Empa Develops Recyclable Flame-Retardant Epoxy Composite for Aircraft Interiors

Updated on September 07, 2026 • 6 min read

Researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa), working with specialty chemicals company Elantas, have developed an epoxy-based sandwich composite that combines flame retardancy with the ability to be dismantled and recovered at end of life.

The work targets a long-standing problem in thermoset composites.

Epoxy resins are widely used because they provide strong bonding, dimensional stability and thermal resistance. Once cured, however, their crosslinked molecular network is difficult to reverse, making it hard to separate valuable fibers, cores and other constituents for recycling.

Empa’s approach modifies that chemistry by introducing a reactive phosphorus-containing additive into the epoxy system.

The additive serves two functions: it contributes to flame retardancy and introduces dynamic chemical bonds that allow the cured network to rearrange or break down under controlled conditions.

Aircraft Sandwich Panels Provide the Test Case

The researchers focused on a lightweight sandwich construction representative of materials used in aircraft and rail interiors.

The structure combines an aramid honeycomb core with flat woven layers of glass or carbon fiber above and below the core.

Epoxy resin impregnates and bonds the reinforcement layers, turning the individual materials into a stiff, lightweight sandwich panel.

This architecture is attractive for transportation because it delivers high stiffness at low mass.

It also demonstrates why composite recycling is difficult.

Once the epoxy cures, the reinforcement fabrics and aramid honeycomb become locked together inside a permanent thermoset network.

Recovering those constituents without seriously damaging them can be technically difficult.

Dynamic Bonds Make the Thermoset Reprocessable

Empa’s chemistry is based on reactive phosphorus-containing molecules incorporated into the epoxy before curing.

Earlier work from the institute used a bis H-phosphonate compound to create a phosphonated epoxy network containing phosphorus-carbon and phosphorus-oxygen bonds.

Under suitable thermal and processing conditions, parts of that network can undergo bond-exchange reactions.

This means the crosslinked structure is no longer completely static.

Instead of remaining permanently fixed until thermal degradation occurs, the network can rearrange through dynamic chemistry.

Empa previously demonstrated that sufficiently modified epoxy formulations could be thermomechanically reprocessed after curing.

The new work with Elantas extends that principle from reshaping the resin itself to disassembling a complete sandwich composite.

Heat and Solvent Separate the Composite Constituents

Using controlled heat together with an appropriate solvent, the researchers were able to break down the epoxy system sufficiently to separate the composite into its major structural components.

The process enabled recovery of the aramid honeycomb and woven reinforcing fabrics.

This is an important distinction.

The current result demonstrates recovery of the valuable structural constituents that would normally remain bonded inside the epoxy matrix.

It does not yet represent a completely closed-loop recycling process for the epoxy itself.

Empa says recovery of the resin from the resulting solution should also be possible and plans to investigate that in follow-up work.

Phosphorus Also Improves Fire Performance

The same phosphorus chemistry used to enable dynamic behavior also contributes to flame retardancy.

That is particularly relevant for aerospace and rail interiors, where recyclability cannot be achieved by compromising fire performance.

According to earlier Empa testing, a formulation containing 2.5% phosphorus reduced peak heat-release rate by 75% and total smoke production by 72.5% compared with the reference thermoset in cone-calorimeter testing.

Higher phosphorus concentrations were needed to achieve effective reprocessing behavior.

Empa reports that the composite developed with Elantas satisfies the required fire-safety regulations while maintaining mechanical properties close to those of conventional epoxy-based materials.

The exact certification framework and full mechanical dataset were not provided in the supplied source, so those performance claims should be interpreted within the reported project scope.

Why This Matters for Aerospace Composites

Aircraft interiors are a logical application for this chemistry.

These structures must combine low mass with mechanical stability, fire resistance and long service life.

At end of life, however, the same durability that makes thermoset sandwich composites useful becomes a recycling obstacle.

Carbon fiber production is energy intensive, while aramid cores and woven reinforcement fabrics are also engineered materials with substantial embedded value.

A process that allows those materials to be separated with less damage could preserve more value than routes that reduce the complete composite to low-grade recyclate.

The challenge is making that process practical at industrial scale.

Professor’s Analysis: Recyclable Epoxy Is Becoming a Chemistry Problem, Not Just a Waste Problem

The important development here is not simply that an epoxy composite can be dismantled.

It is that the end-of-life behavior is being designed directly into the resin chemistry.

Conventional composite recycling usually starts after the structure has already been manufactured.

The question is then how to break apart a material system that was deliberately engineered to remain permanently bonded.

Dynamic thermoset chemistry reverses that logic.

The resin is designed from the beginning with bonds that perform like a conventional thermoset during service but can undergo controlled rearrangement when exposed to a specific recycling environment.

That approach is especially interesting for aerospace because the material cannot sacrifice fire resistance, mechanical properties or long-term stability simply to improve recyclability.

The phosphorus chemistry also illustrates how one molecular modification can influence several functions at once.

It contributes to flame retardancy while also enabling dynamic network behavior.

But several questions remain.

Solvent consumption, recovery efficiency, process temperature, resin recovery, repeated recycling behavior and the quality of recovered fibers and cores will all determine whether the technology is economically and environmentally attractive at commercial scale.

There is also a broader system issue.

A recyclable resin does not automatically make an aircraft interior panel circular.

Adhesives, coatings, inserts, surface films, fasteners and contamination introduced during service can all affect actual recovery.

For this reason, the next stage is less about proving the chemistry works and more about demonstrating a repeatable industrial recycling process for complete components.

Scaling Is the Next Challenge

Empa and Elantas are now working to scale both composite manufacturing and the recycling process.

The chemistry could also have applications beyond aerospace.

Fiber-reinforced epoxy systems are widely used in rail vehicles, automobiles, ships, wind turbine blades, electrical systems and construction.

Empa is already exploring additional applications in the energy and construction sectors.

If the process scales successfully, its larger significance may be that it provides a route toward thermoset composites that remain permanent during use but become deliberately reversible at end of life.

That is a fundamentally different design philosophy from conventional epoxy composites.