Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Utility Patent
1997-06-17
2001-01-02
Rodee, Christopher D. (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S137170, C524S106000
Utility Patent
active
06168895
ABSTRACT:
DESCRIPTION
Pigment for electrophotographic toners and developers
The present invention relates to an improved pigment based on C.I. Pigment Yellow 180 as a colorant in electrophotographic toners and developers.
In electrophotographic recording processes a “latent charge image” is produced on a photoconductor. This “latent charge image” is developed by applying an electrostatically charged toner, which is subsequently transferred to, for example, paper, textiles, films or plastic and is fixed, for example by means of pressure, radiation, heat or the effect of solvent. Typical toners are one- or two-component powder toners (also called one- or two-component developers), but use is also made of specialty toners, for example magnetic or liquid toners and polymerization toners (L. B. Schein, “Electrophotography and Development Physics”; Springer Series in Electrophysics 14; Springer-Verlag, 2nd edition, 1992).
One measure of the quality of the toner is its specific charge q/m (charge per unit mass). In addition to the sign and level of the electrostatic charge, the principal, decisive quality criteria are the rapid attainment of the desired charge level and the constancy of this charge over a prolonged activation period. In addition to this, the insensitivity of the toner to climatic effects such as temperature and atmospheric humidity is a further important criterion for its suitability.
Both positively and negatively chargeable toners are used in photocopiers, laser printers, LED (light-emitting diode) and LCS (liquid crystal shutter) printers or other digital printers based on electrophotographic techniques, depending on the type of process and type of apparatus.
To obtain electrophotographic toners or developers having either a positive or a negative charge, it is common to add charge control agents. The chromophoric component employed in color toners typically comprises organic color pigments. Color pigments have considerable advantages over dyes owing to their insolubility in the application medium, examples of these advantages being better thermal stability and lightfastness.
On the basis of the principle of subtractive color mixing the three primary colors, yellow, cyan and magenta, can be used to reproduce the entire color spectrum which is visible to the human eye. Exact color reproduction is possible only if the respective color meets the precisely defined color requirements. Otherwise, it is not possible to reproduce some shades, and the color contrast is inadequate.
In full-color toners, in addition to the precisely defined requirements in terms of color, the three toners yellow, cyan and magenta must also be matched exactly to one another in respect of their triboelectric properties, since they are transferred in succession in the same apparatus.
It is known that colorants may in some cases have a sustained effect on the triboelectric charge of toners (H.-T. Macholdt, A. Sieber, Dyes & Pigments 9 (1988), 119-127). Because of the different triboelectric effects of colorants and the resulting effect, sometimes very pronounced, on toner chargeability, it is not possible simply to add the colorants to a toner base formulation made available at the start. On the contrary, it may be necessary to make available for each colorant an individual formulation to which the nature and amount of the required charge control agent are tailored specifically. This procedure is, accordingly, laborious and, in the case of color toners for the three-color process, represents a further difficulty in addition to those already described above.
Another important practical requirement is that the colorants should have high thermal stability and good dispersibility. Typical temperatures at which colorants are incorporated into the toner resins, when using kneading equipment or extruders, are between 100° C. and 200° C. Correspondingly, thermal stability at 200° C., and better still at 250° C., is a great advantage. It is also important for the thermal stability to be assured over a relatively long period (about 30 minutes) and in a variety of binder systems. Typical toner binders are addition polymerization resins, polyaddition resins and polycondensation resins, such as styrene, styrene-acrylate, styrene-butadiene, acrylate, polyester, phenolic-epoxy resins, polysulfones and polyurethanes, individually or in combination, which may also contain further components such as charge control agents, waxes or flow assistants, or may have these components added subsequently.
Yellow pigments for electrophotographic toners and developers are in use in numerous forms. In general, azo-based pigments are preferred, above all because of their color, their color strength and their dispersion properties. Yellow azo pigments which are typically employed are C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, C.I. Pigment Yellow 17, C.I. Pigment Yellow 174 and C.I. Pigment Yellow 176.
Disadvantages of these pigments are the fact that some of them lack thermal stability (especially the diarylide pigments), and, on ecological grounds, that organically bonded chlorine or heavy metals are present in the molecule.
One of the few azo pigments which meets the ecological requirements as well is C.I. Pigment Yellow 180. Commercially available Pigment Yellow 180, however, even in its most transparent form has the disadvantage of a level of transparency which is inadequate for use in full-color toners, rendering its use in this sector impossible from the outset. Furthermore, this pigment has a triboelectric effect which influences toner chargeability toward negative polarity.
The transparency is of central importance, since in full-color copying or printing the colors yellow, cyan and magenta are copied or printed on top of one another, the sequence of colors depending on the apparatus. If, then, an overlying color is not sufficiently transparent, the color below it is unable to show through to an adequate extent and the color reproduction is distorted. When copying or printing onto overhead sheets, the transparency is even more important, since a lack of transparency even in only one color makes the entire projected image appear gray.
Fundamentally there is a need for pigments having a minimal inherent triboelectric effect, since these pigments can then be employed without problems for both positively and negatively chargeable toners.
The object of the present invention was to provide a transparent yellow pigment of good color strength which possesses a less negative inherent triboelectric effect and which, furthermore, satisfies the abovementioned requirements from the ecological standpoint.
This object has surprisingly been achieved by the azo pigment detailed below, a characteristic of which is its particularly large surface area. The present invention provides an azo pigment of the formula (1)
which has a specific surface area of the pigment powder of more than 45 m
2
/g, preferably more than 55 m
2
/g and, in particular, more than 60 m
2
/g.
The surface area from 62 to 78 m
2
/g as described in these examples. The surface area can be greater than 78 m
2
/g such as at least 80 m
2
/g up to about 150 m
2
/g. The surface area of about 86 m
2
/g also produces good results.
The pigment of the formula (1) is already known per se (U.S. Pat. No. 4,870,164 and 4,935,502) and is marketed under the name ®Novoperm-Gelb P-HG. This known pigment, however, even in its hitherto most transparent form possesses a specific surface area of not more than 44 m
2
/g, in conjunction with an average particle size d
50
of at least 140 nm. A pigment of the formula (1) having a specific surface area of more than 45 m
2
/g in conjunction with an average particle size d
50
of 120 nm or less has not hitherto been described.
Apart from an improved, larger specific surface area and, in conjunction therewith, an improved, smaller average particle size, the pigment of the invention also has a different particle morphology. Whereas the most transparent form known to date possesses rodlike particles, the pigment of the invention crystallizes in a cuboid
Baur R{umlaut over (u)}diger
Macholdt Hans-Tobias
Metz Hans Joachim
Clariant GmbH
Connolly Bove & Lodge & Hutz LLP
Rodee Christopher D.
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