The leather industry plays an important role in our daily life. And as an important industrial material product, leather is widely used in many fields, including shoes, clothing and furniture upholstery . Tanning is a meaningful process in the leather industry, which improves thermal stability, mechanical properties and porosity of leather. In the leather industry, basic chromium sulphate is preferred as tanning agent, mainly because of its lower cost and better reactivity with collagen fiber. However, it is difficult to dispose the solid waste containing chromium generated in leather manufacturing, and chromium (III) might be converted into chromium (VI), which can cause harm to human skin and lead to some other diseases. In addition, it would take decades or even hundreds of years for chrome tanned leather to be biodegraded in nature, which may cause the ecological imbalance. Therefore, it is imperative to develop chrome-free tanning technology.
Up to now, many chemicals for chrome-free tanning systems have been developed. As a matter of fact, traditional aldehyde tanning agents and vegetable tanning agents can be used to produce chrome-free leather. However, leather tanned with aldehyde tanned leather has the risk of releasing free formaldehyde, and vegetable tannins are not suitable for preparing light leather, which are often used in combination with other tanning agents . Besides, non-chrome metal such as aluminium salt, titanium salt, and zirconium salt can also be applied to the production of chrome-free leather. Nevertheless, the obtained leather suffered from some inherent limitations such as no washability, hardness and poor organoleptic properties. Furthermore, synthetic tanning agents including amino resins and vinyl copolymers also exhibited unsatisfactory tanning effects like low shrinkage temperature, insufficient fiber dispersion and poor mechanical properties, when they were used as main tanning agent. It is worth noting that in recent years, some organic tanning agents based on biomass, such as starch, chitosan and cellulose, especially their derivatives, have been explored and used in the main tanning stage, combined with other chrome-free tanning agents, and showed very good prospect . But there are still lots of issues that need to be addressed. Nevertheless, it can be found that the combination tanning method using different tanning agents with different characteristics is an effective method and achievable strategy.
In recent years, with the development of silicon chemistry and the progress of nanotechnology, the application of silicon-containing materials in chrome-free tanning systems has attracted increasing attention. For example, an environmentally-friendly tanning process based on nano-silica was explored, and it was reported that the obtained leather had shrinkage temperature as high as 98.2 °C. In addition, the leather tanned by using Laponite nano-clay combined with tetrakis (hydroxymethyl) phosphonium sulfate exhibited good physical performance . Interestingly, zeology, a chrome-free, heavy metal-free and aldehyde-free tanning agent based on zeolite, was developed by Smit & Zoon Company, and reported that the tanned leather by zeology had lightfastness, light color, heat resistance and good physical properties. In addition, our team found that silane glycidoxypropyltrimethoxysilane (GPTMS) can increase the porosity of the skin collagen fiber matrix and make the pore distribution more uniform. Especially, we recently developed a new tanning method, which is based on silicic acid derived from sodium silicate and silane containing epoxy group (GPTMS), can make the Ts value of the leather reach 88 °C, and improve its storage stability. However, comprehensive properties of the leather are still insufficient, especially lacking enough softness and hydrophilicity, so it should be improved for further practical application. Therefore, it is still an impending requirement for leather industry to find an efficient chrome-free tanning technology that can not only alleviate the impact on the environment, accelerate sustainable development, but also endow leather with acceptable comprehensive performances for practical application. See details.