Support sheet for photographic printing sheet

A support sheet for a photographic printing sheet having enhanced anti-fogging and anti-yellowing properties comprises a pulp paper substrate sheet, a front coating layer formed on a front surface of the substrate sheet and comprising a cured resinous material produced from electron beam-curable unsaturated organic compound by an electron beam irradiation thereto and mixed with a white pigment, and a back coating layer formed on a back surface of the substrate sheet and comprising a film-forming synthetic resinous material, in which magnesium hydroxide is contained, as an anti-fogging agent, in the substrate sheet.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
The present invention relates to a support sheet for a photographic 
printing sheet. More particularly, the present invention relates to a 
support sheet for a photographic printing sheet having an enhanced 
resistance to fogging and yellowing and thus capable of being printed with 
clear photographic images. 
2) Description of the Related Arts 
Formerly, a baryta paper sheet was used as a support for a photographic 
printing sheet. The baryta paper sheet was produced by coating two 
surfaces of a paper sheet having a good sizing property and mechanical 
property with a coating material containing a white pigment, for example, 
barium sulfate. 
Recently, a waterproof paper sheet composed of a substrate paper sheet and 
coating layers formed on two surfaces of the substrate sheet and 
comprising a polyolefin resin have become widely used as a support sheet 
for a photographic printing sheet, in place of the baryta sheet. 
The photographic printing sheet comprising the waterproof support sheet is 
advantageous in that, in a developing step for the photographic printing 
sheet, the highly hydrophobic polyolefin coating layer obstructs a 
penetration of a developing solution into the support. sheet, and 
accordingly, the time needed for washing and drying the printing sheet can 
be shortened, and the shrinkage and elongation of the support sheet be 
restricted, and thus the photographic printing sheet exhibits a superior 
dimensional stability. 
Nevertheless, the polyolefin resin-coated support sheet is disadvantageous 
in the following items. 
The polyolefin resin coating layer contains an inorganic white pigment, for 
example, titanium dioxide, for enhancing an opacifying power and a 
resolving power of the resultant photographic printing sheet but this 
pigment has a poor dispersion in the polyolefin resin. Also, the pigment 
contains a volatile substance, and in a melt-extruding step of the 
polyolefin resin, the volatile substance forms bubbles in the polyolefin 
resin melt, and thus the resultant polyolefin resin coating layer is 
sometimes cracked. 
To avoid the above-mentioned disadvantages, the amount of the white pigment 
to be added to the polyolefin resin cannot be increased to a high level 
sufficient to obtain a satisfactory opacifying and resolving power of the 
resultant photographic printing sheet. Generally speaking, when the white 
pigment consists of titanium dioxide, it is difficult to add the titanium 
dioxide pigment in an amount of about 20% by weight or more to the 
polyolefin resin. Accordingly, the photographic printing sheet prepared 
from nhe conventional polyolefin resin-coated support sheet does not have 
a satisfactory sharpness of the images printed thereon. 
Recently, a support sheet for a photographic printing sheet having an 
electron beam-cured resin coating layer formed by coating a surface of a 
substrate paper sheet with an electron beam-curable resin composition 
comprising an organic unsaturated compound curable by an electron beam 
irradiation, and irradiating an electron beam to the coated layer of the 
resin composition, was disclosed in, for example, Japanese Examined Patent 
Publication Nos. 60-17,104 and 60-17,105 and Japanese Unexamined Patent 
Publication No. 57-49,946. 
In this type of support sheet, the resin composition to be coated on a 
surface of a substrate paper sheet need not be heated at a high 
temperature, and thus can contain the inorganic white pigment in a large 
amount of 20 to 80% by weight. Therefore, the resultant photographic 
printing sheet produced from this type of support sheet can record thereon 
photographic images with a significantly enhanced sharpness, in comparison 
with those of the conventional polyolefin resin-coated photographic 
printing sheet. 
Nevertheless, this type of photographic printing sheet, in which a 
photo-sensitive layer is formed on an electron beam-cured resin coating 
layer, is disadvantageous in that, when developed with a developing 
solution of photographic chemicals, a portion of the developing chemicals 
is adsorbed by and remains on the electron beam-cured resin coating layer, 
and causes the printing sheet to turn yellow after the development. Also, 
when developed after storage for a long time, non-neglectable fogging 
occurs in the developed photographic printing sheet, or the 
photo-sensitivity of the photographic printing sheet is changed. 
Various attempts have been made to eliminate the above-mentioned 
disadvantages. For example, Japanese Examined Patent Publication No. 
1-21,495 discloses an attempt to form a polyethylene coating layer on an 
electron beam-cured resin coating layer, to thereby restrict the change in 
the photosensitivity during a storage of the photographic printing sheet. 
This attempt is disadvantageous, however, in that, to obtain a 
satisfactory prevention of the change in the photosensitivity, the 
polyolefin coating layer must be formed in a large thickness, and this 
causes the sharpness of the resultant photographic images to become 
unsatisfactory, even though the electron beam-cured resin coating layer is 
employed to increase the sharpness of the printed images. 
Japanese Unexamined Patent Publication No. 60-144,736 discloses an attempt 
to arrange a barrier layer between a substrate paper sheet and an electron 
beam-cured resin coating layer, to thus restrict any change in the 
photographic sensitivity of the photographic printing sheet. The barrier 
layer made from the materials disclosed in the Japanese Publication, 
however, is not satisfactory when trying to prevent the occurrence of 
fogging after storage for a long time. 
Also, Japanese Unexamined Patent Publication Nos. 62-61,049 and 61-141,543 
discloses a specific polymer or monomer for forming the barrier layer, but 
this specific polymer or monomer does not satisfactorily remove the 
above-mentioned disadvantages. 
Further, Japanese Unexamined Patent Publication No. 59-124,336 discloses a 
barrier layer arranged between a substrate paper sheet and an electron 
beam-cured resin coating layer and prepared from at least one member 
selected from aqueous solutions of water-soluble polymeric material and 
dispersions of polyolefin homopolymers and copolymers and polyacrylate and 
polymethacrylate homopolymers and copolymer, to restrict the change in 
photographic sensitivity. 
The barrier layer made from the polymeric material disclosed in the 
Japanese Publication does not provide a satisfactory prevention of fogging 
of the resultant photographic printing sheet after a storage thereof for a 
long time. 
Generally, it is known that the relationship between the energy level of 
the electron beam applied to an electron beam-curable compound composition 
and the fog density of the resultant photographic printing sheet due to a 
developing solution is contrary to the relationship between the energy 
level of the electron beam and the yellowing density. Namely, when the 
electron beam is applied in a large energy level the yellowing caused by 
the developing solution is restricted to a low level but the fogging is 
promoted to a high intensity. Also, when the electron beam is applied in a 
low energy level, the yellowing density is significantly increased, 
whereas the fog density is decreased, and the physical properties, for 
example, adhesive strength and mechanical strength, of the resultant cured 
resin coating layer are poor. 
Accordingly, to prevent or restrict the yellowing and fogging of the 
photographic printing sheet without affecting the physical properties of 
the cured resin coating layer, it is necessary to control the energy level 
of the electron beam to an optimum level. Also, to eliminate all of the 
above-mentioned disadvantages, it is very important to provide a new type 
of support sheet capable of preventing the yellowing and fogging of the 
resultant photographic printing sheet without depending on the quantity of 
the electron beam applied in the formation of the crated resin coating 
layer. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a support sheet for a 
photographic printing sheet having a high surface smoothness, a 
satisfactory water resistance and an excellent resistance to yellowing and 
fogging even after a storage for a long time. 
The above-mentioned object can be attained by the support sheet for a 
photographic printing sheet of the present invention, which comprises, a 
substrate sheet comprising a cellulosic pulp material; 
a front coating layer located on a front surface of the substrate sheet and 
comprising a mixture of an electron beam irradiation-curing product of at 
least one electron beam-curable unsaturated organic compound and a white 
pigment; and 
a back coating layer located on a back surface of the substrate sheet and 
comprising a film-forming synthetic resinous material, the substrate sheet 
containing an anti-fogging agent consisting essentially of magnesium 
hydroxide in an amount of 1 g/m.sup.2 or more and in a proportion of 0.1 
to 70% based on the total weight of the substrate sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Generally, it is known that, when a photographic printing sheet is produced 
by coating a photographic emulsion layer on a support sheet comprising a 
substrate sheet comprising a cellulosic pulp paper sheet and a resin 
coating layer formed by coating an electron beam-curable unsaturated 
organic compound composition on a surface of the substrate sheet and 
applying an electron beam irradiation thereto, a significant fogging 
occurs in the resultant photographic printing sheet during a storage 
thereof. 
Although the mechanism of the fogging phenomenon is not completely clear, 
it is assumed that active radicals are generated in the photographic 
printing sheet due to the electron beam irradiation, and the active 
radicals react with the photographic emulsion layer to create the fogging 
phenomenon. 
Also, it is assumed that the active radicals are generated mainly from 
cellulose in the substrate sheet and the electron beam-cured resin, and 
the fogging phenomenon is mainly influenced by the the active radicals 
generated from the cellulose. 
The inventors of the present invention discovered for the first time that 
the fogging phenomenon can be prevented or restricted by adding magnesium 
hydroxide in an amount of 1 g/m.sup.2 or more, in the substrate sheet. 
The mechanism of preventing or restricting the fogging phenomenon by 
utilizing magnesium hydroxide has not been made completely clear, but it 
is assumed that the penetration of the active radicals generated from the 
cellulose in the substrate sheet by the electron beam irradiation into the 
photographic emulsion layer is prevented or hindered by some actions of 
magnesium hydroxide, and thus the anti-fogging property of the 
photographic printing sheet is improved. 
Accordingly, in the formation of the cured resin coating layer on the base 
paper sheet-containing substrate sheet, even when the electron beam is 
applied at an energy level high enough to prevent the yellowing of the 
photographic printing sheet, the fogging of the photographic printing 
sheet can be satisfactorily restricted. 
Referring to FIG. 1, a support sheet for a photographic printing sheet is 
composed of a substrate sheet 1 consisting of a paper sheet, a front 
coating layer 2 formed on a front surface of the substrate sheet 1, and a 
back coating layer 3 formed on a back surface of the substrate sheet. 
The anti-fogging and yellowing agent consisting essentially of magnesium 
hydroxide is contained in the substrate paper sheet 1. Preferably, the 
magnesium hydroxide is in the form of fine solid particles having an 
average size of 0.1 to 100 .mu.m. 
Optionally, magnesium hydroxide is used in the state of a mixture with an 
additional white pigment, for example, finely divided calcium carbonate, 
titanium dioxide, talc, clay, barium sulfate and aluminum oxide. There is 
no limitation of the amount of the additional pigment to be mixed to 
magnesium hydroxide, and the additional pigment is employed preferably in 
an amount of 40% or less based on the total weight of magnesium hydroxide 
and the additional pigment. If the amount of the additional pigment is 
more than 40%, sometimes the anti-fogging effect of magnesium hydroxide 
becomes unsatisfactory. 
The substrate paper sheet for the substrate sheet is usually selected from 
paper sheets made from softwood pulp, hardwood pulp, and a mixture of the 
softwood and hardwood pulps. Also, the wood pulps are not limited to 
specific types of pulps made by specific pulping methods, but preferably 
are selected from the kraft pulps, sulfite pulps and soda pulps usually 
used for making paper sheets. If necessary, the wood pulps are blended 
with a synthetic pulp or synthetic fibers, to make the paper sheets. 
There is no restriction on the type, basis weight and thickness of the 
substrate paper sheet, but preferably the substrate paper sheet has a high 
surface smoothness enhanced by applying a compressive force thereto by a 
calender or the like, and has a basis weight of from 50 to 300 g/m.sup.2 
and a thickness of 40 to 270 .mu.m. 
The substrate paper sheet usable for the present invention optionally 
contains at least one paper additive, for example, dry paper strength 
reinforcers, for example, cationic starches, cationic polyacrylamides, and 
anionic polyacrylamides, sizing agents, for example, fatty acid salts, 
rosin, maleic acid-modified rosin, cationic sizing agents, and reactive 
sizing agents, fillers, for example, clay, talc, and kaolin, wet paper 
strength reinforcers, for example, melamine-formaldehyde resins and 
epoxidized polyamide resins, fixing agents, for example, aluminum sulfate 
and cationic starches, and pH-adjusting agents, for example, caustic soda 
and sodium carbonate. The paper sheet can be tub-sized or size-pressed by 
a treating liquid containing at least one member selected from 
water-soluble polymeric additives, sizing agents, inorganic electrolytes, 
hygroscopic substances, pigments and pH-adjusting agents. 
In the production of the paper sheet containing the magnesium hydroxide, 
magnesium hydroxide is mixed into a pulp slurry, and the mixed pulp slurry 
is converted to a paper sheet. Alternatively, magnesium hydroxide is 
incorporated into a paper sheet by a press-sizing method in which the 
paper sheet is coated or impregnated with a dispersion containing 
magnesium hydroxide. In the substrate paper sheet, the anti-fogging 
magnesium hydroxide is present in an amount of 0.1 to 70%, preferably 1 to 
40%, based on the total weight of the substrate sheet. If the amount of 
magnesium hydroxide is less than 0.1% by weight, the resultant 
photographic printing sheet exhibits an unsatisfactory anti-fogging 
effect. Note, the use of magnesium hydroxide in an excessively large 
amount of more than 70% by weight is not effective for successively 
enhancing the anti-fogging effect, and sometimes results in a lowering of 
the paper strength, an undesirable powdering, and in a deterioration of 
the photographic performance of the photographic printing sheet. 
In the support sheet of the present invention, a front coating layer is 
formed on a front surface of the substrate sheet. This front coating layer 
comprises, as a principal component, a mixture of a cured resinous 
material with a white pigment. The cured resinous material is produced 
from at least one unsaturated organic compound capable of being cured by 
irradiating an electron beam thereto. 
The electron beam-curable unsaturated organic compound usable for the 
present invention is preferably selected from the group consisting of: 
(1) acrylate compound (or acrylic acid esters) of mono-to hexahydric 
aliphatic, cycloaliphatic and aromatic alcohols and polyalkyleneglycols; 
(2) acrylate compounds (or acrylic acid esters) of addition products of 
mono-to hexahydric aliphatic, cycloaliphatic and aromatic alcohols with 
alkyleneoxides; 
(3) polyacryloylalkylphosphoric acid esters; 
(4) reaction products of carboxylic acids with polyols and acrylic acid; 
(5) reaction products of isocyanates with polyols and acrylic acids; 
(6) reaction products of epoxy compounds with acrylic acid; and 
(7) reaction products of epoxy compounds with polyols and acrylic acid. 
The electron beam-curable unsaturated organic compounds include, for 
example, polyoxyethylene-epichlorohydrin-modified bisphenol A diacrylate, 
dicyclohexylacrylate, epichlorohydrin-modified polyethyleneglycol 
diacrylate, 1,6-hexanediol diacrylate, hydroxypivalic acid 
ester-neopentylglycol diacrylate, nonylphenoxy-polyethyleneglycol 
acrylate, ethyleneoxide-modified phenoxidized phosphoric acid acrylate, 
ethyleneoxide-modified phthalic acid acrylate, polybutadieneacrylate, 
caprolactam-modified tetrahydrofurfuryl acrylate, tris(acryloxyethyl) 
isocyanurate, trimethylolpropane triacrylate, penta-erythritol 
triacrylate, dipentaerythritol hexaacrylate, polyethyleneglycol 
diacrylate, 1,4-butadienediol diacrylate, neopentylglycol diacrylate, and 
neopentyl-glycol-modified trimethylolpropanediacrylate. 
The white pigment in the front coating layer preferably comprises at least 
one member selected from the group consisting of titanium dioxide which 
may be an anatase type or rutile type, barium sulfate, calcium carbonate, 
zinc oxide and aluminum oxide. 
To enhance a dispersing property of the white pigment particles, for 
example, titanium dioxide particles, the surfaces of the pigment particles 
are coated with metal oxide, for example, aluminum oxide. 
The white pigment is preferably present in an amount of 20 to 80%, based on 
the total amount of the front coating layer. 
If the content of the white pigment is less than 20% by weight, the 
resultant front coating layer exhibits an unsatisfactory opacifying power, 
and thus the photographic images recorded on the resultant photographic 
printing sheet have an unsatisfactory sharpness and clarity. If the 
content of the white pigment is more than 80% by weight, the resultant 
front coating layer exhibits an unsatisfactory flexibility, and thus is 
sometimes cracked. 
The front coating layer can be formed by coating a front surface of a 
substrate sheet with a coating liquid containing the electron beam-curable 
unsaturated organic compound and the white pigment, and irradiating an 
electron beam to the resultant coating liquid layer, to cure and solidify 
same. 
The coating liquid for the front coating layer can be prepared by a 
conventional dispersing apparatus, for example, a three roll mill, two 
roll mill, Cowless dissolver, homomixer, sand grinder, Dyno mill, and 
ultrasonic dispersing machine. 
The coating operation of the coating liquid on the substrate paper sheet 
can be carried out by a conventional coating method, for example, bar 
coating method, air doctor coating method, blade coating method, 
squeeze-coating method, air-knife coating method, reverse roll coating 
method or transfer-coating method. Further, a fountain coating method or a 
slit die coating method can be applied to the coating operation. 
The resultant coating liquid layer on the substrate paper sheet can be 
dried by a conventional drying method. 
To obtain a front coating layer having a high surface smoothness and gloss, 
the coating operation is preferably carried out by a coating method in 
which a casting drum is employed. 
The irradiation of the electron beam is carried out to cure and solidify 
the coated coating liquid layer on the substrate sheet. For this electron 
beam irradiation, preferably a curtain type electron beam accelerator, 
which is relatively cheap and can generate a large output, is utilized. In 
this accelerator, the accelerating voltage for electron beam is usually 
from 100 to 300 kV and the energy level is from 0.1 to 6 Mrad, more 
preferably 0.2 to 4 Mrad. 
If the energy level is less than 0.1 Mrad, the reaction of the unsaturated 
bonds in the unsaturated organic compound in the coated coating liquid 
layer sometimes cannot be completed, and thus a non-reacted unsaturated 
organic compound remains in the cured resinous layer and sometimes affects 
the photographic property of the resultant photographic printing sheet. 
Generally, when the energy level is small, the degree of cross-linkage of 
the cured resin becomes low, and thus the anti-yellowing property of the 
resultant photographic printing sheet is affected by the low cross-linkage 
of the cured resin. 
When the support sheet of the present invention is employed, the resultant 
photographic printing sheet exhibits a high anti-fogging property even if 
the electron beam curing is carried out at a high energy level. Also, when 
the electron beam irradiation is carried out at a high energy level, the 
resultant photographic printing sheet exhibits an enhanced anti-yellowing 
property. 
Nevertheless, the energy level of the electron beam must be controlled to 
an appropriate level, because an excessively high energy level results in 
a wasteful consumption of energy, and sometimes causes the resultant front 
coating layer to exhibit an undesirably high hardness and rigidity, and 
thus the resultant photographic printing sheet is curled. 
The electron beam irradiation is preferably carried out in a non-oxidative 
atmosphere containing oxygen in a restricted concentration of 500 ppm or 
less. If the oxygen concentration is more than 500 ppm, the oxygen serves 
as a retarding agent for a polymerization the unsaturated organic 
compound, and thus the curing reaction of the unsaturated organic compound 
becomes poor. 
When the electron beam curing is carried out by a drum curing method, in 
which a coating liquid layer formed on a substrate sheet is brought into 
contact with the peripheral surface of a curing drum and a electron beam 
is irradiated to the coating liquid layer through the substrate sheet, the 
coating liquid layer is not directly exposed to the ambient air 
atmosphere, and thus this electron beam irradiation can be effected 
without lowering the oxygen concentration of the atmosphere. Nevertheless, 
this drum curing operation using the electron beam irradiation may be 
carried out in an inert gas atmosphere, to prevent or hinder a generation 
of ozone due to the electron beam irradiation through the atmosphere, or 
to cool a window through which the electron beam is irradiated and which 
is exothermically heated by the electron beam irradiation. 
The front coating layer preferably has a weight of 2 to 60 g/m.sup.2 more 
preferably 10 to 30 g/m.sup.2. 
In the support sheet of the present invention, a back coating layer is 
formed on a back surface of the substrate sheet. This back coating layer 
comprises a film forming synthetic resin which can be selected from 
conventional synthetic resins usable for coating the substrate sheet for 
photographic printing sheet. 
The synthetic resins include polyolefin resins and the same electron 
beam-cured resins as mentioned above. 
The polyolefin resins include homopolymers of ethylene and .alpha.-olefins, 
for example, propylene, copolymers of at least two of ethylene and 
.alpha.-olefins, and mixtures of at least two of the above-mentioned 
homopolymers and copolymers. 
Preferable polyolefin resins for the present invention are low density 
polyethylene resins, high density polyethylene resins, linear chain type 
low density polyethylene resins, and mixtures of at least two of the 
above-mentioned resins. 
There is no specific limitation of the molecular weight of the polyolefin 
resins, but preferably the molecular weight of the polyolefin resins is 
from 20,000 to 200,000. 
The back coating layer comprising the polyolefin resin can be formed on the 
back surface of the substrate sheet by a customary melt extrusion-coating 
method. 
The back coating layer comprising the electron beam-cured resinous material 
can be formed by the same method as that used for the front coating layer, 
as mentioned above. 
The back coating layer optionally contains at least one additive for 
example, an anti-oxidant or surfactant. 
Preferably, the back coating layer has a weight of 10 to 40 g/m.sup.2. 
EXAMPLES 
The present invention will be further explained by the following specific 
examples, which are only representative and in no way restrict the scope 
of the present invention. 
Example 1 
A mixed pulp slurry with a consistency of 1.0% was prepared from a mixture 
of 20% by weight of a bleached softwood sulfate pulp (NBSP) beaten to a 
Canadian standard freeness of 250 ml and 80% by weight of a bleached 
hardwood kraft pulp (LBKP) beaten to a Canadian standard freeness of 280 
ml determined in accordance with Japanese Industrial Standard P 8121-76. 
To the mixed pulp slurry was added an additive having the following 
composition: 
______________________________________ 
Amount by 
Component weight(*).sup.1 
______________________________________ 
Cationic starch derivative 
2.0% 
Alkylketone dimer resin 
0.4% 
Anionic polyacrylic amide resin 
0.1% 
Polyamidepolyamineepichloro- 
0.7% 
hydrin resin 
______________________________________ 
Note: 
(*) . . . The amount in % is based on the dry weight of the mixed pulp. 
The composition was mixed with an aqueous sodium hydroxide solution, to 
adjust the pH of the composition to a level of 7.5. 
The mixed pulp was uniformly suspended in water, and then to the resultant 
pulp slurry was added finely divided magnesium hydroxide in an amount such 
that after the resultant pulp slurry was converted to a paper sheet the 
content of magnesium hydroxide being 5% based on the dry weight of the 
resultant paper sheet, while agitating the mixture for 5 minutes. 
The consistency of the solid content of the mixed pulp slurry was adjusted 
to 0.05% and then the adjusted slurry was further agitated for 5 minutes. 
A paper sheet having a basis weight of 180 g/m.sup.2 and a density of 1.0 
g/cm.sup.3 was produced from the resultant mixed pulp slurry, by using a 
hand paper-making machine available from Toyo Seiki K.K. 
The resultant paper sheet was employed as a substrate sheet. 
A back surface of the substrate sheet was coated with a polyethylene resin 
by a customary melt extrusion-coating method. 
The resultant back coating layer had a weight of 30 g/m.sup.2. 
Separately, an electron beam-curable resinous composition was prepared in 
the following composition: 
______________________________________ 
Part by 
Component weight 
______________________________________ 
Epoxy acrylate oligomer(*).sub.2 
70 
Di-functional acrylate 
30 
monomer(*).sub.3 
Titanium dioxide 25.0 
(*).sub.4 
______________________________________ 
Note: 
(*).sub.2 . . . Available under the trademark of Viscoat 540, from Osaka 
Yukikayaku K.K. 
(*).sub.3 . . . Available under the trademark of HDDA, from Nihon Kayaku 
K.K. 
(*).sub.4 . . . Available under the trademark of A220, from Ishihara 
Sangyo K.K. 
The composition was mixed and dispersed in a paint conditioner for one 
hour. 
A front surface of the substrate sheet was coated with the electron beam 
curable resinous composition in an amount of 25 g/m.sup.2 by using a wire 
coating bar. Then an electron beam was applied to the composition layer 
under an accelerating voltage of 165 kV at an energy level of 3 Mrad, to 
cure the composition layer. 
A specimen of the resultant support sheet was subjected to an measurement 
of an anti-fogging effect thereof, in the following manner. 
The specimen of the support sheet was superimposed on a specimen of a 
conventional photographic printing sheet so that the front coating layer 
of the support sheet specimen came into contact with a photographic 
emulsion layer surface of the photographic sheet specimen, and the 
resultant test piece was left to stand in a dark room at a temperature of 
70.degree. C. at a relative humidity of 50% for 3 days. Then the support 
sheet specimen was separated from the photographic sheet specimen, and the 
photographic sheet specimen was subjected to a customary development by 
using an automatic developing machine available under the trademark of 
RCP20, from Dast Co. 
The fog density of the developed specimen was measured in a customary 
manner by using a Macbeth densitometer available under the trademark of 
Model No. RD-914, from Kollmorgen Corp. 
The test results are shown in Table 1. 
Example 2 
The same experimental procedures as in Example 1 were carried out except 
that the final content of magnesium hydroxide in the paper sheet after the 
paper-making step was adjusted to 60%, based on the total dry weight of 
the paper sheet. 
The test results are shown in Table 1. 
Comparative Example 1 
The same experimental procedures as in Example 1 were carried out except 
that no magnesium hydroxide was added to the paper sheet. 
The test results are shown in Table 1. 
Referential Example 1 
A resin-coated paper sheet was produced by coating two surfaces of a paper 
sheet not containing magnesium compound with a polyethylene in an amount 
of 25 g/m.sup.2. 
The same test as mentioned in Example 1 was applied to the resin-coated 
paper sheet. 
The test results are shown in Table 1. 
Comparative Example 2 
The same experimental procedures as in Example 1 were carried out except 
that magnesium hydroxide was replaced by magnesium sulfide (MgSO.sub.4) in 
an amount of 5% based on the total dry weight of the resultant paper 
sheet. 
The test results are shown in Table 1. 
Comparative Example 3 
The same experimental procedures as in Example 1 were carried out except 
that magnesium hydroxide was replaced by magnesium oxide (MgO) in an 
amount of 5% based on the total dry weight of the resultant paper sheet. 
The test results are shown in Table 1. 
Comparative Example 4 
The same experimental procedures as in Example 1 were carried out except 
that magnesium hydroxide was replaced by magnesium silicate (Mg.sub.2 
SiO.sub.4) in an amount of 5% based on the total dry weight of the 
resultant paper sheet. 
The test results are shown in Table 1. 
Comparative Example 5 
The same experimental procedures as in Example 1 were carried out except 
that magnesium hydroxide was replaced by calcium carbonate (CaCO.sub.3) in 
an amount of 5% based on the total dry weight of the resultant paper 
sheet. 
The test results are shown in Table 1. 
TABLE 1 
______________________________________ 
Item 
Magnesium Energy 
compound contained 
level of 
in substrate paper sheet 
electron 
Example Amount beam Fog 
No. Type (% by wt) (Mrad) density 
______________________________________ 
Example 1 
Mg(OH).sub.2 
5 3 0.12 
Example 2 
Mg(OH).sub.2 
60 3 0.11 
Comparative 
None -- 3 2.03 
Example 1 
Referential 
RC paper sheet -- 0.14 
Example 1 
Comparative 
Example 
2 MgSO.sub.4 5 3 0.19 
3 MgO 5 3 0.20 
4 Mg.sub.2 SiO.sub.4 
5 3 0.25 
5 CaCO.sub.3 5 3 2.00 
______________________________________ 
In Examples 1 and 2, the fog density of the developed specimens was less 
than 0.15 and it was confirmed that no fogging and yellowing of the 
resultant photographic printing sheets occured even after storage for 12 
months, whereas in Comparative Examples 2 to 4 in which the magnesium 
compounds different from magnesium hydroxide were used in place of 
magnesium hydroxide, the resultant fog density of the developed specimens 
was more than 0.15. 
Also, in Comparative Example 1 in which no antifogging agent was used and 
in Comparative Example 5 in which calcium carbonate was used in place of 
magnesium hydroxide, the resultant developed specimens fogged 
significantly.