Silicone Rubber Types and Distinctions

The most practical way to classify silicone rubber is by its uncured physical form and curing method. From an industrial manufacturing perspective, the main categories are high-consistency silicone rubber, liquid silicone rubber and room-temperature-vulcanizing silicone.

These categories are not simply different hardness grades. They represent different material systems with different processing equipment, curing behavior and manufacturing economics.

They are often treated as equivalent terms, but they emphasize slightly different aspects of the material.

HCR describes the physical consistency of the uncured compound. It is a high-viscosity, gum-like rubber. HTV describes the curing condition because the material is vulcanized at elevated temperature.

In practice, many suppliers use HTV silicone rubber, HCR silicone rubber, solid silicone rubber and high-consistency silicone as overlapping commercial terms. When comparing materials, it is more useful to look at the actual formulation, cure system and processing method than to focus only on the acronym used by the supplier.

Liquid silicone rubber, commonly abbreviated as LSR, is very different from high-consistency silicone in its uncured state. It is normally supplied as two flowable components that are accurately metered, mixed and injected into a heated mold.

LSR is particularly well suited to automated manufacturing. Modern liquid injection molding systems can meter the two components continuously, inject the material rapidly and produce complex parts with short cycle times and limited manual handling.

Because the material flows easily before curing, LSR can fill thin sections and complex mold geometries that may be more difficult for high-consistency rubber. This makes it attractive for precision seals, medical products, electrical connectors and small molded components.

One-component RTV is supplied ready to use. It normally begins curing after exposure to moisture in the surrounding air. This makes it convenient for sealing joints, bonding components and applying silicone directly from cartridges or dispensing systems.

Two-component RTV requires the two parts to be mixed before use. Depending on the chemistry, it may use condensation curing or addition curing. Two-component systems are useful when curing needs to proceed through a larger volume rather than relying mainly on atmospheric moisture.

Silicone materials can also be distinguished by the chemistry used to create the final crosslinked rubber network. Two important systems are peroxide curing and platinum-catalyzed addition curing.

Peroxide curing has long been used for high-consistency silicone. Different peroxide systems can be selected according to molding temperature, product geometry and required properties. Depending on the formulation and application, post-curing may be used after vulcanization.

Platinum-catalyzed addition curing is widely associated with LSR but can also be used with high-consistency silicone compounds. These systems can offer clean curing behavior and are frequently used where tightly controlled processing or specific finished-product requirements are important.

This distinction is especially common when discussing RTV systems.

Condensation curing creates crosslinks through a chemical reaction that produces small molecular by-products. The exact chemistry depends on the formulation. These systems are common in many room-temperature sealants and mold-making silicones.

Addition curing creates the network through an addition reaction, commonly using platinum catalysis. Because there is no equivalent condensation by-product from the crosslinking reaction, addition-cure systems can be advantageous for applications requiring dimensional stability or clean curing behavior.

The correct choice depends on processing conditions, substrate compatibility, required cure speed and final-product performance.

General-purpose silicone compounds are designed to provide a practical balance of hardness, tensile strength, elongation, tear resistance, processability and cost. They are widely used for seals, molded components, profiles, tubing and other industrial products that do not require highly specialized electrical, flame-retardant or thermal performance.

These grades are often available across a broad Shore A hardness range and in different colors. For a straightforward molded component, a general-purpose grade can be more economical than specifying a specialized formulation with properties that the finished part does not actually need.

High-temperature grades are formulated to retain useful mechanical properties during prolonged thermal exposure. This is different from simply measuring the highest temperature a material can survive for a few minutes.

Long-term heat can change hardness, elongation and tensile strength. A suitable heat-resistant formulation therefore needs to slow these changes so that the rubber remains functional for the required service life.

These materials are commonly used around industrial equipment, electrical systems, automotive components, ovens, heating equipment and other environments where conventional elastomers may age too quickly.

Electrical silicone compounds are formulated for applications where dielectric behavior, surface performance and environmental aging are important. They may be used in composite insulators, surge arresters, cable accessories, high-voltage connectors and other electrical products.

In these applications, ordinary mechanical properties are not enough. The material may also need high volume resistivity, suitable dielectric strength, stable hydrophobic behavior and resistance to tracking and erosion.

Outdoor high-voltage insulation creates an especially demanding combination of stresses because moisture, pollution, ultraviolet radiation, corona, leakage current and temperature changes can occur together.

Flame-retardant silicone uses specialized filler and additive systems to improve burning behavior. These formulations can be useful in electrical, transportation, cable and equipment applications where fire performance is part of the product specification.

Flame retardancy must be balanced against mechanical properties and processability. Adding functional fillers can change hardness, elongation, tear strength and viscosity, so the formulation should be evaluated as a complete system rather than by a flame rating alone.

Food-contact silicone is formulated and manufactured with additional attention to raw-material selection, cleanliness and documentation for the intended market.

Depending on where the finished product is sold, buyers may request supporting information associated with FDA 21 CFR requirements, LFGB testing, BfR Recommendation XV or relevant European chemical requirements.

These terms should not automatically be treated as one universal certification. They represent different regulatory frameworks, recommendations, test approaches and compliance requirements.

Food-contact materials are commonly used for kitchenware, seals, tubing, processing components and other products that may contact food during use.

Medical silicone is another specialized category in which material purity, traceability, manufacturing control and supporting documentation may become more important than in ordinary industrial applications.

LSR is widely used for precision medical components because of its suitability for automated molding, but high-consistency silicone is also used in healthcare products. The appropriate material depends on the device, manufacturing process and type of contact with the patient.

A silicone material should never be considered suitable for a medical application based only on the words “medical grade.” The exact formulation, intended use and applicable testing or regulatory requirements need to be reviewed for the finished device.

Fluorosilicone is a specialized silicone elastomer in which fluorinated organic groups are incorporated into the polymer structure. It retains many of the useful temperature and flexibility characteristics associated with silicone while improving resistance to certain fuels, oils and chemicals.

This makes fluorosilicone valuable in aerospace, automotive and fuel-system environments where standard silicone rubber may swell excessively in contact with hydrocarbons.

The trade-off is that fluorosilicone is generally more specialized and more expensive than standard silicone compounds. It should be selected because the application requires its chemical resistance rather than simply because it is considered a higher-performance material.

Standard silicone rubber is normally electrically insulating. However, silicone compounds can be made conductive or semiconductive by incorporating suitable conductive fillers.

These materials are used where controlled electrical conductivity, antistatic behavior, electromagnetic shielding or electrical-field management is required.

This illustrates an important point about silicone classification: the base polymer may remain similar, but changes to the filler system can transform the functional behavior of the finished compound.

Silicone sponge and silicone foam contain a cellular structure rather than being fully dense elastomers. The cells reduce density and change compression behavior, making these materials useful for cushioning, thermal insulation and low-force sealing.

Closed-cell structures are often preferred where moisture resistance and sealing are important, while open-cell structures can be selected for applications where airflow or different compression characteristics are required.

These materials should not be confused with ordinary solid silicone rubber even when the chemical base is similar. Their mechanical response is strongly influenced by the cellular structure.

Two silicone materials can both be 60 Shore A and still behave very differently. One may be optimized for extrusion, another for compression molding and another for high-voltage insulation.

Hardness measures resistance to indentation, but it does not directly describe tear strength, elongation, compression set, dielectric behavior, heat aging or processing viscosity.

For this reason, buyers should avoid selecting a silicone grade only by color and hardness. The material type, cure system, formulation and end-use environment should be reviewed together.

The siloxane polymer provides the foundation of silicone rubber, but the finished compound also contains reinforcing fillers, processing aids, pigments, functional additives and a curing system.

Changing these ingredients can significantly change material performance. Reinforcing silica affects mechanical strength and viscosity. Heat stabilizers influence long-term thermal aging. Electrical fillers can affect tracking and erosion performance. Conductive fillers can transform an insulating elastomer into a conductive material.

This means the distinction between silicone grades often comes from the complete formulation rather than from the silicone polymer alone.