Synthesis and Application of Silane-Modified Castor Oil-Based Waterborne Polyurethane as a Fluorine-Free and Durable Water Repellent

As petroleum resources diminish and public environmental consciousness increases, biobased, fluorine-free waterproof agents have become a research hotspot. However, most reported waterproof agents still suffer from poor water resistance and have the drawback of hardening the fabric’s hand feel. To address these issues, silanated castor oil (SiCO) was synthesized by castor oil and trimethoxysilane and used to prepare a series of biobased waterborne polyurethanes (SiCO-WPU). Then, SiCO-WPU was finished on the polyamide fabric using a pad–dry–cure procedure. The finishing process was optimized by using the Minitab software. The water repellency tests showed that when the SiCO content was 19.92%, the water contact angle of the SiCO-WPU film increased from 74.8° to 95.8°, and the treated polyamide fabric achieved a contact angle of 138.5° along with a water repellency rating of level 4. In addition, the treated fabric exhibited an 11% decrease in the sliding friction coefficient and a 20% enhancement in the crease recovery angle. Even after 20 washes, the treated fabric still maintained good water repellency and improved the smooth and elastic hand feel. This work holds significant importance for the design and application of sustainable, fluorine-free waterproofing agents in the textile industry.