Synthesis and application of a phosphorus-containing waterborne polyurethane based polymeric dye with excellent flame retardancy

Synthesis and application of a phosphorus-containing waterborne polyurethane based polymeric dye with excellent flame retardancy

Progress in Organic Coatings 140 (2020) 105525 Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier...

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Progress in Organic Coatings 140 (2020) 105525

Contents lists available at ScienceDirect

Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Synthesis and application of a phosphorus-containing waterborne polyurethane based polymeric dye with excellent flame retardancy

T

Jun Zhu, Jie Li, Wenyu Cai, Yunjun Luo* School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

ARTICLE INFO

ABSTRACT

Keywords: Waterborne polyurethane Polymeric dye Flame retardancy Anthraquinone

To endow waterborne polyurethane (WPU) with bright color and improve the flame retardancy, a series of flame-retardant and colorful WPU (DOWPU) was synthesized by incorporating anthraquinone chromophore (DV26) and phosphorus-containing flame-retardant (OP550) into the skeleton simultaneously. The migration property, thermal performance and flame retardancy of DOWPU were systematically characterized by a series of experiments, showing that DOWPU was endowed with bright color and satisfactory flame retardancy. Interestingly, the synergistic effect between DV26 and OP550 made the char yield of DOWPU as high as 25.14 %, which increased the limiting oxygen index by 8.1 %. Furthermore, the migration percentage of DOWPU was only 3.9 %, which was attributed to the strong covalent bonding force between DV26 and polymer chains. Additionally, we coated DOWPU on the polyester fabrics to prove the practicality. The coated fabrics not only had bright colors, but also exhibited excellent color fastness and flame retardancy.

1. Introduction Polymeric dyes were a class of self-coloring polymers containing dye chromophore in their skeleton [1,2]. Thus, polymeric dyes exhibited the characteristics of polymer matrix and dye chromophores, such as the excellent thermal stability and solvent resistance properties of polymers, as well as the optical absorption and chromaticity of dye chromophores [3–5]. More importantly, polymeric dyes could overcome the shortcomings of traditional dyes, such as easy migration, poor heat resistance and low color fastness. In addition, they could simplify the process route, improve the utilization rate and conserve energy [6,7]. Therefore, polymeric dyes were the potential candidates for application in textile, coating, optical materials, printing ink and cosmetics, etc [8,9]. It’s well known that WPU had the advantages of easy modification, low-emissions of VOC (organic solvents) and non-pollution. In addition, the physical and chemical properties of WPU are excellent, such as cohesiveness, abrasion resistance and low temperature resistance, etc. As a result, it had been widely used in adhesives, leather, plastics, textiles and industrial coatings, etc [10–12]. Therefore, with the bright color of small molecular dyes and the excellent properties of WPU, WPU based polymeric dyes (WPUD) had attracted extensive attention of researchers for a long time and was developed rapidly [13,14]. However, WPUD were combustible materials seriously threaten the lives and



property of local people. Recently, more and more researches about WPUD focused on the development of novel polymeric dyes applied in coatings, textiles, intelligent materials, etc., which were prepared by altering chromophores or synthesizing novel micromolecular chromophores such as spiropyran, anthraquinone, astragalus and azobenzene [15–18]. In the past three decades, flame retardancy had become an important development direction of material modification [19,20]. WPUD was an flammable material, but the major application fields had high requirements for flame retardancy [21,22]. However, there was no literature report on the flame-retardant modification of WPUD, which was an urgent problem to be solved at present. Two approaches were generally used to improve the flame retardancy of WPU: chemical reaction (intrinsic flame-retardant) and physical blending [23,24]. In chemical reaction method, the flame-retardant groups were chemically bonded to WPU molecular chain, which could overcome shortcomings derived from physical blending method, such as high percentage of migration and inhomogeneous distribution. Besides, chemical reaction method could enhance the compatibility and stability of WPU, and improve its flame retardancy [25]. In recent years, the chemical reaction method had gradually developed to be the main method of flameretardant modification of WPU [26]. Many studies have demonstrated that WPU modified by halogen-free phosphorus-containing flame-retardant using chemical reaction method had excellent flame-retardant

Corresponding author. E-mail address: [email protected] (Y. Luo).

https://doi.org/10.1016/j.porgcoat.2019.105525 Received 3 October 2019; Received in revised form 29 November 2019; Accepted 21 December 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

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properties [27,28]. Zhang et al. synthesized a phosphorus-containing flame retardant and prepared flame retardant waterborne polyurethanes with LOI of 26.6 % [29]. Celebi et al. modified polyurethane with a reactive phosphorus-containing flame-retardant compound and the LOI of the flame-retardant polyurethane was 27 % [30]. Chen et al. synthesized a nitrogen-phosphorus flame-retardant and prepared a halogen-free flame-retardant flexible polyurethane foams, which had a LOI value of 24.1 % [31]. However, most studies individually concerned with either colorful or flame retardant of WPUs, and no effort was reported to improve the flame retardancy of WPU and endow bright color at the same time. In our group’s previous works, a serious of phosphorous-containing flameretardant was synthesized and used to improve the flame retardancy of WPU [20,23,24,32,33]. Fortunately, these highly effective phosphoruscontaining flame-retardants did not affect the optical properties of the WPU emulsion and film, which ensured that these flame-retardants were suitable for the modification of WPUD. In this paper, we synthesized a kind of WPU with both dyeing and flame-retardant properties for the first time. Firstly, a series of WPUDs were prepared by using DV26 as dye chromophore. After that, we modified WPUDs with phosphorus-containing flame-retardant OP550 for achieving excellent flame retardancy. The structure, stability, thermal migration properties, thermal properties and flame-retardant properties of DOWPU were all investigated. The color properties and flame retardancy of the DOWPU coated polyether fabrics were also analyzed. The results showed that the overall performance of DOWPU was better than traditional WPUD, and DOWPU had better development prospects.

DV26 were prepared and named as DxOyWPU, where x and y referred to the weight percentages of DV26 and OP550 in DOWPU, respectively. At the same time, OWPU/D was prepared as the control experiment by physically blended DV26 with WPU. The component of DOWPUs was shown in Table S1. 2.3. Preparation of DOWPU films DOWPU emulsion (25 g) was poured into a Teflon plate (10 cm*6 cm* 1 cm) and naturally dried at room temperature to form a film. Then, the film was dried in a vacuum oven at 60 °C for 48 h to remove residual moisture. Finally, a dry DOWPU film with thickness of 1.5 mm was obtained. 2.4. Preparation of coated polyester fabrics The preparation of coated polyester fabrics was shown in Scheme 2a. 601H (2.5 wt% of WPU) was added to DOWPUs (100 g) and stirred for 5 min at 1500 rpm. The fabric was fixed by needle plate. Then the homogenized sample was evenly coated on the polyester fabrics by a scraper. The scraping was repeated for three times. Finally, the dyed coated polyester fabrics with flame retardancy could be obtained by dried the fabrics at 170 °C for 60 s. 2.5. Characterization Fourier transform infrared spectroscopy (FTIR). The FTIR spectra of DOWPU films was recorded on Nicolet 8700 Fourier transformed infrared spectroscopy (FTIR, Thermo Nicolet Corporation, Waltham, MA, USA) in the range of 4000−400 cm−1 at a 4.0 cm−1 resolution over 64 scans. Particle size and zeta potential. The average particle size and the zeta potential of the emulsions were measured by a Nanosizer (Malvern zetasizer Nano ZS90, Malvern Instruments Ltd., Malvern, UK). Centrifugal stability. Centrifuge the emulsions at 4000 rpm for 15 min, then observe if there was any sediment at the bottom of the centrifuge tube. If the emulsions did not precipitate, it indicated that the storage stability period of the emulsion is more than 6 months. Thermogravimetric analysis (TGA). The thermogravimetric measurement and char yield of DOWPU films were performed using a thermogravimetric analyser (Switzerland Mettler TGA/DSC differential thermal scanners, Mettler-Toledo International Inc., Zurich, Switzerland). The sample ranging from 5−10 mg in weight was heated from 30 to 600 °C at a heating rate of 10 °C/min and under a nitrogen flow of 20 mL/min. TGA-FTIR analysis. The TGA-FTIR measurements were conducted by a thermal analyser system coupled with an FTIR spectrometer (Mettler Toledo TGA/DSC1-Ncolet 6700 FTIR, Mettler-Toledo International Inc., Zurich, Switzerland). The DOWPU film (5−10 mg) placed in the TGA instrument was heated from 30 to 600 °C at a heating rate of 10 °C/min and under a nitrogen flow of 20 mL/min. The FTIR spectroscope was linked to the TGA instrument to measure the gas products. The gas lines between the TGA instrument and the FTIR instrument were heated to 220 °C. The temperature of the IR cell was 230 °C. The infrared spectra of the gas products were collected at a resolution of 4 cm−1. Limiting oxygen index (LOI). LOI was determined using GBT2406-2 critical digital display oxygen analyser (TESTech Instrument Technologies Co., Ltd., Suzhou, China) according to the combustion performance of the oxygen index standard test GB/T 2406.2. The size of DOWPU films was 150 mm*50 mm*2 mm (length * width) and the size of the coated fabrics was 150mm*50 mm (length * width). Cone calorimeter tests (CCTs). CCTs of DOWPU films were carried out on the cone calorimeter (FTT, US) according to the method described in BS ISO 5660-1-2015. Square specimens of DOWPU films (100*100*2 mm3) were irradiated at a heat flux of 35 kW/m2.

2. Experimental 2.1. Materials Isophorone diisocyanate (IPDI), 2,2-dimethylol propionic acid (DMPA), 1,4-butanediol (BDO), trimethylamine (TEA) and dibutyltin dilaurate (DBTDL) were both purchased from Aladdin (Shanghai) Co., ¯

Ltd. Phosphorus-containing flame-retardant OP550 (Mn = 830) was provided by Clariant Chemicals (China) Ltd. Poly propylene glycol 1000 (PPG1000) was supplied by Dawson International Inc., Buffalo Grove, IL, USA. Disperse Violet 26 (DV26) was purchased from LANXESS Chemical (China) Co., Ltd. Ethylenediamine (EDA) and 2Butanone (MEK) were obtained from Beijing Chemical Works. DMPA, PPG1000 and OP550 were vacuum-dried at 80 °C for 12 h prior to use. TEA and MEK were dried with KOH and CaSO4, respectively, and then distilled. IPDI, DBTDL, EDA and DV26 were used as received. Polyester fabrics and 601H (thickener) were supplied by Zhejiang Transfar Co. Ltd. and were used as received. 2.2. Preparation of DOWPU emulsion The emulsion of DOWPU was synthesized according to Scheme 1. IPDI and PPG1000 were added into a dry four-necked flask equipped with a mechanical stirrer, a thermometer, a reflux condenser and a nitrogen inlet, which was heated up to 85 °C. Three drops of DBTDL was added to catalyse the reaction under N2 atmosphere. The reaction was carried out at 85 °C for 1 h. MEK was added to reduce the viscosity of the mixture. Then, the above mixture was reacted with DMPA for another 3 h. Subsequently, OP550 was added at 70 °C and the reaction was conducted for 2 h. Thereafter, DV26 was poured into the flask and further reacted for another 3 h. The reaction system was cooled to 40 °C, and then TEA was added to react with the carboxyl group of DMPA for 0.5 h. Finally, EDA aqueous solution was added with a high shearing speed (3000 rpm) to emulsify the polyurethane for 30 min. The DOWPU emulsion with solid content of 30 % was obtained after removing MEK via vacuum distillation. DOWPUs with different contents of OP550 and 2

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Scheme 1. Synthesis route of DOWPU emulsion.

Scheme 2. The preparation of coated polyester fabrics (a) and the measurement of migration property.

UV-visible absorption spectroscopy. UV–vis spectra of ethanol solutions of DOWPU films were examined by a UV–vis Spectrophotometer (U-3010, Hitachi Limited, Japan) at room temperature in the

wavelength range of 200–800 nm. The scanning speed was 300 nm/ min. Thermal migration property [34]. The migration property of 3

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DOWPU films was evaluated by the percentage of migration (Mp). The measurement of Mp was shown in Scheme 2b. DOWPU emulsion was evenly coated on the glass sheet and then dried at room temperature for 7 days to obtain a DOWPU film. Subsequently, region a was tightly covered with a glass dish, while region b was left uncovered. The glass sheet was then placed in an oven at 60 °C for 48 h. The films of the regions a and b were respectively configured to the same concentration of ethanol solution, and then the UV–vis spectrum was measured with a photometer. Mp was calculated as follows:

Mp =

Aa

Ab Aa

*100%

Table 1 The particle size, zeta potential and thermal migration property of DOWPU samples.

(1)

where Aa and Ab were the absorbance of the maximum absorption wavelength in region a and b, respectively. Weight increments. The samples with the same size (1 cm*1 cm) was cut from polyester fabrics and fabrics coated with DOWPU, respectively, and then weighted. The weight increments ratio of DOWPU coated fabrics was calculated using formula below:

Weight increments = 1000 × (ma

m 0) × 100%

Sample

Particle size /nm

PDI

Zeta potential /mV

Centrifugal stability

Mp/%

WPU O10WPU/D2 O5WPU O10WPU O15WPU O20WPU D1O10WPU D2O10WPU D3O10WPU D4O10WPU

29.12 84.76 43.29 46.19 54.49 67.31 57.18 60.27 63.57 68.62

0.147 0.224 0.097 0.110 0.095 0.143 0.146 0.201 0.201 0.191

−33.9 −22.7 −44.7 −42.4 −38.6 −39.7 −39.8 −42.0 −42.6 −45.0

No precipitate Precipitate No precipitate No precipitate No precipitate No precipitate No precipitate No precipitate No precipitate No precipitate

— 23.56 — — — — 3.48 3.58 3.72 3.90

the stretching vibration of -O-H, CeO (in PeOeC), and PeO (in PeOeC), respectively [36]. As shown in the FTIR spectra of OWPU, the disappearance of the absorption peak at 3410 cm−1 indicated that OP550 had completely reacted with isocyanate. In addition, the absorption bands intensity of CeO (in PeOeC) and PeO (in PeOeC) enhanced with the increase content of OP550, which proved that OWPU had been successfully prepared. Due to the low content of DV26, no obvious characteristic absorption band could be observed in the FTIR spectra of DOWPU (Fig. 1b). The absorption peak of the benzene ring skeleton of DV26 was observed at 1490 cm−1 in a partially enlarged view of the FTIR spectra of DOWPU (Fig. 1c). The appearance of the absorption peaks of benzene ring confirmed that DV26 had been successfully introduced into polyurethane chain. The 1H NMR spectra of WPU and DOWPU were included in Figs. S1–S3.

(2)

where ma and m0 were the masses of DOWPU coated fabrics and polyester fabrics, respectively. Color property. The color property of coated polyester fabrics was measured by SF-300 colorimeter (Datacolor, USA). A black tube and a whiteboard were used to calibrate the instrument. The resulting color parameters included K/S value, L*, a*, b*, c* and h°, which represented the apparent color yield, brightness, green-red color axis, yellow-blue color axis, saturation and hue respectively. Color fastness. The rubbing fastness of coated polyester fabrics was tested using LFY-304 crocking fastness tester (Wenzhou Textile Research Institute, China). The washing fastness was tested using WF517 washing fastness tester (Wenzhou Textile Research Institute, China). Vertical burning test. Based on GB/T5455-1997, the vertical burning test of the coated fabrics (250*13 mm2) was carried out on a YG(B)815D-1-type instrument (Wenzhou Darong Textile Instrument Co., Ltd, China). The vertical burning test classified the materials as B1 and B2 according to the after flame time, after glow time and the damaged length. B1: damaged length within 15 cm, after flame time within 5 s and after glow time within 5 s. B2: dam-aged length within 20 cm, after flame time within 10 s and after glow time within 10 s.

3.2. Particle size, zeta potential and thermal migration property The particle size, zeta potential and migration property of DOWPU were shown in Table 1. It was observed that the particle size of the emulsions increased as the content of OP550 and DV26 increased. After introducing OP550 into the main chain of WPU, a large number of pendant groups (−OC2H5) of OP550 hindered the internal rotation of the molecular chain and increased the steric hindrance, which made the phase transition more difficult during emulsification [37]. As a result, the dispersion was greatly deteriorated, and the particle size of the emulsion was increased. DV26 was a hydrophobic small molecular organic dye containing rigid benzene group and anthraquinone group. The mechanism of which the rigid DV26 increased the particle size of WPU was similar to OP550. In addition, the strong hydrophobicity of DV26 would make the dispersion worse, and the particle size increased more obviously [38]. The absolute values of the zeta potential of all emulsion samples (except O10WPU/D2) were greater than 30 and no precipitation occurred after centrifugation. According to “the spread of electric double layers” theory, the zeta potential had a relationship with electrophoretic velocity, grain size, and the charge of sliding planes [39]. The

3. Results and discussion 3.1. Structure characterization of DOWPUs by FTIR The FTIR spectra of OP550, DV26, WPU and DOWPU were shown in Fig. 1. The absorption bands at 3330 cm−1, 1720 cm−1 and 1240 cm−1 (Fig. 1a) were assigned to the stretching vibration of NeH, C]O and CeO in the urethane group [35], which illustrated that polyurethane had been successfully synthesized. The absorption bands of OP550 (Fig. 1a) near 3410 cm−1, 1032 cm−1 and 977 cm−1 were ascribed to

Fig. 1. FTIR spectra of DOWPU films (a: OWPU, b: DOWPU, c: partially enlarged view of b). 4

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larger the grain size was, the slower the electrophoresis speed was, and the shorter the zeta potential generated from the positive ions absorbed on the surface of the micelle was. Eventually, the stability of the emulsion would be deteriorated. The larger absolute values of zeta potential meant the thicker the hydrated electric double layers, which exhibited better mechanical and chemical stability. Testing results indicated that the emulsion had good stability. As a control experiment, O10WPU/D2 emulsion exhibited large particle size, small zeta potential absolute value, and precipitated after centrifugation. Since DV26 was not chemically bonded to the molecular chain of WPU, it tended to aggregate together, which caused uneven emulsification and affected the stability of the electric double layer, eventually made the particle size larger and the emulsion unstable. Thermal migration property was one of the most important parameters for evaluating dye performance. Dyes tend to migrate from the interior of fabric to the surface due to their weak binding force, or even sublimate to the surrounding environment. Textiles dyed with conventional dyes would show chromatic aberration resulted from dye migration during processing, use and storage. In addition to affect the aesthetics of the material, chromatic aberration also reduced color fastness and deteriorated the quality. As shown in Table 1, the Mp of O10WPU/D2 was as high as 23.56 %, while that of D2O10WPU samples was 3.58 % (only 15.3 % of O10WPU/D2). The physically blended DV26 in O10WPU/D2 had weak binding force and was easy to aggregate, so it is very easy to migrate, which resulted in a high Mp. DV26 chemically bonded to the molecular chain in DOWPU had strong binding force and its movement was restricted, so it was difficult to migrate, which resulted in very low Mp. Compared with O10WPU/D2, DOWPU had excellent thermal migration property.

linear with the concentration: [41]

A=

c l

(3)

where A was the absorbance, ε was the molar absorption coefficient, l was the path length of the beam of light through the material sample, c was the concentration. The intensity of characteristic absorbance peak at 254 nm was plotted against the concentration and linearly fitted to obtain Fig. 2b. According to formula 3, the slope of the fitted line was the molar absorption coefficient. The related parameters were listed in Table S2. Finally, ε of DOWPU at 254 nm was obtained, which was 2.33*104 L∙mol−1 cm−1. 3.4. Thermal property The thermal properties of the flame-retardant polymeric dyes were performed by TGA and DTG. First of all, the effect of flame-retardant on the thermal properties of WPU was investigated. The test results were presented in Fig. 3 and the detailed values were summarized in Table S3. The char yield of WPU at 500 °C was only 0.9 % and the pyrolysis could be considered as two stages. The first stage was the decomposition of carbamate and allophanate in the hard segment., which occurred from 250 to 350 °C, with the maximum decomposition rate at 329.0 °C. The second stage was the decomposition of soft segments, which occurred from 350 to 420 °C, with the maximum decomposition rate at 370 °C [42],43]. After introducing OP550 into WPU, the char yield was significantly increased (the maximum was 5.3 %) and the decomposition of OWPU increased to three stages, in which the decomposition of the hard segment was shifted to an earlier date and the decomposition of the soft segment was delayed. The first stage in the range of 230−320 °C was assigned to the scission of polyphosphate (OP550) in soft segments. Because the bond energy of the PeO bond (149 kJ/mol) was much lower than the CeO bond (257 kJ/mol), the degradation of organic phosphate occurred prior to the hard segment. The second stage from 320 °C to 360 °C was attributed to the decomposition of carbamate and allophanate. The third stage in the range of 360−430 °C was the decomposition of polyether. The decomposition products of polyphosphate could accelerate the decomposition of hard segment, thereby promoted the dehydrogenation of the hard segment to form carbon layer, which could slow down energy transfer and increase the decomposition temperature of the soft segment. Therefore, with the increase amount of OP550, the maximum decomposition temperature of the hard segment gradually decreased, and the decomposition temperature of the soft segment was delayed, besides, the char yield increased from 2.1 % to 5.3 %. Subsequently, the effect of DV26 on the thermal properties of WPUbased polymeric dyes was investigated (Fig. 4 and Table S3). It was

3.3. UV–vis absorption spectroscopy D2O10WPU film was dissolved in ethanol to prepare different concentrations of ethanol solutions. Then the ultraviolet absorption spectra of ethanol solutions were measured to investigate the relationship between the absorbance and the concentration. Fig. 2a showed the UV–vis spectra of DV26 and D2O10WPU solutions. The absorption peaks at 254 nm and 315 nm were attributed to the n-π* and p–π* electronic transition of benzoquinone [40]. The absorption band at 500−600 nm was caused by π–π* the electronic transition of the acetophenone chromophore and the primary amino group. The intensity of the absorption band at 500−600 nm in DOWPU was lower than that of DV26. The reaction of the primary amino group of DV26 with the NCO group weakened the inductive effect, thereby reduced the mobility of electrons and made electronic transition more difficult, eventually resulted in a decrease in the absorbance of DOWPU. According to Beer–Lambert law, the absorbance of the group was

Fig. 2. UV–vis spectra of ethanol solution with different concentrations (a) and the relationship between absorbance and concentration (b). 5

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Fig. 3. TGA (a) and DTG (b) curves of OWPUs.

found that DV26 only slightly improved the thermal stability and increased char yield. The thermal decomposition of DOWPU was also divided into three stages. As the content of DV26 increased from 1 % to 4 %, T5 %, T1max, T2max, T3max and char yield of DOWPU were all improved to a certain extent. Because of the rigid anthraquinone, DV26 had good heat resistance. It absorbed a lot of heat during the decomposition of DOWPU, thus improved the thermal stability of DOWPU, delayed the maximum thermal decomposition temperature and increased the char yield.

parameters, such as the time to ignition (TTI), peak of heat release rate (PHRR), average of heat release rate (AHRR), total heat release (THR), average of effective heat of combustion (AEHC), average mass loss rate (AMLR), and total smoke production (TSP) were summarized in Table 2 [47]. The TTI of WPU was 16 s whereas that of OWPU was reduced at first and then increased. According to the discussion on the result of TGA, OP550 decomposed and induced the degradation of hard segment, thus the resistance to ignition was weakened. However, the degradation products of polyphosphate contributed to charring earlier during combustion, which hindered the transmission of heat and oxygen and enhanced the resistance to ignition. When the content of OP550 was low, the ignition time decreased as OP550 content increased, because only a thin carbon layer was formed. When OP550 was added sufficient to form a dense carbon layer, the ignition time increased from 13 s to 21 s as the OP550 content increased. When OP550 content was 5 %, the TTI of OWPU was the shortest as 13 s. Owing to the better heat resistance of DV26, the higher the content of DV26 in DOWPU, the better the thermal stability. Therefore, TTI of DOWPU was prolonged as DV26 content increased because of the better heat resistance of DV26, as shown in TGA measurement. The curves of heat release rate (HRR) were shown in Fig. 5. The WPU film burned quickly after ignition and had the highest heat release, with a PHRR of 657.8 kW/m2 and AHRR of 246.5 kW/m2, while its char yield was the lowest (Only 0.5 %). After introducing OP550 and DV26 into the molecular chain, the PHRR and AHRR of the modified polymeric dyes decreased sharply, while the char yield increased significantly. Compared with WPU, the PHRR and AHRR of O10WPU decreased by 28.4 % and 30.0 % respectively, and the char yield increased to 19.54 %. This indicated that the OWPU with OP550 had a flame retardancy mechanism in condensed phase. However, as for D4O10WPU, the PHRR and AHRR decreased by 57.5 % and 43 % respectively, the carbon residue rate increased to 25.14 %. The addition

3.5. Flame-retardant property The flame-retardant properties of DOWPU were determined by LOI and Cone calorimeter tests. The detailed data were listed in Table 2. The LOI value of DOWPU gradually increased with the increase amount of OP550 and DV26. The LOI value of WPU was only 22.1 %, while that of O20WPU was 30.7 %. The increase of LOI was related to the increase of char yield [44]. Even if a small amount of OP550 was added to WPU, the char yield increased greatly, thus the flame-retardant property was obviously improved. When the content of OP550 increased from 0 to 10 %, the LOI of OWPU increased from 22.1 % to 28.9 % owing to the increasing char yield. However, when the content of OP550 increased up to 20 %, the LOI only increased to 30.7 %. This was due to the compact carbon layer of O10WPU. As a result, further increasing the content of OP550, the char yield only increased slightly and the flameretardant property was improved limitedly. As suggested by the result of TGA experiment (Fig. 4), the increased char yield indicated that the flame-retardant performance would be improved. Therefore, the LOI value of DOWPU increased from 28.9 % to 30.2 % when the content of DV26 increased from 0 to 4 %. Cone calorimeter test could simulate the real scale fire and could be considered to be an effective test method [45,46]. The characteristic

Fig. 4. TGA (a) and DTG (b) curves of DOWPUs. 6

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Table 2 The LOI and cone calorimeters data of all samples. Sample

LOI (%)

TTI (s)

PHRR (kW/m2)

AHRR (kW/m2)

THR (MJ/m2)

AEHC (MJ/kg)

AMLR (g/s)

TSP (m2)

Char yield (%)

WPU O5WPU O10WPU O15WPU O20WPU D1O10WPU D2O10WPU D3O10WPU D4O10WPU

22.1 26.2 28.9 30.2 30.7 29.1 29.7 29.9 30.2

16 13 16 18 21 16 19 22 24

657.8 564.6 470.5 396.3 254.7 479.7 410.8 328.6 279.1

246.5 201.6 172.5 158.1 136.4 184.2 167.9 148.5 140.4

62.1 50.1 41.3 37.8 33.5 43.5 39.4 35.8 34.9

33.5 29.6 25.8 24.9 23.6 24.7 23.4 22.3 21.6

0.085 0.073 0.058 0.051 0.046 0.079 0.070 0.063 0.053

3.37 3.18 2.93 2.27 2.19 2.87 2.74 2.61 2.53

0.5 11.43 19.54 21.94 22.72 20.97 22.83 24.85 25.14

of DV26 made DOWPU had higher char yield and better flame retardancy. As suggested by the result of TGA experiment (Fig. 4), DV26 could absorb a lot of heat and increased the carbon residue, and then OP550 promoted the carbon residue form a dense carbon layer, which could greatly improve the flame retardancy of DOWPU. Therefore, the synergistic effect of DV26 and OP550 could greatly increase the char yield of DOWPU, which made the flame retardancy in condensed phase more significant. In addition, with the increase content of OP550 and DV26, the THR, AEHC, AMLR and TSP of the modified WPU were significantly decreased. The less combustion heat generated from the burning of volatile gases was attributed to the flame-retardant quenching effect of the gaseous-phase pyrolysis products of OP550 which further confirmed the gas phase flame retardancy of OP550. These results were in accordance with the results of TGA and LOI. Moreover, the addition of OP550 and DV26 suppressed the formation of smoke, which could reduce the risk of suffocation in the event of a fire. In order to further explore the synergistic effect of OP550 and DV26 on flame retardancy, the residual char after the cone calorimeter test was subjected to SEM test (Fig. 6). The residual char of WPU showed a loose fragmentary structure which was insufficient to act as a protective layer to hinder the transmission of oxygen and heat. As the content of OP550 increased, the residual char of OWPU gradually became a denser, continual and complete char layer, which was strong enough to block the release of combustible volatiles and heat transfer. Therefore, the combustion degree of OWPU was reduced, and it showed better flame retardancy. After adding DV26 to OWPU, the synergistic effect of OP550 and DV26 formed a more compact carbon layer with higher strength which greatly improved the flame retardancy of DOWPU. In summary, DOWPU had excellent flame-retardant properties, which were attributed to the synergistic effect of OP55O and DV26.

FTIR-TGA test was adopted to analyze the gaseous products of WPU, OWPU and DOWPU in the pyrolysis. The FTIR spectra of the gaseous products at different temperatures were presented in Fig. 7 and the 3D TGA-FTIR spectrogram of the gaseous products were shown in Fig. S4. As shown in Fig. 7a, only a little of CO2 (2376 cm−1) was released in the range of 200-250 °C, which was caused by the degradation of DMPA-TEA salt. As the temperature kept rising, gaseous products containing CO2, HCN (2295 cm−1) and NH3 (928 cm−1) released at 250-350 °C due to the decomposition of the carbamate (NHCOO-) groups in hard segment. The decomposition of the soft segment between 350 and 450 °C mainly released low molecular weight aldehydes (1020−1245 cm−1), ketones (1800−1650 cm−1), ethers −1 (2820−3000 cm ), hydrocarbons (−CH2, −CH3, 2850−2990 cm−1) and NOx (N2O, NO, NO2, 1320–1550 cm−1) [48] [49]. Compared with WPU, the FTIR spectra of gaseous products of OWPU (Fig. 7b) and DOWPU (Fig. 7c) did not change significantly, but the whole degradation process completed in a shorter period of time. The degradation products of OP550 could promote the decomposition of the hard segment, so that the gas generated from the degradation of carbamate groups in OWPU and DOWPU appeared at lower temperature. This was in accordance with TGA and the residuals analysis results. Combined the results of TGA, TGA-FTIR, SEM and cone calorimeter test, the thermal degradation mechanism of DOWPU was revealed (Scheme 3). The whole degradation process could be divided into four stages: First stage: Degradation of the neutralization chain segment between 200 and 250 °C. The decomposition products were polyether backbone, triethylamine and CO2. Second stage: Degradation of the OP550 from 250 to 320 °C and the decomposition products were phosphate groups (phosphoric acid and polyphosphoric acid) and CO2. Third stage: Degradation of carbamate at the range of 320–360 °C. The phosphate groups produced in the second stage induced the decomposition of hard segment into isocyanates, CO2, NH3 and HCN. Fourth stage: Degradation of the polyether and DV26 from 360 to 430 °C. The decomposition products were low molecular weight ether, ketone, aldehyde, alkene, CO2 and NOx.

3.6. Flame-retardant and thermal degradation mechanism To further study the thermal degradation mechanism of DOWPU,

Fig. 5. The HRR curves of OWPU (a) and DOWPU (b). 7

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Fig. 6. The SEM images of residual chars after cone calorimeter test.

Fig. 7. The FTIR spectra of gas products of WPU (a), O10WPU (b) and D2O10WPU (c).

3.7. Application in coated polyester fabrics

and b* value. This was because DV26 in the physically blended O10WPU/D2 had weak binding force with the polymer chain, which resulted in a high Mp of O10WPU/D2. DV26 would continue to migrate during the storage and usage, eventually making the color of O10WPU/ D2 coated fabric lighter.

In order to prove the practicality of DOWPU, we prepared the polyester fabrics coated with DOWPU and investigated the color properties, color fastness and flame retardancy. The weight increments of coated fabrics were measured (Table S4) to avoid the effect of the amount of DOWPU coating on the performance. The fabrics coated with DOWPU emulsions had a similar weight increment (about 55 g/m2). Fig. 8 presented the SEM images of coated polyester fabrics. Polyester fabric not coated with DOWPU had a rough surface. However, after the fabric was coated with DOWPU, its surface changed from uneven to smooth. This result indicated that the DOWPU was successfully coated on the surface of fabric and had a good compatibility with polyester fabric.

3.7.2. Color fastness Color fastness was one of most important parameters of coated fabrics. In actual use, the coated fabrics were inevitably washed with water and subjected to friction. If the color fastness was unqualified, the dyes on the fabrics would fall off and cause chromatic aberration, which resulted in poor aesthetics of the fabrics. The color fastness of polyester fabrics coated with DOWPU were listed in Table 4. Due to the strong chemical bonding between DV26 and WPU molecular chain in DOWPU, the shedding of the dye could only occur after the DOWPU layer on the surface of the coated fabrics was destroyed. The content of the DV26 did not change the strength of the chemical bonding, so the color fastness of DOWPU coated fabrics did not change with the content of DV26. What’s more, the DOWPU layer was difficult to be destroyed because it had strong interaction with polyester fabrics. Therefore, the polyester fabrics coated with DOWPU had excellent dry rubbing fastness, wet rubbing fastness and washing color fastness. Moreover, the color fastness of O10WPU/D2 coated fabric was lower

3.7.1. Color properties The photographs of fabrics coated with polymeric dyes and the color parameters measured by colorimeter were listed in Table 3. As the content of DV26 increased, the K/S value, a* and b* of the coated fabrics gradually increased, while the L* decreased progressively [50], which demonstrated that the color of the coated fabrics was deepening gradually. Compared with the fabric coated with D2O10WPU, the fabric coated with O10WPU/D2 showed a lighter color with a lower K/S, a* 8

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Scheme 3. The thermal degradation mechanism of DOWPU.

Fig. 8. The SEM images of polyester fabric (a) and fabric coated with DOWPU (b). 9

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Table 3 The color parameter of coated polyester fabrics. K/S

L*

a*

b*

c*

ho

O10WPU/D2

1.770

58.16

25.87

−8.79

26.98

340.73

D1O10WPU

2.284

56.13

25.97

−8.95

27.46

343.13

D2O10WPU

3.869

48.63

32.55

−10.09

34.08

342.78

D3O10WPU

4.600

42.46

35.74

−7.47

36.74

349.28

D4O10WPU

9.575

37.93

40.18

−8.75

41.12

347.71

Sample

photograph

4. Conclusions

Table 4 The color fastness of polyester fabrics coated with DOWPU. Sample

Dry rubbing fastness

Wet rubbing fastness

Washing color fastness

O10WPU/D2 D1O10WPU D2O10WPU D3O10WPU D4O10WPU

4 4∼5 4∼5 4∼5 4∼5

3∼4 4∼5 4∼5 4∼5 4∼5

3 4 4 4 4

In this work, a series of flame-retardant WPU based polymeric dyes were successfully synthesized. By incorporating anthraquinone chromophore and polyphosphate ester, environmentally friendly WPU was endued with bright color and excellent flame retardancy at the same time. By covalently bonding the anthraquinone dye to the molecular chain, the Mp of D2O10WPU was 19.98 % lower than D2O10WPU/D2. The results of particle size, zeta potential and centrifugal stability revealed that the emulsion of DOWPU had very excellent stability. Then, the thermal and flame retardancy of DOWPU films were characterized by TGA, SEM, LOI, TGA-FTIR and cone calorimeter test. Compared with WPU, the thermal stability of OWPU decreased, but the char yield increased significantly, which greatly improved the flame retardancy. The LOI value of D2O10WPU and O10WPU was as high as 29.7 % and 28.9 % respectively, while WPU was only 22.1 %. The synergy between DV26 and OP550 resulted in a higher char yield, which made DOWPU exhibited better flame retardancy than OWPU. In addition, the thermal degradation mechanism of DOWPU was proposed based on the test results. At last, the polyester fabrics were coated with DOWPUs, and the color properties and flame retardancy were measured. The results showed that the DOWPU coated fabrics not only had bright colors, but also obtained excellent color fastness and flame retardancy. Last but not least, this simple yet efficient approach for preparing flame-retardant polymeric dyes could be extended to other waterborne polyurethane systems.

than that of DOWPU coated fabric. The weak physical interaction between DV26 and molecular chains caused the dyes on O10WPU/D2 coated fabrics fall off easily, which resulted in poor color fastness. 3.7.3. Flame retardancy of coated fabrics Flame retardancy of DOWPU coated polyester fabrics was evaluated by LOI and vertical burning test. The detailed data were shown in Table 5. Polyester fabric was combustible material with a LOI value of 17.4 %. The flame retardancy of DOWPU coated fabrics became better and better with the increased of the content of OP550 and DV26, which was consistent with the flame retardancy of DOWPU films. The flame retardancy of D2O10WPU coated fabrics was better than O10WPU coated fabrics, which was attributed to the synergy between OP550 and DV26 (as suggested by the result of Fig. 7). D2O10WPU could reach B2 classification with a damage length of 12.6 cm and after flame time of 5.0 s. Due to the poor compatibility of DV26 with WPU, DV26 in O10WPU/D2 was easy to agglomerate and migrate, which made the flame retardancy of O10WPU/D2 coated fabrics worse than D2O10WPU coated fabrics. To sum up, the polyester fabrics coated with DOWPU not only had bright colors, but also could obtain excellent flame-retardant properties.

Author statement All authors of this manuscript have directly participated in planning, execution, and analysis of this study. The contents of this manuscript have not been copyrighted or published previously. The contents of this manuscript are not now under consideration for publication elsewhere.

Table 5 The flame retardancy of polyester fabrics coated with DOWPU. Sample

Damaged length/ cm

After flame time/s

After glow time/s

Rating

LOI/%

pure fabric O10WPU/D2 O5WPU O10WPU O15WPU O20WPU D1O10WPU D2O10WPU D3O10WPU D4O10WPU

totally damaged 14.2 23.4 14.9 13.8 13.3 13.7 12.6 11.7 11.5

— 8.0 13.8 8.4 7.9 7.6 6.5 5.0 4.5 4.4

— 0 0 0 0 0 0 0 0 0

— B2 — B2 B2 B2 B2 B1 B1 B1

17.4 23.4 19.0 22.6 23.9 23.7 23.9 25.7 26.8 27.1

Declaration of Competing Interest The authors declare no competing financial interests Acknowledgement The authors are grateful for financial support from National Key Research and Development Program of China (2016YFC0204400). 10

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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2019. 105525.

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