What if a material added to polyurethane foam for heat storage could also slow a fire? This study puts paraffin inside chitosan-based microcapsules and tests whether the capsules make rigid foam both thermally functional and more flame resistant.

The flammability of rigid polyurethane foam limits its application. A new type of chitosan phase-change microcapsule, called CS/PCM, was prepared with chitosan and gum acacia as the wall material and paraffin as the core material. CS/PCM was introduced into rigid polyurethane foam as a filler to improve the thermal and flame-retardant properties of polyurethane.

The materials were characterized by scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and cone calorimetry. At thirty weight percent CS/PCM, RPUF-30 has a latent heat of phase transition of 12.308 joules per gram, a limiting oxygen index of 26.1 percent, and reduced fire risk.

Rigid polyurethane foam has many excellent properties, but it is flammable. Its combustion can emit toxic gases and strong olfactory stimulation, while heat released during synthesis can accumulate when the foam is used in large quantities.

Because rigid polyurethane foam is widely used in automotive, mining, machinery, waterproof materials, energy-saving, light industry and aerospace applications, improving flame retardancy and reducing reaction temperature are important unsolved problems. Phase-change materials can store energy, so applying them to rigid polyurethane foam can absorb heat from the reaction that forms polyurethane from polyether polyols and isocyanates, reducing the composite’s reaction temperature.

But solid–solid phase-change materials have low phase-change enthalpy, while solid–liquid and solid–gas materials can leak during phase change, preventing polyurethane from directly combining with them. The proposed solution is to make the phase-change material into a phase-change microcapsule, solving the leakage problem before adding the PCM to rigid polyurethane foam.

Earlier research mainly focused on microcapsule or polyurethane-foam structure, mechanical properties and heat-storage properties. Fewer studies examined the flame-retardant properties of polyurethane foam after PCM was added. This study therefore prepared a new PCM with certain flame-retardant properties and studied its effect on rigid polyurethane foam by adding CS/PCM.

Paraffin wax melts and solidifies at almost constant temperatures, and it has attracted interest for thermal management because of its low subcooling temperature, low cost and wide range of phase-transition temperatures. Paraffin wax could improve rigid polyurethane foam’s thermal management, but directly introducing it would reduce durability and cause leakage, harming the composite’s flame retardance.

Chitosan is a natural polysaccharide polymer that is available, cheap, non-toxic, biodegradable and environmentally friendly. Its carbon-rich molecular framework and hydroxyl and amino groups make it an ideal capsule wall material. During thermal decomposition, carbonization can hinder combustion, while non-toxic and non-corrosive non-flammable gases such as carbon dioxide, ammonia and nitrogen are released, giving chitosan a flame-retardant role.

CS/PCM was prepared from chitosan and gum acacia as the wall material and paraffin as the core material, using the condensation method. The study examined structure, phase transformation, thermal stability and flame retardancy after CS/PCM was introduced into rigid polyurethane foam.

The paper presents the approach as simple, low-cost and efficient, with a feasible solution for thermal management applications and a new idea for functionalizing rigid polyurethane foam composites. The core material was paraffin wax, while gum Arabic and chitosan formed the wall material.

Four grams of paraffin wax and four grams of gum Arabic were melted at sixty-five degrees Celsius and stirred at six hundred revolutions per minute. Chitosan solution and ten percent glacial acetic acid solution were added, the pH was adjusted to four point five, and the condensation reaction lasted twenty minutes before cooling in an ice-water bath.

Glutaraldehyde was then used for cross-linking and curing, followed by heating, filtration, ether washing and drying at forty degrees Celsius for twelve hours to obtain CS/PCM. Figure one schematically shows how the chitosan phase-change microcapsules, or CS/PCM, are formed.

Paraffin-wax PCM droplets are initially suspended in a shell solution, followed by shell cross-linking around each core and then shell curing. This visual matters because it clarifies the core–shell design: the wax provides the phase-change function, while the gum-Arabic and chitosan wall material encapsulates it for incorporation into the composite foam.

The composites used polyether polyol R4110 and polyisocyanate PM200 as the base material, with CS/PCM added alongside catalyst, foaming agent, foam stabilizer and crosslinking agent. CS/PCM was added at five, ten, fifteen, twenty, twenty-five or thirty weight percent of the polyether-polyol mass, producing samples named RPUF-5 through RPUF-30.

Differential scanning calorimetry measured latent heat and phase-change temperature under nitrogen from twenty to one hundred fifty degrees Celsius at a heating rate of ten degrees Celsius per minute. The study also used Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, scanning electron microscopy and thermogravimetric analysis to characterize functional groups, structure, residues, morphology and thermal stability.

Figure two shows the CS/PCM particles in two complementary ways. Panel (a) is a scanning electron micrograph revealing roughly spherical particles with visibly textured, irregular surfaces, while panel (b) summarizes their particle-size distribution as a frequency histogram with error bars and a fitted red curve.

Together, these views establish both the particles’ morphology and the spread of their sizes, which matters because CS/PCM is subsequently incorporated into RPUF and linked to changes in foam cell structure. The CS/PCM particle size ranges from one hundred micrometers to three hundred micrometers, following a normal distribution.

In the composites, the bubble holes are arranged, and increasing CS/PCM addition gradually increases the RPUF cell size. When CS/PCM participated in the synthesis, it was coated by the material and most capsules did not leak out. At thirty weight percent, however, the cell arrangement was disrupted, the cell wall became thin and tearing appeared at the sample boundary.

Pure rigid polyurethane foam has no melting peak, endothermic peak or latent heat of phase transition in its DSC curve. Adding CS/PCM creates an obvious endothermic peak. As CS/PCM content rises, the heat-absorption peak and melting-peak area grow, while the starting temperature and peak temperature of phase transition also rise.

With thirty weight percent CS/PCM, the latent heat of phase transition reaches 12.308 joules per gram. The higher composite peak temperature is attributed to some heat being used for material degradation as temperature rises. Figure eight compares DSC traces for CS/PCM, RPUF with twenty weight percent CS/PCM, and RPUF with thirty weight percent, each measured at heating rates from five to twenty degrees Celsius per minute.

The annotated peaks show phase-transition behavior, while panel d plots the fitting relationships used with the Kissinger, Ozawa, and Starink methods. The lines fit closely, with results near zero point ninety-nine, supporting the reported activation-energy analysis.

To study flame retardancy, the composites were tested with a limiting oxygen index test and a cone calorimetric test. The limiting oxygen index results for different CS/PCM contents are shown in Figure 10. Figure 10 shows the limiting oxygen index, or LOI, of RPUF composites containing different CS/PCM contents, from five to thirty weight percent.

The bars rise from roughly twenty-one percent to about twenty-six percent, while the paper reports that pure RPUF has an LOI of nineteen percent and is classified as a B3 flammable material. The authors use this result to show that CS/PCM raises the oxygen concentration needed to sustain burning, although the improvement is described as relatively small.

After CS/PCM was added, RPUF-30 reached its peak heat release rate after twenty-five seconds, and its peak heat release rate was reduced to 93.57 kilowatts per square meter compared with pure RPUF. The peak heat release rate decreased by 66.5 percent, the peak smoke generation rate decreased by 55.8 percent, and both peak smoke generation rate and total smoke release gradually decreased compared with pure RPUF.

As CS/PCM increased, the fire growth rate index gradually decreased, indicating improved fire safety. The flame-retardant index values for RPUF-10, RPUF-20 and RPUF-30 were 2.0, 6.35 and 10.39, respectively. Figure eleven presents cone calorimetry curves for pure RPUF and composites containing ten, twenty, and thirty weight percent CS/PCM.

It tracks heat release rate and smoke production rate over time, alongside their cumulative measures: total heat release and total smoke release. The figure shows the combustion response and smoke evolution of each formulation, while the paper’s text identifies the corresponding peak values in Table two, making these plots important for evaluating fire behavior beyond the LOI results.

Table two reports cone calorimeter measurements for RPUF containing increasing amounts of CS/PCM. For RPUF-30, the peak heat release rate is ninety-three point five seven kilowatts per square meter, reached after twenty-five seconds, while total heat release is seven point five eight kilojoules per square meter and peak smoke production rate is zero point zero five five eight square meters per second.

These measurements matter because they show how the authors quantify both fire and smoke behavior across the formulations. The flame retardancy of CS/PCM/RPUF composites is mainly affected by CS/PCM. The flammable paraffin core has a negative effect as its content increases, while the chitosan shell has a positive effect on flame retardancy.

When the chitosan shell is attacked by flames, non-flammable gases are released that can dilute combustible gases and oxygen in the vapor phase. In the condensed phase, chitosan can form a carbon–nitrogen bond and promote a protective carbon layer.

At thirty weight percent CS/PCM, the foam reaches a latent heat of phase transition of 12.308 joules per gram, a limiting oxygen index of 26.1 percent, and substantially lower heat and smoke release. The key mechanism is the chitosan shell’s barrier effect.