Preparation and Characterization of High-Performance Composite Coatings Compatible with Near-Infrared Low Reflectivity and Low Infrared Emissivity

Abstract

Polyurethane (PU)/Al-graphene composite coating with low infrared emissivity, a low near-infrared reflectivity of 1.06 μm, and satisfactory mechanical properties was prepared by using Al powder and graphene as composite pigments and PU as a binder, respectively. The study investigated the impact of the mass ratio of graphene to Al powder, as well as the total addition of functional fillers (Al powder and graphene), on infrared emissivity and near-infrared reflectivity, and the mechanical properties of the coating were studied. The results show that a large number of conjugated systems in the molecular structure of graphene can produce strong absorption of near-infrared radiation, allowing for the coating to exhibit low reflectivity at 1.06 μm in near-infrared radiation. The flake Al powder, with good electrical conductivity, can create a strong reflection in the 8–14 μm far-infrared radiation range, resulting in low emissivity and providing the coating with good infrared and laser-compatible stealth performance. By adjusting the mass ratio of graphene to Al powder, the infrared emissivity at 8–14 μm can be tuned from 0.371 to 0.644, and the reflectivity at 1.06 μm can be adjusted from 22.9% to 61.6%. Additionally, the coatings demonstrate satisfactory mechanical properties, with adhesion strength, flexibility, and impact strength reaching grade 1, 2 mm, and 50 kg·cm, respectively, for coatings with different mass ratios of graphene to Al powder. The PU/Al-graphene composite coating can be regarded as a new type of infrared and laser compatible stealth coating with good functional and mechanical properties.

1. Introduction

In recent years, with the rapid development of modern science, there has been continuous progress in military detection. Common single stealth methods, such as laser stealth, radar stealth, infrared stealth, visible-light stealth, and hyperspectral stealth, can no longer adequately address the security threats posed by many detection technologies [1,2,3,4,5].

Particularly, the requirements for stealth equipment with infrared and laser compatible stealth performance continue to rise. Therefore, from a material design perspective, it is crucial to continuously reduce the infrared emissivity and 1.06 μm near-infrared reflectivity of infrared and laser compatible stealth materials [6,7].

In the past few years, there has been significant focus and interest in researching infrared stealth materials [8,9,10,11,12]. Especially in the use of metal powders, such as Al powder and Cu powder, which exhibit strong reflection to infrared light, effectively reducing the target’s infrared emissivity. Through orthogonal experiments, Yan [13] has optimized the formulation design and curing time of waterborne acrylic resin/Al coating, meeting the requirements of infrared emissivity. Zhang [14] designed a low infrared emissivity composite coating with good mechanical properties and salt-water resistance by changing the formulation. Antimony-doped tin oxide (ATO) coated with Al powder was prepared using the coprecipitation method [15]. Al-doped ZnO (AZO) films were prepared by the sol-gel method at different temperatures [16]. The aforementioned materials have low infrared emissivity. An infrared and laser-compatible stealth material was successfully prepared, exhibiting a near-infrared reflectivity of 43.4% and emissivity of 0.708.

While previous research has primarily focused on controlling the infrared emissivity of metal powders in relation to materials, there is a lack of studies exploring the improvement in infrared and laser-compatible stealth performance. Metal powders demonstrate pronounced reflectivity towards near-infrared light due to their inherent absence of absorption characteristics within this specific wavelength range. Therefore, reducing the reflectivity of metal powders in composite coatings holds significant practical implications for enhancing the coatings’ compatibility within the near-infrared spectrum.

The addition of laser absorbers to compensate for the lack of absorption properties of metal powders in the near-infrared band is the traditional method of preparing resin coatings. The selection or preparation of a laser agent is extremely important, as adding a new laser agent should effectively reduce the near-infrared reflectance while ensuring minimal impact on infrared emissivity. Graphene, due to its excellent optical, electrical, and mechanical characteristics, is commonly used to fabricate sensors, transistors, or nanomotors. However, it is not typically utilized as a laser absorber. Given graphene’s zero-band-gap structure, where the conduction band and valence band intersect, and the presence of a large number of conjugate systems (consisting of numerous benzene ring structures) in the graphene molecular structure, graphene can exhibit remarkably broad spectral absorption bands [17,18]. Graphene demonstrates pronounced nonlinear and strong light absorption when subjected to intense illumination [19]. Notably, graphene’s absorption at 1.06 μm in the special near-infrared radiation is remarkably strong, making it a promising candidate for efficient use in 1.06 μm laser applications. Therefore, it can be inferred that the resin-based composite coatings prepared with Al powder and graphene as composite pigments may exhibit a reduced infrared emissivity and near-infrared reflectivity, thereby demonstrating compatibility with the desired characteristics of low infrared emissivity and low near-infrared reflectivity in resin/metal composite coatings.

In this paper, graphene was firstly used as a laser absorber, and a PU/Al-graphene composite coating was prepared using the scraping method. The reflectance in the 1.06 μm band and the emissivity in the infrared band from 8 to 14 μm were systematically investigated. The effects of graphene and Al powder contents on the microstructure, mechanical properties, 1.06 μm near-infrared reflectance, and infrared emissivity of the coatings were investigated. A composite coating with satisfactory mechanical properties and low near-infrared reflectance and emissivity compatibility was obtained, showing wide application potential in the field of multispectral compatible stealth. This achievement holds certain theoretical and practical significance for improving the overall research level of the existing laser stealth materials, and it provides technical support for enhancing the stealth level and defense ability of various types of land, sea, and air equipment.

2. Materials and Methods

2.1. Materials

Graphene (purity 95 wt%, thickness 3.4–8 nm, diameter 5–50 μm, number of layers 5–10), Al Powder (solid content 67 wt%, flake, particle size 20–30 μm), PU (the active ingredient content is 66.67 wt%), curing agent (liquid, the active ingredient content was 33.24 wt%), tinplate substrate (12 cm × 5 cm, thickness 0.28 mm), and anhydrous ethanol (analytically pure) were used. The appropriate amounts of PU, curing agent, graphene, and sheet Al powder were weighed, with PU and curing agent mixed at a 2:1 mass ratio as the coating binder.

2.2. Preparation of the Coating

After sanding the tinplate substrate with sandpaper, we cleaned the surface of the substrate using anhydrous ethanol. Firstly, composite functional pigments of graphene and Al powder were obtained by mixing them in different mass ratios of 5:25, 10:20, 15:15, 20:10, and 25:5. These composite functional pigments were used in the coating formulation with a total mass fraction of 30 wt%. The objective was to establish the optimal ratio of pigments compatible with both near-infrared reflectivity and infrared emissivity. A certain quantity of PU and curing agent were utilized as an adhesive with a mass ratio of 2:1. The graphene and Al powder were mixed in different proportions in a clean plastic cup, while the adhesive and composite pigments were combined in a mass ratio of 7:3. The paint was then diluted with an appropriate amount of specialized polyurethane thinner, followed by even dispersion through mixing with a glass rod. The paint was applied onto the surface of the prepared tinplate substrate using a glass rod scraping method, followed by air-drying for 6 h at room temperature and heating in an oven at 80 °C for 3 h to prepare the coating samples. Based on preliminary experiments, the coating preparation method was employed to investigate the effects of different amounts (10 wt%, 20 wt%, 30 wt%, 40 wt%, and 50 wt%) of graphene and Al powder composite fillers on the coating properties, considering the optimal ratio of functional pigment and adhesive. Ultimately, a highly performing coating formula was obtained.

2.3. Characterization

A scanning electron microscope (SEM, JSM-6510 LV, Japan Electronics Corporation, Tokyo, Japan) was used to examine the surface morphology of the coating. The near-infrared reflection spectrum was measured using an ultraviolet-visible near-infrared spectrophotometer (UV-3600, Shimadzu Corporation of Japan, Tokyo, Japan). The coating’s infrared emissivity at the wavelength of 8–14 μm was evaluated using an infrared emitter (IR-2, Shanghai Wanga Optoelectronic Technology Limited Company, Shanghai, China).

To evaluate the adhesion strength of the coating, a QFZII adhesion tester (Shanghai Gaozhi Precision Instrument Co., Ltd., Shanghai, China) was employed following the Chinese National Standard GB 1720-79(89) [20]. This standard categorizes adhesion into seven levels (level 1 being the highest and level 7 the lowest).

To evaluate the flexibility of the coating, a cylindrical bending tester QTY-10A (Shanghai Rongjida Instrument Technology Co., Ltd., Shanghai, China) was employed, in accordance with the guidelines of the Chinese National Standard GB 1731-93 [21]. The standard classifies flexibility into eight grades based on the diameter of the cylinder: 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, 10 mm, and 12 mm. Among these grades, 2 mm represents the highest level of flexibility while 12 mm indicates the lowest.

The impact strength of coating was determined using a QCJ impact strength testing machine (Tai ding Heng Ye Instrument Co., Ltd., Cangzhou, China), following the Chinese National Standard GB 1732-93 [22]. In this test, a 1 kg heavy hammer was dropped from various heights (up to 50 cm) onto the coating, creating a pit at the impacted area. The coating’s impact resistance was determined by the highest height at which it remained intact, with the measurements obtained by multiplying the height by the weight.

3. Results and Discussion

3.1. Effect of Different Graphene and Al Mass Ratios on Coating Performance

The SEM photos of the coatings with different mass ratios of graphene to Al powder (5:25, 15:15, and 25:5) are depicted in Figure 1. These coatings were prepared using a composite filler with a weight ratio of 30 wt% and cured at a temperature of 80 °C. The microstructure of the coating exhibits significant variations with changes in the mass ratio of graphene to Al powder, as illustrated in Figure 1. When the graphene content in the coating is low, the functional pigment fails to fully cover the entire surface of the coating. Consequently, the coating exhibits a relatively low surface density, with a high content of Al powder dispersed widely, making it difficult to achieve laser stealth capabilities. This condition is also the underlying cause for the conspicuousness of traditional infrared stealth coatings when exposed to laser detection equipment. The increase in graphene content enables a uniform distribution of graphene on the coating surface, resulting in a more compact surface with smaller pores. The density of graphene is relatively low, allowing for it to float above the coating. This significantly enhances the absorption of near-infrared radiation, enabling the coating to exhibit excellent near-infrared low reflectivity and meet laser stealth requirements. Simultaneously, the flake-textured Al powder can align parallel to the coating surface, augmenting the coating’s ability to effectively reflect 8–14 μm far-infrared radiation. Consequently, the coating demonstrates outstanding low emissivity, meeting the criteria for infrared stealth.

In cases where the graphene content is high and the Al powder content is low, graphene tends to float above the Al powder. Due to its wide-ranging absorption property for various wavelengths, it exhibits increased absorption of far-infrared light. However, this phenomenon adversely affects the coating’s low emissivity. Excessive graphene content may result in an increased emissivity, which is detrimental to achieving infrared stealth capabilities.

Figure 2 displays the near-infrared reflectance spectra of the coatings with different mass ratios of graphene to Al powder. Increasing the relative content of graphene gradually weakens the reflectance spectrum in the 800–1200 nm near-infrared wave band while enhancing absorption of near-infrared radiation. Specifically, at the 1.06 μm near-infrared wavelength, the coating’s reflectivity decreased from 61.6% at 5:25 to 22.9% at 25:5, representing a 62.8% decrease. This results in a commendable laser stealth effect. As graphene content increases, its lower density compared to Al powder enables it to float above the Al powder, enhancing the absorption of near-infrared light and demonstrating excellent laser stealth performance.

Based on the data of 1.06 μm near-infrared reflectivity and 8–14 μm infrared emissivity measured at different mass ratios of graphene to Al powder (Figure 3), the findings show that as graphene content increased and Al powder content reduced, the coating’s reflectivity at 1.06 μm gradually declined, while its emissivity at the 8–14 μm band gradually increased. There is a seesaw effect between the infrared emissivity at the 8–14 μm band and the near-infrared reflectivity at 1.06 μm of the coating. The primary factor is the reduction in Al powder and the higher density of Al powder compared to graphene, facilitating easy deposition of Al powder in sheet form on the underlying layer of graphene, thus resulting in high infrared emissivity.

The results indicate that an excessive amount of graphene and insufficient Al powder will result in a higher infrared emissivity, while inadequate graphene and excessive Al powder will increase near-infrared reflectivity. By precisely controlling the appropriate ratio of graphene to Al powder, one can achieve a well-balanced composite coating with both low infrared emissivity and low near-infrared reflectivity. When the amount of graphene and Al powder added to the coating is consistent (M_graphene:M_Al = 15:15), the near-infrared reflectivity of the coating at the wavelength of 1.06 μm is 41.6%, while the infrared emissivity of the 8–14 μm band is 0.506. The coating has low near-infrared reflectivity and infrared emissivity, achieving good infrared and laser compatible stealth.

In addition, good adhesion strength, flexibility, and impact strength are also important technical indicators for coatings to meet engineering application requirements.

Table 1. The adhesion strength, flexibility, and impact strength of the coatings with different mass ratios of graphene to Al powder (30 wt% of the total fillers).

Mgraphene:MAl	5:25	10:20	15:15	20:10	25:5
Adhesion strength/grade	1	1	1	1	1
Flexibility/mm	2	2	2	2	2
Impact strength/kg × cm	50	50	50	50	50

shows the adhesion strength, flexibility, and impact resistance of coatings with different mass ratios of graphene to Al powder. The results demonstrate that the coatings with varying mass ratios of graphene to Al powder exhibited superior adhesion strength (Grade 1), flexibility (2 mm), and impact resistance (50 kg × cm).

In summary, to achieve the desired compatibility of low near-infrared reflectivity and low emissivity, we determined the optimal mass ratio of graphene to Al powder as 15:15 for subsequent experiments.

3.2. Effect of Different Pigment Additions on Coating Performance

The SEM photos of coatings with different contents of total pigments (20 wt%, 30 wt%, 40 wt%, and 50 wt%) are shown in Figure 4. These coatings were prepared using a composite pigment with a mass ratio of graphene to Al powder of 1:1 and cured at a temperature of 80 °C. The diagram clearly demonstrates the influential role of composite pigment content on the microstructure of the coating.

At 20 wt% pigment content, the flake Al powder and graphene fail to fully cover the entire coating, resulting in more resins between the functional pigments and a significantly thicker surface resin. The thicker surface resin increases the absorption of far-infrared radiation, leading to high emissivity, which is not conducive to the infrared stealth of the coating. Moreover, graphene, with lower density, easily distributes in the upper layer of Al powder, producing strong absorption of near-infrared radiation, resulting in low near-infrared reflection and high emissivity.

When the content of the composite pigment increases to 30 wt%, sheet Al powder and graphene in the coating can be densely laid parallel to the coating surface. This feature enhances the coating’s reflection on 8–14 μm far-infrared radiation, significantly reducing emissivity and meeting infrared stealth requirements. Moreover, with an increased pigment content, the density of graphene powder distributed in the coating significantly increases, ensuring a more uniform distribution. These microstructure characteristics allow for graphene to strongly absorb incident near-infrared radiation, resulting in relatively low reflectivity to 1.06 μm near-infrared radiation, meeting laser stealth requirements.

However, when the compound pigment content in the coating increases too much, the coating’s surface becomes noticeably rough and porous. This may increase the absorption of far-infrared light, which is not conducive to maintaining the lowest emission rate performance of the coating [23].

Figure 5 displays the near-infrared reflectance spectra of coatings at various contents of composite pigments. The reflectance of the coating in the 800–1200 nm near-infrared band gradually increases, while the reflectivity at 1.06 μm shows a gradual rise with an increase in the proportion of composite pigments, from 15.2% at 10 wt% to 52.6% at 50 wt%. The coating’s laser stealth performance is compromised due to an excessive presence of composite pigments. This can be attributed to the gradual rise in the absolute content of graphene accompanying an elevated composite pigments concentration, resulting in an enhanced absorption of near-infrared light. Conversely, there is a progressive rise in flake Al powder content within the coating, which exhibits strong reflection towards near-infrared light, inhibiting the absorption of near-infrared light by the coating.

The coating’s laser stealth performance is undermined by the excessive application of composite pigments. This is because, as the content of composite pigments increases in the coating, there is a gradual rise in the absolute content of graphene, enhancing near-infrared light absorption. However, with the gradual increase in the absolute content of flake Al powder, known for its strong reflection of near-infrared light, the coating exhibits a larger surface area covered by Al powder. This results in a more pronounced enhancement of near-infrared reflectivity compared to an increase in the absolute content of graphene. Consequently, as the content of composite pigments increases, the 1.06 μm near-infrared reflectivity of the coating also increases.

The variations in near-infrared reflectivity at 1.06 μm and infrared emissivity within the range of 8–14 μm were observed for different total pigment contents (refer to Figure 6). With the increase in total pigment content in the coating, there is a gradual rise in near-infrared reflectivity at 1.06 μm, while the infrared emissivity within the 8–14 μm band initially decreases and then slightly increases, ranging from 0.915 at 10 wt% to 0.506 at 30 wt%, and finally reaching 0.522 at 50 wt%. The initial decrease and subsequent increase in infrared emissivity are mainly due to the absolute content of flake Al powder increasing with the rise in pigment content in the coating.

The reason behind the initial decrease and subsequent increase in infrared emission is primarily that as the pigment content in the coating increases, the absolute content of flake Al powder in the coating increases, and the resin that can strongly absorb far-infrared radiation on the coating surface is significantly reduced. Consequently, the coating exhibits a strong reflection effect on far-infrared radiation, resulting in a lower emission rate performance.

When the total pigment is added excessively, the horizontal orientation of the flake Al powder is weakened due to mutual extrusion between pigments, leading to an increase in pores in the coating. These structural changes weaken the reflection effect of the coating on far-infrared radiation, causing the infrared emissivity of the coating to increase.

Table 2. The adhesion strength, flexibility, and impact strength of the coatings with different contents of total pigments (M_graphene:M_Al = 15:15).

Contents of Total Pigments	10 wt%	20 wt%	30 wt%	40 wt%	50 wt%
Adhesion strength/grade	1	1	1	1	1
Flexibility/mm	2	2	2	4	8
Impact strength/kg × cm	50	50	50	50	45

presents the adhesion strength, flexibility, and impact strength of coatings with different composite pigments contents. The data in the table indicate that there is essentially no impact on adhesion strength with changes in composite pigment content. However, as the content of composite pigments increased from 30 wt% to 40 wt% and then to 50 wt%, flexibility decreased from 2 mm to 4 mm and further to 8 mm. Simultaneously, the impact strength decreased from 50 kg × cm to 45 kg × cm. These mechanical properties deteriorations are attributed to the elevated proportion of total pigments and the compromised encapsulation of pigments by the resin. After considering the near-infrared reflectivity, emissivity, and mechanical properties of the coating, it was determined that the optimal content of total pigments in the coating is 30 wt%.

4. Conclusions

The PU/Al-graphene composite coating, formulated by incorporating Al powder and graphene as composite pigments, demonstrates lowered infrared emissivity and decreased near-infrared reflectivity at 1.06 μm. By adjusting the mass ratio of graphene to Al powder, the infrared emissivity of the coating can be finely tuned within the range of 0.371–0.644 in the 8–14 μm band, while allowing for adjustable reflectivity to 1.06 μm near-infrared radiation within a range of 22.9%–61.6%. The optimal near-infrared reflectivity, emissivity, and mechanical properties of the coating are achieved with a total addition of 30 wt% Al powder and graphene, using a mass ratio of 15:15 between them. The PU/Al-graphene composite coating exhibits exceptional mechanical properties, with adhesion strength, flexibility, and impact strength reaching grade 1, 2 mm, and 50 kg × cm, respectively. It is anticipated to serve as a novel coating material with good mechanical properties that are compatible with both low near-infrared reflectivity and low infrared emissivity.