Modification of glass and acrylics surfaces by annealing and chemical etching for superhydrophobic coating applications

Abstract

Superhydrophobic surfaces have attracted attention of many materials researchers due to the ability of self-cleaning property. The fabrication of artificial superhydrophobic surface is always combined with high surface roughness and low surface energy. The first and second sections focus on the effects of annealing and chemical etching on the physical and mechanical properties of glass and acrylic substrates. First, the glass surface was modified by hydrofluoric acid (HF) etching with concentrations of 2-8 % v/v for 1 min. Moreover, the glasses were annealed from 100-500 °C for 1 h. The hardness of annealed glass increases while the WCA decrease with increasing of annealing temperature. After that, the annealed glasses were etched by 6% v/v of HF. The glass was annealed at 100 °C and then etched by 6% v/v of HF. This condition was chosen for preparing for superhydrophobic surface in the next section. The residual stress and surface roughness are the results from annealing temperature and chemical etching, respectively. In the second section, the acrylics were modified using annealing temperature at 50, 75 and 100 °C for 1 h. To evaluate the effect of chemical etching on the morphology of acrylic substrates, two different solvents, i.e. tetrahydrofuran (THF) and chloroform, with concentration of 99.8 % v/v were used to etch for 1 min. Then, the annealed acrylics were etched by THF because chloroform has highly hazardous acid and low transparency after etching. This result exhibits that the hardness of acrylic increases with the increase of annealing temperature. Furthermore, the etching has direct affected to the wettability of acrylics due to the increasing of surface roughness. In the third section, annealing and etching techniques were used to increase the surface roughness and hydrophilicity of glass. The glass surfaces were modified by low- temperature annealing at 100 °C for I h and HF etching with a concentration of 6% v/v for 1 min. The modified glass surfaces were made superhydrophobic, evidenced by wettability of the surfaces, after being immersed into methyltrichlorosilane (MTCS) with a concentration of 2.5 % v/v in toluene. The combination of high surface roughness and low surface energy lead to superhydrophobic surface. Increasing surface roughness lead to an increase in the presence of hydroxyl group on the glass surface, which directly affected the wettability. Moreover, surface roughness at different scales affects the superhydrophobicity following the Wenzel and Cassie-Baxter models. Annealing and etching not only played an important role in the modification of the glass surfaces, but also in the formation of polysiloxane spheres and siloxane nanofilaments on the surfaces. Wettability of the annealed and etched glass abruptly reached a superhydrophobic level after being immersed with MTCS with a water contact angle of 154° and a sliding angle of 3°, according to Cassie-Baxter model. In the last section, transparent and water resistance of superhydrophobic acrylic surface was successfully fabricated via tetrahydrofuran/isopropyl alcohol (THF/IPA) etching assisted SiO2 nanoparticles. The coating solution was synthesize by MTCS modified SiO2 nanoparticles in different concentrations of THF/IPA. Whereas acrylic substrates were immersed into the solutions for 1 min, the THF/IPA solutions were used to improve roughness on the acrylic surface. From the results, superhydrophobic surfaces were created on acrylic substrates with high water contact angle (>155°) and low sliding angle (<5°). The average transmittance of the 50/50 (THF/IPA) sample was above 88%. Moreover, it was found to exhibit water resistance over 18000 drops which is necessary for transparent self-cleaning applications.