Effect of sol aging on cross- linking, porosity and stability of glass coating
Sol is a suspension of fine particles and under certain reaction conditions this sol can transform into a three-dimensional polymeric network called gel, through the sequential steps of hydrolysis, polycondensation and dehydration . This is called sol-gel transition and is a relatively low-cost simple technique for making transparent thin film coating on pre-cleaned glass . With the help of appropriate sol-gel formulation and processing technique, properties such as anti-reflectance, nanoporosity, hydrophobicity, scratch resistance, chromogenic nature, photocatalysis, and energy efficiency can be greatly optimized for smart windows, buildings, greenhouse, solar thermal panels and photovoltaic cells. The objective of this study is to understand the underlying molecular phenomenon that occurs during aging of an acid catalyzed sol-gel formulation and hence to optimize the aging time of the sol.
The typical polysiloxane (-Si-O-Si-) type of inorganic polymer gels, produced from sol-gel transition, are much lower in viscosity than the regular organic polymer gels and resins. So, the challenge is to optimize the viscosity as well as other related properties so that the functionality of the glass coating is not compromised.
A sol can be prepared by simply mixing inorganic precursor like tetraethyl orthosilicate (TEOS) or titanium isopropoxide (TIP), alcoholic solvent, water and acid/base catalysis in appropriate molar ratio for a given mixing time and allowing to age for several days. Due to proprietary reason, industry partner’s (NSG Pilkington UK) detailed formulation will not be disclosed in this report, only the characterization work will be highlighted.
In this project Dr. Tania Dey has explored the effect of sol aging time on cross-linking, porosity and stability of the glass coating with the help of shear rheology, contact angle measurement and scanning electron microscopy (SEM) imaging.
Aliquots of sol mixture were withdrawn at certain time intervals and their viscosity were measured. The straightness of the shear stress vs. shear strain curve, with no positive or negative bending (not shown), indicated Newtonian behaviour. The double hump feature observed at the beginning of viscosity (Pa.s) vs. aging time (min) plot (not reported earlier) can be attributed to temporary stabilization of oligomers. The more the cross-linking, the larger the particle size, the higher the viscosity. However, the increase in viscosity showed different rates in hydrolysis and condensation stages with an inflection point at 8.5 mPa.s.
Aliquots of sol mixture at same time intervals were casted on pre-cleaned glass surface and specimens were tested for contact angle. The double-hump feature was again present in the contact angle profile, confirming oligomeric stabilization. The more the cross-linking, the rougher the surface, the higher the contact angle. An increase in contact angle up to 48 degrees followed by a decrease after day 1, indicated an interplay between cross-linking and porosity. Porosity can decrease contact angle to some extent, which possibly predominates the sol aging process at the end.
Therefore, it was necessary to correlate how microstructure of the coating changes with sol aging time for which cross-sectional SEM experiments were performed on the same samples. The results show that no particle is formed in the first 30 mins of sol aging. Particles start to appear after an hour or so (2 µm) and keeps growing into agglomerates (9 µm or more). Exfoliated nanoplate formation (unusual nanostructure, reported in Dr. Tania Dey’s previous project only) of 2 µm thickness imparts porosity which predominates at the end of two-day sol aging.
Wetting tension conveys the force balance between cohesion of liquid to itself and adhesion of liquid to solid surface. A minima in wetting tension profile at 48.11 mN/m at the end of day 1 was observed, indicating better stability or retention capacity of the coating. This is because lower wetting tension implies more soaking or tethering of the coating inside the silica pores and hence better intermolecular interaction.
Viscosity of the sol is the most important characteristics that affects the quality and functionality of the sol-gel coating. After critically reviewing the molecular phenomenon that is occurring during the sol aging of this particular sol composition, an optimum sol aging time of 1 day is being recommended, which will offer sufficient cross-linking but not too much of pores in the coating.
The research grant from Ser Cymru National Research Network (NRN) was broken down into 70% for consumables and 30% for travel and subsistence. This grant award has helped the principal investigator, Dr. Tania Dey, to purchase numerous small equipment and consumables and cover the travel cost for self-initiating external collaboration. Under the general umbrella of this research topic, work has also been expanded from silica sol-aging study to heat-treated titanium dioxide sol-gel coating, a journal publication from which is forthcoming. The application-oriented multi-disciplinary collaborative nature of these projects surely carries a great impact.
 Tania Dey, Daragh Naughton, “Cleaning and Anti-reflective (AR) Hydrophobic Coating of Glass Surface: a review from materials science perspective”, Journal of Sol-Gel Science and Technology 77, 1-27 (2016).  Tania Dey, Daragh Naughton, “Cheap non-toxic non-corrosive method of glass cleaning evaluated by contact angle, AFM, and SEM-EDX measurements”, Environmental Science and Pollution Research 24, 13373-13383 (2017).
Case Study Contacts
Dr Tania Dey