Preparation and Angle-Dependent Optical Properties of Brown Al/MnO2 Composite Pigments in Visible and Infrared Region
© The Author(s). 2017
Received: 12 January 2017
Accepted: 29 March 2017
Published: 8 April 2017
Traditional low infrared emissivity coatings based on aluminum flakes cannot own low IR emissivity and low lightness simultaneously. Herein, a new simple efficient method for the synthesis of brown Al/MnO2 composite pigments with low IR emissivity and low lightness is reported, through forming MnO2 layer on aluminum flakes by thermal cracking, then altering the shape and forming nanoshell by stirring in hot flowing liquid. The results indicate that the MnO2 particles, which have tetragonal structure with high crystallinity, are needlelike and forming a complete shell on the aluminum flakes. The optical properties of composite pigments can be tuned by mass of KMnO4 added in precursor and time of hot flowing. Strong angle-dependent optical effects are observed in five different angles through multi-angle reflectance spectrum, while low lightness and low IR emissivity are preserved. This work is expected to provide a new route for the preparation of colored aluminum effect pigments in low infrared emissivity coatings.
KeywordsAluminum flake Angle-dependent pigments Manganese dioxide Reflectance Infrared emissivity Lightness
Effect pigments which have the angle-dependent optical effect are widely used in many fields [1–4]. Many studies are focused on color pearlescent pigments which are based on mica [5–7]. As a matter of fact, aluminum flaky pigment of single particle  or in a multi-layer structure as substrate are becoming of increasing interest recently in decorative material, heat insulation coatings, and security applications, due to their low infrared (IR) emissivity and other special optical properties [9–11]. As we know, many methods have been developed to prepare chromatic aluminum flakes, such as oxidation , physical/chemical vapor deposition [13–15], and chemical liquid deposition [16, 17]. A kind of aluminum effect pigments which has one metal oxide layer consisting of iron, manganese, copper, vanadium, etc. and an enveloping organic polymer layer is produced by wet chemical oxidation method . But the binding force between the layer and the substrate aluminum is not strong through this sol–gel process. A radical polymerizable resin layer have been coated on aluminum pigment and then adhering coloring pigments by ball milling to fabricate colorful aluminum pigments . However this organic layer results in the drastic increase of infrared emissivity. Aluminum flakes have been decorated by oxidizing in a water-in-oil emulsion comprising a surfactant in the presence of a base . However, the L* value is still up to 96 at 15° in CIELAB. In a word, all of these methods are complicated, device dependent, or unstable in coating quality. Meanwhile, the lightness, gloss, and visible (VIS) reflectance of these pigments are very high which are needed to be as lower as possible in practice .
An efficient way to avoid these problems is to introduce the thermal cracking—hot flowing method which is developed to prepare silver layer on silica spheres . The advantages of this method are that the layer is smooth due to the surface tension and the thickness is controllable. In this paper, this method is applied to fabricate brown Al/MnO2 composite pigments with low lightness, low infrared emissivity, and angle-dependent effects. We systematically discuss the influence of reaction conditions, such as mass of KMnO4 (M KMnO4) added in precursor and time of hot flowing (t hf), on morphology, reflectance of variable angles, lightness, and colors of coatings.
The composition, reaction condition of the samples
S0 (Al flake)
The samples were characterized by X-ray diffraction (XRD) (SHIMADZU, XRD-7000 with CuKa radiation) and field emission scanning electron microscopy (FE-SEM; JEOL JSM-7600 F). For optical characterization, paints containing Al/MnO2 composite pigments were prepared by mixing Al/MnO2 powder 20%, lacquers 60%, and thinner 20%. Then, the films were painted by these mixtures onto microslides. The VIS spectral reflectance and CIE (International Commission on Illumination) L*a*b* with different angles (15°, 25°, 45°, 75°, and 110°) were measured by the angle dependence spectrophotometer (X-Rite, MA98XRB, D65 illuminant). Total infrared reflectance spectrum (3–21 μm) was measured by a Fourier transform infrared spectrometer (BRUKER, Tensor27) with integrating sphere attachment (BRUKER, A562). Total visible and near infrared (VIS/NIR) reflection spectrum (380–2300 nm) was measured by UV/VIS/NIR spectrophotometer (Perkin–Elmer, Lambda 750).
Results and Discussion
That means after being processed, S2 preserve low IR emissivity and low visible reflectance which is beneficial to reduce the light pollution. S4 has lower visible reflectance than S2, but the emissivity of S4 is lower than 0.5, which cannot be used in low IR emissivity coating. In summary, more M KMnO4 and less t hf result in the lower total reflectance of composite pigments, and S2 is supposed to be a good choice of effect pigments in low IR emissivity coatings.
The CIE L*a*b* values of Al/MnO2 composite pigments (S2, S4)
In conclusion, brown Al/MnO2 composite pigments are fabricated through coating MnO2 layer on aluminum flakes by a new thermal cracking and hot flowing method. The composite pigments are termed a brown metallic shade owing to the absorption of MnO2 shell. The variation of reflectance, lightness, and color of composite pigments are huge in different observed angles. When in low M KMnO4 and t hf of 48 h, the Al/MnO2 composite pigments which have strong angle-dependent optical effects and low IR emissivity will supposed to be a good choice of effect pigments in low IR emissivity coatings.
- M KMnO4 :
Mass of KMnO4
- t hf :
Time of hot flowing
- T tc :
Temperature of thermal cracking
The authors would like to thank Wenle Liu from UESTC for the preparation of the paints of samples.
The work presented in this paper was supported by the Open Foundation of National Engineering Research Center of Electromagnetic Radiation Control Materials under Grant (ZYGX2014K009-2).
YL carried out the design of the experiment, fabrication of the Al/MnO2 composite pigments, and writing of the manuscript. JX conceived of this study. ML measured the samples and data analysis. BP and LD carried out the manuscript modification. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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