Electrophotographic liquid developer for the reversal development _of negatively-charged images

An electrophotographic liquid developer is disclosed which is useful in the development of negatively charged charge images. The developer comprises an electrically insulating carrier liquid of high electrical resistivity and a low dielectric constant, a pigment or dye, a resinous binder, a charge controller comprising a copolymer of N-vinyl-2-pyrrolidone and a methacrylic acid ester, and a poly-N-vinyl-2-pyrrolidone which has been alkylated with an .alpha.-olefin. A process for producing the liquid developer is also disclosed.

BACKGROUND OF THE INVENTION 
The present invention relates to an electrophotographic liquid developer 
for the reversal devleopment of negatively charged charge images, 
comprising an insulating carrier liquid of high electrical resistivity and 
a low dielectric constant, in which a pigment, a resinous binder, a charge 
controller comprising a copolymer of N-vinyl-2-pyrrolidone and a 
methacrylic acid ester, and conventional additives are dispersed or 
dissolved, and to a process for the preparation thereof. 
Published European Application No. 0,001,103 (=U.S. Pat. No. 4,243,736) 
discloses a liquid developer containing negatively charged toner particles 
for direct development, i.e., for the development of positively charged 
electrostatic charge images, which comprises, in an electrically 
insulating carrier liquid, a pigment, a resinous binder, and, as the 
charge controller, a copolymer of N-vinyl-2-pyrrolidone and a methacrylic 
acid ester, either as a dispersion or as a solution. This liquid developer 
is, however, not adjusted to, nor suitable for, the reversal development 
of negatively charged charge images, for copies prepared using this 
developer in general show strong scumming and result in unsatisfactory 
image reproductions. 
U.S. Pat. No. 3,542,681 discloses a negative-working electrostatic toner 
composition for producing positive images and reversal images, wherein a 
pigment and an alkylized polymer of a heterocyclic N-vinyl monomer, such 
as N-vinyl-2-pyrrolidone, are dispersed in an electrically insulating 
carrier liquid. It has been found, however, that the quality of images 
produced using liquid developers of this type is not satisfactory, 
especially in the case of reversal development. Therefore, German 
Offenlegungsschrift No. 1,930,783, which is based on U.S. Pat. No. 
3,542,681, proposes a liquid developer containing a controlling agent, but 
which is suitable for a positive-working developing process only. 
Comparative tests have shown that this liquid developer does not yield 
copies of satisfactory quality when it is used for reversal development of 
negatively charged charge images. 
In German Auslegeschrift No. 2,125,668 (=U.S. Pat. No. 3,674,693), a liquid 
developer for the reversal development of negatively charged charge images 
is described, which developer comprises a mixture of a pigment, a 
polycyclopentadiene resin and a phospholipid as controller, dispersed in 
an aliphatic carrier liquid. However, charge control by means of, for 
example, lecithin, cannot always be adjusted precisely enough to ensure 
reproducible results. 
SUMMARY OF THE INVENTION 
It is, therefore, the object of the present invention to provide an 
electrophotographic liquid developer for the reversal development of 
negatively charged charge images, which developer can be produced by a 
simple process, has a long shelf life, and yields a good quality image. 
A further object of the present invention is to provide a process for 
preparing such a liquid developer. 
These objects are achieved according to the invention with an 
electrophotographic liquid developer of the type previously described and 
additionally containing a poly-N-vinyl-2-pyrrolidone alkylated with an 
.alpha.-olefin. Preferably, the liquid developer contains between 0.005 
and 1.0 percent by weight of alkylated poly-N-vinyl-2-pyrrolidone. Thus, a 
liquid developer for reversal development is provided which is similar to 
a liquid developer used for developing positive charge images, but which 
is suitable for use as reversal developer by virtue of diluting the toner 
concentrate with a carrier liquid in which the poly-N-vinyl-2-pyrrolidone 
is dissolved. 
More specifically, there has been provided, in accordance with one aspect 
of the present invention, an electrophotographic liquid developer for the 
reversal development of negatively charged charge images, comprising an 
electrically insulating carrier liquid of high electrical resistivity and 
a low dielectric constant, a pigment, a resinous binder, a charge 
controller comprising a copolymer of N-vinyl-2-pyrrolidione and a 
methacrylic acid ester, and a poly-N-vinyl-2-pyrrolidone which has been 
alkylated with an .alpha.-olefin. 
In accordance with another aspect of the present invention, there has been 
provided a process for preparing an electrophotographic liquid developer 
for the reversal development of negatively charged charge images, 
comprising the steps of (a) producing a homogeneous paste comprising an 
electrically insulating carrier liquid of a high electrical resistivity 
and a low dielectric constant, a pigment or a dye, a resinous binder, and 
a charge controller comprising a copolymer of N-vinyl-2-pyrrolidone and 
methacrylic acid ester; (b) diluting the homogeneous paste using a carrier 
liquid optionally containing a charge controller, producing a toner 
concentrate; and (c) mixing said toner concentrate with a carrier liquid 
in which a poly-N-vinyl-2-pyrrolidone alkylated with an .alpha.-olefin is 
dissolved, producing an electrophotographic liquid developer that is ready 
for use. 
It is unexpected that a liquid developer well suited for use in reversal 
development is obtained by adding a small amount of an alkylated 
poly-N-vinyl-2-pyrrolidone, a known dispersing agent for pigments, to a 
ready-made, negatively controlled, well-dispersed liquid developer used in 
copiers. At present, no explanation for this effect can be given. 
Further objects, features, and advantages of the present invention will 
become apparent from the following detailed description of preferred 
embodiments, considered together with examples presented below.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Poly-N-vinyl-2-pyrrolidone substituted by an .alpha.-olefin is known, as 
shown in U.S. Pat. No. 3,417,054. It is prepared by reacting 
poly-N-vinyl-2-pyrrolidone with the corresponding .alpha.-olefins in the 
presence of organic peroxide catalysts, the reaction being performed at an 
elevated temperature. Poly-N-vinyl-2-pyrrolidones which carry 
.alpha.-olefin substituents having 12 to 20 carbon atoms have proved to be 
particularly advantageous. These substituents include, for example, 
dodecene-1, tetradecene-1, hexadecene-1, heptadecene-1, octadecene-1, 
nonadecene-1, eicosene-1 or low-molecular weight polybutenes. It is also 
possible, however, for various olefinic substituents to be present in a 
mixture. Alkylated poly-N-vinyl-2-pyrrolidones of this kind, which contain 
about 20 percent by weight of vinyl-2-pyrrolidone, can be particularly 
advantageously used in accordance with this invention, because the high 
degree of substitution results in good solubility properties. The 
effective amount of the additive is limited by a decrease in contrast of 
the images produced by an excess amount of the additive, on the one hand, 
and by a decreasing efficiency of the liquid developer, on the other hand. 
It has proved to be adequate to add about 0.005 to 1.0 percent by weight, 
and preferably about 0.02 to 0.5 percent by weight, of alkylated 
poly-N-vinyl-2-pyrrolidone. 
Suitable insulating carriers liquids are, for example, aliphatic 
hydrocarbons, such as n-hexane, ligroine, n-heptane, n-pentane, isodecane 
and isooctane, and the corresponding halogen derivatives, such as 
perchloroethylene, all these compounds exhibiting a high insulating 
capacity (resistivity in excess of 10.sup.9 .OMEGA..cm) and a low 
dielectric constant (of less than 3). Particularly suitable are, for 
example, commercially available hydrocarbons, such as .RTM.Isopar E, G, L, 
H or K, produced by ESSO-Chemie, Hamburg. The may be used alone or in 
combination. 
The pigments employed are known. As a rule, carbon blacks, such as channel 
black, furnace black or lamp black (C.I. No. of all varieties of carbon 
black 77 266), are employed for the preparation of black developers. 
Acidic carbon blacks with a mean particle size of 20-30 nm and a surface 
area of 80-200 m.sup.2 /g (BET) are preferred. Organic pigments, such as 
Phthalocyanine Blue (C.I. No. 74 160), Phthalocyanine Green (C.I. No. 74 
260 or 42 040), Sky Blue (C.I. No. 42 780), Rhodamine (C.I. No. 45 170), 
Malachite Green (C.I. No. 42 000), Methyl Violet (C.I. No. 42 535), 
Peacock Blue (C.I. No. 42 090), Naphthol Green B (C.I. No. 10 020), 
Naphthol Green Y (C.I. No. 10 006), Naphthol Yellow S (C.I. No. 10 316), 
Permanent Red 4R (C.I. No. 12 370), Brilliant Fast Pink (C.I. No. 15 865 
or 16 105), Hansa Yellow (C.I. No. 11 725), Benzidine Yellow (C.I. No. 21 
100), Lithol Red (C.I. No. 15 630), Lake Red D (C.I. No. 15 500), 
Brilliant Carmine 6B (C.I. No. 15 850), Permanent Red F5R (C.I. No. 12 
335) and Pigment Pink 3B (C.I. No. 16 015), a.s.o. are also suitable. 
Inorganic pigments, for example Berlin Blue (C.I. No. Pigment Blue 27), 
are also useful. 
A pigment or pigment mixture is preferably used, according to this 
invention, in the preparation of the toner. In general, pigments have less 
tendency to fade than dyes. In addition, copy images of high contrast can 
be produced using pigments, compared to copies obtained using 
dye-containing toners. 
Suitable resinous binders are known, and are selected from the group of 
natural or synthetic resins. They serve to fix the pigment particles to 
the charge image or, after the image transfer, to fix them to a support 
medium, such as paper, plastic film, or metal foil. Resins which are 
suitable include, for example, resins which are substantially insoluble in 
the aliphatic hydrocarbons used as carrier liquids, such as maleic acid 
resins modified with natural resins, dammar, copal, shellac, colophony, 
hardened colophony, ester gum, maleate resins which have been modified 
with glyceryl esters, copolymers of styrene and butadiene, copolymers of 
vinyltoluene and butadiene, and the like. Dispersions of polymers in 
aliphatic hydrocarbons, so-called organosols or dispersimers, are also 
suitable. 
Conventional additives which may be used include those which influence 
various toner properties and are selected on a case-by-case basis. They 
can influence the charge level, the sedimentation characteristics, the 
useful life, the redispersibility of agglomerates, the transferability and 
the wipe-resistance of the liquid developer or of the toner particles on 
the copy support. Waxes, in particular polyethylene waxes, paraffin waxes 
and chloro-paraffin waxes are suitable additives. A wax or polyethylene 
with a softening point in the range from 60.degree. to 130.degree. C. can 
be used in accordance with the invention. The waxes or polyethylenes 
mentioned possess properties which, with respect to specific weight, are 
similar to those of the carrier liquid used. Polymers which are soluble in 
aliphatic compounds, such as polyvinyl alkyl ethers, polyisobutylene, 
polyvinyl stearate and polyacrylates, or polymethacrylates with a higher 
alcohol radical, or plasticizers, such as, for example, dialkyl 
phthalates, are also suitable. 
The invention also relates to a process for the preparation of an 
electrophotographic liquid developer for the reversal development of 
negatively charged charge images. According to the process, a mixture 
comprising an electrically insulating carrier liquid of a high electrical 
resistivity and a low dielectric constant, a pigment, a resinous binder, a 
charge controller comprising a copolymer of N-vinyl-2pyrrolidone and 
methacrylic acid ester, and conventional additives, is worked into a 
homogeneous paste, diluted into a toner concentrate using carrier liquid 
which optionally contains a charge controller, and further mixed with 
carrier liquid, so that a readymade liquid developer is obtained. The 
toner concentrate used in the process is mixed with a carrier liquid in 
which is dissolved a poly-N-vinyl-2-pyrrolidone alkylated with an 
.alpha.-olefin. The liquid developer produced by the process preferably 
contains between 0.005 and 1.0 percent by weight of alkylated 
poly-N-vinyl-2-pyrrolidone. 
With this process it is possible to produce, in a simple manner, a liquid 
developer suitable for reversal development from a toner concentrate which 
can be employed for direct development, without the need of an additional 
dispersing stage or an additional process step beyond dilution of the 
toner concentrate. 
Homogenizing the mixture to obtain a paste can be performed by means of a 
three-roll mill, a ball mill, a colloid mill, or, preferably, an agitator 
ball mill, at temperatures of from room temperature up to about the 
boiling range of the carrier liquid. The duration of the process depends 
on the milling result. After milling the particles are between about 0.05 
and 10 .mu.m in size. The toner concentrates are adjusted to a solids 
content of about 5 to 10 percent by weight by diluting the paste while 
agitating at high speed. 
Further mixing with carrier liquid to give the ready-for-use liquid 
developer may then be performed, for example, immediately before the 
liquid developer is employed in the copier. In general, the concentrations 
after mixing vary between 0.05 and 5 percent by weight. 
By way of the following examples, the invention will now be explained in 
greater detail. 
COMATIVE EXAMPLE 1 
60 g of carbon black (BET: 96 m.sup.2 /g, pH 4.5) were added to 300 g of a 
10 percent by weight solution, in an aliphatic hydrocarbon (boiling range: 
160.degree.-180.degree. C.), of an alkylated poly-N-vinyl-2-pyrrolidone 
having a chain length of 20 carbon atoms in the alkyl group and about 20 
percent by weight of vinylpyrrolidone (Antaron V220), and mixed with 160 
ml of the same hydrocarbon. This mixture was milled in an agitator ball 
mill for 3 hours at 80.degree. C. and, after it was cooled, diluted to 
give a 5 percent by weight, carbon black-containing toner concentrate. The 
liquid developer was then prepared by mixing 7 ml of the toner concentrate 
with 1 liter of the above-mentioned aliphatic hydrocarbon. With this 
liquid developer, a reversal development was carried out on ZnO paper in a 
microfilm reenlargement apparatus. The images obtained were very scummed 
and inappropriate for practical purposes. 
COMATIVE EXAMPLE 2 
60 g of the carbon black described in Comparative Example 1 were added to 
600 g of an aliphatic hydrocarbon (boiling range: 160.degree.-180.degree. 
C.), mixed with 160 g of a 20 percent by weight solution of a grafted 
terpolymer comprising 76% of dodecylmethacrylate, 20% of 
methylmethacrylate, and 4% of N-vinyl-2-pyrrolidone in the hydrocarbon 
(boiling range 160.degree.-180.degree. C.), 30 g of a polyethylene wax 
(dropping point: 103.degree.-107.degree. C.), and 16 g of a chlorinated 
paraffin wax (softening point: 105.degree. C.), and milled in an agitator 
ball mill under the same conditions as indicated in Comparative Example 1. 
After milling, the dispersion was diluted with 900 g of the aliphatic 
hydrocarbon. The toner concentrate thus obtained was diluted with the 
aliphatic hydrocarbon in a ratio of 1:10. In a copier having a 
photosensitive layer of selenium, i.e., with positive development, 
satisfactory copies were obtained. If, however, this liquid developer was 
used for reversal development on ZnO paper, scummed copies, which were 
unusable for practical purposes, were obtained. Even with a dilution ratio 
of 1:20, no better result was achieved. 
EXAMPLE 1 
A toner concentrate prepared in accordance with Comparative Example 2 was 
diluted, in a ratio of 1:20, with a solution containing 0.075 
percent-by-weight of alkylated poly-N-vinyl-2-pyrrolidone (Antaron V220), 
having a mean chain length of 20 carbon atoms and 20 percent by weight of 
vinyl-2-pyrrolidone, in an aliphatic hydrocarbon (boiling range: 
160.degree.-180.degree. C). When the liquid developer prepared in this 
manner was employed in a reversal development process on ZnO paper, 
performed using a conventional microfilm re-enlargement apparatus, copies 
of good quality with regard to scumming, blackening, and image contrast 
were obtained. The liquid developer has a storage life ranging over 
several weeks. 
EXAMPLE 2 
A liquid developer prepared in accordance with Example 1 was used for 
reversal development in a conventional microfilm re-enlargement apparatus 
on a polyvinylcarbazole photoconductor. Images of good quality were 
obtained. 
EXAMPLE 3 
A toner concentrate prepared in accordance with Comparative Example 2 was 
diluted, in a ratio of 1:30, with a 0.3 percent-by-weight solution of 
alkylated poly-N-vinyl-2-pyrrolidone (mean alkyl chain length of 16 carbon 
atoms) in an aliphatic hydrocarbon (boiling range 175.degree.-190.degree. 
C). When used for reversal development on ZnO paper, the liquid developer 
thus produced gave copies of good quality.