Release Coating for Vacuum Blankets
Many applications of silicone vacuum blankets or inflatable pressure bags require direct contact between the silicone surface and the resin prepreg adhesive. After extended cure cycles the blanket may adhere to the resin/composite lay-up. Some of the more commonly used adhesives contain amine curatives which will degrade the silicone elastomer. In order to protect the silicone from the effects of direct contact with the resin, silicone base release coatings have been developed.
Mosites #14318 – Heat Curable Release Coating
This product is a single component, heat curable, liquid, addition cure system silicone rubber that can be used as a release coating for elastomeric mandrels or pressure bags. The coating can be applied by brushing or spraying, to elastomeric tooling surfaces that come into direct contact with pre-preg resin systems during the composite manufacturing process. The cured #14318 Release Coating allows the mandrel or caul to be easily separated from the cured composite surface to facilitate removal.
Mosites #14318 is compatible with all Mosites high strength silicone compounds and with Mosites #2902 and #10276-B Fluoroelastomer Caul Pad Compounds. When applied to the cured Fluoroelastomer surface and subsequently cured; the coating will function as a release coating to prevent adhesion of epoxy resins and will also allow uncured silicone rubber sheet to be laminated and cured to the Fluoroelastomer. Laminates prepared in this fashion may offer advantages in resistance to volatiles generated by the resin system in the pre-preg that degrade silicone but have no adverse effect on Fluoroelastomers.
Physical Properties of Mosites #14318 Release Coating
The physical properties shown below were obtained on 0.080 inch thick molded ASTM samples. They are typical of Mosites #14318, liquid silicone release coating, but they should not be used to set Quality Control Specification minimum requirements.
|Hardness (Shore A)||22|
|Tensile Strength (psi)||190|
|Elongation at Break (%)||175%|
|Modulus at 100% Elongation (psi)||90|
|Tear Strength- Die C (ppi)||38|
|% Compression set (22 hours at 350°F)||21%|
Mosites #14318 is available as a one component, pre-mixed solution. For shipment overseas, it is difficult to air freight any mixture containing a flammable solvent. For these customers, the components can be shipped separately and mixed with solvent obtained locally. A procedure for blending the solution is shown below.
- Weigh the desired amount of #14318 Base and #14318 Catalyst in a clean, resealable container. The mix ratio is one part of catalyst to ten parts of base, by weight. Mix thoroughly with a spatula or stirring rod; make certain that all base is scraped from the bottom and sides of the container.
- The catalyzed silicone is next diluted with solvent. Mosites recommends VM@ P Naptha, but other aromatic solvents may work also (toluene, zylene, etc.). The solvent content can be varied from 10% to 50%, depending on the type of application. For spraying, a 50% dilution is preferred. The addition of the solvent will help extend the shelf life of the mixture. The stability is still being investigated, but it appears to be in excess of 6 months when stored below 32 degrees F.
Note – One of the by products of addition cured silicone is Hydrogen Gas. At room temperature, in a sealed container, enough gas may be liberated to cause the container to bulge or even force a lid open. Care should be taken to inspect the stored containers periodically, and vent them to remove pressure as required.
The coating contains Naptha, a flammable solvent, and caution must be used in applying the solution to any substrate. Use only in well ventilated areas and keep the containers sealed when not in use. Prevent vapors from contacting sparks or open flames. After the coating has been applied, allow the solvent to evaporate, under ambient conditions, for at least 30 minutes. The coating can be left overnight if desired. Place the coated part in the oven for 30 minutes at 300 degrees F to cure. Higher temperatures will not have an adverse effect on the coating. No post cure is required. A typical coating thickness is 1 – 2 mils. Although the #14318 Release has limited elongation, a sample of cured silicone rubber with a 2 mil thick coating can be stretched 300% before the coating cracks or checks.