Sludge resistant liquid developer for electrostatic images containing a metal-dithiooxamide pigment or coating

A liquid developer composition for visual development of latent electrostatic images in which the pigment particles dispersed in the liquid carrier is a metal-dithiooxamide product or a pigment coated with said metal-dithiooxamide reaction product whereby the liquid developer composition is relatively free of sludging.

This invention relates to the production of copies by electrophotographic 
technique and it relates more particularly to a liquid developer for use 
in the practice of same. 
Processes are well known for forming a latent electrostatic image, existing 
as an electrostatic charge pattern on a layer of material having high 
resistance, and for subsequently converting the latent electrostatic image 
to a visual pattern. 
In one such process, an optical image is produced directly on a film or 
coating containing a photoconductive pigment, such as zinc oxide dispersed 
in an insulating matrix, such as a synthetic resin. The coating may be 
provided on a suitable substrate, such as a flexible sheet of paper, metal 
or other electrically conductive backing. In this process, the 
photoelectrostatic coating is given an overall electrostatic charge while 
being protected from light. Thereafter, it is exposed to a light image of 
the subject to be reproduced. The electrostatic charge on the coating is 
dispersed in the areas struck by light and retained in the unexposed 
areas, thereby to provide an electrostatic reproduction of the optical 
image. This latent electrostatic image is then converted to a visible 
image by a developing composition containing toner particles which are 
attracted to the latent electrostatic image and fixed, as by heat, solvent 
vapors or the like. 
In a plain paper copier, known as the Xerox process, after treatment of the 
latent electrostatic image with the developer, the toner particles are 
transferred from the electrophotographic layer (selenium) onto a copy 
sheet on which the developed image is fixed, as by heat, solvent vapors or 
the like, as described above. 
As described in U.S. Pat. Nos. 2,996,573 and 3,075,859, a latent 
electrostatic image capable of visual development by means of a developer 
composition of the type described can be provided on a suitable substrate 
having a dielectric coating. 
The latent electrostatic images of the type described can be developed in a 
number of ways, such as by cascade development with a dry powdered 
developer composition, as described in U.S. Pat. No. 2,221,726, or by 
magnetic brush development with a dry powdered developer composition, as 
described in U.S. Pat. No. 3,084,043, or by a liquid developer composition 
to which this invention is addressed. 
Liquid developer compositions for use with electrostatic images comprise a 
dispersion of a pigment or toner particles in a volatile liquid having a 
high dielectric strength and a high volume resistivity. The dispersed 
particles may carry either a positive charge or a negative electrical 
charge, depending on their chemical composition for either negative toning 
or positive toning respectively. The non-conductivity and the high 
dielectric strength of the volatile liquid in the liquid developing 
composition preserves the electrostatic image and permits the deposition 
of the dispersed toner particles to form a visible image. Liquid toners 
are described in the Metcalfe U.S. Pat. No. 2,907,674, the Straughan U.S. 
Pat. No. 2,899,335, the Mayer et al. U.S. Pat. No. 2,890,911, the York 
U.S. Pat. No. 3,135,095, the Dirks U.S. Pat. No. 3,155,546 and the Zabiak 
U.S. Pat. No. 3,535,244 and many others. 
One of the problems encountered in the use of liquid developers is the 
tendency for the toner particles to settle in the developer composition, a 
characteristic generally referred to in the trade as "sludging". 
Attempts have been made to overcome sludging by formulation of the liquid 
developer system with gel structures, which are very delicate, or by 
formulation of the liquid developer with a viscosity high enough to 
minimize settling of the toner particles. However, such systems yield 
copies of poor quality due to reduced particle mobility. 
It is an object of this invention to produce and to provide a method for 
producing liquid developer compositions for visual development of latent 
electrostatic images whereby the problem of sludging is greatly minimized, 
if not eliminated, without interfering with the desired mobility of the 
toner particles whereby copy of good quality can be produced. 
In accordance with the practice of this invention, a stable liquid 
developer composition is prepared when use is made of toner particles 
formulated of the reaction product of ions of a metal selected from the 
group consisting of copper, cobalt and iron and preferably nickel with a 
dithiooxamide or derivative thereof in accordance with the following 
equation: 
##STR1## 
in which n is any number and M is nickel, cobalt, copper or iron supplied 
in the form of a compound, such as nickel octoate. The reaction can be 
carried out at room temperature or above with the dithiooxamide or 
derivative dissolved in a suitable solvent such as acetone, or butyl 
Cellosolve (a trademark of Union Carbide Corporation for ethylene glycol 
monobutyl ether). 
The metal-dithiooxamide, in finely divided form, is itself capable of being 
highly charged and is settlement resistant thereby to enable use alone as 
highly colored toner particles in a suitable liquid carrier, such as 
isoparaffinic liquid. It has also been found that combinations of such 
metal-dithiooxamide reaction products with other pigments, such as carbon 
black, become settlement resistant when the reaction to form the 
metal-dithiooxamide is carried out in situ in the presence of such other 
pigment particles. It is believed that the reaction product forms on such 
other pigment particles whereby they also become settlement resistant and 
thus can be used in the formulation of sludge-free liquid toner 
compositions.

The following examples are given by way of illustration, but not by way of 
limitation, of the preparation of liquid toner compositions and toner 
particles employed therein in accordance with the practice of this 
invention. 
EXAMPLE 1 
This example describes the preparation of toner particles formed of the 
metal-dithiooxamide alone. 
Part A 
Dithiooxamide: 15.8 parts by weight 
Staybelite: 15.8 parts by weight 
butyl Cellosolve: 167.5 parts by weight 
Part B 
10% solution of Nickel Octoate--78.9 parts by weight 
Procedure 
Parts A and B are milled together for three hours at room temperature in a 
ball mill or roller mill. The milled product is filtered and the filter 
cake is washed with the same solvent (butyl Cellosolve) dried in an oven 
and ground to a powder. 
The reaction product, having a dark blue color, is taken up in 
isoparaffinic solvent at a concentration of about 2-20 parts by weight per 
100 parts by weight of solvent, preferably containing a solution of 
resinous binder in an amount of about 130 parts by weight of binder per 30 
parts by weight pigment, and then the concentrate is milled to reduce the 
particle size of the toner to 5 microns or less. 
For use in the development of latent electrostatic images, the above 
concentrate is diluted with the same or similar solvent added in an amount 
to dilute 60 grams of the concentrate to a volume of 3 liters. 
EXAMPLE 2 
This sample illustrates the preparation of toner particles formed of carbon 
black and the metal-dithiooxamide. 
Part A 
Dithiooxamide: 15.8 parts by weight 
Staybelite: 15.8 parts by weight 
butyl Cellosolve: 675.5 parts by weight 
Part B 
10% solution of Nickel Octoate--78.9 parts by weight 
Carbon Black (Molacco Black)--23.7 parts by weight 
The procedure for the preparation of the developer composition is the same 
as that of Example 1. By reason of the presence of carbon black, the toner 
is black instead of blue. 
As the liquid carrier or diluent, use can be made of one or more low 
boiling aliphatic solvents having a high volume resistivity, for example 
10.sup.10 ohms-cm, so as to avoid dissipation of charge from the 
electrostatic image. Such aliphatic solvents, as represented by 
isopentane, octane and cyclohexane, serve also to avoid attack on the 
binder present in the photoconductive coating. With the non-settling toner 
particles of the invention. Best use is made of an isoparaffinic solvent 
such as Isopar G as marketed by Exxon Company in the United States, having 
a flash point of about 104.degree. F. and a KB value of about 27. 
The Staybelite functions in the composition in conjunction with other 
charge directors to give polarity in the pigment particles. While not 
necessary, the Staybelite can be replaced, in whole or in part, with other 
charge directors, such as fatty acid soaps of zirconium, manganese and 
cobalt, such as the corresponding metal octoates, resinates, or 
neodecanoates. 
The binder component in Examples 1 and 2 may be selected of polymeric 
materials which are soluble in the carrier solvent, such as methacrylate 
or other alkylacrylate resins such as lauryl methacrylate, isobutyl 
methacrylate, methyl or other alkyl ester of rosin, such as marketed by 
Hercules Chemical Company under its trade name Hercolyn, pentaerythritol 
esters of rosin, such as marketed under the trade name Pentalyn H. The 
amount of binder is not significant. When used, the binder concentration 
can range from 1-10 parts by weight of binder per part by weight of 
pigment. 
The following is a further example of the concentrate representative of the 
practice of this invention: 
EXAMPLE 3 
200 parts by weight Isopar G 
28 parts by weight paraffin oil 
200 parts by weight pigment suspension of Example 1 
25 parts by weight methylmethacrylate polymer 
70 parts by weight zirconium neodecanoate 
In use, the surface containing the latent electrostatic image is wet with 
the liquid developing composition, either by immersion of the sheet in a 
path of developing composition, or by flow coating the composition over 
the imaged surface, or by application of the liquid developing composition 
onto the imaged coating by means of a roller coater or the like. The sheet 
wet with the developing composition is advanced through a squeeze roll to 
remove excess liquid and the toner particles are attracted to the latent 
electrostatic image for visual development of the image. 
When the image is developed on the copy sheet, the sheet is advanced 
through a fusion zone for fixing the image on the sheet. When in a plain 
paper copier, a copy sheet is brought into contact with the surface of the 
selenium drum, after image development, for transfer of the toner 
particles of the developed image from the drum to the copy sheet, on which 
it is subsequently fixed as by heat or the like. 
The term "dithiooxamide", as used herein, includes derivatives thereof as 
represented by: 
N,N-Bis(3-dimethylaminopropyl) dithiooxamide 
N,N-Bis(3-hydroxyethyl) dithiooxamide 
N,N-Bis(carboxymethyl) dithiooxamide 
N,N-diallyldithiooxamide 
N,N-di-sec. butyldithiooxamide 
N,N-di-R-dithiooxamide 
in which R is ethyl, propyl, pentyl, octyl, octadecyl, etc. 
It will be understood that changes may be made from the standpoint of the 
reaction and materials without departing from the spirit of the invention, 
especially as defined in the following claims.