PA Aqueous

Article pubs.acs.org/IECR

Preparation of Water-Tolerant UV-Curable SixOy(OH)z/PA Aqueous Dispersion Films Chenting Cai,† Zhihua Xu,† Xiling Niu,† Jinshan Guo,*,† Zhendong Zhang,‡ and Lunguo Kang‡ †

Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, 730000 Lanzhou, Gansu, China ‡ Technological and R & D Center for Environment-friendly Coating Polymer, Sanshui District, Foshan, 528128, China ABSTRACT: In this experiment, silicon oxide groups (SixOy(OH)z) and carbon−carbon double bonds (CC) were introduced onto the PA molecular chains. In the first step, acrylic resins (PA), grafted by organosilicon groups, were prepared by solution polymerization. In the second step, UV curable SixOy(OH)z/PA aqueous dispersions, with a high silicon content, were prepared by hydrolyzation and condensation of PA and silane coupling agents (KH-570). Aqueous dispersions possess certain stability when the KH-570 consumption ranges from 0 wt % to 30.0 wt %. With increased content of CC, the water absorption of UV cured SixOy(OH)z/PA aqueous dispersion films soaked in the water for 24 h was decreased from 78% to 0%. Furthermore, heat curing further improved the water tolerance of the UV cured aqueous dispersion films. The water absorption change of the films caused by heat curing could reach up to 17%.

1. INTRODUCTION The determining factor that promotes water-based resin to dissolve or disperse in water is the hydrophilic groups of the resin, such as −COOH and −OH. Existence of hydrophilic components, however, would lead to poor water resistance of the water-based resin films. In many studies the water resistance of water-based films is advanced by adding nanoparticles into water-based resin,1−3 grafting hydrophobic groups onto polymer chains,4−6 or increasing the cross-linking degree of the polymer.3,7,8 Siloxane-modified polyacrylates possess many excellent properties and have been used in coatings, adhesives, and textile finishing agents. Siloxane-modified polyacrylates can be prepared by introducing polysiloxane onto polyacrylate chains,9−11 or through polymerization of silane coupling agents12−14 (eg, KH-570). -Si-OR can be hydrolyzed and condensed into −Si−O−Si− in water; thus water-borne acrylic resin chains with −Si−OR may be cross-linked. The emulsion would not be stable if the silane coupling agent used in emulsion polymerization is more than 6%.14 In this work, acrylic resins (PA) grafted by organosilicon groups and carboxyl groups were prepared by solution polymerization. −Si−OR groups were stable in solution polymerization. In the water-based process, KH-570 and PA resin were added into the alkaline aqueous solution simultaneously. Carboxyl anions promoted PA and KH-570 dispersion in alkaline aqueous solution.15 −Si−OR grafted on the PA chains and KH-570 reacted with each other by cohydrolysis and co-condensation. Only a minority of PA chains cross-linked with another PA chain. The aqueous dispersion remained stable while the silane coupling agent used was up to 30 wt % of all the monomers. CC from KH-570 introduced into the SixOy(OH)z/PA aqueous dispersion as KH-570 reacted with PA. The SixOy(OH)z/PA aqueous dispersion containing CC groups was UV-curable. The UV-curing method is a fast, energy-saving, © XXXX American Chemical Society

and pollution-free method. However, activated monomers used in traditional UV curing materials always bring about VOCs, high contractibility rate, and low adhesion.16 UV-curable SixOy(OH)z/PA aqueous dispersion prepared in this work is free of activated monomers. So the performance of SixOy(OH)z/PA films exceeded traditional UV curing materials. The method for introducing CC double bonds into SixOy(OH)z/PA aqueous dispersion can be used in other silicon resin aqueous dispersions. In this work, amphiphilic macromolecules were used to emulsify the oil phase that would react with water. Water-based complexes, including both amphiphilic macromolecules and tiny hydrolysates generated from emulsified oil phase, would be obtained. This method could also be used to design other water-based complexes.

2. EXPERIMENTAL SECTION 2.1. Materials. γ-Methacryloxypropyltrimethoxysilane (KH570) purchased from Nanjing, 2-hydroxy-4-(2-hydroxyethoxy)2-methyl-propiophenone (Irgacure 2959) purchased from Sigma-Aldrich Co., and n-butanol (Tianjin Chemicals Co. Ltd., China) were the analytical reagents. Acrylic acid (AA), methlmethacrylate (MMA), butylmethacrylate (BMA), 1,1bis(tert-buty peroxy)-3,3,5-trimethylcyclohexane (Irgacure 335), and the other reagents were used as received without further purification 2.2. Preparation. 2.2.1. Polymerizations. Polymerizations were carried out in a 250 mL glass flask fitted with a reflux condenser, a stainless-steel stirrer, a nitrogen gas inlet, a calcium chloride tube, and a thermometer. The recipes are given in Table 1. N-butyl alcohol was introduced into the reactor first. Received: August 5, 2013 Revised: September 24, 2013 Accepted: October 1, 2013

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dx.doi.org/10.1021/ie402557r | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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3. CHARACTERIZATIONS Fourier transform infrared (FTIR) spectra were conducted on a Bruker 550 FTIR in the range from 4000 to 400 cm−1 with the KBr pellet method. The transmission electron microscopy (TEM) micrographs of the PA and SixOy(OH)z/PA resin aqueous dispersion particles were taken by a JEM-1200 EX/S transmission electron microscope with an accelerating voltage of 200 kV. A certain quantity of the final latex was diluted properly by deionized water and dried in air before observation. Thermogravimetric analysis (TGA) was used to measure the thermal stability of the films prepared at 600 °C in nitrogen at a rate of 10 °C/min. TGA was performed at a scan rate of 10 °C/ min with a Perkin-Elmer TGA system. Differential scanning calorimetry (DSC) was used to measure the glass temperatures of PA films and SixOy(OH)z/ PA films in nitrogen. DSC was performed at a scan rate of 10 °C/min from −20 to 100 °C with a Mettler Toledo DSC instrument under the protection of nitrogen. The instrument was calibrated by empty cells and the standard indium for heat flow and temperature calibration, respectively. DSC analysis for each sample was repeated twice to record the exothermic curves in order to determine the glass temperature.

Table 1. Compositions of Acrylic Resin (PA) Solutions MMA (g) BMA (g) KH-570 (g) AA (g) Irgacure335 (g) n-butanol (g)

PA-0%

PA-5%

PA-10%

PA-15%

PA-20%

39.0 58.0 0 3.0 3.2 66.6

37.0 55.0 5.0 3.0 3.2 66.6

35.0 52.0 10.0 3.0 3.2 66.6

33.0 49.0 15.0 3.0 3.2 66.6

31.0 46.0 20.0 3.0 3.2 66.6

Then a solution of initiators and monomers was added via drops over a period of 2 h (with an addition rate of 1.3 g/min) to the flask after the system was added to the reaction temperature. Finally, the reaction mixture was kept at the reaction temperature for another 2 h before the acrylic resin (PA) solution was obtained. 2.2.2. PA Water-Based Process. The water-based process was carried out in a 150 mL beaker stirred at a speed of 400 rpm at room temperature. A 5 g sample of PA and 15 g of deionized water was placed into the beaker; with diethanolamine added to adjust its pH value to 9−10. Carboxyl groups turn into negative ions under alkaline conditions prompting PA to disperse in water stably. After the mixture was stirred for 0.5 h, PA aqueous dispersions were obtained. The PA aqueous dispersion was aged at 40 °C for 24 h. 2.2.3. PA Films Preparation. PA films were prepared by spreading the PA aqueous dispersion on a clean glass plate directly and drying it for 24 h at 45 °C in a vacuum oven. The glass plates used were cleaned by washing with nitric acid, rinsed with water, and finally washed with acetone. The filmforming property of PA aqueous dispersions decreased with the increased consumption of KH-570. Films could not be formed when the consumption of KH-570 in PA reached 15%. 2.2.4. SixOy(OH)z/PA Water-Based Process. Because of the excellent film-forming property of PA-5% aqueous dispersion, PA-5% was chosen to prepare SixOy(OH)z/PA . The waterbased process was carried out in a 150 mL beaker under stirring at a speed of 400 rpm at room temperature. The amounts of KH-570, PA-5%, and deionized water charged into the beaker are listed in Table 2. The mixture was stirred for 0.5 h, and diethanolamine was added to adjust its pH value to 9−10. SixOy(OH)z/PA aqueous dispersion was obtained after 24 h aging. The flowchart of the preparation of SixOy(OH)z/PA is shown in Scheme 1. 2.2.5. SixOy(OH)z/PA Films Preparation. SixOy(OH)z/PA films were prepared in the same way as PA films. The thickness of the film was 35 μm, which was measured by the PT-100 C thickness gauge. The UV-cross-linked SixOy(OH)z/PA films were prepared by exposing the latex films under a 1000 W highpressure mercury lamp with the wavelength at 254 nm for 8 min. Finally, the secondary cross-linking films were obtained by heating the UV-cross-linked films for 1 h at 135 °C in a vacuum oven. The obtained films were transparent.

4. RESULTS AND DISCUSSION 4.1. Infrared Analysis. Figure 1 shows the FTIR spectra of films s-0 and s-6. In both spectra a and b, the characteristic stretching peaks of CH3, CH2, and CO occur at 2959 cm−1, 2891 cm−1, and 1728 cm−1, respectively. All of these are the characteristic peaks of PA.17 The absorption peak at 1628 cm−1, due to the CC group, appears in the FTIR spectra of s-6. The characteristic stretching peak of the Si−O−H stretch at 3440 cm−1 is enhanced in s-6 compared with s-0.3 The Si−O− Si stretching peak at 1150 cm−1 indicates that the SixOy(OH)z had been formed in s-6. Thus, FTIR spectra revealed that SixOy(OH)z/PA contained more silica groups than PA, and the CC double bonds from KH-570 was introduced into SixOy(OH)z/PA. This result may possibly provide us with a method for introducing CC double bonds into silicon resin. Figure 2 shows that the FTIR spectra of UV curing s-6 film is unaffected by UV irradiation (a), s-6 film irradiated by UV irradiation (b), and s-6 film irradiated by UV irradiation and heated at 135 °C (c). In spectra a, b, and c, the characteristic PA stretching peaks of CH3, CH2, and CO occur at 2959 cm−1, 2891 cm−1, and 1728 cm−1, respectively. The strong peak of Si−OH at about 3400 cm−1 is reduced in spcectra c, and the peak that occurred at 1150 cm−1 is the peak of Si−O−Si.18 This result shows that the Si−OH groups were greatly diminished after being heated at 135 °C. From the spectrum of SixOy(OH)z/PA films which were unaffected by UV irradiation (a), an obvious absorption peak at 1628 cm−1 can be seen. But there is no obvious peak at the same waveband from the spectrum of SixOy(OH)z/PA irradiated by UV irradiation (b, c). Thus, FTIR spectra reveal that the CC double bonds in

Table 2. . Compositions of SixOy(OH)z/PA Aqueous Dispersions H2O (g) PA-5% (g) KH-570 (g) Irgacure2959(g)

s-0

s-1

s-2

s-3

s-4

s-5

s-6

15.0 5.0 0 0.18

15.0 5.0 0.16 0.18

15.0 5.0 0.33 0.18

15.0 5.0 0.58 0.18

15.0 5.0 0.75 0.18

15.0 5.0 1.0 0.18

15.0 5.0 1.29 0.18

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Scheme 1. Flowchart of the Preparation of SixOy(OH)z/PA

Figure 1. FTIR spectra of film of s-0 (a) and s-6 (b).

Figure 2. FTIR spectra of UV curing s-6 films: unaffected by UV irradiation (a), irradiated by UV irradiation (b), and irradiated by UV irradiation and heated at 135 °C (c).

SixOy(OH)z/PA can also participate in the radical polymerization initiated by UV light.19 The Si−O−H groups in SixOy(OH)z/PA condensed to Si−O−Si after being heated. 4.2. Analysis by XRD. Figure 3 shows the XRD patterns of s-0 and s-5. The results clearly show that there was no characteristic rap peak for pure s-0 and s-6. They also show that no crystal structure existed in either PA or SixOy(OH)z/PA.

However, there was a characteristic peak at 20° for amorphous SixOy in XRD patterns of s-5.20 That is to say, amorphous SixOy was generated in the water-based process of SixOy(OH)z/PA. 4.3. Analysis by TGA. The TGA spectra of PA with different contents of organic silicon groups are shown in Figure C



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Figure 5. TGA spectra of SixOy(OH)z/PA films.

Figure 3. XRD patterns of s-0 and s-5.

weight loss stages in the TGA curve of SixOy(OH)z/PA as in the TGA curve of PA. But the weight losses from 250 to 450 °C display a marked difference. The addition of SixOy(OH)z increases the thermal stability of composites. This can be explained by the existence of SixOy(OH)z which could make the composites possess high temperature stability. In addition, CC double bonds may well be polymerized by radical polymerization as a cross-linker and the higher cross-linking density leads to the higher thermal stability. 4.4. Analysis by DSC. Figure 6 shows the DSC thermograms for PA films. The Tg values of the cured samples

4. The organic silicon groups grafted onto PA came from KH570 in polymerization. So the content of organic silicon is

Figure 4. TGA spectra of PA with different content of organic silicon (a) PA-0%, (b) PA-5%, (c) PA-15%, (d) PA-20%.

judged by proportion of the KH-570 in all the polymerized monomer. In general, there are two weight loss stages in the TGA curve of pure PA. 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 PA and its composites. The second weight loss step, from 250 to 450 °C, is due to the decomposition of polyurethane. The carbon chains of PA usually decompose at a high temperature. Meanwhile, silicon groups in PA may condense to become Si−O−Si which would withstand high temperatures.10 Si−O− Si may be a large part of the remains. The remains of PA increased with the rising consumption of KH-570, but the thermal resistance of PA changed slightly as shown in Figure 4. In Figure 5, the TGA spectra of SixOy(OH)z/PA show the thermal stability of SixOy(OH)z/PA prepared by the addition of different proportions of KH-570 to the water-based process. The proportion of SixOy(OH)z is judged by the content of the KH-570 added in the system. In general, there are also two

Figure 6. DSC thermograms for PA films.

in this study were determined by the peak temperature of tan δ vs temperature graph. By increasing the content of organic silicon groups, the Tg value is increased from 45.77 °C for sample PA-0% to 54.11 °C for sample PA-15%. As is commonly known, a higher rigidity of the chain segment will lead to a higher Tg and a lower cross-link density; resulting in a lower Tg for the cross-linked materials.21 Tg of PA film grows higher with the increased content of organic silicon groups. The reason is that organic silicon groups linked onto a macromolecular group are cross-linked by hydrolytic condensation. Highly crossD



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linked PA aqueous dispersion particles do not possess a good film-forming property, but a PA with a certain content of organic silicon groups may benefit SixOy(OH)z/PA preparation. So we chose PA-5% to prepare SixOy(OH)z/PA. Figure 7 shows the DSC thermograms for SixOy(OH)z/PA films. The Tg of the films have not undergone an obvious

Figure 7. DSC thermograms for SixOy(OH)z/PA films.

change. Most of the organic silicon groups grafted onto PA chains did not cross-link, but reacted with KH-570. So the cross-link density and the DSC thermograms did not change with the content of KH-570. Within a certain range, the SixOy(OH)z/PA aqueous dispersion possesses a good filmforming property regardless of the amount of KH-570 added in it. This result could provide us with important information for the design of a polymer with both a high content of SixOy(OH)z groups and a low cross-linkage. 4.5. Analysis of PA Aqueous Dispersion Particles. The morphologies of PA particles were investigated by TEM as shown in Figure 8a. PA particle sizes and size distributions were tested by dynamic light scattering (DLS) as shown in Figure 8b. In the water-based process, organosilicon groups grafted onto PA chains reacted with each other by hydrolysis and condensation. Cross-linked PA was hard to disperse into small particles. Although the sizes of PA particles are not uniform, they increased with the consumption of KH-570 in polymerization. As shown in Figure 8b, the average particle size of pure PA is 34 nm, and the average particle sizes of silicacontaining latexes are 60, 85, 120, 166, and 190 nm of PA-5%, PA-10%, PA-15%, PA-20%, and PA-25%, respectively. Therefore, with the increase of functionalized silica content, the particle size increased and the granule diameter distribution widened. The morphologies of SixOy(OH)z/PA aqueous dispersion particles were investigated by TEM as shown in Figure 9a. SixOy(OH)z/PA particle sizes and size distributions were tested by dynamic light scattering (DLS) as shown in Figure 9b. KH570 is insoluble in water. Carboxyl groups of PA convert into ions in an alkaline aqueous solution during the process of dispersion, and PA will convert into amphiphilic molecules. PA, performing as an emulsifier, emulsified KH-570. From Figure 9b, we can observe that the sizes of SixOy(OH)z/PA increased with the consumption of KH-570 in polymerization. As shown

Figure 8. (top) TEM of PA aqueous dispersion particles (a) PA-0%, (b) PA-5%, (c) PA-10%, (d) PA-15%, (e) PA-20%,(f) PA-25%. (bottom) Particle size and size distribution of different silicacontaining PA aqueous dispersion particles (a) PA-0%, (b) PA-5%, (c) PA-10%, (d) PA-15%,(e) PA-20%, (f) PA-25%.

Figure 9. (top) TEM of SixOy(OH)z/PA aqueous dispersion particles (a) s-1, (b) s-2, (c) s-3. (bottom) Particle size and size distribution of SixOy(OH)z/PA aqueous dispersion particles (a) s-1, (b) s-2, (c) s-3.

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5. CONCLUSIONS The following conclusions may be drawn from the obtained experimental data: (1) Both PA aqueous dispersion and SixOy(OH)z/PA aqueous dispersion were successfully prepared. (2) Silicon-containing monomer (KH-570) polymerized in PA aqueous dispersion could reach a level (40%) that is impossible for emulsion polymerization. Increased consumption of KH-570 in polymerization, however, leads to a higher cross-link degree of PA aqueous dispersion particles and a lower film-forming property of PA aqueous dispersion. (3) Water tolerant SixOy(OH)z/PA films were successfully prepared from SixOy(OH)z/PA aqueous dispersion via UV irradiation and heating. The film-forming property of SixOy(OH)z/PA aqueous dispersion was excellent. This method for introducing CC double bonds into PA might also be used to prepare other silicon resin aqueous dispersions. (4) The water tolerance of SixOy(OH)z/PA films was enhanced by increased addition of KH-570. The water tolerance of UV cured SixOy(OH)z/PA films was enhanced along with the content of CC, and was further improved after the films were thermocrosslinked.

in Figure 9, with the increased addition of extra KH-570, the proportion of emulsifier decreased, and the sizes of the SixOy(OH)z/PA particles were enlarged. This method could also be used to design compounds including both amphiphilic molecule and hydrolysate from the oil phase. 4.6. Photos of Water Soaked Films. Figure 10 shows the picture of SixOy(OH)z/PA films, soaked in the water for 24 h,

Figure 10. Picture of SixOy(OH)z/PA films soaked in the water for 24 h.

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which were unaffected by UV irradiation (s-0 to s-6), irradiated by UV irradiation (UV-0 to UV-6), and after irradiated by UV irradiation and heated at 135 °C (H-0 to H-6). The water absorption rate of SixOy(OH)z/PA films soaked in the water for 24 h is shown in Table 3. The thickness of films is about 300

*E-mail: [email protected]. Tel.: +86 931 8912516. Fax: +86 931 8912582. Address: Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, 222# Tianshui Nanlu, Lanzhou 730000, Gansu, China.

Table 3. Water Absorption Rate of SixOy(OH)z/PA Films Soaked in the Water for 24 h unaffected by UV irradiation irradiated by UV irradiation irradiated by UV irradiation and heated

s-0

s-1

s-2

s-3

s-4

s-5

s-6

89%

57%

45%

23%

12%

7%

3%

89%

27%

19%

8%

4%s

0%

0%

23%

11%

6%

2%

0%

0%

0%

AUTHOR INFORMATION

Corresponding Author

Notes

The authors declare no competing financial interest.

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REFERENCES

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μm. The existence of SixOy(OH)z has effect on water resisting property of the films. As shown in the picture, most unaffected SixOy(OH)z/PA films (s-0 to s-6) are fogged, and some of them even turn white. More than half of the UV irradiated SixOy(OH)z/PA films (UV-0 to UV-6) are fogged while most heated SixOy(OH)z/PA films (H-0 to H-6) remain transparent. When the SixOy(OH)z content increased, the water resistance of composite films gradually strengthened. Cross-linkage always played an important role in water resistance of composite. UV cured SixOy(OH)z/PA films (UV-0 to UV-6) performed much better in water tolerance than unaffected films (s-0 to s-6). There is also favorable difference between heated films (H-0 to H-6) and UV cured SixOy(OH)z/PA films (UV-0 to UV-6). Changes above may be separately reasoned by a cross-link of CC and Si−OH. The picture confirms water resistant films can be successfully prepared via existence of SixOy(OH)z and UV irradiation. F



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PA Aqueous

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