Thermal Analysis of Polyurethane Coatings Modified with Graphene and Modification Influence on Mechanical Properties of Hybrid Textile Materials Dedicated to Personal Protective Equipment

Abstract

This paper is focused on the modification of polyurethane coating applied to the outer layer of hybrid textile materials dedicated to personal protective equipment. For this purpose, graphene with various weight fractions, i.e., 0.25 and 0.5 wt.%, was introduced into the polyurethane matrix. The prepared pastes were applied to meta-aramid fabric as coating. The results of the thermogravimetric analysis of polymer coating showed a shift in the onset temperature of the polymer coating to higher values after graphene addition, which indicates an improvement in thermal stability. Considering mechanical properties, the implementation of the coating on meta-aramid fabric reduces tear resistance but this may be improved by the addition of 0.5 wt.% of graphene. Such a hybrid textile material meets the tearing force requirements for protective clothing for firefighters according to EN 469:2020.

1. Introduction

In recent years, there has been a particularly rapid increase in the popularity of nanotechnology, which is revolutionizing many fields of science and industry. Nanotechnology has been pushing the boundaries of classical science for many years, focusing on the production, study, and characterization of new nanostructures. Its applications are already observed in personal protective equipment (PPE), with the use of nanostructures in PPE emerging in 2020 during the COVID-19 pandemic being particularly notable. The continuously growing global demand for PPE with high protective parameters, ensuring comprehensive protection in the workplace environment, has led to extensive research on high-performance fabrics, hybrid textile materials, and additional protective measures [1,2,3].

PPE (personal protective equipment) is an extremely important element of daily use, especially in rescue services. On a daily basis, these professions face various types of threats, including chemical, mechanical, biological, or physical hazards, and the risk of injury is particularly high, sometimes even resulting in death in extreme cases. This is why PPE is so crucial, specifically the materials used in it and the entire structures designed to act as a barrier between the person wearing or holding it and the health threat. However, for a material or entire structure to be used in PPE, it must meet the stringent requirements imposed by the European Parliament and Council Regulation (EU) 2016/425 on personal protective equipment. Additionally, it must be tested in accordance with normative documents specifying the level of protection of that material or entire structure. The methodology standard defining the way textile materials used for protective clothing are tested for tearing resistance is EN ISO 13937-2:2000 (PN-EN ISO 13937-2:2002) [4]. The results of this standard must be referenced to other standards (such as EN 469:2020 (PN-EN 469:2020-01) [5], EN ISO 11611:2015 (PN-EN ISO 11611:2015-11) [6], EN ISO 11612:2015 (PN-EN ISO 11612:2015-11) [7]), which qualify the results for use in a given field.

A particular issue with PPE is the trend where higher protection is correlated with higher cost and reduced ergonomics (less flexible materials, increased weight, decreased dexterity, and reduced comfort of use). Work is still ongoing on implementing the latest innovations and technologies which show promising potential to improve PPE. One such possibility lies in the use of nanotechnology [8,9,10]. Nanoparticles, especially graphene seem to be helpful in developing novel solutions in PPE due to their properties [11].

Since its discovery in 2004, graphene has especially caught the attention of both media and scientists worldwide. This two-dimensional honeycomb-structured material has garnered interest due to its exceptional properties that could contribute to innovations in a wide range of fields [12,13]. Its high thermal and electrical conductivity, mechanical strength, and antibacterial properties are considered particularly important, enabling its use in fields such as electronics, biomedicine, and modern power sources [14,15]. However, thanks to its unique properties, we can move beyond conventional applications, leading to initial attempts to use graphene as a modifier in PPE to improve various protective properties [16].

Most studies on the application of graphene in PPE focus on its filtering, antibacterial properties, and electrical conductivity [17,18,19]. There is, however, a lack of research addressing the implications of graphene in coating layers of PPE to determine its effect on the material’s strength properties. Studies dedicated to the use of graphene as a modifier in polymeric materials have confirmed the improved mechanical properties of such nanocomposites. Notably, tribological and strength properties, such as tensile strength and bending strength, have been significantly enhanced. Ali and others observed a 17.8% increase in tensile strength and a 31.4% increase in bending strength with the addition of 1% by weight of graphene in a polypropylene-based composite. However, when the graphene content increased to 2% by weight, the tensile strength improvement was only 4.5% [20]. Similarly, Erdoğdu observed a decrease in the strength properties of jute–epoxy composites as the graphene content increased [21].

An interesting area of research is the evaluation and verification of the impact of using graphene as a modifier in a coating paste for textile materials, focusing on its mechanical properties, particularly tear resistance, in the context of its application in PPE. The authors of this paper conducted experiments applying graphene to the coating paste to analyze the effect of its weight content on the structure and characteristic temperatures of polyurethane, as well as the mechanical properties of hybrid textile materials.

This study involved three variants of hybrid textile material samples, consisting of a textile substrate and a coating material, which included a polyurethane paste and graphene at two different weight contents. The research was conducted with a focus on the material’s potential use in PPE.

This research was divided into two parts. The first part focused on assessing the impact of graphene on the polyurethane paste. Additionally, studies were conducted to evaluate the dispersion of graphene in the polyurethane paste and the coating morphology. Tests such as DSC, TGA, and SEM were performed. The second part of the study assessed the impact of the coating layers on the mechanical properties of hybrid textile materials under tensile testing, with a focus on their potential use as protective materials.

2. Materials and Methods

2.1. Materials

In this research, a polyurethane paste based on the commercially available reagent under the tradename TUBICOAT PU 60 (CHT Germany GmBH, Tübingen, Germany), supplied by Andropol SA (Andrychów, Poland), was applied as a polymer matrix. On the other hand, graphene (CARBON 4LAB, Institute of Carbon Technologies, Toruń, Poland) with 99.9% purity was used as a nanofiller (Figure 1). The selected graphene in a form of flakes was characterized by a particle size of 1.5 µm, a thickness of 3 nm, and a specific surface area of 800 m²/g. Finally, a textile made of meta-aramid fibers with an addition of 2% antistatic fibers characterized by a mass of 215 g/m² was applied as a carrier.

2.2. Preparation of Coating Pastes and Hybrid Textile Materials

In the first stage, the mixture components, namely, the polyurethane coating paste and the graphene nanofiller with different mass contents (0.25 wt.% and 0.5 wt.%), were joined and the obtained two types of coating paste were homogenized via high-energy mixing using a planetary homogenizer (Thinky ARV 930 TWIN, Tokyo, Japan), where 30 steel balls (ø 0.6 mm) were applied for obtaining more effective dispersion of the additive. The mixing speed was 600 rpm and the mixing time amounted to 20 min. Finally, two pastes with nanoadditives were obtained (Figure 2). Subsequently, one part of the prepared pastes was used to obtain hybrid textile materials and the other was formed into foils by applying the pastes to the base via the tape-casting method (Figure 3), cross-linking, and finally removing the cross-linked foils from the base. The cross-linking process was performed using a laboratory dryer at a temperature of 120 °C for 15 min and then at 170 °C within 1 min. The pure PU paste was also formed into the cross-linked foil and applied as a referential specimen. The received foil samples (

Table 1. Foil sample description.

Sample	Material Description
PU	polyurethane foil
PU/0.25%G	polyurethane foil containing 0.25 wt.% graphene nanofiller
PU/0.5%G	polyurethane foil containing 0.5 wt.% graphene nanofiller

), characterized by a thickness of 0.23–0.28 mm, were next analyzed using scanning electron microscopy (SEM), thermogravimetry (TGA), and differential scanning calorimetry (DSC).

The pure polyurethane paste and the polymer pastes with graphene obtained in the first stage of preparation were applied as coatings to the left side of the textile carrier via the tape-casting method (Figure 3). The coating layer thickness was 0.15 mm. The coatings applied to the textile were finally cross-linked similarly to the previously prepared polymer foils, firstly at 120 °C for 15 min and then 170 °C within 1 min. The pure textile, the textile coated with polyurethane, and the textiles coated with polyurethane containing graphene with different contents (

Table 2. Hybrid textile material description.

Sample	Material Description
textile	meta-aramid textile carrier with the addition of 2% antistatic fibers
textile/PU	meta-aramid textile carrier with the addition of 2% antistatic fibers coated with polyurethane
textile/PU/0.25%G	meta-aramid textile carrier with the addition of 2% antistatic fibers coated with polyurethane containing 0.25 wt.% of graphene
textile/PU/0.5%G	meta-aramid textile carrier with the addition of 2% antistatic fibers coated with polyurethane containing 0.5 wt.% of graphene

) were next tested by means of tear resistance and analyzed using SEM.

2.3. Measurements

2.3.1. Microstructure Assessment

Both the foil samples and the hybrid textile materials were analyzed using a scanning electron microscope (SEM) (SU-8000 HITACHI, Tokyo, Japan) with a magnification of approximately 500–10,000× and 5 kV of accelerating voltage, where the microstructure was assessed. In the case of the hybrid textile samples, the measurement was performed before and after the tear resistance test.

2.3.2. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) was performed for the foil samples using a TGA Q500 device provided by TA Instruments. A minimum of two specimens of approximately 9.5 ± 0.5 mg were prepared from different parts of the foil samples and were placed in the platinum crucibles. During the test, the gas flow in the chamber was 10 mL/min and that in the oven amounted to 90 mL/min. The measurement was conducted under a nitrogen and an air atmosphere, where the specimens were heated from room temperature to approximately 600 °C at a heating rate of 10 °C/min. Based on the thermograms obtained thanks to the Universal Analysis program version 4.1D, the influence of graphene on the foil samples’ thermal stability was determined.

2.3.3. Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) was applied to the temperature ranges of the characteristic phase changes of the tested foil specimens. The measurement was performed using a DSC Q1000 instrument provided by TA Instruments. Specimens characterized by a mass range of 7.5–8.5 mg were placed in aluminum crucibles. The test was conducted at a temperature range from −85 °C to 250 °C with a heating/cooling rate of 10 °C/min. The nitrogen flow in the cell amounted to 50 mL/min. The specimens were tested in three cycles, namely, heating–cooling–heating, where two measurements for every foil sample were performed. The results were analyzed using the TA Universal Analysis 2000 program, where the glass transition temperatures of the first and second heating were obtained.

2.3.4. Fourier-Transform Infrared Spectroscopy (FT-IR) Absorbance Spectrum Analysis

Fourier-transform infrared spectroscopy (FT-IR) absorbance spectra were obtained for polyurethane samples cut from hybrid textile materials containing different contents of graphene. Three samples were tested for each material. The test was performed in the range of 4000–500 cm⁻¹ using a Thermo Scientific Nicolet 6700 FT-IR spectrometer. The number of used scans was 32.

2.3.5. Tear Resistance of Hybrid Textile Materials

The tear resistance measurement was carried out in accordance with the EN ISO 13937-2:2000 (PN-EN ISO 13937-2:2002) standard [4] PN-EN ISO 13937-2:2002 using a 4465 INSTRON testing machine applying a head speed of 100 ± 10 mm/min. During the test, the textile-based samples were tested in a form of strips characterized by a length of 200 mm ± 2 mm and a width of 50 mm ± 1 mm, with an incision of 50 ± 5 mm made in the longitudinal direction of the specimen, 25.0 ± 2.5 mm from the edge. The specimens were placed in a tensile tester, where at least 20 mm of each pre-cut strip was clamped in the jaws at least 10 mm apart, which guaranteed a pulling direction parallel to the longitudinal direction of the sample. For every material, 6 specimens, including samples cut into a warp direction (3 specimens) and ones prepared into a weft direction (3 specimens), were examined. The tear resistance was the average tearing force obtained during the test, which was next compared to the results of different protective clothing standards (

Table 3. The minimum value of the test results required to meet the requirements for protective clothing for various occupational environments according to the PN-EN ISO 13937-2:2002 standard.

	The Average Tearing Force Obtained During the Test Based on the PN-EN ISO 13937-2:2002 Standard [4]
The minimum value required based on the EN 469:2020 (PN-EN 469:2020-01) standard (protective clothing for firefighters) [5]	30 N
The minimum value required based on the EN ISO 11611:2015 (PN-EN ISO 11611:2015-11) standard (protective clothing used during welding and related processes) [6]	15 N—level 1 of performance 20 N—level 2 of performance
The minimum value required based on the EN ISO 11612:2015 (PN-EN ISO 11612:2015-11) standard (clothing for protection against heat and flame) [7]	10 N

), including EN 469:2020 (PN-EN 469:2020-01) [5] (specifying minimum performance requirements for protective clothing intended for fire-fighting operations), EN ISO 11611:2015 (PN-EN ISO 11611:2015-11) [6] (specifying minimum requirements for protective clothing used during welding and related processes) and EN ISO 11612:2015 (PN-EN ISO 11612:2015-11) [7] (specifying minimum performance requirements for clothing providing protection against heat and flames).

3. Results and Discussion

The analysis of the prepared materials was carried out in two steps. In the first step, individual coating layers in the form of thin foils without textile support were investigated by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FT-IR). Subsequently, in the case of coating application on the meta-aramid textile, the obtained hybrid textile materials were analyzed by means of tear resistance as well as scanning electron microscopy before and after mechanical testing.

3.1. Foil Sample Analysis

Thermogravimetric analysis (TGA) was applied for the assessment of the thermal stability of the obtained polyurethane-based foils utilized as coatings in the outer layer of hybrid textile materials. The results obtained from thermograms are presented in

Table 4. Temperature values read from TG and DTG curves obtained from thermal analysis test performed in nitrogen and air atmospheres.

Samples	T5%	DTG1	DTG2	DTG3	Residue at 575 °C (Nitrogen Atmosphere)	Residue at 575 °C (Air Atmosphere)
	[°C]	[°C]; [%/°C]			[%]	[%]
PU	292	292; 0.18	357; 0.99	402; 0.92	7.0	28.3
PU/0.25%G	297	299; 0.19	360; 0.80	406; 0.71	27.4	27.8
PU/0.5%G	297	311; 0.26	359; 0.75	403; 0.71	27.9	28.1

, while the TG and DTG curves are depicted in Figure 4 and Figure 5.

The temperature of 5% mass loss (T_5%), corresponding to the start of the degradation process, for PU was 292 °C and was slightly increased (by 5 °C) as a result of the physical modification with graphene, regardless of its share (Table 4). The occurrence of three peaks (DTG1-DTG3) was observed on the derivative curves of mass loss, corresponding to the decomposition of urethane to obtain isocyanate monomers and polyol segments, which then undergo degradation [22]. The individual stages of degradation were also slightly shifted towards higher temperatures, while the decomposition rate for DTG2 and DTG3 was reduced. The amount of residues in the case of PU/0.25%G and PU/0.5%G increased more than 4-fold compared to the unmodified polymer for analyses carried out in nitrogen and was similar to the amount of residues obtained for analyses carried out in air (28%). This means that the applied modification increased the thermal stability of the polyurethane [23].

In DSC analysis, the characteristic temperatures of the polyurethane-based coatings were determined (

Table 5. Characteristic temperature of polyurethane-based coatings obtained during DSC analysis, including first and second heating.

Sample	First Heating			Second Heating
	TIg1, [°C]	Tm, [°C]	ΔH [J/g]	TIg2, [°C]
PU	−60.8	86.2	5.1	−60.1
PU/0.25%G	−59.2	80.3	13.1	−61.2
PU/0.5%G	−60.8	79.2	16.9	−61.2

). According to the DSC curves (Figure 6), it was confirmed that the polyurethane referential sample exhibits two glass transition temperatures, which are responsible for the presence of two types of segments in the structure, a flexible one and a rigid one. The first glass transition, noticed at a temperature of approximately −60 °C, corresponds to the glass transition temperature of the flexible segments. On the DSC curves of all samples from the first heating cycle, one endothermic peak was observed. The heating in the first cycle led to the melting of the soft segments and the ordering of the segment structure in the polyurethane. The peak of melting (T_m) for polyurethane was 86.2 °C; with increasing graphene addition, the T_m values decreased.

The enthalpy of melting (∆H) for polyurethane was 5.1 J/g, while for the nanocomposites with the addition of 0.25 wt.% and 0.5 wt.% of graphene, it was 13.1 and 16.9 J/g, respectively. The low ∆H values determined from the first heating cycle and the absence of endothermic peaks in the second heating cycle curves indicate the slight tendency of the examinated materials to form ordered structures. The nanocomposites with graphene additions were characterized by a higher degree of ordering within the domains of soft segments compared to polyurethane [24,25].

The primary FTIR bands and their respective functional groups in the polyurethane foils are shown in Figure 7. The FT-IR analysis, which was performed to confirm the chemical stability of the polyurethane coating after the application of different graphene contents. All spectra showed characteristic polyurethane bonds: a N-H stretching vibration at 3334 cm⁻¹ and CH₂ asymmetric and symmetric stretching vibrations at 2934 and 2854 cm⁻¹, respectively. The two resolved bands at 1720 and 1740 cm⁻¹ can be attributed to hydrogen bonded and non-hydrogen bonded carbonyl stretching of the urethane group. The bands corresponding to the stretching vibrations of the carbonate group are 1249 cm⁻¹ for the asymmetric one and 996–956 cm⁻¹ for the symmetric ones. The strong infrared band assigned to the asymmetric stretching vibration of the C-N group is expected at 1040 cm⁻¹. This band overlaps with the very strong band at 1104 cm⁻¹, corresponding to the C-O-C stretching vibration of ether groups in polyurethane [23,24,25,26].

The SEM images of the referential polyurethane foil and the samples containing different contents of graphene (0.25 wt.% and 0.5 wt.%) are presented in Figure 8. The SEM micrographs were taken at magnifications of 500× as well as 1000× for fracture surface foil samples, and were used in graphene dispersion analysis and the assessment of its influence on polyurethane coating morphology.

According to Figure 8a,b, the polyurethane foil is characterized by a heterogeneous structure. The green arrows point to the graphene particles in the polyurethane matrix. However, the obtained graphene dispersion was sufficient in the case of both materials, i.e., polyurethane with 0.25 wt.% (Figure 8d) and 0.5 wt.% (Figure 8f) of carbon additive, where high-sized agglomerates were not detected. Nevertheless, the incorporation of the graphene particles into the polyurethane matrix caused changes in homogeneity and appearance.

3.2. Hybrid Textile Material Analysis

Subsequently, the pure textile and the coated textiles were tested by means of tear resistance, where the samples were torn along both directions, namely, the weft direction and warp direction. The influence of polyurethane coating and graphene content on the textile tear resistance was assessed and the results are presented in Figure 9.

According to Figure 9, the results of average tearing force tested along the weft direction were characterized by higher values compared to the results obtained during tearing along the warp direction. The referential sample was characterized by the highest tearing force value of approximately 79.8 N when tearing the material along the weft direction and about 64.8 N when tearing it along the warp direction. The coating process of textiles using polyurethane contributed to a reduction in these values of approximately 50%, both in the weft direction, where the average tearing force decreased to 39 N, and the warp direction, where the same parameter amounted to 33.8 N. The modification of polyurethane coating using 0.25 wt.% of graphene contributed to a slight decrease in average tearing force measured for both directions in comparison to the material coated with polyurethane only. This might be due to the fact that the introduction of a coating onto a textile carrier could also have resulted in a decrease in mobility and effective stress transfer to adjacent fibers. When the fabric is torn, individual threads are damaged. Moreover, tear resistance may be reduced by sticking and immobilizing the threads in the fabric, which strongly adhere to the coating [27].

Interestingly, the application of 0.5 wt.% of graphene in the polyurethane coating caused an increase in tearing force measured when tearing along the weft direction of more than 30% compared to the coated textile with no graphene. However, the same parameter measured as a result of tearing the material along the warp direction was reduced by about 6%. Nevertheless, according to different PPE standards, the obtained materials are characterized by various performance levels and therefore these hybrid textile materials may be used in personal protective equipment (PPE).

The SEM photos of the hybrid textile material samples after the tear resistance test were observed. Figure 10a,b present the non-modified textile carrier. According to the photos, a large number of protruding single threads is observed on the fabric surface. The fiber structure is smooth and uniform. The fiber break area does not exhibit fiber elongation after the tear test. The fiber break locations are brittle in nature.

Figure 11a–d show the teared textile coated the polyurethane with no graphene added. On the surface of the sample, fibers protruding above the coating surface are visible (Figure 11a). Some of them are covered with the coating paste, but some remain uncoated. The presence of fibers not wetted by the paste was found on the coating surface, which introduces discontinuities in the coating layer (Figure 11c,d).

Figure 12a–d show the SEM photos of the textile coated with polyurethane containing 0.25 wt.% of graphene. In the obtained photos, a significant number of fibers protruding above the coating surface which are more wetted by the polyurethane paste can be noticed. The crack in the polyurethane layer created during the tearing test has a similar character to that observed in the case of the sample coated with polyurethane paste without the addition of graphene (Figure 12a).

Figure 13a–d depict SEM microstructure images of a hybrid textile material containing 0.5 wt.% graphene in the coating. It was found that the number of protruding fibers on the coating surface was significantly reduced (Figure 13a,d). By increasing the addition of graphene, the viscosity of the paste increases, which results in better coverage and the adhesion of the paste to the fibers in the tape-casting process.

The common features of all samples of the hybrid textile materials are the brittle nature of the coating and fiber cracking. On the other hand, the number of fibers protruding above the material surface and the degree to which they are covered by the coating paste particularly distinguish them.

4. Conclusions

Currently, it is of considerable importance to develop materials dedicated to personal protective equipment. The presented studies were focused on hybrid textile materials dedicated to PPE, consisting of a polyurethane coating containing graphene and of a textile carrier. The coatings were tested by means of thermal properties and morphology. On the other hand, the impact of the coating on textile average tearing force was also studied.

The results of the thermal tests showed that the addition of graphene slightly affects the polyurethane decomposition process. It was shown that both in nitrogen and air atmospheres, the addition of graphene to polyurethane leads to a decrease in the degradation rate. It was presented that the graphene used may contain built-in oxygen functional groups. These groups may change the decomposition conditions in a nitrogen atmosphere from neutral to oxidizing. This phenomenon was evidenced by the significant amount of carbonized residues formed after the decomposition process. Such amounts were recorded in the degradation process under oxidizing conditions. Furthermore, no significant effect of the graphene addition on the glass transition temperature of polyurethane was detected. Adding graphene to the polyurethane leads to a decrease in the melting temperature of the soft segments (T_m) while significantly increasing the enthalpy of melting. The applied method of mixing graphene with polyurethane paste leads to obtaining evenly distributed particles in the polyurethane matrix.

According to the results, the referential non-coated textile achieved the highest values of average tearing force (64.8–79.8) N, which are higher than required (30 N) based on EN 469:2020 (PN-EN 469:2020-01) (protective clothing for firefighters) [5], but coating implementation reduced this parameter by approx. 50%. In spite of the fact that the obtained materials were characterized by significant tearing force reduction, the hybrid material coated with polyurethane (textile/PU) and the hybrid material with a PU coating with 0.5 wt.% of graphene met the tearing force requirements for protective clothing for firefighters according to EN 469:2020 (PN-EN 469:2020-01) [5]. The only material which did not meet these requirements was a textile coated with polyurethane with 0.25 wt.% graphene (textile/PU/0.25%G), but it may meet other requirements, including the minimum value required based on the EN ISO 11612:2015 (PN-EN ISO 11612:2015-11) (clothing for protection against heat and flame) [7].