Flammability, Toxicity, and Microbiological Properties of Polyurethane Flexible Foams

The aim of the research was to investigate the influence of calcium phosphinate (HPCA) and aluminum phosphinate (HPAL) in synergistic systems with organophosphorus compounds, i.e., diphenylcresyl phosphate (CDP) and trichloropropyl phosphate (TCPP), on the thermal stability, flammability, smoke density, and emission of toxic gases during the thermal decomposition of polyurethane (PUR) foams. Thermogravimetric analysis (TGA), along with cone calorimetry and microcalorimetry, were used to assess the influence of fillers on the thermal stability and flammability of PUR foams. The analysis of toxic gas products was performed with the use of a coupled TG–gas analyzer system. The optical density of gases was measured with the use of a smoke density chamber (SDC). The obtained results showed an increase in thermal stability and a decrease in the flammability of the PUR composites. However, the results regarding smoke and gas emissions, as well as toxic combustion by-products, present ambiguity. On one hand, the applied flame retardant systems in the form of PUR-HPCA-CDP and PUR-HPCA-TCPP led to a reduction in the concentration of CO and HCN in the gas by-products. On the other hand, they clearly increased the concentration of CO2, NOx, and smoke emissions. Microbiological studies indicated that the obtained foam material is completely safe for use and does not exhibit biocidal properties.


Introduction
Polyurethane materials (PUR), due to their potential for mass production, wide range of modifications, and versatile properties, are widely used in many sectors of the industry as well as in everyday life [1][2][3].
An extremely important feature of PUR materials is their ability to be modified by changing the type of raw materials, the relative quantities of the substrates, or the processing conditions.Depending on their chemical composition, the resulting PUR composites can exhibit high mechanical strength, noise-and vibration-damping properties, increased resistance to physico-chemical factors, and resistance to organic solvents and oils [4][5][6].However, a significant factor limiting the application of polyurethane materials is their low fire resistance.The combustion of PUR foams is accompanied by a high release of heat, as well as the intensive emission of smoke and toxic combustion products [7][8][9][10].
Previous research has unequivocally shown that significant amounts of smoke, CO, and CO 2 are emitted during the initial phase of PUR thermal decomposition [11].At higher decomposition temperatures, i.e., above 400 • C, high concentrations of HCN and NO 2 are additionally released [12,13].Nitrogen compounds can cause damage to the respiratory and nervous systems, and with prolonged exposure, they can also affect the cardiovascular system.In extreme cases, they may even lead to death [14][15][16].One of the approaches to enhancing the applicability of PUR (polyurethane) materials involves research aimed at improving their fire resistance, including the use of organophosphorus compounds.A review of the literature indicates that organophosphorus compounds, especially in synergistic systems with mineral or carbon fillers, increase the thermal stability of PUR foam while simultaneously reducing its flammability [17][18][19].
The key mechanism that reduces the fire hazard of polyurethane in the presence of organophosphorus compounds is the formation of a char barrier layer on the surface of the PUR, particularly in the initial stages of thermal decomposition.This barrier prevents the spread of fire both across the surface and into the interior of the polymer [20,21].
In addition to satisfactory fire resistance, PUR materials, due to their widespread contact with the human body, must also be biologically safe.This means they should not contain potentially allergenic or toxic chemical components.To determine whether a composite with the desired functional properties is safe, model in vitro analyses are conducted.These analyses typically involve a wide range of studies using defined cell lines [22,23].
Antibacterial PUR composites are used in hospitals and sanitary wards [24].Additionally, they can be utilized in the production of everyday materials such as exercise mats, sleeping pads, and upholstery foam [25,26].Most antibacterial fillers for PUR, in the form of silver, gold particles, or metal oxides, do not exhibit selective biocidal action.Consequently, the antibacterial activity of polymer materials results in the destruction of bacterial cells [27,28].This effect often leads to the rupture of microorganism cells, releasing toxic cellular components onto the composite surface.These components can be irritating or even toxic due to the presence of numerous enzymes and parts of bacterial cell membranes and walls.The unintentional introduction of these toxins into the human body can have serious consequences, including inflammatory states or even cross-reactions [29,30].
Currently used PUR composites do not exhibit long-term antibacterial properties while maintaining neutrality towards naturally occurring skin bacteria.Therefore, biocompatible PUR materials with long-term effectiveness remain the subject of intensive research.
This article presents the results of studies on the effects of phosphine compounds, also in synergistic systems with organophosphorus compounds, on the properties of PUR composites, with particular emphasis on their fire hazard and biocompatibility.

Preparation of PUR Composites
The PUR foams were synthesized using a one-step reaction with the two-component polyol-isocyanate in a weight ratio of OH:NCO of 2:1 (Table 1).The obtained PUR composites were conditioned to a constant mass at a temperature of 23 ± 2 • C and humidity not exceeding 50 ± 5%, according to ISO 291 [31].Conditioning is considered properly conducted if the sample's mass after 24 h does not exceed an error range of 0.1 g or 0.1%.

Preparation of PUR Composites
The PUR foams were synthesized using a one-step reaction with the two-component polyol-isocyanate in a weight ratio of OH:NCO of 2:1 (Table 1).The obtained PUR composites were conditioned to a constant mass at a temperature of 23 ± 2 °C and humidity not exceeding 50 ± 5%, according to ISO 291 [31].Conditioning is considered properly conducted if the sample's mass after 24 h does not exceed an error range of 0.1 g or 0.1%.

Preparation of Bacterial Culture
Microorganism activity studies against PUR composites were conducted with reference bacterial strains: Staphylococcus aureus ATCC 6538P [32], Escherichia coli ATCC 8739 [33], and Bacillus subtilis PCM486 [34].The bacteria were taken directly from frozen culture (−80 • C in 8% DMSO in LB) onto solid LB medium (5 g yeast extract, 10 g tryptone, 0.5 g NaCl, 1.5 g agar per liter of water).Cultures were incubated for 24 h at 37 • C. The overnight culture was transferred to fresh liquid LB medium and then incubated for 24 h at 37 • C with shaking (160 rpm) (EcoTron, Infors HT).The bacteria were then centrifuged twice using a laboratory centrifuge (5000 rpm, 10 min) and suspended in saline (0.85% NaCl).The bacterial suspension was diluted to an optical density of 1 (OD~1, λ = 550 nm).This is the initial density, assumed to contain between 10 8 and 10 9 CFU bacteria.

Fourier-Transform Infrared Spectroscopy Analysis
Fourier-transform infrared spectroscopy with attenuated total reflection (FTIR-ATR) was conducted using a PerkinElmer Spectrum device (Waltham, MA, USA).The FTIR spectrophotometer was equipped with a diamond ATR crystal, with a single reflection on a ZeSe plate.Measurements were recorded using the PerkinElmer Spectrum software 10 (USA).Spectra were recorded using 4 scans with a resolution of 4 cm −1 in the MIR range of 400-4000 cm −1 in transmission mode.

Thermal Analysis
Thermal analysis was performed using a Netzsch STA 449 F3 Jupiter analyzer (Selb, Germany) in the temperature range from 25 to 650 • C for samples with a mass of 5 ± 1 mg.The samples were placed in an open Al 2 O 3 crucible.Measurements were carried out in an air atmosphere with a heating rate of 10 • C/min.

Classification of Microbial Growth-UV-Vis Analysis
The study of microorganism activity was conducted using a method based on changes in UV-VIS light absorption with a Jasco V-660 spectrophotometer (Tokyo 193-0835, Japan).Measurements were taken at a light wavelength of 550 nm.The results were collected in two stages.In the first stage, the initial bacterial suspension sample was measured, with an optical density (OD) of approximately 1.The second stage of the microorganism activity study in the presence of composites involved collecting absorbance results after overnight incubation of the bacterial suspension with an initial OD of ~1 with the PUR, PUR-HPCA-CDP, PUR-HPAL-CDP, PUR-HPAL-TCPP, and PUR-HPCA-TCPP composites.The background for all results was a 0.85% NaCl solution.The activity measurement in the presence of composites was calculated using the following formula: where: Activity-microbiological activity (%); AbsMeasured-recorded absorption after the incubation stage of the suspension with PUR composites; AbsInitial-recorded initial absorption of the bacterial suspension (OD~1).

Toxicity Tests of Gaseous Decomposition Products
The toxicity of gaseous decomposition products of PUR composites was studied using a coupled TG system (Netzsch TG 209 F1 Libra, Selb, Germany) and an FTIR Omega 5 analyzer (Bruker Omega 5, Billerica, MA, USA).The gas analysis was conducted over a temperature range of ∆T = 30-650 • C in an air atmosphere.The heating rate was 10 • C/min.Gas emissions were determined for samples weighing 5 ± 1 mg.FTIR spectra were recorded every 7-8 s (10 scans).
The concentrations of gases (in ppm) were determined using the Opus GA software (version 8.7.14, Hillview, 18103-6046 USA) gases analyzed included CO, CO 2 , HCl, NO, and NO 2 .The conversion of the concentration of gaseous decomposition products expressed in ppm to concentrations expressed in g/m 3 or mg/m 3 was performed using the following relationships: pV = nRT where: P-pressure (101,325 Pa); V-gas volume; N-number of moles of particles in the gas; R-gas constant (8314 J/mol•K).

50
× 0.0805 where: 0.0805-toxicity constant according to ISO 5659-2 Annex C [35], according to which 0.1 m 2 of exposed product emits gases that disperse in a volume of 150 m 3 ; Es-sampled spectrum corresponding to 4 L of emitted gas (sampled emission); LC 30  50 -lethal concentration for 50% of the test population over 30 min.The flammability of PUR composites was tested using a PCFC (pyrolysis combustion flow calorimetry) microcalorimeter manufactured by Fire Testing Technology Ltd., East Grinstead, UK.The procedure was conducted according to ASTM D 7309 [36].The pyrolyzer temperature was 650 • C with a heating rate of 1 • C/s, and the combustion chamber temperature was 900 • C. The test was carried out under nitrogen/oxygen conditions (80/20 cm 3 /min).The following parameters were recorded during the test: heat release rate (HRR), maximum heat release rate (HRR MAX ) (W/g), time to HRR MAX (s), total heat release (HR) (kJ/g), and heat release capacity (HRC) (J/gK).

Flammability: Cone Calorimetry
The fire hazard of PUR composites was evaluated using a cone calorimeter from Fire Testing Technology Ltd. according to PN-EN ISO 5660 [37].Samples of standardized dimensions 100 × 100 × 50 mm were analyzed in a horizontal position using a heat flux of 35 kW/m 2 .The following parameters were recorded during the test: initial sample mass, sample mass during the test, final sample mass, time to ignition (T i ), time to sample extinction (T f-0 ), total heat release (THR), effective heat of combustion (EHC), average mass loss rate (MLR), heat release rate (HRR), fire growth rate index (FIGRA), and the maximum average heat release rate (MARHE).

Smoke Density
Smoke density was tested using the smoke density chamber (SDC) according to PN-EN ISO 5659-2 [38].Samples of standardized dimensions 75 × 75 × 15 mm were tested with a heat flux of 25 kW/m 2 .The following parameters were recorded during the test: initial sample mass, sample mass during the test, final sample mass, maximum specific optical density of smoke (D sMAX ), optical density at 4 min of testing (Ds(4)), area under the specific optical density curve (VOF4), and light attenuation coefficient after the test.

Surface Morphology and FTIR Analysis of PUR Composites
The spatial structure morphology studies conducted using SEM clearly indicate the porous structure of PUR composites.The introduction of organophosphorus compounds into the PUR matrix, especially in the form of aluminum phosphinate (HPAL), significantly reduces the volume of free spaces in the PUR matrix (PUR-HPAL-TCPP/CDP system) (Figure 2A-C).Based on the obtained SEM images, it can also be concluded that calcium phosphinate (HPCA), in the PUR-HPCA-TCPP/CDP system, reduces the formation of the porous structure of PUR to a much lesser extent compared to the PUR-HPAL-TCPP/CDP system (Figure 2D,E).However, it should be emphasized that in the presence of the PUR-HPCA-TCPP/CDP system, the porosity of the PUR composite is significantly lower compared to the reference composite (Figure 2A,D,E).
The increase in the porosity of the PUR composite structure undoubtedly has a significant impact on improving its elastic properties, and consequently, its ability to dampen vibrations.Highly elastic and inherently porous PUR foams are commonly used for upholstery purposes, including the production of seats for the railway, aviation, and automotive industries [39,40].Upholstery foams used in the broadly understood public transport industry, in addition to satisfactory performance parameters, such as elasticity and resistance to delamination, must also exhibit appropriate fire hazard parameters, such as reduced flammability, smoke emission, and toxic gas emissions.The fire hazard requirements for polyurethane foams are regulated by appropriate standards [35].
The impact of the porosity of the PUR composite on fire hazard parameters is ambiguous.On the one hand, as the porosity of the composite increases, the amount of polymer material per unit volume of the composite decreases, which may result in some reduction in flammability.On the other hand, the free spaces in PUR are filled with air, which catalyzes the degradation and destruction reactions of the polymer material, especially in the initial stage of its decomposition [41].
The SEM results unequivocally indicate that phosphinate compounds reduce the porosity of the PUR structure.In the presence of aluminum phosphinate, the porosity of PUR foam is lower than in the presence of calcium phosphinate.This is likely directly related to the molecular weight of both compounds.The 23% lower molecular weight of calcium phosphinate (HPCA) compared to aluminum phosphinate (HPAL) causes the polyurethane foam to expand much more easily in the presence of HPCA (Figure 2).
PUR foam is lower than in the presence of calcium phosphinate.This is likely directly related to the molecular weight of both compounds.The 23% lower molecular weight of calcium phosphinate (HPCA) compared to aluminum phosphinate (HPAL) causes the polyurethane foam to expand much more easily in the presence of HPCA (Figure 2).During the polyurethane synthesis reaction, the characteristic disappearance of NCO stretching bands at 2270 cm −1 is typically observed.However, in the obtained composites, a minimal peak can still be noticed in this region.The presence of free isocyanate group bonds in the polyurethane composites (PUR-HPCA-CDP, PUR-HPAL-CDP, PUR-HPAL-TCPP, PUR-HPCA-TCPP) suggests the presence of unreacted sites, which may not be adequately During the polyurethane synthesis reaction, the characteristic disappearance of NCO stretching bands at 2270 cm −1 is typically observed.However, in the obtained composites, a minimal peak can still be noticed in this region.The presence of free isocyanate group bonds in the polyurethane composites (PUR-HPCA-CDP, PUR-HPAL-CDP, PUR-HPAL-TCPP, PUR-HPCA-TCPP) suggests the presence of unreacted sites, which may not be adequately filled by the nanofillers [42].In the IR spectrum of the PUR matrix, broad stretching absorption bands are registered at 3337 cm −1 , corresponding to stretching vibrations, as well as at 1538 cm −1 and 1510 cm −1 , corresponding to N-H group vibrations.At 1460 and 1260 cm −1 , signals are present from both symmetric and asymmetric vibrations of the N-C-N group [43].Vibrations characteristic of the methylene group, i.e., symmetric and asymmetric vibrations, are registered at 2955 and 2851 cm −1 , respectively.At 1450 cm −1 , bending vibrations from the C-H group are registered.The IR spectrum of the PUR matrix also shows signals at 1655 cm −1 and 1710 cm −1 corresponding to stretching vibrations of the carbonyl group and a strong, characteristic band at 1095 cm −1 related to stretching vibrations of the C-O-C group (Figure 3) [44][45][46][47].
filled by the nanofillers [42].In the IR spectrum of the PUR matrix, broad stretching absorption bands are registered at 3337 cm −1 , corresponding to stretching vibrations, as well as at 1538 cm −1 and 1510 cm −1 , corresponding to N-H group vibrations.At 1460 and 1260 cm −1 , signals are present from both symmetric and asymmetric vibrations of the N-C-N group [43].Vibrations characteristic of the methylene group, i.e., symmetric and asymmetric vibrations, are registered at 2955 and 2851 cm −1 , respectively.At 1450 cm −1 , bending vibrations from the C-H group are registered.The IR spectrum of the PUR matrix also shows signals at 1655 cm −1 and 1710 cm −1 corresponding to stretching vibrations of the carbonyl group and a strong, characteristic band at 1095 cm −1 related to stretching vibrations of the C-O-C group (Figure 3) [44][45][46][47].The FTIR results of the PUR-HPCA-CDP, PUR-HPAL-CDP, PUR-HPAL-TCPP, and PUR-HPCA-TCPP composites did not show significant changes in the IR spectrum compared to the reference PUR matrix.Composites containing a synergistic flame retardant system in the form of HPCA-CDP and HPAL-CDP exhibit band deformations in the range of 1010 cm −1 to 600 cm −1 , including an increase in the band intensity at 956 cm −1 .For these composites, a new signal was also recorded at 1190 cm −1 , corresponding to the valence vibrations of the phosphate group.In the case of composites containing HPCA-TCPP and HPAL-TCPP, signal deformations occur at 1230 cm −1 and in the range of 700 cm −1 to 550 cm −1 .The recorded changes in the IR spectra of the studied PUR composites may indicate the chemical bonding of organophosphorus compounds with the PUR matrix (Figure 3).The FTIR results of the PUR-HPCA-CDP, PUR-HPAL-CDP, PUR-HPAL-TCPP, and PUR-HPCA-TCPP composites did not show significant changes in the IR spectrum compared to the reference PUR matrix.Composites containing a synergistic flame retardant system in the form of HPCA-CDP and HPAL-CDP exhibit band deformations in the range of 1010 cm −1 to 600 cm −1 , including an increase in the band intensity at 956 cm −1 .For these composites, a new signal was also recorded at 1190 cm −1 , corresponding to the valence vibrations of the phosphate group.In the case of composites containing HPCA-TCPP and HPAL-TCPP, signal deformations occur at 1230 cm −1 and in the range of 700 cm −1 to 550 cm −1 .The recorded changes in the IR spectra of the studied PUR composites may indicate the chemical bonding of organophosphorus compounds with the PUR matrix (Figure 3).

Thermal Analysis and Flammability of PUR Composites
Thermal analysis results unequivocally indicate that the introduction of hybrid flame retardant systems, HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP, into the PUR foam matrix reduces its thermal stability, expressed by both T 5 and T 50 parameters.A significantly greater reduction in the T 5 and T 50 values was recorded in the presence of TCPP than CDP (Table 3).It is possible that the carbon-chlorine bonds present at the ends of the alkyl chains of TCPP (Figure 1) undergo homolytic dissociation at temperatures around 200 • C. The produced chlorine radicals participate in interrupting high-energy combustion reactions occurring in the gas phase and may accelerate the degradation and destruction of the polymer matrix.The increased efficiency of thermal degradation processes of the PUR matrix in the presence of TCPP is also confirmed by the maximum decomposition temperature parameter, T RMAX .In the presence of TCPP, the T RMAX value of the PUR composite was reduced by 16 • C (PUR-HPAL-TCPP) and 19 • C (PUR-HPCA-TCPP) compared to the unfilled PUR composite.It should also be emphasized that the introduction of CDP, containing large aromatic substituents, into the PUR matrix, in the HPCA(HPAL)-CDP system, results in an increase in the T RMAX parameter value (Figure 4, Table 3).The fire hazard of PUR composites is primarily characterized by two thermal stability parameters: the rate of thermal decomposition dm/dt and the residue after thermal decomposition P TD .It is generally assumed that the lower the dm/dt value, the smaller the amount of volatile, including flammable, decomposition products entering the flame zone.In the case of very low dm/dt values, the flame ceases to be fueled, and consequently, the composite self-extinguishes.
The residue after thermal decomposition, P TD , indicates the intensity of processes such as carbonization or cyclization occurring during the thermal decomposition of the composite.These processes directly affect both the thickness and morphology of the boundary layer [48].A homogeneous, insulating boundary layer is crucial in reducing the fire hazard of the studied PUR composites.With increasing homogeneity and insulation of the boundary layer, both mass transport and heat transfer between the sample and the flame are reduced [49,50].
Thermal analysis results indicate that the introduction of both the HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP systems into the PUR matrix reduces both the dm/dt and P TD parameters.Therefore, it can be concluded that both studied systems should be effective in reducing the fire hazard of PUR composites (Table 3, Figures 4 and 5).The results obtained using PCFC microcalorimetry clearly indicate that both the HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP systems effectively reduce the flammability of PUR composites (Figure 6, Table 4).Notably, there was nearly a 40% reduction in the HRRMAX parameter for the PUR-HPCA-CDP sample and a 33% reduction in the THR parameter for the PUR-HPCA-TCPP sample.Both the HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP systems show similar effectiveness in reducing the flammability of the studied PUR composites [51,52].However, it should be noted that in the presence of HPCA(HPAL)-TCPP, the value of the THRRMAX parameter significantly increases.This may indicate that the boundary layer of PUR forms faster, i.e., at a lower temperature range, in the presence of HPCA(HPAL)-TCPP compared to HPCA(HPAL)-CDP (Table 3).Diphenyl cresyl phosphate CDP, due to the presence of large aromatic substituents in its structure, exhibits a much greater tendency towards carbonization processes than tris(1chloro-2-propyl) phosphate, TCPP.The P TD parameter values for both the HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP systems are similar.This indicates that inorganic aluminum (HPAL) and calcium phosphinate (HPCA) significantly influence the residue after thermal decomposition, the P TD parameter, and P 600 (Table 3, Figures 4 and 5).The obtained research results suggest that the boundary layer formed during the thermal decomposition of the PUR composite is primarily stabilized by inorganic phosphorus compounds (HPAL and HPCA).
The results obtained using PCFC microcalorimetry clearly indicate that both the HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP systems effectively reduce the flammability of PUR composites (Figure 6, Table 4).Notably, there was nearly a 40% reduction in the HRR MAX parameter for the PUR-HPCA-CDP sample and a 33% reduction in the THR parameter for the PUR-HPCA-TCPP sample.Both the HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP systems show similar effectiveness in reducing the flammability of the studied PUR composites [51,52].However, it should be noted that in the presence of HPCA(HPAL)-TCPP, the value of the THRR MAX parameter significantly increases.This may indicate that the boundary layer of PUR forms faster, i.e., at a lower temperature range, in the presence of HPCA(HPAL)-TCPP compared to HPCA(HPAL)-CDP (Table 3).The results obtained using PCFC correlate well with the flammability results obtained using cone calorimetry (Tables 4 and 5).The fire hazard parameters obtained under real conditions indicate that both the HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP systems reduce the flammability of PUR composites.The tested flame-retardant systems reduce both kinetic parameters such as HRR MAX and MARHE, which indicate fire kinetics, and the FIGRA parameter, which indicates the rate of fire development.It is also worth noting that the tested systems clearly reduce the total heat released, the THR parameter (Table 5).

Smoke Emission and Toxicity of Combustion Products
The results obtained using the SDC (smoke density chamber) method showed that the synergistic flame retardant system used, HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP, has an ambiguous effect on smoke emission during thermal decomposition (Table 6).For samples containing HPCA-CDA, HPAL-CDA, and HPAL-TCPP, an increase in smoke emission, the DsMAX parameter, was recorded.However, for the HPCA-TCPP sample, the amount of smoke emitted during sample decomposition was significantly reduced.It should also be clearly noted that the amount of smoke emitted for all tested samples increases during the first 4 min of thermal decomposition, parameters Ds(4) and VOF4 (Table 6).
The increase in smoke emissions during the decomposition of the tested samples undoubtedly results from the presence of organophosphorus compounds in their matrix, which intensify carbonization and cyclization processes.The carbonization processes occurring during the thermal decomposition of PUR composites can cause increased smoke emissions.The analysis of the toxicity of gaseous decomposition products during the thermal decomposition of the tested PUR composites showed that the flame retardant systems used, HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP, also reduce the concentration of emitted CO and HCN.For the PUR-HPAL-CDP system, the reduction in emitted HCN concentration was as high as 21% (Table 7).However, it should also be noted that the flame

Smoke Emission and Toxicity of Combustion Products
The results obtained using the SDC (smoke density chamber) method showed that the synergistic flame retardant system used, HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP, has an ambiguous effect on smoke emission during thermal decomposition (Table 6).For samples containing HPCA-CDA, HPAL-CDA, and HPAL-TCPP, an increase in smoke emission, the D sMAX parameter, was recorded.However, for the HPCA-TCPP sample, the amount of smoke emitted during sample decomposition was significantly reduced.It should also be clearly noted that the amount of smoke emitted for all tested samples increases during the first 4 min of thermal decomposition, parameters Ds(4) and VOF 4 (Table 6).The increase in smoke emissions during the decomposition of the tested samples undoubtedly results from the presence of organophosphorus compounds in their matrix, which intensify carbonization and cyclization processes.The carbonization processes occurring during the thermal decomposition of PUR composites can cause increased smoke emissions.
The analysis of the toxicity of gaseous decomposition products during the thermal decomposition of the tested PUR composites showed that the flame retardant systems used, HPCA(HPAL)-CDP and HPCA(HPAL)-TCPP, also reduce the concentration of emitted CO and HCN.For the PUR-HPAL-CDP system, the reduction in emitted HCN concentration was as high as 21% (Table 7).However, it should also be noted that the flame retardant systems used caused an increase in emitted CO 2 concentration and a significant increase in NOx concentration.Only in the PUR-HPCA-TCPP composite can a significant reduction in the emission of toxic gases be observed, which correlates with the results obtained using the smoke density chamber.Consequently, the CIT G toxicometric index for the PUR-HPCA-CDP, PUR-HPCA-TCPP, and PUR-HPAL-TCPP composites turned out to be higher than for the reference PUR material (Table 7).The increase in the CIT G index for the tested composite materials is primarily due to the increase in emitted CO 2 concentration.The increase in carbon dioxide content in the gaseous decomposition products of PUR thermal decomposition, like the increase in smoke emission, is directly due to the presence of organophosphorus compounds in the PUR matrix.However, it should be clearly noted that the increase in CO 2 emission is evident only at high temperature values, i.e., above 420 • C (Figure 7, Table 7).This indicates that the emitted CO 2 is primarily produced as a result of the partial degradation of the carbonized boundary layer.

Analysis of Microbiological Activity
The results of microbiological activity analysis against PUR composites were obtained using the UV-VIS method.The obtained results showed clear changes in the level of bacterial adsorption to the composite surface (Table 8).The bacteria used in the analysis (Escherichia coli, Bacillus subtilis, and Staphylococcus aureus) are reference strains used to assess microorganism activity.Differences in the effectiveness of adsorption from the bacterial suspension to the PUR surface were observed, ranging from 45% to 62% between bacterial species.However, the average degree of bacterial binding to the surface did not show significant differences for PUR-HPCA-CDP 55%, PUR-HPAL-CDP 57.1%, and PUR-HPAL-TCPP 54.6%.The lowest average absorption levels were shown for the original PUR 49% and PUR-HPCA-TCPP 48% (Table 8).It can be observed that composites characterized by smaller pores in SEM imaging (Figure 2), and therefore the largest foam surface, showed the most effective level of bacterial binding to the material (HPAL) (Table 8).

Analysis of Microbiological Activity
The results of microbiological activity analysis against PUR composites were obtained using the UV-VIS method.The obtained results showed clear changes in the level of bacterial adsorption to the composite surface (Table 8).The bacteria used in the analysis (Escherichia coli, Bacillus subtilis, and Staphylococcus aureus) are reference strains used to assess microorganism activity.Differences in the effectiveness of adsorption from the bacterial suspension to the PUR surface were observed, ranging from 45% to 62% between bacterial species.However, the average degree of bacterial binding to the surface did not show significant differences for PUR-HPCA-CDP 55%, PUR-HPAL-CDP 57.1%, and PUR-HPAL-TCPP 54.6%.The lowest average absorption levels were shown for the original PUR 49% and PUR-HPCA-TCPP 48% (Table 8).It can be observed that composites characterized by smaller pores in SEM imaging (Figure 2), and therefore the largest foam surface, showed the most effective level of bacterial binding to the material (HPAL) (Table 8).According to the research assumptions, the developed synergistic systems are intended to be biocompatible for everyday use.Therefore, microscopic observations of bacterial cultures with PUR foams were conducted to confirm bacterial survival during the incubation of the bacterial suspension with PUR composites.The results show clear growth areas around the foams (Figure 8).Additionally, the imprint analysis of the foams after incubation also confirmed that the bacteria transferred from the initial suspension to the foam retained their activity (Figure 9).This result indicates the survival of bacteria during interaction with the modified PUR foams.Therefore, it can be concluded that organophosphorus flame retardants in synergistic systems with phosphinates do not exhibit a toxic impact on material-bacteria interactions.This is an important issue for the safety of using modified materials in daily life.Polyurethane foams can thus be in long-term exposure/contact with humans without the risk of adverse reactions.
the foam retained their activity (Figure 9).This result indicates the survival of bacteria during interaction with the modified PUR foams.Therefore, it can be concluded that organophosphorus flame retardants in synergistic systems with phosphinates do not exhibit a toxic impact on material-bacteria interactions.This is an important issue for the safety of using modified materials in daily life.Polyurethane foams can thus be in longterm exposure/contact with humans without the risk of adverse reactions.

Conclusions
The article presented the use of organophosphorus compounds (CDP and TCPP) in synergistic systems with phosphinate compounds (HPAL, HPCA) in soft polyurethane foam matrices.The obtained results showed increased thermal stability, dm/dt and PTD parameters, and reduced flammability of PUR composites.The results for smoke emission and gaseous, toxic decomposition products are ambiguous.On one hand, the flame

Figure 1 .
Figure 1.Chemical structure of the components.

Figure 1 .
Figure 1.Chemical structure of the components.

Figure 6 .
Figure 6.Flammability results of PUR composites obtained using the PCFC microcalorimetry method.

Figure 8 .
Figure 8. Images of bacterial cultures in the presence of PUR composites.Figure 8. Images of bacterial cultures in the presence of PUR composites.

Figure 8 . 19 Figure 9 .
Figure 8. Images of bacterial cultures in the presence of PUR composites.Figure 8. Images of bacterial cultures in the presence of PUR composites.Materials 2024, 17, x FOR PEER REVIEW 16 of 19

Figure 9 .
Figure 9. Images of imprints of PUR composites covered with live bacteria.

Table 1 .
Composition of the tested composites (parts by weight).

Table 1 .
Composition of the tested composites (parts by weight).

Table 3 .
Results of the thermal stability parameters of PUR composites.

Table 3 .
Results of the thermal stability parameters of PUR composites.

Table 4 .
The flammability parameters of PUR composites obtained with the PCFC microcalorimetry method.

Table 6 .
Smoke density of PUR composites.

Table 5 .
The flammability parameters of PUR composites obtained with the cone calorimetry method.

Table 6 .
Smoke density of PUR composites.

Table 7 .
Toxic gas emissions during thermal decomposition, toxicometric index CIT G .

Table 8 .
Level of bacterial adsorption to the surface of PUR composites obtained by the UV-VIS method.