Impact Foams: Why they’re so important in body armor and how they work

Summary of Key Points:

Foams play a critical, underappreciated role in modern body armor, serving three vital functions:

• Mitigating Deformation: They absorb and dissipate energy from ballistic impacts, reducing trauma to the wearer and protecting brittle materials like ceramics.
• Structural Protection: Foam layers shield armor components (e.g., ceramic strike faces) from shocks, drops, and environmental damage.
• Comfort and Wearability: By distributing forces over time and area, foams reduce pressure points and improve mobility.

Mechanisms of Action:

Absorption: Foams convert kinetic energy into heat/strain via cell collapse, minimizing energy transfer to the body.

Dissipation: They spread impacts over larger areas and prolong collision duration, lowering peak force.

Elastic Rebound: Resilient foams avoid “bottoming out” by partially rebounding, ensuring gradual deceleration.

Material Limitations and Innovations:

Traditional foams (EPS, EPP, shear-stiffening variants) often lack balance—sacrificing rebound, temperature resilience, or energy absorption.

IvoryGuard exemplifies next-gen foam design: closed-cell structure, ultra-lightweight (~0.068 gm/cc), and temperature-stable performance. It optimizes all three mechanisms while maintaining flexibility across environmental conditions.

Why This Matters:

Foam is often the largest component by volume in armor systems. Advancements in foam technology directly enhance protection, comfort, and weight savings.

Foams in Body Armor

When people talk about contemporary armor materials, the focus tends to be on composites or ceramics.  Those are, after all, the strong materials that actually stop bullets in today’s most advanced armor systems.  But body armor – armor that people wear against their bodies – is about more than just stopping bullets.  It’s also about mitigating armor deformation upon ballistic impact, about protecting ceramic tiles that have poor shock and drop resistance, and about comfort.  For these reasons, the average armor plate or soft armor panel is often as much as half foam by volume. 

Lightweight level III/RF1 UHMWPE plates can be more than half foam.  UHMWPE tends to deform a lot, and that deformation usually needs to be mitigated for compliance with ballistic standards or specifications.  So if, for instance, an armor plate is 1.1” thick, roughly 0.6” of that thickness could very well consist of foam padding. 

Heavier ceramic armor plates, built to counter tougher threats, are usually made up of roughly a half-dozen different materials.  From front to back:

• Front cover
• Foam (ceramic strike-face protection)
• The ceramic itself
• A backing layer (usually UHMWPE in a resin matrix)
• Foam again (backface deformation mitigation)
• Rear cover.

And there are adhesive layers between them all, some of which can be nearly 1mm thick. 

Of these, on a volume basis, foam is usually the largest single material component.  There’s almost always more foam than ceramic.

Performance improvements in foam materials therefore translate directly to performance improvements in armor plates and panels.  How foams work, and how to best utilize them, is of paramount importance in body armor design.  (Though, needless to say, foams are far less important in other forms of armor design; they don’t matter much if you’re building an armored vehicle.)  It was also the case that foams were the “low hanging fruit” of body armor system design – there was a lot of room for optimization.  So we’ve worked to optimize them.  To understand how, let’s review how they work.

Protective Foam Mechanisms of Action

Protective foams in armor work by a combination of key physical mechanisms that cushion and manage impact forces. The three primary mechanisms are absorption, dissipation, and elastic rebound.

Absorption: When an impact occurs, foams absorb energy by deforming internally. The foam’s cell structures compress and collapse, converting kinetic energy into other forms (primarily internal strain and heat) instead of letting it transmit to the body behind the armor​.

In effect, the foam soaks up impact energy by crushing cell walls, stretching polymer chains, and generating a bit of heat, rather than transferring that energy to your body.  A foam’s ability to deform plastically under a heavy impact ensures a large portion of the impact energy is absorbed and not returned. Essentially, the more energy the foam absorbs internally, the less reaches you.

In closed-cell foams specifically – such as IvoryGuard – the sealed air bubbles compress, increasing internal pressure, converting impact energy into stored gas pressure and additional heat. This further enhances the foam’s energy-absorbing capability.

Dissipation: Foams also dissipate and distribute the force over a longer time and a larger area. When a foam is compressed, it slows down the impact, stretching out the collision duration. By prolonging the impact event, the foam lowers the peak force.  (Since the same change in momentum over a longer time means less force at any instant.) ​ At the same time, foams spread the load over a larger area by deforming and contacting more surface. A soft layer will distribute a concentrated blow across a broader region, reducing pressure on any single point. The combined effect is that the force is diluted – lower pressure and lower peak acceleration – which greatly reduces injury risk.

A well-designed foam pad behind a hard armor plate will deform and stretch laterally, spreading the force of a bullet impact over the whole pad area rather than a small spot on the body. IvoryGuard foam is engineered to excel at this kind of dissipation. Its closed-cell structure and high ductility allow it to compress predictably without cracking, so it can prolong and spread out an impact even at high strain rates​. Notably, because IvoryGuard is extremely light (density ~0.068 gm/cc, even lighter than most commercial forms of silica aerogel​), one can use a thicker layer of it without adding much weight. Thicker foam means more distance and time to cushion an impact, which further improves dissipation.

Elastic Rebound: Unlike sand or clay which just deform and stay deformed, polymer foams often have elasticity – meaning they spring back to shape. This elastic rebound is the foam returning some energy after absorbing the rest. When an impact compresses the foam, the material stores a portion of the energy as elastic stress, like a compressed spring. After the impact, the foam expands back, releasing that stored energy by pushing the impacting object away in a controlled bounce. A small amount of rebound is useful: It means the foam didn’t completely collapse – it had some “spring” left to give – and it helps separate the impacting object from the protected surface, finishing the impact with a gentle push-off rather than a hard stop. Elastic rebound also allows the foam to recover its shape for the next impact. However, too much rebound, as you’d get with solid rubber, would return a lot of energy to the impactor, potentially causing a ricochet or a second hit. The key is that some energy return is desirable – it indicates the foam is resilient and avoided bottoming out – but it must be combined with absorption. Many impact foams are thus viscoelastic, meaning part springy (elastic) and part lossy (viscous damping). On impact, they absorb most energy as heat but return a fraction as a bounce.

Balancing these mechanisms is crucial for optimal protection. If a foam only absorbed energy but had no rebound at all, it might “bottom out” on a severe impact – compressing completely such that the foam pad eventually becomes a hard unpadded surface. Bottoming out causes a sudden rise in force, as any remaining impact energy transmits directly, negating the benefits of the foam. Conversely, if a foam was very elastic and barely absorbed energy, it would simply spring the object back, leading to high peak forces and not enough cushioning. The best protective foams carefully tune absorption vs. rebound. They absorb as much energy as possible, to minimize what’s transmitted, while being just stiff and resilient enough to slow down the impact gradually and avoid bottoming out. 

Most foams don’t make full use of all three mechanisms.  EPS, though very light, doesn’t rebound at all – it crushes and stays crushed.  EPP is slightly better in this regard, but it’s still permanently spent on impact, if it’s hit hard enough.  Rubber foams, though very elastic, don’t absorb much energy internally.  Shear-stiffening foams like D3O and Poron XRD excel at rapidly stiffening under high strain rates, absorbing significant energy internally through molecular rearrangements and increased internal friction.  However, despite their elastic nature, these foams do not fully leverage elastic rebound, resulting in less-than-optimal performance.

There’s another problem with shear-stiffening foams, and it’s a big one:  Their efficacy is closely tied to their glass-transition temperature (Tg).  These foams work best when their polymer chains are near – but above – their Tg; at ambient conditions they’re soft and flexible, but under rapid impact, the high strain rate temporarily “freezes” molecular motion, yielding a stiff, glassy-like state that dissipates energy.  This response is reversible and not due to a permanent shift in Tg.  The problem is that at temperatures below their Tg – which can be as high as 0°C –  these foams are stuck in the stiff phase and can be brittle and rigid, whereas at high temperatures they’re stuck in the soft phase and lose the ability to stiffen upon impact.  In both cases performance is severely impaired.

Such challenges have inspired innovations in foam technology, with advanced formulations – such as those used in IvoryGuard – designed to optimize the balance of absorption, dissipation, and elastic rebound, over a far broader range of temperatures.

Future posts will go into more detail and will examine how this looks in practice.