Effect of primary driers on oxidative drying of high-solid alkyd binder: Investigation of thickness effects by mechanical tests and infrared spectroscopy

Abstract

Catalytic activity of four primary driers was investigated in high-solid alkyd binder modified with tall oil fatty acids. The drying activity was established by mechanical tests. Infrared spectroscopy was used for detailed investigation of chemical changes during the autoxidation process. Due to strong thickness effect, the infrared study was performed using attenuated total reflectance (ATR) technique, which measures spectra of a thin layer from the interface sample/ATR crystal (down surface of the sample). Coatings of 30 μm-wet thickness were found to be enough thin to be considered as a homogenously dried bulk with negligible effect of oxygen diffusion. It makes the 30 μm-layers very suitable for determination of kinetic parameters of the autoxidation process. For the first time, ATR-IR experiments on several samples of the same composition but different film thickness were used for detailed investigation of the thickness effect.Two well-distinguished modes of film-formation were observed in studied drier/binder systems. Front front-forming drying occurs when the autoxidation is fast and air-oxygen diffusion is slower than its consumption. In extreme case, the film-formation process is practically ceased because impermeable skin is formed and the coating is not through-dried for very long time. The second mode, homogenous drying, is observed when autoxidation is slow and oxygen consumption is not fast enough to establish oxygen gradient in the coating.