ORIGINAL PAPER Sol–gel silica antireflective coating with enhanced abrasion-resistance using polypropylene glycol as porogen Bibo Xia • Qinghua Zhang • Songyuan Yao • Yulu Zhang • Bo Xiao • Bo Jiang Received: 12 March 2014 / Accepted: 9 April 2014 Ó Springer Science+Business Media New York 2014 Abstract Silica antireflective (AR) coatings with high transmittance and enhanced abrasion-resistance were syn- thesized by sol–gel process using polypropylene glycol (PPG) as porogen. The effects of molecular weight of PPG and weight ratio of PPG to SiO 2 on the refractive index and abrasion-resistance of the coating were systematically studied and compared with those of polyethylene glycol (PEG). Experimental data showed that the refractive index decreased with increasing the weight ratio to SiO 2 and molecular weight of both PEG and PPG, but PPG was much more effective than PEG. In the case of same molecular weight, PPG modified coating has the higher porosity than PEG modified one. When the weight ratio of PPG to SiO 2 is in a low level, the PPG- containing silica AR coatings exhibit the good abrasion- resistance. PPG is liquid at room temperature and the better solubility than PEG. These effective and economic AR coat- ings with enhanced abrasion-resistance have potential value in the field of solar thermal collectors. Keywords Sol–gel process  Antireflective coating  Polypropylene glycol (PPG)  Abrasion-resistance 1 Introduction Antireflective (AR) coatings have recently gained great interests for their possible applications in optical and display devices since the AR coatings not only remove ghost images but also enhance the transmittance of light [1–3]. An ideal homogeneous AR coating can achieve near 0 % reflection at a specific wavelength when its refractive index is equal to (n a n s ) 0.5 , where n a and n s are the refractive indices of air and substrate, respectively [4, 5]. The most available thin-film materials are magnesium fluoride, silica, alumina and titania with refractive index of 1.35, 1.44, 1.65 and 2.2 [6, 7]. These condense materials possess excellent abrasion-resistance and ability of adhering to the substrate. It is convenient to prepare AR coatings on high-index substrates, because these substrates have a refractive index sufficiently higher than the available coating materials to enable the design of high-performance AR coatings. Sili- con, for instance, has a refractive index of around 3.5, giving a reflection loss of 31 %. After applying alumina AR coating on silicon surface, the reflection can be reduced to 3 %. However, for substrates with low refractive index, it is difficult to prepare high performance AR coatings by the lack of thin-film materials with very low refractive indices. The refractive index of solar thermal collector cover is about 1.47 in the visible spectral region, which implies that the refractive index of AR coating must be around 1.21. However, the lowest refractive index among homogeneous one-phase material is above 1.35 for mag- nesium fluoride [6]. The refractive index of silica film derived from the typical Sto ¨ber method, named as base-catalyzed silica film, can be lowered to 1.22 [8]. The inter-particle and particles’ interior porosity afford base-catalyzed silica film a very high porosity, and hence a relative low refractive index of 1.22. However, the nature of base-catalyzed silica film is such that it is kept intact only by point contact forces between the individual particles, and therefore the abra- sion-resistance is quite poor. Consequently, base-catalyzed B. Xia  S. Yao  Y. Zhang  B. Xiao  B. Jiang (&) Key Laboratory of Green Chemistry & Technology, College of Chemistry, Sichuan University, Chengdu 610064, China e-mail: jiangbo@scu.edu.cn Q. Zhang Chengdu Fine Optical Engineering Research Center, Chengdu 610064, China 123 J Sol-Gel Sci Technol DOI 10.1007/s10971-014-3362-0