Polypropylene (PP) luggage straps face challenges in improving colorfastness during dyeing due to their material properties. The lack of polar groups and high crystallinity in their molecular structure make it difficult for traditional dyes to adhere effectively, requiring special processes for dyeing. The non-polar molecular chains of PP have a compact internal structure, lacking active sites for dye molecules to bind. This means conventional dyes can only adhere to the fiber surface and cannot penetrate to form a stable bond. Furthermore, the high crystallinity further restricts the diffusion path of dye molecules, resulting in pale colors and easy fading after dyeing. To address this characteristic, the industry has developed various modification technologies to improve dyeing results.
The solution dyeing method involves directly adding masterbatch during the PP polymerization stage, allowing the pigment to be uniformly dispersed within the molecular chains. This method avoids damage to the fiber structure in subsequent dyeing processes, resulting in significantly better colorfastness than post-dyeing processes. Because the pigment is encapsulated within the polymer matrix, its lightfastness, washability, and abrasion resistance are superior to surface dyeing. However, limited by the chromatographic range of the masterbatch, it is difficult to achieve precise mixing of complex colors, and it is mostly used for the production of single-color or basic-color luggage straps.
Surface treatment technologies alter fiber surface properties through physical or chemical means, enhancing their dye adsorption capacity. Physical methods, such as plasma treatment, can create microporous structures on the fiber surface, increasing the dye adhesion area; chemical methods introduce polar groups through reactions like halogenation and sulfonation, constructing dye binding sites. For example, chlorinated polypropylene fibers can be dyed with cationic dyes under weakly alkaline conditions, but the process may reduce fiber strength, and chemical residues may affect the product's environmental performance.
Blending modification technology involves blending additives containing dye sites with polypropylene and then melt-spinning, mechanically mixing to disrupt the tight arrangement of molecular chains. The introduction of additives such as organometallic salts and low-molecular-weight copolymers creates dispersed dye binding sites within the fiber, supporting various dyeing systems such as disperse dyes and cationic dyes. While this method can achieve diverse dyeing effects, the uniformity of additive dispersion directly affects the final color fastness, and some additives are prone to decomposition during high-temperature spinning, requiring strict control of process parameters.
The development of novel disperse dyes targets the hydrophobic properties of polypropylene, introducing long-chain alkyl or mercapto structures through molecular design to enhance the affinity between dyes and fibers. For example, disperse dyes containing dithiol groups can achieve a dyeing rate of around 70% under high temperature and high pressure conditions, and the wash fastness is significantly improved after oxidative fixing. These dyes bind to fibers through intermolecular forces; although light fastness still needs optimization, it already meets the color fastness requirements for everyday use of luggage straps.
Dyeing process parameters have a decisive impact on color fastness. Temperature control is crucial; high temperatures promote dye molecule diffusion, but excessive heating may damage the fiber structure. Time management needs to balance dyeing rate and fiber damage risk. pH adjustment affects the degree of dye ionization and the distribution of surface charge on the fiber. For example, disperse dye dyeing requires a weakly acidic environment to stabilize the dye molecule structure, while cationic dyes require alkaline conditions for optimal dyeing.
Post-treatment processes are essential for consolidating the dyeing effect. Washing removes surface dye, reducing the risk of color fading from friction; soaping further removes residual dye through surfactants; and fixing agent treatment forms a protective film on the fiber surface, improving overall fastness to washing and perspiration. For example, polypropylene fibers modified with hyperbranched polymers can still maintain excellent color fastness after being washed at 60°C, demonstrating the significant role of post-processing in improving product quality.
