Visible-Near Infrared Spectroscopy Simulation of Jungle Camouflage Coating based on Hydrotalcites Containing Magnesium

The rapid development and practical deployment of hyperspectral detection technology render conventional camouflage materials increasingly inadequate for covert protection in complex forest environments, primarily due to their insufficient spectral matching and limited functionality. A critical challenge in forest camouflage lies in the accurate replication of four characteristic spectral signatures of natural vegetation, i.e., the green peak, the red edge, the near-infrared plateau, and the near-infrared water absorption peak. These features form the unique spectral “fingerprint” of vegetation, which serves as a fundamental basis for evading multi-band, high-precision detection. Among them, the green peak governs a visual concealment within the visible region, while the near-infrared water absorption peak acts as a decisive marker for distinguishing vegetation from artificial materials, which are essential for effective camouflage. However, the existing forest camouflage paints and simulation materials are constrained by inadequate structural design and limited functional integration. Most commercially available products can only partially mimic certain spectral traits of vegetation and commonly exhibit deficiencies in core performance. Note that it remains particularly difficult to simultaneously and accurately match both the green peak of healthy foliage and the near-infrared water absorption peak. This shortfall makes such materials susceptible to rapid identification under hyperspectral imaging, thereby severely limiting the practical effectiveness of the existing forest camouflage technologies. Consequently, there is an urgent need for the research and development of novel camouflage materials capable of comprehensive spectral simulation.

Six types of sulfate intercalated Mg/Cr layered double hydroxides (LDHs) were synthesized by a coprecipitation method. The crystalline structure of the as-prepared materials was examined by X-ray diffraction (XRD). The molecular structures and characteristic absorption features were determined by Fourier-transform infrared spectroscopy (FT–IR). The thermal decomposition behavior and mass loss profiles were analyzed by thermogravimetry–differential thermal analysis (TG–DTA). The surface morphology and particle size distribution were characterized by scanning electron microscopy (SEM). These characterization techniques provided a comprehensive structural and physicochemical profile of the synthesized LDHs. Subsequently, to meet the requirement of reproducing the green spectral peak in jungle camouflage, a functional camouflage coating was fabricated via blending the optimized LDHs material with chromium-based green pigment. This formulation enhanced the green-peak characteristics in the visible region and preserved the pronounced water-absorption peak in the near-infrared range. The spectral performance of the coating was evaluated by visible–near-infrared spectroscopy (Vis–NIR), and its thermal camouflage capability was assessed by infrared thermographic imaging.

The results systematically indicate that a series of chromium-containing hydrotalcite materials, prepared by a controlled synthesis process, exhibit typical crystalline structural characteristics of layered double hydroxides (LDHs). The XRD patterns reveal sharp and symmetric diffraction peaks, with the characteristic peaks corresponding to the layered structure displaying a high intensity and a regular shape, indicating a good crystallinity and a well-ordered interlayer arrangement. Among the series of chromium-containing hydrotalcites, sulfate-intercalated magnesium chromium hydrotalcite (MgCr–SO₄^2––LDHs) demonstrates unique structural advantages, i.e., its interlayer spacing is significantly larger than that of other intercalated types of magnesium chromium hydrotalcites. Simultaneously, The TG–DTA thermal analysis indicates that this material exhibits the maximum mass loss of 26.38% at 25–400 ℃. This is directly related to the content of water molecules stored within the interlayers. The spatial configuration and charge distribution characteristics of the sulfate intercalation ions provide a sufficient space for the stable adsorption and storage of water molecules. As a result, MgCr–SO₄^2––LDHs possesses the most abundant interlayer water content among the prepared samples. This rich interlayer water constitutes the core structural basis for simulating the near-infrared water absorption peak of vegetation. The visible to near-infrared spectra (400–2500 nm) further reveal the optical properties of the Mg/Cr hydrotalcite. In the visible region, the material shows a distinct characteristic absorption peak at 490 nm (blue region), attributed to its intrinsic electronic transition behavior. In the near-infrared region, intense and sharp water absorption peaks clearly occur at approximately 1400 nm and 1900 nm, showing a high similarity to the near-infrared water absorption features of natural vegetation leaves. Sulfate-intercalated magnesium chromium hydrotalcite (MgCr–SO₄^2––LDHs) is modified with chromium(Ⅲ) oxide (Cr₂O₃) by an in-situ coating process via leveraging these characteristics. The near-infrared water absorption properties imparted by the abundant interlayer water in MgCr–SO₄^2––LDHs were combined with the green optical characteristics of Cr₂O₃. This complementary interaction successfully offset the influence of the intrinsic blue absorption peak at 490 nm from the magnesium chromium hydrotalcite. Ultimately, a jungle camouflage coating simulation material is prepared, having both green visual characteristics and distinct near-infrared water absorption peaks. This provides a key material foundation for subsequent high-similarity spectral matching with natural vegetation.

The camouflage coating fabricated with an optimal MgCr–SO₄^2––LDHs@Cr₂O₃ : MgCr–SO₄^2––LDHs ratio of 3 : 7 could enable a precise spectral matching with natural vegetation leaves. It accurately replicated both the green peak and the near-infrared water absorption peak, achieving a spectral similarity of up to 95.80%. Furthermore, the results of thermal imaging tests revealed no significant difference between the coating and natural vegetation. This study on dual-functional materials that simulated color and the near-infrared water absorption peak could thus offer a promising direction for the development of advanced vegetation camouflage technologies.