Thinking "Out of the Box"

As a custom silicone formulator, we like to be challenged, and we encourage customers to challenge us with their unique needs. To address these challenges we often think “out of the box” because, to meet previously unmet needs, you have to think differently and almost always have to do something very different. Our business model dictates our direction into new technologies. We have no sales staff or marketing activities beyond a website. As a technology company, we rely on new product development for business growth by meeting customer challenges. The following are some examples of how “out-of-the-box” thinking resulted in some unique products and solved customer problems.

Tactical Vehicle Undercoating

The military required a new tactical vehicle undercoating that could meet the new demands of bio-hazard exposure and would hold up in the desert environment. They required a steam-sterilizable coating that would also resist constant exposure to wind-driven and travel-induced sand impact. The current product, a urethane, was being sand-blasted off. This resulted in loss of underbody metal and loss of armor. It also did not hold up against steam exposure. The coating came off like hot tar, dripping everywhere.

The Office of Naval Research (ONR) testing facility also added the following to their wish list for the new product: gasoline resistance, flame resistance, chemical resistance, low VOCs and impact resistance. No single product on the market today addresses two or three of these requirements.

To address the most demanding aspect of the project, we needed an accelerated test to simulate desert sand exposure. Our staff set up a test method inside a sand-blasting cabinet. Using 90 PSI air to propel sand, and placing the target 10 inches from a fine nozzle, we found that the current product, a very high-modulus, high-durometer (almost crystalline) coating lasted only two seconds in the sand-blasting environment. Due to its very high modulus, it lacked the energy absorbing and dissipating capability in its crosslink network.

Asphalt-based, commercially available products were also tested. For these products the solvent dissipated, resulting in a hard tar-like coating that readily came off in two seconds. We felt that the harder a coating is, the more prone to sand blasting. In softer coatings, sand bounces off. This pointed us in the opposite direction of the current technology and toward a very soft coating. At first glance, it seems counter intuitive to utilize a soft undercoating. How could a soft coating withstand repeated impact from rocks and sand? The answer lies in the crosslink density.

Silicone elastomers can be formulated to have a very low modulus. This, combined with their typical capability of having great energy-absorbing and dissipation capability in their bonds, makes them a perfect choice.

The first coating developed was a low-modulus, electrometric moisture-cure RTV that withstood five minutes in the sandblast cabinet and displayed no damage to the surface. The sand impact energy was absorbed and the particles were repelled. After all of the additional desired properties were included in the product, full testing was undertaken at ONR. This included stone impact testing, where stones are shot out of a cannon into the target. The target here was a steel panel coated to 0.020 inches with the new tactical vehicle undercoating, SS-3000. The impact energy was absorbed and repelled. Much like a suspension bridge, a stiff bridge will fail in a light wind. Flexibility is required in bonds and crosslink networks to survive repeated impact. In March 2006, we passed all tests and have been approved by the ONR. Previously only fluorosilicones had passed gasoline resistance testing.

Sprayable Elastomeric Product

Industrial smokestacks and ducts are often coated with high-temperature organic compositions to extend their useful lifetime. Typical exposure conditions are: cyclic heat to 650 °F, steam/water vapor, acid vapors and particulate exposure. These organic coatings appear crystalline upon curing and are stable when brought up to operating conditions within their temperature capabilities. However, upon the cooling of the stacks in a cyclic heating condition or upon a shut down, the coatings crack. After cracks form, corrosion starts in the metal or masonry materials.

Upon examination, I found that the cracks formed because of the differential in thermal expansion and contraction of the cured coating from the stack or duct material. The crystalline material, while bonded to the stack material, lacked the flexibility required in the application. Again, I felt that the solution lay in a drastically different approach: a non-crystalline, low-modulus formulated silicone elastomer that has very high joint movement capability. This product will stretch and compress while bonded to the substrate through hundreds of heat-up and cool-down cycles, accommodating the changes in dimensions and maintaining coating integrity.

Lab testing was followed by successful field testing. The developed sprayable elastomeric product, SD7000, is also acid resistant and has much higher temperature resistance than any organic coating. Here again, the flexibility of a softer coating resulted in matching the application’s needs with the material’s capabilities.

Energy-Damping Gel

Military officials contacted an associate in Washington about an immediate need in the field. Troops encounter armor-piercing rounds. These bullets are stopped by the Class IV body armor that is worn, however the force of the impact renders troops unconscious. There was an immediate need for a solution. A cushion-like coating material was requested for the armor plates/vest, which would absorb the shock and mitigate the problem. Most organic foams and gels are not well designed to absorb very high shocks.

Silicones are comprised of siloxane bridges, which have alternating silicone and oxygen atoms. Silicones offer a lot of free space between molecules due to the large oxygen interchain species and, the siloxane bond results in a larger bond angle than the carbon – carbon bond, 130° vs. 109.5°. The free rotation around the silicon atoms by the pendant methyl groups also enables very high-energy dissipation. As compared to siloxanes, the carbon-to-carbon bond length is shorter, 1.54 Å vs. 1.63 Å. Also, carbon-carbon bonds have lower dissociation energy than siloxanes, 108 vs. 117. All of these factors combine to give a tougher, high-energy-absorbing substance. This is why silicone polymers are often called “liquid springs”.

In spite of all these favorable molecular stability aspects, an incredibly complex formulation/optimization would still be required to absorb the very high impact forces of a projectile traveling 2800 feet per second (fps). Many gels were lab tested, and several gels were field tested by firing rounds into coated armor plates. SS-1000, an energy-dampening gel with extremely low crosslink density, was successful in absorbing and dissipating the impact forces. This is a situation that fully reinforces the two previous cases where the toughest product was the softest, as this product is very soft and meets the needs of the application.

Conclusion

Silicones offer many more unique characteristics. In this brief article I have addressed only the energy-absorbing and joint movement characteristics. There are many more unique (as compared to organics) characteristics that should be addressed, such as: low surface energies; high temperature stability; high gas transfer capability; low temperature stability; acid resistance; electrical properties; adhesion and abhesion characteristics; cure flexibility capability; resistance to UV, ozone, and sunlight; unique surface effects; and many more. In future articles, I hope to address these other aspects of silicone technology.

For further information contact www.siliconesolutions.com or e-mail Dave@siliconesolutions.com.