Synthesis and characterization of fluorine-containing poly-styrene-acrylate latex with core-shell structure using a reactive surfactant.
Keywords Core-shell, Fluorine-containing, Styrene-acrylate latex, Reactive surfactant
Fluorinated copolymers are known to have many useful and desirable features, such as high thermal, aging, and weather resistance; excellent inertness to solvents, acids, and alkalis; and oil and water repellency due to the low polarizability and the strong electronegativity of the fluorine atom. (1-3) Among numerous fluorinated copolymers, fluorinated acrylate (or styrene-acrylate) copolymer latex has attracted many researchers not only because it is environmental friendly compared to solvent-based coatings, but also because it maintains the excellent properties of both fluorine and acrylate (or styrene-acrylate) copolymer materials. (4-8) However, on the one hand, the industrial application of fluorinated copolymers has so far remained narrow because of the relatively high price of fluorine monomers. One strategy to solve this problem is to prepare the fluorine-containing polyacrylate (PA) or poly-styreneacrylate (PSA) latex particles with a core-shell structure consisting of a fluorine-free core and a fluorine-containing shell and fixing the fluorine in the shell. (9-14) In this way, the excellent properties of PA (or PSA) and fluorinated materials can be preserved while the cost of fluorine-containing materials is decreased considerably. On the other hand, in emulsion polymerization or mini-emulsion polymerization, the residual emulsifier in the latex will bring negative effects to the properties of the products. For reducing disadvantages from the residual emulsifiers' migration during film formation, attempts have been made by some researchers to prepare emulsion latex with reactive emulsifiers. (15-17) Therefore, emulsion polymerization using reactive surfactant is promising for the preparation of environmentally friendly fluorine-containing PA or PSA latex, with excellent properties for industrial application.
Wang et al. (18) and Chen et al. (19) claimed to have prepared core-shell fluorine-containing PA latex with excellent hydrophobic properties using a reactive emulsifier. However, the fluorine monomer they used contained the PFOS (-[C.sub.8][F.sub.17]) group, which has now been proved persistently polluting and toxic to the environment (20) and is restricted from use in many countries. Cui et al. (21) reported a fluorine-containing emulsifier-free PA latex with a core-shell structure, in which the amount of fluorine monomer was beyond 20.0 wt% when the water contact angle (WCA) of the latex film was about 103[degrees], and the solid content of the emulsion was only about 11.3 wt%. Xiao and Wang (22) also claimed to have prepared an emulsifier-free core-shell PA latex with only about 9.1% of fluorine monomer and with a high solid content (about 42.0 wt%), but the WCA of the latex film was only 94.3[degrees].
In this contribution, we present a simple and effective procedure to synthesize fluorine-containing PSA latex with a core-shell structure prepared by seeded semicontinuous emulsion polymerization. By using reactive emulsifier, the resulting emulsion shows excellent stability properties. The crosslinked PSA core and crosslinked fluorine-containing PSA shell structure help to fix the fluorine on the shell. The prepared latex film shows excellent thermal stability, water repellency, and acid/alkaline resistance properties with low fluorine content.
Dodecafluoroheptyl methacrylate [CH.sub.2] = C([CH.sub.3])CO-[OCH.sub.2]CF([CF.sub.3])CFHCF[([CF.sub.3]).sub.2] (Actyflon-G04, 96+%) was obtained from XEOGIA Fluorine-Silicon Chemical Co. Ltd., China; triethylene glycol dimethacrylate (TrEGDMA, 96+ %) and allyloxy polyoxyethyl-ene(10)nonyl ammonium sulfate (DNS-86, 96+%) were purchased from Guangzhou Shuangjian Co. Ltd., China; N-methylolacrylamide (N-MA, 98+%) was purchased from Shanghai Kefeng Chemical Reagent Trade Co. Ltd., China; butyl acrylate (BA, 99+%), styrene (St, 99+%), and methacrylic acid (MAA, 99+%) were purchased from Shanghai Lingfeng Chemical Co. Ltd., China. Ammonium persulfate (APS, 98+%) and sodium bicarbonate ([NaHCO.sub.3], 99.5+%) were purchased from Guanghua Chemical Co. Ltd., China, and used as received. Water was purified by a Milli-Q system (Millipore).
Synthesis of the fluorine-containing PSA latex nanoparticles with core-shell structure
The fluorine-containing soap-free PSA latex particles with core-shell structure were prepared by seeded emulsion polymerization technique. All the polymerizations were carried out in a 500-mL four-neck flask equipped with reflux condenser, mechanical stirrer, and dropping funnels, and heated in the water bath.
Synthesis of PSA seed latex particles
First, the pre-emulsified core monomers were obtained from emulsified core monomers in the mixture of 30.00 g water and 0.50 g DNS-86 at room temperature for about 30 min. Then 20.0 wt% of the pre-emulsified core monomers and 40.0% of the initiator solution (0.64 g APS dissolved in 30.00 g water) were added into the flask containing 70.00 g water, 0.80 g DNS-86, and an appropriate buffer agent ([NaHCO.sub.3]), when the temperature was raised to 77[degrees]C. After an additional 30 min equilibration time, the remaining mixed monomers and initiator were added into the flask drop-by-drop simultaneously within 2 h at the flow rate of 1.0 and 0.3 mL/min, respectively. Then the reaction was continued at 77[degrees] for another 2 h and a small amount of [NaHCO.sub.3] was added to adjust the pH value to the range of 6.0-7.0.
Synthesis of the fluorine-containing PSA latex particles with core-shell structure
The pre-emulsified shell monomers mixture was fed into the flask containing the above PSA seed latex by the starved-feed addition method. The second stage APS aqueous solution was also fed into the flask, keeping pace with the adding of pre-emulsified shell monomers at the flow rate of 0.5 and 0.2 mL/min, respectively. After feeding, the temperature was maintained at 85[degrees]C for another 3 h. Then the obtained latex was cooled to room temperature and [NH.sub.3][H.sub.2]O was used to adjust the pH value of the final latex at around 7.0-8.0. Table 1 shows the recipe for the fluorine-containing PAS latex.
Table 1: Recipes for the fluorine-containing PSA latex Sample [C.sub.4] [F.sub.0] [F.sub.1] [F.sub.2] First stage St 48.00 44.00 44.00 44.00 BA 30.00 30.00 30.00 30.00 MAA 2.00 2.00 2.00 2.00 N-MA 0 2.00 2.00 2.00 TrEGDMA 0 2.00 2.00 2.00 DNS-86 0.60 0.60 0.60 0.60 APS 0.64 0.64 0.64 0.64 Dl water 130.00 130.00 130.00 130.00 Second stage St 6.00 9.80 8.80 7.80 BA 6.00 9.80 8.80 7.80 [G.sub.04] 8.00 0 2.00 4.00 TrEGDMA 0 0.40 0.40 0.40 DNS-86 0.30 0.30 0.30 0.30 APS 0.16 0.16 0.16 0.16 DI water 20.00 20.00 20.00 20.00 Sample [F.sub.3] [F.sub.4] [F.sub.5] [F.sub.6] First stage St 44.00 44.00 44.00 44.00 BA 30.00 30.00 30.00 30.00 MAA 2.00 2.00 2.00 2.00 N-MA 2.00 2.00 2.00 2.00 TrEGDMA 2.00 2.00 2.00 2.00 DNS-86 0.60 0.60 0.60 0.60 APS 0.64 0.64 0.64 0.64 Dl water 130.00 130.00 130.00 130.00 Second stage St 6.80 5.80 4.80 3.80 BA 6.80 5.80 4.80 3.80 [G.sub.04] 6.00 8.00 10.00 12.00 TrEGDMA 0.40 0.40 0.40 0.40 DNS-86 0.30 0.30 0.30 0.30 APS 0.16 0.16 0.16 0.16 DI water 20.00 20.00 20.00 20.00 Note: All reported amounts are in grams
Fourier transform infrared (FTIR) spectra were conducted on a 380 Fourier transform infrared spectrometer (Nicolet Instruments Company, USA) in the range from 4000 to 400 [cm.sup.-1] in transmission. The sample was mixed with KBr powder and pressed into pellets.
Transmission electron microscope (TEM) measurements were performed on a TEM (Model JEM-100CXII, Japan). One drop of the suspension was diluted into water and placed on a carbon-coated copper grid to be stained with 1.0 wt% phosphatolung-stic acid (PTA) for 3 min and dried in air before observation.
X-ray photoelectron spectroscopy (XPS) data were collected in both survey and high-resolution mode on Krafos Axis Ultra DCD systems equipped with an Al Ka source and operating at 150 W. The scanning scope was 700 x 300 [micro][m.sup.2]. Data were recorded at the take-off angle of 30[degrees].
The contact angles of liquid on fluorine-containing PSA latex films were measured by an OCA15 (Data Physics Instruments Company, Germany). The contact angle is the average value of five individual tests' data at different places on the polymer surfaces.
The water absorption ratio was measured by soaking copolymer films at room temperature in water and weighing the amount of water absorbed in 24 h.
Thermo-gravimetric analysis (TGA) was performed by a Q550 TGA System (TA Instruments, USA) in a nitrogen atmosphere at a heating rate of 10[degrees]C/min from 50 to 500[degrees]C.
The [Ca.sup.2+] stability property of latex was tested by adding 1 mL 5 wt% [CaCl.sub.2] solution into 5 mL prepared fluorine-containing PSA latex. The centrifugal stability property was tested by putting the fluorine-containing PSA latex into a centrifugal machine for 30 min at 5000 rpm. The mechanical stability was tested by putting the emulsion into a shear machine for 30 min at 3000 rpm. The stability properties mentioned above were evaluated by observing whether flocculation appeared after the test.
The chemical resistance of the latex films was tested by the following method: the glass slides coated with polymer film were immersed into a solution of 10 wt% [H.sub.2][SO.sub.4], and a solution of 8 wt% NaOH, under room temperature for 24 h, respectively. Then the glass slides were washed with distilled water and dried under atmosphere. Chemical resistance properties of the polymer films were evaluated by the changes of WCA.
Results and discussion
Figure 1 shows the FTIR spectra of the fluorine-containing PSA latex (curve b) and fluorine-free PSA latex (curve a) synthesized by seeded emulsion polymerization. We see in both curves a and b the characteristic stretching peaks of -CH([CH.sub.2]) at 2940 and 2871 [cm.sup.-1], the stretching vibration of C = O at 1730 [cm.sup.-1], the distortion vibration of [-CH.sub.2] at 1452 [cm.sup.-1], and the absorptions from the benzene ring of the styrene monomer units at 1494 and 3027 [cm.sup.-1]. However, the stretching vibration of -CF bonds at 1302 [cm.sup.-1] and the absorption of [-CF.sub.3] groups at 1244 [cm.sup.-1] can be detected only in curve b. Furthermore, the flexing vibration peaks of O = C-O-C at 1068 [cm.sup.-1] in the FTIR spectrum of fluorine-containing PSA are much weaker; and shifts to low wave number 1064 [cm.sup.-1] due to the interactions between fluorine atoms of fluorine carbon chains and ester groups, (23) as well as the absorption peaks at 1000-1260 [cm.sup.-1], are wider and blunter because of the overlap of the stretching vibration absorption of the -CF bond at 1100-1240 [cm.sup.-1] with the stretching vibration absorption of the C-O-C bond of ester groups at 1250 [cm.sup.-1]. Thus, FTIR spectra reveal that fluorine monomer [G.sub.04] was introduced into the latex particles as desired through seeded emulsion polymerization.
[FIGURE 1 OMITTED]
Effect of crosslinking
As shown in Fig. 2, the WCA on the film of uncross-linked fluorinated latex film (sample [C.sub.4]) decreases rapidly with the time of the latex being placed in water. The reason is that the fluorinated groups migrate toward the inside of the film when placed in water. This problem can be resolved by fixing the fluorinated groups on the shell of particles. To restrain the fluorinated groups from migrating to the inside of the film, the core and shell of the particles were crosslinked with crosslinking agent. The crosslinked core can make the shell monomers polymerize on the surface of the core, which is also aids the formation of stable core-shell structure particles. The crosslinked shell helps to restrict the fluorinated groups of shell copolymer from migrating inside. It can be seen from Fig. 2 that the WCA on the crosslinked fluorinated polymer films (sample [F.sub.4]) changes only slightly compared with that of the uncrosslinked ones. This result indicates that the fluorinated groups have been fixed on the shell of particles by crosslinking.
[FIGURE 2 OMITTED]
Micromorphology of the core-shell latex nanoparticles
TEM characterization was employed to observe the micromorphology of the prepared latex in this paper. Figure 3 illustrates the TEM micrographs of fluorine-containing PSA latex nanoparticles (sample [F.sub.4]). A significant contrast between core and shell of the latex particles can be observed clearly in Fig. 3, as a result of the difference of electron penetrability to the core and shell, which unmistakably proves the formation of the core-shell structure. The light and dark regions in the particles correspond to the crosslinked PSA core and the crosslinked fluorine-containing PSA shell, respectively.
[FIGURE 3 OMITTED]
The XPS analysis gives some insight into the chemical composition of the surface of core-shell fluorine-containing PSA latex film (sample [F.sub.4]), and the results are shown in Fig. 4. The survey spectra (Fig. 4a) reveal the characteristic signal of carbon ([C.sub.1s] at 284.60 eV), oxygen ([O.sub.1s] at 532.61 eV), and fluorine ([F.sub.1s] at 288.61 eV). In the [C.sub.1s] spectrum (Fig. 4b), the peaks are attributed to the aliphatic carbon atoms ([C.sub.1s] at 284.60 eV), the ester atoms ([C.sub.1s] at 285.56 eV), and the carbon atoms of -CF ([C.sub.1s] at 289.02 eV) and [-CF.sub.3] ([C.sub.1s] at 293.70 eV) groups, respectively. It is well known that the fluorine-containing group of copolymers has a tendency to migrate to the air-film interface and occupy it during the forming film of latex, which can effectively decrease the surface free energy of the latex film. The XPS test result shows that the wt% of fluorine is 31.13%, much higher than that theoretically calculated by the shell monomer (22.80%) or bulk density (4.56%), which further indicates that the fluorine was fixed on the shell and has a tendency to migrate to the air-film interface of the latex film during film formation.
[FIGURE 4 OMITTED]
Stability of the prepared latex
DNS-86 is a reactive surfactant containing polymeriz-able double bonds which can copolymerize with styrene and acrylate monomers (as can be seen in Fig. 5), and therefore an emulsion could be prepared.
[FIGURE 5 OMITTED]
The stability test results are shown in Table 2. The sample [F.sub.4] shows excellent electrolyte, centrifugal, and mechanical stability. This is because the polar group, [-SO.sub.3.sup.-] of DNS-86, forms electric layers with a negative charge on the surface of the latex particles, which could promote the electrolyte stability of the emulsion. Furthermore, the hydrophilic ether bond of DNS-86 could contribute to the formation of thick water layers on the surface of emulsion particles, which could inhibit coagulation and improve the monomer conversion (22); and therefore the prepared fluorine-containing PSA latex possesses excellent stability.
Table 2: Stability test results of the prepared emulsion (sample [F.sub.4]) Time [Ca.sup.2+] stability Centrifugal stability Mechanical stability 0 [check] [check] [check] 12 [check] [check] [check] 24 [check] [check] [check] 36 + [check] [check] Note: [check] means no flocculation, + means a little flocculation
Water repellency and chemical resistance properties of the latex film
The water repellency and chemical resistance properties of the prepared latex films were tested by water absorption ratio and WCA before and after acid/ alkaline treatment, and the results are shown in Table 3. These results show that the WCAs of the fluorine-containing PSA latex films is increased from 76.2[degrees] to 105.1[degrees], and the water absorption ratio is reduced from 8.93 to 3.66 wt%, as the [G.sub.04] content in the shell increases to 40.0 wt% (sample [F.sub.4]) because of the increment of the fluorine content. Then the incremental tendency of the contact angle begins to weaken and reaches its maximum value when the fluorine content is about 50.0 wt% (sample F5) of the shell monomers. Furthermore, the WCA has obviously not changed before or after acid/alkaline treatment when the fluorine content reaches 40.0 wt% of shell monomers, implying the excellent chemical resistance of polymer film.
Table 3: Water absorption ratio and contact angle of polymer film sample Sample [theta]water ([degrees]) Water absorption Before After 10 wt% After 8 wt% ratio (wt%) treatment [H.sub.2][SO.sub.4] NaOH F0 8.93 76.2 46.4 44.5 F1 5.83 90.5 81.6 78.3 F2 4.79 98.2 92.4 90.6 F3 4.12 101.7 94.4 93.1 F4 3.66 105.1 99.1 98.7 F5 3.43 107.5 100.8 100.4 F6 3.35 107.3 101.3 100.2
Thermal stability of the prepared latex film
The effect of fluorine content on the thermal stability of the latex film was investigated by comparing the TGA curves of the fluorine-containing PSA latex with different content of fluorine monomers, and the results are shown in Fig. 6. We can see that the thermal stability of the latex film improved as fluorine content of the latex film increased. The decomposition of the latex film without fluorine (sample [F.sub.0]) began at about 310[degrees]C and ended at about 410[degrees]C; whereas the latex film of sample [F.sub.2] began to decompose at about 335[degrees]C and decomposed completely at about 430[degrees]C, and sample [F.sub.4] began to decompose at about 350[degrees]C and decomposed completely at about 435[degrees]C. This phenomenon was attributed to long-chain perfluoroalkyl groups. [G.sub.04] contains perfluoroalkyl chains containing -CF and [-CF.sub.3] units with high bond energy, which were able to shield and protect the nonfluorinated segment beneath the fluorinated segment and therefore improved the thermal stability of the latex films.
[FIGURE 6 OMITTED]
Fluorine-containing PSA latex particles with core-shell structure were successfully prepared by pre-emulsified seeded emulsion polymerization using fluorine monomer Actyflon-[G.sub.04], reactive surfactant DNS-86, and a crosslinking agent. FTIR spectra analysis confirmed that fluorine monomer can be introduced into the latex particles through emulsion polymerization. TEM observation proved the formation of the core-shell structure particles. XPS study further indicated that the fluorine can be fixed on the shell copolymer by crosslinking and that it has a tendency to migrate to the film-air surface. Stability tests showed that the prepared latex maintained excellent [Ca.sup.2+], centrifugal, and mechanical stability properties. The WCA, chemical resistance test, and TGA study showed that the core-shell fluorine-containing fluorinated PSA latex films exhibited excellent water repellency, chemical resistance, and thermal stability properties with 8.0 wt% of fluorine monomer content. The excellent properties of the emulsion show that it is a good candidate as a binder for waterborne coatings.
Acknowledgment The authors are grateful for the financial support from Science and Technology Planning Project of Guangdong Province, China (2009B011000010) and the Fundamental Funds for the Central Universities (2009ZM0288).
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G. Xu, L. Deng, X. Wen, P. Pi, D. Zheng,
J. Cheng, Z. Yang (*)
School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, People's Republic of China
[C] ACA and OCCA 2010
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|Author:||Xu, Guilong; Deng, Lili; Wen, Xiufang; Pi, Pihui; Zheng, Dafeng; Cheng, Jiang; Yang, Zhuoru|
|Date:||May 1, 2011|
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