Objective To address the limited expressiveness of traditional leather decorative techniques and the insufficient application of the cyanotype process on leather, this study investigates the feasibility of applying the cyanotype process to sheep leather and expands its application methods in the design of women’s sheep leather bags, thereby broadening the application scope of the cyanotype process and providing a new technical approach for the visual expression of leather materials in bag design.

Methods Response surface methodology (RSM) was employed to design the process optimization experiments. Following single-factor experiments, chrome-tanned sheep leather was used as the experimental material. With exposure time, photosensitizer dosage, photosensitizer (ammonium ferric citrate) concentration, and washing time as independent variables, and ΔE^*_Blue (total color difference between the exposed blue area and the original leather) and ΔE^*_White (total color difference between the masked white area and the original leather) as response values, a four-factor, three-level RSM experiment was designed using the Box-Behnken module in Design-Expert 13.0 software. A quadratic regression equation model was constructed to investigate the optimal process conditions for cyanotype on sheep leather, aiming to maximize ΔE^*_Blue and minimize ΔE^*_White. Subsequently, the cyanotype sheep leather prepared under the optimal conditions was tested for color fastness to rubbing and physical-mechanical properties. At the design practice level, a design method for women’s sheep leather bags based on the cyanotype process was proposed, which specifically includes two parts: the design and production of cyanotype sheep leather and its application in women’s bags.

Results (1) The optimal process parameters determined by model optimization were: exposure time of 27 min, photosensitizer dosage of 1 mL, photosensitizer concentration of 0.123 mol/L, and washing time of 16 min. Under these conditions, the theoretical values of ΔE^*_Blue and ΔE^*_White for cyanotype sheep leather could reach 45.77 and 4.11, respectively. Validation experiments yielded an average ΔE^*_Blue of 43.69 (relative error 4.54%) and an average ΔE^*_White of 4.05 (relative error 1.17%), resulting in clear pattern imaging. Performance tests showed that dry rubbing color fastness reached level 4-5, and wet rubbing color fastness reached level 3-4, meeting the requirements of the QB/T 5087-2017 standard for "Leather for Cases and Bags". The reduction rates of tensile strength (2.84 MPa), elongation at break (41.30%), and softness (5.69 Ri/mm) compared to the original leather were all within 15%, indicating no significant negative impact on the physical properties of the leather. (2) A women’s sheep leather bag featuring blue-and-white porcelain elements was designed, and the design extension of cyanotype sheep leather was achieved through pattern application and multiple decorative techniques.

Conclusions The cyanotype process under the optimal parameters can produce stable and clear blue–white patterns on the sheep leather surface, with good color fastness and physical properties, and can be used for women’s sheep leather bags. In the future, further exploration can be conducted on the application effect of this process on other types of leather or sheep leather tanned by different methods, as well as its color stability under complex temperature and humidity conditions, and the applicability of other patterns in this cyanotype leather bag design.