Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-08-09
2004-04-27
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C526S214000
Reexamination Certificate
active
06727318
ABSTRACT:
The present invention relates to a process for preparing dye-comprising aqueous polymer dispersions by free-radical aqueous emulsion polymerization of ethylenically unsaturated monomers, in which at least some of the monomers are employed in the form of an oil-in-water emulsion E1 whose disperse phase comprises at least one oil-soluble dye.
Dye-comprising polymers can in principle be prepared in two different ways. One approach is to take dyes which are of sufficient solubility in polymers, possibly at elevated temperature and to dissolve them homogeneously in these polymers. The advantage in this case lies in particular in the high level of brilliance of the color produced and in the high color strength, i.e., a low dye requirement. However, this approach does have the disadvantage that, under a thermal load or when the polymers are in contact with solvents, the dyes migrate; this results in reduced properties of fastness with respect to weather and migration, since dye molecules that have migrated to the surface of the substrate are attacked by light or oxygen or leached by solvents. The alternative approach is to disperse insoluble color pigments in the polymer matrix. This avoids the disadvantages which may occur when the colorants are dissolved homogeneously in the polymer matrix. A disadvantage of this approach, however, is that the individual molecules of colorant in the pigment crystals interact with one another, leading to a broadening of the absorption bands and thus to an unwanted reduction in the purity of shade and the strength of coloration. Furthermore, the shape anisotropy of the pigment crystallites gives rise to rheology problems, such as thixotroping, on incorporation into the polymeric matrix.
The disadvantages of both approaches can in principle be circumvented by replacing the color pigments by dye-comprising polymer dispersions in which the dye is distributed homogeneously in the polymer particles. “Pigments” of this kind should on the one hand feature high color brilliance and purity of shade and should on the other hand be largely unaffected by rheological problems, owing to the spherical form of the polymer particles.
With optical brighteners, which should be classed as dyes in the wider sense, and specifically as fluorescent dyes, the problem arises that there is a reduction in the fluorescence yield owing to the formation of relatively high molecular mass associations in the pigment crystals. The danger of formation of relatively high molecular mass associations also exists if the optical brighteners migrate from the interior of the polymer matrix to the surface.
Dye-comprising polymer dispersions are fundamentally known. For instance, EP-A 691 390 and DE-A 44 36 892 describe dye-comprising aqueous polymer dispersions based on ethylenically unsaturated monomers, which are obtainable by preparing, in a first step, a polymeric compound in an organic solvent, one or more soluble dyes being added to said organic solvent before, during or after the polymerization, and by converting, in a second step, the resultant dye-comprising polymer solutions into aqueous polymer dispersions. Dye-comprising polymer dispersions of this kind, however, are still not satisfactory in terms of the migration stability of the dyes.
Dye-comprising polymer dispersions based on polyurethanes are known from DE-A 195 21 500. These are likewise secondary dispersions, with the abovementioned disadvantages.
EP-A 566 448 describes dye-comprising aqueous polymer dispersions based on ethylenically unsaturated monomers, which are obtainable by conducting, in a first step, a free-radical aqueous emulsion polymerization of ethylenically unsaturated monomers and by impregnating the resultant polymer dispersions with a solution of dyes in organic solvents. The polymer dispersions obtainable in this way, however, likewise lack satisfactory migration stability of the dyes. The document also refers to the fact that sufficiently water-compatible dyes can be added even during the polymerization. However, the document gives no concrete references to such an embodiment. The use of relatively large amounts of organic solvents, which is required by this approach, is a disadvantage.
U.S. Pat. No. 4,680,332 likewise recommends preparing dye-comprising polymer dispersions by impregnating aqueous polymer dispersions with solutions of organic dyes in solvents such as toluene or chlorobenzene and also impregnation with oil-in-water emulsions of such dye solutions. This approach also fails to produce satisfactory products.
DE-A 44 24 817 describes “fluorescent pigments” in which a fluorescent dye is present in solution in a polymer matrix based on crosslinked poly(meth)acrylates. Polymers of this kind are obtainable by free-radical suspension polymerization of ethylenically unsaturated monomers, including at least one crosslinking monomer, in the presence of the fluorescent dye. A feature of the so-called “fluorescent pigments” is the reduced migration tendency of the fluorescent dyes. A disadvantage, however, is that the preparation process is limited to pigments having particle sizes >2 &mgr;m. Furthermore, the polymer dispersion obtained in this way is not stable to sedimentation. Consequently, such “fluorescent pigments” are unsuitable for a large number of applications, inkjet processes being one example.
It is an object of the present invention to provide a process for preparing dye-comprising polymer dispersions in which the polymer particles comprise at least one organic dye in homogeneous distribution, i.e., in molecularly disperse form, said polymer particles being distinguished by high migration stability of the dyes present in them. This process should, furthermore, make it possible to obtain fine, stable, dye-comprising polymer dispersions.
We have found that this object is achieved by a process of free-radical aqueous emulsion polymerization using an oil-in-water monomer emulsion where the monomers contain the dye in dissolved form, subject to the proviso that the monomer droplets essentially have a diameter <500 nm. Conventional emulsion polymerization, on the other hand, is unsuccessful.
The present invention therefore provides a process for preparing dye-comprising aqueous polymer dispersions by free-radical aqueous emulsion polymerization of ethylenically unsaturated monomers in the presence of free-radical initiators, at least some of the monomers being employed in the form of an oil-in-water emulsion E1 whose disperse phase comprises at least one oil-soluble dye, wherein the disperse phase of E1 is formed essentially of dye-comprising monomer droplets having a diameter<500 nm.
In accordance with the invention, the oil-soluble dyes include organic optical brighteners, i.e., organic molecules having an extended, conjugated &pgr;-electron system, which unlike the &pgr;-electron system of conventional organic dyes absorbs ultraviolet rather than visible-range radiation and emits it as a bluish fluorescence (see Römpp Chemielexikon, 9
th
edition). Optical brighteners are used, for example, to increase the whiteness of white substrates treated with them, such as paper or fabrics.
The process of the invention differs essentially from conventional emulsion polymerizations in the droplet size of the monomer emulsion to be polymerized. In conventional emulsion polymerization techniques, in contradistinction to the process of the invention, the monomer droplets in the polymerization vessel generally have diameters above 1000 nm. The failure of conventional emulsion polymerization in the preparation of dye-comprising polymers can probably be attributed to the fact that in conventional emulsion polymerization the monomer droplets serve solely as a reservoir for the polymers to be polymerized while the actual polymerization reaction takes place in the aqueous dispersion medium or on the polymer particles that are formed in the course of the polymerization. In that case, the monomers, which generally have a certain solubility in water, diffuse from the monomer droplets to the centers of pol
Böhm Arno
Delavier Paul
Freund Torsten
Hanciogullari Harutyun
Ikeda Takahiro
BASF - Aktiengesellschaft
Lipman Bernard
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