All surfaces should be thoroughly cleaned and/or degreased with naptha, mineral spirits, methyl ethyl ketone (MEK), or other suitable solvent. Light surface abrasion is recommended whenever possible, because it promotes good cleaning and increases the surface area for bonding. A final surface wipe with acetone or IPA is also useful. Some cleaning techniques may provide better results than others; users should determine the best techniques for their particular applications.
Dow Corning silicone adhesives are specially formulated to provide un-primed adhesion to may reactive metals, ceramics, and glass, as well as to selected laminates, resins, and plastics. However, good adhesion cannot be expected on non-reactive metal substrates or non-reactive plastic surfaces such as Teflon, polyethylene or polypropylene. Special surface treatments such as chemical etching or plasma treatment can sometimes provide a reactive surface and promote adhesion to these types of substrates. Dow Corning brand primers can be used to increase the chemical activity on difficult substrates.
Poor adhesion may be experienced on plastic or rubber substrates that are highly plasticized, because the mobile pasticizers act as release agents. Small-scale laboratory evaluation of all substrates is recommended before production trials are made.
In general, increasing the cure temperature and/or cure time will improve the ultimate adhesion.
Due to the wide variety of substrate types and differences in substrate surface conditions, general statements on adhesion and bond strength are impossible. To ensure maximum bond strength on a particular substrate, 100% cohesive failure of the adhesive in a lap shear or similar adhesive strength test is desired. This ensures compatibility of the adhesive with the substrate being considered. Also, this test can be used to determine minimum cure time or can detect the presence of surface contaminants such as mold release agents, oils, greases, and oxide films.
Useful Temperature Ranges
For most uses, silicone elastomers should be operational over a temperature range of -45 to 200°C (-49 to 392°F) for long periods of time. However, at both the low and high temperature ends of the spectrum, behavior of the materials and performance in particular applications can become more complex and require additional considerations
For low-temperature performance, thermal cycling to conditions such as -55°C (-67°F) may be possible, but performance should be verified for your parts or assemblies. Factors that may influence performance are configuration and stress sensitivity of components, cooling rates and hold times, and prior temperature history.
At the high-temperature end, the durability of the cured silicone elastomer is time and temperature dependent. As expected, the higher the temperature, the shorter the time the material will remain usable.
Certain materials, chemicals, curing agents and plasticizers can inhibit the cure of addition cure adhesives. Most notable of these include:
- Organotin and other organometallic compounds
- Silicone rubber containing organotin catalyst
- Sulfur, polysulfides, polysulfones, or other sulfur-containing materials
- Amines, urethanes, or amine-containing materials
- Unsaturated hydrocarbon plasticizers
- Some solder flux residues
If a substrate or material is questionable with respect to potentially causing inhibition of cure, it is recommended that a small scale compatibility test be run to ascertain suitability in a given application. The presence of liquid or uncured product at the interface between the questionable substrate and the cured gel indicates incompatibility and inhibition of cure.
Mixing and De-airing
Upon standing, some filler may settle to the bottom of the liquid containers after several weeks. To ensure a uniform product mix, the material in each container should be thoroughly mixed prior to use.
Silicone adhesive is intended only to survive splash or intermittent exposures to liquid or vapor solvent or fuel that may occur in an application. These adhesives are not suitable for continuous solvent or fuel exposure. Testing should be done to confirm performance of the adhesives under these conditions.
Shelf life is indicated by the "Use Before" date found on the product label.
For best results, RTV adhesives should be stored at or below 25°C (77°F). Special precautions must be taken to prevent moisture from contacting these materials. Containers should be kept tightly closed with head or air space minimized.
One-part Moisture Cure RTV
Dow Corning one-part moisture cure adhesives generally cure at room temperature and in a range of 20 to 80% relative humidity. Greater than 90% of their full physical properties should be attained within 24 to 72 hours depending on the product chosen and application specifics. These materials are not typically used for highly confined or deep section cures. Materials will generally cure about 0.25 inch per seven days from any exposed surface. Cure progresses from the outer surface and is dependent on the moisture in the air. Materials and parts can generally be handled within a few minutes to an hour or two once a surface skin forms. Mild heat acceleration of the cure rate may be possible, but temperatures above 60°C (140°F) are not recommended.