Durable Hydrophobic Cellulose Fabric Finishing By Sol-gel Method Based On Polycarboxlic Acid

Sol-gel technique as a versatile tool to create transparent films on various materials has been applied to the fabrication of superhydrophobic surfaces on cellulose fabrics. Whenever come into use, durability of the material would draw as much attention as its functionality. Therefore, the adhesion of hydrophobic coatings to the fiber surface is crucial for the practicality on textiles. In order to improve the washing durability, silane coupling agents or synthesized silane polymers have been used as cross-linker. However, silane coupling agent will influence the reaction kinetics of the sol-gel polymerization, especially silanes exhibiting bulky organic chains could lead to lower reactivity under both acidic as well as alkaline conditions. Furthermore, water solubility and pH-sensitivity varied greatly with different silane coupling agents, leading to extra requirements of organic solvents and catalysts. Although silane coupling agents are used to improve coating durability on cellulose fabric, the surface of the cellulose fibers is markedly less reactive than that of the inorganic oxides onto which the silanes are usually applied with silane coupling agents. Therefore, alternative approaches in improving washing durability of hydrophobic cellulose fabric finished by sol-gel method are needed.On the basis of our early study, which successfully fabricated superhydrophobic surface on cellulose fabric through sol-gel method, this study aims to improve the washing durability of resulted fabrics. Interaction between silica sol and cellulose fabric was first explored before employing polycarboxylic acids (PAs) to covalently bind both the Si-OH and cellulose-OH so as to improve washing durability of hydrophobic cellulose fabric finished by sol-gel method.The interaction between silica coating and cotton substrate was first investigated to reveal the role of cellulosic hydroxyl groups on coating durability. Cellulose fabrics were acetylated producing fabrics with varied acetyl/hydroxyl group content before being subjected to hydrophobic finishing via sol-gel method. Hydrophobicity and washing durability of these as-finished fabrics were studied. It has been concluded that hydroxyl groups played a twofold role:increasing the hydrophicility of the fabric and strengthen the adhesion of silica sol coating so as to facilitate the durability of this water-repellent finish. The silica coating was the summation of physical and chemical interactions with cellulose fabric, and decreased with the reduction of hydroxyl group content of the fabric. However, the durability of sol-gel-hydrophoic finished cotton fabric was not satisfying without using binding agents, as proved by the coating damage and loss of hydrophobicity after repeated washing.Nine PAs were used to bring in free carboxylic acid groups onto the cotton fabric before carrying out hydrophobic finish via sol-gel method (’PA-cotton route’). The introduced free carboxylic acid groups were to anchor the sol-gel coating onto cellulose fabric so as to strengthen the adhesion of the hydrophobic silica coating. All PAs investigated proved to be effective in improving the washing durability of finished fabrics. Strengthened coating fastness was also confirmed by SEM analysis. XRD analysis showed that neither the modification with PAs nor the sol-gel treatment has changed the crystalline structure of the cotton fabric.The amount of free carboxylic acid groups on the modified cotton fabric increased when PA could esterify cellulose through the formation of cyclic anhydride intermediate. However, the introduced carboxylic acid groups would not come into effect unless there is enough distance between terminal carboxylic acid groups. Besides, more carboxylic acid groups in the PA molecule would largely increase the amount of free carboxylic acid groups on the modified cotton fabric, as proved by sample 1,2,3,4-Butanetetracarboxylic acid (BTCA) which exhibited a high contact angle of 137.6°even after 30 times of washing. Moreover, intermolecular hydroxyl groups in the PAs would also facilitate durability improvement of the resulted fabric, since the increased hydroxyl groups promoted the formation of hydrogen bonds between fabric and the silica coating.Silica sols were prepared using the nine PAs as catalysts, and then applied onto cotton fabrics followed by hydrophobic treatment (’PA-sol route’). All these sols showed great stability that no gelation happened within 80 days. FTIR analysis indicated that PAs played a dual role based on their acidity and multi-functionality. In the stage of sol preparation, the PAs served only as acidic catalysts without changing the structure of resulting silica sol; and the PAs worked as cross-linkers by forming ester bonds simultaneously with cellulose-OH and Si-OH after the PA-sols coated cellulose fabrics were cured. All PAs used in this work have been proved effective in improving washing durability of hydrophobic cellulose fabrics achieved by sol-gel technology. The integrity of the hydrophobic coating was largely preserved after 30 times of washing as proved by SEM analysis.As to the’PA-sol route’, washing durability of finished cotton fabrics would increase with the PA’s capability of forming cyclic anhydride intermediate, and with the number of carboxylic acid groups in the PA molecule. BTCA led to the best durability of hydrophobic cellulose fabric with a water contact angle of 138.6°and spray rating of 80 after 30 times of washing, owing to its largest content of carboxylic acid groups and proper distance in between.Fabrics finished with the’PA-sol route’ showed better hydrophobicity as well as better durability compared with the ones with’PA-cotton route’. Furthermore, the simplified process by’PA-sol route’also decreased fabric yellowing and tensile loss due to the reduction of heating treatment.Four kinds of substrates, i.e. slide, cellulose fabric, HCl-SiO2 sol coated cellulose fabric and BTCA-SiO2 sol coated cellulose fabric were finished with hydrolyzed hydrophobic additive. Hydrophobicity of resulted samples was studied with the Wenzel and Cassie theory. Water contact angle, contact angle hysteresis and shedding angle were compared among these samples. It is suggested the hydrophobic fabric followed Cassie state when water droplet placed on its surface. The hydrophobicity of fabrics finished through sol-gel method surpassed their sol-free counterpart, which implied the advantage of sol-gel process in hydrophobic finishing. In addition, BTCA-sol coating also granted the cellulose fabric with hierarchical rough structure, which dramatically elevated the dynamic hydrophobicity of the finished fabric due to the decreased continuity of three-phase contact line. After finished with hydrophobic additive, the BTCA-sol coated cotton fabric exhibited excellent superhydrophobicity with contact angle of 154.7°and roll-off angle of 5°(droplet volume was 100μL).As a whole, the introduction of PA into cellulose finishing by sol-gel method offered an alternative approach in improving washing durability of hydrophobic cellulose fabric. Particularly, the application of PA catalyzed sols further promoted superhydrophobicity as well as washing durability of finished fabrics, and simultaneously simplified the process. Furthermore, not limited to hydrophobic finish on cotton fabric, this study also gives a hint to various kinds of functional finishing on cellulosic substrates by sol-gel method.