Silicone Rubber Technical Information

Silicone as we know it today began its evolution in the early 1940’s. Differential molecular structure, length of polymer chains and attachment of organic groups give rise to three silicone forms; fluids, resins, and gums. The alternating silicone-oxygen-silicone backbone is the same bond structure found in sand, quartz and glass. This gives the basis for excellent temperature resistance, aging, and weathering characteristics.

Silicone is currently used in chemical and petroleum processing, construction, food and drug processing, medical implants and surgical aids, electronic equipment, mining and metal fabrication, paints and protective coatings, paper production and printing, personal care products, textiles, and transportation (automotive, marine and aerospace).

Silicone Vacuum Bagging in the Aerospace Industry

The aerospace industry pioneered the use of vacuum bagging for fabricating composite articles (originally epoxy resins and fiberglass fabric) to replace selected metal parts. The composite articles had a high strength-weight ratio and usually could eliminate many separate drilling and riveting operations to simplify fabrication. The first vacuum bags were made primarily of nylon film and sealing was achieved by various sticky tape formulations. The film bags were only good for one autoclave cure cycle and were then discarded. The film was not easily drawn into contoured areas and tended to leave wrinkles on the composite articles. Silicone rubber was investigated as a reusable vacuum blanket material and over the years has proven to be extremely effective and cost efficient in this application. Most companies who work with composites are familiar with its advantages and disadvantages.

Aerospace Engineers who are not familiar with silicone rubber may have heard horror stories about contamination of bonding surfaces, transfer of silicone films, etc. This misinformation often prevents silicone rubber from being used in situations where it would be a cost effective problem solver. Solid form vacuum blanket grade silicone rubber is a different material from liquid type RTV silicone elastomers. RTV silicone is used to pour complicated tooling shapes which are then used as thermal expansion forming blocks and in some instances RTV has been coated over reinforcing fabrics to make blankets. The RTV system often utilizes a platinum catalyst. This type of cure system is very sensitive to sulfur, some organic rubber, certain tooling waxes, and machine oil. If an RTV silicone part is contaminated by any of these substances it will not cure and this reverted surface can cause problems. Heat curable vacuum blanket rubber is much less sensitive to these types of contamination. If care is taken in working with clean tooling and the sensitive nature of RTV is taken into consideration, no problems should occur. The important thing to remember is that heat curable vacuum blanket silicone is not the same product as RTV silicone. It has different physical properties and resistance characteristics.

When the high strength, vacuum blanket grade silicone is properly and fully vulcanized, the chances of it causing contamination problems are extremely minimal. If a post bonding operation is required on a part that has been fabricated with a silicone blanket, an effective way of insuring a clean surface is to use on outer nylon fabric peel ply. This peel ply can be removed immediately prior to the bonding step. Most vacuum blanket applications involve several layers of bleederbreather fabric in conjunction with barrier films. This also serves to minimize the chance of any surface contamination occurring.

The surface of composite articles has been examined to determine if silicone contamination was present. In comparison tests of parts made using film bags, both parts showed evidence of silicone being present. It should be noted that adhesion tests performed on these parts were within required limits. It was determined after much investigation that the source of the silicone was not the silicone bag, but a silicone release film that the prepreg adhesive manufacturer used to ship his product on. Subsequent tests with other non-silicone release agents such as TFE showed that it too could be removed from the carrier film by the epoxy resin. The silicone was removed from composite surfaces by vapor degreasing and cleaning more readily than TFE. This highlights the point that epoxy resin adhesive must have something to act as a parting agent in order for it to be removed from the tooling after the part is cured. This parting agent can be silicone mold release in non-critical areas, non-silicone mold release in those shops where silicone mold release is prohibited. Release films such as TFE or FEP should be used where non-silicone release is prohibited. Precautions should be taken to ensure that critical surfaces are protected and cleaned, if necessary, before bonding operations are performed.

Mosites Rubber Company has been a major supplier of silicone rubber to the Aerospace Industry for over thirty years. Our silicone rubber is available in cured roll form, uncured or “B” stage roll form, and fabricated contoured blankets or inflatable bags. Many of our customers have applications that require vacuum bags made on tooling that is too large or otherwise unavailable for shipment to our facility. These situations have led Mosites Rubber Company to offer technical assistance and training to enable our customers to fabricate silicone vacuum blankets in their plants. The silicone elastomer has poor green strength in the uncured form. This softness allows it to be shaped easily into complex configurations. Multiple layers bond easily and uniformly to themselves and the uncured elastomer can be bonded to cured silicone if an area needs to be repaired or built up. High stress areas can be strengthened by applying fabric reinforced, uncured silicone to the area and vulcanizing it in place.

The overall high strength of the silicone vacuum blanket, the retention of physical properties after numerous production cycles, and the inherent release characteristics of the silicone make it an excellent choice for a reusable, cost effective vacuum blanket system.