Classification of Liquid Silicone Rubber (LSR)
Liquid Silicone Rubber (LSR) can be classified based on its chemical composition, curing method, special functionalities, and application fields. Below are the main classification methods and specific categories of LSR:
1. Classification by Curing (Vulcanization) System
This is the most common classification method, categorized based on curing reaction mechanisms and catalyst types:
(1) Addition-Cured Type (Platinum-Catalyzed)
Characteristics: Cures through a hydrosilylation reaction (vinyl reacts with Si-H bond), producing no byproducts, making it environmentally friendly and efficient.
Advantages: Fast curing speed, high transparency, low shrinkage (<0.1%), and excellent biocompatibility.
Applications: Medical devices, baby products, food-contact materials, and other high-end applications.
Subtypes:
Standard Addition-Cured Type: Uses conventional platinum catalysts.
Delayed Curing Type: Contains inhibitors to extend working time.
High-Temperature Curing Type: Can withstand temperatures above 250°C.
(2) Condensation-Cured Type
Characteristics: Cures through condensation reactions (e.g., de-alcoholization, de-oximation), producing small molecular byproducts such as ethanol or acetone.
Advantages: Strong adhesion to substrates, does not require complex equipment, and can cure at room temperature.
Disadvantages: Higher shrinkage (1–3%) and lower heat resistance (generally <150°C).
Subtypes:
De-Alcoholization Type: Byproduct is alcohol, which is low in toxicity.
De-Oximation Type: Fast curing speed, but the byproduct has an odor.
De-Acetic Acid Type: Byproduct is acetic acid, which is highly corrosive and less commonly used.
(3) Peroxide-Cured Type
Characteristics: Cures through free radical reactions and requires high temperature and pressure.
Advantages: Low cost, suitable for large-size products.
Disadvantages: Byproducts may affect purity, making it unsuitable for medical applications.
Applications: Industrial seals, cable insulation layers, etc.
2. Classification by Fillers and Modifications
Based on the added fillers or functional additives, LSR can be categorized into:
(1) Standard LSR
Basic formulation containing only silica as a reinforcing filler, used for general sealing and insulation.
(2) High-Transparency LSR
Contains high-purity nano-silica, achieving >90% light transmittance.
Applications: Optical components, light guides, etc.
(3) Conductive/Anti-Static LSR
Incorporates carbon black, carbon nanotubes, or metal powders to adjust surface resistance (10³–10⁹Ω).
Applications: Electromagnetic shielding, flexible electrodes.
(4) High-Thermal Conductivity LSR
Contains fillers such as boron nitride or aluminum oxide, achieving thermal conductivity of 1–5 W/(m·K).
Applications: Electronic thermal pads, LED encapsulation.
(5) Flame-Retardant LSR
Incorporates aluminum hydroxide or platinum-based flame retardants to meet UL94 V-0 standards.
Applications: Battery seals in electric vehicles.
(6) Biomedical LSR
Purified to meet ISO 10993 biocompatibility standards.
Subtypes:
Implant-Grade: Used for pacemakers, artificial cartilage, etc.
Non-Implant Grade: Used for respiratory masks, catheters, etc.
3. Classification by Viscosity (Processing Performance)
Classified based on viscosity range during injection molding:
| Type | Viscosity (mPa·s) | Characteristics | Applications |
|---|---|---|---|
| Low Viscosity | 1,000–5,000 | High flowability, suitable for microstructures | Precision electronic encapsulation |
| Medium Viscosity | 5,000–20,000 | Balanced flowability and strength | General seals |
| High Viscosity | >20,000 | Anti-sagging, suitable for vertical molding | Large industrial parts |
4. Classification by Special Functions
(1) Self-Lubricating LSR
Contains PTFE or silicone oil microcapsules, reducing friction coefficient by over 50%.
Application: Dynamic seals.
(2) Self-Healing LSR
Incorporates dynamic covalent bonds, allowing it to self-repair when damaged, extending lifespan.
(3) Thermochromic LSR
Contains thermochromic pigments that change color with temperature variations, used for safety indicators.
(4) Foamed LSR
Chemically or physically foamed, reducing density by 30%–70%.
Application: Shock-absorbing materials.
5. Classification by Application Fields
| Field | Typical Requirements | Representative Products |
|---|---|---|
| Medical | Biocompatibility, sterilization stability | Artificial heart valves, IV tubing |
| Automotive | High-temperature resistance, flame retardancy | Battery seals, wire harness sheaths |
| Electronics | Insulation, thermal conductivity | 5G antenna covers, chip encapsulation |
| Consumer Goods | Safety, comfortable touch | Baby pacifiers, smartwatch bands |
| Industrial | Weather resistance, wear resistance | Solar panel sealants |
Summary
The classification system of liquid silicone rubber is complex and overlapping. In practical applications, curing method + functional requirements are often combined for selection. Examples:
Medical implants: Platinum-catalyzed, high-purity addition-cured LSR.
Electric vehicle battery seals: Flame-retardant, high thermal conductivity addition-cured LSR.
Low-cost industrial parts: Condensation-cured or peroxide-cured LSR.
With technological advancements, more specialized LSR types are likely to emerge in the future (e.g., biodegradable LSR, photoresponsive LSR, etc.).