Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
This invention relates to electrostatic recording and
more particularly relates to an image~receiving sheet adapted
to receive on the surface thereof a particle image electro-
statically formed on a photoconductive surface and whereon a
final colored-image is fixed.
The image-receiving sheet of this invention is used Eor
the electrostatic recording process, for example, such as
disclosed in British Patent 1527168. In that proc~ss there are
provided an image composingparticles containing a colorless
sublimable dye. The image-receiving sheet contains a developer
which reacts with the colorless sublimable dye to develop a
color. The particles are ~ormed on a photoconductive insulating
surface by a suitable electrostatic method and -then are
transferred to the image-receiving sheet. The particles are
then heated, whereby the colorless sublimable dye pr~sent in
the particlesis sublimed and reacted with the developer present
in the image-receiving sheet to develop a color. The remaining
parts of the image composing particles are then removed from the
surface of the image-receiviI~g sheet. Thus,the fixed dye-image
is obtained on the image-receiving sheet.
In such process, the following method of forming the
particle image is especially useful. The photoconductive surface
is charged and the image composing particles are laid uniformly
thereon. The image composing particle is transmissive -to a
light of selected color, i.e. functions as a color fil-ter, further
to containing a sublimable dye as above mentioned. The photo-
conductive surface is exposed to imagewise light through the
image composing particles, thereby forming an electrostatic image
corresponding to an image of selected color oE light. Then the
image composing particles in the area where the electrostatic
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attractive force is weakened in accordance with the ~lectrostatic
image are removed, whereby the particle image is obtained. By
utilizing such process, a multicolored dye .image can be formed
through only one exposure as described in sritish Patent 1527168.
As a matter of course the particle image formation
according to the usual xerography may be used.
As an image-receiving sheet applicable to the above-
mentioned process, there has been proposed a s-tructure such that
a color forming layer containing a developer material is disposed
on a support member such as paper and on the color Eorming layer
is disposed a dielectric surface layer which serves to
electrostatically retain the transferred image-composing particles.
The dielectric layer necessarily has a structure such -tha-t the
sublimed dye gas can pass there-through to the color forming layer.
Therefore the surface resistivity of the dielectric layer de-
creases due to moisture absorption under a hi~h humiclity
atmosphere such as a relative humidity of more than 60%. Thus
the electrostatic charge on the dielectric layer is decreased
under high humidity, resulting in low efficiency of transfer of
the image composing particles from the photoconductive surface
to the image-receiving sheet.
- SUMMARY OF T~E INVENTION
It is an object of the invention to provide an
image-receiving sheet which can efficiently retain image
composing particles transferred from a photoconductive surface
over the practical range of humidity in the atmosphere.
: An image-receiving sheet according to this invention
comprises a supporting member, a color forming layer disposed on
~he suppor-ting member and containing a developer material
which reacts wi-th a sublimable dye -to develop a color, and a
dielectric layer coated on the color forming layer, -the
dlelectric layer exhibiting high surEace resistivity in the low
humidity range in the atmosphere and e~hibiting high adhesion in
the high humidity range where the surface resistivity thereof is
lower -than the required value.
More specifically, the image-receiving sheet for
receiving an electrostatically formed image according to the
present invention bears an image comprising particles containing
a sublimable dye-former on which is fixed a colored image
obtained by making said sublimable dye-former react with a
developer contained in the sheet.
Said sheet comprises a suppor-t member, a color
developing layer formed on said support member and containing a
developer ma-terial which reacts with a sublimable dye-former to
develop a color, the surface resistivity thereof being less than
109Q, and a dielectric layer formed on said color developing
layer through which sublimed dye-former gas can pass to said
color developing layer, said dielectric layer comprising a fine
inorganic powder and a transparent binder-resin having high
resistivity.
sRIEF DESCRTPTION OF THE DRAWING
Figure is a sectional view of an image-receiving sheet
according to the inven-tion.
DETAILED DESCRIPTION OF TEIE INVENTION
Referring to the Figure, a supporting paper 1 is
coated with a color forming layer 2, and a dielectric layer 3
is formed on the color forming layer 2. The color -forming layer
2 contains, as a principal component, an activated clay 4 which
acts as a developer for a sublimable dye. The color forming
-- 4 --
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layer 2 also contains fine inorganic powders 5 and 6 having a
white color. The activated clay 4 and the inorganic powders 5
and 6 are dispersed into plastic binder 7. The sur~ace
resistivity of the color forming layer 2 is less than 109~ and
so the layer 2 can act as an electrode for preventing image
composing particles from being scattered by an applied voltage
during an electrostatic transfer of the particles. Thus it is
desirable that the activated clay 4 be of low particle
resistivity.
The inorganic powder 5 serves to prevent yellowing
of the color forming layer 2 due to heating and may be a
calcium carbonate. The inorganic powder 6 serves as a develop-
ing assistant agent and may be a silica. These inorganic
powders are also eEfective for improvinc~ heat resistance. The
binder 7 should have high binding powder and hardly be yellowed
by heating. For example, styrene-butadiene copolymer, acrylic
resin or polyvinyl acetate may be used. Especially, water-based
emulsions of those resins are preferable because they do not
so much conceal
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the activated clay, which is favorable for obtaining high density
color development.
The appropriate ratio of those components is that the
amount of the inorganic powders 5 and 6 is 20 to 80 parts by
weight and the binder 7, 10 to 30 parts by weight to 100 parts by
weight of the activated clay 4. A dispersant such as a surface-
active agent, etc. may be used.
The coating amount of the color forming layer 2 is pre-
ferably 5 to 10 g/m2 for obtaining low surface resistance and
sufficient density of color development. The color forming layer:is preferably subjected to càlendering for making the surface
resistance uniform.
The supporting paper 1 is preferably a fine quality
paper having smooth surface which does not repel coa-ting, whereby
a uniform surface of coated layer is obtained.
The dielectric layer 3 is composed of a release agent
8. a fine inorganic powder 9 and a binder 10. The release agent
8 makes it easy to release the image composing particles from the
surface of the dielectric layer 3 after the heating for develop-
ing color. The inorganic powder 9 has a white color and permitsthe dye-gas sublimed from the image composing particles to pass
through the dielectric layer 3 to the color forming layer 2.
The release agent 8 may be fin~ particles of a trans~
parent or white polyethylene. It is prepared in water-based emul-
sion state. The inorganic powder 9 may be a silica which is
effective for assisting the development, and is preferably pre-
pared by wet process, i.e. a colloidal silica, because it has
high activity. The binder 10 should have the properties of
high resistivity, strong binding power to the color forming
layer and large bending strength. Further the binder 10 should
exhibit, under a high humidity atmosphere such as a relative
humidity of more than 60%, adhesion sufficient to retain the
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image composingparticles through the infuuence of moisture, It
also should have the property that sublimed dye-gas can easily
pass therethrough~ Styrene-butadiene copolymer may be used as
the binder 10 and it is prepared in the form of a water based
emulsion.
The amoun-t of silica used in the dielectric layer 3
should be in the range of from 25 to 70 parts by weight to 100
parts by weight of the total amount of the release agent 8 of
polystyrene and the binder 10 of styrene-butadiene copolymer.
Below that range, the dielectric layer 3 has so low a heat resis-
tance that it is diEficult to remove the image composing
particles after the heat development, because of the softening
of the layer 3. At above that range t:he dielectric layer 3 has
low resistivity and low adhesion unde~ low humidity so as not to
be ab~e to sufficiently retain the image composing particles. The
amount o styrene-butadiene copolymer should be in the range from
0.3 to 1 parts by weight to 1 part by weigh-t of polystyrene. ~t
below that range, the binding power becomes insufficient and at
above that range the adhesion under high humidity becomes so
high that it is difficult to remove the image composing particles
after heating for color development.
A dispersant such as a surface-active agent or a thicken-
ing agent may be used when those components are dispersed and
mixed, The coating thickness of the dielectric layer 3 is
preferably 2 to 5 g/m2. In such range the path from the image
composing particle to the color forming layer 2 through which
sublimed dye gas passes is short enough to obtain desirable
density of developed color, and besides, a suEficient efficiency
of transfer of the image composing particles is obtained.
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The particle diameters of polystyrene and styrene-
butadiene copolymer is preferably from 0.1 to 0.5 ~ and that of
silica is preferably from 0.1 to 0.001 ~.
The thus obtained dielectric layer 3 exhibits a surface
resistivity more than 101 ~ under the atmosphere of relative
humidity below 60% so as to be able to electrostatically retain
the image composing particles. It also exhibits, under a relative
humidity above 60%, an adhesion sufficient to retain the image
composing particles. The relative humidity at above which the
suEficient adhesion is obtained changes in proportion to the ratio
of the components. In above description, sufficient adhesion
means the extent to which particles are retained under a linear
pressure of 100 to 1000 g/cm, the efEiciency of transferring
particles with the diameter of 5 to 50 ~ being more than 80%, and
the retained particles being removed by using fur brush after
heat development for 1.5 seconds at 230~C.
A detailed example is described hereinafter.
According to the following formulation, coatings for
forming a color forming layer and a dielectric layer were pre-
pared. In the following formulation the amount of each compo-
nent is shown by the amount of solid content.
Coating for a color forming layer:
Activated clay : 100 parts by weight
Calcium carbonate : 30 parts by weight
Colloidal silica (in which
the ratio of solid content
~` is 20%) : 20 parts by weight
Styrene-Butadiene copolymer
(in which the ratio oE
solid content is 50~) : 15 parts by weight
Water : ~35 par-ts by weight
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Coating for a dielectric layer;
Water dispersion of low molecular
weight polyethylene (in which
` the ratio of solid content is
20~) : 100 parts by weight
Colloidal silica (in which
the ratio of solid content
is 20~) : 60 parts by weight
Styrene-butadiene copolymer
; 10 (in whi~h the ratio of
solid content is 50%) : 50 parts by weight
The coating for a color forming layer was prepared by
dispersing the components for 30 minutes with an attritor. Cal-
cium carbonate is used after disperslng, with water mixed, for 1
hour with an attritor. The coating i~or a dielectric layer was
prepared by mixing the component with stirrer.
The coating for a color forming layer was coated on a
fine quality paper at 8 g/m2 which represents the value after
drying,and then the coated layer was subjected to calendering. The
thus ormed color forming layer was coated with the coating for
a dielectric layer at 3 g/m2 which represents the value after
drying,and then calendering was carried out.
The thus obtained image-receiving sheet was measured for .
the surface resistivity and the particle transfer eEficiency
under various humidities in a constant temperature bath. The
temperature of the bath was maintained at 30C and the humidity
was varied. The measurement was performed after keeping the image
receiving sheet in the constant temperature bath for one hour.
The particle transfer efficiency was measured by performing a
transfer process such that the dielectric layer surEace of the
image receiving sheet was closely contacted with the image compos-
ing particles which are electrostatically retained onto a photo-
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conductiye plate comp~sed by coating a zinc oxide photoconductivelayer on an aluminum plate, and the voltage oE 1.0 KV was sup-
plied between the conductive layer of the photoconductive plate
and the back surface of the image-receiving sheet, with the
image-receiving sheet being pressed on the particles. The
result of the measurement is shown in the following Table.
.
Relative humidity Surface resistivi~-ty~ Transfer efficiency
( % ) ( 11 ) ( % )
24 2.~9 x 1ol3 90
4.0 x 1012 90
9.2 x 101 85
_
8.2 x 108 85 to 80
I .
93 4.6 x 108 80
.
In the measurement where the humiditles of 80~ and
93~ were employed, the particle transfer was performed by only
applying pressure. <
The image composing particles were scarcely scatter-
ed by the transfer. The transfer properties were similar in both
cases of using a nonconductive image-composing-particle and a
conductive particle.
In order to examine a developed dye image, the image
composing particles transferred to the image-receiving sheet were
pressed by a hot plate with -the temperature of 230C for
1.5 seconds, and then the remainder of the particles were
removed. The thus obtained dye image had fully developed color
and was clear, without fogging.
Further, the image composing particles could be com-
pletely removed even in -the case where -the above-mentioned process
was performed under the rela-tive humidity of ~0~ or 93~.
