Continuous tone diazotype process

A diazotype material useful for making duplicates of continuous tone originals, such as X-ray films, comprising a transparent film support having a coating of a layer thereon consisting essentially of at least two light-sensitive diazo compounds having different photolysis rates, couplers for the diazo compounds, an ultraviolet radiation absorbing material and homogeneously distributed in the layer at least about 7% by weight, based on the weight of lacquer, of an inert particulate material such as silica. The use of this material makes it possible to approximate a logarithmic photolysis, thereby providing sufficient linearity in the corresponding characteristic curve to achieve the desired duplicating results.

This invention relates to a diazo system and process for providing 
duplicates of continuous tone originals such as X-ray films. 
It has long been recognized in the art that the sensitometry of the 
diazotype process does not lend itself, normally, to the duplicating of 
continuous tone originals (M. S. Dinaburg, "Photosensitive Diazo 
Compounds", Focal Press, page 147 (1964)). This is evident when a typical 
characteristic curve of diazo imagery is examined. Thus, in the typical 
characteristic curve for diazo materials, such as shown in FIG. 1, in 
addition to a high contrast or gamma, the linear portion of the 
characteristic curve is extremely abbreviated. However, in order to 
achieve good continuous tone imagery for densities of the order of 1.5 or 
greater, it is desirable to have a gamma in the range of 1 to 1.6. In 
addition, an extended linear portion of the characteristic curve is 
equally important. The present invention makes use of both of these 
parameters in providing an excellent and advantageous diazotype material 
for providing duplicates of continuous tone originals. Such duplicating is 
important, particularly in the medical field, so that a high and 
consistent quality of duplicates of originals, such as X-ray films, can be 
obtained, e.g., for examination and analysis by different medical 
practitioners or staff members. 
Herrick (Journal of the Optical Society of America, pp. 904-910, December, 
1952) shows theoretically that for high initial actinic densities in diazo 
decomposition, the diazo density is a linear function of exposure. Typical 
photolysis curves of a diazo compound are shown in FIG. 2. These curves 
are expressed as density vs. exposure (or relative exposure) as compared 
to characteristic curves, where density is plotted as a function of log 
(relative exposure); compare FIGS. 1 and 2. 
If one takes the photolysis curve of FIG. 2 and calculates its 
corresponding characteristic curve, the curve as shown in FIG. 1 is 
obtained. That is to say, when the photolysis curve is linear, as 
intrinsic with a diazo compound, the characteristic curve does not lend 
itself to the production of good continuous tone characteristics. In order 
to provide such characteristics, an idealized characteristic curve having 
a lengthened linear portion such as shown in FIG. 3 is necessary. In this 
curve, a gamma of 1.0 between density values of 2.0 and 0.15 is observed. 
Thus, the governing linear equation for the logarithmic abscissa can be 
written as 
EQU D = C+KE (1) 
where 
EQU E = log E (2) 
and where D is density, E is exposure and C and K are constants. In terms 
of the photolysis curve needed to provide a linear characteristic curve, 
the photolysis curve can be described by the following equation: 
EQU D = C+K log E (3) 
thus, the important point to note is that in order to achieve a linear 
characteristic curve, a logarithmic photolysis must be simulated. This is 
shown in FIG. 4. Achieving a linear characteristic curve is thus 
accomplished by approximating a logarithmic photolysis curve. This has 
been accomplished unknowingly in the prior art in a number of ways, 
without recognizing the correlation between the desired characteristic 
curve and the required photolysis curve. 
One way, illustrated in FIG. 5, is to approximate the logarithmic 
photolysis by the linear photolysis of a multi-diazo system (three diazos 
in this particular case). In this procedure varying diazo concentrations 
and photolysis rates can be combined to achieve the final approximation. A 
homogeneous mixture of multiple diazo compounds coated in a single layer 
as well as multiple diazo compounds coated in separate layers have been 
employed in order to achieve the desired effect. In some cases, the 
photolysis rate of particular diazos can be varied by the inclusion of 
ultraviolet radiation absorbing compounds having varying ultraviolet 
absorption spectra in the system. This was, in fact, the approach used in 
some of the results of past workers (U.S. Pat. Nos. 3,661,591, 3,069,268 
and 2,793,118). 
A second approach used in the art for simulating the logarithmic photolysis 
curve of FIG. 4 is to approximate it by an exponential photolysis of the 
following form: 
EQU D = D.sub.o exp (-aE) (4) 
where D is density, D.sub.o is the initial density, a is a constant and E 
is exposure. 
This is achieved by the bulk absorption of uniformly distributed 
non-photolyzable attenuating constituents in the diazo layer. Examples of 
such constituents are commercial ultraviolet absorbers, particulates and 
diazos whose products of decomposition themselves have ultraviolet 
absorbing properties. An example of an exponential photolysis is shown in 
FIG. 6. An approach of this type was used in an attempt to extend the 
tonal range of the diazo process by certain workers in the prior art (U.S. 
Pat. Nos. 2,378,583, 2,739,061 and 3,525,618). However, even with this 
approach, only an abbreviated linear portion in the corresponding 
characteristic curve is obained. 
Thus, all of these prior art attempts suffer from one disadvantage or 
another, either in complex preparations or processing or in less than 
acceptable duplicating results. This is particularly critical in medical 
applications where qualities at least equivalent to silver halide 
duplicates are required. However, because of the growing expense of silver 
halide systems, it is quite important to develop a procedure as a 
substitute therefor. 
Accordingly, one of the objects of the present invention is to provide an 
effective and advantageous diazo system which enables the production of 
excellent duplicates of continuous tone originals. 
Another object of the invention is to provide a procedure for obtaining 
duplicates of silver halide X-ray films. 
These and other objects and advantages of the invention will become 
apparent to those skilled in the art from a consideration of the following 
specification and claims, taken in conjunction with the accompanying 
drawings. 
In accordance with the present invention, it has been found that a diazo 
system containing at least two light-sensitive diazo compounds with 
different photolysis rates and particular ultraviolet absorbers which 
selectively attenuate that actinic radiation with respect to one or more 
of the diazos in combination with a bulk, broad actinic 
radiation-absorbing, unreactive inert particulate material provides the 
necessary logarithmic photolysis needed to obtain the linear 
characteristic curve i.e., the resulting photolysis curved obtained by 
plotting the ultraviolet density of the undeveloped or latent image versus 
relative exposure is substantially congruent with respect to a logarithmic 
curve following the equation: 
EQU D = K + C log E 
where D is the ultraviolet density, E is the relative exposure and K and C 
are constants, for a range covering at least one density unit. The 
critical feature of the invention is the use of the bulk (homogeneous) 
inert particulate material in the system. By combining the bulk, 
exponential absorption effect of the particulates and non-photolyzable 
ultraviolet radiation absorber with the logarithmic photolysis 
approximation of a multi-diazo system, the characteristic curve shown in 
FIG. 7 was actually obtained. An extended linear density range of 1.25 was 
observed in this particular case. FIG. 8 confirms that this particular 
characteristic curve has a photolysis curve exhibiting an excellent fit 
with the logarithmic function, 
EQU D = -(1.48 log.sub.10 E + 0.67) (5) 
where D is density and E is relative exposure. 
The support for the diazotype material is normally a transparent film of, 
for example, a polyester or cellulose acetate. This is particularly true 
with duplicates of X-ray photographs where such duplicates are almost 
always observed as transparencies. Suitable copies of X-ray films can, 
however, be produced on diazotype material having a paper support. 
The support may, if desired, be pre-treated to facilitate adhesion of the 
light-sensitive layer. It may also be given a coating of lacquer before 
application of the light-sensitive layer. Alternatively, lacquer may be 
incorporated into the light-sensitive layer itself. However, the preferred 
embodiment is to use a system wherein a solvent solution of the diazo 
compounds, couplers and stabilizers is imbibed into a binder lacquer 
containing the bulk absorbers and optionally some blue dye such as 
Heliogen Violet, DuPont Oil Blue, Crystal Violet or Methyl Violet. 
Examples of suitable lacquers are cellulose acetate, cellulose acetate, 
cellulose acetate propionate, cellulose acetate butyrate, polyvinyl 
acetate and polyvinyl formal. 
Examples of suitable light-sensitive diazonium compounds which may be 
employed in the system of the invention are: 
p-diethylamino-2-ethoxybenzenediazonium fluoroborate; 
2,5-diethoxy-4-morpholinobenzenediazonium fluoroborate; 
p-diethylaminobenzenediazonium fluoroborate; 
1-diazo-4-(N-hydroxyethyl-N-ethylamino)-benzene fluoroborate; 
1-diazo-2-ethoxy-4-(N,N-diethylamino)-benzene fluoroborate; 
1,diazo-2,5-dimethoxy-4-morpholino-benzene fluoroborate; 
1-diazo-4-(N,N-dimethylamino)-benzene fluoroborate; 
1-diazo-3-methyl-4-ethylamino-benzene fluoroborate; 
1-diazo-2,5-diethoxy-4-morpholino-benzene fluoroborate; and 
1-diazo-4-(N,N-diethylamino)-benzene fluoroborate 
Examples of suitable couplers to be employed therewith are 
2,3-naphthalenediol; naphthalene 2,3-dihydroxy-6-sulphinic acid; 
resorcinol; 4,4'-diresorcinol, acetoacetanilide; acetoacet-o-toluidide; 
chlororesorcinol; N,N-bis-.beta.-hydroxyethyl-2-hydroxy-3-naphthoamide; 
N-(2'-methylphenyl)-2-hydroxy-3-naphthoamide; 
N-.gamma.-morpholino-n-propyl-2-hydroxy-3-naphthoamide and 
N,N-bis-.beta.-hydroxyethyl-2-hydroxy-3-naphthoamide. 
Stabilizers for diazos are well known in the art and include substances 
such as tartaric acid, citric acid, sulfosalicylic acid, etc. 
The bulk absorption in the diazo system of the invention results from the 
use of inert particulates such as silica, clays or pigments. Basically, in 
order to obtain the desired linear diazo characteristic curve by means of 
the bulk absorption in the diazotype system, the inert particulates must 
function to provide scattering sites in the diazo layer. Hence, even 
bubbles (vesicules) are suitable if homogeneously scattered. The amount of 
inert particulate employed must be at least about 7% by weight based on 
the amount of lacquer containing the particulate which is coated on the 
support. A lesser amount will not function suitably to give good quality 
duplicates of continuous tone originals. This is particularly true for 
duplicates of X-ray photographs. A suitable range of amount of inert 
particulate material is about 7% to about 20% by weight of particulate 
based on the weight of lacquer. The preferred particulate is silica. 
Also important is the size of the inert particulate material employed. 
Generally, suitable results are obtained with inert particulates, such as 
inorganic pigments, having a particle size of up to approximately 50 
microns. Advantageously, in order to achieve the best results, 
particulates within a size range of about 1 to 50 microns should be used. 
Suitable particulates to be used in the invention include, as noted above, 
various clays and pigments such as silica, aluminum silicates, 
diatomaceous earths, magnesium silicates, barium sulfate, mica, etc. 
Ultraviolet absorbers suitable for use in the diazo system of the invention 
include materials such as 7-diethylamino-4-methylcoumarin, substituted 
benzotriazoles such as 2(2'-hydroxy-5'-methylphenyl)benzotriazole (U.S. 
Pat. Nos. 3,004,896 and 3,189,615), p-methoxybenzylidenemalonic acid 
dimethyl ester, 2-hydroxy-4-methoxybenzophenone, etc. 
It is also important to note that the diazotype material of the present 
invention is contained in a single layer on the support material and not 
in multiple layers as has been done in various prior art systems. Thus, 
the coating procedure for making the diazotype system of the invention is 
much less complex than would be required for multiple layers. 
Development of the exposed diazotype material is carried out by procedures 
well known in the art. Thus, the exposed film is usually developed by 
means of ammonia vapors at a temperature of about 200.degree.-300.degree. 
F. in a conventional diazo developer apparatus.

The following examples are given merely as illustrative of the present 
invention and are not to be considered as limiting. Unless otherwise 
noted, the percentages therein and throughout the application are by 
weight. 
EXAMPLE 1 
A lacquer is prepared consisting of 
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(1) 1/2 second Cellulose Acetate Butyrate 
20 g. 
(2) Polyvinyl Acetate-Polyvinyl Alcohol 
Copolymer 1 g. 
(3) Toluol 50 ml. 
(4) Acetone 40 ml. 
(5) Syloid 72 (silica) 1.7 g. 
(6) Synasol 190 (denaturated ethanol) 
12 ml. 
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This mixture is ball milled for 4 hours and coated on a polyester film 
which has been treated with trichloroacetic acid and silica. 
The lacquered film is then imbibed with the following formulation which 
contains two diazo compounds: 
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(1) Methyl ethyl ketone 40.4 g. 
(2) Synasol 190 47.9 g. 
(3) Formic Acid 9.2 g. 
(4) p-morpholine aceto-acetanilid 
0.07 g. 
(5) 4-N:N-diethylaminobenzene 
diazonium .multidot. BF.sub.4 
0.76 g. 
(6) 4-N:N-diethylamino-2-ethoxybenzene 
diazonium .multidot. BF.sub.4 
0.76 g. 
(7) 7-diethylamino-4-methyl-coumarin 
(u.v. absorber) 0.70 g. 
(8) 2-hydroxynaphthalene-3-carboxylic 
acid ethanol amide 0.91 g. 
(9) Tartaric Acid 240 g. 
(10) Resorcinol 0.35 g. 
______________________________________ 
The 4-N:N-diethylaminobenzene diazonium.BF.sub.4 in conjunction with the 
noted u.v. absorber exhibits a relative photolysis rate of 0.125. The 
4-N:N-diethylamino-2-ethoxybenzene diazonium.BF.sub.4 in conjunction with 
said u.v. absorber exhibits a relative photolysis rate of 0.037. 
The film is exposed through a suitable original X-ray photograph on an 
Arkwright DGS 1400 vacuum frame printer for 20 seconds, high intensity and 
developed on an Arkwright Model 404 developer. Excellent duplicates of the 
original, suitable for medical standards, are obtained. 
EXAMPLE 2 
The following light-sensitive solution is imbibed into the same lacquered 
film as described film in Example 1: 
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Tartaric acid 2.0 g. 
Resorcinol 0.3 g. 
N-(2'-methylphenyl)-2-hydroxy-3- 
0.4 g. 
naphthoamide 
N,N-bis-.beta.-hydroxyethyl-2- 
0.5 g. 
hydroxy-3-naphthoamide 
1-diazo-4-(N,N-diethylamino)- 
0.6 g. 
benzene fluoroborate 
1-diazo-2-ethoxy-4-(N,N-diethylamino)benzene 
0.6 g. 
fluoroborate 
1-diazo-2,5-diethoxy-4-morpholinobenzene 
0.3 g. 
fluoroborate 
7-diethylamino-4-methyl-coumarin 
1.2 g. 
dissolved in: 
Methyl ethyl ketone 50 ml. 
Synasol 190 50 ml. 
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Excellent duplicates of X-ray photographs are obtained using the resulting 
diazotype material in the same manner as described in Example 1. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications are 
intended to be included within the scope of the following claims.