Rigid polyurethane foam is useful in cars, buildings, mining and aerospace—but it burns easily, releases toxic gases, and can trap enough heat to help start a fire. This study asks whether the same material can be made to absorb heat and slow flames.

Rigid polyurethane foam has many useful properties and is used widely in automotive, mining, machinery, waterproofing, energy-saving, light industry and aerospace applications. But rigid polyurethane foam is flammable. Burning can release toxic gases, while heat released during production or use can build up and threaten people’s lives and safety.

So the problem is twofold: the foam needs to resist flames and handle heat more safely. Earlier research mainly examined the capsules’ structure, strength and ability to store heat, while fewer studies examined whether adding them could make polyurethane foam resist fire.

That gap led to a simple question: could a heat-storing filler also improve the foam’s flame-retardant behavior during a fire? The capsules used chitosan and gum acacia as a wall around a paraffin core, prepared through a condensation method in this study. The capsules were then placed inside rigid polyurethane foam, and the resulting material was examined for changes in structure, heat storage, stability and flame resistance.

The intended result was a low-cost way to manage heat and improve the usefulness of polyurethane foam in construction, mining and other fields. Pure rigid polyurethane foam showed no clear sign of melting or heat absorption during a phase change.

Foam containing the capsules did show a clear heat-absorbing peak. As more capsules were added, the heat-absorption peak and the melting area grew, and the temperature at which the phase change began also rose. With thirty percent capsules, the foam stored 12.308 joules of heat per gram during the phase change.

The untreated foam burned readily under ordinary oxygen conditions, with an oxygen requirement of only 19 percent to keep burning. Adding the capsules raised that requirement above twenty-one percent, although the reported improvement in oxygen index was relatively small overall.

At thirty percent capsules, the oxygen requirement reached 26.1 percent, the highest value reported for these composites. Adding CS/PCM makes the foam release less heat and smoke during burning: both the total heat and total smoke produced fall as more is added.

That matters because it points to a fire that is not only harder to sustain, but also creates less dangerous smoke. With thirty percent capsules, the foam’s peak heat-release rate fell by 66.5 percent compared with pure rigid polyurethane foam, while the peak smoke-generation rate fell by 55.8 percent.

The heat-release peak was no longer sharp because the phase-changing capsules absorbed part of the released heat and slowed its release. The study’s flame-resistance index rose from good performance at lower additions to excellent performance at the highest addition, where the index reached 10.39.

There is a catch: the capsules contain flammable wax, so adding more wax can harm flame resistance. But the chitosan shell works in the opposite direction and improves it. When flames attack the shell, it releases gases that cannot burn.

These gases dilute the combustible gases and oxygen around the foam. The chitosan shell has a positive effect on improving the composite’s overall flame-retardant performance within the proposed mechanism described here. In the condensed phase, chitosan can form a carbon-nitrogen bond and promote the formation of a protective carbon layer during burning.

The added capsules did not change polyurethane’s main structure, and the foam still had its bubble-filled form. At the same time, its maximum rate of breakdown decreased. At thirty percent capsules, the foam stored 12.308 joules of heat per gram, needed 26.1 percent oxygen to keep burning, and released 66.5 percent less peak heat than pure foam.

The capsule shell can dilute flammable gas and block heat, smoke, and combustible-gas transfer, improving the composite’s flame-retardant performance overall. The paper presents this as a feasible thermal-management solution for RPUF and a new route toward functionalized foam materials in construction and transportation.

Tiny wax-filled capsules made the foam absorb heat while forming a protective char barrier and reducing heat and smoke release. That could make widely used insulation and structural foam safer in real applications.