Polymer-encapsulated pigments

Some encapsulation methods of polymer-encapsulated pigments often do not perform well for thermal inkjet inks. A common method involves capsules formed with dissolved polymers that are adsorbed onto the pigment surface. 

Since the dissolved polymers are usually not crossUnked, the pol nner surfaces of such known encapsulated pigments t5rpically agglomerate under the high thermal shear and solvent conditions of the pen firing chamber, causing nozzle and ink channel blockages. 

the combined surface areas of the polymer and pigment particles adversely modify the viscosity of the inlqet such that drop ejection rates are limited to lower rates. Moreover, the pigment and the pol5mer often dissociate in the fibers of plain paper, reducing both the otherwise durability and optical density of the print. 

In an in-situ encapsulation process, a dispersion including monomers and the pigment particles can be homogenized. The monomers in the dispersion are then polymerized at the surface of the pigment so as to encapsulate the pigment particles. 6-15% of the monomers are acidic. The acidic monomers can assist the pigment in remaining properly dispersed in the inkjet ink. As a polymerization initiator, potassium persulfate may be used (20). 

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H. Liu and P. Bruinsma, Thermal ink-jet including polymer encapsulated pigment, US Patent 7997713, assigned to Hewlett-Packard Development Company, L.P. (Houston, TX), August 16,2011. 

As mentioned previously, polymer-encapsulated pigments are of major implication in the coating industry and, consequently, a variety of monomers have been used to produce the shell, including vinyl acetate as well as combinations of vinyl acetate, isobutyl acrylate, styrene, -butyl acrylate, methyl methacrylate, acrylonitrile, and... 

Microcapsules containing polymer and pigment were prepared in [299] by dispersing a viscous suspension of pigment and oil-soluble shell monomer forming o/w emulsions. Subsequently, a water-soluble shell monomer was added to the emulsion droplets, encapsulating them via interfacial polycondensation. These microcapsules were then heated for free radical polymerisation of the core monomers. It has been shown that polyvinyl alcohol (PVOH) used as stabiliser reacts with the oil-soluble shell monomers. The decrease of PVOH concentration as result of this interaction leads to coalescence of the particles and to the increase of their equilibrium particle size, however, methods are proposed to prevent the depletion of PVOH. 

Encapsulated pigment 100% Silica (PCR-types), polymer (PCA-types)... 

Although the synthesis of polymer colloids has been known for almost a century, the elaboration of organic/inorganic particles is only in its infancy. Some fifty years ago, early methods focused primarily on pigment encapsulation for the paint industry, and were mainly motivated by the problem of pigment dispersion. Since then, research interest in polymer-encapsulated minerals and the so-called colloidal nanocomposites has exploded such that the subject now relates to a wide variety panel of inorganic particles including mineral oxides, aluminosilicates, metal colloids, and semiconductors. 

The precursors of thermosetting polymers are usually one of the ingredients of complex formulations. They may be present in very small amounts, as in the manufacture of abrasive disks where the thermoset acts as an aggluti-nant in medium amounts, as in the case of filler-reinforced thermosets or as the only components, in formulations used for encapsulation purposes. Apart from fillers, fibers, pigments, etc., some formulations contain rubber or thermoplastic modifiers that phase-separate upon the polymerization reaction (cure). 

For the encapsulation of pigments by miniemulsification, two different approaches can be used. In both cases, the pigment/polymer interface as well as the polymer/water interface have to be carefully chemically adjusted in order to obtain encapsulation as a thermodynamically favored system. The design of the interfaces is mainly dictated by the use of two surfactant systems, which govern the interfacial tensions, as well as by employment of appropriate functional comonomers, initiators, or termination agents. The sum of all the interface energies has to be minimized. 

The most recent developments in pigmenting technology relate to ease of handling, dust free, colour concentrates (73) and dosing on line. Such technology covers encapsulation (385) and the use of polymer carriers (39, 321). 

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