Silicone Bonding Properites

Silicone elastomer bonds readily to properly prepared metal and composite surfaces. Uncured silicone sheet can be used as a bonding agent to adhere previously vulcanized silicone to various substrates or to itself. A number of commercially available RTV Adhesive Silicones are utilized to obtain bonds under ambient conditions or when an assembly is too large to place in a high temperature, pressure curing unit. Listed below are typical procedures used to obtain adhesion under varied conditions. These procedures were developed for our high strength silicone compound, Mosites #1453. They also apply to other Mosites silicone compounds.

Bonding uncured silicone to metal or composite

• Clean the metal or composite surface to remove oil, dust, etc.
• Abrade the metal surface by sandblasting, grinding, or sanding.
• Rinse the surface with an appropriate solvent.
• Prime the metal or composite surface with Thixon 305 primer. Allow to dry for at least 30 minutes.
• Unroll the silicone sheet as per the instructions on the roll. Cut the sheet to the desired size. Lay the sheet into position on the primed metal. Remove the final layer of polyethylene film.
• Use a double layer of clean nylon fabric to work air from beneath the sheet. Start in the center and work air to the outside with firm pressure.
• Inspect the assembly. Patch any thin spots or place extra layers into position if an area needs to be strengthened.
• If the assembly is to be vulcanized in a dry heat oven or autoclave, it must be film bagged. Use bagging film with an FEP barrier and bleeder system.
• If the assembly is to be vulcanized in a steam or inert gas autoclave, no bagging film is required.
• Vulcanize for 30 minutes at 300°F. If an autoclave is used, 60 psi is sufficient pressure. Time the cure cycle after the part reaches 300°F.
• After the initial cure is completed, allow the part to cool to room temperature and inspect. Make any needed patches or repairs and cure these areas by same method.
• Place the assembly in an air circulating oven for 3 hours at 400°F.

Bonding cured silicone to metal or composite with Mosites #1400 RTV adhesive

• Suitable RTV Adhesives are available from most major silicone suppliers. Mosites Rubber Company has had success with General Electric RTV-108 and Dow Corning 732 Adhesive. All three systems rely on moisture present in the air to achieve maximum strength and can not be accelerated by heat. Most assemblies can be handled after one day.
• Clean the metal or composite surface with an appropriate solvent.
• Clean the cured silicone surface. Allow the solvent to evaporate for 30 minutes.
• No primer is necessary to achieve a strong bond. If a primer is needed, Dow Corning 1200 RTV Primer is recommended. Apply a thin uniform coating to the metal or composite surface. Allow at least 30 minutes for the primer to dry.
• Apply the RTV Silicone Adhesive to the metal surface using a spatula or spreader. Try to achieve a coating of approximately 20 mils.
• Lay the substrates together and apply pressure to assure uniform contact. Allow to dry for at least 24 hours. See manufacturer’s recommendation for optimum cure time.
• RTV Adhesive bonding is ideally suited to flat surfaces. Large surface areas may take longer to cure because the RTV is dependent on atmosphere moisture for vulcanization.

Bonding cured silicone to cured silicone using uncured silicone

• As previously described, uncured silicone compound is an excellent bonding agent for adhering cured silicone sheet to itself. This is needed for repair of damaged areas or to fabricate large blankets. Uncured calendered sheet can be used or the Mosites #14206 Adhesive is also available. The advantage of this type of repair or bond system is that it is extremely strong and it eliminates the 24 hour cure cycle needed for RTV. It does require a heat source capable of reaching 300°F to cure the bond layer.
• Clean the cured silicone sheet with solvent and allow the sheet to air dry for at least 30 minutes.
• Apply the uncured silicone or #14206 Adhesive to one surface to be bonded. Use clean, doubled nylon to exclude air.
• Lay the second cured silicone layer into position and use a hand roller to assure uniform contact.
  The assembly can be vacuum bagged with nylon film or clamped between two metal plates to maintain pressure during vulcanization. If an inert gas or steam autoclave is used, no bagging is required.
• Place the assembly in the oven or autoclave for 30 minutes at 300°F. If an autoclave is used, 60 psi is sufficient pressure. Time the cure after the part reaches 300°F.
• Allow the part to cool to room temperature before disassembling. If Mosites #14206 was used, no further cure is necessary. If other silicone was used, post cure the assembly for 3 hours at 400°F.