HTV Silicone Rubber (Heat-Vulcanized)

What Is HTV Silicone Rubber?

HTV (High-Temperature Vulcanization) silicone rubber — also called HCR (High-Consistency Rubber) or mill-mixed silicone — is the solid form of silicone elastomer supplied as a gum compound in slab or strip format. Unlike LSR, which flows as a liquid before cure, HTV has a putty-like consistency that must be milled, calendered, or extruded before compression molding or transfer molding into final parts.

HTV is cured at elevated temperatures (typically 150–200 °C) using either peroxide free-radical initiators or platinum addition catalysts. The choice of cure system has significant implications for extractable profiles, biocompatibility, and post-cure requirements.

HTV Formulation and Cure Chemistry

Base polymer: HTV compounds are built on high-molecular-weight polydimethylsiloxane (PDMS) gum, reinforced with fumed silica or precipitated silica to achieve the desired tensile strength (5–12 MPa) and tear resistance. The polymer molecular weight (typically 500,000–800,000 g/mol) governs processability and green strength.

Peroxide cure: Organic peroxides (e.g., dicumyl peroxide, 2,4-dichlorobenzoyl peroxide) generate free radicals that cross-link polymer chains. Peroxide-cured HTV is cost-effective for industrial applications but leaves peroxide decomposition products that must be removed by post-cure (typically 4 hours at 200 °C in a circulating oven) for food-contact and medical applications.

Platinum cure: Platinum-catalyzed hydrosilylation offers cleaner extractable profiles with no peroxide residues, enabling easier compliance with FDA 21 CFR and food-contact regulations. Platinum-cured HTV costs more but is preferred wherever purity matters.

Processing Methods

Compression molding: The most common HTV processing method. Compound is cut to weight, placed in a heated mold cavity, and pressed at 150–180 °C for 3–10 minutes. Suitable for gaskets, O-rings, seals, and flat membranes.

Extrusion: HTV gum is extruded through a die at room temperature and vulcanized in a continuous salt bath (LCM), hot-air oven, or microwave system. Produces tubing, profiles, cable jackets, and cord stock in continuous lengths.

Calendering: Thin sheets and coated fabrics are produced by passing the compound through heated calendar rolls. Used for diaphragm membranes and silicone-coated glass fiber fabric.

Transfer molding and injection: Less common than for LSR but used for precision parts where flash control is important.

Typical Properties and Grades

HTV Shore hardness spans 20A to 80A. Standard industrial grades target Shore 50A–70A for general sealing applications. Medical and food-contact grades are platinum-cured with post-cure treatment. Flame-retardant grades incorporate alumina trihydrate (ATH) or platinum-based flame inhibitors for cable and aerospace applications. Conductive grades use carbon black loading (typically 30–40 phr) to achieve surface resistivity of 10²–10⁵ Ω/sq for keypad membranes and anti-static applications.

HTV vs LSR: When to Choose HTV

Choose HTV when: (1) processing by extrusion or calendering is required (continuous profiles, cable jacketing); (2) Shore hardness above 70A is needed; (3) the application does not require the complex undercuts achievable by injection molding; (4) tooling cost must be minimized. Choose LSR when: injection molding of complex geometries at high volume is needed, or when the highest purity platinum cure is essential.

Certifications and Compliance

Food-grade HTV: FDA 21 CFR 177.2600 (platinum-cured, post-cured grades). LFGB §30/§31 (Germany). Medical HTV: USP Class VI (systemic, intracutaneous, implantation tests). RoHS: no restricted substances in base polymer; pigments and additives verified separately.

Contact us to verify certification documents and request samples. Inquiries are routed to specialist suppliers in the silicon materials network.