Method for preparing novel waterborne polyurethane foam layer for synthetic leather

A method for preparing a waterborne polyurethane foam layer for synthetic leather, the method comprising:

• (1) preparing a charged cellulose nanofiber, comprising:
• I. preparing a negatively charged cellulose nanofiber, including
• mixing 100 parts by mass of a pulp containing 60% by mass of a wood pulp fiber, 1-10 parts by mass of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), and 10-60 parts by mass of NaBr to be uniform, adding 20-120 parts by mass of a solution of sodium hypochlorite with a mass fraction of 37% thereto while stirring, and subjecting the resulting mixture to an oxidation reaction at a pH value of 8-10;
• after the completion of the reaction, centrifugating the resulting product mixture to obtain a supernatant solution, subjecting the supernatant solution to a dialysis, and freeze-drying to obtain the negatively charged cellulose nanofiber;
• II. preparing a positively charged cellulose nanofiber, including
• adding 2,3-epoxypropyltrimethylammonium chloride and a wood pulp in a mass ratio of (0.1-10):1 to an organic solvent A, adding a certain amount of an alkaline substance B thereto, and subjecting the resulting mixture to a ring-opening reaction at a temperature of 50-90° C. for at least 2 hours;
• after the completion of the reaction, repeatedly washing the resulting product mixture with deionized water to obtain the positively charged cellulose nanofiber;
• (2) preparing an ionic waterborne polyurethane, including
• vacuum drying 30-80 parts by mass of a macromolecular diol at 110° C. for 1 hour, then cooling to 70° C., adding 20-40 parts by mass of a diisocyanate thereto, and subjecting the resulting mixture to a first polyaddition reaction for 1 hour under an anhydrous condition to obtain an isocyanate group-terminated polyurethane prepolymer;
• heating the resulting system containing the isocyanate group-terminated polyurethane prepolymer to 80° C., adding a tertiary amine group-containing (cation) or carboxyl group-containing (anion) diol monomer and a catalyst thereto, and subjecting the resulting mixture to a second polyaddition reaction for another 2-4 hours, during which a certain amount of an organic solvent C is added from time to time to regulate the viscosity of the mixture;
• adding a small molecular chain extender thereto, and subjecting the resulting mixture to a third polyaddition reaction for another 2-4 hours, cooling to 50° C., then adding an acid or alkaline substance D, which is equimolar with tertiary amine groups or carboxyl groups respectively, subjecting the resulting mixture to an acid-base neutralization reaction for 30 minutes, finally dispersing the resulting product mixture in a certain amount of deionized water, and vacuum distilling to remove a residual solvent to obtain the ionic waterborne polyurethane dispersion;
• and
• (3) preparing a Pickering foam and a waterborne polyurethane foam layer including
• adding the charged cellulose nanofiber prepared in step (1) and a certain amount of a crosslinking agent to the waterborne polyurethane dispersion with an opposite charge in step (2), stirring the resulting mixture, forming a bimolecular layer at the gas/liquid interface by a self-assembly of the cellulose nanofiber and waterborne polyurethane nanoparticles through electrostatic interactions, to obtain a stable Pickering foam;
• leaving the stable Pickering foam standing at ambient temperature for a period of time until that the foam structure is cured by the crosslinking agent, and finally removing the solvent water to obtain a cationic waterborne polyurethane foam layer.