Difference Between Addition-Cure and Condensation-Cure Liquid Silicone Rubber
Liquid silicone rubber (LSR) is a versatile material used in numerous industries due to its excellent thermal stability, flexibility, and chemical resistance. There are two primary types of LSR: addition-cure and condensation-cure. While both are silicones, their curing mechanisms, properties, and applications differ significantly. Understanding these differences is crucial for selecting the right material for a specific application.
1. Curing Mechanism
Addition-Cure LSR:
This type cures via a platinum-catalyzed addition reaction between vinyl-functionalized silicone polymers and silicon-hydrogen (Si-H) crosslinkers. The process requires heat (typically 100°C–150°C) and produces no by-products. The reaction is efficient and precise, making it ideal for high-volume manufacturing.
Condensation-Cure LSR:
This type cures at room temperature (RTV) through a condensation reaction, often involving moisture in the air. The reaction releases by-products such as alcohol, acetic acid, or ketones. It does not require heat, making it suitable for applications where heating is impractical.
2. Key Properties Comparison
| Property | Addition-Cure LSR | Condensation-Cure LSR |
|---|---|---|
| Curing Temperature | Elevated temperatures | Room temperature |
| By-Products | None | Alcohol, acetic acid, etc. |
| Cure Speed | Fast (seconds to minutes) | Slow (hours to days) |
| Shrinkage | Low | Moderate to high |
| Substrate Adhesion | Poor (requires priming) | Excellent (self-adhesive) |
| Chemical Purity | High (medical/food grade) | Lower (by-product traces) |
| Heat Resistance | Excellent (up to 200°C+) | Good (up to 150°C–180°C) |
| Depth of Cure | Unlimited | Limited (surface cure) |
3. Applications
Addition-Cure LSR:
Medical Devices: Syringe pistons, implants, respiratory masks.
Automotive: Seals, gaskets, ignition cables.
Consumer Goods: Baby products, kitchenware.
Electronics: Encapsulants, keypads.
Condensation-Cure LSR:
Electronics Potting: Protecting PCBs and sensors.
Construction Sealing: Window and joint seals.
Mold Making: Prototyping and artistic molds.
General Repairs: Adhesives and sealants for DIY use.
4. Advantages and Limitations
Addition-Cure LSR:
Advantages: High purity, fast curing, excellent thermal/chemical resistance.
Limitations: Sensitive to catalyst inhibitors (sulfur, amines), requires heat.
Condensation-Cure LSR:
Advantages: Room-temperature curing, strong adhesion, cost-effective.
Limitations: Slower curing, by-product release, lower heat resistance.
5. Material Handling and Processing
Addition-Cure LSR:
Requires precise metering and mixing (typically 1:1 ratio) and is processed using automated liquid injection molding (LIM) systems. Contamination must be avoided to prevent catalyst inhibition.
Condensation-Cure LSR:
Often supplied in single-part (moisture-activated) or two-part (manually mixed) formulations. It is applied via dispensing guns, brushes, or spraying and cures upon exposure to ambient moisture.
Conclusion
The choice between addition-cure and condensation-cure LSR depends on the application requirements. Addition-cure LSR is preferred for high-precision, high-volume, and high-temperature applications where purity and performance are critical. Condensation-cure LSR is chosen for its convenience, adhesion, and room-temperature curing, making it ideal for sealing, prototyping, and repairs. Understanding these differences ensures optimal material selection for achieving desired performance and efficiency in manufacturing and design.