Synthesis and characterization of scratch resistant polyurethane clear coatings by incorporation of surface modified nanoparticles

The reinforcement of polymers by addition of fillers has been a field of research for some decades. The use of fillers in the nanometer scale, so called nanofillers, is relatively new though and has been intensively studied only since the 1990ies. Due to the high surface to volume ratio this class of fillers provides considerably more surface to interact with the polymer matrix than conventional fillers and significant improvement of properties can be achieved at relatively low filling rates. Additionally these fillers are suitable for application in transparent polymers such as coatings because in the ideal case of uniformly dispersed particles there is no interaction with incident light and thus no turbidity of the resulting composite material. Unfortunately, due to interparticle Van der Waals forces nanofillers show a high tendency to agglomerate with decreasing particle size and these agglomerates may reach several hundred nanometers in size.
In this work the reinforcement of different kinds of nanoparticles in transparent polyurethane coating compositions was studied, particularly with regard to scratch resistance of the resulting nanocomposite. Nanoparticles synthesized by flame spray synthesis, microemulsion polymerization and conventional aerosol process were employed as well as commercially available silica organosols. Some of these nanoparticles were surface modified with organosilanes to improve the interaction with the polymer matrix and provide for a uniform dispersion in the coating system. Surface modification with functional organosilanes bearing amino-, mercapto- or glycidoxy groups allow for reactive integration of the nanoparticles in the polyurethane network and thus enhance the mechanical properties. Also tailor made mixed oxide nanoparticles with a refractive index matched to that of the polymer system were synthesized via flame spray synthesis.
Nanoparticles were characterized by means of Transmission electron microscopy (TEM). The quality of organosilane surface modification was controlled with the help of Solid State 29Si NMR and Thermogravimetric Analysis (TGA). The nanoparticles were incorporated in polyurethane coating formulations and films thereof applied on glass sheets. After curing at 180 °C the transparency of these films was determined using UV/vis spectroscopy. To check the particle dispersion within the coating layer ultramicrotomed cross-sections were investigated by TEM. The scratch behavior of the nanocomposite films was studied with the help of a Nano Scratch Tester (NST). Further investigations like Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Thermal Analysis (DMTA) was performed on selected samples.