Article pubs.acs.org/IECR
Synthesis and Properties of UV-Curable Polyester-Based Waterborne Polyurethane/Functionalized Silica Composites and Morphology of Their Nanostructured Films Lihui Zhang, Hong Zhang, and Jinshan Guo* Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, 730000 Lanzhou, Gansu, China ABSTRACT: In this study, environmentally friendly UV-curable polyester-based waterborne polyurethane (WPU)/ functionalized silica nanocomposites were successfully prepared. The functionalized silica was combined with WPU by chemical bonds and existed as effective cross-link points to form composites. Because of the existence of organic molecule chains from the functionalized silica, the compatibility of WPU with silica was improved obviously. This makes the composite latex possesses certain stability when the silica content rose up to 17.5 wt %. The composite latex particles kept the structure where the functionalized silica was covered by WPU during the film formation process, which could greatly decrease the aggregation of silica. What’s more, the existence of functionalized silica improved thermo-stability, hardness, and water resistance of composite films effectively and the composite films possessed good transparency.
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INTRODUCTION Green chemistry, an inevitable trend of chemical industry development, is the forward position in the chemistry field in the present world. In the polymer field, reducing the use and consumption of organic solvent in the process of synthesis and production is a good path for environmental protection. For this reason, waterborne polyurethane (WPU) has received much attention due to its zero/low volatile organic compounds (VOCs).1 Furthermore, WPU has some advantages, such as good processability, versatile structure property relationships, and excellent elasticity of its films. However, because most WPUs are linear thermoplastic polymers with hydrophilic groups in the main chain, the mechanical properties, water, and solvent resistance of these systems are lower than those of cross-linked solvent-based PUs, the development of WPUs are blocked. For the purpose of improving the properties of WPU, several methods have been researched, for example hybridizing with organics such as casein2 and starch3 and hybridizing with inorganic oxides such as silica,4−7 titania,8 attapulgite,9,10 and clay.11,12 Of which, nanosilica is expected to offer attractive potential in WPU reinforcement. However, because of the agglomeration of inorganic oxide nanoparticles, the doping level is limited. Moreover, most of the polyurethane/silica nanocomposites were prepared in organic solvents by mixing polyol with colloidal silica sol and then reacting with diisocyanate.13 The WPU/silica nanocomposites which are prepared in water are more meaningful to the environment and would have great potential applications in the future. Cross-linking is the other way which can improve the properties of WPU.14−16 On the basis of the fact that UV-light can effectively initiate photopolymerization to form cross-linked systems, UV-initiated polymerization has attracted much attention. Compared with the other initiation methods, the UV-initiated photopolymerization has many excellent characteristics as follows: the reaction is independent of temperature; moreover, by controlling the irradiation wavelength, the © 2012 American Chemical Society
irradiation time, and the intensity of UV-light, polymerization can be easily attained at a high rate.17 What’s more important is that UV-initiated polymerization is environmentally friendly and energy-efficient, which meets with the requirement of green chemistry. In this way, the greatest challenge with cross-linked polymers, difficult manufacturing, could be solved. In this study, the functionalized silica sol was used to blend with waterborne polyurethane to improve the properties of WPU. This kind of functionalized silica contains a large number of CC double bonds which could participate in UV-initiated radical polymerization. In this way, the functionalized silica could be combined with WPU by chemical bonds to form composites. What’s more, the functionalized silica formed an effective crosslink point, which is conducive to better physical properties. Through application of this method, silica doping gets a considerable level due to the existence of organic molecule chains on the surface of functionalized silica particles. Attributed to the cross-linking and silica doping, hardness, thermo-stability, and water resistance of nanocomposite films are increased obviously.
2. EXPERIMENTAL SECTION 2.1. Materials. γ-Methacryloxypropyltrimethoxysilane (KH570) was purchased from Nanjing, and 2-hydroxy-4-(2hydroxyethoxy)-2-methyl-propiophenone (Irgacure 2959) was from Sigma-Aldrich Co. Dimethylol propionic acid (DMPA) and isophorone diisocyanate (IPDI) were dried and degassed at 50 °C for 24 h in a vacuum oven. Polyester (PET, homemade) was heated to 150 °C for 1.5 h in low vacuum, and dimethylbenzene was dried (anhydrous CaCl2) before use. Hydroxyethyl Received: Revised: Accepted: Published: 8434
January 4, 2012 May 15, 2012 June 5, 2012 June 5, 2012 dx.doi.org/10.1021/ie3000248 | Ind. Eng. Chem. Res. 2012, 51, 8434−8441
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The final composite films were obtained by exposing the latex films in the UV light for 8 min. The films we obtained were transparent. The UV source used is a 1000 W high-pressure mercury lamp (the wavelength is at 254 nm), and the light intensity is 20 mW/cm2. The distance between UV light and surface of film is 30 cm.
methacrylate (HEMA), triethylamine (TEA), and the other reagents were used as received without further purification. 2.2. Preparation of Waterborne Polyurethane (WPU). The polyester was prepared as follows: 1.2 mol adipic acid and 1.6 mol neopentyl glycol were introduced into a 500 mL fournecked flask with a mechanical stirrer, thermometer, condenser, and nitrogen in/outlet, and 8 wt % of dimethylbenzene was added as a solvent. This polyester was prepared by heating the mixture at 120−140 °C for 1 h, at 150−160 °C for 2 h, at 160− 180 °C for 2 h, at 200 °C for 1 h, and at 220 °C for 2 h without using any catalyst. The acid value is 8; and the molecular weight is about 750. The waterborne polyurethane was prepared according to the following procedure. In a 250 mL four-necked flask with a mechanical stirrer, thermometer, condenser, and nitrogen in/ outlet, 24.47 g IPDI, 5.36 g DMPA, 30.00 g polyester, and 20 g acetone were charged into the dried flask. The mixture was uniform after being stirred for 10 min, then 0.05 wt % of T-12 was dropped into as catalyst and reacted at 65 °C for 6 h. During the above process, 10 g acetone was added to adjust the viscosity of the mixture. The isocyanate (NCO) content was monitored during the reaction using the standard dibutylamine backtitration method. Upon reaching the theoretical NCO value, 5.20 g HEMA, 0.6 wt % of the inhibitor hydroquinone, and 20 g acetone were added dropwise and reacted at 55 °C for 2 h. The reaction end point was confirmed by the standard dibutylamine back-titration method. At last, TEA was added and stirred for 0.5 h at 40 °C to neutralize the product (pH = 8). The final solid content of waterborne polyurethane dispersion is 56.5 wt %. 2.3. Preparation of Functionalized Silica. The functionalized silica was prepared as follows: In a 250 mL three-necked flask with a mechanical stirrer, thermometer, and condenser, 40.00 g tetraethoxysilane (TEOS), 80.00 g ethanol, and 20.00 g deionized water were charged into the flask. The mixture was stirred for 5 min, and hydrochloric acid was added to adjust its pH value to 4−5. The hydrolysis reaction reacted for 1.5 h under 40 °C. After that, 40.00 g KH-570 was added dropwise and reacted at 60 °C for 3 h. 2.4. Preparation of WPU/Functionalized Silica Nanocomposites. The following describes a procedure for preparing WPU/functionalized silica nanocomposites: first, 19.18 g of WPU was introduced into 50 g of deionized water by mechanical stirring at room temperature. And then, different quantities of functionalized silica were slowly dropped into under vigorously stirring (The recipes are shown in Table 1). After that, the mixture was stirred for another 30 min. The final products were obtained after removing organic solvents by rotary evaporator under vacuum at 40 °C. 2.5. Film Preparation. Latex films were prepared by spreading the composite latexes (mixed with 3 wt % of Irgacure 2959) on a cleaned glass plate directly and allowed to dry for 24 h at 50 °C in a vacuum oven. The glass plates were washed with nitric acid, rinsed with water, and finally washed with acetone.
3. CHARACTERIZATIONS Fourier transform infrared (FTIR) spectra were conducted on a Bruker 550 FTIR in the range from 4000 to 400 cm−1. The samples were mixed with KBr powder and pressed into pellets. The dynamic light scattering (DLS) (Malvern Zetasizer NanoZS) was used to characterize the size and size distribution of product particles. To minimize the adhesion particle by particle, the latex was diluted with deionized water. The result was obtained by using the apparatus with a laser of 660 nm wavelength at room temperature. The transmission electron microscopy (TEM) micrographs of the WPU/functionalized silica latex particles were performed on a TEM. A certain quantity of final latex was diluted properly by deionized water and stained with 1.0 wt % phosphatotungstic acid (PTA) for 1 min and dried in air before observation. Photos were taken to observe the particles’ structure by JEM-1200 EX/S transmission electron microscope with an accelerating voltage of 200 kV. The morphology of WPU/functionalized silica nanocomposites structured films were observed by scanning electron microscope (SEM (S-4800)). Photos were taken with an accelerating voltage of 50 kV. Contact angles (CA) measurements were performed on a contact angle goniometer by the sessile drop method with a microsyringe at room temperature by averaging three fresh points. UV−visible transmission spectra were conducted on a TU1901 UV−visible spectrometer. Thermogravimetric analysis (TGA) was performed by a Perkin-Elmer TGA-7 system in a nitrogen atmosphere at a heating rate of 10 °C/min from room temperature to 800 °C. Water absorption (W%) was measured by immersing composite films in water, and the water absorption was calculated by the following formula: m − mb W% = a × 100% mb “mb” is the mass of the dry film; “ma” is the mass of the film after being immersed-in-water over a specified time interval. Pencil hardness, shock resistance, and adhesion of the films were measured by using a QHQ hardness tester, Paint film impact tester, and Paint film scriber, respectively.
4. RESULTS AND DISCUSSION 4.1. Infrared Analysis. Figure 1 is the FTIR spectra of functionalized silica which were unaffected by UV irradiation (a) and after irradiated by UV irradiation (b). In both a and b, the characteristic stretching peaks of CH3 and CH2 occur at 2959 and 2891 cm−1, respectively. The peak of CO at 1720 cm−1 is attributed to the carboxyl from KH-570. For functionalized silica, symmetric and asymmetric stretching vibration of Si−O−Si bonds could be detected at 808 and 1055 cm−1, respectively. Furthermore, the absorption peak of Si−OH bond overlaps with the peak of −OH (about 3450 cm−1). In the spectrum of functionalized silica which was unaffected by UV irradiation (a),
Table 1. Compositions of WPU/Functionalized Silica Nanocomposites designation WPU (g) SiO2 H2O (g) Irgacure 2959
Si-0%
Si-5%
Si-10%
Si-15%
Si-17.5%
Si-20%
19.18 0 wt % 50.00 3 wt %
19.18 5 wt % 50.00 3 wt %
19.18 10 wt % 50.00 3 wt %
19.18 15 wt % 50.00 3 wt %
19.18 17.5 wt % 50.00 3 wt %
19.18 20 wt % 50.00 3 wt %
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Scheme 1. Flowchart of the Preparation of Polyester-Based WPU/Functionalized Silica Composite Latex
the peak at 1637 cm−1 is the stretching peaks of CC, and the peak at 3099 cm−1 is the stretching peak of CH from CH2C. However, in spectrum b, there is no same peak occurring at 3099 cm−1, and the peak at 1637 cm−1 was greatly diminished. Thus, FTIR spectra revealed that the CC double bonds from the functionalized silica can participate in radical polymerization which was initiated by UV light.
Figure 2 is the FTIR spectra of pure WPU which were unaffected by UV irradiation (a) and after irradiated by UV irradiation (b). The absorption peaks at 2966 and 2889 cm−1 are attributed to CH3 and CH2. The peak that occurred at 1731 cm−1 is the stretching peak of CO, and the strong peak at 1248 cm−1 is the stretching peak of CO. Both of them are the characteristic peaks of PET. Additionally, the peak at 1528 cm−1 is the deformation vibration of NH. The peak at 2720 8436
dx.doi.org/10.1021/ie3000248 | Ind. Eng. Chem. Res. 2012, 51, 8434−8441
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Figure 3. FTIR spectra of pure WPU (a) and Si-17.5% nanocomposite (b) (both were exposed to UV irradiation).
Figure 1. FTIR spectra of functionalized silica (a) unaffected by UV irradiation and (b) after irradiated by UV irradiation.
4.2. Micromorphology of Different Silica-Containing WPU Latex Nanoparticles. Figure 4 shows the particle size and
Figure 2. FTIR spectra of pure WPU (a) unaffected by UV irradiation and (b) after exposure to UV irradiation. Figure 4. Particle size and size distribution of different silica-containing latexes.
cm−1 is from the quaternary ammonium salt. The strong peak at about 3400 cm−1 is the stretching peaks of NH and SiOH. From the spectrum of pure WPU which was unaffected by UV irradiation (a), there is an obvious absorption peak at 1632 cm−1. But there is no obvious peak at same waveband from the spectrum of WPU which was irradiated by UV irradiation (b). Thus, FTIR spectra revealed that the CC groups have been introduced into WPU and the CC double bonds can also participate in radical polymerization which was initiated by UV light. From Figure 3, the characteristic absorption peaks of CH3, CH2, CO, CO, NH, and quaternary ammonium salt could been found at 2957, 2889, 1731, 1248, 1528, and about 2700 cm−1, respectively. Both in a and b, there is no obvious peak of CC at about 1630 cm−1. This result showed that the CC double bonds were greatly diminished after being exposed to UV irradiation. In addition, in the spectrum of Si-17.5% nanocomposite (b), the characteristic absorption peaks occuring at 1068 and 810 cm−1 are the symmetric and asymmetric stretching vibration of SiOSi. Thus, FTIR spectra revealed that functionalized silica could be introduced into WPU as desired through this method and the silica-containing composite could be cured by UV light.
size distribution of WPU and WPU/functionalized silica latexes which were tested by dynamic light scattering (DLS). From Figure 4, we can observe that the average particle size of pure WPU is 74 nm, and the average particle sizes of silica-containing latexes are 90, 132, 279, and 489 nm of Si-5%, Si-10%, Si-15%, and Si-17.5%, respectively. Therefore, with the increasing of functionalized silica content, the particle size increased and the granule diameter distribution widened. The structure of Si-10% (a) and Si-17.5% (b) latex nanoparticles could be proved by TEM (Figure 5). From the TEM micrographs, particle sizes increased with the increasing of silica content. Because of the dehydration, the particle size which was observed from the TEM micrographs is smaller than the results obtained from the DLS. From the TEM micrograph of Si10% latex, the particles have all the shades of color, and the bigger the size of the particle, the darker the color of the particle. However, the color of all the particles of Si-17.5% latex looks darker, and there are some irregular and dark regions in the particles. This phenomenon could be explained in the following way: the more functionalized silica the latex contains, the bigger and 8437
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Scheme 2. Flowchart of the Aggregation Process of Latex
Figure 5. TEM micrographs of Si-10% (a) and Si-17.5% (b) latex particles.
the darker the particle. Furthermore, in the Si-10% latex, some of the particles cannot be filled with enough functionalized silica. The particles could be full of functionalized silica in the Si-17.5% latex, and excess silica causes the aggregation of functionalized silica in the particles. More importantly, this kind of structure, in which the silica was covered by WPU, can effectively suppress the aggregation of silica and improve the stability of latexes. 4.3. Stability of Different Silica-Containing WPU Latexes. Table 2 shows the stability of different silica-containing Table 2. Stability of Different Silica-Containing WPU Latexes designation
Si-0%
Si-5%
Si-10%
Si-15%
Si17.5%
Si20%
stable time
>45 days
>45 days
>45 days
25 days
20 days
10 wt %), the surface of composite films are granular, which would cause the light scattering if the particle size is big enough (about 300 nm of Si-15% and 500 nm of Si-17.5%). This is another reason leading to the reduction of composite films’ transmittance (This inference could be confirmed by SEM). 4.6. TGA Analysis of Different Silica-Containing WPU Composites. Figure 9 shows the TGA curves of pure WPU and
Figure 7. CA pictures of pure WPU and different silica-containing WPU films.
Table 3. Contact Angle of Pure WPU and Different SilicaContaining WPU Films designation
Si-0%
Si-5%
Si-10%
Si-15%
Si-17.5%
contact angle
75.9°
76.1°
75.9°
76.0°
76.2°
films prepared by our method are different from other similar research works.18−20 This phenomenon can be explained through the core−shell structure of nanocomposite particles (This inference can be supported by the TEM micrographs). The functionalized silica was encased in the WPU and the diffusion of Si atoms was restricted during the film forming process due to the interaction between WPU molecule and organic molecule from functionalized silica. So the existence of functionalized silica almost has no influence on surface hydrophobicity of composite films which were prepared by our method. 4.5. UV−Vis Spectra of Composite Films. Figure 8 shows the transmittance of WPU film and WPU composite films with
Figure 9. TGA curves of pure WPU and WPU composites with different silica content.
WPU composites with different silica content. In general, the TGA curve of pure WPU has two weight loss stages and composites exhibit three distinct weight loss stages. The first weight loss stage, in the temperature range of 25−250 °C, is due to the vaporization of residual water and loss of oligomer existing in WPU and its composites. The second weight loss step, from 250 to 350 °C, is due to the decomposition of polyurethane. And the third stage, exhibited in the range of 350−450 °C, which only occurred in the curves of composites attributed to the decomposition of organic molecular chains from functionalized silica and remnant polyurethane. From Table 4, the weight losses of the second Td have few differences among pure WPU and different silica-containing composites. However, with the Table 4. Thermal Properties of Pure WPU and WPU Composites with Different Silica Content Figure 8. Transmittance variation of composite films.
properties
different functionalized silica content, and the thickness of films are about 300 μm. In the ultraviolet band, the existence of functionalized silica has no effect on transmittance of the films. In the wavelength range of 300−800 nm, when the functionalized silica content was lower than 10%, the transmittance of composite films is higher than the pure one. While functionalized silica content was over 10 wt %, the transmittance of composite films is much reduced (compared with the pure one). The transmittance at 800 nm gradually increases from 86.9% of pure
designation
second Td weight lossa (%)
third Td weight lossa (%)
final weight loss (%)
T10b (°C)
T50b (°C)
Si-0% Si-5% Si-10% Si-15% Si-17.5%
6.9 6.4 6.0 6.2 6.2
50.1 40.4 39.3 34.0
97.4 90.0 81.9 82.4 76.1
259.7 265.3 272.1 278.7 290.1
356.5 381.8 401.5 406.8 421.7
a
The weight losses of second and third decomposition temperature (Td). bThe temperature when the weight losses are 10% and 50%.
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dx.doi.org/10.1021/ie3000248 | Ind. Eng. Chem. Res. 2012, 51, 8434−8441
Industrial & Engineering Chemistry Research
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increasing of silica content, the weight losses of the third Td obviously reduce from 50.1% of Si-5% to 34.0% of Si-17.5%, and the final weight loss also remarkably decreased from 97.4% of Si0% to 76.1% of Si-17.5%. What’s more, the temperature when the weight losses are 10% and 50% showed that the decomposition of composites shift toward the higher temperature range than that of the pure one, and the more functionalized silica contain, the better thermal stability could be obtained. Addition of functionalized silica increases the thermal stability of composites; this can be explained that the existence of inorganic oxide could make the composites possess the high temperature stability. In addition, the functionalized silica has a good deal of CC double bonds, and could be polymerized with WPU by radical polymerization as a cross-linkers, and the higher cross-linking density causes to the higher thermal stability. 4.7. Water Absorption of Different Silica-Containing WPU Films. The influence of functionalized silica content on water absorption of silica contain composites is shown in Figure 10. Generally speaking, the water absorption of WPU/function-
Table 5. Influence of Silica Content on the Mechanical Properties of WPU Composite Films designation
pencil hardness
shock resistance(kg·cm)
adhesion (grade)
Si-0% Si-5% Si-10% Si-15% Si-17.5%
H 2H 3H 4H 5H
>50 15 10 10
