**1. Introduction**

At present, leather has a wide range of applications, but its development is limited by its easy aging and poor properties in terms of water proofing and scratch resistance. The main way to improve the performance of leather materials is to coat its surface with a layer of protective film. The common coatings on the market can roughly be divided into three categories, namely polyurethane [1–6], polyacrylate [7–9], and organic silicon [10–12]. Among them, polyurethane (PU) is the first reported and widely studied due to its strong structural designability and excellent wear resistance [13]. However, the waterproof performance of PU material itself is not enough to meet the requirements of leather materials in some special fields, such as in a long term exposure to a humid environment. To further improve the waterproof, oil-resistant and anti-smudge properties of PU coatings, fluorinated polyurethane (FPU) waterproofing agent is prepared often by introducing fluorine-containing groups into the PU molecular structure. On the one hand, the FPU retains the special structural characteristics of PU itself, which can bring lots of excellent properties to the material, for example good film formation, wear resistance and toughness. On the other hand, the introduction of fluorine-containing segments endows polyurethane-based materials with other special properties (like excellent waterproof, oilproof and anti-smudge properties [14–22]) due to its low surface energy [23], which determines that the FPU will have better application prospects in more fields than the traditional PU. The introduction of long-chain perfluoroalkyl (LF) groups to PU is currently the most used method for FPU preparation through implanting fluorinated polyols as

**Citation:** Su, S.; Wang, J.; Li, C.; Yuan, J.; Pan, Z.; Pan, M. Short-Branched Fluorinated Polyurethane Coating Exhibiting Good Comprehensive Performance and Potential UV Degradation in Leather Waterproofing Modification. *Coatings* **2021**, *11*, 395. https://doi.org/ 10.3390/coatings11040395

Academic Editor: Mohor Mihelˇciˇc

Received: 11 February 2021 Accepted: 27 March 2021 Published: 30 March 2021

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soft chain segments [24,25], perfluorinated alcohols as end sealants [26,27], or fluorinated compounds with multi-functional groups as chain extenders [28–30].

To date, fluorinated chemicals containing LF groups [31], the most advanced waterproofing agent on the market, have been verified, demonstrating that their manufacture originated from straight-chain C8 perfluoroalkyl sulfonic acid or perfluoroalkyl acids, such as C8 fluorocarbons, perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) that have potential biotoxicity like bioaccumulation and environmental persistence [32–36]. In view of these worrying dangers, many countries, such as Canada, the United States and China, have adopted certain regulatory measures to prohibit its use in the relevant industrial production. Fortunately, compounds with shorter fluorocarbon (SF) chains are thought to be significantly less toxic, and the subsequent breakdown products are innoxious [37,38]. Although FPU with SF components is more environmentally friendly than FPU with LF components, its hydrophobic, oleophobic and anti-smudge properties are significantly worse than those of long-chain FPU. Thus, it is particularly important to explore a new type of fluorinated polyurethane that is not only environmentally friendly but also has outstanding coating properties in the leather field. As for FPU with short branched fluorocarbon chains, it is unclear whether it can satisfy the above two conditions at the same time, and there have been few reports on this to the best of our knowledge. Therefore, research into the fluorinated polyurethane with short branched chains in the leather field will have strong theoretical and practical significance.

In this work, a novel type of short-branched fluorinated PU was prepared by first synthesizing short-branched chain fluorinated alcohol and then implanting it into the PU structure as an end-capping agent. As a main raw material, hexafluoropropylene trimer is not restricted in use [39]. Thus, we initially synthesized a fluoro-alcohol by reacting hexafluoropropylene trimer with p-hydroxybenzyl alcohol (Figure 1ai) [40]. Next, a new FPU waterproofing agent was prepared by pre-reaction of 1,4-butanediol and toluene diisocyanate, successively end-capping with fluoro-alcohol (Rf-OH) and ethanol respectively (Figure 1 aii). We adjusted the fluorine content in the final mixture by changing the molar ratio of ethanol and fluoro-alcohol, and maintained a molar ratio of approximately 1:1 between isocyanate groups and hydroxy groups. Fluorine-containing short branched chains not only provide FPU with excellent surface performance, low water absorption, and satisfied wear resistance, they also provide it with potential photodegradation. Most notably, the FPU could be compared to the commercial C8 waterproofing agent, and the former also exhibited superior waterproof, oilproof and antifouling properties, prominent solvent resistance, and potential photocatalytic degradation.

**Figure 1.** (**a**) Preparation procedure of (**i**) Rf-OH and (**ii**) FPU; (**b**) 1H-NMR spectrum of Rf-OH in DMSO-d6; (**c**) FTIR spectrum of FPU.
