The effects of particle size distribution on the optical properties of titanium dioxide rutile pigments and their applications in cool non-white coatings

Highlights

• The particle size of Altiris pigments is larger than that of the conventional TiO₂.

• The solar and VIS reflectances of TiO₂ pigments decrease with particle size.

• The NIR reflectance of TiO₂ pigments increases as particle size increases.

• Altiris pigments improve the solar reflectance of colored coatings.

• The addition of Altiris pigments yields a metameric match to the desired color.

Abstract

The particle size distributions of three commercially available titanium dioxide pigments were measured, and their effects on the optical properties of the pigments were investigated. The Altiris 550 and 800 pigments possess larger median particle size and wider size distributions than those of the conventional titanium dioxide rutile. The visible (VIS) reflectance, the solar reflectance and the lightness of the films and coatings singly pigmented with any of these three pigments decrease as the pigment particle size increases. The VIS transmittance and near-infrared (NIR) reflectance both increase as the pigment particle size increases. The main reflection band of the coatings shifts farther into the NIR region with increasing pigment particle size. The partially VIS-transmitting Altiris pigments enable the NIR and the solar reflectance of cool non-white coatings to be improved, thereby yielding metameric matches to the desired colors.

Introduction

Cool-colored pigments play an important role in the manufacture of cool non-white coatings, which are desirable options for consumers from the esthetic and visual considerations [1], [2], [3], [4], [5], [6], [7], [8]. In several related papers, cool-colored pigments were ambiguously termed near-infrared (NIR) reflective pigments [7], [8], [9], while in other papers, the cool-colored pigments were explicitly classified as “NIR-reflecting colorants” or “NIR-transmitting colorants” [10], [11], [12], [13]. A coating pigmented with the former type of colorant is always cool over any substrate, but that pigmented with the latter type of colorant requires a background with moderate-to-strong NIR-reflectance to yield a cool non-white coating [10], [11], [12]. Apparently, the cooling principles of the coatings pigmented with these two types of cool pigments are different.

In a previous informative paper [14], the solar spectral optical properties of 87 predominately single-pigment paint films, with thicknesses ranging from 10 to 37 μm, were characterized with particular emphasis on the NIR properties. Most of the cool pigments were identified as NIR-transmitting colorants, but several pigments, such as titanium dioxide rutile white, were found to be NIR-reflecting colorants, showing both strong NIR backscattering and weak NIR absorption in a binder of refractive index 1.5 [3]. Conventional titanium dioxide rutile with a scattering power of 1.9 is one of the most hiding and the best visible scattering pigments [11]. If used to improve the NIR reflectance of cool non-white coatings, it will modify the reflectance curve [11] and the visual appearance of the coatings.

In addition to the NIR reflectance, color control is an equally important consideration in the manufacture of cool non-white coatings. For certain coating applications, such as building envelope coatings, the coating color is generally fixed. Therefore, significant adjustments in the amount of visible (VIS) light reflected or its wavelength is unlikely to be allowed; slightly modifying the visible absorption might be tolerated [11]. One way to make spectral irradiance-independent non-metameric colors with enhanced NIR reflectance is to use a pigment with high reflectance in the NIR region and high transmittance in the VIS region. An alternative way to produce metameric colors is to replace the absorptive visual color matching pigments with pigments that have similar absorptions in the VIS region but are highly reflective or transparent in the NIR region [11].

For a given value of the scattering power, the wavelength most efficiently scattered by a pigment positively correlates with the diameter of the pigment particles [11]. Therefore, large particle-sized pigment will effectively reflect NIR radiation with longer wavelengths. Commonly used commercial titanium dioxide rutile generally has a particle size ranging from 200 to 300 nm and it reflects well between 400 and 1700 nm, with a peak scattering intensity of 500 nm [11]. Titanium dioxide, with a mean particle size of 10 μm, reflects efficiently between 800 and 2300 nm but poorly between 400 and 800 nm [11]. When used as an extender pigment, it can greatly improve the non-white coatings׳ NIR reflectance, with little or no effect on their visual color. Larger particle-sized titanium dioxide rutile has been used in the manufacture of camouflage coatings; however, to the best of our knowledge, no papers to date have been published on its application in the manufacture of NIR-reflective coatings.

The optical properties of a conventional titanium dioxide rutile and two larger particle-sized commercial titanium dioxide rutile pigments were investigated in this study. In this paper, the measured particle size distributions of the three samples are presented. The spectral reflectance, transmittance and computed absorptance of three paint film samples and the spectral reflectance for coatings pigmented with the three pigments over white and black basecoats are compared. In addition, the effects of pigment concentration on the NIR reflectance of the three pigments are discussed. The applications of two larger particle-sized titanium dioxide pigments in preparation of cool non-white coatings are exemplified.