Effects of crosslinking and nanosilica on waterborne polyurethane dispersions

Abstract

Waterborne polyurethane dispersions (WPUDs) are versatile materials that are a type of waterborne polymer colloids, utilize water as a dispersing medium, and contain little or no organic solvent. WPUDs are increasingly replacing solventborne polyurethanes thanks to their superior properties such as very low or no volatile organic compound (VOC), low viscosity, excellent film properties, ease of use, good adhesion, high abrasion resistance, and flexibility. WPUDs are preferred in a wide range of applications and industries, including coatings, binders, adhesives, sealants, automotive, inks, biomaterials, paper, wood, footwear, textiles, and many more. Different ways are used to improve the weak properties of WPUDs, such as poor mechanical strength, poor water and alkali resistance, relatively low heat resistance, and high raw material costs. The most common of these ways are, using crosslinkers in various structures, hybridization with polymers that contain different groups such as acrylics and silicones, and the creation of composites by adding nano or micron size inorganic fillers to the dispersion such as silica, clay, graphene oxide, SiO2, TiO2, Al2O3, Fe2O3, CaCO3. Nanosilicas, which have different types such as colloidal silica, precipitated silica, fumed silica, are widely preferred for making composite materials with WPUD because of different properties such as superior hardness, low toxicity, good stability and dispersion, and low cost. The main purposes of producing WPUD/nanosilica composites using various techniques including in-situ polymerization, blending, and the sol-gel process are to develop composite materials with superior mechanical, thermal, optical, chemical, rheological, or electrical characteristics than pure WPUDs. This study aims to obtain new waterborne PU dispersion (WPUD)/nanosilica composites by using various nano silicas and crosslinkers with different types such as isocyanate, aziridine, and waterborne PU dispersion, and investigate the effects of crosslinkers and nano silica by forming a film from these WPUD/nanosilica composites. Various tests were carried out to determine the thermal properties, crystallization behaviors, optical properties, swelling degrees of the obtained WPUD/nanosilica composite films, and it was aimed to gain the properties such as high water resistance, homogeneous distribution, and mechanical strength to the films and using them as a surface coating material in the future. In this study, we prepared various formulations using polyether-based aliphatic waterborne polyurethane dispersion with different amounts and types of nanosilicas (hydrophobic fumed silica and aqueous silica dispersion) and different crosslinkers such as polyether modified HDI--based polyisocyanate and polyfunctional aziridine. WPUD ratio was determined as 75% by weight, polyisocyanate ratio of 5%, and polyaziridine ratio of 2%, if present in the formulation. While fumed silica ratio ranged between 0.1 and 3% by weight aqueous silica dispersion ratio ranged between 1% and 25% by weight in the prepared formulations. WPUD/silica nanocomposites were prepared using the blending method by mechanical stirrer. Crosslinkers were added just before the tests due to the limited pot life. After pouring the prepared WPUD/silica nanocomposite dispersion into Teflon molds, cured for 4 hours at room temperature and at 50 °C for 8 hours in the oven to form films with a thickness of approximately 150μm. Fourier transform infrared spectroscopy (FTIR) was used to examine the chemical structure of the waterborne polyurethane dispersion, silicas, and crosslinkers used in the formulations, as well as the films formed after curing, to determine the changes in the peaks with the addition of crosslinker and silica, and to compare with the pure WPUD film. Differential scanning calorimetry (DSC) was used to examine the thermal properties of the films and X-ray diffraction (XRD) was used to examine the crystallization behaviors. Scanning electron microscopy (SEM)-EDX spectrometry was used for the morphological and elemental analysis of the films. The transparency of the films was observed with an optical microscope (OM) under the same conditions. The swelling degrees of the films in water were evaluated over weight changes. Finally, the hardness values of the films were measured with the Shore D Durometer. Characteristic polyurethane peaks were seen in the FTIR spectral analysis of the neat WPUD film. New characteristic peaks formed as a result of crosslinking reactions were determined. In addition, the presence of nano silica in the structure was determined by the appearance of characteristic Si-O-Si peaks. According to the DSC results, it was determined that the films contained crystalline structures, and the thermal stability of the films increased with the addition of fumed silica and aqueous silica dispersion. According to XRD data, it was determined that the crystallinity increased slightly with the addition of polyisocyanate, but the crystallinity decreased significantly with the addition of polyaziridine. In addition to these, nano silica additions were also found to reduce crystallinity. Also, It was determined that the films with the lowest crystallinity were the films containing polyaziridine crosslinker and fumed silica. In the neat WPUD film SEM images, it was observed that the surface was a rough surface consisting of nano and micron-sized polyurethane particles, and phase separations were observed on the surface. While the phase separations decreased and the surface became more uniform with the addition of polyaziridine, the phase separations increased with the addition of polyisocyanate. Also, it was observed that the surface became rougher with the addition of silica, as the silica ratio increased in the samples containing fumed silica, the silica agglomerations increased and the uniform structure did not occur. In the EDX measurements, carbon, oxygen, and nitrogen atoms were determined, and the silicon atom was also included in the structure with the addition of silica. According to the swelling measurement results, it was found that crosslinking and the addition of a certain amount of nanosilica reduced the degree of swelling. It was determined that the films with the lowest swelling degree were the films containing polyaziridine crosslinker and fumed silica. As a result of the transparency tests performed with an optical microscope, it was seen that the transparency of the films decreases as the amount of silica increases. In the hardness test results, it was seen that the hardness of the films increases with the addition of a crosslinker and certain amount silica. Also, It was observed that the addition of polyaziridine increased the hardness more than the polyisocyanate.