Paper coating composition comprising PVA containing acetoacetyl group and a pigment

A paper coating composition comprising a polyvinyl alcohol resin containing acetoacetyl group prepared in the presence of a pigment having a pH of not more than 8. The composition of the invention gives heat sensitive recording paper which has increased color density, excellent plasticizer resistance and low adhesiveness to a thermal print head.

BACKGROUND OF THE INVENTION 
The present invention relates to a paper coating composition suitably 
employed for improving properties of various papers such as printability, 
and more particularly to a paper coating composition employed for 
improving printability, plasticizer resistance, oil resistance, solvent 
resistance and water resistance of a heat sensitive recording paper. 
A recording process utilizing heat has been utilized more and more because 
it is advantageous in comparison with a conventional recording process 
which requires inks, development and fixed process. Particularly, the 
thermal recording paper having a heat sensitive layer in which a color 
forming material such as Crystal Violet lactone and a color developing 
material such as a phenol compound are included and they are thermally 
reacted to develop a color, is highly estimated and has been put to 
practical use. 
As a binder for the above-mentioned color forming material and color 
developing material, a water-soluble binder such as polyvinyl alcohol 
(hereinafter referred to as "PVA") is widely employed. However, the PVA 
has defects that dust of PVA may make dirty a thermal print head during a 
long time recording and that the printed letter disappear and the 
recording paper grows sticky in contact with water or plasticizers. 
Recently, in order to solve the problems, there has been proposed a 
recording paper in which acetoacetyl group-containing PVA is employed as a 
binder of the heat sensitive layer [Japanese Unexamined Patent Publication 
(Tokkyo Kokai) No. 181687/1983], a recording paper in which a protective 
layer composed of a carboxylic acid-modified PVA or acetoacetyl 
group-containing PVA is placed on the heat sensitive layer (Tokkyo Kokai 
No. 106995/1984), and the like, but there is a room for various 
improvements. 
Particularly, the increase of the color density is not always remarkable in 
addition to demands of more improvements in the plasticizer resistance, 
oil resistance and solvent resistance. Also there has been requested a 
paper which does not cause the printed letters to disappear for a long 
time, even if the recording papers are in contact with plastic films or 
sheets including a large amount of plastisizer. 
An object of the invention is to provide a paper coating composition which 
can solve the abovementioned problems. 
The above and other objects of the present invention will become apparant 
from the description hereinafter. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a paper coating 
composition comprising an acetoacetyl group-containing PVA prepared in the 
presence of a pigment having a pH of not more than 8. 
DETAILED DESCRIPTION 
In the present invention, the PVA included in the paper coating composition 
must be prepared in the presence of specific pigments. It has been known 
to employ a mixture of acetoacetyl group-containing PVA and a pigment such 
as clay or calcium carbonate as a binder of heat sensitive layer or a 
protective layer of the heat sensitive recording paper, but such a method 
by mere mixing is hard to give excellent effects as obtained in the 
present invention. 
A method for coexisting the pigment with the PVA is not limited. For 
instance the pigment is made to exist at the time when a vinyl ester such 
as vinyl acetate is polymerized, at the time when polyvinyl ester is 
hydrolysed, or at the time when the PVA is aceto-acetylated by reacting 
with diketene. Among them, it is preferable that the pigment exists at the 
time when the vinyl ester is polymerized, from the points of effective 
preparations and properties of the obtained paper coating composition. 
The PVA employed in the invention includes not only partly hydrolysed 
polyvinyl acetate or completely hydrolysed polyvinyl acetate but also 
hydrolysed copolymer of vinyl acetate and a monomer copolymerizable with 
vinyl acetate, e.g., olefins such as ethylene, propylene, isobutylene, 
.alpha.-octene, .alpha.-dodecene or .alpha.-octadecene; an unsaturated 
acid such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, 
maleic anhydride or itaconic acid, a salt thereof, a monoalkyl ester 
thereof or a dialkyl ester thereof; a nitrile such as acrylonitrile or 
methacrylonitrile; an amide such as acrylamide or methacrylamide; an 
olefin sulfonic acid such as ethylenesulfonic acid, allylsulfonic acid or 
methallylsulfonic acid or a salt thereof; an alkyl vinyl ether, a 
vinylketone, N-vinylpyrrolidone, vinyl chloride, vinylidene chloride, but 
the PVA is not limited thereto. 
It is preferable that the PVA used in the invention has an average degree 
of hydrolysis of 70 to 100% by mole, more preferably 85 to 100% by mole, 
and an average degree of polymerization of 100 to 3,000, more preferably 
400 to 2,000. 
Optional methods can be carried out for introducing acetoacetyl group into 
the PVA. It is preferable that diketene is reacted with the PVA, or the 
PVA is subjected to ester-interchange reaction with acetoacetic acid 
ester. 
The content of acetoacetyl group is from 0.05% by mole based on the PVA to 
the maximum within range where the obtained acetoacetyl group-containing 
PVA is water-soluble. Generally, it is preferable that the content is from 
0.1 to 20% by mole, more preferably 0.5 to 10% by mole. When the content 
of acetoacetyl group is less than 0.05% by mole, the effect of the 
invention is hard to obtain, and if the content is more than 20% by mole, 
the desired effects are hardly improved. 
The pigment used in the invention must have a pH of not more than 8. The pH 
value of the pigment is determined by measuring the pH of a 10% by weight 
aqueous dispersion of the pigment. Examples of the pigment are, for 
instance, silicic acid anhydride (colloidal silica), kaolinite clay, 
calcined kaolinite clay, pyrophyllite, hydrated halloysite, sericite clay, 
sericite, talc, titanium oxide, and the like. When a pigment having more 
than pH 8 such as calcium carbonate, aluminum silicate, calcium silicate 
or aluminum hydroxide is employed, the stability of aqueous coating 
solution lowers, a pot life of the coating solution shortens upon 
employing together with a cross-linking agent, and the smoothness and 
color density of the recording paper become remarkably poor. 
It is preferable that the amount of the pigment is from 5 to 300 parts by 
weight, more preferably 5 to 100 parts by weight, based on 100 parts by 
weight of the acetoacetyl group-containing PVA. When the amount is less 
than 5 parts by weight, the sticking occurs, and on the other hand, when 
the amount is more than 300 parts by weight, the color density and the 
plasticizer resistance become remarkably poor. 
When the paper coating composition of the invention is applied to heat 
sensitive recording papers, the paper coating composition is coated onto a 
heat sensitive layer composed of a color forming material, a color 
developing material and a binder to form a protective layer, and also the 
paper coating composition is employed as a binder in the heat sensitive 
layer, but the former is more efficient than the latter in order to obtain 
the effect of the invention. 
The process in which the coating composition is employed as the protective 
layer is concretely explained. 
When a coating solution for forming the heat sensitive layer is prepared, 
an aqueous dispersion of the color forming material and an aqueous 
dispersion of the color developing material are respectively prepared, the 
aqueous dispersions are finely pulverized by thoroughly stirring, and then 
the dispersions are mixed and stirred to give a uniform costing 
disperison. 
It is preferable that the amounts of the color forming material and color 
developing material are from 3 to 20 parts by weight and from 10 to 60 
parts by weight, respectively, based on 100 parts by weight (whole solid 
matters) of coating dispersion. 
A binder is added to at least one of the aqueous dispersion of the color 
forming material and the aqueous dispersion of the color developing 
material. It is preferable that the amount of the binder is from 10 to 
200% by weight based on the total amount of the color forming material and 
the color developing material. The solid content of the coating dispersion 
is selected from the range of 10 to 40% by weight, from the point of 
workability. Examples of the binder employed in the present invention are, 
for instance, a PVA, a modified PVA such as carboxyl group-containing PVA, 
a acetoacetyl group-containing PVA, a methyl cellulose, a carboxymethyl 
cellulose, a starch, a latex, and the like. 
Examples of the color forming material used in the invention are, for 
instance, a leuco form of a triphenylmethane dye such as 
3,3-bis(p-dimethylaminophenyl)-phthalide, 
3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide [Crystal Violet 
lactone], 3,3-bi(p-dimethylaminophenyl)-6-diethylaminophthalide, 
3,3-bis(p-dimethylaminophenyl)-6-chlorophthalide, 
3-dimethylamino-6-methoxyfluoran, 7-acetoamino-3-diethylaminofluoran, 
3-diethylamino-5,7-dimethylfluoran, 3,6-bis-.beta.-methoxyethyoxyfluoran, 
or 3,6-bis-.beta.-cyanoethoxyfluoran, and the like, but the color forming 
materials are not limited thereto. 
The color developing material employed in the invention includes a compound 
which reacts with the above-mentioned color forming material by heating 
and which is changeable to liquid or gaseous state at more than room 
temperature, preferably at more than 70.degree. C. Examples of the color 
developing material are, for instance, phenol, p-methylphenol, 
p-tert-butylphenol, p-phenylphenol, .alpha.-naphthol, .beta.-naphthol, 
4,4'-isopropylidenediphenol[bisphenol A], 
4,4'-secondarybutylidenediphenol, 4,4'-cyclohexylidenediphenol, 
4,4'-isopropylidenebis-(2-tert-butylphenol), 
4,4'-(1-methyl-n-hexylidene)-diphenol, 4,4'-isopropylidenedicatechol, 
4,4'-pentylidenediphenol, 4,4'-isopropylidenebis-(2-chlorophenol), 
phenyl-4-hydroxybenzoate, salicylic acid, 3-phenylsalicylic acid, 
5-methylsalicylic acid, 3,5-di-tert-butylsalicylic acid, 1-oxy-2-naphthoic 
acid, m-hydroxybenzoic acid, 4-hydroxyphthalic acid, gallic acid, and the 
like, but the color developing material are not limited thereto. 
A paper on which the boating dispersion is coated is not particularly 
limited and optional papers can be employed. When the coating dispersion 
is coated on the paper, conventional and optional methods such as a roll 
coating method, and air-knife coating method and a blade coating method 
can be carried out. It is preferably that a quantity(net)for application 
of the coating dispersion is from about 1 to about 20 g/m.sup.2, more 
preferably from about 3 to about 10 g/m.sup.2. 
The paper coating composition of the invention is coated on the thus 
obtained heat sensitive layer as a protective layer. In coating, any 
methods such as a roll coating method, an air-knife coating method and a 
blade coating method can be adopted. It is preferable that the 
concentration of the paper coating composition is from 1 to 10% by weight 
from the point of the workabiity. It is preferable that the 
quantity(net)for application of the paper coating composition is from 
about 0.5 to about 5 g/m.sup.2. The desired coating layer is fromed by 
air-drying or lightly heating. 
The acetoacetyl group-containing PVA of the invention may be admixed with 
various conventional auxiliaries. Also, the auxiliaries may be coated 
before or after coating of the acetoacetyl group-containing PVA. For 
instance, a conventional compound used for giving the water resistance to 
the aceoacetyl group-containing PVA e.g. glyoxal, methylolmelamine, 
pottassium persulfate, ammonium persulfate, sodium persulfate, a metal 
salt such as ferric chloride or magnesium chloride, ammonium chloride, 
formaline, glycine, a glycidyl ester, a gylcidyl ether, dimethylol urea, 
ketene dimer, boric acid or borax is employed, whereby the water 
resistance of the obtained paper is extremely increased without making 
dirty the thermal print head. Also, conventional additives, e.g. a 
thermoplastic substance such as higher fatty acid amide, a filler such as 
calcium carbonate or titanium dioxide and a dispersing agent may be 
optionally employed. 
Next, the case where the paper coating composition is employed as a binder 
in the heat sensitive layer of the heat sensitive recording paper is 
explained. 
When a coating dispersin for forming the heat sensitive layer is prepared 
by employing the paper coating composition of the invention as a binder, 
an aqueous dispersion of a color forming material and an aqueous 
dispersion of a color developing material are prepared respectively, the 
aqueous dispersions are finely pulverized by thoroughtly stirring, and 
then the dispersions are mixed and stirred to give a uniform coating 
dispersion. the acetoacetyl group-containing PVA is added to at least one 
of the dispersion of the color forming material and the dispersion of the 
color developing material. It is preferable that the amount of the 
acetoacetyl group-containing PVA is from 10 to 200% by weight based on the 
total amount of the color forming material and the color developing 
material. The solid contet of the caoting dispersion is selected from the 
range of 10 to 40% by weight, from the point of the workability. 
there may be optically added a conventional compound used for giving the 
water resistance to the acetoacetyl group-containing PVA, e.g. glyoxal, 
methylolmelamine, pottassium persulfate, ammonium persulfate, sodium 
persulfate, a metal salt such as ferric chloride or magnesium chloride, or 
ammonium chloride, and moreover, a conventional additives, e.g. a 
thermoplastic substance such as a higher fatty acid amide, a filler such 
as calcium carbonate or titanium dioxide and the dispersing agent to the 
coating dispersion. 
Also, as the binder, there are optionally mixed a PVA, a modified PVA such 
as carboxyl group-containing PVA, a methyl cellulose, carboxymethyl 
cellulose, starch, latex, and the like with the acetoacetyl 
group-containing PVA of the invention. 
The paper on which the coating dispersion is coated is not particularly 
limited and optional papers can be employed. When the coating dispersion 
is coated on the paper, conventional and optional methods such as roll 
coating method, air-knife coating method or a blade coating method can be 
carried out. It is preferable that the amount of the coating dispersion is 
from 1 to 20 g/m.sup.2, more preferably 3 to 10 g/m.sup.2. 
The paper coating composition of the present invention can be applied to 
optional papers such as a paper for information industry such as ink-jet 
recording paper, a pasteboard such as Manila board, white board or liner, 
a fine paper or a paper for printing such as photogravure paper. 
Also, the paper coating composition of the invention can be applied to a 
composite composed of a substrate such as film, non-woven cloth or sheet 
as well as papers. 
The present invention is more specifically described and explained by means 
of the following Examples in which all percents and parts are by weight 
unless otherwise noted. It is to be understood that the present invention 
is not limited to Examples, and various changes and modifications may be 
made in the invention without departing from the spirit and scope thereof.

EXAMPLE 1 
[Preparation of acetoacetyl group-containing PVA] 
There were mixed 100 parts of vinyl acetate, 50 parts of methanol and 40 
parts of silicic acid anhydride (pH 6.3) and the mixture was polymerized 
in the presence of azobisisobutyronitrile as a polymerization initiator at 
56.degree. to 58.degree. C. for 6 hours. The polymerization reaction was 
stopped at the time when the polymerization ratio is 75% and thereafter, 
methanol vapor was passed through the mixture in order to remove 
unpolymerized vinyl acetate to give a methanol solution of polyvinyl 
acetate. 
The obtained methanol solution was subjected to hydrolysis reaction 
according to a conventional method to give a PVA having an average degree 
of hydrolysis of 98.5% by mole and an average degree of polymerization of 
1,100. 
A mixer was charged with 196 parts of the obtained PVA powder, and 16.8 
parts of liquid diketene was added with spraying to the mixer for 30 
minutes while stirring at 60 rpm. Then, the mixture was heated up to 
60.degree. C. and reacted for 3 hours to give an acetoacetyl 
group-containing PVA having a content of acetoacetyl group of 3.3% by 
mole. A proportion of the pigment to the acetoacetyl group-containing PVA 
is 100 parts of pigment to 100 parts of the PVA. 
______________________________________ 
[Preparation of heat sensitive recording paper] 
______________________________________ 
Solution (A) 
Crystal Violet lactone 10 parts 
5% Aqueous solution of PVA 
10 parts 
Average degree of hydrolysis: 98% by mole 
Average degree of polymerization: 1,100 
Water 15 parts 
Solution (B) 
Bisphenol A 50 parts 
5% Aqueous solution of PVA 
50 parts 
(the same PVA as the solution (A)) 
Water 75 parts 
______________________________________ 
The solutions (A) and (B) were respectively pulverized by employing a sand 
grinder until an average particle sizes of Crystal Violet lactone and 
bisphenol A were about 3.mu., and then, the solution (A), the solution 
(B), 50 parts of calcium carboxide and 270 parts of 15% aqueous solution 
of PVA having the same average degree of hydrolysis and average degree of 
polymerization as solution (A) were mixed to give a coating solution. 
The obtained coating solution was coated as a heat sensitive layer on a 
substrate having a basis weight of 50 g/m.sup.2 so that a quantity net for 
application was 5.0 g/m.sup.2 and it was dried. 
Further, 10% aqueous solution of acetoacetyl group-containig PVA prepared 
in the presence of silicic acid anhydride was overcoated as a protective 
layer by employing a Dixon coater in a quantity for application of 5 
g/m.sup.2 (net), and it was air-dried to give a heat sensitive recording 
paper. 
With respect to the obtained heat sensitive recording paper, the color 
density, the plasticizer resistance and the state of the thermal print 
head were estimated as follow: 
[(1) Color density] 
Printing is carried out by employing a heat-gradient tester (made by 
Kabushiki Kaisha Toyo Seiki Seisakusho) under a condition of a temperature 
of 120.degree. C., a load of 2 kg/cm.sup.2 and a time of 10 seconds, and a 
color density is measured by employing a densitometer (Model RD-100R, 
product of Macbeth Co., Ltd.) with use of an anber filter. 
[(2) Plasticizer resistance] 
The printed heat sensitive recording paper is interposed with soft 
polyvinyl chloride sheets and it is allowed to stand for 168 hours while 
adding a load of 20 g/cm.sup.2. A percentage of a color density after 168 
hours to an original color density is shown as a ratio of remained color. 
[(3) State of thermal print head] 
After striped pattern is printed on the sensitive recording paper of 100 m 
continuously by facsimile, the sticking or adhesiveness of resin dust is 
observed on the thermal print head. 
The resuls are shown in Table 1. 
COMATIVE EXAMPLE 2 
The procedure of Example 1 was repeated except that a mixture of 100 parts 
of silicic acid anhydride (pH 6.3) and 100 parts of acetoacetyl 
group-containing PVA (a content of acetoacetyl group: 3.5% by mole, an 
average degree of hydrolysis: 98.5% by mole, an average degree of 
polymerization: 1,100) was employed instead of the 10 aqueous solution of 
acetoacetyl group-containing PVA prepared in the presence of silicic acid 
anhydride coated as a protective layer. 
The results are shwn in Table 1. 
EXAMPLES 2 TO 4 AND COMATIVE EXAMPLE 2 
The procedure of Example 1 was repeated except that a coating solution 
shown in Table 1 was coated as a protective layer. 
The results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Protective layer 
Acetoacetyl group-containing PVA 
Thermal recording paper 
Content of aceto- 
Average degree 
Average Plasticizer 
State ofce 
Pigment acetyl group 
of hydrolysis 
degree of 
Color 
(Ratio of thermal 
Kind Content*.sup.1 
(% by mole) 
(% by mole) 
polymerization 
density 
remained color 
print 
__________________________________________________________________________ 
head*.sup.2 
Ex. 1 
Silicic acid 
100 3.3 98.5 1,100 1.53 
100 .circleincircle. 
anhydride 
(pH 6.3) 
Ex. 2 
Kaolinite clay 
200 6.0 88.0 1,800 1.49 
100 .circleincircle. 
(pH 7) 
Ex. 3 
Pyrophellite 
150 4.5 95.0 800 1.51 
100 .circleincircle. 
(pH 6.5) 
Ex. 4 
Sericite clay 
50 1.0 99.0 500 1.55 
100 .circleincircle. 
(pH 6.8) 
Com. 
Silicic acid 
100 3.5 98.5 1,100 1.37 
89 .DELTA. 
Ex. 1 
anhydride 
(pH 6.3) 
Com. 
Calcium 
50 1.5 88.0 500 1.35 
93 .times. 
Ex. 2 
carbonate 
(pH 9.5) 
__________________________________________________________________________ 
*.sup.1 Part based on 100 parts of acetoacetyl groupcontaining PVA 
*.sup.2 Estimation: 
.circleincircle. No sticking 
.DELTA. Considerable sticking 
.times. Marked sticking 
EXAMPLE 5 
The acetoacetyl group-containing PVA prepared in Example 1 was employed as 
a binder to give a heat sensitive recording layer 
______________________________________ 
Solution (A) 
Crystal Violet lactone 
10 parts 
5% Aqueous solution of the 
10 parts 
acetoacetyl group-containing 
PVA prepared in Example 1 
Water 15 parts 
Solution (B) 
Bisphenol A 50 parts 
5% Aqueous solution of the 
50 parts 
acetoacetyl group-containing 
PVA prepared in Example 1 
Water 75 parts 
______________________________________ 
The solutions (A) and (B) were respectively pulverized by employing a sand 
grinder until an average particle size of Crystal Violet lactone and 
bisphenol A about 3.mu., and then, the solution (A), the solution (B) and 
500 parts of 15% aqueous solution of the acetoacetyl group-containing PVA 
(the same PVA as the solutions (A) and (B) were mixed to give a coating 
solution. 
The obtained coating solution was coated as a heat sensitive layer on a 
substrate having a basis weight of 50 g/m.sup.2 so that a quantity for 
application after drying was 7.5 g/m.sup.2 (net) and it was dried to give 
a heat sensitive recording paper. 
The obtained heat sensitive recording paper was overcoated in the same 
manner as in Example 1 and its properties were estimated. 
The obtained heat sensitive recording paper had a color density of 1.55, a 
plasticizer resistance of 100 and a state of the thermal print head of 
.circleincircle. .