In two-component platinum-catalyzed liquid silicone rubber compounding, the processing behaviour of RH-Vi322 depends on its batch-specific viscosity and terminal vinyl equivalent mass; these values are supplied in the certificate of analysis and must be used to calculate the SiH:Vi ratio rather than relying on nominal grade names. RH-Vi322 is metered through a static mixing manifold at a controlled volumetric ratio, commonly
1:1, before injection into a water-cooled barrel maintained at
15–25 °C to prevent premature vulcanization. The terminal vinyl groups react with trimethylsiloxy-terminated polymethylhydrosiloxane crosslinker through hydrosilylation in the presence of a Karstedt platinum catalyst. The molar ratio of silicon hydride to terminal vinyl is held between
1.2:1 and
1.9:1 for most injection-molded seals, grommets, gaskets, and infant-care bottle valves. Below
1.2:1 cure is incomplete and surfaces remain tacky; above
1.9:1 the excess SiH can react with moisture to form silanol groups that disrupt the network and reduce elongation at break. RH-Vi322 does not provide mechanical reinforcement by itself; fumed silica with a BET surface area of
180–300 m²/g is dispersed into the vinyl oil phase before compounding, and the resulting paste is blended with the crosslinker side to minimize shear history. On production-scale injection machines the mold temperature is set between
120 °C and
180 °C, with cure times of
5–60 s depending on part wall thickness and gate location. The low volatility of RH-Vi322 is critical because evaporation in the mold cavity produces gas traps and surface defects; high-volatile fractions can also condense in the mold vacuum system and require more frequent cleaning. Cured compounds are tested according to
ASTM D2240-15 for Shore A hardness,
ASTM D412-16 for tensile strength and elongation,
ASTM D624-12 for tear strength, and
ASTM D395-18 for compression set. Typical injection-molded LSR grades made with vinyl-terminated polydimethylsiloxane span Shore A hardness
5–80, tensile strength
5–11 MPa, elongation
250–700%, tear strength
10–45 kN/m, and compression set after
22 h at
177 °C below
20%. For food-contact seals, cured parts are evaluated under
FDA 21 CFR 177.2600 and
BfR XV because platinum residues and volatile siloxanes are regulated.The critical processing window in LSR injection molding with RH-Vi322 is defined by the inhibitor package rather than by the vinyl oil alone. 1-ethynyl-1-cyclohexanol or tetramethyltetravinylcyclotetrasiloxane inhibitors are used at
10–500 ppm relative to the platinum content to extend pot life. Pot life at
25 °C for a mixed formulation is typically
12–72 h; however, if the inhibitor concentration is increased to protect against heat accumulation in a static mixer during line stoppages, the mold cure time increases disproportionately. A formulation that cures in
15 s at
150 °C may require
60 s or more at
120 °C, which forces process engineers to balance cycle time against scavenger capacity. Injection pressure is normally kept below the flash threshold of the tool; vent gaps below
0.005 mm are used because LSR has a low localized viscosity during injection and will fill microscopic parting-line clearances. Mold vacuum of
–0.08 MPa to
–0.09 MPa is commonly applied before injection to remove trapped air from dynamic seals and undercut regions. RH-Vi322 contributes to the low surface tension required to wet the cavity surface, but it also creates a thin elastomeric film at the parting line if fill pressure exceeds
800 bar, requiring automatic deflashing or cryogenic tumbling. Production experience on electric injection units with closed-loop pressure control shows that batch-to-batch variation in vinyl equivalent weight of
±5% can change the cure onset enough to affect demolding robustness; therefore the certificate of analysis of RH-Vi322 should be reviewed for vinyl content and volatiles before setting the SiH:Vi ratio.
How Does Vinyl-Terminated Polydimethylsiloxane Influence Platinum-Catalyzed Cure in Solventless Release Coatings?
Solventless silicone release coating systems are formulated with RH-Vi322 as the vinyl donor, a methylhydrogen polysiloxane as the hydride source, and a platinum-divinyltetramethyldisiloxane complex as the catalyst. The mixture is applied by multi-roll offset gravure or five-roll direct coating heads to paper, glassine, polyester film, or polypropylene film at coating weights between
0.6 g/m² and
1.5 g/m². Cure ovens operate with web temperatures of
120–180 °C for dwell times of
1–3 s, corresponding to line speeds of
200–800 m/min on modern converting lines. The SiH:Vi ratio in release coatings is typically higher than in LSR, falling between
2.0:1 and
3.5:1, because the release layer must develop a dense network at very low coating weight and because some hydride is consumed by reaction with hydroxyl groups on the paper substrate. RH-Vi322 with a narrow vinyl distribution provides faster initial cohesion development; a broad vinyl distribution can leave uncrosslinked low-molecular-weight chains that migrate to the adhesive surface after lamination. Cured coating release force against a rubber-based pressure-sensitive adhesive is measured according to
FINAT FTM 3 at
300 mm/min and, depending on the control-release additive loading, is adjusted from
5 cN/25 mm to
80 cN/25 mm. Subsequent adhesion retention of the adhesive is checked by laminating the release liner to the adhesive, aging for
24 h at
70 °C, and testing the adhesive bond according to
FINAT FTM 10; values above
70% of the control are required in most release liner specifications.The main operational failure mode in solventless release coating is not insufficient cure after the oven but bath pot life reduction during coating interruptions. When RH-Vi322 is combined with a reactive methylhydrogen polysiloxane and platinum, the mixed bath at
20–30 °C has pot life of
8–24 h without inhibitor addition, but this drops to less than
60 min if the coating pan is heated by a nearby drying hood or if the static mixer is not purged. Because RH-Vi322 has reactive terminal units rather than pendant vinyl groups only, the crosslink density becomes sensitive to the exact hydride distribution of the SiH crosslinker; resins with high internal hydride content create faster gelation but lower release-modifier compatibility. Coating defects such as silicone rub-off occur when the vinyl conversion remains below
95% or when platinum is poisoned by amine-based inks, sulfur-containing paper additives, or heavy metal residues from recycled fiber. A barrier coating or pre-wash is required before applying RH-Vi322-based release coatings to recycled board because the alkalinity and extractable inhibitors in the substrate can shift the cure threshold by
10–20 °C. The viscosity of the coating bath is typically maintained between
150 mPa·s and
500 mPa·s; RH-Vi322 may require blending with a lower-viscosity vinyl-terminated diluent to achieve this target on high-speed gravure units. Process engineers should verify the platinum content of the catalyst batch because over-catalysis causes non-selective cure during application and under-catalysis leaves smear on the liner.When an addition-cure electronic encapsulant is formulated for insulated-gate bipolar transistor modules operating at a junction temperature of
150 °C, RH-Vi322 is used as a low-volatile reactive diluent that lowers the mixed viscosity before thermal fillers are added. The unfilled resin system is adjusted to below
2,000 mPa·s to allow incorporation of α-alumina, boron nitride, or aluminum hydroxide at loadings between
60 wt% and
85 wt%. The filled mixture is degassed under vacuum at
10–50 mbar and transferred into a heated mold or directly into the module housing. Cure is commonly run at
60–100 °C for
30–120 min followed by post-cure at
120–150 °C for
1–4 h to consume residual hydride and reduce outgassing of low-molecular-weight siloxanes. The cured encapsulant is tested for dielectric strength according to
ASTM D149, volume resistivity according to
ASTM D257, and vertical burn classification under
UL 94. Typical dense network formulations achieve dielectric strength above
15 kV/mm and volume resistivity above
1×10^14 Ω·cm after post-cure; values below
12 kV/mm often indicate entrapped moisture or incomplete hydrosilylation. The terminal vinyl groups on RH-Vi322 allow a more regular network than pendant-vinyl fluids, which reduces the free-chain fraction that can migrate to copper leadframes and interfere with wire-bond adhesion. However, the addition-cure reaction is sensitive to platinum catalyst poisoning by sulfur-containing gaskets, amine-based flux residues, and organotin compounds used in some solder masks. Components must be cleaned and dried before potting; baking of metal substrates at
125 °C for
2 h is recommended when condensation humidity exceeds
60% RH. The hardness and thermal cycling performance of the cured encapsulant are adjusted with vinyl-functional MQ silicate resins, not by simply increasing the SiH:Vi ratio, because excess hydride creates post-cure gas generation in closed modules.
Medical Silicone Gel Penetration and Extractables Test Matrix
Silicone gels produced from RH-Vi322 are formed by reacting the vinyl-terminal polydimethylsiloxane with a low ratio of SiH crosslinker so that the network contains a high fraction of free chain segments and exhibits low elastic modulus. The reactive mixture is degassed at
20–50 mbar until no visible foam remains, cast into thermoplastic or metal molds, and cured at
80–120 °C for
30–120 min. Post-cure at
150 °C for
2–4 h increases vinyl conversion and extracts unbound chains. RH-Vi322 with low volatile content is selected because residual cyclic dimethylsiloxanes contribute to extractables and can affect skin contact compliance. In gel formulations the SiH:Vi ratio is often below
1.0:1 to create a soft, tacky network; higher ratios produce a firmer gel but can increase surface bleed. Hardness is reported as penetration depth using a texture analyzer or cone penetration method; formulators avoid durometer measurement because gels fall below the Shore OO range. Medical-grade formulations are subjected to
ISO 10993-5:2009 cytotoxicity testing,
ISO 10993-10:2010 skin sensitization and intracutaneous irritation testing, and extraction testing according to
ISO 10993-12:2021 with extraction ratios specified for surface-contacting devices. If the gel is intended for long-term skin contact, additional genotoxicity testing according to
ISO 10993-3:2014 and subchronic toxicity evaluation according to
ISO 10993-11:2017 may be required. The use of RH-Vi322 in medical gels does not automatically confer biocompatibility; the complete formulation, including platinum catalyst residues, crosslinker byproducts, and fillers, must be validated.
| Biological endpoint | Test method | Typical assessment criterion |
|---|
| Cytotoxicity | ISO 10993-5:2009 | cell viability ≥ 70% relative to control |
| Sensitization | ISO 10993-10:2010 | no grade ≥ 2 erythema or oedema |
| Intracutaneous irritation | ISO 10993-10:2010 | no lasting erythema |
| Acute systemic toxicity | ISO 10993-11:2017 | no mortality or significant clinical signs |
High-consistency silicone rubber downstream processing of RH-Vi322 normally begins with the fluid being added to a partially compounded silicone gum during a second pass in an internal mixer or on a two-roll mill. Typical addition levels are
2–15 phr relative to the silicone gum because lower levels produce no measurable viscosity reduction and higher levels create surface bloom and roll sticking. The vinyl oil is pre-blended with fumed silica to avoid localized plasticization; when added directly to the mill, the fluid can migrate to the roll surface and reduce shear heating in the nip. Mill conditions are set with a roll temperature of
20–50 °C, a friction ratio of
1.1:1–1.3:1, and a nip gap of
0.5–2 mm depending on the gum plasticity. RH-Vi322 modifies both peroxide-cured and addition-cured high-consistency rubber systems. In peroxide-cured compounds the terminal vinyl groups compete with the gum vinyl groups for radical species; omission of this effect in cure kinetics can lead to under-cure at
170 °C with dicumyl peroxide or
115 °C with 2,4-dichlorobenzoyl peroxide. Cured sheets are tested according to
ASTM D412-16 for tensile strength,
ASTM D624-12 for tear strength, and
ASTM D395-18 for compression set. The expected trend is a reduction in Shore A hardness of
5–20 points and an increase in elongation of
50–150% relative to the unmodified gum compound, but published data for RH-Vi322 in high-consistency silicone rubber is limited; the exact response is dependent on the base gum vinyl content and filler surface treatment. Processors should not exceed
15 phr because molded parts exhibit oil exudation and post-cure surface tack.
When Vinyl-Terminated Oil is Selected for Textile Anti-Slip Coatings Instead of Peroxide-Cured PDMS
Textile anti-slip and grip-coating formulations are converted by knife-over-roll coating, rotary screen printing, or scatter coating followed by lamination. RH-Vi322 is selected when the coating must cure without peroxide decomposition byproducts and must retain low odor after high-temperature fixation on hosiery, mattress ticking, upholstery, or cut-resistant gloves. The vinyl oil is compounded with fumed silica to produce a paste with viscosity between
20,000 mPa·s and
80,000 mPa·s; a methylhydrogen polysiloxane crosslinker and platinum catalyst are added before coating. The coated textile is cured in a forced-air oven at
120–150 °C for
1–3 min. Addition-cure systems are more sensitive to residual spinning oils, antistatic agents, and sulfur-containing dyestuffs than peroxide-cured silicone; a fabric pre-wash or plasma treatment is applied when the surface contains amine- or sulfur-containing auxiliaries because these materials poison the platinum catalyst and produce a smearing uncured interface. Cured coatings are evaluated for abrasion resistance on a Martindale machine according to
ASTM D4966-12, wash durability according to
AATCC 135, and coating adhesion to the substrate by a crease-flex test. In textile applications, hardness alone is not sufficient; the grip coefficient must be balanced against the hand feel. RH-Vi322 with low volatile content reduces smoke and oily residue during heat fixation, but the specific coefficient of friction on cotton versus polyester varies with the fabric weave and should be validated on the actual line because published data for RH-Vi322 in textile coating is limited.
Vinyl Terminated Silicone Oil RH-Vi322 is a linear polydimethylsiloxane capped at both chain ends with dimethylvinylsiloxy units, supplied as a clear anhydrous fluid for platinum-catalyzed addition-cure silicone formulations. The designation RH-Vi322 is a supplier-specific grade identifier and does not encode vinyl content or viscosity in a directly readable unit. Representative technical data list a kinematic viscosity of 2,000 mPa·s at 25 °C measured by ASTM D445-21, terminal vinyl content of 0.22 wt% determined by iodometric titration, volatile content below 1.5 wt% after 2 h at 150 °C, and moisture below 200 ppm by ASTM D1533-20. The polymer participates in hydrosilylation with methylhydrogensiloxane crosslinkers and does not release condensation by-products. Because the terminal vinyl groups are the only reactive sites along the chain, the material yields a defined network mesh size in low-modulus gels and release coatings. The anhydrous state reduces platinum catalyst poisoning during metered mixing; preconditioning is nevertheless required when ambient relative humidity exceeds 60%.
How Does Terminal Vinyl Reactivity Alter Cure Kinetics Relative to Chain-Stopped Silicone Oils?
Addition-cure systems containing RH-Vi322 proceed by Pt-catalyzed hydrosilylation between the terminal vinyl group and the Si-H functionality of a methylhydrogensiloxane-dimethylsiloxane copolymer. The kinetic sequence includes Pt(0) oxidative addition across Si-H, coordination of the terminal vinyl group, migratory insertion, and reductive elimination to form an ethylene bridge. Formulation exotherm is controlled by the SiH:Vi molar ratio; typical values are 1.2:1 to 1.8:1. Cure can be initiated at 80 °C with a low-temperature platinum inhibitor, while the practical upper limit is 150 °C because prolonged exposure above this threshold can deactivate platinum and isomerize terminal vinyl groups to unreactive internal alkenes. No by-product is released; linear shrinkage is therefore lower than condensation-cure systems of equivalent molecular weight.
By comparison, silanol-terminated PDMS of equivalent viscosity requires organotin or titanium catalysts and releases water or alcohol. The liberated by-product increases void formation in thick sections, accelerates reversion in closed molds, and makes cure dependent on ambient humidity. Trimethylsilyl-terminated PDMS does not undergo hydrosilylation; it remains a free diluent and is extractable by hexane or toluene. Multi-vinyl silicone oils with pendant vinyl groups along the chain increase crosslink density and modulus, but they are less suitable where terminal chain-end crosslinking is required for low modulus and high elongation. The following comparison is representative for addition-cure formulations based on equivalent polymer molar mass.
| Parameter | RH-Vi322 | Silanol-terminated PDMS | Trimethylsilyl-terminated PDMS | Multi-vinyl PDMS |
|---|
| Reactive site | Terminal vinyl | Terminal silanol | None | Terminal and pendant vinyl |
| Cure mechanism | Platinum hydrosilylation | Tin/titanium condensation | Not reactive | Platinum hydrosilylation |
| By-product release | None | Water/alcohol | Not applicable | None |
| Network integration | Chain-end crosslinks | Chain-end condensation nodes | Free diluent | Multi-site crosslinks |
| Typical cured Shore A hardness at SiH:Vi 1.2:1 | 15–25 | Not applicable | Not applicable | 35–60 |
The Shore A values in the table are representative for a low-modulus addition-cure formulation and are not a product specification; they illustrate the effect of vinyl placement on crosslink density. The comparative behavior is most visible in closed-mold liquid injection molding. In a cold-runner mold fed by a 150 Mg clamp-force injection machine, condensation-cure silanol fluids release volatiles that create gas traps at vent locations. RH-Vi322 formulations do not release such volatiles, but they require scrupulous exclusion of sulfur- and amine-containing compounds because these species coordinate to platinum and produce tacky surfaces.
Material Specifications and Incoming Inspection Limits
Because viscosity and vinyl content define network architecture, the following release ranges are used for incoming inspection at compounding plants. The listed limits are typical release windows, not batch-specific guarantees.
| Property | Test method | Acceptance range |
|---|
| Appearance | Visual inspection | Clear, colorless, no gels |
| Kinematic viscosity at 25 °C | ASTM D445-21 | 1,800–2,200 mPa·s |
| Terminal vinyl content | Iodometric titration, internal method RH-TM-011 | 0.20–0.24 wt% |
| Volatile content, 2 h at 150 °C | ASTM D4559-00 | ≤1.5 wt% |
| Moisture content | ASTM D1533-20 | ≤200 ppm |
| Refractive index at 25 °C | ASTM D1218-21 | 1.402–1.406 |
| Density at 25 °C | ASTM D891-18 | 0.970–0.980 g/cm³ |
Lot-to-lot variation in the manufacturer's ring-opening equilibration can shift molecular weight distribution and residual cyclic siloxane content. Compounding operations therefore verify vinyl content by titration after dilution in toluene and adjust the hydride crosslinker dose accordingly. A shift of 0.02 wt% in terminal vinyl content changes the required SiH mass by approximately 9% at a 1.5:1 SiH:Vi ratio, so gravimetric dosing without density correction is not considered reliable in production.
When the Formulation Window Narrows at High Line Speeds in Solventless Release Coating
Solventless release coating formulations using RH-Vi322 are prepared by combining 100 parts of the vinyl-terminated polymer with 4–6 parts of a methylhydrogensiloxane-dimethylsiloxane crosslinker, 50–100 ppm of platinum as a Karstedt catalyst, and 0.5–1.0 part of ethynylcyclohexanol inhibitor to provide pot life. The fluid is coated on a five-roll differential-speed coater with a head gap of 0.7–1.2 µm to deposit 0.8–1.4 g/m² dry coat weight. Cure at 150 °C for 8–12 s is common on polyethylene-coated kraft; heavier substrates require longer residence time. Release force measured by FINAT FTM 3 at 300 mm/min typically stabilizes after 24 h aging at 23 °C and 50% RH. At catalyst concentrations below 50 ppm, the cure rate at the same oven temperature becomes insufficient at line speeds above 200 m/min, and the coating can exhibit smear or post-cure changes. Published data for this specific product in high-speed coaters is limited; the stated relationships derive from standard platinum-catalyzed addition-cure formulations.
High-consistency liquid silicone rubber stocks based on RH-Vi322 are compounded on a co-rotating twin-screw extruder with an L/D ratio of 40:1. The base polymer is fed into the first barrel, fumed silica is side-stuffed after a melting seal, and the hydride crosslinker is injected downstream of the vacuum vent to prevent premature reaction. Barrel temperatures are held between 20 °C and 50 °C; screw speed is limited to 300 min⁻¹ because higher shear rates can generate local melt temperatures above 70 °C, desorb the inhibitor, and increase viscosity during compounding. The compounded material is injection molded at mold temperatures of 120–160 °C using a clamp force of 150–250 Mg depending on part area. Cure kinetics measured by oscillatory rheometry according to ASTM D6204-19a show tc90 at 150 °C in the range of 20–35 s for a formulation with SiH:Vi 1.5:1; the exact value depends on inhibitor type and catalyst concentration.
Batch-to-Batch Variation in Hydrosilylation Cure and Network Architecture
Because RH-Vi322 contains only terminal vinyl groups, the number-average molecular weight between crosslinks after complete cure is a direct function of polymer chain length rather than the statistical distribution of pendant vinyl groups. This makes low-modulus gels less sensitive to minor hydride dosing errors than multi-vinyl fluids. A dosing error of 0.1 part crosslinker in a terminal-vinyl system changes the fraction of unreacted chain ends; in a multi-vinyl system, the same error can create localized high-crosslink-density domains that raise Shore hardness and reduce elongation at break. For low-modulus gels, the terminal vinyl architecture permits Shore 00 values from 20 to 40 with measurable tear strength. The product should not be combined with amine-based additives or sulfur-containing compounds because these species coordinate to platinum and delay cure or produce exudation.
Batch-to-batch variation in residual cyclic siloxane content and chain termination efficiency can shift low-shear viscosity by ±10% and extractables after cure by ±1.5 percentage points. Incoming inspection should include gel permeation chromatography when the cured network is specified for food-contact or medical use. The manufacturer's certificate of analysis should be requested under EU 1935/2004 or FDA 21 CFR 177.2600 where applicable.
Encapsulant gels formulated with RH-Vi322 are used in power module insulation where soft stress relief is required. In such formulations, the polymer is mixed with a hydrogen crosslinker and a platinum catalyst, degassed at 1–5 kPa for 10 min, and cured at 100 °C for 1 h. The terminal vinyl fluid yields lower volumetric shrinkage than condensation-cure systems and reduces the risk of wire bond damage during thermal cycling from -40 °C to 150 °C. The material is not recommended in contact with strong amines, sulfur compounds, or tin salts because these agents inhibit platinum and can leave a tacky uncured interface.
