Environmental and Economic Advantages of Silicone
Silicone vs. Bagging Film
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.
- 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
- 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|
|100 cycles X $15.00 labor||$1,500.00|
|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|