Abstract:

Objective As an artificial leather product, impregnated microfiber leather owns excellent thermodynamic properties, rebound resilience, softness, and fullness. However, the main material for its production is solvent polyurethane, which causes serious environmental pollution. Whereas waterborne polyurethane, an environmental-friendly material, brings problems of surface cracks and poor texture when applied to the impregnation of microfiber leather. In order to solve these problems, a particular waterborne polyurethane emulsion was designed and synthesized by referring to the solvent polyurethane with large molecular weight and small particle size, and then successfully applied to the production of impregnated microfiber leather.

Method The prepolymer method was adopted to synthesize the target waterborne polyurethane emulsion by using isophorone diisocyanate (IPDI) as the hard segment, polycarbonate diol (PCDL) as the soft segment, 2,2-bis(hydroxymethyl) propionic acid as the hydrophilic chain extender, and 1,4-butanediol as the flexible chain extender, trimethylolpropane (TMP) as internal cross-linking agent, and ethylenediamine as post chain extender. The relative molecular weight and emulsion particle size of waterborne polyurethane were controlled by adjusting the relative molecular weight of PCDL and the content of TMP, the waterborne polyurethane emulsion with the optimal formula was applied to the production of impregnated microfiber leather.

Results Firstly, emulsion tests were conducted to explore the influences of PCDL relative molecular weight and TMP content on the waterborne polyurethane emulsion in different aspects. According to the results, the relative molecular weight and emulsion particle size of the waterborne polyurethane emulsion was increased with the rising of the PCDL relative molecular weight, and the emulsion particle size decreases first and then increases sharply with the growth of TMP content. When the PCDL relative molecular weight reached 500, the emulsion would be cured smoothly to form a film, and when the PCDL relative molecular weight exceeds 3 000, the emulsion doesn't emulsify but disperses in a flocculating form due to the overlong molecular chain segment. When the TMP content surpasses 3%, the waterborne polyurethane emulsion obviously begins to precipitate due to excessive long-chain network structures and seriously intertwined segments. When the PCDL relative molecular weight amounts to 2 000 with a 3% TMP mass fraction, the synthesized waterborne polyurethane demonstrates a strong storage stability with a relative molecular weight of 5.93×10⁴ and a particle size of merely 89.54 nm. After DSC, DMA, and mechanics property analysis tests on the cured films of the above waterborne polyurethane, it was concluded from PCDL molecular weight that the storage modulus and loss modulus of the films at high temperature can reach 2 837.57 MPa and 278.87 MPa, while the breaking strength was 3.725 MPa with an elongation at break of 763.99%. The target product was obtained by adopting the above waterborne polyurethane to produce the impregnated microfiber leather. Its cross-sectional SEM images showed that the internal bonding and curing of waterborne polyurethane were significantly improved with even and stable distribution. It was observed from the tensile diagrams of the microfiber leather that the surface cracks were also resolved.

Conclusion A waterborne polyurethane emulsion which had large molecular weight and small particle size was produced, with both its emulsion state and physical and chemical properties showing stability. While applied to the production of impregnated microfiber leather, it was observed that the obtained waterborne polyurethane demonstrated high performance as well as the possibility of large-scale production. The curing condition inside the microfiber leather has been improved by enhancing the wettability and bonding property of the substrate in the real application, thus effectively reducing surface cracks, and retaining a good texture and elasticity.

Key words: waterborne polyurethane, microfiber leather, impregnating process, emulsion particle size, trimethylolpropane, polycarbonate diol