A blend of amino-functionalized graphene modified with polyaniline plus the phosphorus-nitrogen flame retardant CPDD produces the strongest flame retardant polyurethane foam in this study. The limiting oxygen index rises from 19.1 percent for neat foam to 24.2 percent, vertical burning self-extinguishes within 5 seconds, and the residue left at 800 degrees Celsius climbs from 30.1 to 42.3 percent. Two phases work together: a continuous expanded char layer in the condensed phase and radical capture in the gas phase.
Why rigid polyurethane foam needs help
Rigid polyurethane foam is one of the best organic insulation materials in use, with low thermal conductivity, a high closed-cell content and good specific strength. The same chemistry that makes it useful also makes it burn. The polymer backbone is rich in carbon and hydrogen, and neat foam shows a limiting oxygen index of only 19.1 percent. In a fire it burns quickly, releases heat fast, and gives off large amounts of toxic smoke together with burning drips, which blocks evacuation and makes firefighting harder.
Graphene oxide looks like an obvious fix. Its large specific surface area and sheet structure can form a physical barrier that slows the transfer of heat, oxygen and volatile fuel. In practice, two problems limit it. First, the sheets stack through pi-pi interactions and hydrogen bonding, so they aggregate in the polymer matrix instead of dispersing at the nanoscale. Once aggregated, the barrier function is lost and the clumps become stress concentrators that trigger early failure. Second, the flame retardant action of graphene oxide is mostly physical, so a useful effect needs a high loading, and that loading raises viscosity, complicates processing and degrades mechanical properties.
The route tested here adds two partners to graphene. Polyaniline grows on the sheet surface and holds the layers apart, while the phosphorus-nitrogen flame retardant CPDD supplies the reactive chemistry. The bet is that the three together beat any pair.
What was made, and under what conditions
Three materials were prepared before foaming: the CPDD flame retardant itself, amino-functionalized graphene, and the polyaniline graphene composite NH2-GO/PANI. Table 1 lists the preparation conditions.
Table 1. Preparation conditions for the CPDD flame retardant, NH2-GO, NH2-GO/PANI and the foam.
| Step | Materials | Key conditions |
| CPDD synthesis | Cystamine dihydrochloride, triethylamine, sodium hydroxide, phenylphosphonic dichloride | 12 mmol cystamine dihydrochloride, 6 g triethylamine and 24 mmol sodium hydroxide in 170 mL dry acetonitrile at 0 degrees Celsius; 10 mmol phenylphosphonic dichloride in 30 mL acetonitrile added dropwise; 2 h on ice, then 5 h at 50 degrees Celsius; filtered, washed with petroleum ether and deionized water, dried at 50 degrees Celsius under vacuum; yield 78.5 percent |
| Amino-functionalized graphene (NH2-GO) | Graphene, tert-butyl (4-aminophenyl) carbamate, trifluoroacetic acid | 260 mg graphene with 320 mg tert-butyl (4-aminophenyl) carbamate in a 20 degrees Celsius water bath for 18 h; 20 mL trifluoroacetic acid added at room temperature for 5 h; freeze dried |
| NH2-GO/PANI composite | Aniline, dilute hydrochloric acid, ammonium persulfate | 2.3 mL aniline in 50 mL dilute hydrochloric acid at 300 rpm, cooled to 5 degrees Celsius; NH2-GO added; 12.5 mL ammonium persulfate solution (0.1 g/mL) added slowly; 5 h at 5 degrees Celsius; washed and dried |
| Foam casting | CPDD, NH2-GO/PANI@CPDD, PTMG, MDI, water, catalyst | 15 mass percent CPDD with NH2-GO/PANI@CPDD, 0.01 g deionized water and 0.005 g catalyst in PTMG; MDI added and stirred until the cup wall warmed; free rise and cure at room temperature |
The foam systems compared later are neat PU, PU-CPDD, NH2-GO/CPDD/PU and NH2-GO/PANI@CPDD/PU.
Structure checks before the fire tests
Before any burning test, the structure of the new materials was confirmed by four methods. The reading that matters most is the X-ray diffraction pattern of the composite: the stacking peak of NH2-GO at 10.8 degrees disappears and three new peaks appear at 14.8, 20.5 and 25.2 degrees, which are the signature of polyaniline in its conductive form. Aniline clearly polymerized on the sheets, and that polymerization broke up the original layer stacking.
Table 2. Structural evidence for NH2-GO/PANI.
| Method | Reading | What it shows |
| XRD | NH2-GO peaks at 10.8 and 25 degrees; NH2-GO/PANI loses the 10.8 degree peak and gains peaks at 14.8, 20.5 and 25.2 degrees | Aniline polymerized on the sheets and broke up the original stacking |
| FTIR | Graphene backbone bands at 825, 1210, 1510 and 1620 cm-1 retained; quinoid C-H at 1160 cm-1 and aromatic amine C-N at 1360 cm-1 appear | The amino graphene structure survives and polyaniline is attached |
| Raman | D and G peak positions unchanged but broadened | Polyaniline is loaded uniformly without destroying the graphene skeleton |
| SEM | Polyaniline particles cover the sheets evenly | In situ polymerization succeeded |
Thermogravimetric analysis shows what the polyaniline coating does to thermal stability. The onset of decomposition moves from 152 degrees Celsius for NH2-GO to 178 degrees Celsius for NH2-GO/PANI, and the temperature of maximum mass loss moves from 312 to 335 degrees Celsius. Between 200 and 600 degrees Celsius, the composite loses mass more slowly than the uncoated graphene. The residue at 800 degrees Celsius is 42.3 percent for the composite against 30.1 percent for NH2-GO, a gain of about 12 percentage points. The aromatic rings of polyaniline are simply more thermally stable than the oxygen groups on the graphene surface, and they are the reason the char yield rises.
The fire performance results
The four formulations in Table 3 give the clearest picture of what each component adds to flame retardant polyurethane foam.
Table 3. Limiting oxygen index and vertical burning behavior of the four PU foam systems.
| Formulation | Limiting oxygen index (percent) | Gain | Vertical burning test |
| Neat PU | 19.1 | baseline | Consumed within 10 seconds, no self-extinguishing |
| PU-CPDD | 22.3 | 16.8 percent over neat PU | Self-extinguishes around 15 seconds, still drips |
| NH2-GO/CPDD/PU | 23.8 | 6.7 percent over PU-CPDD | Flame smaller than PU-CPDD |
| NH2-GO/PANI@CPDD/PU | 24.2 | 26.7 percent over neat PU, 8.5 percent over PU-CPDD | Self-extinguishes within 5 seconds, and within 5 seconds again after reignition |
Three readings follow from the table. First, the CPDD flame retardant alone buys 3.2 points of limiting oxygen index and changes a total burn into a 15 second self-extinguishing event with dripping. Second, adding graphene to CPDD buys another 1.5 points, which shows the two components already cooperate. Third, the full three-component system reaches 24.2 percent, and the calculated synergy index is about 1.32, which confirms a positive interaction between NH2-GO/PANI and CPDD rather than a simple additive effect.
The vertical burning test sharpens the difference. Neat foam is gone in under 10 seconds. The PU-CPDD system suppresses the flame but keeps dripping for about 15 seconds. The three-component system shrinks the flame within 5 seconds of ignition and does the same after a second ignition, so the protection survives a first flame exposure instead of being used up.
How the synergy works in flame retardant polyurethane foam
The char left behind explains most of the result. Neat PU leaves a continuous but porous flaky residue with open holes and large gaps. PU-CPDD leaves wrinkled flakes with smaller but still connected pores. The three-component system leaves a continuous, dense, expanded layered char with no open holes at all. Four mechanisms build that char layer and then keep the fire fed less.
- Amino groups improve the interface. The amino groups on NH2-GO form hydrogen bonds or covalent bonds with the P=O group of CPDD. The flame retardant therefore disperses evenly and stays bound to the matrix instead of agglomerating or migrating, so the whole volume chars uniformly when heated.
- Polyaniline feeds the char. The aromatic rings of polyaniline cyclize and condense at high temperature into stable residue, supplying the carbon source for the expanded char layer. This is the same chemistry behind the higher char yield measured by TGA.
- The disulfide bond suppresses smoke. The S-S bond in CPDD breaks during pyrolysis and releases sulfur-containing radicals. These capture gas-phase radicals and interrupt the chain reaction, and they also react with toxic aromatic compounds in the smoke so those compounds crosslink into char instead of leaving as black smoke. The three-component foam visibly released the least smoke of all the samples.
- Phosphorus and nitrogen drive intumescence. The phosphorus in CPDD catalyzes dehydration and charring of both the polyurethane and the polyaniline, while nitrogen leaves as ammonia and nitrogen gas. The released gases swell the melt into a porous char layer, and the graphene sheets then act as a physical barrier that seals the pores and densifies the layer.
Together these give a condensed phase mechanism and a gas phase mechanism at the same time. In the condensed phase, the graphene sheets block heat moving inward and decomposition products moving outward, while polyaniline and the phosphorus component build a continuous expanded char layer and the disulfide decomposition products reinforce it. In the gas phase, nonflammable gases dilute the fuel and phosphorus and sulfur radicals trap the HO and H radicals that carry the chain reaction. Interrupting both heat transfer and the radical chain is what produces the jump in performance.
The counterintuitive part: the reactive chemistry is not doing most of the work
The obvious reading of Table 3 is that the phosphorus-nitrogen system is the flame retardant and the graphene is a filler. The data point the other way. The formulation with 15 mass percent CPDD and no graphene reaches only 22.3 percent and still drips. Swapping in the NH2-GO/PANI carrier adds less than 2 percent of solids yet pushes the index to 24.2 and turns a 15 second burn with drips into a 5 second self-extinguishing one. In other words, dispersion, barrier continuity and char architecture are doing at least as much work as the reactive phosphorus chemistry.
A second surprise sits inside the numbers. A limiting oxygen index of 24.2 percent is below the threshold, usually quoted near 26 to 27 percent, at which a material is classed as self-extinguishing by index alone. This foam still self-extinguishes within 5 seconds in the vertical burning test and repeats that after reignition. The index underrates the system because it measures oxygen demand in a small controlled flame and says nothing about char continuity, which is where this formulation actually wins.
What this means for formulation work
- Do not use graphene oxide alone. Without surface modification it aggregates, and the loading needed for a real effect hurts viscosity and mechanical properties.
- Modify the sheet, then add reactive chemistry. A carrier such as NH2-GO/PANI improves dispersion and raises char yield, so a smaller dose of the phosphorus-nitrogen flame retardant goes further.
- Target char continuity, not char amount. The formulations that drip are the ones with connected pores; the one that self-extinguishes quickly has a sealed expanded layer.
- Pair every condensed phase mechanism with a gas phase one. Radical capture and nonflammable gas dilution explain the fast flame shrink that barrier action alone cannot deliver.
- Check smoke, not just flame. Smoke suppression from the sulfur component turns aromatic fragments into char instead of black smoke, which matters for evacuation time.
Limitations
This is a single laboratory study on one formulation family, and several boundaries belong next to the numbers. The source itself quotes a limiting oxygen index of 17 percent for neat foam in its introduction while reporting 19.1 percent in the results; the results value is used throughout this article. Sample labels vary across the source, so the best-performing system is written here consistently as NH2-GO/PANI@CPDD/PU. The thermal data come from analysis under nitrogen, and char formed in an inert atmosphere is not the same as residue in a real fire. No cone calorimetry, smoke density or mechanical property data are reported, so heat release rate and the cost in strength of the 15 percent flame retardant loading are unknown. The mechanism evidence is spectroscopic and microscopic rather than direct, and scale-up from a paper cup to production foam is untested.
Bottom line
Amino-functionalized graphene carrying a polyaniline coating, combined with 15 mass percent of the phosphorus-nitrogen flame retardant CPDD, lifts the limiting oxygen index of rigid polyurethane foam from 19.1 to 24.2 percent and cuts self-extinguishing time in vertical burning to 5 seconds. The synergy index of 1.32 is real, but the physical assembly, an evenly dispersed sheet network feeding a sealed expanded char layer, is what separates this system from CPDD alone. For anyone formulating flame retardant polyurethane foam, how the components sit in the matrix decides as much as what they are.
FAQ
What is the best flame retardant for polyurethane foam?
There is no single best flame retardant for polyurethane foam, since rigid foam, flexible slabstock and molded parts face different fire tests. For rigid foam the strongest systems pair a reactive phosphorus-nitrogen component with a nanoscale carrier. The flame retardant polyurethane foam built that way here reached a limiting oxygen index of 24.2 percent and self-extinguished within 5 seconds.
How does NH2-GO/PANI CPDD improve flame retardancy?
The carrier keeps graphene sheets separated so they disperse instead of stacking into aggregates. Its amino groups bond with the P=O group of CPDD, holding the flame retardant in place so the whole volume chars uniformly. Polyaniline also cyclizes at high temperature and feeds carbon into the char layer, so the same CPDD loading builds a denser residue.
What limiting oxygen index does self-extinguishing foam need?
About 26 to 27 percent is the usual dividing line for calling a polymer self-extinguishing on the index alone. The flame retardant polyurethane foam measured here sits at 24.2 percent, below that line, yet self-extinguishes within 5 seconds and again after reignition. The index only measures oxygen demand in a small flame, while the vertical test also captures dripping and flame spread.
Does graphene oxide alone stop polyurethane foam from burning?
No. Graphene oxide works only as a physical barrier, and its sheets stack through pi-pi interactions and hydrogen bonding, so in a matrix they clump into aggregates that act as stress concentrators. Its flame retardant action per unit mass is also weak, so a high loading is needed and that raises viscosity and damages mechanical properties, so modification before use is not optional.
What is the CPDD flame retardant made from?
CPDD is a phosphorus-nitrogen flame retardant built from cystamine dihydrochloride, triethylamine, sodium hydroxide and phenylphosphonic dichloride. The cystamine part supplies nitrogen and a disulfide bond, and the phosphonic part supplies phosphorus. Synthesis gave a 78.5 percent yield, and infrared spectroscopy confirmed the product through the loss of the P-Cl band and new P-N and S-S bands.
How does the phosphorus nitrogen synergy work in polyurethane foam?
Phosphorus catalyzes dehydration of the polymer and of polyaniline into char, while nitrogen leaves as ammonia and nitrogen gas that swell the melt into an expanded char layer. That is the condensed phase half. In the gas phase, sulfur radicals from the disulfide bond and phosphorus species trap the HO and H radicals that carry the chain reaction.
Why does polyaniline raise the char yield?
Polyaniline is built from aromatic rings, and those rings cyclize and condense into stable carbon when heated instead of decomposing into volatile fragments. Growing the polymer on graphene sheets also disrupts sheet restacking, which improves dispersion. Thermogravimetric analysis quantified the effect, with the residue at 800 degrees Celsius rising from 30.1 to 42.3 percent.
How does the disulfide bond reduce smoke?
During pyrolysis the S-S bond breaks and releases sulfur-containing radicals. Those radicals trap the gas-phase radicals that drive the chain reaction, and they react with toxic aromatic compounds in the smoke so the aromatics crosslink into char instead of black smoke. The three-component foam visibly released the least smoke of all samples burned.
How much flame retardant does this polyurethane foam need?
The best-performing system used 15 mass percent of CPDD with the NH2-GO/PANI carrier, mixed into PTMG with water and catalyst before adding MDI. That is a substantial loading, and no mechanical data are reported, so the strength cost at 15 percent is unknown. A useful next target is the lowest CPDD level that still gives a sealed expanded char layer.
What does a synergy index of 1.32 mean?
A synergy index compares the measured performance of a mixture with what the components would give if their contributions simply added up. An index near 1.32 means the combined system beats the additive expectation by roughly a third, so NH2-GO/PANI and CPDD genuinely cooperate. In practice the cooperation appears as a higher char yield and a more continuous expanded char layer.
Can this flame retardant system be used in commercial foam?
The chemistry runs on conventional polyol and isocyanate raw materials, and every step is a solution or melt process industry already uses. What is missing is scale-up evidence, fire test certification for the intended end use, mechanical data and standard smoke density numbers. Treat the 24.2 percent index and 5 second self-extinguishing as a laboratory result that justifies pilot work.
What limits halogen free flame retardant polyurethane foam?
Halogen free systems must build performance from phosphorus, nitrogen and nanoscale fillers instead of halogen radical chemistry, so they depend heavily on char quality and dispersion. The usual limits are high loading needs, processing viscosity, lost mechanical properties and stubborn smoke. This study shows a workable direction, but the strength cost at 15 percent loading is the open question.