Cadmium photoconductor with (dialkylpyrophosphato) organic titanate additive

Disclosed is a photosensitive material for electrophotography comprising a conductive substrate and a layer of a composition comprising a dispersion of a photoconductive pigment in a binder resin, which is formed on the conductive substrate, wherein a (dialkypyrophosphato) organic titanate is incorporated into said composition. This photosensitive material is excellent in the ozone resistance and the moisture resistance.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention: 
The present invention relates to a photosensitive material for 
electrophotography. More specifically, the present invention relates to a 
photosensitive material for electrophotography, which is excellent in the 
ozone resistance and the moisture resistance. 
(2) Description of the Prior Art: 
As one kind of a photosensitive plate for electrophotography, there has 
been widely used a photosensitive plate comprising a layer of a 
composition comprising a dispersion of a photoconductor in a binder resin, 
which is formed on a conductive substrate. When this photosensitive plate 
is used for electrophotography, the photosensitive plate is charged by 
corona discharge of a certain polarity and is then subjected to imagewise 
light exposure to form an electrostatic image, the electrostatic image is 
developed with a toner, the toner image is transferred on a copy sheet 
from the photosensitive plate, and the surface of the photosensitive plate 
is cleaned. This reproduction cycle is repeated. 
Among photoconductive pigments used for such photosensitive plates, cadmium 
sulfide is especially excellent in the sensitivity. However, when a 
photosensitive plate comprising cadmium sulfide as the photoconductor is 
used for reproduction repeatedly for a long time under a high-humidity 
condition, the image density is drastically reduced. 
Cadmium sulfide is an n-type photoconductor and hence, it is subjected to 
corona discharge of a negative polarity. Accordingly, the photosensitive 
layer is always attacked by ozone and is rendered sensitive to the 
humidity. It is construed that the initial charge potential is reduced for 
this reason. 
SUMMARY OF THE INVENTION 
We found that when a (dialkylpyrophosphato) organic titanate is 
incorporated in a photosensitive layer comprising a dispersion of a 
photoconductor such as cadmium sulfide in a binder resin such as an epoxy 
resin, the ozone resistance and moisture resistance of the photosensitive 
plate are prominently improved. 
More specifically, in accordance with the present invention, there is 
provided a photosensitive material for electrophotography comprising a 
conductive substrate and a layer of a composition comprising a dispersion 
of a photoconductive pigment in a binder resin, which is formed on the 
conductive substrate, wherein a (dialkylpyrophosphato) organic titanate is 
incorporated into said composition. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The high-humidity deterioration of the photosensitive plate due to the 
attack of ozone is confirmed by subjecting the photosensitive plate to the 
reproduction cycle of negative charging--light 
exposure--transfer--electricity removal repeatedly, allowing the treated 
photosensitive plate to stand under a high-humidity condition and 
measuring reduction of the charge potential. For example, when a 
photosensitive layer formed by dispersing a cadmium sulfide type 
photoconductor in a binder comprising an epoxy resin component and an 
amine type curing agent, which is considered to be most excellent in the 
moisture resistance, is subjected to the above-mentioned reproduction 
cycle 2000 times and then to the humidifying treatment at a temperature of 
30.degree. C. and a relative humidity of 80% for 12 hours, the ratio of 
reduction of the initial saturation charge potential of the photosensitive 
layer is as high as 75%. In contrast, if the (dialkylpyrophosphato) 
organic titanate is incorporated in the above-mentioned photosensitive 
layer, when the photosensitive layer is subjected to the above treatments, 
the ratio of reduction of the charge potential of the photosensitive layer 
is as low as about 5%. Accordingly, it is seen that the ozone resistance 
and the moisture resistance are prominently improved according to the 
present invention. 
The (dialkylpyrophosphato) organic titanate that is used in the present 
invention has a chemical structural feature that is not observed in 
ordinary organic titanate type coupling agents. More specifically, an 
ordinary organic titanate contains an alkyl group or alkyl and acyl groups 
bonded to the titanium atom through the oxygen atom. The organic titanate 
having this structure has no substantial effect of preventing the 
high-humidity deterioration due to the attack of ozone. In contrast, the 
organic titanate used in the present invention is characterized in that it 
contains a dialkylpyrophosphato group bonded to the titanium atom, and by 
dint of this chemical structural feature, the organic titanate of the 
present invention has an excellent effect of preventing the high-humidity 
deterioration due to the attack of oxygen. 
It is preferred that the organic titanate that is used in the present 
invention should have a structure represented by the following general 
formula: 
##STR1## 
wherein n is an integer of rom 1 to 3, R.sub.1 stands for an alkyl group, 
and R.sub.2 stands for an alkyl group, an alkylene group or an 
alkylene-carbonyl group with the proviso that when n is 2, two of groups 
R.sub.2 may be connected together to form an alkylene group or an 
alkylene-carbonyl group. 
In the above general formula (1), it is preferred that the alkyl group 
R.sub.1 be an alkyl group having 6 to 18 carbon atoms, such as an octyl 
group, a decyl group, a dodecyl group or a stearyl group. When the group 
R.sub.2 is an alkyl group, it is preferred that the alkyl group be an 
alkyl group having 2 to 6 carbon atoms, such as an ethyl group, an 
isopropyl group, an n-butyl group, an isobutyl group or a sec-butyl group. 
It is especially preferred that n be 1 or 2, that is, the organic titanate 
be a bis- or tris-(dialkylpyrophosphato) organic titanate. When n is 2 and 
two groups R.sub.2 are connected together to form an alkylene or 
alkylene-carbonyl group, as the alkylene group, there can be mentioned an 
ethylene group (--CH.sub.2 --CH.sub.2 --), and as the alkylene-carbonyl 
group, there can be mentioned a methyl-carbonyl group 
##STR2## 
According to the present invention, a prominent effect can be obtained when 
a photoconductive pigment to be charged by negative corona discharge, 
especially a photoconductor showing extreme reduction of the moisture 
resistance by the ozone treatment, that is, a cadmium sulfide type 
photoconductor, is used as the photoconductor. Any of cadmium sulfide type 
photoconductors known in the field of electrophotography can be used in 
the present invention. It is known that cadmium sulfide can be activated 
or sensitized by copper or chlorine. Particles of a CdS photoconductor 
sensitized by copper or the like is especially advantageously used for 
attaining the objects of the present invention. Instead of cadmium sulfide 
as the single substance, there may be used cadmium selenide sulfide, that 
is, CdS--CdSe solid solution, may be used in the state activated by copper 
or the like. 
As the binder resin, there can be used a known binder resin excellent in 
the resistance to the high-humidity deterioration due to the attack of 
ozone, especially a combination of an epoxy resin component and a curing 
agent component. 
In the present invention, a known epoxy resin component having at least two 
oxirane rings can be used as the epoxy resin component. Preferred examples 
are described below though epoxy resins that can be used in the present 
invention are not limited to those exemplified below. 
Glycidyl ether type epoxy resins: 
Epoxy resins derived from polyfunctional hydroxyl group-containing 
compounds such as bisphenol A, brominated bisphenol A, bisphenol F, 
tetrahydroxyphenylethane, resorcinol, novolak, polyalkylene glycol and 
glycerin and epihalohydrins. 
Glycidyl ester type epoxy resins: 
Glycidyl phthalate, glycidyl hexahydrophthalate and glycidyl ester of dimer 
acid. 
Glycidyl amines: 
Triglycidyl isocyanurate and tetraglycidyldiaminodiphenylmethane. 
Alicyclic epoxy resins: 
3,4-Epoxy-6-methylcyclohexyl 
methyl-3,4-epoxy-6-methylcyclohexane-carboxylate, vinylcyclohexene 
diepoxide, dicyclopentadiene oxide and bis(2,3-epoxycyclopentyl)ether. 
A bisepoxide, especially a bisphenol type epoxide resin, having an epoxy 
equivalent of 150 to 500, particularly 150 to 300, is preferred as the 
epoxy resin component. 
Although a curing agent such as an acid anhydride may be used as the curing 
agent component, it is preferred that a low-temperature or 
medium-temperature curing agent, that is, an amine type curing agent, be 
used as the curing agent component. As preferred examples of the amine 
type curing agent, there can be mentioned aliphatic polyamines such as 
diethylene triamine, triethylene tetramine, diethylaminopropylamine, 
menthene diamine, N-aminoethylpiperazine, m-xylene diamine, 
3,9-bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro[5,5]undecane and 
bis(4-amino-3-methylcyclohexyl)methane, modified aliphatic polyamines such 
as epoxy resin-polyamine adducts, polyamine-ethylene oxide adducts, 
polyamine-propylene oxide adducts, cyanoethylated polyamines and 
ketone-blocked polyamines, and aromatic polyamines such as m-phenylene 
diamine, 4,4'-methylene dianiline and diaminophenylsulfone. 
In the present invention, the epoxy resin component and the amine type 
curing agent component may be used at a known mixing ratio, for example, a 
weight ratio of from 100/1 to 100/200. 
In the present invention, it is preferred that the (dialkylpyrophosphato) 
organic titanate be used in an amount of 0.05 to 5 parts by weight, 
especially 0.1 to 2 parts by weight, per 100 parts by weight of the 
photoconductive pigment. If the amount of the organic titanate is too 
small and below the above range, the desired deterioration-preventing 
effect cannot be obtained, and if the amount of the organic titanate is 
too large and beyond the above range, the sensitivity of the 
photosensitive layer tends to decrease. 
In the present invention, it is preferred that the binder resin be used in 
an amount of 20 to 200 parts by weight, especially 30 to 150 parts by 
weight, per 100 parts by weight of the photoconductive pigment. 
The photosensitive material of the present invention is prepared by 
dissolving or dispersing the epoxy resin component, the amine type curing 
agent component, the organic titanate type ozone deterioration-preventing 
agent and the CdS photoconductor in a solvent, for example, a cyclic ether 
such as tetrahydrofuran, a ketone such as methylethyl ketone, an aromatic 
solvent such as toluene or a halogenated hydrocarbon such as 
dichloroethane or monochlorobenzene and coating the resulting coating 
composition on a known conductive substrate. 
The (dialkylpyrophosphato) organic titanate used in the present invention 
has a surface-active action and exerts a function of dispersing the CdS 
type photoconductor in a good condition in the coating composition. This 
is another prominent advantage of the present invention. 
It is preferred that the solid concentration of the coating composition be 
20 to 50% by weight, though the preferred concentration differs to some 
extent according to the coating method. It is preferred that the thickness 
of the photosensitive layer formed on the substrate be 10 to 100.mu., 
especially 15 to 50.mu., as solids.

The present invention will now be described in detail with reference to the 
following Examples that by no means limit the scope of the present 
invention. 
EXAMPLE 1 
______________________________________ 
Cadmium sulfide powder (PC-108 
8.0 g 
supplied by Silvania Co.) 
Tetrahydrofuran 4.0 g 
Bisphenol type epoxy resin 
2.6 g 
(epoxy equivalent of 192) 
Amine-adduct curing agent 
1.4 g 
Titanate type coupling agent 
0.06 g 
[bis(dioctylpyrophosphato) 
oxyacetate titanate] 
______________________________________ 
The above ingredients precisely weighed were dispersed for 1 minute by an 
ultrasonic dispersing machine. The dispersion was coated on an aluminum 
sheet having a thickness of 80.mu. by a wire doctor blade and subjected to 
a heat curing treatment in an oven at 100.degree. C. for 1 hour to obtain 
a photosensitive plate (No. 1) having a photosensitive layer thickness of 
25.mu. after heating curing. 
For comparison, a photosensitive plate (No. 2) was prepared in the same 
manner as described above except that the titanate type coupling agent was 
not added. Another comparative photosensitive plate (No. 3) was prepared 
in the same manner as described above except that an organic titanium 
compound, that is, tetraoctyl titanate, was used instead of the titanate 
type coupling agent used in Example 1. 
These three photosensitive plates were allowed to stand in a thermostat 
tank maintained at a temperature of 30.degree. C. and a relative humidity 
of 80% for 12 hours, and the charge quantities were measured by using a 
static copying paper tester (Model SP-428 supplied by Kawaguchi Denki 
K.K.) under the following conditions: 
Measuring method: static measurement II 
Applied voltage: -6 KV 
The initial potentials (V1) of the three photosensitive plates measured 
according to the above method are shown in Table 1. 
In order to effect the ozone treatment, each of the three photosensitive 
plates was set at a copying machine (Model DC-15 supplied by Mita 
Industrial Co., Ltd.) from which a development unit zone had been removed 
and the cycle of charging--exposure--transfer--removal of electricity was 
repeated 2000 times without passing a transfer sheet through the copying 
machine. 
The ozone-treated photosensitive plates were allowed to stand in a 
thermostat tank maintained at a temperature of 30.degree. C. and a 
relative humidity of 80% for 12 hours. After this humidifying treatment, 
the charge quantities of the photosensitive plates were measured in the 
same manner as described above. 
The initial potentials (V2) after the ozone treatment and humidifying 
treatment are shown in Table 1. The ratio of reduction of the initial 
potential under the high-humidity condition by the ozone treatment was 
calculated from the values V1 and V2 to obtain results shown in Table 1. 
For convenience, the initial potential reduction ratio is given by the 
following formula: 
##EQU1## 
TABLE 1 
______________________________________ 
Reduction 
Sample No. 
(V1) (V2) Ratio Remarks 
______________________________________ 
1 715V 690V 3.5% present 
invention 
2 700V 215V 69.3% comparison 
3 708V 220V 68.9% comparison 
______________________________________ 
From the results shown in Table 1, it is seen that the charge quantity was 
hardly reduced in the photosensitive plate comprising the titanate type 
coupling agent though the charge quantity was drastically reduced in the 
photosensitive plate free of the titanate type coupling agent or the 
photosensitive plate comprising the organic titanium compound instead of 
the titanate type coupling agent. Thus, it has been confirmed that in the 
photosensitive plate according to the present invention, no substantial 
deterioration under a high-humidity condition by the ozone treatment is 
caused. 
EXAMPLE 2 
______________________________________ 
Cadmium sulfide powder (PCP-MT supplied 
8.0 g 
by Kasei Optonics Co.) 
Tetrahydrofuran 4.0 g 
Bisphenol type epoxy resin 2.9 g 
(epoxy equivalent of 200) 
Amine-adduct curing agent 1.1 g 
Titanate type coupling agent 
0.08 g 
[isopropyltris(dioctylpyro- 
phosphato)titanate] 
______________________________________ 
The above ingredients precisely weighed were dispersed for 1 minute by an 
ultrasonic dispersing machine. The dispersion was coated on an aluminum 
sheet having a thickness of 80.mu. by a wire doctor blade and subjected to 
a heat curing treatment in an oven at 100.degree. C. for 1 hour to obtain 
a photosensitive plate (No. 4) having a photosensitive layer thickness of 
25.mu. after heat curing. 
For comparison, a photosensitive plate (No. 5) was prepared in the same 
manner as described above except that the titanate type coupling agent was 
not added. Another comparative photosensitive plate (No. 6) was prepared 
in the same manner as described above except that an organic titanium 
compound, that is, tributyl chlorotitanate, was used instead of the 
titanate type coupling agent used in Example 2. 
These three photosensitive plates were tested in the same manner as 
described in Example 1 to obtain results shown in Table 2. 
TABLE 2 
______________________________________ 
Reduction 
Sample No. 
(V1) (V2) Ratio Remarks 
______________________________________ 
4 655V 627V 4.3% present 
invention 
5 635V 135V 78.7% comparison 
6 641V 140V 78.1% comparison 
______________________________________ 
From the results shown in Table 2, it is seen that the charge quantity was 
hardly reduced in the photosensitive plate comprising the titanate type 
coupling agent though the charge quantity was drastically reduced in the 
photosensitive plate free of the titanate type coupling agent or the 
photosensitive plate comprising the organic titanium compound instead of 
the titanate type coupling agent. Thus, it has been confirmed that in the 
photosensitive plate according to the present invention, no substantial 
deterioration under a high-humidity condition by the ozone treatment is 
caused. 
EXAMPLE 3 
______________________________________ 
Cadmium sulfide powder (PCP-MT 
8.0 g 
supplied by Kasei Optonics Co.) 
Tetrahydrofuran 4.0 g 
Bisphenol type epoxy resin (epoxy 
4.0 g 
equivalent of 188) 
Amine-adduct curing agent 1.5 g 
Titanate type coupling agent 
0.04 g 
[tris(dioctylpyrophosphato) 
ethylene titanate] 
______________________________________ 
The above ingredients precisely weighed were dispersed for 1 minute by an 
ultrasonic dispersing machine. The dispersion was coated on an aluminum 
sheet having a thickness of 80.mu. by a wire doctor blade and subjected to 
a heat curing treatment in an oven at 100.degree. C. for 1 hour to obtain 
a photosensitive plate (No. 7) having a photosensitive layer thickness of 
25.mu. after heat curing. 
For comparison, a photosensitive plate (No. 8) was prepared in the same 
manner as described above except that the titanate type coupling agent was 
not added. Another comparative photosensitive plate (No. 9) was prepared 
in the same manner as described above except that an organic titanium 
compound, that is, tetrabutyl titanate, was used instead of the titanate 
type coupling agent used in Example 3. 
These photosensitive plates were tested in the same manner as described in 
Example 1 to obtain results shown in Table 3. 
TABLE 3 
______________________________________ 
Reduction 
Sample No. 
(V1) (V2) Ratio Remarks 
______________________________________ 
7 697V 660V 5.3% present 
invention 
8 665V 167V 74.8% comparison 
9 671V 178V 73.5% comparison 
______________________________________ 
From the results shown in Table 3, it is seen that the charge quantity was 
hardly reduced in the photosensitive plate comprising the titanate type 
coupling agent though the charge quantity was drastically reduced in the 
photosensitive plate free of the titanate type coupling agent or the 
photosensitive plate comprising the organic titanium compound instead of 
the titanate type coupling agent. Thus, it has been confirmed that in the 
photosensitive plate according to the present invention, no substantial 
deterioration under a high-humidity condition by the ozone treatment is 
caused.