Nitrogen-Doped Graphene Sheets as Efficient Nanofillers at Ultra-Low Content for Reinforcing Mechanical and Wear-Resistant Properties of Acrylic Polyurethane Coatings

Abstract

The enhancement of the mechanical and wear-resistant properties of polymer coatings plays a vital role for their application in hostile serving environments, and nanofiller is effective for this purpose. Herein, we systematically investigate a new nanofiller, nitrogen-doped graphene sheets (NGSs), which possess a multilayer sheet-like morphology and share a good compatibility with water. After the incorporation of NGS into a waterborne polyurethane (WPU), the mechanical and wear-resistant properties of NGS/WPU composite coatings significantly improve, and wear resistance behaves best at an ultra-low content, reaching up to 0.005 wt% (50 ppm). Furthermore, Young’s modulus is elevated by 52.67% and tensile strength is appreciably boosted by 58.87%. Simultaneously, an apparent reduction of weight loss of 78.74% is observed in the abrasion testing, and the ductility of NGS/WPU composite films is reduced by 48.38%. These make it possible that an ultra-low content of nanofiller efficiently reinforces polymer-based composites, with low cost in the wear-resistance related field.

1. Introduction

Polymer and polymer-based nanocomposites are increasingly widely used in abrasion-related components, such as gears, bearings, clutches, and seals [1]. Waterborne polyurethane (WPU), as an important branch of the polymer, has been broadly used as adhesives and coatings to meet various performance requirements in diverse fields such as construction, automobiles, and textiles [2,3,4,5]. Although polyurethane coatings possess many advantages, such as excellent gloss, hardness, flexibility, chemical resistance, and UV durability, the structure of WPU tends to be damaged when experiencing longer usage time and severe friction [6]. Conventional WPU still has a large loss of quality in the application process. As a result, the performance of the pristine WPU still needs to be further enhanced.

The improvement of wear resistance, which is one of the basic characteristics of coating materials, is able to inhibit the wear of the substrate due to friction [7]. Therefore, it is necessary to enhance the wear resistance of WPU to meet the ever-increasing performance requirements in coatings. Most polymer materials, including polyurethane, are not equipped with sufficient wear resistance, which brings damage for aesthetics and durability under lasting serving processes [4]. In order to further improve the mechanical and tribological properties of polyurethane-based coatings, the commonly adopted method is the incorporation of nanoparticles to enhance wear-resistant properties [8] or increase their hardness [9], toughness [10], and comprehensive strength [3]. For enhancing abrasion resistance or mechanical properties, lubricants such as molybdenum disulfide (MoS₂) [11], boron nitride [12], and clay [13] are usually introduced. At present, nanoparticles have been considered for the preparation of wear-resistant polymer composite coatings. The introduction of nanoparticles can not only enhance the strength and stiffness of the composite coatings, but also bring about less abrasion effects and larger surface area during the friction and sliding process, which helps to improve the tribological properties [7]. Therefore, it is very attractive to improve the wear resistance and the mechanical properties of WPU coatings at the same time.

Compared to such conventional fillers, graphene has drawn great attention recently owing to its exceptional mechanical, electrical, thermal, and structural properties, and has already shown promising results in tribology as a lubricant additive [14,15]. However, as the pristine graphene without modification is introduced into the polymer matrix, it usually results in agglomeration inside the composite material [16]. Therefore, various studies of graphene modification have been carried out to simultaneously improve its dispersion in composites, mechanical properties and wear resistance. Furthermore, graphene decorated with fluorine, phosphorus, and sulfur has been applied on lubrication inside composites. For instance, Ye et al. claimed that the wear rate of polyimide composite coatings decreased by about 37.14% with the concentration of fluorinated graphene at 0.5 wt% under dried sliding conditions [17]. Mu et al. reported that the wear loss and average friction coefficient were reduced by 87% and 9%, respectively, after adding 1 wt% GO-Tr in [CH][P] [18]. Feng et al. showed that the tensile strength of the composite film increased by 46.53% and the wear-resistance of composite films improved significantly after introducing hydrophilic sulfonic groups into the SGO/WPU composites as the added amount of SGO was 0.8 wt% of WPU [19]. Furthermore, as an important part of graphene derivatives, nitrogen-doped graphene sheets (NGSs) can be easily prepared without harm to the environment [20]. In addition, NGS possessing good lubricating properties due to the introduction of nitrogen atoms can form a firm, effective, and protective film on the interface of the friction pair to prevent direct contact between the counterparts [21]. The introduction of nitrogen atoms can also reinforce the interaction between the graphene sheets and the polymer matrix to reduce the damage of the polymer matrix in the friction stage. For instance, Shi et al. found that the wear rate of UHMWPE was reduced by about 59.46%, and the microhardness increased to 115% when the content of nitrogen-doped graphene was 1 wt% [22]. To the best of our knowledge, it is difficult to achieve the large-scale utilization of NGS inside the wear-resistant polymer coatings due to the high cost. Therefore, the concentration of NGS must be limited to ultra-low loading, but should still promote significant progress on the mechanical and wear-resistant properties of composite coatings.

In the present study, a novel NGS prepared via arc discharge on a graphite rod was incorporated into WPU to obviously achieve significant reinforcement on mechanical properties and wear resistance. Wherein, NGS, as an efficient nanoparticle, has an excellent dispersion in WPU matrix, especially at an ultra-low content reaching up to 0.005 wt%, which shows a possibility that NGS, compared to others, possesses an effective function in polymer-based nanocomposites as the content is ultra-low. Simultaneously, the enhancement mechanism of mechanical properties and wear resistance of NGS/WPU composites is investigated based on the interaction between NGS and PU segments and the produced protective film in wearing process.

2. Experimental

2.1. Materials

Hydroxy acrylate (HAR) (solid content 45%), hexamethylene diisocyanate (HDI) were provided by Hubei Double Bond Fine Chemical Co., Ltd., Wuhan, China, The defoamer was purchased from BYK Chemical Technology Consulting Co., Ltd, Shanghai, China. Propylene glycol methyl ether acetate (PMA) was obtained from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China. Graphite rods were provided by Foshan Nanhai Jusheng Graphite Products Co., Ltd., Foshan, China.

2.2. Preparation of NGS

NGS were prepared by using the arc discharge on a graphite rod. First, a graphite rod was put into a sealed container with the air exhausted. Then, nitrogen and hydrogen were introduced in the sealed container to 0.19 bar pressure. After that, arc discharge was produced between the anode (graphite rod) and the cathode for 15 min, which succeeded in peeling graphite into the NGS. When the reaction finished, the power was turned off, and the hydrogen gas was replaced by introducing nitrogen. The NGS obtained in the container was collected for further use.

2.3. Preparation of Acrylic WPU Composites with NGS

NGSs were ultrasonically dispersed in ultrapure water for 0.5 h. The prepared dispersion was mixed with HAR by ball milling at the speed of 200 r min⁻¹ for 2 h. Then HDI diluted with 20 wt% of PMA was added dropwise to the HAR emulsion with NGS and the mixture were mechanically stirred. Finally, the obtained solution was carefully casted on plexiglass plates and aluminum plates until the solvent was fully removed at room temperature for 168 h to obtain dried coatings for testing. The whole process is illustrated in Figure 1. The composite coatings were gained and labeled as 0.01 NG, 0.03 NG, 0.05 NG, 0.07 NG, 0.1 NG, and 0.3 NG, which contain 0.001 wt%, 0.003 wt%, 0.005 wt%, 0.007 wt%, 0.01 wt% and 0.03 wt% NGS respectively. The composite films on the plexiglass plates after drying were soaked in 80 °C water for two hours to obtain peeled composite films, which were then prepared into strips of 80 mm × 10 mm × 0.03 mm for stress and strain testing. The composite coatings on the aluminum plates (60 μm) were adopted for abrasion tests and the following roughness test on AFM.

2.4. Characterization

Morphologies of the samples were observed using Hitachi SU 8220 scanning electron microscopy (SEM). Smartlab Rigaku X-ray powder diffraction (XRD), with Cu Kα radiation (λ = 1.5405 Å) at 40 kV and 30 mA, was adopted to explore the transformation of the degree of crystallinity of the materials, especially the samples of the WPU/NGS composites, which were ground into powder using liquid nitrogen and tested on the glass sheet positioning in the device. Transmission electron microscopy (TEM) images recorded by the TITAN G2 ETEM microscope (FEI, American) at 300 kV were employed to find the dispersion of NGS in the HAR and the micro morphology of NGS. The surface chemical states of NGS were studied through X-ray photoelectron spectroscopy (XPS) with the ESCALAB 250 XI (Thermo Fisher Scientific, Waltham, MA, USA) by a monochromatic Al X-ray source. Fourier-transform infrared (FTIR) spectra of all samples were operated on the Bruker VERTEX 70 infrared spectrometer employing transmission mode in the range 4000–400 cm⁻¹. The roughness of the surface of all samples was measured by an Atomic Force Microscope (AFM), Bruker Dimension Icon, where the roughness calculations were obtained for 500 nm² scan scale and 2 Hz in tapping mode. Thermogravimetry (TGA) was carried out with an Instrument NETZSCH STA 449 F5 (NETZSCH, Selb, Germany) under the atmosphere of nitrogen at a flow rate of 50 mL min⁻¹, heating and cooling process were performed at a rate of 10 min⁻¹. Tensile testing was performed on rectangular-shaped specimens using a universal tensile-testing machine (DZ-106, Zonhow Test Equiment Co., Ltd., Dongguan, China) and the stretching speed was 5 mm min⁻¹; 5 samples were tested in each group. The loss factor (tan δ) was measured by dynamic shear rheometer (DSR), Anton Paar Smart Pave 102, where the shearing strain and frequency were 0.3% and 1 Hz, respectively, in the range between room temperature and 150 °C. The contact angles were carried out on the Attention Theta Lite instrument with deionized water at room temperature, where the volume of dripping was 3 μL and the recording time was 10 s.

According to ISO 7784.2, the wear resistance of the coatings was determined via using the rotating friction wheel on a Taber abrasion testing machine, where the composite coatings were subjected to two paralleled CS-10 rubber wheels rotating for 500 hundred cycles in the opposite directions. The disk on which the samples fixed rotated at 60 r min⁻¹. The pressure loaded on the composite film via the rotating rubber wheels was 750 g. Wear weight loss was employed to characterize the abrasion resistance of composite coatings and calculated according to the formula as follows:

$$M_W=M_1-M_2$$

(1)

where M_W refers to the wear weight loss, M₁ refers to the mass of composite coatings before the Taber abrasion test and M₂ refers to the mass of composite coatings after the test. Reported results were the mean values of at least five measurements.

3. Results and Discussion

3.1. Structural Characterization of NGS

To confirm the compatibility of NGS with water after the defects formed in its structure, the solubility of NGS in water was tested and is shown in Figure 2a. The mixtures were of the same concentration (1 mg mL⁻¹) and were sonicated for 10 min. The NGS solution is stable for a month without precipitate. In addition, the contact angle (Figure 2b) tested between NGS and water was about 30 degrees, revealing that NGS shared good compatibility with water. Based on the excellent dispersion of NGS in ambient water, which brought its great compatibility with waterborne polymer, it was necessary to carry out the characterization of the NGS and analyze this. The morphology of NGS was examined via using SEM and TEM (Figure 2c–e); the samples for TEM were ultrasonically dispersed in absolute ethyl alcohol and the dispersion was dripped to the lacey support films which were then dried for testing. It was found that NGS has a sheet-like morphology, the lattice fringes of which were 0.36 nm after the introduction of nitrogen. The chemical components evaluated by XPS are summarized in

Table 1. The chemical composition of NGS based on XPS calculation.

Samples	Element Content (at%)			Formula 1
	C	N	O
NGS	84.33	9.49	6.18	C1N0.113O0.073
AGS	95.27	0.86	3.87	C1N0.009O0.041

. After introducing N to the structure of graphene, the N content of corresponding NGS increased to 9.49%, which was significantly higher than that of the pristine arc-discharged graphene sheets (AGS).

The detailed high-resolution XPS C1s spectra of NGS are further shown in Figure 2f, which have been deconvoluted into five peaks with binding energy of 284.10 eV, 285.15 eV, 286.30 eV, 287.90 eV and 290.05 eV, corresponding to sp²C−sp²C, N−sp²C, N−sp³C, C=O and O−C=O bonds, respectively [23]. Additionally, the detailed high resolution XPS N1s spectra of NGS are further adopted to distinguish whether the C–N bond is formed. As shown in Figure 2g, the NGS N1s spectra can be fitted into three different peaks of pyridinic nitrogen (397.7 eV), pyrrolic nitrogen (398.8 eV), and quaternary nitrogen (400.7 eV), which directly claims that N successfully formed a linkage with the pristine graphene structure [24].

3.2. Composite Films with NGS

The compatibility of nanofiller with polymer matrix usually determines whether their reinforcing effect is good, which is difficult to quantify, owing to many factors [25]. Before introducing NGS into the WPU matrix, the compatibility of NGS with water was explored via contact angle, exhibited in Figure 2b. It is noticeable that the NGSs share good compatibility with water due to the 30 degrees contact angle. However, when NGSs, as nanofillers, are introduced to WPU, it has always been a problem for their dispersion [26]. Therefore, TEM images of WPU/NGS are adopted to observe the dispersion of the NGSs in the WPU matrix. The samples of WPU/NGS composites, ground into powder (using liquid nitrogen in advance) were firstly dispersed in ultrapure water, and then the upper suspension was dripped on the lacey support films. The samples to be tested were successfully prepared until the ultrapure water dried.

As shown in the TEM images (Figure 3a,b), the morphology of pristine WPU is indicated by its structure of stacking layer-by-layer, looking like steps. The high-resolution morphology shows that virgin WPU is amorphous. Moreover, as plotted in Figure 3c–h, the lattice fringes of NGS can only be observed in the field of edge of the sample as the NGS content is lower than 0.007 wt%. In other words, there is a relationship between the matrix and the filler NGS, where NGS are embraced by matrix or the NGS is inserted into the space between the WPU layers. When the NGS content is as high as 0.007 wt% (see Figure 3i,j), the state observed is different from the former, where the lattice fringes of NGS can be clearly seen in a wide range. NGS, existing on the surface, surround the WPU matrix, owing to the increasing of the NGS content. However, with the continuous increasing of NGS content (see Figure 3k–n), the NGS agglomerate and claim a separated relationship with the matrix, as evidenced by the obvious lattice fringes. As a result of the relationship between the NGS and the matrix, the composites behave robustly in the following tests, as the addition of NGS is no more than 0.007 wt%. While the content is more than 0.007 wt%, the samples exhibit more inferior mechanical properties than the former ones.

3.3. Wear Resistance and Mechanical Properties

As observed in Figure 4a, compared with the pristine WPU coating, the weight loss of the NGS/WPU composite coatings obviously suffered abrasion decreases, and obtained their minimum with the addition of 0.005 wt%. Based on the improvement of wear resistance, the mechanical properties of the coatings need to be investigated. It can be seen from Figure 4b that the ultimate tensile strengths of the composite films increase first and then decrease with the increasing of the NGS content. When the content of NGS reaches up to 0.003 wt%, the strength of the material comes to the peak value, which is about 88.75% stronger than the pristine sample. When the content of NGS further increases, the strengths decrease. However, the strength will have a peak value again while the content of NGS is 0.01 wt%. That can be explained that the property of composites usually has two or more peak values as the content of filler increases, which is caused by the reduction of average filler size and the broadening of the size distribution due to the increased shear stresses during the processing [2].

As indicated in Figure 4c, after incorporating NGS, it is evident that the elongation at break has a noticeable decline compared to the original sample. With the NGS content at 0.003 wt% and 0.005 wt%, the elongation at break reduces by 66.69% and 48.39%, respectively, compared with the original sample. However, compared to the samples with 0.001 wt%, 0.003 wt%, and 0.005 wt% NGS, the elongation of break increases significantly with the content of NGS at 0.007 wt% or more. This can be explained as follows: firstly, when the content of NGS is low, the NGS evenly disperse and do not agglomerate, which can be also confirmed from the morphology in Figure 3c,d and Figure 3e,f, respectively. The NGS exist like nails firmly riveting in the matrix of the material, thereby inhibiting the thermodynamic movement of polymer segments. Secondly, although previous studies have demonstrated that fillers will agglomerate in polymers to form a network structure if they are up to high content [27], for this work, we estimate that the NGS agglomerates to enlarge the interface area among the matrix and NGS, and the composite materials first deform at the larger interface area once the materials suffered pulling or damage. Although the strengths of the composite materials have a certain increase compared with the original material, the dominant role for resisting deformation is not the NGS but the matrix. However, it is interesting to see that, among all the samples, the strength and Young’s modulus of 0.03 NG samples are the best (Figure 3b,d), but the weight loss of 0.05 NG is the least in the actual situations, the reason for which is that the performance of the material is based on its own comprehensive performances in the actual serving environment, rather than relying on just one of its performances [3]. Although the tensile strength and Young’s modulus of 0.05 NG are not the best among all the samples, the good toughness of it means it can avoid being quickly peeled off from the surface of the aluminum plate in the process of contacting with the rubber grinding wheels and moving relatively. In addition, the representative stress-strain curves (Figure 4e) also simultaneously indicate an obvious difference of the mechanical properties of WPU composite films with different NGS contents. As shown in Figure 4f, the cross-section of 0.05 NG composite film with a rough morphology symbolizes that there is a strong interface interaction between the NGS and WPU matrix, which remains to be investigated.

In addition, to more directly verify the enhancement of abrasion resistance, the SEM pictures (Figure 5a–n) of the abrasion surface of the samples display that the wear surface of 0.05 NG was smoother and more complete than that of others. For the pristine sample (Figure 5a,b), the surface of the coating is torn into slightly larger pieces compared with the coatings containing NGS. With the content of NGS gradually increasing, some parts of the surface of the coatings are reinforced, which brings about a smoother morphology, but others, having not been enhanced enough by NGS, are inclined to not be strong enough to resist ploughing and shearing from the process of abrasion. Thereby, the cracks induced by the shearing of harder particles easily propagate, which results in the exfoliation of the coating and rough locality in the weaker area. However, at the time the addition of NGS is over 0.005 wt% (Figure 5i–n), the inhomogeneity of composite coatings is exposed again because of the uneven dispersion of NGS, which can be noted from the deep grooves due to the ploughing of hard particles in the rubber grinding wheels [28]. In addition, the uneven dispersion of NGS is reflected in Figure 3i–n, but the whole process of abrasion still waits for our investigation.

Obviously, with the increasing of the wearing cycles, it is not difficult to speculate that the wear type is changeable, based on the mass loss of the coatings in different cycles, as observed in Figure 6a. In the early stage of wearing, the harder rubber grinding wheels contact the surface of the softer composites and move relatively to form a friction pair, the wear of which is referred to as abrasive wear, which is certificated from the surface change of rubber grinding wheels, as shown in Figure S3. This stage was also the most severe stage of the whole process, which can be seen from the slope in the first 300 cycles in Figure 6a. In this stage, the Young’s modulus and the strength of the material play the main role. Under the action of hard grinding wheel particles, the ability of the material surface to resist deformation depends on the Young’s modulus of the composite, and whether it is easy or not to be peeled off from the surface of the aluminum plate relies on the strength of the composite. With the increasing of wear cycles, parts of the surface of the composites are peeled off by the harder rubber grinding wheels. The friction pair is no longer formed by the direct contact between the grinding wheels and the surface of the composites. Then, the peeled material fragments and the wear debris of the rubber grinding wheels adhere to the surface of the grinding wheels due to periodic wearing. Therefore, adhesive wear in this stage is dominant [29]. In this process, there is a certain shear force due to relative sliding between the composites and the rubber grinding wheels. On the one hand, the toughness of the composite is challenged in this stage, but it is not only limited to the toughness but also the strength and the Young’s modulus [28]. This is the reason that although the toughness of 0.1 NG and 0.3 NG is higher, the weight loss of them in the adhesion wear stage is still large. On the other hand, with the wear cycles increasing, NGS participates in the formation of friction pairs, which can be verified via comparing the roughness (as shown in Figure 6b,c) of the composite coatings bearing abrasion and the weight loss. It is easy to observe that the higher the roughness, the larger the weight loss of composite coatings. The mechanism can conclude that the lubrication of NGS and a protective film forming on the surface of the composite greatly reduces the weight loss [1,29,30]. For pristine sample, the surface of the sample easily becomes rough due to no participation of NGS, and the rougher the sample surface, the larger the friction is, which gives rise to the drastic abrasion. Once the NGS is incorporated in the matrix, the roughness decreases, owing to the lubrication and the protective film in the process of wearing, which weakens the effect of friction on the samples. In addition, the enhanced ultimate tensile strength of composite coatings is strong enough to resist the shearing force from the hard particle of rubber grinding wheels. However, when the content of NGS is more than 0.005 wt%, the agglomerated NGS in the matrix results in the inhomogeneity of the composites, which is also observed in the TEM pictures, as shown in Figure 3i–n. Once NGS agglomerate into bulk, the enhancement of NGS on the mechanical properties is weakened, and coatings are easily inclined to be stripped off, owing to stress concentration occurring on the interface between the NGS and PU matrix. On account of that, there is a little bounce-back of weight loss for samples 0.1 NG and 0.3 NG. Further, the AFM surface height more directly indicates that the improvement of NGS on the surface roughness of coatings, as shown in Figure 6d,e.

Based on the distinct improvement of mechanical properties and wear resistant by comparing the WPU and its composites, it is indicated that there may be interactions between NGS and WPU matrix. According to previous studies, it has been shown that the introduction of nanofiller into the polyurethane can lead to microphase separation, due to the interaction between nanofiller and HS (urea and urethane) or the SS of polyurethane [31,32]. The common method is to use Fourier-transform infrared (FTIR) spectroscopy to investigate the level of microphase separation of WPU and its composites with the NGS. As the plot exhibited in Figure 7a,b, the C=O region in the range of 1600–1720 cm⁻¹ corresponds to the H-boned urea, free urea, H-bonded urethane, and free urethane, respectively. It is shown that addition of NGS, on the one hand, weakens the C=O stretching of H-bonded urea and the peaks of corresponding composites have a right shift; on the other hand, the peak intensity of C=O stretching of H-bonded urethane also recedes or disappears with the participation in the WPU. These comprehensively indicate that the NGS are likely to capture the electron belonging to the H-bonded urea and the H-bonded urethane owing to the high electronegativity of nitrogen [33]. Hence, it is credible that there is an interaction between the WPU matrix and NGS.

In order to further investigate the interaction between the WPU matrix and NGS, TGA is captured to explore the thermal stability of these composites, as illustrated in Figure 7c–e, which different tendency of weight loss of the composites may be due to microphase separation. The decomposition of WPU has two stages: the HS decomposing in the first stage is also an indicator for the level of microphase separation of these composites [31]. Interestingly, compared with the pristine sample, the thermal stabilities of the composites slightly decrease while being heated from 150 °C to 370 °C and then increase from 370 °C to 500 °C. The reason is that the SS decomposes in the second stage [31]. When these two segments are blended, the SS will effectively inhibit the decomposition of the HS. In this study, the presence of NGS contributes to microphase separation, which isolates the segments and weakens the inhibiting effect; thus, the thermal degradation increases as the microphase separation increases in the early temperature range. In the second temperature range, the thermal stabilities increase at higher microphase separation level due to the lack of residual HS.

To further demonstrate the effect of NGS on the microphase separation, the XRD patterns are adopted. The XRD patterns of amorphous diffraction peaks of all samples are shown in Figure 7f. Compared with the virgin sample, the diffraction peaks of the samples with NGS are shifted to smaller angles, which indicates that the domain spacing of the composites are related to the increase in the number of hard segments phases [34]. In other words, the NGS can interact with hard segments of WPU and the nucleation of NGS will induce more fraction of the noncrystalline part of HS to form a crystalline phase along with the NGS, which coincides with the TEM morphology plotted in Figure 3c–n [25]. As direct evidence, this consequence further supports our argument on the results of FTIR spectra and TG analysis.

To detect whether there is an effect for NGS on the soft segments or not, differential scanning calorimetry (DSC) of the WPU and its composites was employed to see the effect of NGS on the thermal properties, such as the melting point of SS (T_m-s) and enthalpy change at T_m-s (ΔH_m-s) [35]. As given in Figure 7g and

Table 2. Thermal properties and value of T_g of WPU and its composites with different content of NGS.

Samples	DSC		DSR
	Tm-s (°C)	ΔHm-s (J/g)	Tg (°C)
0 wt%	55.4	4.45	68.7
0.001 wt%	69.9	6.68	80.0
0.003 wt%	67.3	4.73	81.5
0.005 wt%	68.6	4.61	83.0
0.007 wt%	70.5	5.12	83.5
0.01 wt%	70.6	6.29	84.1
0.03 wt%	73.0	6.28	85.5

, the Tm-s of WPU composites clearly increases with the addition of NGS and the ΔH_m-s of them also has a significant increase. These results suggest that there is a disturbance for SS in the melting process as in the participation of NGS in the WPU matrix. Additionally, we capitalize the DSR of the corresponding WPU composites to further investigate changes in glass-transition temperature (T_g). As the curves depicted in Figure 7h, Tan δ peak decreases as more NGS are incorporated, which means that the existence of NGS within the WPU matrix lowers damping capacity. Further, T_g (see Table 2) of pristine WPU is located at 68.7 °C, while the T_g of corresponding WPU composites has a distinct right shift after the introduction of NGS. Until the NGS content reaches up to 0.03 wt%, the T_g comes to its peak value, 85.5 °C. According to the previous study, the presence of nanofiller in PU not only has a hindering effect on the thermodynamic motion of soft segments but also changes the level of microphase separation [36], and the microphase separation of PU causes fewer hard segments to be alongside the soft segments. If the hindering effect of hard segments plays a less important role than that of nanofiller in determining T_g shift, the T_g will shift to higher temperature [31]. The synergistic effect brings the enhancement to the mechanical properties of the composite polymer material. Herein, we can ensure that the presence of NGS really has a significant suppression on the motion of soft segments.

3.4. The Comparison between This Work and Others

The ultimate tensile strength and Young’s modulus of WPU with 0.005 wt% NGS increased by 58.87% and 55%, respectively, compared to WPU. The performance improvement versus per 0.001 wt% filler content in NGS/WPU composite containing 0.005 wt% NGS is much higher than other works, as shown in

Table 3. Comparisons between this work and others.

Material /Polymer	Filler	Content (wt%)	Result		Performance Improvement versus per 0.001 wt% Filler Content	Reference
			Mechanical Properties	Wear Resistance
Waterborne polyurethane	Nitrogen-doped graphene	0.005 wt%	Ultimate tensile strength and Young’s modulus increased by 58.87% and 55% respectively	Weight loss reduced by 78.74%	Weight loss reduced by 15.75%	This Work
Waterborne polyurethane	MWCNT	0.3 wt%	Hardness increased by 4.4%	Wear rate and friction coefficient reduced by 50.21% and 20.00%	Wear rate and friction coefficient reduced by 0.167% and 0.067%	[37]
Waterborne polyurethane	sulfonated graphene	0.8 wt%	Tensile strength increased by 46.53%; Young’s modulus increased by 38.87%	Weight loss reduced by 88%	Weight loss reduced by 0.11%	[19]
Waterborne polyurethane	Nano silica	6.0 wt%	_ _	Weight loss and friction coefficient reduced by 72.22% and 8.92%	Weight loss and friction coefficient reduced by 0.012% and 0.0015%	[38]
Epoxy resin	Functionalized graphene	0.5 wt%	_ _	Wear rate reduced by 86.34%	Wear rate reduced by 0.173%	[39]
Nylon 66	Graphene /polytetraf- luoroethylene (PTFE)	0.5 wt% graphene and 5% wt% PTFE	Young’s modulus increased by 9.37%; Hardness increased by 6.22%	Wear rate and friction coefficient reduced by 63.29% and 67.18%	Wear rate and friction coefficient reduced by 0.012% and 0.013%	[1]
Epoxy resin	CNTs/GO/MoS2	1.25 wt%	_ _	Wear rate and friction coefficient reduced by 95% and 90%	Wear rate and friction coefficient reduced by 0.076% and 0.072%	[40]
polysiloxane	modified GO	1.0 wt%	Micro-hardness and elastic modulus increased by 57% and 24%	macro-scratch resistance improved by 48%	macro-scratch resistance improved by 0.048%	[41]
Waterborne polyurethane	Boron nitride (BN)	2.0 wt%	_ _	dynamic friction coefficient reduced by 22.6%	dynamic friction coefficient reduced by 0.011%	[12]
Epoxy resin	graphene nanoplatelets (GNPs) and montmorillonite (MMT) nanoclay	0.15 wt% GNPs/0.5 wt% MMT	Micro-hardness increased by 39.06%	weight loss, wear rate and friction coefficient reduced by 29.4%, 29.9%, and 32.8%	weight loss, wear rate and friction coefficient reduced by 0.045%, 0.046%, and 0.051%	[42]
polyurethane/ silicone	graphene oxide (GO) functionalized by aniline oligomer (AOFG)	~1.7 wt% AOFG	_ _	friction coefficient reduced by 73.11%	friction coefficient reduced by 0.043%	[43]

, which make this composite good and feasible to create with a low cost.

3.5. The Model Schematic of Composites

A schematic of the interfacial interaction of the WPU matrix with NGS to reveal the relationship between NGS and WPU segments is shown in Figure 8. The amino group on NGS reacts with the isocyanate group on WPU to form a strong covalent bond connection, providing good dispersion and stability of NGS in the WPU matrix, as shown in Figure 8a. In Figure 8b, it is indicated that the NGS not only accompanies the hard segments of WPU due to interfacial interaction, but also tangle with the soft segments, which agrees well with the rough morphology of the cross-section of 0.05 NG composite film, as shown in Figure 4f. Therefore, for one thing, the hard segments of WPU prefer to unite with NGS and WPU regulates itself along with NGS to form a paralleling structure. For another, once the soft segments form crosslinking knots, the mobility will be heavily limited. Thus, all the relationships among the NGS, the hard segments and the soft segments lead to the improvement in the mechanical and the wear-resistant properties of the corresponding NGS/WPU composites.

4. Conclusions

In this study, we added NGS to the polymer matrix to improve the wear-resistant and mechanical properties of WPU and explored the enhancement mechanism. The wear-resistant tests exhibited that the weight loss of NGS/WPU-composite coating with an ultra-low content of 0.005 wt% NGS under abrasion was reduced by 78.74%, compared with the pristine WPU. In addition, the ultimate tensile strength and Young’s modulus of the film with addition of 0.005 wt% were enhanced by 58.87% and 55%, respectively. However, as the content of NGS increases to over 0.005 wt%, there is an increasing trend for weight loss, and the ultimate tensile strength and Young’s modulus is also reduced compared with 0.005 wt%, which can be attributed to the NGS agglomeration weakening the interaction between NGS and WPU. However, as the addition of NGS is no more than 0.005 wt%, the wear-resistant and mechanical properties significantly improve, benefiting from the even dispersion and interaction among the NGS and hard segments and soft segments of WPU. On the other hand, the firm, effective, and protective film is produced between the friction pair in the wearing process. Well-dispersed NGS not only combines with hard segments and promotes the crystallization of hard segments, but also forms a cross-linking point with the soft segments, which weakens the ductility, while greatly improving the mechanical strength of NGS/WPU composite coatings. Then, the improvement of mechanical strength accompanying with the protective film brings about the rise of wear resistance of the NGS/WPU composite coatings. According to the excellent dispersion of NGS in the WPU matrix, the interaction of NGS with WPU segments at an ultra-low content, and the produced effective protective film (due to the existence of NGS in the wearing process), we believe that the specified NGS or its new derivatives can be designed to achieve tailored properties of composites and be brought into real applications in the future.