New Flame Retardant Systems Based on Expanded Graphite for Rigid Polyurethane Foams

applied sciences

Article New Flame Retardant Systems Based on Expanded Graphite for Rigid Polyurethane Foams

Anna Str ˛akowska 1,* , Sylwia Członka 1, Piotr Konca 2 and Krzysztof Strzelec 1

1 Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland; [email protected] (S.C.); [email protected] (K.S.) 2 Department of Building Physics and Building Materials, Lodz University of Technology, Politechniki 6, 90-924 Lodz, Poland; [email protected] * Correspondence: [email protected]

Received: 29 July 2020; Accepted: 20 August 2020; Published: 22 August 2020

Abstract: The effect of the addition of new flame retardant systems on the properties of rigid polyurethane (RPUF) foams, in particular, reduction in flammability, was investigated. The modification included the introduction of a flame retardant system containing five parts by weight of expanded graphite (EG) (based on the total weight of polyol), one part by weight of pyrogenic silica (SiO2) and an ionic liquid (IL): 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim] [BF4]), in an amount of 3:1 with respect to the weight of added silica. The kinetics of the synthesis of modified foams—including the growth rate and the maximum temperature—were determined and the physicochemical properties, such as the determination of apparent density and structure by optical microscopy, mechanical properties such as impact strength, compressive strength and, three-point bending test were determined. An important aspect was also to examine the thermal properties such as thermal stability or flammability. It has been shown that for rigid polyurethane foams, the addition of expanded graphite in the presence of silica and ionic liquid has a great influence on the general use properties. All composites were characterized by reduced flammability as well as better mechanical properties, which may contribute to a wider use of rigid polyurethane foams as construction materials.

Keywords: RPUF; expanded graphite; ionic liquids; flame retardants; thermal stability; porous material

1. Introduction Nowadays, where materials are increasingly demanding, applications are constantly striving to improve their properties. This is why the purpose of this work was to prepare rigid polyurethane foams (RPUF) with improved performance. RPUFs are materials with very large application possibilities. However, a certain limitation in their use is the flammability of RPUF, which results from their porous structure. For this reason, one of the inseparable elements of any RPUF composition intended for foaming is the flame retardant, also known as an anti-pyrene (FR) [1]. To improve fire resistance, it is necessary to reduce the number of flammable products, as well as to reduce the rate of their release and avoid prolonged burning [2]. Smoke retarders are usually used as reactive and additive additives [3]. These additives have very high requirements for thermal resistance and high resistance to alkaline conditions occurring during the formation of foams. Reactive flame retardants have functional groups capable of reacting with components involved in the foaming process of polyurethane foam [4]. Reactive antipyrine contains elements such as phosphorus in their structure, which are incorporated into the polymer structure, thus causing permanent binding so that they are not susceptible to volatilization during further application. Currently, the most commonly used reactive retardants are phosphoric acid derivatives that contain

Appl. Sci. 2020, 10, 5817; doi:10.3390/app10175817 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 5817 2 of 13 hydroxyl groups [5]. Unlike reactive retarders, additive retarders do not have reactive functional groups capable of being incorporated into the polyurethane structure [6]. One of the most popular additive flame retardants includes halogen-derived liquid phosphates, whose task is to limit the combustion of the substance to a strictly insulating carbon layer, which prevents the access of oxygen and heat energy. In the case of such additives, the synergistic effect of phosphorus and halogen atoms is used to delay or diminish the intensity of the smoking process [7]. In line with the principle of sustainable development and environmental protection, great emphasis is placed on the use of halogen-free phosphorus-based flame retardants, including phosphamides and phosphazenes [8]. Expanded graphite is another anti-pyrene that is also very popular in the production of polyurethane foams. Its operation consists in creating a charred layer that acts as an “insulator” before further degradation of the polymer matrix due to the emerging small air gaps between individual layers of graphite [9]. The addition of graphite greatly reduces heat dissipation, weight loss and smoke production. In industry, low-temperature expanded graphite is most commonly used [10]. In previous studies, we indicated a positive effect of ionic liquids (IL) on the thermal stability and mechanical strength of porous polyurethane composites due to better dispersion of filler particles in the polymer matrix [11–13]. Graphite is a commonly used and effective flame retardant for PU foams, but it does not show reinforcing properties. Therefore, it seems advantageous to combine graphite with a reinforcing silica filler, which can improve the mechanical properties of rigid PU foams and with a small amount of ionic liquid, an improvement in the dispersion of low-molecular weight substances in the polyol can be expected. Therefore, in order to obtain foams with increased fire resistance and improved mechanical properties, we have proposed a modification system consisting of expanded graphite, pyrogenic silica and an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate [emim] [BF4]). The effect of the components of the synergistic flame retardant system on morphology, thermal properties, physical and mechanical properties as well as fire behavior of the resulting PU composites have been thoroughly studied. The results presented in the paper show that the application of the proposed modification methods leads to the production of porous polyurethane composites with improved performance parameters. The modifiers used are generally low-price and available, which will not translate into a sharp increase in the price of the potential product. It is also worth adding that thanks to the proposed modifications it is possible to adapt RPUF to specific applications. In addition to improving the mechanical and functional properties, it was possible to reduce the average intensity of heat release from modified foams during combustion, which can directly increase the application safety of materials used in construction

2. Experimental

2.1. Materials and Manufacturing RPUFs were synthesized from a modified polyisocyanurate rigid foam formulation based on petrochemical components provided by Purinova Sp. z o.o. Izopianol 30/10/C (Purinova Sp. z o.o.) used in the reaction is a fully formulated mixture containing a mixture of polyester polyol (hydroxyl number ca. 230–250 mgKOH/g, the functionality of 2), catalyst (N,N-dimethylcyclohexylamine), flame retardant (Tris(2-chloro-1-methylethyl)phosphate) and chain extender (1,2-propanediol). Purocyn B (Purinova Sp. z o. o.) used in the synthesis is a polymeric diphenylmethane 4, 40-diisocyanate (pMDI) containing 31 wt% of free isocyanate groups. The formulations were modified with expanded graphite [EG] S7 treated with sulfuric acid—(generously donated by the S.Z.T.K. ‘TAPS’—Maciej Kowalski, 2 Poland), fumed silica [SiO2] with a specific surface area of 380 m /g (Aerosil 380, Evonic Industries) and ionic liquid [IL], 1-ethyl-3-methylimidazolium tetrafluoroborate [emim] [BF4] (Fluka). Four series of foams were made. The first reference series without the addition of a modifier and 3 with modifiers. Both graphite and silica were added based on the weight of the polyol, while the Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 14

ionic liquid was added based on the weight of the silica. The compositions of RPUFs are presented Appl. Sci. 2020 10 in Table, , 5817 1. 3 of 13 ionic liquid was addedTable based 1. onFormulations the weight of of rigid the polyurethane silica. The compositions foams (RPUF) of prepared RPUFs in are the presented study. in Table1. Mass Content (by Weight, wt% ) Sample Comments Expanded Fumed Ionic Liquid CodesTable 1. Formulations of rigid polyurethanePolyol foams Isocyanate (RPUF) prepared in the study. Graphite Silica (emim) (BF4) Reference foam Mass Content (by Weight, wt% ) Sample PU–0 100 160 ‐ ‐ ‐ Comments Expanded Fumed Ionic Liquid Codes (unfilled) Polyol Isocyanate PU–EG Foam with graphite 100 160 Graphite5 ‐ ‐ Silica (emim) (BF4) PU–0 ReferenceFoam foam with (unﬁlled) graphite 100 160 - - - PU–EGPU–EG+SiO2 Foam with graphite 100100 160160 55 -1 ‐ - and silica PU–EG+SiO2 Foam with graphite and silica 100 160 5 1 - Foam withFoam graphite, with graphite, silica and PU–EG+SiO2+PU–IL 100 160 5 1 0.33 ionicsilica liquid and ionic 100 160 5 1 0.33 EG+SiO2+IL liquid

All RPUFs were prepared in a plastic container. An adequate amount of ﬁllers (EG/SiO2/IL) were added toAll the RPUFs polyol were and prepared the mixture in a (labeled plastic container. as a component An adequate A) was amount mixed of with fillers the (EG/SiO stirrer at2/IL) were 800 RPMadded for 60 s.to Then,the polyol the isocyanate and the mixture (labeled (labeled as a component as a component B) was added A) was to componentmixed with A the and stirrer the at 800 formulationRPM was for mixed60 s. Then, at 2000 the RPMisocyanate for 10 (labeled s. The PU as mixturea component was pouredB) was intoadded an to open component mold and A and the allowed toformulation expand freely was in mixed the vertical at 2000 direction. RPM for All 10 PU s. The foams PU were mixture conditioned was poured at room into temperature an open mold and for 24 h.allowed A schematic to expand procedure freely of synthesis in the vertical of RPUF direction. is presented All in PU Figure foams1. were conditioned at room temperature for 24 h. A schematic procedure of synthesis of RPUF is presented in Figure 1.

Figure 1. FigureScheme 1. for Scheme drawing-up for drawing the RPUF.‐up the RPUF.

2.2. Characterization Techniques 2.2. Characterization Techniques The absolute viscosities of polyol and isocyanate were determined corresponding to ASTM D2930 The absolute viscosities of polyol and isocyanate were determined corresponding to ASTM (equivalent to ISO 2555) using a rotary viscometer DVII+ (Brookfield, Lorch, Germany). The torque of D2930 (equivalent to ISO 2555) using a rotary viscometer DVII+ (Brookfield, Germany). The torque samples was measured as a range of shear rate from 0.5 to 100 s 1 in ambient temperature. of samples was measured as a range of shear rate from −0.5 to 100 s−1 in ambient temperature. The structure and size distribution of the foam cells was examined on the basis of the images of The structure and size distribution of the foam cells was examined on the basis of the images of the cell structure of the foam, made with the Leica MZ6 optical microscope at two magnifications: the cell structure of the foam, made with the Leica MZ6 optical microscope at two magnifications: 50x and 250x and the JEOL JSM-5500 LV scanning electron microscopy (JEOL, Ltd., Peabody, MA, 50x and 250x and the JEOL JSM‐5500 LV scanning electron microscopy (JEOL, Ltd., USA). All USA). All microscopic observations were made in high vacuum mode and with an accelerating voltage microscopic observations were made in high vacuum mode and with an accelerating voltage of 10 of 10 kV. The samples were scanned for free growth. Mean pore diameter, wall thickness and pore size kV. The samples were scanned for free growth. Mean pore diameter, wall thickness and pore size distribution were calculated with ImageJ software (Media Cybernetics, Inc., Rockville, MD, USA). distribution were calculated with ImageJ software (Media Cybernetics, Inc.). The apparent density of foams (5 specimens per sample were measured and averaged) was The apparent density of foams (5 specimens per sample were measured and averaged) was determined accordingly to ASTM D1622 (equivalent to ISO 845). determined accordingly to ASTM D1622 (equivalent to ISO 845). The compressive strength (σ10%) of foams (averaged 5 measurements per each sample) was The compressive strength (σ10%) of foams (averaged 5 measurements per each sample) was determined accordingly to the ASTM D1621 (equivalent to ISO 844) using Zwick Z100 Testing Machine determined accordingly to the ASTM D1621 (equivalent to ISO 844) using Zwick Z100 Testing (Zwick/RoellMachine Group, (Zwick/Roell Poland, Ł óGroup,d´z).The Poland, compression Łódź). The strength compression was measured strength as was a ratio measured of the load as a ratio of causing athe 10% load deformation causing a of 10% sample deformation cross-section of sample in the parallel cross‐section and perpendicular in the parallel direction and perpendicular to the square surface.direction to the square surface. A three-pointA three bending‐point test bending was carried test was out usingcarried Zwick out using Z100 TestingZwick MachineZ100 Testing (Zwick Machine/Roell Group, (Zwick/Roell Ulm, Germany)Group, atGermany) room temperature, at room temperature, according to ASTMaccording D7264 to (equivalent ASTM D7264 to ISO (equivalent 178). The to obtained ISO 178). The flexural stress at break (εf) results for each sample were expressed as a mean value. The average of 5 measurements per each type of composition was accepted. Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 14 Appl. Sci. 2020, 10, 5817 4 of 13 obtained flexural stress at break (εf) results for each sample were expressed as a mean value. The average of 5 measurements per each type of composition was accepted. TheThe thermal thermal conductivity conductivity of of the the foams foams obtained obtained was was determined determined based based on on the the measurement measurement of of thethe thermal thermal conductivity conductivity ( (λλ).). The The test test was was carried carried out out for for 200 200 × 200200 × 5050 mm mm3 3samplessamples using using a a FOX FOX × × 200200 apparatus apparatus (LaserComp, (LaserComp, Inc., Saugus,MA, USA). MA, Thermal USA). Thermal conductivity conductivity was determined was determined at 10 and at 1020 and°C in20 the◦C range in the from range 20 from to 100 20 mW/(m*K). to 100 mW/(m*K). TheThe fire fire behavior behavior was was analyzed analyzed using using a a cone cone calorimeter calorimeter apparatus apparatus according according to to ISO ISO 5660 5660 in in S.Z.T.K.S.Z.T.K. ‘TAPS’ ‘TAPS’—Maciej ‐ Maciej Kowalski Kowalski Company Company (Poland). (Saugus, Each Poland). specimen Each with specimen dimensions with dimensions of 100 × 100 of 3 2 ×100 50 mm1003 was50 wrapped mm was with wrapped aluminum with aluminum foil and burned foil and at burned an external at an external heat flux heat of flux35 kW of 35 m kW−2. The m− . × × parametersThe parameters were wererecorded recorded during during the time. the time.The measurement The measurement for each for specimen each specimen was repeated was repeated three times,three times,and the and error the errorvalues values of the of typical the typical cone cone calorimeter calorimeter were were reproducible reproducible within within ±5%,5%, and and the the ± reportedreported results results are are the the average average of of three three measurements. measurements. TheThe thermal thermal stability stability testing testing of of rigid rigid polyurethane polyurethane foams foams was was performed performed using using a a Mettler Mettler Toledo Toledo thermogravimetricthermogravimetric analyzer analyzer TGA/DSC1. TGA/DSC1. Thermal Thermal decomposition decomposition was was carried carried out outin the in range the range of 25– of 60025–600 °C in◦ Can in inert an inert gas atmosphere gas atmosphere with witha flow a flowof 50 of mL/min 50 mL /andmin a and heating a heating rate of rate 10 of°C/min. 10 ◦C /Themin. measurementThe measurement consists consists of analyzing of analyzing the thechange change in mass in mass as asa function a function of of temperature temperature during during the the thermalthermal decomposition decomposition of of the the sample. sample. The The initial initial decomposition decomposition temperatures, temperatures, T5%, T5%, T10%, T10%, T50%, T50%, andand T70% T70% of of mass mass loss loss were were determined. determined.

3.3. Results Results and and Discussion Discussion 3.1. Rheological Properties of the Polyol Premixes 3.1. Rheological Properties of the Polyol Premixes The factors determining the course of synthesis—including foaming, and RPUR processing—is The factors determining the course of synthesis—including foaming, and RPUR processing—is the viscosity of the polyol blends. Therefore, the influence of all additives on the dynamic viscosity of the viscosity of the polyol blends. Therefore, the influence of all additives on the dynamic viscosity the polyol was investigated. of the polyol was investigated. Based on the obtained data, a graph of the dynamic change in viscosity as a function of shear Based on the obtained data, a graph of the dynamic change in viscosity as a function of shear was prepared for the tested polyol mixtures. In Figure2, it can be seen that the addition of solids was prepared for the tested polyol mixtures. In Figure 2, it can be seen that the addition of solids increases the viscosity of the system, which may also indicate the reactivity of the modifiers towards increases the viscosity of the system, which may also indicate the reactivity of the modifiers towards the polyol. Moreover, the dependence of the decrease in viscosity of the polyol mixtures as a function the polyol. Moreover, the dependence of the decrease in viscosity of the polyol mixtures as a function of the increasing shear rate is clearly visible, especially in the initial phase, when the shear rates are not of the increasing shear rate is clearly visible, especially in the initial phase, when the shear rates are high. The observed tendency indicates that the tested mixtures are typical for non-Newtonian shear not high. The observed tendency indicates that the tested mixtures are typical for non‐Newtonian thinned liquids. The addition of IL reduces the dynamic viscosity of the polyol due to the addition of shear thinned liquids. The addition of IL reduces the dynamic viscosity of the polyol due to the graphite and silica. However, with increasing shear rate, the viscosity for this system decreases the addition of graphite and silica. However, with increasing shear rate, the viscosity for this system least, and above 2 PRM the highest values are obtained. decreases the least, and above 2 PRM the highest values are obtained. A similar trend of increase in polyol viscosity due to the addition of solid modifiers was also A similar trend of increase in polyol viscosity due to the addition of solid modifiers was also observed in previous studies [14–16]. observed in previous studies [14–16].

950 850 750 650 550 450 350 0246810 RPM [1/S]

WZPU-0 PU-EGM5 M5PU-EG+SiO + A3802 M5PU-EG+SiO + A3802 + IL[EMIM][Cl]

Figure 2. Viscosity of the polyol system modiﬁed with graphite, silica and ionic liquid. Figure 2. Viscosity of the polyol system modified with graphite, silica and ionic liquid. 3.2. Growth Kinetics of PU Systems 3.2. Growth Kinetics of PU Systems Foaming behavior was determined by measuring temperature and processing time—start, growthFoaming and tack-free behavior time. was determined by measuring temperature and processing time—start, growth and tack‐free time. Appl. Sci. 2020, 10, 5817 5 of 13

The reaction of the synthesis of rigid polyurethane foams is classified as highly exothermic, during which large amounts of heat are released. Based on the growth rate of the foams and the temperature during RPUF synthesis, the reactivity of the polyol-isocyanate system can be determined. Analyzing the obtained results, it can be easily seen that for all modified samples the maximum temperature was decreased during the synthesis. The effect of the addition of expanded graphite on the maximum temperature is very visible and additionally intensified by the addition of silica and ionic liquid. Thanks to its very good thermal conductivity, EG has the ability to dissipate part of the heat generated during synthesis to the outside of the foam, which is manifested by a reduction in the maximum temperature of RPUF synthesis. In turn, a further decrease in the temperature of modified systems is associated with the introduction of further fillers containing additional groups that react with isocyanate, leading to side reactions, which release less heat than in the case of polyurethane bond synthesis. The characteristic times in Table2 indicate that the addition of fillers increases the foam start and rise times. This is most likely due to the formation of additional nucleation centers, such as active solid filler particles. This consequently leads to increased bubble formation and to an extended start and rise time. Increased viscosity of mixtures containing solid fillers also inhibits further bubble growth, which significantly determines the foam behavior and extends the synthesis time. Moreover, the rate of PU polymerization itself is slowed down due to the presence of fillers which limit the mobility of the molecules leading to phase separation and an increase in the isocyanate conversion time during the reaction. Despite the fact that the addition of graphite, silica and ionic liquid affects the processing time, the total preparation time of PU foams is still within the permissible operating conditions [17,18]. The presented results are consistent with other materials described in the literature.

Table 2. Growth kinetics and structure parameters of RPUFs.

Processing T Cell Size Wall Total Apparent System max Times [s] [◦C] [µm] Thickness Porosity Density 3 Start Rise Gelling [µm] [% ] [kg m− ] PU–0 140 49 3 262 8 448 14 472 10 62 4 91 41 ± ± ± ± ± PU–EG 134 56 4 277 8 498 13 296 7 67 3 83 46 ± ± ± ± ± PU–EG+SiO 133 54 3 338 9 564 11 282 7 69 3 81 48 2 ± ± ± ± ± PU–EG+SiO +IL 132 58 4 342 8 594 12 304 8 71 4 82 47 2 ± ± ± ± ±

3.3. Apparent Density of RPUFs The apparent density is a parameter that determines the physical properties of foams. It is closely related to such foam properties as dimensional stability, thermal insulation as well as all mechanical properties. By analyzing the results presented in Table3, it can be stated that for all tested foams, the apparent density increased from 41 kg/m3 for the reference foam to a maximum value of 48 kg/m3 for the series containing graphite and silica. The increase in apparent density is the result of increased system viscosity, hindered expansion of bubbles, as well as an additional mass of ﬁllers with higher densities compared to the PU matrix.

Table 3. Mechanical properties of PU foams.

Compressive Compressive Apparent Density Hardness Strength Strength Flexural Strength Impact Strength System 3 2 (kg m− ) (◦Sh) (Parallel) (Perpendicular) (MPa) (kJ/m ) (kPa) (kPa) PU–0 41.1 67.6 162 122 0.41 141 PU–EG 45.7 74.7 243 153 0.51 227 PU–EG+SiO2 47.3 75.6 261 164 0.49 222 PU–EG+SiO2+IL 46.4 74.4 258 163 0.46 215 Appl. Sci. 2020, 10, 5817 6 of 13

3.4. Morphology of RPUFs Figure3 shows the structure parameters of RPUF obtained using an optical microscope. As shown in FigureAppl. Sci.3a, 2020 the, 10 unmodified, x FOR PEER REVIEW foam has a high regularity of the porous structure for which6 of no 14 cell elongation in the direction of foam growth was observed. Their diameter is about 0.47 mm. Moreover, Figure 3 shows the structure parameters of RPUF obtained using an optical microscope. As the rib structures separating individual pores are visible. They are characterized by a fairly large shown in Figure 3a, the unmodified foam has a high regularity of the porous structure for which no thickness,cell elongation which indicates in the direction good stability of foam of growth the entire was structure.observed. Their The photo diameter also is shows about that0.47 mm. standard foamMoreover, has a high the percentage rib structures of closed-cellseparating individual cells. The pores structure are visible. of the They RPUF are modified characterized with by EG a fairly is shown in Figurelarge3 b.thickness, It was noticeablewhich indicates that good the inclusion stability of of the a modifying entire structure. additive The caused photo also less shows uniformity that of RPUFstandard structure. foam The has a structure high percentage of RPUF of closed with‐ thecell cells. addition The structure of EG was of the more RPUF heterogeneous, modified with EG with a simultaneousis shown increasein Figure in3b. the It was number noticeable of damaged that the cells. inclusion A similar of a modifying phenomenon additive was caused visible less for PU uniformity of RPUF structure. The structure of RPUF with the addition of EG was more foams with the addition of EG+SiO2 (Figure3c) and EG +SiO2+IL (Figure3d), in which the structure was moreheterogeneous, irregular with and a damaged simultaneous or defectiveincrease in cellsthe number were visible.of damaged In addition,cells. A similar the phenomenon incorporation of was visible for PU foams with the addition of EG+SiO2 (Figure 3c) and EG+SiO2+IL (Figure 3d), in fillers reduced the porosity of PU foams. As shown in Table3, the total porosity of PU–0 was about which the structure was more irregular and damaged or defective cells were visible. In addition, the 91%, while the porosity of modified RPUFs was slightly reduced and depended on the type of filler incorporation of fillers reduced the porosity of PU foams. As shown in Table 3, the total porosity of was inPU–0 the was range about 81–83%. 91%, while Moreover, the porosity the modified of modified foams RPUFs were was characterized slightly reduced by and a higher depended anisotropy on coeffithecient type due of tofiller the was higher in the viscosity range 81–83%. of the modified Moreover, systems the modified and thefoams more were heterogeneous characterized structureby a of thehigher obtained anisotropy foams. coefficient Summing due up to the the analysis higher viscosity of pore sizeof the and modified surface systems structure and of the synthesized more foams,heterogeneous it could be stated structure that of the the addition obtained of foams. selected Summing modifiers up the significantly analysis of a poreffects size the and morphology surface of the obtainedstructure RPUFs.of synthesized The degree foams, ofit could cell opening be stated dependsthat the addition on many of selected factors—and modifiers among significantly them—the viscosityaffects of the the morphology reaction mixture, of the obtained as well RPUFs. as its The reactivity, degree of was cell of opening great importance.depends on many If the factors— increasing viscosityand ofamong the reaction them—the mixture viscosity was of not the properly reaction correlatedmixture, as with well the as increaseits reactivity, in bubbles, was of theygreat break importance. If the increasing viscosity of the reaction mixture was not properly correlated with the and consequently open pores form. The more irregular structure and elongation in the direction of increase in bubbles, they break and consequently open pores form. The more irregular structure and growth of modified foams relative to the reference foam were due to the nature of the reactions taking elongation in the direction of growth of modified foams relative to the reference foam were due to place,the which nature were of the more reactions violent taking by increasing place, which the were reactivity more violent of the by system increasing [19]. the reactivity of the system [19].

(a) (b)

FigureFigure 3. Images3. Images of of foamfoam using using 250× 250 magnification.magniﬁcation. (a) PU–0; (a) ( PU–0;b) PU–EG; (b) (c PU–EG;) PU–EG+SiO (c) PU–EG2; (d) PU–+SiO ; × 2 EG+SiO2+IL. (d) PU–EG+SiO2+IL. 3.5. Mechanical3.5. Mechanical Properties Properties of RPUFsof RPUFs CellCell morphology morphology is is a a very very importantimportant factor factor that that translates translates into the into later the properties later properties of RPUF. of The RPUF. The closeclose correlationcorrelation with with the the structure structure is isthe the apparent apparent density density of PU of PUfoam, foam, which which is one is oneof the of most the most important parameters describing porous materials. It is directly related to such foam properties as important parameters describing porous materials. It is directly related to such foam properties as dimensional stability, thermal insulation as well as all mechanical properties. By analyzing the results dimensionalpresented stability, in Table thermal 3. It can be insulation stated that as for well all astested all mechanical foams the apparent properties. density By increased analyzing from the 41 results presentedkg/m3 infor Table PU–03 foam. It can to a be maximum stated that value for of all over tested 47 kg/m foams3 for the series apparent containing density EG increased and Si. The from 3 3 41 kgincrease/m for in PU–0 density foam in modified to a maximum systems valuewas the of result over of 47 increased kg/m forsystem the viscosity, series containing impeded bubble EG and Si. expansion, as well as the additional mass of fillers with specific densities. The addition of modifiers Appl. Sci. 2020, 10, 5817 7 of 13

The increase in density in modified systems was the result of increased system viscosity, impeded bubble expansion, as well as the additional mass of fillers with specific densities. The addition of modifiers also increased the hardness of RPUF, which was the result of increased apparent density, because the higher the apparent density of the composite, the more compact it was and thus the higher the hardness. In addition, the presence of reinforcing fillers in the form of expanded graphite and silica capable of creating secondary structure and for polymer–filler interactions, increasing the hardness of synthesized foams. Due to cell anisotropy, the compressive strength (σ10%) was measured in two directions: parallel and perpendicular to the foam growth direction. As shown in Table3, the compressive strength (measured in parallel and perpendicular direction) of modified foams was higher than σ10% PU–0. The results distinctly point out that there is a correlation between σ10% and the density of RPUFs. Samples with higher density had higher values of σ10%. In addition, it was seen that the mechanical strength of rigid polyurethane foams—measured in a direction perpendicular to the growth of the foam—was greater than in the parallel, which was associated with cellular anisotropy manifested in the cell lengthening in the direction of foam growth. The reference foam had a resistance of 162 kPa, which was the minimum compressive strength value declared by PUR manufacturers. In the case of modified foams, a tendency to increase mechanical strength was visible along with the introduction of another modifier. The addition of EG and SiO2 had a positive effect on increasing the compressive strength up to a maximum value of 261 kPa. This value was over 60% higher than in the case of PU–0 foam. The improvement in σ10% for modified foams may be due to the increased content of polar urea groups that can form hydrogen bonds and stiffen the structure. Undoubtedly, the tested mechanical properties were also affected by the increasing viscosity of the systems together with the increase in the content of modifiers that could more stabilize the structure of the resulting foam. An analogous improvement in properties could also be seen in the measurement of impact strength and flexural strength of modified PU foams. This reinforcing effect could also be explained by the mechanism for introducing fillers into the matrix of porous material. Namely, the filler particles, which due to interfacial adhesion were located in the cell struts of the polyurethane matrix, could disperse and transfer stress, which increased the resistance to mechanical stress. A similar trend was also seen in previous studies [20–23].

3.6. Thermal Conductivity of PU Foams Low thermal conductivity (λ) is a key parameter indicating the use of rigid polyurethane foams as insulation materials in construction. The value of thermal conductivity is the result of several factors, namely: apparent density, number of closed cells and cell size [24,25]. To investigate the effect of used modifiers on the insulating properties of PU foams, the value of thermal conductivity was measured at 10 and 20 ◦C. The results in Table4, shows that as a result of the introduction of modifiers, the heat transfer coefficient changes. This was the result of a change in the foam structure due to the addition of additives, which was manifested in a change in foam density and porosity. It could be seen that as the amount of modifiers increased, the density and therefore the thermal conductivity coefficient increased (Figures4 and5).

Table 4. Thermal conductivity coeﬃcient at a temperature diﬀerence of 10 ◦C.

Thermal Conductivity λ (mW/(m*K) System 10 ◦C 20 ◦C PU–0 23.8 24.5 PU–EG 23.4 24.8 PU–EG+SiO2 23.6 24.9 PU–EG+SiO2+IL 24.0 25.3 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 14

Appl.Appl. Sci. 2020,, 10,, 5817x FOR PEER REVIEW 88 ofof 1314 26 60 Thermal conductivity Apparent density 26 60 Thermal conductivity 50 λ Apparent density 50 λ 25 40

[(mW/(m*K)] 25 40 30 Thermal conductivity [(mW/(m*K)] Apparent density [kg*m3] 30 Thermal conductivity 24 20 PU‐0 PU‐EG PU‐EG+SiO2 PU‐EG+SiO2+IL Apparent density [kg*m3] 24 20 PU‐0 PU‐EG PU‐EG+SiO2 PU‐EG+SiO2+IL Figure 4. Dependence of the thermal conductivity coefficient at 20 °C on the apparent density of PU Figurefoams. 4. Dependence of the thermal conductivity coeﬃcient at 20 ◦C on the apparent density of PUFigure foams. 4. Dependence of the thermal conductivity coefficient at 20 °C on the apparent density of PU foams. 26 100 Thermal conductivity Total porosity 26 100 Thermal conductivity 90 λ Total porosity 8090 λ 25 7080 [(mW/(m*K)]

25 Total porosity [%]

Thermal conductivity 6070 [(mW/(m*K)] Total porosity [%] Thermal conductivity 24 5060 PU‐0 PU‐G PU‐G+Si PU‐G+Si+IL 24 50 Figure 5. Dependence of the thermalPU‐0 conductivity PU‐G coeffi PU‐G+Sicient at PU‐G+Si+IL20 ◦C on the total porosity of PU foams. Figure 5. Dependence of the thermal conductivity coefficient at 20 °C on the total porosity of PU Thisfoams. is because the value of the thermal conductivity coefficient depends on the share of individual phasesFigure and their5. Dependence thermal conductivity. of the thermal The conductivity skeleton hascoefficient a much at higher 20 °C on thermal the total conductivity porosity of thanPU the gas component,Thisfoams. is because so the the greater value the of proportionthe thermal of conductivity the skeleton, coefficient the greater depends the thermal on conductivitythe share of ofindividual the porous phases composite. and their The increasethermal in conductivity. thermal conductivity The skeleton caused has by thea much introduction higher of thermal a large This is because the value of the thermal conductivity coefficient depends on the share of amountconductivity of solid than fillers the gas of component, both organic so and the inorganic greater the nature proportion is widely of the described skeleton, in the the greater literature. the individual phases and their thermal conductivity. The skeleton has a much higher thermal Kurańskaetthermal conductivity al. [26] showed of the porous that despite composite. the high The content increase of in closed thermal cells, conductivity the thermal caused conductivity by the conductivity than the gas component, so the greater the proportion of the skeleton, the greater the coeintroductionfficient increased of a large for amount samples of modified solid fillers with of basalt both organic waste in and the inorganic amount of nature 3–40% is by widely weight. described In turn, thermal conductivity of the porous composite. The increase in thermal conductivity caused by the Modestiin the literature. et al. [27 Kura] showedńska theet al. eff [26]ect of showed expanded that flake despite aggregates the high as content carriers of for closed heat transportcells, the throughthermal introduction of a large amount of solid fillers of both organic and inorganic nature is widely described theconductivity solid material, coefficient which increased resulted in for increased samples thermal modified conductivity. with basalt Nevertheless, waste in the amount the value of of 3–40% thermal by in the literature. Kurańska et al. [26] showed that despite the high content of closed cells, the thermal conductivityweight. In turn, (about Modesti 0.025 et (mW al. [27]/(m*K)) showed is in the the effect range of that expanded is acceptable flake aggregates from an application as carriers pointfor heat of conductivity coefficient increased for samples modified with basalt waste in the amount of 3–40% by viewtransport [28]. through A similar the relationship solid material, was which also observed resulted inin theincreased case of thermal our study. conductivity. Nevertheless, weight. In turn, Modesti et al. [27] showed the effect of expanded flake aggregates as carriers for heat the value of thermal conductivity (about 0.025 (mW/(m*K)) is in the range that is acceptable from an transport through the solid material, which resulted in increased thermal conductivity. Nevertheless, 3.7.application Fire Behavior point of of RPUFs view [28]. A similar relationship was also observed in the case of our study. the value of thermal conductivity (about 0.025 (mW/(m*K)) is in the range that is acceptable from an The flame retardant behavior of RPUFs was determined using a cone calorimeter test, which is application point of view [28]. A similar relationship was also observed in the case of our study. recognized3.7. Fire Behavior as the of best RPUFs method to simulate small-scale fire conditions. The results of ignition time (TTI), peak3.7. FireThe heat Behavior flame release retardant of rate RPUFs (pHRR), behavior total of heat RPUFs release was (THR), determined the maximum using a cone average calorimeter rate of heat test, emissionwhich is (MAHRE)recognized as as well the asbest the method total smoke to simulate release (TSR)small‐ andscale char fire residue conditions. are presented The results in Tableof ignition5. time The flame retardant behavior of RPUFs was determined using a cone calorimeter test, which is (TTI),The peak cell heat structure release had rate a great (pHRR), impact total on heat the firerelease behavior (THR), of PUthe foam;maximum however, average the addition rate of heat of a recognized as the best method to simulate small‐scale fire conditions. The results of ignition time filleremission could (MAHRE) change it as significantly well as the [ 26total]. As smoke can be release seen in (TSR) Table and5, the char ignition residue time are (TTI) presented was increased in Table (TTI), peak heat release rate (pHRR), total heat release (THR), the maximum average rate of heat by5. the addition of EG and EG with Si and the peak heat release rate (pHRR), total heat release (THR), emission (MAHRE) as well as the total smoke release (TSR) and char residue are presented in Table totalThe smoke cell release structure (TSR) had decreased a great impact with addition on the fire each behavior modifier. of This PU foam; phenomenon however, probably the addition resulted of 5. froma filler the could effective change heat assimilationit significantly by expanded[26]. As can graphite be seen and in silica, Table which 5, the delay ignition the burning time (TTI) of foams. was The cell structure had a great impact on the fire behavior of PU foam; however, the addition of increased by the addition of EG and EG with Si and the peak heat release rate (pHRR), total heat a filler could change it significantly [26]. As can be seen in Table 5, the ignition time (TTI) was increased by the addition of EG and EG with Si and the peak heat release rate (pHRR), total heat Appl. Sci. 2020, 10, 5817 9 of 13

Table 5. Parameters determined during the combustion process of RPUFs.

pHRR THR MAHRE TSR Char System TTI (s) (kW/m2) (MJ/m2) (kW/m2) (m2/m2) Residue (%) PU–0 2 213 21.9 180.0 37.9 13.1 PU–EG 4 71 17.9 70.8 16.1 28.7 PU–EG+SiO2 5 62 17.2 61.7 15.6 31.8 PU–EG+SiO2+IL 5 64 17.7 63.9 16.3 27.6

The parameter that should be taken into account is the heat release rate—defined by the peak heat release rate, which provides information on the spark ignition of the exposed burned material. Based on the results presented in Table5, it can be noticed that the addition of the modifiers to the RPUFs decreased the pHRR value compared to the unmodified foam. Namely, for PU–EG, PU–EG+SiO2 and PU–EG+SiO2+IL samples, the pHRR value was 71, 62 and 2 64 kW/m− , respectively, which means a reduction of approximately 67, 71 and 70% compared to reference foam. The decrease in pHRR was due to the fact that the expanded graphite exposed to the heat source expands into the carbonaceous layer on the RPUF surface. The formed char layer effectively hinders the diffusion of oxygen and the transfer of mass and heat between the flames and the polyurethane matrix, thus interrupting the self-sustaining combustion of the foam [29]. The addition of an additional filler, i.e., pyrogenic silica, resulted in the creation of an additional thermal barrier, because in addition to the high thermal stability of silica, it also favored the ceramization process, which further enhanced the barrier effect. In contrast, no positive effect of IL on pHRR was observed. Probably, despite the content of anion [BF4]−, the ionic liquid decomposed during the combustion process, as evidenced by a slightly lower content of carbonized residues than in the case of PU–EG+SiO2. However, as could be predicted at the end of combustion, RPUFs with addition only EG and EG in the presence of SiO2 or SiO2+IL retained more carbonized residues than PU–0 foam. This trend was comparable to the residual weight obtained from TGA, although the TGA residue was slightly lower compared to the results of the cone calorimeter. This was probably the result of oxidative decomposition which enhances pyrolytic decomposition under forced-flame conditions. As shown in Table5 the total char residue of the PU–0 sample was about 13.1%, and the addition of flame retardants increased the char yield by up to 31.8% for the PU–EG+SiO2 sample. The increased char yield of PU foams containing expanded graphite may be due to the presence of a protective char layer acting as a swelling, thermal barrier, which contributes to a reduction in the diffusion rate of volatile compounds as well as a significant reduction of oxygen supply to the interior of the sample. As a result of this action, the further distribution of the sample was reduced, and the weight loss of modified foams was less than for a sample without the addition of a graphite flame retardant. It should be noted that 2 for RPUF used as insulation materials the available pHRR value was 300 kW m− [30], while for the 2 analyzed PU foams the pHRR values did not exceed 71 kW m− . In addition, the THR value was obtained much lower for modified foams in comparison with PU–0 foam. The smallest amount of total heat released was obtained for the PU–EG+SiO2 sample, which decreased by over 20% compared to the PU–0 sample. Other modified samples had similar THR values. The most common cause of death in a fire is smoke, therefore total smoke release (TSR) is very important for the safety and fire resistance assessment of materials. Based on the data contained in Table5, it can be seen that the use of fillers e ffectively reduced smoke during the combustion process. All modified PU foams had significantly lower TSR values. For PU–0, the TSR value was 2 2 2 2 about 37.9 m m− and significantly decreased to 15.6 m m− for the PU–EG+SiO2 sample, which was probably the result of the formation of a dense carbon layer on the polymer surface, which may limit the total amount of smoke released during the combustion process. The fire-retardant properties of the test samples can also be well assessed on the basis of the observation of the residual samples after combustion in the calorimetric cone test. Figure6 shows comparative digital photos of PU–0 and PU–EG residues after the cone calorimeter test. Differences Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 14

Appl.Appl.The Sci. Sci. 2020fire 2020‐,,retardant10 10,, 5817 x FOR PEER properties REVIEW of the test samples can also be well assessed on the basis of1010 ofthe of 14 13 observation of the residual samples after combustion in the calorimetric cone test. Figure 6 shows comparativeThe fire digital‐retardant photos properties of PU–0 andof the PU–EG test samples residues can after also the be cone well calorimeterassessed on test. the Differencesbasis of the in the appearance of the two samples can be seen, indicating two different combustion mechanisms. in observationthe appearance of the of theresidual two samples samples can after be combustion seen, indicating in the two calorimetric different combustioncone test. Figure mechanisms. 6 shows In Figure6a it can be seen that the unmodified PU–0 sample was burned almost to the very end. In comparativeFigure 6a it can digital be seen photos that of the PU–0 unmodified and PU–EG PU–0 residues sample after was the burned cone calorimeteralmost to the test. very Differences end. On On the other hand, the photo 6b shows a charred boundary layer that does not let fire into the sample, thein other the appearance hand, the photoof the 6btwo shows samples a charred can be seen,boundary indicating layer twothat different does not combustion let fire into mechanisms. the sample, making the intact part larger than the photo on the left. makingIn Figure the intact6a it can part be larger seen that than the the unmodified photo on the PU–0 left. sample was burned almost to the very end. On the other hand, the photo 6b shows a charred boundary layer that does not let fire into the sample, making the intact part larger than the photo on the left.

(a) (b)

FigureFigure 6. 6. DigitalDigital(a images.) images. (a) ( aPU–0;) PU–0; (b ()b PU–EG) PU–EG after after cone cone calorimeter calorimeter(b) test. test.

ToTo obtain obtain a amore moreFigure detailed detailed 6. Digital structural structuralimages. (a analysis) PU–0; analysis (b )and PU–EG and determine determine after cone the calorimeter the mechanism mechanism test. of of action action of of the the modifiersmodifiers used, used, SEM SEM analysis analysis was was performed performed after after the thecombustion combustion selected selected samples; samples; the images the images are To obtain a more detailed structural analysis and determine the mechanism of action of the presentedare presented in Figure in Figure 7. By analyzing7. By analyzing the following the following pictures, pictures, it can be it seen can that be seen the microscopic that the microscopic pictures modifiers used, SEM analysis was performed after the combustion selected samples; the images are ofpictures the tested of composites the tested compositesdiffer from each differ other. from It each can be other. seen that It can in bethe seen case thatof PU–0, in the the case carbonized of PU–0, presented in Figure 7. By analyzing the following pictures, it can be seen that the microscopic pictures residuethe carbonized has a porous residue structure has a porous with structure broken withcells. broken The PU–0 cells. structure The PU–0 (Figure structure 7a) (Figure had 7rougha) had of the tested composites differ from each other. It can be seen that in the case of PU–0, the carbonized characteristicsrough characteristics and presents and presents an indistinct an indistinct “molten” “molten” network, network, without without clear cell clear structure cell structure features. features. In residu theIn case the case of a ofsample a sample containing containing expanded expanded graphite, graphite, a clear a clear flame flame retardation retardation mechanism mechanism was was visible. visible. TheThe pictures pictures (7b–c) (Figure show7b–c) the show characteristic the characteristic structure structure in the form in the of charred form of ribs charred between ribs between the cells theof thecells porous of the structure, porous structure, which prevented which prevented the spread the of spread fire into of the fire burning into the burningsample. sample.

(a) (b) (c)

Figure(a) 7. SEM images of char residues. ((ba) PU–0; (b) PU–EG; (c) PU–EG+SiO(c2.)

FigureFigure 7. 7.SEM SEM images images ofof charchar residues.residues. ( a) PU–0; ( (b)) PU–EG; ( (cc)) PU–EG+SiO PU–EG+SiO2. . 3.8. Thermogravimetric Analysis (TGA) of PU Foams 2 3.8. Thermogravimetric Analysis (TGA) of PU Foams 3.8.To Thermogravimetric better understand Analysis the role (TGA) of EG of andPU Foamssilica to the flame‐retardant behavior of PU composite, the thermalToTo better better stability understand understand of these thethe composites rolerole ofof EG was and studiedsilica to theby the TGA.flame flame-retardant ‐Thermogravimetricretardant behavior behavior of ofanalysis PU PU composite, composite, (TGA) andthethe differential thermal thermal stabilitystability thermogravimetry ofof thesethese compositescomposites (DTG) analysis was studied of neat by RPUF TGA. (PU–0) Thermogravimetric Thermogravimetric and RPUF filled analysis analysis EG and (TGA) (TGA) EG withandand silica di differentialfferential or silica thermogravimetry thermogravimetry in presence IL on–air (DTG)(DTG) atmosphere analysis of are neat illustrated RPUF RPUF (PU–0) (PU–0) in Figures and and RPUF8 RPUF and 9.filled filled Table EG EG 6 and shows and EG EG thewithwith individual silica silica or or temperatures silica silica in in presencepresence with ILIL a on–airon–airspecific atmosphere weight loss, are namely illustrated 5% (T5%), in in Figures Figures 10% 8 (T10%), and 99.. Table 50%6 (T50%),6 shows shows 70%thethe (T70%) individual individual as well temperatures temperatures as the char withwith residue a specificspecific at 600 weight °C. loss, loss, namely namely 5% 5% (T5%), (T5%), 10% 10% (T10%), (T10%), 50% 50% (T50%), (T50%), 70%70% (T70%) (T70%) as as well well as as the the charchar residue residue atat 600600 ◦°C.C. Thermal degradation of RPUFs took place in three stages. The first stage of decomposition manifested by about 10% loss of initial mass indicated dissociation of urethane bonds occurring at a temperature of 150 to 250 ◦C. The second degradation stage corresponding to a weight loss of about 50% occurs at a temperature between 250 and 400 ◦C and is attributed to the distribution of soft polyol segments [29,31]. At this stage, the hydrocarbon chains in PU foam are initially broken down and EG is activated and begins to expand, creating a protective carbon layer and stabilizing the foam Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 14 Table 6. Results of the thermogravimetric analysis of PU foams. Table 6. Results of the thermogravimetric analysis of PU foams. Appl. Sci. 2020, 10, 5817 T5% T10% T50% T70% Char Residue 11 of 13 System T5% T10% T50% T70% Char Residue System (°C) (°C) (°C) (°C) (%) PU–0 (°C)202 (°C)241 (°C)399 (°C)580 23.4(%) surface. EG pyrolysisPU–EGPU–0 consists of SO2202213, CO 2 and241250 water that399467 is trapped580592 when escaping23.428.3 near the edge of PU–EG 213 250 467 592 28.3 the graphitePU–EG+SiO structure, which2 leads to219 the irreversible257 expansion480 598 of the graphene29.4 structure and the PU–EG+SiO2 219 257 480 598 29.4 formation ofPU–EG+SiO a graphite2 carbonized+IL layer209 on the247 foam surface.420 This591 layer prevents heat27.8 transfer through the solid phasePU–EG+SiO and stops2+IL the fire. This209 effect247 can be seen420 when591 the foam surface27.8 expands during combustionThermal [32]. degradation This is followed of RPUFs by the took third place degradation in three stage,stages. during The first which stage the of fragments decomposition formed Thermal degradation of RPUFs took place in three stages. The first stage of decomposition duringmanifested the second by about stage 10% are loss degraded of initial at mass 500 ◦ C,indicated corresponding dissociation to a of 70% urethane weight bonds loss [ 31occurring,33] and at is a the manifested by about 10% loss of initial mass indicated dissociation of urethane bonds occurring at a resulttemperature of further of breakdown 150 to 250 °C. of The the second RPUF structuredegradation and stage charring. corresponding At this stage,to a weight also other loss of products about temperature of 150 to 250 °C. The second degradation stage corresponding to a weight loss of about that50% come occurs from at a isocyanate temperature groups between (e.g., 250 amines and 400 or °C benzene and is attributed alkyl) decompose to the distribution into volatile of soft products polyol segments50% occurs [29,31]. at a temperature At this stage, between the hydrocarbon 250 and 400 chains °C and in is PU attributed foam are to initially the distribution broken down of soft and polyol EG (HCN, CO2 or NO2)[34]. Compared to the PU–0 reference foam, foams containing only EG and EG issegments activated [29,31]. and begins At this to stage, expand, the creating hydrocarbon a protective chains carbonin PU foam layer are and initially stabilizing broken the downfoam surface.and EG withis activated the addition and ofbegins Si showed to expand, slightly creating less weighta protective loss, carbon suggesting layer increasedand stabilizing carbon the stability. foam surface. In turn, theEG ionic pyrolysis liquid didconsists not a offfect SO and2, CO even2 and reduced water that the thermalis trapped stability when ofescaping the foams near compared the edge toof foamsthe graphiteEG pyrolysis structure, consists which of SO leads2, CO to2 andthe waterirreversible that is expansion trapped when of the escaping graphene near structure the edge and of the containing EG or EG+SiO2, in which the thermal stability of PU foams is observed to increase with graphiteformation structure, of a graphite which carbonized leads to thelayer irreversible on the foam expansion surface. of This the layergraphene prevents structure heat andtransfer the increasing temperature. throughformation the of solid a graphite phase andcarbonized stops the layer fire. onThis the effect foam can surface. be seen This when layer the preventsfoam surface heat expandstransfer throughduring combustion the solid phase [32]. Thisand isstops followed the fire. by theThis third effect degradation can be seen stage, when during the foam which surface the fragments expands Table 6. Results of the thermogravimetric analysis of PU foams. formedduring combustionduring the second[32]. This stage is followed are degraded by the at third 500 degradation°C, corresponding stage, duringto a 70% which weight the loss fragments [31,33] andformed is the during result the of secondfurther stagebreakdown are degraded of the RPUF at 500 structure °C, corresponding and charring. to Chara At70% this weight stage, loss also [31,33] other T5% T10% T50% T70% andproducts is the that result come of furtherfromSystem isocyanate breakdown groups of the (e.g., RPUF amines structure or benzene and charring. alkyl)Residue decomposeAt this stage, into also volatile other products that come from isocyanate( groupsC) ((e.g.,C) amines ( C) or benzene ( C) alkyl) (%) decompose into volatile products (HCN, CO2 or NO2) [34]. Compared◦ ◦ to the PU–0◦ reference◦ foam, foams containing only EG productsand EG with (HCN, the additionCO2 orPU–0 NO of 2Si) [34]. showed Compared 202 slightly 241 to less the weight PU–0 399 loss,reference 580suggesting foam, foamsincreased 23.4 containing carbon stability.only EG andIn turn, EG withthe ionic the addition liquidPU–EG did of notSi showed affect 213 and slightly even 250 less reduced weight 467 the loss, thermal 592suggesting stability increased 28.3 of the foams carbon compared stability. In turn, the ionic liquidPU–EG did+SiO not affect219 and even 257 reduced 480 the thermal 598 stability 29.4 of the foams compared to foams containing EG or EG+SiO2 2, in which the thermal stability of PU foams is observed to increase PU–EG+SiO +IL 209 247 420 591 27.8 withto foams increasing containing temperature. EG or EG+SiO2 2, in which the thermal stability of PU foams is observed to increase with increasing temperature.

100 100 80 80 60 60 PU‐0 PU‐EG 40 PU‐0 Weight loss [%] PU‐EGPU‐EG+SiO2 40 Weight loss [%] PU‐EG+SiO2PU‐EG+SiO2+Il 20 PU‐EG+SiO2+Il 20 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Temerature [°C] Temerature [°C] Figure 8. TGA graph plotted for PU foams. FigureFigure 8. 8.TGA TGA graphgraph plotted for PU PU foams. foams.

0 0 ‐0.001 C] o ‐0.001 C]

o ‐0.002 ‐0.002 ‐0.003 ‐0.003 ‐0.004 PU‐0 ‐0.004 PU‐0PU‐EG Deriv weight [%/ ‐0.005 PU‐EGPU‐EG+SiO2 Deriv weight [%/ ‐0.005 ‐0.006 PU‐EG+SiO2PU‐EG+SiO2+Il ‐0.006 0 100PU‐EG+SiO2+Il 200 300 400 500 600 0 100 200Temperature 300 [°C] 400 500 600 Temperature [°C]

Figure 9. DTG graph plotted for PU foams. Figure 9. DTG graph plotted for PU foams. Figure 9. DTG graph plotted for PU foams. 4. Summary Based on the analysis of the presented results—including both the synthesis and physicochemical tests of RPUFs—it can be concluded that the synergistic flame retardant system based on expanded graphite with the addition of silica and ionic liquid had a positive effect on the properties of the porous Appl. Sci. 2020, 10, 5817 12 of 13 composite. It was observed that the introduction of fillers has a meaningful impact on the structure and further mechanical and thermal properties of RPUFs. Although the addition of graphite expanded alone or in the presence of silica and ionic liquid can interfere with the foam structure and increase apparent density, this can also improve its strength properties. As a result of the introduction of the synergistic system, it was noted that the compressive strength improved by approximately 60% for PU–EG+SiO2 samples compared to the reference sample. Higher three-point bending strength, impact strength, and higher hardness were also observed. The modified RPUFs were characterized by similar values of thermal conductivity and better thermal properties. The RPUFs modification has, above all, a positive effect on the fire resistance of composites. The results showed that the fire resistance of PU foams containing EG and SiO2 is significantly improved, namely the incorporation of expanded graphite and silica significantly reduced the peak heat release rate (pHRR) by about 70% in relation to the reference sample. Based on SEM images, it can be seen that the introduced graphite can form an expansion carbonizing layer during combustion, which can significantly reduce the spread of fire in the foam.

Author Contributions: Conceptualization, A.S.; Methodology, A.S.; Software, A.S.; Validation, A.S., S.C. and P.K.; Formal Analysis, A.S.; Investigation, A.S., S.C. and P.K.; Resources, A.S; Data Curation, A.S; Writing-Original Draft Preparation, A.S; Writing-Review & Editing, K.S.; Visualization, A.S.; Supervision, A.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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