Preparing lignin-based membrane with waterborne polyurethane/ polyvinyl alcohol blend

Environmentally friendly materials have attracted much more attention in agricultural plastic film, instead of traditional petroleum-based materials. Among them, some challenges also remain in the traditional polyvinyl alcohol (PVA) in the field of film, such as low mechanical strength, water resistance, and UV isolation. The film performance and properties can be improved for the better use value of biomass resources. In this study, the raw materials were selected as PVA, Lignin, and WBPU (waterborne polyurethane). Among them, the WBPU was used as the filler to liquefy the lignin, in order to prepare the lignin-liquefied substance L(D). The lignin-based WBPU/PVA blend film was successfully prepared by solution pouring. The single-factor experiment was carried out to compare the addition amount of L(D) and WBPU. A systematic investigation was then implemented to explore the effects of the types and the content of additives on the properties of the blend film, such as tensile strength, elongation at break, light transmittance, opacity, thickness, water absorption, and water solubility. FTIR (Fourier transform infrared spectroscopy) and SEM (scanning electron microscopy) were used to analyze the microstructure and chemical composition of the films. The transmittance and absorbance of the films were identified by UV-VIS spectrophotometer. The size and shape of the grain were characterized by XRD (phase analysis of X-ray diffraction). The mechanical properties of the film were evaluated by a universal material testing machine. The thickness of the film was measured to calculate the opacity using a thickness-measuring instrument. The results show that both L(D) and WBPU were fully mixed with the PVA, where the addition of L(D) improved the tensile strength and elongation at the break of the film. The composite film with 15%L(D) shared the best mechanical properties, while the addition of WBPU only exhibited low mechanical properties. However, the addition enhanced the UV resistance of the film. Furthermore, 8:2L(D)/WBPU/PVA blend film behaved with a transmittance of 1.57% at the UV region of 400 mm, indicating a stronger performance to prevent the ultraviolet light, compared with the pure PVA film in the transmittance of 81.44% under the UV region. The water absorption and solubility of the blended film with L(D) were improved, compared with the pure PVA film. Specifically, the water absorption was reduced from 532% to 362%, and the water solubility was also reduced from 42% to 35%, indicating better waterproof. The water absorption and water solubility of the blended film were further improved after adding WBPU. The water absorption of 5:5L(D)/WBPU/PVA blend film was even reduced to 170% with the increase of WBPU content, and the water solubility was also reduced to 18%. Therefore, the water resistance and UV resistance of the blended film can be further improved under different L(D)/WBPU mass ratios. The blended film can be prepared with a high application potential in the plastic film. The findings can provide a strong reference in the food and pharmaceutical packaging fields.