The study of the coating adhesion on PP surface modified in different plasma/acrylic acid solution

Abstract

The effect of acrylic acid deposition in the plasma-treated polypropylene surface was investigated on the surface adhesion properties. The PP sheets were examined by low frequency LF plasma treatments of the oxygen and argon gas discharge individually followed by acrylic acid dipping. The PP surface topography and hydrophilicity were characterized using surface energy measurement, attenuated total reflection spectroscopy ATR and the atomic force microscopy AFM. An evident improvement in surface energy and the coating adhesion was attributed to the creation of polar groups on PP surface in oxygen and argon plasma treatments. In comparison with argon gas plasma, the AFM images of oxygen plasma-treated specimen revealed the peak and valley structure on the PP surface. The deposition of the plasma-treated surfaces into different acrylic acid solutions resulted to surface water repellency and smoothness. Based on the AFM images, the roughness profile of the plasma treated surface changed by the concentration of acrylic acid solution. The lower surface hydrophilicity may be due to acrylic acid effect in dissolving and then removing off the formed polar groups on PP surface. However, the scanning electron microscopy SEM showed distinct regions of acrylic acid grafting on PP surface. Then, the surface adhesion properties were evaluated in pull-off adhesion test after dipping the modified sheets into an acrylic–urethane coating. The adhesion test indicated for the higher adhesion strength in oxygen plasma treatment than argon treatment and an adverse effect of acrylic acid deposition. The addition of aqueous solution of sodium hydroxide into the acrylic acid solution improved the efficiency of acrylic acid grafting on the plasma-treated PP surface. The results of ATR, water contact angle and the pull-off adhesion test showed increasing degree of acrylic acid grafting on PP surface.