Silicone Rubber Technical Information
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Introduction to Silicone Rubber
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 siliconeoxygensilicone 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).
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 strengthweight 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.
Silicone Chemistry
A polymer is a chain of small repeating units with an end group. With silicone the polymer chains i.e. siloxanes are formed with Si-O bonds. The polysiloxane backbone is a chain consisting of silicone and oxygen. Silicone is part inorganic, part organic. It shares the same Si-O backbone with inorganic materials such as quartz or glass. Silicones also contain organic groups which are attached to the silicon. These organic groups on the polysiloxane can be methyl, vinyl or phenyl groups.
The various silicone polymers are named after the organic side groups attached to the silicon atom. Each silicon atom has four chemical bonds, which is why silicone rubber is often abbreviated with a Q for quaternary groups. The polymers present in rubber can vary in structure e.g. with different chain lengths, with or without branching or with high or low vinyl content. This in turn, affects the ultimate properties of the rubber.
MQ
In polydimethylsiloxanes, the organic groups are methyl groups. They have little importance because they have no double bonds and do not react easily with peroxide crosslinkers. They can however be used as plasticizers.
VM Q
This compound is similar to polydimeythl siloxane but some of the methyl groups have been replaced with vinyl groups. The double bond is a reactive group which is needed for crosslinking.
PMVQ
If a small number of the methyl groups in a VMG are replaced with phenyl groups, the polymer chains have less tendency to pack with adjacent chains at low temperature. This chemical structure of phenyl methyl silicone helps them retain flexibility at temperature as low as -80°C.
Crosslinking Silicone
Crosslinking or curing is initiated by crosslinking chemicals, which react with the vinyl groups present in the VMQ and PVMQ polymer chains. The nature of this reaction depends to a large extent on the chemicals’ properties: it can be fast or slow, complete or incomplete, more sensitive to temperatures or more resilient to external influences.
There are two different curing processes employed: peroxide curing and platinum-catalyzed addition curing. A brief explanation of both is given below.
Liquid silicone rubbers’ are always platinum catalyzed. For solid silicone rubbers, however, either curing system can be employed.
Peroxide Cure
The peroxide group produces an oxygen free radical. A reactive free radical is formed on the vinyl. The free radical attaches itself to another polymer chain and forms a bridge. The free radical chain reaction then continues.
Platinum Cure
The platinum center has one free coordination site. Interaction with the platinum center activates the double bond. The vinyl group crosslinks by transforming the double bond, creating a single bond to a polymer chain; in this case, to a crosslinker molecule containing Si-H groups. The catalyst becomes free and is again available for further crosslinking.
Silicone Cure Procedures
Vacuum Blanket Silicone
The silicone compounds which Mosites Rubber Company normally recommend and use for high strength vacuum blankets contain peroxide curatives. The curing chamber in our plant uses high pressure saturated steam to provide temperature and external pressure during vulcanization. An added benefit of the steam atmosphere is the ability of the silicone surface to cure without the necessity of being protected and sealed with bagging film. If the silicone surface is left uncovered in a dry heat autoclave or oven, the surface tends to be uncured. This is due to the volatility of the catalyst and the inhibiting effect of air.
Most of our customers who purchase uncured or "B" stage silicone do not have steam autoclaves. These customers use inert gas atmospheres such as Nitrogen or film bag and cure in hot air autoclaves or ovens. Regardless of the method used, the cure times and temperatures remain the same.
Initial cure
Mosites Rubber Company recommends that our products receive an initial cure of thirty minutes at 300°F. Time commences after the tooling reaches 300°F. After the initial cure time has elapsed, cool the part to room temperature and remove it from the tool.
Post cure
Lay the initial cured silicone on clean fiberglass and place it in an air circulating oven for post-curing. Folds or cavities in the bag can be packed with clean fiberglass to prevent adjacent surfaces from touching during post cure. The post cure cycle consists of three hours at 400°F.
Molding Compound Silicone
The peroxide catalyst system used to vulcanize Mosites calendered sheet silicone is not recommended for thick section (over 0.5 inches thick) molded articles. These applications require a peroxide that offers good mold fill / flow time, prior to crosslink formation and must not generate acidic cure by products that would cause reversion during subsequent high temperature exposure in service. The organic peroxides used in these applications require closed metal molds in order to effectively cure the silicone polymer. Normally, the molds are placed in presses for the most efficient productivity. A press is not necessary and ovens or autoclaves can be used, but the mold must be securely bolted or held together to withstand the internal pressures exerted by the silicone polymer as it expands during the vulcanization process. A procedure for curing a thick section molded silicone article is shown below.
- Clean the mold thoroughly and apply a uniform coating of mold release. (Teflon Spray or dilute detergent soap solution )
- Calculate the weight or size of the silicone preform required to completely fill the mold cavity and add 3% for optimum mold fill. Calendered sheet is not required for these applications and uncured mill slab is a much more economical choice.
- The mold can be room temperature if the assembly process is time consuming. If molding is performed in a press, where the process has a rapid turn around time, a hot or pre-heated mold is recommended.
- Our recommendation for cure time and temperature is based on the thickness of the rubber part. Cure for 30 minutes at 300 degrees F for the first 1/8 inch of thickness. Cure an additional 5 minutes at 300 F for each extra 1/8 inch of thickness. The cure time should commence after the mold reaches 300 degrees F, if it is loaded at room temperature.
- After the initial vulcanization is complete, remove the part from the mold; trim and inspect. For most Aerospace applications a post cure of 3 hours at 400 degrees F in an air circulating oven is required. Less critical applications may not require the post cure due to the absence of acidic by-products.
Volatile By-Products of Silicone Vulcanization
In compliance with legislative requirements of 29 CFR 1910.1048 the statement must be made that Methylpolysiloxanes (silicone-methyl groups) can generate Formaldehyde when exposed to 300°F in the presence of air. This warning effects products which have the potential to expose workers to formaldehyde above .75 ppm for an 8 hour weighted average. There is no concern if silicone is not used above 300°F or in an oxygen rich environment.
**This product contains methylpolysiloxanes which have the potential to form formaldehyde at 300°F in the presence of air. Formaldehyde is a potential cancer hazard and is a skin and respiratory sensitizer. Vapors irritate eyes, nose and throat.
Methods of monitoring formaldehyde exist in the form of Passive Dosemeters, Detector Tubes, etc. It is considered the users responsibility to specifically determine the actual level in their process and take the appropriate steps to safeguard employees.
The vulcanization reaction of silicone polymer and peroxide curative results in the evolution of
small amounts of volatile gaseous by-products. Proper ventilation is important to minimize employee contact with the gasses. Awareness of their presence can prevent subsequent adhesion problems of bonded assemblies.
Initial cure
- Moisture is liberated – less than 2%.
- Depending on the type of catalyst used, Dichlorobenzene and/or Dichlorobenzoic Acid may be released by decomposition of the catalyst – usually less than 20 ppm.
- Low molecular weight cyclic siloxane polymers are formed, typically Octamethylcyclotetrasiloxane and Hexamethylcylotresiloxane – usually less than 10 ppm.
Oven post cure
- Residual moisture is vaporized.
- The small residual amounts of Dichlorobenzene and Dichlorobenzoic Acid are vaporized.
- The low molecular weight cyclic siloxane polymers are volatilized and become silicon dioxide.
Handling and Storage of Uncured Silicone
Mosites Rubber Company supplies uncured silicone sheet for fabrication into various configurations. When dealing with uncured silicone sheet, shelf life becomes very important. Mosites policy has been to recommend that the sheet be kept under refrigerated storage at a temperature below 40°F. We also recommend that fabrication occur within 8 weeks after our customer receives the shipment. This time frame guarantees that the material will strip properly from its liner, seams and splices can be properly skived or feathered, and that the sheet will cure properly.
Sometimes a customer will have material which has been stored longer than recommended. The following procedure is a quick inspection to determine if the sheet is still suitable for fabrication.
- Remove a portion of the polyethylene film liner and check to see if the silicone adheres to the film. Aged stock often is difficult to strip from the liner.
- Take a spatula or "skive" and try smearing the uncured sheet edge. If the silicone makes a smooth "feathered" edge, it is still usable. If it tends to shred or roll up it is probably aged to the point of causing lay-up problems.
- Take two small pieces of the uncured sheet and lay them together applying light pressure. Observe the degree of adhesion between the pieces. Usable stock should adhere tightly.
The above tests are only guidelines to help determine if aged silicone sheet is still usable. Many customers routinely manufacture lay-ups from sheet that is 6 months old if it has been stored properly. As long as the sheet can be removed without distortion from its polyethylene liners, vulcanization and subsequent physical properties are not affected.
Fabrication of a Contoured Silicone Blanket
The procedures outlined below are general recommendations for film bagging and oven or dry heat autoclave curing of a contoured silicone vacuum blanket.
Preparation of the lay-up tool
- The lay up tool can be a sheet metal mandrel, aluminum, or an actual production composite part. It should be inspected for smoothness and cleaned with solvent.
- Use fillets and felt to prevent bag bridging on inside angles or seams. Mylar tape can be used to secure and cover the fillets.
- The tool should be evenly coated with release agent. Silicone based release agent should not be used. Dry Teflon aerosol spray or Joy detergent solution in water work well.
- If a production composite article is used as a tool, it may be covered with a layer of Tedlar film to prevent uncured silicone transfer to the part. The film can be secured with mylar tape.
Fabrication of the silicone lay-up
- Rolls of uncured silicone should be removed from refrigerated storage and allowed to reach room temperature.
- Unroll the silicone sheet as per instructions on the roll.
- Cut the uncured sheet using a pattern to give a minimum of waste and seams.
- Lay the uncured silicone sheet into place with the exposed surface down. Starting at the top, work around and down using very gentle hand pressure to remove any air that may be trapped under the sheet. Remove the second layer of film and rub gently with a double layer of clean nylon fabric to mold the silicone into place.
- Joints or seams can be overlapped or butt jointed. An overlap is usually preferred for strength and vacuum integrity. The overlap can be skived or feathered using a small spatula.
- Additional layers can be applied to areas of extreme contours or over radii where the sheet may have been thinned. Use the same procedure to remove air that maybe trapped between layers. A hypodermic needle can be used to facilitate air removal. Smear the uncured silicone to fill in and cover the needle hole.
- When the lay-up is completed, check it to make sure there are no areas of foreign matter or seams that have not been feathered.
Film bagging of the lay-up
- Cover the entire lay-up with a layer of release cloth or film. This prevents the silicone from being embedded in the bleeder fabric during cure.
- Place a layer of breather fabric over the lay-up.
- Position any vacuum ports or thermocouple. Bag the entire lay-up with suitable nylon film and seal with sealant tape.
- Attach the tool and lay-up to a vacuum source and pull 24 to 28 inches of mercury vacuum on the lay-up. Verify the integrity of the seal with a vacuum gage.
Curing the lay-up
- Place the tool in an autoclave or oven, while maintaining vacuum pressure.
- Cure the silicone for at least 30 minutes at 300°F. Time the cure cycle after the assembly reaches 300°F. If an autoclave is used for external pressure, 60 psi is usually sufficient. For lay-ups that are thicker than 1/8 inch, cure an additional five minutes at 300°F for each extra 1/8 inch of thickness.
- Allow the lay-up to cool to room temperature before removing it from the tool. Inspect the blanket for any areas that may be thin. Add uncured silicone to the thinned areas and repeat the curing procedure.
- Remove the blanket from the tool and place it in an air circulating oven for 3 hours at 400°F. During the post cure, do not allow silicone surfaces to touch. Use clean fiberglass cloth as a separator.
- After post cure and cooling, the blanket is ready for use.
Release Coatings 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.
| Properties | |
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| Hardness (Shore A) | 22 |
| Tensile Strength (psi) | 190 |
| Elongation at Break (%) | 175% |
| Modulus at 100% Elongation (psi) | 90 |
| Tear Strength- Die C (ppi) | 38 |
| Specific Gravity | 1.1 |
| % Compression set (22 hours at 350°F) | 21% |
| Color | Gray |
Mixing Instructions
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 [email protected] 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.
Application Procedure
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.
Silicone Rubber Pressure Bags and Inflatable Mandrels
The use of composite materials in the manufacture of Aerospace articles has steadily increased and they now are used in structural components, in addition to wing skin coverings. The development of these applications have required shaped elastomeric tooling that can apply pressure to the internal matrix of a composite structure, while the autoclave applies external pressure and elevated temperature to cure the adhesive resin in the lay-up. In these applications, the shaped rubber mandrel is in direct contact with the resin. The inherent resistance of silicone rubber to elevated temperature and adhesion by epoxy type resin makes it an ideal candidate for the elastomer. Some of the early development efforts used liquid silicone rubber, catalyzed for room temperature vulcanization. The obvious advantage was low cost tooling. The disadvantage was in the life expectancy of the rubber part in service. The RTV silicone was not usually a high strength / high tear resistant polymer. It could be damaged during removal from the fabricated article. Shrinkage of the solid rubber blocks, after several cure cycles, could decrease important dimensions and thereby reduce compaction pressure in the composite structure.
In an effort to overcome these disadvantages, Mosites Rubber Company has produced high strength silicone rubber inflatable mandrels. These shaped bags are hollow; usually with an inflation stem in one end; and if the outside dimensions are critical, the bag can be made inside female tooling to yield crisp detail. For less critical applications, the silicone bag can be layed up on inexpensive male sheet metal tools. One advantage of the female tooling is that it allows for variations in the wall thickness of the rubber bag without altering the outside dimensions of the part. If extra material is necessary at a certain location to reinforce the part, it can be added internally. When placed inside the composite article during its cure cycle, the pressure inside the bag can regulated as desired. In some applications the pressure stem is vented to the autoclave atmosphere to maintain exact internal and external pressure equilibrium. Upon completion of the cure cycle, a vacuum can be used to collapse the mandrel and aid in its removal.
This procedure has been utilized to manufacture helicopter rotor blades; to allow the co-cure of entire wing sections with integral stiffeners and hat section channels; to apply erosion coatings to jet engine turbine blades; and to manufacture composite recreational and sporting equipment such as bicycles, golf clubs and fishing rods.
Fabrication of a Silicone Inflatable Pressure Bag
from Uncured Mosites Silicone Sheet
- Most tooling or mandrels for bag fabrication are made from aluminum.
- Apply a TFE aerosol release or use a solution of Joy detergent and water. Do not use a silicone base release.
- If a metal stem is to be used for inflation of the bag, it should be installed now. Use the following procedure for aluminum or carbon steel. Sand blast or abrade the metal surface. Clean with a solvent.
- Prime the metal with one uniform coating of Thixon 305 primer. Allow the primer to dry at ambient conditions for at least 30 minutes.
- Wrap the primed metal with uncured silicone sheet. Use doubled nylon fabric to remove air and skive or feather the edges next to the threaded portion of the stem.
- Place the valve stem into the desired location on the tool. The silicone will be tacky enough to ensure that the stem stays in position. If desired, a cap of uncured silicone can be laid over the stem and extended 1 to 2 inches back on the mandrel. This cap should be feathered or skived to make a smooth transition to the tube.
- Next, lay the uncured silicone sheet into position to form the body of the tube. Feather or skive the edges and allow about 1/4 to 1/2 inch overlap. Use doubled nylon fabric to work all of the air out from underneath. Start at one end and work around and forward to push air to the open end of the tube. Apply extra pressure to the overlap seam area and if desired the excess can be carefully removed. Skive or feather the edge of the sheet.
- This section of the tube is ready to be vulcanized. An inert gas autoclave is normally recommended to cure silicone if it cannot be vacuum bagged. Air will cause a slight reversion of the unprotected silicone surface during its vulcanization. This situation does not occur if nitrogen is used or if a saturated steam autoclave is used. Cure the tube for at least 30 minutes at 300°F and 60 psi. Start timing the cure cycle after the tool reaches 300°F. Allow the part to cool to room temperature before removing it from the mandrel.
- Removing the bag is sometimes difficult, especially if the tube has undercuts or different thicknesses in sections. The easiest way is to apply very slight air pressure to the stem end to slightly expand the bag. By holding the opposite end by hand, the air can be
maintained until the bag slides off. - A more difficult procedure involves wrapping the tube with nylon fabric and peeling it off inside out as you would remove a sock. The tube should then be turned back to the right side out position.
- After the tube is fabricated, the end can be sealed up by several methods. A molded cap can be spliced into place using uncured silicone to bond the cured bag to the cured cap. Uncured silicone can be packed into the tube to affect a seal which is then vulcanized in the autoclave.
- After the end is closed, post cure the bag for 3 hours at 400°F. It is then ready to use.
Adhesion of Silicone
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
- 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.
- Lay a thin sheet of uncured silicone into position on the primed surface. Mosites Rubber supplies an adhesive sheet material, Mosites #14206, which is available in 1 inch wide rolls, 3 inch rolls or 36 inch wide calendered sheet. The #14206 is approximately 0.040 inch thick and can also be used as repair material for damaged, cured, silicone sheet.
- Clean with solvent the cured silicone sheet surface to be bonded. Allow the solvent to evaporate for at least 30 minutes before assembling the layers.
- Lay the cured silicone into position on the uncured silicone layer. Use a hand roller to assure that no air is trapped between the layers.
- The assembly should be vulcanized in an autoclave or oven.
- 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.
- If 14206 was used as the uncured silicone bonding layer, no post cure is necessary. If other silicone sheet was used, a post cure of 3 hours at 400°F is necessary.
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 RTV adhesive
- This method is used to repair torn areas of cured silicone or to bond cured blankets together to form a larger size blanket.
- Clean the silicone surface with solvent. If the area to be bonded is a tear or puncture, remove any abraded silicone with a sharp knife.
- Cut a patch from 1/32 inch thick cured silicone sheet to cover the repair area and extend at least 1 inch beyond all sides.
- Spread the RTV Adhesive over the patch uniformly to give a thickness of approximately 20 mils. Place the patch over the repair area and apply light contact pressure to assure a uniform bond line.
- Allow the assembly to cure at least 24 hours. Turn the repair area over and spread RTV uniformly to fill any voids present. Do not cover this patch area; allow it to cure exposed to the air.
- Bond seams used to adhere sheets of cured silicone during fabrication of large blankets are made in the same fashion. Use strips of 1/32 inch thick cured sheet approximately 3 inches wide to make a batten strip over the joint area.
- Clean the silicone surfaces to be bonded with solvent.
- Do not butt the sheets to be bonded completely together. Leave an approximate 1/8 inch gap between sheets. Apply RTV adhesive to the batten strip and place the batten strip into position on the cured silicone.
- Apply light contact pressure to assure uniform contact and to exclude air that may be trapped between layers.
- Allow the seam area to cure for 24 hours at room temperature. Turn the sheet over and fill the 1/8 inch gap with RTV Adhesive. This material can be spread with a spatula to make a smooth surface.
- Do not cover. Allow the RTV adhesive to cure for at least 24 hours before handling the blanket. The RTV achieves its optimum cure and bond strength after 3 to 7 days at room temperature.
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.
Exposure of Silicone to High Temperature Aging
As an established supplier of silicone vacuum blankets to the Aerospace Industry, Mosites Rubber Company is occasionally asked, "How many cycles can I expect from this blanket?" It is a very difficult question to answer because of the number of factors which must be taken into consideration. Among those factors are; the temperature and duration of the cure cycle, the degree of stretch or movement of the blanket during the vacuum process, how well the blanket can be protected from the volatile gasses given off by some of the epoxy or polyester curatives when heated, and the handling and care taken with the blanket during removal from the tool and storage between cure cycles.
Silicone rubber is affected, to varying degrees, by exposure to amine curatives used in epoxy resins, peroxide curatives used in polyester resins, and formaldehyde liberated by some adhesive compositions used in laminating cements. After prolonged exposure, the physical properties will deteriorate and if direct contact occurs between the silicone and resin, adhesion can result. The use of appropriate bleeder systems and barrier films where applicable will minimize this problem. Silicone rubber has excellent resistance to elevated temperature, but as would be expected, exposure to temperatures in excess of 400°F will reduce the service life of a blanket as compared to the same time and temperatures of 350°F or less.
Our standard vacuum blanket material, Mosites #1453 has been tested under laboratory conditions of elevated temperature and after exposure to several common aerospace adhesive materials. These properties are not intended as specification standards and actual production applications may not duplicate these effects. The tests conditions and physical properties are shown below.
| Resistance of #1453 to high temperature aging | ||||
|---|---|---|---|---|
| #1453 D | 480 hrs | 288 hrs | 100 hrs | |
| Control | @ 350°F | @ 400°F | @ 480°F | |
| Hardness (shore A) | 46 | 49 | 51 | 55 |
| Tensile (psi) | 1430 | 1260 | 1045 | 1020 |
| Elongation (%) | 670% | 510% | 375% | 310% |
| Modulus at 300% (psi) | 490 | 680 | 840 | 960 |
| Tear Strength (ppi) | 229 | 228 | 164 | 154 |
Resistance to amine volatiles – Type A
A sample of #1453 D silicone sheet was used to seal a fixture containing a 3" x 3" square of Hexcel prepreg conforming to BMS-8-79 using di-cyandiamide curative. After each cure cycle of 1 hour at 350°F, new resin prepreg was inserted. Testing was completed after 15 cycles.
| Type A | ||
|---|---|---|
| Control | After 15 cycles | |
| Hardness (shore A) | 47 | 51 |
| Tensile (psi) | 1440 | 1460 |
| Elongation (%) | 540% | 540% |
| Modulus at 300% (psi) | 640 | 740 |
| Tear Strength (ppi) | 212 | 231 |
Resistance to amine volatiles – Type B
A sample of #1453 D silicone sheet was used to seal a fixture containing a 3" x 3" square of Hexcel prepreg conforming to BMS-8-79 using triethylene tetramine curative. After each cure cycle of 1 hour at 350°F, new resin prepreg was inserted. Testing was completed after 25 cycles.
| Type B | ||
|---|---|---|
| Control | After 25 cycles | |
| Hardness (shore A) | 45 | 49 |
| Tensile (psi) | 1275 | 810 |
| Elongation (%) | 600% | 300% |
| Modulus at 300% (psi) | 545 | – |
| Tear Strength (ppi) | 228 | 146 |
Resistance to direct epoxy contact – Type C
A sample of #1453 D silicone was placed in direct contact (no bleeder system) with FM-53 supported Epoxy Adhesive. The cure cycle consisted of 1 hour at 260°F and 50 psi pressure. Testing was completed after 25 cycles.
| Type C | ||
|---|---|---|
| Control | After 25 cycles | |
| Hardness (shore A) | 45 | 46 |
| Tensile (psi) | 1320 | 1310 |
| Elongation (%) | 560% | 560% |
| Modulus at 300% (psi) | 590 | 590 |
| Tear Strength (ppi) | 259 | 253 |
Resistance to direct epoxy contact – Type D
A sample of #1453 D silicone was placed in direct contact (no bleeder system) with FM- 1000 supported Epoxy Adhesive. The cure cycle consisted of 1 hour at 350°F and 50 psi pressure. Testing was completed after 25 cycles.
| Type D | ||
|---|---|---|
| Control | After 15 cycles | |
| Hardness (shore A) | 47 | 49 |
| Tensile (psi) | 1420 | 980 |
| Elongation (%) | 600% | 330% |
| Modulus at 300% (psi) | 620 | 860 |
| Tear Strength (ppi) | 238 | 160 |
Thermal properties of Mosites #1453
Silicone rubber has a linear expansion approximately 17 times greater than carbon steel at 350°F. At the same time, it is a good insulator making it necessary to allow extended heat up times for composite molding. Most users of silicone vacuum blankets monitor both tool surface temperature and composite article temperature using thermocouple wire.
Figures were obtained on molded ASTM sheet samples heated from 72°F to 350°F.
- Linear expansion ………… 1.67 X 10-4 in / in / °F
- Volumetric expansion……….. 5 X 10-4 in / in / °F
- Thermal conductivity…….. 1.7 BTU in / hr / ft2 / °F
Low Temperature Resistance of Silicone
In general, silicone rubber has excellent low temperature performance. Properties of silicone are not adversely affected by exposure to extreme low temperatures. Original properties return when silicone reaches room temperature. The concern is how low of temperature can the material withstand and still remain flexible. This is determined by the polymer type. Below is a comparison of 3 types of silicone; 1) Dimethyl silicone (VMQ), 2) Fluorosilicone (FVMQ), 3) Phenylmethyl-Dimethyl silicone (PVMQ).
| Silicone Type | Brittleness | Young Modulus | TR-10 |
|---|---|---|---|
| General purpose (VMQ) | -100°F | -67°F | -58°F |
| High Strength (VMQ) | -108°F | -76°F | -58°F |
| Extreme Low Temp. (PVMQ) | -180°F | -175°F | -177°F |
| Fluorosilicone (FVMQ) | -90°F | -74°F | -70°F |
- Brittleness Temperature measures the temperature at which rubber becomes so brittle that a sample will break on impact when struck sharply.
- Youngs Modulus in Flexure measures how much a sample, supported by a simple beam, is bent by a measured weight at a measured low temperature. The temperature where modulus
reaches 10,000 psi is called "Youngs Modulus stiffening temperature" for rubber. - Temperature of Retraction (TR-10) measures the temperature at which a frozen sample becomes flexible enough to contract.
Radiation Resistance of Silicone
Exposure of silicone rubber to radiation will cause changes very similar to those caused by heat aging. Continued exposure to radiation will cause an increase in hardness and a decrease in both tensile and elongation. The effects are proportional to the total amount of the radiation level received. General purpose compounds remain flexible after exposure to 108 Rads.
| Theoretical results of exposure of silicone to radiation | ||
|---|---|---|
| Dosage (rads) | Elongation (%) | Tensile (psi) |
| None | 200 | 1200 |
| 5 x 106 | 130 | 100 |
| 5 x 107 | 50 | 900 |
| 5 x 108 | 20 | 600 |
References
1) Dow Corning Corporation (1979) Designing with silicone rubber p19
2) General Electric Company (1987) High performance elastomers p3
Gas Transmission Rate
Permeability through a 25 mil sheet in CC / cm2 / second with 1 Atmosphere of Pressure Differential
| ASTM D1418 | ||||||
|---|---|---|---|---|---|---|
| Silastic | Air | Nitrogen | Oxygen | Hydrogen | Carbon Dioxide | Helium |
| VMQ | 0.382 | 0.307 | 0.660 | 0.729 | 3.87 | 0.436 |
| PVMQ | 0.272 | 0.231 | 0.492 | 0.551 | 2.59 | – |
| FVMQ | 0.077 | 0.064 | 0.129 | 0.215 | 0.82 | 0.145 |
Silicone vs. Bagging Film – Environmental and Economic Concerns
Silicone rubber is considered somewhat expensive compared to the nominal cost of nylon bagging film. In today’s environmental conscious society some thought must be given to the disposal of a product once it has exceeded its useful life. Silicone rubber offers both economic and environmental assets when compared to other bagging methods.
Reduction of solid waste by use of silicone vacuum assemblies
- The composite manufacturing process has been very receptive to reusable silicone vacuum bags as a method of reducing costs when compared to the nylon film and sealant tape process.
- The reusable silicone tools result in less man hours per part and the resultant composite shape requires little or no further sanding to present an acceptable surface finish. The less sanding or finish grinding required, the less worker exposure to nuisance dust.
- The industry recently has become aware of the landfill disposal of solid waste material generated during composite manufacturing. A typical composite lay-up requires a compacting, debulk cycle periodically during the process. This can be required as often as after every ply. The nylon film is not normally reusable due to pin hole vacuum leaks that develop due to the folds and pleats necessary to film a contoured part. The film must then be discarded.
- Some composite structures can contain 50 or more plies. The amount of nylon film and bagging sealant is substantial. These materials do not breakdown when buried in a landfill and at present they are not recycled.
- A reusable silicone vacuum tool is completely recyclable. The tool assemble consists of the silicone membrane bonded to an aluminum frame and attached to a table or a tool surface. When the silicone bag has reached the end of its usefulness, it is removed from the aluminum frame. If the aluminum frame is still structurally sound, a new silicone bag can be attached and the tool returned to production. If the aluminum frame is damaged it can be repaired or scrapped in an aluminum recycling center.
- The scrap silicone blanket can also be recycled. In the United States, there are several companies that purchase the material, reclaim it and use the recycled silicone as an additive to non critical silicone products such as spark plug boots in the automotive industry.
Silicone can be reclaimed by either of two methods:
- cryogenic grinding – Cryogenic grinding consists of freezing the silicone with nitrogen and shattering the polymer to produce a fine powder material
- high pressure and high temperature de-polymerization in a steam autoclave. This method places the silicone in a container in a steam autoclave. The unit is heated by 250 psi steam atmosphere for 16 to 48 hours. The steam attacks the Si-O-Si backbone and results de-polymerization
Work has been carried out on replacing the expendable breather or bleeder cloth with a reusable silicone fabric breather. The breather can also be replaced by using a heavy imprint surface on the silicone sheet as a pathway for air movement during the bagging process.
The Economics of Silicone vs. Bagging Film
Below is a comparison between reusable silicone vacuum blankets and nylon film and tacky tape bagging systems. The comparison involves the manufacture of 100 composite articles. The composite article contains 45 plies of graphite prepreg and requires a debulk cycle every three plies. The shape of the tool does not provide a flat perimeter flange area to accommodate both sealing systems. Vacuum ports are installed in the bagging film and are permanently bonded on the silicone blanket. Costs are shown only for the seal systems. Release film and bleeder fabric would be required for both systems and are not included in this study. Material and labor costs are typical for the industry and have been taken from published data.
Material costs
- Tool size is 48 inches X 72 inches
- Reusable Contoured Silicone Blanket installed on Mosites VCS Tool System. This system is comprised of a silicone blanket bonded to an aluminum framework. The framework provides the vacuum channel to lock the blanket into position on the tool. Vacuum can be applied independently to the silicone blanket.
Total Cost of the VCS Seal System – $1,000.00
- Nylon bagging Film – $0.10 per square foot
- Tacky tape – $0.10 per lineal foot tool would require 24 square feet of film and 20 feet of tape per vacuum cycle.
Film – $0.10 X 24 sq ft = $2.40
Tape – $0.10 X 20 lineal ft = $2.00 - Composite Article Lay-up Procedure will require 15 debulk cycles prior to its autoclave cure. Because of the shape of the part and the chance of perforating the bagging film during the removal, it is discarded each debulk cycle along with the tape.
Film & Tape – $4.40 X 15 = $66.00 per article
Labor costs
- Calculated at 1 man @ $30.00/hour
- Reusable Silicone Blanket – 2 minutes to achieve vacuum.
2 minutes X 15 cycles = 30 minutes = $15.00 per article. - Nylon Film/Tacky Tape – 15 minutes to achieve vacuum.
15 minutes X 15 cycles = 225 minutes = $112.50 per article.
| Silicon – Total Costs Per 100 Articles | |
|---|---|
| Silicone System | $1,000.00 |
| 100 cycles X $15.00 labor | $1,500.00 |
| Total Cost | $2,500.00 |
| Nylon Film – Total Costs Per 100 Articles | |
|---|---|
| Nylon Film System $66.00 X 100 | $6,600.00 |
| 100 cycles X $112.50 labor | $11,250.00 |
| Total Cost | $17,850.00 |
Mosites #14287 Pressure Distribution Pad Rubber
Mosites #14287 silicone compound is a high hardness formulation that is sold in an uncured, calendered sheet form. It is unusual in that it can be co-cured in contact with typical epoxy prepreg reinforcements to fabricate stiff pressure distribution pads or caul plates. Since it is a silicone based product it has excellent resistance to high temperature exposure and natural release properties from accidental or intentional contact with epoxy resin. These properties can be improved by overcoating with Mosites #14232 Release Coating or application of approved release agents such as Freekote #700 before autoclave vulcanization of the composite article. By using epoxy prepreg reinforcement (either carbon or fiberglass fabric ) the elastomer can be stabilized to yield negligible shrinkage values in service.
Lay – up and cure procedure for Mosites #14287 silicone
- Clean and prepare the tool form in the usual manner for a lay-up procedure.
- Apply a release coating to the tool. A dry, uniform coating of Teflon Aerosol Spray is recommended. DO NOT use a silicone base mold release.
- Lay the first layer of Mosites #14287 uncured silicone into position on the tool. Using doubled nylon fabric and starting in the middle of the sheet, apply pressure to remove air that may be trapped under the first layer.
- Lay prepreg reinforcement into position. If desired, a debulk cycle can be applied using standard bagging techniques.
- Apply the top layer of Mosites #14287 silicone sheet and remove any entrapped air in the same fashion as step #3.
- Apply FEP release film to the layup and remove any wrinkles or air pockets.
- Vacuum bag the layup in standard fashion using breather / bleeder system under the vacuum bag.
- Cure the laminate by the epoxy prepreg manufacturer’s recommended cure cycle. Mosites #14287 requires at least 30 minutes at 300 degrees F for proper vulcanization. A pressure of 60 psi is recommended for the autoclave cure pressure. Longer cure times or higher cure temperatures or pressures to facilitate the epoxy resin cure system will not adversely affect the silicone properties.
- Allow the part to cool and remove it from the tool. If any areas require patching they can be repaired using uncured #14287 sheet. Be sure the rubber surface is cleaned to remove any residual release agent and repeat the cure cycle.
- No post cure is necessary.
Repair Methods for Silicone Blankets
Many applications for reusable silicone blankets involve the blanket being bonded or otherwise attached to a metal support frame. When a tear or leak develops in the membrane, a repair that does not require removal of the blanket from the frame is desirable. This can be readily accomplished with uncured silicone patch material when a heat source is available or with RTV Silicone Adhesive if a room temperature cure is necessary. Both methods are outlined below.
Heat Curable Silicone Patch Method
- This method uses uncured Mosites #1453 Silicone as the patch compound. Other
uncured silicone sheet compounds will work in the same manner (#14248 Maroon, #1495 Clear, #14116 Gray, etc.). - Inspect the torn area of the blanket. Remove any shredded or loose material. If possible, make a pattern that will cover all the damaged area. Avoid making square corners – rounded or oval patches reduce stress areas from developing in the corners. Remove the damaged area covered by the pattern with a sharp knife or blade. Transfer the pattern to your uncured silicone patch material and cut it slightly larger – to extend ½ inch on all sides.
- Clean the surface of the cured silicone thoroughly (MEK, alcohol, etc.). Place a piece of Teflon or FEP release film under the repair/patch area. Lay the uncured silicone patch into position and rub down firmly to remove entrapped air. Skive or feather the edges of the patch.
- Cover the patch with a layer of release film. Place a metal plate -aluminum is preferred – on both sides of the repair area. Use a "C”clamp or vise grip pliers to apply light contact pressure to the metal plates.
- Use a heat gun to apply heat to the metal plate. Be careful not to allow the silicone blanket to contact the heat gun nozzle or be exposed directly to the extreme temperature of the gun. A distance of approximately 6 inches from the plates will result in a temperature of 300 – 350 degrees F. Maintain this temperature for at least 15 minutes. (A thermocouple or monitor at the blanket patch is ideal.) After the cure time has elapsed, the clamping fixture can be removed and the blanket inspected and placed back into service.
Mosites TCT-6 Heater Kit for Silicone Repair
The heater kit described in this paragraph consists of a silicone heater blanket, capable of producing a temperature of 300 degrees F, bonded to a layer of thermally conductive silicone rubber with a face cover of 5 mil Teflon film. The heater blankets can be made in various sizes, but TCT-6 refers to a 6 inch diameter heater. The unit works on standard 110 volt electrical outlets and produces the desired temperature without the necessity of a transformer. The Teflon film face cover allows easy clean-up and prevents unwanted adhesion of patch materials. The conductive silicone layer provides uniform dispersion of the temperature supplied by the strip heaters in the silicone heater blanket. This kit can be used in the repair procedure outlined above, and eliminate the need for a heat gun. The metal plates and a source of light clamping pressure are still necessary.
Room temperature cure silicone patch method
- Clean the silicone surface in the area that is to be repaired with solvent (MEK, acetone, alcohol, etc.)
- Examine the damaged area. A linear cut can be repaired with a 2 inch wide "batten strip” of 1/32 inch thick cured silicone sheet. Allow 1 inch of patch material to extend on each side of the cut. Apply the patch on the outside of the blanket to avoid mark-off on the part. Use a uniform coating of GE – RTV -108 Silicone Adhesive on the cured silicone patch material. Lay it into position, press it down to remove entrapped air, remove any excess RTV that exudes from the edges, use a skive or spatula to feather the edges. Allow the repair to cure at ambient temperature for at least 24 hours before using.
- If the tear is jagged or material is missing, make a pattern of the repair area that will remove all the damaged silicone. If possible avoid square corners – rounded corners or oval patches help reduce stress areas from forming. Transfer the pattern to the blanket and to a piece of cured silicone of the same thickness. Use a sharp knife or scissors to cut the rubber. Check the patch for fit in the blanket; mark the side of the patch that will face the tool.
- Lay the patch on the 1/32 inch thick cured silicone sheet. Cut the 1/32 inch thick sheet oversize by 1 inch per side. Use RTV -108 Adhesive to bond the patch to the 1/32 sheet. A bond layer of 15 to 25 mils is sufficient for good adhesion. Apply light pressure to remove air from the RTV. Remove any excess adhesive from the perimeter of the patch and allow the RTV to cure at room temperature for 2-3 hours or until the laminate can be handled.
- Apply RTV-108 Adhesive to the 1 inch border of the 1/32 inch sheet and place the patch into position on the blanket. Use light pressure to exclude air from the patch. Excess RTV can be removed and the patch edges feathered to make a smooth transition.
The RTV-108 Adhesive requires at least 24 hours at room temperature before stress can be placed on the bond joint. Optimum cure usually develops in 3 to 7.
