Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
1999-10-27
2001-08-28
Acquah, Samuel A. (Department: 1711)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C524S082000, C524S086000, C524S088000, C524S092000, C524S096000, C524S107000, C428S304400, C428S402210, C428S402220, C430S109500, C430S111400, C106S031130, C106S031270, C106S031280
Reexamination Certificate
active
06280511
ABSTRACT:
The present invention relates to latently pigmented microbeads formed of a polymeric material, to preferred types of processes for preparing such microbeads, to processes for preparing pigmented microbeads starting from the latently pigmented microbeads and to color toners for electrophotographic printing techniques and inks for ink jet printing.
Latently pigmented polymeric organic materials are already known. Coating compositions based thereon are, for instance, described in EP-A-0 654 711. A latently pigmented material has one or more pigment precursor (also called “a latent pigment” being uniformly dispersed therein. A pigment precursor is defined as a derivative of an organic pigment comprising a sufficient number of solubilizing chemical groups to render the derivative soluble in certain solvents. These solubilizing groups of the pigment precursor can be split off again by applying heat and/or by contacting the precursor with a Broensted acid, thereby re-forming the insoluble pigment from which the precursor has been derived.
Small sized polymeric particles, in particular corresponding microbeads, having on average a diameter of 1000 microns or less which are formed of thermoplastic polymers and comprise high concentrations of organic pigments are also known, and have become of great interest in many fields of technique during the recent years, for instance, for the production of inks, lubricants, paints, cosmetics or as pharmaceutical carriers. Such microbeads are particularly useful as color toners for electrophotographic printing techniques.
The methods known in the prior art for the production of small sized polymeric particles comprising pigments distributed throughout their mass usually include steps for reducing the pigment particles to a suitable small size and for distributing these pigment particles throughout the polymeric material. Size reduction and distribution are generally carried out by grinding or milling techniques. The pigment/polymer mixtures obtained thereby may, for instance, be melt-homogenized thereafter to obtain a pigmented bulk polymer material. This material must be comminuted again to form pigmented polymeric particles of the desired small size. Processes like these have several disadvantages, for instance, the grinding and milling procedures are highly energy consuming and the pigmented polymer particles obtained by a mechanical comminution are rather sharp-edged and not of a substantially spherical shape. A spherical shape of the particles is however desirable for many uses because this is an essential requirement the compositions to which they are added are to display good flow characteristics and a low viscosity.
Pigmented polymer particles of a more spherical shape and comprising sub-micron sized pigment particles can be provided if more sophisticated preparation methods are used. U.S. Pat. No. 5,629,367, for instance, describes a method for making pigment concentrate particles of a controlled morphology and particle size. However, the process is rather complicated and requires six essential steps, even when the pigment material is already of a suitable size: (1) dispersing the sub-micron sized pigment particles in a pigment dispersing liquid to form a first dispersion phase; (2) blending the first dispersion phase with a polymeric binder to produce a second dispersion phase; (3) mixing the second dispersion phase under sufficient shear to produce a suspension of small dispersion phase droplets in an immiscible suspending liquid; (4) allowing the small dispersion phase droplets in the suspension to coalesce to a larger size at which substantially no further coalescence can occur; (5) mixing the suspension of coalesced droplets with an extracting liquid to remove the pigment dispersant liquid from that coalesced droplets; and (6) isolating the resulting pigment concentrate particles.
Therefore, it is an objective of the instant invention to provide polymer particles, in particular microbeads of a regularly spherical shape, which are uniformly filled throughout their entire mass with organic pigment particles, particularly of sub-micron size, using a simple process which does not require grinding, milling or any other comparatively energy consuming mechanical procedure.
The instant invention is based on the surprising finding that pigment precursors can very simply be uniformly incorporated into polymer microbeads and can furthermore be transformed into the corresponding pigments without a substantial impact on the shape of the microbeads.
The invention therefore provides thermoplastic polymer microbeads having distributed throughout the polymer one or more compounds which are pigment precursors, the melting temperature of the lowest-melting polymer of the microbeads being higher than the lowest temperature required to transform all pigment precursors within the microbeads into the respective pigments.
The number of thermoplastic polymers in the microbeads is for example from 1 to 12, preferably from 1 to 4. The number of pigment precursors in the microbeads is for example from 1 to 10, preferably from 1 to 3.
In the instant application the term “microbead” means particles which are substantially regularly curved and which are substantially free of sharp edges. Preferably the microbeads have a substantially spherical shape.
The microbeads according to the invention can be transformed into microbeads which are filled throughout their entire mass with the corresponding pigments of sub-micron size by heating the microbeads to a temperature which is below the melting point of the polymers of which they are formed but which is high enough to transform the pigment precursors into the respective pigments. Previously contacting the microbeads with a Broensted acid in a way that the Broensted acid can penetrate the microbeads, generally reduces the temperature needed for transforming the pigment precursor into the pigment. For example, room temperature (15 to 35° C.) is generally sufficient for carrying out the transformation in presence of a Broensted acid if the pK
a
value of the acid is below about 3.
The microbeads according to the invention have a diameter of 1000 microns (corresponding to 18 mesh according to the US Bureau of Standards) or less, on average, preferably of 600 microns (30 mesh) or less, in particular of 250 microns (60 mesh) or less, for example about 40 microns (400 mesh). The microbeads can also be smaller. For instance, microbeads having, on average, a diameter of 0.05 to 40, in particular 1 to 20 or 1 to 10 microns can be used. A particularly preferred embodiment of the microbeads according to the instant invention has, on average, a diameter of 1 to 250 microns, in particular 3 to 180 microns. The average particle diameters can, for instance, be determined by sieving techniques or, particularly in case of diameters below of 40 microns, by scanning electron microscopy.
The microbeads according to the invention may comprise virtually any type of thermoplastic polymer or copolymer, for instance, one or more of the usual and well-known poly(meth)acrylates, including polymethylmethacrylates, polymethylacrylates, polyethylmethacrylates, polyethylacrylates, styrene-methyl(meth)acrylate copolymers, ethylene-methyl(meth)acrylate copolymers, ethylene-ethyl(meth)acrylate copolymers, polyamides, such as polycaprolactam or polyamide-6,6, or polyamide-6,10, polyesters, polycarbonates, polyurethanes, polyethers, polyvinylalcohols and corresponding copolymers, polystyrenes such as SAN, ASA and ABS, e.g. obtained from monomers like styrene itself, acrylonitrile, methylmethacrylate, butadiene, maleic anhydride, &agr;-methyl styrene, divinylbenzene, or with alkyl and chlorine substituted styrenes, as well as blends of polymers like the aforementioned, such as polyester/polyamide (as described e.g. in U.S. Pat. No. 4,975,220).
Polymers having high melting points are normally preferred because they allow a particularly simple, quick and efficient transformation of the pigment precursors into the corresponding pigments at temperatures which
Bujard Patrice
Schaedeli Ulrich
Acquah Samuel A.
Ciba Specialty Chemicals Corp.
Crichton David R.
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