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Synthesis of opaque and colored hollow polymer pigments.

INTRODUCTION

The white pigments used in coatings such as titanium dioxide, calcite, zinc sulfide, and zinc oxide are inorganic in nature, and may cause paint defects such as settlement and agglomeration because of their higher density than that of organic vehicle. Inorganic pigments are also inherently incompatible with organic binders. In addition, the widely used titania pigments may adversely affect the lifetime of coating as they produce free electrons under UV or solar radiation. Opaque polymer pigment is a very promising alternative to inorganic white pigments and can be used also to reduce the amount of the titania (Ti[O.sub.2]) in surface coating.

Hollow particles as opaque polymer pigments have received great attention because of their low density and compatibility with polymeric binder in the paint. Opaque polymer pigments have core-shell structure with void core and a transparent polymer shell which diffracts the incoming light in different directions. The difference of the refractive index between the polymer shell (e.g., [[eta].sub.polystyrene] = 1.46-1.68, [[eta].sub.PMMA] = 149) and the [[eta].sub.air] = 1.0) inside the voids, produces intensive scatterings of light increasing the intensity of the diffracted light (i.e., diffuse light) and reducing the intensity of the reflected light (i.e., peculiar light), which in turn produces an enhanced opacity [1].

The earliest opaque polymer pigments developed were mainly produced by osmotic swelling [2-4] and solvent swelling methods [5-7]. Some research was also carried out to develop other methods of production involving foamable polymer emulsions and blowing agents [8, 9], water-in-oil-in-water emulsion methods [10, 11], and phase separation of the two polymers in a common solvent [12]. The use of seeding materials and neutralization of the acidic group by alkaline materials improve the quality of hollow pigments [13-17],

In general, the experimental conditions such as the composition of monomer mixture, the crosslinker content, the pH, the acidity of monomer, and the amount of surfactant primarily affect the size of the hollow and the opacity produced. The amount of surfactant in the emulsion is a key parameter as it affects the number distribution of particles, and accordingly the polymerization rate is also affected [18, 19].

The widely used osmotic swelling method involves, (i) synthesis of a core polymer having carboxylic acid groups attached to the backbone, and (ii) encapsulating it with another polymer which functions as shell. The core usually contains 10%-30% ionizable compound which is later neutralized with an alkali and converted into a polyelectrolyte [17, 20]. The polymeric salt thus obtained delivers much higher osmotic pressure than the non-neutralized acidic polymer. The osmotic pressure expands the core, and upon removal of water void is created inside the shell. The hard shell must be thermally stable and be permeable to water. Thermoplastics such as styrene, styrene-methyl methacrylate copolymer or other acrylates are usually used in the preparation of shell [17, 21, 22]. The shell can be coated with another layer such as with silica nanoparticles which improves the mechanical properties and allows functionalization [23]. The light-scattering efficiency depends on the thickness of the shell as well as on the functional molecule on the shell [24-26],

The shape and size of the particle, concentration, size and distribution of the microvoids, refractive index differences between medium and particle, wavelength of the light, and the number density of particles which act as scattering centers are the parameters that all affect the scattering efficiency [27-32]. In hollow particles besides the scattering due to refraction, there is also scattering directly from the surface of the particle, so called the Rayleigh scattering. The Rayleigh scattering is inversely proportional to the fourth power of the wavelength of light. Since the blue light has the smallest wavelength, the Rayleigh scattering may impart a blue shade if the particle size is not optimized properly. Therefore, the size of the particles should not be larger than an upper limit to obtain desired opacity, and should not be lower than a lower limit to minimize the blue shade. There are some studies made to find out the optimum size of hollow pigments from light scattering theory [4, 11], It was calculated that the optimum diameter size of the hollow pigment and the void should be between 0.5 and 0.6 [micro]m and 0.25 and 0.3 [micro]m, respectively. In fact, the visible light more or less has a wavelength spectrum in this range (blue to red, 0.4-0.7 ([micro]m). Coloring the polymeric materials by using master batches is a well-known technique. However, in paint applications instead of dispersing the coloring agent in the vehicle, it is more attractive to incorporate the coloring agent into the hollow particles. The direct use of colored hollow particles can eliminate the inherent dispersion problems of coloring agents. The coloring agent can be encapsulated in the shell of the hollow particle. Since a large variety of the organic dyes have aromatic structure, polystyrene would be a better choice to use as the shell polymer to minimize the possible compatibility problems between the organic pigment and the hollow particle. There are many techniques for encapsulation, such as conventional emulsion, dispersion, and miniemulsion polymerization [33-38].

The aim of this research work was to make hollow and colored polymer pigment. To obtain the most satisfactory coloring properties, it is necessary to work with the hollow particles having the best opacity. Therefore, a systematic study was needed to optimize the process conditions of the production of hollow pigments with the styrene shell. Most of this research was thus devoted to reveal the effects of process parameters to find out the optimum conditions. The effects of process parameters such as (i) surfactant/water ratio below the penetration percolation limit, (ii) acidic monomer content, (iii) crosslinker content, (iv) pH of the medium, and (v) the swelling time, on final particle size were studied. After making colored opaque pigments by using copper phthalocyanine blue pigment, the [L.sup.*][a.sup.*][b.sup.*] values were evaluated for two levels of pigment addition.

EXPERIMENTAL

The synthesis of hollow pigments covers four steps: (i) synthesis of core, (ii) preparation of coloration solution, (iii) synthesis of shell, and (iv) neutralization of core polymer. The step (ii) was excluded in the production of white opaque pigment. The steps involved to obtain hollow pigment are given in Fig. 1.

Materials

Core polymer: Methyl methacrylate (MMA) (Sigma Aldrich) and methacrylic acid (MAA) (Sigma Aldrich).

Shell polymer: Styrene (Sigma Aldrich).

Crosslinker: Ethylene glycol dimethacrylate (EGDM) (Merck).

Surfactant: Sodium dodecyl benzene sulfonate (SDBS) (Sigma Aldrich).

Initiator: Sodium persulphate (SPS) (Merck).

Neutralizing and swelling agents: NaOH (Sigma Aldrich), methyl ethyl ketone (MEK).

Pigment (blue): Copper phthalocyanine blue pigment (Sigma Aldrich).

Synthesis of Core Polymer

After several preliminary experiments three different mixtures of "MMA + MAA + EGDM" which offer sufficient stability and yield hollow particles were used to produce hollow pigments. The compositions of the mixtures are given in Table 1.

Poly (methyl metacrylate-co-methacrylic acid) was produced by emulsion polymerization at 80[degrees]C for 24 h in a four-necked 250 ml round-bottom flask equipped with an inlet of nitrogen gas and a condenser. In the very first step, water and SDBS solution were introduced into the reactor, and the solution was stirred at 250 rpm. Then, MMA and MAA were poured into this solution, and mixed for 30 min. Finally, SPS (0.5 w/w ratio to monomer) was added to the reactor to start up polymerization.

Dispersion of Pigment

Organic copper phthalocyanine blue pigment (2%-3% w/w of monomer) was introduced into styrene, and agitated by a magnetic stirrer at 400 rpm for half an hour. The dispersion thus obtained was added to the aqueous solution having S/W value of 0.45. Then, the mixture was ultrasonically homogenized for 4 min at 90% of maximum power (maximum power: 200W, Bandalin Sonopuls HD2200).

Synthesis of Shell Polymer

The amount of styrene used in the production of shell was taken to be about one third by weight of the monomer mixture used in the production of core polymer. The styrene was added by five equal portions at certain time intervals. This is done due to reason that the polymerization kinetics of styrene is quite low, and polymerization of styrene outside the micelle surface should be avoided. The first portion of water and surfactant solution was added at the very beginning, and the temperature was raised to 90[degrees]C. The reactor was agitated at 350 rpm. After half an hour the first portion of styrene is added into the reactor. Half an hour later the second portion of the initiator was introduced into the reactor, and all steps were repeated 5 times by 1 h intervals.

To produce the colored hollow pigments each portion of the copper phthalocyanine dispersion was introduced at the styrene addition step.

Alkali Treatment

After the production of the core-shell polymer water was added, and the mixture was stirred at 350 rpm under nitrogen atmosphere at 90[degrees]C. The pH of the solution was adjusted to 10 by using NaOH solution (pH 13). The addition of MEK loosens polystyrene shell and facilitates the penetration of alkaline water into core. Samples were taken out each hour and particle size analysis was carried out. Neutralization was stopped when the particle size reached a constant value.

Film Application

Hollow polymer pigment was mixed with water based styrene acrylic resin (Betapol SA-5017B, Betek Paint and Chemical Inc.) which had 50% w/w solid content. The films were cast by using an applicator at a wet thickness of 150 pm. The formulation given in Table 2 is used in casting coat.

The optical properties of the paint, namely opacity and gloss were determined 24 h after drying the film at the ambient temperature.

Characterizations

The dynamic light scattering size measurement (DLS) (Malvern Zetasizer Nano ZS Model No: ZEN3500, A = 633 nm laser), scanning electron microscopy (SEM) (Model: FEI Quanta 200 FEG), transmission electron microscopy (TEM) (Model: FEI Tecnai G2 F30), and opacity and color measurements (color spectrophotometer Model no: X-rite Color 15) were the characterization studies. The color properties were determined by measuring the CEI [L.sup.*][a.sup.*][b.sup.*] values. The white opaque polymer was taken as the standard, and the [L.sup.*][a.sup.*][b.sup.*] values of the phthalocyanine loaded pigment was measured in comparison with it.

The gloss measurements were carried out by using the glossmeter of Novo-Gloss (Model: Rhopoint). Gloss values were measured at 20[degrees], 60[degrees], and 85[degrees] angles of incident light to find out high, medium, and matt gloss, respectively. If the gloss value of film is lower than 10, it is said to be matt whereas a gloss value higher than 70 indicates high gloss.

RESULTS AND DISCUSSION

Particle Size

Effect of S/W (Surfactant/Water) Ratio on Emulsion Stability and Particle Size. The preliminary experiments showed that rapid sedimentation was likely for the systems when S/W ratio was below 0.54 and when surfactant/monomer ratio was above 4.28. A set of experiments were performed at different S/W ratios to understand the effect of the amount of surfactant on emulsion stability, particle size, and particle morphology. These ratios were arranged in such a way that the amount of monomer and water were kept constant, and the amount of surfactant was varied.

The effect of S/W ratio on particle size is shown in Fig. 2. The experiments were conducted at MMA:MAA ratio of 90:10, and at (MMA + MAA):EGDM ratio of 98:2. The increase in the amount of surfactant resulted in a slight decrease in the size of core as expected, and the shell built up around the core naturally followed a similar pattern. The size of particles increased due to swelling after the neutralization stage, and reached to 350 nm at the S/W ratio of 0.65. The increase in the S/W ratio decreased the final size of the hollow particle, and it can be accounted to the increase in the surfactant content on the surface. The increase in the number of surfactant molecules on the surface forms a stronger barrier for the diffusion of alkaline water through the shell into the core. A sharp decrease in the size occurred at the S/W ratio of 1.46. It is likely that the surfactant barrier got quite tight and sharply decreased the penetration of water molecules through the shell. The sharp decrease at S/W = 1.46 may be an indication of exceeded percolation limit of surfactant molecules.

Particle size may be controlled by the amount and type of surfactant. The biggest particle size was observed at S/W ratio of 0.65, and the smallest particle size at S/W ratio of 1.46. SDBS plays a key role for hollow particle production. Greater amounts of surfactant offers better stabilization and produce smaller particle size. As the latex particles grow in both core and shell stage, more surfactant is needed to stabilize the emulsion and so, SDBS should be supplied gradually and slowly. The increase in the amount of surfactant affects both the structure and monodispersity of latex. When it was too low (S/W < 0.54) sedimentation of the particles was observed.

The Ostwald ripening and the coalescence play a predominant role on the polydispersity of particles and emulsion stability. As the S/M ratio increases, the surfactant can reduce monomer diffusion from one droplet to the other. Therefore, S/M ratio affects both emulsion stability and particle size, and smaller particles can be produced at higher S/M ratio.

Effect of MMA Content on Emulsion Stability and Particle Size. The size of polymer particles was measured by DLS technique in core stage, shell stage, and by 1 h intervals in the swelling stage. The change of the size of core, the shell, and the final swelling stage with MAA concentration are shown in Fig. 3. In these sets of experiments the amount of MAA was changed but the (MMA + MAA):EGDM ratio was kept constant as 98:2 by decreasing the MMA. The S/W ratio was changed between 0.65 and 1.46.

The 20% MAA yielded coagulations when S/W was 0.65. The high acidic group content on the polymer chain could cause interactions between particles yielding coagulations when the S/ W ratio was low, because, the surface could not be effectively covered by the surfactant molecules. The smallest core particles were obtained at the highest S/W ratio, i.e., S/W = 1.46. The particle diameters after forming shells exhibited a good correlation with the S/W ratio as seen from the open markers in Fig. 3; the smaller the S/W ratio the larger the particle diameter.

The S/W ratio significantly affected the size of the swollen particles, and the high S/W ratio resulted in smaller particles. The sizes of particles obtained at S/W = 0.65 and S/W = 0.97 ratios were close to each other the former being slightly larger, but the one obtained at S/W = 1.46 had relatively low diameter. As mentioned above, the large surfactant content diminished the diffusion of sodium ions and water molecules into the core. The particles produced at S/W = 0.97 (~1) were pretty much satisfactory as they could be produced in a larger range of MAA content between 10% and 20% without producing any coagulant.

Effect of Crosslinker Content on Particle Size. The effect of the crosslinker (e.g., EGDM) on particle size was investigated for two different monomer mixtures, for MMA/MAA ratios of 90:10 and 85:15. The increase in crosslinker concentration showed a decrease in the particle diameter, but there was not a significant effect of MMA/MAA ratio as seen in Fig. 4. The decrease in the diameter with the increase in the crosslinker concentration was more pronounced in swollen particle until EGDM/ (MMA + MAA) ratio of 4.5. It essentially kept constant on further increase in EGDM even though there was a little decrease in the diameter of the non-swollen core-shell particle.

Effect of pH on the Size of Swollen Particle. The dependence of swollen particle size on pH was investigated by using MMA/MAA ratio of 90:10, EGDM/(MMA + MAA) ratio of 2, and S/W ratio of 0.97. The size of the core and the core-shell particles were 122 and 144 nm, respectively. The change of the particle average diameter with pH is shown in Fig. 5.

The size of the swollen particle increased sharply as the pH was increased beyond 9 and it kept nearly constant after pH = 10. The increase in pH increases the number of carboxyl groups neutralized, and thus increases the number of water molecules in the core due to osmotic effect. The saturation was pretty much reached at pH = 10, such that the increase in the molar concentration of sodium ions beyond this value did not significantly change the total radius.

As the shell behaves as a kind of membrane that allows sodium ions and water molecules to penetrate into core the swelling is a time taking process. Figure 6 shows the effect of duration of alkali treatment on the size of particles. The experiments were carried out at MMA/MAA ratio of 90:10 and EGDM/ (MMA + MAA) ratio of 2, but varying S/W ratios.

In all cases, the diameter increased up to about 5 h, and then it is stabilized. The final diameter achieved is the smallest in S/ W = 1.46 case indicating that excessive use of surfactant prevents the penetration of sodium ions and water molecules as mentioned above. The decrease in S/W to 1.35 showed a large increase in the particle diameter, but it exhibited small increase on decrease in S/W to 0.97 and further to 0.65. So this was the reason why the pH experiments (Fig. 5) were carried out at S/W = 0.97.

Particle Morphology

The SEM micrographs of hollow particles obtained are given in Fig. 7.

All the particles were of spherical shape. The colored hollow particles were also of the same form as seen from the left micrograph of Fig. 8. The right picture is the TEM micrograph of the colored pigment, and the hollow cores can be clearly distinguished.

As the SEM pictures do not give information whether the particles have hollows or not, it is necessary to use TEM micrographs to check for the existence of hollows for sure. It was observed that the samples with the monomer composition MMA/MAA = 90:10 did not have apparent hollows whereas it was possible to create hollows in the samples with the composition MMA/MAA = 80:20 when S/W = 1.46 as seen from Fig. 9.

The samples with larger MAA content naturally had higher number of carboxyl groups to be neutralized and thus higher ability to swell on neutralization, because, as the ionic strength increases more water is gathered in the core. The pH of the medium also has similar effect as the higher sodium ion concentration difference between the medium and the core causes higher amount of penetration of sodium ions into the core, which, in turn, results in higher osmotic pressure for swelling. Another parameter is the S/W ratio which influences the permeability of the shell. It is possible to create hollows if the pH of the medium is increased and the S/W values for the samples were lowered with the MAA content less than 20%. The hollows were not created at low pH values such as pH 8 and pH 9 when MMA/MAA = 90:10, but it was possible to have them when pH 10 and S/W = 0.97 as seen in Fig. 10.

The experiments done at the same conditions (i.e., MMA/ MAA = 90:10, S/W = 0.97) but with different EGDM content in the monomer mixture did not significantly affect the formation of hollows as shown in Fig. 11.

In short we can say that high MAA content, relatively lower S/W ratio such as 0.97 (~1), and high pH ([greater than or equal to] 10) favor the formation of hollows.

Opacity Measurements

The opacity of hollow pigments depends on the chemical compositions of the materials used, the shell thickness, but especially on the diameter of the hollow which must be at some appropriate value compared with the wavelength of the incident light. Table 3 gives information about the opacities of different samples. The opacities were determined in relation to that of titania, opacity of which was taken to be 100.

The numbering of the samples (i.e., first column) was done in the descending order of the diameters of the samples (i.e., last column). The comparison of the first and the second rows and also of sixth and the seventh rows shows that the diameter decreases as the MAA content decreases, which, in tum decreases the opacity. The comparison of the second and third rows and also first and sixth rows tells that the increase in surfactant content decreases both the permeability of the shell and the diameter, and thus the opacity also decreases. Excessive crosslinking decreases the swelling and thus reduces the diameter and the opacity as seen from the first and fourth rows and also from the second and fifth rows. Since all process parameters influence the diameter of the particle, there is a strong dependence of the opacity on the size of the hollow polymer particle. Figure 12 gives the dependence of opacity on particle size.

There is more or less a linear dependence between the particle size and opacity even though the particles have varying compositions. In fact, the particle size and the wavelength of light are two major parameters which affect the scattering of light; the refractive index also plays a role but not as much as the other two [40].

Gloss Measurements

The gloss values of the opaque polymer pigments measured at 20[degrees], 60[degrees], and 85[degrees] are given in Table 4.

The samples numbered in the first column have the same attributes with the ones given in Table 3. The gloss values came out to be very low for all angles of measurement. The gloss value between 0 and 10 is named "flat," and the opaque polymer pigments synthesized fell into this class with values [less than or equal to] 3 as seen from Table 4.

Color Measurements

The measurements were done for 1% and 3% phthalocyanine loadings to the monomer, and the comparative measurement results are shown in Fig. 13.

In color terminology [a.sup.*] is used to designate redness and [b.sup.*] the yellowness. The 2% phthalocyanine content in the monomer mixture imparts green-blue color as seen from (1), but 3% phthalocyanine imparts dark blue color as seen from (2) The broken arrow in Fig. 13 shows the color shift.

The change in lightness ([DELTA][L.sup.*]) for the first (i.e., 2% phthalocyanine), and the second (i.e., 3% phthalocyanine) pigments came out to be -10.74 and -31.49, respectively, while the total color difference ([DELTA][E.sup.*]) was 19.55, and 30.05, respectively.

CONCLUSIONS

Surfactant to water ratio was of critical importance in emulsion stability and coagulation occurred when S/W < 0.54. The large amount of surfactant resulted in smaller particle size as the surfactant molecules formed a strong barrier and made difficult the penetration of sodium ions and water molecules into the core. The particle with the biggest size was obtained at S/W ratio of 0.65, and the smallest one at 1.46. The particle size increased with the increase of the MAA fraction in the monomer mixture, because, the increase in the ionic strength of the core upon neutralization gathers more water molecules around. The increase in the pH of the medium had a similar effect, and pH [greater than or equal to] 10 is needed for effective swelling. High degree of crosslinking of the polymer resulted in smaller polymer particles. The maximum particle size was obtained when the acid (i.e., MAA) content was 20% and the S/W was 0.97. The maximum swelling could be achieved in 5-6 h. Hollows did not form when pH [less than or equal to] 10, and also when the MAA/(MMA + MAA) ratio was below 10%.

When the pigments were mixed with an acrylic resin at 50% by volume an opacity of 93.8% was achieved. The introduction of copper phthalocyanine to the monomer mixture resulted in blue opaque polymer pigment. The addition of 2% phthalocyanine imparted a green-blue color but 3% phthalocyanine imparted blue color.

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Ekin Karakaya, (1) Bora Mavis, (2) Gungor Gunduz (1)

(1) Kimya Muhendisligi Bolumu, Orta Dogu Teknik Universitesi, Ankara, Turkey

(2) Makine Muhendisligi Bolumu, Hacettepe Universitesi, Beytepe, Ankara, Turkey

Correspondence to: G. Giindiiz; e-mail: ggunduz@metu.edu

Contract grant sponsor: Orta Dogu Teknik Universitesi (Scientific Research Projects); contract grant number: BAP-03-04-2012-007.

DOI 10.1002/pen.24468

Caption: FIG. 1. Steps involved in the production of hollow pigment [39].

Caption: FIG. 2. The effect of surfactant/water (S/W) ratio on particle size.

Caption: FIG. 3. The effect of MAA content on the size of the core, the shell, and the swollen panicle at different S/W ratios.

Caption: FIG. 4. The effect of the crosslinker concentration on the size of the core, the shell, and the swollen particle at two different MMA/MAA ratios.

Caption: FIG. 5. Effect of pH on particle size.

Caption: FIG. 6. Effect of the duration time of alkali treatment on particle size.

Caption: FIG. 7. SEM micrograph of swollen particles (left: x20,000, right: x60,000).

Caption: FIG. 8. left: SEM (X80.000), and right: TEM micrograph of colored pigment particles.

Caption: FIG. 9. TEM micrographs, left: 10% MAA. S/W = 1.46; right: 20% MAA. S/W = 1.46.

Caption: FIG. 10. TEM micrographs, left: pH 9, right pH 10.

Caption: FIG. 11. left: (MMA + MAA):EGDM = 98:2, right: (MMA + MAA)/ EGDM = 90:10.

Caption: FIG. 12. Change of opacity with the size of hollow particle.

Caption: FIG. 13. Color property. (1): 2% phthalocyanine. (2): 3% phthalocyanine.
TABLE 1. Composition of the mixtures used.

MMA (a)   MAA (a)   EGDM (b)   S/W (c)   S/M (d)

90          10         2        1.46       11.56
85          15         2        1.46       10.94
90          10         3        0.97        7.71

(a) % w/w based on total monomer.

(b) % w/w of crosslinker to total monomer.

(c) % w/w of surfactant to water.

(d) % w/w of surfactant to total monomer.

TABLE 2. Composition of the coat.

Component                         (% w/w)

Opaque polymer pigment                7.0
Water                                70.5
Styrene-acrylic resin                15.5
Hydroxyelhyl cellulose (2%) (a)       7.0

TABLE 3. Opacities of hollow pigments.

Number   MMA   MAA   EGDM    S/W    Opacity   Size (nm)

1        85    15     2      0.97    93.8        350
2        90    10     2      0.97    91.2        341
3        90    10     2      1.35    90.1        325
4        85    15     4.5    0.97    88.7        310
5        90    10     4.5    0.97    85.2        297
6        85    15     2      1.46    80.1        263
7        90    10     2      1.46    78.6        225

TABLE 4. Gloss values of hollow polymer pigments.

Number   20[degrees]   60[degrees]   85[degrees]

1            1.9           2.7           2.4
2            1.9           2.8           1.6
3            1.8           2.6           1.5
4            1.9           2.9           1.7
5            1.8           2.7           1.8
6            1.9           3.0           1.5
7            1.8           2.9           2.3
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Author:Karakaya, Ekin; Mavis, Bora; Gunduz, Gungor
Publication:Polymer Engineering and Science
Article Type:Report
Date:Sep 1, 2017
Words:5330
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