A New Approach to Graft Siloxanes to Alkyds

The effect of oil length of alkyds and substitution of siloxane backbone has been studied for alkyd–siloxane hybrids. A series of nine alkyd–siloxane hybrids were synthesized by either varying the oil length of the alkyd or the siloxane backbone substitution. Three linseed oil-based alkyds with either a long, medium, or short oil length were grafted with three hydride-terminated siloxanes substituted with methyl, cyclopentyl, or cyclohexyl groups. A hydrocoupling reaction was used to couple the telechelic siloxane with the hydroxyl functionality of the alkyds using Wilkinson’s catalyst. The reaction was monitored by the disappearance of siloxane hydride signal using Fourier transform infrared. Characterization of siloxane–alkyd hybrids was performed using 1H-NMR, 13C-NMR, and gel permeation chromatography. The hybrids were formulated with a Co, Zr, and Ca drier package and auto-oxidatively cured without using any solvent. The tensile, viscoelastic, and coating properties were evaluated for the cured films. The crosslink density, flexibility, and reverse impact resistance were found to increase as a function of oil length. Tensile modulus, elongation-to-break, glass transition temperature, drying time, and fracture toughness decreased with increase in oil length. For the alkyd–siloxane hybrids, the mechanical and rheological properties were dependant on the size of the substituents. The larger-sized cyclopentyl and cyclohexyl groups resulted in better mechanical and rheological properties than the methyl-containing siloxanes