World Journal of Textile Engineering and Technology, 2015 ...

World Journal of Textile Engineering and Technology, 2015, 1, 1-22 1

E-ISSN: 0000-0000/15 © 2015 Synchro Publisher

Current Status and Future Outlook Pertaining to Encapsulation of Organic Pigments: Review Article

O.A. Hakeim*, A.A. Arafa and L.A.W. Abdou

Textile Research Division, National Research Centre, Dokki, Cairo, Egypt

Abstract: Organic pigments have a wide range of commercial applications in coatings, printing and paint industries. Encapsulating pigments with various polymers is a promising approach for improving the quality of the pigment

dispersion. This article gives a summary of the most commonly and new processes along with techniques for encapsulation of organic pigments. It must be noted that a successful encapsulation technique should not impair the original color appearance of pigments, but enhance their dispersion. The review describes a wide variety of polymeric

materials or functional nanomaterials can be used for encapsulation to create nanoscale organic pigments with completely new properties. Many techniques have been reviewed for encapsulating organic pigments such as emulsion or miniemulsion polymerization, phase separation, layer-by layer assembly, sol-gel, and free radical precipitation

polymerization. Additionally, some living materials such as macro-RAFT copolymer and polymerizable dispersant were also reported for pigment encapsulation. The effect of polymer encapsulation on electrophoretic property of organic pigments has been also reviewed.

Keywords: Pigments, Encapsulation, Miniemulsion, Layer by layer and UV-curable resins.

1. INTRODUCTION

Organic pigments have been extensively used in

coating, ink and plastic industries or even in color filters

[1] for electronics and communication apparatus since

they have many advantages such as photosensitivity,

color strength, excellent transparence and etc. Pigment

coatings for textiles in dyeing and printing have many

advantages [2], such as a simple and short product

process, little wastewater, and a low production cost.

However, traditional pigment coatings cannot satisfy

the demand of textile industry for their large particles,

poor stability and color performance that greatly limit

the organic pigment application in textiles [3].

Pigment and polymer latex are the most important

ingredients in water-based paint and ink formulations

[4]. During film formation, latex particles coalesce to

form a polymer film covering the substrate surface while

the presence of pigment particles in the film provides

the final coating with color and influences other appear-

ance properties, such as opacity and gloss [5]. With

such an important role, pigments are typically manufac-

tured with a primary particle size that is designed to

deliver optimum effects in the paint film [4]. However,

due to their surface properties and small size, usually

sub-micrometer, one of the most challenging tasks for

the paint technologist is to disperse pigments to their

primary particle size and to maintain the quality of that

dispersion throughout the manufacturing, storage, and

*Address correspondence to this author at the Textile Research

Division, National Research Centre, Dokki, Cairo, Egypt; Tel: +202-33371499; E-mail: [email protected]

most importantly, through the film formation process to

the final coating [6].

A general problem with latex paints is that pigment

agglomeration occurs during the film formation process,

forming pigment aggregates. The pigment particles are

easy to aggregate to form larger particles due to the

van der waals attractive forces, which represent a big

disadvantage of pigments compared with dyes [6, 7].

Pigment agglomeration thus significantly reduces the

pigment efficiency, resulting in a lower quality product

at higher cost. Moreover; the appearance of aggre-

gates on the film surface reduces surface smoothness

and leads to lower gloss [5]. Thus, the organic pig-

ments must be modified before using .The coating of

organic pigments by polymers may be of great benefit

to improve their processing. [8]. Polymeric dispersants

during the last years have proven good properties in

stabilizing pigments in coating system [9, 10]. In recent

years, more and more copolymers have been synthe-

sized and applied in pigment dispersions.

Recently, researchers reported ultimate solutions to

avoid pigment agglomeration by encapsulating the pri-

mary pigment particles with a layer of binder polymer,

creating polymer shells that ensure that the pigment

particles remain separated during film formation. For

this approach to be effective, the process needs to be

very efficient, ensuring that all pigment particles are

coated and avoiding such problems as pigment agglo-

meration during the encapsulation process [5]. In the

last 10 years a large number of successful encapsula-

tions have been reported, it will be dealt with in this

review.

2 World Journal of Textile Engineering and Technology, 2015, Vol. 1 Hakeim et al.

2. DEFINITION: PIGMENTS–DYES

Colorants are classified as either pigments or dyes.

Pigments are inorganic or organic, colored, white or

black materials, which are practically insoluble in the

medium in which they are incorporated. Dyes, unlike

pigments, do dissolve during their application and in

the process lose their crystal or particulate structure. It

is thus by physical characteristics rather than chemical

composition that pigments are differentiated from dyes

[11, 12]. In many cases, the general chemical structure

of dyes and pigments is the same. Partial solubility of

the pigment is a function of application medium and

processing conditions. Under certain circumstances, it

may even be advantageous to have a pigment dissolved

to some degree in its binder system, in order to

improve certain application properties such as tinctorial

strength and rheological behavior. The performance of

a colorant in its role as a commercial pigment is there-

fore defined by its interaction with the application med-

ium under the conditions that govern its application [12].

3. ORGANIC–INORGANIC PIGMENTS

In some application areas, inorganic pigments can be

used frequently in combination with organic pigments.

Most inorganic pigments exhibit excellent hiding power.

Their rheology is usually an advantage, being superior

to that of most organic pigments under comparable

conditions. However, many inorganic pigments have

much less strength than organic pigments and, the

spectral range that is accessible by inorganic pigments

alone is very limited. Many hues cannot be produced in

this manner by inorganic pigments. Besides, the poor

tinctorial strength and lack of brilliance restricts the use

of inorganic pigments in printing inks. There are areas

of application, however, where it is hardly, at all,

possible to replace the inorganic species by an organic

pigment. The ceramics industry, for example, requires

extreme heat stability, which precludes the use of

organic compounds. Thus, the organic and inorganic

classes of pigments are generally considered comple-

mentary rather than competitive [12].

The application of organic pigments dates to

antiquity. They were used not only for dyeing textiles

but also, due to their ability to adsorb on mineral based

substrate such as chalk and china clay, were used for

solvent resistant coatings for decorative purposes.

4. CLASSIFICATION OF ORGANIC PIGMENTS

Publications have over the course of the years

proposed several classification systems for organic

pigments. Basically, it seems appropriate to adopt a

classification system by grouping pigments either by

chemical constitution, or by coloristic properties. Strict

separation of the two classification systems is not very

practical, because the categories tend to overlap; so it

is useful to list pigments according to chemical

constitution. A rough distinction can be made between

azo and nonazo pigments; the latter are also known as

polycyclic pigments. The commercially important group

of azo pigments can be further classified according to

structural characteristics, as by the number of azo

groups, by the type of diazo or coupling component. On

the other hand, polycyclic pigments may be identified

by the number and the type of rings that constitute the

aromatic structure [12].

5. ENCAPSULATION OF ORGANIC PIGMENTS BY MINIEMULSION POLYMERIZATION

5.1. Miniemulsion Technique

For the preparation of nanoparticles from radically

polymerized monomers, the emulsion polymerization is

often applied. As the process of emulsion polymeriza-

tion is limited because of diffusion processes, the

generation of complex structures is often difficult or

even impossible. An elegant way to circumvent these

problems is the miniemulsion process [13]. Miniemul-

sion polymerization was found to be particularly

attractive to obtain polymeric nanoparticles, which

cannot be achieved by current procedures [14, 15].

Miniemulsions consist of a liquid/liquid dispersion of a

monomer phase in water with diameters in the range of

approximately 50–500nm. The size of the monomer

droplets is usually controlled by shearing the system in

the presence of a surfactant and an hydrophobe

(costabilizer), whose role is to stabilize the emulsion

against diffusion degradation (Ostwald ripening).

Contrary to conventional emulsion polymerization, the

monomer droplets are sufficiently small and numerous

so that the polymerization predominantly occurs by

radical entry into the preexisting miniemulsion droplets

without formation of new particles. Miniemulsion

polymerization is therefore particularly attractive for the

encapsulation reaction of any compound that can be

satisfactorily suspended into the monomer phase [16,

17] as schematically represented in Figure 1.

5.2. Organic Pigments and Carbon-Based Materials

Direct dispersions of carbon black or organic

pigments in the monomer (e.g. styrene) are possible;

however, a limited pigment content of about 10wt% in

the monomer phase can be used for further processing

Current Status and Future Outlook Pertaining World Journal of Textile Engineering and Technology, 2015, Vol. 1 3

due to a drastic increase of the viscosity of the organic

phase, making it difficult to disperse this phase in

aqueous media [18,19]. A great improvement with

respect to the amount, which can be encapsulated, is

offered by the so-called co-sonication process. Instead

of directly dispersing the pigment in the monomer, in

the first step of the process, a dispersion of the

respective pigment in water is generated using a

surfactant [20]. This dispersion is then mixed with a

monomer miniemulsion stabilized with the appropriate

surfactant. A fusion/fission process triggered by ultra-

sonication leads to an encapsulation of the hydropho-

bic or hydrophobized pigment into the monomer drop-

lets. [16]. Subsequent polymerization of the monomer

allows the formation of hybrid nanoparticles. Initially

developed for carbon black [16], this technique was

also successfully applied for other organic pigments

(Figure 2) [20]. Using the cosonication process, the

ratio pigment to polymer can be varied in a wide range

and allowed the formation of hybrid particles with up to

80wt% of pigment. The successful encapsulation could

be shown by TEM and, in the case of using carbon

black, with nitrogen sorption measurements. [16]. The

pigment itself cannot take over osmotic droplet stabili-

zation without the hydrophope as the number of aggre-

gates is too low to be able to create a significant osmo-

tic pressure. Besides, the ultrahydrophobic compo-

nents serve as mediator between the pigment surface

and the monomer or the resulting polymer.

5.3. Encapsulation of Organic Pigment into UV-Curable Resins via a Miniemulsion Technique

The preliminary investigations on encapsulation of

nano-scale Pigment Red 122 into a UV-curable system

containing polyester acrylate (oligomer) and 1,6-hexa-

nedioldiacrylate (monomer) using the mini-emulsion

technique have been reported by Hakeim et al. [21].

TGA, SEM and ultracentrifuge sedimentation results

showed that CI Pigment Red 122 is successfully encap-

sulated into polyester acrylate/HDDA resins (Figure 3).

The particles of encapsulated pigment exhibit spherical

shape heterogeneously distributed with an average

size close to 100nm (Figure 4). The oligomer (polyester

acrylate) in the presence of organic pigment could

stabilize the mini-emulsion droplets without introducing

any other hydrophobes (co-stabilizer) in the formula-

tion. The most stable emulsions, as shown by the long-

est shelf-life results, were those obtained in the absence

Figure 1: Schematic representation of the encapsulation reaction of organic pigments through miniemulsion polymerization [19].

Figure 2: Encapsulate of materials by the cosonication process: (a) carbon black in PS; (b) nanotubes in PS; (c) azo-pigment in PS [13].


of known hydrophobe. Polyester acrylate oligomers

may be used as a costabilizer as long as they meet the

criteria of hydrophobe that being essentially insoluble in

water, but soluble in the monomer mixture, and are

innocuous in the final product. Conventional hydroph-

obes such as hexadecane goes through the polymeri-

sation reaction unchanged [19,20] as a residue of the

organic compound. Therefore, it is preferably removed,

requiring an additional step, which is costly and difficult.

Besides, the acrylate oligomers based on polyester

acrylate in the presence of water-soluble initiator stabi-

lizes the mini-emulsion droplets and ink formulation

with time. This trial offers a great benefit of industrial

importance in application of pigmented UV-curable inks

for textiles and, could find numerous applications in

surface coating. Using the encapsulated pigment in

UV-curable inkjet printing on textiles is a much simpler

and cheaper and environment friendly method. The

printed fabrics had soft handle and very good fastness

properties [22, 23]. The US Environmental Protection

Agency views UV-curable inks as a green technology

that it deems preferable to conventional solvent-based

ink systems [24].

6. ENCAPSULATED ORGANIC PIGMENT DISPER- SION BY LIQUID PHASE SEPARATION TECHNIQUE

This preparation method is on the basis of the liquid

droplet coalescence method followed by phase separa-

tion belonging in the physicochemical method category

[25].

Figure 4: Statistics graph of particle size distribution for encapsulated CI Pigment Red 122 [21].

Namely, the oil droplets dissolving a shell material

are forced to collide and coalesce with the core oil

Figure 3: SEM micrographs: (a) Nano-scale CI Pigment Red 122 dispersion that used SDS as dispersant; (b) mini-emulsion droplets of HDDA/ polyester acrylate encapsulated CI Pigment Red 122; (c) UV curable film of HDDA/polyester acrylate

encapsulated CI Pigment Red 122 [21].


droplets in the continuous water phase. Here, the core

oil droplet phase is a poor solvent for the shell material

but a good solvent for the oil droplet phase dissolving

the shell material. When the oil droplets dissolving the

shell material are forced to collide and coalesce with

the core oil droplets, the shell material has to be

separated in the newly formed oil droplets by phase

separation (Figure 5), because the core oil phase is a

poor solvent for the shell material [26].

If interfacial energy between the continuous phase

and the shell material is lower than that between the

continuous phase and the newly formed oil phase, the

shell material separated in the newly formed oil

droplets has to transfer to the outer region of the newly

formed oil droplet and form the microcapsule shell [26].

Microencapsulation of organic pigment, can improve

the applicable properties of pigments by modifying sur-

face properties. It includes solvent evaporation, spray

drying, liquid phase separation, and suspension cross-

linking and monomer polymerization. Copper phthalocy-

anine (CuPc) was microencapsulated by liquid phase

separation in an organic solvent using a polystyrene

(PS) wall [27]. In this method the wall material was

dissolved in solvent (A); (2) CuPc powder was well

dispersed into the above solution; and (3) solvent (B),

called a coacervation agent, which dissolves solvent

(A) to partially desolvate the wall material but does not

dissolve the wall material, is added to the dispersion

(Figure 6).

The wettability, flowability and dispersibility level for

the microcapsule was improved significantly, compared

to untreated pigment. The molecular weight of wall

material, PS was found to be influential and required

optimization. The higher molecular weight of PS has an

adverse effect on the dispersing level of CuPc. The

longer polymer chains may adsorb two or more

pigment particles and connect them together, leading

to flocculation (Figure 7). Based on these data, the

amount of PS should be carefully controlled; otherwise

a reduction in tinctorial strength may occur.

Figure 6: Schematic representation of microencapsulation mechanisms by coacervation [26].

Microcapsules containing a PS wall were analyzed

by scanning electronic microscopy (SEM, Figure 8).

When PS/CuPc was 5%, a few microcapsules were

observed, but the particle distribution was not uniform

and many small particles existed. When PS/CuPc was

20%, all particles were nearly microcapsules and

particle distribution was uniform.

Ongoing research along this line, Xu and his

workers fabricated colloidal nanoparticles consisting of

oxidized carbon black (CB) encapsulated by poly(vinyl

pyrrolindone) (PVP) by a simple phase separation

method [28]. PVP and CB particles were first dispersed

in water, then acetone, served as a nonsolvent of PVP,

and was added to induce precipitation of the PVP to

Figure 5: Microencapsulation mechanism with liquid droplet coalescence method followed by phase separation [25].


absorb onto the CB particle surface. Finally, initiator

was added to make the particles shell-crosslinked

(Figure 9). TEM indicated a core-shell structure for the

composite (Figure 10).

Excessive amount of acetone can cause the precipi-

tation of CB/PVP nanoparticles before crosslinking re-

action while inadequate amount also cannot sufficiently

precipitate the PVP to induce encapsulation layer on

the surface of CB particles [28].

The crosslinking of PVP shell with loading adequate

amount of initiator provides an effective way to prepare

stable CB nanoparticles in aqueous media (Figure 10).

Copolymers of styrene and maleic acid (PSMA)

were synthesized by free radical polymerization and

used as encapsulating agent for pigment red 122 by

sedimentation, milling and phase separation method

[29]. The pigment dispersion prepared by phase

separation method had higher stability, smaller particle

size, and narrower particle size distribution than those

prepared by milling method with dispersant using molar

content of maleic acid was at 0.43. The results of

contact angle confirmed that the pigment was

successfully encapsulated by phase separation method

and showed that encapsulated pigment can be wetted

by water more easily than those without modification

(Figure 11).

Figure 7: A possible model for bridging flocculation caused by very long PS chain [27].

Figure 8: The SEM of PS microencapsulated CuPc. A: PS/CuPc=5%; B: PS/CuPc=20% [27].

Figure 9: Schematic diagram for the fabrication process of CB/PVP nanoparticles via the phase separation method [28].


Further hydrolyzing of encapsulated C.I.P.R122

(Figure 12) lead to fine and small uniformly dispersion

in aqueous media than without hydrolyzing. This

attributed to the negative charges on pigment surface,

which hinder the pigment particles to combine together.

Based on the aforementioned results a schematic

representation was given for the illustration of the

whole encapsulation process divided into four steps

(Figure 13).

With the same manner, Fu et al. [30] investigated

the encapsulation of CuPc with a copolymer PSMA via

Figure 13: Schematic of encapsulation course by phase separation method [29].

Figure 10: TEM image of CB/PVP nanoparticles obtained by adding 2ml of acetone, with and without crosslinking reaction [28].

Figure 11: Wettability of pigment powder; (a) starting C.I.P.R122; (b) encapsulation C.I.P.R122 by milling method; (c)

encapsulation C.I.P.R122 by phase separation method [29].

Figure 12: TEM photo of pigment in different stage, (a) encapsulated C.I.P.R122 (enlarge about 40,000 times); (b) hydrolyzing

of encapsulated C.I.P.R122 (enlarge about 70,000 times) [29].


a phase-separation technique. Here too, the particles in

the PSMA-encapsulated pigment dispersion were dis-

tributed in aqueous media more uniformly than those of

the PSMA-dispersed pigment in the dispersion (Figure

14).

The apparent viscosity of the PSMA encapsulated

CuPc dispersion was lower than that of the PSMA-

dispersed pigment dispersion. In addition, max of the

PSMA-encapsulated pigment dispersion was lower

than that of the PSMA-dispersed pigment dispersion

[30], and this indicated that the color of the former was

purer than that of the latter (Figure 15).

Figure 15: Absorbency of the PSMA-encapsulated pigment dispersion and PSMA-dispersed pigment dispersion [30].

The effects of process conditions on properties of

pigment red 122 encapsulated by PSMA have been

also evaluated [31]. Hydrolyzing of encapsulated pig-

ment red 122 using optimum amount of sodium hydrox-

ide is accompanied by small particle size, highest and

superior performance of dispersion stability compared

with the other additives, ammonia and triethanolamine.

The effect of sodium hydroxide concentration can be

indicated by the relationship between the amount of

sodium hydroxide and Zeta potentials (Figure 16). The

encapsulation process by PSMA dispersion was

optimized by hydrolysis in 15 minutes at 45ºC; with the

molar amount of sodium hydroxide being about 0.60–

0.68 [31]. The morphology image of pigment red 122

encapsulated by PSMA and particle size distribution

(Figures 17,18) indicated that the dispersing property

was significantly improved after poly (styrene-maleic

acid) encapsulation of the pigment surface under the

set of conditions.

Figure 16: The effect of the amount of sodium hydroxide on Zeta potentials of pigment particle: hydrolysis temperature 45ºC; hydrolysis time 30 minutes; stirring rate 10000r/min [31].

Fu and Wang [32] investigated the rheological pro-

perties of Azo pigment yellow 14 (P.Y.14) encapsulated

Figure 14: Scanning electron microscopy images of the particles in (a) the PSMA-encapsulated pigment dispersion and (b) the PSMA-dispersed pigment dispersion [30].


into copolymer of styrene and maleic acid (PSMA) via

phase separation technique followed by preparation of

composite dispersions. The results revealed that all the

dispersions show shear-thinning behavior at molar

content of maleic acid (FM) in PSMA about 0.38 and

Figure 17: TEM photo of composite particle in the dispersion: MNaOH% 0.64; hydrolysis temperature 45ºC, stirring rate 10000r/min [31].

Figure 18: Particle size distribution of composite dispersion: M NaOH% 0.64; hydrolysis temperature 45ºC, stirring rate 10000r/min [31].

0.68. The composite dispersion was closer to Newton-

ian fluid when FM was about 0.53 than that of 0.38 and

0.68 [32] (Figure 19). At a small amount of PSMA, the

particles could not completely be encapsulated by

PSMA, where the pigment can be easily combined via

van der walls forces, thus resulted in large apparent

viscosity (na). Additional, when PSMA amount was high

enough, the PSMA that was not encapsulated onto the

pigment dissolved into water, which would greatly in-

crease na [32].

Figure 19: Effect of FM on rheological properties of PSMA-encapsulated pigment dispersion, PSMA structure: the amount of initiators to monomers: 1.8 wt%, RC=P%, 12%; pigment content: 10% [32].

Fu et al. [33] reviewed the ink formulations and

color performance of encapsulated pigment yellow 74

(PY74) prepared by the phase-separation technique

and by the milling method using copolymer PSMA. The

encapsulation of pigment onto PSMA layer increases

the wettability and hindered the PY74 to combine with

each other; this led to a small particle size and a

narrow particle size distribution compared to surface –

modified PY74 dispersion (Figure 20).

The nozzle-clogging rate of the ink made from the

PSMA-encapsulated PY74 dispersion was lower than

Figure 20: Particle size distributions of the dispersions [33].


that of the surface-modified PY74 dispersion. These

poor stability and printing performance [33] might have

been due to the larger particle size and the easier

clogging the nozzle of the printhead (Figure 21). The

color strength, rubbing fastness, and washing fastness

of the fabrics were better when they were printed with

the PSMA-encapsulated PY74 ink.

Figure 21: Printing performances of the pigment ink [33].

7. SURFACE CHARGE MODIFICATION USING

LAYER-BY-LAYER DEPOSITION FOR ORGANIC PIGMENT ENCAPSULATION

Layer-by-layer deposition is a simple and versatile

method to construct multiple layer thin films on the

surface of a substrate [34]. Electrostatic interactions

facilitate the adsorption of a polyelectrolyte on the

surface, consequently imparting its charge to the

substrate. This cycle can be repeated many times to

increase film thickness with the final charge on the

substrate surface being decided by the charge of the

outermost adsorbed polyelectrolyte.

Layer-by-layer deposition has been successfully used

to form polymer thin films on a number of substrates

such as silica nanoparticles [34], polymer latex and

carbon nanotubes [35-37]. Yuan and his worker [38]

investigated the nano silica-encapsulation of organic

pigment Yellow 109 by depositing multilayer polyelect-

rolyte films. Two kinds of polyelectrolytes, poly-(diallyl-

dimethylammonium chloride) (PDADMAC) and poly-

(sodium 4-styrenesulfonate) (PSS), adsorbed onto the

surfaces of the organic pigment and then coated by

colloidal nano-SiO2. The mechanism of coating of

colloidal nano-SiO2 particles onto the surfaces of the

organic pigment can be schematically described as

shown in Figure 22. The first layer of nano-silica

assembly increased the particle size of the organic

pigment, causing a relatively rough surface and a broad

pore size distribution, but the second and the third

layers of nano-silica assembly preferred to fill the pores

caused by the first layer of silica, causing a relatively

smooth surface, slight increases in shell thickness and

a narrow pore size distribution [38] (Figure 23).

This study proved that the encapsulated nano-SiO2

pigment acquire the UV scattering property [38], after

the second and the third layers of nano-SiO2 assembly,

in turn enhanced the weather durability and dispersion

ability for the organic pigment in waterborne systems.

Benzidine yellow G was applied as organic pigments

since this pigment has good color strength, good trans-

parency, and low cost and is widely used in various

types of printing inks while the poor light fastness and

weatherability limit its applications in coatings, rubbers

and plastics [39]. The properties of the organic pigment,

Benzidine yellow G particles encapsulated with nano-

silica particles via multi-step layer-by-layer self-assemb-

ly technique was further investigated by Yuan et al.

Figure 22: Mechanism for preparation of organic pigment particles coated with colloidal silica particles [38].


[40]. The encapsulation of nano-silica onto the pigment

surfaces improved the dispersibility of organic pigment

in water-borne system (Figure 24). Besides that, the

nano-silica coating could depress the decomposition

rate of organic pigment. Additionally, the acid and alkali

resistance performances of the encapsulated organic

pigment were enhanced.

Figure 23: SEM images of uncoated organic pigment (a), PE6-SiO2-coated organic pigment (b), PE6-2SiO2-coated organic pigment (c), and PE6-3SiO2- coated organic pigment (d) [38].

Figure 24: benzidine yellow G (b); PE6e2SiO2-20 nm coated benzidine yellow G (c); PE6e3SiO2-20 nm coated benzidine yellow G (d); PE6e3SiO2-10 nm coated benzidine yellow G (e); PE6e3SiO2-5 nm coated benzidine yellow G (f) [39].


Ageing test revealed that encapsulated pigment

could shield 254 nm UV ray remarkably, which was

consistent with the nano-silica particle itself [39].

The layer-by-layer deposition method was also

explored by Nguyen et al. [5] to modify the surface

charge of Paliotol Yellow pigment for its eventual

encapsulation with polymer. Cationic polyeletrolyte poly

(allylamine hydrochloride) (PAH) was first adsorbed

onto the negatively charged surface of the pigment.

After PAH adsorption, the pigment was dispersed in a

negatively charged macro-RAFT copolymers solution,

resulting in adsorption of the macro-RAFT copolymers

via electrostatic interactions. The adsorbed living

copolymers by emulsion polymerization successfully

encapsulate the pigment particles (Figure 25).

Figure 25: Polymer encapsulated Paliotol yellow pigment [5].

8. ENCAPSULATION OF ORGANIC PIGMENT PART- ICLES VIA SOL-GEL PROCESS

The sol–gel method seems to be more conventional

for application of nano silica on textile materials, due to

easy processing and acceptable treatment conditions

[41]. In this method, hydrolysis and condensation

reactions of the precursor material, tetraethoxysilane

(TEOS) are carried out to form a nano-colloidal

solution, and a network of nanoparticles will be formed

on the substrate through the gradual evaporation of the

solvent as shown in the reaction shown below:

Si (OC2H5)4 +H2O Si (OH) 4 + 4C2H5OH (1)

2Si (OH) 4 2 (Si O Si) + H2O (2)

A simple approach to encapsulate the organic

pigment, Yellow 109 (2GLTE) particles with silica shell

via sol-gel process using tetraethylorthosilicate (TEOS)

as the precursor was investigated [42]. The surfaces of

the organic pigment were first orderly modified by

poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyl-

dimethylammonium chloride) (PDADMAC) to endow

enough affinity of organic pigment with silica, and give

positively charged surfaces of particles, then coated by

silica via sol-gel process of TEOS. The polyvinyl-

pyrrolidone (PVP) adsorbed on the surfaces of organic

pigment could not offer enough affinity for organic

pigment to silica (Figure 26b). Comparing with the

original organic pigment (Figure 26a), the relatively

Figure 26: TEM images of original organic pigment (a) and uncharged organic pigment particles modified by PVP (b), positively

charged organic pigment coated with silica (c), positively charge organic pigment coated with silica in the presence of PVP (d) [42].


much darker surfaces of the organic pigment particles

indicated that silica was successfully coated on the

surfaces of the organic pigment particles. However, it

seemed that the silica coated the organic pigment

particles was very inhomogeneous. The high amount of

silica coating corresponded to thicker silica coating,

indicated that the thickness of silica coating could be

easily modulated with various TEOS loading (Figure

27).

Figure 27: Variation of silica content coated organic pigment with different TEOS loading [42].

Yuan et al. [43] further investigated the organic pig-

ment coating with titania to improve the weatherability

and dispersion ability in waterborne system via sol-gel

process, then coated by titania via sol-gel process from

titanium n-butoxide (TBOT) (Figure 28).

It is interesting to mention that, the two polyelectro-

lyte layers adsorption could guarantee the titania

coating on the surfaces of the organic pigment particles

efficiently [43], probably because more polyelectrolyte

layers resulted in more hydration on the surfaces of

pigment particles. The water and TBOT content

strongly influences the morphology of the titania-coated

organic pigment particles (Figure 29). At higher water

level, more titania particles formed on the surfaces of

the organic pigment particles due to the faster hydro-

lysis and condensation of TBOT. Coating of titania

could considerably improve the UV shielding property

and thermal stability of organic pigment. It was claimed

that this method was more efficient than the organic

pigment particles coated with colloidal nanosilica via

layer-by-layer assembly.

Organic pigments have been successfully used as

photosensitizers for TiO2 [44]. Thus, without special pre-

cautions, organic pigments might fade quite significant-

ly with time if used in conjunction with a photo catalyst

[45].

Figure 28: TEM images of original organic pigment (a) titania-coated organic pigment with two polyelectrolyte layers (b); four polyelectrolyte layers (c) [43].

Figure 29: TEM images of the Titania coating organic pigment with different content water: (a) 3.8 mol/L (b) 4.5 mol/L); (c) 6.0 mol/L [44].


Recently Fabjan and his workers [44] succeeded in

protection of organic pigments, (ß copper phthalocy-

anine) against photocatalysis by encapsulation process

via sol-gel process. The aim of their approach is to

prepare a thin transparent layer (inorganic shell)

around each individual pigment particle. The thin

protective layer should not only be resistant against the

reactive species released during photocatalysis but

also extremely compact (non-porous) to prevent

penetration of such species through it.

The obtained silica shells served as an efficient

protection against the highly reactive products of photo-

catalysis (Figure 30). The thickness of silica shells

depend mainly on the temperature and pH of prepara-

tion (Figure 31). The degree of protection depends not

only on the thickness of silica shells but also on their

porosity. The thin and compact silica shell around

individual pigment particles may be a good trial for

protection of organic pigment that found in the vicinity

of photocatalytically active paint additives. Thicker and

smaller mesoporosit silica shells showed a better

protection. It is essential that porosity of the protective

shell around the pigment be minimized.

9. ENCAPSULATION OF ORGANIC PIGMENT USING A POLYMERIZABLE DISPERSANT VIA EMULSION POLYMERIZATION

The dispersants play the role in building voluminous

shells and intensifying charges on the pigment surface

that help resist flocculation and coagulation of pigments

in media [46-48]. Nowadays, many structured polymeric

dispersants have been developed for pigment disper-

sions [49-51]. These polymeric materials can also be

employed to encapsulate pigments by techniques such

an emulsion or mini-emulsion polymerisation [19, 20,

21, and 52], phase separation [53, 54], in situ polymeri-

zation [55, 56], layer-by-layer assembly [40], sol-gel

[42], and free-radical precipitation polymerisation [57].

Few researches are investigated the polymerizable

dispersant, which contained allyl groups in emulsion

Figure 30: Evaluation of the efficiency of pigment protection against photocatalysis by fast irradiation method. The protection efficiency is expressed as the total colour change (DE) of TiO2-pigment mixture. This colour change is monitored as a function of time for the unprotected (UP) and differently encapsulated pigment particles (EP). Two UV light intensities were used: a 60 and b 180 W m-2 [44].


polymerization for encapsulation the pigment. Nowa-

days, some pigments that are encapsulated by poly-

merizable dispersant were also developed [58, 59].

A novel polymer-encapsulated C.I. Pigment blue

15:3 dispersion by a polymerizable dispersant has been

developed [58]. The encapsulation process involves

three steps: predispersing C.I. Pigment blue 15:3 using

allyloxy nonyl-phenoxypropanol-polyoxyethyleneethe-

rammonium sulfonate (ANPS, Figure 32) as dispersant

and emulsifer, emulsifying with the addition of comon-

omers, and polymerizing comonomer and ANPS.

(CH2)8

O

H2C O

OH

O C2H4 CH2 SO3NH4

n

Figure 32: Chemical structures of ANPS [62].

The similarity of the hydrophobic benzene ring of

styrene to the aromatic structure of pigment promotes

the attractive forces between pigment and styrene, thus

reduces the desorption of commoners and ANPS from

pigment surface, facilitating to form smaller particles

and larger amount of copolymer content. The thickness

of core–shell encapsulation layer can be adjusted

according to the mass ratio of monomer to pigment

(Figure 33). The encapsulated pigment dispersion had

improved wettability and enhanced stabilities to centri-

fugal force, temperature, pH value and electrolytes.

Encapsulation of phthalocyanine blue pigment with

a polymerizable dispersant via emulsion polymerization

was also studied by Fu et al. [60]. Small particles was

obtained when the mass ratio of ANPS to phthalocy-

anine blue pigment, styrene (St) to phthalocyanine blue

pigment, and ammonium persulfate (APS) to St was

Figure 31: TEM micrographs of a Sample encapsulated pigment at 70ºC -pH10-one shell, b Sample encapsulated pigment- at 70ºC -pH8-one shell and c Sample encapsulated pigment- at 70ºC -pH10 with two number of shell [44].

Figure 33: TEM imagines of (a) original C.I. Pigment blue 15:3, (b) polymer encapsulated pigment dispersion with a mass ratio of styrene to pigment of 0.1, and (c) polymer- encapsulated pigment dispersion with a mass ratio of styrene to pigment of 0.2 [60].


about 0.2, 0.2, and 0.01, respectively. The emulsion–

polymerization process for encapsulation of phthalocy-

anine blue pigment with a polymerizable dispersant

includes four steps was illustrated in Figure 34.

The polymer-encapsulated phthalocyanine blue

pigment dispersion had an enhanced stability com-

pared with the conventional phthalocyanine blue pig-

ment dispersion.

Fu et al. [61] further investigated the encapsulation

of phthalocyanine blue pigment with a polymerizable

dispersant for inkjet printing inks. The analyses proved

the polymer encapsulation layer onto phthalocyanine

blue pigment surface and had a narrow particle size

distribution. XRD indicated that the crystal structure of

phthalocyanine blue pigment was not changed during

the encapsulation process. The wettability, dispersion

stability to temperature and centrifugal forces were

improved after polymer encapsulation. Its rheological

behavior was close to Newtonian fluid. This method

provided a novel and practical solution for preparing

the encapsulated phthalocyanine blue pigment disper-

sion for formulation of inkjet printing ink.

10. PIGMENT ENCAPSULATION USING MACRO-RAFT COPOLYMERS

Recent advancements in the use of Reversible

Addition Fragmentation chain Transfer (RAFT), tech-

nology have demonstrated that nanoparticles can be

successfully encapsulated using macro-RAFT copoly-

mers [5, 62]. The method is based on the adsorption of

macro-RAFT copolymers on the particle surface

followed by the growth of each individual RAFT copoly-

mer during polymerization as was illustrated in Figure

35.

Figure 34: Emulsion polymerization process for encapsulation of phthalocyanine blue pigment [59].

Figure 35: Schematic diagram for RAFT technology [5].


For particles with a hydrophilic positively charged

surface such as titanium dioxide pigment [5] and

Gibbsite nanoclay [62], the dispersed particles carry a

net negative charge that helps to stabilize them in

water. However, there is a large range of particle types

such as silica nanoparticles [34], nanoclays [63] and

pigments, which carry a net negative charge, the

dispersion and encapsulation of such particles in water

requires the macro-RAFT copolymers to have a

positive charge on the polymer backbone, inquiring the

design and synthesis of a whole new range of macro-

RAFT copolymers.

The encapsulation of both hydrophilic inorganic,

zirconia and alumina coated titanium dioxide and

hydrophobic organic, phthalocyanine blue pigments

with poly (methyl methacrylateco- butyl acrylate) using

living amphipathic random macro-RAFT copolymers

has been reported [5]. Encapsulated organic pigments

by thick polymer shells were formed that had core-shell

morphology (Figure 36).

The encapsulated particles by this trend were stabi-

lized in the aqueous phase by an anchored hydrophilic

layer of negatively charged carboxyl groups on the

surface. The method was found to be incredibly effi-

cient in that 100% of the pigment particles were

encapsulated and almost all of the polymer growth was

within the encapsulating polymer shells. This is first

attributed to the use of macro-RAFT copolymers as

stabilizers that do not self-assemble in the aqueous

phase and/or do not form centers for secondary particle

nucleation, leading to the formation of a uniform coat-

ing over the entire particle surface. This encapsulation

approach is simple and can be readily scaled up for

industrial production.

11. ENCAPSULATED ORGANIC PIGMENTS VIA FREE-RADICAL PRECIPITATION POLYMERIZATION

Nanoscale azo pigment yellow 13 (PY13) was

encapsulated by PSMA with a free-radical precipitation

polymerization [57]. The four step schematic diagram

for the encapsulation course by free-radical precipita-

tion polymerization was shown in Figure 37. The

results revealed that, the molar ratios of the feeding of

maleic acid, styrene, initiator and pigment have an im-

pact on particle size and the stability of the PY13/

PSMA dispersions.

Figure 36: TEM images of poly (methyl methacrylate-co-butyl acrylate) encapsulated phthalocyanine particles: (A) poly (methyl methacrylate-co-butyl acrylate) encapsulated phthalo-

cyanine blue latex particles and (B) an even coating over a particle with large aspect ratio. Macro-RAFT RAFT was used as a dispersant. Emulsion polymerization was carried out at 70 °C using MMA/BA (7:3 by weight) monomer mixture and V-501 initiator [5].

The morphological structure indicated that the

particle size of encapsulated pigment (Figure 38b) was

uniformly distributed and was a little larger than that of

PSMA-dispersed PY13 (Figure 38a) in the dispersion

[57].

Figure 37: Schematic diagram of the encapsulation course by free-radical precipitation polymerization [57].


The particle size of the encapsulated pigment

mainly distributed in the range 50–700 nm and was

smaller than that of PSMA-dispersed PY13 dispersion.

12. ENCAPSULATION OF ORGANIC PIGMENTS FOR ELECTROPHORETIC COLOR DISPLAY

Electrophoretic displays (EPDs) have attracted a

great deal of interest because of their merits of good

brightness and contrast, wide viewing angles, and low

power consumption [64-68]. Organic pigments due to

their brilliant colors, great brightness, low-specific

gravity and easiness of being charged, are suitable to

use for EPDs. The research progress on using pigment

particles as electrophoretic particles have shown that

the pigment particles in the microcapsules used for

EPDs can move reversibly at a constant velocity in a

DC electric field.

The effect of polymer encapsulation on electro-

phoretic properties of organic pigment nanoparticles in

low dielectric medium was investigated [68]. The blue

organic pigments were encapsulated with poly (methyl

methacrylate) via two-step dispersion polymerization of

methyl methacrylate in presence of organic pigments.

Vinyl imidazole was adopted as a functional co-

monomer to enhance the electrophoretic properties of

the pigments. The results indicated that the polymer

coated organic pigments possess an average net

charge of plus 12mV on their surface, while the

uncoated organic pigments exhibit an average charge

of minus 1mV on their surface. The surface charge of

P(MMA-co-vinyl imidazole) coated organic pigments

changed from negative to positive upon coating with

organic pigments, pertaining to the comonomer, vinyl

imidazole, affected the charge of particle. The charge

control agent (CCA), OLOA 1200 improves the only

minus charge of the nanoparticles and more effective

to the colored particles than to the white particles when

the two particle systems were investigated for their

electrophoretic properties. The electrophoretic response

was observed to be much better than the neat organic

pigment [69]. Nanoscale red organic pigment was

encapsulated with poly (acrylamide-co-methylmetha-

crylate) (PAAm-co-MMA) using in-situ dispersion

polymerization to enhance the electrophoretic move-

ment [70]. The results clearly demonstrated that the

irregular shape of organic pigment nanoparticle was

still maintained through the coating process (Figure

39). Zeta potential and mobility value indicated that

Figure 39: SEM image of (a) unencapsulated pigment and (b) Poly (AAm-MMA) encapsualted organic pigment [70].

Figure 38: Transmission electron micrographs of the (a) encapsulated pigment dispersion and (b) PY13/PSMA dispersion [57].


polymer encapsulated organic pigment had average

net charge of plus 6mV on their surface, while the

uncoated red pigment has no specific value, demon-

strating that the introduced acrylamide functional group

helps to enhance the plus charge on their surface. The

electrophoretic movement of the electrophoretic parti-

cles can be detected by observing color transition when

the product particles were dispersed with the white

pigment particles (Figure 40). The color difference for

polymer coated pigment red was observed to be more

distinct than for raw pigment red in the presence of

charge transfer agent. It showed also that the polymer

encapsulation enhance the electrophoretic mobility of

organic pigment.

Diarylide yellow pigments/modified SiO2 core/shell

hybrid composite particles were fabricated via a newly

developed two-step procedure. The surface of diarylide

yellow pigments was coated with SiO2 by the hydrolysis

of Na2SiO3. The obtained particles were then modified

with in situ generated 3-aminopropylsilanetriol or

succinic acid to form composite particles containing

amino or carboxyl groups.

The results showed that the obtained composite not

only have organic pigments’ brilliant color and inorganic

pigments’ good solvent resistance, but also have a

core-shell structure with smooth surface morphology

and a high chargeability, which can be applied in EPDs

triumphantly [71].

13. SUMMARY AND OUTLOOK

It is shown that the encapsulation of organic pig-

ments can be used to create numerous hybrid nano-

particles have a large number of many different

materials and compounds in a wide variety of different

(functional) polymeric shells using different techniques.

The most commonly and new processes and tech-

niques for encapsulation of organic pigments are

reviewed in the article. A compatibilization step is often

required between the organic pigments and monomer/

polymer shell so that the pigment is easily to be

incorporated in the shell prior to polymerization. Special

intention is paid to the encapsulation of pigment using

miniemulsion polymerization, the most appropriate

particularly attractive method to obtain polymeric nano-

particles which would ideally offer the nanocapsules

with the stated high requirements compared with the

other techniques. Besides, encapsulated organic pig-

ment with a copolymer via a phase-separation tech-

nique was found to be distributed in aqueous media,

more uniformly than those milled and dispersed pig-

ments. The thickness of silica shell played a deter-

mined role in the properties of encapsulated pigment,

irrespective of the method of encapsulation. The encap-

sulation of pigment using macro-RAFT copolymers

have shown to be incredibly efficient in that 100% of

the pigment particles can be encapsulated within the

polymer shell. We are convinced that many more

functional encapsulated organic pigments for a wide

field of applications, inks, paints and electrophoretic

displays can still be created in the future.

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Received on 01-09-2015 Accepted on 15-09-2015 Published on 05-10-2015

© 2015 Hakeim et al.; Licensee Synchro Publisher. This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

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