Article pubs.acs.org/IECR
Synthesis of a Waterborne Polyurethane-Fluorinated Emulsion and Its Hydrophobic Properties of Coating Films Jing Zhao,† Tao Zhou,*,† Jihai Zhang,† Hongmei Chen,† Canyao Yuan,‡ Weidong Zhang,‡ and Aiming Zhang*,† †
State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute, Sichuan University, Chengdu, Sichuan 610065, China, and ‡ The Technology Research Center of Polymer Materials Engineering of Tai’an, Longteng Polymer Materials Co., Ltd., Tai’an, Shandong 271000, China ABSTRACT: In this study, a novel waterborne polyurethane-fluorinated acrylic (WFPU) hybrid emulsion was synthesized and introduced. The cross-linker aziridine was used to enhance the film-forming capacity and mechanical property of WFPU. FTIR and XPS results confirmed that WFPU latex was successfully prepared. WFPU has an excellent hydrophobicity, resulting in an obvious increasing of water contact angle from 58.5° to 92.5°. AFM measurements observed that a continuous polymer film was formed after adding the cross-linker, indicating an excellent film-forming capacity of the cross-linked WFPU. The influence of the amount of the cross-linker on the hydrophobicity and mechanical properties was also studied. Cotton coating experiments visually approved that the cotton hydrophobicity is effectively enhanced by WFPU and the cross-linked WFPU. These results indicated that the cross-linked WFPU containing 30 wt % of fluorinated acrylate and 2.93 wt % of cross-linker has a potential as a coating material for water repellent applications due to providing fantastic hydrophobicity, excellent film-forming capacity, and outstanding mechanical properties.
1. INTRODUCTION Waterborne polyurethanes (WPUs) emulsion is a nontoxic material with excellent adhesion, good chemical resistance, outstanding flexibility, and high weather resistance,1−5 and it has been widely applied in the adhesive for fabrics, metals, wood, and coating materials, particularly in the field of coating.5−8 In order to disperse or dissolve in water, most WPUs contain ionic groups in their molecular structure. Unfortunately, dried WPUs films are generally hydrophilic because of the presence of ionic groups, which limits the application of WPUs as hydrophobic materials.9,10 Moreover, the WPUs are deficient in both chemical resistance and mechanical property. This is because the WPUs are linear thermoplastic polymers without cross-linking, which is allowed to dissolve in solvents.11 The hydrophilic segments of WPUs adversely affect the water and soil repellency due to its relatively high surface energy.12 Commonly, strong hydrophobic components (such as fluorinated monomers) can be incorporated into the WPUs structure to compensate for this deficiency. The lower critical surface energy of many polymers containing fluorocarbon side chains is mainly attributed to the higher content of CF3 groups on their surfaces.3,13 Owing to the low polarizable and the strong electronegativity of fluorine atom, fluorinated polymers have many useful and desirable features, such as high thermal, chemical, aging, and weather resistance, unique surface properties, and low surface energy.14−17 Fluorine-containing polymers have been used in widespread applications such as microelectronics, antifogging, and antifouling applications. In particular, fluorinated acrylic can adhere well to various substrates with the acrylic groups.18,19 Therefore, fluorinated polyacrylate (FA) latexes have been widely used as surface © 2014 American Chemical Society
coating for cotton, paper, and leather. The modified waterborne polyurethane-fluorinated acrylic emulsion was expected to combine some virtues of fluorinated acrylic and polyurethane, such as water resistance, excellent mechanical properties, attractive adhesion, and good chemical resistance.20−23 Some studies have reported the preparation and properties of these hybrid materials. Wang et al.5 prepared the waterborne fluorinated polyurethane−acrylate hybrid emulsion (WFPUA) by two steps, including the preparation of the fluorinated alcohol blocked polyurethanes (FBPU) and the free-radical polymerization. The effects of hydrophobic monomers on the surfactivity and the emulsifiability of the FBPU were also reported. Hua et al.24 obtained WFPUA by radical polymerization with polyurethane macromonomer having quaternary ammonium groups. Wang25 investigated a series of waterborne fluorinated polyurethane-acrylate with different FA content from polyester polyol (NJ-330), isophorone diisocyanate (IPDI), dimethylolpropionic acid (DMPA), and different content of hexafluorobutyl acrylate. The film-forming ability is as important as hydrophobicity for WFPU as coating materials. Unfortunately, the above studies only focused on WFPU polymerization methods and some certain factors that affect the synthesis of the polymerization. There was little literature reporting the film-forming capacity which is crucial for WFPU to apply to coating materials. A good film-forming capacity is a key factor for the application of WFPU as a high-performance hydrophobic coating material. Received: Revised: Accepted: Published: 19257
October 14, 2014 November 21, 2014 November 22, 2014 November 23, 2014 dx.doi.org/10.1021/ie5040732 | Ind. Eng. Chem. Res. 2014, 53, 19257−19264
Industrial & Engineering Chemistry Research
Article
Scheme 1. Preparation Process of CCWPU and WFPU
Therefore, the research on the coating ability of WFPU has a practical value. In this study, we prepared the cationic WFPU hybrid emulsion by soap-free emulsion polymerization in a two-step process. In the first step, the end-capped polyurethane with double bonds in aqueous (CCWPU) was prepared. The second step is the polymerization of CCWPU and trifluorethyl methacrylate (TFMEA). Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), water contact angle (WCA), cotton processing effect, particle size distribution (PSD), and mechanical properties were used to characterize their structure and properties. Our study focuses on the hydrophobic and filmforming capacity of WFPU and the cross-linked WFPU. In the present study, the hydrophobic WFPU was successfully prepared, but its film-forming capacity and mechanical strength could not meet the requirement for a coating material. So we introduced a cross-linker into the WFPU hybrid emulsion to expect to gain a high mechanical strength of the hydrophobic coating. In addition, the film-forming ability of the in situ crosslinked WFPU was greatly improved.
obtained from the Beijing Branch of special reagents joint development center. Trifluoroethyl methacrylate (TFMEA) was bought from Harbin Xuejia fluorosilicone Chemical Co., Ltd. Aziridine cross-linker [trimethylolpropane tris(2-methyl-1aziridinepropionate)] was purchased from Shanghai Youen Chemical Co., Ltd. Hydroxyethyl acrylate was produced by Shanghai Gaoqiao Petrochemical Company. Triethylamine (TEA) was obtained from Shanghai Chemical Reagent Company. All other ingredients, such as toluene diisocynate (TDI) and ammonium persulfate (APS), were analytical grade, kindly supplied by Chengdu Kelong Reagent Co., Ltd. 2.2. Sample Preparation. 2.2.1. Preparation of CCWPU and WFPU. The end-capped polyurethane with double bonds in aqueous (shown in Scheme 1) based on TDI, PBA, DMPA, and HEA was prepared with DMPA as a chain extender. The solvent and the catalyst are DMF and DBTDL, respectively. Briefly, a 500 mL four-necked flask with round-bottom was fitted with a mechanical stirrer, condenser, thermometer, and a dropping funnel. TDI, PBA, and DMPA as a solution in DMF (2−5 wt % of PU) were put into a reactor with DBTDL as the catalyst (0.5 wt % of PU). TDI was dropwise added at 40 °C and ended in 30 min. The condensation reaction was carried out at 80 °C for 3 h. Di-n-butylamine titration was used to calibrate the content of −NCO. Then, the temperature was cooled down to 40 °C, and HEA was added sequentially into the reaction vessel until the content of −NCO was unchanged. If the viscosity of the reaction system was too high, acetone could be used. TEA was added to neutralize −COOH and reacted for 30 min. After cooling, polyurethane prepolymer was dispersed into deionized water with a high-speed shearing. Finally, EDA was used as a chain extender, and acetone was removed in a vacuum.
2. EXPERIMENTAL SECTION 2.1. Materials. Poly(1,4-butylene adipate) (PBA, numberaverage molecular weight was 2900 g/mol, Polyurethane Co., Ltd. of Liaoyang) was dried at 120 °C under −0.09 MPa for 3 h. Dimethylformamide (DMF, Tianjin Bodi Chemical Co., Ltd.) and acetone (Shanghai Institute of Organic Chemical Reagents) were used after dipped with 0.3 mm molecular sieves for 7 days. Dimethylol propionic acid (DMPA) was bought from Run Shun Chemical Co. Ethylenediamine (EDA, chain extender) was produced by Chengdu Guanghua Chemical Reagent Company. Dibutyltindilaurate (DBTDL, initiator) was 19258
dx.doi.org/10.1021/ie5040732 | Ind. Eng. Chem. Res. 2014, 53, 19257−19264
Industrial & Engineering Chemistry Research
Article
recorded in the range of 4000−400 cm−1 at a resolution of 4 cm−1 with 32 scans. 2.3.2. X-ray Photoelectron Spectroscopy (XPS). The film surface was analyzed by XPS (XSAM-800 KRATOS). The spectrometer was equipped with a Al Kα achromatic X-ray source (14.86 kV), and a takeoff angle of 90° was used with Xray source. The testing depth was 3−5 nm. The sample for XPS (
