Development of IR-reflective pigments for coatings and polymers

Interior temperature of automobiles and buildings may reach an uncomfortable level during hot weather, which requires the use of air-conditioners. The solar heating of automobiles and buildings can be partly reduced if the incident solar radiations can be reflected back to the atmosphere. One way to achieve this is to replace the traditional pigments used in surface coatings by NIR (near infrared)-reflective pigments as they keep the surface cool by rejecting significant amounts of NIR-radiations of the solar spectrum.

There are many commercially available NIR-reflecting pigments to date but most of them are inorganic, heavy metals based and are not environment friendly. Among the organic pigments, the use of perylene pigments in the NIR-reflective coatings has been described in few patents. However there is no systematic study in the open literature regarding the influence of different factors such as particle size, pigment concentration, electronic influence of different substituents and degree of crystallinity on their NIR-reflectance. The aim of this study is to develop understanding of the effect of these factors on the NIR-reflectance of perylene based pigments which would help to develop perylene based pigments with significant reflectance in the NIR-region.

Therefore five different perylene derivatives P1, P2, P3, P4 and P5 were synthesized and their NIR-reflectance was measured and compared with each other as well as with some commercially available pigments such as carbon black, lumogen and titanium dioxide after their dispersion in polypropylene.

The relative reflectance of different diimide derivatives (P1-P4) varied from 38 to 65 at 980nm wavelength and from 37 to 54 at 1300 nm wavelength; P1 has shown the highest NIR-reflectance among these derivatives. On the other hand derivative P5 has shown the least reflectance in the NIR-region. The reflectance of synthesized black coloured derivative P1 is comparable to that of Lumogen® (LG) at 1300nm, whereas reflectance of LG drops radically in the wavelength region of 700-1000nm.

The particle size and particle size distribution do not seem to be the only contributing factor for this observed difference as all the perylene diimides derivatives (P1-P4) have similar particle size and particle size distribution, yet they show significant variation in their NIR-reflectance. Similarly PTCA and P5 have similar particle size range but exhibit dramatic difference in their NIR-reflectance.

Computational study performed on these derivatives has shown that two of the derivatives P2 and P5 have a considerable amount of net dipole moments and low reflectance compared to that of P1, P4 and PTCA which have zero net dipole moment and higher NIR-reflectance. This demonstrates the direct impact of the development of net dipole moment in reducing the NIR-reflectance.

The results of degree of crystallinity for these derivatives indicate the following order of increasing crystallinity:

PTCA>P3>P2>P1>P4>P5

A combination of the effect of three factors i.e. particle size, dipole moment and degree of crystallinity, means that PTCA having smallest particle size, zero dipole moment and high degree of crystallinity has the highest ability to reject NIR-radiations.