Color-developing sheet for pressure-sensitive recording sheets

A color-developing sheet for pressure-sensitive recording sheets, said sheet comprising as a color-developing agent at least one 2,2'-bisphenolsulfone zinc salt selected from compounds of the general formula ##STR1## wherein R.sub.1 and R.sub.2 are identical or different, and represent a hydrogen atom, a halogen atom, an alkyl group containing 1 to 10 carbon atoms, a cycloalkyl group containing 3 to 10 carbon atoms, an aralkyl group containing 7 to 11 carbon atoms, or a phenyl group.

BACKGROUND OF THE INVENTION 
This invention relates to pressure-sensitive recording sheets, and more 
specifically, to a color-developing sheet for pressure-sensitive recording 
sheets which contains a novel color developing agent. 
Pressure-sensitive recording sheets are also known as carbonless copying 
paper. They produce a color upon the application of a mechanical or impact 
pressure by writing or by pounding a typewriter, thus permitting 
duplication of several copies simultaneously. The color is based on a 
color forming reaction between an electron-donating colorless dye and an 
electron-accepting color developer. 
The structure of a pressure-sensitive recording sheet and the mechanism of 
color formation are illustrated generally with reference to the 
accompanying drawing. The back surface of each of a top (CB: Coated Back) 
sheet 1 and a middle (CFB: Coated Front and Back) sheet 2 is coated with 
microcapsules 4 having a diameter of several microns to ten and several 
microns and composed of a shell of a polymeric film such as gelatin and a 
solution of a colorless color-forming pressure-sensitive dye in an 
involatile oil enclosed therein. The surface of each of the middle (CFB) 
sheet 2 and the bottom (CF: Coated Front) sheet 3 is coated with a layer 5 
containing a color developing agent having the property of reacting with 
the colorless dye upon contact therewith and thus producing a color. A 
sheet coated with a coating composition containing a color developer, such 
as the CFB sheet and the CF sheet, is called a color-developing sheet. 
When a localized pressure is applied by a writing instrument 6 (or a 
typewriter or the like) to a pressure-sensitive recording sheet composed 
of a multiply structure of the CB sheet 1, CFB sheet 2 and CF sheet 3 so 
that the microcapsule-coated surface faces the surface coated with the 
color developer-containing layer 5, the microcapsules 4 under the applied 
pressure break and the solution of the colorless dye moves to the color 
developer-containing layer 5. Thus, the dye reacts with the color 
developer to form a colored image 7 in the desired pattern of recording. 
In the pressure-sensitive recording sheet illustrated in the drawing, only 
one CFB sheet is interposed between the CB sheet 1 and the CF sheet 3, but 
if desired, two or more CFB sheets may be interposed. 
Conventionally known electron-accepting color developing agents include (1) 
inorganic solid acids such as acid clay (Fuller's earth) or attapulgite, 
(2) substituted phenols and diphenols, (3) p-substituted phenol 
formaldehyde polymers, and (4) metal salts of aromatic carboxylic acids. 
These color-developing agents, however, are not entirely satisfactory. For 
example, the inorganic solid acids adsorb gases or moisture in the air and 
cause yellowing of the sheet or are deteriorated in color-forming 
property. With the substituted phenols and diphenols, the ultimate density 
of color is insufficient. The p-substituted phenol formaldehyde polymers 
(for example, p-phenyl phenol novolak resin) have superior color-forming 
properties, but have the defect that the coated sheet undergoes yellowing 
upon exposure to sunlight. The aromatic carboxylic acid metal salts are 
superior in color-forming ability, light fastness of the colored image and 
resistance to yellowing under light, but their water resistance and 
plasticizer resistance are not entirely satisfactory. 
SUMMARY OF THE INVENTION 
It is a first object of this invention to provide a color-developing sheet 
which exhibits an equivalent or higher color-developing ability to or than 
developing sheets containing conventional inorganic solid acids or 
p-phenylphenol novolak resin as a color developing agent, and which give 
colors having superior water resistance and plasticizer resistance. 
A second object of this invention is to provide a color developing sheet 
which gives a colored image having superior light fastness and undergoing 
little decrease in density with time, and whose tendency to yellowing 
under sunlight or the like is drastically reduced. 
A third object of this invention is to provide a color-developing sheet 
which lends itself to very advantageous handling and storage. 
According to this invention, there is provided a color-developing sheet for 
pressure-sensitive recording sheets, said sheet comprising as a 
color-developing agent at least one 2,2'-bisphenolsulfone zinc salt 
selected from the group consisting of compounds of the formula 
##STR2## 
wherein R.sub.1 and R.sub.2 are identical or different, and represent a 
hydrogen atom, a halogen atom, an alkyl group containing 1 to 10 carbon 
atoms, a cycloalkyl group containing 3 to 10 carbon atoms, an aralkyl 
group containing 7 to 11 carbon atoms, or a phenyl group. 
In a preferred embodiment, the present invention provides a 
color-developing sheet for pressure-sensitive recording sheets, said 
color-developing sheet comprising both the aforesaid 2,2'-bisphenolsulfone 
zinc salt and at least one polyvalent metal compound selected from the 
group consisting of oxides, hydroxides and carbonates of zinc, magnesium, 
aluminum, lead, titanium, calcium, cobalt, nickel, manganese and barium. 
The 2,2'-bisphenolsulfone zinc salts of formula (I) are known compounds. 
However, it has not been known that these compounds form a color upon 
contact with a colorless pressure-sensitive dye to form a colored image 
having superior fastness characteristics, and are thus suitable as a 
color-developing agent for pressure-sensitive recording sheets. In 
addition, it has been difficult to anticipate the suitability of the 
2,2'-bisphenolsulfone zinc salts as a color-developing agent. 
Free bisphenolsulfone, a precursor for the zinc salts of formula (I), has 
been found to have little or no color-forming ability when used as a 
color-developing agent for pressure-sensitive recording sheets. 
The color-developing sheet of this invention containing the zinc salt of 
formula (I) has an equivalent or greater color-forming ability to or than 
color-developing sheets containing inorganic solid acids or p-phenyl 
phenol novolak resin as a color-developing agent, and the formed image has 
good resistance to fading under the action of water, plasticizers, light, 
etc. The color-developing sheet of this invention also has much improved 
resistance to yellowing under exposure of sunlight or the like, and lends 
itself to very advantageous handling and storage. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The subject matter of this invention is a color-developing sheet for 
pressure-sensitive recording sheets, said color-developing sheet 
comprising as a color-developing agent at least one 2,2'-bisphenolsulfone 
zinc salt selected from the group consisting of compounds of the general 
formula 
##STR3## 
wherein R.sub.1 and R.sub.2 are identical or different, and represent a 
hydrogen atom, a halogen atom, an alkyl group containing 1 to 10 carbon 
atoms, a cycloalkyl group containing 3 to 10 carbon atoms, an aralkyl 
group containing 7 to 11 carbon atoms, or a phenyl group. 
2,2'-bisphenolsulfone used to produce the zinc salt of formula (I) is 
produced, for example, by (i) oxidizing with hydrogen peroxide a 
2,2'-bisphenol sulfide compound obtained by the reaction of a 
p-substituted phenol with sulfur dichloride, or (ii) oxidizing with 
hydrogen peroxide a 2,2'-bisphenol sulfoxide compound obtained by the 
reaction of a p-substituted phenol with thionyl chloride. Examples of the 
p-substituted phenol that can be used in the above reaction include 
p-halophenols such as p-chlorophenol and p-bromophenol; p-alkylphenols 
such as p-cresol, p-amylphenol, p-octylphenol, p-tertiary butylphenol and 
p-nonylphenol; p-cycloalkylphenols such as p-cyclohexylphenol; 
p-(.alpha.,.alpha.-dimethylbenzyl) phenol; and p-phenylphenol. Among 
these, p-octylphenol, p-nonylphenol, p-cyclohexylphenol, and 
p-(.alpha.,.alpha.-dimethylbenzyl)phenol are preferred. These 
p-substituted phenols may be used alone or in combimation with one 
another. When two or more p-substituted phenols are used, there can be 
obtained a mixture composed of two or more 2,2'-bisphenolsulfones having 
the formula 
##STR4## 
in which two R groups are different from each other. 
Some known methods can be applied to the production of the zinc salt of 
formula (I) from the resulting 2,2'-bisphenolsulfone or its mixture. 
One method comprises reacting an alkali metal salt of the 
2,2'-bisphenolsulfone with a water-soluble zinc salt in a solvent capable 
of dissolving both. According to this method, the 2,2'-bisphenolsulfone is 
first reacted with an alkali metal hydroxide or alkoxide to form the 
alkali metal salt of 2,2'-bisphenolsulfone or its solution in water, an 
alcohol or in a mixture of water and an alcohol. Then, the product is 
reacted with the water-soluble zinc salt. 
Specifically, when the 2,2'-bisphenolsulfone zinc salt of formula (I) is to 
be produced, at least 2 moles of the alkali metal hydroxide or alkoxide 
and at least one mole of the water-soluble zinc salt are used per mole of 
the 2,2'-bisphenolsulfone. Examples of the water-soluble zinc salt that 
can be used are inorganic acid salts such as zinc chloride, zinc sulfate 
and zinc nitrate and organic acid salts such as zinc oxalate and zinc 
acetate. 
The zinc salt of formula (I) can also be produced by reacting 
2,2'-bisphenolsulfone with an organic acid salt as zinc oxalate, or zinc 
acetate in an organic solvent at an elevated temperature. Useful organic 
solvents for this reaction include methanol, ethanol, butanol, acetone, 
benzene, toluene, and methylene chloride. It is especially effective to 
dissolve the 2,2'-bisphenolsulfone compound in such a solvent. 
The zinc salts of general formula (I) are used as color-developing agents 
either alone or in combination with each other according to the purpose of 
use. 
The color-developing agent in accordance with this invention can be used 
jointly with known color-developing agents, such as inorganic solid acids 
(e.g. activated acid clay), organic polymers (e.g., phenolformaldehyde 
resin), and aromatic carboxylic acid metal salts, without any deleterious 
effect. 
In one preferred embodiment of this invention, 0.01 to 10 parts by weight, 
preferably 0.2 to 5 parts by weight, per part by weight of the 
color-developing agent of at least one polyvalent metallic compound 
selected from the group consisting of oxides, hydroxides and carbonates of 
zinc, magnesium, aluminum, lead, titanium, calcium, cobalt, nickel and 
manganese is coated together with the color-developing agent of formula 
(I) on a sheet. Examples of the polyvalent metal compounds are zinc oxide, 
magnesium oxide, aluminum oxide, calcium oxide, lead oxide, titanium 
oxide, manganese oxide, magnesium hydroxide, aluminum hydroxide, calcium 
hydroxide, zinc hydroxide, and calcium carbonate. When these polyvalent 
metal compounds are used, the resulting color-developing sheet has 
improved resistance to yellowing under light, and the rate of color 
development becomes faster and the color density increases. Moreover, the 
light fastness, plasticizer resistance and storage stability of the formed 
image are improved at the same time. Among the aforesaid polyvalent metal 
compounds, zinc oxide is most practical. 
The color developing sheet of this invention can be prepared by various 
methods. A typical method comprises coating a sheet such as paper with a 
water-base coating color containing the color-developing agent suspended 
therein. The coating color is conditioned to have a suitable viscosity and 
suitable coating characteristics by incorporating a kaolin clay, calcium 
carbonate, starch, etc. in a synthetic or natural latex, and desirabably 
contains 10 to 70% by weight of the color-developing agent based on the 
total solids content. When the content of the color-developing agent is 
less than 10% by weight, the color-developing agent cannot exhibit 
suffices color-forming properties. When it exceeds 70% by weight, the 
characteristics of the surface of the resulting color-developing sheet are 
deteriorated. The amount of the coating color to be coated on the base 
sheet is at least 0.5 g/m.sup.2, preferably 1 to 10 g/m.sup.2, as solids 
content. In other words, at least 0.05 g/m.sup.2, preferably 0.1 to 7 
g/m.sup.2, of the color-developing agent is applied to the sheet. 
The color-developing sheet of this invention can also be produced by an 
alternative method which comprises dissolving or suspending the 
color-developing agent in an organic solvent such as alcohols, 
hydrocarbons or ketones, optionally incorporating a kaolin clay, calcium 
carbonate, etc., and coating the resulting solution or suspension on a 
sheet such as paper. The amount of the coating solution or suspension is 
at least 0.05 g/m.sup.2, preferably 0.1 to 7 g/m.sup.2, as the 
color-developing agent. 
Still another method for producing the color-developing sheet of this 
invention comprises incorporating the color-developing agent into a 
sheet-forming stock, and subjecting the mixture to a sheet-forming 
process. 
The amount of the color-developing agent can be reduced in the present 
invention, and the concentration, viscosity, etc. of the coating 
composition can be varied over relatively wide ranges. Accordingly, both 
on-machine coating and off-machine coating are possible, and great 
advantages can be obtained not only in the properties of the 
color-developing sheet, but also in the process steps of sheet production.

The following Examples specifically illustrate the present invention. 
SYNTHESIS EXAMPLE 1 
In a mixture of carbon tetrachloride and n-hexane, p-tert-octyl phenol was 
reacted with sulfur dichloride to form 
2,2'-bis(p-tert-octylphenol)sulfide. The sulfide was oxidized with 
hydrogen peroxide in glacial acetic acid to form 
2,2'-bis(p-tert-octylphenol)sulfone. Recrystallization from carbon 
tetrachloride/n-hexane afforded a purified product having a melting point 
of 142.degree. to 144.degree. C. 
Then, 9.48 g (0.02 mole) of the resulting 2,2'-bisphenolsulfone was added 
to a solution of 1.6 g (0.04 mole) of sodium hydroxide in 100 ml of ethyl 
alcohol. The solution was stirred for 1 hour under reflux, and then cooled 
to 30.degree. C. A solution of 2.73 g (0.02 mole) of zinc chloride in 50 
ml of ethyl alcohol was added, and the mixture was refluxed for 1 hour. 
The reaction mixture was poured into 500 ml of ice water. The precipitated 
white crystals were collected by filtration, and dried to afford 10.6 g of 
2,2'-bis(p-tert-octylphenol)sulfone zinc salt (to be referred to as 
compound No. 1) corresponding to general formula (I). 
SYNTHESIS EXAMPLE 2 
Pale yellow 2,2'-bis(p-tert-butylphenol)sulfone was prepared in the same 
way as in Synthesis Example 1 using p-tert-butylphenol. 
To a solution of 1.38 g (0.06 mole) of metallic sodium in 180 ml of ethyl 
alcohol was added 14.2 g (0.03 mole) of the aforesaid 
2,2'-bisphenolsulfone. The mixture was stirred for 1 hour under reflux, 
and then cooled to 30.degree. C. The reaction mixture was added to a 
solution of 4.09 g of zinc chloride in 100 ml of ethyl alcohol. The 
mixture was stirred for 4 hours at 30.degree. C. The precipitated crystals 
were separated by filtration, and ethyl alcohol was distilled off under 
reduced pressure from the filtrate to afford 13.6 g of pale yellow 
crystals of 2,2'-bis(p-tert-butylphenol)sulfone zinc salt (to be referred 
to as compound No. 2) corresponding to general formula (I). 
SYNTHESIS EXAMPLE 3 
In dichloroethane, 2 moles of p-cyclohexylphenol was reacted with 1 mole of 
thionyl chloride to form 2,2'-bis(p-cyclohexylphenol)sulfoxide. The 
product was then oxidized with hydrogen peroxide in glacial acetic acid to 
form 2,2'-bis(p-cyclohexylphenol)sulfone. Recrystallization from carbon 
tetrachloride afforded white crystals having a melting point of 
176.degree. to 178.degree. C. Using this product, 
2,2'-bis(p-cyclohexylphenol)sulfone zinc salt (to be referred to as 
compound No. 3) corresponding to general formula (I) was prepared in the 
same way as in Synthesis Example 1. 
SYNTHESIS EXAMPLE 4 
Using p-phenylphenol, 2,2'-bis(p-phenylphenol) sulfone zinc salt (to be 
referred to as compound No. 4) was prepared in the same way as in 
Synthesis Example 1. 
EXAMPLES 1 to 4 
Using each of compounds Nos. 1 to 4 in Synthesis Examples 1 to 4 as a 
color-developing agent, a suspension of the following formulation was 
prepared by means of a sand grinding mill. 
______________________________________ 
Parts by weight 
______________________________________ 
Color-developing agent 
24.5 
Sodium ligninsulfonate 
3.0 
Water 42.5 
______________________________________ 
A coating composition of the following formulation was prepared by using 
the above suspension. 
______________________________________ 
Parts by weight 
______________________________________ 
Suspension 50 
Kaolin 100 
Styrene/butadiene latex 
(concentration 50%) 
15 
Oxidized starch 15 
Water 180 
______________________________________ 
The coating composition was coated on a sheet of fine paper and dried so 
that the amount of the coating composition applied was 6 g/m.sup.2 upon 
drying. Thus, a color-developing sheet was obtained. 
COMATIVE EXAMPLE 1 
A suspension and a coating composition were prepared in accordance with the 
same formulations as in Example 1 using 
2,2'-bis(p-tert-octylphenol)sulfone which is a precursor of the 
2,2'-bis(p-tert-octylphenol)sulfone zinc salt (compound No. 1) obtained in 
Synthesis Example 1. The coating composition was coated on a sheet of fine 
paper and dried so that the amount of the coating composition was 6 
g/m.sup.2 upon drying. Thus, a color-developing sheet was obtained. 
COMATIVE EXAMPLE 2 
A color-developing sheet was prepared in the same way as in Comparative 
Example 1 using 
2,2'-bis[(p-(.alpha.,.alpha.-dimethylbenzyl)phenol]sulfone. 
COMATIVE EXAMPLE 3 
A glass reactor was charged with 170 g of p-phenylphenol, 22.5 g of 80% 
para-formaldehyde, 2.0 g of p-toluenesulfonic acid and 200 g of benzene, 
and with stirring, the contents were heated. While water generated as a 
result of the reaction was distilled off out of the reactor as an 
azeotrope with benzene, they were reacted at 70.degree. to 80.degree. C. 
for 2 hours. Then, 320 g of a 10% aqueous solution of sodium hydroxide was 
added, and benzene was distilled off by steam distillation. Dilute 
sulfuric acid was added dropwise, and the precipitated 
p-phenylphenol/formaldehyde polymer was collected by filtration, washed 
with water, and dried to afford 176 g of a white powder. A 
color-developing sheet was produced in the same way as in Comparative 
Example 1 using the resulting p-phenylphenol/formaldehyde polymer. 
COMATIVE EXAMPLE 4 
A color-developing sheet was prepared in the same way as in Comparative 
Example 1 except using zinc 
3-[4'-(.alpha.,.alpha.-dimethylbenzyl)phenyl]-5-(.alpha.,.alpha.-dimethylb 
enzyl)salicylate. 
TEST EXAMPLES 
The properties of the color-developing sheets produced in Examples 1 to 4 
and Comparative Examples 1 to 4 were tested by the following methods. The 
results are tabulated below. 
(1) Color-developing rates of the color-developing sheet 
A commercially available CB sheet (NW-50T, a product of Jujo Paper Co., 
Ltd.) containing crystal violet lactone as a main pressure-sensitive dye 
and a sample color-developing sheet are laid so that the coated surfaces 
of the sheets contacted each other. A pressure is applied to the multiply 
sheet by an electric typewriter to form a cobalt blue color. The 
reflectance I.sub.o of the sheet before color formation, the reflectance 
I.sub.1 of the sheet two minutes after color development, and the 
reflectance I.sub.2 of the sheet 24 hours after color development are 
measured by using a Hunter Reflectmeter (using an amber filter). The 
initial color-developing rate (J.sub.1) and the ultimate color-developing 
rate (J.sub.2) are calculated in accordance with the following equations. 
##EQU1## 
Higher initial and ultimate color-developing rates are preferred. 
(2) Retention of color intensity (Light-fade resistance) 
The color-developing sheet having a color formed by the method described in 
(1) is exposed to a fade-o-meter for 2 hours and 6 hours, respectively. 
The color-developing rates of the sheets are calculated in the same way as 
in (1) above as J.sub.3 and J.sub.4. The retention of color is calculated 
in accordance with the following equations. 
##EQU2## 
Higher color retention values are preferred. 
(3) Retention of whiteness (Resistance to yellowing) 
The color-developing sheet before color formation is exposed to a 
fade-o-meter for 6 hours. The reflectance K.sub.1 of the sheet before the 
exposure, and its reflectance K.sub.2 after the exposure are measured by 
using a Hunter Reflectmeter (using a blue filter). The retention of 
whiteness (H) is calculated in accordance with the following equation. 
##EQU3## 
Higher whiteness retention values show less yellowing of the sheet under 
light. 
(4) Plasticizer resistance of a colored image 
A small amount of dioctyl phthalate (a plasticier for vinyl chloride 
resins) is coated on the colored surface of the color-developing sheet 
after color formation by the method described in (1) above. A change in 
density of the color was observed with unaided eyes immediately after 
coating and two weeks after coating. 
(5) Water resistance of colored image 
The color-developing sheet after color formation by the method described in 
(1) is dipped in water. A change in the density of the color was observed 
with unaided eyes two 1 hours after dipping. 
__________________________________________________________________________ 
Plasticizer resis- 
Color tance of colored 
Water 
developing agent White- 
image resistance 
Amount 
Color ness 
Immedia- of colored 
Color-de- (parts 
developing 
Retention of 
reten- 
tely 2 weeks 
image after 
veloping 
Compound 
by rates (%) 
color (%) 
tion 
after 
after 
2 hour 
Sheet* 
No. weight) 
J.sub.1 
J.sub.2 
2 hrs 
6 hrs 
(%) coating 
coating 
dipping 
__________________________________________________________________________ 
Ex. 1 
1 24.5 40.9 48.0 
52.1 
15.8 
90.7 
No Slight 
No decrease 
decrease 
decrease 
in color 
in color 
in color 
density 
density 
density 
No Slight 
Ex. 2 
2 24.5 30.6 46.4 
52.3 
16.1 
90.3 
decrease 
decrease 
No decrease 
in color 
in color 
in color 
density 
density 
density 
No Slight 
Ex. 3 
3 24.5 39.0 43.4 
52.0 
15.2 
86.5 
decrease 
decrease 
No decrease 
in color 
in color 
in color 
density 
density 
density 
No Slight 
Ex. 4 
4 24.5 35.0 43.5 
52.3 
15.5 
86.5 
decrease 
decrease 
No decrease 
in color 
in color 
in color 
density 
density 
density 
CEx. 1 
Free 24.5 no color 
-- -- -- -- -- -- 
sulfone formed 
CEx. 2 
Free 24.5 no color 
-- -- -- -- -- -- 
sulfone formed 
Cex. 3 
Known 24.5 41.0 47.0 
50.7 
10.7 
82.0 
Marked 
Marked 
No decrease 
color decrease 
decrease 
in color 
develop- in color 
in color 
density 
ing agent density 
density 
CEx. 4 
Known 24.5 23.5 46.2 
68.3 
13.4 
90.0 
Slight 
Marked 
No decrease 
color decrease 
decrease 
in color 
develop- in color 
in color 
density 
ing agent density 
density 
__________________________________________________________________________ 
*Ex. = Example; 
CEx. = Comparative Example 
It can be seen from the table that the color-developing sheets of this 
invention have a marked improvement in color-forming properties than the 
color-developing sheets of Comparative Examples 1 and 2 (prepared in the 
same way as in Example 1 using free sulfones which are precursors of the 
color-developing agents of the present invention). In other words, while 
the free sulfones have no color-forming properties, the zinc salts thereof 
have very good color-forming properties. Furthermore, the color-developing 
sheets of this invention have equivalent or slightly better color-forming 
properties (the color-developing rate) and water resistance to or than the 
color-developing sheets of Comparative Examples 3 and 4 (color-developing 
sheets containing known color developing-agents), but have much better 
light fading resistance (color retention), resistance to yellowing under 
light (whiteness retention) and plasticizer resistance than the latter. 
Since the color produced by the color-developing sheet of this invention is 
stable to light and plasticizers or the like and scarcely undergoes a 
decrease in density, it can be used in applications for which conventional 
color-developing sheets are unsuitable because of the need for long-term 
storage stability. Hence, the color-developing sheet of this invention has 
a very great practical significance.