| Waterborne polyurethane (WPU) usually refers to nano-sized aqueous dispersion or emulsion of polyurethane resin containing hydrophilic segment or hydrophilic group. Compared to solvent-based polyurethane, waterborne polyurethane not only retains excellent mechanical properties, flexibility and elasticity, but also has some unique properties, such as non-toxicity and non-pollution. Therefore, synthesis and application of waterborne polyurethane in coatings, adhesives, synthetic leather and other industries has received widespread attention. However, synthesis of waterborne polyurethane in industry mainly adopts acetone method which needs a large number of organic solvents in the synthesis process. Thus method not only causes serious environmental pollution, but also low economics and poor security. Hence, it is necessary to develop green synthesis process of waterborne polyurethane.First, to address solvent-free synthesis of waterborne polyurethane dispersion, a conception of critical number-average molecular weight Mnc was put forward and used to control the molecular weight and viscosity of polyurethane prepolymer. Through incorporating moderate hydrophilic monomer and controlling the molecular weight below Mnc, stable WPU dispersions were successfully prepared in a solvent-free manner from isophorone isocyanate, various polyether diols and DMBA (or DMPA). The value of Mnc varies in the range of1800～2400g/mol, depending on the loading of hydrophilic monomer, the structure of soft segment and the existence of solvent or not. The value of Mnc increases with increasing the loading of hydrophilic monomer, using high molecular weight, high flexible and low-crystalline macromolecular diol or adding some solvent. Four percent loading of hydrophilic monomer is sufficient for stabilizing WPU dispersion, but more loading will lead to high water absorption of PU film.Synthesis process and raw materials have certain influence on the properties of WPU dispersion and PU film. To this end, a series of WPU dispersions were prepared from different raw materials and by different synthesis processes and properties of WPU dispersion and PU film were compared. The results show that solvent-free synthesis technology is characterized by environmental benignancy as well as small particle diameter, low water absorption and improved tensile strength of PU film. Although particle diameter of WPU dispersion increases slightly, one-step synthesis technology is beneficial to simplify process, shorten reaction time and reduce the usage of NMP. When DMBA was used instead of DMPA, WPU dispersion with small particle diameter and better stability was obtained, and the resultant PU film had slightly low tensile strength and water absorption and slightly high elongation at break. PU film with high tensile strength was prepared using polyester diol PBA2000, PU film with high water absorption was prepared using polyether diol PPG2000, and PU film with high strength and low elongation was prepared using PTMG1000diol. Diisocyante mixture of IPDI and HDI can also used to synthesize WPU. At IPDI/HDI molar ratio7/3, the resulting PU film has improved flexibility and low water absorption.Finally, to improve tensile strength and lower down water absorption of PU film, crosslinked WPU was prepared using three external crosslinkers, namely, trimethylolpropane tris (2-methyl-l-aziridinepropionate)(XR-100), ethylene glycol diglycidyl ether (EGDE) and tetrahydrophthalic acid diglycidylester (711). The influences of type and dosage of crosslinkers on tensile properties and water absorption of PU film were investigated and discussed. The results show that suitable dosage of XR-100can improve water absorption and tensile properties of PU film. EGDE can hardly improve high water absorption of PU film, but EGDE/XR-100complex can improve it though improvement of mechanical properties is not satisfactory. Suitable dosage of711can not only improve water resistanceof PU film, but also reduce100%modulus and improve flexibility of the PU film. |
PDF Full Text Request