Effect of Methyltrimethoxy Silane Modification on Yellowing of Epoxy Coating on UV (B) Exposure

The exterior durability of epoxies is severely affected due to its poor weathering resistance. Epoxies exhibit chalking and discoloration under UV exposure caused as a result of photodegradation. The present work aims at studying the extent to which the color change and yellowing caused due to weathering under accelerated weathering conditions, of DGEBA epoxy, could be lowered by in situ modification of the epoxy polymer backbone with a silane, namely, MTMS. The epoxy resin and silane-modified epoxy resin were formulated into a TiO2-based white coating, applied on mild steel panels, and exposed in a UV (B) weatherometer. The color change (dE) and yellowness index (YI) values of weathered panels were evaluated using a spectrophotometer. The weathered samples were also characterized using FTIR-imaging technique to study the effect of weathering on the structural backbone of the formulated coatings. The silane-modified epoxy coatings showed lowered yellowing by 45% on UV exposure and the enhanced resistance to yellowing of the modified coatings was indicated by lowered dE and YI values. The enhanced resistance to yellowing by the silane-modified epoxy was attributed to the strengthening of the epoxy backbone by introduction of Si–O–C linkage onto the epoxy polymeric chain. 1. Introduction Epoxies as binders are extensively used for corrosion protection in coating industry because of their excellent corrosion resistance and adhesion to substrates. However, their outdoor usage is restricted due to their poor weathering resistance. On exterior exposure, conventional epoxy coatings exhibit chalking and discoloration and lose most of their gloss in three to six months. It is generally due to the presence of aromatic moiety in epoxy polymeric backbone; they absorb at about 300？nm wavelength of light, comprising of UV light and degrade resulting in discoloration and chalking [1, 2]. Malshe and Waghoo partially modified the diglycidyl ethers of bisphenol A (DGEBPA) and bisphenol F (DGEBPF) with various linear and aromatic dibasic acids [2]. The paints prepared using these resins and cured with amine terminated dimer fatty acid-based polyamide (FPA) were evaluated for their weathering characteristics by subjecting them to accelerated and environmental weathering. It was observed that the chalk resistance and the gloss increased with the increase in the carbon length of the linear acids. Weatherable, corrosion resistant epoxy siloxane hybrid binders can be formulated with silicone intermediates, oxysilanes, and aminosilanes using the organic-inorganic chemistry. The