Key Takeaways & Executive Findings
- •• • Light fastness improved to Grades 3–4 (ISO 105 B02), enabling stable thermochromic function for several months of sunlight exposure, a significant upgrade from typical Grade 2–3 for unprotected systems. • • Dual UV-shielding strategy blocks nearly the entire UV spectrum, reducing degradation of thermochromic three-component compounds; maximum decolorization rate controlled within 10% under benzophenone/benzothiazole protection. • • Coaxial wet-spinning process creates core-sheath structure, minimizing impact on mechanical properties compared to excessive UV absorber dosage, which can compromise fiber wearability. • • C12-Azo stores absorbed UV energy via trans-to-cis photoisomerization, enabling on-demand heat release, adding functional value beyond mere UV blocking.
Abstract
Leuco dye-based thermochromic fibers suffer from poor light fastness, limiting their cyclability. Here, a hydrogen bond dissociation/structural isomerization-based dual ultraviolet (UV)-shielding strategy is proposed to develop enhanced light-resistant thermochromic elastomer fibers (Azo/TCM@Abs/TPU) employing UV absorbers and 4-dodecyloxyazobenzene (C12-Azo) through a coaxial wet-spinning process. The integration of UV absorbers and C12-Azo enhances UV protection, effectively blocking nearly the entire UV spectrum. The light fastness of Azo/TCM@Abs/TPU has been improved to Grades 3–4, enabling a stable thermochromic function to withstand several months of sunlight exposure. Additionally, the absorbed UV light is stored as chemical energy within C12-Azo via trans-to-cis photoisomerization. This stored energy can be released as heat on demand. The coaxial dual-protection concept using photoisomerization offers an efficient method to enhance light resistance in thermochromic fibers.
1. Introduction
Thermochromic fibers, particularly those based on leuco dye three-component systems, face a critical bottleneck: poor light fastness. Ultraviolet (UV) radiation in sunlight penetrates conventional polymer encapsulation, causing photodegradation of the color former and loss of thermochromic function. Even with protective coatings, noticeable color changes occur after 400 hours of accelerated aging, as reported in prior studies. Existing UV absorber strategies, such as benzophenone and benzothiazole derivatives, can limit decolorization rates to below 10%, but excessive dosages often compromise the mechanical integrity and wearability of the fiber. This trade-off between photostability and mechanical performance has hindered commercial adoption in textiles and smart wearables.
This work introduces a coaxial wet-spinning approach that integrates a dual UV-shielding mechanism: conventional UV absorbers combined with 4-dodecyloxyazobenzene (C12-Azo), which undergoes trans-to-cis photoisomerization. The coaxial architecture confines the UV-protective layer to the sheath, preserving the core's mechanical properties while blocking nearly the entire UV spectrum. This design not only enhances light fastness to Grades 3–4 but also stores absorbed UV energy as chemical potential, releasable as heat on demand. By decoupling UV protection from bulk mechanical properties, this strategy addresses the core bottleneck of light resistance without sacrificing fiber performance, offering a scalable route for durable thermochromic textiles.
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YU Wei-dong, FEI Liang, TAN Jia-ling, ZHANG Zi-nuo, ZHANG Chen, WANG Chao-xia (2026). Azobenzene/UV absorber dual-protected thermochromic elastomeric fibers toward enhanced light-resistance. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3766-0
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Frequently Asked Questions
What is the maximum UV-blocking efficiency achieved by the dual-protection system, and how does it compare to single UV absorber systems?
The dual-protection system blocks nearly the entire UV spectrum, as stated in the abstract. While exact percentage is not provided, the light fastness improvement to Grades 3–4 (ISO 105 B02) indicates a significant reduction in photodegradation. In comparison, single UV absorber systems typically control decolorization rates within 10%, but the dual system's photoisomerization adds a second pathway for UV energy dissipation, likely enhancing overall protection.
How does the coaxial wet-spinning process affect the mechanical properties of the fibers compared to conventional blending of UV absorbers?
The coaxial design confines UV absorbers and C12-Azo to the sheath layer, minimizing their impact on the core's mechanical properties. This is critical because excessive UV absorber dosage in blended fibers can compromise wearability. The paper notes that spinning offers potential to minimize impact on comfort, and the coaxial structure is specifically designed to preserve mechanical integrity while providing effective UV shielding.
What is the mechanism by which C12-Azo stores UV energy, and how is it released?
C12-Azo undergoes trans-to-cis photoisomerization upon UV absorption, storing energy in the metastable cis configuration. This energy can be released as heat on demand, likely through thermal relaxation or photochemical back-isomerization. This dual functionality not only shields UV but also provides a heat-release capability, which could be exploited for thermal management applications.
What is the expected lifetime of the thermochromic function under real-world sunlight exposure?
The paper states that the improved light fastness (Grades 3–4) enables stable thermochromic function to withstand several months of sunlight exposure. This is a qualitative estimate, but the grade improvement suggests a substantial increase in durability compared to unprotected systems, which may fail within weeks or even days under intense UV.
Are there any scalability challenges in the coaxial wet-spinning process for industrial production?
The paper does not discuss scalability in detail, but coaxial wet-spinning is a well-established technique for producing core-sheath fibers. The main challenges likely involve precise control of sheath thickness and uniformity, as well as the dispersion of UV absorbers and C12-Azo in the sheath solution. However, the process is inherently scalable, and the use of thermoplastic polyurethane (TPU) as the matrix suggests compatibility with existing melt-spinning or wet-spinning infrastructure.
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