Dual-end Hydrogen Silicone Oil RH-H45

Dual-end Hydrogen Silicone Oil RH-H45 is a linear polydimethylsiloxane bearing reactive silicon-hydride moieties at both terminal positions of the polymer chain. The product is supplied as a colourless to pale yellow transparent liquid and is used as a controlled-functionality chain extender in platinum-catalysed hydrosilylation systems. The supplier-stated active hydrogen content falls within 0.045–0.065 wt%, kinematic viscosity at 25 °C is specified at 35–65 mm²/s, and volatile content at 150 °C/3 h is held below 1.2 wt%. Terminal Si–H placement differentiates RH-H45 from random copolymer-type methylhydrosiloxane fluids, in which hydride is distributed along the siloxane chain. In vinyl-functional addition-cure formulations, the telechelic hydride position supports near-linear chain extension rather than dense heterogeneous crosslink nodes, producing lower modulus and higher elongation at equivalent Si–H:Si–vinyl stoichiometry. The molecular architecture also reduces the probability of cyclic by-product entrapment during cure because no pendant hydride clusters are present to generate localised over-crosslinking. Batch release records include an acid number below 0.01 mg KOH/g and a refractive index at 25 °C of 1.400–1.406.

How does terminal Si–H influence network topology compared with pendant or side-chain hydride fluids?

The kinetic consequence is observed in platinum-catalysed hydrosilylation with divinyl-terminated polydimethylsiloxane. Pendant hydride fluids introduce multiple reaction loci per molecule; terminal hydride fluids provide a maximum of two reactive hydride sites per chain. At a stoichiometric ratio r = Si–H/Si–vinyl of 1.0–1.2, pendant systems tend toward densely crosslinked domains with higher Shore A hardness and reduced tensile elongation. RH-H45 under the same ratio extends linear segments and delays gelation, as measured by moving-die rheometer studies in which minimum torque ML and scorch time ts2 shift toward longer flow windows. Addition-cure formulations using RH-H45 as the sole hydride species typically exhibit lower complex viscosity during the first 30 s of the rheometer trace. This effect is relevant for narrow-gap injection moulding where premature gelation produces flow lines and knit-line defects. The chain-extension behaviour does not eliminate crosslinking; residual vinyl groups on polyfunctional silicone resins or T-branched siloxanes provide the final network density. Formulation hardness is calibrated by controlling the molar ratio of terminal hydride to pendant hydride if a mixed hydride system is required.

Inhibitability is another practical difference. Because the terminal Si–H structure offers fewer hydride reaction sites per molecule, the addition of 0.01–0.05 wt% of 1,3-divinyltetramethyldisiloxane or 1,3,5,7-tetravinyltetramethylcyclotetrasiloxane extends pot life at 25 °C from 4–8 h to 24–72 h in solventless formulations, with less drift in complex viscosity than is observed with higher-hydride pendant fluids. The inhibition is overcome by heating above 110–120 °C; cure speed then remains sufficient for injection cycles under 60 s. This makes RH-H45 suitable for moulders requiring a single-component pre-mixed compound with ambient stability or two-component systems with long mixed reservoir life.

On production-scale liquid silicone rubber compounding lines, RH-H45 is pre-mixed with the vinyl-terminated base polymer before addition of the platinum catalyst. In high-shear planetary mixers with tip speeds above 5 m/s, the hydride component is added after fumed silica has been fully dispersed; premature hydride addition to a hydrophobic silica surface can sequester Si–H through hydrogen bonding with residual silanol groups and shift rheological response between batches. Batch-to-batch variance in complex viscosity is controlled by maintaining the Si–H/Si–vinyl ratio at 1.0–1.2 and by verifying actual hydride content of each incoming lot. A twin-screw extruder with L/D ratio 44:1 and barrel temperatures from 20 °C to 80 °C is suitable for continuous addition-cure compounding, provided the screw configuration includes low-shear conveying elements in the final third of the barrel. The terminal hydride architecture produces a lower viscous heating rise than pendant fluids under comparable shear rates, reducing the risk of scorch in high-output lines. Capillary rheometry at 25 °C and apparent shear rate 100 s⁻¹ shows that addition of 2.5 wt% RH-H45 to a 10,000 mPa·s vinyl-terminated PDMS base reduces extrusion die swell relative to a pendant hydride fluid at the same active hydrogen contribution.

Specification envelope, reference methods, and batch-release parameters

Incoming quality-control testing for RH-H45 is performed according to the following methods. The reference values are supplier batch-release targets for the standard grade; custom narrow-range specifications may be available depending on the application.

For food-contact release liners, the formulated film rather than the hydride fluid alone is tested under 21 CFR 177.2600 for rubber articles intended for repeated use, or under EU Regulation (EU) No 10/2011 where migration limits apply. RH-H45 is not intended as a direct food additive. The raw material contains no intentionally added substances of very high concern above 0.1 wt%. REACH registration duties should be confirmed with the manufacturer for the registered tonnage band. No volatile organic solvent is included in the standard grade.

When low outgassing and optical clarity become critical in electronic encapsulation

In LED encapsulation and conformal coating for photovoltaic junction boxes, the terminal hydride fluid can replace part or all of a pendant crosslinker in a two-part addition-cure silicone. The benefit is a more uniform crosslink distribution, which lowers haze after thermal ageing because local regions of high crosslink density are less prone to refractive index mismatch. A formulation using RH-H45 at 0.5–1.0 wt% of total hydride-active material shows improved optical transmittance at 450 nm after 1,000 h at 125 °C, with retained transmittance typically above 95% relative to initial. Outgassing under vacuum at 150 °C/24 h is below 0.3 wt% when low-volatile vinyl silicone is used. For potting applications requiring UL 94 V-0, RH-H45 is evaluated with the selected filler package because silicone hydride contributes to char formation but does not itself confer flame retardancy. The product is not recommended for systems containing amine-based adhesion promoters; primary and secondary amines can deactivate platinum catalysts and trigger involuntary hydrogen evolution during storage of uncured compounds. Published data for LED-specific reliability testing of this exact hydride fluid is limited; therefore qualification under IEC 62717 or ANSI/UL 8750 should be performed on the fully compounded encapsulant.

Solventless thermal-cure release coatings benefit from terminal Si–H because the low viscous contribution permits the use of higher coating line speeds without excessive misting. In pilot-scale five-roll coaters running silicone release liners at 300 m/min, replacement of a pendant hydride fluid with RH-H45 at equal hydride molar loading reduced transfer coating weight variability from ±0.05 g/m² to ±0.02 g/m² across a 1,200 mm web width. The cure response with platinum catalysts is activated above 110 °C, and typical oven dwell at 150 °C for 8–12 s is sufficient to reach a stable release value. Release force against a standard acrylic adhesive tape is formulation-dependent and can be tuned from 5 cN/20 mm to 50 cN/20 mm using controlled stoichiometry and release-modifying resin addition. Because RH-H45 contains no epoxy or aryl functional groups, adhesion modification is achieved by blending with phenyl- or epoxy-functional silicone components rather than by in-situ reaction on the hydride chain ends. Residual unreacted Si–H after cure can continue to oxidise in humid storage environments; release liners stored at 40 °C/90% RH for 14 days may shift release values upward by 10–20% if the coating is formulated with excess hydride above r = 1.2. Therefore excess hydride is minimised or consumed by post-cure at 30–60 min/120 °C before slitting.

In textile coatings and elastic films, RH-H45 can be formulated with fumed silica and vinyl-functional MQ resin to produce soft-touch surfaces. The terminal hydride contribution reduces the Shore A hardness of a 10,000 mPa·s base at 20 parts per hundred filler loading from 25 Shore A to 15 Shore A without increasing plasticity, as measured by ASTM D2240-15. This property shift is valuable where fabric drape or film flexural modulus is critical, but it is balanced against a possible reduction in tensile strength measured by ISO 37:2017 when the formulation lacks branched resin reinforcement.

Comparative performance data separate RH-H45 from random hydride fluids in low-hardness addition cure

The table below summarises formulation-relevant differences observed in addition-cure liquid silicone rubber. The data are based on standard bench-scale formulations with a 10,000 mPa·s divinyl-terminated PDMS and fumed silica filler at 15 wt%. All values are typical rather than guaranteed; direct substitution should be validated on the target production line.

Terminal hydride fluids are not a direct replacement for vinyl-terminated base polymers, because they do not provide the main siloxane backbone length. They are used as stoichiometric chain extenders between vinyl end groups. The choice between RH-H45 and a random hydride crosslinker affects not only Shore A hardness but also compression set after 22 h at 175 °C, with terminal hydride formulations typically producing lower compression set in low-hardness compounds due to reduced clustered crosslink domains. For high-tear applications, a mixed hydride system is often required because pure terminal hydride chain extension can reduce tear strength if the base polymer lacks branching. Published data for this specific configuration is limited for tear-strength optimisation; lab-scale evaluation per ASTM D624-20 is recommended before production commitment.

Unlike hydroxy-terminated polydimethylsiloxane, RH-H45 does not require condensation catalysts such as dibutyltin dilaurate and does not release water or alcohol during cure. This is a critical difference for void-free encapsulation and for bonding interfaces that are sensitive to condensation by-products. Where a manufacturing process is already set up for tin-catalysed RTV condensation cure, direct substitution of RH-H45 is not possible without changing the cure chemistry to platinum-catalysed addition cure.

Storage stability is specified at 12 months in unopened original drums under nitrogen blanketing at 5–35 °C. Exposure to atmospheric moisture initiates slow hydrolysis of Si–H bonds and generates hydrogen gas; containers remain sealed and fitted with pressure relief if stored above 30 °C. Viscosity rises slowly after exposure to trace amounts of platinum, tin, or titanate condensation catalysts; dedicated stainless steel or polyethylene-lined dispensing equipment is required. In injection moulding of addition-cure liquid silicone rubber, accumulated RH-H45 residues in static mixers exposed to air can form gelatinous particles after 8–24 h shutdown; purging with anhydrous solvent or low-molecular-weight vinyl silicone is recommended before restart. The product is not classified as hazardous under GHS when managed in closed systems, although hydrogen evolution during decomposition requires explosion-proof ventilation in bulk storage areas. The product should not be combined with alkali metal salts of fatty acids used in some rubber release sprays; these can destabilise the Si–H bond and accelerate gas evolution during heating cycles above 150 °C.