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Silicone Materials in Rubber Goods Manufacturing

The rubber goods industry covers a broad range of finished products — seals and O-rings, hoses and tubing, gaskets, vibration dampers, bellows, diaphragms, and consumer goods such as grips, footwear soles, and kitchenware. Silicone rubber occupies a defined segment of this market: where temperature resistance, biocompatibility, electrical insulation, UV stability, or specific regulatory compliance (food contact, medical) justify its 3–5x cost premium over organic rubber (EPDM, NBR, CR, SBR).

Silicone is not a commodity rubber — it is a performance material. Procurement requires selecting the correct polymer type (HTV, LSR, or RTV-2), specifying the reinforcing filler system, and in many cases qualifying silane coupling agents to achieve target compound properties.

HTV Silicone Rubber: Compounds and Processing

High-temperature vulcanization (HTV) silicone rubber is the dominant form for molded and extruded silicone goods. It is supplied as a high-viscosity gum compound, processed on standard rubber equipment (two-roll mills, Banbury mixers, extruders) and cured by peroxide crosslinking at 150–200 °C.

ZEMSIL SB-HTV is supplied as unfilled polymer gum. Compounders add:

Fumed silica (FS-200, FS-300) at 30–50 phr as reinforcing filler
Peroxide curing agent (2,4-dichlorobenzoyl peroxide for extrusions; dicumyl peroxide or DBPH for molded parts)
Silane coupling agent (CS-550 or CS-560 for surface-treated silica, or CS-570/CS-580 for peroxide-compatible systems)
Specialty additives: iron oxide (heat stabilizer), carbon black (conductive grade), metal oxide flame retardants, pigments

Typical mechanical properties of FS-200 reinforced SB-HTV compound after cure and post-cure:

Tensile strength: 8–12 MPa
Elongation at break: 300–600%
Shore A hardness: 30–80 (adjusted by silica loading and polymer MW)
Compression set (22 h at 175 °C): <25%

Secondary post-cure: peroxide-cured HTV parts must be post-cured (typically 4 hours at 200 °C) to remove peroxide decomposition byproducts (cumyl alcohol, acetophenone), which otherwise outgas in service and cause odor or certifications failures in food or medical applications.

Precipitated Silica as Reinforcing Filler

Fumed silica (FS series) is preferred in premium silicone rubber for its superior reinforcement efficiency at lower loading. However, precipitated silica (PS series) is used where cost reduction is prioritized, particularly in:

General industrial seals without high-transparency requirements
Compression molded gaskets with Shore A >60
Silicone sponge and foam compounds (precipitated silica foams more evenly)

Property	Fumed Silica (FS-200)	Precipitated Silica (PS-160)
BET surface area	~200 m²/g	~160 m²/g
Primary particle size	10–20 nm	15–30 nm
Reinforcement efficiency	High	Moderate
Transparency contribution	High (low refractive index)	Moderate
Cost	Higher	Lower
Typical loading in silicone	30–45 phr	40–60 phr

For optimum performance with precipitated silica, PS-200 or PS-230 (higher surface area grades) deliver reinforcement closer to fumed silica than standard PS-120/PS-160, at intermediate cost.

Silane Coupling Agents: Si-69 Equivalent and Surface Treatment

In silicone rubber, silane coupling agents serve two roles:

Silica surface treatment (structural reinforcement): untreated silica forms strong silanol–silanol hydrogen bonds with the PDMS polymer chains (the "silica–polymer interaction"), causing stiffening on aging (crepe hardening). Treating with hexamethyldisilazane (HMDS) or short-chain silanes such as CS-151 (methyltrimethoxysilane) passivates silanol groups, reducing crepe hardening and improving processability without significantly reducing reinforcement.

Bifunctional silane for organic rubber compounds: in carbon-black or silica-filled organic rubber (SBR, EPDM, NR), bis-(triethoxysilylpropyl)tetrasulfide (TESPT, commonly known as Si-69) and bis-(triethoxysilylpropyl)disulfide (TESPD, Si-75) are the standard coupling agents for silica-reinforced tire tread compounds. ZEMSIL CS-910 (3-mercaptopropyltrimethoxysilane) provides similar silane–rubber coupling functionality for non-tire rubber goods applications where mercapto-functional silane is preferred.

Liquid Silicone Rubber (LSR) Processing

LSR (ZEMSIL SB-LSR) is a two-component platinum-cured system with viscosity 1,000–200,000 mPa·s, injection-moldable at room temperature and cured at 150–180 °C in seconds to minutes. LSR enables:

Complex thin-wall geometries (membrane switches, valve diaphragms, infant feeding products)
Overmolding onto plastic substrates (co-injection with PA, PC, ABS)
High-volume automation with no post-cure required (platinum-cured LSR does not leave volatile byproducts)

Critical LSR processing concern: platinum catalyst is poisoned by sulfur, tin, amine, and phosphorous compounds. Mold release agents and mold cleaning materials must be platinum-compatible — silicone-based mold release (DM-series silicone oil emulsion or SE-DM emulsion) is the safe choice.

Mold Release in Rubber Goods Manufacturing

Silicone rubber itself requires mold release during HTV compression and transfer molding because fresh silicone compound is inherently tacky and bonds to steel molds. Mold release agents:

SE-DM silicone emulsion: water-based mold release for semipermanent application; suitable for food-contact certified molds
DM-50 to DM-350 PDMS diluted in solvent or water: applied by spray, wipe, or dip; dilution ratio determines film thickness and release performance
Pure PDMS on organic rubber molds: standard industrial rubber compression molds use PDMS silicone spray as release — this is the largest volume mold-release application in the rubber goods sector

Avoid over-application: excess silicone mold release on rubber goods causes adhesion problems in bonded rubber-metal assemblies.