Coating: Silicone-based
Description
A water-resistant, highly durable, flexible adhesive or coating. Most commercially available silicone sealant/caulks are sold as liquids, pastes (tube) or paints. The formulations for silicones are very abundant. They are most often available as uncured mixtures containing about 80-90% of the silicone prepolymer. To understand the application process it is very important to understand the composition and characteristics.
Silicone-based coatings provide good adhesion to fabric, low flammability, along with resistance to UV and visible light. Although they are water-repellent, they do allow oxygen, moisture, and other gaseous elements to pass. Silicone-based coatings can be applied via spraying, dipping, brushing or roller depending on the desired thickness of the coating. Any application process requires prior surface preparation to ensure proper adhesion.
- Uncured Silicone: In the pre-cured form, the silicone backbones contain easily hydrolyzable end groups, such as acetates, that are designed to crosslink when exposed to water vapor in the air. Since they crosslink at room temperature (within 24 hours), they are often call RTV silicones (Room Temperature Vulcanization). Some types of one-part silicone curing systems include: acetoxy-cure cure; alkoxy neutral-cure and oxime neutral-cure. Note that a high percentage of the commercial silicone caulks and sealant are acetoxy-based. These emit acetic acid upon curing and should not be used in museum environments. For more information see silicone cure systems.
- Cured Silicones: This is a highly durable material. Once cured, it is typically a solid. It can be reprocessed by dissolving it in a solvent to make a paint that dries with the evaporation. It can also be ground into a powder, and used as a heated coating.
| Polymer type | Polymer form at room temperature | General Characteristics | Applications |
|---|---|---|---|
| Pure Silicone resin | Solid, usually a pre-cured material | Excellent weather resistance; highly durable; hard to reprocess (must be dissolved or melted) | Molds, gaskets |
| Pure Silicone resin | Liquid or paste, may be uncured or cured | Excellent fluidity, can be sprayed or dipped (then air dried) after which it is highly durable and wear resistant | Oil, lubricants, precursor for coating or adhesive, mold release agents |
| Silicone resin mixed with solvents | Liquid | Mixture controls fluidity and viscosity to make it uniform and change drying speed, other additives easily added to make paints | Paints, weatherproof coatings, caulks |
| Silicone resins mixed with other polymers | Variable | Control adhesion and processing with amount and type of other resin (alkyd, acrylic, epoxy, polyester) | Wide selection of applications |
Commercial Products
The following names for commercially available Silicone Coatings were pulled from the AIC Materials Testing page (search for Silicone)
- Dow Corning: 9-1374 Clear Silicone Sealant (2016); 739 Silicone Sealant (2016); Transparent Noncorrosive Silicone Adhesive Sealant (1999)
- Industrial Control Development: Opacicoat-300 Silicone water based Elastomer (2018)
- Selleys: Selleys Brick And Concrete Silicone Sealer, Opaque (1999); Selleys Roof And Gutter Silicone Sealer, Clear (1999)
Physical and Chemical Properties
- Low surface energy providing non-stick surface once dry
- Conformal; readily seals any penetrations, gaps and seams
- Excellent chemical resistance; poor abrasion resistance
- Low glass transition temperatures; wide range of continuous use temperatures
- High reflectivity, good weathering, water repellent,
- Fast curing, single component
- High gas permeability (breathability for clothing)
- Provides electrical insulation and fire resistance
Resources and Citations
- Irving Skeist, Handbook of Adhesives, Van Nostrand Reinhold Company, New York, 1977
- Silicone Technologies: Silicone cure systems
- ShinEtsu: Silicone Resins and Oligomers
- ShinEtsu: Silicones for Coatings
- Romakk: What are Silicone Coatings
- Dowsil: 3140 RTV Coating