Rigid polyurethane foam is everywhere, but it burns easily. This paper shows how wrapping wax in chitosan shells creates a material that absorbs heat and stops fires. Rigid polyurethane foam has excellent properties but is unfortunately a flammable material that emits toxic gases when it burns.

Heat released during its synthesis can cause accumulation, posing a great threat to people's lives and safety. While many studies focus on mechanical properties, there are few investigations into the flame-retardant properties of polyurethane foam after adding phase-change materials.

To address this gap, the authors prepared a new type of phase-change microcapsule with specific flame-retardant properties. Figure 1 illustrates the schematic preparation procedure for chitosan phase-change microcapsules, or CS/PCM. The diagram depicts a three-step process where paraffin wax cores are first suspended in a shell solution, followed by shell cross-linking around the core, and finally shell curing to form the complete microcapsule.

This encapsulation strategy is critical because it allows the authors to integrate these thermal storage materials into rigid polyurethane foam composites while preventing leakage of the core material. The process involves melting four grams of paraffin wax and four grams of gum Arabic at sixty-five degrees Celsius.

After adjusting the pH to four point five, glutaraldehyde is added to cross-link the solution for curing. Figure 2 provides a detailed look at the physical characteristics of the CS/PCM additive. Panel (a) presents a scanning electron microscope image, revealing that the particles are roughly spherical with textured surfaces and appear to range in size up to several hundred micrometers.

Complementing this visual inspection, panel (b) displays a histogram of the particle size distribution, where the frequency peaks around two hundred micrometers. Establishing these specific dimensions is crucial because the text notes that adding these particles directly influences the cell size of the resulting foam material.

As observed in the images, the particle size of the microcapsules ranges from one hundred micrometers to three hundred micrometers following a normal distribution. When thirty weight percent of the microcapsules is added, the cell structure arrangement is disrupted and the cell walls become thin.

Figure 7 presents the thermogravimetric analysis (TG) and derivative thermogravimetry (DTG) curves for the CS/PCM capsules and RPUF composites containing varying amounts of these additives. The TG data on the left illustrates a three-stage weight loss process, where an initial drop near one hundred degrees Celsius corresponds to surface water evaporation, followed by rapid pyrolysis between two hundred and three hundred fifty degrees Celsius as the core materials degrade.

The DTG plot on the right highlights the rate of this mass change, showing that the addition of CS/PCM shifts the degradation profile compared to pure RPUF, indicating how the composite's thermal stability evolves with increased additive content. For the composite with five weight percent microcapsules, the starting degradation temperature is two hundred fifty-eight degrees Celsius.

Increasing the content to thirty weight percent lowers the starting temperature to two hundred three degrees Celsius. Figure 8 presents Differential Scanning Calorimetry curves for the CS/PCM and RPUF composites at four distinct heating rates, revealing how thermal peaks shift with temperature.

The authors then utilize these peak temperatures to calculate activation energy using three different kinetic models: Kissinger, Ozawa, and Starink. As shown in panel (d), the linear fitting of these methods yields correlation coefficients close to 0.99, confirming that the experimental data is reliable for determining the material's thermal stability parameters.

According to the differential scanning calorimetry curve, the latent heat of phase transition for the composite with thirty weight percent is twelve point three zero eight joules per gram. After adding the microcapsules, the peak phase transition temperature of the composites increased because some heat is used for material degradation.

Table 1 presents the dynamic parameters of phase transformations for three distinct samples: CS/PCM, RPUF-20, and RPUF-30. The authors calculated activation energies using three separate methods—Kissinger, Ozawa, and Starink—yielding values like 404.65 kJ/mol for the first sample and 869.66 kJ/mol for the third.

These calculations are supported by high correlation coefficients, with R-squared values reaching 0.99, which confirms the reliability of the experimental data across all tested materials. Thermal conductivity measurements show that values increase with the addition of microcapsules at both thirty and sixty degrees Celsius.

At sixty degrees Celsius with twenty weight percent loading, conductivity is twenty-five point eight seven percent higher than pure foam. Figure 10 displays the Limiting Oxygen Index, or LOI, for RPUF composites as the additive amount of CS/PCM increases from five to thirty weight percent.

The bar chart shows that the LOI value rises steadily with each increment in additive content, reaching its highest point at the thirty weight percent level. This trend indicates that incorporating more CS/PCM into the material requires a higher concentration of oxygen to sustain combustion, suggesting an enhancement in flame retardancy.

Pure rigid polyurethane foam is classified as a class B3 flammable material that burns readily with a low limiting oxygen index of only nineteen percent. When the addition amount reaches thirty weight percent, the limiting oxygen index of the composite reaches a maximum of twenty-six point one percent.

Figure 11 presents cone calorimetry data tracking the fire behavior of RPUF composites with varying CS/PCM content. The four panels display the change curves for heat release rate, total heat release, smoke production rate, and total smoke release over time.

These metrics allow the authors to evaluate how adding the additive alters the material's combustion profile compared to pure RPUF. Table 2 presents the cone calorimeter data for RPUF samples containing different contents of CS/PCM, tracking metrics like time to ignition and peak heat release rate.

As the sample composition changes from pure RPUF to RPUF-30, the peak heat release rate drops from two hundred seventy-nine point five eight kilowatts per square meter to ninety-three point five seven. The table also records a corresponding decrease in total heat release, which falls from seventeen point five seven kilojoules per square meter down to seven point five eight as the additive content increases.

The composite with thirty weight percent took twenty-five seconds to reach peak heat release rate, which dropped to ninety-three point five seven kilowatts per square meter. This represents a sixty-six point five percent decrease in peak heat release rate compared with pure rigid polyurethane foam.

Figure 12 presents scanning electron microscopy images of the charred residue left after burning RPUF composites with varying amounts of CS/PCM. The authors use these visuals to compare the surface morphology, noting that the pure material exhibits an uneven texture.

In contrast, the samples supplemented with the additive display a more compact and continuous structure, which helps illustrate how the material changes during combustion. Raman spectral analysis detected ordered graphitized carbon and disordered carbon peaks near thirteen hundred fifty-nine and fifteen hundred eighty-three wavenumbers.

The ratio of these peaks decreased from one point one three for pure foam to zero point nine eight for the composite with twenty weight percent loading. Figure 14 illustrates the dual flame-retardant mechanism of the CS/PCM/RPUF composite, operating in both the vapor and condensed phases.

In the vapor phase, the material releases non-flammable gases like water, carbon dioxide, and nitrogen to dilute flammable gases and oxygen. Simultaneously, in the condensed phase, chitosan promotes a protective barrier that blocks heat transfer and prevents smoke precursors from escaping while cutting off oxygen supply.

Adding thirty percent of these microcapsules boosts the oxygen index to twenty-six point one percent and cuts peak heat release by over sixty-six percent, making the foam significantly safer.