Improving Weatherability and Dye Compatibility of Shoe Materials! Total Solution for SCF TPU Midsoles

24 May, 2023

Author : Specialty Chemical Business Unit

Everlight provides a series of solutions to enhance weatherability and high-efficiency dyeing for low-carbon eco-friendly shoe materials, maintaining the attractive appearance of products and increasing their value!





New breakthrough in shoe-making technology—SuperCritical Foaming


The popular “ETPU Popcorn” or “Hyper Burst” midsole technology for sports shoes do not only look trendy, but also feel light and breathable when you put them on! Why do these newly launched shoes feel lighter and more shock-absorbing while giving a more obvious sense of springiness? That’s precisely because of a new breakthrough in the foaming technology used for making midsole of sports shoes—the “SuperCritical Foaming” which, compared with conventional chemical foaming, makes the midsole cells shrink to less than 10nm in diameter and enables the midsole to provide up to 75% of spring-back performance even at a foam density of 0.2g/cm3. In addition, finished products are 100% recyclable, which nicely meets the trend of environmental protection.


Later, the industry introduced such innovative physical foaming technologies as MUCELL and NEXCELL, which can be directly injected into molds to speed up production processes and reduce energy consumption.


Trendy shoes no longer look so trendy; yellowing is really troublesome!


Among the shoe materials made with foaming technologies, EVA (ethylene-vinyl acetate) offers a price advantage, yet relatively weaker in elasticity, aging resistance, flexure resistance, and wearability; PEBAX (polyether block amide) is made with relatively higher material cost, but its support is poor and the scrap rate is high.


Hence, mainstream foam midsoles are still generally made with TPU (thermopolyurethane) or TPEE (thermoplastic polyester elastomer) for their relatively higher stability. However, even the relatively stable TPU is prone to yellowing and discoloration after being exposed to UV light outdoors due to the benzene and ester structure in its chemical structure.



Eversorb® light stabilizer keeps shoes spotlessly white


To solve the problem of TPU yellowing and fading, Everlight has specially brought out Eversorb® yellowing-prevention light stabilizer exclusively for TPU, effectively slowing down the deterioration of TPU materials after exposure to sunlight. The following experiment used polyether TPU for SCF foaming, and then employed ultraviolet light to perform accelerated aging tests. After 48 hours of continuous irradiation, the △YI (difference in yellowing index) of the TPU in the control group significantly increased to 57.8 (below level 1) and yellowing was obvious. The △YI of the sample added with Eversorb® light stabilizer was about 1.44 (Level 4-4.5). Thus, this experiment serves as proof that, compared with the test specimen of the control group, the one added with Eversorb® light stabilizer can effectively improve weatherability of foamed polyether TPU, delay the yellowing of products for up to approximately 6 months to 1 year, and maintain the attractive spotlessly white appearance of shoe midsoles over long periods.




Evertint® reactive polymeric colorants combine qualities as vivid colors, eco-friendliness and convenience into one


It’s also noteworthy that, for making SCF TPU foam midsoles, TPU granules are usually pre-mixed with organic or inorganic pigments before being injected into a mold; the key to maintaining fine foam is to precisely control the nucleation speed and gas supersaturation in the molten state. However, due to large differences in surface tension between TPU molecules and pigments, plus the fact that a small amount of pigment or carbon black will be converted into crystallization after agglomeration during the nucleation process, interfering with the proliferation and merging of voids. That results in large-sized voids which are likely to be generated during depressurization and lead to obvious flow stains and a grainy finish on the surface of the product, making it rough to touch as well as affecting the aesthetic quality and wearing comfort of the finished product.


To tackle TPU foam’s dyeing problem, Everlight has developed Evertint® reactive polymeric colorants, featuring outstanding compatibility with TPU. They can be added at the particle synthesis stage to quickly disperse and form a structurally stable chemical bond. Because they do not contain tiny physical particles and have low viscosity, they will not affect the processes of nucleation or bubble proliferation and merging during the foaming process. Evertint® reactive polymeric colorants are easy to work with, high in color vibrance and come in a variety of colors! Further, they do not contain dimethylformamide (DMF-Free) or other harmful solvents. This is currently the dyeing method with the highest benefit and the easiest to carry out. These are the products that offer both vivid colors and environmental safety!





Everlight has long been cultivating the innovation of weather-resistant products and production process improvement, and puts great effort into the practice and promotion of green chemistry. For that reason, we have also brought out a series of enhanced weatherability and dyeing solutions for emerging low-carbon eco-friendly shoe materials, featuring higher quality and greater environmental friendliness. Light stabilizers and polymer dyes can effectively improve the aesthetics and weatherability of eco-friendly shoe materials, and hugely increase their service life and product value! Should you have any questions about our products or project cooperation suggestions, please do not hesitate to contact us



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