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A novel approach to the preparation of powder coating—Manufacture of polyacrylate powder coatings via one step minisuspension polymerization
Progress in Organic Coatings 57 (2006) 282–287
Short communication
A novel approach to the preparation of powder coating—Manufacture of polyacrylate powder coatings via one step minisuspension polymerization Erjun Tang a,∗ , Guoxiang Cheng b , Qing Shang a , Xiaolu Ma b a
School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang, Hebei 050018, China b School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Received 2 January 2006; received in revised form 8 August 2006; accepted 19 August 2006
Abstract A novel method for manufacturing powder coating through one step minisuspension polymerization is described. The conventional production of powder coating includes six steps—synthesizing resins, mixing the raw material, extrusion, cooling, pre-crushing and pulverization. Comparatively, the present method can simplify the complicated processes, reduce equipment and save energy. Before polymerization, the TiO2 particles were treated with a reactive silane coupling agent 3-methacryloxypropyltrimethoxysilane (MPTMS) to obtain enough compatibility with the monomers. The powder coating was directly synthesized through employing one step minisuspension polymerization in the presence of titanium dioxide white particles. The powder coating was characterized using Fourier transform infrared spectra (FT-IR) and thermogravimetric analysis (TGA). The results show that TiO2 particles and polymer are successfully linked up via MPTMS in the powder particles. The morphology of powder coatings produced via different methods was observed by scanning electron microscope (SEM). The powder coatings obtained from minisuspension polymerization consist of regular spherical morphology particles with narrow particle size distribution. The powder flowability and surface film smoothness were enhanced compared to the pulverization powder coating. © 2006 Elsevier B.V. All rights reserved. Keywords: Powder coatings; Minisuspension polymerization; Particle morphology and size distribution; Powder flow; Film smoothness
1. Introduction In coating application, pollution controls have become increasingly stringent, which forces to move away from the use of organic solvents towards alternative coating technologies. Volatile organic compound (VOC) emission limits are being reduced in many industrialized countries, for example new European Union (EU) legislation came into force in 2000. It has been estimated that ca. 6% of man-made VOC emissions across the EU emanate from coating operations [1]. Moves to reduce VOC emission have accelerated the development of environmentallyfriendly coating systems such as aqueous and powder coatings. Although water is an attractive alternative, it does have drawbacks. The properties of waterborne polymers are often inferior to those of their solvent-borne analogues. Powder coating has almost no solvent emission during the course of coating preparation and application and overcomes the drawbacks of water-
∗
Corresponding author. E-mail address: [email protected] (E. Tang).
0300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2006.08.013
based coatings. Therefore, much interest has been focusing on powder coatings to eliminate VOC emission to the environment. Recently, the Big Three auto manufacturers, together with coating and application system suppliers, have been working to develop powder coatings systems [2–6]. However, the traditional production process of powder coatings includes synthesizing resins, blending resin and pigment, melting extrusion, cooling, pre-crushing and pulverization. It is evident that the production process is very complicated and employs a lot of energy and equipment. Thus, it is necessary to produce powder coatings by a novel method to simplify the complicated process and to save energy. In addition, the powder coatings obtained from the conventional pulverization normally possess irregular morphology and a wide size distribution, which gives rise to application problems such as poor flow, irregular feed rates and powder accumulation in transport hose and gun. From the results of application, they are not satisfactory in terms of the smoothness of finished films. Powder coatings have a rather long history in automotive applications. However, for a long time all applications have been limited to underbody and interior trim components. For aesthetic
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reasons, related mainly to the poor flowability, the car producers were reluctant to use powder as a full-body coating for any of the layers [7]. To improve the appearance, it is necessary to produce powder coatings with fewer coarse particles and a regular morphology to improve both film appearance and powder flow [8,9]. There are a number of references [10–17] addressing the manufacture of powder coatings. One interesting example is the use of supercritical or liquid carbon dioxide as a medium for delivering powder coating system [18,19]. Nevertheless, nothing has been reported that employs the one step minisuspension process to directly synthesize powder coatings particles in the presence of titanium dioxide white particles. In this paper, a powder coating with spherical morphology was prepared via the minisuspension polymerization process in which the titanium dioxide white particles were encapsulated by polyacrylate. The suspension method was developed from a study of suspension polymerization. The production method was invented to obtain powder coating particles having a narrow particle size distribution by using a water-soluble polymer as the suspension stabilizer [20]. A surface chemical phenomenon was utilized in which the oil droplets were suspended in an aqueous solution of stabilizer. Using this process, we have tried to produce polyacrylate powder coating particles with spherical morphology and a narrow particle size distribution. In order to compare this preparation with the conventional method, the powder coating was also prepared through conventional pulverization with the same powder composition. Particle size distribution and powder flow were measured. Surface roughness of the films formed was also examined.
mixture was first dispersed for 20 min through an ultrasonic instrument (HQ-50,100W, China) at room temperature and then the mixture was heated to reflux for at least 4 h. At the end of the reaction, the mixture was cooled down and diluted four or five times with n-propanol to improve the solubility of the homocondensates. This sample was centrifuged at 15,000 rpm for 2 h at room temperature. The clear supernatant, that contains the homocondensates and unreacted MPTMS, was decanted from the deposit composed of the particles with the grafted MPTMS. The deposit was dried at 50 ◦ C in vacuum for at least 8 h.
2. Experimental
A Fourier transform infrared (FT-IR) analysis was performed using a spectrophotometer (Nicolet Co., NEXUS, USA). FTIR was used to characterize the functional groups of the TiO2 , treated TiO2 and TiO2 /coplymer composite powders. The thermal characterization of powder coatings was determined by thermogravimetric analysis (TGA, TG 204, Netzsch Co., Germany). Samples were heated to 550 ◦ C from room temperature at the speed of 10 ◦ C/min The mean size and size distribution of powder coatings particles were determined by particle size analyzer (Marwen Instruments Co., Mastersilersion, England). The morphology of powder coatings particles was observed using scanning electron microscope (SEM, Phlilps Ltd. Co., XL30ESEM, Netherlands).
2.1. Materials Methyl methacrylate (MMA, CP), -hydroxyethyl methacrylate (HEMA, CP) and butyl acrylate (BA, CP) were purified by distillation under nitrogen at reduced pressure. The initiator, 2,2 -azobis(isobutylonitrile) (AIBN, analytical grade), (Tianjin Chemical Co., Ltd., Tianjin, China) was re-crystallized in ethanol. Poly(vinyl alcohol) (PVA 1799) was purchased from Shijiazhuang Chemical Fibre Co., Ltd. (Hebei, China). 3-Methacryloxypropyltrimethoxysilane (MPTMS) and polyoxyethylene nonylphenyl ether (OP-10) were supplied by Tianjin Research Institute of Synthetic Material (China) without further purification. TiO2 powder (R2310) was donated from Shijiazhuang GoldFish Coatings Co., Ltd. (Hebei, China). TEM images showed spherical particles of about 1 m. Aerosil R-812 obtained commercially (from Nippon Aerosil Co.) was used as an external additive to improve flow of powder coatings.
2.2.2. TiO2 /polymer powder coatings prepared through suspension polymerization Initiator (AIBN) was dissolved in a monomer mixture solution of MMA, BA and HEMA. Then the treated TiO2 particle was redispersed in the mixture of monomer and initiator. The sample was dispersed by ultrasonic instrument for 30 min at room temperature. An aqueous solution of non-ionic surfactant (OP-10, 0.3 g/l) was introduced to PVA aqueous solution. The TiO2 particle dispersion in the mixed monomer solution was poured slowly into 2 wt% PVA aqueous solution and emulsified with a homogenizer at 5000 rpm for 20 min. Polymerization was then carried out under stirring in a four-neck glass reactor equipped with a stirrer, a reflux condenser and a nitrogen gas inlet system. The reactor was heated to 80 ◦ C through a water bath and maintained for 6 h at this temperature. The particles produced were washed repeated by deionized water to remove PVA and dried under vacuum at ambient temperature. 2.3. Characterization
2.2. Preparation of powder coating by minisuspension polymerization
2.3.1. Powder application Powders were applied electrostatically using a corona charging gun and powder coating equipment (manufactured by Nihon Parkerizing Co.). Standard 1 mm × 100 mm × 300 mm steel test panels with an electrodeposited primer and a solvent-based primer surface were coated with powder coatings. A standard baking condition was controlled at 180 ◦ C for 30 min.
2.2.1. TiO2 particle treated with MPTMS TiO2 particles were mixed with a mixture of water and methanol, and then MPTMS was added to the system. The
2.3.2. Powder flow In this study, angle of repose was adopted as a representative value of powder flow and was measured using a Powder Tester
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Fig. 2. The scheme of grafting MPTMA onto the surface of TiO2 particles and copolymerization with monomers. Fig. 1. Measurement of powder flow by angle of repose.
(manufactured by Hosokawa Micron Co.). This value is the angle between the horizontal and the slope of a pile of powder dropped from a defined elevation as shown in Fig. 1. 2.3.3. Film surface smoothness Film appearance was assessed using surface roughness (Ra ), the arithmetic average deviation of the surface profile of a finished film, and was measured using a Surfcom 470A manufactured by Tokyo Seimitsu Co. 3. Results and discussion 3.1. TiO2 /polymer powder coating composite synthesis by suspension polymerization The surfaces of TiO2 particles were treated with a reactive silane coupling agent MPTMS to obtain the functionalization of TiO2 surface. The reaction scheme for grafting of MPTMS onto the TiO2 particles is given in Fig. 2. It shows that the grafting reaction has taken place between the Si OH groups of MPTMS and the hydroxyl groups of TiO2 surface. So MPTMS is grafted onto the surface of TiO2 particles. The terminal group of MPTMS is an organic polymer chain, which can provide steric hindrance between inorganic particles. The surface of TiO2 particle became sufficiently hydrophobic to exhibit affinity to the monomers of MMA and BA. Therefore, the functional TiO2 particles could be perfectly dispersed in the monomer droplets suspended in the PVA aqueous solution. In order to ensure droplet suspension stability in aqueous systems, the water solution of non-ionic surfactant (OP-10, 0.3 g/l) was introduced to PVA aqueous solution. The amount of OP-10 is lower than that corresponding to saturation of the surface so as to avoid the formation of emulsifier micelles. MPTMS contains a polymerisable group (unsaturated double bond), which can copolymerize with
monomers to obtain covalent grafting of at least a part of the polymer during the suspension polymerization (in Fig. 2). Consequently, encapsulated powder coating was prepared via the minisuspension polymerization. MPTMS, as a bridge, links up the polymer chains and TiO2 particles through chemical bonds in the powder particles. Thus, the interfacial adhesion and compatibility between inorganic TiO2 and organic polymer can be enhanced. 3.2. FT-IR analysis Fig. 3a–c shows the spectrum of original TiO2 particles, modified TiO2 particles and the powder coatings, respectively. In the spectrum of TiO2 particle, the appearance of a peak at 3410 cm−1 (Fig. 3a) indicates the presence of –OH. The strong absorption bands of 1725 cm−1 in the spectrum of the modified TiO2 particle (Fig. 3b) should correspond to C O
Fig. 3. FT-IR spectrum of original TiO2 particle (a), modified TiO2 particle (b) and the powder coating (c).
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appeared mainly at 320 ◦ C, while that of suspension powder was shifted to a higher temperature range (365 ◦ C). It indicates that the temperature of thermal decomposition for powders was elevated by over 40 ◦ C. This result demonstrates that the thermal stability of powder coating film could be improved. This thermal behavior of suspension powders provided a strong support for MPTMS linking up the polymer and TiO2 particles through chemical bonds. It is the cross-linking role of the TiO2 particles in the polymerization process that leads to a higher decomposition temperature. 3.4. Particle size distribution and morphology of powders The particle size distribution of the suspension powder coatings and the pulverized powders are shown in Fig. 5. Although the mean particle size of the two powders is almost 50 m, the powder coating prepared by minisuspension polymerization presents a narrower particle size distribution. The conventional powder coatings have some undesirable big and small particles. In the conventional method, the pulverizing process results in a higher fraction of smaller and bigger particles. The SEM of two powder coatings is shown in Fig. 6. It can be clearly seen that the conventional powder coatings have more coarse particles and fine particles. In addition, the morphology of pulverized powder is irregular with triangular and diametric shapes
Fig. 4. The thermal characterizations of two powder coatings: (a) pulverized powder and (b) suspension powder.
of MPTMS. The appearance of a peak at 1640 cm−1 indicates the presence of C C in the molecule of MPTMS. The peak at 1100 cm−1 is assigned the Si O groups. The absorption peaks in the region 2800–3000 cm−1 correspond to the CH2 and CH3 group of MPTMS. In the FT-IR spectrum of the powder coating composites (Fig. 3c), the absorption bands at 1730 cm−1 are characteristic of C O stretching vibration from poly(MMABA-HEMA). The absorption bands at 1150, 1192, 1240 and 1270 cm−1 represent C O C in the copolymers. The absorption peaks in the region 2900–3000 cm−1 correspond to the CH2 and CH3 group stretching vibration from the copolymers. The peak at 1640 cm−1 (characteristic of unsaturated double bond) presented in the modified particles disappeared in the spectrum of the composite particles, implying that MPTMS had been covalently copolymerized with the monomers through the unsaturated double bond. It can be inferred from the above results that MPTMS linked up the polymer and TiO2 particles through chemical bonds. 3.3. Thermal characterization The thermal characterizations of two powder coatings were determined by TGA. It can be seen from Fig. 4 a and b that the polymer decomposition temperature of the pulverized powders
Fig. 5. The particle size distribution of the pulverized powder (a) and the suspension powder (b).
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Fig. 6. The scanning electron microscope (SEM) image of pulverized powder particles (a) and suspension powder particles (b).
and so on. However, the encapsulated powder prepared by suspension polymerization presents a regular spherical structure. In the present suspension method, particle size was controlled by the size of the suspended monomer drops containing the TiO2 particles. Therefore, the powder coatings have fewer bigger and finer particles. As can be found in Fig. 7, the dispersion state of TiO2 particles is different in both powder particles. In pulverized powder, some TiO2 particles lie in the polymer phase, but there are some TiO2 particles outside of the polymer phase. In contrast, in encapsulated powders, TiO2 particles are embedded in the polymer microsphere. From the above discussion, the polymer chains and TiO2 particles in the powder composite particles are linked up through chemical bonds. Therefore, the interfacial adhesion and compatibility between inorganic TiO2 and organic polymer can be enhanced. 3.5. Powder flow of powder coatings The angle of repose of suspension powder coatings and pulverized powder coatings were measured using a Powder Tester. The result was 50◦ and 38◦ , respectively. Without
Fig. 7. The dispersion state of TiO2 particles in pulverized powder (a) and suspension powder (b).
addition of external additives, suspension powder coating exhibited a smaller angle of repose than the pulverized powder coating (adding Aerosil R-812). This behavior is attributed to the spherical shape of suspension powder coating particles. In contrast to the pulverized powder, it is easy to obtain uniform encapsulated powder coatings with spherical shape particle and narrow particle size distribution. No matter how they are produced, it is difficult to obtain uniform spherical powder particles using a conventional pulverization process [21]. The irregular structure (seen in Fig. 6) would inevitably prevent its flow. 3.6. Film surface smoothness of powder coatings The relationship between surface roughness Ra and film thickness is shown in Fig. 8. The surface roughness of suspension powder coating is superior to that of pulverized powder coating. In addition, the Ra of suspension powder coating varies only slightly variable with an increment in film thickness, but the Ra of pulverized powder coating was found to reduce dramatically
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were successfully linked up via MPTMS. TGA measurements indicated that the thermal stability of powder film was enhanced. Compared to the conventional pulverization method, the present method can simplify the complicated processes and save energy. The powder particles possess regular spherical morphology and a narrow particle size distribution. This encapsulated powder coating had excellent film surface smoothness at a relatively low film thickness. References [1] [2] [3] [4] [5] Fig. 8. The relationship between surface roughness Ra and film thickness.
with an increment in film thickness. This variety in pulverized powder coating is due to the presence of coarse particles producing large irregularities in the film that cause it to coalesce and degas slowly, resulting in a poor appearance [22]. The suspension powder coating exhibited regular spherical structure and narrow particle size distribution, which would result in a perfect appearance. 4. Conclusions Powder coatings were prepared through a one step minisuspension polymerization. Before polymerization, the TiO2 particles were treated with a reactive silane coupling agent (MPTMS) to obtain enough compatibility with the monomers. The FT-IR results showed that TiO2 particles and polymer
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
J.N. Hay, A. Kan, J. Mater. Sci. 37 (2002) 4743. T. Meschievitz, Y. Rahangdale, R. Pearson, Met. Finish. 93 (10) (1995) 26. P.M. Koop, Powder Coat. 5 (1994) 53. J. Desimone, Z. Guan, C. Elsbernd, Science 257 (1992) 945. J. Kendall, D.A. Canelas, J. Young, J. Desimone, Chem. Rev. 99 (1999) 543. D. Peck, K. Johnston, Macromolecules 26 (1993) 1537. T.A. Misev, R. van der Linde, Prog. Org. Coat. 34 (1998) 160. J.C. Kenny, T. Ueno, K. Tsutsui, J. Coat. Technol. 68 (855) (1996) 35. H. Satoh, Y. Harada, S. Libke, Prog. Org. Coat. 34 (1998) 193. C. Wadenpohl, Int. J. Miner. Process 74 (2004) 155. K. Okada, K. Yamaguchi, H. Takeda, Prog. Org. Coat. 34 (1998) 169. A.I. Cooper, J. Mater. Chem. 10 (2000) 207. A. Wenning, Macromol. Symp. 187 (2002) 597. C. Flosbach, M. Herm, H. Ritter, P. Gl¨ockner, Prog. Org. Coat. 48 (2003) 177. K. Grundke, S. Michel, M. Osterhold, Prog. Org. Coat. 39 (2000) 101. M. Oksuz, H. Yildirim, J. Appl. Polym. Sci. 94 (4) (2004) 1357. M. Rusu, P.C. Mindrila, Mater. Plast. 41 (2004) 29. J.J. Shim, M.Z. Yates, K.P. Johnston, Ind. Eng. Chem. Res. 38 (1999) 3655. M. Ghadiar, J.N. Hay, R. Kamal, A. Khan, in: Proceedings of the Eighth Meeting on Supercritical Fluids, ISASF, April 2002, Bordeaux. H. Sato, N. Yabuuchi, S. Seo, T. Ojima, Manufacturing Method of Resin Granules, US Patent No. 5,395,880 (1995). E.J. Tang, Y.Z. Geng, Chinese Patent No. ZL 03123552.2 (2003). P.G. de Lange, J. Coat. Technol. 56 (1984) 23.
Short communication
A novel approach to the preparation of powder coating—Manufacture of polyacrylate powder coatings via one step minisuspension polymerization Erjun Tang a,∗ , Guoxiang Cheng b , Qing Shang a , Xiaolu Ma b a
School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang, Hebei 050018, China b School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Received 2 January 2006; received in revised form 8 August 2006; accepted 19 August 2006
Abstract A novel method for manufacturing powder coating through one step minisuspension polymerization is described. The conventional production of powder coating includes six steps—synthesizing resins, mixing the raw material, extrusion, cooling, pre-crushing and pulverization. Comparatively, the present method can simplify the complicated processes, reduce equipment and save energy. Before polymerization, the TiO2 particles were treated with a reactive silane coupling agent 3-methacryloxypropyltrimethoxysilane (MPTMS) to obtain enough compatibility with the monomers. The powder coating was directly synthesized through employing one step minisuspension polymerization in the presence of titanium dioxide white particles. The powder coating was characterized using Fourier transform infrared spectra (FT-IR) and thermogravimetric analysis (TGA). The results show that TiO2 particles and polymer are successfully linked up via MPTMS in the powder particles. The morphology of powder coatings produced via different methods was observed by scanning electron microscope (SEM). The powder coatings obtained from minisuspension polymerization consist of regular spherical morphology particles with narrow particle size distribution. The powder flowability and surface film smoothness were enhanced compared to the pulverization powder coating. © 2006 Elsevier B.V. All rights reserved. Keywords: Powder coatings; Minisuspension polymerization; Particle morphology and size distribution; Powder flow; Film smoothness
1. Introduction In coating application, pollution controls have become increasingly stringent, which forces to move away from the use of organic solvents towards alternative coating technologies. Volatile organic compound (VOC) emission limits are being reduced in many industrialized countries, for example new European Union (EU) legislation came into force in 2000. It has been estimated that ca. 6% of man-made VOC emissions across the EU emanate from coating operations [1]. Moves to reduce VOC emission have accelerated the development of environmentallyfriendly coating systems such as aqueous and powder coatings. Although water is an attractive alternative, it does have drawbacks. The properties of waterborne polymers are often inferior to those of their solvent-borne analogues. Powder coating has almost no solvent emission during the course of coating preparation and application and overcomes the drawbacks of water-
∗
Corresponding author. E-mail address: [email protected] (E. Tang).
0300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2006.08.013
based coatings. Therefore, much interest has been focusing on powder coatings to eliminate VOC emission to the environment. Recently, the Big Three auto manufacturers, together with coating and application system suppliers, have been working to develop powder coatings systems [2–6]. However, the traditional production process of powder coatings includes synthesizing resins, blending resin and pigment, melting extrusion, cooling, pre-crushing and pulverization. It is evident that the production process is very complicated and employs a lot of energy and equipment. Thus, it is necessary to produce powder coatings by a novel method to simplify the complicated process and to save energy. In addition, the powder coatings obtained from the conventional pulverization normally possess irregular morphology and a wide size distribution, which gives rise to application problems such as poor flow, irregular feed rates and powder accumulation in transport hose and gun. From the results of application, they are not satisfactory in terms of the smoothness of finished films. Powder coatings have a rather long history in automotive applications. However, for a long time all applications have been limited to underbody and interior trim components. For aesthetic
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reasons, related mainly to the poor flowability, the car producers were reluctant to use powder as a full-body coating for any of the layers [7]. To improve the appearance, it is necessary to produce powder coatings with fewer coarse particles and a regular morphology to improve both film appearance and powder flow [8,9]. There are a number of references [10–17] addressing the manufacture of powder coatings. One interesting example is the use of supercritical or liquid carbon dioxide as a medium for delivering powder coating system [18,19]. Nevertheless, nothing has been reported that employs the one step minisuspension process to directly synthesize powder coatings particles in the presence of titanium dioxide white particles. In this paper, a powder coating with spherical morphology was prepared via the minisuspension polymerization process in which the titanium dioxide white particles were encapsulated by polyacrylate. The suspension method was developed from a study of suspension polymerization. The production method was invented to obtain powder coating particles having a narrow particle size distribution by using a water-soluble polymer as the suspension stabilizer [20]. A surface chemical phenomenon was utilized in which the oil droplets were suspended in an aqueous solution of stabilizer. Using this process, we have tried to produce polyacrylate powder coating particles with spherical morphology and a narrow particle size distribution. In order to compare this preparation with the conventional method, the powder coating was also prepared through conventional pulverization with the same powder composition. Particle size distribution and powder flow were measured. Surface roughness of the films formed was also examined.
mixture was first dispersed for 20 min through an ultrasonic instrument (HQ-50,100W, China) at room temperature and then the mixture was heated to reflux for at least 4 h. At the end of the reaction, the mixture was cooled down and diluted four or five times with n-propanol to improve the solubility of the homocondensates. This sample was centrifuged at 15,000 rpm for 2 h at room temperature. The clear supernatant, that contains the homocondensates and unreacted MPTMS, was decanted from the deposit composed of the particles with the grafted MPTMS. The deposit was dried at 50 ◦ C in vacuum for at least 8 h.
2. Experimental
A Fourier transform infrared (FT-IR) analysis was performed using a spectrophotometer (Nicolet Co., NEXUS, USA). FTIR was used to characterize the functional groups of the TiO2 , treated TiO2 and TiO2 /coplymer composite powders. The thermal characterization of powder coatings was determined by thermogravimetric analysis (TGA, TG 204, Netzsch Co., Germany). Samples were heated to 550 ◦ C from room temperature at the speed of 10 ◦ C/min The mean size and size distribution of powder coatings particles were determined by particle size analyzer (Marwen Instruments Co., Mastersilersion, England). The morphology of powder coatings particles was observed using scanning electron microscope (SEM, Phlilps Ltd. Co., XL30ESEM, Netherlands).
2.1. Materials Methyl methacrylate (MMA, CP), -hydroxyethyl methacrylate (HEMA, CP) and butyl acrylate (BA, CP) were purified by distillation under nitrogen at reduced pressure. The initiator, 2,2 -azobis(isobutylonitrile) (AIBN, analytical grade), (Tianjin Chemical Co., Ltd., Tianjin, China) was re-crystallized in ethanol. Poly(vinyl alcohol) (PVA 1799) was purchased from Shijiazhuang Chemical Fibre Co., Ltd. (Hebei, China). 3-Methacryloxypropyltrimethoxysilane (MPTMS) and polyoxyethylene nonylphenyl ether (OP-10) were supplied by Tianjin Research Institute of Synthetic Material (China) without further purification. TiO2 powder (R2310) was donated from Shijiazhuang GoldFish Coatings Co., Ltd. (Hebei, China). TEM images showed spherical particles of about 1 m. Aerosil R-812 obtained commercially (from Nippon Aerosil Co.) was used as an external additive to improve flow of powder coatings.
2.2.2. TiO2 /polymer powder coatings prepared through suspension polymerization Initiator (AIBN) was dissolved in a monomer mixture solution of MMA, BA and HEMA. Then the treated TiO2 particle was redispersed in the mixture of monomer and initiator. The sample was dispersed by ultrasonic instrument for 30 min at room temperature. An aqueous solution of non-ionic surfactant (OP-10, 0.3 g/l) was introduced to PVA aqueous solution. The TiO2 particle dispersion in the mixed monomer solution was poured slowly into 2 wt% PVA aqueous solution and emulsified with a homogenizer at 5000 rpm for 20 min. Polymerization was then carried out under stirring in a four-neck glass reactor equipped with a stirrer, a reflux condenser and a nitrogen gas inlet system. The reactor was heated to 80 ◦ C through a water bath and maintained for 6 h at this temperature. The particles produced were washed repeated by deionized water to remove PVA and dried under vacuum at ambient temperature. 2.3. Characterization
2.2. Preparation of powder coating by minisuspension polymerization
2.3.1. Powder application Powders were applied electrostatically using a corona charging gun and powder coating equipment (manufactured by Nihon Parkerizing Co.). Standard 1 mm × 100 mm × 300 mm steel test panels with an electrodeposited primer and a solvent-based primer surface were coated with powder coatings. A standard baking condition was controlled at 180 ◦ C for 30 min.
2.2.1. TiO2 particle treated with MPTMS TiO2 particles were mixed with a mixture of water and methanol, and then MPTMS was added to the system. The
2.3.2. Powder flow In this study, angle of repose was adopted as a representative value of powder flow and was measured using a Powder Tester
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Fig. 2. The scheme of grafting MPTMA onto the surface of TiO2 particles and copolymerization with monomers. Fig. 1. Measurement of powder flow by angle of repose.
(manufactured by Hosokawa Micron Co.). This value is the angle between the horizontal and the slope of a pile of powder dropped from a defined elevation as shown in Fig. 1. 2.3.3. Film surface smoothness Film appearance was assessed using surface roughness (Ra ), the arithmetic average deviation of the surface profile of a finished film, and was measured using a Surfcom 470A manufactured by Tokyo Seimitsu Co. 3. Results and discussion 3.1. TiO2 /polymer powder coating composite synthesis by suspension polymerization The surfaces of TiO2 particles were treated with a reactive silane coupling agent MPTMS to obtain the functionalization of TiO2 surface. The reaction scheme for grafting of MPTMS onto the TiO2 particles is given in Fig. 2. It shows that the grafting reaction has taken place between the Si OH groups of MPTMS and the hydroxyl groups of TiO2 surface. So MPTMS is grafted onto the surface of TiO2 particles. The terminal group of MPTMS is an organic polymer chain, which can provide steric hindrance between inorganic particles. The surface of TiO2 particle became sufficiently hydrophobic to exhibit affinity to the monomers of MMA and BA. Therefore, the functional TiO2 particles could be perfectly dispersed in the monomer droplets suspended in the PVA aqueous solution. In order to ensure droplet suspension stability in aqueous systems, the water solution of non-ionic surfactant (OP-10, 0.3 g/l) was introduced to PVA aqueous solution. The amount of OP-10 is lower than that corresponding to saturation of the surface so as to avoid the formation of emulsifier micelles. MPTMS contains a polymerisable group (unsaturated double bond), which can copolymerize with
monomers to obtain covalent grafting of at least a part of the polymer during the suspension polymerization (in Fig. 2). Consequently, encapsulated powder coating was prepared via the minisuspension polymerization. MPTMS, as a bridge, links up the polymer chains and TiO2 particles through chemical bonds in the powder particles. Thus, the interfacial adhesion and compatibility between inorganic TiO2 and organic polymer can be enhanced. 3.2. FT-IR analysis Fig. 3a–c shows the spectrum of original TiO2 particles, modified TiO2 particles and the powder coatings, respectively. In the spectrum of TiO2 particle, the appearance of a peak at 3410 cm−1 (Fig. 3a) indicates the presence of –OH. The strong absorption bands of 1725 cm−1 in the spectrum of the modified TiO2 particle (Fig. 3b) should correspond to C O
Fig. 3. FT-IR spectrum of original TiO2 particle (a), modified TiO2 particle (b) and the powder coating (c).
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appeared mainly at 320 ◦ C, while that of suspension powder was shifted to a higher temperature range (365 ◦ C). It indicates that the temperature of thermal decomposition for powders was elevated by over 40 ◦ C. This result demonstrates that the thermal stability of powder coating film could be improved. This thermal behavior of suspension powders provided a strong support for MPTMS linking up the polymer and TiO2 particles through chemical bonds. It is the cross-linking role of the TiO2 particles in the polymerization process that leads to a higher decomposition temperature. 3.4. Particle size distribution and morphology of powders The particle size distribution of the suspension powder coatings and the pulverized powders are shown in Fig. 5. Although the mean particle size of the two powders is almost 50 m, the powder coating prepared by minisuspension polymerization presents a narrower particle size distribution. The conventional powder coatings have some undesirable big and small particles. In the conventional method, the pulverizing process results in a higher fraction of smaller and bigger particles. The SEM of two powder coatings is shown in Fig. 6. It can be clearly seen that the conventional powder coatings have more coarse particles and fine particles. In addition, the morphology of pulverized powder is irregular with triangular and diametric shapes
Fig. 4. The thermal characterizations of two powder coatings: (a) pulverized powder and (b) suspension powder.
of MPTMS. The appearance of a peak at 1640 cm−1 indicates the presence of C C in the molecule of MPTMS. The peak at 1100 cm−1 is assigned the Si O groups. The absorption peaks in the region 2800–3000 cm−1 correspond to the CH2 and CH3 group of MPTMS. In the FT-IR spectrum of the powder coating composites (Fig. 3c), the absorption bands at 1730 cm−1 are characteristic of C O stretching vibration from poly(MMABA-HEMA). The absorption bands at 1150, 1192, 1240 and 1270 cm−1 represent C O C in the copolymers. The absorption peaks in the region 2900–3000 cm−1 correspond to the CH2 and CH3 group stretching vibration from the copolymers. The peak at 1640 cm−1 (characteristic of unsaturated double bond) presented in the modified particles disappeared in the spectrum of the composite particles, implying that MPTMS had been covalently copolymerized with the monomers through the unsaturated double bond. It can be inferred from the above results that MPTMS linked up the polymer and TiO2 particles through chemical bonds. 3.3. Thermal characterization The thermal characterizations of two powder coatings were determined by TGA. It can be seen from Fig. 4 a and b that the polymer decomposition temperature of the pulverized powders
Fig. 5. The particle size distribution of the pulverized powder (a) and the suspension powder (b).
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Fig. 6. The scanning electron microscope (SEM) image of pulverized powder particles (a) and suspension powder particles (b).
and so on. However, the encapsulated powder prepared by suspension polymerization presents a regular spherical structure. In the present suspension method, particle size was controlled by the size of the suspended monomer drops containing the TiO2 particles. Therefore, the powder coatings have fewer bigger and finer particles. As can be found in Fig. 7, the dispersion state of TiO2 particles is different in both powder particles. In pulverized powder, some TiO2 particles lie in the polymer phase, but there are some TiO2 particles outside of the polymer phase. In contrast, in encapsulated powders, TiO2 particles are embedded in the polymer microsphere. From the above discussion, the polymer chains and TiO2 particles in the powder composite particles are linked up through chemical bonds. Therefore, the interfacial adhesion and compatibility between inorganic TiO2 and organic polymer can be enhanced. 3.5. Powder flow of powder coatings The angle of repose of suspension powder coatings and pulverized powder coatings were measured using a Powder Tester. The result was 50◦ and 38◦ , respectively. Without
Fig. 7. The dispersion state of TiO2 particles in pulverized powder (a) and suspension powder (b).
addition of external additives, suspension powder coating exhibited a smaller angle of repose than the pulverized powder coating (adding Aerosil R-812). This behavior is attributed to the spherical shape of suspension powder coating particles. In contrast to the pulverized powder, it is easy to obtain uniform encapsulated powder coatings with spherical shape particle and narrow particle size distribution. No matter how they are produced, it is difficult to obtain uniform spherical powder particles using a conventional pulverization process [21]. The irregular structure (seen in Fig. 6) would inevitably prevent its flow. 3.6. Film surface smoothness of powder coatings The relationship between surface roughness Ra and film thickness is shown in Fig. 8. The surface roughness of suspension powder coating is superior to that of pulverized powder coating. In addition, the Ra of suspension powder coating varies only slightly variable with an increment in film thickness, but the Ra of pulverized powder coating was found to reduce dramatically
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were successfully linked up via MPTMS. TGA measurements indicated that the thermal stability of powder film was enhanced. Compared to the conventional pulverization method, the present method can simplify the complicated processes and save energy. The powder particles possess regular spherical morphology and a narrow particle size distribution. This encapsulated powder coating had excellent film surface smoothness at a relatively low film thickness. References [1] [2] [3] [4] [5] Fig. 8. The relationship between surface roughness Ra and film thickness.
with an increment in film thickness. This variety in pulverized powder coating is due to the presence of coarse particles producing large irregularities in the film that cause it to coalesce and degas slowly, resulting in a poor appearance [22]. The suspension powder coating exhibited regular spherical structure and narrow particle size distribution, which would result in a perfect appearance. 4. Conclusions Powder coatings were prepared through a one step minisuspension polymerization. Before polymerization, the TiO2 particles were treated with a reactive silane coupling agent (MPTMS) to obtain enough compatibility with the monomers. The FT-IR results showed that TiO2 particles and polymer
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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