Silicone Rubber Adhesives (Bottom Coating) RH-GZ-05

Silicone Rubber Adhesives (Bottom Coating) RH-GZ-05 is a one-component, neutral alkoxy-cure room-temperature vulcanizing elastomer supplied as a thixotropic black paste for bottom coating, edge sealing, and vibration-damping applications on non-porous substrates. The uncured compound is packaged in 300 mL aluminium cartridges and 20 L pail containers and is formulated for moisture-triggered crosslinking at 23 °C and 50 % RH. Density measured per ISO 1183-1 ranges from 1.04 g/cm³ to 1.08 g/cm³. Brookfield rotational viscosity at 20 rpm using spindle 7 falls between 35,000 mPa·s and 55,000 mPa·s. The cure reaction releases 0.3 wt% to 0.6 wt% of mixed alcohol condensate, allowing the material to be handled in enclosed coating cells where acetic acid generated by acetoxy-cure silicones is not acceptable.

Why Does Alkoxy-Cure Chemistry Suppress Corrosion on Tin-Plated and Copper Alloy Surfaces?

The alkoxy-cure mechanism releases a neutral alcohol condensate rather than acetic acid. In contrast, acetoxy one-part silicones generate acetic acid during moisture cure, and condensed acidic droplets may lower local pH below 4.0 on copper traces, tin-plated terminals, and silver-plated contacts. That corrosion mechanism is eliminated with RH-GZ-05 because the neutral condensation product does not produce the same electrochemical attack on mixed-metal assemblies. The formulation is qualified for use in mixed-metal enclosures under flowing mixed gas testing per IEC 60068-2-60. Compared with oxime-cure systems, RH-GZ-05 exhibits lower risk of stress cracking and softening of polycarbonate and polycarbonate/ABS blends used in coated housings and bezels. The difference is operationally significant in bottom coating lines where the bead contacts both metal chassis components and transparent thermoplastic covers.

On FR-4 printed circuit assemblies, the adhesive is applied directly to cleaned solder mask edges, aluminium baseplates, glass-filled PBT connectors, and tin-plated terminal blocks. When aluminium 6061-T6 coupons are degreased with isopropanol and dried before dispensing, lap shear strength per ISO 4587 stabilises after 7 days at 0.9–1.4 MPa. For copper alloy leadframes, adhesion values are typically 0.8–1.3 MPa under the same surface preparation. Polyolefin substrates are not directly bondable without plasma, corona, or a primer; published adhesion data for untreated polyethylene and polypropylene is insufficient for production use.

Processing Window and Humidity-Dependent Cure Response

Dispensing is performed with positive-displacement piston robots or pneumatic cartridge guns at 0.2–0.6 MPa. Needle inner diameter is selected between 0.25 mm and 1.19 mm to control bead width, with wet film thickness typically 0.1–0.5 mm. At 23 °C / 50 % RH, tack-free time is 8–18 min, and cure through a 2 mm section requires 24 h. At 10 % RH, tack-free time can extend beyond 45 min; a controlled chamber at 60 % RH or pre-humidification is required to maintain cycle time. At RH above 60 %, condensation on cold substrates can produce interfacial blisters and adhesion loss. Pre-drying of substrates at 60 °C for 30 min is recommended before applicator contact when dew-point margins are less than 3 °C. Ambient temperatures below 5 °C retard condensation cure, while temperatures above 35 °C accelerate skin-over and may trap volatile condensate in thick sections, leading to cavitation or internal voids. Amine-containing primers and accelerators are not recommended because alkaline residues can destabilise the tin condensation catalyst at the bond line. In production-scale bottom coating cells, no static mixer is required, but drip control at the nozzle is maintained using a suck-back setting of 0.02–0.05 mL per dispense cycle.

When RH-GZ-05 Replaces Two-Part Addition-Cure Silicones in Bottom Coating Lines

One-part alkoxy chemistry removes meter-mix ratio errors, static mixer purge, and short pot life after mixing. Two-part platinum-cure silicone systems require precise metering and are vulnerable to catalyst poisoning by sulfur residues, amine compounds, and some organometallic stabilisers. RH-GZ-05 tolerates ordinary residues on production assemblies because condensation cure proceeds by moisture diffusion and does not depend on a platinum catalyst. The main limitation is depth of cure. Addition-cure silicone can be vulcanised through a 2 mm section at 80 °C in 10–20 min, whereas RH-GZ-05 requires 24–48 h at 23 °C for the same thickness. Heat-assisted cure at 50 °C and 60 % RH reduces through-cure time to 12–18 h but does not eliminate the moisture-diffusion limitation. Where process cycle time is critical and the substrate can tolerate elevated temperature, a two-part addition-cure system remains preferable. Where the line converts to single-component ambient cure and requires compatibility with sulfur-bearing FR-4 residues, RH-GZ-05 substitutes directly.

For existing bottom coating lines converting from acetoxy or oxime products, replacement is most predictable when substrate cleaning, bead geometry, and cure-chamber humidity are held constant. Production-scale observation indicates that bead width variation below ±0.1 mm is achievable with piston dispensing robots, whereas pneumatic cartridge guns can introduce wider shot-to-shot variation if cartridge temperature fluctuates. Cartridge stabilisation at 20–25 °C for 4 h before dispensing reduces viscosity drift and improves bead repeatability in uncontrolled plant environments.

Thermal Degradation Proceeds by Siloxane Chain Randomization Above 250 °C

The manufacturer’s thermal aging data for RH-GZ-05 lists continuous service from −60 °C to 200 °C, with short excursions to 250 °C permitted without immediate reversion. Above 250 °C, siloxane chain randomization and cyclosiloxane evolution become measurable, and long-term exposure at that temperature is not recommended. Thermal shock testing per IEC 60068-2-14 from −40 °C to 125 °C for 100 cycles has been used to confirm adhesion retention on aluminium and FR-4 interfaces above 0.7 MPa. Hydrolytic stability is consistent with neutral-cure silicone networks, but continuous immersion in 5 % NaOH solution at 23 °C can reduce bond strength by more than 30 % within 30 days in screening tests. The product is not recommended for continuous exposure to strong alkaline cleaning baths, chlorinated solvents, or concentrated mineral acids. Storage life in unopened containers is 12 months from date of manufacture at 5–25 °C. Opened cartridges should be consumed within 30 days or sealed with moisture-impermeable caps to prevent premature skin-over in the cartridge neck.