Pigmented Polymer Particles with Controlled Morphologies - American

Chapter 25

Pigmented Polymer Particles with Controlled Morphologies Wei-Hsin Hou, Thomas B. Lloyd, and Frederick M. Fowkes

Downloaded by COLUMBIA UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch025

Center for Polymer Science and Engineering, Department of Chemistry, Lehigh University, Bethlehem, PA 18015

In this study, mechanisms of phase separation as well as microencapsulation of pigments from polymer solutions are presented. Two types of pigmented polymer particles with different morphologies have been prepared by the microencapsulation process : 1) particles with pigment enriched on the surface and 2) particles with more fully encapsulated pigment. The morphologies as well as the surface properties of the encapsulated particles are controlled by the interactions between pigment, polymer and solvent which are dependent upon the functional groups on the pigment surface, the chemical structure of the polymer and the solvency. Photon correlation spectroscopy and scanning electron microscopy showed that the pigmented polymer particles have narrow size distribution and the size varied from 0.5µm to2µmdepending upon the pigment content and the solvency. Different techniques such as BET adsorption, densitometry, and microelectrophoresis are used to characterize the surface properties of the pigmented polymer particles. These results support a proposed mechanism for the microencapsulation and delineate morphologies of the encapsulated particles.

The process of microencapsulation started with nature's creation of the first living cell. However, no technique had been successfully developed to copy this ingenious process of nature until aboutfiftyyears ago. The first process of microencapsulation was developed by Barry Green at National Cash Register Company in the 1930s. Since then, more and more scientists have entered this fascinating field and are producing new processes and new applications which encompass fields from graphic arts to agriculture to medicine. Microencapsulation is the development of small solid particulates, 0097-6156/92/0492-0405$06.00/0 © 1992 American Chemical Society In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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liquid droplets, or gas bubbles with a coating. Basically three different microcapsule structures; namely mononuclear capsules, microcapsule aggregates and double-wall structures have been prepared for different applications^). There are many approaches to cany out microencapsulation. In general, these approaches can be classified into five categories : polymerization(2,3), phase separation^), emulsification(5, - _ , »•>

Adsorption

"> T . c .

*»

Precipitation

/

#

(

b) G N / M O / E t O H

Precipitation

Figure 2 : Mechanisms of the microencapsulation of (a) Regal L 330 and (b) Monarch 1000 carbon black from Griltex/ethanol solutions at an initial cooling rate of l°C/sec.

In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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We suspect that each primary carbon black floe acts as a nucleus during phase separation, but that thesefloesmay differ in size as their concentration was increased. However,flocculationof embryonic particles at an increasing rate with increasing concentration is also probable. Accordingly, in this model the final capsule size would not only depend on the nucleation due to the pigment but also onflocculationof nuclei or embryonic particles (wherein the R L particles would be relatively free to migrate to the surface). Surface Area. Table III shows that the surface area of the R L encapsulated particles increases significantly with increasing pigment content. However, the surface area of the MO encapsulated particles demonstrate a somewhat lower increase. Table IV shows that the M O pigment has a surface area about three times larger than the RL. Therefore, if Monarch 1000 is on the surface, G M particles should have larger area than equally loaded particles with R L on the surface. However, we found the reverse to be true, i.e., the surface area of the Regal L 330 pigmented encapsulates is larger than those pigmented with Monarch 1000 at each level of pigment content. The PCS and SEM results show us that the particle size of GR and G M encapsulates are almost the same, and therefore, the higher area of the GR particles over the G M is due to more R L roughening the particle surface.

Table III: Particle size and surface area of different pigmented polymer particles which were made by the precipitation technique Pigment Particle Diameter Content (wt%) By PCS (nm)

Surface Area Blackness By BET (m /g) Density 2

GR-l GR-5 GR-10 GR-25 GR-50

1.0 4.8 9.1 16.7 33.3

1600 960 840 670 580

17.3 28.4 32.5 36.3 47.4

0.37 0.81 1.16 1.44 1.71

GM-1 GM-5 GM-10

1.0 4.8 9.1

1560 930 770

17.4 27.1 28.1

0.24 0.60 0.76

* 0 = total reflection of incident light; 2 = total adsorption of light

If we take the particle diameter of these pigmented polymer particles measured by the PCS and convert them to surface area by assuming that the particles are spherical and have smooth surfaces and that the density of the pigments and polymer are 1.83 and 0.8 g/cm , respectively, then we find that 3


25. HOU ET AL.

Pigmented Polymer Particles with Controlled Morphologies

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415

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POLYMER LATEXES

the ratio of the surface areas measured by the B E T adsorption to that calculated from the PCS diameter is revealing. The ratio, an indication of roughness, increases for the R L system but is almost constant or slightly decreases with an increase of pigment content for the M O system (see Table V). So, for the R L system, the higher relative roughness is an indication of more pigment on the particle surface.


Table IV : Surface area of carbon black pigments measured by single point BET adsorption and blackness

Pigment

Surface Area (nr/g) Reported by Measured by Cabot Corporation BET Adsorption

Blackness Density Measured by Densitometer

RL

94

71.7

1.65

MO

343

211.4

1.87

Table V :: Relative surface roughness and amount of pigment on the particle surface Relative Pigment on Surface Roughness s /s (%)

Pigment Content (wt%)

Surface Area calculated from PCS (m /g)

GR-1 GR-5 GR-10 GR-25 GR-50

1.0 4.8 9.1 16.7 33.3

4.63 7.35 7.99 9.23 9.05

3.74 3.86 4.07 3.93 5.24

100 87 62 75

GM-1 GM-5 GM-10

1.0 4.8 9.1

4.75 7.59 8.72

3.66 3.57 3.22

25 23

2

B E T

P C S

Moreover, using the particle size from PCS results and the surface area measured by the BET adsorption (from Table III), the amount of pigment on the surface of the encapsulated particles was calculated based on the following assumptions: 1) The density of the pigments and Griltex nylon are 1.83 g/cm and 0.8 g/cm , respectively, 2) In a given sample, each particle has the same size and is spherical, 3) For samples, GR-1 and GM-1, which contain 1 wt% 3

3


25. HOU ET AL.


417


of pigment, the amount of pigment on the particle surface is negligible, 4) The surface roughness is independent of the particle size, and 5) Nitrogen can not diffuse into the particles. The calculations are summarized in Table V which shows that more than 60% of Regal L but only 25% of Monarch 1000 pigments are on the surface of the capsules. This is another piece of evidence to support the proposed morphologies of the two different encapsulated particles. Blackness Index. It is known that, for carbon blacks, light adsorption is more important than light scattering in developing tinting strength, although the later is not negligible. The blackness of carbon black is related to its particle size as well as its aggregate size. Small particle size and/or low degree of aggregation of carbon blacks favor blackness. For a pigmented polymer particle, the blackness of the particles is dependent upon the amount of carbon black in the particles, the morphology of carbon black in the particles, the refractive index of the polymer and carbon black, the path of incident light passed before or after being reflected/scattered or adsorbed by the polymer and the size of the particles. The blackness of the pigments and pigmented polymer particles were measured with a Macbeth RD514P Densitometer and the results are listed in Table III and IV. They show that the blackness of both carbon black pigmented polymer particles increases with the increase of pigment content. Since the M O exhibits higher blackness density than the R L pigment, the M O pigmented particles should have higher blackness density than the R L counterpart given the same morphology. However, the blackness density of the R L pigmented polymer particles is higher and increased more significantly than the M O pigmented polymer particles as the pigment content was increased. This provides two bits of information. Firstly, in the G M series, the pigment was coated with a thick polymer layer, so incident light was reflected/scattered by the polymer layer before being adsorbed by the pigment. This caused the M O pigmented particles to have lower blackness density than the R L pigmented particles which have relatively more pigment located on the particle surface. Secondly, the M O pigment particles are aggregated in the center of particles reducing their adsorption efficiency relative to the R L pigmented particles whose pigment particles are less aggregated. Accordingly, the proposed morphologies of the different encapsulated particles are further supported by the blackness measurements. Zeta potential. According to Fowkes' proton charging mechanism for non aqueous colloidspi,), the charge formation is directly related to the acid-base interactions between the charging species and the functional sites on the particle surfaces. An acidic particle in a basic medium receives a negative charge, and a basic particle in an acidic medium receives a positive charge. For example, rvX\\t(22) and anatase(23,), which have both acidic and basic surfaces, are positive in n-butanol and n-pentanol, which are acidic liquids. However, rutile become negative in more basic liquids such as butylamine^) and nitrobenzene^).


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Lloyd et a\.(25) titrated rutile pigments with different surface treatments by flow microcalorimetry and found that silica-coated titanium oxide has a more acidic surface and alumina-coated titanium oxide has a more basic surface. They further found that the silica-coated sample had a larger negative zeta potential than the alumina-coated sample in xylene solutions containing lecithin which is considered to be a basic dispersant. The alumina-coated titanium oxide, however, had a larger positive zeta potential than the silicacoated titanium oxide in xylene solutions containing alkyd resin, a primarily acidic dispersant. From zeta potential measurements, surface characteristics of particles can be qualitatively evaluated. Microelectrophoretic mobility results (see Table VI)

Table V I : Zeta potential (mV) of R L and M O carbon blacks in different solvents and Isopar G solutions containing 0.025wt% of basic barium petronate (BaPB) and cupric naphthenate measured by Coulter DELSA at 25°C

THF

CHC1

Regal L 330

-165

+90

-143

+45

Monarch 1000

-150

+30

-150

+15

3

BaPB

Isopar G / Cu Naphthenate

Pigment

show that R L and M O carbon black pigments have very similar negative zeta potential values in basic tetrahydrofuran and basic barium petronate/Isopar G solution. However, in acidic chloroform and cupric naphthenate/Isopar G solution, R L pigment has positive zeta potentials about three times larger than M O pigment. These results indicate that both pigments have similar acidic characteristics, but R L is considerably more basic than MO, which is consistent with the flow microcalorimetry data as shown in Table I. Accordingly, the dramatic difference of the zeta potential for R L and MO pigments in acidic liquids and solutions containing acidic charge control agents is a good approach to differentiate the two pigmented polymer particles. Figure 4a shows that the MO pigmented particles have positive zeta potential values very close to the polymer, but very different from the Monarch 1000 itself. However, Figure 4b shows that the R L pigmented particles have positive zeta potential values almost identical to Regal L 330 itself but different from the polymer. The electrophoretic results give strong evidence that the R L pigmented polymer particles have a large amount of the pigment on the surface, and therefore, have surface characteristics similar to the pigment. However, the MO pigmented polymer particles have the pigment mostly fully



25. HOU ET AL.


120-

80-

40-

0.001

0.1

0.01

Cu Naphthenate wt*

(b) Figure 4 : The zeta potential of polymer, pigments and pigmented polymer particles in Isopar G solutions containing different concentrations of cupric naphthenate measured by Coulter DELSA at 25°C.


419

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encapsulated, and therefore the surface characteristics are similar to the polymer itself. Once again, the proposed encapsulation mechanisms and encapsulated particle morphologies are strongly supported.


Conclusions The interactions between polymer, pigment and solvent play an important role in controlling the morphologies of pigmented polymer particles during phase separation from polymer solutions. Two types of pigmented polymer particles with different morphologies were prepared : a) particles with pigment largely on the surface and b) particles with more fully encapsulated pigment. Pigmented polymer particles with different surface characteristics have different physical properties such as surface area, optical density and surface potential.

Postscript Professor Frederick M. Fowkes passed away on October 17, 1990; this work was done with his advice.

Acknowledgement This research project was sponsored by Coulter Systems Corporation, Bedford, MA. The encouragement of Dr. Kenneth A. Lindblom is highly appreciated.

Literature Cited 1. Thies, C.; Encyclopedia of Polymer Science and Engineering; Wiley-Interscience: New York, NY, 1987; Vol. 9, pp. 724. 2. Huang, T. C. Ph.D. Dissertation; Lehigh University, 1986. 3. Luzzi, L. A.; Zoglio, M. A.; Maulding, H. V. J. Pharm. Sci., 59, 338(1970) 4. Green, B. K.; Schleicher, L. U.S. Patent 2,800,457 (1957) 5. Micale, F. J. European Patent 238,035 (1987) 6. Micale, F. J. U.S. Patent 4,665,107 (1987) 7. Landa, B.; Hall, J.; Gibson, G. A. U.S. Patent 4,842,974 (1989) 8. Vollmann, H.; Soden, B.; Herrmann, H. U.S. Patent 4,594,305 (1986) 9. Brenner, J. Perfum. Flavour, 8, 40(1983) 10. Siegel, B. M.; Johnson D. H.; Mark H. J. Polym.Sci.5,111(1950) 11. Kumaki, J. Macromolecules, 21, 749(1988) 12. Brown, H. R.; Wignall, G. Macromolecules, 23, 683(1990) 13. Der, R. K. J. Colloid Interface Sci., 51, 388(1975) 14. Moffatt, W. C.; Bowen, H. K. J. MaterialSci.Letters, 6, 383(1987) 15. Joslin, S. T.; Fowkes, F. M.; Ind. Eng. Chem. Prod. Dev., 24(30), 69(1985)



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16. Fowkes, F. M.; Chen, W. J.; Fluck, D. J.; Hou, W. H. Particulate Science and Technology (1990 in press) 17. Chessick, J. J.; Young, G. J.; Zettlemoyer, A. C. Trans. Faraday Soc., 50, 587(1954) 18. Harkins, W. D.; Jura, D. J. Amer. Chem. Soc., 66, 919(1944) 19. Gupta, A.; Patel, M. J. MaterialsSci.Letters, 7, 1021(1988) 20. Hou, W. H.; Lloyd, T. B.; J. Appl. Polym.Sci.(in press 1991) 21. Fowkes, F. M. Discuss. Faraday Soc., 42, 243(1966) 22. Romo, L. A. J. Phys. Chem., 67, 386(1963) 23. Griot, O. Trans. Faraday Soc., 62, 2904(1966) 24. Micale, F. J.; Lui, Y. K.; Zettlemoyer, A. C. Discuss. Faraday Soc., 42, 238(1966) 25. Lloyd, T. B.; Li, J.; Fowkes, F. M.; Brand, J. R.; Dizikes, L. J. Coatings Technology (1991 in press) RECEIVED December 4, 1991


Pigmented Polymer Particles with Controlled Morphologies - American

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