Waterborne UV-curable polyurethane acrylate/silica nanocomposites for thermochromic coatings

Abstract

Waterborne UV-curable polyurethane acrylate/silica (PUA/SiO₂) nanocomposites were prepared by a sol–gel method. SiO₂ nanoparticles were modified by γ-methacryloxypropyltrimethoxysilane (KH-570) and then introduced to the ends of the PUA main chains through radical polymerization. According to the transmission electron microscopy (TEM) results, the size of the PUA/SiO₂ nanocomposite particles was approximately 70–100 nm. It was easier to get a uniform emulsion by the sol–gel method than by the physical blending method. Surface tension and contact angle tests both demonstrated the good wettability of the nanocomposites. Besides, the kinetics of the curing process of the PUA/SiO₂ films were analyzed by ATR-FTIR and gel content. This revealed that the modified SiO₂ could accelerate the curing speed of PUA coatings. Scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA) indicated that the nanosilica were well dispersed in the PUA matrix and the soft and hard segments of PUA/SiO₂ were well phase mixed. Furthermore, the nanocomposite films displayed enhanced rigidity, hardness, abrasion resistance and good weather resistance. Finally, waterborne UV-curable PUA/SiO₂ nanocomposites were applied to thermochromic coatings, showing excellent temperature sensitivity and reversibility.

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1. Introduction

Thermochromic coating, as an intelligent material, has been studied and widely used in the fields of high temperature alarms, temperature sensors, anti-counterfeiting marks, etc.^1,2 However, most solvent-based thermochromic coatings are harmful to human health and the living environment. Growing attention to the environmentally friendly coating industry has led to the development of waterborne UV-curable coatings that can be cured at ambient temperature by short exposure to UV light. Waterborne UV-curable thermochromic coatings will be a new development direction for intelligent coatings.

Among the film formers of waterborne UV-curable coatings,^3,4 polyurethane acrylate (PUA) has received increasing interest due to its excellent comprehensive performance, such as its versatility, environmental friendliness, nice mechanical performance and toughness as well as its good resistance to chemicals and solvents.^5–9 However, there are some limits to its applications in terms of weather resistance, hardness and abrasion resistance. In recent years, organic–inorganic hybrids have attracted considerable attention due to their enhanced coating properties, such as improved abrasion resistance, thermal stability, etc.^10–14 On the basis of the previous studies, the shortcomings of PUA can be improved by adding reinforcement such as inorganic oxides,^15–17 clay^18,19 and fiberglass²⁰ to the polymer matrix, under the proper conditions. Currently, research about waterborne UV-curable PUA/SiO₂ hybrids is becoming more and more popular. As the diisocyanate monomer is more sensitive to water, the hybrids are usually prepared by a physical blending method²¹ or a sol–gel method^22–25 instead of in situ polymerization.^26,27 However, SiO₂ is easy to aggregate in the PUA matrix by physical blending. When the sol–gel method is used, the precursor is first dissolved in the PUA dispersion, and then hydrolyzed under the catalytic action of an acid, alkali or some salt, forming nanoparticles uniformly dispersed in the polymer matrix. But during the UV curing process, these catalytic substances may influence the curing effect.

In this study, waterborne UV-curable PUA/SiO₂ nanocomposites incorporating nanosilica obtained from colloidal silica as reinforcement were prepared with a new form of sol–gel method. With this method, the nanosilica connected PUA chains with chemical bonds by a crosslinking reaction (gel process) between C=C bonds. These double bonds on nanosilica particles were achieved by the hydrolysis and condensation of γ-methacryloxypropyltrimethoxysilane (KH-570). The morphology, kinetics of the curing process, dynamic mechanical properties, optical properties, hardness and abrasion resistance of the PUA/SiO₂ hybrid films as well as the temperature sensitivity of the thermochromic coating were studied. To the best of our knowledge, this is the first time that PUA/SiO₂ nanocomposites have been successfully applied to thermochromic coatings and the coating showed good temperature sensitivity and reversibility.

2. Experimental

2.1. Materials

Polyethylene glycol (PEG, M_n = 400) was distilled at 110 °C under −0.1 MPa for 4 h before use. 2,2-Dimethylol propionic acid (DMPA, Aladdin Reagents Co., LTD) was dried at 80 °C for 3 h in a vacuum oven. Acetone was dried over 4 Å molecular sieves before use. 2,4-Diisocyanatotoluene (TDI), N-methyl-2-pyrrolidone (NMP, as the solvent), hydroxyethyl acrylate (HEA), triethylamine (TEA), dibutyltin dilaurate (DBTDL, as the catalyst), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methyl-propiophe (photoinitiator 2959, TCI Shanghai), HgI₂, KI and AgNO₃ were used as received. Colloidal silica (solid content: 30%, particle size: 15 nm) and γ-methacryloxypropyltrimethoxysilane (KH-570) were obtained from WD SILICONE CO., LTD. All reagents were from Sinopharm Chemical Reagent Co., Ltd., unless otherwise noted.

2.2. Preparation of waterborne UV-curable PUA/SiO₂ nanocomposites

Synthesis of waterborne UV-curable PUA was carried out in a three-step process and the network structure of the PUA/SiO₂ hybrid was formed by a sol–gel method (shown in Fig. 1). PEG (9.84 g), DBTDL (1 drop) and TDI (9.42 g) were introduced into a four-neck flask fitted with a mechanical stirrer, thermometer and reflux condenser. Specifically, TDI was added dropwise at a constant flow rate to avoid imploding and agglomeration. The prepolymerization of polyurethane was kept at 60 °C under a nitrogen atmosphere for 2 h. Next, DMPA (1.65 g) dissolved in NMP (5 g) was added and reacted at 75 °C for 1.5 h. Subsequently, HEA (2.85 g) was added into the flask. The mixture was heated to 80 °C for 1.5 h, and then waterborne UV-curable PUA was acquired. At that moment, the NCO content that was tested by the standard dibutylamine back-titration method was close to zero. After cooling to room temperature, TEA (1.5 g) was rapidly added to neutralize the carboxyl of the PUA chain. Distilled water (30 ml) was added dropwise at a constant flow rate under high speed mechanical stirring to make the PUA prepolymers fully emulsified. And 30 min later, a stable PUA emulsion with a solid content of 36.6% was obtained. Afterwards, PUA/SiO₂ nanocomposites were formulated by the addition of colloidal silica and KH-570 successively to the PUA emulsion at the mass ratio of 10 : 3. The sol–gel process started at this stage and the system showed a semi-crosslinked (semi-IPN gel) state.

Fig. 1 Synthesis scheme to prepare PUA and PUA/SiO₂ nanocomposite.

Then, the PUA/SiO₂ emulsion was mixed with 2 wt% photoinitiator 2959 and cast onto a Teflon mould or a glass plate at room temperature, followed by drying the coating at 105 °C for 0.5 h. Finally, the coating was irradiated using a full-automatic UV solidifying machine at room temperature, then demoulded for properties measurement.

2.3. Preparation of thermochromic coating

The thermochromic coating was prepared by mixing PUA/SiO₂ binders, photoinitiator 2959, HgI₂, KI, AgNO₃ and deionized water. The synthesis reaction equations²⁸ of thermochromic pigments are as follows:

2.4. Characterizations

A Fourier transform infrared spectroscopy (FTIR) spectrometer was used to characterize the chemical structure of PUA and PUA/SiO₂. Samples for infrared analysis were prepared by drying the emulsion on a KBr wafer, which was analyzed from 4000 cm⁻¹ to 400 cm⁻¹. For cured coatings, an affix of attenuated total reflectance (ATR) was necessary. The morphology of PUA/SiO₂ nanocomposite emulsion particles was examined by transmission electron microscopy (TEM). The testing was performed on a JEM-2100 (HR) operating at 200 kV. The wettability of the PUA/SiO₂ dispersion was tested by a surface tension meter and a contact angle meter.

The surface morphology of the hybrid films was investigated with a field emission scanning electron microscope (Quanta200, FEI Corp., Holland). Dynamic mechanical analysis (DMA) of the PUA/SiO₂ films was carried out using a dynamic mechanical analyzer (Diamond DMA, PerkinElmer Instruments, Shanghai) under a dry nitrogen atmosphere, at a heating rate of 3 °C min⁻¹ from −80 to 150 °C and a frequency of 1 Hz. The kinetics of the curing process were studied by ATR-FTIR and a gel content test.²⁹ The gel content was calculated by the following formula:

where “m₀” is the mass of the cured coating and “m₁” is the mass of the cured coating after extracting for 48 h in a soxhlet extractor with acetone.

Optical properties were characterized by UV-visible spectra in the 325–1000 nm region. Pencil hardness of the coating was measured according to GB/T 6739-2006 by a QHQ hardness tester. A paint film scriber was used to test the adhesion to the substrate. Abrasion resistance was studied through the rotating abrasive rubber wheel method and calculated as the loss in mass after 500 abrasion cycles with a loading of 500 g. The temperature sensitivity of the thermochromic coating was detected by observing the color changes of the coating in warm water (T = 40 °C) and room temperature water (T = 20 °C).

3. Results and discussion

3.1. Infrared analysis

The FTIR spectra of PUA and PUA/SiO₂ nanocomposites are shown in Fig. 2. From the spectrum (a) of the PUA emulsion, there was no absorption peak at 2273 cm⁻¹, suggesting that all –NCO were consumed. The peaks at 3440 cm⁻¹ and 1723 cm⁻¹ were respectively ascribed to the stretching vibration peaks of N–H and C=O. The characteristic peaks of CH₃, CH₂, C=C, N–H, and C–O could be found at 2947 cm⁻¹, 2871 cm⁻¹, 1635 cm⁻¹, 1531 cm⁻¹ and 1224 cm⁻¹, respectively. But the characteristic absorption of C=C at 1635 cm⁻¹ disappeared from the spectrum (b) of PUA cured film. Therefore, this revealed that PUA prepolymers had been synthesized and C=C completely participated in radical polymerization after being exposed to UV irradiation. From the spectrum (c) of the cured hybrid film with 4 wt% SiO₂, the peaks at 1105 cm⁻¹ and 810 cm⁻¹, which are the characteristic absorptions of the asymmetric and symmetric stretching vibrations of Si–O–Si, became stronger. Moreover, a new peak appeared at 471 cm⁻¹ which was attributed to the blending vibration absorption of Si–O–Si. It indicated that SiO₂ could be introduced into PUA to form a network structure with the help of KH-570 as desired through this method.

Fig. 2 FTIR spectra of PUA and hybrid film: (a) PUA emulsion, (b) PUA cured film, (c) PUA/SiO₂ cured film with 4 wt% SiO₂.

3.2. Morphology analysis

The morphology of PUA and the PUA/SiO₂ nanocomposite was observed by TEM (shown in Fig. 3). The micrograph showed that the PUA particles were well dispersed and the average particle size was approximately 50 nm (Fig. 3a). When colloidal silica and KH-570 were introduced into the PUA matrix, SiO₂ particles modified by KH-570 dispersed uniformly on the surface of PUA and the composite nanoparticle sizes became larger (Fig. 3c and d). When the SiO₂ content rose from 4 to 6 wt%, the average size of the composite particles increased from 70 to 100 nm, since the increase in KH-570 led to a more hydrophobic structure and higher cross-linking density in the macromolecular chains. When only colloidal silica was added into the PUA matrix by the physical blending method, SiO₂ nanoparticles with hydroxyl groups enclosed the polymer and showed a tendency to aggregate together (Fig. 3b). The results implied that it was easier to get a uniform composite emulsion by the sol–gel method than the physical blending method, which would be of great importance in practical applications.

Fig. 3 TEM micrographs of PUA and PUA/SiO₂ hybrid particles: (a) pure PUA, (b) PUA with 4 wt% unmodified SiO₂, (c) PUA with 4 wt% modified SiO₂ and (d) PUA with 6 wt% modified SiO₂.

SEM (shown in Fig. 4) was used to evaluate the dispersion of SiO₂ nanoparticles in the PUA matrix. A smooth PUA film was observed in Fig. 4a. In the cases of 2 and 4 wt% SiO₂ content, the SiO₂ nanoparticles were well dispersed in the matrix, implying strong interactions between PUA and SiO₂ nanoparticles (Fig. 4b and c). But for the hybrid films with 6 wt% SiO₂ content, agglomeration phenomena appear (Fig. 4d), which arise from the aggregation among free SiO₂ particles.

Fig. 4 SEM images of the hybrid films: (a) pure PUA, (b) PUA with 2 wt% modified SiO₂, (c) PUA with 4 wt% modified SiO₂ and (d) PUA with 6 wt% modified SiO₂.

3.3. Wettability of PUA/SiO₂ composites

The wettability of a polymer is usually measured by surface tension (σ) or contact angle (θ). The smaller σ or θ is, the better the wettability of the polymer. The surface tension curve and contact angle images of PUA/SiO₂ nanocomposites are shown in Fig. 5. These show that σ decreased first, then became basically flat and finally rose with the increase of SiO₂ content. The nanocomposites with 2 and 4 wt% SiO₂ content had better wettability than pure PUA. The contact angle showed the same change and confirmed the phenomenon. The reason for this phenomenon might be that σ is related to the volume and quantity of polar groups in polymer chains.⁶ The addition of SiO₂ led to more polar groups which made σ smaller. But when the SiO₂ content reached 6 wt%, σ increased due to SiO₂ coagulation (as shown by SEM).

Fig. 5 Surface tension (σ) curve and contact angle (θ) images of PUA/SiO₂ nanocomposites with different SiO₂ content.

3.4. Curing kinetics analysis

The kinetics of the curing process were analyzed by ATR-FTIR (Fig. 6a) of the PUA film and gel content (Fig. 6b) of the hybrid coatings at different curing times. The film thickness was 1 mm. As shown in Fig. 6a, the absorption area of C=C bonds decreased with the extension of curing time. Also, the gel content of all the cured coatings rose rapidly at first, and then slowly approached a constant value with the extension of curing time (Fig. 6b). These measurements both proved that C=C bonds participated in radical polymerization by UV radiation constantly until they were almost used up. In addition, the curing speed and gel content of the hybrid films increased with the increasing of SiO₂ content. The maximum gel content could reach 98%. When the SiO₂ content was increased to 6 wt%, the curing speed slightly declined and its final gel content was close to that of the hybrid film with 4 wt% SiO₂. This phenomenon might be explained as follows, with the increase of SiO₂ content, the larger amount of C=C bonds led to a faster curing speed and greater cross-linking density, resulting in the increase in gel content. When the SiO₂ content reached 6 wt%, excessive free silica had a shielding effect for UV light, which might slow down the curing speed to some degree. Thus we might conclude that a moderate amount of modified nanosilica could improve the curing speed.

Fig. 6 The ATR-FTIR (a) of the PUA film and gel content (b) of the hybrid coatings at different curing times.

3.5. Dynamic mechanical analysis

It could be observed that the storage modulus (E′) of the PUA/SiO₂ nanocomposite films was higher than that of the pure PUA film in Fig. 7. And, the storage modulus rose as the amount of SiO₂ increased. The reason for this might be that the incorporation of modified SiO₂ nanoparticles led to an increase of cross-linking density, which strengthened the rigidity of the hybrid coatings.

Fig. 7 Storage modulus E′ versus temperature for the PUA/SiO₂ nanocomposite films.

Fig. 8 depicts the tan δ curve for the PUA and PUA/SiO₂ nanocomposite films. The nanocomposite films with 2 wt% and 4 wt% SiO₂ showed a single tan δ peak as pure PUA, which implied that the soft segments and hard segments of the nanocomposite were well phase mixed. This was probably due to the excellent interaction between the hard and soft segments of PUA and SiO₂. For the nanocomposite film with 6 wt% SiO₂, two damping peaks appeared. The two peaks could be ascribed to the glass transition temperature (T_g) of the soft and hard segments of PUA and free SiO₂ particles (as shown by SEM). Furthermore, the tan δ of the PUA/SiO₂ nanocomposite films was shifted to a slightly higher temperature. This was because the presence of SiO₂ confined the mobility of the PUA chains. And, the peak value of tan δ was found to decrease with an increased amount of SiO₂, which was due to the interfacial interaction between the PUA and SiO₂ nanoparticles. In conclusion, PUA and SiO₂ were well phase mixed and the incorporation of SiO₂ increased the T_g and the rigidity of PUA.

Fig. 8 Isochronal temperature dependence of tan δ for the PUA/SiO₂ nanocomposite films.

3.6. Optical properties

Fig. 9 illustrates that the transmittance of the cured films (thickness: 300 μm) decreased with increasing SiO₂. Especially in the UV light region (below 400 nm), the transmittance dropped to 15%. This was because the surface roughness of the films increased with the increase of SiO₂ content, which led to higher light scattering and lower transmittance. In addition, SiO₂ nanoparticles had strong UV absorption and showed a good UV shielding effect. Therefore, SiO₂ could improve the weather resistance of the PUA film, which might provide theoretical basis for the application of PUA/SiO₂ hybrid films in the fields of painting or ink.

Fig. 9 UV-vis spectra of PUA and PUA/SiO₂ films with the thickness of 300 μm.

3.7. Surface properties

Table 1 lists the surface properties of PUA and PUA/SiO₂ films, such as pencil hardness, abrasion resistance and adhesion. With the increasing of SiO₂ content, the pencil hardness rose from HB for pure PUA films to 4H for PUA/SiO₂ films with 6 wt% SiO₂, the abrasion resistance became better and the adhesion was slightly reduced from grade 0 to 1. That might be because with the increasing of SiO₂ content, both the inorganic component and cross-linking density increased which improved the hardness and abrasion resistance. However, when SiO₂ was excessive, the volume shrinkage of cured films was reduced, which led to the adhesion being slightly reduced.²⁴

Table 1 Influence of SiO₂ content on surface properties of PUA/SiO₂ films

Sample	Pencil hardness	Abrasion resistance (500 g, 500 r)	Adhesion (grade)
PUA	HB	0.04	0
PUA-2%	2H	0.03	0
PUA-4%	3H	0.02	0
PUA-6%	4H	0.01	1

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3.8. Properties of the thermochromic coatings

3.8.1 Temperature sensitivity. When the thermochromic coating was put in warm water at a temperature of 40 °C, it rapidly turned from yellow to orange, then returned to yellow immediately when cooled to room temperature (shown in Fig. 10). This phenomenon could be explained as follows. At room temperature, the Ag₂HgI₄ (pigment) showed a yellow colour with a quartet sphalerite structure. However, when the temperature increased to 40 °C, Ag₂HgI₄ showed orange colour with cubic crystal structure. Therefore, the thermochromic coating changed from yellow to orange when heated to 40 °C and vice versa. This demonstrated that the thermochromic coating based on PUA/SiO₂ nanocomposites had good temperature sensitivity and reversibility.

Fig. 10 Thermochromic effect of the waterborne UV-curable thermochromic coating.

3.8.2 The effect of SiO₂ content on temperature sensitivity. Fig. 11 shows the thermochromic effect of the waterborne UV-curable thermochromic coatings with 0, 2, 4 and 6 wt% SiO₂ content. When all the thermochromic coatings were heated to 40 °C, they rapidly turned from yellow to orange, then returned to yellow immediately when cooled to room temperature (T = 20 °C). This demonstrated that the addition of SiO₂ had no effect on the temperature sensitivity of the waterborne UV-curable thermochromic coatings.

Fig. 11 Thermochromic effects of the waterborne UV-curable thermochromic coatings with different SiO₂ content.

4. Conclusions

Waterborne UV-curable PUA/SiO₂ nanocomposites were prepared and applied to thermochromic coatings. TEM images implied that it was easier to get a uniform emulsion by the sol–gel method than by the physical blending method. The size of PUA/SiO₂ nanocomposite particles was approximately 70–100 nm, and increased slightly with the incorporation of modified SiO₂. SEM examinations of hybrid films showed that the nanosilica were well dispersed in the matrix. Surface tension and contact angle tests both demonstrated the good wettability of the PUA/SiO₂ nanocomposite, which was conducive to its dispersion in thermochromic coatings. DMA indicated that PUA and SiO₂ were well phase mixed and the incorporation of SiO₂ increased the rigidity and T_g of PUA. Meanwhile, the nanocomposite films displayed enhanced hardness, abrasion resistance and good weather resistance. It was particularly necessary to point out that a moderate amount of modified SiO₂ could improve the curing speed of PUA coatings. Consequently, the resulting thermochromic coating showed good temperature sensitivity and reversibility. As we know, thermochromic coating is a temperature sensitive material, which can be used as a sensor with the function of anti-counterfeiting or overtemperature reminding, is usually printed on the surface of an object by inkjet printing or other methods. This study laid a good foundation for research of environmental temperature sensors.

Acknowledgements

This work was partially supported by the program of seventh batch of “3551 Optics Valley talent plan” of Wuhan East Lake High-tech Development Zone, Combination Project of Guangdong Province and the Ministry of Education (2011B090400397), Independent research project of the Fundamental Research Funds for the Central Universities (2042014gf037).

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Footnote

† These authors contributed equally to this work.

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