Silicone rubber is an elastomer built on a repeating siloxane (Si-O-Si) backbone with methyl, vinyl, phenyl, or trifluoropropyl groups pendant to the silicon atoms; the Si-O bond dissociation energy of 433 kJ/mol versus 355 kJ/mol for a C-C bond is the structural reason behind its 300°C upper service limit and useful elasticity down to roughly -55°C [S2][S4].
Polydimethylsiloxane (PDMS) is the largest subset of polysiloxanes in commercial use, and it is the chemistry that almost every HTV, LSR, and RTV grade is built from; grades are differentiated by side-group chemistry, filler system (typically high-purity fumed silica), and cure package rather than by a new backbone [S1][S2].
Backbone, Bond Energy, and Why the Numbers Look the Way They Do
The siloxane backbone gives the polymer an inorganic Si-O-Si repeat unit with side methyl groups; the methyls sit on the outside of a helical coil, so intermolecular force stays low and the chains can rotate freely, which is the structural reason behind the elastomer's high compressibility and cold flexibility [S4]. Comparing bond energies, Si-O at 433 kJ/mol sits well above C-C at 355 kJ/mol, and that delta is the underlying reason silicone rubber holds elongation at 150°C where chloroprene rubber deteriorates visibly between 150°C and 250°C in the JIS K 6261 Section 5 ageing test data shown in Shin-Etsu's compound catalogue [S4].
Because the chain is inorganic, the polymer is non-reactive, stable, and resistant to extreme environments, with the widely cited operating envelope of -55°C to 300°C (-70°F to 570°F) maintained without losing useful properties, and that same chemistry drives the water repellency, low surface energy, and physiological inertness that show up in food-contact, medical, and outdoor HV insulator applications [S2][S4]. Silicone is technically a polysiloxane, not a true "silicone" (a misnomer based on an early incorrect assumption about double-bonded oxygen in the backbone), and this naming distinction matters when reading ISO or FDA documents that reference PDMS or dimethyl polysiloxane by their correct chemical name [S2].
How Silicone Rubber Is Built: Polymer, Filler, Additives
Uncured silicone rubber is a highly adhesive gel or liquid composed of linear silicone polymers of comparatively low molecular weight, reinforced with high-purity silica (typically fumed) and combined with crosslinkers, cure catalysts, pigments, and stabilizers; one- or two-part systems are common, and the compound is shaped then post-cured [S2][S3]. For a fuller reference on the elastomer family see the silicone rubber entry, and for downstream high-temperature parts the same chemistry is often specified into the PEEK comparison set when seals run next to PEEK components in fluid handling.
Fillers carry most of the mechanical property tuning: fumed silica raises tensile and tear strength, extending grades add heat stability, and conductive fillers (carbon black, silver-coated fillers) make the polymer electrically conductive for EMI gaskets and fuel-cell plate seals; specialty additives tune flame retardancy, thermal conductivity, and radiation resistance on top of the base gum [S4]. Curing converts the gum into a crosslinked network and is the axis along which the three commercial types (HTV/HCR, LSR, RTV) are defined, since the gum and filler system are broadly similar across the family [S1][S3].
The Three Main Types: HTV (HCR), LSR, and RTV
High Temperature Vulcanizing silicone rubber (HTV, also called HCR, High Consistency Rubber) is supplied as a millable gum with very high molecular weight; it is shaped by compression molding, extrusion, or calendering, and is most often cured with organic peroxides (2,4-dichlorobenzoyl peroxide, dicumyl peroxide) at roughly 150-180°C, which gives the highest mechanical strength of the three types and is the default for HV insulator sheds, automotive turbo hoses, and large molded gaskets [S1][S2].
Liquid Silicone Rubber (LSR) is a two-part, pumpable grade with lower molecular weight, shorter chains, and lower viscosity that flows under injection pressure; it uses a platinum-catalyzed addition cure where a hydride-functional siloxane and a vinyl-functional siloxane react at the platinum complex to form an ethyl bridge with no byproducts, which is the reason LSR is favored for medical and baby-care parts [S1][S2]. Room Temperature Vulcanizing silicone rubber (RTV) cures at ambient temperature by a condensation mechanism, with one-part systems (RTV-1) crosslinking on exposure to atmospheric humidity and two-part systems (RTV-2) crosslinking on mixing of silanol polymer with a crosslinker; common crosslinkers are alkoxy, acetoxy, ester, enoxy, or oxime silanes such as methyl trimethoxy silane (alkoxy) and methyl triacetoxysilane (acetoxy) [S2].
Beyond these three, functional specialty grades include fluorosilicone (FVMQ) with trifluoropropyl side groups for fuel and solvent resistance, and phenyl-silicone (PVMQ) with phenyl substitution for very low temperature flexibility down toward -100°C; both keep the same Si-O-Si backbone and differ only in the side group [S2].
Cure System Comparison: Peroxide, Platinum Addition, Condensation
Selection is driven by cure chemistry first, then viscosity and mechanical targets; the three principal systems and their decision criteria are compared below using only the public chemistry facts in the research [S1][S2].
Peroxide cure (typical for HTV/HCR) handles high-viscosity gums in compression and transfer molds, leaves a peroxide residue that usually needs a post-cure to remove volatiles, and is the workhorse for thick-section industrial rubber parts where platinum catalyst cost cannot be justified. Platinum addition cure (LSR) gives clean parts with no cure byproducts, fast cycle times in injection molding, and is required for medical and food-contact parts, but the reaction is easily inhibited by elemental tin, sulfur, and many amine compounds that can poison the catalyst, so mold and feedstock hygiene are critical. Condensation cure (RTV) is the only system that reliably cures at room temperature without external heat, which makes it the right pick for field-applied sealants, potting, and in-place gasketing on assembled equipment, with the acetoxy variant giving acetic acid as a byproduct (the classic vinegar-smelling bathroom sealant) while alkoxy and oxime variants release alcohol or oxime instead [S1][S2].
Property Envelope: Heat, Cold, Flame, and Electrical
Heat resistance: the Si-O backbone plus 433 kJ/mol bond energy lets general-purpose silicone rubber run indefinitely at 150°C with almost no change in properties, and specialty grades push the upper continuous service limit toward 250-300°C, while chloroprene rubber deteriorates and discolours between 150°C and 250°C in the same test [S2][S4]. Cold resistance: standard dimethyl grades remain flexible to roughly -55°C, and low-temperature grades formulated with phenyl substitution or modified backbone geometry reach into the -100°C range, so silicone is the default rubber for aerospace and polar-cable jackets where EPDM and natural rubber go glassy [S2][S4].
Flame retardancy: silicones are widely credited with better fire resistance than most carbon-based polymers because the inorganic backbone chars rather than combusts readily, and the material passes common UL 94 V-0 and equivalent tests when formulated with minimal or zero halogen additives; this is a key reason silicone cable insulation is specified in mass-transit, nuclear, and offshore projects [S5]. Electrical insulation: the Si-O-Si backbone is highly stable and the polymer is non-conductive, which makes silicone the standard for HV insulator sheds on transmission lines and for flexible heating-element mats rated to several kV; the same inertness supports the medical, food-contact, and potable-water applications listed in the silicone product family [S2][S4].
Selection Criteria and Where Each Type Fits
Pick HTV/HCR when you need high mechanical strength, thick sections, extrusion profiles, and the lowest cost per kilogram on long production runs; pick LSR when you need flashless injection molding, tight tolerances on small medical or consumer parts, and zero byproduct; pick RTV when you need in-place gasketing, field repair, potting of electronics, or ambient-temperature cure on substrates that cannot be heated [S1][S2]. Avoid standard silicone in dynamic, hydrocarbon-soaked service unless you switch to fluorosilicone (FVMQ), because dimethyl grades swell in non-polar fuels and oils; also avoid peroxide-cured HTV for optical or food-contact parts unless a post-cure removes residual peroxide breakdown products, and avoid platinum-cured LSR if the tool or feedstock has even trace sulfur, tin, or amine contamination, since the cure will be inhibited and parts will be undercured [S1][S2].
For engineers cross-shopping silicone against PEEK, polyimide, or PTFE on a high-temperature, high-purity fluid handling job, the practical decision comes down to sealing versus structural duty: silicone is the elastomeric seal while PEEK and PTFE are the rigid seats and back-ups; for insulation versus dielectric duty in HV gear, silicone rubber provides the resilient weather shed while epoxy and porcelain handle the rigid body, and the silicone family spans gasket, insulator, and medical implant parts across a single chemistry platform [S2][S4]. Related industrial news coverage on the broader polymers and composites supply chain sits in the polymeric FR replaces HBCD in EPS and XPS foam note, which documents the same inorganic-flame-retardant logic that governs silicone compound selection.
Failure Modes, Standards, and Sourcing
Known failure modes include reversion (depolymerization back to cyclic siloxanes) in confined, high-temperature steam above roughly 200°C, inhibition of platinum cure by sulfur/tin/amine contamination, and outgassing of cyclic siloxanes in vacuum or space-grade applications unless a post-cure has been specified and validated [S1][S2]. For sealing duty in steam, specify post-cured peroxide-cured HTV or platinum-cured LSR with controlled cyclic content; for food contact, reference FDA 21 CFR 177.2600 and BfR XV silicone listings; for medical implants, reference USP Class VI and ISO 10993 biocompatibility on the specific grade; for electrical insulation, reference ASTM D1418 (the VMQ/FVMQ designation system) and ASTM D2000 line call-outs for rubber properties.
For sourcing, the global silicone rubber supply base remains concentrated among a handful of compounders and raw gum producers (Shin-Etsu, Dow, Wacker, Elkem, Momentive, KCC, along with a long tail of regional compounders), and the technology is stable: HCR for extruded and molded mechanical parts, LSR for high-purity injection-molded parts, RTV for sealants and field-applied gasketing, with fluorosilicone and phenyl-silicone specialty grades covering the extreme-cold and fuel-resistance niches [S1][S3][S4]. Engineers specifying a new silicone component should pin gum type (VMQ, FVMQ, PVMQ), cure system, hardness (Shore A), tensile/tear targets, and a test method from ASTM D1418 / D2000 / D2240 rather than relying on generic "silicone" labels, since the same chemistry name covers a 5-Shore-A gel and a 70-Shore-A structural rubber depending on filler loading and cure package [S2][S4].
The underlying component specifications are covered under dry type transformer.