Is Silicone Rubber Impression Material Made of Liquid Silicone Rubber?
Introduction
Silicone rubber impression materials are widely used in dentistry for capturing precise impressions of teeth and oral tissues. However, a common question arises: Are these materials made of liquid silicone rubber (LSR)? The answer is no-while both belong to the silicone family, dental silicone impression materials are specially formulated pastes, distinct from industrial LSR in composition, handling, and application.
1. Composition and Form: Dental vs. Industrial Silicones
A. Dental Silicone Impression Materials
These materials are supplied as two-part paste systems (base + catalyst) and are categorized into two types:
Condensation-Cure Silicones:
Base: Hydroxy-terminated polydimethylsiloxane (PDMS) + silica fillers.
Catalyst: Crosslinker (e.g., tetraethyl orthosilicate) + tin catalyst.
Byproduct: Ethanol (causes slight shrinkage over time).
Addition-Cure Silicones (Polyvinyl Siloxane, PVS):
Base: Vinyl-terminated PDMS + platinum catalyst.
Catalyst: Hydrogen-containing siloxane (crosslinker).
Advantage: No byproduct, minimal shrinkage (<0.1%), high accuracy.
Both types are high-viscosity pastes (available in light-, medium-, and heavy-body consistencies) optimized for dental workflows.
B. Industrial Liquid Silicone Rubber (LSR)
Form: Low-viscosity, injectable liquid (typically two-part LSR).
Curing: Platinum-catalyzed addition curing (similar to PVS).
Applications: Mold making, electronics, automotive parts.
Unsuitable for Dentistry:
Too fluid for intraoral use (lacks controlled viscosity).
Not optimized for biocompatibility or tear strength.
2. Key Differences Between Dental and Industrial Silicones
| Property | Dental Silicone Impression Material | Industrial LSR |
|---|---|---|
| Physical Form | Paste (light/heavy-body) | Liquid (low viscosity) |
| Curing Mechanism | Condensation or addition | Primarily addition-cure |
| Shrinkage | 0.1–0.5% (condensation) / <0.1% (PVS) | ~0.1% (varies by formulation) |
| Biocompatibility | ISO 10993-certified | Not always medical-grade |
| Tear Strength | High (prevents distortion) | Lower (designed for molding) |
| Working Time | 2–6 minutes (adjustable) | Seconds to minutes (machine-dependent) |
3. Why Dental Impressions Aren't Made with LSR?
Controlled Viscosity: Pastes allow easy mixing and adaptation to tooth anatomy.
Extended Working Time: LSR cures too quickly for clinical manipulation.
Dimensional Stability: Dental silicones minimize shrinkage for accuracy.
Biocompatibility: Industrial LSR may contain non-medical additives.
4. Exceptions: "Liquid-Like" Dental Silicones
Some light-body wash materials (e.g., for syringe dispensing) have low viscosity but are still dental-grade formulations, not industrial LSR.
Conclusion
Dental silicone impression materials are not liquid silicone rubber but specialized pastes engineered for precision, biocompatibility, and ease of use. While both share silicone chemistry, their formulations and purposes differ significantly. For dental applications, always use ISO 4823-compliant silicones to ensure optimal results.