaning or barrier coatings before dispensing. Coefficient of linear thermal expansion is characterised by ASTM E831, and glass transition temperature by ASTM E1356; in power modules, low stress on bond wires is achieved when the cured silicone remains below 0.6 W/(m·K) unless a filled grade is required for heat transfer. Published data for this specific configuration with RH-Vi302 is limited, so the crosslinker adjustment should be re-validated on the production batch rather than transferred from generic vinyl-terminated silicone oil data.

For polyethylene terephthalate and glassine release liners, a solvent-free addition-cure silicone coating is prepared from vinyl-terminated PDMS, a hydride-functional siloxane crosslinker, a platinum catalyst, and an MQ release resin for controlled peel force. The coater applies the mixture at 0.8 g/m² to 1.5 g/m² using five-roll offset gravure or multi-roll film splitting, then passes the web through a forced-air oven at 120 °C to 150 °C for 5 s to 15 s, depending on line speed and duct length. Release force is measured by FINAT FTM 3 at 300 mm/min peel and 180° angle, subsequent adhesion by FINAT FTM 1, and loop tack by FINAT FTM 9; the control target is adjusted by changing the MQ resin fraction and SiH:SiVi ratio rather than by post-cure processing. A key production limitation is residual volatile siloxane migration into the pressure-sensitive adhesive, which lowers subsequent adhesion and is quantified by solvent extraction followed by GC-FID according to the supplier method. Condensation-cure systems, tin catalysts, and sulfur-containing paper coatings must be excluded from the coating room because they poison the Karstedt catalyst and produce uncured bands or release force drift. The coating bath pot life is controlled by keeping the mixed material below 25 °C and by adding an alkyne inhibitor; excessive inhibitor causes oven temperature fall-off and migration of unreacted crosslinker into the adhesive.

When α,ω-Divinyl Fluids Serve as Reactive Diluents in Low-Crosslink-Density Gels

Low-crosslink-density silicone gels are compounded at substoichiometric SiH:SiVi ratios, commonly below 0.8:1, so that the cured network retains unreacted vinyl groups and a large distance between crosslink nodes. RH-Vi302 is used as a reactive diluent to reduce mixed viscosity during filler addition and to control the gel’s cone penetration without adding non-reactive plasticizer that would migrate during service. Crosslink density is governed by the molar ratio, the functionality of the hydride crosslinker, and the chain length of the vinyl-terminated fluid; chain extension competes with crosslinking, so a small change in the vinyl fluid’s molecular weight from batch to batch shifts penetration, tack, and modulus more than it shifts viscosity. Gel properties are measured by cone penetration using ASTM D217 or ISO 2137, with penetration values frequently specified from 30 to 250 tenths of a millimetre at 25 °C; tensile and elongation are measured by ISO 37 where specimens can be cut. Medical and skin-contact gel grades are evaluated under ISO 10993-5 for cytotoxicity, ISO 10993-10 for irritation and delayed-type hypersensitivity, and ISO 10993-23 for irritation in reconstructed epidermis models where applicable; extractable low-molecular-weight siloxanes are limited because they are suspected of altering barrier function and sensitisation potential. Curing is performed at 80 °C for 30 min to 60 min or at 25 °C for 24 h depending on inhibitor loading; thick sections require a programmed ramp rather than a direct high-temperature cure because the exotherm can exceed the boiling point of low-boiling inhibitors and create bubbles. Production-scale defects include gel separation from the housing, creep at elevated temperature, and tack transfer to adjacent surfaces when the SiH:SiVi ratio is too low or when the hydride crosslinker is not thoroughly incorporated by high-shear mixing.

Textile LSR Ink Systems, Fabric Adhesion, and Wash Durability Thresholds

In garment-printing liquid silicone rubbers, a printable paste is prepared by high-shear dispersion of fumed silica, pigment masterbatch, and an adhesion promoter such as vinyltriethoxysilane or 3-glycidoxypropyltrimethoxysilane at 0.5 phr to 2.0 phr. The paste is screen-printed or rotary-printed onto cotton, polyester, or nylon textiles, then cured in a tunnel oven at 130 °C to 160 °C for 60 s to 120 s. The addition-cure mechanism forms a flexible film with low oil separation and high elongation, but catalyst poisoning from amine-containing textile finishes and sulfur-dyed fabrics is a known batch failure mode; pre-treatment with a barrier coat or heat cleaning is required on contaminated substrates. Adhesion is evaluated after cure by ISO 13934-1 for coated fabric tensile strength, ISO 105-C06 for colour fastness to washing, and AATCC 61-2A where accelerated laundering is specified by brand requirements. A key formulation conflict exists between crosslink density and film softness: increasing the SiH:SiVi ratio above 1.5:1 raises hardness and abrasion resistance but reduces stretch and elastic recovery, while low ratios below 1.0:1 leave tacky surfaces that attract lint and exhibit poor screen release. Volatile cyclic siloxanes in the cured film are reduced by a brief post-cure at 180 °C for 30 s to 60 s on the production line, and the residual content is verified by solvent extraction according to the supplier method before shipment.