The Limitations and Drawbacks of One-Component Liquid Silicone Rubber (1C-LSR)
Abstract: One-component liquid silicone rubber (1C-LSR or RTV-1) is renowned for its ease of use, excellent flexibility, and high-temperature resistance. However, despite its widespread adoption across industries, a thorough understanding of its inherent limitations is crucial for engineers and designers to make informed material selection decisions. This article provides a detailed examination of the key drawbacks associated with 1C-LSR.
Introduction
One-component liquid silicone rubber cures upon exposure to atmospheric moisture at room temperature, making it incredibly convenient for sealing, bonding, and encapsulation applications. Its properties, such as weatherability, electrical insulation, and biocompatibility, are highly valued. Nonetheless, several technical and practical challenges can limit its effectiveness or render it unsuitable for certain projects.
Key Disadvantages of One-Component Liquid Silicone Rubber
1. Curing Limitations
Depth of Cure: This is the most significant limitation. The curing reaction depends on the diffusion of atmospheric moisture into the material. As a result, the cure proceeds from the surface inward. For thick beads (typically over 10mm/0.4 inches), the center may remain uncured indefinitely, leading to a tacky surface and poor mechanical properties.
Sensitivity to Environmental Conditions: Curing speed is highly dependent on temperature and humidity. Low temperatures and/or low humidity levels can drastically slow down the skin-over time and full cure, delaying production processes. Conversely, very high humidity can cause rapid surface curing, potentially trapping air bubbles underneath.
2. Handling and Storage Challenges
Limited Pot Life: Once the cartridge or tube is opened, the material is exposed to moisture and begins to cure. If not used promptly, the entire container can solidify, leading to significant material waste. This contrasts with two-part systems that only begin curing after mixing.
Stringing and Messiness: 1C-LSRs often have a paste-like viscosity that can lead to "stringing" or "webbbing" during automated dispensing, requiring precise equipment calibration and potentially causing mess and defects on the assembly line.
Strict Storage Requirements: Unopened containers must be stored in a cool, dry place and often require sealing to prevent premature moisture ingress. Their shelf life, typically 9-12 months, is shorter than that of many two-part system base polymers.
3. Performance and Chemical Limitations
Weak Adhesion to Certain Substrates: While 1C-LSR adheres well to many materials, achieving strong, durable bonds to certain plastics (like polypropylene, polyethylene, PTFE, and nylon) often requires the use of a dedicated primer. This adds an extra step to the manufacturing process.
Poor Tear Strength: Standard 1C-LSR formulations generally exhibit lower tear strength compared to their two-component (2C-LSR) counterparts or organic rubbers. This makes them susceptible to damage from sharp objects or point loads.
Chemical Vulnerability: Although resistant to many chemicals, silicone rubber is susceptible to swelling and degradation when exposed to concentrated acids and bases, steam (>121°C), and certain organic solvents (e.g., ketones, concentrated aromatic hydrocarbons).
4. Economic and Processing Considerations
Higher Cost per Volume: 1C-LSR is generally more expensive on a cost-per-volume basis than two-component systems, polyurethane sealants, or epoxy adhesives. The convenience of a single-component system often comes at a premium price.
Byproduct Release During Cure: Most 1C-LSR formulations cure by releasing a byproduct, most commonly acetic acid (vinegar smell), but also methanol, ethanol, or oxime. These byproducts can be problematic:
Acetic Acid: Corrosive to sensitive metals (e.g., copper, brass), electronic components, and certain catalysts.
Odor: Can be unpleasant and require ventilation in confined spaces.
Food/Medical Concerns: Requires specially formulated "neutral-cure" grades (e.g., alkoxy-based) for sensitive applications, which are often more expensive.