Quantification of RTV Intumescence using X-ray Imaging

When spacecraft plunge into the atmosphere of a planet, the air in front of them compresses so violently that it turns into a searing plasma, heating the vehicle's surface to temperatures that can melt steel. To survive this, engineers wrap these vehicles in a thermal protection system, a shell made of special tiles designed to absorb and shed that intense heat. However, these tiles cannot be placed edge-to-edge; they must be separated by small gaps to allow for the expansion and contraction of the materials as they heat and cool. These gaps are filled with a soft, rubber-like sealant known as room-temperature vulcanizing silicone. This material acts as a flexible glue, holding the tiles in place and preventing the superheated air from leaking through the cracks. The problem is that when this silicone gets too hot, it does not just sit there; it reacts chemically, releasing gases and swelling outward like a rising loaf of bread. If this swelling becomes too large, the material can push into the flow of air, disrupting the smooth layer of air that protects the spacecraft and causing even more heat to build up, potentially leading to failure.

Scientists have long known that this swelling, or intumescence, happens, but they have struggled to see exactly how it occurs inside the material while it is being heated. To understand this hidden process, a team of researchers from the University of Illinois and Lawrence Berkeley National Laboratory decided to watch the silicone change shape in real time. They used a powerful X-ray machine, similar to the ones used in hospitals but far more intense, to take thousands of three-dimensional pictures of the material as it was heated. By combining these images with chemical analysis, they were able to track how the rubber bubbled, cracked, and shrank under different conditions, revealing that the speed at which the material heats up and the environment around it dramatically change how much it swells.

The researchers began by studying the chemical breakdown of the silicone. They heated small samples in a controlled environment and measured the gases escaping as the material decomposed. They found that the silicone does not break down all at once. Instead, it goes through several distinct stages as the temperature rises. First, it releases trapped water and air. Then, as the heat intensifies, the long molecular chains that make up the rubber begin to snap, releasing heavier gases and leaving behind a porous, carbon-rich char. The speed at which the material is heated matters immensely. When heated slowly, the gases have time to escape through tiny cracks that form in the material, allowing the silicone to shrink back down after it swells. But when the heat is applied rapidly, mimicking the conditions of a spacecraft entering a planet's atmosphere, the gases get trapped inside. This trapped pressure builds up quickly, forcing the material to expand much more than it would under slower conditions.

To visualize this expansion, the team placed samples of the silicone inside a special chamber and heated them while taking X-ray images. In one set of experiments, they heated the samples slowly, taking three-dimensional scans every minute. They saw that the material first expanded slightly due to simple thermal expansion, then swelled significantly as gases formed inside closed pockets within the rubber. Eventually, as the material developed cracks and pores, these gases escaped, and the sample shrank back down. However, when they constrained the sample so it could only expand in one direction, it swelled even more and shrank more slowly, because the gases could not escape as easily. This confirmed that the physical shape of the material and how it is held in place play a major role in how it reacts to heat.

The team then turned to the more extreme conditions relevant to actual spaceflight, heating samples at rates far faster than before, reaching temperatures that rise by nearly two thousand degrees Celsius per minute. In these rapid tests, they observed a dramatic difference. The silicone swelled to more than twice its original height before it could shrink back. They also tested samples that had been pre-treated to remove trapped air and water. Surprisingly, these "degassed" samples swelled even more than the untreated ones. This counterintuitive result happened because the untreated samples already had tiny open pathways for gas to escape, while the degassed samples had fewer escape routes, causing the internal pressure to build up to higher levels before the material finally cracked and released the gas.

The environment surrounding the material also changed the outcome. When the researchers heated the silicone in air instead of an inert gas like argon, a hard, glass-like layer formed on the surface of the sample. This layer acted like a skin, trapping the gases inside and preventing the material from shrinking back down after it swelled. As a result, the samples in air remained swollen and rigid, whereas those in the inert gas eventually shrank. This finding suggests that the chemical reaction with oxygen in the atmosphere could significantly alter how the sealant behaves during a real mission.

Finally, the researchers looked at how the silicone behaves when it is not just sitting in a container but is actually bonded to the porous tiles it is meant to protect. They soaked the liquid silicone into the tiny holes of a carbon-fiber tile and a similar porous material before letting it harden. When they heated these bonded samples, the swelling was much less than in the free-standing samples. The porous tile acted like a sponge, soaking up the gases that the silicone tried to release and providing a path for them to escape. This "porous relief" kept the internal pressure lower, preventing the dramatic swelling seen in the other tests. This is a crucial discovery for spacecraft design, as it suggests that the way the sealant interacts with the tiles it bonds to can naturally limit the swelling and keep the heat shield safe.

By combining these X-ray images with chemical data, the researchers built a clear picture of how this vital sealant behaves under the stress of atmospheric entry. They found that the swelling is driven by trapped gases that build up pressure when the material heats up too fast to let them escape. The extent of this swelling depends on how quickly the heat is applied, whether the material has been pre-treated to remove air, and whether it is bonded to a porous surface that can help vent the gases. These insights provide engineers with the data they need to predict how the sealant will behave on future missions, ensuring that the gaps between heat shield tiles remain safe even as the spacecraft faces the extreme heat of entering a new world.