Polydimethylsiloxane (PDMS)-based silicone elastomers are supplied as moisture-cure one-part RTV-1 sealants, two-part platinum-catalysed RTV-2 encapsulants, peroxide-catalysed high-consistency rubber (HCR), and liquid silicone rubber (LSR). The product is identified by grade family rather than a single model number because cure chemistry, mixed viscosity, and post-cure performance define processing and end-use selection. RTV-2 products are commonly mixed at 10:1 or 1:1 volume ratios with viscosities from 3 to 60 Pa·s; LSR grades are shipped as paired A/B components with mixed viscosity from 200 to 800 Pa·s for injection molding. Across commercial grades, Shore A hardness spans 5 to 80 under ASTM D2240, specific gravity spans 1.05 to 1.55 under ASTM D792, tensile strength ranges from 3.5 to 11 MPa, and elongation at break ranges from 100 to 700 percent under ASTM D412. The siloxane backbone gives a glass transition near −125 °C, but the low intermolecular forces reduce tear resistance relative to polyurethane and nitrile elastomers. Silicone also differs from organic rubbers in its high gas permeability, excellent ozone resistance, and retention of flexibility over a wider temperature interval.

What Limits Addition-Cure Work Life in High-Volume Liquid Injection Molding?

Platinum-catalysed LSR is processed on injection molding machines with heated platens at 110–180 °C; for wall sections between 2 and 5 mm, cure cycle times are typically 30–90 s. The principal production risk is cure inhibition. Sulfur-containing gloves, amine-based mold releases, natural rubber residues, certain PVC plasticizers, and contact with tin-catalysed condensation silicone can poison the hydrosilylation catalyst and leave a tacky, uncured surface even when the interior is crosslinked. Metering is performed by positive-displacement pumps through a static mixer; on a 100-tonne clamp machine, LSR cavity pressure remains lower than 35 MPa because the material flows in thin sections without the packing force required by thermoplastic melts. Mixed A/B work life at 25 °C is commonly 24–72 h, but at 40 °C the same material can gel within 4–8 h. Cold-runner blocks are therefore maintained below 60 °C upstream of the gate, while the mold surface is kept above 110 °C to initiate cure. When mold gaps exceed 0.02 mm, LSR flash can generate parted-line contamination that must be removed before the next cycle.

Process audits on liquid silicone molding lines identify three recurring causes of batch-to-batch variance: A/B mixing ratio drift from worn pump seals, moisture uptake in the filler resin, and storage of mixed material in warm plant environments. Mixing ratio drift of more than 2 percent often shifts tensile strength by 5–10 percent and changes cured hardness by 2–4 Shore A points. Because LSR has no solvent carrier, shrinkage during cure is low, but differential shrinkage between thin and thick sections can still produce internal stress if the cavity is packed too early. De-molding is normally performed at 60–80 °C part surface temperature to avoid reversion in confined sections above 200 °C.

Peroxide-cured high-consistency rubber is delivered as milled slab stock with Mooney viscosity ML 1+4 at 100 °C between 20 and 80 MU. It is mixed on two-roll mills at 20–30 °C to disperse fumed silica and to re-soften filler network after storage; storage beyond 6 months typically requires re-milling because stiffening changes flow into compression and transfer molds. Press cure is conventionally 170–180 °C for 5–10 min, followed by air post-cure at 200 °C for 4 h to remove volatile peroxide decomposition residues and to complete crosslinking. HCR is specified for seals that operate above 125 °C in dry air but should not be used in submerged hydrocarbon or ketone service until immersion testing under ASTM D471 confirms acceptable volume swell and property retention. Compared with addition-cure LSR, HCR has higher green strength and can be compression molded as thick blocks; the trade-off is longer cycle time and greater sensitivity to storage-induced hardening.

Compression Set Controls Seal Life in High-Temperature Enclosure Applications

Compression set measured by ASTM D395 Method B after 22 h at 177 °C is typically 15–40 percent for heat-stabilized silicones, while general-purpose EPDM grades often exceed 50 percent. This difference determines gasket performance in enclosures cycling between −40 °C and 150 °C. The low glass transition prevents low-temperature stiffening, and the siloxane network resists permanent deformation at high temperature. The design constraint is low tear strength. Sharp metal edges, installation radii below 0.5 mm, and flange gaps above 1 mm can initiate cuts because hot tear strength of silicone is lower than that of FKM or HNBR. Durometer is matched to flange flatness: 30–40 Shore A for rough castings and 60–70 Shore A for precision-machined faces. For applications with a processing window narrower than ±5 °C, such as cure reversion in confined sections above 200 °C, published data for long-term seal stability is limited and component-level validation is required.

Silicone is specified as a gasket material when compression set and broad temperature range matter more than tensile strength. Compared with PTFE, silicone is conformable and resilient at low torque, but it cannot match chemical inertness or low friction; PTFE exhibits service temperatures from −200 °C to 260 °C and tensile strength above 20 MPa, but its cold flow requires higher flange loads. Compared with EPDM, silicone has better high-temperature and ozone stability but lower tear resistance and higher gas permeability. Compared with polyurethane, silicone has better UV and ozone resistance but poorer abrasion resistance; polyurethane tensile strength can exceed 40 MPa, while general-purpose silicone typically remains below 11 MPa.

Typical comparative property ranges for elastomer families used in gaskets and seals are provided below. Values are obtained from published supplier data sheets using the listed methods.

Long-term hot-air aging of silicone rubber at 150 °C for 1000 h generally retains more than 70 percent of initial tensile strength and elongation when measured to ASTM D573. At 200 °C, retention depends on the stabilizer package and filler type; some iron oxide and cerium oxide stabilized compounds show acceptable elongation retention, while unstabilized grades may harden and lose flexibility within 500 h. The brittle point under ASTM D2137 remains below −60 °C for general-purpose grades, but phenyl-modified silicones extend low-temperature flexibility to −100 °C and below at the expense of higher viscosity and narrower processing windows.

When Silicone Encapsulants Replace Urethane Potting Compounds in Electronics

Addition-cure RTV-2 silicones are used as potting and conformal coatings where bond-line temperatures exceed 125 °C. Typical dielectric strength is 20–25 kV/mm under IEC 60243-1, volume resistivity is 1×10¹⁴ to 1×10¹⁶ Ω·cm under ASTM D257, and dissipation factor at 50 Hz is below 0.001. Compared with rigid polyurethane potting resins, silicones generate lower cure exotherm and lower stress on solder joints during thermal cycles between −40 °C and 125 °C. Lap shear strength of unfilled grades is generally below 2 MPa; structural bonding requires a silane primer or plasma treatment. Dispensing through 18–25 gauge needles at 2–6 bar is standard, but mixed pot life must be re-verified when ambient relative humidity exceeds 60 percent because moisture can destabilize the platinum catalyst. The product also has lower thermal conductivity than epoxy alternatives, typically 0.2–0.3 W/m·K, so high-power modules may require filled thermal grades or a thinner bond line.

Medical and food-contact silicone parts are specified as platinum-catalysed, post-cured grades to avoid peroxide residuals and condensation-cure by-products. Cytotoxicity is screened under ISO 10993-5, sensitization under ISO 10993-10, and systemic biocompatibility under USP <88> Class VI. Food-contact grades are tested under FDA 21 CFR 177.2600 or BfR Recommendation XV. Tubing formulated to 50–70 Shore A with tear strength above 30 kN/m under ASTM D624 is used in peristaltic pumps because the material recovers from repetitive compression. Silicone is not appropriate for continuous transfer of nonpolar solvents or silicone-swelling fluids unless immersion testing to ASTM D471 at the service temperature produces acceptable volume swell and property retention data.

Typical compliance matrix for silicone grades used in regulated applications includes the following test designations.

Surface-Free-Energy Constraints and Primer Chemistry in Bonded Assemblies

Unfilled silicone surfaces have low surface free energy, typically 22–24 mN/m, which limits direct adhesion to metals, glass, and engineering thermoplastics. Bonding processes use plasma, corona discharge, or organosilane primers applied at a dry film thickness below 5 µm before molding or bonding. In lap shear coupons prepared from 2 mm slabs adhered to aluminum with a silane primer, cohesive failure at 1–2 MPa under ASTM D1002 is typical, while untreated surfaces may fail adhesively below 0.1 MPa. The specific primer selection depends on substrate chemistry: methacryloxy and glycidoxy silanes are used on glass and metal, and amino silanes are used on some filled polyamides. Each additional primer in a medical device is assessed under ISO 10993-1 because the extractable profile changes after surface treatment. Manufacturers must also control humidity during primer application; moist conditions above 60 percent RH promote condensation of silanol groups and reduce bond reproducibility.

If Tubing Must Satisfy BfR and USP Simultaneously, Addition-Cured LSR Is the Usual Choice

Extruded and injection-molded LSR tubing is produced with wall-thickness tolerances near ±0.15 mm for diameters up to 12 mm and ±0.25 mm for larger sizes, measured according to ISO 3302-1. Pump trials on peristaltic platforms at 10–20 percent occlusion show that silicone tubing often maintains flow stability over 500 h continuous running, but performance depends on pump geometry, roller speed, and fluid backpressure. Compared with PVC, silicone contains no phthalate plasticizers and offers higher temperature tolerance; compared with thermoplastic elastomer tubing, silicone exhibits broader low-temperature flexibility but lower tensile strength and higher gas permeability. Oxygen and carbon dioxide permeation through silicone can be 10–20 times higher than through butyl rubber, which makes silicone appropriate for respiration circuits but unsuitable for long-term beverage gas-barrier lines.

Chemical resistance boundaries are determined by solubility parameter mismatch. General-purpose PDMS swells in nonpolar aliphatic and aromatic hydrocarbons, ketones, chlorinated solvents, and some ester-based hydraulic fluids; volume swell in mineral oil at 100 °C typically exceeds 20 percent. Silicone is resistant to hot water, steam, dilute alkali, salt solutions, and many polar organic compounds. For aggressive fluid seals, fluorosilicone or FKM should be evaluated rather than general-purpose PDMS. When condensation-cure RTV-1 is specified for electrical enclosures, acetic acid-releasing grades should not be used on copper traces or brass fittings; neutral oxime or alcohol-cure RTV-1 grades are selected for those substrates.