Preparation And Application Of UV Curable Hybrids Of Hyperbranched And Linear Polyurethane Dispersions

Due to increasingly restrictive environmental regulations regarding volatile organic chemicals (VOCs), environment friendly coatings attract more and more academic and industrial attentions. The application of these products like high-solid coatings, waterborne coatings, and UV curable coatings is greatly enhanced. As polyurethanes (PUs) offer very good elasticity with reasonably high mechanical strength and abrasion resistance, and controllable hardness, PUs are widely used in coatings. Regarding to environmental regulations, PUs are being replaced by UV-curable polyurethane dispersions (PUDs) for their fast curing, environment friendliness, low energy consumption, etc. The UV-curable PUDs can combine the flexibility of high molecular weight polymers with the hardness of cross-linked polymers. It is of great practical importance to develop useful UV-curable PUDs and it has been studied extensively in recent years. Although the acrylated PUDs are superior to conventional one-component PUDs, there are still some problems to resolve for current UV curable PUDs:Firstly, the C=C content and the conversion is too low, which make UV curable PUDs inferior to solvent based PUs in terms of chemical resistance and mechanical durability; Secondly, the solid content is usually below30%, so more energy is required to evaporate the water in the formulations. Recently, many research efforts have been devoted towards new molecular composition and structure, nanocomposites, involvement of nature materials and organic silicon resins. Due to the unique three-dimensional molecular architecture and high density of functional terminal groups, hyperbranched polymers (HBPs) are excellent candidates for the modification of UV curable PUDs.It has been confirmed by many reports that hybrids of PUDs could combine high performance with low cost. In the hybrids, HBPs can be used as cross-linkers in UV-curable PUDs in view of their high density of terminal groups. The hybrids are expected to exhibit high flexibility and moderately high hardness. In the hybrids, only a fraction of hyperbranched component is needed as cross-linkers in the PU matrix. It can be reasonably expected that incorporation of a fractional amount of such hyperbranched cross-linkers into UV curable waterborne PU systems could significantly enhance the mechanical properties and their UV curing behavior.In this work, important variables are optimized in the preparation of aqueous PUDs such as the order of material addition, the temperature and time of the reactions, etc. In the end, we explored a new kind of hybrids. The advantages of the hybrids are as follows:1) high unsaturation conversion can be obtained;2) the tensile strength can be improved greatly without decreasing the elongation at break too much;3) the ratio of HBP in the hybrids is small, and thus hybrids can be cost-effectively and massively produced. Different wt%of hyperbranched polyurethane acrylate (HBPUA) was added into linear polyurethane acrylate (LPUA) as cross-linker. The properties of the hybrids were compared with conventional LPUA to investigate the effect of the hyperbranched cross-linker on the dispersion stability and physical properties of the resulting cured PU coatings. Films prepared from these dispersions were thoroughly characterized by Fourier-transform infrared (FT-IR) spectroscopy, differential scanning calorimeter (DSC), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA) and universal testing machine. Furthermore, the properties of the hybrids films were compared with that of the solvent-based UV coatings. The water resistance and gel content of the cured films was also studied.